P.F. Smith

Review of mathematics, numerical factors, and corrections for dark matter experiments based on elastic nuclear recoil

Abstract We present a systematic derivation and discussion of the practical formulae needed to design and interpret direct searches for nuclear recoil events caused by hypothetical weakly interacting dark matter particles. Modifications to the differential energy spectrum arise from the Earths motion, recoil detection efficiency, instrumental resolution and threshold, multiple target elements, spin-dependent and coherent factors, and nuclear form factor. We discuss the normalization and presentation of results to allow comparison between different target elements and with theoretical predictions. Equations relating to future directional detectors are also included.

Limits on WIMP cross-sections from the NAIAD experiment at the Boulby Underground Laboratory

The NAIAD experiment (NaI Advanced Detector) for WIMP dark matter searches at the Boulby Underground Laboratory (North Yorkshire, UK) ran from 2000 until 2003. A total of 44.9 kg x years of data collected with 2 encapsulated and 4 unencapsulated NaI(Tl) crystals with high light yield were included in the analysis. We present final results of this analysis carried out using pulse shape discrimination. No signal associated with nuclear recoils from WIMP interactions was observed in any run with any crystal. This allowed us to set upper limits on the WIMP-nucleon spin-independent and WIMP-proton spin-dependent cross-sections. The NAIAD experiment has so far imposed the most stringent constraints on the spin-dependent WIMP-proton cross-section.

NaI dark matter limits and the NAIAD array – a detector with improved sensitivity to WIMPs using unencapsulated NaI

Re-analysis of published data from the UKDMC NaI Tl dark matter experiment is presented using latest spin factors and comparison is made with the sensitivity predicted for NAIAD, a 100 kg NaI detector concept based on unencapsulated . NaI Tl . We present experimental results and Monte Carlo simulations for NAIAD and show that a factor of 1.5-2 improvement in energy threshold is achievable over conventional NaI dark matter detectors with consequent ; 50% improvement in nuclear recoil discrimination at 10 keV. An overall improvement in sensitivity to spin dependent WIMP interactions of factor 50, based on 100 kg = yrs of data, is predicted relative to previous UKDMC limits. q 2000 Published by Elsevier Science B.V. All rights reserved.

The scintillation efficiency of sodium and iodine recoils in a NaI(Tl) detector for dark matter searches

Abstract Searches for weakly interacting massive particles that may constitute the Galactic dark matter can be based on the detection of nuclear recoil events in NaI(Tl) scintillation detectors. For this purpose it is necessary to know the relative scintillation efficiency for nuclear recoil events. Presented here are the results of measurements of the efficiency for conversion of low energy I and Na nuclear recoil events into scintillation light in NaI(Tl). The experiments were performed using elastic scattering of monoenergetic neutrons of energy 3.2–5.5 MeV. The relative scintillation efficiency was found to be about 30% for Na recoils, down to 15 keV, and 8% for I recoils, down to 27 keV.

The scintillation efficiency for calcium and fluorine recoils in CaF2 and carbon and fluorine recoils in C6F6 for dark matter searches

Abstract Results are presented of neutron scattering experiments to measure the relative scintillation effeciency of low energy calcium and fluorine recoils in a CaF 2 (Eu) scintillator, and carbon and fluorine recoils in a C 6 F 6 scintillator. These measurements are needed to calibrate these fluorine-containing scintillators for uses as possible detectors of weakly interacting massive particles (WIMPs) that may constitute the dark matter of the Universe. The relative scintillation efficiency of CaF 2 (0.5% Eu) below 100 keV was found to be about 12% for fluorine recoils and 8% for calcium recoils. In C 6 F 6 quenching of the scintillation was found to be very severe, limiting the efficiency relative to electrons for carbon recoils to 0.7%, with a limit for fluorine recoils of

Nuclear recoil scintillation and ionisation yields in liquid xenon from ZEPLIN-III data

Scintillation and ionisation yields for nuclear recoils in liquid xenon above 10 keVnr (nuclear recoil energy) are deduced from data acquired using broadband Am–Be neutron sources. The nuclear recoil data from several exposures to two sources were compared to detailed simulations. Energy-dependent scintillation and ionisation yields giving acceptable fits to the data were derived. Efficiency and resolution effects are treated using a light collection Monte Carlo, measured photomultiplier response profiles and hardware trigger studies. A gradual fall in scintillation yield below ∼40 keVnr is found, together with a rising ionisation yield; both are in agreement with the latest independent measurements. The analysis method is applied to the most recent ZEPLIN-III data, acquired with a significantly upgraded detector and a precision-calibrated Am–Be source, as well as to the earlier data from the first run in 2008. A new method for deriving the recoil scintillation yield, which includes sub-threshold S1 events, is also presented which confirms the main analysis.

OMNIS: An improved low cost detector to measure mass and mixing of mu tau neutrinos from a galactic supernova

Abstract OMNIS is an observatory for multiflavour interactions from Galactic supernovae. It has evolved from the Supernova Neutrino Burst Observatory (SNBO) proposed by Cline et al., based on the detection of the neutrinos by neutral current nuclear excitation in natural rock. This would result in the emission of neutrons, which could be captured by counters embedded in the rock. This scheme would be sensitive principally to the higher temperature μ and τ neutrinos and would allow time-of-flight measurement of neutrino mass if in the cosmologically interesting range 10–100 eV. The present paper proposes a new neutron detection arrangement which improves detection efficiency and reduces cost by a factor ∼ 30, allowing the time profile of over 2000 ν μτ events to be recorded with only 200 tons scintillator. This new scheme represents the optimum configuration for a natural underground rock target. A Galactic supernova signal would be a factor 10–30 above neutron background (from cosmic ray muons and alpha activity) in sites with depth > 500 mwe. Another major improvement results by supplementing the rock with more favourable target elements such as iron or lead, giving sensitivity to all three neutrino types and mixing between these. Some calculations are included on the speculative possibility of extra-Galactic supernova neutrino detection using this principle.

Liquid xenon as a dark matter detector. Prospects for nuclear recoil discrimination by photon timing

Abstract Two mechanisms lead to liquid xenon scintillatlon, and are excited differently by incident electrons and by heavy charged nuclei. This could be used to discriminate nuclear recoil events from radioactive background in a search for hypothetical Galactic dark matter particles. To achieve low energy threshold a likelihood analysis is proposed using the time intervals between detected photons. Results are presented of Monte Carlo simulations, showing the background rejection achievable as a function of the number of detected photons. Some comments are included on background discrimination by simultaneous measurement of scintillation and ionization signals.

Demonstration of nuclear recoil discrimination for low temperature dark matter detectors, by measurement of simultaneous ionization and thermal pulses in silicon

Abstract We report experiments using a silicon target exposed to a neutron source, giving both nuclear recoil events from neutron scattering and electron recoil events from photon scattering. We show that simultaneous measurement of ionization and thermal energy for each event allows the neutron and photon events to be separated. The degree of separation of the nuclear recoil events, and their absolute event rate, are consistent with theoretical expectation.

Position reconstruction in a dual phase xenon scintillation detector

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 γ -ray source. Using the techniques described, the performance of the ZEPLIN-III dark matter detector was studied for 122 keV γ-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.