Measurement of low level neutron fluxes: status and prospects
John McMillan
University of Sheffield
John McMillan IDM2004
Applications
Neutral current detectors for neutrino oscillation studies
Dark matter (etc…) background studies
Fission studies
Supernova observatories
Cold fusion…
John McMillan IDM2004
Neutron capture reactions
John McMillan IDM2004
1keV
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)
John McMillan IDM2004
Neutral Current Detectors at SNO
3He proportional counters
- But with a radiopurity requirement <12 parts per 1012 by weight Thorium!!
John McMillan IDM2004
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
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.
John McMillan IDM2004
Disadvantage? Cost!
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
John McMillan IDM2004
John McMillan IDM2004
John McMillan IDM2004
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
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.
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
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)
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
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.
John McMillan IDM2004
Can we build large, high efficiency, low background, low cost thermal neutron detectors using polycrystalline lithium salicylate with wavelength shifted readout?