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Muon-induced Neutron as a Background for Double Beta Decay Experiments

Muon-induced Neutron as a Background for Double Beta Decay Experiments. Dong-Ming Mei, Andrew Hime, Steve Elliott, Vic Gehman P-23, Los Alamos National Laboratory. 2615 keV  from 208 Pb(n,n’  ). 2615 keV  : A common background observed by many double beta experiments at underground

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Muon-induced Neutron as a Background for Double Beta Decay Experiments

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  1. Muon-induced Neutron as a Background for Double Beta Decay Experiments Dong-Ming Mei, Andrew Hime, Steve Elliott, Vic Gehman P-23, Los Alamos National Laboratory PANIC05, Santa Fe

  2. 2615 keV  from 208Pb(n,n’ ) 2615 keV  : A common background observed by many double beta experiments at underground A common explanation for this background is that it comes from 208Tl decay from 232Th decay chain Really? All of them are from 208Tl decay? What about 208Pb(n,n’ ) ? PANIC05, Santa fe

  3. AmBe activation data and background data comparing to the MC Observed lead excitation lines on the surface 2615 keV from 208Pb(n,n’ ) PANIC05, Santa fe

  4. Clover detector • 4 detectors PANIC05, Santa fe

  5. Zoom-in spectrum • AmBe and background data comparing to the MC: 570 keV from 207Pb(n,n’ ),803 keV from 206Pb(n,n’ ) PANIC05, Santa fe

  6. Determination of production rate of 2615 keV • Using the measured rates: • Rambe(692), Rambe(2615), Rback(692),Rback(2615) PANIC05, Santa fe

  7. Determination of production rate • Formulas • Rback(2615) = R208Tl + R208Pb • Rback(2615)/Rback(692) = R208Tl/Rback(692) + R208Pb/Rback(692) • R208Pb/Rback(692) = Rambe(2615)/Rambe(692) • Production rate • R208Pb = 667/day, R208Tl = 613/day PANIC05, Santa fe

  8. R208Pb(2615) = 667 /day Equivalent to ~17 Bq/kg 232Th in lead Equivalent to ~1 Bq/kg 232Th in cryostat Required 232Th activity for germanium-based detector: <10 µBq/kg for lead <1 µBq/kg for Cryostat ~106 reduction is needed Required n-flux reduction >106 wrt to the surface What do these numbers mean? PANIC05, Santa fe

  9. Using 692 keV line and capture 139.68 keV from 75mGe Ψn = (4.9±0.9)X10-2 cm-2 s-1 Los Alamos Фn = 1.22X10-2 cm-2 s-1 Sea level Made from NIM A 476 (2002) 42 Measured neutron fluxes at Los Alamos PANIC05, Santa fe

  10. Estimate the production rate underground • Simulated neutron flux underground • Rock/cavern boundary • Shielding/detector boundary • Total neutron flux: • Excitation function as a function of neutron energy Ref. Phy. Rev. C 50(1994) 1952 • Multiple (n,n’ ) production by single neutron PANIC05, Santa fe

  11. Depth required for germanium-based detector (preliminary) • If Pb is used as inner shielding 570 keV 207Pb(n,n’ ) in the KKDC spectrum? NIM A 522 (2004)371 PANIC05, Santa fe

  12. Conclusion • Inelastic scattering on germanium is a significant background • Lead excitation may represent a background depending on the depth • It is better to not use lead as the inner shielding, Cu is an alternative PANIC05, Santa fe

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