Monthly Archives: September 2020


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.

References:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1520288/pdf/califmed00247-0028.pdf

http://large.stanford.edu/courses/2011/ph241/birer2/

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 arrythmia 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.

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.