Revisiting external dose rate and ground deposition of Chornobyl fallout in the Gävle region in Sweden: comparison between estimates from soil sampling vs in situ measurements using a field portable NaI(Tl) gamma spectrometer.

A field survey was conducted in Sweden with the purpose to revisit the ground deposition of Chornobyl 137Cs and the associated ambient dose equivalent rate, H* (10), at various measuring locations in the Gävle and Älvkarleby municipalities. The specific purpose was to compare the 137Cs deposition values as measured in situ using a field portable gamma spectrometer [3″(Ø) × 3″ NaI(Tl)-crystal] with the ex situ estimates obtained from gamma spectrometry of soil cores taken at the measuring locations. An additional purpose was to re-assess the effective ecological half-times of the Cs-contribution to the H* (10) and compare with a previous assessment done for data until 2013. The results show that ex-situ soil sample data exceed the 137Cs deposition values from the field portable NaI(Tl) gamma spectrometer by, on average, 50%. The discrepancy could mainly attributed to the difference between the actual field-of-view seen by the portable device at 1 m above ground compared with the calibration geometry of an infinite planar surface with a Cs ground penetration at 3 g cm-2. The H* (10) measurements collected in 2022 could indicate that the effective ecological half-time of 137Cs attributed ambient dose equivalent rate in the area ranges between 10 and 20 y instead of the 6.8 y obtained from the previous assessment of data between 1987 and 2013. The findings demonstrate, however, the usability of the portable field gamma spectrometer when used to characterize the long-term time dynamics of external doses from 137Cs in cases where the 137Cs dose contribution is well below normal background levels.