Scintillation Pulse Research Articles

New thermal neutron scintillators: Cs/sub 2/LiYCl/sub 6/:Ce/sup 3+/ and Cs/sub 2/LiYBr/sub 6/:Ce/sup 3+/

The thermal neutron detection properties of Cs/sub 2/LiYCl/sub 6/:0.1%Ce/sup 3+/ and Cs/sub 2/LiYBr/sub 6/:1%Ce/sup 3+/ single crystals are presented. The compounds show high scintillation photon yields of 70 000 and 88 200 photons emitted per absorbed thermal neutron, respectively. Events caused by primary electrons with energy below 2.9 and 3.4 MeV, respectively, can be separated from neutron-induced events by pulse-height discrimination. For Cs/sub 2/LiYCl/sub 6/:0.1Ce/sup 3+/, due to the presence of a 4-ns fast core-valence luminescence, thermal neutrons and gamma rays can also be discriminated by means of the shape of the scintillation pulse. The main part of the scintillation pulse in Cs/sub 2/LiYCl/sub 6/:0.1%Ce/sup 3+/ is relatively slow, however, Cs/sub 2/LiYBr/sub 6/:1%Ce/sup 3+/ has a relatively intense component with /spl tau/=85;ns decay time.

A fast digital filter algorithm for gamma-ray spectroscopy with double-exponential decaying scintillators

Scintillators like CsI(Na), having double-exponential decay times, typically cannot be used in high-count rate applications due to the complicated pulse shapes created by the convolution of their light output decay curves with the decay constant of charge integrating preamplifiers. We present here a novel digital filtering algorithm that is capable of using CsI(Na) at input count rates exceeding 250 kcps, while still achieving good energy resolution. We used a 2.54 cm diameter and 2.54 cm long CsI(Na) crystal, whose scintillation light can be best described by a short component with a 550 ns decay time and a long component with a 4 /spl mu/s decay time. The crystal was coupled to a 2.86 cm diameter photomultiplier tube. The digital filtering algorithm was implemented in XIA's all-digital Polaris spectrometer, in which five running sums were captured from each digitized scintillation pulse and the Polaris's on-board DSP read these sums and used a set of precomputed coefficients to reconstruct the pulse's total light output as a measure of the deposited energy. The algorithm was tested at different input count rates, ranging from 19 kcps to 270 kcps using a 1 mCi /sup 137/Cs source. The energy resolution (full-width at half-maximum) at 662 keV was 10.7% at 19 kcps and 11.7% at 270 kcps with a filter rise time of 1.0 /spl mu/s, and improved to 7.0% and 8.4%, respectively, with a filter rise time of 3.2 /spl mu/s. The energy peak shifted by less than 0.3% for input count rates below the maximum throughput point. Output count rates of 65.3 and 17.8 kcps were obtained with filter rise time of 1.0 and 3.2 /spl mu/s, respectively, at an input count rate of 270 kcps. This algorithm can be easily adapted to other double-exponential decaying scintillators by changing the decay times used in the energy reconstruction formula.

Structure and scintillation properties of CsI(Tl) epitaxial layers

CsI(Tl) layers of microcolumnar morphology were grown on LiF substrates by vacuum deposition. Structure and scintillation properties of layers were examined using X-ray diffraction, scanning electron microscopy, scintillation pulse height spectrometry. Single crystalline structure of constituent blocks and increased light yield of films was observed.

A PET system with free running ADCs

A small PET system has been built up with two multichannel photomultipliers, which are attached to a matrix of 64 single LSO crystals each. The signal from each multiplier is being sampled continuously by a 12 bit ADC at a sampling frequency of 40 MHz. In case of a scintillation pulse a subsequent FPGA sends the corresponding set of samples together with the channel information and a time mark to the host computer. The data transfer is performed with a rate of 20 MB/s. On the host all necessary information is extracted from the data. The pulse energy is determined, coincident events are detected and multiple hits within one matrix can be identified. In order to achieve a narrow time window the pulse starting time is refined further than the resolution of the time mark (=25 ns) would allow. This is possible by interpolating between the pulse samples. First data obtained from this system will be presented. The system is part of developments for a much larger system and has been created to study the feasibility and performance of the technique and the hardware architecture.

Rare-earth-activated wide bandgap materials for scintillators

Open f-shell rare-earth (RE) ions in wide bandgap host materials are usually characterized by closely spaced electronic levels due to various electron configurations and charge states. These levels provide convenient luminescent transitions that can be excited by efficient recombination of charge carriers generated in the host material by ionizing radiation. Therefore, it is the area of ionizing radiation detectors, where search for new, fast and efficient scintillator materials for high-energy physics and nuclear medicine, has yielded much of the recent advances in the understanding of radioluminescence and scintillation mechanism in some solid state, UV and VUV luminescent, RE-activated materials. In this paper we shall present selected results of basic experiments such as radioluminescence, VUV spectroscopy, time profiles and thermoluminescence, on barium fluoride (activated with Ce, Pr, Nd, Tb) and two aluminum perovskites, YAlO 3 and LuAlO 3, activated with Ce. We shall demonstrate that these results point to consecutive carrier capture and recombination at RE ions as the basic mechanism of radioluminescence and scintillation in these materials, despite the strong self-trapping and poor charge transport properties. Consequently, various electron and/or hole traps that intercept and retain for some time the recombining charge carriers play an active role influencing both the scintillation light yield and time profiles of scintillation pulses in these and many other wide bandgap RE-activated luminescent materials.

LiCaAlF 6:Ce crystal: a new scintillator

Scintillation properties of LiCaAlF 6:Ce crystal, well known as the effective UV laser material, is reported. Ce 3+ emission at 286–305 nm with a single exponential decay time of 35 ns provides a scintillation pulse. Radiation damage in pure and Ce-doped crystals is studied. In contrast to the majority of fluoride crystals, cerium is responsible for the ultradeep traps formation revealing thermostimulated luminescence. Overlapping of color center absorption and Ce 3+ ion emission bands limits the scintillation efficiency of LiCaAlF 6:Ce at high radiation doses.

Study of the nuclear recoil response of a NaI(Tl) scintillator detector with a 252Cf neutron source

This paper describes the behavior of the scintillation pulses induced in a NaI(Tl) crystal scintillator under excitation of a 252Cf neutron source and a 137Cs gamma source and the possible identification of the incident particle by the study of the pulse shape. Furthermore, the energy dependence of the mean decay time is presented.

Measurements of the proton light output function of the organic liquid scintillator NE213 in several detectors

When using an organic liquid scintillator such as NE213 for neutron spectrometry, the light output as a function of proton energy is needed in order to unfold the neutron spectrum from the scintillator's pulse height distribution. We have measured this function for several detectors over the range 1.5–16 MeV approximately, using monoenergetic neutrons from the Harwell 5 MV Van de Graaff accelerator. Results were obtained for a wide variety of sizes and shapes of the scintillator cell, and were found to be essentially in agreement within errors. The results were also compared with those of several other workers (amongst whom there is considerable disagreement). Below 10 MeV, there is excellent agreement with one worker and moderate or poor agreement with others; above 10 MeV, agreement is moderate in all cases. We conclude that workers wishing to unfold neutron spectra from NE213 pulse height distributions would be advised to make measurements with their own particular detector configuration, rather than use published functions.

Measurements of scintillation efficiency and pulse shape for low energy recoils in liquid xenon

Results of observations of low energy nuclear and electron recoil events in liquid xenon scintillator detectors are given. The relative scintillation efficiency for nuclear recoils is 0.22 +/- 0.01 in the recoil energy range 40 keV - 70 keV. Under the assumption of a single dominant decay component to the scintillation pulse-shape the log-normal mean parameter T0 of the maximum likelihood estimator of the decay time constant for 6 keV < Eee < 30 keV nuclear recoil events is equal to 21.0 ns +/- 0.5 ns. It is observed that for electron recoils T0 rises slowly with energy, having a value ~ 30 ns at Eee ~ 15 keV. Electron and nuclear recoil pulse-shapes are found to be well fitted by single exponential functions although some evidence is found for a double exponential form for the nuclear recoil pulse-shape.

CsI(Tl) for WIMP dark matter searches

We report a study of CsI(Tl) scintillator to assess its applicability in experiments to search for dark matter particles. Measurements of the mean scintillation pulse shapes due to nuclear and electron recoils have been performed. We find that, as with NaI(Tl), pulse shape analysis can be used to discriminate between electron and nuclear recoils down to 4 keV. However, the discrimination factor is typically 10–15% better than in NaI(Tl) above 4 keV. The quenching factor for caesium and iodine recoils was measured and found to increase from 11% to ∼17% with decreasing recoil energy from 60 to 12 keV. Based on these results, the potential sensitivity of CsI(Tl) to dark matter particles in the form of neutralinos was calculated. We find an improvement over NaI(Tl) for the spin-independent WIMP–nucleon interactions up to a factor of 5 assuming comparable electron background levels in the two scintillators.

HYBRID RADIOASSAY OF MULTIPLE RADIONUCLIDE MIXTURES IN WASTE SOLUTIONS BY USING LIQUID AND NaI(Tl) SCINTILLATION MONITORS

A new analytical technique for radioactive waste solutions has been developed by using a combination of a liquid and a NaI(Tl) scintillation monitor, which enables beta-emitter mixtures to be radioassayed using one calculation process with the method of least squares. This hybrid system can facilitate the analysis of beta-emitter mixtures with very similar liquid scintillation pulse height distributions, such as 3H, 51Cr, and 125I. All that is required for the technique are sets of quench standards of the nuclides to be analyzed and calibration standards for the gamma-emitters. Detection limits for seven nuclides were estimated to be about 0.005 Bq mL(-1), which are sufficiently low compared with the values of authorized safety guidelines.

Radon-implanted 214Po and anomalous pulses in sodium iodide detectors for dark matter

214Po is implanted into surfaces exposed to radon. The implantation rate could be sufficient to explain the rate of low-energy events with anomalously fast scintillation pulses that has been observed in sodium iodide crystals used in a search for WIMP dark matter. The rate should be closely coupled in time to the radon concentration.

Digital alpha/beta pulse shape discrimination of CsI:Tl for on-line measurement of aqueous radioactivity

A digital pulse shape discrimination data acquisition system developed around a commercially available digital oscilloscope card and data acquisition software was applied to on-line measurement of aqueous radioactivity. The radiation detector consisted of a flow-cell containing granular 63-90 /spl mu/m Parylene C polymer coated CsI:Tl and two Burle 8850 photomultiplier tubes (PMTs). The PMTs were connected in a software-realized coincidence mode. The digital data acquisition system consisted of a GaCe CompuScope 8012A/PCI 12-bit dual channel 50-MHz digital oscilloscope card residing in a Pentium III 450 MHz personal computer and the LabVIEW 5.1 data acquisition software. The individual fast anode scintillation pulses were digitized by the oscilloscope, transferred from on-board memory to computer memory, and processed by the LabVIEW Virtual Instrument (VI). The coincidence resolving time was set to 100 ns to reduce background. Pulse shape discrimination was achieved using the charge integration technique. Experimental tests showed very good pulse shape discrimination between alpha and beta particles. The alpha to beta and beta to alpha spillover were 2.1% and 1.4% respectively for single parameter analysis which yielded a figure of merit (FOM) of 1.65. The alpha to beta and beta to alpha spillover for dual parameter analysis were 1.3% and 0.3%, respectively. The alpha and beta detection efficiencies for the CsI:Tl flow-cell were typically 31% and 26%, respectively. This system was also coupled to a liquid ion chromatography system for online measurement of aqueous alpha/beta radionuclides.

Investigation of pulse shapes and time constants for NaI scintillation pulses produced by low energy electrons from beta decay

The scintillation pulse shape in NaI crystals of electrons from beta decay is investigated, as a possible explanation of anomalous pulses seen in dark matter searches based on NaI detectors. The pulse shapes and time constants from beta decay electrons are found not to be significantly different from recoil electrons from Compton scattering and thus do not account for the anomalous shorter pulses.

Dual parameter analysis of CsI:Tl/PMT with a digital oscilloscope

Abstract Scintillation pulses from alpha-particle and gamma-ray excited CsI:Tl were recorded with a photomultiplier tube and digital oscilloscope card in single parameter (pulse shape or pulse height) and dual parameter (simultaneous pulse shape and pulse height) modes. A 2.8×1.3×1.3 cm 3 CsI:Tl crystal was coupled to a Burle 8850 photomultiplier tube (PMT) and excited with 148 Gd ( E α =3.18 MeV) and 137 Cs ( E γ =0.662 MeV). The timing pulses (50 Ω terminated dynode signal) from the PMT base were directly input to a GaGe CompuScope 8012A/PCI TM (12-bit, 50 MHz dual channel digital oscilloscope card) that resided in a P6-300 MHz personal computer. Individual digitized pulses were processed via a LabVIEW TM V 4.1 interface to the oscilloscope. The ratio of the charge collected over a short time interval to the total charge collected was used for the single parameter pulse shape spectrum while the total charge collected was used for the single parameter pulse height spectrum. The dual parameter spectrum was produced by displaying the pulse shape and pulse height information for individual pulses on a 2-D grid. The digital oscilloscope with LabVIEW TM interface offers a versatile data acquisition system for pulse shape, pulse height and dual-parameter analyses.

A hybrid radioassay technique for multiple beta-emitter mixtures using liquid and NaI(Tl) scintillation spectrometers

Multiple pure-beta and beta-gamma emitter mixtures can be reliably radioassayed using a combination of a liquid and a NaI(Tl) scintillation spectrometer. The method of least squares has been applied to the radioassay of relatively low-activity samples. This system removes the difficulty of analysing beta-emitter mixtures with very similar liquid scintillation pulse height distributions, such as 3 H , 51 Cr and 125 I . All that is required for the technique is a quenched standard sample set of the nuclides to be analyzed and calibration standard samples for the gamma-emitters.

Characteristics of alpha, gamma and nuclear recoil pulses from NaI(Tl) at 10–100 keV relevant to dark matter searches

Measurements of the shapes of scintillation pulses produced by nuclear recoils, alpha particles and photons in NaI(Tl) crystals at visible energies of 10–100 keV have been performed in order to investigate possible sources of background in NaI(Tl) dark matter experiments and, in particular, the possible origin of the anomalous fast time constant events observed in the UK Dark Matter Collaboration experiments at Boulby mine [P.F. Smith et al., Phys. Rep. 307 (1998) 275]. Pulses initiated by X-rays (via photoelectric effect close to the surface of the crystal) were found not to differ from those produced by high-energy photons (via Compton electrons inside the crystal) within experimental errors. However, pulses induced by alpha particles (degraded from an external MeV source) were found to be ∼10% faster than those of nuclear recoils, but insufficiently fast to account for the anomalous events.

Radiation Damage of NaI(Tl) by Fast Neutron Irradiation: Blocking of the Energy Transfer Processes fromVkCenters and Electrons to the Activator of Tl

The scintillation pulse heights and decay curves from fast-neutron irradiated NaI(Tl) crystals up to the integrated fast neutron flux of 2.4×10 15 cm -2 were measured under excitation by electrons, alpha particles and fission fragments. For Tl concentrations of 0.01, 0.07 and 0.22 mole%, decreases by 20∼40% in scintillation pulse heights are observed for the irradiated crystals compared with those of non-irradiated crystals. The decreases are enhanced for electron excitation compared with those for alpha particle and fission fragment excitation. The decrease in scintillation pulse height is attributed to blocking of the energy transfer processes of V k centers and electrons by the traps and lattice disorders which were produced by fast neutrons. The effect of this blocking of the energy transfer processes is clearly observed as a difference in decay curves for the crystals before and after fast neutron irradiation; a flat top which appeared before the irradiation is not present after irradiation.

Current limits on the cold dark matter interaction cross section obtained by the UK collaboration

The UK Dark Matter Collaboration has run a 5–6 kg NaI detector in a well-shielded underground environment for about a year. Signatures of Cold Dark Matter (CDM) interactions are sought using pulse shape discrimination techniques by searching for the relatively short scintillation pulses arising from nuclear recoils among the residual, longer, background events from residual radioactivity in the detector and its environment. Here we report on an improvement to the limits on the CDM cross section for spin dependent interactions using our latest data from a crystal with improved detection efficiency and taking into account recent improved estimates of the spin factor correction between the nucleus and nucleon-supersymmetric particle cross sections.

A thermal-neutron scintillator with n/γ discrimination LiBaF [formula omitted]Ce,Rb

A new method to discriminate thermal neutrons against γ-rays using a radiation damaged LiBaF 3 : Ce, Rb scintillation crystal is presented. Discrimination is based on the scintillation pulse shape which for thermal neutrons is very different than that for γ-rays because different scintillation mechanisms are involved. Efficient suppression of γ-rays with an energy from a few keV to a few MeV is possible.