HgI2 Detectors Research Articles

Optimization of the HgI2 Crystal Preparation for Application as a Radiation Semiconductor Detector

The effect of HgI2 crystal encapsulation using different polymer resins, with the intent of avoiding the oxidation of the crystal surface, was evaluated in this work. The crystal was purified and grown by the physical vapor transport (PVT) technique modified. Systematic measurements were carried out for evaluating the stoichiometry, structure orientation, surface morphology and impurity of the crystal grown. The purer region of the crystal grown was selected to be prepared as a radiation detector, applying water-based conductive ink contacts and copper wire on the crystal surfaces. After that, the crystal was encapsulated with a polymeric resin which insulates atmospheric gases, aiming to improve the stability of the HgI2 detector. Four resins were used for crystal encapslation and the performance of the detector depended on the composition of the resins used. Among the four resins studied to evaluate the influence of encapsulation on the performance of crystals, as a radiation detector, the best result of resistivity and energy spectrum was obtained for the resin #3 (50% - 100% of Methylacetate and 5% - 10% of n-butylacetate). The encapsulation of crystals with polymer resins, performed with the intent of avoiding the oxidation of the crystal surface, did not compromise the measurements and were fully capable of detecting the presence of gamma radiation. The stability of the encapsulated HgI2 crystal detector was of up to 78 hs, while the stability found for HgI2 detector no encapsulated was in order 3 ~4 hs.

HgI2 nanostructures obtained hydrothermally for application in ionizing radiation detection

The compound semiconductor HgI2 has been widely studied and employed as a material for ionizing radiation detection. Monocrystal growth is an intricate method for obtaining materials for this application. With the aim of finding a simpler and more effective way to develop ionizing radiation detectors, we employed HgI2 nanostructures subjected to a hydrothermal treatment and then pressed for this purpose. In the synthesis procedure, aqueous solutions of Hg(NO3)2 and NaI were mixed until their reaction completed and the suspension obtained was then placed in a homemade autoclave and heated at 120 °C for 2, 10 or 24 h. We confirmed the HgI2 tetragonal phase by powder XRD in all cases, independently of the synthesis conditions employed. Nanoparticles were characterized by their size and morphology by TEM. We used the HgI2 nanostructures to obtain a pellet by applying 0.7 GPa of pressure at room temperature. The pellet was then used to construct the detector, and we studied the electrical properties of the detector and its response to 241Am sources of different exposure rates. The resistivity and signal-to-noise ratio obtained are of the order of those reported for HgI2 detectors assembled with monocrystals. The results obtained in this work encourage us to work further on this topic, improving the method, scaling the detector’s size and studying its spectrometric grade.

Simulation and characterization of polycrystalline mercuric iodide film detector based on coplanar grid electrode

Polycrystalline mercuric iodide films are being developed as a new detector technology for X-ray imaging at room temperature. Polycrystalline mercuric iodide (HgI2) films have been grown on ITO-coated glass substrates using hot-wall physical vapor deposition (HWPVD) with the different source temperatures. The electrical properties of high quality polycrystalline HgI2 detector based on coplanar-grid electrode are investigated at room temperature. The HgI2 device of high quality polycrystalline films has low leakage current before X-ray radiation and stable capacity under low frequency. Different coplanar-grid electrode patterns are designed by varying the widths of grid and gap. The finite elements analysis is used to simulate the potential distribution of the coplanar-grid HgI2 device. The electrode design of pattern A (the width of grid is 150 μm and the width of gap is 500 μm) is the best. The electrical characteristic of coplanar-grid device using mask A verifies the feasible simulation of electrode design for pattern A.

Coplanar-grid electrode design of HgI2 detector by finite elements simulation

Polycrystalline mercuric iodide films are being developed as a new detector technology for X-ray imaging at room temperature. In this paper, different electrode patterns were designed by varying the widths of grid and gap. The finite elements analysis was used to simulate the potential distribution of the coplanar-grid detectors. The electrode design of pattern A (the width of grid was 150μm and the width of gap was 500μm) was the best. In addition, the influence of edge effects resulting from the asymmetrical electrodes on the potential distribution was discussed. In this way, the geometry of electrodes for coplanar-grid detectors was optimized by widening the second grids near the edges as shown in pattern N.

광도전체 물질의 치료 방사선 선량계 적용을 위한 가능성 연구

The use of the dosimetry have been increasingly recognized as high radiation energy and radiation treatment planning(RTP) have rapidly developed in radiotherapy. There are many types of detectors for the dosimetry such as ionization chamber, film, TLD, diode, and etc. Among such detectors, the diode detector uses a photoconductor materials that generate electrical signals by the incident radiation energy. Though many research groups are recently interested in such materials, there is few experimental results except for silicon in the radiation therapy field. In this study, the feasibility of photoconductor materials was verified as a dosimeter through the evaluation of response properties at a high radiation energy. For the fabricated detectors based on HgI2 and PbI2, reproducibility, linearity, and pulse-rate response were analyzed. Such evaluations are essential factors for the use of dosimeter. From results, linearity and reproducibility of the fabricated HgI2 detector indicated about 7% error. The fabricated PbI2 detector showed 1.7% error in linearity, and 12.2% error in reproducibility.

Modelling of detective quantum efficiency of direct conversion x-ray imaging detectors incorporating charge carrier trapping and K -fluorescence

A cascaded linear system model is developed for calculating the frequency-dependent detective quantum efficiency, DQE(f), of a direct conversion x-ray imaging detector by incorporating the effects of charge carrier trapping and reabsorption of K-fluorescent x-rays. The present model considers a combination of series and parallel processes and interactions between them. The modulation transfer function for K-fluorescent x-ray reabsorption is modelled by determining the line spread function and subsequent one-dimensional Fourier transform. The DQE model is applied to amorphous selenium (a-Se) and polycrystalline mercuric iodide (poly-HgI2) detectors. The charge carrier trapping has a significant effect on DQE in both a-Se and poly-HgI2 detectors. The charge carrier transport properties have higher influences on DQE performance in a-Se detectors than that in HgI2 detectors, because of relatively low conversion gain in a-Se detectors. High conversion gain can minimise the adverse effect of incomplete charge collection. A simplified model for the calculation of zero spatial frequency detective quantum efficiency, DQE(0), under parallel cascaded system is also proposed in this study. There exists an optimum photoconductor thickness, which maximises the DQE(0). The proposed model is compared with the published measured data and shows good agreement.

Assessment of the Radiation Tolerance of CdZnTe and HgI$_{2}$ to Solar Proton Events

Single crystals of CdZnTe of dimensions 10 times 10 times 3 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> and HgI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> of dimensions 10 times 10 times 2 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> were fabricated into planar detectors. Four sets of detectors were then exposed to simulated solar proton events of varying magnitude at the Kernfysisch Versneller Instituut in the Netherlands. Events were simulated by generating protons over the energy range 60 MeV to 200 MeV in 10 energy bands which approximated the spectral shape of the August 1972 solar particle event. One pair of detectors (a CdZnTe and a HgI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) was exposed to an integral fluence of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , a second to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , a third to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> and the fourth to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . The latter corresponds to an absorbed dose in silicon of 100 krad or in SI units, 1 kGy. The detectors were characterized both before and after the irradiations in terms of background count rate and spectral performance using <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">55</sup> Fe, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">109</sup> Cd, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">241</sup> Am, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Co and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs radioactive sources. Typical pre-irradiation FWHM energy resolutions of 3 keV and 4 keV were recorded at 60 keV for the CdZnTe and HgI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> detectors, respectively. It was found that HgI <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is very radiation tolerant showing little degradation in energy resolution or charge collection efficiency (CCE) for input fluences up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . CdZnTe, on the other hand, showed a clear degradation in energy resolution and CCE for input fluences above 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . For example, the energy resolution degraded by a factor of 3 between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> protons cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> while the CCE decreased by a factor of 2.

Systematic investigation of the signal properties of polycrystalline HgI2 detectors under mammographic, radiographic, fluoroscopic and radiotherapy irradiation conditions

The signal properties of polycrystalline mercuric iodide (HgI2) film detectors, under irradiation conditions relevant to mammographic, radiographic, fluoroscopic and radiotherapy x-ray imaging, are reported. Each film detector consists of an ∼230 to ∼460 µm thick layer of HgI2 (fabricated through physical vapour deposition or a screen-print process) and a thin barrier layer, sandwiched between a pair of opposing electrode plates. The high atomic number, high density and low effective ionization energy, WEFF, of HgI2 make it an attractive candidate for significantly improving the performance of active matrix, flat-panel imagers (AMFPIs) for several x-ray imaging applications. The temporal behaviour of current from the film detectors in the presence and in the absence of radiation was used to examine dark current levels, the lag and reciprocity of the signal response, x-ray sensitivity and WEFF. The results are discussed in the context of present AMFPI performance. This study provides performance data for a wide range of potential medical x-ray imaging applications from a single set of detectors and represents the first investigation of the signal properties of polycrystalline mercuric iodide for the radiotherapy application.

Array of HgI2 detectors on the HL-1M Tokamak

An array of seven HgI2 detectors, which are suitable for detecting the time and space evolution of the x-ray radiation flux and energy spectrum in the range between 15–150 keV relevant to suprathermal electron and runaway electrons was installed on the HL-1M Tokamak (R0=1.02 m, a=0.26 m, BT=3.5 T, IP=350 kA, ne=3–6×1013 cm−3, Te=0.8–1.5 keV). Our iodine mercury (HgI2) semiconductor detectors possess the properties of low noise level, high detection efficiency, high counting rate, high energy resolution, and working at room temperature. Every detector is a high quality, high purity detector cut out of a big size HgI2 crystal. A new type of multichannel energy spectrometer with a high speed acquisition system was developed. Therefore, the spatial and temporal distribution of the x-ray energy spectrum induced by thermal and superthermal electrons can be observed.

Comparison of 5 and 10 mm thick HgI2 pixelated γ-ray spectrometers

HgI2 detectors fabricated with pixelated anodes were tested as potential room temperature spectrometers. Spectra were obtained with 5 and 10mm thick devices, and their results were compared. The 5mm thick devices showed some effect of the anode weighting potential, while the 10mm thick devices showed the effects of charge diffusion/sharing and electron trapping. A resolution near 1.4% FWHM using a 137Cs source was achieved after correcting for the interaction depth within the detector. The resolution from individual depths was as low as approximately 1.1% FWHM.

Direct conversion X-ray sensors: sensitivity, DQE and MTF

Imaging characteristics of photoconductive flat panel X-ray image detectors, such as X-ray sensitivity, detective quantum efficiency (DQE), and resolution in terms of the modulation transfer function (MTF) are examined with applications to amorphous selenium (a-Se), polycrystalline HgI2 and CdZnTe detectors. A theoretical model has been developed for the calculation of X-ray sensitivity of a pixellated detector by using the Shockley–Ramo theorem, the weighting potential of the individual pixel and the final trapped charge distribution across the photoconductor. The X-ray sensitivity of pixellated X-ray detectors mostly depends on the mobility and lifetime product of charges that move towards the pixel electrodes, and the extent of dependence increases with decreasing pixel per unit detector thickness. A cascaded linear system model that includes incomplete charge collection and interaction depth dependent conversion gain and charge collection stages is considered for the calculation of the zero spatial frequency detective quantum efficiency DQE(0) of a direct conversion X-ray image detector. The DQE(0) performance of a-Se, HgI2 and CdZnTe detectors is examined for fluoroscopic applications. It is shown that, in addition to high quantum efficiency, both high conversion gain and high charge collection efficiency are required to improve the DQE performance of an X-ray image detector. The factors affecting the MTF of a-Se flat-panel detectors for digital mammography are investigated. Both theoretical and experimental methods are examined. A theoretical model has been developed based on cascaded linear system analysis with parallel processes to take into account the effect of K-fluorescence. This model has been used to understand the performance of a small-area prototype detector with 85 μm pixel size. The presampling MTF of the prototype has been measured and compared to a theoretical calculation based on the model. The calculation shows that K-fluorescence accounts for a 15% reduction in the MTF at the Nyquist frequency of the prototype detector. The measurement of presampling MTF of the prototype detector reveals an additional source of blurring, which is probably due to charge carrier trapping in the blocking layer at the interface between a-Se and the active matrix. This introduces a drop in presampling MTF at high spatial frequencies.

Thick films of X-ray polycrystalline mercuric iodide detectors

Polycrystalline HgI2 thick film detectors are among the leading semiconductor materials to be used as direct converters in X-ray digital radiography. Their properties along with a survey of the properties of alternative materials, such as PbI2 or A-Se, will be given. The preparation of HgI2 detector plates, both by direct evaporation (Physical vapor deposition, (PVD)) and by binding the individual crystallites with polymeric glue, forming screen-printed (SP) detector plates, will be described. The microstructure of the PVD thick films showing a columnar morphology, as determined by SEM measurements, will be shown. The X-ray response to radiological X-ray generator of 85kVp using the current integration mode will be reported for both PVD and SP films. Finally, some X-ray images taken at Xerox–Parc using HgI2 polycrystalline detectors will be shown.

Gamma-ray imaging and spectroscopy system using room-temperature semiconductor detector elements

We report on the design, construction, and testing of a gamma-ray imaging system with spectroscopic capabilities. The imaging system consists of an orthogonal strip detector made from either HgI2 or CdZnTe crystals. The detectors utilize an 8×8 orthogonal strip configuration with 64 effective pixels. Both HgI2 or CdZnTe detectors are 1 cm2 devices with a strip pitch of approximately 1.2 mm (producing pixels of 1.2 mm × 1.2 mm). The readout electronics consist of parallel channels of preamplifier, shaping amplifier, discriminators, and peak sensing ADC. The preamplifiers are configured in hybrid technology, and the rest of the electronics are implemented in NIM and CAMAC with control via a Power Macintosh computer. The software used to readout the instrument is capable of performing intensity measurements as well as spectroscopy on all 64 pixels of the device. We report on the performance of the system imaging gamma-rays in the 20–500 keV energy range and using a pin-hole collimator to form the image.

Evaluation of mercuric iodide ceramic semiconductor detectors

Mercuric iodide ceramic radiation detectors, which can act as nuclear particle counters, have been fabricated with single continuos electrical contacts and with linear strip contacts. They have been tested with different kinds of γ and β sources as well as in a high energy beam at CERN. The detectors were also successfully tested for radiation hardness with irradiation of 5*10 14 neutrons/cm 2 . The ratio of detected photons over the number of absorbed photons has been measured with T sources of different energies, and it ranges from 20% at 44 keV up to about 30% at 660 keV. An absolute efficiency of 70% has been measured for a 350 gm thick detector for β particles emitted by a 90 Sr source. Charge collection efficiency, defined as the amount of charge induced on the electrodes by a Minimum Ionizing Particle (MIP) traversing the detector, has been measured in two samples. The average collected charge fits well with a linear curve with slope of 35 electrons/(kV/cm) per 100 pro. This result is well described by a dynamic device simulation, where the free carrier mean lifetime is used as a free parameter, adjusted to a value of 1.5 ns, i.e. about 1/100 of the corresponding lifetime in single crystal HgI2 detectors. The response to MIP has also been studied with a high energy (100 GeV) muon beam in CERN. A preliminary beam profile is presented while a more detailed analysis is still in progress and will be presented elsewhere. These results together with the low cost of the material make ceramic HgI 2 detectors excellent candidates for large area particle tracking and imaging applications, even in a radiation harsh environment.

Improved method for HgI2 crystal growth and detector fabrication

Abstract Instead of the temperature oscillation method (TOM), a modified vapor growth method was applied in this paper to grow large HgI 2 crystals with fewer lattice defects by providing a relatively stable temperature field during growth. And a new processing technique of HgI 2 crystals was developed for detector fabrication by merging solution string-sawing and hand-cleaving. The lattice deformations caused during cleaving were reduced greatly by eliminating passivated layers on the sawed crystal platelets before cleaving.

Defects and Impurities in Mercuric Iodide Processing

AbstractIn the fabrication of mercuric iodide (HgI2) room temperature radiation detectors, as in any semiconductor process, the quality of the final device is very sensitive to the impurities and defects present. Each process step can change the effects of existing defects, reduce the number of defects, or introduce new defects. In HgI2 detectors these defects act as trapping and recombination centers, thereby degrading immediate performance and leading to unstable devices. In this work we characterized some of the defects believed to strongly affect detector operation. Specifically, we studied impurities that are known to be present in typical HgI2 materials. Leakage current measurements were used to study the introduction and characteristics of these impurities, as such experiments reveal the mobile nature of these defects. In particular, we found that copper, which acts as a hole trap, introduces a positively charged center that diffuses and drifts readily in typical device environments. These measurements suggest that Cu, and related impurities like silver, may be one of the leading causes of HgI2 detector failures.

Electrical Characterization of Impurities in and Contacts on Mercuric Iodide

ABSTRACTMercuric iodide (HgI2) is a semiconductor that has shown great promise for use in roomtemperature high-resolution x-ray and gamma-ray spectrometers. Its widespread usage has been limited, however, by low yield and long-term reliability problems. The processing of this material is still in its infancy compared to silicon, so research efforts continue to pursue the root causes of device failures. Two likely sources of performance limitations are impurities and poor contacts. Significant efforts have been expended in developing and using various purification schemes. However, quantitative chemical analyses have shown that several metallic impurities still exist at the high ppb level. In addition, it has not yet been definitively determined which impurities are most problematic and at what level they have a detrimental effect. Leakage currents and currentvoltage measurements have been used to study the movement of mobile impurity-related defect centers in the bulk mercuric iodide. In particular, this method has been used to quantify the drift of metallic impurities, such as Ag and Cu, which are known to or believed to degrade HgI2 detectors. Four-point-probe sheet resistance measurements have been used to study the stability of contacts and the formation of reaction layers. In particular, such measurements have revealed that the Pd contacts currently used for the highest quality detectors are not as stable as previously thought, as the films of Pd react with the HgI2 to form the amalgam PdHg.

X‐Ray fluorescence escape peaks in HgI2 detectors

AbstractA mathematical model for the estimation of the relative intensities of the escape peaks in HgI2 detectors is presented. Comparison of calculated results with experimental values for various x‐ray energies shows satisfactory agreement. The method can be used to evaluate directly the sensitive thickness of an HgI2 detector through the escape to total peak intensity ratio.

X-Ray Fluorescence Analysis Capability of HGI2 Detector Systems

ABSTRACTA laboratory XRF analysis system employing an HgI2 spectrometer detector has been built. The system consists of an x-ray generator tube, a rotating carousel holding multiple XRF targets, and a probe head containing the HgI2 detector. Tests have been performed on several samples, under vacuum, helium and nitrogen ambient atmospheric conditions. This is of particular interest, since the applications for such an XRF system include operation under different ambient conditions. For applications in terrestrial and space planetary exploration, the expected temperatures and gas ambient will vary widely. Comparative analyses using the various gases were made on spectra obtained from standard geological samples.

Thickness Gauging of Organic Films on Large Plastic Body Parts With an Xrf Probe Based on a Roomtemperature Mercuric Iodide Detector

AbstractThe variability of adhesion promoter (primer) film thickness has been identified at Ford Motor Company as an important factor in maintaining the quality of paint on exterior plastic body parts. XRF offers unique advantages for non-destructive film monitoring. Monitoring of large parts for process control requires a non-contact approach using a small mobile probe. Previous attempts to implement a practical XRF gauging probe have been hampered by the choice of available x-ray detection technologies; proportional counters cannot resolve the layer's fluorescent lines properly and suffer from instability, while Si(Li) or Ge detectors need cooling to low temperatures, which makes their usage impractical for mobile probe.The recent commercial availability of state-of-the-art Mercuric Iodide (HgI2) x-ray detectors led to the development of a lightweight XRF probe that contains an x-ray tube and a HgI2 detector. Thickness measurements of primer films on plastic substrates were characterized, and 15 sec. acquisition yielded precision of 0.225 and 0.525 pm for film thicknesses of 5 and 20 µm respectively. Ti was added to the substrate and Cr to the film as tagging elements. A ratiometric technique, taking into account absorption of the Ti signal in the film, yields excellent linearity and compensates for instrumental errors such as sampleto- probe distance variation and x-ray source fluctuations.