Construction of Low-Granularity counters

LGPS Matrix

PMT Testing

Expectations

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:

Photomultiplier Tube Assembly (H7415)
Dynode Structure	Linear-focused
Dynode Stages	10
Max. Rating) Anode to Cathode Voltage	-2000 V
Anode to Cathode Supply Voltage	-1500 V
Anode) Gain Typ.	5E+06
(Time Response) Rise Time Typ.	1.7 ns
(Time Response) Transit Time Typ.	16 ns

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

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 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.

	PMT Amplitudes (mV)
PMT #	-1.1 kV	-1.2 kV	-1.3 kV	-1.4 kV	-1.5 kV
1	171	303	513	833	1320
2	124	219	365	583	906
3	187	330	553	892	1400
4	213	376	627	1010	1575
5	216	387	657	1060	1670
6	178	313	522	839	1310
7	171	302	507	812	1256
8	181	322	542	877	1380
9	159	284	473	755	1160
10	168	299	504	811	1260
11	160	289	489	785	1236
12	210	370	624	1000	1556
13	214	385	650	1050	1660
14	203	359	600	968	1500
15	159	287	483	780	1215
16	170	299	504	810	1300
17	195	348	607	970	1500
18	216	376	627	997	1550

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. Gain Slope histogram.

	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

Figure 6. Applying glue to test light guides.

Figure 7. Curing test light guides with UV light.

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.

Typical Properties of NOA 87
Refractive Index	1.52
Temperature Range	-30 C to 90 C
Viscosity @ 25C	900-1500cps
Elongation at Failure	13%
Modulus (PSI)	209,700
Tensile Strength	4,880
Hardness - Shore D	50

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

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

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 Ray Detecting

We tested the wrapped PS detector in the same manner as with the PMTs with one exception. Instead of using light from an LED, we let cosmic ray muons trigger the scintillator.

Nate, edit this however you'd like. I just thought I would upload some files.