Fast Event Survival as a Function of Cut Position

The validity of a wpar cut and the subsequent efficiency correction can be verified by

examining its effect on populations of events with known rise-times. These populations are available not just from data generated using a pulser, but from well understood physical features within the dataset. For example, events that occur in the K and L-shell capture peaks around 1 and 10 keV are fast rising bulk events. The number of events remaining in these populations after a wpar cut and signal acceptance efficiency correction should remain

constant. As shown in Figure 3.16, most events that occur between the 1 keV peak and the detector threshold in the 241_{Am dataset are slow surface events, so the number of events}

remaining in this energy region after a rise-time cut should decrease with increasingly strict cut values.

Three datasets were used for this study, the flood 241_{Am source measurement, 40 days}

of data collected with lead shims inside of the cryostat, and 89.5 kg-d of data collected within the shield once the lead patches were removed. The wpar cuts used for this study

are constant with energy, resulting in a fast event acceptance efficiency that drops off with decreasing energy. The acceptance efficiency of a given wpar cut value is calculated using

a set of fast pulser generated signals. For each of the three datasets, constant wpar cuts

were applied with values ranging from 10 to 31 and the resulting spectra were efficiency corrected. Beyond a wpar cut value of 31, the acceptance efficiency of the cut drops off

rapidly. The resulting spectra were then fit with a flat Compton continuum, an exponential to parametrize the slow pulse contamination, and two gaussians at the65_{Zn (1.096 keV) and}

the68Ge (1.299 keV) L lines. An example fit to the 89.5 kg-d dataset and the fit residual are shown in Figure 3.19. The number of counts remaining the L-shell capture peaks are then extracted from the fit.

energy (keV) 1 1.5 2 2.5 3 -1 d -1 kg -1 counts keV 5 10 15 energy (keV) 1 1.5 2 2.5 3 residual counts -5 0 5

Figure 3.19: 89.5 kg-d spectrum after removing events with wpar < 15. Results from a

fit to a flat Compton continuum (dotted), an exponential to parametrize the slow pulse contamination (dashed), and two gaussians (long dash) at the 65_{Zn (1.096 keV) and the} 68_{Ge (1.299 keV) L lines are shown along with residuals of the fit.}

The counts remaining in the241_{Am dataset as a function of thew}

parcut value are shown in

Figure 3.20. Below 60 keV, this dataset is composed primarily of slow events. The efficiency corrected counts remaining in the region from 0.6 to 0.9 keV decreases with an increasingly strict wpar cut value. This is the expected behavior in an energy region that is dominated

by slow events. The same behavior is seen in the region from 1.6 to 3.6 keV. The efficiency corrected number of events decreases as a function of the wpar cut value, but this time only

until a wpar cut value of 26 is reached. At this point, most of the slow events in the region

have been removed and the remaining rate is due to fast events. The fact that the number of counts remains constant suggests that the efficiency correction is properly accounting for fast signal events removed by the cut. The two populations of events from the 241_{Am data}

are shown in contrast to the combined counts remaining in the65Zn and68Ge lines extracted from the 89.5 kg-d dataset. The number of efficiency corrected counts in these peaks, which are all fast bulk events, remain constant with cut value, albeit with increasing uncertainty

due to larger efficiency corrections. These results demonstrate that the efficiency correction based on the fast pulser dataset is correctly weighting the remaining counts while the wpar

cut is removing slow events.

cut value Par w 10 15 20 25 30 normalized counts 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Am: 0.6 - 0.9 keV 241 Am: 1.6 - 2.1 keV 241 89.5 kg-d: L lines

Figure 3.20: Counts remaining in the region from 0.6 to 0.9 keV (black circles) and 1.6 to 3.6 keV (black triangles) in the 241Am source data as function of wpar cut value. The

efficiency corrected counts in the 89.5 kg-d dataset L-shell capture peaks (red circles) remain constant as a function of the cut position. The error bars on the points include statistical uncertainty as well as uncertainty from the efficiency correction.

Figure 3.21 shows a similar analysis performed on the 40 days of data taken with lead shims inside the cryostat. Again, the number of counts in the region from 0.6−0.9 keV drops with an increasingly strict cut. Because the wpar calculation doesn’t perform as well

at the lowest energies, not all of the slow events are removed, even when a wpar cut value

of 31 is used. The counts in the region above the L lines decreases with increasingwpar and

become constant at cut values greater than 25, when the majority of the slow events are removed. The counts in the L lines from the 89.5 kg-d dataset are shown in comparison.

Finally, Figure 3.22 shows the counts remaining between 0.5 and 0.9 keV as a function of wpar cut value for the 89.5 kg-d dataset. The number of counts in this region decrease until

a wpar cut value of 25. At this point, most of the slow events have been removed from the

cut value Par w 10 15 20 25 30 normalized counts 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Pb: 0.6 - 0.9 keV 210 Pb: 1.6 - 2.1 keV 210 89.5 kg-d: L lines

Figure 3.21: Counts remaining in region from 0.6 to 0.9 keV (black circles) and 1.6 to 3.6 keV (black triangles) in the lead source data as function ofwpar cut value. The efficiency

corrected counts in the 89.5 kg-d dataset L-shell capture peaks (red circles) remain constant as a function of the cut position.

constant valuedwpar cuts, and that the efficiency correction calculated using the fast pulser

dataset correctly accounts for the fast events removed by the cut.