Energy resolution of CsI(Na) scintillators

The presented paper on CsI(Na) scintillators is a continuation of systematical studies of alkali halide crystals exhibiting more than one exponential decay time. The CsI(Na) crystals are known to have short (～550 ns) and long (few microseconds) components in the scintillation pulse. Previous studies showed that integration of slow components of the light pulse improved the light output, non-proportionality and energy resolution of CsI(Tl), NaI(Tl) in lower temperatures and undoped NaI crystal at liquid nitrogen temperature. In this work, an influence of the shaping time in the spectroscopy amplifier on the light output, non-proportional response to γ-rays, and energy resolution of two different size CsI(Na) scintillators is examined. Each crystal was coupled to the Photonis XP5212 PMT with a photocathode blue sensitivity of 12.2 μA/l mF. The data analysis showed improved proportionality of the crystal response, higher number of photoelectrons/MeV-γ, and consequently, a better overall energy resolution obtained for 12 μs shaping time constant. Finally, the CsI(Na) characteristics are compared to that obtained previously for NaI(Tl).     

Size-induced effect on cathode luminescence spectra of CsI(Na) and CsI(Tl) crystals

We investigated the cathode luminescence characteristics of CsI(Na)and CsI(Tl)crystals by the spectrum and structure properties at room temperature.We fabricated three different sizes of CsI(Na)and CsI(Tl)crystals and measured their luminescence spectra under cathode rays.We found that CsI(Na)cathode luminescence peaks appear at 420 and 305 nm,and CsI(Tl)cathode luminescence peaks are 540 and 410 nm,the grain size affects CsI(Na)luminescence significantly,and the Na-related420 nm luminescence intensified relatively when the average grain size reaches~20μm,which becomes weak when the grain size is down to nano-scale.But the cathode luminescence spectra of CsI(Tl)crystals with different size have no obvious changes.Our explanations for these phenomena are that the different impurities in the same host material CsI lead to different luminescence mechanisms.These cathode luminescence characteristics indicate the suitability of CsI(Na)and CsI(Tl)crystals to match photomultiplier tube for large area crystal detector development.     

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

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.     

Thermo-and photostimulated luminescence of CaI2: Tl and CaI2: Pb crystals

Thermo-and photostimulated luminescence of CaI₂: Tl and CaI₂: Pb scintillation crystals under optical and X-ray excitation is studied. It is shown on the basis of the results obtained with account for the data of studies of photo-and X-ray-luminescent properties of these scintillators that Tl⁺ and Pb²⁺ ions form complex capture centers with host defects. These centers are responsible for the thermostimulated luminescence in the temperature range of 150–295 K, and the centers of charge carrier trapping are spatially separated from the centers of recombination emission. An assumption is made that thermo-and photostimulated luminescence of CaI₂: Tl and CaI₂: Pb crystals under optical excitation is observed mainly due to the delocalization of charge carriers from hydrogen-containing centers responsible for the excitation band at 236 nm and the photoluminescence of CaI₂ with a maximum at 395 nm. The luminescence of CaI₂: Tl crystals in the 510-nm band and CaI₂: Pb crystals in the 530-nm band is determined by the radiative decay of near-activator excitons.     

Possible implications of the channeling effect in NaI(Tl) crystals

The channeling effect of low energy ions along the crystallographic axes and planes of NaI(Tl) crystals is discussed in the framework of corollary investigations on WIMP dark matter candidates. In fact, the modeling of this existing effect implies a more complex evaluation of the luminosity yield for low energy recoiling Na and I ions. In the present paper related phenomenological arguments are developed and possible implications are discussed at some extent. PACS 95.35.+d     

Radiation background due to radioactive sources in a luminescence dating laboratory

For practical reasons, usually, luminescence dating laboratories contain in the same room (or in adjacent rooms) the equipment and the radioactive sources, as well as the storage cabinet for the samples. It is generally assumed that the absorbed dose due to ambient radioactivity is null or at least negligible in terms of human health (below the dose limit).To test the actual dose rate inside our laboratory room, an investigation combining portable dose rate meter, portable NaI(Tl) gamma spectrometry, and blue-OSL dosimetry using Al₂O₃:C pellets was made. Although our measurements show the presence of X-rays as far as 3 m from the sources, the dose rates are below the safety regulation and does not affect the dating of the sample stored in the same room as the radioactive sources.     

Tl 0 and Tl 2+ centers as paramagnetic probes for the 74 K phase transition in Rb 2 ZnCl 4

Tl 0 (6 p 1 ) irradiation centers are proposed as new paramagnetic probes for structural phase transition investigations. The P2 1 cn M C1 c1 phase transition in Rb 2 ZnCl 4 single crystals is monitored using the Tl 0 and the well-known Tl 2+ (6 s 1 ) paramagnetic probes. The anomalous temperature dependence exhibited by the EPR spectra of both types of centers in the C1 c1 phase was analyzed considering a soft mode contribution.     

Scintillation properties of pure and Ce3+-doped SrF2 crystals

In this paper results of scintillation properties measurements of pure and Ce³⁺-doped strontium fluoride crystals are presented. We measure light output, scintillation decay time profile and temperature stability of light output. X-ray excited luminescence outputs corrected for spectral response of monochromator and photomultiplier for pure SrF₂ and SrF₂-0.3 mol% Ce³⁺ are approximately 95% and 115% of NaI–Tl emission output, respectively. A photopeak with a 10% full width at half maximum is observed at approximately 84% the light output of a NaI–Tl crystal after correction for spectral response of photomultiplier, when sample 10 × 10 mm of pure SrF₂ crystal is excited with 662 keV photons. Corrected light output of SrF₂-0.3 mol% Ce³⁺ under 662 keV photon excitation is found at approximately 64% the light output of the NaI–Tl crystal.     

The nature of emission centers in CdS nanocrystals

Photoluminescence (PL) of CdS nanocrystals (NCs) grown in different polymeric matrices is investigated. It is shown that chemical nature of the matrix does not affect the nature of PL centers in NCs. PL centers of NCs are identical to the emission centers in CdS single crystals and are related to their intrinsic defects.     

Photo- and radiation-chemical transformations of carbonate ions in CsI and CsI(Tl) crystals

It is shown that irradiation of CsI and CsI(Tl) crystals containing carbonate and hydroxyl ions induces radiation defects there in the form of color centers, HCO ₃ ^- ions, and H₂O molecules. HCO ₃ ^- ions are formed in the bulk of crystals, whereas water molecules are formed only in the surface layer. IR spectra offered no evidence of decomposition of CO ₃ ^2- ions into CO and CO ₂ ^- and of formation of CO ₃ ^- ions in the course of growth nor in the course of irradiation of CsI(CO₃) and CsI(Tl, CO₃) crystals. Electron activator color centers in CsI(Tl, CO₃) crystals are likely to be stabilized by hole near-activator centers and by HCO ₃ ^- ions produced in radiation-induced chemical reactions.     

Distribution of Blue and Green Luminescence Centers in Barrier Regions of the Grains of Zinc Sulfide Electroluminophors

Based on the investigation of the spectra of two-photon excited luminescence of powder zinc sulfide electroluminophors, their electroluminescence spectra measured under different conditions at different stages of degradation, and also the data known from the literature dealing with the study of the microstructure of ZnS:Cu crystals, a conclusion has been drawn that the centers of green and blue luminescence are nonuniformly distributed over the luminophor grains. It is presumed that close by the surface of the grains, near the second phase–zinc–sulfide interface (the regions of energy barriers responsible for impact multiplication of charge carriers in prebreakdown electroluminescence) the centers of green luminescence predominate, while the blue luminescence centers are located at the periphery of the barriers, in deeper regions of the crystals.     

Possible measurement of the characteristics of stacked up oscillators

Two methods are proposed for measuring the absorption coefficient of the inherent luminescence in scintillating detectors (for instance in stacked up single crystals of NaI(Tl) without perturbing the crystals. These methods are based on the use of an approximation formula relating the light yield and the absorption in the detector. Different methods for the determination of are compared.     

Luminescence centers in cubic boron nitride

Complex and multiband photoluminescence spectra for GB and HBN centers in single crystals of cubic boron nitride (cBN) were recorded in the wavelength ranges 385–400 nm and 365–395 nm and the nature of these centers was studied. The use of models involving resonance vibrations and strongly shifted configuration diagrams for the electronic ground state and excited state made it possible to associate formation of the GB-1 center with the presence of tungsten impurity in cBN. It was established that the HBN band in the 300–350 nm range of the cathodoluminescence spectra of cBN polycrystals, single crystals, and micropowders is associated with luminescence centers present in microinclusions of graphite-like boron nitride (hBN). The nature of the hBN band is tentatively interpreted within the model of recombination of donor and acceptor defects in hBN: respectively nitrogen vacancies and carbon atoms in positions substituting for nitrogen. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 2, pp. 241–246, March–April, 2007.     

Portable gamma spectrometry with cerium-doped lanthanum bromide scintillators: Suitability assessments for luminescence and electron spin resonance dating applications

Cerium-doped lanthanum bromide (LaBr₃:Ce) crystals offer a range of improved scintillation properties over traditional NaI:Tl crystals for in situ gamma spectrometry. At present, however, it remains unclear whether the internal radioactivity of LaBr₃:Ce detectors compromises their suitability for low-level activity radioisotope measurements of natural sedimentary deposits, such as those required in luminescence and electron spin resonance (ESR) dating. In this study we investigate the suitability of a commercial LaBr₃:Ce detector for measuring individual concentrations of ⁴⁰K, ²³⁸U and ²³²Th using predefined ‘energy windows’ from gamma ray spectra. Performance tests have been undertaken using reference materials with well-constrained radioisotope concentrations (the Oxford calibration blocks) and compared with results obtained for a NaI:Tl detector of the same geometry. These tests reveal that the LaBr₃:Ce detector has a non-negligible intrinsic activity that needs to be accurately quantified prior to measuring any gamma ray spectra in the field. Compared to the NaI:Tl detector, the energy resolution of the LaBr₃:Ce detector is improved by a factor of two, or more, for the main indicator isotope photopeaks in the ⁴⁰K, ²³⁸U and ²³²Th decay series. Signal-to-noise ratios for the LaBr₃:Ce detector show a 25-35% improvement over those of the NaI:Tl detector. In addition, the LaBr₃:Ce detector is characterised by suitable energy linearity over the full spectral range of interest for the ⁴⁰K, ²³⁸U and ²³²Th decay series. Replicate gamma ray measurements made with the LaBr₃:Ce and NaI:Tl detectors for 20 natural sedimentary samples from the Lower Tejo River basin, Portugal, and the Duero River basin, Spain, yield consistent radioisotope concentrations and gamma dose rate estimates. These results are encouraging and suggest that LaBr₃:Ce detectors can provide suitable estimates of individual radioisotope concentrations in low-level activity (0.5-1.5 Gy/ka) environments, providing that their intrinsic activity is adequately measured and subtracted from field spectra. Our comparison also reveals that subtraction of the intrinsic activity from LaBr₃:Ce spectra produces a significant reduction in the precision with which radionuclide concentrations can be determined using the ‘energy windows’ approach. This shortcoming necessitates longer counting times in natural sedimentary environments and overshadows the practical advantages that LaBr₃:Ce detectors might otherwise offer for luminescence and ESR dating applications.     

Luminescence of bound excitons in NaI and RbI crystals

X-ray induced luminescence spectra were for the first time investigated at 4.2–100 K in the vicinity of fundamental absorption of NaI crystals cleaved in liquid helium. Besides the luminescence of the free exciton and its LO replicas an additional band due to radiative annihilation of bound excitons appears in NaI crystals, as was earlier observed in RbI crystals. The obtained value of the binding energy of excitons is 50 and 60 meV for NaI and RbI, respectively. The nature of the shallow trap in the crystals of wide-gap insulators is discussed.     

Luminescence Centers of Pr3+ in Gallium Sulfide Single Crystals

The luminescence and luminescence excitation spectra of GaS:Pr single crystals are investigated. It is shown that in the single crystals of gallium sulfide the luminescence centers can be at two positions. The symmetry of the environment and the structure of these centers are considered.     

On the role of hole trapping centers in the interactive mechanism of the trap interaction in anion-defect alumina single crystals

The effect of deep hole traps on the intensity and shape of the dosimetric peak of thermoluminescence (TL) has been studied at 450 K in anion-defect alumina single crystals. It has been shown that filling of deep hole centers leads to a decrease in the sensitivity to radiation of crystals with a small half-width of the TL peak and has no effect on the sensitivity of crystals with a broadened peak. It has been assumed that traps responsible for the TL dosimetric peak broadening are of hole nature, which can be caused by the presence of Ti3+ ions in the corundum lattice. The results obtained have been interpreted within the modified model of the interactive system of traps.     

Features of gamma background variations in the surface air layer

An increase in the gamma radiation intensity (20–2500 keV) in the surface air layer during precipitation is studied. Measurements were performed at two high-latitude stations: Apatity (Murmansk region) and Barentsburg (Spitzbergen). Scintillation detectors based on NaI(Tl) crystals were used as sensors. More than 500 events of a gamma background increase during precipitation were recorded. Averaged profiles of the increases in the gamma background and related intensities of precipitation found by using superimpose method are compared; the time gap is ～30–40 min. The barometric coefficient is zero for the surface gamma radiation, indicating it is of local origin.     

Experimental Studies on Radiation Damages of CsI(Tl) Crystals

The results of experimental studies on eadiation damage of CsI(Tl) crystal were reported.There are radiation damage effects on CsI(Tl) crystal.Experimental studies on recovery of damaged CsI(Tl) crystals were made.It seems that after heating at 200℃ for 4 hours,the damaged crystals could be recovered completely.     

Color centers in heavily irradiated CsI(Tl) crystals

The absorption and luminescence properties of CsI(Tl) crystals colored by irradiation are studied by the method of the time-resolved spectroscopy. The scheme of the electron transitions in CsI(Tl) crystal is suggested to explain the appearance of the color centers under exposure to the near-UV light. It is established that either of the two types activator color centers holds the charge carrier with opposite sign. The model of the hole Tl²⁺v_c⁻ activator color center is suggested. According to the model the positive charge of Tl²⁺ ion is compensated by the negative charge of a close cation vacancy v_c⁻. The color center emission reveals in the cathode-luminescence spectrum of the colored CsI(Tl) crystal. The high-dose irradiation of CsI(Tl) crystal results in the reduction of the decay time of the near-thallium self-trapped excitons (STE) emission. The decay kinetics of Tl²⁺v_c⁻ emission contains the time components typical for the decay kinetics of near-thallium STE emission. The reason of the observed effects is the energy transfer from the near-thallium STE excitons to the color centers via the inductive-resonant mechanism.