Response of TLD-albedo and nuclear track dosimeters exposed to plutonium sources

Abstract

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...

Validation of Hanford Personnel and Extremity Dosimeters in Plutonium Environments

“…3.16 measurements in the PFP work environments have shown significant under-response for TEDs in highly moderated neutron fields (Brackenbush, Baumgartner and Fix 1991;Endres et al 1996). Based on previous Hanford measurements, a range of acceptable values for each of the albedo TLD chip ratios R2 and R4 has been determined, that indicates sufficient hardness in the neutron spectrum for energy-independent dose measurement with CR-39.…”

“…Detailed dosimeter specifications are given in PNL-MA-842. The development of the dosimeter design is described by Brackenbush, Baumgartner, and Fix (1991) and Endres et al (1996). A HCND plastic holder is used to contain all three of these dosimeters, along with the Hanford personnel nuclear accident dosimeter (PNAD).…”

Retrospective assessment of personnel neutron dosimetry for workers at the Hanford Site

“…Personnel neutron dosimetry continues to the present day to pose significant technical challenges (Fix et al 1991). In fact, field calibration and verification of the personnel neutron dosimetry system are critically important to ensure accurate dose results for the range of neutron spectra encountered in DOE work environments (Brackenbush et al 1980)., The most recent effort to validate dosimeter performance in Hanford work environments is documented by Endres et al (1996).…”

“…The new system utilizes a more complicated field-based calibration procedure (Endres et al 1996). Field studies of the neutron dose have been conducted on several occasions(") , Brackenbush et al 1991). …”

“…During the 1990s field measurements were performed with a new Hanford prototype personnel neutron dosimeter design (Brackenbush et al 1991) and, subsequently, with the commercially procured Hanford Combination Neutron Dosimeter (HCND) (Endres et al 1996), which' was implemented on January 1,1995. Plutonium tetrafluoride (PUF,), plutonium oxide mod, and plutonium metal sources were used at the Hanford PFP facility to characterize the response of Hanford dosimeters in several exposure configurations involving bare and moderated sources.…”