John McMillan IDM2004 3 He proportional counters 3 He + n = 3 H + 1 H + 0.764MeV Typical 3 He counters - Reuters Stokes 630mm long 25mm diameter 6 atmospheres 3 He Kr(11%) CO 2 (1%) Stainless steel casing Efficiency depends on geometry (but 30% with 20 tubes)
John McMillan IDM2004 Neutral Current Detectors at SNO 3 He proportional counters - But with a radiopurity requirement <12 parts per 10 12 by weight Thorium!!
Body of detector is 50mm diameter chemical- vapour-deposited (CVD) nickel tube Electropolished, acid etched to remove Rn progeny Laser welded Gas 3 He + CH 4 (15%) at 2.5 atmospheres Alpha activity of first detectors (1998) 48 counts/m 2 /day
John McMillan IDM2004 “3 He Neutral Current Detectors at SNO”, Elliot, S.R. et al., DOE/ER/41020-42, 1998. “Low-background 3 He 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 6 LiF-ZnS detectors 6 Li + n = 4 He + 3 H + 4.8MeV 6 LiF powder mixed with ZnS(Ag) scintillator plus binder Spread into thin (100 m) layers Read out to photomultipliers using wavelength shifting lightguides Insensitive to gammas
High efficiency Eight detectors in cylindrical geometry gave 37% for 252 Cf 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 A rediscovered thermal neutron scintillator, 6 Li-salicylate “A new liquid scintillator for thermal neutron detection”, Ross, H.H. and Yerick, R.E., Nucl Sci Eng, 20, p23-27, 1964. “6 Li-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 Polycrystalline 6 Li-salicylate scintillator Efficiency 10-15% of anthracene for alphas Polycrystalline layers have optimal thickness at 250 m – thicker layers absorb light Such layers (95% 6 Li) have 5% total efficiency for detecting thermal neutrons max = 421nm Low atomic number Z eff = 6.8 so insensitive to gammas Not hygroscopic
John McMillan IDM2004 Synthesis of 6 Li-salicylate Add enriched lithium metal to distilled water to form hydroxide. Add Salicylic Acid to solution Dehydrate and crystallise 6 Li metal chunks (95% 6 Li) 10g £210 (Aldrich)
John McMillan IDM2004 Purification 6 Li metal probably very pure – lithium has been isotopically separated (mass spectrometer beam) Salicylic Acid manufactured using Phenol, NaOH, CO 2, H 2 SO 4 Large scale production for pharmaceuticals – Aspirin Can be further purified by sublimation at 76 o C
John McMillan IDM2004 Production of layers of 6 Li-salicylate Deposition from solution in alcohol. Layers need overcoating in transparent material to prevent radon progeny emitting alphas into the scintillator. Epoxy or silicone. 6 Li-salicylate could be mixed with transparent epoxy or silicone binder. The mixture could be spread to the desired thickness then overcoated.
John McMillan IDM2004 Can we build large, high efficiency, low background, low cost thermal neutron detectors using polycrystalline lithium salicylate with wavelength shifted readout?