246 results found
Quenby JJ, Webber WR, 2015, Transient heliosheath modulation, Monthly Notices of the Royal Astronomical Society, Vol: 453, Pages: 1297-1304, ISSN: 1365-2966
Voyager 1 has explored the solar wind–interstellar medium interaction region between the terminal shock and heliopause, following the intensity distribution of Galactic cosmic ray protons above 200 MeV energy. Before this component reached the expected galactic flux level at 121.7 au from the Sun, four episodes of rapid intensity change occurred with a behaviour similar to that found in Forbush Decreases in the inner Solar system, rather than that expected from a mechanism related to models for the long-term modulation found closer to the Sun. Because the mean solar wind flow is both expected and observed to be perpendicular to the radial direction close to the heliopause, an explanation is suggested in terms of transient radial flows related to possible heliopause boundary flapping. It is necessary that the radial flows are of the order either of the sound speed found for conditions downstream of the terminal shock or of the fluctuations found near the boundary by the Voyager 1 Low Energy Charged Particle detector and that the relevant cosmic ray diffusion perpendicular to the mean field is controlled by ‘slab’ fluctuations accounting for about 20 per cent of the total power in the field variance. However, additional radial drift motion related to possible north to south gradients in the magnetic field may allow the inclusion of some diffusion according to the predictions of a theory based upon the presence of 2D turbulence. The required field gradients may arise due to field variation in the field carried by solar plasma flow deflected away from the solar equatorial plane. Modulation amounting to a total 30 per cent drop in galactic intensity requires explanation by a combination of transient effects.
Webber WR, Quenby JJ, 2015, Unusual Galactic Cosmic Ray Intensity and Spectral Changes Observed by V1 Near The Heliopause, Astrophysical Journal, Vol: 806, ISSN: 1538-4357
We discuss here two unusual increases of cosmic ray intensity that were observed by V1 in the last 1.1 AU before itcrossed the heliopause in 2012 August, at 121.5 AU. These two increases are roughly similar in amplitude andresult in a total increase in ∼1 GV cosmic ray nuclei of over 50% and 0.01 GV electrons of a factor ∼2. During thefirst increase the changes in the B field are small. After the first increase the B field changes become large andduring the second increase the B field variations and cosmic ray changes are correlated to within ± one day. Duringthese time intervals, the rigidity dependence of the increases of GCR H and He nuclei from 100–600 MeV/nucresemble those used to describe the solar modulation near the Earth during a large transient decrease but the ratiobetween the intensity changes of H, He, and electrons are different. The magnitude of these increases at Voyager is∼1/3 of the modulation that is required to produce the total modulation of protons, helium nuclei, and electronsbetween the local interstellar intensities and those observed at the Earth at the 2009 sunspot minima. This mayimply that a significant part of the residual solar modulation at times of sunspot minima occurs near the heliopauseitself.
Quenby JJ, Webber WR, 2013, The structure of the Heliopause, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 436, Pages: 3306-3308, ISSN: 0035-8711
Reichhart L, Lindote A, Akimov DY, et al., 2013, Measurement and simulation of the muon-induced neutron yield in lead, ASTROPARTICLE PHYSICS, Vol: 47, Pages: 67-76, ISSN: 0927-6505
Reichhart L, Lindote A, Akimov DY, et al., 2013, A measurement of the muon-induced neutron yield in lead at a depth of 2850 m water equivalent, 4th International Workshop on Low Radioactivity Techniques (LRT), Publisher: AMER INST PHYSICS, Pages: 219-222, ISSN: 0094-243X
Quenby JJ, Mulligan T, Blake JB, et al., 2013, Diffusion Coefficients, Short-Term Cosmic Ray Modulation, and Convected Magnetic Structures, ADVANCES IN ASTRONOMY, Vol: 2013, ISSN: 1687-7969
Solovov VN, Belov VA, Akimov DY, et al., 2012, Position Reconstruction in a Dual Phase Xenon Scintillation Detector, IEEE Transactions on Nuclear Science, Vol: 59, Pages: 3286-3293, ISSN: 1558-1578
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.
Reichhart L, Akimov DY, Araujo HM, et al., 2012, Quenching factor for low-energy nuclear recoils in a plastic scintillator, Physical Review C, Vol: 85, ISSN: 1089-490X
Plastic scintillators are widely used in industry, medicine, and scientific research, including nuclear and particle physics. Although one of their most common applications is in neutron detection, experimental data on their response to low-energy nuclear recoils are scarce. Here, the relative scintillation efficiency for neutron-induced nuclear recoils in a polystyrene-based plastic scintillator (UPS-923A) is presented, exploring recoil energies between 125 and 850 keV. Monte Carlo simulations, incorporating light collection efficiency and energy resolution effects, are used to generate neutron scattering spectra which are matched to observed distributions of scintillation signals to parameterize the energy-dependent quenching factor. At energies above 300 keV the dependence is reasonably described using the semiempirical formulation of Birks and a kB factor of (0.014±0.002) g MeV−1 cm−2 has been determined. Below that energy, the measured quenching factor falls more steeply than predicted by the Birks formalism.
Majewski P, Solovov VN, Akimov DY, et al., 2012, Performance data from the ZEPLIN-III second science run, Journal of Instrumentation, Vol: 7, ISSN: 1748-0221
ZEPLIN-III is a two-phase xenon direct dark matter experiment located at the Boulby Mine (U.K.). After its first science run in 2008 it was upgraded with: an array of low background photomultipliers, a new anti-coincidence detector system with plastic scintillator and an improved calibration system. After 319 days of data taking the second science run ended in May 2011. In this paper we describe the instrument performance with emphasis on the position and energy reconstruction algorithm and summarise the final science results.
Akimov DY, Araújo HM, Barnes EJ, et al., 2012, WIMP-nucleon cross-section results from the second science run of ZEPLIN-III, Physics Letters B, Vol: 709, Pages: 14-20, ISSN: 0370-2693
Santos E, Edwards B, Chepel V, et al., 2011, Single electron emission in two-phase xenon with application to the detection of coherent neutrino-nucleus scattering, The Journal of High Energy Physics, Vol: 2011, Pages: 1-21, ISSN: 1029-8479
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 appli-cations 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 ~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.
Horn M, Belov VA, Akimov DY, et al., 2011, Nuclear recoil scintillation and ionisation yields in liquid xenon from ZEPLIN-III data, PHYSICS LETTERS B, Vol: 705, Pages: 471-476, ISSN: 0370-2693
Neves F, Akimov DY, Araujo HM, et al., 2011, ZE3RA: the ZEPLIN-III Reduction and Analysis package, Journal of Instrumentation, Vol: 6, ISSN: 1748-0221
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 using calibration data 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.
Ghag C, Akimov DY, Araujo HM, et al., 2011, Performance of the veto detector incorporated into the ZEPLIN-III experiment, Astroparticle Physics, Vol: 35, Pages: 76-86, ISSN: 1873-2852
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 γ-ray induced background events, allowing the sampling of the dominant source of background in ZEPLIN-III – multiple scatter γ-rays with rare topologies. Since WIMPs will not be tagged by the veto detector, and tags due to γ-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.
Meli A, Becker JK, Quenby JJ, 2011, Active Galactic Nuclei and Gamma Ray Bursts: Relativistic shocks and the origin of ultra high energy cosmic rays, 2nd Roma International Conference on Astro-particle Physics, Publisher: ELSEVIER SCIENCE BV, Pages: 95-98, ISSN: 0168-9002
Solovov VN, Belov VA, Akimov DY, et al., 2011, Position Reconstruction in a Dual Phase Xenon Scintillation Detector, IEEE Nuclear Science Symposium/Medical Imaging Conference (NSS/MIC)/18th International Workshop on Room-Temperature Semiconductor X-Ray and Gamma-Ray Detectors, Publisher: IEEE, Pages: 1226-1233, ISSN: 1095-7863
LIEU RICHARD, QUENBY JOHN, BONAMENTE MASSIMILIANO, 2010, NON-THERMAL INTERCLUSTER MEDIUM: A SIMULTANEOUS INTERPRETATION OF THE CENTRAL SOFT EXCESS AND WAMP's DETECTION OF REDUCED SUNYAEV-ZEL'DOVICH EFFECT, The Astrophysical Journal: an international review of astronomy and astronomical physics, Vol: 721 NUMBER 2, Pages: 1482-1491
We present limits on the WIMP–nucleon cross section for inelastic dark matter from a reanalysis of the2008 run of ZEPLIN-III. Cuts, notably on scintillation pulse shape and scintillation-to-ionisation ratio, givea net exposure of 63 kg day in the range 20–80 keV 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 haloassumptions a small region of parameter space consistent, at 99% CL, with causing the 1.17 ton yr DAMAmodulation signal is allowed at 90% CL: it is in the mass range 45–60 GeVc−2 with a minimum CLof 87%, again derived from the maximum patch. This is the tightest constraint yet presented using xenon,a target nucleus whose similarity to iodine mitigiates systematic error from the assumed halo.
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 γ-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 10−8 pb to be probed.
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.
Lebedenko VN, Araujo HM, Barnes EJ, et al., 2009, Limits on the Spin-Dependent WIMP-Nucleon Cross Sections from the First Science Run of the ZEPLIN-III Experiment, PHYSICAL REVIEW LETTERS, Vol: 103, ISSN: 0031-9007
Lebedenko VN, Araujo HM, Barnes EJ, et al., 2009, Results from the first science run of the ZEPLIN-III dark matter search experiment, Physical Review D- particles, fields, gravitation, and cosmology, Vol: 80, ISSN: 1550-7998
The ZEPLIN-III experiment in the Palmer Underground Laboratory at Boulby uses a 12 kg two-phasexenon time-projection chamber to search for the weakly interacting massive particles (WIMPs) that mayaccount for the dark matter of our Galaxy. The detector measures both scintillation and ionizationproduced by radiation interacting in the liquid to differentiate between the nuclear recoils expected fromWIMPs and the electron-recoil background signals down to 10 keV nuclear-recoil energy. An analysisof 847 kg days of data acquired between February 27, 2008, and May 20, 2008, has excluded a WIMPnucleonelastic scattering spin-independent cross section above 8:1 10 8 pb at 60 GeVc 2 with a 90%confidence limit. It has also demonstrated that the two-phase xenon technique is capable of betterdiscrimination between electron and nuclear recoils at low-energy than previously achieved by otherxenon-based experiments.
Mulligan T, Blake JB, Shaul D, et al., 2009, Short-period variability in the galactic cosmic ray intensity: High statistical resolution observations and interpretation around the time of a Forbush decrease in August 2006, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 114, ISSN: 2169-9380
Alner GJ, Araujo HM, Bewick A, et al., 2009, ZEPLIN-II limits on WIMP-nucelon interactions, 8th UCLA Symposium on Sources and Detection of Dark Matter and Dark Energy in the Universe, Publisher: AMER INST PHYSICS, Pages: 218-+, ISSN: 0094-243X
Meli A, Becker JK, Quenby JJ, 2008, On the origin of ultra high energy cosmic rays: subluminal and superluminal relativistic shocks, ASTRONOMY & ASTROPHYSICS, Vol: 492, Pages: 323-336, ISSN: 0004-6361
Quenby JJ, Mulligan T, Blake JB, et al., 2008, Local and nonlocal geometry of interplanetary coronal mass ejections: Galactic cosmic ray (GCR) short-period variations and magnetic field modeling, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 113, ISSN: 2169-9380
Alner GJ, Araujo HM, Bewick A, et al., 2008, The ZEPLIN II dark matter detector: Data acquisition system and data reduction, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, Vol: 587, Pages: 101-109, ISSN: 0168-9002
Mulligan T, Blake BJ, Shaul D, et al., 2008, Heliospheric Transient Structures Associated with Short-Period Variations in the GCR, mexico City, 30th International Cosmic Ray Conference, Publisher: Universidad Nacional Autónoma de México, Mexico City, Mexico, Pages: 359-343
Meli A, Becker JK, Quenby JJ, et al., 2008, Particle acceleration in relativistic subluminal shock enviroments, Mexico City, 30th International Cosmic Ray Conference, Publisher: Universidad Nacional Autónoma de México, Mexico City, Mexico, Pages: 287-291
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