Cortes, M. L.

Gerl, J.

Goel, N.

Habermann, T.

Kojouharov, I.

Pietralla, N.

GSI Repository

Study of Neutron Damaged Germanium Detectors with Pulse ShapeComparisonM. L. Cortes1,2, T. Habermann3,2, N.Goel2, I. Kojouharov2, J.Gerl2, and N. Pietralla11Technische Universita¨t Darmstadt, Darmstadt, Germany; 2GSI, Darmstadt, Germany; 3Goethe Universita¨t Frankfurt,Frankfurt, GermanyNeutron damage of Germanium detectors is very fre-quent in nuclear physics experiments. It decreases theenergy resolution of the detector and it is necessary toanneal it or to characterize its damage to recover itsperformance. The neutron damage of an EUROBALLcrystal was studied using a 511 keV source and the pulseshape comparison technique.Experimental Set-upA neutron-damaged EUROBALL crystal was studied us-ing a coincidences scanning approach at the scanning tableof GSI [1]. In this set-up a 22Na source is placed betweena position sensitive detector (PSD) and the Germanium de-tector to be scanned. The PSD is a LYSO scintillator gluedto a Hamamtsu R2486 position sensitive photomultiplierand the source is surrounded by a Tungsten shielding withconical opening. Two 511 keV γ rays are emitted in oppo-site directions by the source. One going to the Ge detectorand the other one to the PSD, allowing a 2D image of theGe detector to be generated. With the setup it is possibleto obtain an image from the front and an image from theside of the detector. Event by event the pulse shape fromthe Ge detector is recorded as well as the energy depositedin it. By using a novel analysis technique, the images canbe used to select particular interaction positions inside thedetector and to study the signals coming from that position.ResultsFig. 1 shows the FWHM of the photopeak as a functionof the radial distance in the detector. Near the surface of thedetector the width of the peak is smaller, indicating a higher 1.5 2 2.5 3 3.5 4 4.5 5 0  0.5  1  1.5  2  2.5  3  3.5FWHM (channels)Radial distance (cm)i l  (c )FWHM (arbitrary units)Figure 1: FWHM of the 511 keV photopeak as a functionof the radial position.resolution. In the central part of the detector the photopeakbecomes wider.For a coaxial Ge detector is expected that the rise timeof the signals increase with the radial distance. For theEUROBALL crystal two cases were studied: Events dom-inated by full charge collection, in the central part of thephotopeak and events with incomplete charge collection,recorded in the low energy tail. In both cases the behaviorwas as expected and no clear difference between the caseswas found.Signals associated with the same position in the detectorwere compared by pairs and the χ2 value of this compar-ison was calculated. The distribution obtained is shownin Fig. 2. A comparison with the result obtained for anAGATA crystal is shown. Differences in the shape of theAGATAEuroballcrystal2χχCountsAGATAEUROBALL 1002003004001 221 10 10Figure 2: χ2 distribution in the EUROBALL crystal com-pared to the distribution in an AGATA crystaldistributions, may be related to an effect of neutron dam-age, nevertheless a comparison with a non-damaged EU-ROBALL crystal is necessary.ConclusionsThe neutron damaged EUROBALL crystal shows a ra-dial dependence of the neutron damage. The shape of theχ2 distribution suggest the possibility to quantify the neu-tron damage, but a comparison with a non-damage detectoris needed to make a final conclusion.References[1] C. Domingo-Pardo, et. al.. NIM A643 (2011) 79-88PHN-ENNA-EXP-56 GSI SCIENTIFIC REPORT 2012186

Study of Neutron Damaged Germanium Detectors with Pulse Shape Comparison

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Study of Neutron Damaged Germanium Detectors with Pulse Shape Comparison

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