Neutron Thermalization Optimization

[chapmanja@ornl.gov (Jeff Chapman)]

Anthony,

Here's how we'd solve this problem:

1.  Use IAEA Technical Report Series No. 318 for neutron spectra.
2.  Input the source distribution into MCNP and establish all corresponding
MCNP input for the specific case: materials, geometry, etc.
3.  Use the code to predict at what point the thermal flux is optimized.

By measurement:
1.  Time tag a Cf252 source and measure the resulting time distribution of
fission neutrons (the so-called Rossi alpha distribution).  We have
time-tagged Cf-252 neutrons in two ways:  a.  using 2 NaI detectors to
measure coincident gamma-rays (a bit sloppy but it works), then open the
"gate" for detecting subsequent neutrons; or b. using a 4pi ion chamber with
a ng of Cf252 placed inside---the neutrons are tagged at a time when the
highly charged fission fragments are detected by the ion chamber.
2.  By tagging the neutrons to the Cf-252 source, you will get an "exact"
representation of system efficiency.  Use a bare He3 or BF3 tube to measure
the thermal flux in various geometries.  Make sure your discriminator is set
high enough to discriminate out the gobs of gamma-rays emitted from the source.
3.  If you can't afford or don't have the time to time-tag the neutrons, you
can just time-integrate the output of the neutron detector and experiment
with your proposed design.

I don't know what the spectrum is for AmBe or PuBe, but I would guess that
the poly will be no thicker than 15 cm, and probably around 7 to 10cm.

Jeff Chapman


Date: Tue, 04 Feb 1997 12:01:28 +1000 (EST)
From: Anthony Barber <a.barber@qut.edu.au>
Subject: Thermalisation optimisation

I am trying to repost this message, I think it failed last time.


I am looking for information to help optimise the thermal neutron flux in
the cavity around a 370GBq Am/Be and 370GBq Pu/Be source.

What thickness of polyethelene will produce optimised thermal neutron flux
in this configuration?

The TVL for polyethelene and 4.5MeV neutrons is 20cm

I'm considering polyethelene moderator surrounded by a Be reflector, then a
1m thick concrete bunker. (if practical)

All help and suggestions greatfully accepted.

Regards.

 
  Anthony Barber                                         
  Queensland University of Technology 
  a.barber@qut.edu.au  
  61 7 3864 3566
  fax: 61 7 3864 3993
***************************************************************
 Jeff Chapman, CHP, PE
 Applied Radiation Measurements Group
 Oak Ridge National Laboratory
 PO Box 2008, MS6388
 Oak Ridge, TN  37831-6388
 CHAPMANJA@ORNL.GOV,  423-574-5729, FAX 423-576-7954
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