Polarimeter Construction for measuring the depolarization of neutrons in Deuterium
Matthew Bowers, Orlay Sint, Dr. Alexander Komives
Abstract:
Each of the fundamental forces has constants associated with its strength. One of the constants for the weak force is
the r meson coupling coefficient There is a proposed experiment to measure this constant using deuterium capturing
polarized neutrons to make tritium and polarized gamma rays. The calculation of the r meson coupling coefficient
requires that the neutrons be polarized when they are captured. But it is known that the absorption is not very
efficient so there is a possibility that before it is captured the neutron will lose its spin. The degree of neutron
polarization can be measured by passing the polarized gamma rays through devices containing polarized electrons
in the form of magnetized metal. Two of these devices, called polarimeters, have been built and initial tests have
been performed.
Polarimeter Construction and Testing:
d
u
d
u
u
d
The detector is a Cesium Iodide Crystal (CsI) with a
photo multiplier tube (PMT) on each end. CsI is a
scintillator, which will release flashed of light when
photos enter it. The gamma rays in this experiment
have enough energy that the electrons of the crystal
are actually ionized, recaptured, and de-excite
producing a visible photon. These photons then travel
down the length of the crystal and into the PMT. The
PMT has a clear lens with a transparently thin coating
of metal. The photos knock electrons off this layer as
they pass. The remained of the tube is a series of metal
plates with a high voltage. The result is the electrons
accelerate to a plate, collide, producing more electrons,
then accelerate to the next plate. By the end enough
electrons have cascaded that there is a discernable
current which can be measured and tabulated. The
larger this current the larger the initial gamma ray was.
Polarimeters are essentially large and powerful electromagnets designed to maintain a high magnetic field on the inside, but minimize leakage to the outside. The metal for our polarimeters was chosen for its properties to
hold a very strong field. To reduce field leakage both top and bottom at flat plates and an iron ring was placed around the outside. The effect is an approximately 2 Tesla magnetic field which cannot be detected by a
compass from more than a foot away and barely keeps a metal washer from falling off.
This polarimeter has 493 turns of 16 gauge Hyslik coated magnet wire. To produce the field we have 6 amps flowing at about 20 volts. This amount of current produces
a great deal of heat, so in amongst the wire are 6 turns of 1/8 inch refrigerator tubing to act as a cooling system. The wire has a heat tolerance of 200 degrees but we
will be running for up to days at a time during which quite a bit of heat can build up. After an hour or so the outside of the polarimeter is warm to the touch but to this
point we have not melted the insulation off of our wires.
Background:
Additionally, around the core there are 4 turns of 18 gauge wire to act as a field detector. When the main field is turned on or off a current is induced across the 18 gauge
wire. This is a simple and effective way to check the strength of the field and how long it takes to change its polarity.
:p
The magnetic field produced by the polarimeter is key to all of our measurements, but at the same time it can potentially sabotage them. The photomultiplier tubes use
charges plates to accelerate electrons, so by introducing another field we cause the electrons to travel with a different trajectory. This would not be too much of a
problem if the field were going to be constant but it will be switched back and forth. Moreover the nature of the experiment is to find the difference in the number of
gamma counts independent of PMT effects. To this end we tested extensively to find what kind of effect the magnetic fields would have on our count rates. Then to try to
nullify these effects we used magnetic shielding to protect the PMT from the magnetic field of the polarimeter. The results of these tests are highlighted below.
W+
PMT time dependency and the 16 bit test:

d
u
300
PM T Time Dependency
250
u
d
Weak
coupling
u
d
:n
2000
2000
Strong
coupling
200
1st
1800
1800
4th
6th
Counts
This is not actually the experiment we are
n:
interested in, but this diagram is useful to
show the weak and strong couplings. These
couplings are always present when the weak
interaction is involved.
This diagram is an example of proton and
neutron scattering. It is more than a proton and
neutron simply bouncing off one another.
Rather by way of weak interaction by the W+
particle. This interaction changes one of the
down quarks in the proton to become an up
quark, making the proton a neutron, and one of
the up quarks in the neutron to become a down p:
quark, turning it into a proton. The W+ is a very
short range particle so when the proton and
neutron move apart a  particle takes over
and completes the scattering.
Cesium Iodide Scintillator and Photo Multiplier Tubes:
1600
1600
7th
150
10th
11th
13th
n + d => t +(6.2 MeV)
Parity says that when an experiment is run you should get the same results when it is run in mirror image. For this
experiment this implies reversing the spin of the neutrons. It is not known why the weak interaction violates this
symmetry that every other force conserves.
600
600
400
400
200
200
0
0
0
50
100
150
200
250
50
100
150
200
250
300
Channel
Above are histograms of a Cs 137 source with the Crystal and PMT covered with black felt. Though they look the same you can see at left the purple and blue points match up much better than at right where the
blue is to the right of the purple. This comes from the positioning and shielding on the polarimeter. To left the polarimeter is about 34-40 cm away with magnetic shielding surrounding it. On right the polarimeter is
about 10 cm away and there is no shielding. This is the type of effect we can see from the magnetic field. Quantitatively we measured the centroids of the curves. The change in centroid on the left is .07 channels,
where at right the change is 1.83 channels.
are absorbed and de-excite they produce a gamma with a spin that will preserve the momentum.
This shows the general set up
of our experiment. Though the
reaction is the same as that
above we are not concerned
CsI in iron box
with the asymmetry due to the
weak interaction but only the
Polarimeter
degree of polarization of the

Sn
neutrons.
Polarized neutrons will be
Deuteriu
directed toward the Deuterium
Vn
n
m
Target
Polarized
target where they will be

neutron
captured producing tritium and
beam
Polarimeter
6.2 MeV gamma rays. The
gamma will then enter one of
CsI in iron box
the polarimeters.
In the polarimeter the gamma will collide with the electrons of the metal and undergo Compton scattering.
Because of the strong magnetic field (2 Tesla), some of the electrons will be spinning in the same direction. If the
electron is spinning parallel to the spin of the gamma it will be scattered away from the CsI detector. If the spins
are anti-parallel the gamma will pass through to the CsI scintillator where they will be detected and measured.
DePauw University, Science Research Fellows, Dr. Komives
Help: Andy Smith
Equipment: Dr. Kertzman, IU, NIST, Rea Magnet Wire
Papers:
97
93
89
85
81
77
73
69
65
61
57
53
49
45
41
37
33
29
25
2000
The data analysis for this test will be given to show how we are drawing results. Just despite the consistency of the
centroid shift, we do not know why it would happen. It is thought that if we could run more faster tests with less dead
time we would be able to see the asymmetry after 50 some measurements. This is obviously not practical until the
process is automated.
Felt electromag #1
1800
1600
1400
Run #1
1200
1000
800
+
-
1+
12+
23+
3counts
centroid
counts
centroid
counts
centroid
counts
centroid
counts
centroid
counts
centroid
127049
127.258
124897
123.922
123353 127.3481
123388 123.8433
127755 127.2577
122757 124.0295
centroid diff
3.335987
difference
stat error
centroid diff
3.504767
Asymmetry
2152 A= (A+ - A-)/(A+ + A-)difference
501.9422 A=
0.008542 stat error
stat error
0.001992
600
400
200
Average Asymmetry
STD
Asymmetry
-35 A= (A+ - A-)/(A+ + A-)difference
496.7303 A=
-0.00014 stat error
stat error
0.002013
centroid diff
3.228263
Asymmetry
4998 A= (A+ - A-)/(A+ + A-)
500.5117 A=
0.019951
stat error
0.001998
Numbers of Interest:
The asymmetry is the
primary number of interest
though, as seen on the
chart is consistent with
zero. We anticipated a
non-zero value because of
the strength of the
magnetic field.
average centroid change
STD
662
It appears as though the crystals have not degraded too much to be useful for our measurements. They are in fact large enough to pick up multiple instances simultaneously. The
Photomultiplier tubes work but in such a way that a 16 bit sampling method is best to limit time dependent detection efficiency. The crystals are most effective when the radiation source is near
to their center. The cause of this is unknown but thought to be geometric effect as the gammas are more likely to hit the crystal if they occur near the center.
With current measuring techniques we are unable to detect asymmetries, though error has been as low as 4 x 10-3. Because of this we were not able to see if the magnetic field of the
polarimeter has any effect on the asymmetry or not. However, we were able to remove almost all of the centroid change inherent to the presence of a magnetic field using a combination of
spacing and magnetic shielding. It is important to note that the asymmetries for these test should be zero, as the gammas are not polarized. It was expected, and observed on previous test that
the polarimeters would cause asymmetries in the counts taken. It is also possible that these polarimeters were well enough constructed that those effects are not being seen with this set up. It is
much more likely, however, that we have too much systematic error to be able to see what is there.
To further improve upon these results an automated data collection system is being constructed. With this we will be able to take more shorter samples faster. These samples should allow us to
be more precise in the amount of time we are collecting data, as well as eliminate some of the time dependencies of the PMT.
CsI Crystal Histogram
6000
3.356339
0.139371
The centroid is the center
0
50
100
150
200
250
300
of mass of the area under
Channel
the curve. Away from the
presence of a magnetic field (such as above left) the centroid does not move. The stronger and nearer the field to the PMT the more the centroid moves. Interestingly enough, the centroid always moves in the same
direction when there is a field even when the polarity of the field is changed. This is possibly caused by a residual magnetic field on the magnet that is not quite overcome by changing the polarity. Perhaps testing with
identical permanent magnets would yield a result where the negative polarity and positive polarities would be on opposite sides of no field.
0
7000
0.00945
0.010077
Conclusions:
Acknowledgments:
21
For our experiment to be a success we will need to measure the asymmetry of the gamma rays. For this reason we started these tests measuring the asymmetry caused not by the filtering of gammas but only by
the changing magnetic field on the PMT. Unfortunately after much analysis we decided that our error was too large and we could not take enough data fast enough with our current equipment to see the changes
that should be there. For this reason we started to examine the change in centroid from trial to trail. However in a last attempt to see a non-zero asymmetry because of the magnetic field we put aside the polarimeter
and used a completely unshielded solenoid at about 5 cm from the PMT. Amazingly the asymmetry was still consistent with zero when error was taken into account. This trial however was useful in reaffirming the
was the magnetic field changes the centroid.
Counts
Proposed Setup:
The CsI crystals were chosen for this project not only because they were available, but also because they are a very
fast crystal meaning that more counts will be taken than with a slower crystal such as NaI. The sacrifice of a faster
crystal however is a loss of energy resolution. This is a measure of how well the crystal can discern between gamma
energies. In the charts below a clear picture of the energy resolution is shown. These histograms were produced with
a Nucleus brand multi-channel analyzer. A MCA recieves the signals from the PMT and assigns a channel to each
different energy, this is known as digitizing because the analog pulse comes in from the PMT and all that is left is a
digital number.
At top is a chart from a NaI crystal with peaks for 137Cs, 22Na, and 60Co. At bottom is the same measurements taken
on the CsI crystal. Notice how the NaI peaks are much sharper. The most obvious difference is the fact that the 2
60Co peaks merge into one on the CsI chart.
The spin of the particle creates a rotational momentum, which must be preserved. For this reason, when the neutrons
According to theory the asymmetry of this experiment when run forward and in mirror image will lead to the
calculation of the rmeson coefficient. There is an asymmetry between these two measurements because the weak
interaction violates parity.
17
Energy Resolution and Crystal Degradation Testing:
0
300
9
Channel
Channel
The Asymmetry which is expressed as (N1 – N2) / (N1 + N2) where N1 is the number of counts with the neutrons
polarized one way and N2 polarized the opposite.
Nucleons, neutrons and protons, are Fermions and have spin, or angular momentum of + or - ½. Naturally the spins
would occur randomly and evenly. It is possible, however, to polarize the neutrons by forcing them to spin in the same
way.
800
0
101
800
+
-
13
p
+
-
1000
5
p
1000
Counts
n
6.2MeV 
50
5000
511
Counts
Vn
n
Counts
n
1200
1200
n
100
1
Sn
16th
1400
1400
The plot to left shows eight ten second trials all with a
positive polarimeter polarity. All of these lines should be
all but identical but they vary quite substantially. It
seems that Photo Multiplier Tubes have a regular time
dependency. To get around this measurements are
taken in a regularly osculating pattern. We found the
most success with a 16 bit pattern. To do this we took
16 consecutive measurements but the polarity of the
polarimeter went as follows: + - - + - + + - - + + - + - - +.
To complete the analysis the +’s and –’s are added
according to their kind and uses as a single
measurement. The hope here is that the variation in the
PMT will average out over all of these trial. We are
confident that it has.
1173
Na22
1332
4000
Cs137
Co60
3000
2000
1785
1274
2505
1000
0
0
50
100
150
200
250
Channel
Part of the degradation of the crystals is that they will absorb water out of the air. This water will cloud the crystal and
reduce its effectiveness at detecting peaks. According to previous testing over the last 10 years these crystals have
lost about 10% of their energy resolution. This should not be a problem because of the nature of our project but it is
possible to repair them by polishing off the clouded layer with very fine sand. This would result in a clearer crystal, but
a loss in size which is also important for gamma detection.