Author Archives: Dr. Zoomie

Are Power Lines and Other Electromagnetic Fields Dangerous?

Dear Dr. Zoomie – I keep hearing about how dangerous electromagnetic fields are. Should I be worried about the power lines near my house? And the wiring in my house? And my electric razor? And all that other stuff? I don’t want to be Amish – but I don’t want to get sick either!

The first time I came across this question was over 20 years ago – and it was about a decade old even then. In my case, my father was trying to sell a house that was close to some high-voltage power lines and he couldn’t understand why people didn’t want to buy it. Someone finally told him that they were worried about electromagnetic fields around the power lines. My dad asked me what I could tell him about the science behind these worries. The short version is that these fears were unfounded and the risks from power lines – and electromagnetic fields in general – is vastly overstated. Here’s the longer version of the story.

Are Power Lines Dangerous?

Are Power Lines Dangerous?

This whole story starts in the late 1970s with the publication of a paper suggesting that overhead power lines (and the electromagnetic fields they produce) were associated with cases of childhood leukemia. Although nobody was ever able to show how these fields could cause this disease, some scientific studies received a lot of publicity. There were dramatic videos of people holding up fluorescent light bulbs near high-voltage power lines – the electromagnetic field was enough to cause the bulbs to light up. Ever since these first studies came out people have been worried about electromagnetic fields and their possible health effects.

The problem is that the initial studies have all been discredited and subsequent studies have very clearly shown that electromagnetic fields aren’t dangerous – at least, not at the levels we find near power lines or in our homes. There’s a great summary article online (written by physics professor John Farley) about this on the Quackwatch website, summarizing the history of this debate – it also features prominently in a great book called Voodoo Science by physicist Robert Park. And there have been any number of reports in the intervening years – including some by the National Research Council – that have shown this to be a non-issue. Part of the problem, though, is that one side shows photos of kids dying of cancer while the other side shows calculations and academic studies – the emotional impact of the one side far outweighs the scientific impact of the other. But first, let’s look a little at the science.

First, you’ve got to understand that the Earth has its own electromagnetic field and every creature that has ever live on Earth – including humans – has been exposed to these fields from birth. As with radiation, we have to remember that any exposure to man-made fields is in addition to our exposure to natural fields – if the magnitude of the man-made fields is small compared to the natural ones then we have to consider that the man-made fields might not be that dangerous.

Earth's Magnetic Field

Earth’s Magnetic Field – The Earth’s magnetic field varies from about 300-500 milliGauss (unit of measurement of magnetic field strength) while the magnetic fields from power lines are only a few milliGauss.

According to both Park and Farley, the strength of the electromagnetic fields produced by power lines is very small compared to natural fields. For example, the human body is electrically conductive – we’re pretty much filled with salt water and salt water conducts electricity quite well (pure water, by comparison, is a lousy conductor). If you move any conductor through a magnetic field you’ll induce an electrical current – as we walk (or drive or fly) through the Earth’s magnetic field we induce electrical currents in our own bodies. The fact is that the electrical and magnetic fields induced by high-voltage power lines are much smaller in magnitude than are the natural fields we’re exposed to on a regular basis. To put some numbers on it – the Earth’s magnetic field varies from about 300-500 milliGauss (unit of measurement of magnetic field strength) while the magnetic fields from power lines are only a few milliGauss. If small variations in magnetic field strength can cause health effects then we’d expect to see much greater health effects among people moving from place to place on Earth. The fact that we don’t see these changes (for example, cancer rates are about the same in the North and in mountainous states as they are in the South and in low-lying states) suggests that the much smaller variability from power lines won’t be harmful.

Something else to consider is what I touched on earlier – there is no plausible mechanism for how external magnetic fields can cause cells to become cancerous. Think, for example, if someone gave you a can of gas and told you it could get you home. Without a vehicle of some sort the gas isn’t going to get you anywhere – you need a car to turn gasoline into mileage. At present we can’t find any way to turn external electrical or magnetic fields into genetic damage – we’ve seen nothing in human experience or in animal studies, including those of mice exposed to as much as 10,000 milliGauss.

It also turns out that the original study had some problems, the biggest one being that the authors of the study never actually measured magnetic field strength. Once follow-up studies were done that did make this rather important measurement it turned out that there was no correlation at all. Not only that, but the initial studies looked at only relatively small numbers of people. When larger studies were performed, including measuring magnetic fields, the apparent correlations melted away.

On top of all this, we’ve got to look at what’s possibly the most important piece of information – age-adjusted cancer incidence rates have been dropping steadily for several decades in spite of the fact that our exposure to electromagnetic fields has increased astronomically in those same decades. Think about it – we live our entire lives surrounded by electromagnetic fields – the wiring in our homes, overhead power lines, our computers and monitors, electric razors, televisions and entertainment systems, microwave ovens, and so forth and so on. Seventy years ago, many Americans didn’t even have electricity in their home, and those who did used it primarily for lighting – today, we use it for everything.

All this being said, are there things about the health effects of electromagnetic fields that we don’t know? Of course. We don’t – we can’t – know everything. And, let me add, the fact that we don’t know everything is often used as rationale for continuing to be frightened while further studies are being performed. The question, though, shouldn’t be “Do we know everything?” so much as “Do we know enough?” All of the evidence to date tells us that we know enough to conclude that these levels of electromagnetic fields are not causing harm. There is an awful lot of scientific evidence and scientific reasoning that tells us that electromagnetic fields aren’t nearly the hazard they’re portrayed to be.

At the same time, we know that we get a lot of benefits from the use of electricity – if we’re going to look at the potential downside then we also have to look at the benefits and the billions of lives that have been made better by its use. Let’s think about it for a moment – even if there’s a small risk from using electricity, it makes possible things like street lights, x-ray machines, computer control systems, aluminum and steel manufacture, air conditioners, elevators, and so much more. Electrical power makes our lives better, longer, and healthier – stopping (or even scaling back) its use would certainly add risk to all of our lives. We know that driving puts us all at risk – in the US, almost 1% of us will die in a traffic accident – but we accept this risk because of the benefits we derive from cars and trucks. Similarly, even if (against all scientific evidence) electromagnetic fields turns out to carry with them a small risk, I would argue that we derive far more benefit than risk from their use – if our goal is to make our society as safe as it can be then we should continue our use of electricity.

Finally, for what it’s worth…I have read up on this topic, if only to find out if my father (and those who eventually bought his house) faced any risk. While I’m neither a physicist nor a physician, I understand the science well enough to follow the scientific papers I’ve read and they make sense to me. After taking a look at the science and the epidemiological evidence and after reading the conclusions of scientists I respect (Park and Farley) I’ve decided that this is something I’m not going to be worried about. So I use my electric razor, my computers, my microwave, and all of the other electrical and electronic stuff in my apartment. I guess you’ve got to decide for yourself what you feel comfortable with, but I’d suggest that your concerns about electromagnetic fields might be misplaced.

How to Do Radiation Surveys and Contamination Surveys

Dear Dr. Zoomie – I’m trying to figure out what goes into a radiation survey program and I have to admit I’m drawing a blank. Can you tell me how often I should be doing radiological surveys? Also, can you tell me how to do a survey? Thanks!

I should start by saying that I can often make a pretty good guess about the quality of a radiation safety program by looking at the quality of their surveys. In general, if I audit your facility and see that you’re doing your surveys properly then it’s pretty safe to say that the rest of your radiation safety program is likely to be squared away; at the same time, if you’re missing surveys or being slipshod in your survey technique I’m probably going to find other problems as well. Now, let’s talk about what goes into a survey program and how surveys are performed.

When to survey

We’ll start with when you should be surveying. First, there is no regulatory requirement on this – your survey requirements will be set by your internal procedures, your license application, and your license conditions. You’ll have to use your own judgement as to how often various areas need to be surveyed – if you don’t have the experience to make this decision on your own it’s not a bad idea to ask a consultant for suggestions, or even to ask your regulators what they recommend. Here are a few things to think about:

  • Will people be using unsealed sources of radioactivity? For example, are they working with radiopharmaceuticals or radiolabeled compounds that can cause a spill? If so, you might want to ask people to survey workbenches or fume hoods for contamination daily when the area is in use and to survey the entire room (laboratory, hot lab, etc.) monthly.
  • Are there activities taking place that can be expected to cause contamination? For example, in a rad waste storage room, are you compacting waste, crushing vials, or moving a lot of packages? If so then you should consider surveying for contamination at least weekly, as well as after any potentially contaminating activities.
  • Are you storing radioactive materials in the area? If so then you should consider surveying for radiation at least every six months, as well as after any movement of radioactivity into or out of the area.
  • Do you have radiation-generating machines (x-ray, electron microscopes, etc.)? If so, you might be required to survey annually for radiation leakage, scattered radiation, and/or the effectiveness of your shielding. If the device will be used for medical diagnosis or treatment there will be other requirements as well, including routine quality insurance checks on a daily, weekly, monthly, quarterly, or semi-annual basis.
  • Have you had maintenance on anything that could affect your radiation shielding? Have you had an earthquake, flood, or anything else that could damage your shielding? If so, you should perform a radiation survey as soon as conditions stabilize to make sure the shielding is still intact and doing its job.

These are some of the most common circumstances and your facility might not fit into any of these categories. The important things are to think about how frequently people are using radiation and radioactivity, how potentially dangerous it can be, and what can happen that could cause radiation or contamination to be higher than what you’d normally expect (or want) them to be.

The next part of this discusses (briefly) how to perform radiation and contamination surveys. But please not – this is not the same as receiving formal training – this is a guide, but you should really receive appropriate training and develop a formal survey procedure before you do your own surveys. And remember – before you start ANY survey, make sure your instrument is in calibration, check to make sure the batteries are OK, ensure the meter, probe, and cable are all in good condition, and (for count-rate instruments) make sure to response check against a source of known strength.

Performing radiation surveys

Performing radiation surveys isn’t too difficult – mostly you’ll be walking around with your radiation dose-rate meter watching the dial; make a note of the dose rate on your survey map anyplace from time to time, especially in places where dose rates are higher than the rest of the area being surveyed. As a default, hold your meter about waist-high unless you’re measuring a specific location (say, in front of a source storage safe or a low-temperature freezer). Finally, you’re most interested in dose rates in “accessible areas” – that’s about one foot (30 cm) from any surface, and only in areas where a person could actually be expected to enter. So you don’t need to survey inside of refrigerators or fume hoods unless you expect people to spend a lot of time inside of them. Oh – and make sure that your meter has been calibrated within the last year so that your survey counts! All of your meters have to be calibrated annually (according to regulations) and you can’t meet a legal requirement with an illegal meter.

Chuck Surre, a University of Rochester radiation safety technician is shown here performing a radiation survey around some drums of just-compacted radioactive waste in the waste storage room.

Chuck Surre, a University of Rochester radiation safety technician is shown here performing a radiation survey around some drums of just-compacted radioactive waste in the waste storage room.

Performing contamination surveys

The key to any contamination survey is “low and slow.” You want to keep the detector as close as possible to whatever it is that you’re surveying without being so close that you contaminate the detector. And you want to move the detector no more than about 2-3 inches (about 5-8 cm) per second. If you hold the detector too far away you can miss some contaminated areas, and if you’re surveying too quickly then the probe might not be over a contaminated area long enough to pick up any counts. You should also try to survey as much of the surface as possible – 100% of the surface if you can – to avoid missing any contamination.

Alternately, you might want to perform a smear wipe survey to look for removable contamination (contamination that could come off on your hands or feet) – especially if you’re looking for radionuclides (such as tritium or carbon-14) that aren’t easily detected by hand-held radiation detectors. For a smear wipe survey you’ll need to use a piece of dry filter paper (a Watman or Milipore filter will do the trick) and you’ll need to wipe an area of 100 square centimeters (about 4”x4”). Apply enough pressure to the wipe to pick up any loose contamination, but not so much that you tear the wipe.

After you’re done with your survey you’ll have to records your results on a survey map; your survey map will have to be filed and maintained for three years (under normal circumstances) or longer if your survey is used to reconstruct the radiation dose to one of your workers. And – again – remember that there’s more to doing surveys than what’s provided here; this will give you a start, but there’s a lot more to the topic than what’s written here.

Sample Forms

  1. radiation_and_contamination_survey_procedure (PDF)
  2. radiation_and_contamination_survey_procedure (DOC)
  3. sample_radiological_survey_report_form (DOC)
  4. sample_radiological_survey_report_form (PDF)

Smugglers Selling Radioactive Materials to ISIS – What If They Succeed

Dear Dr. Zoomie – I just read in the news today that they caught criminals trying to sell radioactive and nuclear materials to ISIS in Moldova. What gives? I thought it took a nation to make a nuclear weapon – I’m assuming I should be scared; how scared should I be? Do I need to update my life insurance?

The threat of radiological and nuclear terrorism has been a concern for a number of years and this article is only the latest of a number of articles on this threat. There’s a lot in here so let’s take things one at a time, starting with the easiest.

The article mentions the isotope Cs-135 (cesium-135) as possibly being available for purchase by terrorist organizations to use to make a radiological dispersal device (the so-called “dirty bomb”).  There’s been a lot written about dirty bombs and I won’t add to that here except to note that ISIS has mentioned their pursuit (or possession) of radioactive materials to use in making such a device. This particular isotope (Cs-135) is an odd one to mention – it’s neither as widely used nor as dangerous as Cs-137 – it’s produced in nuclear reactors but is usually disposed of as radioactive waste. It also has a fairly long half-life (so it’s not intensely radioactive) and it emits beta radiation, which is not terribly dangerous. So this nuclide could be used as a nuisance, but it poses much less health risk than Cs-137. The fact that this nuclide is being offered suggests that either the person doesn’t know exactly which nuclide they have, that they’re making something up, or that they have access to spent (or reprocessed) reactor fuel or nuclides that can be scavenged from it.


Members from the Oregon National Guard’s 102nd Civil Support Team approach a mock crash scene, ready to take readings and assess levels of contamination by a “dirty bomb” at a joint-agency exercise held Feb 15, 2006 at the Portland Fire Department’s training facility in north Portland.

The article also mentions plutonium. There is a fissionable isotope of plutonium that’s used in nuclear weapons (Pu-239) but this is produced in nuclear reactors and then must be chemically separated from the spent fuel using some fairly complex chemistry. Pu-239 is too valuable as a fissionable material to be used in a dirty bomb, and it’s a lot harder to fashion into a nuclear weapon than is uranium – chances are that, if plutonium is being sold as a potential RDD material, it’s more likely to be Pu-238. Pu-238 is fairly common – although it’s not fissionable (so it can’t be used to make nuclear weapon) it is fairly highly radioactive. Its primary use is in spacecraft in the form of radioisotopic thermal generators (RTGs) due to the high levels of heat emitted by the decay of Pu-238 atoms. It’s also toxic, although (contrary to common misconception) it’s not the most toxic substance known to science. In any event, an RDD made with Pu-238 would be a great way to make a mess, but it’s not likely to put many (if any) people at risk since the radiation it gives off (alpha radiation) is fairly innocuous unless it’s inhaled or ingested. It would, however, make a huge mess – it could contaminate a large area, cause a huge financial impact, and would likely cause a degree of panic – but the perceived health risk would very likely far outstrip the actual danger.

Finally we get to the weapons-grade uranium that was mentioned.  The nuclear weapon that was dropped on Hiroshima contained a little over 60 kg of highly enriched uranium; this story mentions that the seller had 100 grams of weapons grade uranium with him and promised to deliver it in 1-kg batches at a cost of about $32 million per batch.  To make a Little Boy-type device would require over 60 transactions and a total cost of two billion dollars. The financial part of it is beyond the reach of most (but not all) of our enemies – this narrows the list of potential buyers. Equally important is the sheer number of transactions that would need to take place for a terrorist group (or rogue nation) to get their hands on a bomb’s—worth of material. Sixty transactions greatly increases the odds of being caught by law enforcement or intelligence agencies and having the whole plot unwind. This isn’t to say that this would be impossible – but the more complex a plan is, the less likely it is that it will be successful. It’s certainly possible that a criminal organization might have access to large quantities of weapons-grade uranium, but it’s also possible (perhaps more likely) that the seller has just enough of the material to get rich and that the supply would dry up after just a handful of sales. So, while we have to take this report seriously, I’m not sure that it’s time to start buying nuclear bomb insurance.

Taking all of this together, what have we got?

Well, first we now that there’s a continuing interest in putting radiological or nuclear materials into the hands of groups that wish us ill. And if organized crime organizations really have found sources of radiological or nuclear materials – and if they have found buyers – the possibility of an attack of some sort is quite possibly higher today than in the past. On the other hand, there have been attempts at radiological and nuclear smuggling for at least a decade – what makes this report different is the apparent involvement of organized crime.  The biggest unknown is whether this group has access to enough uranium to make a weapon and whether or not a terrorist group has access to enough money to buy it all.  Right now we just don’t know the answers to these questions.

Is Nuclear Energy the Solution to Global Warming, and is it Safe?

Dear Dr. Zoomie – I’m worried about global warming because I live near the beach. The pro-nucks tell me that nuclear reactors don’t give off any greenhouse gases, but the anti-nukes tell me that you can’t make a safe reactor and we’ll just keep having more meltdowns will contaminate the whole world. What’s the real story?

Thanks – it’s been awhile since I’ve been asked a question that deals with the fate of the world. I guess, for starters, I have to say that I don’t think that nuclear energy is the solution to our problems – but I think that it’s part of the solution. I should also say up front that, while I have never worked for a commercial nuclear power plant (or for any sort of commercial power plant), I did spend 8 years in Naval Nuclear Power, including two years as an instructor and four years on a nuclear submarine. I also spent nearly two weeks in Japan after the Fukushima accident, helping to train emergency responders and medical responders in how to care for patients coming from the contaminated areas. So I think I’ve seen both some of the good and bad that nuclear power can offer – I hope that this helps me to be objective.

The Fukushima I Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. Reactor 1 to 4 from right to left.

The Fukushima I Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. Reactor 1 to 4 from right to left.

So – this is one of those “Do you want the good news or the bad news first?” sort of things. I normally go for the bad news first, so let’s start there. The bottom line is that, in the 70 years since Fermi oversaw the world’s first reactor criticality, the world has had three serious nuclear accidents in the nuclear age as well as a number of near-misses or less serious accidents. The result of these accidents is that large swathes of land in Japan, Ukraine, and Belorussia are currently evacuated and might not be reoccupied for decades. Huge amounts of radioactivity were put into the environment – the fallout cloud from Chernobyl and Fukushima blanketed the Northern Hemisphere, and Fukushima also dumped large amounts of radioactivity into the oceans. The accidents themselves have cost hundreds of billions of dollars and have had a global impact on the way that we view nuclear energy. And – at least in the case of Chernobyl – people have died. I should add that other accidents have killed people as well.

Chernobyl Disaster Aftermath:very extensive damage to the main reactor hall (image center) and turbine building (image lower left)

Chernobyl Disaster Aftermath:very extensive damage to the main reactor hall (image center) and turbine building (image lower left)

On the other hand, I should also point out that the World Health Organization conducted an extensive study of the people around Chernobyl in 2006 – on the 20th anniversary of the accident – and concluded that fewer than 100 people have died of radiation sickness or from radiation-induced cancer from that accident, and they also concluded that nobody in Japan will die from radiation-induced disease (including cancer) from Fukushima. Nobody got sick or died from the Three Mile Island accident, and only a handful of people have died from any of the other nuclear reactor accidents that have taken place. When we compare the number of deaths per gigawatt-hour of energy produced, nuclear actually stacks up quite favorably to other forms of energy – coal mining, hydrocarbon extraction and processing, and transportation of these fossil fuels are dangerous activities that cause hundreds to thousands of deaths annually; far more than the deaths from these accidents. And that doesn’t even get into the health effects of smokestack emissions, acid rain, and so forth. I’m not trying to say that nuclear power is harmless – just that we have to remember that no form of energy is without risk.

The Diablo Canyon Power Plant in San Luis Obispo County, California. This electricity-generating nuclear power plant near Avila Beach has operated safely since 1985. Photo by marya from San Luis Obispo, USA: http://

The Diablo Canyon Power Plant in San Luis Obispo County, California. This electricity-generating nuclear power plant near Avila Beach has operated safely since 1985. Photo by marya from San Luis Obispo, USA: http://

Ah – I hear you say – but if we build more nuclear reactors it’s only a matter of time until the next accident, and the more reactors we build the more accidents there will be. Well – yes and no. If we assume that the risk of an accident is the same for every reactor built then yes, this logic makes sense. But reactors have been getting safer with time – reactors built now are much less likely to suffer catastrophic accidents than those built in the 1980s. And – in case you’re wondering – factors that make reactors less likely to melt down or to have catastrophic accidents include operating at lower pressures (we can’t avoid high temperatures), simplifying the engineering designs, and making use of basic laws of physics to provide cooling in an emergency rather than pumps and other reengineered safeguards that require power to operate properly. Reactors powered by thorium, for example, have a number of features that make them almost impossible to melt down (sorry – this is far too much to get into here, but maybe at a later time; in the interim, is a site where you can find some information if you’re interested). So if today’s reactors are safer than those built in the past, doubling the number of reactors won’t necessarily double the number of accidents. The bottom line is that some of the newest designs are virtually melt-down proof – not completely so, but pretty close. The bottom line is that the risk of meltdowns might increase, but not as much as one might expect. In fact, if we replace older reactors with the newer designs (in addition to building new ones), the risk of a meltdown might actually drop somewhat.

One last thing that’s touted as being a risk from nuclear reactors is the radioactive waste that they produce – more specifically, where it can be stored safely. At the moment the US has no long-term radioactive waste solution, but this is more a function of politics than of science. Again, there’s not enough space here to go into a full-blown discussion about radioactive waste disposal, but here’s something to consider: about two billion years ago a particularly rich uranium deposit achieved criticality and operated as a reactor for at least 100,000 years in what’s now West Africa. In the time since then, all of the fission products (the nuclear waste) have stayed put, in spite of the fact that the reactor zones are in porous and fractured sandstone and below the water table for most of that time. This bodes well for our ability to safely contain radioactive waste in an engineered facility set in much less-porous rock well above the water table.

The bottom line is that nuclear energy has its drawbacks, as does every form of energy. But these drawbacks are understood and can largely be managed – they certainly don’t call for abandoning nuclear energy altogether. Incidentally, you might also be interested to know that fossil fuels (coal, oil, natural gas) are associated with natural radioactivity due to the geochemistry of uranium. The radiation dose from burning any of these fuels is actually higher than the radiation dose from nuclear energy for every GW-hour of energy produced.

Now let’s look at the other side – the part of your question regarding nuclear energy and global warming. Here the argument is pretty simple – nuclear energy produces a lot of power and virtually no greenhouse gas at all; pretty attractive in a world increasingly worried about global warming. And – yes – some greenhouse gases are produced by the mining, processing, and transportation of uranium. But this pales in comparison to the greenhouse gas produced by fossil fuel plants.

There are other forms of low-carbon energy – solar, wind, tidal, and geothermal power come to mind. But each of these has its limitations – solar power doesn’t work well at night for example, the strongest winds are usually distant from the cities that use the most power, geothermal is only useful in a few location, and tidal power isn’t very useful in the continent’s interior. As of today, nuclear energy is the only form of non-fossil fuel power that can be used anywhere at any time. That’s not to say that nuclear energy is the answer to all of our problems (it isn’t) or that there’s no place for alternative energy sources (there is) – just that for reliable baseline power that can be put virtually anywhere on Earth and that doesn’t emit any greenhouse gases we just can’t get any better than nuclear at the moment. One of these days we might have fusion reactors, solar power satellites, or other exotic forms of energy – but not today and not in the plannable future.

How Do Sodium Iodide (Scintillation) Detectors Work?

Dear Dr. Zoomie – your last posting discussed how gas-filled detectors work, but I’ve got a different type. I think it’s called a sodium iodide detector. Can you tell me how this works and when I should use it? Thanks!

You’ve got one of my favorite detectors (and yes, I know that this raises me way up on Geek sale)! There are two fundamental families of detectors – the gas-filled detectors I wrote about the last time, and the scintillation detectors, of which the sodium iodide (abbreviated NaI) is one. The vast majority of radiation detectors out there – and virtually every detector used in general situations – fall into one of these two families. So since I wrote about the gas-filled detectors last time, this is a good chance to write about the other major family. Here’s how the scintillation-type detectors work.

First, an important point. Most scintillation detectors are only sensitive to one type of radiation. So NaI detectors will pick up gamma radiation, but not alpha or beta, zinc sulfide (ZnS) will only pick up alpha radiation, and so forth. In actuality, there might be some sensitivity to other radiations – NaI, for example, will sometimes pick up high-energy betas – but you should only use a detector for the type of radiation it’s designed to pick up. Now, with that out of the way, on to how the things work!


The basic principle is the same for every scintillation-type detector: when radiation strikes the scintillator it causes it to give off photons of visible light (that’s the scintillation part). These photons pass through the crystal and they strike a thin metal foil called a photocathode – when this happens the light enters the second part of the detector, called a photo-multiplier tube (PMT). When the photon hits the photocathode it causes an electron to be ejected from the photocathode. Just past the photocathode there is a set of metal cups, each with a voltage applied to it (typically several hundred to a thousand volts) – the electron is accelerated by this voltage to a high energy and it strikes the cup with enough energy that it knocks loose a number of other electrons. Each of them, in turn, is accelerated towards the next metal cup, where each of the “new” electrons knocks loose a number of additional ones – by the end of the PMT the initial signal has been multiplied by a factor of a million or so. From there, it’s up to the instrument manufacturer to figure out how they’re going to use the light that’s emitted.

As one example, every time a gamma hits the crystal it starts this whole process that culminates in a pulse of electrons arriving at the far end of the detector. The simplest way to deal with this is to simply count the pulses of electrons as they arrive at a counting circuit – this is a great way to measure contamination (which we normally record in terms of counts per minute or counts per second). We can also use this mode to measure radiation dose rates, but only by assuming that every count carries with it a specific amount of energy. If you remember the posting on gas-filled detectors, this is the same way that Geiger counters work and it’s one of the reasons that Geiger counters aren’t always accurate for measuring radiation dose rates. We’ll get back to that in a moment. Oh – one other thing to be careful about with an NaI detector is that the larger crystals (say, 2”x2”) can be sensitive to drastic temperature changes – they can thermally stress the crystal, eventually breaking it. So if you’re working outside on a blazing hot (or bitingly cold) day, you probably want to leave the crystal outside, rather than bring it into a much colder (or warmer) office a few times a day.

Something else that we can use NaI detectors for is identifying specific radionuclides by measuring the energy of each individual gamma that enters the crystal – this process is called gamma spectroscopy, or can also be called multi-channel analysis (and the instrument set up for this purpose is called a gamma spectroscopy device, or a multi-channel analyzer – abbreviated MCA). The basic principle behind gamma spectroscopy is that every gamma-emitting radionuclide emits a gamma ray (or a few gammas) with very specific energies – like a fingerprint – and if we can identify the gamma energies precisely enough then we can identify the radionuclide(s) present. For example, cesium-137 (Cs-137) gives off a gamma with an energy of 662 thousand electron volts (abbreviated keV) – if we analyze a gamma ray spectrum and find a peak with an energy of 662 keV then we know that Cs-137 is present. Along the same lines, seeing twin gamma peaks at about 1.1 and 1.3 million electron volts (MeV) tells us that we’ve found cobalt-60 (Co-60).


The problem is in figuring out how much energy is in each gamma photon – luckily we can do this with a scintillation detector. When a gamma ray interacts with the NaI crystal it deposits energy – this energy is what causes the photons to be given off. Not only that, but a predictable number of photons are emitted depending on the energy deposited – in a sodium iodide crystal, depositing 1 MeV of energy will cause about 42,000 scintillation photons to be emitted. We know the number of photons that it takes to eject a single electron from the photocathode, and we know the amount of amplification (and the size of the electrical pulse) for each electron ejected. So if we can measure the size of the output pulse then we know how much energy was deposited in the crystal – and we can know what radionuclide emitted the gamma that we just detected.

So these are two uses of NaI detectors – measuring contamination levels (in counts per minute or counts per second) and identifying radionuclides by measuring gamma energies – now for a third, measuring radiation dose rate.

Radiation dose rate is a measure of the amount of energy deposited in an object. You’d think that this would be fairly simple with an NaI detector since the number of electrons coming out of the detector is proportional to the energy deposited in the crystal. But that’s not how most manufacturers handle things – most of the time they simply count the pulse rate and, as with a Geiger counter, assume that they are all coming from Cs-137. So, for example, the manufacturer might determine that 175 counts per second are equivalent to a dose rate of 1 mR/hr. This means that NaI detectors set up to measure dose rate this way have the same limitations as a Geiger counter – unless you are measuring exactly what they were calibrated with the dose rates you measure are going to be wrong. Thus, if you’re using a scintillation detector to measure radiation dose rate you need to make sure that either you’re measuring the same nuclide to which it was calibrated or you need to have a set of correction factors that will let you convert the meter reading to the correct dose rate (assuming that you know the nuclide that’s actually present). This graphic, which is for a sodium iodide scintillation detector, show that the meter will show only half the actual dose rate if the radiation is from a Co-60 source, but will over-respond by a factor of 6 or 7 if the radiation is coming from the Am-241 in, say, a box of smoke detectors.


OK – so that covers how the NaI detector works and what it can be used for (again, to measure gamma contamination, for nuclide identification, or – with caveats – to measure radiation dose rate). Now a little on other types of scintillators.

Zinc sulfide (ZnS) is used to measure alpha contamination. The ZnS crystals are razor-thin – only about as thick as a single human hair (give or take a little). But since alpha particles can’t penetrate very far into any materials the crystals don’t need to be any thicker than this. For a number of reasons we don’t worry about dose rate from alpha radiation, so the only thing we need to measure is count rate – a fairly simple matter of counting pulses. The biggest problem with ZnS detectors is that they can be fragile (remember how thin the crystals are). It can also take a long time to do a proper alpha contamination survey since alphas are so easily shielded and have such a short range in air. But for alpha counting, ZnS is about as good as it gets, in spite of its limitations.

Finally, there are also beta scintillators. Liquid scintillation counting is normally performed in the laboratory using fairly expensive (and large) machines – chances are that you won’t have to use one of these unless you work in a laboratory. There are also beta scintillation crystals that you use the same way you use an NaI detector – these tend to be made of plastic (called organic scintillators). While not as fragile as ZnS or NaI, the photo-multiplier tube is the same in all of these detectors and is not very sturdy – no matter what kind of scintillation detector you’re using, you need to treat it gently.

So to sum all of this up, scintillation detectors have their limitations, but they are essential pieces of equipment for just about anyone making radiation measurements. The big things are to treat them gently to keep from breaking them, and to use them properly and within their limitations.

How Do Geiger Counters Work?

Dear Dr. Zoomie – I’ve got all these Geiger counters but I have no idea how they work. Plus, somebody told me that they can only be used for a few types of measurements. Is it true that I can’t use my Geiger counter to measure radiation dose rate? And how DO they work, anyhow?

Geiger counters are part of a family of radiation detectors called “gas-filled detectors.” These detectors – as suggested by the name – are filled with gas. They have an electrode in the middle of the chamber and are set up so that there’s an electrical voltage between the electrode and the metal wall of the chamber – in a Geiger counter, for example, the voltage difference between these is about 900 volts. When radiation hits the molecules of gas in the tube it strips electrons off of the atoms – this process is called ionization. The electron is attracted to the positive charge of the anode and the rest of the atom (a positively charged ion) rushes towards the wall of the tube. Then the electron travels through the wires that make up the electrical circuit and recombines with the ion – part of this electrical circuit is a device that measures the flow of electrons.

Geiger-Mueller Tube

Geiger-Mueller Tube

Of course, it’s hard to measure a single electron – luckily the instruments don’t have to do so. When the electron and ion are accelerated towards the electrode and chamber walls they gain a LOT of energy because of the high voltage – they bump into other atoms and knock electrons off of them in a process called secondary ionization. Those electrons and ions, in turn, cause even more ionizations and so on – this amplifies the original signal by a huge degree, to a point where it can be measured.

Creation of Discrete Avalanches Proportional Counter

Creation of Discrete Avalanches in a Proportional Counter

In a Geiger counter, the voltage is so high that the entire chamber of gas becomes ionized – this gives the highest sensitivity to incoming radiation. In other detectors (called ion chambers and proportional counters) the voltage is lower and the amount of gas amplification is lower as well; only some of the gas atoms are ionized in these detectors. The amount of amplification from increasing voltage is shown in the graphic.

Geiger-Mueller Region

To make sense of this graph – especially when it comes to understanding some of the limitations of Geiger counters – it helps to understand that alpha particles have a lot more energy than do beta particles. It also helps to understand that radiation dose is a measure of the amount of energy that’s deposited by radiation in an object – more energy means more dose. So looking at this graph, we can see that high-energy radiation hitting a detector leaves a bigger signal than does low-energy radiation. Now look to the right, in the Geiger-Muller region – in this region the high-energy radiation produces exactly the same signal as the low-energy radiation. So we can’t tell the difference between high-energy and low-energy radiation, which means that we can’t necessarily tell how much radiation dose a person was exposed to if we’re just measuring with a Geiger counter. This is one reason that we can’t always use a Geiger counter to measure radiation dose rate accurately. If a Geiger counter, for example, is calibrated to measure radiation dose rate from the radionuclide Cs-137 it will be right on the money as long as you’re trying to measure radiation dose from this nuclide. But what if you’re trying to measure radiation from cobalt-60 (Co-60)? Well, then you’re in a bit of trouble – radiation from Co-60 is twice the energy as radiation from Cs-137 so whatever your detector reads will be only half the actual radiation dose rate. On the other hand, a lot of radionuclides are lower-energy; in this case, your meter is going to read a higher dose-rate than is actually the case. The bottom line is that a Geiger counter will only give an accurate radiation dose-rate reading if it’s measuring the same radioactive material it was calibrated with. This is why Geiger counters aren’t always the best instruments to use to measure radiation dose rates.

It turns out that there is one type of Geiger tube that will give fairly accurate readings from a wide range of radiation energies – they’re called energy-compensated Geiger counters. These are designed to give a fairly constant reading across a wide range of radiation energies, so if you’re using an energy-compensated GM you actually can make accurate dose-rate readings. Otherwise, it’s probably best to make your dose-rate measurements with an ion chamber.

One more thing about Geiger counters – they are great general-purpose radiation detectors because they can measure alpha, beta, and gamma radiation. If you’re measuring gamma radiation you’re probably most interested in measuring dose-rate (measured in mR/hr or, for really low levels, in µR/hr – microR per hour); if you’re measuring alpha or beta radiation then you’re probably looking for contamination and you should be measuring counts per minute (cpm) or counts per second (cps), depending on the type of meter you’re using.

So let’s put all of this together:

  • Gas-filled detectors (such as Geiger counters and ion chambers) are filled with a gas that has an electrical voltage applied across it.
  • When radiation interacts with the gas it causes ionizations, and this small signal is amplified by the electrical voltage – the amount of amplification depends on the voltage.
  • Ion chambers can measure the difference in radiation energy – for this reason they are ideal for measuring radiation dose-rate.
  • Geiger counters, on the other hand, have full amplification of the signal for any radiation energy that strikes them. For this reason, they don’t always give accurate dose-rate readings, especially if the energy of the radiation is different than what they were calibrated with.
  • Having said that, energy-compensated GM detectors are designed to help accommodate this problem – they give fairly accurate dose-rates across a wide range of radiation energies.
  • And finally, Geiger counters can also measure not only gamma radiation dose (measured in mR/hr or µR/hr), but also alpha and beta contamination (measured in cps or cpm).

I know this is a long and fairly complicated answer – I hope it helps!

Do I Need To Provide Dosimetry For My Rad Workers?

Hi, Dr. Zoomie. Quick question for you – I’ve got a small radiation safety program and I’m wondering if I need to get dosimetry for my rad workers. How do I know what I need to do?

This question is either really simple or fairly complicated depending on how you approach it. The full answer is going to take a little time, but let me start with the easy version.

According to regulatory requirements, you are required to provide dosimetry to any radiation workers who can reasonably be expected to receive a dose of 10% of allowable limits – that’s 500 mrem (or 5 mSv) in a year. So in theory all you have to do is to evaluate the radiation dose rates in the areas where your workers spend their time and multiply that by the amount of time they spend in that area per year. For example, if the highest radiation dose rate is, say, 0.2 mR/hr and a person works in that area full-time then they can receive an annual dose of 400 mrem (40 hours per week times 50 working weeks per year times 0.2 mR/hr = 2000 hours x 0.2 mR/hr = 400 mR annually). So these workers are not required to be issued radiation dosimetry. Having said that, you might choose to give them dosimeters since they’re close to the level where badging is required – that’s up to you.

Example of a Luxel Badge

Example of a Luxel Badge

Another easy answer is to look at the model procedure for a radiation dosimetry program. This is found in the volume of NUREG 1556 that’s applicable to your type of a program, or in corresponding state regulatory guidance documents. For example, many model dosimetry procedures call for badging anyone who handles millicurie quantities of gamma or high-energy beta emitting radionuclides. Using this criterion, a lab worker (for example) who works with more than 1 mCi of P-32 (a high-energy beta emitter) is required to be issued a dosimeter, even if he or she never works with them long enough to receive an appreciable dose during the year. Other types of radiation safety programs – industrial radiography for example – are also required to badge specific categories of workers (the radiographer and radiographer’s assistant, in this case).

A third (and final, as far as I know) easy answer is just to give a dosimeter to everyone, but this only makes sense if your company is small enough that this won’t cost a fortune, and if your workers are worried about working around radiation.

OK – those are the easy answers, now let’s take a look at what happens when things get a little more complicated. Let’s look at a few specific types of radiation safety programs and see whether or not dosimetry might be called for. And please note – these are suggestions! You HAVE to follow the regs, but you’re not required to do anything in excess of what they require.

  • Industrial radiography – the model procedures are almost certainly going to call for badging the radiographer and his or her assistant regardless of the amount of dose they normally receive. You might also consider putting a dosimeter inside the room where the camera is stored to measure radiation dose in this area – this is known as an area monitor.
  • Blood bank irradiator (also research irradiators) – these are normally very well-shielded and dose to those working with them is typically fairly negligible. However, you will probably want to give dosimetry to those who operate the irradiator, just in case the shielding becomes cracked or damaged. It’s also a good idea to put at least one area monitor in the irradiator room and in adjacent spaces.
  • Industrial gauges (tank level, density monitoring, process control, etc.) – these sources are typically relatively low-activity and well-shielded and workers usually receive very low exposures. However, it’s not a bad idea to badge anyone who works directly with the gauges – especially if they do maintenance on them or work near the beam that emanates from them.
  • Soil density and soil moisture content gauges – dose rates here are also very low, but the operator often has the opportunity to expose the source directly to the air, so it usually makes sense to badge those who operate these gauges.
  • Nuclear medicine technologists – even if the administered doses are low, these workers are exposed to patients (and sometimes to syringes or capsules) containing radionuclides and they certainly have the potential to receive higher exposures. Nuclear medicine techs (and radiology techs) should be badged unless they have been removed from duties involving any exposure to radiation or radioactivity.
  • Small x-ray devices (lead paint analyzers, gauges to measure coating thickness, etc. – these, too, emit relatively low levels of radioactivity and are usually well-shielded. If they are used in a fixed location it makes sense to install area dosimeters at the operator’s station (or wherever workers stand during operation). If the devices are hand-held it is reasonable to badge the operators – this might include giving them extremity dosimeters (ring badges) if they have to hold the objects that are being tested.
  • Veterinary clinics, podiatrists, dental offices, and similar places using diagnostic x-ray machines – a lot depends on where the x-ray machines are located. If they are in dedicated x-ray rooms then only the operator needs to have a dosimeter. Oh – if it’s necessary to have a person hold an animal during an x-ray then that person should be badged also if they’re a member of your staff (if the owner holds the animal then dosimetry isn’t needed since they’re only going to hold their pet once or twice a year). If you don’t have a dedicated x-ray room then you should consider having everyone leave the room during x-rays; if staff are going to be in the room routinely during x-rays then they should probably be badged also.

There are a lot of other possibilities – these only scratch the surface, but they should help to give you an idea as to who to consider badging at your facility.

Finally – there’s more to running a dosimetry program than just handing out dosimetry. You have to remember to exchange the badges from time to time (this could be monthly, quarterly, or semi-annually depending on your program), you have to remember to notify your badged personnel of their badge readings at least annually (and preferably after every read), and you have to remember to hang onto your dosimetry reports as long as you have your license. There’s more as well, but if you can attend to these bits then you’ll be off to a good start.

If you’d like to learn more about running a personnel dosimetry course you may want to consider Nevada Technical’s 2-day Personnel Dosimetry course which is given once a year in Las Vegas, NV.

Information Resources for Radiation Safety

Dear Dr. Zoomie – I’m new to this whole radiation safety business and I’ve got a lot of questions about the right way to do things. Can you tell me where I can find the information I need to do my job right? Thanks!

If you’re looking for information there are three categories of resources – people, websites, and documents. Let’s take each of them in turn.

The best way to get an answer to your questions is often to ask someone. This can be a consultant, but it can also be someone who works in radiation safety and is willing to lend a hand. If you want to retain a consultant we can modestly put ourselves forward (although there are other people who do this sort of work as well). But before you call us up, there are some other people you can check.


For example, the Health Physics Society ( is the nation’s premier radiation safety professional societies. It might not make sense for you to join the HPS, but joining your local chapter almost always makes sense – local chapters meet at least once annually (some have monthly or bi-monthly meetings), and chapter meetings are a great way to get to know others in your area who also work with radiation safety – you can find out how to join your local chapter by going to the HPS web page.

You can also try to contact people directly. For example, if you have a large research university nearby (or a big hospital) there’s a good chance that the Radiation Safety Officer is a full-time health physicist. So if you call their radiation safety office you should be able to be put in touch with a radiation safety professional who likely has the time and ability to give you a hand. Even if not, he or she can most likely put you in touch with someone – one of their colleagues – who can help out.

Electronic resources

While there are a ton of websites that include – or are even dedicated to – information on radiation, few of them are really good. You have to beware of the great number of anti-nuclear websites that are out there; the information that they have is usually wrong and is almost always incomplete. If you’re looking for scholarly analysis, practical help, or regulatory information your best bet is to go to one of the regulatory agencies, or a professional organization that serves the radiation safety community. Some of these are:

Health Physics Society (includes a great deal of information for the general public, and even more information for members – also has an “Ask the Experts” feature where you can post questions)

International Radiation Protection Association (IRPA) (more of an international resource, including links to information documents and the Proceedings from some IRPA meetings.

Nuclear Regulatory Commission (includes regulations, fact sheets, and regulatory guidance documents)

Environmental Protection Agency (the home page for EPA’s radiation regulations and information on radiation-related topic)

Centers for Disease Control (CDC) (contains a great deal of information on the health effects of radiation and on responding to radiation emergencies)

National Council on Radiation Protection and Measurements (NCRP) (NCRP has published nearly 200 reports on various aspects of radiation safety, many of which are likely relevant for your work)

International Commission on Radiation Protection (ICRP) (the ICRP is an international body that makes recommendations on various aspects of radiation safety)

United Nations Science Committee on the Effects of Atomic Radiation (UNSCEAR) (UNSCEAR has published a number of definitive reports on the sources of radiation – both natural and man-made – and their health effects)

International Atomic Energy Agency (IAEA) (IAEA is best known for conducting nuclear safeguards inspections, but they also have a large number of documents on various aspects of radiation safety, including model procedures, suggested regulations, and incident reports)

American College of Radiology (ACR) (the ACR is primarily a society for clinicians, but they maintain a great deal of information on the safe use of radiation in medicine

Radiation Event Medical Management (a very useful site with information, downloads, and calculators, mostly aimed at emergency response, but with a lot that can be used everyday)

Rad Pro Calculator (includes calculators for radiation unit conversions, dose, decay, and so forth)

In addition to all of these, there are a number of software packages and smartphone apps that might be useful. But since these come and go so rapidly I’ve decided not to list them here – you should do a search to see what is out now (one of my favorites is The Effects of Nuclear Weapons, but the IAEA isotope browser is also useful).

Printed resources

Those of you who are (like me) still a fan of hardcopy references will also find a great deal to make you happy. In addition to the reports of the NCRP, ICRP, IAEA and UNSCEAR (all of which you can download in PDF or purchase hard copies), here are a few of the very many references that you might find useful.

Health Physics and Radiological Health (Johnson and Birky) – if you are going to have only one professional reference in your library it should be this one – it’s the single most comprehensive one-volume reference out there.

Basic Radiation Protection Technology (Gollnick) – a classic text aimed at the technician; presents material that is complete and easy to understand.

Introduction to Health Physics (Cember and Johnson) – a higher-level text aimed at the college student; unless you’re a professional health physicist you probably don’t need this level of detail

Environmental Radioactivity from Natural, Industrial, and Military Sources (Eisenbud and Gesell) – if you’re working with environmental radiation safety or in industries that generate naturally occurring radioactive materials (NORM) then you should have a copy of this book; it’s the definitive text on radiation in the environment

Radiological Risk Assessment and Environmental Analysis (Till and Grogan) – a somewhat higher-level book that will be of most use if you are working on environmental projects or for a company that produces a great deal of radioactive byproducts (e.g. from mineral processing)

Radiation Protection and Dosimetry (Stabin) – an introductory college-level introduction to the science and profession of health physics

This list just scratches the surface. I have two floor-to-ceiling bookshelves filled with professional references and journals, but the majority of these are very specialized and would probably not be of much interest to you. But between the books listed here and appropriate reports from NCRP, ICRP, IAEA, and UNSCEAR you should be in pretty good shape.

Plutonium – The Power Behind New Horizon’s Trip to Pluto

Dear Dr. Z – I heard something about plutonium powering the New Horizons spaceship that just visited Pluto. And come to think of it, I’ve heard that other spaceships are powered by plutonium. Do we have nuclear reactors up there? How do they use plutonium to power spaceships? And is there any risk to us if one of these blows up (or reenters) during launch?

Color image of Pluto, photographed by the New Horizons spacecraft on 13 July 2015

Color image of Pluto, photographed by the New Horizons spacecraft on 13 July 2015

I’ve been following the New Horizons mission as well – I remember reading about the discovery of Pluto when I was just a kid (I even sent a letter to Clyde Tombaugh at one point, trying to hook him up with my grandmother). I’ve also followed many of the other space missions, including reading up on their power supplies. First, I can tell you that there are no nuclear reactors on any US spaceships today – we designed a few nuclear-powered spacecraft in the early years of the space program, and NASA was looking into them more recently for prolonged missions to Jupiter and Saturn. But at the moment the US has no nuclear reactors in space. But let’s back up a little bit and talk about where our spaceships get their power from.

One of the most notable aspects of many of our spacecraft is their solar panels – for anything operating on this side of the asteroid belt there’s enough solar energy to power a spacecraft and it will be there as long as the Sun is still shining. But when we go past the asteroid belt into the outer solar system sunlight falls off quite a bit – solar intensity drops by a factor of 4 if you double your distance to the Sun – so solar power just won’t hack it at Jupiter, Saturn, and beyond. We need something different. That “something different” comes from radioactivity.

When any radioactive atom decays away it carries with it some energy – in the case of plutonium (the isotope Pu-238 to be precise) each decay contains about 5.5 million electron volts (MeV) of energy. This is not a huge amount of energy in and of itself, but if you get enough plutonium atoms decaying at the same time it can really add up. If you have one curie of Pu-238 you have 37 billion atoms decaying every second – that comes out to about 200 billion MeV per second, or about 0.00003 BTUs per second. This isn’t a whole lot of energy, but Pu-238 packs a lot of activity into a gram – one gram of Pu-238 puts out about a half watt of power, one kilogram will produce 500 watts – enough to do something with.

A glowing cylinder of Pu-238

A glowing cylinder of Pu-238

Pu-238, in fact, puts out so much energy from its radioactive decay that a large enough chunk (about a kilogram) will actually glow red from the internal heat. Spacecraft use thermocouples to turn this heat into electrical power in what’s called an RTG – a radioisotopic thermal generator.

Diagram of an RTG

Diagram of an RTG

Plutonium-238 is not the only isotope that’s been used to make RTGs – the Soviet Union used to use huge quantities of Sr-90 (strontium-90) for this purpose. Polonium has been used as well, although with a short half-life Po-210 RTGs don’t last very long. Plutonium and strontium are ideal because they have high-energy radioactive decays coupled with relatively long half-lives – at 84 years, Pu-238 can power a spacecraft for decades. In fact, the doughty little Pioneer craft beamed back data for 30 years before running out of energy and the Voyager probes are still operational after nearly 40 years.

New Horizon's RTG

This is the plutonium-powered radioisotope thermoelectric generator (RTG) for the New Horizons spacecraft, which is also seen in the background. Source: NASA

As far as blowing up goes, that can’t happen with Pu-238. There are a number of isotopes of plutonium and only one is used in nuclear weapons – Pu-239. The isotope used in RTGs simply can’t explode (it really doesn’t fission well at all) so it cannot explode as a nuclear weapon.

Lastly, you asked about the risk from launching all of this plutonium into orbit – or, rather, the risk should there be a problem with the launch. Sadly, rockets have blown up on the Launchpad and in the air; others have reentered the atmosphere and burned up unexpectedly. Plutonium is a toxic heavy metal (although, contrary to popular opinion, it is NOT the most toxic substance known to science) – do we have to worry about an accident poisoning people on Earth?

We actually have a few test cases for this and the answer is a resounding no. First, the RTGs are swaddled in protective layers designed to keep them safe from just such an explosion – these are tested extensively before a device is space-rated, and nothing can be loaded onto a rocket unless it has been so tested. As far as reentry goes, there have actually been a few Pu-powered RTGs that have experienced this tribulation with interesting results. One (from a failed 1964 American satellite launch) burned up in the atmosphere, distributing its plutonium into the stratosphere – while this is not ideal, the plutonium was eventually spread globally and no single location was contaminated to the point of causing a health risk. Other RTGs have survived reentry – an RTG in the Apollo 13 lunar module and one on an aborted Russian mission to Mars – without apparently losing any of their plutonium. The bottom line is that RTGs are designed to be safe, even in the event of a launch failure – and this design has proven to be safe in a handful of instances.

One other comment about RTGs – in this case, terrestrial ones. During the Soviet Union’s existence a large number of very high-activity RTGs were built using Sr-90; these were used to power meteorological stations as well as lighthouses along the Arctic coast. While all of these seem to have been accounted for now, this was not always the case. In 2001 some woodcutters in the nation of Georgia found an abandoned RTG in the Georgian mountains. Noticing that snow near the RTG was melting the men carried it into their camp for heating. During the course of the night they received serious radiation exposure leading to radiation sickness and skin burns. All survived, but they were seriously ill for quite some time.

Oklo Natural Nuclear Reactor

Dear Dr. Zoomie – I saw something the other day that scientists found a natural nuclear reactor that’s over a billion years old. Is this for real? Or was this left behind by aliens?

Never fear – there was an actual natural nuclear reactor found on Earth and aliens had nothing to do with it! It was in a place called Oklo, in what is now the nation of Gabon in western Africa. And what happened is pretty cool.

There’s not enough room here for all the details; if you’re interested in some of the nitty-gritty you can check out an on-line article written by a scientist who is conversant with both geology and nuclear reactors. But here’s the big picture – let’s start with the geochemistry of uranium.

Uranium dissolves easily into oxygen-rich water but not at all into water that lacks oxygen. Until about two billion years ago the Earth’s atmosphere was oxygen-deprived which meant that surface water couldn’t dissolve uranium. Ancient algae was producing oxygen, but all the oxygen that was produced was immediately sucked up by iron and other metals in the Earth’s crust – in effect, the Earth was rusting. About two billion years ago the iron was all oxidized and oxygen began to accumulate in the atmosphere – as soon as it did so it also started to accumulate in rainwater (and streams, lakes, and rivers) and when that rain fell on granite rocks (granite tends to have elevated levels of uranium compared to other rocks) it began to dissolve the uranium out of the water.

As the uranium-rich water flowed along it sometimes came to areas where, due to decaying organic matter (more of that algae) the water was oxygen-deprived; when that happened the uranium came out of solution. And in one place in particular, apparently a fluke, enough uranium collected in one place in a configuration that resembled that of a nuclear reactor – a number of lumps of uranium dispersed in a sandstone formation. And when that area became flooded with water (water slows down neutrons, which makes them more efficient at causing fissions) these lumps of uranium began to fission. As they did so they produced heat, neutrons, and fission products (when a uranium atom fissions it produces two radioactive atoms). Of course, there’s more to making a reactor than mobilizing uranium and precipitating it out of solution in lumps. The uranium also has to be enriched so that it will sustain a nuclear chain reaction – that can’t happen today because there simply isn’t enough of the fissionable U-235 in uranium to sustain a chain reaction. At least, not today. But in the past things were different.

Oklo Reactor Zones

Oklo Reactor Zones

Uranium today has a very specific composition – if you take a uranium sample from anywhere on Earth and count the atoms you’ll find that 99.2% of the uranium atoms have a weight of 238 atomic mass units (it’s abbreviated U-238), and U-238 doesn’t fission very well. Virtually all of the rest (0.72%) of the uranium atoms are a little lighter with a mass of 235 – U-235 fissions quite nicely, but uranium that only has seven tenths of one percent of U-235 will not sustain the chain reaction necessary to keep a reactor operating. This is why we need to enrich uranium; so that there’s enough U-235 to achieve and maintain criticality. And, incidentally, in a nuclear reactor “critical” simply means that the reactor is operating at a constant power – all reactors are critical when they are operating. Anyhow – natural uranium today can’t sustain a chain reaction unless we use something like heavy water or graphite to help coax things along. But two billion years ago, things were different.

U-238 has an incredibly long half-life – it takes almost 4.5 billion years for half of it to decay away, which means that over the entire life of our planet only half of the U-238 it formed with is left. On the other hand, U-235 has a half-life of “only” about 700 million years – there was around 75 times as much U-235 on Earth when it first formed compared to today. And two billion years ago – at the time that the Oklo reactor formed – U-235 comprised about 5% or so of natural uranium; this is about the same amount that’s found in the fuel for commercial nuclear reactors today.

Oklo could not have happened at many times in history. Earlier in Earth’s history there was enough U-235 but not enough oxygen to mobilize the uranium; later in Earth’s history there was plenty of oxygen but not enough U-235 to sustain a chain reaction. But for one brief moment – actually for about a half-billion years – conditions were about perfect for uranium to dissolve, move, and concentrate in a manner that would sustain a criticality. During that time, a body of water-saturated sandstone with a number of uranium deposits achieved criticality.

The reactor operated only sporadically – it depended on water to sustain the chain reaction and, as the reaction progressed, the water would boil away, shutting down the reactor. So the reactor would operate and then shut down; while shut down it would cool off until water could re-saturate the sandstone and the reaction would start up again. Each time this happened a little more U-235 would fission and the concentration dropped slightly – eventually there was too little left to sustain a chain reaction and the reactor shut down for good. All in all scientists estimate that the Oklo reactor operated for about 100,000 years.

Now let’s fast-forward a few billion years to the mid-1970s. French geologists located a rich body of uranium ore and they started evaluating it as a source of reactor fuel. But when they started feeding the uranium into their uranium enrichment system they found they weren’t getting the amount of enrichment they expected – some good scientific detective work showed them that the ore itself was deficient in U-235, something that had never been seen before. Eventually they accepted that this uranium ore body was different than any other on Earth. Then they saw evidence of fission products in the ore. It seemed reasonable to assume that the presence of fission products was linked to the lack of U-235 – when they looked at the geology they had to accept the conclusion that they had found a fossil nuclear reactor – nature (not aliens!) had preempted Enrico Fermi (the Italian Nobel laureate who built the first artificial nuclear reactor) by a couple of billion years.

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