¹³⁷Cs

Spectrum:

Figure 1: Energy histograms of ¹³⁷Cs γ-radiation. The spectra have been normalized to coincide in intensity at the high-energy Compton-scatter shoulder.

We can define a peak-to-Compton (P/C) ratio as the ratio of intensities at 661.7 keV and 440 keV. As can be seen the 2-inch NaI(Tl) detector has the least stopping power and its P/C ratio is 6.3. On the other hand, a 3-inch CsI(Na) achieves a P/C ratio of 11.7