An Analysis of Residual Radiation in Thoriated Camera Lenses

Abstract: The usage of the naturally occurring radioactive isotope thorium-232 in older camera lenses was implemented to provide better optical properties and a cheaper production. In this thesis, the resulting radioactivity in camera objectives containing such lenses is studied via gamma spectroscopy. A number of camera objectives that potentially contain thorium-232 were measured using a germanium semiconductor photon detector. The spectra were compared to the background radiation whereby it is possible to determine whether any inherent radiation from the thorium-232 decay chain came from the lenses themselves. For one lens, the gamma spectra are then complemented with beta emission spectroscopy. From this, it is finally possible to establish the activity of the lenses and the radiation dose a photographer would receive from them. The radiation dose can then be compared to national standards of allowed doses. The measurements showed that though there clearly was a presence of decaying thorium and daughter nuclides in the objectives, the activity was small enough not to be hazardous.