The 241 Am Front Face Scan

The A006241Am scan is also used to determine the crystal axis. The photopeak gated Fold-1 position intensity are generated from the MWD spectra for this scan as well. Only the Ring 1 spectra show significant events from the 241Am scan due to absorption of the lower energy -ray (60 keV). The position intensity generated from the 241Am scan is shown in Fig. 4.25.

The position spectra generated from the A006 241Am scan are gated on the 60 keV photon energy. An additional feature observed are the regular

Figure 4.25: The position intensity of the 241Am scan of capsule A006. This spectrum is gated on Fold-1, 60 _± 2 keV events.

rectangular features in each segment within the ring one position intensity. This is observed due to the absorption of the 60 keV -rays within the A006 capsule. It has been determined that this feature indicated a spacer between the crystal front face and encapsulation material.

T30 and T90 risetime maps can be produced for the 241Am scan from the baseline di↵erence calculation in the same way as well, shown in Fig. 4.26.

Figure 4.26: The T30 (left) and T90 (right) risetime maps generated from the

241_{Am scan.}

The 241Am scan is useful due to the complete deposition of energy for the low energy -rays in ring 1. At 60 keV photoelectric absorption is the domi-

nant interaction process of photons within germanium as described in Fig. 2.1. Compton scattering, which leads to uncertainty in the137_{Cs measurement does}

not a↵ect the241Am results. The spectra generated from this scan are the result of single site interactions, resulting in the uniform position intensity detector response. Both the T30 and T90 risetime are useful for the crystallographic axis calculation. The 241Am T30 and T90 risetime maps for A006 Ring 1 show the same pattern of risetime as observed in the 137Cs A006 Ring 1 map.

4.6.2

Crystal Axis Orientation Results

The crystal axis is measured quantitatively by plotting the T30 risetime as a function of azimuthal angle for 360 degrees as shown in Fig. 4.27. The crystal axis for crystals A004 and A006 show good agreement for the direction of the h100_i axis. The A006 measurements on both the137_{Cs and}241_{Am spectra show}

agreement with the curve maxima and minima. The T90 measurement shows a smooth curve indicating the strong e↵ect of the crystal axis on the charge collection of the 60 keV -ray. The lower -ray energy in the241Am scan results that the spectra from this scan are more susceptible to noise e↵ects indicated in the T30 risetime distribution.

360 0 40 80 120 160 200 240 280 320 40 60 80 100 120 140 160 180 200 Angle (degrees) Risetime (ns) A004137Cs T30 A006137Cs T30 A006241Am T30 A006241Am T90 A004h100iaxis A006_h100_iaxis

Figure 4.27: The crystallographic axis calculation for the A004 and A006137Cs and the A006 241_{Am risetime measurements. The fast h100i axis is indicated}

The agreement with the manufacturer, Canberra, is that the fast h100iaxis direction lies along the corner of sector A (the centre of the triplet) with an accuracy of _± 3 degrees [35]. The sector A corner is at the centre right of the T30 risetime profiles (see Fig. 4.11 for sector orientation). The results shown in Fig. 4.27 indicate o↵sets of 25 and 27 degrees from corner A for capsule A004 and A006, respectively.

The fastest risetime occurs at the 25 and 27 degree angles, which is in the h100i axis direction. The slowest risetime occurs in the h111i axis direction which is o↵set from the h100i by 30 degrees. Although detector crystals A004 and A006 are closely matched, they do not agree with the specification.

4.7

Reduced Bias Voltage Scans

Photon scans were made at voltages less than the operating voltage (+ 5000 V) in order to study the detector performance at these reduced voltages using the 137Cs source. These scans were performed for a shorter time per position, 30 s, and on a 2 mm xy grid in order to acquire data more rapidly, reducing the collection time from ⇠4 days for full bias to ⇠20 hours acquisition time. The scans are useful for determining the rate at which the depletion surface (the detection region) grows as applied bias voltage is increased. Also studied in detail is the change in energy resolution and shift in peak position (gain) at reduced bias voltages. The energy resolution and peak shift are indicators of detector performance.

Detector A004 was scanned with 8 reduced bias voltage values. Detector A006 was scanned with 10 reduced bias voltage values. As the bias voltage de- creased the preamplifier signal amplitude decreased such that the CFD thresh- old was lowered for each bias. To produce the reduced bias intensity matrices the core MWD energy gate had to be adjusted for each bias voltage. The e↵ects of the bias voltage on the photo peak centroid position are discussed in Section 4.7.2.

In order to present uniform data for the depiction of the depletion surface growth a coaxial part of the detector, Ring 4, is plotted in Fig. 4.28 and Fig. 4.29 for all voltages. The active area of the detector at each voltage is indicated by an intensity of counts greater than _⇠200 counts per position. At very low voltages (50 - 250 V) the outer ring of the detector is slightly depleted. This depletion at the pn-junction is discussed within Section 3.2.2. As the voltage is increased the sensitive region also grows, dependant upon Equation 3.5 where the depletion depth is proportional to the voltage, as described in Equation 3.8. The detector is most sensitive to changes in voltage at lower biases (from 0 to + 2000 V). At + 3000 V the detector is nearly completely depleted through its volume. The intensity, however, is lower due to the lack of electric field uniformity throughout the volume at the lower voltages.

A measurement of the proportion of the detector depleted at each voltage for each ring is made in the next section, Section 4.7.1. The performance of the detector at the voltage between + 50 and + 5000 V is studied in Section 4.7.2.

Figure 4.28: Intensity matrices for the A004 detector at full bias and reduced bias voltages gated on segment Fold-1 events and 137_{Cs 662 keV photo peak}

energy. The key feature of these images is the growth of the depletion region from the outer contact to the core at this depth.

Figure 4.29: Intensity matrices for the A006 detector at full bias and reduced bias voltages gated on segment Fold-1 events and 137Cs 662 keV photo peak energy. The key feature of these images is the growth of the depletion region from the outer contact to the core. Also apparent at a bias of 3000 V the detector is nearly totally depleted at this depth.