[BLAST_ANAWARE] new collimator monte carlo results

From: Aaron Joseph Maschinot (ajmasch@MIT.EDU)
Date: Fri Oct 24 2003 - 10:54:18 EDT


here are some new collimator monte carlo results. the setup for each of
these plots is as follows:

  an areally-constant distribution of beam-halo electrons were generated
  within rings of varying radii about the BLAST z-axis. the different
  rings had the following radii:

     5mm < r_beam < 15mm
    15mm < r_beam < 25mm
    25mm < r_beam < 35mm
    35mm < r_beam < 45mm

  these beam-halo electrons were then allowed to propagate through (and
  physically interact with) the collimator and target. the amount of
  bremsstahlungh reaching a radius of 60cm away from the center of the
  target was then recorded. the scattering angle (defined relative to the
  center of the target, NOT the entrance plane of the collimator)
  histogrammed and normalized (via the total number of incident beam-halo
  electrons)

i repeated this analysis for the collimator that is currently in BLAST as
well as the one proposed at the collaboration meeting last week (the
proposed collimator has the downstream outer recess completely filled and
the upstream inner recess halfway filled):

  current collimator

    ***************
    * *
    * *
    *********** **********************************
              * *
              **************************************

    -- beam -->

              **************************************
              * *
    *********** **********************************
    * *
    * *
    ***************

  proposed collimator

    ************************************************
    * *
    * *
    ****** *
         * *
         *******************************************

    -- beam -->

         *******************************************
         * *
    ****** *
    * *
    * *
    ************************************************

from the plots, i concluded the following:

1) the proposed collimator should reduce a significant portion of the
   bremsstrahlungh going straight down the beam pipe
   (i.e. 0 deg < scattering angle < 5-10 deg).

2) for beam-halo radii less than about 25mm, the proposed collimator will
   significantly reduce the amount of bremsstrahlungh going into the
   detector (i.e. 10-20 deg < scattering angle < 80-90 deg). however, as
   beam-halo radii increase beyond 25mm, the amount of bremsstrahlungh
   reduction diminishes. for 25mm < r_beam < 35mm, the proposed
   collimator is still noticably more effective than the current one, but
   it is less noticeable than in the 5mm < r_beam < 25mm region. for
   35mm < r_beam < 45mm, the proposed collimator will essentially do no
   better than the current one.

3) the proposed collimator with produce about the same amount of
   backscattered bremsstahlungh as the current one
   (i.e. 160 deg < scattering angle < 180 deg).

i would thus conclude that the proposed collimator should replace the
existing one. in addition, one of the two following options (or
both) should be done:

a) the scrapers should be used to eliminate as much high-beam-halo radii
   electrons as possible (this option is already done, of course)

b) if physically possible, another centimeter or so should be added in
   overall radius to the collimator.

both of these options will be effective in reducing the amount of
bremsstrahlungh reaching the detectors.

aaron


















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