Re: Neutron Quality Factor

["Scott O. Schwahn" <schwahn@CEBAF.GOV>]

At 01:01 PM 4/11/96 -0500, Tom O'Brien wrote:
>Greetings Radsafers;
>
>I'm inquiring about the neutron QF decrease above 1 MeV. Can someone provide
>the basis for this?  I have yet to find/hear a good accounting.
>

O.K., I'll be Mr. Smarty-pants.  The values you refer to are the neutron
MEAN QUALITY FACTORS for the energy groups specified, and are the maximum
value (with respect to depth) in a 30-cm phantom.  

The most important interaction of fast neutrons in tissue is the elastic
scattering interaction with hydrogen nuclei.  This produces energetic
protons that deposit energy very locally in the tissue.  For thermal and
low-energy neutrons, the capture reaction becomes more important.
H-1(n,g)H-2 produces a 2.2 MeV gamma ray which interacts with the body; this
is the most important reaction at low energies w.r.t. delivering dose to the
body.  Near the surface of the body, the N-14(n,p) reaction is somewhat more
effective than the hydrogen capture.  

As the cross sections for these various reactions vary with neutron energy,
the total dose to the body maxes out around 1 MeV, drops, and flattens out
at very high energies (with some resonances around 20-40 MeV).  Hope this
helps, and any fine-tuning by other RADSAFERS is welcome.

Scott O. Schwahn, CHP
Operational Health Physicist
Continuous Electron Beam Accelerator Facility
(804)249-7551 (w)
(804)249-7363 (fax)
schwahn@cebaf.gov