Re: line source calc.

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

At 11:03 AM 1/27/98 -0600, you wrote:
>Your point is made and looks quite acceptable except that for such a
>geometry, I am not sure that you can apply a point source assumption (1/r2).
>For this to be applied, you must have d> that 6 times the greatest distance
>of the source.

Actually, my point was not stated well enough. I was trying to say that
each point along the line source contributes to the total dose rate at the
target, which is 1 foot away from the center of the line source. The
contribution from the center of the line will be the greatest, and the dose
rate from each point not at the center will be smaller because all points
not at the center are farther from the target. For photons, this 1/R^2
decrease in contribution with distance from the center is the only effect
on the photon dose rate. However, for beta radiation, the 1/R^2 effect is
supplemented and actually surpassed by attenuation in air, making the beta
contribution to the total shallow dose rate a smaller fraction than would
be caused by a true point source. Did I say that right?

I also saw some words about brem from the beta contributing to the photon
field. The energy of Co-60 beta is pretty weak (< 100 keV average, 300 keV
max). I don't know that such beta energies would cause much brem, but I
have no specific calculation to offer.