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Letters to Nature
Nature 217, 629-631 (17 February 1968) | doi:10.1038/217629a0; Received 3 November 1967
Ultra-low Levels of Terrestrial Ionization
ABRAHAM R. LIBOFF & MORRIS H. SHAMOS
- Physics Department, New York University, New York.
Abstract
NEHER has suggested1 that the approximate lower limit of the exposure rate resulting from soil radioactivity be taken as 1.4 
r./h, an unpublished value measured by Millikan in Manitoba. Subsequent surveys in Europe and the United States2,3 reported minimum values for the intensity of terrestrial ionization which averaged approximately 4.5 r./h. Millikan's value was also less than that found in the exhaustive study by Hultqvist4, in which the lowest indoor gamma level, as measured in wooden houses, was about 5.5 r./h.
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