BEGe detector response to α-radiation near its p + electrode

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1 BEGe detector response to α-radiation near its p + electrode Matteo Agostini, Marik Barnabé-Heider, Tobias Bode, Dušan Budjáš, Andrea Lazzaro and Stefan Schönert for the GERDA collaboration

2 Outline Introduction to Broad Energy Germanium (BEGe) detectors and radioactive backgrounds The α-scanning setup TUBE First results Conclusion

3 GERDA phase II Background index aim: < cts/(kev kg y) Employ Pulse Shape Discrimination (PSD) Broad Energy Germanium detectors well suited for PSD of possible backgrounds (α,β,γradiation) p + electrode for signal read out (grounded) groove n + electrode HV ~mm thick

4 Pulse shapes of different event topologies p + electrode n + electrode bb single site event p + surface event internal g multi site events surface event 42 K beta

5 Pulse shapes of different event topologies p + electrode e - n + electrode bb h single site event p + surface event internal g multi site events surface event 42 K beta

6 current charge [a.u.] charge [a.u.] charge [a.u.] Technische Universität München Pulse shapes of different event topologies SSE time [ns] p + electrode MSE SSE bb single site event p + surface event internal g multi site events surface event time [ns] n+ surface pulse: NSP 42 K beta n + electrode time [ns] time [ns]

7 current charge [a.u.] charge [a.u.] charge [a.u.] Technische Universität München Pulse shapes of different event topologies SSE time [ns] p + electrode MSE SSE bb single site event p + surface event internal g multi site events surface event time [ns] n+ surface pulse: NSP 42 K beta n + electrode time [ns] time [ns]

8 Pulse shape disrcimination work so far Multiple site events treated by D. Budjaš et. al (JINST, 4 (2009) P10007) & M. Agostini et. al (JINST, 6 (2011) P03005 n + surface events: talk by A. Lazzaro T113.2 Study of p + and groove events: this talk

9 Pulse shape disrcimination work so far Multiple site events treated by D. Budjaš et. al (JINST, 4 (2009) P10007) & M. Agostini et. al (JINST, 6 (2011) P03005 n + surface events: talk by A. Lazzaro T113.2 Study of p + and groove events: this talk Critical surface events only by α- and β-particles Phase I α-background index: 10-3 cts/(kev kg y) (T 109.3) Potentially harmful for aim of phase II Behaviour of α-particles not studied before Dedicated setup for scanning the p + & groove region

10 Scanning setup TUBE (TUM Up-side down BEGe) Study needs data sample rich in p + & groove events collimated beam of α-particles (Am-241) source

11 Scanning setup TUBE (TUM Up-side down BEGe) Study needs data sample rich in p + & groove events collimated beam of α-particles (Am-241) In common vacuum cryostat p + contact shielded by holder For movable collimator large vacuum tube neccessary source

12 Scanning setup TUBE (TUM Up-side down BEGe) Study needs data sample rich in p + & groove events collimated beam of α-particles (Am-241) In common vacuum cryostat p + contact shielded by holder For movable collimator large vacuum tube neccessary Cooling power to be considered Two IR shields for reduction of radiative heat load source

13 Scanning setup TUBE (TUM Up-side down BEGe) BEGe p + contact common IR shield common vacuum flange n + contact Signal readout pin

14 Scanning setup TUBE (TUM Up-side down BEGe) custom IR shield with slit collimator vacuum tube rail system new vacuum flange

15 The TUBE build up in the Underground Laboratory at TUM heater for outbaking pumping system

16 Complete setup with lead shield and muon veto panel

17 Results of scanning the p + & groove region With small collimated α-beam a very accurate scan of the p + & groove region was possible Signals can be easily discriminated ( rise time etc..) Suppression factor of 10

18 Conclusion and outlook Current α -background index of 10-3 cts/(kev kg y) potentially harmful for GERDA phase II Pulse shapes of α-particles in BEGe s never studied before Dedicated setup needed Model of signal formation (M.Agostini et.al) verified With achieved suppression factor > 10, α-backgrounds are within specifications of GERDA phase II Use another BEGe Use β-source