Position Sensitive Radiation Detector Research Articles

Optimization With Simulated Annealing and Experimental Study of a Discretized Position Circuit (DPC) Network Using an Array of SiPMs

Arrays of tens or even hundreds of Silicon Photomultipliers (SiPMs) coupled to scintillators to build compact and robust position sensitive radiation detectors are becoming ubiquitous in nuclear medicine devices. In order to reduce the number of read-out electronic channels, and still be able to keep position information, multiplexing the SiPM signals with 2D resistor networks is fairly common. Good results are obtained with the DPC (Discretized Positioning Circuit) configuration. Even though it is a simple topology with a minimum number of discrete components, it yields good peak/valley ratios. However, it has the disadvantage that the network is asymmetric in X and Y directions, leading to different paths for the signal to reach the output depending on the position of the event. This causes the pulse shape to be also dependent on the position of the interaction in the detector, and creates a distortion (pincushion or barrel) of the position determined with center of energy algorithms. This effect can be partially remedied by a non-homogeneous choice of resistors in the network, but finding the best resistor values, in the case of large SiPM aggregations is very time consuming. We have developed a framework to optimize resistor values in DPC configurations to minimize position distortion and then impose that every SiPMs see an impedance to ground as similar as possible. We solved the DPC circuit and employed a simulated annealing algorithm to explore thousands of resistor combinations. The best combinations found in the computer were implemented in the laboratory confirming the results of the simulated annealing algorithm. In addition, we have verified that the optimized DPC network preserves the pulse shape to the extent that pulse shape discrimination makes it possible to disentangle two different scintillator layers (phoswich) of LYSO and GSO.

Monte Carlo Model of the Large Modular Array for Positron Emission Particle Tracking

Positron emission particle tracking (PEPT) is a non-invasive technique used to study fluid, granular, and multi-phase systems of interest to academia and industry which employs position-sensitive radiation detectors to record gamma rays in coincidence and track the movement of discrete sources. A modular detector array, the Large Modular Array (LaMA), has been constructed at the University of Birmingham’s Positron Imaging Centre (PIC) to enable custom detector geometries. In order to estimate the LaMA’s performance characteristics prior to experimentation, assist in developing optimised camera geometries, and determine ideal PEPT tracer characteristics, a Monte Carlo model of LaMA is created and subsequently validated with experimental measurements. Validation is achieved through comparisons of the spatial resolution and count-rate response following the National Electrical Manufacturers Association (NEMA) industry standard protocol. Notably, the model’s pulse-processing chain is autonomously calibrated to match experimental measurements using a recently developed technique which applies an evolutionary algorithm. Ultimately, the simulated spatial resolution matches the experiment to within 5%. In addition, the total, true, and corrupted count-rates are reproduced to a mean error of 3.41% over the range of source activities tested. This calibrated detector model strengthens the PIC’s modelling capabilities. To facilitate future research, this model has been made publicly available through the PIC’s GitHub repository.

Cherenkov radiation-based three-dimensional position-sensitive PET detector: A Monte Carlo study.

Cherenkov radiation has recently received attention due to its prompt emission phenomenon, which has the potential to improve the timing performance of radiation detectors dedicated to positron emission tomography (PET). In this study, a Cherenkov-based three-dimensional (3D) position-sensitive radiation detector was proposed, which is composed of a monolithic lead fluoride (PbF2 ) crystal and a photodetector array of which the signals can be readout independently. Monte Carlo simulations were performed to estimate the performance of the proposed detector. The position- and time resolution were evaluated under various practical conditions. The radiator size and various properties of the photodetector, e.g., readout pitch and single photon timing resolution (SPTR), were parameterized. The single photon time response of the photodetector was assumed to be a single Gaussian for the simplification. The photo detection efficiency of the photodetector was ideally 100% for all wavelengths. Compton scattering was included in simulations, but partly analyzed. To estimate the position at which a γ-ray interacted in the Cherenkov radiator, the center-of-gravity (COG) method was employed. In addition, to estimate the depth-of-interaction (DOI) principal component analysis (PCA), which is a multivariate analysis method and has been used to identify the patterns in data, was employed. The time-space distribution of Cherenkov photons was quantified to perform PCA. To evaluate coincidence time resolution (CTR), the time difference of two independent γ-ray events was calculated. The detection time was defined as the first photon time after the SPTR of the photodetector was taken into account. The position resolution on the photodetector plane could be estimated with high accuracy, by using a small number of Cherenkov photons. Moreover, PCA showed an ability to estimate the DOI. The position resolution heavily depends on the pitch of the photodetector array and the radiator thickness. If the readout pitch were ideally 0 and practically 3 mm, a full-width at half-maximum (FWHM) of 0.348 and 1.92 mm was achievable with a 10-mm-thick PbF2 crystal, respectively. Furthermore, first-order correlation could be observed between the primary principal component and the true DOI. To obtain a coincidence timing resolution better than 100-ps FWHM with a 20-mm-thick PbF2 crystal, a photodetector with SPTR of better than σ = 30 ps was necessary. From these results, the improvement of SPTR allows us to achieve CTR better than 100-ps FWHM, even in the case where a 20-mm-thick radiator is used. Our proposed detector has the potential to estimate the 3D interaction position of γ-rays in the radiator, using only time and space information of Cherenkov photons.

Electrical delay line multiplexing for pulsed mode radiation detectors

Medical imaging systems are composed of a large number of position sensitive radiation detectors to provide high resolution imaging. For example, whole-body Positron Emission Tomography (PET) systems are typically composed of thousands of scintillation crystal elements, which are coupled to photosensors. Thus, PET systems greatly benefit from methods to reduce the number of data acquisition channels, in order to reduce the system development cost and complexity. In this paper we present an electrical delay line multiplexing scheme that can significantly reduce the number of readout channels, while preserving the signal integrity required for good time resolution performance. We experimented with two 4 × 4 LYSO crystal arrays, with crystal elements having 3 mm × 3 mm × 5 mm and 3 mm × 3 mm × 20 mm dimensions, coupled to 16 Hamamatsu MPPC S10931-050P SiPM elements. Results show that each crystal could be accurately identified, even in the presence of scintillation light sharing and inter-crystal Compton scatter among neighboring crystal elements. The multiplexing configuration degraded the coincidence timing resolution from ∼243 ps FWHM to ∼272 ps FWHM when 16 SiPM signals were combined into a single channel for the 4 × 4 LYSO crystal array with 3 mm × 3 mm × 20 mm crystal element dimensions, in coincidence with a 3 mm × 3 mm × 5 mm LYSO crystal pixel. The method is flexible to allow multiplexing configurations across different block detectors, and is scalable to an entire ring of detectors.

X’tal Cube PET Detector Composed of a Stack of Scintillator Plates Segmented by Laser Processing

We have developed a three-dimensional (3-D) position-sensitive radiation detector named X'tal cube, which can be applied to a PET detector. The X'tal cube is composed of a scintillation crystal block and multi-pixel photon counters (MPPCs). The crystal block is segmented three-dimensionally into small cubes by optical discontinuity (3-D segmentation) and no reflector is inserted inside. Scintillation light originating in a segment then spreads three-dimensionally so that the MPPCs are set on all six surfaces of the crystal block to detect the light. Regarding the 3-D segmentation of the crystal block, we have already succeeded in getting the segmentation inside of a monolithic scintillator block by a laser processing technique instead of the general way, arranging small cubic scintillator elements into a 3-D array. We have confirmed that utilizing the laser processing technique not only eliminates the difficulty of handling the small scintillator elements but also improves detector performance. As a new trial, we considered fabrication of the crystal block by stacking the scintillator plates which were segmented two-dimensionally by the laser processing technique (2-D segmentation). Plates are also easier to handle and for the laser processing, 2-D segmentation is simpler than 3-D segmentation. In this study, we prepared the X'tal cube with the scintillator plates (Plate-XC) and evaluated its performance to confirm its technical feasibility. For the Plate-XC, we segmented 18 mm × 18 mm × 2.0 mm LYSO plates two-dimensionally by laser processing so as to make a 9 × 9 array of 2.0 mm × 2.0 mm segments. The crystal block was composed of 9 stacked LYSO plates without using coupling material but with air gaps. The Plate-XC showed sufficient crystal identification performance when 662 keV gamma-rays were irradiated. Furthermore, to understand the characteristics of the Plate-XC, we also analyzed the scintillation light distribution in the crystal block. Results indicated that light spread from outer segments was influenced by the segment boundary conditions, air gaps or laser-processed gaps, while the spread from the center segment did not seem to have such an influence. Regarding energy performance, we obtained around 10% energy resolution for the outer segments as well as for the center segment.

Development of the X'tal Cube: A 3D Position-Sensitive Radiation Detector With All-Surface MPPC Readout

We have developed a new three-dimensional (3D) position sensitive radiation detector, called the X'tal cube. The X'tal cube is composed of a scintillation crystal block and a number of multi-pixel photon counters (MPPCs) which are coupled on all six sides of the block. The block is segmented into cubes and no reflector is used between the segments. Scintillation light originating in a crystal segment accordingly propagates three-dimensionally along the alignment of the crystal segments and is efficiently detected by the MPPCs. The X'tal cube could be used as the detector element of a PET system, for instance. We constructed two prototypes of the X'tal cube and evaluated their performance using gamma-ray sources. The crystal block of each prototype is composed of a 3D array of Lu2xGd2(1-x)SiO5: Ce (LGSO, x = 0.9) crystal segments. Each crystal volume is 3.0 mm × 3.0 mm × 3.0 mm. MPPCs of a 3.0 mm × 3.0 mm active area are coupled to each surface of the crystal block. In this work, we show that all crystal segments are identified by a simple Anger-type calculation performed on the MPPC signals for both prototypes. The X'tal cube provides high spatial resolution in three dimensions regardless of the incident angle of the radiation.

In vivo dosimetry for gynaecological brachytherapy using a novel position sensitive radiation detector: Feasibility study

In gynecological radiotherapy with high dose rate (HDR)(192)Ir brachytherapy, the treatment complexity has increased due to improved optimization techniques and dose constraints. As a consequence, it has become more important to verify the dose delivery to the target and also to the organs at risk (e.g., the bladder). In vivo dosimetry, where dosimeters are placed in or on the patient, is one way of verifying the dose but until recently this was hampered by motion of the radiation detectors with respect to the source. The authors present a novel dosimetry method using a position sensitive radiation detector. The prototype RADPOS system (Best Medical Canada) consists of a metal oxide field effect transistor (MOSFET) dosimeter coupled to a position-sensor, which deduces its 3D position in a magnetic field. To assess the feasibility of in vivo dosimetry based on the RADPOS system, different characteristics of the detector need to be investigated. Using a PMMA phantom, the positioning accuracy of the RADPOS system was quantified by comparing position readouts with the known position of the detector along the x and y-axes. RADPOS dose measurements were performed at various distances from a Nucletron(192)Ir source in a PMMA phantom to evaluate the energy dependence of the MOSFET. A sensitivity analysis was performed by calculating the dose after varying (1) the position of the RADPOS detector to simulate organ motion and (2) the position of the first dwell position to simulate errors in delivery. The authors also performed an uncertainty analysis to determine the action level (AL) that should be used during in vivo dosimetry. Positioning accuracy is found to be within 1 mm in the 1-10 cm range from the origin along the x-axis (away from the transmitter), meeting the requirements for in vivo dosimetry. Similar results are obtained for the other axes. The ALs are chosen to take into account the total uncertainty on the measurements. As a consequence for in vivo dosimetry, it is determined that the RADPOS sensor, if placed, for example, in the bladder Foley balloon, would detect a 2 mm motion of the bladder, at a 5% chance of a false positive, with an AL limit of 9% of the dose delivered. The authors found that source position errors, caused by, e.g., a wrong first dwell position, are more difficult to detect; indeed, with our single RADPOS detector, positioned in the bladder, dwell position errors below 5 mm and resulting in a dose error within 10%, could be detected in the tandem but not in the colpostats. A possible solution to improve error detection is to use multiple MOSFETs to obtain multiple dose values. In this study, the authors proposed a dosimetry procedure, based on the novel RADPOS system, to accurately determine the position of the radiation dosimeter with respect to the applicator. The authors found that it is possible to monitor the delivered dose in a point and compare it to the predetermined dose. This allows in principle the detection of problems such as bladder motion/filling or source mispositioning. Further clinical investigation is warranted.

Color coding of the two-dimensional coordinates of events in position-sensitive radiation detectors

A new approach to determining the coordinates of events is proposed, which allows the point coordinates on a plane to be determined on line using a virtually single-channel readout system. According to the proposed method of color coding, each element of the area of a detecting target is assigned its color defined as a ratio of the component radiation wavelengths. Defined in this way, the color of the ith point on the mapping plane is in one to one correspondence with the (xi, yi) coordinates of this point on the target.

Development of position sensitive radiation detectors using gas electron multipliers

Gas electron multipliers (GEM) were introduced to develop a radiation detector which is applicable to medical imaging or luggage inspection systems at the airport or harbor. Two GEM foils were used in the amplifier, and an Ar/CO 2 mixed gas was inserted into the chamber at a mixing ratio of Ar:CO 2=80:20. A two-dimensional X-ray image was taken with a 64-channel GEM detector from an Fe-55 radiation source. We also constructed a 256-channel GEM detector in which 4 charge sensitive preamplifiers were used in a daisy chain. With linear array type readout electrodes, we were able to realize a position sensitive radiation detector.

Position-Sensitive Radiation Detector with Flexible Light Guide and Liquid Organic Scintillator to Monitor Distributions of Radioactive Isotopes

Position-sensitive detectors consisting of optical fibers such as scintillating fibers, normal optical plastic fibers, and liquid light guides have been developed as radiation sensors to monitor long-range distributions of radioactive isotopes. Because of losses of optical transmission due to radiation damage in high-level radiation environments, however, both scintillating fibers and normal optical plastic fibers are less applicable to areas of high dose rate. In order to develop a flexible fiber-type radiation sensor with good radiation hardness, we propose to use a liquid light guide, the core of which is replaced by a liquid organic scintillator. A liquid light guide consists of flexible cladding material, and can be easily bent. A liquid organic scintillator can generate a much larger number of photons than the original liquid material, leading to a better detection efficiency than a simple liquid light guide; furthermore, it is not significantly damaged by incident radiation. In preliminary experiments using a fast neutron beam, we confirmed that our prototype system can be operated as a position-sensitive radiation detector. Fundamental characteristics such as spatial resolution, detection efficiency and attenuation length of light propagation were examined with two detectors of 2 m and 10 m length. Optical transmission losses due to radiation damage were examined using 60Co gamma rays; no significant transmission loss was observed up to a dose of 1.5 × 104 Gy.

Evaluation of Single-Electron Transistor as Nanoscale Thermometer for a Cryogenic Radiation Detector

We have proposed a position-sensitive radiation detector, which is a microcalorimeter that uses single-electron transistors (SETs) as its electrical circuit to readout the temperature changes of an absorber, which are due to the energy deposition caused by incident radiation. In order to demonstrate the principle of the detector, we used numerical calculations with the orthodox theory to estimate the required performance of the SETs. Our results indicate that several SETs with different dynamic temperature ranges are necessary to span the entire temperature range found within the detector. Moreover, we fabricated a nanometer-scale tunneling junction using electron-beam lithography with a bilayer resist and a two-angle shadow evaporation technique. The current-voltage characteristics of the fabricated tunneling junction agree with Simmons’ theory. In addition, a current modulation of about 10 pA by the gate voltage was successfully observed at 4.4 K. Comparisons with numerical calculations estimated that the total capacitance of the SET is 180 aF. Thus, the SET should be able to operate as a thermometer from 0.8 K to 2.2 K. Although we could not observe the decreased conductance around the zero bias voltage at 4.4 K, the decreased conductance should be observed if the SET is cooled below 2.4 K.

Fabrication of a Si Photodiode for Position Sensitive Radiation Detection

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We have fabricated a position sensitive radiation detector by creating radiation damage regions in a Si photodiode. Radiation damage was created in layers at a depth of several <formula formulatype="inline"><tex>$\mu$</tex> </formula>m by <formula formulatype="inline"><tex>$^{7}$</tex></formula>Li and <formula formulatype="inline"><tex>$^{16}$</tex></formula>O ion beams which had an energy between 2 and 4 MeV. Ions were focused and scanned using a nuclear microprobe facility. The ion beam induced charge (IBIC) signals created in diodes during irradiation were simultaneously measured to monitor the exact ion beam fluence and analyzed to estimate the amount of defects created. By controlling the ion microbeam scanning system, graduated and position dependent radiation damage was produced in different micro-patterns. Such structures may be used as a simple position sensitive radiation sensor. </para>

Conceptual Design of High-Spatial-Resolution Position-Sensitive Radiation Detector with Single-Electron Transistors

We proposed a new microcalorimetric detector with four single-electron transistor (SET) read-out circuits to act as a real-time position-sensitive radiation detector with a spatial resolution of 100 nm. The proposed detector has micro- calrimeters in a two layer configuration, where the top and second layers work for position and energy determination, respectively. The position sensitive layer has four SETs that work as thermometers to decide the radiation incident position by the differences in SET temperatures caused by the heat flow in the layer. We investigated the SET performance and the heat flow in the substrate at cryogenic temperatures and, in addition, estimated the noise of the SET through simulation studies. It was founded that a temperature resolution of 10 mK could be achieved by using a pico-ammeter with a precision of 5.0 × 10−9 S. We successfully designed a detector configuration that would achieve the spatial and time resolution of 100 nm and 8 μs, respectively.

Conceptual Design of High-Spatial-Resolution Position-Sensitive Radiation Detector with Single-Electron Transistors

We proposed a new microcalorimetric detector with four single-electron transistor (SET) read-out circuits to act as a real-time position-sensitive radiation detector with a spatial resolution of 100 nm. The proposed detector has micro- calrimeters in a two layer configuration, where the top and second layers work for position and energy determination, respectively. The position sensitive layer has four SETs that work as thermometers to decide the radiation incident position by the differences in SET temperatures caused by the heat flow in the layer. We investigated the SET performance and the heat flow in the substrate at cryogenic temperatures and, in addition, estimated the noise of the SET through simulation studies. It was founded that a temperature resolution of 10 mK could be achieved by using a pico-ammeter with a precision of 5.0 × 10−9 S. We successfully designed a detector configuration that would achieve the spatial and time resolution of 100 nm and 8 μs, respectively.

Position sensitive regions in a generic radiation sensor based on single event upsets in dynamic RAMs

Modern integrated circuits are highly complex systems and, as such, are susceptible to occasional failures. Semiconductor memory devices, particularly dynamic Random Access Memories (dRAMs), are subject to random, transient single event upsets (SEUs) created by energetic ionizing radiation. These radiation-induced soft failures in the stored data of silicon based memory chips provide the foundation for a new, highly efficient, low cost generic radiation sensor. The susceptibility and the detection efficiency of a given dRAM device to SEUs is a complicated function of the circuit design and geometry, the operating conditions and the physics of the charge collection mechanisms involved. Typically, soft error rates measure the cumulative response of all sensitive regions of the memory by broad area chip exposure in ionizing radiation environments. However, this study shows that many regions of a dynamic memory are competing charge collection centres having different upset thresholds. The contribution to soft fails from discrete regions or individual circuit elements of the memory device is unambiguously separated. Hence the use of the dRAM as a position sensitive radiation detector, with high spatial resolution, is assessed and demonstrated.

Position sensitive detectors for medical imaging

The application of position sensitive radiation detectors in medical imaging is fundamental to improvements in image quality and quantification that underwrites the search for better diagnosis and treatment. Developments in particle physics, nuclear physics and astronomy could have a profound impact on medical imaging by overcoming the limitations of existing technology. The advent of new scintillators and semi-conductors allied to the enormous advances in micro-electronics and microprocessing should lead to substantial improvements in autoradiography, digital radiography, positron emission tomography, single photon emission tomography and other imaging methods relevant to medicine. In this paper we discuss recent developments which may be relevant to improved medical imaging on a broad front.

Detection mechanisms employing single event upsets in dynamic random access memories used as radiation sensors

A hardware system is being designed and constructed for the detection of neutrons, with a view to using it in neutron imaging and elemental analysis. A feasibility study was initially carried out to demonstrate that dynamic Random Access Memories (dRAMs) can be used as heavy charged particle detectors and furthermore be made sensitive to neutrons. We are interested, however, in constructing a detector that will be position sensitive, and hence carried out experiments to investigate the relative sensitivity of specific elements within the dRAM chips. The findings from these initial system tests highlight the usefulness of such a device as a position sensitive radiation detector. This paper aims to explain and give a review of most aspects concerning the soft error (SE) performance using dRAM as a radiation sensor.

Radiation position-sensitive detector based on plastic scintillating fibers

The applicability of plastic scintillation fibers as a radiation position detector is evaluated for low-energy x-ray irradiation. A continuous x-ray generator with a copper anode is used as a source radiating very intense lines: Kα2=8.02 keV, Kα1=8.04 keV, Kβ1=8.9 keV. A microchannel intensified tube followed by a vidicon camera is used as the photodetector for scintillation light guided to both extremities of a fiber detector. A one-dimensional position-sensitive radiation detector is constructed as an application. The irradiation position is derived independently of the system operating conditions (gain, sensitivity, efficiency) with a relative error smaller than 4%. Using this device, the optical attenuation length is measured with a relative error less than 10%. The differences in the photon distribution at fiber terminating faces are discussed for light and for x-ray excitations with respect to their incidence angles.

Utilization of a-Si:H Switching Diodes for Signal Readout from a-Si:H Pixel Detectors

Two-dimensional arrays of amorphous silicon photodiodes can be used as position-sensitive radiation detectors when they are coupled to an appropriate phosphor. We have developed signal readout schemes from amorphous silicon photodiode arrays utilizing one or two switching diodes attached to each pixel photodiode. Individual cells and prototype arrays of amorphous silicon photodiodes with single- and double-diode switching readout were fabricated and tested. A charge storage time and a readout time were measured. The measurement results were analyzed by simple circuit theory.

Radiation-induced hot spots in superconducting Sn strips

The thermal propagation of hot spots in superconducting strips was investigated. Thin Sn strips were irradiated with 5.5 MeV α-particles and the rise times of the detected voltage pulses were measured for various bias currents and temperatures. The results are discussed and compared to numerical simulations of thermal propagation of hot spots in superconductors. It is shown that superconducting strip detectors may serve as position-sensitive radiation detectors.