Current Projects

Geological survey

Rock compositions at Aberdeen Tunnel Laboratory were analysed with XRF and ICP-MS techniques. Granite rock type is expected to contain high levels of U-238 and Th-232 series and K-40

Overburden distribution Spectrum of radioisotopes

Overburden distribution was simulated using contour maps and mountain profiles. Comparisons show similar rock properties and overburden between Daya Bay and Aberdeen sites.

Background measurements

Gamma spectra (up to 10 MeV) inside cross tunnel and laboratory were obtained following 20-hour measurements using a HPGe gamma spectrometer. Activities of radioisotopes were determined using characteristic energies of respective radiations. K-40 was found to be the major γ source.

Measured gamma count rate Calculated isotope activities

Sea-level muon flux was measured using a portable muon telescope which consists of plastic scintillators connected to photomultiplier tubes with lead shielding.

Sea level muon flux Muon flux at different threshold

Fluxes were studied at different angles and signal discrimination thresholds. Angular distribution shows good agreement with earlier Monte Carlo results.

Neutron flux along the Aberdeen Tunnel was measured by a portable He-3 probe with polyethylene moderator. Simulation using Geant4 is being done to study the muon-induced and alpha-particle induced neutron background.

Simulation of gamma background

Ambient gamma background due to surrounding rocks was simulated using GLG4sim event generator in Geant4. Three sources, U-238, Th-232 and K-40 were included independently at measured concentrations. The radioisotopes were assumed to be homogenously distributed in the rocks and the emissions were isotropic.

Gamma absorption by rocks Gamma energy spectrum

The gamma photons were generated from a 50cm thick layer of rock and the energy spectrum, zenith and azimuthal distributions of gamma that came out of the rock surface were calculated for each radioisotope based on 10 million events. Results were compared to the measured gamma spectrum and will be used as input to other Geant4 simulations.

Zenith distribution of gamma Azimuthal distribution of gamma

Above 4 plots correspond to K-40.

LED calibration system of liquid scintillator

Diffuser balls made of epoxy and zinc oxide nano-particles are being tested at different concentrations of ZnO.

Diffuser ball, 0.05 g/ml ZnO Diffuser ball, 0.10 g/ml ZnO

They act as isotropic light sources when connected to LEDs. Photomultiplier tubes in the neutron detector will be calibrated using controlled light pulses of width < 10ns which emulate annihilation photons from inverse β decay and γ bursts from the subsequent neutron capture.

Measurements of radon emanation from Daya Bay near-site rocks

Radon concentrations inside Daya Bay near-site borehole were measured at depths 10m to 26m using a portable radon dosimeter.

Radon dosimeter Radon emanation rates at different borehole depths

Borehole measurements were subjected to back diffusion of radon. This effect was estimated in the laboratory by analyzing the emanation of radon from bricks under the same area-to-volume ratio as that in the borehole. Average emanation rate in air was found to be 0.58 x 10-3 Bq m-2 s-1

Setup for radon emanation measurement Radon emanation rate

Radon emanation from rock to water was measured by placing the borehole rock samples in distilled water and subsequently measuring the radon concentration in the water. The emanation rate in water was found to be 5.0 x 10-5 Bq m-2 s-1

Simulations on neutron detector design

Quantum efficiency and photo-coverage of the neutron detector are being studied at different configurations using Geant4 simulations. Variables include geometry and size of acrylic tank and steel tank, employment of aluminium reflectors, arrangement of photomultiplier tubes, thickness and orientation of lead shielding, PMT efficiency and concentration of Gadolinium in liquid scintillator.

PMT configuration PMT configuration, side view PMT configuration, top view

Distribution of neutron capture time at Gadolinium concentrations 0.1% to 1% and neutron energy 10 MeV is also being studied using Geant4 simulations.

Neutron capture time, 0.1% gadolinium Neutron capture time, 0.2% gadolinium Neutron capture time, 0.4% gadolinium