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Why are calculated values
for stopping powers different than ICRU 37?
To calculate average stopping power, you must divide the energy
deposited during transport steps (IARG = 0) by the pathlength of the
transport step (IARG = 0). Then you must add a "track-length" term to
account for the particles that stopped at the EGS transport threshold.
The estimate of track-length for these stoppers under equilibrium
conditions is Delta_E(E)/L(E) where Delta_E(E) is the kinetic energy of
the stopper and L(E) is its stopping power.
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If you take just total energy deposition and divide by IARG = 0 track
length, then you make an overestimate because you miss the track-length
of the stoppers in the divisor. The size of the overestimate will depend
on the transport thresholds.
This procedure is spelled out in the paper by Malamut et. al in Med Phys
18 (1991) p. 1222-1228. The original calculations of this kind were
done by Alan Nahum. His work is referenced in the above paper. Alan did a lot
of work to clarify the &track-end& contribution.
FAQ answer provided by Alex F. Bielajew
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last updated 10/04/01