Comparison of Sample Sizes

In order to determine the effect of scintillator sample size on the visible energy peak, a single non-gadolinium doped scintillator sample (1-70-2) was machined into three samples of differing length. The shortest was 10 mm thick, the middle 13 mm, and the longest 20 mm. These were compared to a different sample (1-66) of length 50 mm. 1-66 is also a non-gadolinium doped sample. Each run was conducted with 2“ PMT R1332 biased at 2200 V in the new dark room. The details of each run are shown in Table 1, below.

Table 1: Information about data runs
Run	Bias	Sample Size	Source	Number Events	Total Time	Percent Saturated	Data File
1	2200 V	10 mm	background	10007	343 s	1.249%	20140731_R1332_10mm-sample_2200V_background_90mV-threshold.dat
2	2200 V	10 mm	Am-241	10513	245 s	1.589%	20140731_R1332_10mm-sample_2200V_am241_90mV-threshold.dat
3	2200 V	13 mm	background	9998	627 s	3.901%	20140729_R1332_2200V_background_90mV-threshold.dat
4	2200 V	13 mm	Am-241	10005	229 s	2.009%	20140729_R1332_2200V_am241_90mV-threshold.dat
5	2200 V	20 mm	background	10061	358 s	1.967%	20140731_R1332_0.75inch-sample_2200V_background_90mV-threshold.dat
6	2200 V	20 mm	Am-241	10032	228 s	1.495%	20140731_R1332_0.75inch-sample_2200V_am241_90mV-threshold.dat
7	2200 V	50 mm	background	10062	417 s	1.898%	20140801_R1332_2200V_scint1-60_background.dat
8	2200 V	50 mm	Am-241	10014	368 s	2.087%	20140801_R1332_2200V_scint1-60_am241.dat

Individual Runs Compared to Background

Shown below are a comparison of each Am-241 run to the run of background for each sample length. The Am-241 peak is clearly visible over the background for each length.

Figure 1: Comparison Background and Americium-241 spectra for the 10 mm Sample (weighted by time)

Figure 2: Comparison Background and Americium-241 spectra for the 13 mm Sample (weighted by time)

Figure 3: Comparison Background and Americium-241 spectra for the 20 mm Sample (weighted by time)

Figure 4: Comparison Background and Americium-241 spectra for the 50 mm Sample (weighted by time)

Comparison of Americium runs for each length

Figure 5: Comparison of Am-241 spectra for each size (weighted by peak area)

The peaks in Figure 5 are weighted by peak area. The area of each peak between 1 and 5.5 nVs has been normalized to match that of the 10 mm run in order to compare the shapes of the spectra. Clearly the length of the sample (from 10 mm to 50 mm) has no affect on the location and shape of the Americium peak. The 20 mm run is not considered to be contradictory to this statement because the condition of that scintillator is much worse than the other samples. The 20 mm sample had considerably worse crazing and cracking along the edges as well as one blemish on the flat surface farthest from the PMT.

Figure 6: Comparison of Am-241 spectra for each size (weighted by rate)

The spectra is Figure 6 are weighted by rate (Number of events / run time). Ignoring the 20 mm run for the reasons mentioned above, this shows that as the length of the scintillator increases, the ratio between background and americium events increase. The background peak is comparatively higher for a larger scintillator sample, meaning the americium peak appears flatter, smaller, and more difficult to find.