arXiv subjects

J. J. Quenby

Publications and source records attributed to J. J. Quenby.

32 records · Page 2Linked to original sources

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↗

arXiv2007

Particle acceleration in relativistic subluminal shock environments

The understanding of the particle spectra resulting from acceleration in relativistic shocks as they occur in extragalactic sources, is essential for the interpretation of the cosmic ray spectrum above the ankle ($E_p>3\cdot 10^{18}$ eV). It is believed that extragalactic sources like Active Galactic Nuclei and Gamma Ray Bursts can produce particle spectra up to $E_p\sim 10^{21}$ eV. In this contribution, subluminal shocks are investigated with respect to different shock boost factors $Γ$ and the inclination angle between the shock normal and the magnetic field $ψ$. A correlation between the boost factor and the spectral behavior of the emitted particles is found. The results are compared to Active Galactic Nuclei and Gamma Ray Burst diffuse cosmic ray contribution and the observed cosmic ray spectrum at the highest energies.

astro-ph↗

arXiv2007

First limits on WIMP nuclear recoil signals in ZEPLIN-II: a two phase xenon detector for dark matter detection

Results are presented from the first underground data run of ZEPLIN-II, a 31 kg two phase xenon detector developed to observe nuclear recoils from hypothetical weakly interacting massive dark matter particles. Discrimination between nuclear recoils and background electron recoils is afforded by recording both the scintillation and ionisation signals generated within the liquid xenon, with the ratio of these signals being different for the two classes of event. This ratio is calibrated for different incident species using an AmBe neutron source and Co-60 gamma-ray sources. From our first 31 live days of running ZEPLIN-II, the total exposure following the application of fiducial and stability cuts was 225 kgxdays. A background population of radon progeny events was observed in this run, arising from radon emission in the gas purification getters, due to radon daughter ion decays on the surfaces of the walls of the chamber. An acceptance window, defined by the neutron calibration data, of 50% nuclear recoil acceptance between 5 keVee and 20 keVee, had an observed count of 29 events, with a summed expectation of 28.6+/-4.3 gamma-ray and radon progeny induced background events. These figures provide a 90% c.l. upper limit to the number of nuclear recoils of 10.4 events in this acceptance window, which converts to a WIMP-nucleon spin-independent cross-section with a minimum of 6.6x10^-7 pb following the inclusion of an energy dependent, calibrated, efficiency. A second run is currently underway in which the radon progeny will be eliminated, thereby removing the background population, with a projected sensitivity of 2x10^-7 pb for similar exposures as the first run.

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↗

arXiv2005

A Study of the Scintillation Induced by Alpha Particles and Gamma Rays in Liquid Xenon in an Electric Field

Scintillation produced in liquid xenon by alpha particles and gamma rays has been studied as a function of applied electric field. For back scattered gamma rays with energy of about 200 keV, the number of scintillation photons was found to decrease by 64+/-2% with increasing field strength. Consequently, the pulse shape discrimination power between alpha particles and gamma rays is found to reduce with increasing field, but remaining non-zero at higher fields.

physics.ins-det↗

arXiv2003

The NAIAD experiment for WIMP searches at Boulby mine and recent results

The NAIAD experiment (NaI Advanced Detector) for WIMP dark matter searches at Boulby mine (UK) is described. The detector consists of an array of encapsulated and unencapsulated NaI(Tl) crystals with high light yield. Six crystals are collecting data at present. Data accumulated by four of them (10.6 kg x year exposure) have been used to set upper limits on the WIMP-nucleon spin-independent and WIMP-proton spin-dependent cross-sections. Pulse shape analysis has been applied to discriminate between nuclear recoils, as may be caused by WIMP interactions, and electron recoils due to gamma background. Various calibrations of crystals are presented.

hep-ex↗

arXiv2002

Particle acceleration in ultra-relativistic oblique shock waves

We perform Monte Carlo simulations of diffusive shock acceleration at highly relativistic oblique shock waves. High upstream flow Lorentz gamma factors are used, which are relevant to models of ultra relativistic particle shock acceleration in Active Galactic Nuclei (AGN) central engines and relativistic jets and Gamma Ray Burst (GRB) fireballs. We investigate numerically the acceleration properties -in the ultra relativistic flow regime of $Γ\sim 10-10^{3}$- such as angular distribution, acceleration time constant, particle energy gain versus number of crossings and spectral shapes. We perform calculations for sub-luminal and super-luminal shocks, using two different approaches respectively. The $Γ^{2}$ energization for the first crossing cycle and the significantly large energy gain for subsequent crossings as well as the high 'speed up' factors found, are important in supporting the Vietri and Waxman models on GRB ultra-high energy cosmic ray, neutrino, and gamma-ray output.

astro-ph↗

arXiv2001

Study and suppression of anomalous fast events in inorganic scintillators for dark matter searches

The status of dark matter searches with inorganic scintillator detectors at Boulby mine is reviewed and the results of tests with a CsI(Tl) crystal are presented. The objectives of the latter experiment were to study anomalous fast events previously observed and to identify ways to remove this background. Clear indications were found that these events were due to surface contamination of crystals by alphas, probably from radon decay. A new array of unencapsulated NaI(Tl) crystals immersed either in liquid paraffin or pure nitrogen atmosphere is under construction at Boulby. Such an approach allows complete control of the surface of the crystals and the ability to remove any surface contamination. First data from the unencapsulated NaI(Tl) do not show the presence of anomalous fast events.

hep-ex↗

arXiv2001

Measurements of Scintillation Efficiency and Pulse-Shape for Low Energy Recoils in Liquid Xenon

Results of observations of low energy nuclear and electron recoil events in liquid xenon scintillator detectors are given. The relative scintillation efficiency for nuclear recoils is 0.22 +/- 0.01 in the recoil energy range 40 keV - 70 keV. Under the assumption of a single dominant decay component to the scintillation pulse-shape the log-normal mean parameter T0 of the maximum likelihood estimator of the decay time constant for 6 keV < Eee < 30 keV nuclear recoil events is equal to 21.0 ns +/- 0.5 ns. It is observed that for electron recoils T0 rises slowly with energy, having a value ~ 30 ns at Eee ~ 15 keV. Electron and nuclear recoil pulse-shapes are found to be well fitted by single exponential functions although some evidence is found for a double exponential form for the nuclear recoil pulse-shape.

hep-ex↗

arXiv2001

Particle Energy And Acceleration Efficiencies In Highly Relativistic Shocks

In this paper we present an investigation of numerical Monte Carlo simulations of the diffusive shock acceleration in the test particle limit. Very high gamma flow astrophysical plasmas, have been used, from $γ_{up}$ $\sim50$ up to $γ_{up}$ $\sim1000$, which could be relevant to the suggested models of AGNs Jets and their Central Engines as well as the ultra-relativistic shock particle acceleration in Gamma Ray Burst (GRB) fireballs. Particularly the energy gain per shock crossing and the time constant for the above high relativistic processes is numerically calculated. We explicitly find a considerable $γ^{2}$ energy boosting in the first shock cycle, and in all subsequent shock cycles the particle energy is multiplied by a large factor. Also a noted acceleration speed-up for the same acceleration process has been observed. Both of those results are connected with theoretical suggestions that a significant enhancement of the acceleration process is possible due to relativistic effects not present at lower plasma flow speeds. The acceleration speed-up and the mean energy-gain per shock cycle found could efficiently justify the theory of the origin of Ultra High Energy Cosmic Rays (UHECR) from the sites of GRBs.

astro-ph↗

arXiv2000

Particle Propagation Upstream of CIR Shocks

The first high solar latitude pass of the Ulysses spacecraft revealed the presence of MeV particle increases up to latitudes above 60 degrees, well outside the CIR belt but associated in time with the regular passage of these plasma interfaces at more equatorial latitudes. The particle increases have been explained variously as due to diffusion from a field line connection with a CIR at a greater distance, perpendicular diffusion in latitude, propagation from the inner Corona or an acceleration process on a connecting field line. Numerical solutions to the 1-D propagation equation upstream of a CIR shock for reasonable diffusion mean free paths are shown here to limit the source of the increases to within about 2 AU of the CIR's, asssuming the CIR's either trap or accelerate energetic particles and they diffuse from the nearby CIR interface. The problem of allowing propagation from the CIR's is eased if the CIRs are located according to the predictions for the current sheet with a solar source surface at $2.5 R_{s}$, rather than at $3.25 R_{s}$. Energetic electron observations showing delays with respect to the CIR-associated ions suggest possibly an acceleration in the inner corona, rather than in the CIR-associated shock. A more likely explanation, however, is the ability of electrons to take a longer a longer route, from beyond the point of observation, than for that of the ions and yet arrive within a few days of CIR closest approach time.

physics.space-ph↗

arXiv1999

The Very Energetic Radiation Imaging Telescope Array System (VERITAS)

We give an overview of the current status and scientific goals of VERITAS, a proposed hexagonal array of seven 10 m aperture imaging Cherenkov telescopes. The selected site is Montosa Canyon (1390 m a.s.l.) at the Whipple Observatory, Arizona. Each telescope, of 12 m focal length, will initially be equipped with a 499 element photomultiplier camera covering a 3.5 degree field of view. A central station will initiate the readout of 500 MHz FADCs upon receipt of multiple telescope triggers. The minimum detectable flux sensitivity will be 0.5% of the Crab Nebula flux at 200 GeV. Detailed simulations of the array's performance are presented elsewhere at this meeting. VERITAS will operate primarily as a gamma-ray observatory in the 50 GeV to 50 TeV range for the study of active galaxies, supernova remnants, pulsars and gamma-ray bursts.

astro-ph↗

arXiv1997

The Origin of the Cosmic Soft X-Ray Background: Optical Identification of an Extremely Deep ROSAT Survey

We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was made with the ROSAT PSPC and reaches a flux limit of 1.6x10^-15 erg cm^-2 s^-1 (0.5--2.0 keV). Above a flux limit of 2x10^-15 erg cm^-2 s^-1 we define a complete sample of 70 sources of which 59 are identified. Some (5) other sources have tentative identifications and in a further 4 the X-ray error-boxes are blank to R=23 mag. At the brighter flux levels (>= 10^-14 erg cm^-2 s^-1) we confirm the results of previous less deep X-ray surveys with 84% of the sources begin QSOs. At fainter fluxes, however, the survey is dominated by a population of galaxies with narrow optical emission lines (NELGs). In addition, a number of groups and clusters of galaxies are found at intermediate fluxes. Most of these are poor systems of low X-ray luminosity and are generally found at redshifts of > 0.3. Their numbers are consistent with a zero evolutionary scenario, in contrast to the situation for high luminosity clusters at the same redshift. We discuss the significance of these results to the determination of the cosmic soft X-ray background (XRB) and show that at 2x10^-15 erg cm^-2 s^-1, we have resolved more than 50% of the background. We also briefly consider the probable importance of NELG objects to the residual background and look at some of the properties of these unusual objects.

astro-ph↗