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

Transcription

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

2 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)

3 1. Motivation 1.2 Comparison to GeMPI Corrado at MPI-K GeMPI at LNGS Overburden (mwe.) Active mass (kg) Counts/day/mass [ kev] (1/d/kg) Sensitivity U/Th (mbq/kg) ~ 1 ~ orders of magnitude discrepancy in sensitivity

4 1. Motivation 1.3 Background Components Gerd Heusser, 1993

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

6 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

7 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

8 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

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

10 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

11 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

12 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)

13 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)

14 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)

15 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

16 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 < ± 3 53 ± ± 18 Th-228 < ± 2 9 ± 2 50 ± 9 K-40 < ± 7 33 ± 8 70 ± 32 about 7 cm of inner copper layer needed for sufficient gamma suppression

17 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