Background Detection Research Articles

Bayesian electron density determination from sparse and noisy single-molecule X-ray scattering images.

Single molecule x-ray scattering experiments using free-electron lasers hold the potential to resolve biomolecular structures and structural ensembles. However, molecular electron density determination has so far not been achieved because of low photon counts, high noise levels, and low hit rates. Most approaches therefore focus on large specimen like entire viruses, which scatter sufficiently many photons to allow orientation determination of each image. Small specimens like proteins, however, scatter too few photons for the molecular orientations to be determined. Here, we present a rigorous Bayesian approach to overcome these limitations, additionally taking into account intensity fluctuations, beam polarization, irregular detector shapes, incoherent scattering, and background scattering. We demonstrate using synthetic scattering images that electron density determination of small proteins is possible in this extreme high noise Poisson regime. Tests on published virus data achieved the detector-limited resolution of 9 nm, using only 0.01% of the available photons per image.

Impact of spin in compact binary foreground subtraction for estimating the residual stochastic gravitational-wave background in ground-based detectors

Impact of spin in compact binary foreground subtraction for estimating the residual stochastic gravitational-wave background in ground-based detectors

Pushing the Limits of the Cosmic Origin Spectrograph (COS) with an Optimized Background Correction

Observations utilizing the ultraviolet capabilities of the Cosmic Origin Spectrograph (COS) onboard the Hubble Space Telescope are of unique value to the astronomy community. Spectroscopy down to 900 Å with COS has enabled new science areas. However, contrary to the situation at longer wavelengths, these observations are limited by detector background noise. The background correction currently applied by the standard calibration pipeline (CalCOS) is not optimized for faint targets, limiting the scientific value of low signal-to-noise ratio (S/N) observations. In this work we investigate a possible dependence of the variations of the dark rate in both segments of the COS far-ultraviolet detector on time, detector high voltage (HV), and solar activity. Through our analysis we identified a number of detector states (on a configuration basis, e.g., HV and segment) characterizing the spatial distribution of dark counts, and created superdarks to be used in an optimized two-dimensional (2D) background correction. We have developed and tested Another COS Dark Correction (ACDC), a dedicated pipeline to perform a 2D background correction based on statistical methods, producing background-corrected and flux-calibrated spectra. While our testing of ACDC showed an average improvement in S/N values of ∼10%, in a few cases the improvements in S/N reached 60% across the whole wavelength range of individual segments.

The sensitivity of innovative techniques for measuring low levels of radon in the environment using passive detectors

In recent years, there has been a significant surge in interest in measuring low radon levels in the environment. These measurements are valuable, particularly for identifying radon priority areas as required by the European Council Directive 2013/59/EURATOM and for research related to climate change. Due to the limited sensitivity of existing radon detectors/monitors in addressing these challenges, substantial efforts have been devoted to developing new designs. This report compares the sensitivity of several innovative designs with that of existing passive radon monitors. These novel designs incorporate alpha track detectors, including large area low background detectors, with activated carbon fabric used as an efficient radon adsorber/radiator. Recent innovative solutions to mitigate the impact of temperature and humidity on detectors using adsorbers are also discussed. The background signal of detectors intended for use in these novel designs is examined, and their sensitivity is evaluated. The findings demonstrate that these novel designs have the potential to significantly enhance the sensitivity of long-term radon measurements, surpassing the detectors currently in widespread use by more than an order of magnitude.

From optimal observables to machine learning: an effective-field-theory analysis of e+e−→ W+W− at future lepton colliders

We apply machine-learning techniques to the effective-field-theory analysis of the e+e− → W+W− processes at future lepton colliders, and demonstrate their advantages in comparison with conventional methods, such as optimal observables. In particular, we show that machine-learning methods are more robust to detector effects and backgrounds, and could in principle produce unbiased results with sufficient Monte Carlo simulation samples that accurately describe experiments. This is crucial for the analyses at future lepton colliders given the outstanding precision of the e+e− → W+W− measurement (~ 10−4 in terms of anomalous triple gauge couplings or even better) that can be reached. Our framework can be generalized to other effective-field-theory analyses, such as the one of e+e− → tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t} $$\\end{document} or similar processes at muon colliders.

First detailed calculation of atmospheric neutrino foregrounds to the diffuse supernova neutrino background in Super-Kamiokande

The diffuse supernova neutrino background (DSNB)—a probe of the core-collapse mechanism and the cosmic star-formation history—has not been detected, but its discovery may be imminent. A significant obstacle for DSNB detection in Super-Kamiokande (Super-K) is detector backgrounds, especially due to atmospheric neutrinos (more precisely, these are foregrounds), which are not sufficiently understood. We perform the first detailed theoretical calculations of these foregrounds in the range 16–90 MeV in detected electron energy, taking into account several physical and detector effects, quantifying uncertainties, and comparing our predictions to the 15.9 live time years of pre-gadolinium data from Super-K stages I–IV. We show that our modeling reasonably reproduces this low-energy data as well as the usual high-energy atmospheric-neutrino data. To accelerate progress on detecting the DSNB, we outline key actions to be taken in future theoretical and experimental work. In a forthcoming paper, we use our modeling to detail how low-energy atmospheric-neutrino events register in Super-K and suggest new cuts to reduce their impact. Published by the American Physical Society 2024

A high-sensitivity radon emanation detector system for future low-background experiments

Radioactive radon atoms originating from the primordial 238U and 232Th decay chains are constantly emanated from the surfaces of most materials. The radon atoms and their radioactive daughter isotopes can significantly contribute to the background of low-background experiments. The 222Rn progeny 214Pb, for example, dominates the background of current liquid xenon-based direct dark matter detectors. We report on a new detector system to quantify the 222Rn surface emanation rate of materials. Using cryogenic physisorption traps, emanated radon atoms are transferred from an independent emanation vessel and concentrated within the dedicated detection vessel. The charged radon daughter isotopes are collected electrostatically on a silicon PIN photodiode to spectrometrically measure the alpha decays of 214Po and 218Po. The overall detection efficiency is ∼ 36 % for both polonium channels. The radon emanation activity of the emanation vessel was measured to be (0.16± 0.03) mBq, resulting in a detection sensitivity of ∼ 0.06 mBq at 90 % C.L.

Development of low-radon ultra-pure water for the Jiangmen Underground Neutrino Observatory

The Jiangmen Underground Neutrino Observatory (JUNO) is a state-of-the-art liquid scintillator-based neutrino physics experiment under construction in South China. To reduce the background from external radioactivities, a water Cherenkov detector composed of 35 kton ultra-pure water and 2,400 20-inch photomultiplier tubes is developed. Even after specialized treatment, ultra-pure water still contains trace levels of radioactive elements that can contribute to the detector background. Among which 222Rn is particularly significant. To address this, an online radon removal system based on the JUNO prototype has been developed. By integrating micro-bubble generators to enhance degasser’s radon removal efficiency, the radon concentration in water can be reduced to 1 mBq/m3 level, meeting the stringent requirements of JUNO. Additionally, a highly sensitive online radon concentration measurement system capable of detecting concentrations ∼1 mBq/m3 has been developed to monitor the radon concentration in water. In this paper, the details regarding both systems will be presented.

Discovering long-lived particles at DUNE

Long-lived particles (LLPs) arise in many theories beyond the Standard Model. These may be copiously produced from meson decays (or through their mixing with the LLPs) at neutrino facilities and leave a visible decay signal in nearby neutrino detectors. We compute the expected sensitivity of the DUNE liquid argon (LAr) and gaseous argon near detectors (NDs) to light LLP decays. In doing so, we determine the expected backgrounds for both detectors, which have been largely overlooked in the literature, taking into account their angular and energy resolution. We show that searches for LLP decays into muon pairs, or into three pions, would be extremely clean. Conversely, decays into two photons would be affected by large backgrounds from neutrino interactions for both near detectors; finally, the reduced signal efficiency for e+e− pairs leads to a reduced sensitivity for ND-LAr. Our results are first presented in a model-independent way, as a function of the mass of the new state and its lifetime. We also provide detailed calculations for several phenomenological models with axionlike particles (coupled to gluons, electroweak bosons, or quark currents). Some of our results may also be of interest for other neutrino facilities using a similar detector technology (e.g., MicroBooNE, SBND, ICARUS, or the T2K near detector). Published by the American Physical Society 2024

Background experiment of the low energy x-ray telescope detectors on insight-HXMT

The low energy x-ray telescope (LE) is one of the main instruments of the insight-hard x-ray modulation telescope, the first x-ray astronomical satellite of China. The scientific objectives of the LE focus on the scanning and pointed observations of the x-ray sources in the soft x-ray band (1–13 keV). In order to complete the observation tasks and accurately analyze the background information of the LE, it is essential to obtain the background data of the detectors. Therefore, we designed an LE background experiment. The experiment began with an underground background experiment in the China Jinping Underground Laboratory with a rock of a thickness of 2400 m, followed by a ground background experiment. These two experiments lasted for a long time, and through comparison and analysis of the background data, it was found that underground laboratories significantly shielded cosmic rays. In addition, the background of detectors in the underground experiment was more than one order of magnitude lower than that in the ground experiment. The experiments also revealed multiple x-ray fluorescence peaks of various elements in the background, including silicon from the detector itself, erbium in the ceramic substrate, and copper in the mounting plate. The anti-coincidence design of detectors was observed to reduce the x-ray fluorescence peaks of silicon. By comparing the background flux obtained with the background flux in the orbit, it was found that the background generated by radioactive substances inside the LE detector is very low. Within the energy range of less than 7.5 keV, the flux in the orbit is about 0.012 counts/s/keV, the ground flux is ∼3 × 10−3 counts/s/keV, and the underground flux is about 1.5 × 10−4 counts/s/keV. However, the flux in the orbit significantly increases above 7.5 keV, which does not occur in both the ground and underground background experiments. These results provide reference and guidance for scientific teams and instrument teams to analyze the data of the low energy x-ray telescope.

Beam optics distortion caused by orbit deviation at strong sextupole magnets in SuperKEKB

The SuperKEKB is an electron-positron double-ring asymmetric-energy collider at the High Energy Accelerator Research Organization (KEK) in Japan. It uses a novel collision method referred to as the nano-beam scheme to avoid the so-called hourglass effect. The highest peak luminosity achieved thus far is 4.65 × 1034 cm-2s-1, which was reached on June 8th, 2022. Although the SuperKEKB holds the record for the world's highest instantaneous luminosity, several unresolved issues prevent a further increase in luminosity. One of the major issues is inadequate stability and reproducibility of the beam orbit and optics. This paper discusses optics distortion caused by orbit deviation near strong sextupole magnets. Numerical estimations indicate that a horizontal orbit deviation of a few tens of micrometers induces a sizable distortion of the optical functions at the interaction point. Analyses of the amount of betatron tune feedback and orbit at sextupole magnets used for crab waist and vertical local chromaticity correction indicate that the beam-current dependence of the vertical tune shift is attributable to the change of the beam orbit at the sextupoles. The orbit fluctuation at the sextupoles causes beta-beating and makes stable operation at high beam currents more difficult. The results show that orbit tuning at the sextupoles improves both the injection efficiency and the detector background level. Optics deterioration within a few days is another urgent issue in high-beam-current operation. A vertical orbit change of a few tens of microns at sextupole magnets installed in arc cells is a matter of concern according to numerical estimations.

Mass spectrometry developments of 232Th and 238U radiopurity measurements for LEGEND

The LEGEND collaboration has been developing a 76Ge-based double-beta decay experimental program where precise radiopurity measurements of ultraclean materials are crucial. Ultralow concentrations of thorium and uranium, the main contributors to the detector background via their decay products, can be determined by inductively coupled plasma mass spectrometry (ICPMS) and accelerator mass spectrometry (AMS). Here we shall present recent developments in thorium and uranium mass spectrometry methods, together with basics of separation chemistry applied to process different samples. The new possibilities to measure 232Th and 238U by ICPMS and AMS at the Comenius University in Bratislava are discussed as well.

Low-energy radiative backgrounds in CCD-based dark-matter detectors

The reach of sub-GeV dark-matter detectors is at present severely affected by low-energy events from various origins. We present the theoretical methods to compute the single- and few-electron events that arise from secondary radiation emitted by high-energy particles as they pass through detector materials and perform a detailed simulation to quantify them at (Skipper) CCD-based experiments, focusing on the SENSEI data collected at Fermilab near the MINOS cavern. The simulations account for the generation of secondaries from Cherenkov and luminescent recombination radiation; photo-absorption in the bulk, backside layer, pitch adapter, and epoxy; the photon reflection and refraction at interfaces; thin-film interference; the roughness of the interfaces; the dynamics of charges produced in the highly doped CCD-backside-layers; and the partial charge collection on the CCD backside. We consider several systematic uncertainties, notably those stemming from the backside modeling, which we estimate with a “fiducial” and an “extreme” charge-diffusion model, with the former model being preferred due to better agreement with partial-charge collection data. We find that Cherenkov photons constitute about 30% of the observed single-electron events for both diffusion models; radiative recombination contributes negligibly to the event rate for the fiducial model, although it can dominate over Cherenkov for the extreme model. We also estimate the fraction of 2-electron events that arise from 1-electron event coincidences in the same pixel, finding that the entire 2-electron rate can be explained by coincidences of radiative events and spurious charge. Accounting for both radiative and non-radiative backgrounds, we project the sensitivity of future Skipper-CCD-based experiments to different dark-matter models. For light-mediator models with dark-matter masses of 1, 5, and 10 MeV, we find that future experiments with 10-kg-year exposures and successful background mitigation could have a sensitivity that is larger by 9, 3, and 2 orders of magnitude, respectively, when compared to an experiment without background improvements.

A plastic scintillation muon veto for sub-Kelvin temperatures

Rare-event search experiments located on-surface, such as short-baseline reactor neutrino experiments, are often limited by muon-induced background events. Highly efficient muon vetos are essential to reduce the detector background and to reach the sensitivity goals. We demonstrate the feasibility of deploying organic plastic scintillators at sub-Kelvin temperatures. For the NUCLEUS experiment, we developed a cryogenic muon veto equipped with wavelength shifting fibers and a silicon photo multiplier operating inside a dilution refrigerator. The achievable compactness of cryostat-internal integration is a key factor in keeping the muon rate to a minimum while maximizing coverage. The thermal and light output properties of a plastic scintillation detector were examined. We report first data on the thermal conductivity and heat capacity of the polystyrene-based scintillator UPS-923A over a wide range of temperatures extending below one Kelvin. The light output was measured down to 0.8 K and observed to increase by a factor of 1.61 ± 0.05 compared to 300 K. The development of an organic plastic scintillation muon veto operating in sub-Kelvin temperature environments opens new perspectives for rare-event searches with cryogenic detectors at sites lacking substantial overburden.

Ultra-low background germanium assay at the Boulby Underground Laboratory

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.

Intrinsic background of EJ-309 liquid scintillator detectors

In deep-underground experiments, intrinsic detector background can exceed the external one, limiting the detection sensitivity. Intrinsic background in liquid scintillator neutron detectors consists of α particles emitted from radioactive impurities in the detector housing (and possibly in the liquid). We present the results of long background measurements of 12 EJ-309 liquid organic scintillators detectors in the deep-underground Gran Sasso National Laboratory of the Italian Institute of Nuclear Physics. These detectors are a part of the detection array for the SHADES ERC project that aims to measure the very low cross section (down to picobarn) of the astrophysically important reaction 22Ne(α,n)25Mg. With an exposure of ∼35 days we identify an actinide concentration in the detector housing of 8.4(1.8)sta(1.4)sys×10−2ppm and 1.62(0.57)sta(0.03)sys×10−1ppm of 238U and 232Th, resulting in 64 α particle counts per hour. The half-life of 214Po was also determined as 163.7(11)μs.

Cosmic muons as PMT and SiPM detector background signals

Photomultiplier tube (PMT) and Silicon Photo Multiplier (SiPM) are often used for detecting small number of photons or very weak radiations. A light guide usually connects these photodetectors to the test space. In this article, we investigate the effect of background signals caused by cosmic muons scintillation or interactions with PMT and SiPM, their light guide or input window materials. We study such interactions by making simulations using GATE software package and undertaking experiments using detector circuits developed as in [1]. The background cosmic muons can generate photons which will lead to errors in low radiation and single photon detection experiments especially if standard scintillators are not used. For such experiments, we conclude that the most useful method for cutting down the cosmic muons background should be by conducting the experiments deep underground or inside tunnels with several tens of meters of materials above it.

Reconstruction of point events in liquid-scintillator detectors subjected to total internal reflection

The outer water buffer is an economic option to shield the external radiative backgrounds for liquid-scintillator neutrino detectors. Since the refractive index of the liquid scintillator is larger than that of water, the consequential total internal reflection of scintillation light at the media boundary introduces extra complexity to the detector optics. This paper develops a precise detector-response model by investigating how total internal reflection complicates photon propagation and degrades reconstruction. We first parameterize the detector response by regression, providing an unbiased energy and vertex reconstruction in the total internal reflection region while keeping the number of parameters under control. From the experience of event degeneracy at the Jinping prototype, we identify the root cause as the multimodality in the reconstruction likelihood function, determined by the refractive index of the buffer, detector scale and PMT coverage. To avoid multimodality, we propose a straightforward criterion based on the expected photo-electron-count ratios between neighboring PMTs. The criterion will be valuable for the success in future liquid-scintillator detectors by guaranteeing the effectiveness of event reconstruction.

Low-background experiment to search for double beta decay of 106Cd using 106CdWO4 scintillator

An experiment to search for 2ε-, εβ+- and 2β+-decays of 106Cd, using a 215 g cadmium tungstate scintillation crystal enriched at 66 % by 106Cd (106CdWO4) is carried out at the Gran Sasso underground laboratory (Italy). Events in the 106CdWO4 detector are recorded in (anti)coincidences with two large-volume CdWO4 scintillation counters. The design of the detector system, calibration and background measurements, methods, and results of data analysis to determine key detector characteristics are described. The experimental data are compared with Monte Carlo simulation results, and a background model is constructed. The radioactive contamination of the setup components is studied. The sensitivity of the experiment approaches the level of theoretical predictions for the 2νεβ+-decay channel, while for other possible 2β-decay channels it is already on the level of lim T1/2 ∼ 1021-1022 years.

Ultra-low radioactivity flexible printed cables

Flexible printed cables and circuitry based on copper-polyimide materials are widely used in experiments looking for rare events due to their unique electrical and mechanical characteristics. However, past studies have found copper-polyimide flexible cables to contain 400-4700 pg 238U/g, 16-3700 pg 232Th/g, and 170-2100 ng natK/g, which can be a significant source of radioactive background for many current and next-generation ultralow background detectors. This study presents a comprehensive investigation into the fabrication process of copper-polyimide flexible cables and the development of custom low radioactivity cables for use in rare-event physics applications. A methodical step-by-step approach was developed and informed by ultrasensitive assay to determine the radiopurity in the starting materials and identify the contaminating production steps in the cable fabrication process. Radiopure material alternatives were identified, and cleaner production processes and treatments were developed to significantly reduce the imparted contamination. Through the newly developed radiopure fabrication process, fully-functioning cables were produced with radiocontaminant concentrations of 20-31 pg 238U/g, 12-13 pg 232Th/g, and 40-550 ng natK/g, which is significantly cleaner than cables from previous work and sufficiently radiopure for current and next-generation detectors. This approach, employing witness samples to investigate each step of the fabrication process, can hopefully serve as a template for investigating radiocontaminants in other material production processes.