Chad Whitney

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.

Radiation Effects on a Potential Scintillation-Based Solid-State Spectrometer Prototype for Compact Monitoring of Space Radiation/Weather Satellite Conditions

New and emerging scintillators, such as DPA (Diphenylanthracene) and CLYC (Cs2LiYCl6:Ce), coupled with Solid-State Photomultipliers (SSPM) provide a lightweight, low voltage, potentially radiation hard spectrometer for real-time monitoring of space weather conditions. This paper presents the results from proton radiation-hardness tests conducted for candidate scintillators and for individual cells of 1 cm2 SSPMs. Decreases from original light outputs were observed for samples of DPA and CLYC by 4% and 28% respectively with a dose of 1 kGy . Following 1 kGy exposure to the SSPMs, the dark current increased by a factor of ~ 32 for a reverse bias of 30 V. The effects of room temperature annealing on the dark current were also monitored. Decreases in quantum efficiency (22% for 420 nm and 7% for 535 nm) and light response (33% for 420 nm and 26% for 535 nm) were observed as a possible result of the formation of surface recombination-generation centers. The energy resolution of a LYSO scintillator sample coupled with a SSPM and measured for a 22Na gamma source (E = 511 kev) decreased from 15% pre-irradiation to 34% post 1 kGy irradiation.

DPA-Based Fast Neutron Dosimeter for the Space Environment

The space environment is inherently complicated with multiple sources of radiation, and within a spacecraft, these radiation fields are further complicated by the production of secondary particles (i.e., γ, e±, n, π±, π0), where the generation of neutrons represent a significant contribution to the dose received by astronauts. The signals in most detectors resulting from neutron interactions are difficult to discriminate from other types of interactions such as the incident energetic protons and gamma rays, making it difficult to provide accurate dose equivalent information. The results presented here demonstrate the capability of Diphenylanthracene (DPA) scintillation materials to detect and discriminate fast neutrons from gamma rays using pulse shape discrimination (PSD) techniques. The new scintillation sensors generate amplitude and emission-time signatures that provide information regarding the neutron dose and linear energy transfer (LET). This information can then be used to determine appropriate quality factors and the dose equivalent or biological effect. Considerations for a DPA based dosimeter design will be presented along with optimization of the detector signal processing steps for discriminating neutrons from gamma rays. The emission time and amplitude signatures from a new scintillation material, crystalline DPA, are characterized for proton, neutron, and electron (from gamma-ray irradiation) irradiation. An estimation of Birks parameters for DPA, which is necessary to describe the light yield as a function of LET, is presented.

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.

CLYC in gamma -Neutron imaging system

RadCam™ is a detection system developed by RMD for gamma ray imaging. The system images and tracks radiation contamination and its distribution. The current system is based on a CsI:Na scintillator coupled to a position-sensitive photomultiplier tube (PS-PMT). The image is obtained either using MURA coded aperture or pin-hole masks. Detected signals are decoded to produce radiation distribution that is overlaid on an image from a video camera (real-time). Due to increased focus on neutron detection, we recently started upgrading the system with a new scintillating material capable of simultaneous gamma ray and neutron detection - Cs2LiYCl6 (CLYC). Due to excellent pulse-shape discrimination, CLYC provides dual-mode detection of gamma-rays and neutrons while cleanly separating the signals from these two types of radiation. In addition, CLYC delivers better energy resolution than classical scintillators, such as CsI:Na. In this paper we present our initial results obtained with a system in which the original CsI:Na crystal was replaced with a CLYC scintillator.

CMOS solid-state photomultipliers for high energy resolution calorimeters

High-energy, gamma-ray calorimetry typically employs large scintillation crystals coupled to photomultiplier tubes. These calorimeters are segmented to the limits associated with the costs of the crystals, photomultiplier tubes, and support electronics. A cost-effective means for construction of a calorimeter system is to use solid-state photomultipliers (SSPM) with front-end electronics, which is at least half the cost, but the SSPM must provide the necessary energy resolution defined by the physics goals. One experiment with plans to exploit this advantage is an upgrade to the PRIMEX experiment at Jefferson Laboratories. We have developed a large-area SSPM (1 cm × 1 cm) for readout of large scintillation crystals. As PbWO4 has excellent properties (small Molière radius and radiation hard) for high-energy gamma-rays (>1 GeV) but low light yields (~150 photons/MeV at 0 °C), evaluation of the SSPM and support readout electronics with LaBr3 provides a measure of the device performance. Using the known detection efficiency and dark current of the SSPM, an excess noise factor associated with after pulsing and cross talk is determined. The contribution to the energy resolution from the detector module is calculated as <1% for gamma rays greater than ~2.5 GeV (0.7% at 4.5 GeV).

6-Li Enriched Cs2LiYCl6:Ce Based Thermal Neutron Detector Coupled with CMOS Solid-State Photomultipliers for a Portable Detector Unit

For detecting neutrons, 3-He tubes provide sensitivity and a unique capability for detecting and discriminating neutron signals from background gamma-ray signals. A solid-state scintillation-based detector provides an alternative to 3-He for neutron detection. A real-time, portable, and low cost thermal neutron detector has been constructed from a 6Li-enriched Cs2LiYCl6:Ce (CLYC) scintillator crystal coupled with a CMOS solid-state photomultiplier (SSPM). These components are fully integrated with a miniaturized multi-channel analyzer (MCA) unit for calculation and readout of the counts and count rates. CLYC crystals and several other elpasolites including Cs2LiLaCl6:Ce (CLLC) and Cs2LiLaBr6:Ce (CLLB) have been considered for their unique properties in detecting neutrons and discriminating gamma ray events along with providing excellent energy resolution comparable to NaI(Tl) scintillators. CLYCs slower rise and decay time for neutrons (70ns and 900ns respectively) relative to a faster rise and decay time for gamma ray events (6ns and 55ns respectively) allows for pulse shape discrimination in mixed radiation fields. Light emissions from CLYC crystals are detected using an array of avalanche photodiodes referred to as solid-state photomultipliers. SSPMs are binary photon counting devices where the number of pixels activated is directly proportional to the light output of the CLYC scintillator which is proportional to the energy deposited from the radiation field. SSPMs can be fabricated using standard CMOS processes and inherently contain the low noise performance associated with ordinary photomultiplier tubes (PMT) while providing a light and compact solution for portable neutron detectors.

Proton-Electron Discrimination Detector (PEDD) for space weather monitoring

Electronics used for space applications (e.g. communication satellites) are susceptible to space weather, primarily consisting of electrons and protons. As more critical equipment is used in space, a comprehensive monitoring network is needed to mitigate risks associated with radiation damage. Compact detectors suited for this requirement have been too complicated or do not provide sufficient information. As the damage from electrons (e.g. total ionizing dose effects) is significantly different compared to protons (e.g. displacement damage effects), monitors that can provide unique measurements of the dose and/or spectral information for electrons and protons separately are necessary for mission assessment to determine strategies for maintaining function. Previously, we demonstrated that the Proton-Electron Discrimination Detector (PEDD) is space-compatible and can discriminate fast electrons from protons using a diphenylanthrecene (DPA) scintillator coupled to a CMOS silicon photomultiplier (SiPM). The SiPM has a temperature dependence, and a circuit has been developed to provide a stable response as a function of temperature. The PEDD detector is scheduled to participate on the RHEME experiment to be flown on the ISS, scheduled for launch in 2016.

Neutron and gamma ray discrimination for CLYC using normalized cross correlation analysis

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. Some of the well-known pulse shape discrimination techniques fail to produce the desired results in a mixed radiation environment, particularly at high event rates. In the work presented here, we have applied a normalized cross correlation (NCC) approach to real neutron and gamma ray pulses produced by exposing CLYC scintillators to a mixed radiation environment generated by 137Cs and 252Cf/AmBe at different event rates. The cross correlation analysis produces distinctive results for measured neutron pulses and gamma ray pulses when they are cross correlated with reference neutron and/or gamma templates even at high event rates where the pileup is significant. With the NCC analysis shown to be a valid approach, efforts are continuing to develop a suitable algorithm to automatically process the NCC results to not only count the pulses but also to provide rudimentary neutron and gamma energy spectra. Ultimately, the technique will be implemented in an FPGA.

Li-Ion Batteries Used as Ubiquitous Neutron Sensors for Nuclear Forensics

With an increasingly complex worldwide nuclear environment, nuclear forensics provides a deterrent through proper attribution. Identifying the type of event is important in the attribution process, and the lithium battery has the potential to provide significant information for this purpose. The incident neutron spectral fluence is related to the type of nuclear event. The lithium battery is built using materials that can be used to assess the number of incident neutrons, as well as provide threshold detection dependant on the neutron energy. This study looks at correlating the production of various long-lived radioactive isotopes to the incident neutron spectrum. The analysis uses known nuclear cross sections and battery properties to calculate the number of long-lived radioactive elements that can be produced in the solid winding of the active battery elements, which include the cathode, separator, and anode. Some isotopes are only produced above a threshold energy, such as 58Co , which requires 10.6 MeV neutrons. Other isotopes such as 3H are produced through various reaction channels, mostly 6Li(n,α)3H, which is an exothermic reaction, and is not associated with a threshold energy.