Saturday, April 5, 2008

Space (radiation) Madness

It is time to put on my nuclear physicist hat.

A recent article by Astroprof and a related comment by Elias address questions raised in the media about radiation exposure to astronauts. I will direct you to Astroprof's article for a discussion of the magnetosphere and other areas where he has expertise, but just about every paragraph in his article reminded me of some things that need to be said. I'll put them here (below the fold) rather than fill up his comments section. Italicized quotations are from Astroprof unless otherwise identified.

By the way, it starts with the earliest research of interest to Dr. Pion!

Concerning Physicists have known for many years that radiation is coming to Earth form space.

Ah, the good old days when particle physicists were mountain climbers. Seriously. Most of the early research in particle physics was done by sending equipment up with a high altitude balloon or by hauling it up onto a mountain. The pion was discovered in cosmic ray experiments done on top of a mountain. It is really quite remarkable how much can be accomplished with data acquired under circumstances where you don't have any control of the energy or intensity of the source.

I should also add that the nuclear emulsions (basically thick photographic film) developed for that research are still used to monitor radiation exposures, both on earth and during space flight.

Units used to measure adiation exposure used below: There are two systems in wide use. The "correct" (SI) one uses the Sievert (Sv), while I was raised on the rem (roentgen equivalent man). They differ by a factor of 100, so a typical background radiation exposure can be stated as either 250 millirem/year or 2.5 mSv/yr. Both measure the same thing, the effect of ionizing radiation (x rays, gamma rays, alpha particles, protons, etc) on a human body by correcting for the different biological effects of energy and the mass and charge of the particle.

Concerning People living at high altitudes, therefore, receive more radiation exposure than people living at lower altitudes.

In addition, local geology will also affect background radiation levels. Simply living in Denver will increase your radiation exposure (higher altitude), but not nearly as much as some places. Natural background levels can be as much as 10 times higher than the average value listed above: e.g. 2.5 rem per year or 25 mSv/yr. For comparison, 5 rem (50 mSv) per year is the maximum allowed for radiation workers [in the US, less in some other countries].

Concerning I was surprised in my research to find that airline crew members can receive more annual radiation exposure than many people working with radioactive materials.

Not widely known, but important for a number of reasons. The most important one is that the exposure from regular air travel could be comparable to what might result from a minor "dirty bomb" in a big city. Objective measures of such things might be essential to help manage the subjective response (terror) to an involuntary exposure that is not significantly more risky than many voluntary ones.

Anecdote. I know of one case where a theoretical physicist working at a national lab as a regular visitor (consultant) was ordered to leave his radiation monitoring badge at the lab when he left. It seemed that he was getting measurable exposures at a level that would trigger administrative interest, except he was never in an area of the lab with any radiation. The problem: He flew. A lot. Really a lot. (Quite possibly more than flight crews are allowed to fly.) He commuted across the country from university to lab to Washington, and his badge was always in his brief case. It also went through airport x-ray machines, but that was a small part of the total. There are probably corporate executives who get more radiation exposure than a radiation worker at a nuclear plant.

Concerning But we don’t really know just how much radiation exposure astronauts might get outside of Earth’s magnetosphere.

You are selling short all of the work done by astrophysicists studying the output from the sun. Surely they have been measuring the charged particle flux in the solar wind? The really high energy stuff has been studied on or near the earth by physicists for almost a century. The complications are that the flux does change due to solar dynamics and the actual radiation exposure depends on the design of the spacecraft. Much of the radiation in a spacecraft (and an airliner) comes from secondary particles produced when the cosmic ray hits the skin of the craft.

What results is not exactly unknown (there is a vast collection of data on what happens when particles of various energies interact with matter, and it is easy to fill in any gaps in the data), but does require serious modeling specific to an actual design. That, along with acquiring additional experimental data, is probably where the research money needs to be spent. Since this is basically what high energy experimentalists do for a decade or so while simulating what data will be produced by a new detector (like the ones at the LHC in CERN), there is even a barely-employed bunch of physicists ready to do the job. This makes it a financial, not a physical, problem.

Concerning the comment by Observer about a conspiracy nut case [*] ignorant of the exposure rate in rem (or any other unit for that matter) for a human in the Apollo capsule:

I spent some entertaining time [**] browsing what can be Googled out of the various data bases available on the web. The answer is that the exposures (for an entire mission) ranged from 0.4 rem (4 mSv) to 1.4 rem (14 mSv). This is well under the annual exposure rate and comparable to common medical procedures. A whole-body CT scan is, IIRC, in the middle of this range (and a single large dose is more risky than one obtained over more than a week). Remember, the "radiation worker" dose is based on estimates that getting 5 rem (50 mSv) every year of your working life will increase your cancer risk by 1 to 2 percent.

[*] Simple question for the nut cases: If we did not go to the moon, how can we bounce a laser beam off of retroreflectors left by the Apollo astronauts? What would motivate so many people to lie about those observations?

[**] Maybe you don't think it is entertaining to read about independent estimates of radiation exposure of Apollo 16 astronauts made by comparing the isotope distribution of elements found in the urine and feces before and after the trip, but I do. There appears to be almost no limit to what nuclear chemists will study!

Exposures can sometimes be higher for long duration missions (radiation exposure is one of the limits for workers in the ISS) and were fairly significant (170 millirem or 1.7 mSv) for an Apollo-Soyuz mission, if I read the report correctly. By the way, NASA sets its occupational exposure levels significantly (10 times) higher than those for terrestrial radiation workers. I suppose the rationale is that (1) they are volunteers who know the risks, (2) they don't make that many trips over a career, and (3) they really want to do it. Sort of like the way the military discounts the risk of bullets when it needs to go to war.

Closing remark:

I had to go through several different training sessions regarding radiation exposure and the work environment. Even a theorist who works in a lab needs to know what else is going on there, what behaviors are not safe, and why and when monitoring devices of various types must be used. At the time this was annoying, but in the long run it provided valuable information about a wider range of risks we are exposed to and the ability to comment professionally about matters of public interest, like this one.


Rebecca said...

At my lab, we wear dosimeters, and they do tell us to leave them at home when we go on business travel, even if we're traveling to another lab site.

Strangely, our annual reports always show zero radiation exposure, despite the existence of background radiation. Maybe they subtract that out?

Doctor Pion said...

You should ask. They are required by law to tell you what is done.

My understanding (this was years ago) is that they keep a few blank dosimeters in what is expected to be a background only area. I seem to recall that ours would report an "M" to indicate a valid reading with no excess dose.

Storing them in a rack at the lab was the normal procedure where I worked as a grad student. The place I was referring to had the dosimeter mounted on the access badge, which you always took with you (employee or consultant). They had to modify some procedures because of that case involving x-rays and flying.