Cosmogenic activation of xenon and copper

Rare event search experiments using liquid xenon as target and detection medium require ultra-low background levels to fully exploit their physics potential. Cosmic ray induced activation of the detector components and, even more importantly, of the xenon itself during production, transportation and storage at the Earth's surface, might result in the production of radioactive isotopes with long half-lives, with a possible impact on the expected background. We present the first dedicated study on the cosmogenic activation of xenon after 345 days of exposure to cosmic rays at the Jungfraujoch research station at 3470m above sea level, complemented by a study of copper which has been activated simultaneously. We have directly observed the production of 7Be, 101Rh, 125Sb, 126I and 127Xe in xenon, out of which only 125Sb could potentially lead to background for a multi-ton scale dark matter search. The production rates for five out of eight studied radioactive isotopes in copper are in agreement with the only existing dedicated activation measurement, while we observe lower rates for the remaining ones. The specific saturation activities for both samples are also compared to predictions obtained with commonly used software packages, where we observe some underpredictions, especially for xenon activation.Comment: 9 pages, 11 figure

Dark matter sensitivity of multi-ton liquid xenon detectors

We study the sensitivity of multi ton-scale time projection chambers using a liquid xenon target, e.g., the proposed DARWIN instrument, to spin-independent and spin-dependent WIMP-nucleon scattering interactions. Taking into account realistic backgrounds from the detector itself as well as from neutrinos, we examine the impact of exposure, energy threshold, background rejection efficiency and energy resolution on the dark matter sensitivity. With an exposure of 200 t x y and assuming detector parameters which have been already demonstrated experimentally, spin-independent cross sections as low as $2.5 \times 10^{-49}$ cm$^2$ can be probed for WIMP masses around 40 GeV/$c^2$. Additional improvements in terms of background rejection and exposure will further increase the sensitivity, while the ultimate WIMP science reach will be limited by neutrinos scattering coherently off the xenon nuclei.Comment: 21 pages, 8 Figures; matches version accepted by JCA

A Dual-phase Xenon TPC for Scintillation and Ionisation Yield Measurements in Liquid Xenon

A small-scale, two-phase (liquid/gas) xenon time projection chamber (Xurich II) was designed, constructed and is under operation at the University of Zurich. Its main purpose is to investigate the microphysics of particle interactions in liquid xenon at energies below 50 keV, which are relevant for rare event searches using xenon as target material. Here we describe in detail the detector, its associated infrastructure, and the signal identification algorithm developed for processing and analysing the data. We present the first characterisation of the new instrument with calibration data from an internal 83m-Kr source. The zero-field light yield is 15.0 and 14.0 photoelectrons/keV at 9.4 keV and 32.1 keV, respectively, and the corresponding values at an electron drift field of 1 kV/cm are 10.8 and 7.9 photoelectrons/keV. The charge yields at these energies are 28 and 31 electrons/keV, with the proportional scintillation yield of 24 photoelectrons per one electron extracted into the gas phase, and an electron lifetime of 200 $\mu$s. The relative energy resolution, $\sigma/E$, is 11.9 % and 5.8 % at 9.4 keV and 32.1 keV, respectively using a linear combination of the scintillation and ionisation signals. We conclude with measurements of the electron drift velocity at various electric fields, and compare these to literature values.Comment: 11 pages, 14 figure

Online 222Rn^{222}Rn removal by cryogenic distillation in the XENON100 experiment

We describe the purification of xenon from traces of the radioactive noble gas radon using a cryogenic distillation column. The distillation column was integrated into the gas purification loop of the XENON100 detector for online radon removal. This enabled us to significantly reduce the constant $^{222}Rn$ background originating from radon emanation. After inserting an auxiliary $^{222}Rn$ emanation source in the gas loop, we determined a radon reduction factor of $R\,>\,27$ (95% C.L.) for the distillation column by monitoring the $^{222}Rn$ activity concentration inside the XENON100 detector

Search for Electronic Recoil Event Rate Modulation with 4 Years of XENON100 Data

We report on a search for electronic recoil event rate modulation signatures in the XENON100 data accumulated over a period of 4 yr, from January 2010 to January 2014. A profile likelihood method, which incorporates the stability of the XENON100 detector and the known electronic recoil background model, is used to quantify the significance of periodicity in the time distribution of events. There is a weak modulation signature at a period of $431^{+16}_{−14}$ day in the low energy region of (2.0–5.8) keV in the single scatter event sample, with a global significance of 1.9$\sigma$; however, no other more significant modulation is observed. The significance of an annual modulation signature drops from 2.8$\sigma$, from a previous analysis of a subset of this data, to 1.8$\sigma$ with all data combined. Single scatter events in the low energy region are thus used to exclude the DAMA/LIBRA annual modulation as being due to dark matter electron interactions via axial vector coupling at 5.7$\sigma$

Simple strategy for simulation of large area of axially symmetric metasurfaces

Metalenses are composed of nanostructures for focusing light and have been widely explored in many exciting applications. However, their expanding dimensions pose simulation challenges. We propose a method to simulate metalenses in a timely manner using vectorial wave and ray tracing models. We sample the metalens' radial phase gradient and locally approximate it by a linear phase response. Each sampling point is modeled as a binary blazed grating, employing the chosen nanostructure, to build a transfer function set. The metalens transmission or reflection is then obtained by applying the corresponding transfer function to the incoming field on the regions surrounding each sampling point. Fourier optics is used to calculate the scattered fields under arbitrary illumination for the vectorial wave method and a Monte Carlo algorithm is used in the ray tracing formalism. We validated our method against finite difference time domain simulations at 632 nm and we were able to simulate metalenses larger than 3000lambda0 in diameter on a personal computer.Comment: 10 page

Meta-analysis of molecular imaging of serotonin transporters in ecstasy/polydrug users

We conducted a meta-analysis on the available data from studies investigating SERTs in ecstasy users and polydrug using controls. From 7 studies we compared data from 157 ecstasy users and 148 controls across 14 brain regions. The main effect suggested ecstasy/MDMA related SERT reductions (SMD = 0.52, 95% CIs [0.40, 0.65]; Z = 8.36, p < .01, I2 = 89%). A significant effect of subgroups (X2 = 37.41, df = 13, p < .01, I2 = 65.3%) suggested differential effects across brain ROIs. Ecstasy users showed significant SERT reductions in 11 out of the 14 regions, including every neocortical and limbic region analysed. Greatest effects were observed in the occipital cortex (SMD = 1.09, 95% CIs [0.70, 1.48]). No group effects were observed in subcortical areas of the caudate, putamen and midbrain. Literature on Postsynaptic 5HT2A receptor imaging was synthesised with these results. We conclude that, in line with preclinical data, serotonin axons with the longest projections from the raphe nuclei appear to be most affected by ecstasy/MDMA use