T h i s report documents response characteristics and the development of dose algorithms for the Hanford Combination Neutron Dosimeter (HCND) implemented on January 1, 1995. The HCND was accredited under the U.S. Department o f Energy (DOE) Laboratory Accreditation Program (DOELAP) during 1994. The HCND employs two neutron dose components consisting o f 1) an albedo thermoluminescent dosimeter (TLD) , and 2) a track-etch dosimeter (TED). Response characteristics of these two dosimeter components were measured under the low-scatter conditions of the Hanford 318 Building Calibration Laboratory, and under the high-scatter conditions in the workplace a t the Plutonium Finishing Plant (PFP). The majority of personnel neutron dose a t Hanford (currently and historically) occurs a t the PFP. National Institute of Standards and Techno1 ogy (NIST) 'traceable sources were used t o characterize dosimeter response i n the laboratory. A t the PFP, neutron spectra and dose-measuring instruments, including a mu1 tisphere spectrometer, tissue equivalent proportional counters, and specially calibrated rem meters, were used t o determine the neutron dose under several configurations from three different plutonium sources: 1) plutonium tetrafluoride, 2) plutonium metal , and 3) plutonium oxide. In addition,
This paper describes an approach to quantifying errors in recorded estimates of external radiation dose obtained from personal dosimeters and applies the approach to dose estimates of workers at the Hanford site. Because a major objective of this evaluation is to provide the information needed for adjusting epidemiologic dose-response analyses of worker data for errors in dose estimates, the paper addresses the extent that errors for different workers are correlated, focuses on recorded doses as estimates of organ doses, and focuses on recorded doses as estimates of organ doses, and gives consideration to both annual and cumulative doses. The evaluation emphasizes errors resulting from the fact that dosimeters are limited in their ability to respond accurately to all radiation energies to which workers are exposed or to radiation coming from all directions. For each of several sources of error, systematic bias factors are estimated for two energy ranges (100--300 keV and 300--1,000 keV), two geometries (anterior-posterior and rotational), and four calendar year periods. These are then combined using information provided by health physicists on energies and geometries in Hanford exposure environments. Except for the period before 1958, deep dose, the objective of modern dosimetry systems, was found to be fairly accurately estimated. Lung dose was found to be overestimated by about 10%, and bone marrow dose was found to be overestimated by about 50%. However, many aspects of this evaluation relied heavily on subjective judgments, and, thus, these factors are subject to considerable uncertainty. Estimates of uncertainty in the bias factors and uncertainty reflecting random error are provided.
Worker dose estimatesare used in epidemiologicstudiesof nuclear workers. A major objectiveof these studies is to provide a direct assessment. of the carcinogenicrisk of exposure to ionizingradiationat low doses and dose rates. If dose estimatesused in analyses of worker data are biased, • then risk estimatesexpressedper unit of dose will also be biased. In addition,random error in dose estimatesmay lead to underestimationof risk coefficientsand can also distort dose-responseanalyses. Analyses of data from nuclearworker studies, includingHanford, have typicallynot been ' adjusted for biases and uncertaintiesin dose estimates in part because of the lack of adequate informationon the nature and magnitude of these biases and uncertainties. 10%, and bone marrow dose was overestimatedby about 50%. However, these overall bias factors are applicableonly to the "average"worker. Also, they reflect subjectivejudgments and thus are subjectto uncertainty.
Neutron dosimetry has been extensivelystudied at Hanford since the g inception of operationsin the mid-J940s (Wilson et al. 1990). At the present r, time, Hanford contractorsuse thermoluminescentdosimeter (TLD)-albedo , dosimeters to record the neutron dose equivalentreceived by workers. The energy dependenceof the TLD-albedodosimeter has been recognizedand documented since introducedat Hanford in 1964, and numerous studies have been performed to help assure the accuracy of dosimetersused in the work place. Historically,the largest pe,'_onnel neutron exposures occurred at the fluorinatorglove boxes used for processingplutonium. In the past, the Hanford MultipurposeDosimeterwas calibratedto accuratelyrecord doses from this source. This has resulted in conservativeassessments for many other neutron sources. With the recent change in Hanford'smission, there has been a significant decrease in the handling of plutoniumtetrafluoride,and an increase in the handling of plutonium metal and plutonium oxide sources. Also, the introduction of U.S. Departmentof Energy LaboratoryAccreditationProgram (DOELAP) requires the dosimeter to accuratelyassess the dose equivalentfrom 2SZCf sources under the low scatter conditionsof the calibration laboratory. This study was initiated to document the performanceof the current Hanford TLD-albedodosimeter under the low scatter conditions of the callbration laboratoryand under the high scatter conditions in the work place under carefully controlledconditionsat the Plutonium Finishing Plant (PFP). The neutron fields at the PFP facility were measured using a variety of instruments,includinga multispherespectrometer,tissue equivalentproportional counters,and specially calibratedrem meters. The dose equivalent rates from plutonium fluoride, plutoniumoxide, and plutoniummetal sources were determinedusing these instruments. The plutonium tetrafluoridesource is of 'importance because it is the same source that was used to calibrate o instrumentsand neutron dosimeters at Hanford from 1964 to 1981. The dosimeter results from the plutonium tetrafluorideexposures may be useful in interpretingpast dosimeter records. A variety of neutron dosimeterswere iii exposed on an acrylic plastic phantom (the same phantoms used in DOELAP accreditation) in these measured fields. Measurements were also performed with selected thickness of acrylic plastic positioned between the neutron source and the detectors to simulate glove boxes and neutron shielding windows. Various algorithms were used to evaluate the TLD-albedo dosimeters, and q the results are given in Section 7 of this report. Using current algorithms, the dose equivalents evaluated for bare sources and sources with less than 2.5 cm (1 in.) of acrylic plastic shielding in high scatter conditions typical of glove box operations are reasonably accurate, as expected. But the TLDalbedo dosimeters significantly overestimate the dose equivalent when exposed to moderated neutrons from sources with 5 cm to 15 cm (2 in. to 6 in.) of acrylic plastic shielding. The re...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.