Tl Crystals Research Articles

Direct searches of dark matter with the SABRE South experiment

The SABRE (Sodium-iodide with Active Background REjection) South experiment is a direct dark matter detector, made of radio-pure NaI(Tl) crystals surrounded by a liquid scintillator veto. The achievement of ultra-low background rate is essential to provide a model independent test of the signal observed by the DAMA/LIBRA experiment whose claim has not been verified yet. The SABRE South experiment will be located at the Stawell Underground Physics Laboratory (SUPL), Australia, the first deep underground laboratory in the Southern Hemisphere. The laboratory will not only house rare event physics searches but also measurement facilities to support low background physics experiments and applications like radiobiology and quantum computing. The SABRE South detector commissioning is expected to occur in 2023. This paper details the setup and projections for the experiment, and a brief description of the underground laboratory.

A CsI(Tl) hodoscope on CSHINE for Bremsstrahlung [formula omitted]-rays in heavy ion reactions

Bremsstrahlung γ production in heavy ion reactions at Fermi energies carries important physical information including the nuclear symmetry energy at supra-saturation densities. In order to detect the high energy Bremsstrahlung γ rays, a hodoscope consisting of 15 CsI(Tl) crystals read out by photo multiplier tubes has been built, tested and operated in experiment. The resolution, efficiency and linear response of the units to γ rays have been studied using radioactive source and (p,γ) reactions. The inherent energy resolution of 1.6%+2%/Eγ1/2 is obtained. Reconstruction method has been established through Geant 4 simulations, reproducing the experimental results where comparison can be made. Using the reconstruction method developed, the whole efficiency of the hodoscope is about 2.6×10−4 against the 4π emissions at the target position, exhibiting insignificant dependence on the energy of incident γ rays above 20 MeV. The hodoscope is operated in the experiment of 86Kr+124Sn at 25 MeV/u, and a full γ energy spectrum up to 80 MeV has been obtained.

Tangentially viewing hard X-ray imaging system and its measurement simulation on the HL-2M tokamak

A tangentially viewing Hard X-ray Real-Time Imaging (HXR-RTI) system and its measurement simulation on the HL-2M tokamak are presented. The HXR-RTI system mainly consists of a CsI:Tl scintillator crystal, a pinhole mechanism and a high-speed camera. When the CsI:Tl crystal is struck by the X-ray photons, certain wavelengths of visible or near-visible photons will be scintillated, which could be collected by the lens and imaged by the high-speed camera. This system can measure HXR with energy up to 300 keV, spatial resolution of less than 10 mm, and temporal resolution of several tens of microseconds. To understand the emission profile, we have developed a Geant4 model. The simulation results confirm that the spatial asymmetry will eventually lead to radiation asymmetry distribution. This system is designed for real-time exhibition of hard X-ray radiations from fast electron forward bremsstrahlung and will be used to study the dynamics of fast electrons.

Pulse shape analysis of LiI:Tl crystal scintillators

The LiI:Tl crystal is a candidate for thermal neutron measurements and rare event searches due to the advantage of the lithium and iodide isotopes in its composition. This study shows the pulse shape discrimination (PSD) capability of thermal neutron irradiation. Particle discrimination is essential for improving neutron measurement and rare event searches to reduce unwanted backgrounds. To investigate the pulse-shape characteristics of LiI:Tl crystals with different radiations, digital analysis of pulse shape has been considered. The raw pulses were fitted with a suitable pulse form. The fitted pulses were studied in both the time and frequency domains which improve the Figure-of-Merit over the charge comparison methods. Our results show the change of PSD for LiI:Tl crystals with different Thallium concentrations. The surface effect is discussed, and other related scintillation characteristics were also studied.

Comparative study on gamma-ray detectors for in-situ ocean radiation monitoring system

Large-sized crystals and state-of-the-art photosensors are desirable to cope with low environmental radioactivity (e.g., 1–2 Bq∙m−3137Cs in surface seawater) for homeland security purposes. We compared the performances of two different gamma-ray detector assemblies, GAGG crystal + silicon photomultiplier (SiPM) and NaI(Tl) crystal + photomultiplier tube, for our mobile in-situ ocean radiation monitoring system. We performed energy calibration, followed by water tank experiments with varying the depth of a137Cs point source. Experimental energy spectra were compared with MCNP-simulated spectra with identical setup and the consistency was validated. We finally assessed the detection efficiency and minimum detectable activity (MDA) of the detectors. Both GAGG and NaI detectors exhibited favorable energy resolutions (7.98 ± 0.13% and 7.01 ± 0.58% at 662 keV, respectively) and MDAs (33.1 ± 0.0645 and 13.5 ± 0.0327 Bq∙m−3 for 24-h 137Cs measurement, respectively). Matching the geometry of the GAGG crystal with that of the NaI crystal, the GAGG detector outperformed the NaI detector. The results demonstrated that the GAGG detector is potentially advantageous over the NaI detector in detection efficiency and compactness.

Spatial Recognition: A Temporal Tale of Multiple Window Spatial Registration.

The performance ability of position logic circuitry of gamma-camera to superimpose the X-Y gains obtained from different pulse height analyzers is multiple window spatial registration (MWSR). This study was conducted to evaluate the feasibility of dual-tracer method using 99mTc and 131I to assess MWSR. The procedure was carried out on a dual-head gamma-camera with rectangular NaI (Tl) crystal using a point source containing 99mTc pertechnetate and 131I-NaI. Pixel coordinates, full width at half-maximum of point spread function at 364 keV and 140 keV, and pixels with maximum count of images for dual detectors were analyzed and compared. The experiment demonstrated an excellent correlation between the X and Y coordinates identified on 364 keV and 140 keV images (Spearman's coefficient of rank correlation: 0.9, P < 0.0001). Intraclass correlation of studies performed on two separate occasions showed a good correlation (k = 0.87) demonstrating the reproducibility of the procedure. Dual-tracer method for evaluation of MWSR is a reproducible, accurate, and logical alternative to the routine National Electrical Manufacturers Association procedure.

Exploring coherent elastic neutrino-nucleus scattering using reactor electron antineutrinos in the NEON experiment

Neutrino elastic scattering observation with NaI (NEON) is an experiment designed to detect neutrino-nucleus coherent scattering using reactor electron antineutrinos. NEON is based on an array of six NaI(Tl) crystals with a total mass of 13.3 kg, located at the tendon gallery that is 23.7 m away from a reactor core with a thermal power of 2.8 GW in the Hanbit nuclear power complex. The installation of the NEON detector was completed in December 2020, and since May 2021, the detector has acquired data at full reactor power. Based on the observed light yields of the NaI crystals of approximately 22, number of photoelectrons per unit keV electron-equivalent energy (keVee), and 6 counts/kg/keV/day background level at 2–6 keVee energy, coherent elastic neutrino-nucleus scattering (CEnu NS) observation sensitivity is evaluated as more than 3sigma assuming 1-year reactor-on and 100 days reactor-off data, 0.2 keVee energy threshold, and 7 counts/keV/kg/day background in the signal region of 0.2-0.5 keVee. This paper describes the design of the NEON detector, including the shielding arrangement, configuration of NaI(Tl) crystals, and associated operating systems. The initial performance and associated sensitivity of the experiment are also presented.

Production and suppression of delayed light in NaI(Tl) scintillators

We investigate a hypothesis that energy accumulation and the subsequent release in NaI(Tl) may lead to pulse-like events in the few-keV energy regime, a phenomenon suggested by the crystal manufacturing company Saint-Gobain, who provided the crystals for DAMA-LIBRA. While we observed delayed long-lasting light emission in a 3" NaI(Tl) crystal after exposing it to UV light, the delayed light consists primarily of single photons that are uncorrelated with each other. We also observe delayed light emission in NaI(Tl) following gamma radiation and large ionization events like cosmic-ray muons. We found that irradiating the crystal with red light after UV exposure significantly suppressed delayed photon emissions.

Performance of an ultra-pure NaI(Tl) detector produced by an indigenously-developed purification method and crystal growth for the COSINE-200 experiment

The COSINE-100 experiment has been operating with 106 kg of low-background NaI(Tl) detectors to test the results from the DAMA/LIBRA experiment, which claims to have observed dark matter. However, since the background of the NaI(Tl) crystals used in the COSINE-100 experiment is 2–3 times higher than that in the DAMA detectors, no conclusion regarding the claimed observation from the DAMA/LIBRA experiment could be reached. Therefore, we plan to upgrade the current COSINE-100 experiment to the next phase, COSINE-200, by using ultra-low background NaI(Tl) detectors. The basic principle was already proved with the commercially available Astro-grade NaI powder from Sigma-Aldrich (now Merck) company. However, we have developed a mass production process of ultra-pure NaI powder at the Center for Underground Physics (CUP) of the Institute for Basic Science (IBS), Korea, using the direct purification of the raw NaI powder. We plan to produce more than 1,000 kg of ultra-pure powder for the COSINE-200 experiment. With our crystal grower installed at CUP, we have successfully grown a low-background crystal using our purification technique for the NaI powder. We have assembled a low-background NaI(Tl) detector. In this article, we report the performance of this ultra-pure NaI(Tl) crystal detector produced at IBS, Korea.

Technology for the Nondestructive Reprocessing of Sodium Iodide Technogenic Solutions into a Charge for Single Crystal Growth

Introduction. The production of NaI-based single crystals results in accumulation of great amount of liquid waste. To bring valuable constituents of this waste back, the NaI technogenic solutions are treated to precipitate iodine that is further used in the synthesis of high-pure NaI. However, the least contaminated part of the waste may be brought back into the main technological process without the intermediary stage of iodine destruction, which allows a considerable cost saving.Problem Statement. There have been no data on the effective purification coefficients for NaI solutions subsequently treated with barium hydroxide, sodium carbonate, and by mass crystallization. Requirements to impurity content, which NaI solutions have to comply with in order to be used in the nondestructive treatment have mot been not formulated.Purpose. The purpose of this research is to develop a technology for reprocessing NaI technogenic solutionsinto sodium iodide of high purity without NaI destruction at the intermediate stage.Material and Methods. The materials that have been used for this research are as follows: aqueous solutions of NaI-based crystal production waste, active coal, barium hydroxide, sodium carbonate. The methods that have been employed are the treatment of NaI solutions with barium hydroxide, sodium carbonate, and by mass crystallization. Results. The technology of NaI solution purification by treatment with barium hydroxide and sodium carbonate with further mass crystallization of the purified salt has been developed. This technology is more environment friendly and cheaper than the conventional one that includes obtaining pure iodine. A series of NaI technogenic solutions has been treated in laboratory conditions and on industrial equipment, and the efficiency of their purification by the chemical treatment and mass crystallization hasbeen estimated. The impurity content in the obtained salt and the scintillation parameters of grown NaI : Tl crystals meet the requirements for these products.Conclusions. The technology has been implemented in the manufacturing process at the pilot plant of the Institute for Scintillation Materials of the NAS of Ukraine. It may be used at other enterprises that deal with alkali metal iodide waste treatment.

A novel YAG:Ce/CsI:Tl phoswich detector consisting of transparent ceramic scintillator for α–γ discrimination

Developing a ceramic scintillator operating in a phoswich detector is a feasible way for pulse shape discrimination. A YAG:Ce transparent ceramic scintillator has been synthesized by using the vacuum sintering method. In this work, we present a new phoswich detector mainly developed for the determination of low radioactivity levels. The system was constructed by combining a transparent ceramic scintillator, for the detection of α particles, and a CsI:Tl crystal, for the detection of γ-rays. With the device proposed here, the particle identification in a mixed α–γ field has been achieved with the rise time discrimination method. Furthermore, the figure of merit can also be improved using the phoswich detector. The higher reliability, simplicity, and low cost of the design that combines ceramics and crystals make the YAG ceramic have extremely good application prospects.

Full spectrum estimation of helicopter background and cosmic gamma-ray contribution for airborne measurements

The airborne radiation monitoring has been used in geophysics for more than forty years and now it also has its important role in emergency monitoring. The aircraft background and the cosmic gamma-rays contribute to the measured gamma spectrum on the aircraft board. This adverse effect should be eliminated before the data processing. The paper describes two semiparametric methods to estimate the full spectrum aircraft background and cosmic gamma-ray contribution from spectra measured at altitudes where terrestrial contribution is negligible. The methods only assume to know possible peak positions in spectra and their full width at half maximum, that can be easily obtained e.g. from terrestrial measurement. The methods were applied to real experimental data acquired on Mi-17 and Bell 412 helicopter boards. The IRIS airborne gamma-ray spectrometer, with 4 × 4 L NaI(Tl) crystals, produced by Pico Envirotec Inc., Canada, was used on helicopters’ boards. To obtain valid estimate of the aircraft background and the cosmic contribution, the measurements over sea and large water areas were carried out. However, the satisfactory results over inland were also achieved comparing with those acquired over large water areas.

Pulse Shape Discrimination of low-energy nuclear and electron recoils for improved particle identification in NaI:Tl

The scintillation mechanism in NaI:Tl crystals produces different pulse shapes that are dependent on the incoming particle type. The time distribution of scintillation light from nuclear recoil events decays faster than for electron recoil events and this difference can be categorised using various Pulse Shape Discrimination (PSD) techniques. In this study, we measured nuclear and electron recoils in a NaI:Tl crystal, with electron equivalent energies between 2 and 40 keV. We report on a new PSD approach, based on an event-type likelihood; this outperforms the charge-weighted mean-time, which is the conventional metric for PSD in NaI:Tl. Furthermore, we show that a linear combination of the two methods improves the discrimination power at energies greater than 10 keV.

Growth and Evaluation of Improved CsI:Tl and NaI:Tl Scintillators

Scintillators play an important role in radiation detection and imaging. Thallium-doped cesium iodide (CsI:Tl) and sodium iodide (NaI:Tl) are two of the major scintillators that have been used for many applications for many decades. In this paper, we will present an improved scintillation performance of Bridgman method-grown CsI(Tl) and NaI(Tl) crystals developed by Xtallized Intelligence, Inc. (XI, Inc.), and we will compare the performance with commercially available CsI:Tl and NaI:Tl. In a preliminary testing using MicroFJ−60035−TSV silicon photomultipliers (ON Semiconductor), the newly developed and improved 12.5 × 12.5 × 6 mm3 CsI:Tl crystal has shown an energy resolution of 4.8% (FWHM) at 662 keV, compared to 7.2% obtained by a commercially available CsI:Tl with a size of 12.5 × 12.5 × 25 mm3. The energy resolution of 5.4% (FWHM) at 662 keV is obtained for the newly improved NaI:Tl crystal, compared to 7% obtained by a commercially available ∅1″ × 1″ NaI(Tl). The comparison of the photo peak channel locations shows that the improvements in both CsI:Tl and NaI:Tl, developed by XI, Inc., have produced much larger signal amplitudes (roughly double), when compared to the commercially available CsI(Tl) and NaI:Tl. Improved decay time constants are presented as well.

Status of the SABRE South experiment at the Stawell underground physics laboratory

The SABRE (Sodium iodide with Active Background REjection) experiments aim to detect an annual rate modulation from dark matter interactions in ultra-high purity NaI(Tl) crystals. The SABRE South experiment is located at the Stawell Underground Physics Laboratory (SUPL), Australia, and is the first deep underground laboratory in the Southern Hemisphere. SABRE South is designed to disentangle seasonal or site-related effects from the dark matter-like modulated signal first observed by DAMA/LIBRA in the Northern Hemisphere using an active veto and muon detection system. It is a partner to the SABRE North effort at the Laboratori Nazionali del Gran Sasso (LNGS). SABRE South is instrumented with ultra-high purity NaI(Tl) crystals immersed in a linear alkyl benzene (LAB) based liquid scintillator veto, further surrounded by passive steel and polyethylene shielding and a plastic scintillator muon veto. Significant work has been undertaken to understand the experimental backgrounds and performance of the crystals. The SABRE South experiment is under construction, and will be commissioned from late 2022 to early 2023. We will present the final design of SABRE South, the status of its construction, its expected background, and its sensitivity to a DAMA/LIBRA like modulation. We will also present recent quenching factor measurements of sodium nuclear recoils in NaI(Tl) crystals measured at the Heavy Ion Accelerator Facility, and a report on the status of SUPL.

NaI(Tl) crystal scintillator encapsulated in two organic-scintillator layers with pulse shape data analysis

Thallium-doped sodium iodide (NaI(Tl)) crystals are widely used in radiation detection applications, from gamma-ray spectroscopy to particle dark matter searches. However, if the crystal is exposed to relative humidity of even a few percent, its light emission degrades, making the crystal impractical as a detector. Surrounding the crystal with organic scintillators not only protects the surface of the crystal from humid air but also offers a new capability to tag backgrounds such as external gamma rays and surface contaminations. We developed a detector that is constructed by fully encasing a NaI(Tl) crystal in a plastic scintillator and then immersing the plastic-crystal assembly in liquid scintillator. Using data collected from this triple phoswich detector, a pulse shape analysis is able to identify the various radiation signals from the three scintillators. Additionally, we find that the crystal's emission quality is maintained for a month.

Comparative analysis of gamma ray spectrometers applied to Irati Formation, Paraná basin, São Paulo State, Brazil

This paper describes the use of gamma ray spectrometry in the study of rock samples from the Irati Formation, Paraná sedimentary basin, São Paulo State, Brazil. This technique allowed to measure the natural radiation emitted by 40K, as well the radionuclides belonging to the decay series of 238U (eU = 226Ra = 214Bi) and 232Th (eTh = 228Th = 208Tl) which occur in the analyzed samples. Four gamma ray spectrometers have been utilized for comparing the results obtained: a portable sodium iodide [NaI(Tl)] scintillation detector (Digidart), a handheld bismuth germanate oxide (BGO) detector and two bench NaI(Tl) crystals, differing in their geometry (Planar and Well types). This study involved the calibration of the spectrometers, except in the case of the BGO that is factory calibrated. Afterwards, gamma ray analysis was done for 122 rock samples colleted at Partecal Quarry located at Assistência District, Rio Claro city, which have been of interest for the oil and gas sector. For comparison purposes, the obtained datasets were subjected to different statistical tests, including the analysis of variance (ANOVA) that proved to be of great value for checking the differences of the mean concentration values of eU, eTh and K. The results pointed out several factors that affect the gamma ray analysis for the natural radioelements uranium, thorium, and potassium such as the samples size, shape and geometry, detector type, shielding and counting time. These factors are of difficult control in order to get reliable radiometric measurements by this technique.

Large-Area SiPM Pixels (LASiPs) in SPECT

A standard camera for Single Photon Emission Computed Tomography (SPECT) contains 50–100 photomultiplier tubes (PMTs) that occupy at least 50% of its volume. It is shielded by a thick layer of lead which makes it heavy and bulky. Replacing PMTs by silicon photomultipliers (SiPMs) could significantly reduce the weight and size of a SPECT camera. However, the main obstacle is the limited size of SiPMs: even with the largest commercially available SiPM of 6 × 6 mm2 a few thousand channels would be needed to fill a camera. As a solution, we propose to use Large Area SiPM Pixels (LASiPs) which are built by summing the currents of several SiPMs into a single output. We developed a LASiP prototype summing 8 SiPMs using the MUSIC ASIC. To test the feasibility of using this solution in SPECT, we built a proof-of-concept micro camera that consisted of four of our LASiP prototypes coupled to a 40 × 40 × 8 mm3 NaI(Tl) crystal. We were able to reconstruct simple images of a 99m Tc capillary with an intrinsic spatial resolution of ∼2 mm and an energy resolution of ∼11.6%. We also simulated the system with Geant4, finding a good agreement with our experimental results. The simulations were extended to a larger camera, aiming to study the impact of pixel size, shape and noise.

Study on NaI(Tl) crystal at −35 [formula omitted]C for dark matter detection

We present the response of a NaI(Tl) crystal in terms of the light yield and pulse shape characteristics at two different temperatures: 22 °C (room temperature) and −35 °C (low temperature). The light yield is measured using 59.54keV γ-rays from a 241Am source relative to the mean charge of single photoelectrons. At the low temperature, we measure a 4.7 ± 1.3% increase in the light yield compared to that at room temperature. A significantly increased decay time is also observed at the low temperature. The responses to nuclear recoil events are measured using neutrons from a 252Cf source and compared to those to electron recoil events. The measured pulse shape discrimination (PSD) power of the NaI(Tl) crystal at the low temperature is found to be improved in the entire energy range studied because of the increased light yield and the different scintillation characteristics. We also find a 9.23 ± 0.26% increased quenching factor of α-induced events, which is the light yield ratio of α recoil to electron recoil, at the low temperature. This supports the possibility of an increased quenching factor of the nuclear recoil events that are known to have similar processes of dark matter interaction. The increased light yield and the improved PSD power of nuclear recoil events enhance the sensitivity for dark matter detection via dark matter–nuclei interactions.

The light response of CsI:Tl crystal after interaction with gamma radiation study using analysis of single scintillation pulses and digital oscilloscope readout

Experimental research on light response of CsI:Tl, that was performed in the last 30 years, have shown that spectrometry parameters of this crystal are influenced by existence of slow scintillation components and could be improved by increasing the peaking time. In this study authors analysed and discussed the non-proportionality, number of photoelectrons and energy resolution (with their intrinsic, statistical and noise contributions) as a function of light pulse integration time (adequately to the peaking time known from analogue readouts). The study of light responses of CsI:Tl crystal after interaction with gamma radiation was performed by acquisition of raw single scintillation signals using a photomultiplier and a high class digital oscilloscope. The analysis was performed off-line using Python scripts. The crystal was excited using X-ray and gamma-ray sources with energies from 22 keV up to 835 keV at 293 K (＋20 °C). The data was processed with integration time up to 150μs, which was not achievable so far with analogue electronics commonly used in gamma-ray spectrometry with scintillation detectors. This study considers the problem of the energy resolution as good as other spectrometry parameters improvement using a digital light pulse processing.