J.D. Lewin

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.

Dark Matter Detection

Abstract There is increasing interest in the possibility of setting up terrestrial experiments for the direct detection of galactic dark matter on the hypothesis that this may consist of new types of elementary particle. The past two years have seen an increased level of development work directed towards dark matter detectors for several types of candidate particle. Existing low background detectors have set some limits on certain types of dark matter particle and a number of new experiments are now planned which will be capable of improving these limits. This review discusses the principal particle candidates for the galactic dark matter, and summarizes the various experimental ideas which have been proposed for their detection, in particular electromagnetic detectors for light axions and nuclear recoil detectors for heavy neutral particles. For the latter, a variety of low temperature and ionization techniques are being studied. Progress in the development of each of these is summarized, together with prospects and plans for specific experiments in the near future.

The ZEPLIN-III dark matter detector: Instrument design, manufacture and commissioning

We present details of the technical design, manufacture and testing of the ZEPLIN-III dark matter experiment. ZEPLIN-III is a two-phase xenon detector which measures both the scintillation light and the ionisation charge generated in the liquid by interacting particles and radiation. The instrument design is driven by both the physics requirements and by the technology requirements surrounding the use of liquid xenon. These include considerations of key performance parameters, such as the efficiency of scintillation light collection, restrictions placed on the use of materials to control the inherent radioactivity levels, attainment of high vacuum levels and chemical contamination control. The successful solution has involved a number of novel design and manufacturing features which will be of specific use to future generations of direct dark matter search experiments as they struggle with similar and progressively more demanding requirements.

Measurement of scintillation efficiencies and pulse-shapes for nuclear recoils in NaI(Tl) and CaF2(Eu) at low energies for dark matter experiments

Abstract Measurements have been performed with a 2.85 MeV mono-energetic neutron beam of relative scintillation efficiency and pulse-shape for nuclear and electron recoils in NaI(Tl) and CaF2(Eu). Scintillation efficiencies in NaI(Tl) relative to 60 keV gamma events were found to be 27.5±1.8% for Na recoils (recoil energy Erec>4 keV) and 8.6±0.7% for I recoils (Erec>10 keV). Relative scintillation efficiencies in CaF2(Eu) for Ca and F recoils show some evidence for a fall with energy (17% to 8% for F) for 10 keV 〈 t i 〉 of 263±15 ns for Na events (visible energy Evis in the range 2–8 keV and 272±10 ns for I events (2 keV 〈 t i 〉 of 4 keV 〈 t i 〉 for 2 keV

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

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.

Rates for inelastic nuclear excitation by dark matter particles

We calculate rates for the inelastic scattering of dark matter particles X on nuclei to produce low-lying excited nuclear states. Assuming a maxwellian velocity distribution for the dark matter particles X, the inelastic two-body phase space suppresses all rates by factors > 10 for mx⩽100 GeV unless the excitation energy ΔE<100 keV. We catalogue all stable nuclei with excited states in this range, and we estimate inelastic scattering matrix elements by relating them to M1 transitions, after correcting for internal conversions. Our calculated rates are typically ≲ 10−4 events/kg·day, with the least unfavourable rates being for 16969Tm and 18775Os. Problems of natural radioactivity and expense disfavour these and many other materials, leaving 12753I, 18374W and 20180Hg as the least unpromising isotopes.

The ZEPLIN-III dark matter detector: Performance study using an end-to-end simulation tool

We present results from a GEANT4-based Monte Carlo tool for end-to-end simulations of the ZEPLIN-III dark matter experiment. ZEPLIN-III is a two-phase detector which measures both the scintillation light and the ionisation charge generated in liquid xenon by interacting particles and radiation. The software models the instrument response to radioactive backgrounds and calibration sources, including the generation, ray-tracing and detection of the primary and secondary scintillations in liquid and gaseous xenon, and subsequent processing by data acquisition electronics. A flexible user interface allows easy modification of detector parameters at run time. Realistic datasets can be produced to help with data analysis, an example of which is the position reconstruction algorithm developed from simulated data. We present a range of simulation results confirming the original design sensitivity of a few times 10−8 pb to the WIMP-nucleon cross-section.