At the BLAST meeting on Wednesday, Adrian showed the effect of
proton energy loss on the BLAST acceptance for protons. The
GEANT results indicate that it takes protons of >/= 35 MeV,
momentum = 0.26 GeV/c, to reach and fire one of the TOF
scintillators.
Someone asked where the most significant loss occurs. We were
not able to answer that at the meeting, but I have now looked
at that. The most significant energy loss occurs in the 0.2 cm
aluminum entrance and exit plates of the Cerenkov boxes. These
contribute roughly twice as much as the 10 mil sheet of Pb in
front of the scintillators. This is roughly consistent with
the fact that each aluminum plate is about 10X as thick as the
Pb with about 1/5 the density. The 1/8" acrylic exit plate on
the wire chambers contributes an energy loss of about 75% of
that from each aluminum plate, again roughly consistent with
the ratios of thickness and density.
We have not yet put in the TOF backbones to see what energy
protons make it to the neutron bars. However, the backbones
are constructed of aluminum "honeycomb" with the cells lined
up parallel (approximately) to the trajectory. The skin of
each backbone is 40 mil. Since there are 4 skins, the total
thickness is approximately the same as for the sum of the two
plates on the Cerenkov box, 0.4 cm. The energy loss would be
comparable except that by that time the protons have now lost
a larger fraction of their initial energy, which makes dE/dX
larger.
-- John R. Calarco Dept. of Physics Univ. of New Hampshire Durham, NH 03824 phone: (603)862-2088 FAX: (603)862-2998 email: calarco@unh.edu
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