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Measurement of low level neutron fluxes: status and prospects

Measurement of low level neutron fluxes: status and prospects. John McMillan University of Sheffield. Applications. Neutral current detectors for neutrino oscillation studies Dark matter (etc…) background studies Fission studies Supernova observatories Cold fusion….

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Measurement of low level neutron fluxes: status and prospects

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  1. Measurement of low level neutron fluxes: status and prospects John McMillan University of Sheffield

  2. Applications Neutral current detectors for neutrino oscillation studies Dark matter (etc…) background studies Fission studies Supernova observatories Cold fusion… John McMillan IDM2004

  3. Neutron capture reactions John McMillan IDM2004

  4. 1keV John McMillan IDM2004

  5. 3He proportional counters 3He + n = 3H + 1H + 0.764MeV Typical 3He counters - Reuters Stokes • 630mm long 25mm diameter • 6 atmospheres 3He Kr(11%) CO2(1%) • Stainless steel casing Efficiency depends on geometry (but 30% with 20 tubes) John McMillan IDM2004

  6. Neutral Current Detectors at SNO 3He proportional counters - But with a radiopurity requirement <12 parts per 1012 by weight Thorium!! John McMillan IDM2004

  7. John McMillan IDM2004

  8. Body of detector is 50mm diameter chemical-vapour-deposited (CVD) nickel tube Electropolished, acid etched to remove Rn progeny Laser welded Gas 3He + CH4 (15%) at 2.5 atmospheres Alpha activity of first detectors (1998) 48 counts/m2/day John McMillan IDM2004

  9. “3He Neutral Current Detectors at SNO”, Elliot, S.R. et al., DOE/ER/41020-42, 1998. “Low-background 3He proportional counters for use in the Sudbury Neutrino Observatory”, Browne, M.C., IEEE Trans NS-46(4), p873-876, 1999. "Neutral Current Detectors for the Sudbury Neutrino Observatory“, Thornewell, P., PhD thesis, Oxford University, 1997. "Preparation for Deployment of the Neutral Current Detectors (NCDs) for the Sudbury Neutrino Observatory“, Browne , M., PhD thesis, North Carolina State University, 1999. John McMillan IDM2004

  10. Disadvantage? Cost! John McMillan IDM2004

  11. 6LiF-ZnS detectors 6Li + n = 4He + 3H + 4.8MeV 6LiF powder mixed with ZnS(Ag) scintillator plus binder Spread into thin (100mm) layers Read out to photomultipliers using wavelength shifting lightguides Insensitive to gammas John McMillan IDM2004

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  15. High efficiency Eight detectors in cylindrical geometry gave 37% for 252Cf fission neutrons “A novel neutron multiplicity detector using lithium fluoride and zinc sulphide scintillator”, Barton, J.C. et al, J Phys G 17, p1885-1899, 1991. But… Background not as low as expected ~0.01Hz => ZnS(Ag) contaminated with U, Th John McMillan IDM2004

  16. A rediscovered thermal neutron scintillator, 6Li-salicylate “A new liquid scintillator for thermal neutron detection”, Ross, H.H. and Yerick, R.E., Nucl Sci Eng, 20, p23-27, 1964. “6Li-salicylate neutron detectors with pulse shape discrimination”, Greenwood, L.R. et al, NIM 165, p129-131, 1979. “Properties of a new class of organic scintillators: derivatives of salicylic acid”, Mandzhukov, I.G. et al, Instrum Exp Tech, 24(3), p605-611, 1981. “Possibilities to use polycrystalline lithium salicylate as a thermal neutron scintillator”, Mandzhukov, I.G. et al, Bulg J Phys, 8(4), p349-354, 1981. John McMillan IDM2004

  17. Polycrystalline 6Li-salicylate scintillator Efficiency 10-15% of anthracene for alphas Polycrystalline layers have optimal thickness at 250mm – thicker layers absorb light Such layers (95% 6Li) have 5% total efficiency for detecting thermal neutrons lmax = 421nm Low atomic number Zeff= 6.8 so insensitive to gammas Not hygroscopic John McMillan IDM2004

  18. Synthesis of6Li-salicylate Add enriched lithium metal to distilled water to form hydroxide. Add Salicylic Acid to solution Dehydrate and crystallise 6Li metal chunks (95% 6Li) 10g £210 (Aldrich) John McMillan IDM2004

  19. Purification 6Li metalprobably very pure – lithium has been isotopically separated (mass spectrometer beam) Salicylic Acid manufactured using Phenol, NaOH, CO2, H2SO4 Large scale production for pharmaceuticals – Aspirin Can be further purified by sublimation at 76oC John McMillan IDM2004

  20. Production of layers of 6Li-salicylate Deposition from solution in alcohol. Layers need overcoating in transparent material to prevent radon progeny emitting alphas into the scintillator. Epoxy or silicone. 6Li-salicylate could be mixed with transparent epoxy or silicone binder. The mixture could be spread to the desired thickness then overcoated. John McMillan IDM2004

  21. Can we build large, high efficiency, low background, low cost thermal neutron detectors using polycrystalline lithium salicylate with wavelength shifted readout? John McMillan IDM2004

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