Shielding at large depths

From: Joel DeWitt <>
Date: Fri, 3 Oct 2008 12:16:22 -0500


I have a 956 MeV/n 56-Fe beam passing through a target-detector system
as shown in System.eps. Interspersed between the Al targets are CR-39
PNTDs, which are modeled as water in FLUKA. For simplicity I have
surrounded my system with vacuum. In the past I needed to know the
momentum distribution of the beam upon reaching the target-detector
system, so I ran a separate FLUKA simulation by passing a 1 GeV/n
56-Fe beam through a known depth of air. I chose to score
longitudinal momentum in the c.m.s. frame.

Does this seem like a good approach? This is a minor point, but one I
need to ask about.

After obtaining this I chose (again for simplicity) a beam size equal
to the size of the scanned area of my CR-39 (the "read" portion of the
detector). I chose the number of primaries based on the fluence
measured in the front detector (furthest left in System.eps--if you
can see it). In scoring the double differential yield with respect to
LET and charge, I have applied a normalization factor that compensates
for the LET and charge bin widths, while dividing by the scanned area
to obtain fluence. A sample result is shown in Al30.eps.

Obviously FLUKA will pick up many low-LET contributions over the
detector, as shown.

Can this be responsible for the height of the main peak?

(I do realize there could be losses due to scattering at the periphery
of the detector, especially at large depths.)

My main concern is the position of the primary ionization peak under
30 g/cm2 Al. The theoretical comparison to our experimental data is
based on the semi-empirical range-energy code by Weaver and Westphal
(NIM B 187, 285 (2002)). This code was found to agree with the FLUKA
prediction of the position of the primary ionization peak to within
2-3%. (We looked at LET in basic materials of various 1 GeV beams
when doing this comparison; it is unknown if this agreement is still
good after passing through matter.) In the case of 30 g/cm2 Al, the
Weaver code says ~342 keV/um. As shown in Al30.eps, both the FLUKA
and experimental result disagree with this prediction. Our
disagreement is still being investigated. This is surprising since
for lower depths there is much better agreement up to about 15 g/cm2.
It should also be noted that FLUKA also predicts more range straggling
for larger depths than that shown by experiment.

Could it be that the Weaver code underestimates the final energy of a
heavy ion primary under large depths of shielding, giving a higher LET

So in addition to passing along a sample of current results in
shielding using FLUKA, I would like to poll those here for suggestions
or advice as to a future direction with this comparison. If
interested, I can also send the other results for other smaller depths
of aluminum.

With much appreciation,
Joel DeWitt
Oklahoma State University

Received on Sat Oct 04 2008 - 17:15:22 CEST

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