Troxler 3411-B, Manual Of Operation And Instruction

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Appendix B-8
Figure 14. Effects of Distance on Exposure
SHIELDING
Shielding is any material used to reduce the radiation reaching
the user from a radioactive source. While some types of
radiation such as alpha particles may be stopped by a single
sheet of paper, other particles such as neutrons and photons
require much more shielding. Materials containing large
amounts of hydrogen, such as polyethylene, are used to shield
neutrons. Dense materials, such as lead, are used to shield
photons. The 3400 series gauges have shielding built into the
system which reduces the exposure.