Measurement of low level neutron fluxes: status and prospects

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…


Neutron capture reactions


1keV


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)


Neutral Current Detectors at SNO

3He proportional counters

- But with a radiopurity requirement <12 parts per 1012 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 3He + CH4 (15%) at 2.5 atmospheres

Alpha activity of first detectors (1998)

48 counts/m2/day


“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.


Disadvantage? Cost!


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





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


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.


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


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)


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


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.


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

Documents

Measurement of low level neutron fluxes: status and prospects