The influence of 134Cs on the 137Cs gamma-spectrometric peak-to-valley ratio and improvement of the peak-to-valley method by limiting the detector field of view

The peak-to-valley method was investigated under laboratory conditions and in situ with respect to both 134Cs perturbation of the 137Cs valley and use of collimation. The 134Cs perturbation is significant down to 134Cs:137Cs activity ratios of 1:100. In these cases the full energy peaks from 134Cs (796 and 802 keV) and associated valley should be used instead of the peak and valley from the 137Cs 662 keV peak. Use of collimators in situ outside Fukushima Daiichi significantly increased PTV for 134Cs.