RE: Beam Correction Factors

[Lester Slaback <Lester.Slaback@NIST.GOV>]

"Lieskovsky, Miro" said....
Dick Olsher of LANL has used MCNP runs with the following response for the
9" REM ball; results are normalized to unit incidence fluence and use a
certain neutron spectrum.
1" dia beam/9" beam = 0.0187,
2" dia beam/9" beam = 0.0733,
4" dia beam/9" beam = 0.274
The factors assume that if the fluence fields are expanded to fully
irradiate the REM ball, ratios would be unity.

Measurements at NIST for a 'soft' spectrum transmitted via a glass neutron
guide, translated into the above format,  are...
          side-on     end-on  area fraction
1" dia  .046         .028        .012
3" dia.  .327        .219        .111
5" dia   .638        .569        .309
7" dia   .935        .844        .605

Side-on means with the BF3 detector oriented perpendicular to the beam.
End-on has the detector pointed towards the beam (at a downwards angle as
in the Eberline design).
The were obtained with a 2.8x2.8 cm beam scanned across the detector
equator in 1" increments, and then converted to annular ring responses.
Interpolating, the agreement is not bad, particular given the unspecified
spectrum.

Note that the response is well in excess of the area fraction.
Disclaimer:  the above are the personal musings of the author, and do not
represent any past, present, or future position of NIST, the U.S. government,
or anyone else who might think that they are in a position of authority. 
Lester Slaback, Jr.  [Lester.Slaback@NIST.GOV] 
NBSR Health Physics 
Center for Neutron Research 
NIST
100 Bureau Dr.  STOP 3543 
Gaithersburg, MD  20899-3543 
301 975-5810 voice
301 921-9847 fax
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