Construction of Low-Granularity counters

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

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