I saw some questions in another thread, so I thought I'd also provide some explanation from the radiological report (pp. 371-377) on the biological samples.
First, some background:
Being in the presence of radioactive sources results in an external dose.
For alpha radiation, that dose is zero. Alpha particles are stopped by the dead layer of skin covering our bodies, so they won't damage any living cells.
Beta radiation typically will get absorbed in the skin. It typically cannot penetrate very far. It is possible to get a "beta burn". High doses of beta to the eye can also harm one's vision. (And the beta contamination on the hikers' clothing is many orders of magnitude lower than what would be needed to cause any of this.)
Gamma radiation is typically what one considers when one looks at external dose because it causes the most health risk. Gamma can penetrate deep into the body and damage cells in organs.
With external dose, once you leave the area, the dose stops accumulating.
Internal dose is different. This refers to contamination that gets inside the body. Typically, this happens either through breathing contamination into one's lungs, or eating or drinking something that is contaminated. In this case, the contamination is a gift that keeps on giving. It stays in your body, even when you leave the contaminated environment.
Alpha emitters can cause lots of damage to living lung tissue. This is why radon exposure is the second leading cause of lung cancer.
Sr-90 is particularly dangerous because strontium is chemically similar to calcium. It incorporates in your bones and will irradiate the marrow with beta radiation. Sr-90 released in atmospheric nuclear tests found its way into milk that people drank. It then showed up in children's baby teeth.
Cs-137 and Cs-134 (a shorter half-life fission product) chemically resemble potassium to the body. They accumulates in muscles and organs. Cs-137 is a gamma and beta emitter.
Iodine-131 is a fission product with a very short half life (8 days) that accumulates in the thyroid. A lot of thyroid cancers occurred after Chernobyl.
On my Chernobyl trips, we had dosimeters to monitor external gamma dose. But that was never really much to be alarmed about. The key concern was preventing internal contamination. This means being very mindful of hygiene. Washing hands before eating. Scanning clothing and shoes to make sure contaminated dust doesn't follow you home. When we went inside the Sarcophagus, we were given a full change of clothing, boots, a cap for our hair, gloves, and a dust mask. All this was left behind as we changed back into our own clothes, and we had to pass additional contamination scans to leave.
In addition to analyzing the clothing of the four hikers from the ravine, samples from their bodies were tested as well.
Beta is easily absorbed by tissue and bone. This actually presents a problem when attempting to measure contamination that is distributed throughout a piece of tissue. The betas emitted by contamination close to the center can get absorbed before they can escape to the outside of the sample. Betas originating closer to the outside of the sample have a higher likelihood of escaping and being measured. This is known as self-shielding, causing the sample to appear much less radioactive than it actually is.
The solution here is to burn the sample to ash and then measure the ash. This eliminates all the water and much of the mass, however all the heavier radioactive isotopes should still be left behind in the ash. If we look at Table 1 Row 3, a 42.820 g sample of Kolevatov's brain was burned to produce 0.710 g of ash. Of that ash, 0.2 g was counted in the detector. This small amount is unlikely to self-shield, so it should give a much more accurate result.
This is definitely a different type of detector. It's listed in the report as a BFA-25, also in a "lead house" to reduce background. I have not been able to find any info on what this is. But the conversion factor and background radiation numbers are very different from the detector used to measure the clothing, so I don't think they are the same.
The background radiation for the detector ranges from 22-34 counts per minute in table 1. It appears the background was measured either before or after counting each sample. For line 3, background is 26, and the sample of brain tissue was 4 counts per minute over the background.
Now if 0.2 g of ash were measured out of 0.710 g of ash total, the sample should be (0.710 / 0.2) * 4 = 14.2 counts per minute.
The detector isn't counting every beta. The conversion ratio is 5.5. So the sample is really emitting 14.2 * 5.5 = 78.1 betas per minute.
Because the samples are all variable sizes, one needs to standardize the activity as counts per minute per kilogram. We know that the original brain sample, before burning, was 42.820 g. So the actual sample activity is 78.1 * (1000 / 42.820) = 1824 cpm / kg. The table rounds it to 1850.
1 Curie = 3.7e10 Bequerel. And 1 Bq = 1 count / second. So to convert this to Ci/kg, we take 1824 / (60 * 3.7e10) = 8.2e-10 or 0.82e-9 or 0.82 nCi/kg. The table lists 0.85e-9.
One thing I'd like to note: the activity levels range from 2 to 7 cpm above background. And, as I said, background fluctuates from 22-34 cpm. So many of these measurements are very close to background. A number of rows do not have anything listed; my guess is that 1 cpm or less above background was considered no activity. A small discrepancy in cpm above background translates into a much larger activity.
Table 3 shows the results, for comparison, from a motor vehicle accident victim from Sverdlovsk. In this table, a single background number is used for all samples.
The report concludes that: "Thus, the results of the studies in Tables 1 and 3 do not exceed the averaged data on the content of radioactive substances in human organs and may be due to natural radioactive Potassium-40." I think this makes sense. The most radioactive sample was Kolevatov's heart, at 3.8 nCi/kg, which is very close to the control sample, at 3.6 nCi/kg. So while some of the hikers' bone and tissue samples were above background, none of them were abnormally above background.
What does this say about what happened?
Well, I don't see any discrepancy between the clothes being contaminated and their bodies being normal. In order for the bodies to be contaminated internally, they would first need to be exposed to radioactive contamination, and then it would need a way to enter their bodies.
I think Sr-90 is an excellent candidate for the contamination. But someone getting Sr-90 dumped on them shortly before they died probably won't be internally contaminated. It would need to get inside them (e.g. from the contamination getting into their food) and then I am guessing their body would need time to incorporate the Sr-90 into bone and other cells.
I also saw a question in another thread about why their thyroids were not analyzed for I-131. This would not be possible. Even if they were exposed and there was time for I-131 to enter their bodies and be absorbed by their thyroids before they died, the half life of I-131 is only 8 days. If we assume they died on Feb 2 and were found on May 5, more than 11 half-lives would have passed. Less than 0.05% of the I-131 would be left.
The one issue I have with the biological testing is that no lung tissue of any of the hikers was analyzed. But lung tissue of the Sverdlovsk motor vehicle accident victim was analyzed. This is one place where I think internal exposure may have been possible. If the hikers' lungs were substantially more radioactive than the lungs of the control sample, that might be an indication that they were alive and breathing when they encountered radioactive dust. That said, tobacco contains radon daughters Pb-210 and Po-210 that are absorbed in the lungs. So the amount that someone smoked may also influence how much beta their lung tissue emits.
Topic: Measuring contamination in biological samples - Tables 1 and 3 (Read 898 times)