Number 38 March, 2001
Shielding
Selecting the appropriate sheilding material for unsealed sources is important. The basic rule is to use lead to shield gamma photons and plastic to shield beta particles. Some radionuclides emit high-energy beta particles that may require both types. The following table summarizes shielding for commonly used radionuclides. Keep in mind that large or bulky shields can make the work more difficult and lead to other problems. Also remember to use your other strategies: maximize the distance from sources and minimize the time near them.
|
radionuclide
|
type
of radiation
|
energy
(MeV)*
|
shielding |
comment
|
| H-3 | beta | 0.0186 | none | never an external exposure hazard in any quantity because of the extremely low beta energy |
| C-14 | beta | 0.157 | none | not an external exposure hazard in the quantities typically used because of the low beta energy |
| P-32 | beta | 1.71 | plastic | Use 1/4 inch or greater for small quantities of activity |
| plastic and lead |
When using several mCi or more use layered shielding--plastic shielding next to the P-32 source, and then a lead shield to absorb bremsstrahlung photons created by interactions of the high-energy beta particles in the plastic shield. | |||
| P-33 | beta | 0.248 | none | not an external exposure hazard in the quantities typically used because of the low beta energy |
| S-35 | beta | 0.167 | none | none for small quantities |
| plastic | plastic for multi-millicurie quantities | |||
| Ca-45 | beta | 0.252 | none | not an external exposure hazard in the quantities typically used because of the low beta energy |
| Cr-51 | gamma | 0.320; others of low energy | none/lead | The gamma photon is higher energy than I-125 photons. However, it is emitted in only a small fraction of decay events so the unit exposure rate is lower than that of I-125. 1.7 mm of lead will stop one-half of the photons. |
| I-125 | gamma | <0.070 | lead foil | Lead foil is sufficient for most applications with this very low-energy radionuclide; lead bricks are not needed for the quantities typically used |
Use this NUtrino as a training tool. Circulate it among the radiation workers in your group and have them sign and date the training form on the back. File it with your authorization and other radiation safety documents for review during regulatory inspections. Discuss it during laboratory meetings. We have back issues, or you can print them off the Web.