SIXTH ERDA WORKSHOP ON PERSONNEL NEUTRON DOSIMETRY

Abstract

This workshop was the sixth of a series and was held on July 11 and 12, 1977, at the Oak Ridge National Laboratory in Oak Ridge, Tennessee. Those presenting papers at the Sixth Workshop prepared summary reports of their recent work for inclusion in this document. The reports are reproduced here as submitted by the participants, with only minor editing. This year's Workshop took a decidedly international flavor, with participants from seven countries in addition to the United States. The significance of this group's contributions has raised the possibility that the next Neutron Dosimetry Workshop may be held in Europe. Of particular interest at the Workshop was the keynote address by Dr. Harald Rossi. He commented that there is evidence that 1) accepted values of RBE for low absorbed doses of neutrons may be low by an order of magnitude or more and 2) the risk of leukemia is significant at 0.5 rad to the bone narrow. A reduction of the limit for permissible neutron exposure, which could result from consideration of this information, would necessitate major improvements in our "middle ages" neutron dosimetry. A number of participants reported conversions to thermoluminescent dosimeter (TLD) systems. This move has not been unanimous, however, as there were several reports of apparently satisfactory fission fragment, activation foil, and NTA film dosimeters. While thementionof NTA film resulted in the usual discussion of energy cut off and humidity effects, it seems the use of NTA in accelerator environments still has some merit. Discussion of fission fragment dosimeters centered around track etching techniques, which have shown some improvement. Of particular interest was Tommasino's report on the use of polycarbonate centrifuge tubes as the sensitive element. Thermally stimulated exoelectron emission (TSEE), never very popular for personnel dosimetry, has lost additional ground with the report that the neutron/gamma response ratio is much less than earlier work had indicated. Initial work with an experimental Harshaw CaF:thulium TLD shows some promise. This TLD exhibits a double-humped glow peak (150°C and 240°C), which may permit separation of neutron and gamma components from a single TLD chip. There was a plea for uniformity in neutron-to-dose conversion factors. A typical example of 120% differences between commonly used factors was cited. Perhaps it i s time for a new look at the problem by an international standards-setting group. Measurement of neutron spectra around facilities would be easier with a good, portable multichannel analyzer. Dr. Harry Ing may have one. It has 64 channels in a 6 - in. x 8 - in. x 2 1/2-in. package (including the display) and is battery powered. A new intermediate energy neutron source is available at Berkeley Nuclear Laboratories (UK). An SbBe source in a water-filled boron ball was reported to produce a neutron spectrum centered around 1/2 keV. Additional efforts of this type coupled with the "monoenergetic" sources available at the National Bureau of Standards will permit significant improvement in neutron dosimeter calibrations.