Rules of Thumb

TIt requires an alpha particle of at least 7.5 MeV to penetrate the dead layer of skin, 0.07 mm thick.

It requires a beta particle of at least 70 keV to penetrate the dead layer of skin, 0.07 mm thick.

The range of beta particles in air is about 4 meters per MeV; for example, a 3 MeV beta has a range of about 12 meters in air.

The bremsstrahlung x-rays from a 10 mCi ³²P aqueous solution in a glass bottle cause an exposure rate of about 0.1 to 0.2 mR/hr at 1 foot.

For a point source of beta radiation which can travel at least 1 foot in air, the beta skin dose can be determined as follows: Multiply the activity, in mCi, by 300. The result is the beta skin dose at 1 foot from the source in mR/hr.

For a point source gamma emitter with an energy between 0.07 and 3 MeV, the exposure rate (mR/hr) at 1 foot is 6 CEN, when C is the activity of the source in millicuries, E is the gamma energy in MeV, and N is the number of gammas per disintegration. If more than one gamma is emitted per disintegration, this equation becomes 6CE_iN_i.

The activity of any radionuclide is reduced to less than 1 percent after 7 half-lives.

For ³²P, 8,300 cpm is about equal to 1 mR/hr beta skin dose when using a Ludlum Model 3 with a pancake probe, which has been calibrated with an electronic pulser.

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