The influence of 134 Cs on the 137 Cs 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 both134 Cs perturbation of the137 Cs valley and use of collimation. The134 Cs perturbation is significant down to134 Cs:137 Cs activity ratios of 1:100. In these cases the full energy peaks from134 Cs (796 and 802keV) and associated valley should be used instead of the peak and valley from the137 Cs 662keV peak. Use of collimators in situ outside Fukushima Daiichi significantly increased PTV for134 Cs.