Beta- and Gamma-comparative Dose Estimates on Enewetak Atoll

Abstract

Enewetak Atoll is one of the Pacific atolls used for atmospheric testing of U.S. nuclear weapons. Beta dose and gamma-ray exposure measurements were made on two islands of the Enewetak Atoll during July-August 1976 to determine the beta and low energy gamma-contribution to the total external radiation doses to the returning Marshallese. Measurements were made at numerous locations with thermoluminescent dosimeters (TLD), pressurized ionization chambers, portable NaI detectors, and thin-window pancake GM probes. Results of the TLD measurements with and without a beta-attenuator indicate that approx. 29% of the total dose rate at 1 m in air is due to beta- or low energy gamma-contribution. The contribution at any particular site, however, is somewhat dependent on ground cover, since a minimal amount of vegetation will reduce it significantly from that over bare soil, but thick stands of vegetation have little effect on any further reductions. Integral 30-yr external shallow dose estimates for future inhabitants were made and compared with external dose estimates of a previous large scale radiological survey (En73). Integral 30-yr shallow external dose estimates are 25-50% higher than whole body estimates. Due to the low penetrating ability of the beta's or low energy gamma's, however, several remedial actions can be taken to reduce the shallow dose contribution to the total external dose.

Utirik Atoll Dose Assessment

“…The unshielded beta contribution to the external dose was estimated for Enjebi Island at Enewetak Atoll in 1980 (Crase et al, 1982). The average beta dose at 1-m height over open ground was 29% of the external gamma dose.…”

An updated dose assessment for Rongelap Island

“…The buildings provide shielding so that maximum effective dose occurs in the first year the exposure rate is reduced compared to the and is about 0.11 mSvy -1 (11 mrem y-l). The unshielded beta contribution to the Airborne concentrations of 239+240pu and external dose was estimated for Enjebi Island at 241Am are estimated from data developed in Enewetak Atoll in 1980 (Crase et al, 1982).…”

A dose assessment for a U.S. nuclear test site -- Bikini Atoll

“…The unshielded beta contribution to the external dose was estimated at Enewetak Atoll in 1980 [5]. More recent studies at Bikini Atoll using new, thinner thermoluminescent dosimeters (TLDs) indicate that the dose over open ground at 1-c,-a height is about three times that of 1-m height [6].…”