Protection from external exposure to ionizing radiation involves the use of three basic methods: time, distance, and shielding. It is important to understand the principles of time, distance, and shielding so that they may be implemented in the laboratory.
Dose is proportional to time for a constant dose rate. Often, minimizing time will reduce your accumulated dose to near background levels.
Distance is often the best attenuator because of the inverse square law. This law is an approximation for point sources where the dose rate decreases by four with twice the distance from the source. Even for source-to-detector geometry that doesn't meet this criterion, the inverse square law provides a good general guideline.
Shielding is used to attenuate radiation. The half value layer is the thickness of shielding material required to reduce the dose rate by a factor of one half. For instance, I-125 x-rays will have a half value layer in lead that is less than a tenth of a millimeter. On the other hand, Cr-51, whose principal photon energy is 0.32 MeV will have a half value layer in lead of about 3 mm. For example, a 1 cm thick piece of lead sheet will be a bit more than about 3 half-value layers. In this case, the shield will reduce the dose rate to 1/10 th (equivalent to a tenth-value layer).
|
Radionuclide
|
Energy (MeV)
|
HVL (mm Lead)
|
|---|---|---|
|
I-125
|
0.027-0.40
|
0.1
|
|
Cr-51
|
0.320
|
3
|
Half value layers are not used for shielding beta radiation. Rather, the beta has a maximum range in a given shielding material. This range can be approximated by 0.5 cm per MeV for materials like plastic or water (but is not linear). For example, P-32 has maximum beta energy 1.7 MeV, which requires about 1 cm of plastic to shield the beta.
When beta radiation is shielded, it sometimes gives rise to x-rays through a process called "bremsstrahlung" (braking radiation). These x-rays are produced in much greater abundance with increasing beta energy and with increasing atomic number of the shield. Bremsstrahlung is a significant consideration in designing shielding for larger quantities (milliCuries or greater) of higher-energy beta emitters such as P-32. The best shielding materials for high-energy beta emitters are those of low atomic number like plastic, with an optional outer layer of lead foil or sheet.
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