Re: line source calc.

[Bob Flood <bflood@SLAC.Stanford.EDU>]

At 06:59 AM 1/27/98 -0600, you wrote:
>Make sure you have not underestimated the betas from Co60 (96 keV and 626
>keV average energy) and especially exposure to the skin.

If this thread mentioned anything about encapsulation of the Co-60, I don't
recall it. If the source is encapsulated, it is probable that none of the
Co-60 beta radiation will escape the source.

A massless point source (no self absorption) of Co-60 would produce a
shallow dose rate at 1 foot that is about 5 percent beta, 95 percent
photon. If the geometry is changed to a line source, the beta contribution
is smaller because the photon contribution from points away from the
source's center are reduced by 1/r^2, while the beta contribution is
reduced by 1/r^2 and attenuation in air. However, even if the source is not
encapsulated, self-absorption of the rather low energy beta spectrum within
the source itself will reduce actual emissions to only those betas emitted
at or extremely close to the surface of the source. After attenuation in
air (1 foot and more), the beta contribution to the total dose rate will be
very small, probably well under one percent, and much less than the
uncertainty in the photon dose rate estimates.

I believe that the beta contribution to skin dose could be neglected in
this case.