Validation of Hanford Personnel and Extremity Dosimeters in Plutonium Environments

Abstract

A study was performed to assess the performance of Hanford personnel neutron dosimetry. Measurements were performed in the Plutonium Finishing Plant (PFP), which is the facility with the most significant worker exposures to neutrons on the Hanford site. The study was conducted in two parts: one assessing whole body dosimetry and one assessing extremity dosimetry. For both parts of the study, the Tissue-Equivalent Proportional Counter (TEPC) was used as the principle instrument for characterizing workplace neutron fields.In the whole body study, 12.7-cm-diameter TEPCs were used in ten different locations in the facility. At each location, TLD and TED personnel dosimeters were exposed on a water-filled phantom at the same location to enable a comparison of neutron dose equivalents as measured by the TEPC and as determined from the personnel dosimeters.In the extremity study, 1.27-cm-diameter TEPCs were exposed inside the fingers of gloves worn in gloveboxes. Hanford extremity dosimeters (gamma sensitive ring TLDs) were wrapped around the TEPCs. The glove, loaded with TEPCs and dosimeters, was exposed to six different cans of plutonium, simulating the exposure that a worker's fingers would receive in a glovebox. The comparison of TEPC-measured neutron dose equivalent to TLD-measured gamma dose equivalent provided neutron-to-gamma ratios that can be used to estimate the neutron dose equivalent received by a worker's finger based on the gamma readings of an extremity dosimeter.The study also utilized other neutron instruments including a Snoopy and detectors based on bubble technology for assessing neutron exposures, providing a comparison of the effectiveness of these instruments for workplace monitoring.The study concludes that the TLD component of the Hanford Combination Neutron Dosimeter (HCND) performs adequately overall, with a positive bias of 30%, but exhibits excessive variability in individual results due to instabilities in the algorithm. The electrochemically etched TED response was less variable but only 20% of the TEPC reference dose on average because of the low neutron energies involved. The neutron response of the Hanford Standard Dosimeter (TLD) was more variable than the TLD component of the HCND and biased high by a factor of 8 overall due to its calibration to unmoderated 252 Cf. The study recommends further work to correct instabilities in the HCND algorithm and to explore the potential shown by the bubble-based dosimeters.iii EXECUTIVE SUMMARYThis report describes measurements performed in the Plutonium Finishing Plant (PFP) work environment to validate the recorded neutron dose for Hanford workers. The responses of Hanford dosimeter components, including the Hanford standard dosimeter (HSD), and both the thermoluminescent dosimeter (TLD) and track-etch dosimeter (TED) components of the Hanford combination neutron dosimeter (HCND) have been well characterized under the low-scatter conditions of the Hanford 318 Radiation Standards Laboratory. Hanford whole body personnel dosimeter respon...

Hanford External Dosimetry Technical Basis Manual PNL-MA-842

“…(a) The factor of 2 compensates for the ring's lack of response to neutrons while taking into account the ring's over response to 59 keV photons, and is predicated on typical neutron to gamma ratios observed when working with plutonium compounds. This practice has been validated by field measurements (Scherpelz, Fix, and Rathbone 2000).…”

“…The factor also takes into account the fact that there is no beta radiation in PFP glove box operations to which the ring might under-respond. More recently, direct measurements inside leaded gloves used in PFP glove boxes produced average neutron to gamma ratios between 0.09 and 0.55 for a variety of plutonium oxide and metal sources in sealed cans (Scherpelz, Fix, and Rathbone 2000). In this study, for the purpose of calculating neutron to gamma ratios, the gamma reading from a TLD 700 chip in a Hanford ring was used as the gamma dose without correction for over response to low energy photons.…”

“…self shielding in the source), shielding in the cans, lead loading in the gloves used, orientation of the ring on the finger, age of source material, and others (DOE 1998b). For example, in the Scherpelz, Fix and Rathbone (2000) study, ring response was measured with and without gloves to determine the photon attenuation affect of the glove. The glove thickness was shown to be equivalent to almost a half value layer for some of the sources used.…”

“…For multi-element dosimeters such as the 8816 dosimeter, energy dependence is less severe with energy compensating algorithms but field measurements may still be necessary to verify dosimeter accuracy. Technical reports of field measurements (Fix et al 1981(Fix et al , 1982Brackenbush et al 1980Brackenbush et al , 1991Endres et al 1996;Scherpelz, Fix, and Rathbone 2000), along with numerous letter reports to HEDP files, have been prepared.…”

Hanford Radiological Protection Support Services Annual Report for 1999

“…A study was performed to validate HSDs, HCNDs (including the track-etch component), and extremity neutron dosimeters in various work environments at the PFP (Scherpelz, Fix, and Rathbone 2000). Neutron doses from the aforementioned dosimeters were compared with simultaneous measurements obtained with TEPCs.…”