Measurements of the neutron background from rock at Boulby mine Eirini Tziaferi University of Sheffield, UK JRA1 meeting, ILIAS, Paris, 14/02/2006

n thermal + 155,157 Gd → 156,158 Gd* → 156,158 Gd + γ´s Detection Principle: 2 pulse signature ~ 0.01% Gd-loaded liquid scintillator Large thermal neutron capture cross section of the isotopes 155 Gd, 157 Gd 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 ( 60 Co, 57 Co, 137 Cs) Red: data Black: simulation

Neutron Calibration Cf Energy Spectra Data vs. Simulation data sim data sim

Neutron Calibration – 252 Cf Time delay distribution τ = 68 ± 6 μsec 60 Co with coincidences sim data

Sources of Background Afterpulses from PMTs

Sources of Background Correlated background: 238 U ( 214 Bi → 214 Po → 214 Pb) 214 Po: t 1/2 = 164 μs τ = 237 μs 0.8 MeV (7.68 MeV) Pulse shape analysis

Time delay distribution of the neutron background no. of neutrons = 124 ± 50  rate = 1.01 ± 0.41 d -1 February – June 2005 July – October 2005 shielded run

Simulations First step: Input → 238 U, 232 Th 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 boundary Output → Proton recoil energy, Captured energy, time delay distribution In progress … 10 ppb U, 10 ppb Th 60ppb U, 130ppb Th