Re: Radiation weighting factor for neutrons

["H.B. Knowles" <hbknowls@ix.netcom.com>]

At 05:28 02/16/1999 -0600, Sunil C Nair wrote:
>
>Hi All
>
>I have this doubt for some time. Coud'nt clear it properly in spite of
>searching the library. Someone in this group may have an answer to this.
>
>The radiation weighting factor for neutron peaks at 20 in the
>energy range 100KeV-2MeV and then falls off for higher energies. Why high
>energy neutrons has less QF ?  Is it because the secondary particles
>produced has less LET, or something else ? How will you explain the graph
>showing radiation weighting factor and neutron energy ? (fig 1, page 7,
>ICRP publication 60). I read something about "overkill" by which high LET
>particles may "waste" energy so far as biological effects are
>concerned.(Radiation dosimetry, Vol 1,1968, edited by,Attix and Roesch
>Academic Press). Is that a(the) reason ?
>
>Do you mind sharing your thoughts about this ?
>
>Thanks to all 
>Sunil Nair

Dr. Nair:

You have raised a question about one of the most poorly explained (and named)
features of radiation dosimetry.
        1. First of all, let's look at the nomenclature. What is now called
the
"radiation weighting factor" used to be called"the quality factor" (symbol Q)
and before that, up to about 1965.. " the Relative Biological Effect" (symbol
RBE). Now note the word "relative".
In those days, the RBE was a number that was used to multiply the phsical
dose 
(measured in rem or Gray) by to make the biolopgical effect of the radiation
dose (of ant kind-not just neutrons ) the same as that of the same amount of
dose deposited by energetic photons. The resulting "dose erqiovalent (or
biological dose) was/is expressed in rem or Sieverts.
        2. Now pause for a minute here, since the above is, or should be,
common
knowledge. First of all, which  gamma rays or photons are the standard? At one
time, it was the x-rays from a 70 keV electron beam. later, it seems to have
been a higher-energy kind of photon (2 MeV, I heard). Subsequent research on
the Hiroshima survivoirs has reveled that there is some distinction betreen
the
effects of the low-energy and high-energy radiation wilk the resultiong
confusion in the (to many, indefensible) attemts by ICRP to increase the
RBEs/Quality factors/radiation weighting facors of neutrons. I pass on this
controversy. Howevr, the reference standard is not well-defined or
questionable. 
        3. The next point to be raised is, what effect is being used to
evaluate
the RBE or whatever iy is called? Primitive experiments of cells in Prtri
dishes tend to show that if the effect one is lookuing for is , say 50%
survival, then the RBE is low- like 1.5 to 2.5 for most "modalities. This
means, to restate the definition, that if 1 kilorad of reference photons
causes
a 37% survival of some bacterium, ifonly 500 rad need be deposited by say,
fission specreum neutrons to achieve the same mortality on those cells under
identicl conditions,  it would be stated that they have an RBE of 2.0. But it
is generally found that, as the survival fraction increases- say to 80%. the
RBE increases- maybe to 5.0 (I am making up these numbers,to illustrate the
discussion; I never heard of a bacteium that came anywhere cxlose to this
behavior.). In effect, the high-survival cases are clearly what is desired and
these have quite high experimantal RBE's. It is only a matter of bvureacratic
caution to defien the protection RBEs higher still, like: 10 for protons and
neutrons, and 20 for alpha- particles. The name RBE is lost, together with the
relative (to photons) concept, and no one evern points out that, when you want
to eradicate a tumor by some type of raduation other than x-rays, thre goal is
low survival of the tumor cels, and the measured RBE gets down to 1.3 or so. 
        4 Physically, why shpold a neutron do more damage than a photon id
eqal
physical doses? A neutron interacts with bodily tissue by causing nuclear
reactions and-probably more important-nuclear recoils of heavy ions. Rather
simple biophysical arguments show that the effect omn cells moght be expected
to depend on Linear Energy Transfer (LET) which is almost equal to what
physicists call stopping power, but modiified to exclude the effect of
high-energy electrions (:"delta rays" ) which leave the track core. These
cause
an ionization density, which is supposed to predict the lethality on scells.
Protons, and electrons have the same peal LET (as one of the other respondents
has piointed out) , so why areprotons so much worse? Because they can cause
nuclear reactions and recoils. The slow moving, highly charged recoil atoms
have a tremendously high LET, so a high lethality is imputed to thgem, but, as
showm by Howard-Flanders in two incredibly brilliant papers in the late
1950's,
the very high LETs basically cause "local Overkill" and can exibit RBEs which
are LESS THAN 1.0 by quite a bit. Neutrons are still more complex because of
the numerous particles they can create, and, even as they die, by capture,
kick
off a gamma ray, usually.

        5. The actual physical dose deposited  by a neutron  increases
dramatically as the initial energy increases, as shown in the excellent review
by Belogorov, et al, published in the 1980s But at the higher energies, they
produce frewer low-energy particles of t5he kind that really damage (fewer
peak
iionizations) and the RBE/Q/weightingfactor must decrease. Make no mistake: a
1000 MeV neutron is many times more dangerous than a 1 MeV neutron.
Incidentally, the suggestion that the low weighting factor is responsible for
thew lower exposure at high-energy accelwerators is unjustified: the personnel
are mostly exposed to the lower-energy neutrons that make it through the
shield
walls and also to the (roughly) fission-spectrum and lower-energy neutrons
that
leak thru mazes, ducts, and the like. The principal neutron background at the
old LAMPF accelerator (now LANCSE) in the A area was the 80 keV neutrons which
had leaked through the famous "hole" in the mostly iron shielding. 
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