Eric A. Wulf

Three-Compton telescope: theory, simulations, and performance

The advent of highly segmented gamma-ray detectors with good energy resolution has made a new class of gamma-ray detectors possible. These instruments record the positions and energies of each individual gamma-ray interaction with high precision. Analysis of the individual interactions can provide energy and directional information, even for events with only partial energy deposition. Advantages over traditional gamma-ray detectors include enhanced efficiency, background rejection, gamma-ray imaging, and sensitivity to polarization. Consider those gamma-rays that interact three or more times in the detector. The energy of the gamma-ray that initiated one of these events is uniquely determined by measuring the energies of the first two interactions and the scatter angle of the second interaction. The precision of this measurement is limited by the energy and position resolution of the detector, but also from Doppler broadening that results from gamma-ray scattering off-bound electrons in the detector. It is also essential to correctly sequence the first three interactions. The importance of Doppler broadening is greater in higher Z-materials, thus silicon becomes a good choice for the detector material. We discuss performance and simulations of the multiple Compton telescope. Possible applications include an advanced Compton telescope (ACT) for astrophysics, a medical multiple-gamma detector high-energy imaging survey instrument, and a gamma-ray tracking detector for future low-energy nuclear physics experiments.

Germanium strip detector Compton telescope using three-dimensional readout

Compton telescopes using two germanium strip detectors with depth resolution have been demonstrated at the Naval Research Lab. Depth resolution allows interactions to be located to less than 1 mm, down from 1 cm with no depth resolution, which improves the imaging resolution of the telescope substantially. Compton images and reconstructed energy spectra of events in which gamma rays interact three times in the two detectors but did not deposit their full energy (i.e. Three-Compton) are examined. Finally, the multiple modes of the system including a standard Compton telescope and a Three-Compton telescope are compared.

Development and Applications of Position-Sensitive Solid-State Gamma Ray Detectors

The development of high-resolution position-sensitive, solid-state detectors will enable gamma ray detectors with improved sensitivity and imaging capabilities. The gamma ray astrophysics group at NRL has been developing germanium strip detectors for several years. We have shown that three-dimensional locations for gamma ray interactions can be determined with sub-millimeter accuracy, and have also demonstrated imaging capability within a single germanium strip detector. We have also initiated work on thick, silicon strip detectors. This was based on the fact that three sequential interactions can enable the energy and direction cone of the incident gamma ray to be determined, even without total energy deposition of the incident gamma ray. We are also working on low-power ASICs that are required to handle the large number of channels associated with arrays of strip detectors. Progress on this work will be presented, along with applications to high-energy astrophysics, medical imaging, nuclear physics, detection of fissile materials, and monitoring of environmental radioactivity.

Depth measurement in a germanium strip detector

We have demonstrated the ability to determine the depth of a gamma-ray interaction point over the full active volume of a thick germanium strip detector. This capability provides depth resolution of less than 0.5-mm full width half maximum (FWHM) at 122 keV in a device 11 mm thick with a 2-mm strip pitch. Fifty channels of electronics have been developed and tested with a 25 /spl times/ 25 germanium orthogonal strip detectors. Experiments examining the capabilities of the system and demonstrating a simple Compton telescope using a single detector have been performed.

Thick silicon strip detector Compton imager

We present initial results obtained with double-sided, thick (2 mm) silicon strip detectors used as a Compton imager. A reconstructed image of a gamma ray source and a spectrum of the gamma ray energy are produced at room temperature using the multiple Compton technique. Multiple Compton interactions allow the energy and Compton scattering angle to be reconstructed without having to absorb the energy of the incident gamma ray completely. This extends our work on multiple Compton imagers using germanium strip detectors to silicon detectors that operate at higher temperatures. The detectors are 57 /spl times/ 57 mm in active area and are 2 mm thick with 64 strips per side with a pitch of 0.9 mm. The energy resolution of the detectors for 60 keV gamma rays are 3-4 keV at room temperature and 2.1 keV at -20/spl deg/C. Simulations of the imager are performed in GEANT4, including Doppler broadening, and show agreement with the real data.

Mobile imaging and Spectroscopic Threat Identification (MISTI): System overview

The Mobile Imaging and Spectroscopic Threat Identification (MISTI) system developed to locate radiological threats in urban and rural environments is currently undergoing characterization activities. MISTI is a mobile source detection and imaging system designed to identify and localize a radiological source to within +/- 10 m in range. This requirement is based on a 1 mCi Cs-137 source at 100 m in 20s, while maintaining a false alarm rate of less than one per day. MISTI utilizes the cost effective collection power of NaI for imaging and the sensitivity of high resolution HPGe for spectroscopy. MISTIs data acquisition system was developed with the latest commercially availed hardware that met MISTIs requirements. The performance of crucial software and hardware components is presented along with overall system performance. A synopsis and example of the initial characterization results are presented here.

Designing SWORD--SoftWare for Optimization of Radiation Detectors

The software for the optimization of radiation detectors (SWORD) is an integrated system (based on MCNPX [Pelowitz, DB, 2005] and GEANT4 [Agostinelli, S, et al., 2003] 3D Monte Carlo radiation transport codes) useful for the optimization of high energy radiation detection systems. A set of usable sample inputs and analysis algorithms are integrated into the system. Sample inputs include special nuclear material (SNM) targets, nuisance sources, and industrial and marine backgrounds. Analysis algorithms include spectroscopy and imaging for coded aperture detectors and Compton imagers. The system is designed from the start to be easy to use and to be deployable to detector design and system architecture study groups who are its ultimate users.

Front-End ASIC for a Silicon Compton Telescope

We describe a front-end application specific integrated circuit (ASIC) developed for a silicon Compton telescope. Composed of 32 channels, it reads out signals in both polarities from each side of a Silicon strip sensor, 2 mm thick 27 cm long, characterized by a strip capacitance of 30 pF. Each front-end channel provides low-noise charge amplification, shaping with a stabilized baseline, discrimination, and peak detection with an analog memory. The channels can process events simultaneously, and the read out is sparsified. The charge amplifier makes uses a dual-cascode configuration and dual-polarity adaptive reset. The low-hysteresis discriminator and the multi-phase peak detector process signals with a dynamic range in excess of four hundred. An equivalent noise charge (ENC) below 200 electrons was measured at 30 pF, with a slope of about 4.5 electrons / pF at a peaking time of 4 mus. With a total dissipated power of 5 mW the channel covers an energy range up to 3.2 MeV.

HX-POL—A Balloon-Borne Hard X-Ray Polarimeter

We report on the conceptual design and estimated performance of a balloon-borne hard X-ray polarimeter called HX-POL. The experiment uses a combination of Si and Cadmium Zinc Telluride detectors to measure the polarization of 50 keV-500 keV X-rays from cosmic sources through the dependence of the angular distribution of Compton scattered photons on the polarization direction. On a one-day balloon flight, HX-POL would allow us to measure the polarization of strong Crab-like sources for polarization degrees down to 5%. On a longer (15–30 day) flight from Australia or Antarctica, HX-POL would be be able to measure the polarization of strong sources down to polarization degrees of 1%. Hard X-ray polarization measurements provide unique venues for the study of particle acceleration processes by compact objects. In this contribution, we discuss the overall instrument design and performance. Furthermore, we present results from laboratory tests of the Si and CZT detectors.

Mobile imaging and spectroscopic threat identification (MISTI)

Characteristic gamma-radiation can be used to identify radiological threats, however gamma-ray detection and imaging is extremely difficult due to the low interaction probability and inability to focus high energy photons. MISTI’s hybrid system combines the exceptional spectroscopic capabilities of germanium with the cost effective collection power of a large volume sodium iodide imaging array. The system is a mobile, self contained, gamma-ray spectroscopy and imaging system for detecting radiological threats. While moving, the MISTI system is designed to detect sources of nuclear materials, such as a 1mCi Cs-137 source at distances up to 100m in 20s. The spectroscopic identification is performed using a 28 detector germanium array, which in turn triggers imaging using a 10x10 sodium iodide array, when a source is detected. The project is composed of commercial off the shelf technology, allowing a quicker transition from the design phase to the construction phase, reducing total cost. The data from the sensor will be analyzed in real time on board the vehicle and is combined with images and data from other instruments to provide users with a visual location of the source. MISTI’s unique design reduces false alarms, while improving weak source location and identification in urban and rural environments.