As a form or radiation, neutrons come from
nuclear fission in reactors, and from spontaneous fission from certain Uranium,
Plutonium and Californium isotopes.
Cf-252 is one such isotope that gives off a lot of
neutrons, and is used as a neutron source in industry.
Another neutron source commonly used in industry
utilizes an alpha emitter and the reaction between alpha particles and low
atomic weight nuclei, such as Oxygen, Carbon, Lithium or
Beryllium.
Sources made with Am-241/ Beryllium (Am/Be),
Cm/Be, Ac/Be, Pu-238/Be, Pu-239/Beryllium (Pu/Be), Radium/Beryllium (Ra/Be) and
Po-210/beryllium (Po/Be) are made, with Am/Be being the most widely
deployed today.
Am/Be sources with 40 milliCuries of Am-241 are
part of the so-called nuclear gauges made by Troxler, and used in road
construction etc.
The reaction is:
9Be(a,n)12C.
Various neutron generator tubes are out there in
use, these are electronically driven devices that create neutrons in the
reaction between
Deuterium or Tritium atoms in a simple form of an
electronic accelerator.
Gamma Rays can create neutrons too, called
PhotoNeutron sources.
Lastly neutrons come indirectly from Galactic
Cosmic Rays. When a very powerful cosmic particle, mostly hyper energetic
protons, enters the earth's atmosphere, it collides with atomic nuclei,
shattering them into pieces with tremendous energy. Neutrons fly on and
eventually collide with other nuclei, creating "neutron
showers".
Some of the resulting neutrons are detectable here
on the earth's surface.
Neutrons are neutral of course, and that makes
them problematic to detect. In a gas filled detector, the neutron interaction
with Boron or Helium-3 is capitalized to create a secondary charged particle,
which is then detected by normal ionization techniques. Currently the most
efficient of these methods uses He3, and the higher the pressure within the
tube, the higher the chance of an interaction, therefore the higher the
efficiency.
Solid detectors made up of material high in
hydrogen atoms uses a reaction between neutrons and the proton in the hydrogen
to induce a Proton Recoil, which can be easily detected by ZnS(Ag), converting
the neutron into an optical light photon, in a manner similar to an Alpha
Scintillator. The resulting light pulse is converted to an electrical pulse by a
Photo Multiplier Tube in both instances.Some plastic scintillation material
provides both the Hydrogen atoms and the Proton detection in a single material,
making for a simple detector scheme, needing only a PMT and dynode
string.
Among present state-of-the-art techniques, high
pressure He-3 tubes are the most efficient available, being 50 times more
sensitive than the previous king of the hill, BF3 tubes.
Almost all of the detection schemes mentions will
only respond to very slow neutrons. At the point of generation, neutrons are
very fast indeed and must be slowed down to very low speeds, referred to as
"thermal", a reference to room temperature.
Think of bowling balls, marbles and billiard balls
for a moment. There are all similar in rigidity and shape, but differ greatly in
size, also called mass. A marble
slammed into a bowling ball will not affect the
bowling ball, nor would a billiard ball. Another bowling ball would of course
have a great affect.This same principle is at work on the sub atomic scale as
well. Nothing will affect a neutron more than something about it's own size. For
this reason, hydrogen atoms are used to interact with neutrons, slowing the
neutron down by robbing some of it's energy art each interaction.
Hydrogen in the form of water, wax of
high-density-polyethylene (HDPE) is the material of choice to slow down or
"moderate" neutrons so that they can be more easily detected by the tubes in the
probe.
For mechanical reasons HDPE was chosen as the
moderator for this project.
Pictures of home made neutron detector using 20
atm He3 proportional neutron detector tube. Tube is 1/2" x 4 " active length,
He3 is 300 PSI, moderator is 6.5" X 12.5" HDPE. Compare this to the N. Wood BF3
tube in the 9" Remball @ 1" active length. and .78 ATM or roughly equiv. to LND
20126.
A handle will be fitted when the final arrangement is
decided.
Pickup tube is a GE/ Reuter-Stokes RS-94-0404-253
and is equiv. to LND 25143, with thermal neutron sensitivity of 12.7
cps/nv.
An N. Wood G-5-1 as used in the Remball is rated at .25 cps/nv making
the home made probe some 50 times more sensitive as the Remball and 4 times the
active length. .
Shown is a very small, temporary Ra/Be neutron
source which was constructed for testing purposes then quickly
dismantled.
Have
Fun
George Dowell, “Geo”
NLNL/ New London Nucleonics
Laboratory
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2007
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