E.N. Gimenez

Characterization of Medipix3 With Synchrotron Radiation

Medipix3 is the latest generation of photon counting readout chips of the Medipix family. With the same dimensions as Medipix2 (256 × 256 pixels of 55 μm × 55 μm pitch each), Medipix3 is however implemented in an 8-layer metallization 0.13 μm CMOS technology which leads to an increase in the functionality associated with each pixel over Medipix2. One of the new operational modes implemented in the front-end architecture is the Charge Summing Mode (CSM). This mode consists of a charge reconstruction and hit allocation algorithm which eliminates event-by-event the low energy counts produced by charge-shared events between adjacent pixels. The present work focuses on the study of the CSM mode and compares it to the Single Pixel Mode (SPM) which is the conventional readout method for these kind of detectors and it is also implemented in Medipix3. Tests of a Medipix3 chip bump-bonded to a 300 μm thick silicon photodiode sensor were performed at the Diamond Light Source synchrotron to evaluate the performance of the new Medipix chip. Studies showed that when Medipix3 is operated in CSM mode, it generates a single count per detected event and consequently the charge sharing effect between adjacent pixels is eliminated. However in CSM mode, it was also observed that an incorrect allocation of X-rays counts in the pixels occurred due to an unexpectedly high pixel-to-pixel threshold variation. The present experiment helped to better understand the CSM operating mode and to redesign the Medipix3 to overcome this pixel-to-pixel mismatch.

Study of charge-sharing in MEDIPIX3 using a micro-focused synchrotron beam

X-ray photon-counting detectors consisting of a silicon pixel array sensor bump-bonded to a CMOS electronic readout chip offer several advantages over traditional X-ray detection technologies used for synchrotron applications. They offer high frame rate, dynamic range, count rate capability and signal-to-noise ratio. A survey of the requirements for future synchrotron detectors carried out at the Diamond Light Source synchrotron highlighted the needs for detectors with a pixel size of the order of 50?m. Reducing the pixel size leads to an increase of charge-sharing events between adjacent pixels and, therefore, to a degradation of the energy resolution and image quality of the detector. This effect was observed with MEDIPIX2, a photon-counting readout chip with a pixel size of 55?m. The lastest generation of the MEDIPIX family, MEDIPIX3, is designed to overcome this charge-sharing effect in an implemented readout operating mode referred to as Charge Summing Mode. MEDIPIX3 has the same pixel size as MEDIPIX2, but it is implemented in an 8-metal 0.13?m CMOS technology which enables increased functionality per pixel. The present work focuses on the study of the charge-sharing effect when the MEDIPIX3 is operated in Charge Summing Mode compared to the conventional readout mode, referred to as Single Pixel Mode. Tests of a standard silicon photodiode array bump-bonded to MEDIPIX3 were performed in beamline B16 at the Diamond Light Source synchrotron. A monochromatic micro-focused beam of 2.9?m x 2.2?m size at 15keV was used to scan a cluster of nine pixels in order to study the charge collection and X-ray count allocation process for each readout mode, Single Pixel Mode and Charge Summing Mode. The study showed that charge-shared events were eliminated when Medipix3 was operated in Charge Summing Mode.

Precision scans of the Pixel cell response of double sided 3D Pixel detectors to pion and X-ray beams

Three-dimensional (3D) silicon sensors offer potential advantages over standard planar sensors for radiation hardness in future high energy physics experiments and reduced charge-sharing for X-ray applications, but may introduce inefficiencies due to the columnar electrodes. These inefficiencies are probed by studying variations in response across a unit pixel cell in a 55μm pitch double-sided 3D pixel sensor bump bonded to TimePix and Medipix2 readout ASICs. Two complementary characterisation techniques are discussed: the first uses a custom built telescope and a 120GeV pion beam from the Super Proton Synchrotron (SPS) at CERN; the second employs a novel technique to illuminate the sensor with a micro-focused synchrotron X-ray beam at the Diamond Light Source, UK. For a pion beam incident perpendicular to the sensor plane an overall pixel efficiency of 93.0±0.5% is measured. After a 10o rotation of the device the effect of the columnar region becomes negligible and the overall efficiency rises to 99.8±0.5%. The double-sided 3D sensor shows significantly reduced charge sharing to neighbouring pixels compared to the planar device. The charge sharing results obtained from the X-ray beam study of the 3D sensor are shown to agree with a simple simulation in which charge diffusion is neglected. The devices tested are found to be compatible with having a region in which no charge is collected centred on the electrode columns and of radius 7.6±0.6μm. Charge collection above and below the columnar electrodes in the double-sided 3D sensor is observed.

Detector optimization of a small animal PET camera based on continuous LSO crystals and flat panel PS-PMTs

We have built a PET camera for small animals based on continuous block LSO crystals coupled to a PS-PMT flat panel. When working with continuous crystals, surface treatment is an important factor that strongly determines the main characteristics of the detector module. As a part of the work done for the development of our small animals PET camera, we have investigated the effect of the scintillator crystal surface treatment on the PET detector module performances, in order to optimize crystal configuration. We present the results for spatial resolution, image compression and energy resolution obtained when using different surface treatments in continuous LSO crystals. These results are compared with those obtained from simulations that have been carried out using DETECT2000 package.

Edge pixel response studies of edgeless silicon sensor technology for pixellated imaging detectors

Silicon sensor technologies with reduced dead area at the sensors perimeter are under development at a number of institutes. Several fabrication methods for sensors which are sensitive close to the physical edge of the device are under investigation utilising techniques such as active-edges, passivated edges and current-terminating rings. Such technologies offer the goal of a seamlessly tiled detection surface with minimum dead space between the individual modules. In order to quantify the performance of different geometries and different bulk and implant types, characterisation of several sensors fabricated using active-edge technology were performed at the B16 beam line of the Diamond Light Source. The sensors were fabricated by VTT and bump-bonded to Timepix ROICs. They were 100 and 200 μ m thick sensors, with the last pixel-to-edge distance of either 50 or 100 μ m. The sensors were fabricated as either n-on-n or n-on-p type devices. Using 15 keV monochromatic X-rays with a beam spot of 2.5 μ m, the performance at the outer edge and corners pixels of the sensors was evaluated at three bias voltages. The results indicate a significant change in the charge collection properties between the edge and 5th (up to 275 μ m) from edge pixel for the 200 μ m thick n-on-n sensor. The edge pixel performance of the 100 μ m thick n-on-p sensors is affected only for the last two pixels (up to 110 μ m) subject to biasing conditions. Imaging characteristics of all sensor types investigated are stable over time and the non-uniformities can be minimised by flat-field corrections. The results from the synchrotron tests combined with lab measurements are presented along with an explanation of the observed effects.

Excalibur: A three million pixels photon counting area detector for coherent diffraction imaging based on the Medipix3 ASIC

A three million pixels photon counting area detector for the coherent diffraction imaging beam line (113) of Diamond Light Source has been developed by a joint team of Diamond and STFC staff. The detector is the state of the art of X-ray detection technology since it exploits the latest generation of Medipix ASICs family that introduced a number of innovations. The specifications required by the beam line represented a severe challenge to all of the components of the detector. The frame rate of Excalibur is up to 1,000 frames per second when stored in local RAM or up to 100 frames per second when streamed to storage. Tests with an X-ray set show the imaging capabilities of the detector as well as the data acquisition speed.

Depth of interaction measurement in gamma ray imaging detectors with continuous scintillation crystals

A design for an inexpensive depth of interaction (DOI) detector for gamma rays, suitable for nuclear medical applications, especially positron emission tomography (PET), has been developed, studied by simulations and tested experimentally. The detector consists of a continuous LSO-scintillator of dimensions 42/spl times/42/spl times/10 mm/sup 3/ and a new compact large-area (49/spl times/49 mm/sup 2/) position sensitive photo-multiplier (PSPMT) H8500 from Hamamatsu. Since a continuous crystal is used, the scintillation light distribution is not destroyed and its first 3 moments can be used to determine the energy (0th moment), the centroids along the x- and y-direction (1st moments) and the depth of interaction (DOI), which is strongly correlated to the distributions width and thus its standard deviation (2nd moment). The simultaneous computation of these moments allows a three-dimensional reconstruction of the position of interaction of the /spl gamma/-rays within the scintillating crystal and will be realized by a modified position sensitive proportional (PSP) resistor network. No additional photo detectors or scintillating crystals are needed. According to previous Monte Carlo simulations which estimated the influence of Compton scattering for 511 keV /spl gamma/-rays, the transport of the scintillation light within the detector assembly and also the behavior of the modified PSP resistor network, we expect a spatial resolution of /spl lsim/ 2 mm and a DOI resolution of /spl ap/ 5 mm. The first experimental results presented here yield an intrinsic spatial resolution of /spl lsim/ 1.8 mm and 2.6 mm for the x- and y-direction respectively and a DOI resolution /spl lsim/ 1 cm. Further we measured an energy resolution of 12%-18%.

Medipix3RX: Characterizing the Medipix3 Redesign With Synchrotron Radiation

The Medipix3RX is the latest version of the Medipix3 photon counting ASICs, which implements two new operational modes, with respect to the Medipix2 ASIC, aimed at eliminating charge shared events (referred to as Charge Summing Mode (CSM)) and at providing spectroscopic information (referred to as Colour Mode (CM)). The Medipix3RX is a redesign of the Medipix3v0 ASIC and corrects for the underperformance of CSM features observed in the previous version. This paper presents the results from synchrotron X-rays tests to evaluate the Medipix3RX ASIC performance. The newly implemented CSM algorithm eliminates the charge sharing effect at the same time as allocating the event to the readout pixel corresponding to the sensor pixel where the X-ray photon impinged. The new pixel trimming circuit led to a reduced dispersion between pixels. Further results of the linearity for all the gain modes, energy resolution and pixel uniformity are also presented.

Synchrotron tests of 3D Medipix2 and TimePix X-ray detectors

In this article we report on the use micro-focus synchrotron X-ray radiation and pion beams to compare the detection efficiencies and charge sharing properties of novel 3D detectors to that of the current planar technology. Detector substrates are bump-bonded to the Medipx2 and Timepix chips. 55μm square pixel maps of the detection efficiencies have been produced using X-ray and MIP beams. For X-rays, a drop of 3-4% detection efficiency over the pixel area was found due to the central electrode. The corner electrodes show no degradation in efficiency compared to that of the planar device. For MIPs a drop of 0.5% in efficiency due to the central electrode was observed. Evidence of a considerable reduction in charge sharing in the 3D detectors compared to the planar devices is also shown.

Probing Defects in a Small Pixellated CdTe Sensor Using an Inclined Mono Energetic X-Ray Micro Beam

High quantum efficiency is important in X-ray imaging applications. This means using high-Z sensor materials. Unfortunately many of these materials suffer from defects that cause non-ideal charge transport. In order to increase the understanding of these defects, we have mapped the 3D response of a number of defects in two 1 mm thick CdTe sensors with different pixel sizes (55 μm and 110 μm) using a monoenergetic microbeam at 79 keV. The sensors were bump bonded to Timepix read out chips. Data was collected in photon counting as well as time-over-threshold mode. The time-over-threshold mode is a very powerful tool to investigate charge transport properties and fluorescence in pixellated detectors since the signal from the charge that each photon deposits in each pixel can be analyzed. Results show distorted electrical field around the defects, indications of excess leakage current and large differences in behavior between electron collection and hole collection mode. The experiments were carried out on the Extreme Conditions Beamline I15 at Diamond Light Source.