Within ARMS Reach
An SwRI-developed technique enhances the capability of portable gamma ray imaging devices
Dr. Roland Benke is a principal engineer and certified health physicist at the Center for Nuclear Waste Regulatory Analyses in SwRI’s Geosciences and Engineering Division. He has more than 15 years of experience in radiological dose assessment, radiation detection and measurement, radiation transport modeling and risk analysis.
Photo courtesy of Air Photo Services Co. Ltd., Japan.
A Japanese unmanned aerial vehicle (UAV) was able to take detailed photos of damage to the Fukushima Dai-ichi nuclear power plant on March 20, 2011 (from left: partial view of Unit 1 and view of Unit 2, Unit 3 and Unit 4). Similarly, UAVs equipped with radiation detectors rapidly assess the distribution of radioactive contamination on the ground surrounding damaged facilities. By providing finer spatial resolution compared to existing technologies, ARMS technology can generate tomographic maps of radionuclide concentrations at the surface and improve the detection of radioactive hot spots.
ARMS determines energy dependent angular flux of gamma rays, which indicates their direction of origin. Red shading corresponds to the maximum fractional responses and incident directions. Black shading corresponds to negligible contributions.
The ARMS instrument is shown atop a platform in a laboratory test while exposed to low-level radiation emitted from bags of potassium chloride salt and other sources placed at several locations around the instrument.
Hand-held radiation survey instruments provide important, real-time information about radiation fields and nearby radioactive materials. Detection instruments can be simple or complex, depending on the specific application and data requirements. Simple survey meters, which respond to ionizing radiation without distinguishing the radiation type or its energy, are common. Gamma-ray spectrometers yield a spectrum of count rates over numerous channels that correspond to specific gamma-ray energies, or photon wavelengths. Their spectroscopic capability allows for differentiation of characteristic emissions, which is critical to identifying and quantifying contributions from multiple radioactive materials.
The Advanced Radiation Method for Surveys (ARMS), an emerging technology developed under internal research funding at Southwest Research Institute (SwRI), adds a third level of capability by generating radiation source images from existing handheld detection instruments, without need for shielding or collimation. ARMS data requirements are minimal: only instrument position and detector output are needed. The resulting images provide a visual indication of the energy-dependent angular flux, or direction, of gamma rays, a quantity that is rarely measured for a solid angle of 4p steradians. (A steradian is the three-dimensional angle created by the sides of a cone whose apex is at the center of a sphere and whose curved base covers a certain area on the sphere’s surface.)Laboratory demonstration
Using a commercially available gamma-ray spectrometer in a laboratory demonstration, SwRI engineers positioned naturally occurring radioactive material and low-intensity radioactive sources at different locations in a room to create a crisscrossing field of gamma rays with low, medium and high energies. Because radiation measurements are more challenging with weak sources in the presence of natural background, the experiments were intentionally performed at background levels to test the robustness of the approach. In fact, at the central location where the survey measurement was performed, the dose rate attributed to all added sources of radiation was less than the background dose rate. Potassium chloride (salt), available from hardware stores, is a naturally occurring source of radioactive material due to the presence of the radioactive isotope 40K at about one-hundredth of 1 percent in natural potassium.
Several bags of potassium chloride were grouped together in the laboratory to create weak volume sources of high-energy gamma rays. Given that the same radionuclide, 40K, also provides a significant component of the natural gamma-ray background, there was a potential for natural background interference, not only at the highenergy photopeak, but also throughout the remainder of the lower energy portions of the spectrum. The ARMS approach overcame these challenges easily. Based on a set of measurements acquired at arm’s length from one central location in the room, ARMS produced a suite of three-dimensional images over the gamma-ray energy spectrum.
Although determining the three-dimensional angular flux was highlighted as an essential intermediate step, SwRI researchers initially had not foreseen or sought to generate images of the radiation source from those data. Had they not embarked on research toward the end-point, they would not have discovered something potentially more important along the way.
Other gamma-ray imaging approaches rely on detection arrays (multiple discrete detector modules or position-sensitive detectors and signal processing) and/or collimation (a process that uses shielding material to block, or significantly attenuate, incoming radiation except for that within a proscribed field of view). Because collimation and coded apertures reduce the instrument’s detection efficiency and add significant weight, longer measurement times are required and portability is limited, especially for hand-held systems. ARMS is an attractive option because it does not require gamma-ray shielding or specialized detectors.Additional potential benefit
In the mid-1990s, the International Commission on Radiological Protection and the International Commission on Radiation Units and Measurements adopted updated human models for converting ionizing radiation to radiological dose. These external dose conversion coefficients are based on radiation type, radiation energy and direction of the incoming radiation relative to the forward-facing direction of the individual. Radiological dose is determined by accounting for the dose (the energy absorbed per unit mass) received by individual organs within the body. Certain organs are more radiosensitive than others, so organ weighting factors are applied to determine the effective dose for the whole body. Because deleterious health effects can vary for the same absorbed dose from different kinds of ionizing radiation, radiation weighting factors are applied to represent the whole-body dose that is equivalent to the radiological risk of health effects from exposure. Organs more sensitive to radiation can be shielded by less sensitive organs and tissues. An inherent aspect of updated external dosimetric modeling, this intrabody shielding effect is responsible for the published sensitivity of external dose conversion coefficients to the incoming direction of radiation.
Therefore, the ability to measure the angular flux, or directionality, of gamma rays or X-rays with a portable instrument can improve survey measurements of radiological dose rates. Using ARMS, effective dose rate at a location can be reported based on the direction an individual is facing in the radiation field, which is a new provision for radiological surveys.Advancing the state of the art
Although it advances the state of the radiation detection art, ARMS faces a number of practical barriers to widespread adoption. Many facilities and institutions rely on simpler and less expensive instruments without spectroscopic capability for routine survey measurements. Also, for many ionizing radiation fields the effective dose is not greatly sensitive to the direction from which the radiation originates. For X-rays and gamma rays at energy levels at or below 100 keV (kiloelectron volts), the effective dose varies by less than a factor of three from different irradiation directions. At higher energies, the radiation is more penetrating, which produces a more homogeneous irradiation within the body and diminishes the organ shielding effect. For neutron radiation over a broad energy range, different irradiation directions can change the effective dose by a factor of three, and irradiation of the front of the body (anterior-to-posterior directed radiation) results in the highest effective doses compared to other irradiation directions.
As a long-standing practice, radiation workers customarily wear their personal radiation dosimeters on the front of their body, and institutions simply use the most conservative, anterior-to-posterior dose factors for all irradiation scenarios rather than accounting for different radiation directions. For situations in which such overestimates of radiological dose are acceptable, simplified approaches may continue to be the preferred option. Higher-fidelity dose rate information is enticing in situations requiring more realism in radiological dose estimation. Even though ARMS probably has a niche role in dose estimation, its abilities for improving gamma-ray imaging and radioactive material characterization hold even more promise.Potential future applications
ARMS is a general technique, suitable to many mobile radiation detection measurements, including environmental monitoring, cargo screening and in-plant measurements where radiation readings are collected from different positions. Although demonstration measurements acquired with a spectrometer allowed for full implementation of the method, benefits also exist for ARMS applications using instruments that yield gross-count rate data instead of energy-dependent information. Medical applications, where source-to-detector distances are much shorter, also may be feasible. For beta particle detecting probes used in surgical procedures with radiopharmaceuticals to intraoperatively identify cancerous nodes and confirm their complete removal, improvements in spatial resolution and differentiation of nearby radionuclide foci can be expected with ARMS compared to current methods.
SwRI was awarded U.S. Patent No. 8,183,523, “System and Method for Acquiring Radiation Spectral Data in a Radiation Field and Determining Effective Dose Rate and Identifying Sources of Localized Radiation,” on May 22, 2012. ARMS technology can be commercialized and readily applied to various existing systems for portable detection measurements, including hand-held, mobile vehicle and remotely operated unmanned systems. Other potentially promising applications include homeland security and radiological protection. Recent interest has related to aerial measurement of environmental radioactive contamination released from damaged nuclear power plants.
Questions about this article? Contact Benke at (210) 522-5250 or firstname.lastname@example.org.