The method for determining optimum conditions of a liquid scintillation counter (LSC) for lowlevel counting of tritium was studied. To compare the lower limit of detection, the figure of merit EM/√B was used. The measurement system is a LSC connected with a multichannel pulse-height analyzer (MCA) . Both of tritium spectra and background spectra were searched for the condition that maximizes the figure of merit. The optimum condition obtained by using the MCA was fitted for a single channel pulse-height analyzer of the LSC. The figure of merit obtained by this method was 20.6% greater than that obtained by the balance point method.
Absence of plasmid DNA sequences in non-tumorigenic (crown gall tumor) strains of Agrobacterium tumefaciens was confirmed by DNA-DNA hybridization between purified tumor-inducing (Ti) plasmid DNAs isolated from the progenitor tumorigenic strains and3H-labeled whole cell DNAs from tumorigenic strains and their non-tumorigenic derivatives. The genes controlling tumorigenicity in plant and utilizability of nopaline as their sole nitrogen source were confirmed to locate on Ti-plasmids.