Manipulating Neutrons in the Home Rad Lab
Neutrons can be bent,
reflected, moderated (slowed down) and absorbed
Most neutron detectors respond
only to "thermal" or slowed neutrons.
Most neutron reactions and therefore most neutron sources generate "fast"
or highly energetic neutrons. Very few detection systems can pick up fast
neutrons directly. The BTI Bubble Detector is one such fast neutron
detector.
The He3 tube is currently the
most sensitive and popular instrument to detect neutrons and available to the
Home Rad Lab, it too requires Thermal Neutrons to perform.
So keep in mind when reading
the following article describing experiments with neutrons, that they must first
be generated in their native FAST condition, then slowed down to detect and/or
measure them.
Materials used in these
experiments are:
1 small neutron source, lab
made from radium watch hands and beryllium sheets
1 Lot of HDPE (High Density
Poly Ethylene) sheets, 6" X 6" X ½" thick. Cut from kitchen cutting
boards.
1 Lot of pure graphite (carbon) sheets 6" X 6" X ½" thick. Army surplus electric motor brush stock.
1 1 lb. container of 100%
Boric Acid (Roach Powder)
1 He3 detector tube with
associated electronics (BNC SAM 935)
HDPE is used for its hydrogen
atoms, well know to be key players in
the slowing down of fast
neutrons (moderator)
Graphite is used for its
carbon atoms, also key players in neutron moderation, the first and all early
?PILES? used graphite moderators.
100% Boric Acid is used for
its boron atoms, well know to react with neutrons.
About 20% of natural boron is
the isotope B-10, the rest being B-11, both are stable (non radioactive)
isotopes. B-10 has a particular affinity towards neutrons and is even used in
some neutron detector tubes.
Photo credit:
http://en.wikipedia.org/wiki/Boron
The boron N> B-10 reaction
captures a neutron , emits an alpha particle and a recoil Li-7 ion.
Above photo credit:
http://web.mit.edu/nrl/www/bnct/info/description/description.html
The instrument package
used is a BNC SAM 935 with internal
He3 tube in a lightweight HDPE moderator.
Shown below in detail is the
internal He3 tube in its moderator. Our lab tests have shown that this moderator
is insufficient to slow down many neutrons. This makes it ideal for our purposes
since we want to test the affects of various moderators, reflectors and
absorbers.
For the first step, we measure
the bare neutron source with the SAM 935 with no additional apparatus. The
distance chosen for ALL tests was 4" from source to He3 tube
center.
The
source:
Without any external apparatus, the SAM 935 reads the source as a steady 6 CPM:
Next we try
adding reflectors BEHIND the source:
GN- 1 Graphite,
then the source
GGN- 2 Graphite, then the source ( this setup reduced the readings to near zero):
Trying next the HDPE as a
reflector, we added one sheet at a time and carefully took readings. More HDPE
was better but only up to a point. Fine tuning the reflector resulted in
1 inch as being the most efficient for this neutron energy. Note> In all
cases, the source is the same 4" from the He3 tube center. Apparatus is added
behind and in front as needed.
PPN- 2 HDPE, then the
source.
Fine tuning reflector results
in a doubling of the readings:
Next we tried many combinations of reflector and moderator:
GGGNGGG- 3 graphite reflectors, source, graphite moderators (very poor performer)
Next we tried various HDPE combinations:
PNP- 1 HDPE, source, 1 HDPE
Fine tuning an all HDPE
arrangement, the very best combination for this energy neutron seems to be
PPN__PP.
2 HDPE Neutron Source Space
Space 2 HDPE.
Putting the moderator right
next to the source was not as helpful
as placing them right next to
the detector:
By fine tuning the distances we could increase the reading from the original 6 CPM to a steady 48 CPM.
Once in a while we would get
readings of more than 50 to over 100 CPM.
Analyzing our methodology lead to the conclusion the extra hydrogen atoms IN MY HAND made that difference. I had decided that if anyone walked in to see me waving my hands and fingers around, I would tell them I was practicing my Theremin concert!!
Finally with the best
reflector/moderator scheme in place, we tried applying the boron atoms. The
results indicate that it does not matter much where the boron is placed, it will
suck up the neutrons. Shown below is an arrangement of apparatus that gives a
near zero reading. The boron does not seem to have to be in the direct path
all.
Conclusions:
A) From observation, neutrons
do not travel in a straight line when in the presence of light nuclei as in
hydrogen.
B) Carbon atoms affect the
travel of neutrons.
C) Boron atoms are an
effective absorber of neutrons.
Soon I will be in possession
of some new BTI BND Neutron Bubble Detectors.
I will repeat and fine
tune the fission project using this
new data:
http://www.qsl.net/k/k0ff//Fission%20in%20the%20Home%20Rad%20Lab/
Have Fun
George Dowell, "Geo"
NLNL/ New London
Nucleonics Laboratory
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2007
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