mucous membranes are tissue biopsied often in science due to cell generation in epithelial mucosa outer tissues . I could imagine anus and underside of tongue being uniform harvest sites in dropped cattle.

airborne radioactive particles are incorporated into cells through open wounds or mucosal surfaces

one layperson ref : https://www.uptodate.com/contents/clinical-manifestations-evaluation-and-diagnosis-of-acute-radiation-exposure#!