The high energy spectrum of alpha particles emitted from a single isotope uniformly contaminating a bulk solid has a flat energy spectrum with a high end cutoff energy equal to the maximal alpha kinetic energy ($T_α$) of the decay. In this flat region of the spectrum, we show the surface rate $r_b\text{\,(Bq/keV-cm}^{2})$ arising from a bulk alpha contamination $ρ_b$ (Bq/cm$^3$) from a single isotope is given by $r_b =ρ_b ΔR/ 4 ΔE $, where $ΔE = E_1-E_2>0\ $ is the energy interval considered (keV) in the flat region of the spectrum and $ΔR = R_2-R_1$, where $R_2$ ($R_1$) is the amount of the bulk material (cm) necessary to degrade the energy of the alpha from $T_α$ to $E_2$ ($E_1$). We compare our calculation to a rate measurement of alphas from $^{147}$Sm, ($15.32\%\,\pm\,0.03\%$ of Sm($nat$) and half life of $(1.06\,\pm\,0.01)\times\,10^{11} \text{yr}$, and find good agreement, with the ratio between prediction to measurement of $100.2\%\pm 1.6\%\,\text{(stat)}\pm 2.1\%\,\text{(sys)}$. We derive the condition for the flat spectrum, and also calculate the relationship between the decay rate measured at the surface for a [near] surface contamination with an exponential dependence on depth and an a second case of an alpha source with a thin overcoat. While there is excellent agreement between our implementation of the sophisticated Monte Carlo program SRIM and our intuitive model in all cases, both fail to describe the measured energy distribution of a $^{148}$Gd alpha source with a thin ($\sim200μ$g/cm$^2$) Au overcoat. We discuss possible origins of the disagreement and suggest avenues for future study.
Low counting experiments (search for double $β$ decay and dark matter particles, measurements of neutrino fluxes from different sources, search for hypothetical nuclear and subnuclear processes, low background $α$, $β$, $γ$ spectrometry) require extremely low background of a detector. Scintillators are widely used to search for rare events both as conventional scintillation detectors and as cryogenic scintillating bolometers. Radioactive contamination of a scintillation material plays a key role to reach low level of background. Origin and nature of radioactive contamination of scintillators, …
The performance of high-precision mass spectrometry of radioactive isotopes can often be hindered by large amounts of contamination, including molecular species, stemming from the production of the radioactive beam. In this paper, we report on the development of Collision-Induced Dissociation (CID) as a means of background reduction for experiments at TRIUMF's Ion Trap for Atomic and Nuclear science (TITAN). This study was conducted to characterize the quality and purity of radioactive ion beams and the reduction of molecular contaminants to allow for mass measurements of radioactive isotopes …
Measurement of radioactive contamination in the high-resistivity silicon CCDs of the DAMIC experiment
我们目前的测量放射性污染的高电阻率硅电荷耦合器件（CCD）的DAMIC实验用于寻找暗物质粒子。开发了利用CCD独特空间分辨率的新分析方法来鉴定$α$和$β$颗粒。通过$α$光谱法测量CCD块体中的铀和钍污染，其中$^{238}$ U（$^{232}$ Th）衰变速率的上限为5（15）kg $^{-1}$ d $^{-1}$（95%CL）。我们还搜索了在时间上间隔长达数十天的空间相关电子径迹对，正如从$β$衰变的$^{32}$ Si- $^{32}$ P或$^{210}$ Pb- $^{210}$ Bi序列所预期的那样。发现$^{32}$ Si的衰减速率为$80^{+110}_{-65}$ kg $^{-1}$ d $^{-1}$（95% CI）。通过$α$光谱学和$β$衰变序列搜索独立获得了$\sim$ 35 kg $^{-1}$ d $^{-1}$（95% CL）对$^{210}$ Pb衰变速率的上限。这些放射性污染水平足够低，可以使即将推出的100 g DAMIC探测器中的CCD成功运行。
We present measurements of bulk radiocontaminants in the high-resistivity silicon CCDs from the DAMIC at SNOLAB experiment. We utilize the exquisite spatial resolution of CCDs to discriminate between $α$ and $β$ decays, and to search with high efficiency for the spatially-correlated decays of various radioisotope sequences. Using spatially-correlated $β$ decays, we measure a bulk radioactive contamination of $^{32}$Si in the CCDs of $140 \pm 30$ $μ$Bq/kg, and place an upper limit on bulk $^{210}$Pb of $< 160~μ$Bq/kg. Using similar analyses of spatially-correlated bulk $α$ decays, we set limits…
The design, construction and performance characteristics of a simple axial-field ionization chamber suitable for identifying ions in a radioactive beam are presented. Optimized for use with low-energy radioactive beams (< 5 MeV/A) the detector presents only three 0.5 $μ$m/cm$^2$ foils to the beam in addition to the detector gas. A fast charge sensitive amplifier (CSA) integrated into the detector design is also described. Coupling this fast CSA to the axial field ionization chamber produces an output pulse with a risetime of 60-70 ns and a fall time of 100 ns, making the detector capable of su…
As low as possible radioactive contamination of a detector plays a crucial role to improve sensitivity of a double beta decay experiment. The radioactive contamination of a sample of $^{116}$CdWO$_4$ crystal scintillator by thorium was reduced by a factor $\approx 10$, down to the level 0.01 mBq/kg ($^{228}$Th), by exploiting the recrystallization procedure. The total alpha activity of uranium and thorium daughters was reduced by a factor $\approx 3$, down to 1.6 mBq/kg. No change in the specific activity (the total $α$ activity and $^{228}$Th) was observed in a sample of ZnWO$_4$ crystal prod…
Low radioactive material screening is becoming essential for rare event search experiments, such as neutrinoless double beta decay and dark matter searches in underground laboratories. A gaseous time projection chamber (TPC) can be used for such purposes with large active areas and high efficiency. A gaseous TPC with a Micromegas readout plane of approximately 20$\times$20 cm$^2$ is successfully constructed for surface alpha contamination measurements. We have characterized the energy resolution, gain stability, and tracking capability with calibration sources. With the unique track-related ba…
Radioactive contaminants on the surfaces of detector components can be a problematic source of background events for physics experiments searching for rare processes. Exposure to radon is a specific concern because it can result in the relatively long-lived $^{210}$Pb (and progeny) being implanted to significant subsurface depths such that removal is challenging. In this article we present results from a broad exploration of cleaning treatments to remove implanted $^{210}$Pb and $^{210}$Po contamination from silicon, which is an important material used in several rare-event searches. We demons…
The next generation low-background detectors operating deep underground aim for unprecedented low levels of radioactive backgrounds. The deposition and presence of radon progeny on detector surfaces is an added source of energetic background events. In addition to limiting the detector material's radon exposure in order to reduce potential surface backgrounds, it is just as important to clean surfaces to remove inevitable contamination. Such studies of radon progeny removal have generally found that a form of etching is effective at removing some of the progeny (Bi and Pb), however more aggres…
Analysis of the 40K contamination in NaI(Tl) crystals from different providers in the frame of the ANAIS project
利用NaI（Tl）大晶体在探测器中的闪烁信号，通过探测器中目标核的弹性散射，寻找银河系中的暗物质粒子。然而，以核反冲形式沉积的能量很小，这增加了将该能量转换成闪烁的低效率，使得处于或非常接近能量阈值的事件，归因于放射性背景或伪噪声（非本体NaI（Tl）闪烁事件），可以损害这样的实验的灵敏度目标。DAMA/LIBRA实验，使用250公斤碘化钠（铊）目标，报告了第一个证据的存在，每年调制的检测率与预期的暗物质信号在6 keVee（电子当量能量）以下的区域兼容。在ANAIS（Annual modulation with NaI Scintillators）暗物质搜寻计划的框架内，为了理解大型碘化钠探测器中极低能量事件的起源，并开发方便的滤波器以滤除那些与大块NaI（Tl）晶体中的闪烁无关的事件，进行了大量而长期的努力。40 K可能是NaI（Tl）探测器中最相关的放射性污染物，因为它在非常低的能量下对本底有重要的贡献。ANAIS的目标是达到或低于20 ppb的天然钾水平。在本文中，我们将报告我们的努力，以确定40 K污染的几个NaI（Tl）晶体之间的符合测量，其中两个（或更多）。本文将比较不同供应商的晶体40 K含量的结果，并简要回顾ANAIS暗物质搜索实验的前景。
Radioactive $^{233}$U alpha recoil sources are being considered for the production of a thorium ion source to study the low-energy isomer in $^{229}$Th with high-resolution collinear laser spectroscopy at the IGISOL facility of the University of Jyväskylä. In this work two different $^{233}$U sources have been characterized via alpha and gamma spectroscopy of the decay radiation obtained directly from the sources and from alpha-recoils embedded in implantation foils. These measurements revealed rather low $^{229}$Th recoil efficiencies of only a few percent. Although the low efficiency of one …
We present new results on the radiopurity of a 3.4-kg NaI(Tl) crystal scintillator operated in the SABRE proof-of-principle detector setup. The amount of potassium contamination, determined by the direct counting of radioactive $^{40}$K, is found to be $2.2\pm1.5$ ppb, lowest ever achieved for NaI(Tl) crystals. With the active veto, the average background rate in the crystal in the 1-6 keV energy region-of-interest (ROI) is $1.20\pm0.05$ counts/day/kg/keV, which is a breakthrough since the DAMA/LIBRA experiment. Our background model indicates that the rate is dominated by $^{210}$Pb and that a…
Cs$_2$LiLaBr$_6$:Ce$^{3+}$ (CLLB) is an elpasolite scintillator that offers excellent linearity and gamma-ray energy resolution and sensitivity to thermal neutrons with the ability to perform pulse-shape discrimination (PSD) to distinguish gammas and neutrons. Our investigation of CLLB has indicated the presence of intrinsic radioactive alpha background that we have determined to be from actinium contamination of the lanthanum component. We measured the pulse shapes for gamma, thermal neutron, and alpha events and determined that PSD can be performed to separate the alpha background with a mod…
Large liquid argon detectors offer one of the best avenues for the detection of galactic weakly interacting massive particles (WIMPs) via their scattering on atomic nuclei. The liquid argon target allows exquisite discrimination between nuclear and electron recoil signals via pulse-shape discrimination of the scintillation signals. Atmospheric argon (AAr), however, has a naturally occurring radioactive isotope, $^{39}$Ar, a $β$ emitter of cosmogenic origin. For large detectors, the atmospheric $^{39}$Ar activity poses pile-up concerns. The use of argon extracted from underground wells, deprive…
The Advanced Mo-based Rare process Experiment (AMoRE)-Pilot experiment is an initial phase of the AMoRE search for neutrinoless double beta decay of $^{100}$Mo, with the purpose of investigating the level and sources of backgrounds. Searches for neutrinoless double beta decay generally require ultimately low backgrounds. Surface $α$ decays on the crystals themselves or nearby materials can deposit a continuum of energies that can be as high as the $Q$-value of the decay itself and may fall in the region of interest (ROI). To understand these background events, we studied backgrounds from radio…
