Measurements of the neutron background

from rock at Boulby mine

Eirini TziaferiUniversity of Sheffield, UK

JRA1 meeting, ILIAS , Paris, 14/02/2006

nthermal + 155,157Gd → 156,158Gd* → 156,158Gd + γ´s

Detection Principle: 2 pulse signature

~ 0.01% Gd-loaded liquid scintillator

• Large thermal neutron capture cross section of the isotopes 155Gd, 157Gd

• Release of a high energy (8 MeV) gamma cascade after thermal neutron capture on Gd

LS+0.01%Gd

Circuit for neutron measurement(2 pulses with delayed coincidences)

Energy calibration gamma sources (60Co, 57Co, 137Cs)

Red: dataBlack: simulation

Neutron Calibration - 252Cf

Energy SpectraData vs. Simulation

data

sim data

sim

Neutron Calibration – 252CfTime delay distribution

τ = 68 ± 6 μsec

60Co with coincidences

simdata

Sources of Background Afterpulses from PMTs

Sources of Background Correlated background: 238U (214Bi → 214Po → 214Pb)

214Po:t1/2 = 164 μs

τ = 237 μs

0.8 MeV(7.68 MeV)

Pulse shapeanalysis

Time delay distribution of the neutron background

no. of neutrons = 124 ± 50 rate = 1.01 ± 0.41 d-1

February – June 2005 July – October 2005shielded run

SimulationsFirst step:

Input → 238U, 232Th spectra from SOURCES

Output → Energy spectrum, position, momentum

of neutrons which pass the rock/lab boundary

Second step:Input → Energy spectrum, position, momentum of neutrons which pass

the rock/lab boundaryOutput → Proton recoil energy,

Captured energy, time delay distribution

In progress …

10 ppb U, 10 ppb Th

60ppb U, 130ppb Th