We report on a search for particle dark matter with the XENON100 experiment, operated at the Laboratori Nazionali del Gran Sasso for 13 months during 2011 and 2012. XENON100 features an ultralow ...electromagnetic background of (5.3 ± 0.6) × 10(-3) events/(keV(ee) × kg × day) in the energy region of interest. A blind analysis of 224.6 live days × 34 kg exposure has yielded no evidence for dark matter interactions. The two candidate events observed in the predefined nuclear recoil energy range of 6.6-30.5 keV(nr) are consistent with the background expectation of (1.0 ± 0.2) events. A profile likelihood analysis using a 6.6-43.3 keV(nr) energy range sets the most stringent limit on the spin-independent elastic weakly interacting massive particle-nucleon scattering cross section for weakly interacting massive particle masses above 8 GeV/c(2), with a minimum of 2 × 10(-45) cm(2) at 55 GeV/c(2) and 90% confidence level.
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.
We present new experimental constraints on the elastic, spin-dependent WIMP-nucleon cross section using recent data from the XENON100 experiment, operated in the Laboratori Nazionali del Gran Sasso ...in Italy. An analysis of 224.6 live days×34 kg of exposure acquired during 2011 and 2012 revealed no excess signal due to axial-vector WIMP interactions with 129Xe and 131Xe nuclei. This leads to the most stringent upper limits on WIMP-neutron cross sections for WIMP masses above 6 GeV/c², with a minimum cross section of 3.5×10(-40) cm² at a WIMP mass of 45 GeV/c², at 90% confidence level.
We report experimental upper limits on WIMP-nucleon elastic scattering cross sections from the second science run of ZEPLIN-III at the Boulby Underground Laboratory. A raw fiducial exposure of 1344 ...kg⋅days was accrued over 319 days of continuous operation between June 2010 and May 2011. A total of eight events was observed in the signal acceptance region in the nuclear recoil energy range 7–29 keV, which is compatible with background expectations. This allows the exclusion of the scalar cross-section above 4.8×10−8 pb near 50 GeV/c2 WIMP mass with 90% confidence. Combined with data from the first run, this result improves to 3.9×10−8 pb. The corresponding WIMP-neutron spin-dependent cross-section limit is 8.0×10−3 pb. The ZEPLIN programme reaches thus its conclusion at Boulby, having deployed and exploited successfully three liquid xenon experiments of increasing reach.
The next generation of very-short-baseline reactor experiments will require compact detectors operating at surface level and close to a nuclear reactor. This paper presents a new detector concept ...based on a composite solid scintillator technology. The detector target uses cubes of polyvinyltoluene interleaved with 6LiF:ZnS(Ag) phosphor screens to detect the products of the inverse beta decay reaction. A multi-tonne detector system built from these individual cells can provide precise localisation of scintillation signals, making efficient use of the detector volume. Monte Carlo simulations indicate that a neutron capture efficiency of over 70 % is achievable with a sufficient number of 6LiF:ZnS(Ag) screens per cube and that an appropriate segmentation enables a measurement of the positron energy which is not limited by γ-ray leakage. First measurements of a single cell indicate that a very good neutron-gamma discrimination and high neutron detection efficiency can be obtained with adequate triggering techniques. The light yield from positron signals has been measured, showing that an energy resolution of 14%/√E(MeV) is achievable with high uniformity. A preliminary neutrino signal analysis has been developed, using selection criteria for pulse shape, energy, time structure and energy spatial distribution and showing that an antineutrino efficiency of 40% can be achieved. It also shows that the fine segmentation of the detector can be used to significantly decrease both correlated and accidental backgrounds.
Abstract
As we move to an era where next generation ultra-low
background particle physics experiments begin to be designed and
constructed, the ability to assay materials with high sensitivity
and at ...speed with a variety of techniques will be key.
This paper describes the Mirion Technologies (Canberra) specialty
ultra-low background detectors installed and commissioned at the
Boulby Underground Laboratory between 2017 and 2021. The low
background levels of the detectors combine with low background
shielding and a radon-reduced dry nitrogen purge system to give
sensitivity approaching the best in the world without the need for
intricate shielding solutions.
For an optimised sample geometry, run for 100 d, it would be
possible to reach close to 10 μBq kg
-1
(10
-12
g/g) for background radionuclides of interest in
neutrinoless double-beta decay.
Abstract
Photomultiplier tubes (PMTs) are traditionally an integral part of large underground experiments as they measure the light emission from particle interactions within the enclosed detection ...media. The BUTTON experiment will utilise around 100 PMTs to measure the response of different media suitable for rare event searches. A subset of low-radioactivity 10-inch Hamamatsu R7081 PMTs were tested, characterised, and compared to manufacture certification. This manuscript describes the laboratory tests and analysis of gain, peak-to-valley ratio and dark rate of the PMTs to give an understanding of the charge response, signal-to-noise ratio and dark noise background as an acceptance test of the suitability of these PMTs for water-based detectors. Following the evaluation of these tests, the PMT performance agreed with the manufacturer specifications. These results are imperative for modeling the PMT response in detector simulations and providing confidence in the performance of the devices once installed in the detector underground.
A measurement is presented of the neutron production rate in lead by high energy cosmic-ray muons at a depth of 2850m water equivalent (w.e.) and a mean muon energy of 260GeV. The measurement ...exploits the delayed coincidences between muons and the radiative capture of induced neutrons in a highly segmented tonne scale plastic scintillator detector. Detailed Monte Carlo simulations reproduce well the measured capture times and multiplicities and, within the dynamic range of the instrumentation, the spectrum of energy deposits. By comparing measurements with simulations of neutron capture rates a neutron yield in lead of (5.78-0.28+0.21) ×10-3neutrons/muon/(g/cm2) has been obtained. Absolute agreement between simulation and data is of order 25%. Consequences for deep underground rare event searches are discussed.
Scintillation and ionisation yields for nuclear recoils in liquid xenon above 10 keVnr (nuclear recoil energy) are deduced from data acquired using broadband Am–Be neutron sources. The nuclear recoil ...data from several exposures to two sources were compared to detailed simulations. Energy-dependent scintillation and ionisation yields giving acceptable fits to the data were derived. Efficiency and resolution effects are treated using a light collection Monte Carlo, measured photomultiplier response profiles and hardware trigger studies. A gradual fall in scintillation yield below ∼40 keVnr is found, together with a rising ionisation yield; both are in agreement with the latest independent measurements. The analysis method is applied to the most recent ZEPLIN-III data, acquired with a significantly upgraded detector and a precision-calibrated Am–Be source, as well as to the earlier data from the first run in 2008. A new method for deriving the recoil scintillation yield, which includes sub-threshold S1 events, is also presented which confirms the main analysis.
Changes induced by perturbed aerosol conditions in moderately deep mixed-phase convective clouds
(cloud top height ∼ 5 km)
developing along sea-breeze convergence
lines are investigated with ...high-resolution numerical model simulations. The
simulations utilise the newly developed Cloud–AeroSol Interacting Microphysics (CASIM) module for
the Unified Model (UM), which allows for the representation of the two-way interaction between cloud and
aerosol fields. Simulations are evaluated against observations collected during the COnvective
Precipitation Experiment (COPE) field campaign over the southwestern peninsula of the UK in 2013.
The simulations compare favourably with observed thermodynamic profiles, cloud base cloud droplet
number concentrations (CDNC), cloud depth, and radar reflectivity statistics. Including the
modification of aerosol fields by cloud microphysical processes improves the correspondence with
observed CDNC values and spatial variability, but reduces the agreement with observations for
average cloud size and cloud top height. Accumulated precipitation is suppressed for higher-aerosol conditions before clouds become organised
along the sea-breeze convergence lines. Changes in precipitation are smaller in simulations with
aerosol processing. The precipitation suppression is due to less efficient precipitation production
by warm-phase microphysics, consistent with parcel model predictions. In contrast, after convective cells organise along the sea-breeze convergence zone, accumulated
precipitation increases with aerosol concentrations. Condensate production increases with the
aerosol concentrations due to higher vertical velocities in the convective cores and higher cloud
top heights. However, for the highest-aerosol scenarios, no further increase in the condensate
production occurs, as clouds grow into an upper-level stable layer. In these cases, the reduced
precipitation efficiency (PE) dominates the precipitation response and no further precipitation
enhancement occurs. Previous studies of deep convective clouds have related larger vertical
velocities under high-aerosol conditions to enhanced latent heating from freezing.
In the presented simulations changes in latent heating above the 0∘C are negligible, but latent
heating from condensation increases with aerosol concentrations. It is hypothesised that this
increase is related to changes in the cloud field structure reducing the mixing of environmental air
into the convective core. The precipitation response of the deeper mixed-phase clouds along well-established convergence lines
can be the opposite of predictions from parcel models. This occurs when clouds interact with a
pre-existing thermodynamic environment and cloud field structural changes occur that are not
captured by simple parcel model approaches.
PDF
