Both ¹⁹²Ir and ¹³⁷Cs are used in brachytherapy, the use of radioactive isotopes to treat various cancers. Both are beta emitters.

(a) Explain how this observation is consistent with the information presented in Figure 14.4.

Figure 14.4

(b) How do the N/Z ratios for these two medical isotopes compare to each other? How do they compare to their nearest stable isotopes?

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Atornic number, Z 110 100 90 80 70 60 50 30 undergo positron 40 emission or electron capture. 20 10 0 Stable isotopes = Radioactive isotopes 0 Nuclei with N/Z ratios above the band of stability often 10 20 30 40 50 60 Beyond Z = 83, all isotopes are unstable and undergo « decay. N = Z Nuclei with N/Z ratios below the band of stability often undergo beta decay. 70 80 90 Neutron number, N 100 110 120 130 140 150 160