Difference between revisions of "Construction of Low-Granularity counters"

Revision as of 10:16, 20 June 2014

Pair Spectrometer Counters.

LGPS Matrix

PMT Testing

Expectations

PMT 1

PMT 2

PMT 3

PMT 4

PMT 5

PMT 6

PMT 7

PMT 8

PMT 9

PMT 10

PMT 11

PMT 12

PMT 13

PMT 14

PMT 15

PMT 16

PMT 17

PMT 18

The photomultiplier tubes (i.e. 18 in all) should, in principle, all exhibit the same characteristics. All 18 should have the same change in output amplitude with a change in input potential, as well as identical time responses regardless of the input potential. Therefore, one would expect a histogram of these characteristics to show a count of 18 at a single value on the x-axis. Consequently, a logarithmic plot of the overall gains of each PMT should be 18 identical lines of positive slope.

The Hamamatsu Photomultiplier Tube (H7415) was chosen because it has the characteristics required for this pair spectrometer. This PMT will be attached to the EJ-200 organic plastic scintillator and light guide fabricated by Eljen Technology. The properties of the tube are in the table below:

Rise time listed here was arrived at using a laser. In the testing of the PMT's at Jefferson Lab, an LED will be used, which will not produce a clean signal elimination like the laser.

Testing

To test the PMT's, connections had to be soldered on to the wires in order to have quick connection ability to both the oscilloscope and high-voltage power source. Once this was done, the PMT was placed in a previously fabricated dark box with mounts for the tube and LED. The LED was powered by a wave generator which was set to run 8V square waves at 10 kHz with a 8 ns pulse width. Figure 1 shows the set up used for each PMT.

Figure 1. Testing configuration for PMTs.

The LED was positioned behind a diffracted lens to spread the light out. Careful attention had to be made that each PMT was positioned at the same place on the mount in order to eliminate fluctuations in beam intensity. The anode-to-cathode potential would range from -1.1 kV to -1.5 kV.

Data

The data gathered from the oscilloscope were the amplitude and rise time. Figure 2 shows what those values represent on the oscilloscope screen. Amplitudes were measured in millivolts and the rise time was in nanoseconds.

Figure 2. Oscilloscope analysis was done using the rise time and amplitude.

The data for the PMT amplitudes is in the table below. At -1.1 kV, the minimum amplitude was 124 mV. At -1.5 kV, the maximum amplitude was 1660 mV.

The data in the table above produced the plots located at the right side of the page.

PMT # Slope of gain 1 0.00287 2 0.00196 3 0.00303 4 0.00341 5 0.00364 6 0.00283 7 0.00271 8 0.00300 9 0.00250 10 0.00273 11 0.00269 12 0.00337 13 0.00362 14 0.00324 15 0.00264 16 0.00283 17 0.00326 18 0.00334

Figure 3. Histogram of change in gain from -1.1 kV to -1.5 kV.

Rise Time (ns) PMT # -1.1 kV -1.5 kV 1 4.57 4.33 2 4.76 4.33 3 4.58 4.28 4 4.66 4.33 5 4.61 4.31 6 4.79 4.43 7 4.58 4.33 8 4.70 4.32 9 4.81 4.31 10 4.72 4.39 11 4.70 4.40 12 4.82 4.43 13 4.78 4.33 14 4.86 4.36 15 4.70 4.46 16 4.57 4.36 17 4.85 4.42 18 4.74 4.38	Figure 4. Rise time histogram for -1.1 kV.
	Figure 5. Rise time histogram for -1.5 kV.

Conclusions

From the data gathered here, one can see that the original hope that all PMT's would behave identically must be abandoned. The histograms in Figure 3, Figure 4 and Figure 5 do not show a significant number of counts for any particular bin. However, the solution to this is simply to run the PMT's all at fixed anode-to-cathode potential. This would eliminate nearly all fluctuations between the various tubes. Finally, it should be noted that, like expected, the average rise times of the PMT's with an LED source is over twice the characteristic rise time of the device. This was what was expected. However, just to be sure, one PMT was tested with a laser and the rise time was 1.97 ns at -1.4 kV.

There were some sources of error that should be considered. The first is the soldering of connections. Every time testing of one device was complete, the connectors were unsoldered from the tested PMT and re-soldered to the untested PMT. Therefore, after being heated and cooled 36 times, there was bound to be some degradation after time. This was done to ensure that there were 16 brand new connectors for the final detectors. The other source of error was the distance from the PMT to the LED in the black box. Alignment was done visually and so there may have been small differences is distance between tests. This would have affected the intensity of light hitting the PMT.

Gluing

After some trial and error, the glue chosen was Norland Optical Adhesive 87. It is an adhesive that cures clear when exposed to ultraviolet light.

Figure 8. Spectral Transmission of NOA 87 (Measured through a bond thickness of 1 mil.)

Once the glue was chosen, testing could begin. The goal was to determine the minimal amount of glue necessary for complete covering of the surfaces as well as to determine the length of time the UV light was needed in order to cure properly. During testing, we noticed that if the glue was not cured properly, clouds or bubbles would appear after 48 hours. The optimal time for curing was found to be 25 seconds from 4 sides of the light-guide and 20 seconds from the top.

Figure 6. Applying glue to test light guides.

Figure 7. Curing test light guides with UV light.

Figure 8. Gluing the scintillator/light-guide to the PMT.

File:PresentationNOA87.pptx

Wrapping

Because we wish to minimize wrinkling, a custom template has been made. Using Mylar, the light guide was wrapped using a trial and error process. From the finalized wrapping a template was constructed from non-corrugated cardboard to make it more practical to use.

The wrapping template next to the light guide/scintillator assembly

The wrapping from which the template was made

Another picture of the wrapping

Figure 9. This is one of the completed wrapped counters with a layer of Mylar, Tedlar, and Electrical tape.

Once a template was completed, the light guide was wrapped with Mylar after being cleaned with distilled water and lent free cloth. Over the Mylar, a layer of Tedlar was added using the same template. The PS counter was then covered with a layer of black electrical tape and ready to be tested.

Cosmic Particle Detecting

Expectations

Because muons easily penetrate well into the Earth's surface, they will serve as a suitable means through which one can test a scintillator detector. Therefore, we hope to test these PS detectors in the same manner as we did with just the PMTs as outlined in Section 1. The difference will be that instead of the LED pulses, we will observe muon-interactions with the scintillator and amplified by the PMT. Each muon should, in theory, generate a readable pulse on the oscilloscope.

Testing

The set up for the testing, as was previously stated, is very similar to the testing of the PMTs. The image below portrays how each PS detector will be situated in the dark box. The dark box is used, of course, to minimize the all outside sources of radiation in an attempt to isolate only the cosmic radiation we are looking for.

Figure 10. PS detector set up for cosmic ray detection. (Note: Despite their proximity in this picture, the laser and LED were not used in our test.)

Data

The oscilloscope was set to keep a history of pervious pulses in an attempt to find an average pulse amplitude and the trigger in each was set to -900 mV with a potential across the PMT of -1.3 kV.

Cosmic Particle Oscilloscope Readings

The data extracted from the readings in the link above is located in Figures 11-12.

PMT # Avg Rise Time (ns) 1 3 2 4 3 4 4 3 5 4 6 3 7 4 8 3 9 4 10 4 11 4 12 3 13 3 14 4 15 3 16 4 17 4 18 4

Figure 11. Histogram of Average Rise Time for Cosmic Particle Detection.

PMT # Avg Amplitude (V) 1 1.25 2 1.25 3 1.5 4 1.25 5 1.25 6 1.25 7 1.25 8 1.25 9 2 10 1.25 11 1.25 12 1.5 13 1.5 14 2 15 1.25 16 1.25 17 1.25 18 1.25

Figure 12. Histogram of Average Amplitude for Cosmic Particle Detection.

Conclusions

From looking at the plots from the oscilloscope readings, one might notice an irregularity in the pulse shape of counters 2 & 8. The shape is unlike the other 16 counters in that they have a cleaner shape without the residual after-pulse that existed in the others. In these two counters, the oscilloscope data was collected after the 3 foot wires were cut down to about 12 inches. Now, this was done with all counters, but these two were the only ones tested after they were cut down. The extra wire seemed to add a secondary reflection pulse from the PMT that showed up even in the early testing of the PMTs by themselves (See Figure 2).

The data gathered from the testing of cosmic particles is satisfying in that the rise times appear relatively consistent between 3-4 ns and the average pulse amplitude is just over 1 V. Furthermore, that abnormal pulse shape that once existed in all counters is now eliminated since the shortening of the wires. There were significant sources of error here, in that the data was gathered visually from the images produce by the oscilloscope and not read digitally by electronics. This was done partially out of the need to ensure all counters were complete within the timeline, but mainly because the precise data would have been little use in the operation of the pair spectrometer once it was installed, calibrated, and running.

Assembly

Once gluing, wrapping and testing were complete, magnetically sensitive parts of the detector (i.e. the PMTs) were wrapped in a mu-metal foil (See Figure 13) and placed in a mu-metal casing, held in place by a plastic collar, and sealed on the other end with a mu-metal end cap and mounting plate (See Figure 14). This assembly will serve as the PMT's primary magnetic shielding as well as facilitate installation in the hall. Once fully assembled, only the wrapped scintillator, part of the light guide, and connecting wires from the PMT are exposed beyond the mu-metal casing.

Figure 13. Wrapping the PS counter in mu-metal foil.

Figure 14. Encasing the PS counter in mu-metal tube.

Figure 15. Soldering the connectors to the bracket.

Figure 16. Pair Spectrometer Counters.

During assembly, there were a few instances where stress might be put on the glue joint, which may cause it to separate from the PMT. There have been 3 counters where this occurred during assembly, thus care must be taken not to put any excess strain on the glued joint.

Once each counter was complete, it was again tested for cosmics. During this test it was found that small yet chronic stress could also cause the PMT to separate from the joint. Therefore, it is imperative to use caution during assembly to assure that the joint remains free even from the mu-metal casing to avoid any unnecessary pressure.

Figure 17. Retesting the counters.

Figure 18. Testing the complete pair-spectrometer counter.

To facilitate alignment during installation, the centers of scintillators were marked using a set of digital calipers and printed labels. The scintillators are 60 mm x 44 mm but the layers of mylar, tedlar, and electrical tape add length to the overall measurement. Therefore all measurements are likely off by a fraction of a millimeter. This error should be trivial because the detectors overlap by 2 millimeters on each side.

Note: Many labels were printed off center so it is the actual "+" mark that should be used to identify the center.

Figure 19. Marked center of the detector.

Figure 20. The completed counter.

Figure 21. View of connectors.

Figure 22. View from scintillator.

Coincidence Counting

In the final testing of the low granularity pair spectrometer counters, a coincidence counting was done. Here, one counter was held 30 cm above another counter and at an angle of 90 degrees from each other where only the scintillators were overlapping. Here, the purpose was to ensure that there was no noise and that all counters were light tight. This is accomplished by using one counter as a trigger to initiate the reading on the second counter. Therefore, only cosmic particles were read on the oscilloscope and any other signal would be an indicator that the counter had problems either with not being light tight or with electrical noise.

Coincidence Counting Oscilloscope Readings

The data from the oscilloscope readings is displayed in Figures 23-24. As in Section 4, the data was gathered visually for the same reasons. However, the goal will eventually get precise coincidence data from ADC and TDC electronics before the counters are installed in Hall D. Unlike precise data from cosmics, the precise data from coincidence will assist in the calibration of the pair spectrometer.

PMT # Avg Rise Time (ns) 1 5 2 5 3 6 4 5 5 6 6 6 7 5 8 5 9 3 10 5 11 6 12 5 13 6 14 5 15 5 16 6 17 6 18 5

Figure 23. Histogram of Average Rise Time for Coincidence Counting.

PMT # Avg Amplitude (mV) 1 700 2 500 3 700 4 600 5 800 6 800 7 700 8 900 9 700 10 600 11 700 12 900 13 900 14 900 15 800 16 600 17 800 18 800

Figure 24. Histogram of Average Rise Time for Coincidence Counting.

Testing (2014)

The PSCs, after being stored for a year, were retrieved and tested to account for possible wear during storage and transport. The first task was to test all eighteen PSCs for background noise using cosmic rays. The background check has a dual purpose; it allows us to lower the signal threshold on the oscilloscope to check for light leaks and possible PMT noise.

Data

Data was collected from both background and coincidence tests. Background testing was used to isolate possible light leaks and PMT signal noise. Coincidence testing largely eliminates the background, therefore, it was used to obtain amplitude, rise time, and full width at half maximum. All non-variable potential testing took place at 1.35kV.

Background Test Data

To start the testing of the PSCs we first had to check for background noise. The types of noise we had a chance of encountering were thermionic emission from the cathode and dynodes, leakage currents, ionization phenomena, and light phenomena. Our main focus was to make sure each PSC was light tight and had no visible damage from the year of storage. We had a considerably lower threshold then the group last year which made any background more noticeable. After testing all of the PSCs we determined there was no light phenomena noise, however, there may have been leakage currents. The way we solved this will be discussed in the section "Background Voltage Tests." Below is a link to all the background tests at an input voltage of 1.35kV.

Cosmic Particle Oscilloscope Readings (Lower Threshold)

Figure 25. Shows the scintillator and light guide wrapped in black electrical tape so that it is light tight and the mu-metal screen encasing the PM so that no magnetic fields will effect the cascade of electrons. Figure 26. Shows the set-up to test for background. The device in the upper leftmost portion of the picture is an oscilloscope and below it is the high voltage supply we used throughout testing. The utmost care was used in handling the PSCs as seen in the picture.

Figure 25. Mu-metal tube encasing the Photomultiplier.

Figure 26. Layout for the PSC background testing

Coincidence Test Data

The coincidence testing was started after the respective PSC was tested for background. Each test consisted of two PSCs, one which was the trigger or control variable (PSC-18) and one which was the experimental variable (Test PSC). This was a key task to complete in that it eliminated most background noise and gave data about each PSC. The data collected was signal amplitude, rise time, and full width at half maximum. Each of these were used to determine which 16 of the 18 PSCs will be used in installation in Hall-D, this will be covered in "Choosing PSCs." Below is link to the pictures of each PSC coincidence test with an input voltage of 1.35kV.

Cosmic Particle Oscilloscope Coincidence Test

Figure 27. Shows the set-up for the coincidence testing. The PSC on the mount is the trigger PSC and the PSC on the pink foam pad is the test PSC.

Figure 27. Layout for the PSC coincidence testing

Figures 28, 29, and 30. Graphs of the three sets of data we obtained from the PSC coincidence tests.

PMT # Avg Amplitude (mV) 1 960 2 700 3 760 4 680 5 1080 6 960 7 840 8 1000 9 920 10 860 11 960 12 1160 13 1120 14 960 15 880 16 920 17 800 18 1000

Figure 28.Amplitude for Coincidence Counting at input potential 1350V.

PMT # Rise Time (ns) 1 3.2 2 2.8 3 2.4 4 2.8 5 3.6 6 2.8 7 3.2 8 1.6 9 2.4 10 2.8 11 2.8 12 2.4 13 2.4 14 2.4 15 2.4 16 2.4 17 2.8 18 2.0

Figure 29.Rise times for PSCs at an input potential of 1350V.

PMT # FWHM (ns) 1 8.4 2 9.6 3 8.0 4 9.2 5 8.0 6 8.0 7 7.6 8 8.0 9 7.6 10 8.0 11 8.0 12 7.2 13 8.0 14 7.6 15 7.6 16 8.0 17 8.4 18 7.6

Figure 30.Full Width Half Maximum of PSCs at an input potential of 1350V

Background Voltage Test

PMT 4

PMT 8

PMT 13

PMT 15

PMT 18

PSC-4 Background Voltage Test

PSC-13 Background Voltage Test

PSC-18 Background Voltage Test

Coincidence Voltage Test

PSC-4 Coincidence Voltage Test

PSC-8 Coincidence Voltage Test

PSC-13 Coincidence Voltage Test

PSC-15 Coincidence Voltage Test

PSC-18 Coincidence Voltage Test

Choosing PSCs

Using the statistics from the previous tables we constructed a weighted scoring system for determining which PSCs would be best for the detectors. We aimed for the most consistent data collection, with the most points being distributed to those within one standard deviation from the category mean value. In the table below three points were given if the values were within one standard deviation.

Installation

EXPECTED JUNE-JULY 2014