TAM-D.2 - External Dosimetry Woodrow Wilson D 09:45 - 12:15
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| Chair(s): Rusi Teleyark
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TAM-D.2.1
09:45 RENEWABLE, LOW-COST PLA BIO-POLYMER BASED SOLID-STATE GAMMA-NEUTRON-ALPHA RADIATION SENSOR FOR MEDICAL, HEALTH PHYSICS & NUCLEAR INDUSTRY APPLICATIONS W Jiang, Purdue University
; N Boyle*, Oak Ridge Institute of Science and Education; T Barlow, Purdue University; Y Ota, Purdue University; D DiPrete, Savannah River National Laboratory; RP Taleyarkhan, Purdue University; Na Boyle
Abstract: Polylactic-acid (PLA) as a “green”, renewable corn-soy based polymer resin is being assessed for potential as a novel solid-state detector for rapid-turnaround gamma-neutron-alpha radiation dosimetry in the 0.1 mGy to-100 kGy range – of significant interest in general health physics, bio-medical and general nuclear industry applications. Co-60 was used as the source of gamma photons whereas, the 10 kW Purdue University’s PUR-1 research reactor was used for assessing for mixed neutron-gamma fields. It was found that PLA resin responds well in terms of a multitude of readily determined simple figures of merit: molecular structure, dissolution rate, grinding efficiency, rheology and porosity with absorbed gamma dose (Dg). This presentation focuses on response of PLA to gamma radiation, and secondary impact of neutron radiation. It was found that readily derived values on relative viscosity (RV), molecular structure (via FTIR), molecular weight (via GPC), and embrittlement (via particulate spectra from grinding) correlated very well with gamma dose. Studies also focused on ascertaining rheological changes via measuring the differential mass loss ratio (MLR) of irradiated PLA placed within PTFE framed (40mm x 20mm) cavities bearing ~0.9g PLA resin and pressed for 12-16 min. in a controlled force hot press under ~6.6 kN loading and platens heated to 2270C for the low Dg range: 0-11 kGy; and to 193oC for the extended Dg range: 11-120 kGy. MLR varied quadratically from 0.05 to ~0.2 (1 ~0.007) in the 0-11 kGy experiments, and from 0.05 to ~0.5 (1 ~0.01) in the 0-120 kGy experiments. Rheological changes from gamma irradiation were modeled and simultaneously also correlated with void-pocket formations which increase with Dg. A single PLA resin bead (~0.4g) was compressed 5 minutes at 216℃ in 0-16 kGy experiments, and compressed 2 minutes at 232℃ in the 16-110 kGy experiments, to form sturdy ~100 µm thickness wafers in the same press. Aggregate coupon porosity was then readily measurable with a conventional optical microscope imaging, and analyzed with standard image processing; this provided complementary data to MLR. Average porosity vs dose varied quadratically from ~0 to ~15% in the 0-16 kGy range, and from ~0 to ~18% over the 16-114 kGy range. These results provide evidence for utilizing “green”/ renewable (under $0.01)PLA resin beads for rapid, and accurate (+/-5-10%) gamma dosimetry over a wide 0-120 kGy range, using simple to deploy mass and void measuring techniques using common laboratory equipment. PUR-1 irradiated (mixed neutron-gamma) samples were also assessed and initial results indicate negligible impact of neutron dose relative to gamma dose on MLR and porosity metrics. The presentation will include discussions on results from recent ongoing studies pertaining to gamma blind thermal-to-fast neutrons and alpha radiation detection and dosimetry in the low (0.1 to 10 mRem/h) dose rate range.
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TAM-D.2.2
10:00 A one-year-old anthropomorphic phantom organ dose assessment using BeO optically stimulated luminescence dosimeters in computed tomography. E Kara*, Helmholtz Zentrum München, Neuherberg, Germany
; C Woda, Helmholtz Zentrum München, Neuherberg, Germany; El Kara
Abstract: Optically stimulated luminescence (OSL) detectors offer several potential advantages for radiation dosimetry for industrial and medical purposes and have sparked a lot of attention with the introduction of several new materials. The most common OSL dosimeters are Aluminum Oxide (Al2O3:C) and Beryllium Oxide (BeO) because of their appropriate dosimetric characteristics. Moreover, BeO is also a nearly tissue-equivalent material, making it more feasible to assess the applied doses in medical dosimetry by using the OSL technique. However, there is a lack of literature on the use of BeO OSL dosimeters in the medical environment. Characterization and the organ dose assessment in total body irradiation (TBI) has been carried out by using BeO OSL dosimeters and a one-year-old anthropomorphic phantom (CIRS ATOM). The results present that BeO may be considered as an alternative dosimeter to verify delivered doses in medical applications.
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TAM-D.2.3
10:15 Investigation of the radiographic imaging volume and occupational dose of radiologic technologists before and during the COVID-19 pandemic NA Shubayr*, Department of Diagnostic Radiography Technology, Collage of Applied Medical Sciences, Jazan University
Abstract: This study aimed to assess occupational radiation doses for radiologic technologists (RTs) in Saudi Arabia shortly before and during the COVID-19 pandemic, considering changes in imaging volume during that time. This retrospective study included the imaging volume data and the RTs' occupational dose records from a central hospital for 2019 and 2020. The occupational dose—in terms of annual and quarterly mean effective doses [(AMEDs) and (QMED)] —was estimated for 115 RTs using the thermoluminescent dosimeter records. There was a 22% increase in the AMED in 2020 compared with 2019, though the overall imaging volume decreased by 9% in 2020. The percentage changes in AMEDs between 2019 and 2020 for general radiography (GR), computed tomography (CT), interventional radiology (IR), nuclear medicine (NM), and mammography (MG) were 45%, 56%, 9%, 18% and -2%, respectively. The highest contribution to AMEDs in 2020 for modalities was due to GR and CT procedures, accounting for 0.50 mSv and 0.58 mSv, respectively. The percentage change in imaging volumes between 2019 and 2020 depicted a slight decrease in Q2 (-1%) and a substantial decrease in Q1 (-10%), Q3 (-12%), and Q4 (-11%) for 2020. The overall percentage changes in imaging volumes in 2020 for GR (conventional and mobile), CT, IR, NM, and MG were -7% (-19% and 48%), -11%, 13%, -26%, and -46%, respectively. Investigating the changes in 2020 by comparing Q1 of 2020 (before the pandemic restrictions) with Q2 (during the pandemic restrictions and changes in workflow) revealed that the QMED during Q2 increased by 5% and a 17.4% decrease in the imaging volume. However, CT procedures were increased by 11.1% during the pandemic restrictions in Q2 of 2020, with an increase in the corresponding QMED of 66%. Moreover, mobile GR procedures increased by 21% in Q2 of 2020 compared to Q1. This study indicated the impact of the COVID-19 pandemic on imaging volume and occupational dose. Overall, the study observed a decrease in the imaging volume and an increase in RTs’ effective doses by 2020. However, there was an increase in mobile GR and CT examinations during the COVID-19 pandemic restrictions in 2020. This study suggested that the increased mobile GR and CT examinations contributed to greater RTs’ effective doses in 2020.
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TAM-D.2.4
10:30 Development of an Energy-Dependent Neutron Shielding Model for VARSKIN+ ZG Tucker*, Renaissance Code Development, LLC
Abstract: A new method of applying Fermi age theory has been developed to model the degradation of neutron energy flux after traversing a simple shield. This model is to be integrated into the neutron dosimetry code VARSKIN+ as a means of rapidly and easily modeling the effects of shielding on neutron dosimetry, as a prelude to more advanced calculations.
The basic tenets of Fermi age theory have been applied to produce a simple expression for determining the probability of a particular neutron energy loss. This is then combined with the familiar expression for probability of attenuation to produce a deterministic, first-principles flux calculator which is dependent on the isotopic composition and depth of the shield, and the initial energy flux. Different segments of shielding, of various depth and composition, can be modeled.
Such a model provides a rapid, simple to use method of determining the relative effectiveness of various shielding configurations, before a method such as MCNP --- which is more accurate but requires more expertise and computational power to use --- is applied.
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TAM-D.2.5
10:45 Validation of VARSKIN+ Neutron Model Using MCNP CT Rose*, Renaissance Code Development
; ZG Tucker, Renaissance Code Development
Abstract: A new method of applying Fermi age theory has been developed to model the degradation of neutron energy flux after traversing a simple shield and is to be integrated into the skin dosimetry code VARSKIN+ as a means of rapidly and easily modeling the effects of shielding on neutron dosimetry.
Output of this neutron energy degradation model was verified by comparison to an MCNP model using the same assumptions used to reproduce the aforementioned flux model. The MCNP model comprised a sphere separated into “shells” of increasing radii simulating the skin depth, where neutrons were emitted by an isotropic point source at the center of the sphere and only forward-moving neutrons were counted. Neutron fluence was tallied at the surface of each shell, binned into the same 40 energy bins that VARSKIN’s neutron energy model uses. To provide a straightforward comparison between the two models, the average neutron energy traversing each shell of the MCNP model was compared to the average neutron energy calculated by the VARSKIN+ model for an equivalent thickness of shielding.
Average energy calculated by MCNP for various depths in water shielding was found to be in good agreement with the results from VARSKIN+’s energy model.
Percent error for the high-energy graph is less than 20% at and below 16 cm. The 955 keV results are below 31% error until 4 cm, before increasing to 75% error by 10 cm. Error then remains below 95%. The low-energy results are within 26% at and below 4 cm, before increasing up to about 100%.
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TAM-D.2.6
11:00 Monte Carlo Estimates of Secondary Photon Contribution to Effective Dose from External Exposure to Beta-Emitting Radionuclides Concentrated in Environmental Soil EA Asano*, Georgia Institute of Technology
; KF Eckerman, Oak Ridge National Laboratory; SA Dewji, Georgia Institute of Technology
Abstract: External exposure to secondary photons generated from electron source emissions in environmental soil are of concern regarding their contribution to ionizing energy deposited in organs and tissues within the body. The “condensed history method” employed in many modern Monte Carlo (MC) codes may be used to simulate secondary photon radiation fields with relatively few assumptions regarding the electron/photon production mechanisms and their energy/angular distributions. The condensed history method is, however, often computationally burdensome for many practical problems even with available parallel computing resources. Consequently, use of the method was prohibitive for prior applications that required innumerable MC simulations for deriving radiation protection quantities (i.e., effective dose rate coefficients). A method has been proposed for estimating the dose contribution of secondary photons from electron source emissions in environmental soil using the condensed history method and the Monte Carlo N-Particle version 6.2 (MCNP6.2) radiation transport code. The method was demonstrated with radiation transport models of idealized external exposure scenarios patterned of ICRP Publication 144 and Federal Guidance Report No. 15 (FGR 15). The secondary photon effective dose rate coefficients derived were compared with previously adopted analytical method and those full run MCNP simulations of beta-emitting radionuclides. The proposed method demonstrated significant improvement in levels of effective dose rate convergence with those derived from full-run MCNP simulations, in addition to enabling simulations with the condensed history method in a computationally feasible manner.
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TAM-D.2.7
11:15 Practical Importance of Dose Accuracy in Criticality Accident Emergency Response KG Veinot, Y-12 National Security Complex
; KJ McMahon, Y-12 National Security Complex; AE Detweiler*, Y-12 National Security Complex; JM Hayes, Oak Ridge Associated Universities (ORAU)
Abstract: How important is accuracy of dose following a criticality accident? Experience from past criticality events indicates that prompt medical attention to persons receiving greater than 50 rad (0.5 Gy) of whole body absorbed dose can significantly increase their chance of survival. But is the pinpoint accuracy of dose estimates, distinct from this kind of broad screening criteria, a priority in an emergency situation?
The Y-12 external dosimetry program participated in a criticality dosimetry intercomparison conducted at the Godiva-IV critical assembly in August 2022. Many teams utilized sophisticated methods and equipment, including measurements of simulated hair and teeth, to obtain dose estimates. Y-12 employed two simple methods: utilizing field instruments to measure copper activation of the foil found in standard-issue (not neutron specific) TLDs, as well as utilizing handheld gamma spectrometers to measure blood sodium activation. These methods are useful for quickly identifying those persons exposed to a significant fluence of neutrons, but they provide no spectral information, upon which neutron dosimetry is highly dependent.
Despite much simpler methods, Y-12 performed competitively when compared to other groups. Sophisticated methods such as activation foils and sulfur pellet analysis require significant funds and knowledge in order to maintain. Complex systems invite complex error and require personnel that have expertise in counting and analysis of count results. In addition, due to the vast variability of the population’s size, build, age, and radiological sensitivity, an identical dose may have very different medical consequences from one person to the next. Positive identification of persons receiving a dose above some benchmark is vital to bringing care to those who need it, yet after that, actual medical decisions and treatments will be informed by symptomatic assessments and biological dosimetry that account for radiation effects more than reported estimates of dose quantities.
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TAM-D.2.8
11:30 A Method to Detect Radioactive Contamination in Metal Filters for Dosimetry AK Mirza*, Landauer
; Ab Mirza
Abstract: Optically stimulated Luminescence (OSL) material in dosimeters respond uniquely to various types of radiation, but insufficiently characterizes radiation energy or type on its own. A filter system for dosimeters is therefore necessary to correctly assess a wearer's radiation environment and accurately estimate the dose registered on a dosimeter. Landauer's current Inlight and Luxel dosimeters contain a set of 3 filters comprised of plastic, aluminum and copper. When obtaining metals for filters, it is commonly beyond a supplier's capability to assess the metals for low levels of radioactivity. Although low levels of fixed metal contamination may move from supplier to customer undetected, the presence of radioactive contamination in metals processed as filters and in close proximity to the OSL material can disrupt the correct assessment of occupational dose. In fact, prior studies at Landauer showed evidence for the presence of low levels of Pb-210 contamination in tin metal. This has led to the development of contamination analysis procedures a threshold level for the various metallic filters prior to affixing them in new dosimeters. The subject of the presentation will describe the history behind the contamination investigation, methods developed to assess levels of contamination in pre-processed metallic filters and activity threshold limits placed as a quality control method to prevent the accumulation of dose on dosimeters from metal filters.
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TAM-D.2.9
11:45 Significant Reduction in X-ray Dose with a Simple Tube Housing Shield PC Steege*, University of Wisconsin - Madison
; JP Gainor, Egg Medical; RF Wilson, University of Minnesota - Twin Cities
Abstract: X-ray imaging exposes hospital staff near the device to scatter radiation from the patient, however, x-ray exposure from tube housing leakage is not usually considered We measured the x-ray intensity at points on a grid placed on the sides of tube housings of 4 different manufacturers. We found that all x-ray tube housings had significant leakage, with the greatest at the top (average of 786 ± 25.9 μSv/hr, range 430 μSv/hr to 1470 μSv/hr) and less at the bottom (average of 114 ± 3.8 μSv/hr). An additional shield was designed and fabricated to reduce the leakage radiation from the tube housing. The shield was created with 1 mm lead on the walls and 2 mm lead on the top and an aperture in the top for the primary beam. We then studied the effect of the tube housing shield on scatter radiation dose. We measured x-ray dose levels using a calibrated anthropomorphic phantom in fluoroscopy (15 f/s) at 6 points around the lab representing the standard locations of hospital staff during procedures. Measurements were taken from 20 cm to 200 cm above the floor at 20 cm increments, with and without the tube housing shield in place. With the tube housing shield, the total scatter x-ray dose to staff members reduction was 16.3 ± 0.9% (mean ± SD, p < 0.01), 480.1 ± 15.8 μSv/hr to 401.7 ± 13.2 μSv/hr. The average scatter x-ray dose reduction was above the table of 19.6 ± 1.1% (198.3 ± 6.5 μSv/hr to 159.4 ± 5.3 μSv/hr) was similar to the average dose reduction below the table of 15.5 ± 0.8% (761.8 ± 25.1 μSv/hr to 643.9 ± 21.2 μSv/hr ). Moreover, the doses measured at each of the 6 positions were significantly reduced with the addition of the housing shield. These results suggest that x-ray leakage, primarily from the top of the tube housing, contribute significantly to scatter radiation around the x-ray table. Adding additional shielding to the x-ray tube housing may give a significant reduction in the exposure to the medical personnel in the lab.
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TAM-D.2.10
12:00 Debunking Myths and Urban Legends in Ionizing Radiation Dosimetry CN Passmore*, Radiation Detection Company
Abstract: Urban legends and myths regarding various dosimetry technologies have been promulgated throughout the health physics community. This presentation deals head-on with several widely quoted myths about various external dosimetry technologies and limitations of various dosimeters associated with accreditation testing. The presentation examines data from scientific studies and published papers in relation to key type testing parameters of dosimetry systems including fade and lower limit of detection. In addition, NVLAP scope of accreditation will be compared for various dosimetry technologies and what this could mean to a Radiation Safety Officer in selecting an appropriate dosimeter for the radiation fields encountered by the workers.
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