The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a Probe-class mission concept that will provide essential contributions to multimessenger astrophysics in the next decade. AMEGO operates both as a Compton and pair telescope to achieve unprecedented sensitivity between 200 keV and >5 GeV. The instrument consists of four subsystems. A double-sided strip silicon Tracker gives a precise measure of the first Compton scatter interaction and tracks of pair-conversion products. A novel CdZnTe Low Energy Calorimeter with excellent position and energy resolution surrounds the bottom and sides of the Tracker to detect the Compton-scattered photons which enhances the polarization and narrow-line sensitivity. A thick CsI High Energy Calorimeter contains the high-energy Compton and pair events. The instrument is surrounded by a plastic anti-coincidence detector to veto the cosmic-ray background. We have performed detailed simulations to predict the telescope performance and are currently building a prototype instrument. The AMEGO prototype, known as ComPair, will be tested at the High Intensity Gamma-Ray Source in 2021, followed by a balloon flight in Fall of 2022. In this presentation we will give an overview of the science motivation, a description of the observatory, and an update of the prototype instrument development.

AMEGO: exploring the extreme multimessenger universe

Recent progress in CdZnTe based room temperature detectors for nuclear radiation monitoring

Thallium bromide (TlBr) is a semiconductor material and, simultaneously, a good Cerenkov radiator. The performance of a TlBr detector that integrates two different readouts, the charge induction readout and the detection of Cerenkov light, was evaluated. A TlBr detector with dimensions of 4  ×  4  ×  5 mm3, with a monolithic cathode and an anode segmented into strips, was manufactured. One of the bare and polished 4  ×  4 mm2 faces of the detector was coupled to a silicon photomultiplier (SiPM) to read out the Cerenkov light. Simultaneous timing and energy resolutions of  <400 ps full width at half maximum (FWHM) and ~8.5% at 511 keV were measured using the Cerenkov detection and charge induction readouts, respectively. A coincidence time resolution of 330 ps was obtained when selecting Cerenkov events with amplitudes above 70 mV. The combination of both readouts showed the potential to resolve the depth-of-interaction (DOI) positioning, based on the improvement of energy resolution when selecting events with similar electron drift times. This manuscript sets the stage for a new family of semiconductor detectors that combine charge induction readout with the Cerenkov light detection. Such detectors can provide, simultaneously, outstanding timing, energy, and spatial resolution, and will be an excellent fit for applications that require the detection of high-energy gamma photons with high timing accuracy, such as time-of-flight positron emission tomography (TOF-PET) and prompt gamma imaging (PGI) to assess the particle range in hadron therapy.

https://iopscience.iop.org/article/10.1088/1361-6560/ab35c4/pdf

First Cerenkov charge-induction (CCI) TlBr detector for TOF-PET and proton range verification

Arrays of position-sensitive virtual Frisch-grid CdZnTe (CZT) detectors offer an economical approach to making high efficiency and high energy resolution gamma cameras for spectroscopy and imaging of radioactive sources. There are many application areas for such instruments including gamma-rays astronomy , medical and industrial imaging, nonproliferation and nuclear safeguards . Here we present the test results from a 4 × 4 array module coupled to the front-end ASIC. The array houses 16 detectors made of 6 × 6 × 20 mm3 CZT crystals. Each crystal is encapsulated inside an ultrathin polyester shell and furnished with four 5-mm-wide charge-sensing pads placed near the anode on each side of the detector. The pad’s signals are used to measure X–Y coordinates while the cathode signals give the interaction depths ( Z coordinates). Combined together the signals provide 3D position information of the interaction points which can be used to correct the detector response non-uniformity. This allows developers to use standard grade (unselected) CZT crystals while retaining high spectroscopic performance comparable with that of the H3D pixelated detectors. The array’s design provides flexibility to replace individual detectors and extend their sizes, potentially increasing upwards of 4 cm in height. We started development of position-sensitive VFG detectors several years ago and today this technology has reached a high-maturity level and is ready to use in practical applications.

A 4 × 4 array module of position-sensitive virtual Frisch-grid CdZnTe detectors for gamma-ray imaging spectrometers

The traditional method for electron lifetime measurements of CdZnTe (CZT) detectors relies on using the Hecht equation. The procedure involves measuring the dependence of the detector response on the applied bias to evaluate the μτ product, which in turn can be converted into the carrier lifetime. Despite general acceptance of this technique, which is very convenient for comparative testing of different CZT materials, the assumption of a constant electric field inside a detector is unjustified. In the Hecht equation, this assumption means that the drift time would be a linear function of the distance. This condition is not fulfilled in practice at low applied biases, where the Hecht equation is most sensitive to the μτ product. As a result, researchers usually take measurements at relatively high biases, which work well in the case of the low μτ-product material, <10−3 cm2/V, but give significantly underestimated values for the case of high μτ-product crystals. In this work, we applied the drift-time method to measure the electron lifetimes in long-drift-length (4 cm) standard-grade CZT detectors produced by the Redlen Technologies. We found that the electron μτ product of tested crystals is in the range 0.1–0.2 cm2/V, which is an order of the magnitude higher than any value previously reported for a CZT material. In comparison, using the Hecht equation fitting, we obtained μτ = 2.3 × 10−2 cm2/V for a 2-mm thin planar detector fabricated from the same CZT material.

Use of the drift-time method to measure the electron lifetime in long-drift-length CdZnTe detectors

We investigated the feasibility of long-drift-time CdZnTe (CZT) gamma-ray detectors, fabricated from CZT material produced by Redlen Technologies. CZT crystals with cross-section areas of 5 × 5 mm2 and 6 × 6 mm2 and thicknesses of 20-, 30-, 40-, and 50-mm were configured as 3D position-sensitive drift detectors and were read out using a front-end ASIC. By correcting the electron charge losses caused by defects in the crystals, we demonstrated high performance for relatively thick detectors fabricated from unselected CZT material.

CdZnTe position-sensitive drift detectors with thicknesses up to 5 cm

Position-sensitive virtual Frisch-grid (VFG) CdZnTe (CZT) detectors offer a unique capability for correcting the response nonuniformities caused by crystal defects. This allowed us to achieve high energy resolution, while using typical-grade commercial CZT crystals with relaxed requirements to their quality, thus reducing the overall cost of detectors. Another advantage of the VFG detectors is that they can be integrated into arrays and used in small compact hand-held instruments or large-area gamma cameras that will enhance detection capability for many practical applications, including nonproliferation, medical imaging, and gamma-ray astronomy. Here, we present the results from testing small array prototypes coupled with front-end application-specified integrated circuit. Each detector in the array is furnished with 5-mm-wide charge-sensing pads placed near the anode. The pads signals are converted into XY coordinates, which combined with the cathode signals (for Z coordinates) provide 3-D position information of all interaction points. The basic array consists of a number of detectors grouped into $2\times 2$ subarrays, each having a common cathode made by connecting together the cathodes of the individual detectors. These features can significantly improve the performance of detectors while using typical-grade low-cost CZT crystals to reduce the overall cost of the proposed instrument.

Arrays of Position-Sensitive Virtual Frisch-Grid CdZnTe Detectors: Results From a $4\times 4$ Array Prototype

We evaluated the X-Y position resolution achievable in 3D pixelated detectors by processing the signal waveforms readout from neighboring pixels. In these measurements we used a focused light beam, down to 10 μ m, generated by a ∼1  mW pulsed laser (650  nm) to carry out raster scans over selected 3 × 3 pixel areas, while recording the charge signals from the 9 pixels and the cathode using two synchronized digital oscilloscopes.

Using a pulsed laser beam to investigate the feasibility of sub-pixel position resolution with time-correlated transient signals in 3D pixelated CdZnTe detectors

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CdZnTe position-sensitive drift detectors with thicknesses up to 5 cm

Arrays of Position-Sensitive Virtual Frisch-Grid CdZnTe Detectors: Results From a $4\times 4$ Array Prototype

Using a pulsed laser beam to investigate the feasibility of sub-pixel position resolution with time-correlated transient signals in 3D pixelated CdZnTe detectors