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.

Background:

This is not actually the experiment we are 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 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.

n: p: d d u d u u Weak coupling S n n V n n p n p n + d => t +  (6.2 MeV) n

W+

 Strong coupling

Polarimeter Construction for measuring the depolarization of neutrons in Deuterium Matthew Bowers, Orlay Sint, Dr. Alexander Komives

d u

6.2MeV

 u u d :p d :n 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. 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. The spin of the particle creates a rotational momentum, which must be preserved. For this reason, when the neutrons are absorbed and de-excite they produce a gamma with a spin that will preserve the momentum.

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.

Proposed Setup:

Polarized neutron beam n S n V n

CsI in iron box

 

Polarimeter

Deuteriu m Target

Polarimeter CsI in iron box

This shows the general set up of our experiment. Though the reaction is the same as that above we are not concerned with the asymmetry due to the weak interaction but only the degree of polarization of the neutrons.

Polarized neutrons will be directed toward the Deuterium target where they will be captured producing tritium and 6.2 MeV gamma rays. The gamma will then enter one of 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.

Polarimeter Construction and Testing:

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.

2000 1800 1600 1400 1200 1000 800 600 400 200 0 0 50 100 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.

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. 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.

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Channel

200 250 + 300 2000 1800 1600 1400 1200 1000 800 600 400 200 0 0 50 100 150

Channel

200 250 + 300 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.

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. 2000 1800 1600

Felt electromag #1

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.

1400 Run #1 1+ counts 127049 centroid 127.258

1 counts 124897 centroid 123.922

2+ counts 123353 centroid 127.3481

2 counts 123388 centroid 123.8433

3+ counts 127755 centroid 127.2577

3 counts 122757 centroid 124.0295

Numbers of Interest: 1200 1000 800 600 400 + centroid diff 3.335987

centroid diff 3.504767

centroid diff 3.228263

difference stat error Asymmetry 2152 A= (A+ - A-)/(A+ + A-)difference 501.9422 A= stat error 0.008542 stat error 0.001992

Asymmetry -35 A= (A+ - A-)/(A+ + A-)difference 496.7303 A= stat error -0.00014 stat error 0.002013

Asymmetry 4998 A= (A+ - A-)/(A+ + A-) 500.5117 A= stat error 0.019951

0.001998

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. 200 Average Asymmetry STD 0.00945

0.010077

0 0 50 100 150

Channel

200 250 300 average centroid change STD 3.356339

0.139371

The centroid is the center of mass of the area under 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.

Cesium Iodide Scintillator and Photo Multiplier Tubes: PMT time dependency and the 16 bit test:

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PM T Time Dependency

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Channel

1st 4th 6th 7th 10th 11th 13th 16th 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.

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.

Energy Resolution and Crystal Degradation Testing:

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 137 Cs, 22 Na, and 60 Co. 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 60 Co peaks merge into one on the CsI chart. 7000 6000 5000 4000 3000 511 662

CsI Crystal Histogram

1173 1332 Na22 Cs137 Co60

Acknowledgments:

DePauw University, Science Research Fellows, Dr. Komives Help: Andy Smith Equipment: Dr. Kertzman, IU, NIST, Rea Magnet Wire Papers:

Conclusions:

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. 2000 1000 0 0 50 100 1274

Channel

150 1785 200 2505 250 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.