Lee Mitchell

MISTI imaging and source localization

MISTI (Mobile Imaging and Spectroscopic Threat Identification) provides both excellent gamma-ray spectroscopic and imaging capabilities to maximize the detection sensitivity of localized sources while at the same time minimizing the false alarm rate. The truck-based instrument is currently under construction and will detect and locate, in both position and range, a 1 mCi 137Cs at 100 m in 20 s with typical background levels. This performance is achieved using a hybrid system that combines the superb spectroscopic capabilities of germanium detectors for sensitive detection with the large collecting power available using scintillation detectors for producing gamma-ray images. Once an isotope of interest is detected by the germanium system, the NaI imager generates gamma-ray images for that energy that are overlaid based on the truck location at the time of acquisition. The overlayed images are calculated at a number of distance from the truck and the location and range of the source is determined by locating the reconstructed position and range with the maximum flux. The fully coded field of view, where all detectors contribute to the image, covers 92° × 29° using 9 × 3 pixels with the larger coverage in the horizontal direction.

Cross country background measurements with high purity germanium

Background measurements were made with twenty-eight high purity germanium detectors (HPGe) on the Mobil Imaging and Spectroscopic Threat Identification (MISTI) system across a portion of the Continental United States (CONUS). MISTI was developed by the Naval Research Laboratory(NRL) for the Domestic Nuclear Detection Office (DNDO) as part of the Stand off Radiation Detection Systems (SORDS) program. Measurements were made from Washington D.C. to Pocatello, Idaho, during the dates of Mar 25, 2011 to Mar 28, 2011. The effect of variable backgrounds on detection sensitivities will be discussed. An opportunity was also afforded to measure the radioactive materials released from Japans Fukushima I Nuclear Plant across a significant portion of CONUS. A description of the instrument and results are discussed.

Strontium Iodide Radiation Instrumentation (SIRI)

The Strontium Iodide Radiation Instrumentation (SIRI) is designed to space-qualify new gamma-ray detector technology for space-based astrophysical and defense applications. This new technology offers improved energy resolution, lower power consumption and reduced size compared to similar systems. The SIRI instrument consists of a single europiumdoped strontium iodide (SrI2:Eu) scintillation detector. The crystal has an energy resolution of 3% at 662 keV compared to the 6.5% of traditional sodium iodide and was developed for terrestrial-based weapons of mass destruction (WMD) detection. SIRI’s objective is to study the internal activation of the SrI2:Eu material and measure the performance of the silicon photomultiplier (SiPM) readouts over a 1-year mission. The combined detector and readout measure the gammaray spectrum over the energy range of 0.04 - 4 MeV. The SIRI mission payoff is a space-qualified compact, highsensitivity gamma-ray spectrometer with improved energy resolution relative to previous sensors. Scientific applications in solar physics and astrophysics include solar flares, Gamma Ray Bursts, novae, supernovae, and the synthesis of the elements. Department of Defense (DoD) and security applications are also possible. Construction of the SIRI instrument has been completed, and it is currently awaiting integration onto the spacecraft. The expected launch date is May 2018 onboard STPSat-5. This work discusses the objectives, design details and the STPSat-5 mission concept of operations of the SIRI spectrometer.

Low-altitude measurements of the neutron background

Low-altitude neutron measurements were made to better define the neutron background in airborne radiation detection systems. An early publication shows a little-known effect that occurs in the thermal and epithermal component of the neutron flux at approximately 150-300 ft (~90m). Both components of the neutron flux decrease as they approach 150 ft in altitude and increase with further increases in altitude. This work better defines the altitude dependence of the neutron flux near the ground. Unmoderated He-3 tubes were used to measure the thermal neutron background component, and a EJ-299 plastic pulse shape discrimination (PSD) detector was used to measure the fast neutron background component. The direct energy measurements were also some of the first altitude measurements made with this type of neutron detector. Measurements were made using a 6 ft tethered helium-filled balloon at altitudes of 50 to 500 ft (~150 m) in 50 ft increments. Measurements above 500 ft were made on board the Navys MZ-3A (Figure 1) airship. The results show reasonable agreement to simulations performed in SoftWare for Optimization of Radiation Detectors (SWORD), which interfaces with MCNP6. The simulated results for the thermal neutron rates agreed with the experimental measurements showing an increase in the count rate near the air/ground interface.