Warnick J. Kernan

Optimized geometry and limitations of compton cameras with LaBr 3

LaBr3(Ce) scintillators have a high atomic number, high light yield, and fast decay. Thus, detectors with LaBr3(Ce) crystal can achieve high energy resolution with a significantly improved efficiency over Nal(Tl) scintillators of the same geometry. In this work, LaBr3 was investigated for use in a Compton camera with a two-plane array design, as the material for both the scattering and absorbing detector arrays. We calculated and modeled the following properties of a LaBr3 Compton Camera: 1) The probability of single Compton scatter; 2) Doppler broadening effects; 3) The relationships between intrinsic efficiency and angular uncertainty; 4) Optimized efficiency for a required angular resolution by selecting various geometries, such as the thickness of detectors and distance between detectors. Our study showed that Doppler broadening effects induced an intrinsic limitation of ~0.05 radian angular uncertainty for Compton cameras with LaBr3 as the scattering detector. In comparison of LaBr3 with other traditional scatter detector materials such as Si, we found that below thickness of ~2 cm LaBr3 has a higher efficiency for observation of single Compton scattering events, including the hits of the recoil electron and scattered photons, than that of Si. Based on these investigations, a prototype of Compton imaging system was proposed and evaluated with Monte Carlo simulation (Geant4) also.

Measuring Concentrations of Particulate 140La in the Air.

AbstractAir sampling systems were deployed to measure the concentration of radioactive material in the air during the Full-Scale Radiological Dispersal Device Field Trials. The air samplers were positioned 100–600 m downwind of the release point. The filters were collected immediately and analyzed in a field laboratory. Quantities for total activity collected on the air filters are reported along with additional information to compute the average or integrated air concentrations.

Using Atmospheric Dispersion Theory to Inform the Design of a Short-lived Radioactive Particle Release Experiment.

AbstractAtmospheric dispersion theory can be used to predict ground deposition of particulates downwind of a radionuclide release. This paper uses standard formulations found in Gaussian plume models to inform the design of an experimental release of short-lived radioactive particles into the atmosphere. Specifically, a source depletion algorithm is used to determine the optimum particle size and release height that maximizes the near-field deposition while minimizing both the required source activity and the fraction of activity lost to long-distance transport. The purpose of the release is to provide a realistic deposition pattern that might be observed downwind of a small-scale vent from an underground nuclear explosion. The deposition field will be used, in part, to study several techniques of gamma radiation survey and spectrometry that could be used by an On-Site Inspection team investigating such an event.

PRex: An Experiment to Investigate Detection of Near-field Particulate Deposition from a Simulated Underground Nuclear Weapons Test Vent.

AbstractA radioactive particulate release experiment to produce a near-field ground deposition representative of small-scale venting from an underground nuclear test was conducted to gather data in support of treaty capability development activities. For this experiment, a CO2‐driven “air cannon” was used to inject 140La, a radioisotope of lanthanum with 1.7‐d half-life and strong gamma-ray emissions, into the lowest levels of the atmosphere at ambient temperatures. Witness plates and air samplers were laid out in an irregular grid covering the area where the plume was anticipated to deposit based on climatological wind records. This experiment was performed at the Nevada National Security Site, where existing infrastructure, radiological procedures, and support personnel facilitated planning and execution of the work. A vehicle-mounted NaI(Tl) spectrometer and a polyvinyl toluene-based backpack instrument were used to survey the deposited plume. Hand-held instruments, including NaI(Tl) and lanthanum bromide scintillators and high purity germanium spectrometers, were used to take in situ measurements. Additionally, three soil sampling techniques were investigated and compared. The relative sensitivity and utility of sampling and survey methods are discussed in the context of on-site inspection.

Modeling and measurements for mitigating interference from skyshine

Skyshine, the radiation scattered in the air above a high-activity gamma-ray source, can produce interference with radiation portal monitor (RPM) systems at distances up to even many hundred meters. Pacific Northwest National Laboratory (PNNL) has been engaged in a campaign of measurements, design work and modeling that explore methods of mitigating the effects of skyshine on outdoor measurements with sensitive instruments. An overview of our work with shielding of skyshine is being reported by us in another paper at this conference. This paper will concentrate on two topics: measurements and modeling with Monte Carlo transport calculations to characterize skyshine from an iridium-192 source, and testing of a prototype louver system, designed and fabricated at PNNL, as a shielding approach to limit the impact of skyshine interference on RPM systems.

Particle Release Experiment (PRex) Final Report

...................................................................................................................................................... iii Executive Summary .................................................................................................................................... v Acknowledgments ..................................................................................................................................... vii Acronyms and Abbreviations ................................................................................................................... ix 1.0 Introduction ............................................................................................................................ 1 1.1 Background .............................................................................................................................. 1 1.2 Objective .................................................................................................................................. 2 1.3 Scope of Document .................................................................................................................. 2 2.0 Experimental Considerations ................................................................................................ 3 2.1 Overview of PRex .................................................................................................................... 3 2.2 Constraints ............................................................................................................................... 3 2.3 Assumptions ............................................................................................................................. 4 2.4 Pre-Execution Release Parameters ........................................................................................... 4 3.0 PRex Experiment Preparation .............................................................................................. 6 3.1 Experiment Plan ....................................................................................................................... 6 3.2 Site Selection............................................................................................................................ 6 3.3 Background Measurements, September and October 2012 ..................................................... 7 3.4 Source Production .................................................................................................................... 9 3.4.1 Preparation of La2O3 Powder .................................................................................................................. 9 3.4.2 Irradiation at Washington State University ........................................................................................... 12 3.4.3 Source Transport ................................................................................................................................... 14 3.5 Release Mechanism ................................................................................................................ 19 3.6 Meteorology Planning and Modeling ..................................................................................... 22 3.7 NNSS Experiment Site Preparation ....................................................................................... 22 3.7.1 Posting Contamination and High Contamination Areas ....................................................................... 22 3.7.2 Prepositioned Sampling ........................................................................................................................ 22 4.0 PRex Experiment Execution ............................................................................................... 28 4.1 Time and Location ................................................................................................................. 28 4.2 Meteorological Conditions during Release ............................................................................ 28 4.3 Sampling Network ................................................................................................................. 32 4.3.1 Collection of Witness Plates ................................................................................................................. 32 4.3.2 Soil Sampling ....................................................................................................................................... 33 4.3.3 Air Sampling ......................................................................................................................................... 36 4.4 Radiation Survey .................................................................................................................... 36 4.4.1 Vehicle Survey...................................................................................................................................... 37 4.4.2 Handheld/Man-portable ........................................................................................................................ 38 4.4.3 Aerial Survey ........................................................................................................................................ 39 4.5 Field Laboratory ..................................................................................................................... 40

Simulations of material-dependent detector responses for airborne gamma-ray spectroscopy

This work aims to create a library of predicted detector responses used for real-time background estimation during airborne gamma-ray spectroscopy. The simulated spectra are a function of naturally occurring radioactive potassium, uranium, and thorium (KUT) and radioactive daughter products which are present in terrestrial-based materials. Self-attenuation within these materials varies, such that the gamma spectra emitted from the surfaces can differ, despite all having KUT origins. This study also compares various simulated detector responses among materials to determine if some could be accurately scaled from another across reasonable helicopter altitudes and orientations relative to the ground. Results show that materials with comparable sample thicknesses can be scaled consistently across altitude. This work allows for efficient background estimation via aggregation of simulated detector responses in real time.

Shielding of skyshine interference with radiation detection systems

Skyshine, the radiation scattered from the air above a source, such as radiography or x-ray imaging systems, can be a problematic source of background gamma rays that interfere with radiation detection systems even at very large distances. The effects of skyshine have been studied for many years, but almost entirely within the context of nuclear power and radioactive waste storage. These earlier studies show that modeling of skyshine is made difficult because it must take into account huge volumes of space in the models that slow analysis significantly. This paper reports on results from various modeling and measurement efforts that examine shielding approaches to the skyshine interference problem with radiation detection systems.

Shielding and build-up considerations for radiation detection

Shielding for gamma radiation has traditionally focused on the reduction of dose effects. For these applications, reducing the energy of the radiation is important along with reducing the actual number of photons, and therefore large masses of high Z material are typically used. However, for measurements requiring low backgrounds or for detecting low activity signals, such as in homeland security applications, the primary use of shielding is to decrease the total number of background photons (perhaps in a region of interest), and therefore the processes of buildup and down scattering become important. In these applications, where the important measure is count rate instead of dose and low background are important, improved reduction in counts from background radiation may be achieved with specially designed configurations of thin layers of different materials instead of a single thick layer. This paper describes recent modeling and experimental investigations in layered-shielding methodology, provides results with comparison to single shielding material such as Pb, and discusses applications of detector systems for homeland security.

Editorial Conference Comments by the Editors

The Symposium on Radiation Measurements and Applications (SORMA) convened for the third time on the West Coast, May 22–26, 2016, at the Clark Kerr Campus of the University of California, Berkeley, CA, USA. With radiation detectors increasing in number, variety, and societal importance, we are alternating between SORMA (in Ann Arbor, MI, USA) and SORMA West so that the forum will be available every two years.