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Measurement of low level neutron fluxes: status and prospects
John McMillan
University of Sheffield
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John McMillan IDM2004
Applications
Neutral current detectors for neutrino oscillation studies
Dark matter (etc…) background studies
Fission studies
Supernova observatories
Cold fusion…
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John McMillan IDM2004
Neutron capture reactions
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John McMillan IDM2004
1keV
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John McMillan IDM2004
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)
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John McMillan IDM2004
Neutral Current Detectors at SNO
3He proportional counters
- But with a radiopurity requirement <12 parts per 1012 by weight Thorium!!
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John McMillan IDM2004
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John McMillan IDM2004
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
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John McMillan IDM2004
“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.
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John McMillan IDM2004
Disadvantage? Cost!
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John McMillan IDM2004
6LiF-ZnS detectors
6Li + n = 4He + 3H + 4.8MeV6LiF powder mixed with ZnS(Ag) scintillator
plus binder
Spread into thin (100m) layers
Read out to photomultipliers using wavelength shifting lightguides
Insensitive to gammas
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John McMillan IDM2004
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John McMillan IDM2004
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John McMillan IDM2004
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John McMillan IDM2004
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
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John McMillan IDM2004
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.
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John McMillan IDM2004
Polycrystalline 6Li-salicylate scintillator
Efficiency 10-15% of anthracene for alphasPolycrystalline layers have optimal thickness at
250m – thicker layers absorb lightSuch layers (95% 6Li) have 5% total efficiency
for detecting thermal neutrons
max = 421nm
Low atomic number Zeff= 6.8 so insensitive to gammas
Not hygroscopic
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John McMillan IDM2004
Synthesis of 6Li-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)
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John McMillan IDM2004
Purification
6Li metal probably 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
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John McMillan IDM2004
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.
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John McMillan IDM2004
Can we build large, high efficiency, low background, low cost thermal neutron detectors using polycrystalline lithium salicylate with wavelength shifted readout?