Monthly Archives: January 2021

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.