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Subject: Re: Gamma Ray Spectrometer?
From: (Bill Higgins-- Beam Jockey) 
Date: Aug 08 1995

In article <408d6h$enp@nntp.Stanford.EDU>, pln@egret0.Stanford.EDU (Patrick L. Nolan) writes:
> Henry Spencer ( wrote:
> : My understanding is that it can be done, somewhat indirectly:  what you
> : observe is not the hydrogen itself, but gamma rays produced as a side
> : effect of cosmic-ray neutrons interacting with the hydrogen.  (I'm vague
> : on the details, and would appreciate it if someone who knows could supply
> : them.)
> I haven't heard of any measurements being done, but it's pretty clear
> how it would have to work.  The hydrogen nucleus is just a proton.  
> A proton is a simple object with no excited states that are accessible
> under normal circumstances.  Thus no gamma spectrum.  However, a proton
> will readily capture a slow neutron to form a deuteron, releasing a
> gamma with an energy of 2.223 MeV.  That's quite easy to detect.

If memory serves, this is not the main mechanism for detecting hydrogen.

> There aren't many neutrons in cosmic rays because free neutrons are
> unstable.  When cosmic-ray protons interact with any form of matter,
> secondary neutrons are formed.  

True. The neutrons induced by cosmic rays tend to have a "fast"
spectrum. Hydrogen is also an excellent moderator, which means that
the low mass of its nuclei makes them efficient at slowing down fast
neutrons.  I think slow neutrons trigger a different set of nuclear
reactions in the heavy nuclei of the soil.  Details elude my memory.
Nuclear physics is not my strong suit; in our work here, E always
equals p*c pretty exactly.

The idea of using a GRS to look for lunar water depended upon
identifying characteristic gammas from slow-neutron absorption (in
heavy lunar soil elements) rather than from fast-neutron absorption,
if I'm not mistaken.  This scheme was improved by one other gimmick
which I'll discuss in a moment.

> If the matter is dense enough, the
> neutrons will slow down through collisions before they decay.  

No, if the matter is made of heavy nuclei, the neutrons will not
quickly lose their energy in collisions.  They'll just bounce off and
retain most of their kinetic energy.  (Recall that this is a problem
in building a fission bomb or reactor.)  Light nuclei will slow the
neutrons down after a small number of collisions.

> Some
> will be captured by any protons (hydrogen) that happen to be present.

I may be wrong about this, but as I said, I don't think this is the
major signal that (say) Lunar Prospector expects to see.

The gimmick that will help LP detect hydrogen is a shell of some
just-right element surrounding the gamma-ray spectrometer.  This
element has a good cross-section for absorbing a slow neutron, and
such an event will kick it into an excited state which quickly emits a
gamma of a particular energy.  Thus the strength of the spectrum at
this energy is a measure of the flux of slow neutrons in the
spacecraft's vicinity.  You don't have to depend only upon a signal
from neutron-induced reactions in the soil;  you have a way of
measuring slow neutrons more or less directly.

Imagine LP is flying over an ordinary part of the Moon.  

1. Cosmic-ray protons hit the soil.  
2. The collisions generate showers of fast neutrons.  
3. The fast neutrons go a long way in the soil, making elastic
   collisions with soil nuclei. 
4. Some gamma rays, typical of states excited by elastic collisions in 
   the soil elements, are given off.  They go in all directions, but
   some go straight up into the sky.
5. The spacecraft's gamma-ray spectrometer detects these.  But it
   doesn't see any photons at the energy triggered by slow neutrons.

Now imagine LP is flying over a few square kilometers of ice, mingled
with lunar regolith, in those shady spots near the poles.

1. Cosmic-ray protons hit the soil.  
2. The collisions generate showers of fast neutrons.  
3. The fast neutrons bounce off lightweight hydrogen nuclei in the
   water molecules. 
4. They lose energy rapidly and become slow neutrons.
5. Some slow neutrons are absorbed by iron or uranium or whatever in
   the soil.  The excited nuclei kick out gamma photons of certain
   energies.  Some go upwards.  The GRS aboard the spacecraft detects
6. Other slow neutrons are scattered upwards from the soil.  They
   interact in the special shield around the GRS (sorry, I've forgotten
   what this magic material is).  Gammas are detected that say "Slow
   neutrons here!"

For details, see papers on planetary gamma-ray spectrometers by such
people as Al Metzger, William Boynton, and Jack Trombka in various
journals (*Nuclear Instruments and Methods* comes to mind).  Late
Seventies, early to middle Eighties.  Mars Observer had a GRS aboard
and I think it used this slow-neutron gimmick to try to detect ice in
the Martian regolith.

I don't suppose they bothered putting neutron measurement into the
NEAR GRS-- not much point in looking for ice on Eros.

> Measurement of the capture gamma rays is easy, but understanding the
> results is not.  The strength of the line depends not just on the
> quantity of hydrogen, but on the density of other matter, the
> incident cosmic ray flux, and probably several other details.

Very true.  All the stuff I just described was very simplified;
actually the signal is on top of a large background of photons and
neutrons from the Sun and the galaxy.  This paragraph makes me suspect
that Patrick's understanding of this subject is more subtle than my

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