Erik B. Johnson

New developments for CMOS SSPMs

A high fill factor SSPM built using a standard CMOS fabrication process can provide an energy resolution of 12.4% at 511 keV using CsI(Tl) crystals. The SSPM was operated at an excess bias of 2 V and 0 °C. The magnitude of the noise terms of the SSPM under these conditions are provided. This is compared to the energy resolution of 11.7% using a PMT at room temperature and the identical crystal. CMOS SSPMs can provide PMT-like energy resolution. Additional developments in back-illuminated and position-sensitive SSPMs devices are provided. A back-illuminated device has the promise of a low-noise, high fill-factor design, and the initial results of the quantum efficiency of back-illuminated, thinned devices, fabricated with an existing SSPM design, are provided. For position-sensitive SSPMs, an image of a 3 × 3 CsI array has been made with an SSPM based on a resistive-network configuration to provide position information has been made with minimal distortions.

Energy Resolution in CMOS SSPM Detectors Coupled to an LYSO Scintillator

SSPMs fabricated using CMOS technology consisting of arrays of 30- and 50-mum square pixels in 1.5 mm times 1.5 mm total area with high, 61%, and low, 29%, fill factors (packing density) were used to measure the photon intensity resolution for a pulsed laser light source. Different sources of noise (i.e., cross talk, dark counts, and electronic) have a deleterious effect on the energy resolution, and this work looks at the relative sizes of these contributions. Even though noise effects increase with larger fill factors and active area, this work examines the trade-off between these noise terms and the improvement in the detection efficiency with increased excess bias or fill factor. The energy resolutions from various gamma rays, including 122 keV, 511 keV, 662 keV, and 1275 keV, measured with an LYSO scintillation crystal (1.5 mm times 1.5 mm times 3 mm) were compared to the pulsed laser results to examine all noise contributions to the energy resolution. The SSPM response can be described by a binomial function. When greater than 70% of the pixels are triggered, the energy resolution contains a substantial contribution from the detector response that arises from a binomial detector response of the SSPM, which contains a finite number of pixel elements.

Digital scintillation-based dosimeter-on-a-chip

A reliable, low-cost, real-time dosimeter is constructed from a scintillation crystal mounted to a CMOS solid-state photomultiplier. Determination of the minimum silicon area that can provide reliable dose information is important to minimize dosimeter cost, and allow pervasive deployment. The purpose of this paper is to examine the effect of errors from event statistics and temperature variations on the dose measured in the crystal and the calculated human equivalent dose calculated using principal component analysis (PCA). Measured spectra are written as a linear combination of the calibration data sets, or principal components, and weighting factors from the calibration data are used to calculate the human equivalent dose. Applying this method to data measured by a 1.2times1.2times 0.2-mm3 LYSO scintillator coupled to a 100-pixel SSPM allows a predicted dose sensitivity better than 1 muSv, with far less than 40% error in determination of the dose from several unknown sources outside the calibration set. The measured sensitivity is approximately 1/300 the radiation dose from natural background in one month, making possible use as a potential replacement for a monthly film badge. Statistical fluctuations, temperature-induced gain fluctuations, and number of calibration sources required are investigated with respect to the dose calculated using the PCA method.

Nuclear material detection techniques

Illicit nuclear materials represent a threat for the safety of the American citizens, and the detection and interdiction of a nuclear weapon is a national problem that has not been yet solved. Alleviating this threat represents an enormous challenge to current detection methods that have to be substantially improved to identify and discriminate threatening from benign incidents. Rugged, low-power and less-expensive radiation detectors and imagers are needed for large-scale wireless deployment. Detecting the gamma rays emitted by nuclear and fissionable materials, particularly special nuclear materials (SNM), is the most convenient way to identify and locate them. While there are detectors that have the necessary sensitivity, none are suitable to meet the present need, primarily because of the high occurrence of false alarms. The exploitation of neutron signatures represents a promising solution to detecting illicit nuclear materials. This work presents the development of several detector configurations such as a mobile active interrogation system based on a compact RF-Plasma neutron generator developed at LBNL and a fast neutron telescope that uses plastic scintillating-fibers developed at the University of New Hampshire. A human-portable improved Solid-State Neutron Detector (SSND) intended to replace pressurized 3He-tubes will be also presented. The SSND uses an ultra-compact CMOS-SSPM (Solid-State Photomultiplier) detector, developed at Radiation Monitoring devices Inc., coupled to a neutron sensitive scintillator. The detector is very fast and can provide time and spectroscopy information over a wide energy range including fast neutrons.

High event rate, pulse shape discrimination algorithm for CLYC

CLYC is a scintillation material that is a viable alternative to 3He for neutron detection because the shape of the scintillation emission depends on the linear energy transfer producing the event, e.g., electrons from gamma-rays versus charged ions from neutrons. Analyzing the pulse shape on an event-by-event basis discriminates the neutron events from the gamma-ray event, which is called pulse shape discrimination. The long decay time associated with the scintillation emission of CLYC can result in pulse pile-up for event rates exceeding 100 kHz. One method to address this issue is to develop digital signal processing algorithms to remove pile up, while providing gamma-neutron discrimination and gamma spectra. Research on the algorithm development using saved CLYC pulses processed offline on a personal computer with C++ code indicate that CLYC can provide pulse shape discrimination exceeding 600,000 events per second, primarily from gamma events. The algorithm digitally suppressed the decay tail, allowing for unique temporal identification of a pulse (pile-up removal). Additional filtering of the pulse is done to reduce noise, and peak-to-tail information is obtained on filtered data within a time frame less than 1 μs to provide the pulse shape discrimination.

Next generation CMOS SSPMs for scintillation detection applications

Early CMOS SSPM pixel designs utilize a highly doped layer near the surface as a component for the Geiger junction, which limits the collection of charge from the surface and the UV response of the high gain solid state photodetector. To address these limitations, we are developing a new generation of CMOS SSPMs using pixel elements with a buried layer as a component of the Geiger junction in a process with smaller feature sizes. The new SSPM, an array of newly designed Geiger photodiode elements, is designed and fabricated to provide improvements in blue light response and dark noise performance. This work compares the performance of the early and new CMOS SSPM designs. Results showed ~2-4× improvement of detection efficiency in the blue/shallow UV region (350nm to 450nm), and a 10× reduction in detector dark count rate. Due to higher operating bias, the after pulse multiplier is no larger than a factor of 1.5 larger than the previous design. Inter-pixel cross-talk is similar to previous SSPM designs at comparable Geiger probabilities.

Optical and noise performance of CMOS solid-state photomultipliers

Solid-state photomultipliers (SSPM) are photodetectors composed of avalanche photodiode pixel arrays operating in Geiger mode (biased above diode breakdown voltage). They are built using CMOS technology and can be used in a variety of applications in high energy and nuclear physics, medical imaging and homeland security related areas. The high gain and low cost associated with the SSPM makes it an attractive alternative to existing photodetectors such as the photomultiplier tube (PMT). The capability of integrating CMOS on-chip readout circuitry on the same substrate as the SSPM also provides a compact and low-power-consumption solution to photodetector applications with stringent area and power requirements. The optical performance of the SSPM, specifically the detection and quantum efficiencies, can depend on the geometry and the doping profile associated with each photodiode pixel. The noise associated with the SSPM not only includes dark noise from each pixel, but also consists of excess noise terms due to after pulsing and inter-pixel cross talk. The magnitude of the excess noise terms can depend on biasing conditions, temperature, as well as pixel and inter-pixel dimensions. We present the optical and noise performance of SSPMs fabricated in a conventional CMOS process, and demonstrate the dependence of the SSPM performance on pixel/inter-pixel geometry, doping profile, temperature, as well as bias conditions. The continuing development of CMOS SSPM technology demonstrated here shows that low cost and high performance solid state photodetectors are viable solutions for many existing and future optical detection applications.

Tissue-equivalent solar particle dosimeter using CMOS SSPMs

A dosimeter-on-a-chip (DoseChip) comprised of a tissue-equivalent scintillator coupled to a solid-state photomultiplier (SSPM) built using CMOS technology represents an ideal technology for a space-worthy, real-time solar-particle monitor for astronauts. It provides a tissue-equivalent response to the relevant energies and types of radiation for low-Earth orbit and interplanetary space flight to the moon or Mars. The DoseChip will complement the existing Crew Passive Dosimeters by providing real-time dosimetry and as an alarming monitor for solar particle events (SPEs). A prototype of the DoseChip was exposed to protons at three incident energies at the NASA space radiation laboratory at Brookhaven National Laboratory. The prototype provides an unambiguous, proportional response for 200, 500, and 1000 MeV protons. The measured response produced a detector response function that was used to model the behavior of an improved instrument. The data presented here indicate that a 3 × 3 × 3 mm3 piece of BC-430 plastic scintillator coupled to a 2000-pixel SSPM can accommodate the needed dynamic range for protons with an incident energy of 20 MeV and greater.

Using decay time to discriminate neutron and gamma ray pulses from a CLYC detector

The reduced availability of 3He is a motivation for developing alternative neutron detectors. 6Li-enriched CLYC (Cs2LiYCl6), a scintillator, is a promising candidate to replace 3He. The neutron and gamma ray signals from CLYC have different shapes due to the slower decay of neutron pulses. The long decay time associated with the scintillation emission of CLYC often results in pulse pileup for event rates exceeding 100 kHz. Discriminating neutrons in a mixed field of gamma rays and neutrons is a challenging task especially when the event rate is high. There have been other methods that successfully distinguish neutrons at less than 100 kHz event rates, but separating them at higher event rates has not been satisfactory. In this work, we propose an algorithm that discriminates the neutron events directly based on their decay time and energy spectral density (ESD). The approach is assessed with data collected for different event rates (13 kHz to 1660 kHz) for an average data record length of about 750 ms in each case, providing more than 100,000 events to analyze. The results show accurate clusters of neutron events in the pulse shape discrimination (PSD) plot even during high event rates, and the approach gives a uniform figure of merit (FOM) ranging between 1.28-1.35 for all event rates.

Al 0.8 Ga 0.2 As Avalanche Photodiodes for Single-Photon Detection

We report Al0.8Ga0.2As recessed-window single-photon avalanche photodiodes with high internal single-photon detection efficiency and low dark count probability. External quantum efficiency was increased by a factor of 2 at λ = 405 nm. Annealing in arsine with hydrogen carrier gas reduced the dark count probability by a factor of 100, to ~10-6/gate with a ~5 ns gate, at room temperature. The activation energies of primary carrier traps, which give rise to afterpulsing, are extracted in a temperature range from 150 to 200 K.