Isotope
|
Emission
|
Energy (MeV)
|
Radio toxicity
|
Shielding
|
Instrument
|
|
---|---|---|---|---|---|---|
Counting
|
Survey
|
|||||
H-3
|
Beta
|
0.018
|
Low
|
None
|
LSC
|
None
|
C-14
|
Beta
|
0.156
|
Low to Moderate
|
Plexiglass
(<1 mm) |
LSC
|
Thin-window GM pancake
|
P-32
|
Beta
|
1.71
|
High
|
Plexiglass
(8 mm) |
LSC or Cherenkov
|
Thin-window GM pancake
|
P-33
|
Beta
|
0.249
|
Low to Moderate
|
Plexiglass
(<1 mm) |
LSC
|
Thin-window GM pancake
|
S-35
|
Beta
|
0.166
|
Low to Moderate
|
Plexiglass
(<1 mm) |
LSC
|
Thin-window GM pancake
|
I-125
|
Gamma
|
0.027-0.04
|
High
|
Lead
(<1 mm) |
LSC or AutoGamma
|
Crystal Scintillation
|
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.
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.
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.