Scintillation Yield Research Articles

Geiger mode APD performance in a cryogenic two-phase Ar avalanche detector based on THGEMs

Characteristic properties of a Geiger Mode APD (G-APD) in a THGEM-based cryogenic two-phase Ar avalanche detector were studied in view of potential applications in rare-event experiments. G-APD signal amplitude and noise characteristics at cryogenic temperatures turned out to be superior to those at room temperature. The effective detection of avalanche scintillations from THGEM-multiplier holes in two-phase Ar has been demonstrated using a G-APD without wavelength shifter. At an avalanche gain of 60, the avalanche scintillation yield measured by the G-APD was as high as 0.9 photoelectrons per avalanche electron, extrapolated to 4 π acceptance.

Fundamental Limitations in the Performance of ${\rm Ce}^{3+}$–, ${\rm Pr}^{3+}$–, and ${\rm Eu}^{2+}$–Activated Scintillators

The fundamental limits on the scintillation decay, scintillation yield, and X-ray or γ-ray energy resolution obtainable with Ce3+, Pr3+, and Eu2+ activated inorganic scintillators are determined. Those limits are compared with what has been achieved today with scintillators like YAlO3:Ce, Lu2SiO5:Ce, LaCl3:Ce, LaBr3:Ce, Lul3:Ce, and emerging scintillators like Lu3Al5O12:Pr3+ and SrI2:Eu2+. Higher scintillation yield not necessarily leads to better energy resolution; nonproportional scintillation yield with deposited amount of ionization energy is an important aspect too that will be addressed.

Scintillation Screen Investigations for High-Current Ion Beams

At GSI, the Helmholtz Centre for Ion Research, the properties of scintillation screens, irradiated by an ion beam, were studied. For various materials, the different ion beams H+, C2+, Ar10+, Ni9+, Ta24+, and U28+ in the energy range from 4.8-11.4 MeV/u were applied with currents ranging from nA to some mA, delivered by the heavy ion LINAC at GSI. Scintillation screens are widely used for qualitative ion beam profile monitoring. However, precise measurements of the beam profile yield ambivalent results, especially for high beam currents. Thus, the properties (light yield, beam width, and higher statistical moments) of well-known scintillators, ceramic materials, and different quartz glasses are compared. The image of each ion beam pulse was recorded by a digital CCD camera and individually evaluated. A change of the imaged ion beam shape was observed for some materials. The recorded beam profile shows dependence on the scintillator material. Even for low beam intensities (17 nA) a difference in the beam width of about 25% was measured. Additionally, the light yield and beam width depend significantly on the screen temperature, which is increased by the ion impact. For ZrO2 : Al the influence of the screen temperature on the statistical moments was investigated. Furthermore the spectra of scintillation screens were studied in the region from 350 to 750 nm for the irradiation with H+ and Ta24+ ions. Empirical results are discussed and give rise to further investigations on the materials.

Nonproportional scintillation response of NaI:Tl to low energy x-ray photons and electrons

Nonproportional response of the scintillation yield of NaI:Tl was measured using highly monochromatic synchrotron irradiation ranging from 9–100 keV. Special attention is paid to the x-ray escape peaks. They provide us additional information about photon response in the range 0.9–12 keV. A rapid variation in the photon response curve is observed near the iodine K-electron binding energy. A dense sampling of data is performed around this energy and that data are used to apply a method, which we call K-dip spectroscopy. This method allows us to derive the electron response curve of NaI:Tl down to energies as low as 30 eV. A comparison of our data with data of others employing different methods is made. Advantages, limitations, and peculiarities of presented techniques and methods are discussed.

Effects of Nitrogen contamination in liquid Argon

A dedicated test of the effects of Nitrogen contamination in liquid Argon has been performed at the INFN-Gran Sasso Laboratory (LNGS, Italy) within the WArP R&D program.A detector has been designed and assembled for this specific task and connected to a system for the injection of controlled amounts of gaseous Nitrogen into the liquid Argon. The purpose of the test is to detect the reduction of the Ar scintillation light emission as a function of the amount of the Nitrogen contaminant injected in the Argon volume. A wide concentration range, spanning from ∼ 10−1 ppm up to ∼ 103 ppm, has been explored.Measurements have been done with electrons in the energy range of minimum ionizing particles (γ-conversion from radioactive sources). Source spectra at different Nitrogen contaminations are analyzed, showing sensitive reduction of the scintillation yield at increasing concentrations. Direct PMT signal acquisition exploiting high time resolution by fast waveform recording allowed high precision extraction of the main characteristics of the scintillation light emission in contaminated LAr. In particular, the decreasing behavior in lifetime and relative amplitude of the slow component is found to be appreciable starting from \U0001d4aa(1 ppm) of Nitrogen concentrations. The rate constant of the quenching process induced by Nitrogen in liquid Ar has been found to be kQ(N2) = 0.11 ± 0.01 μs−1ppm−1, consistent with a previous measurement of this quantity but with significant improvement in precision. On the other hand, no evidence for absorption by N2 impurities has been found up to the higher concentrations here explored.

Radiation hardness of polysiloxane scintillators analyzed by ion beam induced luminescence

The radiation hardness of polysiloxane based scintillators has been measured by ion beam induced luminescence (IBIL). The light intensity as a function of the irradiation fluence with an He + beam at 1.8 MeV (1.0 μA/cm 2) has been measured on undoped polymers synthesized with different amounts of phenyl units and on polysiloxanes doped with two different dye molecules (BBOT and Lumogen Violet) sensitizing the scintillation yield.

Doping of polysiloxane rubbers for the production of organic scintillators

Polysiloxane rubbers have been produced with different concentrations of phenyl groups and of dye molecules in order to find the best synthesis conditions for reaching a high light yield. In particular, two different polymer compositions were examined, namely with 15% and 22% of phenyl units in the starting resin. 2,5-Diphenyl oxazole (PPO) as a primary dopant and Lumogen F Violet 570 as secondary dopant were dispersed in the polysiloxane. Ion beam induced luminescence (IBIL) technique was employed for studying radioluminescence and radiation hardness properties. The α and γ scintillation yields were analyzed by measuring the pulse height spectra from 241Am and 60Co radioactive sources. First tests on the suitability of these materials to the detection of fast neutrons were also performed with a TOF procedure. Preliminary results indicate that these materials exhibit a scintillation yield comparable with NE102 plastic scintillator.

Optical and Scintillation Properties of Polydimethyl-Diphenylsiloxane Based Organic Scintillators

Polysiloxane based scintillators with high light yield have been synthesized. The polymer consists in cross-linked polydimethyl-co-diphenylsiloxane with different molar percentages of phenyl units. 2,5-diphenyl oxazole (PPO) and 2,5-bis(5-ter-butyl-2-benzoxazolyl)thiophene (BBOT) have been dispersed in the polymer as dopants. The energy transfer and scintillation capabilities have been investigated, for two different amounts of phenyl groups in the polymer network and for different concentrations of dye molecules, by means of fluorescence spectroscopy, ion beam induced luminescence (IBIL) and scintillation yield measurements with ¿ particles from an <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup> Am source. The luminescence features and the scintillation yields have been correlated to the composition of the scintillators.

Performance of scintillation materials at cryogenic temperatures

An increasing number of applications of scintillators at low temperatures, particularly in cryogenic experiments searching for rare events, has motivated the investigation of scintillation properties of materials over a wide temperature range. This paper provides an overview of the latest results on the study of luminescence, absorption and scintillation properties of materials selected for rare event searches so far. These include CaWO4, ZnWO4, CdWO4, MgWO4, CaMoO4, CdMoO4, Bi4Ge3O12, CaF2, MgF2, ZnSe and AL2O3-Ti. We discuss the progress achieved in research and development of these scintillators, both in material preparation and in the understanding of scintillation mechanisms, as well as the underlying physics. To understand the origin of the performance limitation of self-activated scintillators we employed a semi-empirical model of conversion of high energy radiation into light and made appropriate provision for effects of temperature and energy transfer. We conclude that the low-temperature value of the light yield of some modern scintillators, namely CaWO4, CdWO4 and Bi4Ge3O12, is close to the theoretical limit. Finally, we discuss the advantages and limitations of different materials with emphasis on their application as cryogenic phonon-scintillation detectors (CPSD) in rare event search experiments.

Secondary scintillation yield in high-pressure xenon gas for neutrinoless double beta decay [formula omitted] search

The search for neutrinoless double beta decay (0νββ) is an important topic in contemporary physics with many active experiments. New projects are planning to use high-pressure xenon gas as both source and detection medium. The secondary scintillation processes available in noble gases permit large amplification with negligible statistical fluctuations, offering the prospect of energy resolution approaching the Fano factor limit. This Letter reports results for xenon secondary scintillation yield, at room temperature, as a function of electric field in the gas scintillation gap for pressures ranging from 2 to 10 bar. A Large Area Avalanche Photodiode (LAAPD) collected the VUV secondary scintillation produced in the gas. X-rays directly absorbed in the LAAPD are used as a reference for determining the number of charge carriers produced by the scintillation pulse and, hence, the number of photons impinging the LAAPD. The number of photons produced per drifting electron and per kilovolt, the so-called scintillation amplification parameter, displays a small increase with pressure, ranging from 141±6 at 2 bar to 170±10 at 8 bar. In our setup, this parameter does not increase above 8 bar due to non-negligible electron attachment. The results are in good agreement with those presented in the literature in the 1 to 3 bar range. The increase of the scintillation amplification parameter with pressure for high gas densities has been also observed in former work at cryogenic temperatures.

Scintillation yield of Bi 4Ge 3O 12 (BGO) pixel crystals

BGO crystals have been grown using the Czochralski method by the West Pomeranian University of Technology in Szczecin, Poland. We report the measurements of scintillation yield and low temperature thermoluminescence, performed on polished 2×2×10 mm 3 pixel and 2×2×2 mm 3 cube samples. The yield of the pixels placed horizontally on the PMT window, as well as of the cubes, is about twice higher than the yield of the pixels placed vertically. Such a distinct light loss is interpreted in the frame of a simple two-ray model, describing quantitatively the scintillation yield of pixel samples depending on the measurement geometry. Related to this effect, the common overuse of “BGO units” for expressing the light output of any scintillator is discussed. Thermoluminescence glow curves consist of several peaks attributed to the existence of traps, which turn out to be responsible for a slight decrease of the scintillation yield measured at room temperature in a short time window. To improve the performance of BGO crystals, efforts should be aimed at reducing both the internal light losses and the trap concentration, with the main emphasis laid on the former.

Energy resolution and non-proportionality of scintillation detectors – new observations

Abstract According to the present knowledge, the non-proportionality of the light yield of scintillators appears to be the fundamental limitation of energy resolution and it is related to the intrinsic properties of the crystals. However, several observations collected in the last 10 years suggest more complex processes in the scintillators. First, the study of undoped NaI and CsI crystals showed that the non-proportionality and energy resolution are very sensitive to the accidental traces of impurities. For some crystals, like CsI(Tl), ZnSe(Te), undoped NaI at liquid nitrogen temperature and finally for NaI(Tl) at temperatures reduced below 0 °C, an influence of slow components of the light pulses on energy resolution and non-proportionality is observed. A common conclusion of these observations is the fact that the highest energy resolution, and particularly the intrinsic resolution measured with scintillators, characterized by two components of the light pulse decay, is obtainable when the spectrometry equipment integrates the whole light of both components. In the limiting case, the afterglow could be considered also as a very slow component destroying the energy resolution. The aim of this work is to summarize all above observations looking for their origin.

Luminescence and scintillation characteristics of YAG:Ce single crystalline films and single crystals

Abstract The detailed comparative analysis of luminescent and scintillation properties of the single crystalline films (SCF) of YAG:Ce garnet grown from melt-solutions based on the traditional PbO-based and novel BaO-based fluxes, and of a YAG:Ce bulk single crystal grown from the melt by the Czochralski method, was performed in this work. Using the 241 Am (α-particle, 5.49 MeV) excitation we show that scintillation yield and energy resolution of the optimized YAG:Ce SCF is fully comparable with that of the YAG:Ce single crystal analogue.

Tungstate and Molybdate Scintillators to Search for Dark Matter and Double Beta Decay

Results are presented on our latest research, aimed at the development and study of oxide scintillation crystals (ZnWO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , ZnMoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , PbWO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , PbMoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , and MgWO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) with high scintillation yield and low intrinsic radioactivity. We report on the improvement of these properties for conventional scintillators, as well as on new promising crystals based on metal tungstates and molybdates. The results are discussed in view of applying these materials in cryogenic experiments searching for dark matter and/or neutrinoless double beta decay.

Effect of SSPM surface coating on light collection efficiency and optical crosstalk for scintillation detection

As part of a project to develop a complete stochastic model of the solid state photomultiplier (SSPM), we simulated the effects of a reflective layer in the non-active region which reflects photons back into the crystal for a second chance at detection. The scintillation photon collection efficiency of the proposed device is greater than that predicted by the scintillation yield and the ratio of active area. The added reflectivity also increases external optical crosstalk from hot carrier emissions and the collection efficiency for these photons was simulated to be of the same order as the scintillation photon collection efficiency. As a function of excess bias, the rate of hot carrier emissions will rise more rapidly than the photon detection efficiency (PDE), setting a practical upper limit on the applied bias. Due to spectral mismatch between detection and avalanche emission, a blue-sensitve SSPM will be largely transparent to its own emissions. The spatial dependence of externally reflected hot carrier emissions was determined to be approximately uniform.

Computer simulation of the light yield nonlinearity of inorganic scintillators

To probe the nature of the physical processes responsible for the nonlinear scintillation light yield of inorganic scintillators, we have combined an ab initio based Monte Carlo code for calculating the microscopic spatial distributions of electron-hole pairs with an atomistic kinetic Monte Carlo (KMC) model of energy-transfer processes. In the present study, we focus on evaluating the contribution of an annihilation mechanism between self-trapped excitons (STE) to the scintillation response of pure CsI and Ce-doped LaBr3. A KMC model of scintillation mechanisms in pure CsI was developed previously and we introduce in this publication a similar model for Ce-doped LaBr3. We show that the KMC scintillation model is able to reproduce both the kinetics and efficiency of the scintillation process in Ce-doped LaBr3. Relative light output curves were generated at several temperatures for both scintillators from simulations carried out at incident γ-ray energies of 2, 5, 10, 20, 100, and 400 keV. These simulations suggest that STE-STE annihilation can account for the initial rise in relative light yield with increasing incident energy for both types of materials. This is due to the fact that the proportion of high-density regions decreases as the incident energy increases, thus reducing the likelihood for STE-STE encounter. In addition, the simulations clearly show a lack of temperature dependence of the relative light output, in agreement with a majority of experimental work on the temperature dependence of nonlinearity in inorganic scintillators. The collective modeling tool is a fundamental advance over phenomenological modeling approaches because it has its foundation in first-principles physics of scintillation. While the KMC simulations here are parametrized largely by empirically derived rate constants, this study suggests that combining ab initio based electron-hole pair distributions with ab initio derived rate constants for a select set of energy transfer processes is in principle sufficient to detach this tool from experiment entirely, yielding a holistic predictive simulation framework useful for exploring a wide range of scintillator performance characteristics.

Suppression of Afterglow in Microcolumnar CsI:Tl by Codoping With Sm$^{2+}$: Recent Advances

Microcolumnar CsI:Tl remains a highly desirable sensor for digital X-ray imaging due to its superior spatial resolution, bright emission, high absorption efficiency, and ready availability. Despite such obvious advantages, two characteristic properties of CsI:Tl undermine their use in clinical and high speed imaging: a persistent afterglow in its scintillation decay, and a hysteresis effect that distorts the scintillation yield after exposure to high radiation doses.In our earlier work we have discovered that the addition of 0.05 to 0.5 mol percent of Sm(2+) to crystals of CsI:Tl suppresses their afterglow by a factor of up to 50, even when subjected to a very high exposure of 120 R. This additive also diminishes hysteresis by an order of magnitude, which is a major accomplishment. Consequent- ly, our work is now focused on developing codoped microcolumnar CsI:Tl, Sm films that can potentially combine excellent properties of the current state-of-the-art CsI:Tl films with the reduced afterglow and hysteresis observed in codoped crystals. While our earlier attempts in CsI:Tl, Sm film fabrication, reported at the previous IEEE meeting, demonstrated obvious advantages of the approach, the recent work has succeeded in producing films that show improvement by at least a factor of 7 in afterglow and 150% in brightness compared to the standard CsI:Tl films. We report these important results in this paper, along with other recent advances in film growth and new imaging results.

Temperature dependent scintillation properties of pureLaCl3

The scintillation yield, scintillation decay, and x-ray excited emission of pureLaCl3 was studied as a function of temperature between 80 and 600 K. Twobroad band emissions centered around 325 nm and 400 nm were identifiedand correlated to emissions from two localized exciton states namedSTE1 andSTE2, respectively. Different temperature dependences were observed for the short and longwavelength band intensity. From 80 to 150 K, the 400 nm band intensity increases at theexpense of the 325 nm band intensity. Above 150 K almost all emission is in the 400 nmband. From 150 to 600 K, the intensity of this band decreases and its lifetime shortens.These results are analyzed and interpreted with a model that comprises the creation ofSTE1 andSTE2 self-trapped excitons, thermally activated quenching ofSTE1 andSTE2 emission, and thermally activated transfer of excitation energy fromSTE1 toSTE2.

Secondary scintillation yield from gaseous micropattern electron multipliers in direct Dark Matter detection

Efforts are being made in direct Dark Matter search experiments to detect the primary ionisation in the liquid by extracting the electrons to the gas phase and use the secondary ionization produced in the micropattern electron multipliers for signal amplification in noble-liquid dual-phase TPCs. We have studied the secondary scintillation yield of a single Gas Electron Multiplier (GEM) and of a Micro-Hole & Strip Plate (MHSP) for xenon at room temperature. Values for secondary scintillation yield between 5.0×103 and 1.3×103 photons per primary electron were obtained for the GEM and between 7.2×104 and 1.8×103 photons per primary electron for the MHSP, as the pressure increased from 1.0 to 2.5 bar in the GEM-setup and from 1.0 to 3.3 bar in the MHSP-setup, respectively. These values can be more than one order of magnitude higher than what has been obtained in the uniform-field scintillation gaps of the XENON and ZEPLIN-III experiments. Although in the present setups the amount of secondary scintillation obtained is sufficient in view of the use of PMTs, if a different type of readout is considered, such as large area avalanche photodiodes, it will be important to increase the amount of secondary scintillation. The attained results demonstrate the clear advantage of reading the secondary scintillation instead of the charge produced in the electron avalanches of micropattern electron multipliers, in low-background and low-rate experiments, as is the case in direct Dark Matter search.

Theoretical investigations on the high light yield of the LuI 3:Ce scintillator

The extremely high scintillation efficiency of lutetium iodide doped by cerium is explained as a result of at least three factors controlling the energy transfer from the host matrix to activator. We propose and theoretically validate the possibility of a new channel of energy transfer to excitons and directly to cerium, namely the Auger process when Lu 4f hole relaxes to the valence band hole with simultaneous creation of additional exciton or excitation of cerium. This process should be efficient in LuI 3, and inefficient in LuCl 3. To justify this channel, we perform calculations of density of states using a periodic plane-wave density functional approach. The second factor is the increase of the efficiency of valence hole capture by cerium in the row LuCl 3–LuBr 3–LuI 3. The third one is the increase of the efficiency of energy transfer from self-trapped excitons to cerium ions in the same row. The latter two factors are verified by cluster ab initio calculations. We estimate either the relaxation of these excitations and barriers for the diffusion of self-trapped holes (STH) and self-trapped exciton (STE). The performed estimations theoretically justify the high LuI 3:Ce 3+ scintillator yield.