Ce Scintillator Research Articles

Determination of LaBr3(Ce) internal background using a HPGe detector and Monte Carlo simulations

The presence of 138La and 227Ac impurities in LaBr3(Ce) scintillator crystals is a drawback for their use in environmental radiation monitoring. A method is presented to evaluate the internal 138La activity. Firstly, an experimental set-up is prepared with the LaBr3(Ce) crystal acting as the radiation source and an HPGe detector that acquires the photon spectrum. Then, the internal background spectrum is simulated with a modified version of the PENELOPE/penEasy Monte Carlo code. The simulated spectra agree with measurements conducted at ultra-low-background facilities.

Front-end chip for Silicon Photomultiplier detectors with pico-second Time-of-Flight resolution

A mixed-mode readout Application Specific Integrated Circuit (STIC3) has been developed for high precision timing measurements with Silicon Photomultipliers (SiPM) for medical imaging and particle physics applications. The STiC3 is a 64-channel chip, with fully differential analog front-end for cross-talk and electronic noise immunity. The time and charge information from the SiPM signals are encrypted into two time stamps generated by integrated Time to Digital Converter (TDC) modules with 50ps time binning. The TDC data is stored in an internal memory and transferred to a PC via a 160MBit/s serial link using an 8/10 bit encoding. The chip provides an input bias tuning in a range of 0–900mV to compensate the breakdown voltage variation of individual SiPMs. The TDC jitter together with the digital part is around 37ps. A Coincidence Time Resolution (CTR) of 213.6ps FWHM has been obtained with 3.1×3.1×15m2 LYSO:Ce scintillator crystals and Hamamatsu SiPM matrices (S12643-050CN(X)). Characterization measurements with the chip and its integration into the external plate of the EndoTOFPET-US prototype are presented.

Luminescence characteristics of Lu2SiO5:Ce3+ (LSO:Ce) ceramic scintillators under VUV–UV excitation

The luminescent properties of LSO:Ce (1mol%) scintillators were characterized systematically under vacuum ultraviolet (VUV)–UV excitation. Under 185nm VUV excitation, a broad emission band was detected in the wavelength range from 350nm to 600nm at different temperatures, which is ascribed to the 5d→4f electron transitions of Ce3+. Excitation wavelength and temperature effects on luminescent properties of LSO:Ce ceramic scintillators were investigated in this paper. At 50K, the emission intensity of Ce1 (Ce3+ locating in seven-oxygen-coordinated site) was much stronger than that of Ce2 (Ce3+ locating in six-oxygen-coordinated site), and the total emission intensity excited by 355nm (Ce1) was higher than that excited by 185nm (host). The total luminescence intensity increases as the rising temperature in the temperature range of 150–300K, which is originated from thermal enhancement of transportation of free carriers dominating Ce1 emission.

Atomic origin of 3d94 f 1 configuration in La3+ solids

We have studied the excited electronic structure of LaBr3(Ce) scintillator by soft x-ray spectroscopy such as x-ray absorption spectroscopy (XAS) and x-ray emission spectroscopy (XES). The La 3d XAS and XES spectra of LaBr3(Ce) are compared with those of other La3+ solids (LaF3, La2O3, and La metal). From this comparison, it turns out that the La 3d XAS and XES spectra from all the La3+ solids considered here appear at almost the same energy, even though the corresponding binding energies of the 3d core holes determined by XPS (x-ray photoelectron spectroscopy) are very different. As a result, we argue that the atomic nature of the configuration created by x-ray absorption process in La3+ solids is maintained via the localized 4 f 1 state, which screens the 3d core holes differently from one La3+ solid to another.

Response of the Li-7-enriched Cs2LiYCl6:Ce (CLYC-7) scintillator to 6–60 MeV neutrons

We discuss a test campaign designed to irradiate the 7Li-enriched Cs2LiYCl6:Ce3+ (CLYC-7) scintillator with 6–60MeV neutrons using the cyclotron located at the Crocker Nuclear Laboratory in Davis, CA. CLYC-7 is a newly developed scintillator that exhibits the ability to make good γ-ray measurements and has the ability to detect and discriminate fast neutrons via pulse shape discrimination (PSD) while functioning as a spectrometer. This allows a single detector to make good measurement of both stimuli simultaneously. The response of this scintillation detector has been investigated below 20MeV [1] but has yet to be explored for energies greater than 20MeV. Understanding the spectral and pulse shape response across a broad energy range is important for any radiation detection instrumentation capable of detecting multiple species. At the highest energies sampled, the CLYC-7 PSD demonstrated not only the standard electron/proton separation expected in a mixed γ/n field but the ability to discriminate locally produced deuterons, tritons and α particles. We show the results from the four different neutron beam energies sampled during the experiment. Lastly, we present the results obtained for relating the light output equivalence between electrons and protons/deuterons.

Comparison of LSO and BGO block detectors for prompt gamma imaging in ion beam therapy

A major weakness of ion beam therapy is the lack of tools for verifying the particle range in clinical routine. The application of the Compton camera concept for the imaging of prompt gamma rays, a by-product of the irradiation correlated to the dose distribution, is a promising approach for range assessment and even three-dimensional in vivo dosimetry. Multiple position sensitive gamma ray detectors arranged in scatter and absorber planes, together with an imaging algorithm, are required to reconstruct the prompt gamma emission density map. Conventional block detectors deployed in Positron Emission Tomography (PET), which are based on Lu2SiO5:Ce (LSO) and Bi4Ge3O12 (BGO) scintillators, are suitable candidates for the absorber of a Compton camera due to their high density and absorption efficiency with respect to the prompt gamma energy range (several MeV). We compare experimentally LSO and BGO block detectors in clinical-like radiation fields in terms of energy, spatial and time resolution. The high energy range compensates for the low light yield of the BGO material and boosts significantly its performance compared to the PET scenario. Notwithstanding the overall superiority of LSO, BGO catches up in the field of prompt gamma imaging and can be considered as a competitive alternative to LSO for the absorber plane due to its lower price and the lack of intrinsic radioactivity.

Distribution of the 134Cs/137Cs ratio around the Fukushima Daiichi nuclear power plant using an unmanned helicopter radiation monitoring system

Large amounts of radioactive substances were released into the environment by the Fukushima Daiichi nuclear power plant (FDNPP) accident. Several research institutes have mapped the distribution of nuclides with long half-lives, such as 134Cs and 137Cs. Although the ratio of 134Cs and 137Cs has been believed to be equal without depending on the location of the contaminated area, several researchers report that it is different depending on places quite a little. We measured the energy spectrum of gamma rays in high resolution within an approximately 3-km radius of the FDNPP by using an unmanned helicopter equipped with a LaBr3(Ce) scintillation detector. Then, we analyzed the 134Cs/137Cs ratio in the area from these measured data in detail. The results show that the 134Cs/137Cs ratio is different between the plume trace extending north and the other plume traces. We have obtained valuable data for identification of which radioactive substances were released by individual reactor units.

Feasibility study on signal separation for spontaneous alpha decay in LaBr3: Ce scintillator by signal peak-to-charge discrimination.

A novel analysis method named peak-to-charge ratio (V(p)/Q(total)) discrimination, aiming at background rejection especially for alpha decay self-activity in LaBr3: Ce scintillators has been developed. This method is based on a waveform analysis using the peak-to-charge ratio in the output waveform of a photomultiplier tube. The discrimination of alpha-induced events was achieved by using a threshold function based on the error propagation of the V(p)/Q(total) value. The accidental rejection ratio of gamma-induced events was evaluated to be 0.17%. Furthermore, a total absorption peak spectrum processed with the V(p)/Q(total) discrimination method for (68)Ga 1.883 MeV gamma rays, where the energy was overlapped with background alpha events, reproduced exactly the same result as that of the background subtraction method. The difference in measured peak counts of both methods was 0.716%, and the statistical error in the V(p)/Q(total) discrimination method and background subtraction was 4.81% and 8.70%, respectively. Thus a higher-accuracy measurement could be achieved using the V(p)/Q(total) discrimination method. The present study demonstrates that the V(p)/Q(total) discrimination method is a promising method for background rejection of the spontaneous alpha decay in LaBr3:Ce scintillators.

Estimation of Fano factor in inorganic scintillators

The Fano factor of an integer-valued random variable is defined as the ratio of its variance to its mean. Correlation between the outputs of two photomultiplier tubes on opposite faces of a scintillation crystal was used to estimate the Fano factor of photoelectrons and scintillation photons.Correlations between the integrals of the detector outputs were used to estimate the photoelectron and photon Fano factor for YAP:Ce, SrI2:Eu and CsI:Na scintillator crystals. At 662keV, SrI2:Eu was found to be sub-Poisson, while CsI:Na and YAP:Ce were found to be super-Poisson.An experiment setup inspired from the Hanbury Brown and Twiss experiment was used to measure the correlations as a function of time between the outputs of two photomultiplier tubes looking at the same scintillation event. A model of the scintillation and the detection processes was used to generate simulated detector outputs as a function of time for different values of Fano factor. The simulated outputs from the model for different Fano factors was compared to the experimentally measured detector outputs to estimate the Fano factor of the scintillation photons for YAP:Ce, LaBr3:Ce scintillator crystals. At 662keV, LaBr3:Ce was found to be sub-Poisson, while YAP:Ce was found to be close to Poisson.

Relationship between Ca2+ concentration and the properties of codoped Gd3Ga3Al2O12:Ce scintillators

Codoping is a method of current interest for modifying the properties of scintillators. The study reported here explores the effect of codoping on cerium doped Gd3Ga3Al2O12 (GGAG:Ce) crystals with various concentrations of Ca. These single crystals were grown via the Czochralski technique with Ce concentrations fixed at 0.2at% and Ca concentrations ranging from 0.1at% to 0.4at% in the initial melt. The relationship between dopant concentration and light yield, rise time, and scintillation and photoluminescence decay times was determined. In addition, the absorbance, photoluminescence, radioluminescence, afterglow and thermoluminescence dependence on the dopant concentration are presented. In some of the Ca codoped crystals, an additional luminescence center was observed with an excitation wavelength of 350nm and an emission wavelength of 400nm and a photoluminescence decay time of ~3.5ns.

Verification of the method of average angular response for dose measurement on different detectors

At present most radiation dose meters have serious problems on aspects of energy response and angular response. In order to improve the accuracy of dose measurements, a method of average angular response has been proposed. The method can not only correct the energy response, but also the angular response. This method has been verified on NaI(Tl)(50 mm× 50 mm) scintillation detectors, but has not been proved on other types and sizes of detectors, In this paper the method is also verified for LaBr3(Ce) scintillation detectors and HPGe detector To apply the method, first of all, five detectors are simulated by Geant4 and average angular response values are calculated. Then experiments are performed to get the count rates of full energy peak by standard point source of 137Cs, 60Co and 152Eu. After that the dose values of five detectors are calculated with the method of average angular response. Finally experimental results are got. These results are divided into two groups to analyze the impact of detectors of various types and sizes. The result of the first group shows that the method is appropriate for different types of detector to measure dose, with deviations of less than 5% compared with theoretical values. Moreover, when the detector's energy resolution is better and the count rate of the full energy peak is calculated more precisely, the measured dose can be obtained more precisely. At the same time, the result of the second group illustrates that the method is also suited for different sizes of detectors, with deviations of less than 8% compared with theoretical values.

On-line experiments on rapid detection of radionuclides based on sequential Bayesian analysis

A rapid detection method based on sequential Bayesian analysis provides a new perspective on national security in preventing the smuggling and illegal transportation of nuclear materials. In this paper, a sequential Bayesian analysis system, which mainly consists of a LaBr3(Ce) scintillator detector, a pulse analyzer based on a field-programmable gate array technique, and a sequential Bayesian analysis processor, is developed to directly validate the feasibility of sequential Bayesian analysis. The detection ability of 60Co, 137Cs, 133Ba, and 152Eu for a specific radioactivity is studied and quantified using the maximum detection distance and the equivalent minimum detection activity.

Comparative Study of Co-Doped and Non Co-Doped LSO:Ce and LYSO:Ce Scintillators for TOF-PET

Scintillation properties of LSO:Ce, LSO:Ce,0.2%Ca, LYSO:Ce, LYSO:0.11%Ce,0.2%Mg, and LYSO:0.2%Ce,0.2%Ca were studied for application in time-of-flight positron emission tomography. To find the scintillation material with the best timing performance among those, we measured intrinsic scintillation pulse shapes–defined as the number of photons emitted per unit time at the ionization track as a function of time - by means of 100 ps x-ray excitation in the temperature range from 193 K to 373 K, from which intrinsic rise and decay time constants were derived. Photoelectron yields were measured by 662 keV gamma excitation. From these scintillation properties we calculated the corresponding lower bound on the timing resolution for each material.

LaBr3:Ce small FOV gamma camera with excellent energy resolution for multi-isotope imaging

The simultaneous administration of radiopharmaceuticals labeled with more than one radioisotope is becoming of increasing interest in clinical practice. Because the photon energies of the utilized radioisotopes could be very close (less than 15% difference), a gamma camera with adequate energy resolution is required. The availability of scintillation crystals with high light yield, as lanthanum tri-bromide (LaBr3:Ce), is particularly appealing for these applications.In this work, a new small field of view gamma camera prototype is presented, based on a planar LaBr3:Ce scintillation crystal with surfaces treatment typical of spectrometric devices, in order to enhance energy resolution performances. The crystal has round shape and has been optically coupled with a position sensitive photomultiplier tube with high quantum efficiency. The presented gamma camera shows outstanding energy resolution results in the investigated energy range (32–662 keV). These relevant performances have been obtained through the application of uniformity correction on the raw data, necessary due to the presence of position sensitive phototube, characterized by a spread of anodic gain values. In spite of position linearity degradation at crystal edges, due to reflective treatment of surfaces, intrinsic spatial resolution values are satisfactory on the useful field of view.The characterization of the presented gamma camera, based on a continuous LaBr3:Ce scintillation crystal with reflective surfaces, indicates good performances in multi-isotope imaging due to the excellent energy resolution results, also in comparison with similar detectors.

Effect of oxidation annealing on optical properties of YAG:Ce single crystals

YAG:Ce single crystals were annealed in air atmosphere at different temperatures. Their optical properties, such as optical absorption, cathodoluminescence spectra, cathodoluminescence decay and thermoluminescence were studied. Growth of UV absorption with increasing annealing temperature is explained by ionization of Ce3+ ions during the oxidation annealing and consequent ligand-to-metal charge transfer to the Ce4+ ions. Near-UV luminescence emission caused by presence of the antisite defects is undesirable for fast scintillation applications. After oxidation annealing, the emission related to the antisite defects is partially quenched due to the spectral overlap of the charge transfer absorption and the antisite defects emission. Together with suppression of electron traps due to the oxidation annealing, the described mechanism effectively decreases luminescence afterglow and improves YAG:Ce scintillation characteristics.

Development of a measurement system for the determination of ([formula omitted]) cross-sections using multi-nucleon transfer reactions

We have installed a new experimental apparatus to measure γ-rays from highly excited states populated by the multi-nucleon transfer reactions with heavy-ion projectiles to determine the (n, γ) cross-sections by means of the surrogate reaction method. An apparatus consists of two anti-Compton LaBr3(Ce) spectrometers to measure the γ-rays and a Si ΔE–E detector system to detect outgoing projectile-like particles. Reactions of 153-MeV 18O beams with 155Gd and 157Gd targets were used to study the performance of apparatus. By using the LaBr3(Ce) scintillators with relatively large volume (101.6mm in diameter and 127mm in length), we have successfully measured γ-rays from the compound nuclei, which have excitation energy above neutron separation energy, populated by 155Gd(18O, 16O)157Gd and 157Gd(18O, 16O)159Gd two-neutron transfer reactions. To measure in-beam γ-rays through heavy-ion-induced transfer reaction, it is important to assign the reaction channel clearly, since the cross-sections of the transfer reactions are much smaller than those of competing reactions such as Coulomb excitation and fusion reactions. The Si ΔE–E detector system was used to separate outgoing particles. The present study has demonstrated high capability of apparatus to measure the de-excitation γ-rays in the compound nuclei produced by the multi-nucleon transfer reactions for determination of the (n, γ) cross-sections by using the surrogate reaction method.

Si and Cu ablation with a 46.9-nm laser focused by a toroidal mirror.

Ce scintillator (cerium-doped YAG crystal) was used to image the variations of the laser spots. We discuss the shape and damage mechanics of the measured patterns. Melting of the target material was observed in the ablation region on Cu, but not on Si.

A practical fan-beam design and reconstruction algorithm for Active and Passive Computed Tomography of radioactive waste barrels

Active and Passive CT (A&PCT) of waste barrels is mostly carried out in parallel beam configuration due to its relative ease of implementation. This necessitates either using a single detector–source pair and translating the barrel or using multiple detector–source pairs for increasing the scanning speed. Additionally, because the use of bulky HPGe detectors may limit the number of detectors used in both active and passive modes, we propose to use 1″×1″ LaBr3(Ce) scintillators. This paper describes a practical fan-beam reconstruction for A&PCT imaging of waste barrels. A fan beam system model has been computed analytically and reconstruction done using MLEM algorithm. The results are compared with analytical reconstruction.

Measurement of β−-decay continuum spectrum of 138La

The LaBr3:Ce scintillator offers the unique opportunity to study the β− radioactive decay of 138La. The 138La isotope is one of the rarest isotopes on Earth (it is present as 0.09% in natural lanthanum) and because of its extremely long lifetime, of the order of 1011 years, large amounts of 138La are needed for the measurement of the β−-decay spectrum. In the literature, only one dedicated experimental measurement is present and the results seem not to be reproduced by the nuclear theory. A second measurement of the β− continuum spectrum is presented in this work. For this measurement, two LaBr3:Ce scintillators and two different experimental techniques were used. The shape of the β− continuum spectrum, measured down to the energy of 10 keV, is found to be very similar to the one previously measured and published, but it is different from the theoretical published spectrum.

First test of O(α) correction on the energy spectrum in the orthopositronium decay

Positronium(Ps) is an ideal system for the precision test of quantum electrodynamics (QED) in bound state. Orthopositronium(o-Ps) decays into three gamma rays dominantly, and their energy spectrum has detail information on bound-state QED. The purpose of this study is to check the spectrum at the O(α) level accuracy. We constructed a setup for the precise measurement. Ps is created at silica aerogel in a vacuum chamber, and its decay gamma rays are measured with a LaBr3 (Ce) scintillator. In this setup, the materials around the Ps assembly and the detector are well-controlled, and all their responses are coded in a GEANT4 simulator in detail. After the measurement for 100 days, the obtained spectrum is compared with the expected spectrum from the QED calculations and the GEANT4 simulator. The obtained spectrum is consistent with O(a) calculation and excludes the lowest- order calculation at 92% C.L.