Direct and indirect detectors for X-ray photon counting systems

Flexible direct conversion X-ray detectors enable a variety of novel applications in medicine, industry, and science. Hybrid organic-inorganic perovskite semiconductors containing elements of high atomic number combine an efficient X-ray absorption with excellent charge transport properties. Due to their additional cost-effective and low-temperature processability, perovskite semiconductors represent promising candidates to be used as active materials in flexible X-ray detectors. Inspired by the promising results recently reported on X-ray detectors that are based on either triple cation perovskites or inkjet-printed perovskite quantum dots, we here investigate flexible inkjet-printed triple cation perovskite X-ray detectors. The performance of the detectors is evaluated by the X-ray sensitivity, the dark current, and the X-ray stability. Exposed to 70 kVp X-ray radiation, reproducible and highly competitive X-ray sensitivities of up to 59.9 μC/(Gyaircm2) at low operating voltages of 0.1 V are achieved. Furthermore, a significant dark current reduction is demonstrated in our detectors by replacing spin-coated poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) with sputtered NiOx hole transport layers. Finally, stable operation of a flexible X-ray detector for a cumulative X-ray exposure of 4 Gyair is presented, and the applicability of our devices as X-ray imaging detectors is shown. The results of this study represent a proof of concept toward flexible direct conversion X-ray detectors realized by cost-effective and high-throughput digital inkjet printing.

The most heavily investigated detector concept for medical photon-counting X-ray imaging, in particular photon-counting X-ray computed tomography (CT), is based on the semiconductors CdTe and CdZnTe (CZT). Alternatively, a detector design based on Si is under investigation. Both approaches have advantages and disadvantages, which leaves room for the development of other types of detector. In this chapter, we investigate the feasibility of scintillation detectors based on silicon photomultipliers (SiPMs) for this application. We first summarize the potential clinical advantages of energy-resolving photon-counting X-ray detectors, the associated challenges, and the working principles and drawbacks of detectors based on CdTe/CZT and Si. We then introduce the concept of SiPM-based scintillation detectors and discuss the scintillator and SiPM properties that determine two fundamental performance metrics, viz. the rate capability and the energy resolution. We have previously developed a pulse shape model that takes all of these properties into account. We use the model to estimate the expected detector performance as a function of pixel size, for LaBr3:Ce and YAP:Ce scintillators coupled to currently available SiPM technology and to a more advanced SiPM prototype. The model calculations show that the short decay time constant of the scintillators, combined with the reduced recharge time constant of the new SiPM prototype, results in pulses short enough that a pixel size of 400 µm (LaBr3:Ce) or 333 µm (YAP:Ce) yields comparable rate capability as a CdTe/CZT detector with a pixel size of 500 µm. The high light yield and nearly proportional response of the investigated scintillators results in good energy resolution, provided that the SiPM does not suffer from excessive saturation when the pixel size is reduced to such small dimensions. We show that the level of saturation remains limited with the new SiPM prototype and that energy resolutions of 12% (LaBr3:Ce) and 15% (YAP:Ce) at 60 keV may be achieved. Our work indicates that it may be feasible to develop SiPM-based scintillation detectors for medical photon-counting X-ray imaging with comparable rate capability and energy resolution as CdTe/CZT detectors. Remaining challenges include a dose-efficient optical isolation between pixels. However, the fact that the detector is based on light transport rather than charge transport means that issues typically associated with CdTe/CZT detectors, such as charge sharing and polarization, do not play a role.

As the extent of the shortage becomes clear, an interagency task force is giving scientific users priority, but some say the material is not available at any price.

The boron-coated straw (BCS) neutron detection technology has been proposed as an attractive 3He replacement solution in a series of applications in homeland security, neutron science, and safeguards. In order to improve the neutron detection efficiency of the BCS, while maintaining manufacturability, Proportional Technologies, Inc. is introducing a modified BCS with significant performance benefits. The proposed pie-like shape consists of any desired number of arc segments, and an equal number of septa walls, which are radially oriented, and extending a desired distance towards the center of the geometry. Each septum is essentially a segment of the wall that has been collapsed onto itself, resulting in a rigid surface, coated on both sides. The area coated on the septa is added to the area coated on the arc segments, thus offering a significant benefit in sensitivity. Compared to a round straw of the same diameter, the coated wall area of the proposed shape with 12 septa is 3 times larger, assuming the septa are equal in length to each of the arc segments. We present laboratory measurements with two prototype detectors that demonstrate stable operation, a neutron sensitivity that is nearly 3 times higher than that measured in a simple BCS with no septa walls, and adequate energy resolution to effectively discriminate against gamma rays.

Applications of micro total analysis systems (μTAS) span basic-science research, clinical medicine, and field work. Assay devices designed for these applications offer improvements to existing methods or provide fundamentally new strategies. Both mature methods and novel techniques have benefited from the increased throughput, integration and miniaturization afforded by μTAS. Traditional assays such Western blots and binding assays are recapitulated in a μTAS format but with reduced reagent usage, decreased performance times and added capabilities. An increasingly vibrant area is the performance of drug screening and toxicology assays on-chip, enabling the efficient screening of very large numbers of molecules. Similarly, recent μTAS reactors demonstrate greater chemical synthetic yields and novel product synthesis compared to macro-systems, often as a result of accurate control over reaction conditions including precision reagent dispensing. These exciting systems are now enabling on-site production of short-lived radioactive compounds for medical applications. The greatest impact of μTAS may very well be the ability to perform massively parallel laboratory experiments, for example, the use millions of reaction vessels or the analysis of hundreds of thousands of single cells. Another strength of μTAS lies in the creation of multicellular communities, for example, the combination of many cell types into an interacting system to explore intercellular communication. Devices with multiple layers of co-cultured tissues benefit from precise placement of molecules, such as extracellular matrices or growth factors, in both space and time. Similarly, the complexity and variety of organ-on-chip and organism-on-chip technologies continues to escalate rapidly. Impressively, the types of organisms cultured on-chip now range from the simplest bacteria to complex animals such as fish. Automation, reliability, and integration must all increase as a device moves from the specialist environment of a lab to usage by non-expert personnel in the outside world, for example, at a clinical point-of-care or in environmental monitoring. Key innovations in recent months result in devices that operate with minimal external equipment, error-free operation, and unambiguous readouts, all critical for operation by untrained personnel. Lightweight, portable devices are increasingly used to identify chemical and biological toxins in water, air and soil with applications in public health, defense, and homeland security. Perhaps most exciting is the development of μTAS with sufficient robustness for operation in challenging environments, such as the ocean and outer space. A central component of these systems is the ability to withstand the unexpected. These systems push the boundaries of current integration principles and spur rapid growth of new design philosophies. This review focuses on advances in the area of μTAS or “lab-on-a-chip” systems over the time span of May 2011 through September 2012 with a focus on applications in basic research, clinical medicine and field usage. A range of journals with 2011 impact factors from 2.0 to 36.3 were screened to cover publications with highly specialized content as well as those directed at multidisciplinary audiences. These publications included discipline-specific journals such as Analytical Chemistry and Lab on a Chip as well as general scientific publications, e.g. Science and Nature. To identify material beyond the individually examined journals, extensive key word searches in databases such as PubMed, SciFinder, and Web of Science were performed. Recent reviews in the area of μTAS were also examined for appropriate references. Care was taken to identify impactful and exciting work from across the globe. Well over a thousand papers in the three target areas were identified and discussed. Due to space limitations, we were unable to include all papers but instead incorporated those most fitting into the review scheme and those reporting innovations in basic microdevice technology as well as in applications to biological, physical and engineering sciences. We apologize in advance for omitted papers and welcome feedback regarding any oversights on our part.

Several families of optical fibers are highly transparent in the mid-IR. These fibers can be used for non-contact thermometry, chemical sensing and many other sensing applications in science, industry, environmental protection, homeland security and medicine.

This is a time of great opportunity in nuclear spectroscopy. The development of radioactive beam capabilities around the world is opening a new landscape for discovery, and the connections between nuclear structure studies and astrophysics, neutrino physics, and physics beyond the standard model are stronger than ever. New detector technologies are evolving which can meet the challenges of the new generation of experiments. Leading these is the technology of ''{gamma}-ray tracking'' which can revolutionize {gamma}-ray spectroscopy in a way that large arrays of {gamma} detectors did a decade ago. During the last few years this technology has been shown feasible and GRETINA a 1{pi} detector is under construction. However, the momentum in developing this technology to its full potential must continue towards GRETA, a full 4{pi} calorimeter. GRETA will carry {gamma}-ray spectroscopy into the next generation where it will be needed to fully exploit the science opportunities at radioactive beam facilities and increase the reach of stable beam facilities. In addition, {gamma}-ray tracking technology will have important applications for science, medicine, and homeland security.

This paper describes a new patent-pending digital-ultrasonic sensor network technology, which provides a “security protection sphere” around the authorized user(s) and the infrastructure system or system(s) to achieve C3I in Homeland Security and Law Enforcement Applications. If the system device, such as a firearm, a secure computer, PDA, or vehicle is misplaced, stolen or removed from the security protective sphere, an alarm is activated. A digital-ultrasonic sensor/tag utilizes the system’s physical structure to form a 2 to 20 Meter programmable protection sphere around the device and the authorized user. In addition, the system allows only authorized users to utilize the system, thereby creating personalized weapons, secure vehicle access or secure computer hardware. If an unauthorized individual accesses the system device, the system becomes inoperative and an alarm is activated. As the command and control, the authorized individual is provided a secure wristwatch/PDA. Access control is provided by touch and is controlled through the wristwatch/PDA/smartcard with a unique digital-ultrasonic coding and matching protocol that provides a very high level of security for each wireless sensor.

Raman spectroscopy is an analytical technique with vast applications in the homeland security and defense arenas. The Raman effect is defined by the inelastic interaction of the incident laser with the analyte molecule’s vibrational modes, which can be exploited to detect and identify chemicals in various environments and for the detection of hazards in the field, at checkpoints, or in a forensic laboratory with no contact with the substance. A major source of error that overwhelms the Raman signal is fluorescence caused by the background and the sample matrix. Novel methods are being developed to enhance the Raman signal’s sensitivity and to reduce the effects of fluorescence by altering how the hazard material interacts with its environment and the incident laser. Basic Raman techniques applicable to homeland security applications include conventional (off-resonance) Raman spectroscopy, surface-enhanced Raman spectroscopy (SERS), resonance Raman spectroscopy, and spatially or temporally offset Raman spectroscopy (SORS and TORS). Additional emerging Raman techniques, including remote Raman detection, Raman imaging, and Heterodyne imaging, are being developed to further enhance the Raman signal, mitigate fluorescence effects, and monitor hazards at a distance for use in homeland security and defense applications.

The silicon photomultiplier (SiPM) is a promising semiconductor device for low-level light detection. The recovery time, or the photon-counting rate of the SiPM is essential for high-flux photon detection in such applications as photon counting computer tomography (CT). A SiPM with epitaxial quenching resistors (EQR SiPM) has advantages in fabricating small APD microcells connected in series with lower quenching resistors, therefore, APD cells with a low RC time constant and a short recovery time can be expected. In this report, the recovery time of EQR SiPM has been investigated using both the double light pulse method and the waveform analysis method. The results show that the recovery time of EQR SiPM is strongly dependent on the size of the active area and the number of fired pixels. For a 3 × 3 mm2 device, while total about 90,000 pixels were fired, the recovery time was 31.1 ± 1.8 ns; while fired pixels were controlled to about 2000, the recovery time decreased significantly to 6.5 ± 0.4 ns; and the recovery time of one fired pixel was 3.1 ± 0.2 ns. For 1.4 × 1.4 mm2 device, the recovery time was 15.2 ± 0.5 ns, while a total of about 20,000 pixels were fired. Effects that may affect the recovery time of the SiPM, including strength of the pulse light, signal transmission time delay, and the readout electronics are discussed.

Measurements of thermal neutrons and gamma rays using multiple Cs <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> LiYCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> : Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> (CLYC) crystals with photomultiplier tube (PMT) and silicon photomultiplier (SiPM) are compared. The first set of measurements used three single-sided crystals (two made from 95% enriched <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> Li and one made with 99% enriched <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> Li) mated with the PMT and SiPM to compare the energy resolutions and figures of merit (FOM) for the pulse shape discrimination of gamma rays and neutrons. All crystals and photomultipliers were able to resolve full-energy photopeaks for <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co and distinguish gamma-ray interactions from neutron interactions (FOM > 1 in all cases) while operating in a thermal neutron environment. Measurements made with the PMT had better energy resolution, and switching to the SiPM degraded the energy resolution by an average of 34.0 ± 0.7% (at 662 keV). The second set of measurements used a CLYC crystal with two optical windows to enable simultaneous measurements with the PMT and SiPM. Once again, both photomultipliers were able to distinguish gamma rays from neutrons, and the PMT had the best energy resolution. However, in this case, the SiPM was not able to resolve the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs full-energy photopeak, and neither photomultiplier could resolve any photopeaks if <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">60</sup> Co was used simultaneously. The degradation in energy resolution was due to the relative splitting of scintillation light intensity between each photomultiplier depending on the position of the interaction site within the crystal. Each pair of waveforms recorded from the PMT and SiPM, corresponding to a single interaction within the double-sided crystal, were combined to improve the energy resolution at 662 keV from 15.9 ± 0.9% to 12.7 ± 0.3% and the FOM from 1.850±0.004 to 2.117±0.003 when compared to the data just from the PMT.

— At the Department of Physics of the University of Pisa and INFN of Pisa, we have assembled a high-resolution CT scanner prototype for small animals. The prototype will be integrated with the YAP-(S)PET scanner, in order to realize a combined PET\SPECT\CT scanner for small animals. We have characterized the microCT scanner prototype in terms of spatial resolution and noise using three different X-ray detectors. The first detector (Hamamatsu S7199) is composed by a CsI:Tl layer coupled to a CCD via a 1:1 fiber optic plate; the second detector (Radicon Rad-Eye 4) consists in a flat-panel CMOS sensor, directly coupled to a Ga2O2S:Tb scintillator; the third detector is a 1mm thick Si detector, bump-bonded to the Medipix2 readout chip. The first two detectors are based on indirect conversion of X-rays and operate in charge integration mode, while the third is a direct conversion detector and operates in photon counting mode. The characterization was carried out using dedicated phantoms for the measurement of spatial resolution and image noise. The results obtained with the three detectors are compared, in order to evaluate the differences in tomographic performances due to the different characteristics of each detector.

Low light detection, down to the single photon, has been possible so far relying on photo-multiplier tubes (PMT). PMT are certainly a reliable, solid rock technology with decades of improvements and refinements. Their architecture is certainly beautiful but irreducibly complex. Biasing electronics can be sophisticated but the operational voltage necessarily exceeds 1000 V. Operation in magnetic fields may be possible but it does not come for free. Last but not least, miniaturisation below a matchbox size is hard to imagine. These figures certainly constrain their integration in apparatus and instruments, with an impact on design and cost. Silicon PhotoMultipliers (SiPM) stand to PMT like transistors stand to thermoionic valves. In essence, SiPM are an array of p-n junctions operated beyond the breakdown voltage, with every cell in the array ready to trigger an avalanche with 106 gain as long as the absorption of a photon generates a charge carrier. With the simplicity and cost of a Silicon sensor, operational voltage not exceeding 80V, magnetic field immunity and miniaturization down to 1 mm2, SiPM are state-of-the-art sensors, featuring an unprecedented photon number resolving capability and introducing the digital revolution in low light detection. In my talk, I will address the fundamentals of SiPM, identifying the key figures of merit, their measurements and presenting the state of the art and future directions. Moreover, I will consider exemplary applications in nuclear particle detection and dosimetry, homeland security, medical imaging and environmental science.

An ASIC front-end for fluorescence and Cherenkov light detection with SiPM for space and ground applications

Rapid standoff spectroscopic characterization of plastic waste using quartz tuning fork