Andrey Gueorguiev

Concept study of a two-plane Compton camera designed for location and nuclide identification of remote radiation sources

The concept of a two-plane planar Compton camera, consisting of scintillation detector elements, is presented. Several materials as C<inf>9</inf>H<inf>10</inf>, CaF<inf>2</inf>, YAlO<inf>3</inf>, NaI, and LaBr<inf>3</inf> are considered for operation in the scatter and/or absorption plane. The performance of the Compton camera is optimized by means of Monte Carlo simulations to meet the requirements for Homeland Security applications. For a low-threshold detector system we propose to utilize C<inf>9</inf>H<inf>10</inf> or CaF<inf>2</inf> for the scatter plane and NaI or LaBr<inf>3</inf> for the absorption plane. Particular effort must be focused to achieve low energy thresholds in particular for the detectors of the scatter plane if photons of incident energies below 200 keV are to be detected with reasonable efficiencies.

Novel timing method using IEEE 1588 and Synchronous Ethernet for Compton telescope

ICx Radiation, Inc. has implemented a novel timing method for use in a Compton telescope that is capable of nanosecond timing resolution. A critical task in Compton telescope design is to minimize the timing variance between detectors in a large array in order to reduce the background. The voxelSPEC has been developed to combine precise timing with pulse processing electronics in a single device, where all timing, communication, and power is transmitted over non-proprietary Ethernet hardware. The IEEE 1588 Precision Timing Protocol (PTP) combined with Synchronous Ethernet (SyncE) performs both phase and frequency locking of the individual detectors clock to a master clock. PTP phase locks each node to the absolute time recorded by the master clock. Hardware assist PTP uses the Ethernet PHY to time stamp the actual receiving and sending times of each PTP packet with sub-nanosecond accuracy, removing the time variance contribution of the CPU and other hardware, which can be on the order of microseconds to milliseconds. PTP-compatible network switches remove the unequal buffering delays in the switch hardware by performing a hardware assisted time correction to the PTP packet arrival and departure times directly (transparent clock mode) or by synchronizing the node to the network switch with hardware assistance (boundary clock mode). In 100MBps SyncE, the transmitters frequency is provided by the master clocks oscillator and propagated through the network switches to each individual node. Each Ethernet PHY connected to this frequency recovers the clock signal for the internal PTP clock. Using the master clock frequency minimizes the frequency drift. Results using both PTP (version 1 without PTP hardware assisted network switch) and SyncE show a node to master drift of about 8ns and a timing resolution of about 25ns between nodes. The implementation of hardware assisted PTP version 2 network switches will further reduce the timing resolution between nodes.

New generation plastic scintillators for fast neutron spectroscopy and Pulse Shape Discrimination

New generation of plastic scintillators have been developed at RMD for fast neutron detection technology. These plastics have peak emission wavelength ~ 440 nm, fast scintillation decay <; 10 ns, light output ~ 13,000 photons/MeV, and excellent Pulse Shape Discrimination (PSD) between gamma rays and neutrons. We have achieved a Figure-of-Merit (FOM) of 2.3 at 1.0 MeVee electron energy threshold for a 2 inch diameter right cylinder sample. At RMD, comparative measurements were made between the plastic scintillator and Eljen liquid scintillator EJ309 both 1 inch diameter × 1 inch length. RMD plastic showed competitive performance. Additionally, in an experiment performed at the University of Kentucky 7 MV Van De Graaff accelerator, RMD plastic scintillator was irradiated with mono-energetic fast neutron beam energies up to 20.8 MeV. The results from this experiment confirm fast neutron spectroscopy capabilities. These results and effects of different electronic systems on the PSD measurements are discussed in this paper.

A novel method to determine the directionality of radiation sources with two detectors based on coincidence measurements

ICx Radiation has developed a novel method to determine the direction of radiation with a device containing only two detectors by comparing and analyzing the paired-energy distributions of coincident events. The method does not require complex image reconstruction but rather extracts the directionality from the means and skewness of the two coincidence spectra. All energy data contribute to the energy spectrum, while events that occur within the coincidence time window are also added to a separate time-dependent buffer that represents the “reduced” spectra from each detector. These spectra contain counts only from true Compton events. The mean and skewness of each detectors reduced spectra subset is then calculated. Equal skewness and means correspond to the source being in front of the device. Opposite polarity skewness correspond to the source being located to the left or right of the device. For low count rates due to weak sources or sources located far away from the detectors, the skewness comparison gives fast indication for the hemisphere in which the source is located, while increased count rates or increased acquisition times reduce the uncertainties and allow a detailed angular detection of the source position. The simulations, experimental results and the angular resolution as a function of the strength, source energies and distance of the radiation source are discussed.

Unseeded growth of CdZnTe:In by THM technique

Travelling heater method (THM) has been a great success lately for the growth of large CdZnTe crystals. In this presentation, indium doped CdZnTe crystals have been grown adapting travelling heater method (THM) in vertical configuration, using three zone custom designed muffle furnace. Crystals have been grown with different ampoule diameter and size to study the grain growth. Seedless single crystalline CdZnTe:In crystals have been gown with 4 cm diameter weighing about 650 grams. Crystals have been characterized by near IR imaging, both microscopic and full wafer. The average resistivity along the length of the ingot was found to be about 109 ohm-cm. A resolution 3.2% was obtained at 662 keV. The effect of annealing of the whole wafer in Cd-Zn alloyed vapor on the resistivity and on the Te precipitations will be discussed.

Composite neutron gamma detector

In recent years, a number of new inorganic scintillating materials have been discovered that offer simultaneous thermal neutron and gamma ray detection. One of the first such scintillators is Cs2LiYCl6 (CLYC) [1], which offers (1) efficient thermal neutron detection (2 x higher cross section than 3He at 10 atmospheres); and (2) excellent separation between gamma and neutron events (>10-7). However, the cost of the new single-crystal scintillators is still relatively high, compared to traditional scintillators, such as NaI and CsI. Significant progress in the development of organic scintillators with neutron and gamma sensitivity has also been recently achieved [2]. They have very low production cost, but have relatively low detection and photopeak efficiencies due to the low density and low-Z constituents. They also do not provide thermal neutron detection.

Semiconductor neutron detectors

Lithium Indium Selenide (LiInSe2) has been under development in RMD Inc. and Fisk University for room temperature thermal neutron detection due to a number of promising properties. The recent advances of the crystal growth, material processing, and detector fabrication technologies allowed us to fabricate large detectors with 100 mm2 active area. The thermal neutron detection sensitivity and gamma rejection ratio (GRR) were comparable to 3He tube with 10 atm gas pressure at comparable dimensions. The synthesis, crystal growth, detector fabrication, and characterization are reported in this paper.

Temperature behavior of CLYC/MPPC detectors

He-3 tubes are the most popular thermal neutron detectors. They are easy to use, have good sensitivity for neutron detection, and are insensitive to gamma radiation. Due to low stockpiles of the He-3 gas, alternatives are being sought to replace these devices in many applications. One of the possible alternatives to these devices are scintillators incorporating isotopes with high cross-section for neutron capture (e.g. Li-6 or B-10). Cs2LiYCl6:Ce (CLYC) is one of the scintillators that recently has been considered for neutron detection. This material offers good detection efficiency (~80%), bright response (70,000 photons/neutron), high gamma ray equivalent energy of the neutron signal (>3MeV), and excellent separation between gamma and neutron radiation with pulse shape discrimination. A He-3 tube alternative based on a CLYC scintillator was constructed using a silicon photomultiplier (SiPM) for the optical readout. SiPMs are very compact optical detectors that are an alternative to usually bulky photomultiplier tubes. Constructed detector was characterized for its behavior across a temperature range of -20°C to 50°C.

Pulse shape discrimination for CLYC based handheld instruments

Neutron detectors are used in homeland security to improve the identification of special nuclear materials (SNM). The detection of any neutrons above the natural background is a strong indication of SNM presence. Currently, there is a significant dependence on He-3 tubes for neutron detection. These devices are simple and effective detectors; however they have a number of drawbacks. He-3 tubes are pressure vessels and cannot be brought on board an airplane. They also exhibit micro-phonic effects, which are a serious issue for the handheld instrumentation. The last but not least is a diminishing stockpile of the He-3 gas, which stimulated the recent push to create new neutron detection technologies. In the last few years, RMD has developed a Cs2LiYCI6 (CLYC) scintillator that offers (1) efficient thermal neutron detection, which is higher on a per-volume basis than that of He-3; (2) excellent gamma rejection ratio (GRR), better than 1x10-7; and (3) gamma-ray energy resolution as good as 4% at 662 keV.