Browsing by Author "Pinto da Cunha, J."
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- Position Reconstruction in a Dual Phase Xenon Scintillation DetectorPublication . Solovov, V.N. et al. (36 authors); Chepel, V.; DeViveiros, L.; Lindote, A.; Lopes, M.I.; Neves, F.; Pinto da Cunha, J.; Silva, C.We studied the application of statistical reconstruction algorithms, namely maximum likelihood and least squares methods, to the problem of event reconstruction in a dual phase liquid xenon detector. An iterative method was developed for in-situ reconstruction of the PMT light response functions from calibration data taken with an uncollimated gamma-ray source. Using the techniques described, the performance of the ZEPLIN-III dark matter detector was studied for 122 keV gamma-rays. For the inner part of the detector (R<100 mm), spatial resolutions of 13 mm and 1.6 mm FWHM were measured in the horizontal plane for primary and secondary scintillation, respectively. An energy resolution of 8.1% FWHM was achieved at that energy. The possibility of using this technique for improving performance and reducing cost of scintillation cameras for medical applications is currently under study.
- Radioactivity Backgrounds in ZEPLIN-IIIPublication . Araujo, H.M. et al. (37 authors); Chepel, V.; DeViveiros, L.; Lindote, A.; Lopes, M.I.; Neves, F.; Pinto da Cunha, J.; Silva, C.; Solovov, V.N.We examine electron and nuclear recoil backgrounds from radioactivity in the ZEPLIN-III dark matter experiment at Boulby. The rate of low-energy electron recoils in the liquid xenon WIMP target is 0.75$\pm$0.05 events/kg/day/keV, which represents a 20-fold improvement over the rate observed during the first science run. Energy and spatial distributions agree with those predicted by component-level Monte Carlo simulations propagating the effects of the radiological contamination measured for materials employed in the experiment. Neutron elastic scattering is predicted to yield 3.05$\pm$0.5 nuclear recoils with energy 5-50 keV per year, which translates to an expectation of 0.4 events in a 1-year dataset in anti-coincidence with the veto detector for realistic signal acceptance. Less obvious background sources are discussed, especially in the context of future experiments. These include contamination of scintillation pulses with Cherenkov light from Compton electrons and from $\beta$ activity internal to photomultipliers, which can increase the size and lower the apparent time constant of the scintillation response. Another challenge is posed by multiple-scatter $\gamma$-rays with one or more vertices in regions that yield no ionisation. If the discrimination power achieved in the first run can be replicated, ZEPLIN-III should reach a sensitivity of $\sim 1 \times 10^{-8}$ pb$\cdot$year to the scalar WIMP-nucleon elastic cross-section, as originally conceived.
- WIMP-nucleon cross-section results from the second science run of ZEPLIN-IIIPublication . Akimov, D.Yu. et al. (37 authors); Chepel, V.; DeViveiros, L.; Lindote, A.; Lopes, M.I.; Neves, F.; Pinto da Cunha, J.; Silva, C.; Solovov, V.N.We report experimental upper limits on WIMP-nucleon elastic scattering cross sections from the second science run of ZEPLIN-III at the Boulby Underground Laboratory. A raw fiducial exposure of 1,344 kg.days was accrued over 319 days of continuous operation between June 2010 and May 2011. A total of eight events was observed in the signal acceptance region in the nuclear recoil energy range 7-29 keV, which is compatible with background expectations. This allows the exclusion of the scalar cross-section above 4.8E-8 pb near 50 GeV/c^2 WIMP mass with 90% confidence. Combined with data from the first run, this result improves to 3.9E-8 pb. The corresponding WIMP-neutron spin-dependent cross-section limit is 8.0E-3 pb. The ZEPLIN programme reaches thus its conclusion at Boulby, having deployed and exploited successfully three liquid xenon experiments of increasing reach.