Silicon Sensor Layer Research Articles

Beam test performance of the highly granular SiW-ECAL technological prototype for the ILC

The technological prototype of the CALICE highly granular silicon-tungsten electromagnetic calorimeter (SiW-ECAL) was tested in a beam at DESY in 2017. The setup comprised seven layers of silicon sensors. Each layer comprised four sensors, with each sensor containing an array of 256 $5.5\times5.5$ mm$^2$ silicon PIN diodes. The four sensors covered a total area of $18\times18$ cm$^2$, and comprised a total of 1024 channels. The readout was split into a trigger line and a charge signal line. Key performance results for signal over noise for the two output lines are presented, together with a study of the uniformity of the detector response. Measurements of the response to electrons for the tungsten loaded version of the detector are also presented.

Subpixel response of SOI pixel sensor for X-ray astronomy with pinned depleted diode: first result from mesh experiment

We have been developing a monolithic active pixel sensor, “XRPIX”, for the Japan led future X-ray astronomy mission “FORCE” observing the X-ray sky in the energy band of 1–80 keV with angular resolution of better than 15′′. XRPIX is an upper part of a stack of two sensors of an imager system onboard FORCE, and covers the X-ray energy band lower than 20 keV . The XRPIX device consists of a fully depleted high-resistivity silicon sensor layer for X-ray detection, a low resistivity silicon layer for CMOS readout circuit, and a buried oxide layer in between, which is fabricated with 0.2 μm CMOS silicon-on-insulator (SOI) technology. Each pixel has a trigger circuit with which we can achieve a 10 μs time resolution, a few orders of magnitude higher than that with X-ray astronomy CCDs. We recently introduced a new type of a device structure, a pinned depleted diode (PDD), in the XRPIX device, and succeeded in improving the spectral performance, especially in a readout mode using the trigger function. In this paper, we apply a mesh experiment to the XRPIX devices for the first time in order to study the spectral response of the PDD device at the subpixel resolution. We confirmed that the PDD structure solves the significant degradation of the charge collection efficiency at the pixel boundaries and in the region under the pixel circuits, which is found in the single SOI structure, the conventional type of the device structure. On the other hand, the spectral line profiles are skewed with low energy tails and the line peaks slightly shift near the pixel boundaries, which contribute to a degradation of the energy resolution.

The Dosepix detector—an energy-resolving photon-countingpixel detector for spectrometric measurements

The Dosepix detector is a hybrid photon-counting pixel detector based on ideas of the Medipix and Timepix detector family. 1 mm thick cadmium telluride and 300 μm thick silicon were used as sensor material. The pixel matrix of the Dosepix consists of 16 x 16 square pixels with 12 rows of (200 μm)2 and 4 rows of (55 μm)2 sensitive area for the silicon sensor layer and 16 rows of pixels with 220 μm pixel pitch for CdTe. Besides digital energy integration and photon-counting mode, a novel concept of energy binning is included in the pixel electronics, allowing energy-resolved measurements in 16 energy bins within one acquisition. The possibilities of this detector concept range from applications in personal dosimetry and energy-resolved imaging to quality assurance of medical X-ray sources by analysis of the emitted photon spectrum. In this contribution the Dosepix detector, its response to X-rays as well as spectrum measurements with Si and CdTe sensor layer are presented. Furthermore, a first evaluation was carried out to use the Dosepix detector as a kVp-meter, that means to determine the applied acceleration voltage from measured X-ray tubes spectra.

Medipix in space on-board the ISS

On 16 October 2012, five active radiation detectors (referred to by NASA as Radiation Environment Monitors, or REMs) employing the Timepix version of the technology developed by the CERN-based Medipix2 Collaboration were deployed on-board the International Space Station (ISS) using simple USB interfaces to the existing ISS laptops for power, control and readout [ 1– 3]. These devices successfully demonstrated the capabilities of this technology by providing reliable dose and dose-equivalent information based on a track-by-track analysis. Figure 1 shows a sample comparison of the output from all five devices with respect to the on-board tissue equivalent proportional counter (TEPC) for both absorbed dose (top) and dose-equivalent (bottom) as defined in NCRP 142. The lower graph in each set is the TEPC. Several issues were identified and solutions to adjust for them have been included in the analysis. These include items such as the need to identify nuclear interactions in the silicon sensor layer, and to separate penetrating from stopping tracks. The wide effective range in fluence and particle type of this technology was also verified through the highest rates seen during the South Atlantic Anomaly passes and the heavy ions nominally seen in the Galactic Cosmic Rays. Corrections for detector response saturation effects were also successfully implemented as verified by reference to ground-based accelerator data taken at the Heavy-Ion Medical Accelerator Center (HIMAC) facility at the National Institute for Radiological Sciences in Japan, and at the NASA Space Radiation Laboratory (NSRL) at the Brookhaven National Laboratory in New York. Flight hardware has been produced that will be flown on the first launch of the new Orion spacecraft, and flight hardware development is ongoing to accommodate the next generation of this technology as a baseline for radiation monitoring and dosimetry on future operational manned missions. Fig 1.Five ISS REM units compared with ISS IVTEPC in absorbed dose (a) and dose-equivalent (b).

Initial results on charge and velocity discrimination for heavy ions using silicon-Timepix detectors

The Timepix ASIC is a version of the hybrid pixel detector technology developed by the Medipix2 Collaboration [1]. Within the 256 by 256 pixel matrix, the electronics for each of the 55μm individual pixels are contained in the footprint of that pixel. The Timepix has a charge-sensitive pre-amp and an associated discriminator attached to a logic unit capable of being employed in one of several different modes. For the present work, the Time-Over-Threshold mode was used to allow measurement of deposited energy in the silicon sensor layer. The general properties of a Timepix-based device with a silicon sensor have been described in [2,3], and [4].Ionization along heavy ion particle tracks in the silicon sensor results in the production of free charge carriers in the detector. The charge carrier motion under the influence of an applied bias voltage leads to charge collection at the Timepix-sensor interface in one or more pixels. Signatures within the pixel cluster patterns are currently being examined, and initial results indicate that such signatures, when coupled with stopping power information, provide enough discrimination capability to begin to resolve heavy ion charge and velocity. Here we present the salient characteristics that have been identified for heavy ion charge and velocity discrimination using Timepix Silicon detectors and discuss the application of this method for particle track characterization.

Toward quantifying the composition of soft tissues by spectral CT with Medipix3

To determine the potential of spectral computed tomography (CT) with Medipix3 for quantifying fat, calcium, and iron in soft tissues within small animal models and surgical specimens of diseases such as fatty liver (metabolic syndrome) and unstable atherosclerosis. The spectroscopic method was applied to tomographic data acquired using a micro-CT system incorporating a Medipix3 detector array with silicon sensor layer and microfocus x-ray tube operating at 50 kVp. A 10 mm diameter perspex phantom containing a fat surrogate (sunflower oil) and aqueous solutions of ferric nitrate, calcium chloride, and iodine was imaged with multiple energy bins. The authors used the spectroscopic characteristics of the CT number to establish a basis for the decomposition of soft tissue components. The potential of the method of constrained least squares for quantifying different sets of materials was evaluated in terms of information entropy and degrees of freedom, with and without the use of a volume conservation constraint. The measurement performance was evaluated quantitatively using atheroma and mouse equivalent phantoms. Finally the decomposition method was assessed qualitatively using a euthanized mouse and an excised human atherosclerotic plaque. Spectral CT measurements of a phantom containing tissue surrogates confirmed the ability to distinguish these materials by the spectroscopic characteristics of their CT number. The assessment of performance potential in terms of information entropy and degrees of freedom indicated that certain sets of up to three materials could be decomposed by the method of constrained least squares. However, there was insufficient information within the data set to distinguish calcium from iron within soft tissues. The quantification of calcium concentration and fat mass fraction within atheroma and mouse equivalent phantoms by spectral CT correlated well with the nominal values (R(2) = 0.990 and R(2) = 0.985, respectively). In the euthanized mouse and excised human atherosclerotic plaque, regions of calcium and fat were appropriately decomposed according to their spectroscopic characteristics. Spectral CT, using the Medipix3 detector and silicon sensor layer, can quantify certain sets of up to three materials using the proposed method of constrained least squares. The system has some ability to independently distinguish calcium, fat, and water, and these have been quantified within phantom equivalents of fatty liver and atheroma. In this configuration, spectral CT cannot distinguish iron from calcium within soft tissues.

Spectral CT of carotid atherosclerotic plaque: comparison with histology

To distinguish components of vulnerable atherosclerotic plaque by imaging their energy response using spectral CT and comparing images with histology. After spectroscopic calibration using phantoms of plaque surrogates, excised human carotid atherosclerotic plaques were imaged using MARS CT using a photon-processing detector with a silicon sensor layer and microfocus X-ray tube (50 kVp, 0.5 mA) at 38-μm voxel size. The plaques were imaged, sectioned and re-imaged using four threshold energies: 10, 16, 22 and 28 keV; then sequentially stained with modified Von Kossa, Perl's Prussian blue and Oil-Red O, and photographed. Relative Hounsfield units across the energies were entered into a linear algebraic material decomposition model to identify the unknown plaque components. Lipid, calcium, iron and water-like components of plaque have distinguishable energy responses to X-ray, visible on spectral CT images. CT images of the plaque surface correlated very well with histological photographs. Calcium deposits (>1,000 μm) in plaque are larger than iron deposits (<100 μm), but could not be distinguished from each other within the same voxel using the energy range available. Spectral CT displays energy information in image form at high spatial resolution, enhancing the intrinsic contrast of lipid, calcium and iron within atheroma. Spectral computed tomography offers new insights into tissue characterisation. Components of vulnerable atherosclerotic plaque are spectrally distinct with intrinsic contrast. Spectral CT of excised atherosclerotic plaques can display iron, calcium and lipid. Calcium deposits are larger than iron deposits in atheroma. Spectral CT may help in the non-invasive detection of vulnerable plaques.

A Novel Experiment to Search for the Decay μ→ eee

Design studies for an experiment searching for the lepton flavor violating decay μ→ eee are presented. The detector concept is based on thin layers of silicon sensors with fast readout. The aim of this experiment is to reach a sensitivity of B(μ→ eee) ≈ 10-16 corresponding to an improvement by a factor 10000 compared to previous experiments.

X-ray polarimetry by means of Compton scattering in the sensor of a hybrid photon counting pixel detector

For the first time a hybrid semiconductor photon counting pixel detector has been used for measurements of linear X-ray polarization by exploiting the Compton effect in the silicon sensor layer. The time-to-shutter mode of the X-ray imaging detector Timepix was used to identify Compton-scattering events in the sensor layer by the analysis of coincidences. For irradiation with polarized X-ray photons of energies between 27 and 84 keV we were able to measure a large modulation factor of μ meas = ( 68.1 ± 16.4 ) % for this type of X-ray polarimeter. Degree and orientation of linear polarization can be determined. This publication describes the experimental setup, data analysis method, measurement and simulation results, and gives first estimations on the polarimetric performance for an application in X-ray astronomy.

Hadronic particle identification with silicon detectors by means of d E/d x sampling

In order to study Generalized Parton Distributions (GPD) at future fixed-target Deep Inelastic Scattering (DIS) experiments, a detector is needed to observe particles at angles larger than can be covered by the usual forward-angle spectrometer used in this type of experiments. This additional (recoil) detector will determine the exclusivity of the measurements, which cannot be guaranteed by the main spectrometer due to its insufficient energy resolution. Within the limited space available in the target area of such an experiment, the recoil detector must accomplish particle identification as a stand-alone detector, up to particle momenta of 1.3 GeV c −1. Using Monte Carlo simulations it has been investigated to what extend an energy loss measurement with a small barrel shaped detector consisting of a few layers of silicon sensors can be used to realize this goal. It turns out that a rough estimate of the momentum is necessary in this case, which can be obtained by measuring the curvature of the tracks in the strong magnetic field employed by the polarized targets used in this type of experiments. Because these studies rely heavily on calculated energy loss distributions as produced by the Monte Carlo code, a beam test with a mixed proton-pion beam has been carried out at CERN to verify the results of the simulation. The beam test results presented here confirm the predicted particle identification capabilities of a detector based on multiple sampling of ionization tracks in silicon up to 1.3 GeV c −1.

Using the Medipix2 detector for energy weighting

Simulations in Giersch et al. [Nucl. Instr. and Meth. A 531 (2004) 68–74] have shown that the concept of energy weighting can result in improved image quality or significant dose reduction. In this work, the main aim is to show that energy weighting is possible in measurements. The Medipix2 detector can be operated in energy window mode. This offers the possibility to emulate energy resolving measurements and to apply the energy weighting technique. The Medipix2 detector has a 3-bit threshold equalisation for both thresholds. For energy sensitive measurements it is important to optimise the threshold masks, because thresholds within the incident X-ray spectrum are necessary and small energy bin size is desirable. We compared masks generated by different methods with respect to image quality in energy window mode. Detector effects entail a loss of energy information. For the Medipix2 detector with 300 μ m silicon sensor layer, especially the charge-sharing effect has to be considered. We studied if energy weighting is possible under these circumstances. Measurements were performed with a phantom consisting of water and PMMA. For this example, the improvement of image quality by energy weighting is conspicuous.

‘In-situ’ observation of dynamical diffraction by means of Medipix2 sensor crystal fulfilling Bragg condition

Hybrid pixel detectors, with the X-ray conversion layer and the readout electronics realised in two separate semiconducting crystals, offer the unique possibility to observe effects of dynamical diffraction directly in a crystal sensor. The sensor can be used to Bragg diffract X-rays in transmission geometry and simultaneously to observe the wave fields generated inside the sensor through absorption thus producing signals in the pixel readout. A monochromatic beam of sufficiently high energy is used to obtain a large absorption length inside the sample. This allows to use a setup geometry with the narrow incident beam hitting from the side of the platelet-shaped sensor, parallel to the readout electronics, so that the whole Borrmann triangle can be observed inside the sample. We used a Medipix2 detector with a 700 μ m silicon sensor layer to observe diffraction in both transmission and reflection geometry.