Author Archives: Dr. Zoomie

The Radiation Propeller

When I was in the Navy, at one point I got to stand watch for four hours every day for nearly 2 months – I happened to be sitting directly in front of a cabinet that was marked with the radiation symbol (or, as we called it, the radiation propeller) which meant that I got to spend four hours every day staring at the sign. Three magenta blades and a magenta dot in the center, all on a yellow background – thanks to our training I could tell you the size of the blades, the size of the dot in the middle, and the spacing of the “propeller” blades, I knew that this was all specified by regulations, but I didn’t have a clue as to who chose this particular color scheme or why they decided to go with the three-bladed design. And, yeah, I know that in the giant overall scheme of things this isn’t that big a deal – but when you’re staring at this thing for a half shift every day for two months one just starts to wonder about that sort of thing.

Later I was able to find out where the radiation symbol was described in the regulations (in case you’re wondering it’s in 10 CFR 20.1901, but not much more than that. I was starting to resign myself to the possibility I might go to my grave without ever learning the origin of the radiation symbol…I pretty much decided that I’d be OK with that. So imagine my surprise when, sometime in the 1990s, I stumbled across a paper in the scientific journal Health Physics entitled “A Brief History of a 20th Century Danger Sign” by Stephens and Barrett that appeared in Health Physics Vol. 36 (May) pp. 565-571, describing where the design came from. While I can’t say that this made my life complete, it was sort of neat!

Without getting into all the nitty-gritty detail, I thought it was interesting to read that the design was (somehow) supposed to represent energy radiating out from a radioactive source. More importantly, it was also interesting to find out that there’s more to the radiation symbol than the propeller shape – if it doesn’t have a yellow background and a black or magenta symbol then it’s not compliant with the regulations and it doesn’t “count.” But instead of retracing the history of the symbol, let me instead tell you a few stories about people getting it wrong.

Early radiation warning tags used at the University of California Radiation Laboratory.

At one university I worked at in the early 1990s I was walking down a corridor and saw a radiation sign on the door to a lab that I knew didn’t use radioactivity. When I asked, I found out that that particular lab had lost a lot of analytical balances – they were stolen by drug dealers – so they started putting a radiation sign on the door. Drug dealers didn’t want to break into the labs because they were scared of the radiation they thought was there so they stopped stealing equipment. The only problem was that it wasn’t quite legal – we were cited for misuse of the radiation symbol. A few years later, when I was working as a regulator, we cited an environmental consulting company for the same thing – they were hanging radiation symbols on their equipment trailers when they had to leave them overnight in a location, again to discourage theft. Good in theory – expensive in practice (at least for the environmental consulting firm – they were fined).

Several years later I was working at a different university and we were putting up a new research building. I went to a meeting that included our interior designers and they were eager to show me their idea for a less-garish radiation sign – apparently, the yellow-and-magenta clashed with their color scheme. I agree with them that pastel green-and-cream looked much nicer, but then had to break the news to them that no matter how nice it looked, wasn’t legal. They ignored me and put up the more soothing signs all over the new building. And they were disappointed when they came back to do a walkthrough as the building began to fill with labs, seeing that their tasteful and subdued color scheme was ruined by the garish (but regulatorily compliant) signs. But I do have to admit that their signs were a lot nicer than the legal ones….

Around this same time, I was going through some of the old records at my university and I found some old radioactive materials tags – apparently ones that pre-dated the official colors. These had a green background and a red radiation propeller. I’m guessing that these dated back to the mid or late 1940s when the university had done a lot of research on radiation health effects as part of the Manhattan Project. I was tempted to use these in the new building…but we really did have to follow the regs. So I just put them in a folder instead (well…except for a few I kept for myself).

An early version of the radiation symbol used on a gummed tape label for chemical ware

Coming back to my time in the Navy, I remember one day that I was sitting at my watch station, bored out of my mind and staring at the radiation sign. I let my eyes unfocus and was just staring at the dot in the center of the symbol, letting my mind go sort-of blank. And I noticed that one of the blades on the propeller was starting to dissolve and vanish. The only thing I could figure was that it was at the edge of my blind spot. So I shifted my gaze a little and the blade returned, but the next blade began to dissolve into my blind spot. Then I did it again with the third blade. At the end of the watch, I mentioned this to one of the other mechanics with who I shared the watch. His reply? “You too, huh?” It’s probably the most fun I’ve had with the radiation symbol.

Atoms for…pace?

At 2 PM local time on January 19, 2006, an Atlas V rocket lifted off from Cape Canaveral’s Pad 41, carrying a craft that was bound for the outermost part of the Solar System – the New Horizons spacecraft. The payload included an RTG – a radioisotopic thermal generator – powered by 7.8 kg of plutonium (Pu-238 to be precise). How this works is that the plutonium gives off a lot of heat – enough to make it glow red-hot – when it radioactively decays and this heat is turned into electricity through devices called thermocouples. There are plutonium-powered RTGs scattered throughout the Solar System – in orbit around Saturn and Jupiter, gathering scientific information on the Moon, orbiting the Sun, roaming Mars, and entering interstellar space as part of the Voyager probes. And, in the early 1970s, this same technology was being implanted into the chests of heart patients to help keep their hearts beat steadily.

At Kennedy Space Center’s Payload Hazardous Servicing Facility, technicians prepare the New Horizons spacecraft for a media event (2005).

RTGs are actually pretty interesting devices. The whole thing starts off with a heat source – a chunk of radioactive material that gives off enough energy to produce serious amounts of heat. For reasons that go beyond the scope of this piece (which is another way of saying that I don’t quite understand the physics…) when you put two different metals together and heat one end to a high temperature then electrical current starts to flow. If the temperature difference is high enough and the radioactive source is producing enough heat then you can use the current to power a meteorological station, a lighthouse…or a spaceship.

Diagram of an RTG used on the Cassini probe.

The USSR used to make RTGs – they were used to power the meteorological stations and lighthouses I noted earlier, as well as some of their space probes. The US sent most of its own RTGs into space and, with an 88-year half-life, the Pu-238 is able to power the craft for decades (the Voyager RTGs have lost a fair amount of their activity after 40+ years but are still putting out enough energy to keep their radio transmissions reaching Earth). On the more negative side, a number of the Soviet RTGs were abandoned in their former subject nations when the Soviet Union collapsed at the end of the Cold War; one of these caused severe radiation burns in a number of woodcutters who found an RTG in the woods in 2001.

The pacemaker is a device with a much longer history than most would guess. As early as 1889 British physician John Alexander MacWilliam had reported that applying electricity to the heart of a person with a serious arrhythmia could shock the heart back into a normal rhythm. This idea was refined over the next few decades and, in 1928 the American physician Alfred Hyman invented an electro-mechanical pacemaker (even using that name), although the idea didn’t really start to take off until after the Second World War.

The next decade or so saw continued improvements, although some of the earlier efforts delivered the current (painfully) through the skin and had to be plugged into the wall. Batteries helped, although the batteries of the day didn’t last very long and, while the cardiologists appreciated what they had they couldn’t help but hope for something better. In the 1970s, that “something better” turned out to be plutonium “batteries” in a device compact enough to be implanted into the patient’s body. And for the next decade – until lithium-based batteries proved to be sufficiently reliable and long-lived – plutonium-powered pacemakers were regularly implanted into heart patients.

Before and during their use the radiation dose and dose rate from these devices was evaluated and turned out not to be too bad. And while the dose rate on the surface of the pacemaker could be as high as 15 mrem/hr, the radiation they emitted was quickly attenuated by the body so the dose to the patient was only about 100 mrem annually, with even lower doses to their spouse. On the other hand, they contained as much as 4 Ci of radioactivity, which is enough to require a radioactive materials license – I wasn’t able to find out if the patient had to be a licensee (although I doubt it), but the plutonium itself needed to be turned over to the government upon the patient’s death.

There were concerns of course – especially about potential radiation dose to the patient. Luckily, Pu-238 emits alpha radiation almost exclusively and its gamma emissions are so rare and so low in energy as to be insignificant. On the other hand, plutonium is highly toxic, requiring careful encapsulation to ensure the patient could not be accidentally poisoned. Thermal shielding was another concern – a large enough plutonium-238 source will glow red-hot – making these pacemakers a bit larger and heavier than most; on the other hand, the plutonium “battery” would last for the patient’s entire lifetime so it would never be necessary to operate again to change the batteries. The biggest downside, however, was the plutonium itself – when the patient died the pacemaker had to be removed and sent off for proper disposal – burying a patient with their pacemaker wasn’t too bad, but cremation was out of the question.

Eventually, the plutonium pacemaker was superseded by non-radiological versions and the plutonium-powered devices are being turned over to the government so they can be disposed of properly.

What strikes me about the plutonium pacemakers is their similarity to other RTG-powered devices. And while their time has come and gone, I think it’s neat to think of a device that can be used to steady a heartbeat, to illuminate a lighthouse on the Arctic Ocean, or to power craft on Mars, circling Saturn and Jupiter, and making humanity’s first push into interstellar space.


I’ve been working in radiation safety and related fields my entire adult life (well…except for a short stint during which I worked the night shift running copier machines on campus). I got used to hearing the standard jokes about glowing in the dark (for example, “Do you need a night light?”), and I have to admit that I came up with some myself. For that matter, one of the nicknames for Navy Nukes (those of us who worked in Naval Nuclear Power) was glow-worms. The thing is – things (including people) who are exposed to radiation don’t glow in the dark. Don’t get me wrong – radioactivity has been used in things that glow in the dark for over a century – but the glow doesn’t come from the radiation directly; it’s a little more interesting than that.

What’s happening is that some materials give off light when they’re hit by the radiation. Looking at the science behind it can get fairly complicated – the short version is that the radiation deposits energy into a substance and that energy goes into (among other things) bumping electrons up to higher energy levels. They only stay there for a short period of time before they drop back down to what’s called the “ground state” – when the electrons drop back down, they give off a greenish or bluish photon. That’s where the glow comes from.

This phenomenon – called radioluminescence – was discovered over a century ago and it was put to use almost immediately. We’re all familiar with watches with luminous dials, making it possible to tell time at night. Radioluminescent paint was also used for aircraft instruments, compasses, gunsights…during the First World War soldiers would even put a small radioluminescent patch on their uniform to help the person behind them follow a little more easily when marching at night. This spread to the private sector as well – luminous paints, clock dials, even fishing tackle and golf balls were painted with radium compounds to make them easier to see in the dark.

Interestingly, this phenomenon had been used even before the war by Ernest Rutherford, a New Zealander whose graduate students spent many long hours with their attention glued to a screen made of zinc sulfide. They were looking for tiny flashes of light that were caused by alpha particles scattering off of atoms in a piece of gold foil, by measuring the scattering angle Rutherford hoped to figure out how atoms were structured. To Rutherford’s great surprise, they noticed that every now and again an alpha particle would bounce directly back from the foil – with what was known about atoms at the time this was as unexpected “as if you fired a 15” naval shell at a piece of tissue paper and it bounced back.” (

So by the time WWI rolled around, radioluminescence was well-known to scientists, the public, and to any number of manufacturers and their customers. The problem is that the people making these things (and those buying them) didn’t know that there were dangers.

One of the things, of course, was the minor factor that these self-luminous products were radioactive. And – yes – they knew that radium was radioactive, but a century ago they didn’t know that radiation caused cancer, or how much radioactivity it took to give someone radiation sickness. In fact, so little was known about the effects of ingested radioactivity that people were prescribed medicines that contained radium that they’d drink for their “health” – to enhance their libido, for example. It wasn’t until an industrialist and amateur golfer by the name of Eben Byers ( died as a result of taking a radium-based patent medicine that the public began taking this sort of thing seriously; the deaths of the radium watch-dial painters ( added to the public’s concerns about radiation and radioactivity. The fact that it was younger girls – chosen because of their steadier hands and attention to detail – who were getting sick and (in some cases) dying made for a tragic story that captured the public’s attention.

So…radioluminescence can be a good thing – making it possible to read a watch or instrument dials or to remain in marching formation in the dark. But it can be bad as well – as attested to by the Radium Girls. Because in the days before automation all of those watch dials were painted by hand, and the great majority of those hands belonged to the young women who were employed in hand-painting watches, aircraft instruments, compasses, and the like.

Young women were chosen for this role because of their steady hands and attention to detail, and the ladies did a great job, as anybody who’s owned one of their watches or other instruments can attest. But much of what they were painting was tiny and making it look good required a fine point on the brushes. To get that fine point, the ladies would “point” the brushes – rolling the tip of the brush quickly on their tongue. In doing so, they ingested a small amount of radium each time and, over the weeks and months that radium became deadly. There are accounts of some of these ladies having so much radium in their bodies that they could cause the luminescent paint to sparkle just by exhaling on it (due to the presence of radon, which forms from the decay of the radium).

A photograph of the “radium girls” who worked at the U.S. Radium Corporation in Orange, New Jersey. This historical event was also recently popularized in the 2018 movie, Radium Girls, directed by Lydia Dean Pilcher and Ginny Mohler.

In the 1920s and 1930s, many of these women became ill and some died from various radiation-related illnesses. This, naturally, garnered media attention and eventually led to some regulatory changes – including contributing to the passage of laws regarding worker compensation for job-related illness and injury. Radium continued to be used for watches into the 1960s (later in some nations) before being phased out.

For the last few decades, the focus has shifted to cleaning up the sites where these watches and other self-luminous products were once made; in fact, I spent a few years managing aspects of a radium remediation in central Illinois. While much of the work was fairly routine, we did find a radium source inside the wall of one home, found some areas where contamination had spread further than expected, and even had to characterize the amount of radium paint used on a flag pole that was about 10 meters high. It was an interesting project; it was also neat to be working on a project that had such an interesting historical aspect.

Radioluminescence has an interesting history – but it’s also useful even today. In fact, I have a few examples in my radiation instrument case – in what are called scintillation detectors. One of my detectors, the one I use for measuring alpha radiation, uses a zinc sulfide crystal – the crystal emits photons of visible light each time it’s struck by an alpha particle. And sometimes when I’m performing an alpha survey it strikes me that the detector I’m using is based on the same principles as the scintillation screen used by Ernest Rutherford when he was teasing out the structure of the atom and the same principles as the phosphor that the Radium Girls were meticulously painting onto watch, compass, and instrument dials at such great cost to so many.

Curies and Sieverts and REM – Oh my!

So how do you measure something that’s invisible? Something that weighs nothing, that we can’t smell or taste, that we can’t feel – how can we know how much is there? And just as important – how can we know how much is dangerous? Somehow we need to find a way to measure it – whatever it is – and then we have to be able to put a number to that measurement. That means that we have to come up with a system of units.

Think for example if you’re impressed by an athlete and ask your friend how fast they are. “Nine-five!” is the enthusiastic reply. And you’re left wondering what in the world that means. Is it 9.5 minutes to run a mile or two? Is it 95 seconds to run a half-mile? Does your friend mean 9.5 seconds to run 40 yards, or maybe to run 100 yards? Some of these times are genuinely impressive, others are remarkably ordinary or unimpressive. Without knowing the units (seconds, minutes, yards, miles) you have no way to evaluate the number your buddy gave you.

So now think about the early days of studying and learning about radiation – there was this phenomenon that scientists knew was important, something that could injure or even kill, but that we had no way to sense.

This was a problem in the early days of using radiation. Early workers found out that x-ray machines could burn the skin and that carrying radium around could cause skin burns as well. There had to be a link between, say, the amount of radium somebody was holding, the strength of an x-ray beam, and the ability of both of these to burn skin.

Part of the problem was finding a unit for radioactivity that made sense, and the committee that was assembled to find a way to quantify radioactivity decided to let Marie Curie solve this problem for them, and to honor her by naming the unit of radioactivity after her. Since radioactivity is characterized by the decay of unstable atoms, it seemed reasonable that the Curie should measure the rate at which these atoms were decaying – Curie eventually decided that the decay rate of one gram of Ra-226, her discovery of which led to her first Nobel Prize, would be an appropriate basis for a unit of radioactivity. Thus, one curie (Ci) of radioactivity is the amount of any material that undergoes 37 billion decays every second – the decay rate of Ra-226[1].

With time the curie has become the American unit of radioactivity and has been replaced with the Becquerel as the SI unit of radioactivity. The Bq was defined as the amount of radioactive material that undergoes a decay rate of 1 decay per second; since 1 Ci undergoes 37 billion decays per second, 1 Ci = 37 billion Bq (or 37 GBq). But what both of these units have in common is that they’re both a measure of the decay rate, which we can only measure with our instruments.

It’s also important to mention that we can’t look at a radioactive source and tell by looking at it how quickly it’s decaying away. One gram of Ra-226 is about 1 Ci of radioactivity, one gram of Co-60 has more than 1000 Ci of activity, one gram of tritium (H-3) contains about 10,000 Ci of activity. On the other extreme, 1 Ci of depleted uranium (U-238) weighs about three tons. A source we can barely see can be deadly while a source the size of a steamer trunk poses only a minor radiological risk.

But simply quantifying the decay rate is only part of the issue of determining the risk a radioactive source poses – we also have to figure out how much those radioactive decays will damage our bodies. This gets into radiation dose.

Ionizing radiation can strip the electrons from atoms, creating two charged particles (the negatively charged electron and the positively charged atom) called an ion pair. Measuring this ionization – the amount of electrical charge created in the air by a source of radiation – was the first way that scientists used to measure radiation dose, resulting in a unit called the Roentgen. One problem with the Roentgen, though, was that it measured the electrical charge produced in air, but we were interested in what was happening to organisms – especially in people – who were exposed to radiation.

This led to still another unit, the rad (the gray in the SI system), which simply measured the amount of energy that was deposited in any substance by the radiation – the rad. A rad was defined as depositing 100 ergs of energy for every gram of the material absorbing the radiation. This made it much easier to determine radiation dose, although the details are beyond the scope of this particular piece.

But there was one more piece of the puzzle that needed to be solved – scientists realized that some types of radiation were more effective at damaging our DNA than were other types. Alpha radiation, for example, turned out to cause about 20 times as much DNA damage as did beta or gamma radiation for the same amount of energy deposited, and neutrons caused anywhere from 5-20 times as much damage. The amount of energy deposition is important, but the amount of DNA damage is even more so.

Consider, for example, a bowling ball versus a ping pong ball – even if they have the same amount of kinetic energy the bowling ball is going to do much more damage to the pins while it’s going to be hard to roll a strike with the ping pong ball. By the same token, an alpha particle that goes zipping through the DNA is more likely to cause the sort of damage that might one day lead to cancer. This factor is known as the Quality Factor (QF) or the Relative Biological Effectiveness (RBE). And this takes us to the final unit – the rem (or the sievert in the SI system). To find out the dose in rem we just measure (or calculate) the energy deposition – rads – and multiply by the RBE. The RBE for beta and gamma radiation is equal to one, so one rad of beta or gamma radiation gives a biological dose of one rem. The RBE of alpha radiation is 20 so one rad of alpha radiation gives a biological dose of 20 rem.

The units we use for measuring radiation, radioactivity, and the biological damage from radiation exposure were developed over the course of several decades, evolving as we’ve learned more about radiation and its effects and growing somewhat more sophisticated at the same time. And, in fact, our units today (rem and sievert) are measuring the amount of damage inflicted on DNA, a molecule that hadn’t even been discovered at the time Marie Curie was defining the unit named in her honor. We’ve come quite a long ways.

[1] Although, with more precise measuring technology we’ve found out that one gram of Ra-226 actually has a slightly different decay rate; the specific activity of one gram of Ra-226 turns out to be 0.986 Ci.

Thorium – From Lenses to Fuels

A little over a decade ago I got a phone call asking me what I knew about thorium. Not much as it turned out – that part of the call didn’t last long. But it led to an interesting consulting project that let me delve into the manufacture of high-quality optical glass and the Second World War (among other things).

Some older photographic lenses were produced with glass elements containing small amounts of thorium. If stored in the dark for prolonged periods of time, these lenses can undergo what is commonly called “yellowing”.

In the first part of the 20th century the highest-quality optics were made in Germany, and even American military suppliers were buying their optical glass from the Germans – lenses that went into periscopes, telescopes, and pretty much anything else that depended on lenses. Part of what made German glass so good was its clarity and its refractive index, both of which were enhanced by adding materials to the glass. Just as dissolving lead into molten glass gives it the clarity, shine, and sparkle of lead crystal, the addition of thorium does the same, making it possible to fabricate thinner and lighter lenses. At the same time, the properties of thorium help to reduce the dispersion of different wavelengths of light (the different colors) as they pass through the glass. The first patents for thoriated glass lenses were issued in 1939 or so with additional patents issued through the 1940s and 1950s. And that led to my phone call, albeit a half-century later.

In the 1930s, when Germany was starting to get a little frisky the American military realized that they might need to find a more reliable supplier for their high-quality optics, so they began contracting with American lens-makers. This led to our becoming self-sufficient in this area. And, in the case of my client, it required a bit of cleanup about 15 years ago.

See, in order to make a lens the manufacturer starts off by pouring glass to form a blank – a piece of glass with the general shape of a lens, just as baseball bat starts with a piece of cylindrical wood of roughly the correct dimensions. And, just as the excess wood is removed by a lathe, the extra glass is ground off using a sort of slurry that contains the grinding compound. The glass that was removed, though, contains thorium. In the 1940s this wasn’t much of a problem – the regulations pertaining to thorium were not very strict. But in the last few decades of the 20th century this began to change and places that hadn’t required much attention earlier were now a potential concern. Which meant that my client needed to have their site surveyed and some subsurface tanks checked to see if they contained any thoriated glass residue (to cut to the chase, they did).

Of course, any lens could benefit from the addition of thorium, and after the war ended it started finding its way in to consumer products – primarily into lenses for high-end cameras. Over the years a number of manufacturers began making thoriated lenses, a practice that continued into the 1980s. And, while such lenses are no longer being made, there are still some around for those who are lucky enough to find them.

My client’s thorium-contaminated site needed to be cleaned up, even though the thorium posed no health risk to anybody. Similarly, while it’s easy enough to get radiation measurements from a thoriated camera or periscope lens from that era, the radiation dose rates are not high enough to cause harm to those using them – with the possible exception of workers who spend many hours weekly looking through thoriated glass eyepieces on microscopes, telescopes, and the like.

It also turns out that thorium is fairly versatile. I have a small container of thoriated welding electrodes that I use for demonstrations when I teach radiation safety classes – the thorium is used for its metallurgical properties just as it used to be alloyed in the metal from which jet turbine blades were made. I also have some gas lantern mantles that contain thorium; these aren’t made much in the US today, although there are still some overseas manufacturers. And then there’s the thoriated toothpaste that I wrote about in an earlier posting. These have all led to lengthy cleanups around the world, including a number in the US.

My last contact with thorium came about 7 or 8 years ago when I was characterizing a thorium-contaminated site for the NYC Health Department – this time working for the City. In this case the thorium was an afterthought – the facility used to produce rare earth elements (back in the 1930s and 1940s), the ores of which were contaminated with thorium. It turns out that thorium is very similar geochemically to rare earths so most rare earth ores also have some thorium in them. Interestingly, in this case the company began producing thorium to sell to the Atomic Energy Commission – thorium went from being waste to being a product. For awhile the government was looking into using thorium for nuclear reactor fuel. That idea was never really pursued vigorously in the US, although I’ve heard rumors that India and China might be looking into it.

Thorium reactors, though, start to get a little complicated and this piece has already covered enough ground – from Germany to the US; from military optics to cameras, welding rods, and jet engines; and from additive to waste to fuel.

If the shoe fits….

When I was younger I remember my father telling me about when my grandparents took him to get shoes when he was a kid – one of the things that really made an impression on him was this device that he could stand on and push a button and he could see an x-ray image of his feet inside the shoes – not only was it a good way to see if his feet were going to be cramped inside his new shoes, but he could wiggle his toes and see his toes and toe bones move in the image he was looking at. I got the impression he’d sometimes hit the button a few times in a row because the image was so neat to look at. The way he described it, it sounded pretty cool – it made me wish I had something like that to play with myself. Alas, I was born too late and these shoe-fitting fluoroscopes were no longer in use.

Awhile later – actually, a few decades later – I found out a little more about the things; in particular, that they’d been outlawed sometime between my father’s childhood and my own. But as I started working in health physics I started hearing occasional tales of the shoe-fitting fluoroscopes, usually with an undercurrent of “Whatever were they thinking?”

When I was in my 30s I worked in radiation safety at two hospitals, one in Ohio and one in Upstate New York. Here, I started learning more about medical fluoroscopes and learned more about how high the dose rate can be in the beam with the machine turned on. In fact, as I found out, people working regularly with fluoroscopes tend to get more radiation exposure than even most nuclear power plant workers, and fluoroscopes cause more radiation injuries than any other device of which I’m aware.

Shoe-fitting fluoroscope
A decommissioned Adrian Fluoroscope showing operator’s controls and three viewing screens on the top of the device. On the opposite side is a low platform with two half-circle holes cut into the vertical back surface for inserting the fronts of the shoes.

At one hospital I worked at there was an older fluoroscopy machine that had a dose rate of 90 rem per minute in the beam. Think about that – it takes about 300 rem to cause skin burns, so leaving that machine turned on for just a little more than three minutes could start to cause burns to the patient. Interestingly, the dose rate from the shoe-fitting fluoroscopes was to be limited to 2 rem to the feet for a five-second exposure, or 24 rem per minute, which is comparable to the dose rate from medical fluoroscopy machines in the 1990s and 2000s (newer machines have lower dose rates). On the other hand, a lot of machines didn’t comply with the guidelines – some were measured as having dose rates of up to 75 rem per minute. On one of those machines, every time my father wiggled his toes, he was getting several rem to his feet. If he used one monthly, he’d have been close to today’s annual 50-rem dose limit to the extremities.

In the 1950s there were also concerns that irradiating the feet and legs of children might affect the growth plates in their leg bones, affecting their future height – given the fact that my father is above-average in height, however, it seems that he didn’t suffer from this. Although, I guess I should add that I was about four inches taller than him, so who knows how tall he might have been….

I suppose that’s what a little more personally interesting to me is the waist-level dose – the estimate there was anywhere from about 30-170 mrem to the pelvis for each 20-second exposure. So if my father had spent 20 seconds wiggling his toes each month he’d have received as much as 2 rem of gonad dose every year. And it’s reasonable to wonder if this might have had any effect.

It turns out it didn’t. And I’m not basing this on what I see in the mirror every day (as impressive as that view might be!) but, rather, on the understanding we’ve accumulated over the years on the reproductive effects of ionizing radiation. And it turns out that there’s no evidence that pre-conception radiation exposure is linked to any sort of health effects at all in children conceived later. So none of the kids who were wiggling their toes back in the 1930s, 1940s, and 1950s had to worry about their future kids.

In fact, it turns out that the kids (and adults) who were playing with the shoe-fitting fluoroscopes don’t appear to have been affected at all – but some of the sales staff were because they might be using the machines repeatedly every day, with their hands in the beam as they squeezed the shoes (and the feet within) to show how the shoes might fit under different circumstances. One saleswoman, for example, developed a case of dermatitis from having her hands in the fluoroscope beam so often; there was also an incident in which a shoe model had her feet in the machine so often that she developed skin burns and other radiation damage – her lower leg eventually had to be amputated.

As we learned more about the effects of radiation on health, as public concerns about radiation grew, and as regulations started ratcheting down these devices eventually went away – by 1960 or so they were pretty much extinct in the US. The consensus is that the health effect on the customers was negligible; likely because even kids don’t get new shoes all that often.


From the Chest to the Cosmos

A solid chunk of plutonium-238 (Pu-238) is so hot that it glows red-hot. Where the heat comes from is the radioactive decay of the plutonium – every time a plutonium atom decays it gives off a high-energy (5.6 MeV) alpha particle and all of this energy is deposited in the chunk of plutonium. A single alpha doesn’t have enough energy to make plutonium glow – but every single gram of Pu-238 undergoes enough radioactive decays to produce slightly more than a half-watt of heat; one kilogram will generate more than 500 watts – in two hours this piece of plutonium will produce 1 kilowatt-hour of energy, enough to light a small home. Incidentally, 1 kg of Pu-238 will fit inside a typical shot glass. This isn’t the place to go through the detailed calculations, but it’s fairly easy to see that it doesn’t take much plutonium to produce a fair amount of energy – if we can figure out how to use it.

Luckily, there are these things called thermocouples! It turns out that if a thermocouple is heated on one side and cooled on the other – more specifically if there is a temperature differential from one end of the thermocouple to the other – it will generate electricity. Which means that if we simply let the plutonium do what it wants to do – undergo radioactive decay and heat up – we can use thermocouples to extract some of that energy to produce electricity. Then the question is what to do with it.

One of the things we can do with this energy is to power spacecraft. We’ve sent plutonium-powered RTGs to Mars to help search for evidence of past or present life. Other RTGs are currently orbiting Jupiter, RTG-powered spacecraft have explored Saturn, zipped past Uranus, Neptune, and Pluto, and the RTG-powered Voyager craft are currently humanity’s first probes to enter interstellar space. Plutonium power has carried, if not humans, then our robot explorers throughout the Solar System – the heat from Pu-238 alpha radiation, even after a half-century of decay, is still powering our most-distant probes.

Interestingly, we’ve used plutonium to power things much closer to home.

Over a century ago, in 1889 Scottish physiologist John Alexander MacWilliam realized that applying electrical current to the heart could help to keep it beating regularly. Over the next 50 years his observations were refined and provided the foundation for what evolved into the modern pacemaker.

The beating of our hearts, something that happens tens of thousands of times daily from before we are born and through the entirety of our lives, is triggered by a tiny electrical impulse to a small cluster of cells called the sinoatrial node, this propagates through the heart, causing the four chambers to contract in the proper sequence to send blood flowing through our bodies. The problem is that this system can malfunction, causing erratic patterns that can range from annoying to life-threatening. What MacWilliam discovered was that a stable heartbeat could be triggered by electrical signals from outside the body as well as through the natural system. Over the years the equipment needed to produce this regular current has become increasingly smaller, to the point where it can now be carried inside the body; the devices have also become increasingly sophisticated and able to not just produce a regular series of electrical pulses, but to adjust to different levels of physical activity and even to recognize and respond to unanticipated arrhythmia as they arise.

The problem is that it takes energy to produce this electrical signal and even more energy to power the electronics that give modern pacemakers so much capability. Early pacemakers had to be wired into an external power supply because batteries didn’t hold enough energy to power a pacemaker for an entire day. But in the 1960s and 1970s engineers realized that the heat of radioactive decay could be used to power pacemakers just as easily as they could power spacecraft. This led to the development of a plutonium RTG-powered pacemaker that could be implanted into the body, requiring no battery changes, no recharging, and that would last for decades.

Pictured here is a Medtronic plutonium-powered pacemaker from 1974.

Using plutonium raised some concerns of course, because most people aren’t quite comfortable with the idea of implanting radioactivity into their bodies. But it turns out that the 2-5 curies of plutonium they used doesn’t emit high levels of radiation – the radiation dose rates at the surface of the pacemaker were only about 5-15 mrem/hr and the patient would only receive about 100 mrem annually to their whole body – even over a lifetime this is not enough radiation dose to cause problems.

Plutonium-powered pacemakers were only used for a short time before being replaced with rechargeable lithium batteries; at present, there’s still a small number of them in the chests of patients here and there, but only a handful. Like so much other technology, they had their time, but that time has passed. But it’s sort of cool to think that, for awhile, there were people walking around with the same technology implanted in their chests that was also roaming the outer reaches of the Solar System.

A Glowing Smile

About a decade ago I was reading an account of Allied activities in Germany at the tail end of World War 2. The Allies knew that Germany was working on a nuclear program of some sort – there was evidence that it was a nuclear weapons program – and we were trying to find out how far along they were since a Nazi nuclear weapon would have been able to turn the war back in Germany’s favor. Consider, for example, the effect of a nuclear attack against the landing forces on D-Day, or against Russia’s forces as they advanced towards Berlin – or a nuclear weapon arriving over London and Moscow, delivered by V2 rockets with the attacks ordered by a leader with no compunctions about visiting utter destruction upon his foes.

As part of their operations, the Allies came to realize that Germany seemed to be buying up the world’s supply of thorium and they grew concerned that Germany might have found a way to make nuclear weapons out of thorium. It’s not out of the question – the primary isotope of thorium is Th-232 which, if it captures a neutron, will end up as U-233, an artificial isotope of uranium that fissions as easily as the U-235 used in the Little Boy nuclear weapon dropped at Hiroshima. So when Leslie Groves, the general overseeing the Manhattan Project, learned about German thorium purchases, he launched an investigation to find out if the focus on uranium and plutonium had somehow overlooked another viable path to nuclear weapons.

If thorium really did provide such a path then it could be important to the American effort as well. The main reason – and one of the reasons that there are so many proponents of thorium as a fuel for nuclear reactors – is that there is four times as much thorium on Earth as there is uranium and, unlike uranium (in which only one atom in 139 is fissionable), every single atom of natural thorium can become an atom of fissionable U-233. So instead of having to laboriously separate U-235 from U-238, a thorium weapon would only have to bombard Th-232 with neutrons and then chemically separate the resulting U-233. That being said, there were some difficulties in working with this process but if Germany has mastered them, perhaps we could as well. So an investigation was launched.

What they found had nothing to do with nuclear weapons – not even with nuclear energy. Rather, it had to do with cosmetics.

During the first part of the previous century, radium was all the rage, and radium was added to patent medicines, toothpaste, and there was even a line of condoms named “Radium” (although it doesn’t look as though the condoms were impregnated with radium!). A German cosmetics company, understanding that the war was coming to an end, was trying to figure out how to increase their business in peacetime and they hit upon a variation of this trend – if radium could be added toothpaste, why not thorium? So they set out to stock up on thorium for their post-war toothpaste line.

Doramad Advertisement for Doramad Radioactive Toothpaste

Doramad Radioactive Toothpaste (Doramad Radioaktive Zahncreme) was produced in Germany by Auergesellschaft of Berlin from the 1920s through World War II.

In hindsight, it seems their idea and their imagined product line were both somewhat overly optimistic. Oh – and as far as using the Th-232 → U-233 method for making nuclear weapons…the US (1955), Soviet Union (1955), and India (1998) have all detonated weapons that used U-233 for some or all of the fissionable mass. However, for a number of reasons, U-235 and Pu-239 continue to be the materials of choice for nuclear weapons.

Image reference:

Title: “Doramad Advertisement”, Creator:, Source:, License: This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license.

How Dangerous is 100 Micro Roentgen/hour?

There are actually two issues here – one is about the safety of the dose rate (1 microGy or 100 microR per hour); the other is whether or not that dose rate is accurate.

Let’s tackle the first one first. A dose rate of 100 microR/hr (1 microGy/hr) is not dangerous. If this dose rate is accurate, living in it continuously (8760 hours per year) will give you a radiation dose of about 0.9 R/hr (9 mGy/yr). This is not a trivial dose – it’s about three times as much as what we’re normally exposed to in a year from natural sources (on average). At the same time, it’s less than 20% as much as nuclear workers in the US are permitted to receive in a year and a little less than half of what radiation workers in Europe are permitted to receive in a year. In addition, it’s less than half the radiation dose rate I measured in Ramsar Iran, which has the highest natural radiation levels of any inhabited place in the world. The residents in Ramsar do not appear to be suffering any ill health effects from their exposure there – it seems unlikely that the dose rate you mention will cause any harm to you.

Now – on to the second question!

One thing that you have to determine is whether or not the readings you note are accurate, and a lot of that depends on the exact kind of radiation instrument you’re using. I used a GM instrument in the Navy and I continue to use them today – they’re incredibly useful. But I also recognize their limitations; one of which is that they’re not very accurate at measuring radiation dose rate – especially from low-energy gamma radiation and even more so at low dose rates. One of the first questions I’d have to ask is whether it’s a digital display, or an electro-mechanical one with a needle pointing at the dose rate. If it’s the latter, I’d also be interested in knowing if the meter is on the very lowest scale with the needle pointing at the very lowest tick mark on the meter face – if this is the case then I would take that reading with a considerable grain of salt; in general, I try to use a meter only when the reading is somewhere between about 20–80% of the range of the scale.
Another question to ask is the size of the GM tube – a larger (and more expensive) tube is more accurate than a smaller, cheaper one.

But the main factor is that GM tubes – unless they are a type of tube called “energy-compensated” – are not accurate at measuring radiation from more than one specific energy. So if your GM was calibrated (for example) using Cs-137 – which has a gamma energy of 662 keV (1 keV = 1000 electron volts) then it can only accurately measure radiation dose from gammas of that exact energy. If you’re using that to measure natural background radiation – with a lower average energy – then the reading is going to be off by a factor of up to 10. This is because the meter “expects” that every bit of radiation entering it has the same energy as Cs-137; if the radiation is lower-energy then the reading will be higher than the actual dose rates.

Anyhow – my best guess is that the dose rate displayed by your instrument is likely not accurate for the reasons given here. But even if it is accurate, this level of radiation exposure should not be harmful.

How Do You Receive Radioactive Materials?

Hi, Dr. Zoomie – I’m working on a radioactive materials license application and it says I need to have a procedure for receiving radioactive materials. What are they looking for?

Virtually every radioactive materials license is going to require you to tell the regulators how you plan to receive radioactive materials at your facility – what precautions you plan to take, what checks you’re going to perform, and so forth. You might only receive radioactive materials once a month – maybe only once a year. Or, on the other hand, if you are at a nuclear pharmacy, a large hospital, or a large research university then you might be receiving multiple packages daily. However frequently you receive shipments, though, you’ve got to have a procedure to make sure it’s done correctly.

The easy way to do it is to commit to using the model procedure that your regulator has almost certainly developed. For example, one of my consulting clients (they had what’s called a broadscope radioactive materials license) had a line in their license application that simply stated “For receipt of radioactive materials we commit to using the model procedure found in Appendix I of NUREG 1556 vol. 11 (Consolidated Guidance About Licenses of Broad Scope).” And that’s all you really need. You can certainly draft your own receipt procedure, but if you do so then you have to be able to show that your procedure is at least as good as the model procedure.

There are a couple of things that have to be part of your procedure – whether you write your own or use the model procedure.

  • All radioactive packages should be delivered directly to the RSO if at all possible.
  • If the RSO is not available (vacation, illness, travel, restroom, etc.) then the package should be placed in a secure location until the RSO can retrieve it.
  • Alternately, the RSO may designate qualified radiation workers to receive radioactive packages in his/her absence.
  • Each package needs to be visually inspected for damage or evidence of leaking contents, surveyed for radiation dose rates (and possibly contamination), and the contents checked against the shipping papers. Most of these checks are required to be performed within three working hours of the package delivery.
  • All of these checks and surveys must be documented and you are required to maintain these records.
  • And if any contamination limits or radiation dose rates are excessive, you need to let the carrier and your regulators know as soon as possible.

With regards to the first point (delivery directly to the RSO), this is important. I worked in radiation safety at one university where a radioactive package was somehow lost between being signed for by University Receiving and delivery to Radiation Safety. In another, a man was ordering radioactive materials to be delivered to him personally, then sending them out to colleagues of his overseas. In both cases, the problems was solved by requiring all radioactive materials to be delivered only to Radiation Safety (and in the latter case, the man was arrested).

Finally, one last thing to consider….

If you regularly receive packages of radioactive materials you should consider having a dedicated location for this purpose. For example, perhaps you can take a corner of a workbench to cover with a benchpad (e.g. plastic-backed absorbent paper). In addition, you should have a secure storage location where the packages can be stored until you can perform the receipt inspection and surveys – and where you can store the materials until they’re moved to their permanent storage or use location.