Tl Crystals Research Articles

Modelling of a Single-Channel Beta–Gamma Coincidence Phoswich Detector Using Geant4 for the Conversion Electron Energy Peak Resolution and Beta–Gamma Coincidence Efficiency Improvement

In this study, an optimized single-channel phoswich well detector design has been proposed and assessed in order to improve beta–gamma coincidence measurement sensitivity of xenon radioisotopes. This newly designed phoswich well detector consists of a plastic beta counting cell (BC404) embedded in a CsI (Tl) crystal coupled to a photomultiplier tube. The BC404 is configured in a cylindrical pipe shape to minimise light collection deterioration. The CsI (Tl) crystal consists of a rectangular part and a semi-cylinder scintillation part as a light reflector to increase light gathering. Compared with a PhosWatch detector, the final optimized detector geometry showed 15 % improvement in the energy resolution of a 131mXe 129.4 keV conversion electron peak. The predicted beta–gamma coincidence efficiencies of xenon radioisotopes have also been improved accordingly.

Growth of CsI:Tl crystals in carbon coated silica crucibles by the gradient freeze technique

Single crystals of CsI having different Tl concentrations have been grown by a gradient freeze technique. Carbon films were deposited on the inside surface of fused silica crucibles to avoid sticking of the grown crystals. The crystals could be extracted easily from the crucibles without involving the inversion process at higher temperatures. Effects of varying growth parameters and after-growth thermal treatment on crystal properties like daylight coloration and radiation hardness were studied. Characterization techniques including high resolution X-ray diffraction, induced absorption, photoluminescence, afterglow and thermally stimulated luminescence were employed to evaluate the grown crystals. Gamma-ray detectors were fabricated using the grown crystals that showed good linearity and nearly 7.5% resolution at 662keV. This established a very simple and low cost method to grow small to medium size (35mm in diameter and 25mm in length) CsI crystals for various applications.

Acceleration tests of a 3 GHz proton linear accelerator (LIBO) for hadrontherapy

This paper describes the acceleration tests performed at the Catania LNS Laboratory on a 3GHz linac module of the side coupled type, which boosts the proton energy of a beam extracted from a cyclotron from 62 to 72MeV. The output proton energy was measured with two devices: a NaI(Tl) crystal and a bending magnet. The experimental spectra are in good agreement with the calculated ones. From their shape it is obtained that (18±3.0)% of the transmitted protons fall in a ±2MeV interval centered around 72MeV. This result is in good agreement with the 20% value derived from the simulation of the acceleration process. The measured energy of the accelerated protons was used to check that the shunt impedance of the structure is equal to the computed one within 3%. This was the first time that a 3GHz structure has been used to accelerate protons, and the results of the tests have demonstrated that a high frequency linac can be used as a cyclotron booster.

Activity measurement of electron-capture radionuclides by a KX-gamma-coincidence method at VNIIM

The article describes the development of a KX-γ-coincidence method implemented in the prototype setup developed at the ionizing radiation department of the D.I. Mendeleyev Institute for Metrology. The first part was presented at Third European IRPA Congress (2010 June 14–18, Helsinki, Finland). NaI(Tl) crystals of 100μm×40mm and 80mm×80mm are used for photon radiation detection. A set of “ORTEC NIM” units and ADC “MKGB-01” are used for coincident pulse processing. The activity of electron-capture radionuclides 54Mn, 57Co, 65Zn, 85Sr, 88Y, 139Ce and 119mSn was measured. The expanded uncertainty (k=2) of activity was estimated in the range of 1–2%. The main metrological characteristics of the setup were determined: long-term stability, background, dead time, resolving time. The results obtained are in good agreement with international comparisons results and the Russian national standard of radioactivity.

Development of a large-angle pinhole gamma camera with depth-of-interaction capability for small animal imaging

A large-angle gamma camera was developed for imaging small animal models used in medical and biological research. The simulation study shows that a large field of view (FOV) system provides higher sensitivity with respect to a typical pinhole gamma cameras by reducing the distance between the pinhole and the object. However, this gamma camera suffers from the degradation of the spatial resolution at the periphery region due to parallax error by obliquely incident photons. We propose a new method to measure the depth of interaction (DOI) using three layers of monolithic scintillators to reduce the parallax error. The detector module consists of three layers of monolithic CsI(Tl) crystals with dimensions of 50.0 × 50.0 × 2.0 mm3, a Hamamatsu H8500 PSPMT and a large-angle pinhole collimator with an acceptance angle of 120°. The 3-dimensional event positions were determined by the maximum-likelihood position-estimation (MLPE) algorithm and the pre-generated look up table (LUT). The spatial resolution (FWHM) of a Co-57 point-like source was measured at different source position with the conventional method (Anger logic) and with DOI information. We proved that high sensitivity can be achieved without degradation of spatial resolution using a large-angle pinhole gamma camera: this system can be used as a small animal imaging tool.

Experimental and simulation studies for the optimization of dedicated scintimammography cameras

The main drawback of scintimammography with conventional gamma cameras is the large distance between detector and breast, which limits imaging performance and adds significant background noise from the heart and other organs. These limitations can be minimized by dedicated gamma cameras placed in tight contact with the breast. The purpose of this study was to determine the lowest Tumor-to-Background (T:B) ratio for detectable lesions and the smallest lesions that can be imaged with a dedicated system. The system is based on an R3292 position sensitive photomultiplier tube (PSPMT) from Hamamatsu, coupled to a 10 cm in diameter CsI(Tl) crystal and a general purpose collimator with hexagonal holes. Two crystals were assessed, with pixel size of 2 × 2 mm2 and 3 × 3 mm2. The experimental setup consists of a cylindrical breast phantom, cylindrical tumor phantoms positioned inside the breast phantom at various depths, and a heart phantom. The same setup, as well as some additional tumor phantoms, were modeled with the GATE simulation toolkit, following the validation of the entire system. A 20% energy window and the Dual Energy Window Subtraction Method (DEWSM) for scatter correction were applied in experimental and simulation data. In all measurements 99mTc was used. Signal to Noise Ratio (SNR) and contrast were measured in all experiments and used for quantification of results. Experimental and simulation results show that the lowest T:B ratio that can be detected is 2:1 with tumor phantom volume ⩾ 0.25 ml at 1 cm depth. The smallest volume of a tumor phantom that can be detected is 0.04 ml with T:B ratio equal to 9:1 at 1 cm depth.

Fast light of CsI(Na) crystals

The responses of different common alkali halide crystals to alpha-rays and gamma-rays are tested in this research. It is found that only CsI(Na) crystals have significantly different waveforms between alpha and gamma scintillations, while others do not exhibit this phenomena. The rise time of the fast light is about 5 ns and the decay time is 17±12 ns. It is suggested that the fast light of CsI(Na) crystals arises from the recombination of free electrons with self-trapped holes of the host crystal CsI. Self-absorption limits the emission of fast light of CsI(Tl) and NaI(Tl) crystals.

Ground Radiometric Method as a Tool for Determining the Surface Boundary of a Buried Bauxitic Karst

Forty two ground radiometric measurements along nine traverses within a rectangular network area were taken across a bauxitic karst within the Ubaid Formation (Lower Jurassic) in the Western Desert of Iraq. A 4-Channel Gamma Ray Spectrometer (GAD-6) with sodium iodide NaI (Tl) crystal (GSP-4S) was used in the field to measure the total radioactivity of the surface soil. Soil samples collected from the surface at each measurement point and core samples collected from a test well penetrating the karst were analyzed by Gamma ray spectrometer. The main objective of this study was to detect the hidden bauxitic karst and determine its surface boundary. The radioactivity on the surface of the karst was ranging between 60 and 80 count per second (c/s), while the background radioactivity of the Ubaid Formation, which hosts the karst, was ranging between 100 and150 c/s. Chemical weathering, especially dissolution and leaching moved uranium (238U) and thorium(232Th) from the overburden downward. Accordingly, these elements have been adsorbed on the surface of clay minerals and bauxite buried at a depth of about 5m causing enrichment with radioactivity. The leached overburden lack radioelements, so its radioactivity was less than background radioactivity level. The gamma ray spectroanalysis showed that the radioactivity of 238U and 232Th in the overburden was 0.5 and 3 Bq/Kg, whereas, in the bauxite and flint clay bed, it was 240 and 160 Bq/Kg respectively. Based on the radioactivity anomaly contrast on the surface, an isorad map was plotted and the karst diameter which represents low anomaly was determined to be ranging from 150 to 200m. The current study demonstrates that the ground radiometric method is quite useful for detecting the bauxitic karst and inferring its surface boundaries.

External bremsstrahlung spectra of the 90Sr source in some lead compounds measured using NaI detector

Abstract The spectral distributions of external bremsstrahlung (EB) excited by beta particles from a 90 Sr/ 90 Y source in thick target compounds PbCl 2 , PbF 2 , Pb(NO 3 ) 2 and CdO were measured using a 3.8 cm × 3.8 cm NaI(Tl) crystal. The spectra, unfolded using the Liden-Starfelt procedure, showed fairly good agreement with theory (Tseng and Pratt) at low energies and some deviation (less than 15%) at higher energies. The discrepancy between theory and experiment increases with modified atomic number of the target compound and photon energy. The application of the measured/theoretical spectrum to estimate the bremsstrahlung dose is also discussed.

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

Abstract 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.

Development of a technique using MCNPX code for determination of nitrogen content of explosive materials using prompt gamma neutron activation analysis method

Abstract Nuclear-based explosive detection methods can detect explosives by identifying their elemental components, especially nitrogen. Thermal neutron capture reactions have been used for detecting prompt gamma 10.8 MeV following radioactive neutron capture by 14 N nuclei. We aimed to study the feasibility of using field-portable prompt gamma neutron activation analysis (PGNAA) along with improved nuclear equipment to detect and identify explosives, illicit substances or landmines. A 252 Cf radio-isotopic source was embedded in a cylinder made of high-density polyethylene (HDPE) and the cylinder was then placed in another cylindrical container filled with water. Measurements were performed on high nitrogen content compounds such as melamine ( C 3 H 6 N 6 ) . Melamine powder in a HDPE bottle was placed underneath the vessel containing water and the neutron source. Gamma rays were detected using two NaI(Tl) crystals. The results were simulated with MCNP4c code calculations. The theoretical calculations and experimental measurements were in good agreement indicating that this method can be used for detection of explosives and illicit drugs.

A study of the active thermal control for the high energy detector on the HXMT

A thermal control system (TCS) based on the resistance heating method is designed for the High Energy Detector (HED) on the Hard X-ray Modulation Telescope (HXMT). The ground-based experiments of the active thermal control for the HED with the TCS are performed in the ambient temperature range from −15 to 20°C by utilizing the pulse width to monitor the interior temperature of a NaI(Tl) crystal. Experimental results show that the NaI(Tl) crystal's interior temperature is from 17.4 to 21.7°C when the temperature of the PMT shell is controlled within (20±3)°C with the TCS in the interesting temperature range, and the energy resolution of the HED is maintained at 16.2% @122 keV, only a little worse than that of 16.0% obtained at 20°C. The average power consumption of the TCS for the HED with a low-emissivity shell is about 4.3 W, which is consistent with the simulation.

Experimental study on neutrinoless double beta decay of

Abstract An experiment for the detection of 0 ν β + / EC in 92 Mo nuclei has been carried out with a newly developed scintillating crystal, CaMoO 4 , surrounded by CsI(Tl) crystals. We study the background events inside the event selection window for 0 ν β + / EC decays of CaMoO 4 detector. The 92 Mo 0 ν β + / EC decay half-life limit was set to 2.3 × 10 20 years with a 90% confidence level. A perspective on the current experiment is discussed.

SU‐E‐I‐151: Performance Evaluation of a New Commercially Available Portable Pixelated Gamma Camera for 99mTc‐Scintimammography

Purpose: To characterize the imaging performance of a pixelated gamma camera (Ergo, Digirad) for 99mTc‐scintimammography. Methods: The 31×40cm̂2 Ergo detector consists of 6mm thick 3.31×3.24mm̂2 CsI(Tl) crystals coupled to silicon photodiodes. The sensitivity and resolution was measured according to NEMA NU 1‐2007. Scintimammography performance was evaluated as hot tumor detection capabilities. We acquired images of different 99mTc‐spheres (inner diameters 3.95, 4.95, 6.23, 7.86, 9.89, 12.43mm) suspended in a 6.6cm thick 500mL 99mTc‐water bath. Images were acquired at tumor depths of 1.7 and 3.7mm with different sphere‐to‐background ratios (SBR: 5.0, 8.0, 10.1, 12.5, 18.8) for 10min acquisition using low‐energy collimation (LEHR). Acquisitions for 10, 5, 2.5, 1.25min were performed at SBR=5 where the 99mTc‐activity concentration (uCi/mL) in spheres/background was 34/6.8. The sphere‐CNR was estimated as: Sphere‐CNR = (SphereCounts_Max ‐ BackgroundCounts_Mean)/ BackgroundNoise. BackgroundNoise was the average standard deviation of 10 different 9‐pixel ROIs distributed across the image. BackgroundNoise was modeled as a power‐law function of BackgroundCounts_Mean and used to predict the sphere‐CNR at different activity levels and acquisition times. A preliminary visual assessment of sphere visibility was used to establish the threshold‐CNR for detection. Results: The sensitivity and resolution at 10cm using LEHR was measured to be 128cpm/uCi and 7.5mm. BackgroundNoise was found to vary as BackgroundCounts_Mean̂−0.67 suggesting a Poisson noise dominated imaging system. Threshold‐CNR of ∼8 corresponded to visible spheres. For SBR=5, depth=3.7cm, acquisition=10min, the measured sphere‐CNR for diameters 6.23, 7.86, 9.89, 12.43mm were 12.1, 15.9, 22.8, 33, respectively; that decreased to <3, 6.6, 10.1, 14.6 for acquisition=2.5min. The CNR predicted by the power‐law model for BackgroundNoise was in good agreement (differences of 9‐26%) with measured sphere‐CNR at lower acquisition times. Conclusions: The Ergo system is suitable for 99mTc‐ scintimammography with predictable performance at different imaging conditions. Small tumors (<1cm) could be visualized at a range of SBR under tested conditions.

Results from an investigation of the physical origins of nonproportionality in CsI(Tl)

Abstract The relative scintillation response per energy deposited by Compton electrons, or nonproportionality, has traditionally been considered an intrinsic scintillator property. However, such an interpretation is inconsistent with recent results that show nonproportionality to depend on external factors such as shaping time, temperature and supplier. Apparently, at least some of the overall nonproportionality has an extrinsic origin. In this work we describe the results from a suite of measurements designed to test the hypothesis that nonproportionality in CsI(Tl) material has an extrinsic component that correlates with impurity levels. Our choice of material was motivated by the excellent energy resolution observed in one bulk crystal (6.4%)—a marked departure from that measured with conventional CsI(Tl) stock (8–8.5%). Six bulk CsI(Tl) crystals were procured and diced into 44 wafers. Using X-ray fluorescence techniques no conclusive evidence for impurities was found in any of the wafers at the 1–50 ppm level. One crystal exhibited a distinct correlation among energy resolution, decay lifetimes, nonproportionality and a very low level of Tl doping.

Neutron–gamma discrimination of CsI(Na) crystals for dark matter searches

The luminescent properties of CsI(Na) crystals are studied in this report. Using a TDS3054C oscilloscope with a sampling frequency of 5 GS/s, we find out that nuclear recoil signals are dominated by a very fast light pulse with a decay time of ∼20 ns, while γ-ray signals have a decay time of ∼600 ns. The study of n/ γ separation shows that the rejection factor can reach an order of 10 −7 with a signal efficiency of more than 80% at an equivalent electron recoil energy of 20 keV or more, and the quality factors are ∼10 times smaller than that for CsI(Tl) crystals in the corresponding energy region. It is suggested that the wavelength of nuclear recoil and γ-ray signals is also different. Such a property makes CsI(Na) an ideal candidate for dark matter searches.

The NATALYA-2M spectrometer of high-energy radiations for the CORONAS-PHOTON space project

The NATALYA-2M high-energy radiation spectrometer is an element of the complex of scientific equipment of the CORONAS-PHOTON satellite. The instrument intended for registering gamma radiation of solar flares in the broad energy range of 0.2–1600 MeV as well as neutrons of solar origin with energies of 20–300 MeV represents itself as a scintillation spectrometer based on CsI(Tl) crystals with a total area of 32 × 38 cm2 and the thickness of 18 cm. The spectra and time profiles of the gamma quanta count rates are measured in four subranges: R (0.2–2 MeV), L (1–18 MeV), M (7–250 MeV), and H (50–1600 MeV). Depending on the gamma radiation energy, the effective area of the instrument varies within the range from 750 to 900 cm2, and the energy resolution at the Cs-137 line (662 keV) is 10%, it being about 30% at energies higher than 50 MeV. A system of stabilization based on the signal from the generator of reference light pulses is used to provide stability and automated adjustment of the parameters of spectrometric modules. The measuring channels of the instrument are calibrated during the flight using a source of “tagged” gamma quanta on the Co-60 radioactive isotope. Polystyrene scintillation counters are used to provide protection from the background of charged particles. The “CORONAS-PHOTON” spacecraft (SC) was launched from the Plesetsk spaceport on January 30, 2009, to a low circular near-Earth orbit (the altitude is 550 km, the inclination is 82.5°). On February 27, the first scientific data were obtained from the NATALYA-2M instrument. The results of the flight calibration of the instrument detectors in different energy channels demonstrated good agreement with the ground measurements. The paper describes the instrument and observational potentials of the NATALYA-2M spectrometer, gives the results of the adjustment and calibration, and exemplifies the registration of gamma-ray bursts (GRBs)on the orbit.

The Optimization of CsI(Tl)-PIN Photodiode for High-Energy Gamma-Ray Detection

A Monte Carlo simulation was performed to induce optimized parameters for CsI(Tl) crystals and large area PIN phohtodiode detectors for applications in high energy gamma ray spectroscopy. In order to derive the optimum parameters we used a Trade-Off procedure between the deposition energy and light transmission efficiency, while varying the detector thickness. Moreover, it was found that the surface treatment of the crystal in the photodiode is also an important factor for good light transmission. The tests were performed using a low noise charge-sensitive preamplifier and a pulse shape amplifier. Concerning the operating condition of the system, it is necessary to shape the time and reverse the bias voltage in order to provide another minimization of noise for the system. In order to maximize the light output from the crystal, the crystal geometry, crystal wrapping, optical bonding and matching to the PIN photodiode were all optimized. With a fixed crystal height of 5 cm, it was best to grind the sides and the top surface of the crystal on the front facing sides of the external reflectors, resulting in about a 14.5% enhancement compared with polishing all the sides and top.

Scintillation Efficiency of Inorganic Scintillators for Intermediate-Energy Charged Particles

We carried out experiments to investigate the light output response of NaI(Tl), CsI(Tl), GSO(Ce) and LYSO(Ce) crystals for intermediate-energy 4 He, 12 C and 40 Ar beams from HIMAC at National Institute of Radiological Sciences. And we investigate the light output of these crystals for several-energy gamma-ray for comparison. From these light output responses, we obtained the relationships between the scintillation efficiency (dL/dx) and the specific energy loss (dE/dx) for each crystal. The scintillation efficiency curves of NaI(Tl) and CsI(Tl) crystal have the peak in a particular dE/dx. On the other hand, the scintillation efficiency curves of GSO(Ce) and LYSO(Ce) crystal decrease with increasing dE/dx. The light output curves of these crystals were systematically reproduced using obtained scintillation efficiencies.

Low-dose megavoltage cone-beam CT imaging using thick, segmented scintillators

Megavoltage, cone-beam computed tomography (MV CBCT) employing an electronic portal imaging device (EPID) is a highly promising technique for providing soft-tissue visualization in image-guided radiotherapy. However, current EPIDs based on active matrix flat-panel imagers (AMFPIs), which are regarded as the gold standard for portal imaging and referred to as conventional MV AMFPIs, require high radiation doses to achieve this goal due to poor x-ray detection efficiency (∼2% at 6 MV). To overcome this limitation, the incorporation of thick, segmented, crystalline scintillators, as a replacement for the phosphor screens used in these AMFPIs, has been shown to significantly improve the detective quantum efficiency (DQE) performance, leading to improved image quality for projection imaging at low dose. Toward the realization of practical AMFPIs capable of low dose, soft-tissue visualization using MV CBCT imaging, two prototype AMFPIs incorporating segmented scintillators with ∼11 mm thick CsI:Tl and Bi4Ge3O12 (BGO) crystals were evaluated. Each scintillator consists of 120 × 60 crystalline elements separated by reflective septal walls, with an element-to-element pitch of 1.016 mm. The prototypes were evaluated using a bench-top CBCT system, allowing the acquisition of 180 projection, 360° tomographic scans with a 6 MV radiotherapy photon beam. Reconstructed images of a spatial resolution phantom, as well as of a water-equivalent phantom, embedded with tissue equivalent objects having electron densities (relative to water) varying from ∼0.28 to ∼1.70, were obtained down to one beam pulse per projection image, corresponding to a scan dose of ∼4 cGy–-a dose similar to that required for a single portal image obtained from a conventional MV AMFPI. By virtue of their significantly improved DQE, the prototypes provided low contrast visualization, allowing clear delineation of an object with an electron density difference of ∼2.76%. Results of contrast, noise and contrast-to-noise ratio are presented as a function of dose and compared to those from a conventional MV AMFPI.