Measurement of induced activity to estimate personnel radiation exposures received from accelerator beams.

A method has been developed at the Lawrence Radiation Laboratory in Berkeley to estimate the dose received by an individual accidentally exposed to an accelerator particle beam. The dose estimate is based on measurement of the gross induced activities, primarily W, resulting from nuclear interactions of the particle beam with the body tissues of the exposed individual. The activities are measured with a portable NaI crystal p ray scintillation detector. To achieve maximum sensitivity and counting statistics, the activity measurements are made in the LRL Health Physics Group’s Low-Background Counting Enclosure soon after exposure. Doses have been estimated by exposing two tissue-equivalent phantoms, one simulating an arm and the other simulating a torso, to particle beams (protons, and secondary beams of protons, n+ mesons, and neutrons) of various energies. Results indicate that independent of the part of the body exposed, the beam size (6-81 cme) or the beam energy from 50 MeV to -6 GeV, the actual dose can be estimated within a factor of 2. Under favorable conditions the lower limit of sensitivity in estimating the dose is less than 1 rem. INTRODUCTION EARLY in 1969 we noted that -150nCi of 59Fe could be detected in an individual by using a 3x 3-in. portable NaI crystal y-ray scintillation detector in the LRL Health Physics Group’s Low-Background Counting Enclosure.(l) The method of examination consisted of placing the NaI crystal a t the midsection of the individual and noting the net counting rate (background subtracted) of the y rays. Based upon the observation that this detector system could measure very low levels of radioactivity in an individual, we felt that the same technique could be applied to measure the induced radioactivities resulting from an accidental exposure to an accelerator-produced particle beam. The activity induced(2) within the body tissues from nuclear interactions is primarily 1% with a 20.4-rnin half-life, I3N with a 9.96-min half-life, and 150 with a 123-sec half-life. To determine the sensitivity of this technique we constructed two tissue-equivalent phantoms. A 5-gal polyethylene bottle (29 cm diameter and 40 cm high) simulating the torso of a man and a 500 ml polyethylene bottle (10 cm diameter and 18 cm high) simulating the arm were filled with a tissue-equivalent fluid.(3) Each phantom was exposed to primary proton beams in the energy range of 50 MeV-6 GeV and to mixed-particle secondary beams of protons, T+ mesons, and neutrons in the 400 MeV-1.4 GeV energy range. After irradiation, the phantoms were returned to our Low-Background Counting Enclosure where the gross induced radioactivity was measured. With each phantom exposure a polyethylene or polystyrene disc of known thickness was also exposed. Subsequent counting of the IIC activity in the disc provided a monitor from which the particle fluence of the beam was calculated. Knowing the following additional information, (1) particle composition, (2) energy, (3) irradiated area, (4) time duration of the exposure, and (5) decay time after the exposure, we calculated the absorbed dose and the dose equivalence for each phantom irradiation. By combining the dose equivalence and absorbed dose values with the respective phantom activity data, we were able to establish sensitivity of detection values for the different beams employed in this study. 671 672 MEASUREMENT OF INDUCED ACTIVITY Table 1. Physical properties of particle beams studied Particle beam Particle energy Beam integral particles Beam area ______ _ _.