To record Mossbauer spectrum of Fe containing samples, a source 'X' is used. X after a nuclear transformation (Y), gives $\gamma$-radiation used in Mössbauer spectroscopy.
(a)${ }^{57} \mathrm{Fe}, \beta$-emission
(c)${ }^{57} \mathrm{Co}$, electron capture
(b)${ }^{57} \mathrm{Co}, \beta$-emission
(d)${ }^{57} \mathrm{Fe}$, electron capture
Answer
Answer: B ✓ checked by 4AB · confidence high
The source book printed C; on checking, B is correct — see the explanation.
Explanation
The source is ⁵⁷Co, which decays by electron capture to the 14.4 keV excited state of ⁵⁷Fe, whose γ-ray is used.