NUtrino 10

Edition 10, March 1998

Selecting the appropriate shielding 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 needs for commonly used radionuclides. Shielding must be evaluated after any change of location or radionuclide, or increase in activity.

radionuclide	type of radiation	energy (MeV)*	shielding needed	comment
H-3	beta	0.0186	none	never an external exposure hazard even in large quantities because of the 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, or plastic and lead	Use 1/4 inch thick or greater. 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/plastic	none for quantities commonly used/plastic for multimillicurie 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; lead bricks are not needed for the quantities typically used.

*average beta energy is approximately 1/3 of the maximum energy

Take a conservative approach toward shielding P-32 sources. If you aren't sure, then the prudent thing to do is shield them. 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.

Erratum: There is an error in some printed versions of the Radiation Safety Handbook, Appendix A, in the tritium table. The correct half-life is 12.26 years, not 2.26 years.

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