Data Analysis

Several GAT processors were developed in order to analyze the data from both the NaI[Tl] and MALBEK detectors, one for calibrating digitizer timestamps and the other for the Digital Signal Processing (DSP) of the NaI[Tl] waveforms. The timestamps were calibrated to take into account delays associated with the pretrigger and buffer wrap delays as well as the delay caused by shaping the NaI[Tl] signal. For both the NaI[Tl] and MALBEK signals, since the timestamp marked the start of the digitization window, the location in time that the signal reached 50% of its maximum (t50) was added to each timestamp. Once the timestamps were

calibrated, we calculated the time since last 356 keV peak event in the MALBEK signal chain. This time difference was taken as the charge carrier drift time. This resulted in a significant amount of accidental coincidences; this is is clearly shown in Figures 6.13 and 6.14 as the horizontal band at t10−90∼400 ns.

6.3.4 Results

The results from this experiment were consistent with the hypothesis outlined in Section 6.1.2, showing that events with long drift times are directly related to slow-signals, see e.g. Fig- ures 6.13 and 6.14. It was also found that the maximal drift time (Figure 6.16), rise time (Figure 6.15) and spectral shape (Figure 6.12) depend significantly on the position of the source. This was expected since the distance to the collecting electrode and electric field change as a function of interaction position; see Ref. [172] for a detailed review of position dependence within germanium detectors. Perhaps this is better illustrated by examining the rise time energy dependence for various positions, as shown in Figure 6.11. This figure shows the rise time energy dependence for the collimated data only – again, it is clear that the source location greatly affects the rise time energy dependence. With this in mind, re- call the cosmogenic backgrounds discussed in the previous chapter. These backgrounds are uniformly distributed throughout the germanium crystal, creating a uniform distribution of potential slow-signal sources. This is a drastically different scenario than placing a calibration source above or beside a detector. Due to attenuation, these external gamma-rays will have a preference towards interactions near the outer parts of the detector. Therefore, it can be argued that if one’s background spectrum is dominated by slow-signals from cosmogenics, then a source measurement cannot be used to determine an expected spectral shape due to slow-signals.

The shape of Figure 6.16 merits an in depth explanation. Consider for example the runs in which the source was uncollimated and located on top of the cryostat. The entire top face of the crystal was bathed in gamma-rays from133Ba, even the corners. An interaction taking place in the corner of the crystal will have a longer distance to drift as well as a weaker field to drift through (slower drift velocity). Conversely, an interaction taking place directly in the

(a) Collimated Side

(b) Collimated Top

Figure 6.11: 133Ba rise time energy dependence for (a) a collimated source illuminating the side of MALBEK, (b) a collimated source illuminating the top of MALBEK.

middle of the top face of the detector will have the shortest drift path and strongest field to drift through. Therefore, the drift time peak in the top uncollimated measurement was not symmetric, and appeared to be a superposition of the collimated top and uncollimated side measurements. The distributions observed in Figure 6.15 can easily be understood by comparing them with Figure 6.11. It is clear that when the source is located on the side of the detector, the maximal rise time appears to be less than the maximal rise time from when the source is located on top of the detector.

These results highlight the need for a model that takes into account the physical processes taking place in the RDR and DDR. The following sections will present a simple model, initially developed by David Radford, that, after taking into account diffusion and recombination, reproduces to first order several experimental measurements.

Energy (keV)

Count Rate (Hz)

Uncollimated Side Uncollimated Top Collimated Top Collimated Side

Figure 6.12: The energy spectra from MALBEK during the drift time measurements (see legend). The SIS3302 was configured so as to not trigger on events with energy greater than ∼82 keV. It is clear that the shape of the low-energy rise due to slow-signals is dependent upon the position of the source. Only the inhibit and preliminary cuts have been applied to these data (see Section 5.3.4).

Figure 6.13: t10−90 versus drift time from above the cryostat, collimated (top panel), uncol-

limated (bottom panel). See text for explanation of the distributions observed. However, it is clear that an event’s rise time is proportional to its drift time.

Figure 6.14: t10−90 versus drift time from the side of the cryostat, collimated (top panel),

uncollimated (bottom panel). See text for explanation of the distributions observed. However, it is clear that slow-signals are directly correlated to events with long drift times.

(ns)

t

Count Rate (Hz)

Uncollimated Side Uncollimated Top Collimated Top Collimated Side

Figure 6.15: MALBEK t10−90 histogram from various source positions. Only events with

energies between 4 keV and 81 keV have been plotted. See text for explanation.

Drift Time (ns)

Count Rate (Hz)

Collimated Side Uncollimated Side Collimated Top Uncollimated Top

Figure 6.16: MALBEK drift time histogram from various source positions with energies greater than 4 keV but less than 81 keV. See text for explanation.