What are the radionuclides that may exist in the bone samples and emitted alpha particles when using the cup technique and CR-39 detectors? And what is ideal time of exposure we must use it to detect the tracks of alpha in the detectors ?
The sealed cup technique with track-etch detectors is a simple way to measure radon exhalation from samples of soil, building materials or even water. The dimensions of the cup must ensure that all alpha particles emitted within the sample are stopped in air before reaching the detector film. Thus the detector can only measure alpha particles emitted by radon in the enclosed air, or by radon progeny deposited on the wall of the detector and on the detector itself. Radon is produced by radium in the sample, more precisely 226Ra (giving 222Rn) and 224Ra (giving 220Rn known as thoron), and a variable fraction of it exhalates from the sample, depending on several physical properties of the sample. 226Ra is in the progeny of 238U, 224Rn in that of 232Th, but the presence of Ra is not a proof of the presence of the parent nuclide, depending on possible processes that might have separated Ra from the parent.
A porous filter on top of the sample can avoid the thoron contribution.
The time of exposure is long for low-activity samples, generally several weeks, in order to get enough tracks for acceptable statistical uncertainty.
thank you very much Dr. Tondeur for your answer
in my experiment I used to measure only alpha emitters by put the detector in touch with the samples in the cup , by this I canceled any possibility to record alpha particles emitted from radon .
my question is :
1. what the radionuclides that may exist in the bone samples and emit alpha which make the tracks in the detectors?
2. how can I cancel the weight effect of the samples on that increase or decrease of the number of tracks ? is there any equation includes this factor?
I have no personal experience of this technique or of the analysis of bones in general. I know that the sample can be prepared as a powder put on the sensitive film, and then covered by another polymer permeable to Rn to avoid any contribution from radon.
If you can discard artificial contamination, e.g. by enriched U or by Pu, the main alpha-emitters present in bones should belong to the 232Th family and the 238U family. Their presence depends on the diet, and on the age of the bone.
It is generally considered that the transfer of Th itself through the food chain is small, in particular because of the low solubility of its compounds. Among the Th progeny elements present in the soil, 228Ra (alpha, 5.7 y, chemically similar to Ca) could be transferred to crops or grass and then to the animal or human bones, followed by the slow growth of 228Th (alpha, 1.9 y half-life) and its progeny 224Ra (alpha). The next member of the chain, 220Rn, is expected to escape from the powdered sample and will not contribute, and its progeny rapidly disappears if initially present.
Although not very strong, the transfer of 238U (alpha) and its progeny 234U (alpha) through the food chain is not negligible. Because of long half-lives, they will not themselves generate alpha-emitting progeny in the bone, but 226Ra (alpha) can itself be transferred through the food chain to the bones. As above, its progeny 222Rn will escape from the powder sample, and its short half-life progeny will rapidly disappear, but this is not true for 210Pb that generates 210Po (alpha).
My guess is thus: 3 alpha-emitters of the Th-chain, and 4 from the U-chain, could be present in the bones. Old bones (>50 y) should only contain the U-chain.
thank you Tondeur for your interest and for the detail explanation
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