Victor Negut

Routine Surveys for Gamma-Ray Background Characterization

In gamma-ray spectroscopy and imaging, the natural gamma-ray background can significantly reduce detection sensitivity, especially when the source is weak and the background varies substantially. This project aims to systematically measure and characterize the spatial and temporal variations of the background in order to assess their impact on detection sensitivity and specificity for homeland security applications. An extensive survey of typical backgrounds found in the San Francisco bay area was performed, and initial measurement results are presented here.

Effects of Background on Gamma-Ray Detection for Mobile Spectroscopy and Imaging Systems

The presence of gamma-ray background significantly reduces detection sensitivity when searching for radioactive sources in the field, particularly in mobile systems which must contend with a variable background that is not known a priori . An extensive survey of the background was performed in the San Francisco Bay Area using both sodium iodide and high-purity germanium detectors, covering a wide variety of environments that might be encountered in an operational scenario. This data was used as a basis for source injection in a moving detector scenario in order to assess the effects of the background on different detection approaches. Both imaging and spectroscopic algorithms were implemented for the sodium iodide array, and their performances are compared for a variety of source energies and stand-off distances in the presence of the measured background.

Measurements of Fukushima fallout by the Berkeley Radiological Air and Water Monitoring project

The massive earthquake and tsunami off the coast of Japan on March 11, 2011 caused extensive damage at the Fukushima Daiichi nuclear power plant. During subsequent venting and explosions at the reactor site, there were releases of fission products such as 131I, 134Cs, 136Cs, 137Cs, and 132Te. Trace amounts of these isotopes were detectable in California around March 17. In the days after the disaster, the Berkeley Radiological Air and Water Monitoring (BRAWM) Project was started to measure the amounts of radioisotopes in the local environment around Berkeley. BRAWM has detected radioactive isotopes from Fukushima Daiichi in the air, rainwater, creek runoff, milk, soil, berries, and leafy vegetables. The team continues to monitor fallout levels in order to understand the nature of the radioactive releases from Fukushima as well as quantify the dilution or accumulation of the radioisotopes as they make their way through the environment and food chain.

MO-FG-CAMPUS-JeP1-01: Prompt Gamma Imaging with a Multi-Knife-Edge Slit Collimator: Evaluation for Use in Proton Beam Range Verification

PURPOSE To evaluate and characterize a multi-slit collimated imaging system for use in prompt gamma range verification of proton therapy. METHODS Acrylic (PMMA) targets were irradiated with a 50 MeV proton beam. With the collimator placed 13 cm from the beam axis, photons of energy from 2-7 MeV were measured. Image reconstruction provided 2-dimensional distribution of gamma rays. Estimated Bragg peak location was compared with 1-dimensional profiles of photon images. Shifts in Bragg peak were simulated by physically moving the targets in 1 mm increments. RESULTS The imaging system measured prompt gamma emissions resulting from a 50 MeV proton beam, at currents up to 2 nA, incident on a PMMA target. Overall system detection efficiency was approximately 2.6×10-5 gamma/proton. With delivery of 1×1011 protons, shifts of 1 mm in the target location were detected in 2D prompt gamma images and 1D profiles. With delivery of 1×108 protons, shifts of approximately 3 mm were detectable. CONCLUSION This work has characterized the performance of a prototype multi-slit collimated imaging system. The system can produce 2D images of prompt gamma distributions and detect shifts in Bragg peak location down to 1 mm. These results encourage further development and optimization of the system for clinical proton beam applications. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number: DENA0000979 through the Nuclear Science and Security Consortium.

Status of the High Efficiency Multimode Imager

The High Efficiency Multimode Imager (HEMI) is an instrument to detect, locate, and spectroscopically characterize radioactive sources with gamma-ray emissions at long-range standoff distances. HEMI consists of modular cubic-centimeter coplanar-grid CdZnTe detector elements configured in a two-plane array that allows for both Compton scattering and coded aperture imaging. Measurements and simulations have been performed using a variety of radioactive sources at different positions with respect to a prototype HEMI array in order to demonstrate the source identification and localization capabilities of the system. This paper will provide an overview of the HEMI instrument as well as present results from simulations and measurements using the completed detector array.