G.J. Homer

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.

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.

Dark matter experiments at the UK Boulby Mine UK Dark Matter Collaboration

Abstract The current status is summarised of dark matter searches at the UK Boulby Mine based on pulse shape discrimination in NaI, together with further plans for international collaboration on detectors based on nuclear recoil discrimination in liquid and gaseous xenon.

Tests on low temperature calorimetric detectors for dark matter experiments

Abstract Tests are reported on low temperature calorimetric particle detectors consisting of Si and LiF crystals of mass 1–32 g fitted with commercial Ge temperature sensors, operating in the temperature range 50–70 mK. The objective was to demonstrate energy thresholds in the 1–10 keV range for dark matter detection. Using the energy spectra from 57 Co and 137 Cs photon sources for calibration, the measured RMS energy sensitivities were 1.5 keV in 1 g Si, 2.5 keV in 4 g Si, 4 keV in 8 g LiF, and 12 keV in 32 g LiF. Spectra were also observed for the photon and cosmic ray muon background, and for thermal neutron reactions in LiF. In all cases, observed energy spectra and absolute event rates were in agreement with Monte Carlo estimates, showing that the whole crystal volume was uniformly sensitive. Energy calibration and operating conditions remained stable with time and thermal cycling.

Results from the first stage of a UK Galactic dark matter search using low background sodium iodide detectors

Abstract Low-energy background spectra from 1.3 kg and 6.2 kg NaI(TI) crystal scintillators operating in the shielded Boulby underground facility were measured. Upper limits to the scattering interaction rates and cross-sections of Galactic dark matter in the form of Weakly Interacting Massive Particles (WIMPs) are calculated from these data. This work provides an improved limit for spin-dependent interactions for WIMP masses above 10 GeV.

A search for fractional electric charge on levitated niobium spheres

Abstract A new magnetic levitation system has been used to investigate the residual electric charge on iron-coated 0.28 mm niobium spheres. A total of 64 measurements on 46 different samples show no evidence for the apparent ±e 3 values observed in the Standard experiments using samples of similar size and material, but differing in pre-test treatment and environment. Further experiments to investigate the possible effect of the latter are in progress.

Observation of nuclear recoil events from neutron scattering in low temperature calorimetric detectors

Abstract Energy spectra are obtained for low temperature calorimetric detectors exposed to a neutron source to produce nuclear recoil events simulating those which would occur in a “dark matter” detector. The targets used were a 4 g Si crystal and a 32 g LiF crystal, fitted with Ge temperature sensors to observe the particle scattering events as thermal pulses at a temperature ∼ 50 mK. Monte Carlo calculations were used to predict the spectra expected from both the neutrons and photons emitted by the Am/Be neutron source. Comparison with observed spectra shows that the nuclear recoil events are being registered by the detectors with the correct rate and energy spectrum.

New limits to Galactic cold dark matter interaction rates

Abstract Analysis is reported of a 6 month run with a 6.2 kg NaI detector deployed in a well shielded underground laboratory to search for signatures of cold dark matter interactions. By using pulse shape information to reject most background events, a significant improvement is obtained on existing upper limits to the cold dark matter spin-dependent interaction rates.

Searches for fractional electric charge in meteorite samples

The magnetic levitation technique has been used to test for fractional electric charge on samples of several types of meteorite material, thus providing the first mass-independent search for free quarks or other fractionally-charged particles in natural nonterrestrial materials. The tests were made on diced samples of the Hoba and Forsyth County iron-nickel meteorites, and the Murchison stony meteorite, the latter being plated with iron to allow magnetic levitation. No evidence for fractional charge was found, the quantities tested being 1.3 mg (Hoba), 1.1 mg (Forsyth), and 0.4 mg (Murchison).