Re: Radiation weighting factor for neutrons

[Jim Durham <jsdurham@lamar.colostate.edu>]

The radiation weighting factor is an attempt to explain the observations in
radiobiology.  It is observed that, for the same amount of energy deposition in a volume
of tissue, the biological effect (survival, mutation, etc.) of the cells is less for
higher energy neutrons compared to those in the 100 keV to 2 MeV range.  The term
relative biological effect (RBE) is used to compare the observed effect at the same
doses to a standard (I believe it is X-Ray irradiation).  The RBE seems to be closely
correlated to the LET of the secondary particles formed by neutron interactions in
tissue, especially at dose rates associated with radiation protection.  Consequently,
the radiation weighting factor is introduced to correlate the observed biological
effects to the measured absorbed dose.  Explaining the reason for these effects is the
purpose of cellular biology and probably has a direct bearing on cancer induction by
radiation.  It is interesting to note that the RBE of radiation is not the same at all
doses and all dose rates.  However, in order to make the profession of health physics
somewhat manageable, the ICRP has simplified the process (in ICRP-60) by condensing
broad categories of biological effect into radiation weighting factors that are meant to
be used for risk.  Thus, since the average LET of recoils generated at high neutron
energy is less than the LET of recoils for fast neutrons, Wr decreases.

Jim Durham
Tom Borak
Department of Radiological Health Sciences
Colorado State University

Scott Schwahn wrote:

> No, you're sharp as a tack.  My main point was lost in the details of my poorly
> worded response and also due to my assumptions.  The main point I was trying to make
> was that the reactions types change with increasing energy, in this case, to a
> not-as-biologically-damaging set of mechanisms.  However, I am used to working with
> the conversion from neutron flux (neutrons/cm2) to dose equivalent (Sv), not the
> radiation weighting factor (Sv/Gy).  The latter is too much for me to quickly think
> about at the moment, so I will bow out of the discussion.
>
> Sorry for the confusion I caused.
> Scott
>
> --
> Scott O. Schwahn, CHP
> Thomas Jefferson National Accelerator Facility
> schwahn@jlab.org
>
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