Author Archives: Dr. Zoomie

Cosmic Rays

I’m a geek. I’ve gotten used to this fact, as have my children, my wife, and other family and friends. And it’s why, when I fly, sometimes I monitor radiation levels. I’ve learned that this can sometimes make the person sitting next to me a little nervous – I’ve learned not to talk about it much. Luckily my radiation detector has a cell phone app so I can leave it in my carry-on bag and monitor it via Bluetooth from my seat…I just need to remember to turn off the alarm so my bag doesn’t beep annoyingly in the overhead storage bin.

What’s interesting is that, as we ascend after takeoff, radiation levels drop steadily. Actually, this part isn’t very surprising because when we’re on the ground most of the radiation I’m measuring comes from radioactivity in the rocks and soils and, as we gain in altitude we’re getting further and further from the surface, reducing the radiation coming from the rocks and soil. But then when we get to about 10,000 feet the readings stopped dropping – interesting. And by 12,000 feet or so they started rising again – even more interesting. By the time we were at our cruising altitude they were several times higher than they’d been at ground level – not only that, but I was also seeing many more neutrons than I’d seen at the surface. Curious.

What I was seeing was nothing new – except to me the first few times I made these measurements. The first time it was noticed was over a century ago, in 1911 when Austrian physicist Victor Hess was taking a balloon flight, and he had his own radiation instruments with him (albeit without Bluetooth or a phone app) when he noticed the same thing I was to see a century later. Hess was more curious than I was because he was the first person to see or hear of this effect, while I had the benefit of knowing about his work. Hess recognized the physics behind the lowering dose rates that happened at first, but the rising dose rates as he ascended still further had him puzzled. What he finally realized was that he was seeing radiation coming in from outer space – cosmic rays. Then he just needed to figure out what they were.

Hess first thought that the Sun might be the source of this cosmic radiation. But when he arranged to make measurements during a solar eclipse he saw the same effect – while he still didn’t know their source, he could rule out the Sun – cosmic rays apparently originated from somewhere outside of our Solar System. Hess went on to be awarded the 1936 Nobel Prize in Physics (https://www.nobelprize.org/prizes/physics/1936/summary/) for this discovery.

(https://phys.org/newman/gfx/news/hires/2012/1_1911_1912_hess_ballon.jpg)

It turns out that I know more about cosmic rays than Hess did. That’s not because I’m smarter than he was (and goodness knows, I’m not!). Rather, it’s because I’ve been able to read over a century of research on the subject, much of it using instruments that Hess never even dreamed of. Not only that, but cosmic rays have been studied by physicists and astronomers – even by geologists – who have teased out details far beyond the science of Hess’ day.

One of the things we’ve discovered, for example, is that most of the cosmic radiation we’re exposed to originates elsewhere in our galaxy in the form of high-energy particles blasted into space by exploding stars. These are the nuclei of atoms that have had all of their electrons stripped away and that have sailed through space until they encounter our planet – they have so much energy that they punch through the Earth’s magnetic field and stream into our atmosphere. There, they’re likely to smash into an atom in the atmosphere, initiating a chain reaction of events giving rise to a cosmic ray air shower (https://scied.ucar.edu/image/cosmic-ray-air-shower) – a cascade of gamma rays and particles, some of which can create radioactive tritium (H-3) and carbon-14. Other particles can smash into airplane fuselages (where, among other things, they show up on my radiation detector) and still more filter down to sea level to give us radiation dose of between 20-30 mrem annually.

Something else we’ve learned over the decades is that cosmic radiation accounts for about 10% of our annual exposure to natural background radiation – and that this remains relatively constant throughout the solar cycle (the decadal waxing and waning of solar activity). The reason for this is that, when Solar activity is low, the relatively anemic charged particles from the Sun can’t penetrate to sea level, but the much higher-energy galactic cosmic rays can. On the other hand, when the Sun’s activity is high the solar cosmic rays are more energetic and more of them can penetrate more deeply into the atmosphere, exposing us to more radiation from the Sun; at the same time the stronger solar wind helps to sweep the galactic cosmic rays from the inner solar system, reducing our exposure to them. These two factors tend to cancel each other out with the net result that our exposure to cosmic radiation remains relatively constant throughout the Solar cycle.

We’ve also found out that cosmic rays can induce radioactivity in rocks that are exposed at the surface – cosmic ray air showers produce neutrons and when neutrons are captured by stable atoms they can become radioactive. My MS advisor made use of this fact – it turns out that you can tell how long ago a rock first became exposed to cosmic rays by studying the induced radioactivity and this can be used to determine the rate at which glaciers are retreating. By analyzing rocks from Antarctica, for example, my advisor was able to tell when rocks were last covered by glaciers; using similar analyses a friend of mine was able to tell when the glaciers melted back from his front yard in Kansas.

Another Kansan, astronomer Adrian Melott has other speculations about cosmic rays – he thinks they might be linked to the manner in which life has evolved on Earth. He notes that our Solar System bobs up and down through the galactic disk every 40-50 million years or so and postulates that when we’re on “top” of the disk (the side facing the direction of our travel through intergalactic space) we might be exposed to more cosmic radiation than when we’re on the “bottom” of the disc, and that this extra smidgeon of radiation might be enough to trigger faster rates of evolution. And there does seem to be some correlation between the fossil record and the timing of our excursions above the plane of our galaxy…so he might have a point.


But then there’s the stuff we’re still trying to figure out….

On October 15, 1991 a cosmic ray observatory (the Fly’s Eye camera – https://en.wikipedia.org/wiki/High_Resolution_Fly%27s_Eye_Cosmic_Ray_Detector) in the Utah desert recorded the debris of a cosmic ray with a staggering amount of energy – in physics terms it was about 300 billion billion (3×1020) electron volts; in more prosaic terms this was a single atom that packed the same wallop as a Little League pitcher’s fastball. Not only was this unprecedented, but it was also beyond anything science could explain.

The first thing that science was unable to explain was how an atom could have been endowed with so much energy. First, even exploding stars – supernovae – lack the power to impart so much energy to a single atom, and scientists were having problems thinking of any other phenomena that could do the trick. But then there was another problem – even if they could find a mechanism to accelerate an atom to so high an energy, it should have been whittled away while the atom was in transit across the galaxy through collisions with the rare atoms of hydrogen or even by running into photons – for the most energetic particles in the universe, anything they encounter will have a lower energy that will slow them down ever so slightly. As far as scientists could understand there was no way particles with so high an energy could exist…and yet they did.

As things stand now there are a few candidates that might be able to boost particles to so high an energy – the super-massive black holes in the cores of quasars are a candidate, as are the huge lobes of energetic gas near some galaxies that emit powerful beams of radio-frequency radiation. Another hypothesis dates back to the earliest days of the universe – speculating that cosmic-scale shock waves from the epoch of galaxy formation might have accelerated particles to these velocities. This latter is the most ancient, but there are some that are even more exotic, including the decay products from supermassive particles formed by what are called topological defects in the fabric of space itself (and no, I don’t really know exactly what they mean by that either, but it sounds pretty cool). But we still have to figure out how these particles can hold on to their energies for so long – especially when we consider that they seem to originate from outside of our own galaxy.

(a radio galaxy – the jets that form the large lobes might be one source of ultra-high energy cosmic rays https://www.nasa.gov/images/content/709514main_hs-2012-47-a-print.jpg)

Interestingly, these highest-energy particles provide a degree of comfort to the designers of our highest-energy particle accelerators. Every now and again when a new accelerator pushes to higher energy levels there’s a concern that it might produce new types of particles (https://phys.org/news/2014-02-chances-particle-collider-strangelets-earth.html) or miniature black holes or that it might even damage the structure of space – any of which could be dangerous. So physicists examine the matter – to date they’ve always decided that the new accelerator was safe to operate. Of course, they might have made a mistake, or there might be gaps in our understanding of the physics. But then they remember the ultra-high energy cosmic rays – boosted by mechanisms we still don’t understand to energies a million times greater than any accelerator we can yet make. The fact that the Earth still exists tells us that even these highest-energy particles can’t do extensive harm, which tells us that our accelerators are most likely safe to operate.

Sometimes when I’m flying and looking at my radiation instruments I think about a star that exploded somewhere in space – maybe in our galaxy, maybe in a galaxy halfway across the universe – and that spat out a flurry of atoms at incredible velocities. I think about these atoms speeding through space for eons – maybe since before our planet was even born – deflected time after time by the magnetic fields of stars, of clouds of interstellar gas, and that are woven into the fabric of our galaxy and in the spaces between the galaxies. And after so long a journey, to come to a rest in my radiation detector…just seems too mundane an end for so long and exotic a trip.

Oklo’s Natural Nuclear Fission Reactors

A bit over six billion years ago a star exploded somewhere in our galaxy – we can’t be sure where the star was so long ago, but we know that it was within shouting distance (on a galactic scale) of a cloud of gas and dust. A shock wave from the supernova slammed into the cloud, compressing it and setting in motion a series of events that would, millions of years later, lead to the birth of a star and the formation of a number of planets – we’re currently all sitting on the surface of one of those planets.

When stars are active they produce energy by fusion – first of hydrogen, then of helium, and working up to iron. But fusing iron doesn’t produce energy, it sucks it up – when a star starts burning iron it collapses, forming a neutron star or a black hole and the outer layers rebound out into space, blowing the rest of the star apart into space. During this explosion, every element heavier than iron is formed, including gold, lead, and uranium. This debris emerges as a shock wave that can trigger the collapse of existing clouds of gas and dust; whatever is left will eventually become part of such a cloud itself. Thus, the cloud from which the Solar System formed contained uranium, and the shock wave that caused its collapse contained still more.

Over time, as the Earth cooled it began to solidify, the first rocks forming a thin skin that floated like pond scum atop the underlying magma as the magma itself circulated, driven by convection. As the world churned the various elements began sorting themselves out with the large atoms (including uranium) partitioned into the solid rocks of the nascent crust, iron and nickel sinking to form the core, with the mantle lying in between.

Over the eons, the rocks that contained the uranium began to weather and the grains bounced their way down the stream beds, collecting in areas where the speed of the water slowed, just as grains of gold do. But the early atmosphere lacked oxygen, as did the water and since uranium is insoluble in anoxic water the grains just sat there. And then, two billion years ago, that changed – photosynthesis produced oxygen that flooded the Earth’s atmosphere and saturated the water and the uranium began to dissolve, precipitating out again in places where chemical reactions had again stripped oxygen from the water.

One such location was in a part of the planet that would one day become part of the nation of Gabon. Here, the water was percolating through sandstone and depositing nodules of uranium in the vicinity of some hydrocarbon deposits, bathed with groundwater. And this is where things start to get interesting!

Nuclear reactors consist of clumps of uranium surrounded by water – when a uranium atom fissions it emits (among other things) neutrons. The neutrons bounce off the hydrogen atoms in the water molecules and slow down, just as a cue ball slows down as it bounces off the other balls of the same size – this is important because slow neutrons are more likely to be absorbed by a uranium atom and to cause it to fission. If one of those neutrons is absorbed by another uranium atom and causes a second fission the reactor is said to be critical. In our sandstone, the uranium nodules were surrounded by water…just as in a nuclear reactor. And, just as in a nuclear reactor, the water slowed down the fission neutrons to the point where they could be absorbed by another uranium atom.

Uranium is comprised of two isotopes – one has a mass of 235 atomic mass units (AMU) and the other is slightly heavier with a mass of 238 AMU. The lighter atoms of U-235 fission fairly easily, but they account for less than 1% of uranium atoms found in nature – too few to sustain a criticality. That’s why we have to enrich uranium – to increase the amount of U-235 to somewhere around 3-6% of the uranium atoms. And this is the last piece of the puzzle.

U-235 and U-238 have astonishingly long half-lives – about 4.5 billion years for U-238 and a mere 700 million years for U-235. This means that the fissile U-235 decays more rapidly than does U-238; calculating the amount of each that was present 2 billion years ago reveals that natural uranium at that time was around 3.5%…at the lower end of the band of concentrations we have in reactor fuel today. Thus, in one place there were lumps of uranium containing enough U-235 to sustain a chain reaction that was sitting in a water-saturated sandstone formation. And about 1.8 billion years or so ago, enough uranium had precipitated that the occasional spontaneous fission sparked a chain reaction – the uranium ore deposit had become a reactor.

There was one moment in time when this could have happened. Before about 2.2 billion years ago there wasn’t enough oxygen in the environment to mobilize the uranium and after about 1.5 billion years ago there was too little U-235 to sustain a chain reaction. But for about 700 million years the Earth could make it work.

Fast-forwarding to the 1970s, French geologists located a rich uranium ore deposit in Gabon that they started to extract at the Oklo uranium mine. As the uranium was extracted and enriched the radiochemists carefully tested the uranium enrichment at various stages of the process and were surprised to see that there was less U-235 than they expected. Through some impressive investigatory work they came to realize that what they were mining was the remnants of a natural nuclear reactor – the first (and, to date, only) ever found. And in the 40-odd years since it was discovered further study has helped to tease out some of the details of its operation.

As the reactor operated the fissions heated up the water percolating through the sandstone; as it warmed up it became less dense – it might even have boiled off entirely from time to time. When this happened, the neutron moderation ground to a halt and the reactor shut down. When the rocks and the water cooled the chain reaction restarted and the reactor would fission some more. It seems to have continued in this vein for about 100,000 years. And that leads us to another intriguing detail – with some implications for our storage of nuclear waste.

The natural nuclear fission reactors of Oklo: (1) Nuclear reactor zones (2) Sandstone (3) Uranium ore layer (4) Granite

We have a very good understanding of how fission works and what’s produced when uranium atoms split. And when examined in detail, physicists realized that virtually all of the fission products formed during the millennia of operation are still present in the rock – in spite of being located in porous and fractured rocks that were saturated with water for the better part of 2 billion years. Such a location would never be approved for radioactive waste disposal today yet, in spite of the rocks’ unsuitability, the fission products are still there. With no planning, no engineering, and a lousy (by our standards) location Nature managed to store the waste safely for eons – this bodes well for our ability to store radioactive waste in a well-designed and well-constructed disposal site located in an impermeable rock formation.

And, interestingly, the conditions under which the reactor formed and the rock formation that hosted it are hardly rare…what’s rare is the preservation of such a formation for so long. Earth might have once been littered with natural reactors and we might never know. And think about it – uranium from a dying star flew through space, causing the collapse of the pre-solar nebula (itself the detritus of other dying stars) and formed the Sun and the Earth. There it sat, gradually moving into the continental crust, eroding and collecting in streams…and waiting for oxygen levels to increase to the point where it could form an ore, and waiting until there was enough uranium in a suitable rock formation to achieve criticality. We don’t know how many times it might have happened on Earth. But we do know that it happened once – and that’s pretty cool.

China’s Taishan Nuclear Power Plant

Why we should care about noble gases

So a friend sent me an email on Monday (June 14, 2021) that linked to a news story reporting that China’s Tianshan nuclear reactor seemed to have elevated levels of noble gases in the reactor coolant. This caused some mental alarms to sound – muted at this point, but there nevertheless. But the reason for these alarms isn’t very obvious to the vast majority of the population – and why noble gases should cause any sort of reaction calls for a little explanation. So let’s see if we can figure out where these noble gases come from and what they might portend. But to do that we’ve got to get into things like how fission works, how reactor fuel is made, and how we can use this knowledge. So…let’s get started!

Where noble gases come from

When uranium atoms fission they split into two radioactive atoms that are called fission products (also called fission fragments). These aren’t equal in size – most are clustered around masses of about 135 and 95 atomic mass units (AMU). And as it turns out, there are two noble gases (krypton and xenon) that fall into these peaks. This means that fission causes (among other things) these fission product noble gases to be produced and they accumulate in the fuel as the reactor operates. Most have relatively short half-lives, but some (such as Kr-85) stick around for a while. Luckily they’re normally trapped in the fuel matrix – we’ll talk about that next.

How reactor fuel is made

Reactor fuel is made up of uranium oxide that’s compressed into a pellet that’s an inch or so in diameter and about the same length:


These fuel pellets are loaded into fuel rods that are a few tens of feet in length, clad with an alloy of zirconium, a tough and corrosion-resistant metal. These fuel rods are then assembled into fuel bundles that are arranged in the reactor core in a pattern that will sustain a critical reaction while permitting the fuel to be cooled and that leaves room for neutron-absorbing control rods to be inserted to help control the chain reaction. As long as the cladding remains intact the fission products are contained safely in the fuel pellets. But if the cladding cracks or otherwise becomes compromised then these fission products can be released into the reactor coolant. And the cladding isn’t made of pure zirconium. The process of making it and loading it with fuel results in traces of uranium (called “tramp uranium”) to be present in the cladding.

What it all means

Radioactive krypton and xenon are not found in nature, and they’re certainly not found in the ultra-purified water circulating through the reactor plant. So if we find evidence of these in the reactor fuel it tells us that there might be a crack of some sort that’s letting the fission products into the reactor coolant.

But here’s the thing – the tramp uranium is also fissioning and since it’s in the cladding some of the fission products can shoot out of the cladding and into the coolant. This means that there are always traces of fission product noble gas in the reactor coolant. So simply finding them in the coolant doesn’t necessarily mean that there’s a problem with the fuel – before we can determine that we need to know how much noble gas is in the coolant and how the levels we’re measuring differs from what we normally see.

Something else we have to be aware of is that the amount of this noble gas in the coolant depends on the reactor power – if reactor power increases then we see more in the coolant than we do at low powers. But even more than that – since these radionuclides don’t decay immediately, they can stick around for hours after reactor power drops. So we need to calculate a “power-corrected” fission product activity to see how what’s measured compares to what we expect to see. The power-correction calculations aren’t necessarily highly complex; at the same time, it’s easy to make a mistake, especially if power is changing frequently. On the nuclear submarine I was on power changed frequently – every time we changed speed – and commercial reactor plants change power as well, responding to changing electrical demand during the day.

What’s the situation at Tianshan?

And that brings us to Tianshan. The Tianshan reactors were built by a consortium that includes the French utility EDF (Électricité de France), Framatome (a French design and construction firm), and the Chinese government. Framatome got word that the concentrations of fission product noble gases were higher than expected and they asked the US government for help in interpreting the laboratory results.

The problem is that there’s very little information that’s been released to date. The Chinese government notes that fission product noble gases are normally found in reactor coolant, which is true, and that the levels that have been found are more or less normal. The problem is that we don’t know what the normal levels are on the Tianshan reactor, we don’t know the levels that have been found, and we don’t know the power history so we can’t do the power corrections. So the information that’s available isn’t sufficient to know if the noble gasses are really elevated or if they just seem that way. And without knowing that we don’t know how significant these lab results might be.

What comes next?

This is another question that we can’t really answer at this time, but there are a few possibilities.

If there really is a defective or compromised fuel rod then we’ll likely continue to see noble gas concentrations rise for a while, after which they should stabilize. We might also start to see other radionuclides show up, especially if the crack or defect gets worse over time. And the radionuclides we see ought to follow a predictable pattern – first we can expect to see volatile elements such as iodine and cesium, which are in the fission product peak close to 135 AMU. This is what I saw when I was in the Fukushima area after the accident there – I-131, Cs-134, and Cs-137 were all present, and they were exactly what I expected to see (the device I was using wasn’t the correct instrument to measure noble gases).

At the moment we can be reasonably confident that the reactor isn’t melting down – if that were the case then we’d see a wider variety of radionuclides, including some that have higher melting and vaporization temperatures such as strontium, molybdenum, and so forth. The fact that these have not been reported suggests that the reactor is not suffering a meltdown.

I should also note that fuel element defects are not unheard-of – they don’t happen frequently, but they do happen and they’re not necessarily a disaster. In fact, they happen often enough that the International Atomic Energy Agency has even written a document titled Review of Fuel Element Failures in Water-Cooled Reactors (https://www-pub.iaea.org/MTCD/Publications/PDF/Pub1445_web.pdf). On the other hand, if there’s a problem with a fuel element then the utility will have to do its best to limit future damage to the fuel element and might need to try to locate the exact fuel rod that’s damaged to that it can be removed and replaced. This can be a long and expensive process – if the defect isn’t too bad then the best course of action might simply be to operate the reactor carefully until the next refueling when the damaged fuel can be removed and replaced with a new one.

The bottom line

At the moment we just don’t know very much, which means that there’s far more speculation than fact. It is entirely possible that the levels of fission product noble gases that so concerned Framatome are actually no more than what’s to be expected for the Tianshan reactor. It’s also possible that there is a fuel element defect similar to what a number of other reactors have experienced. Or this could be the prelude to something more serious – there’s just no way to know. But it seems fairly reasonable to assume that, as of now, the reactor is not melting down or we’d be seeing a lot more than noble gas in the coolant.

So at the moment, I’m interested – but I’m not worried. At the same time, I’m going to keep looking for more information to see what else I can learn.

My Day With Radiation

So…I’m a radiation safety professional, which means that I have a bunch of radiation detectors at my home. And every now again I turn on my meters to see what they read – sometimes I’m teaching a class via Zoom, sometimes I’m checking to make sure the instruments are working properly, I might be checking my own radioactive materials that I use for teaching, or sometimes I’m just curious. The other day I was making some measurements and I noticed they were a little higher than I’m used to seeing and it made me think about all the ways that I encounter radiation on a regular basis. And, being a writer, it occurred to me that it might be worthwhile to share with you the sorts of things I run across.

We can start with natural background radiation – every minute of every day we are all exposed to radiation from nature. Potassium, for example, is vital to the proper operation of our bodies (including our hearts and other muscles)…and about one potassium atom in 10,000 is radioactive, exposing us to radiation from within our own bodies. Not only that, but we also have small amounts of radioactive carbon and hydrogen in our bodies – these are formed by cosmic ray interactions in the upper atmosphere and they filter on down to sea level where we breathe them, drink them, and eat them. Incidentally, a colleague of mine once calculated the radiation he received from potassium in his wife’s body due to, as he put it, “spending about 25% of his time at a distance of less than 1 meter from her” (when I asked him about installing lead shielding, he pointed out that the toxicity of lead would be more dangerous than that extra radiation exposure).

The potassium in bananas, salt substitutes, and other high-potassium foods gives us a little radiation as well – as do the traces of radium found in Brazil nuts. Add to that scant amounts of uranium, thorium, radium, and a few other natural heavy elements that lodge in our bones (mostly from breathing and ingesting dust) and we get about 40 mrem every year from radioactivity that’s a part of our bodies.

There’s also cosmic radiation – some from our Sun, but most that originate in exploding stars elsewhere in our galaxy. In fact, every time I fly I can see cosmic radiation exposure increase as we climb to cruising altitude – in 2019 I flew from NYC to Seoul South Korea on a flight that took us over the North Pole and I saw cosmic radiation levels climb even higher as we flew increasingly northward.

Then there’s still more radiation from the rocks and soils as well as from things made from rocks and soil (granite countertops, bricks, concrete, and so forth) – these each account for just under 30 mrem annually. And the radon emanating from the ground exposes us to another 200 mrem a year, although this is variable, depending on the amount of uranium in the soil and the underlying bedrock. All told, we get about 300 mrem (more or less) from natural sources and from things that are built or made of natural materials. And that’s just the start!

In my apartment I’ve got a lot of radiation sources – some of these are pretty common, some are not, but I don’t need to have a radioactive materials license for any of them. There’s the granite countertop that my landlord installed, for example (granite contains potassium as well as uranium and thorium), as well as the brick my building is made of (brick is made of clays that often contain potassium). I’ve got my collection of radioactive rocks and minerals as well – I picked up most of these at rock and mineral shows or shopping online – and I also have a bunch of consumer products, most of which I bought online. Thoriated welding electrodes, “Vaseline glass” and Fiestaware plates colored with uranium, a stainless-steel soap dispenser contaminated with radioactive cobalt, and a few other things.

There used to be even more than this – I recently got a “Revigator” that’s almost 75 years old. The Revigator is a ceramic crock that’s lined with what looks like concrete…except that the concrete is impregnated with radium-bearing rocks. The premise was that people would fill this with water and, overnight, the radium would “invigorate” the water with energy that, when drunk the next day, would improve one’s health. I only made a few measurements on my new acquisition, but it looks like the most radioactive thing in my collection. Having said that, it still gives off too little radiation to pose a risk to me – especially since it sits about six feet from my desk. Another “back in the day” source of radiation were the cathode ray-type television sets and computer monitors – they never gave off enough radiation to cause problems, but they did give off radiation.

Interestingly, a few months ago I turned on one of my radiation detectors and noticed that dose rates were higher than I’m used to seeing. At the time I was borrowing a gamma spectroscopy device from a colleague – I identified the nuclide as I-131, which is commonly used for treating thyroid cancer and other thyroid diseases. My guess is that one of my neighbors was having thyroid problems – I probably could have figured out which one by checking the walls, floor, and ceiling…but decided to leave my neighbors with a bit of privacy, especially since the dose rate wasn’t at all high enough to be a concern (for me or for them). Of course, nuclear medicine is hardly rare – when I was working for the police (as a civilian scientist) I made hundreds of radiation surveys, both on the ground and from the helicopter, and we picked up nuclear medicine patients on our instruments all the time.

This is a photo of my radiation detector display when we were flying circles over the Brooklyn Bridge and the East River. The high readings showed up when we were over the bridge.

Getting back to building materials, granite’s a big one – due to the geochemistry of uranium, thorium, and potassium (and due to the way that minerals crystallize in magma chambers – https://en.wikipedia.org/wiki/Bowen%27s_reaction_series) many light-colored igneous rocks, including the gray, pink, and red granites, have more radioactivity than many other types of rock. Enough, in fact, to sometimes set off radiation alarms for ground-based surveys and to show elevated dose rates from the air. Flying over the granite Brooklyn Bridge, we always saw higher readings than when we flew over the East River; flying over cemeteries gave us higher readings due to all of the granite headstones.

I’ve also been called on to respond to other sources of radiation – loads of ceramic tiles for example that were coated with glaze that included uranium for the bright colors (mostly yellow and orange) it could produce. And then there was a time we detected radiation from industrial radiography – using radioactivity to take images of pipes, welds, structural steel, and the like – anyplace with a lot of construction and a lot of welding is likely to have radiography taking place on a regular basis. Here, too, the radiation levels aren’t nearly high enough to cause problems, providing the radiographer is doing their job properly – in the US and Europe that’s likely to be the case, but there have been radiography accidents in a number of nations over the years.

I teach a lot of classes on radiation safety – many of my students work in industries that use radioactive sources to gauge the levels of tanks or to control various manufacturing processes. If you put anything between a radiation source and a detector, the radiation levels drop – the more material, the more the levels go down. So radiation levels from a source at the top of one side of, say, a tank filled with caustic chemicals will suddenly drop when the tank fills too high, letting operators (or automatic systems) know it’s time to stop filling the tank; a source at the bottom of the tank will keep the tank from emptying out and possibly ruining a pump. Similarly, I’ve seen radioactive sources used to check the levels of beer bottles on an assembly line, to control the thickness of paper or steel, to check the density of soil, even to detect clogged conveyor belts at a gold mine in Nevada. While none of these expose me on a daily basis, they’re examples of how radiation and radioactivity are used on a daily basis in the nooks and crannies of industry.

In fact, radiation is present in all sorts of society’s nooks and crannies, whether it’s the natural radiation we’re exposed to the mountains, in our basements, at high altitudes, or in the bunch of bananas we stash in our kitchens; in our workplaces and in the workplaces of others; or in the hospitals and clinics and contained within the patients who have visited them.

What’s interesting is that people are exposed to more or less radiation depending on where they live, where they work, what they buy, what they do for a living, and so forth…but these don’t seem to affect the rates of cancer. This suggests that the radiation we run into on a daily basis isn’t likely to hurt us.

Looking for U

The 1950s were optimistic years in the U.S. – the economy was booming, we were (more or less) at peace, and the undisputed leader of the free world; only the USSR challenged us militarily. We were optimistic in other areas as well – our scientific and technical capabilities had helped us to win WWII, especially our work on nuclear weapons, and at the time, nuclear energy was still touted as one day being “too cheap to meter.” In January 1954, National Geographic published an article titled Man’s New Servant, the Friendly Atom (https://archive.org/details/jishankhan_hotmail_1954/1954-01/page/n72/mode/2up), touting the many ways that radiological and nuclear science was going to benefit humanity – while it’s hard to picture any magazine today running such a headline, it fit the spirit of the times perfectly. Later that year the National Geographic ran another piece in the same vein – Hunting Uranium around the World (https://archive.org/details/jishankhan_hotmail_1954/1954-10/page/n95/mode/2up); it’s this piece and the sentiment behind it that’s the basis for this article.

To understand the uranium craze we have to put ourselves in the world of the era – before the environmental movement really took off, when atmospheric nuclear weapons testing was seen as a sign of national pride rather than a source of massive environmental impact, and when nuclear energy was seen as the energy source of the future. In the world that was envisioned uranium was going to take the place of oil and coal…if only we could find enough of it to power the world. And in fact, the October 1954 article was even subtitled: “Lured by Rich Rewards, Prospectors Search from Desert to Arctic for New Supplies of This Magic Fuel of the Atomic Age.” Uranium prospecting in the 1950s became a craze; according to author Catherine Caulfield, Americans purchased over 35,000 in 1953 alone. In her book Multiple Exposures, Caulfield quotes Gordon Dean, who chaired the Atomic Energy Commission in the early 19050s:

The security of the free world may depend on such a simple thing as people keeping their eyes open. Every American oilman looking for “black gold” in a foreign jungle is derelict in his duty to his country if he hasn’t at least mastered the basic information on the geology of uranium. And the same applies to every mountain climber, every big game hunter, and, for that matter, every butterfly catcher.

Uranium fever made its way into popular culture as well – I remember, as a child, reading about uranium prospecting in some of the young adult books of the day (in the 1960s it was the Hardy Boys as well as a few other series that I just can’t remember offhand). In my early teens, this made a big impression on me – it wasn’t until the 1976 accident at Three Mile Island that I realized there were people out there who didn’t really agree that nuclear energy was a Good Thing.

Prospecting for uranium is actually not all that difficult – at least, not in principle. It’s the radiation – while uranium only gives off relatively low-energy gamma and alpha radiation (neither of which travel far in the air), in nature, uranium isn’t found all by itself; it’s found in the company of another dozen or so radionuclides that form as the uranium “parent” decays towards stability. While uranium itself is only weakly radioactive, the collection of nuclides in the decay series adds to the collective radioactivity – in addition, many of the progeny nuclides emit higher-energy radiation that’s easier to detect at a distance. This made it relatively easy for prospectors walking over the ground to pick up elevated readings, even on a hand-held Geiger counter – better yet, it can even be picked up from the air. In fact, when I was making aerial flights with the police we would always get higher readings when we flew over cemeteries, some bridges, and some buildings – granite tends to have higher levels of uranium that would always bump our readings up high enough to see, even from 1500 feet in the air.

A uranium miner works underground near Montrose County, Colorado. Photo dated 1972. (https://commons.wikimedia.org/wiki/File:Underground_uranium_mining.jpg)

So as long as the uranium-bearing rocks are near the surface, they’re fairly easy to pick up on the surface and even from the air. And even uranium-bearing rocks that are somewhat deeper down can often be picked up as well – soil forms from the underlying bedrock so if the bedrock contains elevated levels of uranium then so will the soil. This will not only cause higher levels of radiation at the surface (and in the air) but will also lead to higher radon levels in the air because radon is yet another of the uranium decay series. If there’s a lot of uranium in the rocks then there will be more uranium in the soil and more radon in the air.

In fact, when I worked for the State of Ohio a number of years ago I remember looking at some maps –radiation levels, radon concentrations, and bedrock geology. The contours matched very closely – places where the Ohio Shale (a dark, organic-rich shale with elevated uranium concentrations) was close to the surface there was more radon in the air, and radiation levels were higher than in places where the first layer of bedrock was made of sandstone, limestone, or even from less-radioactive shales. But then there was an area that had elevated radiation levels, without the radon we expected to see, and in an area where the Ohio Shale was deeply buried. It turned out to be an agricultural part of the state – and it turns out that there are some fertilizers made with phosphate rock from Florida that’s rich in uranium; one of these facilities was in the news recently when it threatened to release water with radioactivity in it to the environment (https://www.bbc.com/news/world-us-canada-56629013). The same radioactivity that raised concerns at the phosphate rock processing plant in Florida also caused anomalous readings in Central Ohio.

What the early prospectors didn’t realize is that uranium can be found even when deep underground – by sampling groundwater, geologists can get an idea of the elements that are present in the rocks the water has flowed through. So if geologists grab a water sample that shows elevated levels of radioactivity, tracing it back to the source rocks can often lead the geologists to the source of the radioactivity…and sometimes to a uranium deposit.

By the end of the 1950s prospectors had located rich uranium deposits in the American West and Southwest as well as in Canada, Australia, and throughout Africa. Not only that, but they had realized that uranium was also associated with a number of mineral deposits – rare earth elements, for example, are chemically very similar to uranium and thorium so many rare earth element ores sometimes contain enough of these elements to extract. But the most interesting example of this was found in South Africa, where some gold mines were found to contain higher levels of uranium as well as the gold – I and my fellow grad students learned about this in a class on economic geology. What I thought was most interesting was that the rocks weren’t worth the expense of mining for the uranium, but it had enough gold to be worth the trouble. And once the rock was at the surface, it was worthwhile to remove the uranium.

Interestingly, technology developed for, among other things, aerial uranium prospecting is finding a use in counterterrorism today. Most notable was the radiation detection system I used to fly within the helicopter, a direct descendant of the systems developed for geologists to look for radioactive mineral deposits a generation ago (albeit with better software). Which actually completes the circle – instruments developed for an earlier generation’s quest to find uranium to help arm America are now being used to help locate and identify nuclear threats posed by terrorists.

The gamma ray spectrum of uranium with its decay products. This is a unique pattern that belongs only to uranium. I collected this spectrum with the same instrument we used for our interdiction surveys.

The Little Neutral One

A century ago was a heady time to be a physicist – new phenomena, new particles, new laws of nature, and more were being discovered, it seemed, daily as physicists appeared to be determined to refute an earlier contention that “the future truths of physical science are to be looked for in the sixth place of decimals.” And, of course, every discovery brought with it additional questions to be answered – but for a few decades, one of the most significant of these questions involved beta radiation. To explain why, though, we need to back up a little bit and get into some of the background.

One of the bedrock principles of physics is the conservation of energy – often stated along the lines of “Energy is neither created nor destroyed, but only changes in form.” We see this in a pendulum – at the top of its arc (maximum potential energy) the pendulum is momentarily motionless (zero kinetic energy); as it begins its downward swing it loses potential energy (it’s closer to the ground) but gains kinetic energy (it’s moving faster). If we add the potential and kinetic energies at any point in the pendulum’s cycle the sum will be the same – the total energy is the same at all times.

Two spectra I collected, Cs-137 on the top and Co-60 on the bottom. Note the distinct peaks that are unique to each of these radionuclides.

Conservation of energy works at the atomic level as well. A radioactive atom has a specific amount of energy contained within, as does the atom to which it decays. Thus, every radioactive decay involves an atom losing a very specific amount of energy – energy that goes into the radiation the atom has just emitted. So when early researchers analyzed the energy of the alpha and gamma radiation given off by the radioactive atoms they were studying they were pleased to note that the radiation had distinct energies – so distinct, in fact, that they are used to identify the radionuclides that emitted them. If I see gamma radiation with 662,000 electron volts (abbreviated as 662 keV) I know there is Cs-137 in the area; 1460 keV is the signature of potassium (specifically, the naturally radioactive isotope K-40), and so forth. Alpha radiation is the same – an alpha with 4871keV of energy was almost certainly emitted by a Ra-226 atom.

So imagine the surprise of physicists a century ago when they realized that beta decays have a continuous spectrum and not the sharp peaks they expected to see. This left them in a quandary. One possibility was that that conservation of energy was not as rigid a rule as physicists thought – perhaps it was only accurate when averaged across a large number of atoms, but not with every individual atom; that was the suggestion of the great Niels Bohr. Or, as suggested by another great physicist, Wolfgang Pauli, maybe beta decays were associated with the ghost of a particle that we just couldn’t find a way to detect – that, together, both particles contained the total energy difference between parent and progeny, divvying it up between them and conserving energy.

Bohr, a future Nobel laureate (as was Pauli), was highly respected and any suggestions he made had to be taken seriously. On the other hand, the thought that energy might not be conserved in every single interaction – that some radioactive decays might simply involve more or less energy than other identical decays – was anathema. But by 1934, as beta spectra became more accurately determined, physicists noticed that they were unique to each beta emitter – they did not behave the way that Bohr’s hypothesis required. It turned out to be easier to believe in a ghost particle that we hadn’t yet learned to detect than to believe that the conservation of energy might sometimes be violated. So, however reluctantly, physicists began to try to puzzle out what this particle might be like.

Beta energy spectra – note that, unlike the earlier gamma spectrum, there is no peak energy. Source: https://ocw.mit.edu/courses/nuclear-engineering/22-02-introduction-to-applied-nuclear-physics-spring-2012/lecture-notes/MIT22_02S12_lec_ch1.pdf

One thing seemed likely – since it was invisible to all of the detectors of the day it likely had no electrical charge; it was likely electrically neutral. For this reason, Pauli suggested naming it a neutron (following the convention set by the names electron and proton). The problem was that, around the same time, British physicist James Chadwick wanted to use “neutron” to name the heavy neutral particle in the atomic nucleus he’d just discovered. To resolve this small impasse, Italian physicist Edoardo Amaldi joked that they should name Pauli’s smaller particle “neutrino,” the “-ino” an Italian diminutive making the word mean “little neutral one” (in comparison to Chadwick’s much larger particle). The name was adopted – now physicists just had to find the thing. And that raised an interesting question – how do you find a particle that doesn’t want to interact with your detectors?

Consider ionizing radiation for example. Gas-filled detectors work because incoming radiation slams into an atom and strips off an electron; this sets into motion the process of gas amplification that leads to a pulse of electrons passing through an electrical circuit that includes an analyzer capable of extracting information about the radiation. Or a scintillation detector, in which the radiation interacts with the detector medium (usually a crystal, but sometimes liquid or plastic) – this interaction deposits energy into the scintillator and leads to the emission of photons that, passing through a photomultiplier tube, produce a signal that can be amplified and studied. In fact, even our eyes work this way with non-ionizing radiation (light) – the photons interact with the rods and cones of our retinas which lets them be detected, sending signals to our brains. But something that doesn’t seem to interact with anything? That’s a bit more difficult.

In 1942, Chinese physicist Wang Ganchang suggested that, since neutrinos are emitted along with beta particles (which are electrons ejected from the nuclei of unstable atoms), perhaps they could be detected by looking for evidence of neutrinos interacting with beta particles during a form of radioactive decay called beta capture. Fourteen years later a team of American physicists announced their detection of neutrinos in the journal Science, earning the 1995 Nobel Prize for their achievement.

We’ve learned a lot in the decades since neutrinos were first confirmed to exist, and the ghostly little particles turn out to give us some huge insights about the universe – in fact, for some time it was thought that the fate of the universe hinged on whether neutrinos weighed nothing…or almost nothing. Neutrino observations from the Sun have raised questions about the inner workings of the solar furnace and, in 1986, a burst of neutrinos that appeared in the world’s neutrino telescopes were associated with a supernova in a satellite galaxy to our own Milky Way. In fact, some astrophysicists have even suggested (erroneously – https://what-if.xkcd.com/73/) that neutrinos from gamma ray bursts and supernovae might even be responsible for mass extinctions throughout the galaxy. We’ve also discovered that there is an entire family of neutrinos out there, associated with not only beta particles (electrons), but also with the more exotic tau particles and muons – along with their respective anti-neutrinos. These are all still quite ghostly – but they are no longer invisible or unknown…and they’re fascinating.

A neutrino observatory in Antarctica. Neutrinos are so elusive that they pass completely through the Earth before they’re detected here. Source: https://research.msu.edu/23m-nsf-grant-funds-to-upgrade-antarctic-icecube-neutrino-observatory/

About That Tritium…

I noticed recently a flurry of articles about Japan’s recent announcement that it planned to release millions of gallons of water from the site of the Fukushima reactor into the Pacific Ocean. Most of the “flurry” was about the response to this announcement – from environmental groups, some of Japan’s neighbors, and other concerned parties. And it made me wonder why so many were so exercised about so little a risk. But before getting into that, let’s take a look at this tritium stuff.

The first time I came across tritium was when I was in the Navy – we had a tritium monitor in the Torpedo Room of my submarine and I asked why. “It’s for the special weapons” was the reply. Which makes sense – tritium is used in some forms of nuclear weapon and if any of it leaked out then we needed to know. I read up on it and realized it wasn’t much of a threat, so I stopped giving it much thought.

Fast-forward to after I got out of the Navy and was working for an academic radiation safety program to help pay for my college. I noticed that a lot of our research labs used tritium so I learned a little more about it. I found out it emitted a very low-energy beta particle – so low-energy that we couldn’t even detect it with our normal Geiger-Mueller detectors; so low-energy that the tritium beta would barely (if ever) even penetrate through the outer layers of skin to reach the living cells underneath. In fact, I remember holding a small plastic vial that contained (according to the label) over 20 curies of tritium – a large amount of radioactivity – and I couldn’t get a single peep on my Geiger counter. And even if someone ingested or inhaled some of it, this same low-energy beta particle made it about the most innocuous type of radioactivity any of us could take into our bodies. It just wasn’t a big deal.

A few years later I was working for the state government and I learned a little more about tritium – mostly that the same low-energy beta that made it such a minor threat also made it hard to clean up since nobody could survey for it directly. Not only that, but tritium, as a nuclide of hydrogen, would simply dissolve into water and would go wherever water or water vapor traveled. My boss, for example, had needed to deal with a concrete wall that was contaminated with tritium; concrete is somewhat porous and the tritium had permeated the entire thickness of the wall and the only way to “decontaminate” the wall was to tear it down entirely. Not because the tritium posed a risk to anybody – the beta particles couldn’t even escape from the concrete to expose anybody (and, one can hope, nobody was nibbling on the concrete itself) – but because the amount of tritium contamination exceeded regulatory contamination limits. So the wall was ripped down, loaded into waste containers, and shipped off to be buried in a waste disposal site. A lot of money was spent to mitigate a non-risk.

Let’s jump ahead a little more – to another job I had that included oversight of the radiation safety at a laser fusion research facility. They had more tritium than I’d ever seen at a single location – enough to kill anyone who ingested or inhaled any of it – and even there we didn’t get any readings from their stores. On the other hand, if they took a tiny (1 mm) plastic sphere, filled it with frozen tritium, and slammed it with a powerful laser a tiny fraction of the tritium would fuse, releasing enough energy and enough radiation to give a fatal dose to anyone unlucky enough to be in the target chamber at the time. Enough of anything can be dangerous – we just have to know where the boundary between “most likely safe” and “possibly unsafe” lies.

Somewhere along the line I also came to understand where tritium comes from – in nature it’s formed when cosmic rays collide with atoms in the atmosphere. The Earth actually has a lot of natural tritium; every glass of water we drink, every lake or ocean or pool we wade into, every tub or hot spring in which we soak has tritium dissolved in it, to the tune of a few tens of picoCuries (pCi) per liter of natural water. Globally, cosmic rays produce over one and a quarter million curies of tritium every year; couple that production rate with the rate at which it decays and we find that our planet has a total inventory of tritium of about 26 million curies, including the tritium that’s in our own bodies from eating, drinking, and breathing.

Tritium is also, of course, produced in nuclear reactors – the core of a reactor is a neutron-rich environment and as water passes through it’s exposed to those neutrons. When they strike a hydrogen atom, sometimes a neutron will stick, forming deuterium (a stable isotope of hydrogen with one neutron and a proton). If another neutron hits a deuterium atom and is captured then it will form tritium (one proton and two neutrons) – this is where the tritium at the Fukushima site originated.

And that brings us to the water to be discharged from Fukushima into the ocean. It turns out that there are about 20,540 curies of tritium contained in the 820,000 cubic meters of water stored at the Fukushima site. Most of this is groundwater from the area, contaminated with reactor coolant that leaked from the broken reactors. But the Pacific Ocean is huge – it contains about 400 billion times as much water as is being stored on the Fukushima site. So adding the water from Fukushima into the Pacific Ocean increases the tritium in the ocean by only a minuscule amount – by less than a trillionth of a curie (or about 1 pCi) for every cubic meter of water in the ocean. And because tritium emits so low-energy a beta particle, this increases radiation exposure by so little (less than 1 microrem annually) that it poses no risk to anybody – or to any creature – in that water. It’s like adding a few more grains of sugar to a pitcher of KoolAid. This means that seafood lovers can continue eating their sushi or their fish-and-chips without worrying that it’s going to hurt them.

Something else to keep in mind is that seawater also contains many other natural radionuclides that also swamp any radiation dose from the waters of Fukushima. Uranium, rubidium, and potassium are also dissolved in this water, and each of these contributes far more radioactivity (and produces far more radiation dose) than the tritium from Fukushima. The bottom line is that, while discharging this water might be something that we can measure and calculate, it’s just not worth worrying about – in fact, the stress from worrying is going to be more dangerous than the cause of that worry.

Reference: https://www.whoi.edu/multimedia/source-of-radioactivity-in-the-ocean/

Nuclear Medicine

This month marks the 80th anniversary of doctors injecting radioactivity into their patients – not as part of evil experiments, but to help to diagnose and treat disease. And in spite of being only 80 years old, the field of nuclear medicine has become impressively versatile and valuable over the decades, giving physicians the ability to, in effect, “see” within their patients’ bodies to learn what was ailing them. Here’s how it works.

Tracking tagged molecules

Every organ, every tissue in our bodies has a different biochemistry and uses different molecules and elements. The thyroid, for example, uses iodine to function and to produce the hormones that help to regulate our metabolism; the brain thrives on an exclusive diet of glucose; our bones incorporate calcium, phosphorus, and a handful of other elements. So if we want to learn about how the thyroid is functioning – or to learn where in the body metastatic thyroid tissue has migrated – injecting radioactive iodine will cause the thyroid tissue to become radioactive, making it easy to find. Similarly, adding radioactive atoms to glucose molecules can help us to see what’s going on in a patient’s brain while adding radioactivity to molecules that are used by growing bone can tell a physician where there is a small fracture that has begun to knit together.

Physicians have know for decades that certain organs and different tissues have different biochemical characteristics. But until the middle of the last century this knowledge, while interesting and important, was of no help whatsoever in diagnosing illness – but with the advent of nuclear technology (specifically, nuclear reactors and accelerators) this began to change. Radioactive atoms emit radiation (of course!) and that includes gamma rays that can penetrate through the overlying tissues to be detected from outside the body.

So say a patient has metastatic thyroid cancer – when I-131 is injected into the patient the iodine will seek out thyroid tissue, including any metastases where cancerous thyroid cells have spread through the body; when the patient is scanned with a gamma camera (more on this shortly) these clumps of now-radioactive thyroid tissue will show up as bright spots on the scan. Give the patient a higher dose and, instead of revealing their locations, the radioactive iodine will deliver a high enough dose of radiation to destroy the cancerous tissue.

In 1934, Frederick and Irene Joliet Curie formed radioactive P-30 by bombarding stable aluminum with alpha particles. Not long after, Enrico Fermi, speculating that neutrons’ lack of an electrical charge might make them even more effective, began slamming neutrons into as many elements as he could get his hands on to see what would happen. Four years later Fermi was awarded the Nobel Prize for this work.

The Second World War slowed non-war-related research; at the same time, it provided wonderful new tools for producing artificial radionuclides – nuclear reactors and particle accelerators – as well as new instruments with which to measure and characterize the radiation they gave off. After the war, the stage was set for putting these to use in medicine.

While there had been some early work with nuclear medicine in the 1930s, the first bona fide treatment came about in 1946 with the use of I-131 to stop the growth of a thyroid tumor and the subsequent realization that lower doses could be used for imaging rather than destroying the diseased tissue. Over the ensuing years and decades, nuclear medicine physicians learned to insert radioactive atoms into molecules that would be taken up by other organs, by cancerous tumors, by healing bone, and more – the current list of nuclear medicine procedures published by the Nuclear Medicine Technologist Certification Board lists 78 procedures using radiopharmaceuticals not even dreamed of in 1946 (https://www.nmtcb.org/exam/procedureslist.php). And the most recent additions – positron-emitting radionuclides (often used in conjunction with CT scanners) – make use of a particle of antimatter, most commonly encountered in science fiction.

Collecting images

It’s possible to find out where the radioactivity is collecting without creating an image at all. If I give someone I-131, for example, I can start scanning their body with my Geiger counter – as I survey I’ll notice I get a low count rate at their feet, higher when surveying their pelvis and abdomen, and highest when my detector is over their throat. This would tell me that there’s not much in the feet or legs that absorbs iodine, that it collects in the bladder and colon while waiting for the person to urinate or defecate, and that the thyroid collects iodine more effectively than any other part of the body. On the other hand, if I got a high count rate from, say, the underarm, groin, or liver then I might suspect a metastatic tumor that had spread to lymph nodes or to the liver.

The thing is, while many scientists and engineers are comfortable with numbers, they’re a little unusual in that; most people can get more information from a picture than from a table of numbers. Finding a way to produce images of where the radioactivity was collecting was a way to make this technique accessible to a much wider range of physicians

The Moritz Orthodiagraph – devised by Friedrich Moritz, of Munich (1861-1938)

The problem is that gamma rays are given off in all directions – so if I’m holding my radiation detector over, say, an I-131 patient’s nose, it’s going to show a high count rate. This isn’t because some iodine ended up in the person’s nose – it’s because the iodine in the patient’s thyroid will emit gamma radiation in all directions, including into my radiation detector. Unless I can find a way to screen out all of the gammas except for those that are directly beneath my detector.

One way to do this is to take a piece of lead and drill an array of holes through it, putting an array of radiation detectors on the other side of the lead. If a particular detector registers a “hit” then I can be sure that the gamma was emitted directly beneath that hole in the lead. Doing this with the entire array of holes and detectors gives us an image of the part of the body beneath the array; scanning the array over a part (or all) of the body can tell us where the radionuclide has collected. The former (parking the array over, say, the neck) can help us to locate small nodules within the thyroid while the latter (assembling an image of the entire body) can show us where metastatic thyroid tissue has landed.

Newer methods

One of the problems with nuclear medicine is that it shows where the radionuclide ends up, but not necessarily what structures it ends up in. For example, a “hot spot” in the abdomen might be the result of radioactivity in the liver, the gall bladder, one of the ducts between the liver, gall bladder, and digestive tract, the connective tissue in the area, or maybe even in the muscles that line the abdominal cavity. Taking “pictures” from different angles can help to narrow down the possible location – a hot spot halfway between the front and back walls of the abdomen, for example, is more likely to be in the liver than in the abdominal muscles. But even the best gamma camera still has limits to their ability to precisely resolve the location – precise information requires a higher-resolution image; we can get that sort of precise three-dimensional imaging from a CT scan. Thus, the PET-CT – a device that can be used to show not only metabolic or biochemical activity (where the radiopharmaceutical collects) but also the structures in which the hot spots lie.

Images from a Ga-68 PSMA PET-CT in a man with prostate cancer shows tumors in lymph nodes in the chest and abdomen. Credit: Adapted from Int J Mol Sci. July 2013. doi: 10.3390/ijms140713842. CC BY 3.0.

Radiation safety

Injecting a patient with radioactivity raises some safety concerns – for the patient, for the medical staff, for their family, and for others with whom they might come in contact.

Consider the I-131 patient we discussed earlier. I-131 gives off both beta and gamma radiation; the beta radiation is not much of a concern because it’s trapped within the body, but gamma rays can (and do) expose others. The key is to limit that exposure.

When a patient is in the hospital, workers can use the principles of Time, Distance, and Shielding to reduce their exposure and that of others. Reducing the amount of time anyone is near a patient reduces their exposure, as does increasing the distance to the patient. Nurses, for example, can stand at the foot of the bed instead of at the head to be further from the thyroid; if they must be at the head of the bed, they can stand back a step instead of standing at the bedside. Simply taking one step away from the patient can reduce radiation exposure by a factor of four or greater. In addition to this, many hospitals install lead shielding in the walls surrounding the rooms where their I-131 patients remain during their stay; this reduces radiation exposure to those in adjacent rooms.

Radiation safety for the patient primarily takes the form of carefully calculating the amount of I-131 that’s administered – a high enough dose to obtain a good image or to ablate the thyroid tissue, but not much more than that to avoid excessive radiation exposure to healthy tissues. In addition, nuclear medicine patients are usually given instructions on how to reduce radiation exposure to their family and friends when they return home.

These instructions usually include cautions to use a separate bathroom than other family members (if possible), not to share cups or cutlery with others, not to hold babies and young children on their lap, to sleep in a separate bed from spouse and children, and so forth – the idea is to not only reduce radiation exposure from the gamma radiation emitted from their bodies but also to reduce the chance for family members to come in contact with radioactive contamination. In cities with widely used mass transportation (or patients who rely on mass transit) there is also a common admonition to try not to sit or stand too close to other passengers if possible. These precautions might last for only a few hours (with most PET radionuclides), for a few days (in the case of Tc-99m), or for a few weeks (in the case of I-131).

One other consideration in recent years is that many cities have established networks of radiation detectors as part of their counterterrorism efforts, and nuclear medicine patients cause the majority of radiation alarms that come in. In the decade or so that I’ve worked with law enforcement on radiological interdiction, I’ve seen far more alarms from nuclear medicine patients than from all other sources combined. For this reason, many nuclear medicine physicians will give notes or cards to their patients so that, if they do trigger a radiation alarm, they can show the cards to the police.

After 80 years nuclear medicine is a mature methodology with a solid technological and scientific foundation. Over the years it’s been developed to a high level, and in recent years there have been about 15 million procedures annually – both diagnostic and therapeutic. For what it’s worth, I account for some of those procedures – I’ve had a number of scans over the last decade or so, so I can personally confirm that it doesn’t hurt, it doesn’t make one dangerously radioactive, and it doesn’t put others at risk. And the information that it provides can either be used to help guide treatment – or to put one’s mind at ease. Either way, it’s useful to both patients and physicians – probably one of the reasons it’s still in use.

The Demon Core

On May 21, 1946 physicist Louis Slotin was conducting a dangerous experiment with a plutonium sphere – designed and constructed to the exacting standards required to achieve criticality and to explode with the force of tens of thousands of tons of TNT. The experiment Slotin was conducting involved bringing the two halves of the sphere close together – close enough for neutrons in one half to start to cause plutonium atoms in the other to start fissioning, but barely far enough apart to keep those fissions from increasing uncontrollably. To do this, Slotin had placed one hemisphere on top of the other, using a screwdriver to hold the two pieces a safe distance apart. To cause the fissions to start he’d move the screwdriver slightly to lower the top half ever so slightly, noting the increase in neutron levels; moving the screwdriver in the other direction increased the separation and shut the reaction down.

And then his hand slipped.

The top hemisphere moved too close to the bottom and everyone in the room saw a blue flash and felt a pulse of heat. Realizing what had happened, Slotin quickly used the screwdriver to flip the top hemisphere away from the bottom, stopping the reaction.

Slotin died nine days later, his estimated radiation exposure was well over the 1000 rem (10 Sv) that is invariably fatal. Of the other seven people in the room at the time of the accident, the highest radiation exposure was 182 rem and nobody else exceeded a dose of 62 rem. Two later died of cancer – both of different forms of leukemia – but their doses were low enough and the cancer appeared late enough (19 and 29 years after the accident) that it’s unlikely that radiation from this accident played a role in their illness.

The image above is a re-creation of the 1946 experiment. The half-sphere is seen, but the core inside is not. The beryllium hemisphere is held up with a screwdriver. Reference: Los Alamos National Laboratory – Taken from “A Review of Criticality Accidents”, LA-13638, Figure 42, page 75, Los Alamos National Laboratory

The thing is, this same plutonium core had killed before – in August, 1945. That time its victim had been a physicist name Haroutune (Harry) Daghlian.

While Daghlian was performing a different experiment than Slotin he, too, was working by hand, manually moving pieces of beryllium around the core to change the number of neutrons that were reflected off the beryllium atoms back into the plutonium. Like Slotin, Daghlian’s hand slipped, dropping a beryllium block onto the experiment and causing the core to become critical. Also like Slotin, Daghlian was able to disassemble the critical configuration, shutting down the reaction, and receiving a fatal dose of radiation in the process. Receiving a dose of about 310 rem (3.1 Sv), Daghlian died 25 days later.

I had known about the accidents and the deaths; I learned about them both in the 1990s during a class I took on nuclear criticality safety. But it turns out that the history of this particular core – re-nicknamed the “Demon Core” in the aftermath of Slotin’s death (its original nickname had been “Rufus”) – is far more interesting than this, and extends over a longer period of time.

Weighing a mere 14 pounds and measuring 3 ½ inches in diameter, this particular bit of plutonium had actually been manufactured in 1945 and it was originally intended to be placed in the third weapon to be dropped on Japan in the closing days of the Second World War – tentatively scheduled to be used on August 19, 1945…or on the first day after August 18 with good weather over the target area. As it was being prepared for shipment, Japan surrendered and it remained in the US, at the Los Alamos Laboratory site in New Mexico – the same site at which both Daghlian and Slotin worked.

In the aftermath of Slotin’s accident, the core received its nickname. But, more importantly, the lab also implemented a number of new safety measures that were intended to reduce the risk of such an accident occurring as well as moving the scientists far enough away to keep them safe even if a criticality did occur. The criticality safety training I attended in the mid-1990s was a continuation of these changes, as was a series of criticality safety inspections and audits I conducted at a uranium enrichment facility several years later.

But the history of the Demon Core didn’t end with Slotin’s death. As the US moved into the post-war years it realized that it needed to better understand the effects of these powerful new weapons; when the Soviet Union set off its first nuclear weapon in 1949 these tests gained a new urgency. One series of tests, dubbed Operation Crossroads, was planned for 1946 and was intended to learn about the impact of nuclear weapons on naval ships – the Demon Core was to be shipped to a balmy tropical island where it would be placed into a nuclear weapon and detonated.

The second accident put an end to that plan. The reason for this is that, when plutonium and uranium fission they split roughly in half, and the two fission fragments that are formed are radioactive. Not only that, but some of these fission fragments absorb neutrons quite efficiently – in these early days of nuclear weapons design and fission research, nobody was quite sure if these fission products and “poisons” would affect the test explosions. Why, the thinking went, use an uncertain weapon core when they could use a new one with no such issues instead? So instead of a working vacation on a tropical island, the Demon Core remained locked up in New Mexico.

Unable to be used in Operation Crossroads, the Demon Core (or, if you prefer, Rufus) was eventually melted down and mixed in with other plutonium, eventually ending up in a number of nuclear weapons cores in the nation’s nascent, but growing, nuclear stockpile.

References:

https://web.archive.org/web/20160524122659/

https://www.newyorker.com/tech/annals-of-technology/demon-core-the-strange-death-of-louis-slotin

Nuclear Death in the Desert: the SL-1 Accident

On my very last day at Naval Nuclear Power School, in addition to doing a lot of paperwork and receiving our orders and still more paperwork to bring to our next duty stations, we were sent to the only room at Nuke School that had room for our entire class. There, we heard about the loss of the USS Thresher (SSN 593) and what happened to cause it to sink. And then we were shown a movie about a nuclear reactor accident in the desert of southern Idaho. The discussion of the Thresher got my attention – I had already volunteered for submarine duty (and, as it turned out, I ended up on one of Thresher’s sister ships – but that’s a story for another time) – but there was something about the reactor accident that riveted my attention, including the fact that, since all three operators died, nobody was quite able to figure out exactly why the accident happened. Even today, in spite of the engineering investigations, scientific publications, and popular accounts, we’re still not quite sure about what, exactly, caused the accident – the reason is that it was due to the actions of a single person, and that person is dead and can’t tell us why he took the actions he took. It was this human factor that caught my attention.

But I’m getting ahead of myself – let me go back to the start.

In the 1950s it was clear that the Navy’s nuclear power program was going to be a success and other branches of the military became interested in what nuclear energy might do for them. The Air Force began experimenting with, among other things, nuclear-powered aircraft and the Army looked into developing small land-based nuclear power plants that could be easily transported by truck or airplane and used to power remote bases – the SL-1. As an aside, there were some non-military programs aimed at developing spaceships powered by nuclear reactors or driven by nuclear explosions – which sounds like a good topic for a future blog posting!

The SL-1 reactor was the Army’s first design of a “stationary, low-power” nuclear reactor. The prototype of this reactor was built at a government facility in the desert of southern Idaho, not far from the Navy’s prototype submarine and carrier reactors, the Air Force’s nuclear airplane project, and a few other military or military-adjacent nuclear projects.

US Atomic Energy Commission image of SL-1 reactor core being removed from National Reactor Testing Station facility in Idaho

Along with its other specifications, the Army wanted a reactor that could be operated by a small number of people – the design that was constructed in Idaho was fueled with uranium enriched to the same level as the uranium used in nuclear weapons, arranged in an array that included nine control rods. In a nuclear reactor, neutrons caused by one fission fly out of the fuel rod and into the water in which the core is immersed; while in the water the neutrons lose energy and slow down, becoming more effective at causing fission. Neutron-absorbing control rods can be inserted into the reactor core to soak up these neutrons, shutting down the reactor; pulling the rods from the core will allow the reactor to start up. Normally a reactor’s control rods are moved by a motor, hydraulics, or some other sort of mechanism – in the case of the SL-1 the reactor sat in a pool on the main floor of the containment structure, beneath some radiation shielding. The operators could actually walk out on top of the reactor when the reactor was shut down and radiation levels were low.

So…with that as a prelude, here’s what we know about the events of January 3, 1961.

Following a holiday shut-down and maintenance period the three operators, Army Specialists John A. Byrnes (age 22) and Richard Leroy McKinley (age 27), and Navy Seabee Construction Electrician First Class (CE1) Richard C. Legg (age 26) were starting the reactor. At 9:01 PM Byrnes abruptly pulled one of the control rods, causing a sudden and dramatic increase in power. In the next few seconds, much of the fuel either melted or was vaporized, the sudden input of thermal energy caused some of the water in the core to flash to steam and the steam explosion shot jets of water out of the reactor core and caused the entire reactor to jump upwards by about nine or ten feet and spraying the inside of the containment structure with radioactive water. The pressure also shot the #7 shield plug out of the top of the reactor vessel; standing above it was Richard Legg, who was impaled by the plug and pinned to the ceiling of the containment structure.

The other two operators were apparently standing nearby at the time of the accident and were also killed by the accident – it’s possible that they were killed by the shock when the reactor jumped upwards so violently and the impact of the operators with the reactor. Byrne and Legg appear to have died instantly, McKinley seems to have survived for an hour or two before succumbing to his injuries. Although all three men died of physical trauma from the explosion, they received a dose of radiation that would have been fatal even in the absence of their injuries.

The aftermath of the accident unfolded over the next several years and included an extensive investigation as well as decontamination and demolition of the reactor plant along with radiation surveys and decontamination of areas surrounding the reactor site. But – and this is the part that fascinated me – there is no definitive way to know why Byrnes was standing over the reactor, manually pulling the control rod.

The story I heard was anecdotal; one of my classmates said he’d heard that Byrnes had learned that his wife was having an affair with one of the other two operators – that he had deliberately caused the explosion to either commit suicide, to try to kill his wife’s lover, or both. That was the conclusion of one popular account of the accident as well. The problem is that there’s no way to prove – or to disprove – this hypothesis. Not only that, but there are plausible explanations other than murder and/or suicide.

As one example, the control rods sometimes stuck in their channels; it’s possible that Byrnes was trying to free a stuck rod and, when it finally came free, he was pulling so strongly that the rod traveled far past the point at which the reactor became critical, causing it into a dangerous condition known as “prompt criticality” in which power increases uncontrollably.

The rod might have been pulled inadvertently, perhaps by an operator expecting some resistance and finding none.

Or the operators, noticing the rod having a tendency to stick when moved, might have been trying to move it up and down by hand until it was able to move smoothly, accidentally jerking it upwards.

Unfortunately, the operators failed to make any notes in their operating logs that they were having problems with any of the control rods or that they would be working on them.

Thus, my fascination with SL-1. First, at the very start of my stint in the Navy’s nuclear power program, it was sobering to hear of an accident that had cost the lives of three military operators; just as hearing the technical details was fascinating. Hearing how the accident was re-created and what the investigators were able to learn was fascinating, sparking that part of my mind that had so enjoyed reading Sherlock Holmes stories when I was younger. And adding in the fact that we might never know exactly why it happened – plus a little bit of spice with the suggestion that it might have been the result of a love triangle…that made it irresistible.