Analysis of tissue samples containing colloidal thorium dioxide (Thorotrast) or zirconium dioxide (Zirconotrast): radiochemical preparation and alpha-spectrometry.

The aim of the present study is to assess the activity of the thorium isotopes, Th-232, Th-230, and Th-228 in tissue samples containing Thorotrast or Radiozirconotrast, i.e. Th-230 and Th-228 enriched Zirconotrast, in order to enable calculations of tissue doses. Therefore: 1. a procedure was developed for radiochemical processing of these samples, and 2. a computerised alpha-spectroscopy system has been constructed for routine activity measurements. The procedure developed for sample preparation permitted complete radiochemical processing of the above tissue samples. By subsequent electrodeposition of the thorium isotopes as well as the internal isotopic tracer Th-229, almost massless sources were obtained for alpha-spectrometry. The recovery amounts to a mean of 85% and 70% for tissues containing Thorotrast and Radiozirconotrast. With regard to the great number of samples to be measured and the expected counting times of more than 1500 min, a computerised alpha-spectroscopy system with a sample conveyor has been constructed. The energy resolution is between 45 keV and 75 keV FWHM. The detection efficiency amounts to 12%. The values of the detection level for the thorium isotopes are between 1,4 mBq and 3,1 mBq.