CsI Scintillator Research Articles

Development of FPGA-based digital signal processing system for radiation spectroscopy

Abstract We have developed an FPGA-based digital signal processing system that performs both online digital signal filtering and pulse-shape analysis for both particle and gamma-ray spectroscopy. Such functionalities were made possible by a state-of-the-art programmable logic device and system architectures employed. The system performance as measured, for example, in the system dead time and accuracy for pulse-height and rise-time determination, was evaluated with standard alpha- and gamma-ray sources using a CsI(Tl) scintillation detector. It is resulted that the present system has shown its potential application to various radiation-related fields such as particle identification, radiography, and radiation imaging.

Multi-Layer Organic Squaraine-Based Photodiode for Indirect X-Ray Detection

The paper presents an organic-based photodiode coupled to a CsI(Tl) scintillator to realize an X-ray detector. A suitable blend of an indolic squaraine derivative and of fullerene derivative has been used for the photodiode, thus allowing external quantum efficiency in excess of 10% at a wavelength of 570 nm, well matching the scintillator output spectrum. Thanks to the additional deposition of a 15 nm thin layer of a suitable low electron affinity polymer, carriers injection from the metal into the organic semiconductor has been suppressed, and dark current density as low as has been obtained, which is comparable to standard Si-based photodiodes. By using a collimated X-ray beam impinging onto the scintillator mounted over the photodiode we have been able to measure current variations in the order of 150 pA on a dark current floor of less than 50 pA when operating the X-ray tube in switching mode, thus proving the feasibility of indirect X-ray detection by means of organic semiconductors.

Gamma camera with a two-layer diverging-slat collimator for radioisotope monitoring

The purpose of this study was to develop and evaluate a gamma camera with a newly designed diverging collimator for monitoring radiation fields in nuclear medicine. Simulations using the Geant4 Application for Tomographic Emission (GATE) were performed to model the gamma camera system designed to monitor Tc-99m radioactive isotopes usually used in nuclear medicine. A gamma camera consists of a diverging collimator, a CsI(Na) scintillation crystal with dimensions of 50.0mm×50.0mm×6.0mm and Hamamatsu H8500 PSPMT. The diverging collimator is composed of two layers of diverging slats stacked directly above each other, and the front layer is rotated by 90° with respect to the back layer. The point source at different positions was simulated, and the optimal slat thickness and slat height were determined by evaluating the spatial resolution and sensitivity. The slat thickness is 1.0mm, the slat height is 40.0mm and the angle of slats ranges from 0 to 22.5°. The front and back layers are composed of 40 and 18 slats, respectively, to achieve equal spatial resolution in the x and y directions. The diverging collimator improves the uniformity of the spatial resolution and sensitivity across the field of view and the count rate better than the pinhole collimator. Experimental measurements were performed, and the results agreed well with simulations in terms of spatial resolution and sensitivity. The results demonstrated that the two-layer diverging-slat collimator is suitable for large area monitoring of the radiation fields.

The recent results from KIMS experiment

In this paper, the recent results from KIMS (Korea invisible mass search) experiment are presented. KIMS has searched for WIMPs (Weakly Interactig Massive Particles) scattering off the nucleus by using the CsI(Tℓ) scintillator. The detector is an array of 12 CsI(Tℓ) scintilltors, whose total mass is 103.4 kg. The results reported here used the exposure of 24524.3 kg-days. With pulse shape discrimination (PSD) analysis, we estimated the nuclear recoil (NR) event rate and no meaningful excess of NR events rate were found. From this, we derived the improved cross section limit for WIMP–nucleon interaction.

Optimization of a gamma imaging probe for axillary sentinel lymph mapping

Sentinel lymph node (SLN) mapping is a technique for assessing whether early-stage invasive breast cancer has metastasized, thus determining prognosis and treatment options. SLN identification is achieved using the blue-dye and radioactive colloids techniques, which are sometimes combined with lymphoscintigraphy. Furthermore, intra-operative gamma acoustic probes, as well as gamma imaging probes are used during surgery. The purpose of this study is the construction of a gamma probe for sentinel lymph node imaging and its optimization in terms of sensitivity with respect to spatial resolution. The reference probe has small field of view (2.5 × 2.5 cm2) and is based on a position sensitive photomultiplier tube (PSPMT) coupled to a pixellated CsI(Tl) scintillator. Following experimental validation, we simulated the system using the GATE Monte Carlo toolkit (GATE v6.1) and modeled various collimator geometries, in order to evaluate their performance and propose the optimal configuration. The constraints of the proposed gamma imaging probe are i) sensitivity close to 2 cps/kBq and ii) spatial resolution equal to 6 mm at 2 cm source-to-collimator distance and ∼ 10 mm at 5 cm. An integrated structure that achieves those requirements is a tungsten collimator with 2 × 2 mm2square holes, 16 mm thickness, 0.15 mm septa, where each CsI(Tl) 2 × 2 × 5 mm3 crystal pixel is placed inside the collimator.

A Compton-vetoed germanium detector with increased sensitivity at low energies

The difficulty to directly detect plutonium in spent nuclear fuel due to the high Compton background of the fission products motivates the design of a gamma detector with improved sensitivity at low energies. We have built such a detector by operating a thin high-purity Ge detector with a large scintillator Compton veto directly behind it. The Ge detector is thin to absorb just the low-energy Pu radiation of interest while minimizing Compton scattering of high-energy radiation from the fission products. The subsequent scintillator is large so that forward-scattered photons from the Ge detector interact in it at least once to provide an anti-coincidence veto for the Ge detector. For highest sensitivity, additional material in the line of sight is minimized, the radioactive sample is kept thin, and its radiation is collimated. We will discuss the instrument design, and demonstrate the feasibility of the approach with a prototype that employs two large CsI scintillator vetoes. Initial spectra of a thin Cs-137 calibration source show a background suppression of a factor of ~2.5 at ~100 keV, limited by an unexpectedly thick 4 mm dead layer in the Ge detector.

Atomic layer deposition α-Al2O3 diffusion barriers to eliminate the memory effect in beta-gamma radioxenon detectors

Well designed scintillator detectors, including such examples as ARSA, SAUNA, and XIA’s “PhosWatch”, can readily achieve the state of the art radioxenon detection limits required for nuclear explosion monitoring. They are also reliable, robust detectors that do not require cryogenic cooling for operation. All three employ the principle of beta-gamma coincidence detection to reduce background counting rates, using a BC-404 plastic scintillator to detect the betas and a CsI or NaI scintillator to detect the gamma-rays. As a consequence of this commonality of design, all three also display a “memory effect” arising from the diffusion of Xe into BC-404. Thus, when one sample is pumped out of the detector, a fraction remains behind, embedded in the BC-404, where it artificially raises the signal counting rate for the next sample. While this is not a fatal flaw in scintillator detectors, developing a method to eliminate the memory effect would significantly enhance their utility. This paper reports efforts to develop thin, amorphous Al2O3 films, deposited by atomic layer deposition (ALD) to act as diffusion barriers on the BC-404 surfaces exposed to radioxenon. Using radon as a convenient substitute for Xe, film thicknesses between 2 and 10 nm were originally investigated and found to show a memory effect to varying degrees. A second set of 20 and 30 nm films was then produced, which appeared to completely eliminate the radon memory effect, but, when consequentially tested with radioxenon, were found to exhibit xenon memory effects that were approximately half of the effect found on uncoated BC-404. We draw two conclusions from this result. The first is that it will be necessary to develop an improved method for depositing thicker ALD Al2O3 films at lower temperatures while still retaining high film quality. The second is that, since xenon is required to test for the xenon memory effect, we need a test method that does not require xenon radio-isotopes in order to facilitate screening large numbers of samples.

Investigation of the Dependence of CsI(Tl) Scintillation Time Constants and Intensities on Particle's Energy, Charge and Mass Through Direct Fitting of Digitized Waveforms

The dependence of the CsI(Tl) scintillation time constants and intensities on the particle ionization profile is at the basis of the Charge and Mass identification technique for Light Charged Products in Intermediate Energy Nuclear Physics multi-detector arrays. The possibility of storing the digitized waveforms in true beam experiments allows performing further offline analyses and makes available a data set related to the interaction of different particle types in a wide energy range.

Design optimization of a 2D prompt-gamma measurement system for proton dose verification

To verify in-vivo proton dose distribution, a 2-dimensional (2D) prompt-gamma measurement system, comprised of a multi-hole collimation system, a 2D array of CsI(Tl) scintillators, and a position-sensitive photomultiplier tube (PS-PMT), is under development. In the present study, to determine the optimal dimension of the measurement system, we employed a series of Monte Carlo simulations with the MCNPX code. To effectively measure the high-energy prompt gammas while minimizing background gammas, we determined the collimator hole size, collimator thickness, and scintillator length to be 0.4 × 0.4 cm2, 15 cm, and 5 cm, respectively. Thereafter, the performance of the optimized measurement system was estimated for monoenergetic proton pencil beams. The peak locations of the prompt-gamma distributions for 80- and 150-MeV proton beams were clearly distinguished, and the correlation between the beam range and the peak location was confirmed by using the measurement system. For a 200-MeV proton beam, however, the peak location could not be determined due to the dominance of background gammas and the lateral dispersion of the proton beam at the end of the beam range. Based on these simulation results, a prototype 2D prompt-gamma measurement system currently is under construction and, upon completion, will be tested with therapeutic proton beams.

SU-E-J-09: Performance Optimization of Thick, Segmented Scintillators for Radiotherapy Imaging.

Thick segmented scintillators, incorporating a 2-dimensional matrix of optically-isolated scintillator elements, have shown considerable potential for improving the performance of megavoltage active matrix, flat-panel imagers (MV AMFPIs). While over a factor of 20 improvement in DQE at zero spatial-frequency has been demonstrated for prototypes incorporating CsI(Tl) and BGO scintillators, less-than-optimal element-to-element alignment (misalignment) as well as mis-registration to the underlying AMFPI array pixels can result in spatial resolution loss, reducing DQE improvement at higher spatial frequencies. In this presentation, a method to restore spatial resolution and DQE, based on the use of a high resolution AMFPI array along with special binning techniques, is investigated. The effect of misalignment and mis-registration of segmented scintillators on imaging performance was investigated theoretically and empirically through determination of the modulation transfer function (MTF) and DQE, as well as through realization of reconstructed images of a phantom in a cone-beam CT geometry. The empirical investigation, which was conducted using a 6 MV photon beam, employed a prototype BGO segmented scintillator consisting of 120×60 elements separated by 50 μm-thick septal walls and an element-to-element pitch of 1016 μm. The scintillator was coupled to a higher resolution 127-μm-pitch AMFPI array. Misalignment and mis-registration result in significant degradation of spatial resolution, leading to DQE reduction at non-zero spatial frequencies. While mis-registration for a well-aligned scintillator can be overcome through 8×8 binning of the array pixels to match the scintillator elements, any misalignment will affect such binning and lead to spatial resolution loss. However, the use of 'selective' binning, consisting of the selection of those pixels corresponding to the interior locations of each element, improves resolution while preserving DQE. The use of high-resolution AMFPI arrays combined with selective binning allows prototype AMFPIs incorporating thick, segmented scintillators to achieve imager performance limited only by scintillator performance. Work Supported by NIH grant R01-CA051397.

TU-E-217BCD-06: Cone Beam Breast CT with a High Resolution Flat Panel Detector-Improvement of Calcification Visibility.

To investigate the advantage of a high resolution flat panel detector for improving the visibility of microcalcifications (MCs) in cone beam breast CT Methods: A paraffin cylinder was used to simulate a 100% adipose breast. Calcium carbonate grains, ranging from 125-140 μm to 224 - 250 μm in size, were used to simulate the MCs. Groups of 25 same size MCs were embedded at the phantom center. The phantom was scanned with a bench-top CBCT system at various exposure levels. A 75μm pitch flat panel detector (Dexela 2923, Perkin Elmer) with 500μm thick CsI scintillator plate was used as the high resolution detector. A 194 μm pitch detector (Paxscan 4030CB, Varian Medical Systems) was used for reference. 300 projection images were acquired over 360° and reconstructed. The images were reviewed by 6 readers. The MC visibility was quantified as the fraction of visible MCs and averaged for comparison. The visibility was plotted as a function of the estimated dose level for various MC sizes and detectors. The MTFs and DQEs were measured and compared. For imaging small (200 μm and smaller) MCs, the visibility achieved with the 75μm pitch detector was found to be significantly higher than those achieved with the 194μm pitch detector. For imaging larger MCs, there was little advantage in using the 75μm pitch detector. Using the 75μm pitch detector, MCs as small as 180 μm could be imaged to achieve a visibility of 78% with an isocenter tissue dose of ∼20 mGys versus 62% achieved with the 194 μm pitch detector at the same dose level. It was found that a high pitch flat panel detector had the advantages of extending its imaging capability to higher frequencies thus helping improve the visibility when used to image small MCs. This work was supported in part by grants CA104759, CA13852 and CA124585 from NIH-NCI, a grant EB00117 from NIH-NIBIB, and a subcontract from NIST-ATP.

Low energy fast events from radon progenies at the surface of a CsI(Tℓ) scintillator

In searches for rare phenomena such as elastic scattering of dark matter particles or neutrinoless double beta decay, alpha decays of 222Rn progenies attached to the surfaces of the detection material have been identified as a serious source of background. In measurements with CsI (Tℓ) scintillator crystals, we demonstrate that alpha decays of surface contaminants produce fast signals with a characteristic mean-time distribution that is distinct from those of neutron- and gamma-induced events.

Development of X-ray/gamma-ray imaging spectrometers using reach-through APD arrays

We present spectroscopic capability of a position sensitive detector using a large area reach-through avalanche photodiode (APD) array, mainly for astronomical applications. It is quite important to obtain wide band spectra of high energy astrophysical phenomena simultaneously in order to probe emission processes or structures. Especially observations of transient objects, such as gamma-ray bursts of active galactic nuclei, require detectors with wide energy band coverage for the sake of an efficient spectroscopy within limited time windows. An APD is a compact semiconductor photon sensor with an internal gain which is often up to ∼ 100. A reach-through type APD has a thicker depletion layer thus higher efficiency for direct X-ray detection compared to a reverse type APD. We have developed 1-dimensional reach-through APD arrays which consist of 8 and 16 segments with a pixel size of 2.2 × 16 and 1.1 × 16 mm2. We demonstrated quite uniform gain and energy resolution for 5.9 keV X-ray over the pixels of these arrays. Subsequently we constructed X-ray/gamma-ray detector using the APD array optically coupled to a conventional CsI(Tl) scintillator which demonstrated energy coverage typically from 1 keV to 1 MeV.

A comparative analysis of the sensitivity of CsI (Tl), ZnO(Ga), and YAG(Ce) scintillators to the plasma background radiation under operating conditions of the ITER tokamak reactor

Problems related to neutral particle flux measurements on the ITER tokamak reactor under intense plasma background radiation conditions are considered. The results of measuring a background sensitivity with respect to neutron and γ-radiation for the scintillation detector, which is based on three different crystals (CsI (Tl), ZnO(Ga), and YAG(Ce)), are presented. The scintillators are compared and conclusions about the possibility of their applications in detectors of neutral particle analyzers currently created at the Ioffe Institute for the ITER tokamak reactor, are drawn.

Characterization of prototype full-field breast tomosynthesis by using a CMOS array coupled with a columnar CsI(Tl) scintillator

We have developed a prototype full-field digital breast tomosynthesis (DBT) system by using a complementary-metal-oxide semiconductive (CMOS) array coupled with a columnar CsI(Tl) scintillator. The imaging system consists of a matrix with an active detector area of 3072 × 3888 pixels and a pixel pitch of 74.8 µm. For tomosynthesis imaging, the X-ray tube is automatically rotated in 3° increments in the shoot mode to acquire projection images at 15 different angles over a ±21° angular range in less than 10 s. The digital detector is stationary during image acquisition. In this research, we also carried out evaluation studies to characterize the performance of the system in different operational modes designed for the DBT system, e.g., binning mode and the range of view angles, in terms of the modulation transfer function (MTF), the normalized noise power spectra (NNPS), and the detective quantum efficiency (DQE): The MTF value measured at the Nyquist frequency was 18.49%, the NNPS value at zero frequency was about 1.93 × 10−5 (mm2), and the maximum value of DQE was about 47.09% for the full resolution. For the pixel binning mode, the MTF decreased more than it did for the full resolution mode due to the increased effective pixel size. However, the full resolution mode was more sensitive to noise than the pixel binning mode. For the scan angle of the DBT system, oblique incidence of X-rays on a detector caused blurring that reduced resolution. These results seem to be promising for the use of the DBT system in potential clinical applications and will provide important information when comparisons with other DBT systems are made.

低線量領域におけるDigital Radiographyの物理的画質特性の検討

We measured physical image properties of a flat panel detector (FPD) system and a computed radiography (CR) system, targeting to a low dose range (reference dose: 2.58×10(-7) C/kg: to 1/20 dose). Input-output properties, pre-sampled modulation transfer functions (pre-sampled MTFs), and normalized noise power spectra for an FPD system equipped with a CsI scintillator (FPDcsi) and a CR system with an imaging plate coated with storage phosphor (CR) were measured at the low dose range for radiation quality of RQA3 (≍50 skV) and RQA5 (≍70 kV), and detective quantum efficiencies (DQEs) were calculated. In addition, in order to validate the DQE results, component fractions of Poisson and multiplicative and additive noise were analyzed using relative standard deviation analysis. The DQE values of FPDcsi were decreased with dose decrease, and contrarily to these, those of CR were increased. At the 1/10 and 1/20 doses for RQA3, the DQEs of FPDcsi and CR became almost the same. From the results of RSD analysis, it was proved that the main cause of DQE deterioration on FPDcsi are non-negligible additive (electronic) noise, and the DQE improvement on CR was caused by both of significant multiplicative (structure) noise and very low electronic noise. The DQE results were validated by comparing burger phantom images of each dose and radiation quality.

Neutron Background Predictions and Measurement at ATF2 Beamline

Abstract In electron machines, neutrons near the interaction region are dominantly produced by photonuclear processes in electromagnetic showers initiated by lost particles in dense materials. The photonuclear cross-section and the high multiplicity of low-energy photons make the low-energy regime vastly dominating this neutron production. ATF2, operating at 1.3 GeV, offers most of the phase space to assess the widely used Geant4 toolkit with this respect. The experiment beam dump is used to mimic the above mentioned high density region: the flux of neutron is initiated by the electron beam showering in the dump; which then scatters up to exiting the dump. The measurement of the time dependent flux is sensitive to both the neutron production and transport. Measurements of neutron fluxes performed with plastic and CsI scintillators will presented. They will be compared to a Geant4 simulation of the setup. The simulation makes use of biasing techniques to boost the simulated flux exiting the beam dump which results will be discussed.

種々の大きさのCsI(Tl)シンチレーション検出器のγ線に対するスペクトル―線量変換演算子 (<i>G</i>(<i>E</i>) 関数) の計算と評価

The spectrum-dose conversion operators (hereafter, G(E) function) were calculated for CsI(Tl) scintillation counters based on the response functions obtained by a Monte Carlo simulation code, MCNP5, in the photon energy range from 40 to 10,000 keV. The G(E) functions were determined in the cases of crystal sizes 5.5 × 5.5 × 5.5, 10 × 10 × 10, 13 × 13 × 20 and 38 × 38 × 25 mm3 for ambient dose, H*(10). It was found that the values of H*(10) deduced from the pulse-height spectra and the G(E) function agreed with those of H*(10) evaluated in the calibration field in Japan Atomic Energy Agency (JAEA) within 2% in the case of irradiations by 137Cs, 226Ra and 60Co source, by the measurements using a scintillation counter whose crystal size was 13 × 13 × 20 mm3.

種々の大きさのCsI(Tl)シンチレーション検出器のγ線に対するスペクトル―線量変換演算子 (<i>G</i>(<i>E</i>)関数) の計算と評価

種々の大きさのCsI(Tl)シンチレーション検出器のγ線に対するスペクトル―線量変換演算子 (<i>G</i>(<i>E</i>)関数) の計算と評価

Feasibility Study of a Lens-coupled Charge-Coupled Device Gamma Camera

A charge-coupled device (CCD) is generally used in a digital camera as a light-collecting device such as a photomultiplier tube (PMT). Because of its low sensitivity and very high dark current, CCD have not been popularly used for gamma imaging systems. However, a recent CCD technological breakthrough has improved CCD sensitivity, and the use of a Peltier cooling system can significantly minimize the dark current. In this study, we investigated the feasibility of a prototype CCD gamma camera consisting of a CsI scintillator, optical lenses, and a CCD module. Despite electron-multiplying (EM) CCDs having higher performance, in this study, we built a cost-eective system consisted of low-cost components compared to EMCCDs. Our prototype detector consists of a CsI scintillator, two optical lenses, and a conventional Peltier-cooled CCD. The performance of this detector was evaluated by acquiring the sensitivity, resolution, and the modulation transfer function (MTF). The sensitivity of the prototype detector showed excellent linearity. With a 1 mm-diameter pinhole collimator, the full width at half-maximum (FWHM) of a 1.1 mm Tc-99m line source image was 2.85 mm. These results show that the developed prototype camera is feasible for small animal gamma imaging.