Simulation and verification of the cosmogenic background at the shallow depth GIOVE detector

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1 Simulation and verification of the cosmogenic background at the shallow depth GIOVE detector J. Hakenmüller, G. Heusser, M. Lindner, W. Maneschg, M. Weber Max-Planck-Institut für Kernphysik, Heidelberg (GER) TAUP 2015 Torino, September 7-11, 2015 Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

2 GIOVE = Germanium Spectrometer with Inner and Outer Veto Scopes of GIOVE: What is GIOVE? New low background Ge spectrometer at shallow lab of MPIK (15 m w.e.) Improved design compared to earlier generation Ge detectors in same lab: Improved muon veto efficiency Neutron suppression within shield Stronger suppression of bg from Rn and Rn progenies Material screening: Sensitivities for U and Th: Corrado at MPIK (15 m w.e.): 1 mbq/kg GIOVE s aimed sensitivity: 0.1 mbq/kg GeMPI at LNGS (3800 m w.e.): 0.01 mbq/kg Beyond material screening: Bg n tio la lcu ca f. g ef in l t. el De od m n io at Te st ne w Im m pr at ov er e ial se s ns itiv ity Material screening Measurements lid Va Shield optimisation MC simulation Prediction Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

3 GIOVE: Background reduction via passive and active shield Optimisation of muon veto system Simulation with tool LuxIter: validated with data of a scanning muon telescope to optimize number/inclination of PMTs plastic scintillator plate 1 x1 PMT Very good γ/µ discrimination: Muon veto efficiency: 99% Dead time loss: 2% y -1 ] -1 kg -1 counts [kev unshielded passive shield passive shield + muon veto Gamma-rays from natural radioactivity Optimum threshold Muon-induced signals energy [kev] Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

4 GIOVE: Continuous background and comparison with other detectors y -1 ] -1 kg -1 counts [kev Corrado muon veto (MPIK, 15 m w.e.) GIOVE muon veto (MPIK, 15 m w.e.) GeMPI (LNGS, 3800 m w.e.) m Ge 75m Ge 71m Ge Annihil energy [kev] Integral count rate in (0,2700) kev Detector m act Location Depth Count rate [kg] [m w.e.] [d 1 kg 1 ] Corrado 0.94 MPIK ±12 [1] GIOVE 1.81 MPIK ±3 [1] Ge HADES ±3 [1] Gator 2.2 LNGS ±1 [2] GeMPI 2.06 LNGS ±1 [1] Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

5 GIOVE: Threshold activity and comparison with other detectors Measurement of a copper sample with GIOVE and GeMPI-1 GIOVE GeMPI-1 [3] Cu mass [kg] 65.2 kg 7 kg Live time 32.8 d 7.6 d Isotope/decay chain [mbq kg 1 ] [mbq kg 1 ] 226 Ra 0.13± Th K Co 0.04± ±0.004 Threshold activity and benchmarks: GIOVE achieves threshold activities for U and Th of 50-0 µbq kg 1-20 better than earlier Ge spectrometers at MPIK only 5- higher than GeMPI detectors * Sensitivity comparable with detectors in underground labs of several 0 m w.e. At such depths: natural radioactivity and cosmogenic bg compete GIOVE: home-based lab with easier and more flexible access * For more information about the construction and achieved sensitivity of the GIOVE detector see arxiv: [astro-ph.im], Ref. [4]. Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

6 GIOVE: Detector modelling with the Geant4-based toolkit MaGe 1. Diode position determination: Method: fine-grained scan with robotic arm and collimated 241 Am source n+ p+ borehole 3. Verification of MC in the energy interval [88,1275] kev: MaGe [5], based on Geant4 [6,7] Use (mono)-energetic γ-ray sources: 54 Mn, 22 Na, 57 Co, 65 Zn, 9 Cd Detector efficiency curve: 2. Active volume determination: Comparison calibration data - MC: 1. Gamma-line ratio method with 241 Am or 133 Ba to fix n+ layer; 2. Absolute count rate method with 60 Co to fix eff. bore hole dimensions n+ p+ borehole detector efficiency rel. difference data simulation energy [kev] Mean absolute deviation of simulation and measurements: (2.3±1.3)% AV=(339±23) cm 3, m=(1807±124) g (main syst. uncertainties included) (diff. expressed in terms of: (data-mc)/mc ) Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

7 GIOVE: Modelling of cosmic background at shallow depth Flux diagramm: 15 m w.e Our approach (J. Hakenmüller M.Sc. thesis [8]): Analytic calculations: 1 Muon flux above ground: - Extrapolation formulas by Bogdanova [9] & Bugaev/Reyna [,11] - Altitude effect corrected (305 m a.s.l.) 2 Muon and neutron flux at 15 m w.e.: - Muons energy loss: de µ/dx = a + be µ - Primary neutron flux: exponential decrease with λ= g/cm 2 MC simulations with MaGe: 3 Simulation of muons through shield: - Muon starting positions: on lab walls - µ + /µ ratio 1.3, similar to sea level 4 Simulation of muon-induced neutrons through shield and Ge diode: - Propagation in different shield layers - Backscatter effects considered Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

8 GIOVE: Calculation of muon flux and spectrum at 15 m w.e. Analytic calculation of muon flux at MPIK shallow lab: Integral muon flux according to Bugaev/Reyna parametrisation: - Flux from ceiling (15 m w.e.): 57.0 m 2 s 1 - Flux from walls (18 m w.e.): 37.2 m 2 s 1 Adopted angular distribution: - Overall cos 2 (θ), projection on source plates Starting positions of µ + and µ : - from laboratory walls; see red plates Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

9 MC validation: Prompt muon-induced gamma-ray spectrum Count rates in: A) (40,2700) kev [d 1 kg 1 ] B) 511 kev [d 1 ] data MC diff A) 31125± ±0 2% B) 1113±17 20±30 8% differences expressed as: (data-mc)/data Comparison of data vs. MC simulation: Data collection: muon veto off; muonic background dominates over other bg Muonic background: muon-induced bremsstrahlung continuum dominates Excellent agreement above 80 kev between data and MC µ + and µ contribute in similar way to the spectrum Similar results for measurements/simulations for empty sample chamber (12.4 L) or filled with neutron moderators/absorbers Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

10 GIOVE: Simulation of cosmogenic neutron production Propagation through shield Muon induced neutrons in diff. layers Neutrons arriving at diode MC simulation with MaGe based on Geant 4.9.6: Separate simulation of: 1,, 0 GeV µ + /µ ; full µ spectrum at MPIK lab Neutron flux due to µ + and µ : similar for and 0 GeV, smaller at 1 GeV for µ +, since muon capture (induced only by µ ) dominates at these energies n flux at diode: Sample n entering diode n entering diode for chamber [m 2 s 1 ] 1st time [m 2 s 1 ] Empty Pure PE B-doped PE Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

11 MC validation: Prompt gamma lines from neutron captures in samples Counts/bin GIOVE Ge spectrum SC: filled with B-doped PE B(n,α )7Li* (477.6 kev) Counts/bin GIOVE Ge spectrum SC: filled with PE H(n,p)D* ( kev) MC n in shield 1. MC n capture in sample γ-ray emission (478, 2223 kev) Outside diode Data Intensity of 478 & 2223 kev MC 478 & 2223 kev γ-ray absorb. inside diode Inside diode Energy [kev] Energy [kev] Simulations vs. measurements: Data collection: muon veto off; Sample chamber filled with moderators/absorbers Results: Sample Energy Count rate [d 1 ] chamber [kev] data MC diff Pure PE ±5 24±6 54% B-doped PE ±18 59±16 36% differences expressed as: (data-mc)/data Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

12 MC validation: Delayed neutron-induced Ge isomeric lines counts/bin Ge73m ( kev) GIOVE Ge spectrum SC empty; Muon veto on Ge75m (139.9 kev) Ge71m (197.9 kev) 1. Analytic calculation 2. MC simulation MC n in shield MC 1. n arriving at diode Data Intensity of Ge75m in diode 2. MC n-induced Ge75m in diode energy [kev] Outside diode Inside diode Simulations vs. measurements: Focus on kev line: high efficiency, neutron cross section data available Two approaches: 1. Reverse calc. of n flux at diode acc. to formula in [12], extended by [8] 2. Implement neutron cross section in Geant4 for metastable 75m Ge Results: Sample n at diode [m 2 s 1 ] 139 kev [d 1 ] chamber data MC diff data MC diff Empty 4.8± % 8.8± % Pure PE 4.0± % 14.6± % B-doped PE 3.7± % 8.7± % differences expressed as: (data-mc)/data Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

13 Design optimisation using the current MC code A road map for optimisation Parameters to be varied: - Thickness of single layers and/or permutation of their order - Exchange of materials (e.g. PE with higher B concentrations) Boundary conditions: - Radiopurity of high density materials and/or neutron moderators - Handling and construction: mechanical stability, toxicity, flammability etc. Example: Permutation of layers, all 15 cm of Pb to the outside PE layers with varying B concentrations (3% 0%, 3% %) MC results for both configurations: - count rate of prompt µ-induced spec. - number of n s arriving at Ge diode both consistent within stat. uncert. standard modified Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

14 GIOVE: Summary and outlook GIOVE construction and performance: - Inner and outer myon veto: µ rejection efficiency of 99%, dead time 2% - Lowest bg levels achieved in a shallow depth laboratory: Threshold activity for large, high density materials: 50-0 µbq/kg for U and Th Bg levels similar for low bg Ge detectors in semi-deep labs of 500 m w.e. Full MC simulation of cosmic-ray induced bg: - Ab initio calculation of µ flux at MPIK shallow underground lab Validation of calculated values with literature data: very good agreement - Geant4-based MC simulation of µ s, µ induced n s & their interactions in the Ge detector Validation of MC results with different bg and calibration data sets: - Prompt muonic spectrum: very good agreement between MC and data - Prompt/delayed signals induced by cosmic n s: discrepancy of 40-80% Indications: muonic neutron generation underestimated by MC Next steps: - Continue study of bg induced by cosmic n s in MC and data: External 252 Cf neutron source calibration to study neutron propagation Careful determination of all relevant systematics Potentially usage of other simulation tools (e.g. FLUKA) - MC based optimisation of low bg Ge detector setups Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

15 BACK - UP Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

16 Material screening for the construction of GIOVE Sample Quantity Activity concentration [mbq Unit 1 ] Unit 226 Ra 228 Th 228 Ra 40 K plastic scintillator NE-2A 7.30 kg 0.96 ± ± ± ± 0.7 plastic scintillator EJ kg 0.09 ± ± ± ± 0.5 GALLEX carbon steel 51.2 kg 0.13 ± 0.03 < ± ± 0.3 high-density PE (HDPE) 7.76 kg 0.27 ± 0.08 < 0.14 < 0.50 < 3.2 PMT R MOD 7 pc 0.36 ± 0.21 < 0.4 < ± 1.7 B 2 O 3, for silic. anal. Merck 4.0 kg < 1.2 < 1.9 < ± 8 black Lexan scint. coverage 1.43 kg 11 ± 2 < 4 < 116 ± 18 opt. cement EJ-500 resin 296 g 39 ± 22 < 73 < 122 < 194 opt. cem. EJ-500 hardener 65 g < 52 < 170 < 135 < 730 Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

17 Benchmark comparison of different low background Ge detectors Integral count rate in (0,2700) kev Detector m act SC vol Location Depth µ flux red. Count rate Ref. [kg] [liter] [m w.e.] (comp. to s.l.) [d 1 kg 1 ]* Corrado MPIK ±12 [1] GIOVE ±3 [1] Ge HADES ±3 [1] Gator 2.2 n.n Soudan ±3 [2] GeOroel n.n. LSC ±2 [13] 148±1 [14] Pasquito 1 n.n 79±2 [13] HPGe coax 1.9 n.n. Boulby ±1 [15] Gator LNGS ±1 [2] GeMPI ±1 [1] Count rates are normalized to kg of active mass. Prior installation at LNGS, Gator was operated in Soudan underground lab. Count rate was obtained with a more recently improved passive shield design. Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

18 Extrapolation: Muon flux and mean muon energy in underground location Vertical integral intensity J v (X, 0 ) [m 2 s 1 sr 1 ]: Location Depth Data Bogda- Bugaev/ Barbouti (height above sl) [m w.e.] nova Reyna & Rastin Heidelberg (305 m) Garching (482 m) Stanford (-20 m) 17±1 29.2± Belgrade (78 m) 25±0.6 25± London, Holborn (35 m) ± Bern (524 m) Janossy Pit 17.5± ± near Csillebérc ( 0 m) 38.4± ± ± ± Nottingham (sl) 8.95± ± ± ± ± ± Potential overestimation of flux at low energies; measurement under Pb, not rock overburden Mean muon energy E [GeV]: Location (height above s.l.) Depth Mei et al. Bogdanova Bugaev/Reyna Heidelberg (305 m) 15 m w.e Stanford (-20 m) 20 m w.e Gypsum mine (Russia) (? m) 25 m w.e Palo Verde/Wintersburg (307 m) 32 m w.e no exact location stated in publication Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

19 Discrepancy of neutron-induced background in MC and data Possible origins: 1 Underestimation in MC: a Muon-induced neutron generation (e.g. not all production channels included) b Neutron propagation (e.g. backscattering not correctly taken into account) 2 Overestimation in data due to: a Cosmic neutrons at surface : 0.1 m 2 s 1 [16] excluded b Neutrons from natural radioactivity in non-granitic rock s. below c Neutrons produced by fast cosmic muons inside overburden s. below Intensity [cm -2 s -1 ] 15 m w.e. 15 m w.e. Depth [m w.e.] Figure adopted from [16] Neutron flux (2 a+b+c) measurement at Standford underground lab [17]: (0.81±0.06) m 2 s 1 ( kev-20 MeV) Standford vs. MPIK lab overburden similar: composition: sandy soil/gravel vs. sandstone density: 1.6 vs. 2.3 g cm 3 depth: 17 vs. 15 m w.e. similar neutron flux expected at MPIK Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20

20 Bibliography 1 Background and calibration spectra available to authors 2 L. Baudis et al., JINST 6 P080, M. Laubenstein, private communication 4 G. Heusser et. al, arxiv: [astro-ph.im], submitted to EPJ-C, July M. Boswell et al., IEEE Trans. Nucl. Sci. 58, 1212, S. Agostinelli et al., Nucl. Instrum. Meth. A 506, 250, J. Allison et al., IEEE Trans. Nucl. Sci. 53, 270, J. Hakenmüller, master thesis, University of Heidelberg, L.N. Bogdanova et al., Phys. Atom. Nucl. 69, , 2006 E.V. Bugaev et al., Phys. Rev. D 58, , D. Reyna, arxiv:hep-ph/ , G.P. Skoro et al., Nucl. Instr. Meth. A, Volume 316, p S. Cebrián et al. (NEXT coll.), JINST P05006, S. Cebrián (for NEXT coll. & LSC Radiopurity Service), Talk at LRT X.R. Liu, and J. Mott, J. Phys.: Conf. Series , G. Heusser, Annu. Rev. Nucl. Part. Sci. 45, pp , A.J. Da Silva, phd thesis, University of British Columbia, 1996 Gerd Heusser (MPIK Heidelberg) GIOVE cosmic bg: data vs simulation TAUP 2015, Sept / 20