Section 7 - Properties of Radionuclides

It is important to be familiar with the properties of the radionuclides with which you work. The above table lists the most common radionuclides used at UMSL and their properties.

a. Beta Emitters

Tritium (H-3) is a very low energy ("soft") beta emitter. It is very low in radiotoxicity, but is also very difficult to detect. It cannot be detected by any Geiger-Mueller (GM) instrument. The only practical way H-3 can be detected is to prepare a sample for liquid scintillation counting.

Carbon-14 (C-14) and Sulfur-35 (S-35) are low energy ("soft") beta emitters of low to moderate radiotoxicity, which can be detected by a GM instrument if it has a thin window. Maximum beta energy is about 0.16 MeV. Remember that there is a distribution of beta energies emitted and the average beta energy is approximately one third of the maximum (skewed distribution of beta energies). Even the maximum energy beta from C-14 and S-35 is easily stopped by 1 mm of Plexiglas ä and the average energy beta will have a range in air of less than 5 centimeters.

Phosphorous-33 (P-33) and Calcium-45 (Ca-45) are moderate energy beta emitters of low to moderate radiotoxicity, which can be detected by a GM detector if it has a thin window. Maximum beta energy is about 0.25 MeV. The beta range of P-33 and Ca-45 is about twice that of C-14 or S-35. A couple of millimeters of Plexiglas ä will stop the beta and the average energy beta range in air is less than 10 cm.

Phosphorous-32 (P-32) is a high-energy beta emitter that needs to be carefully shielded with Plexiglas ä. P-32 is highly radiotoxic if ingested since phosphorous incorporates into bone and will dose the bone marrow. P-32 beta maximum energy is about 1.7 MeV and can penetrate up to 8 millimeters depth in tissue. This penetration depth is certainly not enough to reach any major organs, but is a skin or eye dose concern, unless appropriate precautions are taken to shield.

b. Gamma Emitters

Chromium-51 is a moderate-energy gamma emitter that has a low gamma constant because so few of its atomic disintegrations emit gamma (emission frequency is about 9%). The half-value layer for the 0.32 MeV gamma from Cr-51 is about 3 mm of lead, therefore it requires substantial amounts of lead to shield the gamma. Also, Cr-51 is very difficult to detect with a GM detector because it does not have a beta or any other electron radiation of sufficient energy to be seen by a GM. It is much more easily detected with a crystal scintillation detector that has high detection efficiency for the gamma rays. Although we survey with a portable crystal scintillation detector, we rely heavily on smear surveys because the LSC has higher count efficiency. Counting Cr-51 in liquid scintillation will provide the best count efficiency since it also counts the auger or conversion electrons emitted. Cr-51 is very low in radiotoxicity.

Iodine-125 is a low-energy gamma and x-ray emitter that is very easy to shield with thin sheet of lead. It is highly radiotoxic because it concentrates into the thyroid. It is very difficult to detect with a GM, for the same reasons as Cr-51. It is much more easily detected with a proper scintillation detector. The unbound form of the radioactive iodine atom is not covalently bound to a non-volatile macromolecule and therefore poses an inhalation risk. Sodium iodine forms are extremely hazardous with respect to inhalation and skin absorption.

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