CdTe Double-sided Strip Detectors Research Articles

2-mm-thick large-area CdTe double-sided strip detectors for high-resolution spectroscopic imaging of X-ray and gamma-ray with depth-of-interaction sensing

We developed a 2-mm-thick CdTe double-sided strip detector (CdTe-DSD) with a 250μm strip pitch, which has high spatial resolution with a uniform large imaging area of 10 cm2 and high energy resolution with high detection efficiency in tens to hundreds keV. The detector can be employed in a wide variety of fields for quantitative observations of hard X-ray and soft gamma-ray with spectroscopic imaging, for example, space observation, nuclear medicine, and non-destructive elemental analysis. This detector is thicker than the 0.75-mm-thick one previously developed by a factor of ∼2.7, thus providing better detection efficiency for hard X-rays and soft gamma rays. The increased thickness could potentially enhance bias-induced polarization if we do not apply sufficient bias and if we do not operate at a low temperature, but the polarization is not evident in our detector when a high voltage of 500 V is applied to the CdTe diode and the temperature is maintained at −20 °C during one-day experiments. The “Depth Of Interaction” (DOI) dependence due to the CdTe diode’s poor carrier-transport property is also more significant, resulting in much DOI information while complicated detector responses such as charge sharings or low-energy tails that exacerbate the loss in the energy resolution.In this paper, we developed 2-mm-thick CdTe-DSDs, studied their response, and evaluated their energy resolution, spatial resolution, and uniformity. We also constructed a theoretical model to understand the detector response theoretically, resulting in reconstructing the DOI with an accuracy of 100 μm while estimating the carrier-transport property. First, we evaluated the energy resolution using 22Na, 133Ba, and 57Co reconstructing the DOI effect from the energy correlation on both the anode and cathode sides. We obtained an energy resolution of 4.3 keV (FWHM) at 356 keV. Second, we formulated a theoretical detector-response model that reproduced the experimental data. Using the model, we determined the mobility-lifetime products of carriers μτe,h=(4±1)×10−3,(1.15±0.05)×10−4 [cm2/V] with the space charge density of nsp=−6.0×1010 [1/cm−3] and then reconstructed the DOI with an accuracy of 100 μm. Finally, we evaluated the spatial resolution and demonstrated to resolve patterns of 250-μm-width slits with an X-ray test chart. We found no positional variation in the detector response. We realized the detector that has high energy resolution and high 3D spatial resolution with a uniform large imaging area.

Gamma-ray computed tomography system with a double-sided strip detector

We have developed a γ-ray computed tomography system using a CdTe double-sided strip detector. Owing to a 250 μm fine strip pitch and high energy resolution with photon-counting capability, the system provides highly accurate images, with which the materials and their distributions inside the target can be determined according to the photon transmittances. We evaluated the key performance of the system, conducting transmission measurements for Al, Cu, and Pb plates and also for Al, Fe, Cu, and Pb rod-phantoms, both using X-rays (∼30 keV) and γ-rays (∼80 keV) from a 133Ba source. The measured transmittances agreed well with the calculated values from simulations. We successfully reconstructed the three-dimensional structure of the rod-phantom and distinguished the elements inside the phantom. Compared with the simulated photon transmittances, we found that material identification based on tomographic images obtained with the system is efficient as long as the target object does not contain thick high-Z elements.

Non-destructive 3D imaging method using muonic X-rays and a CdTe double-sided strip detector

Elemental analysis based on muonic X-rays resulting from muon irradiation provides information about bulk material composition without causing damage, which is essential in the case of precious or otherwise unreachable samples, such as in archeology and planetary science. We developed a three-dimensional (3D) elemental analysis technique by combining the elemental analysis method based on negative muons with an imaging cadmium telluride double-sided strip detector (CdTe-DSD) designed for the hard X-ray and soft gamma-ray observation. A muon irradiation experiment using spherical plastic samples was conducted at the Japan Proton Accelerator Research Complex (J-PARC); a set of projection images was taken by the CdTe-DSD, equipped with a pinhole collimator, for different sample rotation angles. The projection images measured by the CdTe-DSD were utilized to obtain a 3D volumetric phantom by using the maximum likelihood expectation maximization algorithm. The reconstructed phantom successfully revealed the 3D distribution of carbon in the bulk samples and the stopping depth of the muons. This result demonstrated the feasibility of the proposed non-destructive 3D elemental analysis method for bulk material analysis based on muonic X-rays.

Imaging and spectral performance of a [formula omitted] pitch CdTe Double-Sided Strip Detector

We have evaluated the performance of a fine pitch CdTe Double-sided Strip Detector (CdTe-DSD), which was originally developed for the focal plane detector of a hard X-ray telescope to observe the Sun. The detector has a thickness of 750 μm and has 128 strip electrodes with a 60 μm strip pitch orthogonally placed on both sides of the detector and covers an energy range 4 keV to 80 keV. The study of the depth of photon interaction and charge sharing effects are of importance in order to provide good spectroscopic and imaging performance. We study the tail structure observed in the spectra caused by charge trapping and develop a new method to reconstruct the spectra based on induced charge information from both anode and cathode strips. By applying this method, energy resolutions (FWHM) of 0.76 keV and 1.0 keV can be obtained at photon energies of 14 keV and 60 keV, respectively, if the energy difference between the anode and cathode is within 1 keV. Furthermore, the tail component at 60 keV is reduced, and the energy resolution of the 60 keV peak is improved from 2.4 keV to 1.5 keV (FWHM) if the energy difference is greater than 1 keV. In order to study the imaging performance, we constructed a simple imaging system using a 5 mm thick tungsten plate that has a pinhole with a diameter of 100 μm. We utilize a 133Ba radioisotope of 1 mm in diameter as a target source in combination with a 100 μm slit made from 0.5 mm thickness tungsten. We imaged the 133Ba source behind the 100 μm slit using a 30 keV peak, with a 100 μm pinhole placed at the center of the source-detector distance. By applying a charge sharing correction between strips, we have succeeded in obtaining a position resolution better than the strip pitch of 60 μm.

Development of 60 [formula omitted] pitch CdTe double-sided strip detectors for the FOXSI-3 sounding rocket experiment

FOXSI-3 (Focusing Optics X-ray Solar Imager) is an international sounding rocket experiment to observe hard X-rays from the Sun. The previous two flights successfully demonstrated the efficacy of the concept of direct solar imaging in hard X-ray band. For the third launch scheduled in the summer of 2018, we have fabricated a prototype of the CdTe Double-sided Strip Detector.To evaluate the basic performance, laboratory tests were conducted. Energy resolution (FWHM) of 0.8 keV at 13.9 keV and 1.3 keV at 59.5 keV are confirmed. Since the optic angular resolution is finer than the strip pitch of the detector at the focal plane, sub-strip position determination is important to make full use of the high precision of the optic. To test the possibility of sub-strip resolution, we developed a new method of investigating the detector strips with a fine multi-pinhole collimator. The results of the analysis were highly favorable and we confirmed the sub-strip resolution by making sub-strip images of multi-pinholes and flat-irradiation. The spectral uniformity over the detector is also confirmed using the sub-strip image of flat-irradiation.

Long-term test of a stacked CdTe mini-HXI setup

This work presents a long-term operation of two stacked CdTe double sided strip detectors that are comparable with the CdTe detectors onboard Hitomi’s HXI. The goal of this test is to study the evolution of the spectroscopic performance of the detectors during a one year operation cycle which resembles the in-orbit operation cycle of Hitomi HXI. Crystal defects inside CdTe cause a degradation of the spectroscopic performance (polarization effect) of the crystal which is becoming worse during detector operation. In order to prevent crystal polarization, the detectors are reset (switch-off of the depletion voltage) once a day. Our main investigation was to study if a long-term degradation can occur as a result of incomplete depolarization during the reset. We present the hardware setup and the analytical steps that were used to investigate the detector stability during each day and over the whole testing period. For the anode signals our results show at 60 keV: a daily line drift of (−2.8±0.7) eV/ks while the long-term drift is (−1.5±1.2) eV/day. The degradation of the energy resolution is measured to be (+2.4±0.3) eV/ks FWHM and the loss of efficiency is (−0.29±0.02) %/ks.

A compact imaging system with a CdTe double-sided strip detector for non-destructive analysis using negative muonic X-rays

A CdTe double-sided strip detector (CdTe-DSD) is an ideal device for imaging and spectroscopic measurements in the hard X-ray range above 10 keV. Recent development enables us to realize an imager with a detection area of ∼10 cm2. An energy resolution of 1–2 keV (FWHM) and a position resolution of a few hundred μm are available from the detector. This type of imager has been long awaited for non-destructive elemental analysis, especially by using negative muons, because energies of characteristic X-rays from muonic atoms are about 200 time higher than those from normal atoms. With the method that uses negative muons, hard X-ray information gives the spatial distribution of elements in samples at a certain depth defined by the initial momentum of the muon beam. In order to study three-dimensional imaging capability of the method, we have developed a compact imaging system based on CdTe-DSD and a ϕ 3 mm pinhole collimator as the first prototype. We conducted experiments with samples which consist of layers of Al, BN and LiF irradiated by negative muon beams in MUSE/J-PARC and successfully reconstruct hard X-ray images of muonic X-rays from B, N and F at various depths.

A high-resolution CdTe imaging detector with multi-pinhole optics for in-vivo molecular imaging

High-resolution CdTe semiconductor detectors with both fine position resolution and high energy resolution hold great promise to improve measurement in various hard X-ray and gamma-ray imaging experiments. In the field of in-vivo molecular imaging, where the molecules labeled with radioisotopes are injected into a small animal and their distribution in a body is detected by gamma-ray imaging detectors externally, a large detection area is also required to obtain a number of projections from multiple pinholes. We have developed a prototype imaging system consisting of a 32 mm-wide CdTe double-sided strip detector equipped with multi-pinhole optics. We describe its imaging capability, as studied through experiments with radioisotopes for medical use (125I and 111In) and a 3D phantom.

The Si/CdTe semiconductor camera of the ASTRO-H Hard X-ray Imager (HXI)

The Hard X-ray Imager (HXI) is one of the instruments onboard the ASTRO-H mission [1–4] to be launched in early 2016. The HXI is the focal plane detector of the hard X-ray reflecting telescope that covers an energy range from 5 to 80keV. It will execute observations of astronomical objects with a sensitivity for point sources as faint as 1/100,000 of the Crab nebula at >10keV. The HXI camera – the imaging part of the HXI – is realized by a hybrid semiconductor detector system that consists of silicon (Si) and cadmium telluride (CdTe) semiconductor detectors. Here, we present the final design of the HXI camera and report on the development of the flight model. The camera is composed of four layers of Double-sided Silicon Strip Detectors (DSSDs) and one layer of CdTe Double-sided Strip Detector (CdTe-DSD), each with an imaging area of 32mm×32mm. The strip pitch of the Si and CdTe sensors is 250μm, and the signals from all 1280 strips are processed by 40 Application Specified Integrated Circuits (ASICs) developed for the HXI. The five layers of sensors are vertically stacked with a 4mm spacing to increase the detection efficiency. The thickness of the sensors is 0.5mm for the Si, and 0.75mm for the CdTe. In this configuration, soft X-ray photons will be absorbed in the Si part, while hard X-ray photons will go through the Si part and will be detected in the CdTe part. The design of the sensor trays, peripheral circuits, power connections, and readout schemes are also described. The flight models of the HXI camera have been manufactured, tested and installed in the HXI instrument and then on the satellite.

Applications and Imaging Techniques of a Si/CdTe Compton Gamma-Ray Camera

Abstract By using a new Compton camera consisting of a silicon double-sided strip detector (Si-DSD) and a CdTe doublesided strip detector (CdTe-DSD), originally developed for the ASTRO-H satellite mission, an experiment involving imaging radioisotopes was conducted to study their feasibility for hotspot monitoring. In addition to the hotspot imaging already provided by commercial imaging systems, identification of various radioisotopes is possible thanks to the good energy resolution obtained by the semiconductor detectors. Three radioisotopes of 133 Ba (356 keV), 22 Na (511 keV) and 137 Cs (662 keV) were individually imaged by applying event selection in the energy window and the gamma-ray images were correctly overlapped by an optical picture. Detection efficiency of 1.68 ×10 −4 (effective area: 1.7×10 −3 cm 2 ) and angular resolution of 3.8 degrees were obtained by stacking five detector modules for a 662 keV gamma ray. The higher detection efficiency required in specific use can be achieved by stacking more detector modules.

High Energy Resolution Hard X-Ray and Gamma-Ray Imagers Using CdTe Diode Devices

We developed CdTe double-sided strip detectors (DSDs or cross strip detectors) and evaluated their spectral and imaging performance for hard X-rays and gamma-rays. Though the double-sided strip configuration is suitable for imagers with a fine position resolution and a large detection area, CdTe diode DSDs with indium (In) anodes have yet to be realized due to the difficulty posed by the segmented In anodes. CdTe diode devices with aluminum (Al) anodes were recently established, followed by a CdTe device in which the Al anodes could be segmented into strips. We developed CdTe double-sided strip devices having Pt cathode strips and Al anode strips, and assembled prototype CdTe DSDs. These prototypes have a strip pitch of 400 mum. Signals from the strips are processed with analog ASICs (application specific integrated circuits). We have successfully performed gamma-ray imaging spectroscopy with a position resolution of 400 mum. Energy resolution of 1.8 keV (FWHM: full width at half maximum) was obtained at 59.54 keV. Moreover, the possibility of improved spectral performance by utilizing the energy information of both side strips was demonstrated. We designed and fabricated a new analog ASIC, VA32TA6, for the readout of semiconductor detectors, which is also suitable for DSDs. A new feature of the ASIC is its internal ADC function. We confirmed this function and good noise performance that reaches an equivalent noise charge of 110 e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> under the condition of 3-4 pF input capacitance.