arXiv subjects

R. Preece

Publications and source records attributed to R. Preece.

At least 37 records · Page 2Linked to original sources

arXiv2011

Single electron emission in two-phase xenon with application to the detection of coherent neutrino-nucleus scattering

We present an experimental study of single electron emission in ZEPLIN-III, a two-phase xenon experiment built to search for dark matter WIMPs, and discuss applications enabled by the excellent signal-to-noise ratio achieved in detecting this signature. Firstly, we demonstrate a practical method for precise measurement of the free electron lifetime in liquid xenon during normal operation of these detectors. Then, using a realistic detector response model and backgrounds, we assess the feasibility of deploying such an instrument for measuring coherent neutrino-nucleus elastic scattering using the ionisation channel in the few-electron regime. We conclude that it should be possible to measure this elusive neutrino signature above an ionisation threshold of $\sim$3 electrons both at a stopped pion source and at a nuclear reactor. Detectable signal rates are larger in the reactor case, but the triggered measurement and harder recoil energy spectrum afforded by the accelerator source enable lower overall background and fiducialisation of the active volume.

physics.ins-det↗

arXiv2011

Radioactivity Backgrounds in ZEPLIN-III

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 $β$ 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 $γ$-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.

physics.ins-det↗

arXiv2011

ZE3RA: The ZEPLIN-III Reduction and Analysis Package

ZE3RA is the software package responsible for processing the raw data from the ZEPLIN-III dark matter experiment and its reduction into a set of parameters used in all subsequent analyses. The detector is a liquid xenon time projection chamber with scintillation and electroluminescence signals read out by an array of 31 photomultipliers. The dual range 62-channel data stream is optimised for the detection of scintillation pulses down to a single photoelectron and of ionisation signals as small as those produced by single electrons. We discuss in particular several strategies related to data filtering, pulse finding and pulse clustering which are tuned to recover the best electron/nuclear recoil discrimination near the detection threshold, where most dark matter elastic scattering signatures are expected. The software was designed assuming only minimal knowledge of the physics underlying the detection principle, allowing an unbiased analysis of the experimental results and easy extension to other detectors with similar requirements.

astro-ph.IM↗

arXiv2011

Performance of the veto detector incorporated into the ZEPLIN-III experiment

The ZEPLIN-III experiment is operating in its second phase at the Boulby Underground Laboratory in search of dark matter WIMPs. The major upgrades to the instrument over its first science run include lower background photomultiplier tubes and installation of a plastic scintillator veto system. Performance results from the veto detector using calibration and science data in its first six months of operation in coincidence with ZEPLIN-III are presented. With fully automated operation and calibration, the veto system has maintained high stability and achieves near unity live time relative to ZEPLIN-III. Calibrations with a neutron source demonstrate a rejection of 60% of neutron-induced nuclear recoils in ZEPLIN-III that might otherwise be misidentified as WIMPs. This tagging efficiency reduces the expected untagged nuclear recoil background from neutrons during science data taking to a very low rate of ~0.2 events per year in the WIMP acceptance region. Additionally, the veto detector provides rejection of 28% of gamma-ray induced background events, allowing the sampling of the dominant source of background in ZEPLIN-III - multiple scatter gamma-rays with rare topologies. Since WIMPs will not be tagged by the veto detector, and tags due to gamma-rays and neutrons are separable, this population of multiple scatter events may be characterised without biasing the analysis of candidate WIMP signals in the data.

astro-ph.CO↗

arXiv2010

The ZEPLIN-III Anti-Coincidence Veto Detector

The design, optimisation and construction of an anti-coincidence veto detector to complement the ZEPLIN-III direct dark matter search instrument is described. One tonne of plastic scintillator is arranged into 52 bars individually read out by photomultipliers and coupled to a gadolinium-loaded passive polypropylene shield. Particular attention has been paid to radiological content. The overall aim has been to achieve a veto detector of low threshold and high efficiency without the creation of additional background in ZEPLIN-III, all at a reasonable cost. Extensive experimental measurements of the components have been made, including radioactivity levels and performance characteristics. These have been used to inform a complete end-to-end Monte Carlo simulation that has then been used to calculate the expected performance of the new instrument, both operating alone and as an anti-coincidence detector for ZEPLIN-III. The veto device will be capable of rejecting over 65% of coincident nuclear recoil events from neutron background in the energy range of interest in ZEPLIN-III. This will reduce the background in ZEPLIN-III from ~0.4 to ~0.14 events per year in the WIMP acceptance region, a significant factor in the event of a non-zero observation. Furthermore, in addition to providing valuable diagnostic capabilities, the veto is capable of tagging over 15% for gamma-ray rejection, all whilst contributing no significant additional background. In conjunction with the replacement of the internal ZEPLIN-III photomultiplier array, the new veto is expected to improve significantly the sensitivity of the ZEPLIN-III instrument to dark matter, allowing spin independent WIMP-nucleon cross sections below 1E-8 pb to be probed.

hep-ex↗

arXiv2010

Limits on inelastic dark matter from ZEPLIN-III

We present limits on the WIMP-nucleon cross section for inelastic dark matter derived from the 2008 run of ZEPLIN-III. Cuts, notably on scintillation pulse shape and scintillation-to-ionisation ratio, give a net exposure of 63 kg.days in the range 20-80keV nuclear recoil energy, in which 6 events are observed. Upper limits on signal rate are derived from the maximum empty patch in the data. Under standard halo assumptions a small region of parameter space consistent, at 99% CL, with causing the 1.17 ton.year DAMA modulation signal is allowed at 90% CL: it is in the mass range 45-60 GeV with a minimum CL of 88%, again derived from the maximum patch. This is the tightest constraint on that explanation of the DAMA result yet presented using a xenon target.

hep-ex↗

arXiv2010

The LAT Low-Energy technique for Fermi Gamma-Ray Bursts spectral analysis

Fermi Large Area Telescope (LAT) data analyses based on event reconstruction and classification are so far restricted to events of measured energy larger than 100 MeV. We present a new technique to recover the signal from Gamma-Ray Bursts' (GRB) prompt emission between ~30 MeV and 100 MeV, which differs from the standard LAT analysis. Filling the gap between the energy ranges where the Gamma-ray Burst Monitor (GBM) and LAT operate is important to better constrain the high-energy spectra of GRBs. The LAT Low-Energy (LLE) technique is described, first performance studies are presented, as well as preliminary spectral re-analyses of two Fermi GRBs.

astro-ph.HE↗

arXiv2009

Identification and properties of the photospheric emission in GRB090902B

The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B, lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover the spectral range from 8 keV to >300 GeV is covered. Here we show that the prompt burst spectrum is consistent with emission from the jet photosphere combined with non-thermal emission described by a single power-law with photon index -1.9. The photosphere gives rise to a strong quasi-blackbody spectrum which is somewhat broader than a single Planck function and has a characteristic temperature of ~290keV. We derive the photospheric radius Rph = (1.1 \pm 0.3) x 10^12 Y^{1/4} cm and the bulk Lorentz factor of the flow, which is found to vary by a factor of two and has a maximal value of Gamma = 750 Y^{1/4}. Here Y is the ratio between the total fireball energy and the energy emitted in the gamma-rays. We find that during the first quarter of the prompt phase the photospheric emission dominates, which explains the delayed onset of the observed flux in the LAT compared to the GBM. We model the photospheric emission with a multi-color blackbody and its shape indicates that the photospheric radius increases at higher latitudes. We interpret the broad band emission as synchrotron emission at R ~ 4x10^15 cm. Our analysis emphasize the importance of having high temporal resolution when performing spectral analysis on GRBs, since there is strong spectral evolution.

astro-ph.HE↗

arXiv2009

Results from the First Science Run of the ZEPLIN-III Dark Matter Search Experiment

The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses a 12kg two-phase xenon time projection chamber to search for the weakly interacting massive particles (WIMPs) that may account for the dark matter of our Galaxy. The detector measures both scintillation and ionisation produced by radiation interacting in the liquid to differentiate between the nuclear recoils expected from WIMPs and the electron recoil background signals down to ~10keV nuclear recoil energy. An analysis of 847kg.days of data acquired between February 27th 2008 and May 20th 2008 has excluded a WIMP-nucleon elastic scattering spin-independent cross-section above 8.1x10(-8)pb at 55GeV/c2 with a 90% confidence limit. It has also demonstrated that the two-phase xenon technique is capable of better discrimination between electron and nuclear recoils at low-energy than previously achieved by other xenon-based experiments.

astro-ph↗

arXiv2009

Multi-wavelength observations of the energetic GRB 080810: detailed mapping of the broadband spectral evolution

GRB 080810 was one of the first bursts to trigger both Swift and the Fermi Gamma-ray Space Telescope. It was subsequently monitored over the X-ray and UV/optical bands by Swift, in the optical by ROTSE and a host of other telescopes and was detected in the radio by the VLA. The redshift of z= 3.355 +/- 0.005 was determined by Keck/HIRES and confirmed by RTT150 and NOT. The prompt gamma/X-ray emission, detected over 0.3-10^3 keV, systematically softens over time, with E_peak moving from ~600 keV at the start to ~40 keV around 100 s after the trigger; alternatively, this spectral evolution could be identified with the blackbody temperature of a quasithermal model shifting from ~60 keV to ~3 keV over the same time interval. The first optical detection was made at 38 s, but the smooth, featureless profile of the full optical coverage implies that this originated from the afterglow component, not the pulsed/flaring prompt emission. Broadband optical and X-ray coverage of the afterglow at the start of the final X-ray decay (~8 ks) reveals a spectral break between the optical and X-ray bands in the range 10^15 - 2x10^16 Hz. The decay profiles of the X-ray and optical bands show that this break initially migrates blueward to this frequency and then subsequently drifts redward to below the optical band by ~3x10^5 s. GRB 080810 was very energetic, with an isotropic energy output for the prompt component of 3x10^53 erg and 1.6x10^52 erg for the afterglow; there is no evidence for a jet break in the afterglow up to six days following the burst.

astro-ph.HE↗

arXiv2009

Prospects for GRB Science with the Fermi Large Area Telescope

The LAT instrument on the Fermi mission will reveal the rich spectral and temporal gamma-ray burst phenomena in the > 100 MeV band. The synergy with Fermi's GBM detectors will link these observations to those in the well explored 10-1000 keV range; the addition of the > 100 MeV band observations will resolve theoretical uncertainties about burst emission in both the prompt and afterglow phases. Trigger algorithms will be applied to the LAT data both onboard the spacecraft and on the ground. The sensitivity of these triggers will differ because of the available computing resources onboard and on the ground. Here we present the LAT's burst detection methodologies and the instrument's GRB capabilities.

astro-ph.HE↗

arXiv2009

Calibration of Photomultiplier Arrays

A method is described that allows calibration and assessment of the linearity of response of an array of photomultiplier tubes. The method does not require knowledge of the photomultiplier single photoelectron response model and uses science data directly, thus eliminating the need for dedicated data sets. In this manner all photomultiplier working conditions (e.g. temperature, external fields, etc) are exactly matched between calibration and science acquisitions. This is of particular importance in low background experiments such as ZEPLIN-III, where methods involving the use of external light sources for calibration are severely constrained.

physics.ins-det↗

arXiv2009

Limits on the spin-dependent WIMP-nucleon cross-sections from the first science run of the ZEPLIN-III experiment

We present new experimental constraints on the WIMP-nucleon spin-dependent elastic cross-sections using data from the first science run of ZEPLIN-III, a two-phase xenon experiment searching for galactic dark matter WIMPs based at the Boulby mine. Analysis of $\sim$450 kg$\cdot$days fiducial exposure revealed a most likely signal of zero events, leading to a 90%-confidence upper limit on the pure WIMP-neutron cross-section of $σ_n=1.8\times 10^{-2}$ pb at 55 GeV/$c^2$ WIMP mass. Recent calculations of the nuclear spin structure based on the Bonn CD nucleon-nucleon potential were used for the odd-neutron isotopes $^{129}$Xe and $^{131}$Xe. These indicate that the sensitivity of xenon targets to the spin-dependent WIMP-proton interaction is much lower than implied by previous calculations, whereas the WIMP-neutron sensitivity is impaired only by a factor of $\sim$2.

hep-ex↗

arXiv2008

Radiation from relativistic jets

Nonthermal radiation observed from astrophysical systems containing relativistic jets and shocks, e.g., gamma-ray bursts (GRBs), active galactic nuclei (AGNs), and Galactic microquasar systems usually have power-law emission spectra. Recent PIC simulations of relativistic electron-ion (electron-positron) jets injected into a stationary medium show that particle acceleration occurs within the downstream jet. In the presence of relativistic jets, instabilities such as the Buneman instability, other two-streaming instability, and the Weibel (filamentation) instability create collisionless shocks, which are responsible for particle (electron, positron, and ion) acceleration. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the jet head. The ``jitter'' radiation from deflected electrons in small-scale magnetic fields has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation, a case of diffusive synchrotron radiation, may be important to understand the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants.

astro-ph↗

arXiv2008

Broadband Spectral Properties of Bright High-Energy Gamma-Ray Bursts Observed with BATSE and EGRET

We present the spectral analysis of duration-integrated broadband spectra (in $\sim30 $keV$-200 $MeV) of 15 bright BATSE gamma-ray bursts (GRBs). Some GRB spectra are very hard, with their spectral peak energies being above the BATSE LAD passband limit of $\sim$2 MeV. In such cases, their high-energy spectral parameters (peak energy and high-energy power-law indices) cannot be adequately constrained by BATSE LAD data alone. A few dozen bright BATSE GRBs were also observed with EGRET's calorimeter, TASC, in multi-MeV energy band, with a large effective area and fine energy resolution. Combining the BATSE and TASC data, therefore, affords spectra that span four decades of energy ($30 $keV$-200 $MeV), allowing for a broadband spectral analysis with good statistics. Studying such broadband high-energy spectra of GRB prompt emission is crucial, as they provide key clues to understanding its gamma-ray emission mechanism. Among the 15 GRB spectra, we found two cases with a significant high-energy excess, and another case with a extremely high peak energy (\epeak $\gtrsim$ 170 MeV). There have been very limited number of GRBs observed at MeV energies and above, and only a few instruments have been capable of observing GRBs in this energy band with such high sensitivity. Thus, our analysis results presented here should also help predict GRB observations with current and future high-energy instruments such as AGILE and GLAST, as well as with ground-based very-high-energy telescopes.

astro-ph↗

arXiv2006

Observations of the intense and ultra-long burst GRB041219a with the Germanium Spectrometer on INTEGRAL

GRB041219a is the brightest burst localised by INTEGRAL. The intense burst occurred about ~250s after the precursor and the long delay enabled optical and near infrared telescopes to observe the prompt emission. We present comprehensive results of the temporal and spectral analyses, including line and afterglow searches using the spectrometer, SPI, aboard INTEGRAL, BAT on Swift and ASM on RXTE. We avail of multi-wavelength data to generate broadband spectra of GRB041219a and afterglow. Spectra for the burst and sub-intervals were fit by the Band model and also by the quasithermal model. The high resolution Germanium spectrometer data were searched for emission and absorption features and for gamma-ray afterglow. The overall burst and sub-intervals are well fit by the Band model. The photon index below the break energy shows a marked change after the quiescent time interval. In addition the spectra are well described by a black body component with a power law. The burst was detected by BAT and ASM during the long quiescent interval in SPI indicating the central engine might not be dormant but that the emission occurs in different bands. No significant emission or absorption features were found and limits of 900 eV and 120 eV are set on the most significant features. No gamma-ray afterglow was detected from the end of the prompt phase to ~12 hours post-burst. Broadband spectra of the prompt emission were generated in 7 time intervals using gamma-ray, x-ray, optical and near-infrared data and these were compared to the high-redshift burst GRB050904. The optical and gamma-ray emission are correlated in GRB041219a. The spectral lag was determined using data from the BAT and it changes throughout the burst. A number of pseudo-redshifts were evaluated and large dispersion in values was found.

astro-ph↗

arXiv2006

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

We present details of the technical design and manufacture 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.

astro-ph↗

arXiv2006

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.

astro-ph↗