Neutron dose factor, comments

[Les Slaback <lester.slaback@nist.gov>]

The reactions involved are well documented in various ICRU references (ICRU
10e, 46), but a few highlights......

1.  Take care to differentiate between absorbed dose and DE/ED.  A process
that dominates the former may not control the latter.

2. Interactions with hydrogen dominate overall, either as n-capture or via
scattering.

3. The kerma dose factors do not  track the people dose factors (whichever
quantity you want to choose).  Specifically, at low energies (< 1 keV)
where the kerma follows the increasing cross-section the ED factor is
relatively constant.

4.  At less than 1 keV life is simply in terms of reactions, but complex in
terms of why the dose factor does not change much.
    - the only reactions of interest are the N(n,p) and the H(n,g), where
the N delivers a local dose and the H creates a distributed dose.
    - the H(n,g) controls the absorbed dose but the N(n,p) controls the ED
    - the cross-section is increasing at 1/v, but the ED dose factor does
not change much.
    - N is present primarily in protein (see ICRU 44) so for low energy
neutrons the major portion of the dose distribution is controlled by the
protein distribution. [Interestingly ICRP in its latest reports computes
the DE factors by doing the absorbed dose calculation, which is controlled
by the H(n,g) reaction, and then simply applying a QF.  This totally
ignores the dose distribution influence of the protein.]
    - with the much higher cross-section of cold neutrons I originally
thought that they would represent a skin dose issue.  Two factors made this
presumption false.  First, the distribution of low energy neutrons in the
body is in fact controlled by the much larger scattering cross-section, not
the absorption cross-section.  Hence the increasing absorption
cross-section does not have much effect in moving the peak dose to the
surface. (And also note that skin has a low nitrogen content which also
suppresses that dose factor.)  Second, cold neutrons in a warm environment,
e.g., a living human, warm up very quickly so that in fact they become in
essence thermal neutrons.  But the main point of this cold neutron mention
is that even at this energy extreme shallow dose does not become a
controlling issue. [For those unfamiliar with the topic cold neutrons are
those in partial equilibrium with an environment colder than room
temperature, e.g., typically one at 5-20 Kelvin.  These neutrons are used
at research facilities like the NIST Center for Neutron Research and at ILL
(France).]
  
More than most wanted to read, but a few of the recent comments needed to
be expanded upon.

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 
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