HEXITEC 2 × 2 tiled hard X-ray spectroscopic imaging detector system

HEXITEC is a spectroscopic imaging X-ray detector technology developed at the STFC Rutherford Appleton Laboratory for X-ray and γ-ray spectroscopic imaging applications. Each module has 80 × 80 pixels on a 250 μm pixel pitch, and has been implemented successfully in a number of applications. This paper presents the HEXITEC 2 × 2 detector system, a tiled array of 4 HEXITEC modules read out simultaneously, which provides an active area of 16 cm2. Systems have been produced using 1 mm thick Cadmium Telluride (CdTe) and 2 mm thick Cadmium Zinc Telluride (CdZnTe) sensor material. In this paper the system and data processing methods are presented, and the performance of the systems are evaluated. The detectors were energy calibrated using an 241Am sealed source. Three types of charge sharing correction were applied to the data-charge sharing addition (CSA), charge sharing discrimination (CSD), and energy curve correction (ECC) which compensates for energy lost in the inter-pixel region. ECC recovers an additional 34 % of counts in the 59.5 keV peak in CdTe compared to the use of CSD; an important improvement for photon-starved applications. Due to the high frame rate of the camera system (6.3 kHz) an additional End of Frame (EOF) correction was also applied to 6.0 % of events to correct for signals that were readout whilst the signal was still forming. After correction, both detector materials were found to have excellent spectroscopic performance with a mean energy resolution (FWHM) of 1.17 keV and 1.16 keV for CdZnTe and CdTe respectively. These results successfully demonstrate the ability to construct tiled arrays of HEXITEC modules to provide larger imaging areas.

Single source-based energy and efficiency calibration method for the assessment of an alpha imaging detector

An alpha imaging detector acquires a two-dimensional distribution of a sample that emits alpha particles. For the quantitative analysis of the image of an alpha-emitting sample, the individual energies of the alpha particles must be identified, which can be achieved using the spectrometric method after detector calibration. In this study, an energy and efficiency calibration method was investigated to assess an alpha imaging detector. The calibration was performed using a single standard source of Am-241 based on the energy loss characteristic of an alpha particle. The feasibility of the calibration method was evaluated using another source, Ac-225. The calibrated alpha imaging detector was evaluated in terms of energy resolution and detection efficiency, and the alpha imaging detector was found to be efficiently calibrated using a single standard source. The calibrated alpha imaging detector appears promising for the quantitative analysis of samples that emit alpha particles.

RIPTIDE: a novel recoil-proton track imaging detector for fast neutrons

Neutron detectors are an essential tool for the development of many research fields, as nuclear, particle and astroparticle physics as well as radiotherapy and radiation safety. Since neutrons cannot directly ionize, their detection is only possible via nuclear reactions. Consequently, neutron-based experimental techniques are related to the detection of charged particle or electromagnetic radiation originating from neutron-induced reactions. The study of fast neutrons is often based on the neutron-proton elastic scattering reaction. In this case, the ionization induced by the recoil protons in a hydrogenous material constitutes the basic information for the design and development of neutron detectors. Although experimental techniques have continuously improved and refined, so far, proton-recoil track imaging is still weak in laboratory rate environments because of the extremely small detection efficiency. To address this deficiency, we propose a novel recoil-proton track imaging system in which the light deriving from a fast scintillation signal is used to perform a complete reconstruction in space and time of the event. In particular, we report the idea of RIPTIDE (RecoIl Proton Track Imaging DEtector): an innovative system which combines a plastic scintillator coupled to imaging devices, based on CMOS technology, or micro channel plate sensors. The proposed apparatus aims at providing neutron spectrometry capability by stereoscopically imaging the recoil-protons tracks, correlating the spatial information with the time information. RIPTIDE intrinsically enable the online analysis of the ionization track, thus retrieving the neutron direction and energy, without spoiling the overall efficiency of the detection system. Finally, the spatial and topological event reconstruction enables particle discrimination — a crucial requirement for neutron detection — by deducing the specific energy loss along the track.

The X-ray Sensitivity of an Amorphous Lead Oxide Photoconductor

The photoconductor layer is an important component of direct conversion flat panel X-ray imagers (FPXI); thus, it should be carefully selected to meet the requirements for the X-ray imaging detector, and its properties should be clearly understood to develop the most optimal detector design. Currently, amorphous selenium (a-Se) is the only photoconductor utilized in commercial direct conversion FPXIs for low-energy mammographic imaging, but it is not practically feasible for higher-energy diagnostic imaging. Amorphous lead oxide (a-PbO) photoconductor is considered as a replacement to a-Se in radiography, fluoroscopy, and tomosynthesis applications. In this work, we investigated the X-ray sensitivity of a-PbO, one of the most important parameters for X-ray photoconductors, and examined the underlying mechanisms responsible for charge generation and recombination. The X-ray sensitivity in terms of electron–hole pair creation energy, W±, was measured in a range of electric fields, X-ray energies, and exposure levels. W± decreases with the electric field and X-ray energy, saturating at 18–31 eV/ehp, depending on the energy of X-rays, but increases with the exposure rate. The peculiar dependencies of W± on these parameters lead to a conclusion that, at electric fields relevant to detector operation (~10 V/μm), the columnar recombination and the bulk recombination mechanisms interplay in the a-PbO photoconductor.

A massively scalable Time-to-Digital Converter with a PLL-free calibration system in a commercial 130 nm process

A 33.6 ps LSB Time-to-Digital converter was designed in 130 nm BiCMOS technology. The core of the converter is a differential 9-stage ring oscillator, based on a multi-path architecture. A novel version of this design is proposed, along with an analytical model of linearity. The model allowed us to understand the source of the performance superiority (in terms of linearity) of our design and to predict further improvements. The oscillator is integrated in a event-by-event self-calibration system that allows avoiding any PLL-based synchronization. For this reason and for the compactness and simplicity of the architecture, the proposed TDC is suitable for applications in which a large number of converters and a massive parallelization are required such as High-Energy Physics and medical imaging detector systems. A test chip for the TDC has been fabricated and tested. The TDC shows a DNL≤1.3 LSB, an INL≤2 LSB and a single-shot precision of 19.5 ps (0.58 LSB). The chip dissipates a power of 5.4 mW overall.