FW: Bremsstrahlung photon energy spectra

["David Scherer" <scherer@mirlink.wustl.edu>]

The following message is being forwarded from the Medical Physics listserver. 
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David Scherer
_______________________________________________________________________________
From: Medical Physics Network on Tue, May 30, 1995 12:58 PM
Subject: Bremsstrahlung photon energy spectra
To: Multiple recipients of list MEDPHYS

A grad student of mine is running some Monte Carlo
simulations and ran into the following interesting situation.
If anyone can offer some insight, we would be very
grateful.  Please respond directly to me, and I'll compile the
responses into a single message for reposting to the servers.

Thanks,

James B. Stubbs
Radiation Internal Dose Information Center
************************************************
I've been running a series of electron transport problems
using MCNP.  The problems consist of a uniform,
homogeneous, isotropic beta spectrum source located in a
tissue equivalent volume and surrounded by an additional
volume of tissue equivalent material.  I've been calculating
the absorbed dose vs. depth profile for different
radioisotopic beta spectra (eg. I-123, I-131, etc.).
To do this I've been using the energy deposition tally (ie.
tally 8) with the source physics set up for an electron
spectrum source and with the photon physics turned on.
Hence, tally 8 is calculating energy deposition from
electrons and photons.  The results are interesting.  Dose
drops off "exponentially", initially, vs. depth into tissue;
however, once the max beta energy range is reached the
dose levels off at a "plateau" value and then very slowly
decreases with continuing depth.  In the case of I-131, this
plateau occurs 4 orders of magnitude below the initial dose
(ie. dose @ zero depth). This sort of dose behavior is
expected since bremsstrahlung photons are being created by
the electrons.  Thus the dose plateau is due to
bremsstrahlung, and the results of MCNP confirm that this
is indeed the case.

The typical result of 3-4 orders of magnitude difference
between the initial dose and the bremsstrahlung plateau
dose is not intuitive (for me).  So my question is:  Is there an
analytic calculational method that predicts the
bremsstrahlung ENERGY SPECTRUM emitted from a
monoenergetic electron passing through tissue?  If so, would
integrating this bremsstrahlung spectrum over a
radioisotopic beta spectrum yield an appropriate
"radioisotopic bremsstrahlung spectrum?  If such a method
exists, I could then check MCNP's calculated results.

Thanks for your help.

Jeff Evans

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Date:         Tue, 30 May 1995 11:36:00 -0500
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From: Stubbs T <STUBBSJ%ORAU.GOV@WUVMD.WUSTL.EDU>
Subject:      Bremsstrahlung photon energy spectra
Comments: To: dose-net@ORAU.GOV, nucmed@largnet.uwo.ca
To: Multiple recipients of list MEDPHYS <MEDPHYS@CMS.CC.WAYNE.EDU>