2014/05/27

While warming up the fridge, we collected ionization only data at a range of higher temperatures in order to determine if flashing the detector at a higher base temperature can still neutralize the crystal and whether or not the crystal deneutralizes sooner when at these higher temperatures. The base temperatures studied, as determined by the tower thermometer, are 75 mK, 105 mK, 290 mK, 455 mK, 630 mK, 850 mK, and 1.68 K. However, the data taken at 1.68 K was neutralized differently than expected.

We used detector G106a, which is a 100 mm iZIP with 12 phonon channels and 4 charge channels. There are six phonon channels and two charge channels on each side. The charge channels are patterned with an inner (Qi) and outer (Qo) ring. The fridge was protected from external neutral and gamma-ray sources by an external shield of 8 inches of polyethylene and 1.25 inches of lead on each side of the fridge.

At each temperature, the crystal was biased once at +/- 3.5V and once at -/+ 3.5 V. At the start of each data series, the detector was flashed with 4 LEDs for six seconds. Data was acquired for a period of 30 minutes at each bias and temperature. The base temperature was increased by increasing the power to the mixing chamber heater.

For the +/- 3.5 V bias, side 1 was biased to +3.5 V and collected electrons. Side 2 was biased to -3.5 V and collected electron holes. The -/+ 3.5 V bias is opposite. The collimated, 20 Bq Americium-241 source was always placed on side 1. Because the 60 keV gammas from the Americium source have a mean-free-path of around 1 mm, in order to see an event on side 2, the charge carriers must cross the width of the crystal. A 10 Bq Lead-210 source was also present on the same side as the Americium-241.

The Q-sum (Qi+Qo) spectra show whether or not the 60 keV peak is visible on side 2. These plots were created with a low energy cut such that only data with Q-Sum > 0.25 mV is considered. This cut is used to remove events that arise from noise in the electronics. A charge partition cut is also used to remove events that occur near the surface of the crystal (the data used is at least 90% in the inner charge channel). The data is also cut by the chi-squared of the fit of the pulse. The q-sum spectra on side 2 also only includes events that contribute to the 60 keV peak on side 1 with all of the aforementioned cuts as well. An example of this cut is shown in Figure 1 by the violet lines.

Figure 1: Example of side 1 spectra to show cut on 60 keV peak. (T = 75 mK, -/+ 3.5V bias)

The q-sum versus time plots were used to show whether or not the crystal was losing its neutrality. For example, the 60 keV peak is usually visible as a dense line of points around 0.002 V. If the crystal stayed neutral, this line would have a fairly constant thickness and would stay around 0.2 V. If the crystal is becoming deneutralized, the thickness of this line would increase and turn down from 0.2 V.

Since side 2 was collecting electrons, the 60 keV peak wasn't really visible in the q-sum vs. time plots even though it was visible in the first q-sum spectra in the first 20 minutes. An example of a q-sum spectra with the 60 keV peak visible on side 2 is given in Figure 2. Figure 3 shows the corresponding q-sum versus time plot. An example of a q-sum spectra from side two with no 60 keV peak visible is given in Figure 4 with Figure 5 showing the corresponding q-sum versus time plot.

Figure 2: Example of q-sum spectra of side 2 with 60 keV peak visible (T = 75 mK).

Figure 3: q-sum versus time of side 2 for the same data series as Figure 2 (T = 75 mk).

Figure 4: Example of q-sum spectra of side 2 without 60 keV peak visible (T = 455 mK).

Figure 5: q-sum versus time of side 2 for the same data series as Figure 4 (T = 455 mK).

In general, before 455 mK, the 60 keV peak is somewhat visible, but at 455 mK and higher, the peak does not show up in the q-sum spectra.

Because side 2 is now collecting electron holes, the 60 keV peak is much more visible on that side. The 60 keV peak was visible for all temperatures measured on the q-sum spectra after even just five minutes and remained clear up to at least 20 minutes. Generally, the 60 keV peak broadened and dropped on the q-sum versus time plots around 15 or 20 minutes ( 900 or 1200 s). Figure 5 shows the q-sum spectra for the lowest temperature (75 mK) with Figure 6 showing the corresponding q-sum versus time plot. Figure 7 shows the q-sum spectra for the highest temperature (850 mK) with Figure 8 showing the corresponding q-sum versus time plot. The lowest and highest temperature plots are shown to show the consistency throughout all temperatures between.

Figure 5: Q-Sum Spectra of 75 mK.

Figure 6: Q-Sum versus Time for 75 mK.

Figure 7: Q-Sum Spectra of 850 mK.

Figure 8: Q-Sum versus Time for 850 mK.

When the crystal is biased such that the side furthest from the source (side 2) collects electrons, the 60 keV peak from Americium-241 is not visible on the Q-sum vs. time plots (Figures 4, 12, 20, 28, 36, and 44) even when the 60 keV peak can be somewhat seen in the first 20 minutes on the Q-sum spectra (Figures 2, 10, and 18).

For Q-Sum vs time plots when electron holes are collected on the side furthest from the source (side 1), the 60 keV peak is visible near 0.002 V. In all cases (shown in Figures 8, 16, 24, 32, 40, and 48), the peak begins to thicken or drop somewhere around 20 minutes. This suggests that the deneutralization of the crystal did not change with temperature for this temperature range (75 mK to 850 mK). It appears that we are able to successfully neutralize the crystal within this entire temperature range using the standard flashing times.

Included below are are all of the Q-sum Spectra and Q-Sum vs. Time plots for each bias and temperature.

These runs occurred at a tower temperature of 75 mK. DIB 1 (side 1) was biased to -3.5 V and would therefore collect electron holes, and DIB 2 (side 2) was biased to +3.5 V and would collect electrons. The source was located on side 1.

Figure A1: Q-Sum spectra for zip 1

Figure A2: Q-Sum spectra for zip 2

Figure A3: Q-Sum vs. Time for zip 1

Figure A4: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 75 mK. DIB 1 (side 1) was biased to +3.5 V and would therefore collect electrons, and DIB 2 (side 2) was biased to -3.5 V and would collect electron holes. The source was located on side 1.

Figure A5: Q-Sum spectra for zip 1

Figure A6: Q-Sum spectra for zip 2

Figure A7: Q-Sum vs. Time for zip 1

Figure A8: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 105 ± 5 mK. DIB 1 (side 1) was biased to -3.5 V and would therefore collect electron holes, and DIB 2 (side 2) was biased to +3.5 V and would collect electrons. The source was located on side 1.

Figure A9: Q-Sum spectra for zip 1

Figure A10: Q-Sum spectra for zip 2

Figure A11: Q-Sum vs. Time for zip 1

Figure A12: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 105 ± 5 mK. DIB 1 (side 1) was biased to +3.5 V and would therefore collect electrons, and DIB 2 (side 2) was biased to -3.5 V and would collect electron holes. The source was located on side 1.

Figure A13: Q-Sum spectra for zip 1

Figure A14: Q-Sum spectra for zip 2

Figure A15: Q-Sum vs. Time for zip 1

These runs occurred at a tower temperature of 290 ± 5 mK. DIB 1 (side 1) was biased to -3.5 V and would therefore collect electron holes, and DIB 2 (side 2) was biased to +3.5 V and would collect electrons. The source was located on side 1.

Figure A17: Q-Sum spectra for zip 1

Figure A18: Q-Sum spectra for zip 2

Figure A19: Q-Sum vs. Time for zip 1

Figure A20: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 290 ± 5 mK. DIB 1 (side 1) was biased to +3.5 V and would therefore collect electrons, and DIB 2 (side 2) was biased to -3.5 V and would collect electron holes. The source was located on side 1.

Figure A21: Q-Sum spectra for zip 1

Figure A22: Q-Sum spectra for zip 2

Figure A23: Q-Sum vs. Time for zip 1

Figure A24: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 455 ± 5 mK. DIB 1 (side 1) was biased to -3.5 V and would therefore collect electron holes, and DIB 2 (side 2) was biased to +3.5 V and would collect electrons. The source was located on side 1.

Figure A25: Q-Sum spectra for zip 1

Figure A26: Q-Sum spectra for zip 2

Figure A27: Q-Sum vs. Time for zip 1

Figure A28: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 455 ± 5 mK. DIB 1 (side 1) was biased to +3.5 V and would therefore collect electrons, and DIB 2 (side 2) was biased to -3.5 V and would collect electron holes. The source was located on side 1.

Figure A29: Q-Sum spectra for zip 1

Figure A30: Q-Sum spectra for zip 2

Figure A31: Q-Sum vs. Time for zip 1

Figure A32: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 630 ± 5 mK. DIB 1 (side 1) was biased to -3.5 V and would therefore collect electron holes, and DIB 2 (side 2) was biased to +3.5 V and would collect electrons. The source was located on side 1.

Figure A33: Q-Sum spectra for zip 1

Figure A34: Q-Sum spectra for zip 2

Figure A35: Q-Sum vs. Time for zip 1

Figure A36: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 630 ± 5 mK. DIB 1 (side 1) was biased to +3.5 V and would therefore collect electrons, and DIB 2 (side 2) was biased to -3.5 V and would collect electron holes. The source was located on side 1.

Figure A37: Q-Sum spectra for zip 1

Figure A38: Q-Sum spectra for zip 2

Figure A39: Q-Sum vs. Time for zip 1

Figure A40: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 850 ± 5 mK. DIB 1 (side 1) was biased to -3.5 V and would therefore collect electron holes, and DIB 2 (side 2) was biased to +3.5 V and would collect electrons. The source was located on side 1.

Figure A41: Q-Sum spectra for zip 1

Figure A42: Q-Sum spectra for zip 2

Figure A43: Q-Sum vs. Time for zip 1

Figure A44: Q-Sum vs. Time for zip2

These runs occurred at a tower temperature of 850 ± 5 mK. DIB 1 (side 1) was biased to +3.5 V and would therefore collect electrons, and DIB 2 (side 2) was biased to -3.5 V and would collect electron holes. The source was on side 1.

Figure A45: Q-Sum spectra for zip 1

Figure A46: Q-Sum spectra for zip 2

Figure A47: Q-Sum vs. Time for zip 1

Figure A48: Q-Sum vs. Time for zip2