(a) 2mm Copper collimator (b) 1mm Brass collimator
Figure 6.12 Collimators with241Am source. The 2mm Copper collimator creates a beam size of 5 mm and the 1mm Brass collimator a beam size of 1.5 mm on the detector surface.
(a)90Sr scan collimator (b) MC implementation
Figure 6.13 Collimator Beta2000 designed for 90Sr source measurements with small Bremsstrahlung component. The polyethylene layers for blocking beta particles with a soft Bremsstrahlung spectrum and the copper layers for shielding the remaining Bremsstrahlung are shown on the left. A sketch of the MC implementation showing the collimator holes with increasing width along with the source and the cryostat interior is shown on the right.
6.5
Immediate Characterization Results
The data taken with the upside-down mounting of GD91C will be used in the following chapters. InSec. 7.3the dead layer of this detector will be compared between the top, the bottom and the lateral side. InChap. 8, the data is used to develop a more comprehensive model for the n+ electrode. Here, a few important conclusions are presented that are not connected with the rest of this thesis.
6.5.1 PSD in Vacuum Cryostat with and without Passivation Layer
The HADES characterization of the PSD performance found large anomalies in the A/E distributions. A poor A/E resolution at 208Tl DEP energies was observed along with oc- casional double peak structures. A previous characterization of depleted BEGe detectors
that followed the same general production line4 did not show anomalies. In both cases the groove was covered with a passivation layer.
One hypothesis for the anomaly assumes that charges are collected on the passivation layer which then distort the electric potential inside the BEGe. Pulse shape simulations with an asymmetric charge in the groove could qualitatively reproduce double peak structures in the A/E spectra [136]. This hypothesis can be cross-checked with the upside-down mounting of GD91C without passivation layer.
(a)208Tl DEP with and without PL (b) 56Co DEPs
Figure 6.14 Comparison of DEPs as proxy for single-site 0νββ events. Left: 208Tl DEP comparison
with and without passivation later in HADES and LNGS, respectively. Right: Two DEPs from56Co.
Fig. 6.14a shows the A/E spectra for 208Tl DEP events in GD91C from HADES (green) and from the upside-down mounting (red). The double peak structure in the HADES mea- surement is not present after removal of the passivation layer. The FWHM determined in the histogram is 2.5 % with passivation layer and 0.45 % without.
An additional measurement was performed with a56Co source which has multiple DEPs e.g. at 2231.4 keV and 1576.5 keV compared to 1592.5 keV for208Tl. The source was produced
at TU Munich to cross-check the PSD performances in LAr in Gerda with a DEP closer to Qββ at 2039 keV.Fig. 6.14bshows the A/E spectra for the two56Co DEPs measured with
GD91C in the vacuum cryostat. The histograms are normalized to the counts in the peak window and the A/E values are calibrated to the 1576.5 keV peak. The 2231.4 keV peak shows a larger low A/E tail due to Bremsstrahlung. The higher energetic Bremsstrahlung created by a larger electron / positron energy can deposit energies further away from the primary interaction point and create MSE features.
6.5.2 Pulse Shapes on the p+ Electrode and the Groove
Circular241Am Scan
The groove has been scanned on the outer edge with the 1mm Brass collimator at different positions. The scan was performed to test the assumption that the pulse shapes are consis- tent throughout the groove. The measurement positions are illustrated inFig. 6.15a with the red labels in terms of angular coordinates. It was found that the groove was slightly
4The diode production of depleted and enriched detectors lies several years apart and small changes in
6.5 Immediate Characterization Results 79
misaligned with respect to the endcap center and the groove illustration in the picture. The measurement points were chosen accordingly. The resulting A/E spectra within the 59.5 keV peak window are shown inFig. 6.15b. The fraction of the beam hitting the groove was estimated with the peak count rate and is shown in the legend.
(a) 1mm Brass collimator scan positions (b)241Am A/E 55− 65 keV
Figure 6.15 Left: Illustration of the scanning points for241Am circular (red) and radial measurement (light blue). Right: A/E spectrum for 59.5 keV events for all circular points. The legend shows the measurement position along with the estimated covering fraction of the groove.
The A/E spectra show small differences in the high A/E maxima and in the count rate which can be correlated with the coverage of the groove: A larger groove coverage implies more events close to the p+ electrode which increase the A/E maximum as will become clear later. It also implies less events on the n+ electrode resulting in less attenuation and a larger count rate. Otherwise, there is no evidence that the A/E reconstruction of groove events is dependent of the angular position inside the groove for this detector.
Radial 241Am Scan
A radial scan has been performed with the 1mm Brass collimator from the n+ electrode over the groove onto the p+ electrode. The measurement points are illustrated inFig. 6.15a
with the light blue labels. Fig. 6.16ais showing the energy spectra of selected measurement points andFig. 6.16b shows the corresponding A/E spectra for the 59.5 keV peak region. The legends show the distance from the endcap center. The circular scan suggest a slight misalignment of ≈ 1 mm so that the p+ electrode reaches up to roughly 8.5 mm followed
by the groove reaching up to 11.5 mm.
The furthest 14 mm point is fully collimated on the n+ electrode surface which can be seen in a large fraction of slow pulses and missing high A/E events inFig. 6.16b. Additionally the low energy γ-lines of 241Am and X-rays are not visible for this point in Fig. 6.16a suggesting a thicker layer of dead germanium starting just outside the groove. For all other measurement points the low energy γ-lines are visible. They are highly sensitive to attenuating material. Qualitatively, the count rate for those peaks does not appear sig- nificantly different between the groove and p+ electrode positions; however, note that the center point at 1 mm is shadowed by the teflon bar and an attenuation is visible.
(a)241Am energy spectra (b) 241Am A/E 55− 65 keV
Figure 6.16 Selected positions of the radial241Am scan with 1mm Brass collimator from n+ electrode (n) over the groove (g) onto the p+ electrode (p) in 1 mm steps. The beam size is 1.5 mm. The
positions denote the radial distance from the endcap center. The p+ electrode reaches up to roughly
8.5 mm followed by the groove up to 11.5 mm.
Fig. 6.16b. The full set of A/E vs E scatter plots is shown in Fig. B.3 andB.4 in the ap- pendix. Interactions on the inside edge of the groove at 9 mm show the largest A/E values of up to 1.9 when normalized to the 59.5 keV bulk events. Towards the outer edge of the groove the A/E distribution shifts to smaller values. Towards the p+ electrode the A/E values are also decreasing. This is consistent with the calculated strength of the weighting field shown inFig. 4.8which is strongest at the inner edge of the groove. The largest A/E values are created when holes and electrons drift simultaneously in the largest weighting field. Towards the center of the p+ electrode the A/E distribution shifts to lower values.
At the center position at roughly 1 mm the A/E values are smallest but still significantly larger than bulk events.
The dependence of A/E values on the interaction position around the p+ electrode po- tentially allows to determine the position for point-like events. This may give a handle to pin-point background contributions on the thin dead layer surfaces such as alpha emitters: Larger A/E-values would indicate a position inside the groove whereas smaller A/E-values would indicate a position more centered on the p+ electrode. Qualitatively comparing the A/E values of alpha events above 3 MeV in the Gerda Phase I BEGe dataset (Fig. 5.5) with the measured A/E distributions in the scan (Fig. 6.16b) suggest that the alpha events originate from the p+ electrode and not from the groove5.
Wide circular 90Sr and 241Am Scan
A wider area of the n+ electrode surface has been scanned with Beta2000 collimator using
a90Sr and an241Am source. The scan positions are illustrated in Fig. 6.17a. The energy spectra of the 90Sr source on the 0 deg position and the p+ electrode position along with a background and Bremsstrahlung measurement are shown in Fig. 6.17b. The Brems- strahlung measurement was performed with the90Sr source placed misaligned with respect to the collimator hole. Thus, this spectrum only accounts for the Bremsstrahlung contribu-
5
Note that this is only an indication. The number of alpha events in the Phase I BEGe dataset is limited. Also the difference in electronics between the Gerda setup and the vacuum cryostat potentially introduces a bias for maximum A/E values.
6.5 Immediate Characterization Results 81
tion going through the collimator material and not for the contribution being created inside the germanium or along the collimator hole. It can be seen that the 90Sr spectra contain a large fraction of beta interactions up to 2 MeV which is enhanced for the p+ electrode spectrum. The energy range below 600 keV is dominated by Bremsstrahlung.
(a) Beta2000 collimator scan positions (b)90Sr energy spectra
Figure 6.17 Left: Illustration of scanning points for90Sr and241Am circular scans with the Beta2000
collimator. Right: Energy spectra for the90Sr source at the n+ electrode and p+ electrode position as well as for a background and Bremsstrahlung measurement.
A/E spectra are shown for the energy ranges of 650− 1000 keV and 1000 − 1450 keV in Fig. 6.18aand 6.18b respectively for all measurements. Due to the scattering of electrons the n+ electrode measurements contain some p+ electrode events and vice versa. The A/E distribution are qualitatively equal for all n+ electrode points suggesting a homogeneous n+ electrode surface on the bottom of the BEGe. Note that the measurements at 90 deg and 270 deg are partly shadowed by the teflon bar and show a reduced rate. In all 90Sr spectra two gaps can be seen between low A/E values, A/E=1 and high A/E values en- abling a strong separation between those event types.
(a)90Sr 650
− 1000 keV (b) 90Sr A/E 1000
− 1450 keV Figure 6.18 A/E spectra for the 90Sr circular scan points in two different energy regions.
The241Am scan measurements, obtained with the same collimator and a subset of the same n+ electrode positions as the90Sr measurements, are shown inFig. 6.19a (energy spectra)
(a)241Am energy spectra (b) 241Am A/E 55− 65 keV
Figure 6.19 241Am energy spectra (left) and A/E spectra for peak events (right) for circular scan measurements on the n+ electrode.
and Fig. 6.19b (A/E distribution for peak energies). No difference for the n+ electrode contact surface section is observed either in the energy spectra nor in the A/E distribution. Both90Sr and241Am measurements strengthen the conclusion that the n+ electrode surface is homogeneous on the bottom side of GD91C.