Crystalline Detectors Research Articles

Development and characterization of CsI(Tl) crystal for use as a radiation detector

Cesium iodide crystal activated with thallium (CsI(Tl)) is used as radiation sensor because of its favorable characteristics as scintillator, when excited by gamma radiation. This crystal has good mechanical strength and it is relatively little hygroscopic. In the present work, the CsI(Tl) crystal was grown in the Nuclear Energy Research Institute (IPEN/CNEN/SP) by Brigdman technique, in two different formats: (a) cylindric (∅ 20.1 mm x ­ 11.9 mm) and (b) parallelepiped ( c 12.3 mm x ­ 19.5 mm). The scintillator response was studied through five gamma radiation sources: 99mTc (140 keV), 137Cs (662 keV), 60Co (1173 keV, 1333 keV) and 22Na (511 keV, 1275keV). The crystals were coupled to a photomultiplier tube using 0.5 McStokes viscosity silicone grease as the optical interface. All electronics for signal measurements were developed at IPEN. Luminescence property of the CsI(Tl) crystal was excited by the radiation from a 99mTc source. The energy resolution of the crystalline detector was determined by the FWHM parameter, corresponding to the photopeak width at half of its height. Gamma radiation was spectrometry performed with the following sources: 99mTc, 241Ba, 137Cs, 60Co and 22Na.

Removal of [formula omitted]Pb by etch of crystalline detector sidewalls

A potential source of dominant backgrounds for many rare-event searches or screening detectors is from radon daughters, specifically 210Pb, deposited on detector surfaces, often during detector fabrication. Performing a late-stage etch is challenging because it may damage the detector. This paper describes a late-stage etching technique that reduces surface 210Pb and 210Po by >99× at 90% C.L.

Advanced Robotic Angiography Systems for Image Guidance During Conventional Transarterial Chemoembolization

The aim of this study was to compare 2 advanced robotic angiography systems for real-time image guidance in terms of radiation dose and image quality (IQ) during conventional transarterial chemoembolization (C-TACE) of hepatic malignant tumors. One hundred six patients (57 women/49 men; mean age, 60 ± 11 years) who had undergone C-TACE using 2 generations of robotic angiography platforms for image guidance were included in this retrospective study. Patients were divided into 2 groups (n = 53, respectively): group 1 (first generation) and group 2 (second generation). Radiation dose for fluoroscopy and digital subtraction angiography (DSA) was compared between first-generation and second-generation angiography equipment, respectively. Among several features of the second-generation compared with the first-generation system, improvements included a refined crystalline detector system for enhanced noise reduction and advanced CARE filter software for lowering radiation dose. Radiation dose was measured using an ionization chamber. Image quality was assessed by 3 radiologists using 5-point Likert scales. Both groups were comparable in terms of number and location of lesions, as well as body weight, body mass index, and anatomical variants of feeding hepatic arteries (all P > 0.05). Dose-area product (DAP) for fluoroscopy was significantly lower in group 2 (1.4 ± 1.1 Gy·cm) compared with group 1 (2.8 ± 3.4 Gy·cm; P = 0.001). For DSA, DAP was significantly lower (P = 0.003) in group 2 (2.2 ± 1.2 Gy·cm) versus group 1 (4.7 ± 2.3 Gy·cm). Scores for DSA IQ indicated significant improvements for group 2 by 30% compared with group 1 (P = 0.004). Regarding fluoroscopy, scores for IQ were 76% higher in group 2 compared with group 1 (P = 0.001). Good to excellent interrater agreement with Fleiss kappa coefficients of κ = 0.75 for group 1 and κ = 0.74 for group 2 were achieved. Most recent generation robotic angiography equipment allows for considerable radiation dose reductions while improving IQ in fluoroscopy and DSA image guidance during C-TACE treatment.

Alpha radiation dosimetry using Fluorescent Nuclear Track Detectors

To answer the need for better tools for alpha radiation radiobiology and microdosimetry research, a novel irradiation setup based on a honeycomb collimator, in combination with Fluorescent Nuclear Track Detectors (FNTD) for alpha radiation dosimetry and spectroscopy, was introduced. FNTDs are a novel type of small, crystalline detector that can visualize individual alpha particles and simultaneously measure their location, velocity direction and energy with good accuracy. The performance of FNTDs for alpha radiation dosimetry was evaluated for the first time and the results were compared to extrapolation chamber measurements and simulations. The surface dose rate to water of the irradiation setup for two different honeycomb collimators, measured using FNTDs, agreed with the extrapolation chamber measurements within 6%. The simulations underestimated the surface dose rate to water for the first collimator and overestimated the dose for the second collimator, indicating the sensitivity to manufacturing errors in the collimators of this irradiation setup. The dose homogeneity in the setup was measured using radiochromic film and showed variations of less than 5%, making this setup, in combination with the rich information obtained regarding the spatial, angular and energy distributions of the alpha particles, obtained using the FNTDs, ideal for microdosimetry and radiobiology experiments. The accuracy and ease-of-use of FNTDs in addition to the surface or absorbed dose and fluence of the radiation field indicate that these detectors are prime candidates for research applications in the field of alpha radionuclide therapy.

Thermally induced fading of Mn-doped YAP nanoceramics

Thermally induced fading of the thermoluminescent (TL) detectors in the form of YAlO3:Mn ceramics prepared from the nanocrystalline materials synthesized by the solution combustion method have been measured and analyzed in comparison with the single crystalline YAlO3:Mn detectors studied previously. The observed differences in thermoluminescent and dosimetric properties of nanoceramic and single crystalline detectors are discussed in terms of peculiarities of the nanoceramic material.

TU-FG-209-03: Exploring the Maximum Count Rate Capabilities of Photon Counting Arrays Based On Polycrystalline Silicon

Purpose: Photon counting arrays (PCAs) offer several advantages over conventional, fluence-integrating x-ray imagers, such as improved contrast by means of energy windowing. For that reason, we are exploring the feasibility and performance of PCA pixel circuitry based on polycrystalline silicon. This material, unlike the crystalline silicon commonly used in photon counting detectors, lends itself toward the economic manufacture of radiation tolerant, monolithic large area (e.g., ∼43×43 cm2) devices. In this presentation, exploration of maximum count rate, a critical performance parameter for such devices, is reported. Methods: Count rate performance for a variety of pixel circuit designs was explored through detailed circuit simulations over a wide range of parameters (including pixel pitch and operating conditions) with the additional goal of preserving good energy resolution. The count rate simulations assume input events corresponding to a 72 kVp x-ray spectrum with 20 mm Al filtration interacting with a CZT detector at various input flux rates. Output count rates are determined at various photon energy threshold levels, and the percentage of counts lost (e.g., due to deadtime or pile-up) is calculated from the ratio of output to input counts. The energy resolution simulations involve thermal and flicker noise originating from each circuit element in a design. Results: Circuit designs compatible with pixel pitches ranging from 250 to 1000 µm that allow count rates over a megacount per second per pixel appear feasible. Such rates are expected to be suitable for radiographic and fluoroscopic imaging. Results for the analog front-end circuitry of the pixels show that acceptable energy resolution can also be achieved. Conclusion: PCAs created using polycrystalline silicon have the potential to offer monolithic large-area detectors with count rate performance comparable to those of crystalline silicon detectors. Further improvement through detailed circuit simulations and prototyping is expected. Partially supported by NIH grant R01-EB000558. This work was partially supported by NIH grant no. R01-EB000558.

Monte Carlo Modeling of Phonons at Crystal Interfaces

One of the strategies in the effort to directly detect dark matter particles is to measure the phonon and charge signals produced in an incidental collision of a weakly interacting massive particle with a nucleon in a crystalline detector. Proper calibration of the detected phonon energy relies on detailed models of phonon propagation through the crystal to the instrumented surface. Previous Detector Monte Carlo incorporate probabalistic anharmonic decay and mass defect scattering but neglect the mode dependent scattering at crystal boundaries. We calculate mode specific reflection and transmission at detector interfaces with a simple acoustic mismatch model. We find that mode preserving transmission is the most probable outcome at germanium-aluminum detector interfaces, but the probability of reflection is not negligible. The average phonon reflection probability at near normal angles of incidence at a Ge/Al interface is near 20 %, but grows dramatically for oblique incidence. We develop a code using Geant4, which should allow modeling extensions to all phonon mediated dark matter detection schemes. Our models are adaptable to other crystal materials and are generally useful in any phonon interface problem.

Cascaded‐systems analyses and the detective quantum efficiency of single‐Z x‐ray detectors including photoelectric, coherent and incoherent interactions

Theoretical models of the detective quantum efficiency (DQE) of x-ray detectors are an important step in new detector development by providing an understanding of performance limitations and benchmarks. Previous cascaded-systems analysis (CSA) models accounted for photoelectric interactions only. This paper describes an extension of the CSA approach to incorporate coherent and incoherent interactions, important for low-Z detectors such as silicon and selenium. A parallel-cascade approach is used to describe the three types of x-ray interactions. The description of incoherent scatter required developing expressions for signal and noise transfer through an "energy-labeled reabsorption" process where the parameters describing reabsorption are random functions of the scatter photon energy. The description of coherent scatter requires the use of scatter form factors that may not be accurate for some crystalline detector materials. The model includes the effects of scatter reabsorption and escape, charge collection, secondary quantum sinks, noise aliasing, and additive noise. Model results are validated by Monte Carlo calculations for Si and Se detectors assuming free-atom atomic form factors. The new signal and noise transfer expressions were validated by showing agreement with Monte Carlo results. Coherent and incoherent scatter can degrade the DQE of Si and sometimes Se detectors depending on detector thickness and incident-photon energy. Incoherent scatter can produce a substantial low-frequency drop in the modulation transfer function and DQE. A generally useful CSA model of the DQE is described that is believed valid for any single-Z material up to 10 cycles/mm at both mammographic and radiographic energies within the limitations of Fourier-based linear-systems models and the use of coherent-scatter form factors. The model describes a substantial low-frequency drop in the DQE of Si systems due to incoherent scatter above 20-40 keV.

Radiation damage of heavy crystalline detector materials by 24 GeV protons

Samples of three heavy crystalline materials: PbWO4, Bi4Si3O12, and PbF2 were irradiated in a high-intensity 24GeV proton beam at the CERN PS to fluencies of 3.8×1013protons/cm2. The optical transmission radiation damage was measured and all crystals show a shift of the cutoff in the transmission spectrum that is not observed when the crystals are irradiated with γ radiation. This shift of the cutoff under proton irradiation seems to be a general property of the heavy crystalline materials. A mechanism for this proton-induced transmission damage is discussed.

Recoiling Ion-Channeling in Direct DM Detectors

The channeling of the recoiling nucleus in crystalline detectors after a WIMP collision would produce a larger scintillation or ionization signal in direct detection experiments than otherwise expected. I present estimates of channeling fractions obtained using analytic models developed from the 1960's onwards to describe channeling and blocking effects. We find the fractions to be too small to affect the fits to potential WIMP candidates. I also examine the possibility of detecting a daily modulation of the dark matter signal due to channeling.

Daily modulation due to channeling in direct dark matter crystalline detectors

The channeling of the ion recoiling after a collision with a weakly interacting massive particle (WIMP) in direct dark matter crystalline detectors produces a larger scintillation or ionization signal than otherwise expected. Channeling is a directional effect, which depends on the velocity distribution of WIMPs in the dark halo of our Galaxy and could lead to a daily modulation of the signal. Here we compute upper bounds to the expected amplitude of daily modulation due to channeling using channeling fractions that we obtained with analytic models in prior work. After developing the general formalism, we examine the possibility of finding a daily modulation due to channeling in the data already collected by the DAMA/NaI and DAMA/LIBRA experiments. We find that even the largest daily modulation amplitudes (of the order of 10% in some instances) would not be observable for WIMPs in the standard halo in the 13 years of data taken by the DAMA Collaboration. For these to be observable, the DAMA total rate should be $1/40$ of what it is or the total DAMA exposure should be 40 times larger. The daily modulation due to channeling will be difficult to measure in future experiments. We find it could be observed for light WIMPs in solid Ne, assuming no background.

SU‐C‐220‐05: Investigation of Novel, Focused, Segmented Scintillator Geometries for High DQE Megavoltage Active Matrix Imagers

Purpose: Megavoltage active matrix, flat‐panel imagers (AMFPIs) have become ubiquitous in modern radiotherapy environments. However, their imaging performance is severely constrained by very low DQE (∼1%). While significant performance improvements have been demonstrated through introduction of thick, precision‐machined matrices of optically‐ isolated crystalline detector elements, the benefits are constrained by geometric beam divergence. It is therefore of interest to examine the potential performance of focused, segmented scintillator geometries specifically designed to circumvent DQE losses caused by divergence. Methods: The potential detection efficiency and DQE performance of hypothetical detector geometries was examined by means of Monte Carlo simulation of radiation transport. The choice of geometries examined was governed by considerations including: maximization of DQE through geometrically focused elements; manufacturability of the focused matrix; and feasibility of the underlying active matrix of indirect detection imaging pixels. Calculations were performed for a variety of candidate scintillator materials up to 6 cm thick. Results: The most promising detector geometries identified in the study consisted of matrices of scintillator elements located along the surface of a plane, as well as along the surface of a spherical cap. For cap detectors, elements arranged in a warped‐rectangular matrix, concentric rings, and a geodesic are of interest — and geometries employing a single element shape are particularly favored, although at the cost of fill factor. While focused planar detectors would be compatible with conventional AMFPI arrays, cap detectors would involve active matrix arrays in the shape of an approximately spherically curved surface created through cut and bend techniques. Overall, depending upon detector geometry, scintillator material and scintillator thickness, calculations indicate that DQE improvements of up to a factor of ∼50 are conceivable. Conclusions: This initial theoretical study suggests that the promise of very high DQE performance from thick, segmented scintillators could be realized through novel, focused detector geometries. Work supported by NIH grant R01‐CA051397

Relevance of ion-channeling for direct DM detection

The channeling of the recoiling nucleus in crystalline detectors after a WIMP collision would produce a larger scintillation or ionization signal in direct detection experiments than otherwise expected. I present estimates of the importance of this effect for the total direct detection rate and the daily modulation of the signal using analytic models produced in the 1960's and 70's to describe the effects of channeling and blocking in crystals.

Confocal fluorescent imaging of tracks from heavy charged particles utilising new Al2O3:C,Mg crystals

A completely optical, non-destructive imaging of tracks in a fluorescent crystal provides a new way to detect and to assess doses from heavy charged particles and neutrons. The technique combines confocal fluorescent microscopy with a new radiation-sensitive, luminescent material based on aluminium oxide single crystals doped with carbon, magnesium and having aggregate oxygen vacancy defects (Al2O3:C,Mg). Radiation-induced colour centres in the new material have an absorption band at 620 nm and produce fluorescence at 750 nm with a high quantum yield and a short, 75 +/- 5 ns, fluorescence lifetime. Three-dimensional spatial distribution of fluorescent intensity allows one to obtain depth-dose distributions and to discriminate between high- and low-linear energy transfer radiations. Images of single tracks produced by different types of radiation have been obtained. Irradiations with a calibrated 241Am alpha source showed high efficiency for track detection. Thermal neutrons were detected using a nuclear reaction with a 6LiF radiator and production of alpha particles and tritium ions. Fast neutrons were detected using recoil protons produced in a polyethylene radiator installed in front of the crystalline detector. Three-dimensional reconstruction of a recoil proton propagating through the crystal was demonstrated.

Different shapes of tracks in muscovite mica

Ion tracks shape, in irradiated and etched crystalline detectors, is ruled mainly by the material structure. Abundant data of rhombic tracks in muscovite mica etched with HF under several conditions are reported in the literature. In the present paper, rhombic and hexagonal tracks in muscovite mica for different ions and energies are reported.

Current instability and polarization phenomena in lead iodide crystalline detectors

The study of the evolution of the leakage current after suddenly reversing the bias has been used to investigate the origin of polarization phenomena in lead iodide single-crystal detectors. The presence of a peak directly related to the accumulated charge and the temperature in the current vs. time curves can be explained using a model previously proposed for mercuric iodide. This behaviour results from the presence of a trap level, in the bandgap whose energy has been estimated to be 0.41 eV from current vs. time and temperature measurements. Actually, Thermally Stimulated Current measurements indicated that this value corresponds only to an average and that several levels are present in the bandgap, the main ones being situated at 0.29, 0.45 and 0.50 eV. Correlation with the time behaviour of the detector characteristics (noise and pulse height) indicates that polarization is due to the modification of the charge of these levels with time.

Geometry of etched charged particle tracks in crystalline detectors

Studies on the shape of etched fission tracks in crystalline detectors show that the track geometry carries valuable information on the crystal structure of the detector involved. The geometry of the etched tracks in crystalline detectors is a prism formed of certain crystal planes. In mica detectors, for given etching conditions the type and number of the planes depend on the incident angle and the extent of the radiation damage along the track. In a single track the type of crystal planes forming the track prism may change along the track resulting in dramatic variations of the track geometry at certain depth of focus.

State of the art of wide-bandgap semiconductor nuclear radiation detectors

The leading materials which operate as room temperature nuclear radiation detectors are HgI2, CdTe, and Cd1−xZnxTe (0.05>x>0.25). However, additional materials have also been developed, such as semi-insulating GaAs and PbI2. A comparison of the charge transport properties of all these materials will be made, followed by a discussion of each of the materials separately. Crystal growth methods of spectrometer-grade materials will be mentioned, and defects which limit their performance will be discussed. Nuclear spectra measurements with detectors fabricated from these materials, for low X-ray energies as well as for high-energy gamma-rays, will be shown. Polarization effects which occur in some detectors such as HgI2 will also be discussed. Correlation between crystalline perfection and detector performance will be shown. Results of quantitative chemical analysis of various detector materials and problems encountered in determining accurate values ofx in Cd1−xZnxTe and its homogeneity in the bulk will be presented. Finally, the present state of the art and developments for the near future will be discussed.

Microwave detectors based on granular high-T/sub c/ thin films

The detecting and mixing properties of microstrip superconducting Y-Ba-Cu-O and Bi-Ca-Sr-Cu-O thin-film structures deposited on various substrates have been investigated. The device performance was tested in the 22-, 55-, and 110-GHz frequency bands at temperatures ranging from 100 K to about 50 K. The sensitivity obtained at 110 GHz was comparable to that of crystalline detectors. Mixing experiments were performed in the 25-GHz frequency band and indicated that the detector response time is less than 40 ps. The intermediate frequency was varied from 50 MHz to 5 GHz without any decrease in the mixer output up to 3 GHz. Auxiliary emission measurements performed at 12 GHz and down to 4.2 K revealed that the detector low-temperature performance limit is associated with microwave radiation from clusters of intergrain weak links arranged in multiloop quantum interferometers. >

Discontinuous fission tracks in crystalline detectors

It is widely recognized that a key to the more complete exploitation of solid state nuclear track detectors lies in knowledge of the ways in which a particle is brought to rest, and how its energy is stored as lattice defects. One indirect way of looking at latent tracks — in an effort to bridge the micro/macro — has been small angle X-ray scattering, and the subsequent, multi-parameter model has spawned others for different solids. This work led to the so-called “gap model”, in which intermittent extended defects are separated by point defect rich regions. Latent intermittent fission fragment tracks were seen as long ago as 1962, using direct transmission electron microscopy (TEM). It is shown, in more detail, that the intermittency arises from periodic bursts of electronic energy loss along the particle trajectory, and that this makes it possible to measure the thickness of a crystal, using TEM, to an accuracy of one atomic (or molecular) layer. In other words anisotropy of the basic crystal lattice is responsible also for the latent track structure, as it is for the well-known angular variations in particle ranges evidenced in etched track polar plots, for which “channelling” and “quasi-channelling” are responsible. Simple probability theory, isotropic fragment emission, and the lattice structures of muscovite mica and molybdenite are used to predict the most probable extended defect length for at least an order of magnitude comparison with the X-ray work, since crystal orientations are not specified. Speculation is made on the consequences of this work for the etching of latent tracks.