Internal dose assessment of (148)Gd using isotope ratios of gamma-emitting (146)Gd or (153)Gd in accidently released spallation target particles

The pure alpha emitter (148)Gd may have a significant radiological impact in terms of internal dose to exposed humans in case of accidental releases from a spallation source using a tungsten target, such as the one to be used in the European Spallation Source (ESS). In this work we aim to present an approach to indirectly estimate the whole-body burden of (148)Gd and the associated committed effective dose in exposed humans, by means of high-resolution gamma spectrometry of the gamma-emitting radiogadolinium isotopes (146)Gd and (153)Gd that are accompanied by (148)Gd generated from the operation of the tungsten target. Theoretical minimum detectable whole-body activity (MDA) and associated internal doses from (148)Gd are calculated using a combination of existing biokinetic models and recent computer simulation studies on the generated isotope ratios of (146)Gd/(148)Gd and (153)Gd/(148)Gd in the ESS target. Of the two gamma-emitting gadolinium isotopes, (146)Gd is initially the most sensitive indicator of the presence of (148)Gd if whole-body counting is performed within a month after the release, using the twin photo peaks of (146)Gd centered at 115.4 keV (MDA < 1 Bq for ingested (148)Gd, and < 25 Bq for inhaled (148)Gd). The corresponding minimum detectable committed effective doses will be less than 1 µSv for ingested (148)Gd, but substantially higher for inhaled (148)Gd (up to 0.3 mSv), depending on operation time of the target prior to the release. However, a few months after an atmospheric release, (153)Gd becomes a much more sensitive indicator of body burdens of (148)Gd, with a minimum detectable committed effective doses ranging from 18 to 77 µSv for chronic ingestion and between 0.65 to 2.7 mSv for acute inhalation in connection to the release. The main issue with this indirect method for (148)Gd internal dose estimation, is whether the primary photon peaks from (146) and (153)Gd can be detected undisturbed. Preliminary simulations show that nuclides such as (182)Ta may potentially create perturbations that could impair this evaluation method, and which impact needs to be further studied in future safety assessments of accidental target releases.