GERDA Meeting, September 2009, LNGS. News from Low-Level-Lab: Attempt to Reduce Neutron Background at Shallow Depths

GERDA Meeting, September 2009, LNGS News from Low-Level-Lab: Attempt to Reduce Neutron Background at Shallow Depths

1. Motivation 1.1 Gamma Spectroscopy at MPI-K Gentner Building Bruno Corrado Low-level-laboratory under 15 m we. overburden 2 detectors with active muon veto (proportional counting chambers)

1. Motivation 1.2 Comparison to GeMPI Corrado at MPI-K GeMPI at LNGS Overburden (mwe.) 15 3800 Active mass (kg) 0.94 2.15 Counts/day/mass [40-2700 kev] (1/d/kg) 3830 30 Sensitivity U/Th (mbq/kg) ~ 1 ~ 0.01 2 orders of magnitude discrepancy in sensitivity

1. Motivation 1.3 Background Components Gerd Heusser, 1993

1. Motivation 1.3 Background Components proportional counters lead Ge N2 copper

1. Motivation 1.3 Background Components cosmic ray μ proportional counters proportional chambers provide anti-coincidence veto for e.m. components lead γ e Ge n N2 copper but delayed neutrons from nuclear interactions difficult to detect

1. Motivation 1.3 Background Components γ-lines from neutron interaction Ge-73m Ge-75m annihilation peak Ge-71m Bremstrahlung & Compton neutron activation of surrounding copper Co-60

1. Motivation 1.4 GIOVE: a New Detector Preliminary design study! Goal: increase of sensitivity by a factor of 5-10 by efficient neutron background reduction GeMPI-oriented construction plus Inner and outer veto system Use of plastic scintillators Borated polyethylene as a neutron shield designed by G.Heusser, B.Mörk

2. Experiments 2.1 Investigation of Neutron Flux Reduction by Borated PE (Geiger, K.W. and Hargrove, C.K.)

2. Experiments 2.1 Investigation of Neutron Flux Reduction by Borated PE No PE 5 cm borated PE 15 cm borated PE 8 cm unborated PE

2. Experiments 2.1 Investigation of Neutron Flux Reduction by Borated PE Ge(n,γ) capture reactions Ge(n,n ) scattering reactions mostly induced by thermal neutrons mostly induced by fast neutrons No PE 5 cm borated PE 15 cm borated PE 8 cm unborated PE

2. Experiments 2.1 Investigation of Neutron Flux Reduction by Borated PE borated (10% mass) unborated Relative reduction (a.u.) integral flux moderated neutrons fast neutrons Absorber thickness (cm)

2. Experiments 2.1 Investigation of Neutron Flux Reduction by Borated PE borated PE (10% mass) Relative reduction (a.u.) Best fit with correction for pedestal due to experimental setup yields mean attenuation length of λ= (6.2 ± 0.6) cm integral flux (data) exp() - fit with pedestal exp() - fit Absorber thickness (cm)

2. Experiments 2.2 Current Test Stand for PE/Lead Configurations varying sequence of borated PE and lead layers pure PE for neutron signal creation (2223 kev gamma line from 1 H(n,γ)reaction)

2. Experiments 2.2 Current Test Stand for PE/Lead Configurations Sequence on top Peak Area at 2223 kev (counts/day) Ratio (%) lead 90±6 100 lead PE 68±4 76 lead PE 67±4 74 lead PE 69±3 77

Attempt to Neutron Background Reduction at Shallow Depths 2. Experiments 2.3 Radio-Purity detector chamber filled with PE Ge PE Activity (mb/kg) Pure PE 5% B-PE 10% B-PE B 2 O 3 Ra-226 < 4 130 ± 3 53 ± 3 573 ± 18 Th-228 < 203 12 ± 2 9 ± 2 50 ± 9 K-40 < 17 22 ± 7 33 ± 8 70 ± 32 about 7 cm of inner copper layer needed for sufficient gamma suppression

3. Conclusions 3.1 The Giove Design Preliminary design study! GeMPI-oriented construction plus Inner and outer veto system Use of plastic scintillators Borated polyethylene as a neutron shield designed by G.Heusser, B.Mörk