JEE Advanced · 2015 · 24 May 2015 · Paper 1 · Modern Physics

**(A) Nuclear fusion $\to$ (R), (T).** The conversion of hydrogen to helium that powers stars *is* fusion, so (R). The proton-proton chain converts protons to neutrons by $p\to n+e^{+}+\nu$, so neutrinos are emitted, giving (T). The official key accepts either (R) alone or (R) and (T); the bank stores (R) and (T).

**(B) Fission in a nuclear reactor $\to$ (P), (S).** The chain reaction is sustained by ${}_{92}^{235}\mathrm{U}$ absorbing thermal neutrons, which is (P), and heavy water is the moderator that slows the fast fission neutrons to thermal energies, which is (S). The Resonance booklet also darkens (T), on the grounds that the fission fragments are $\beta$-active; the official key gives (P) and (S) only, and that is what is stored here.

**(C) $\beta$-decay $\to$ (Q), (T).** ${}_{27}^{60}\mathrm{Co}$ is a ${\beta }^{-}$ emitter, giving (Q), and every $\beta$ decay emits a neutrino (an antineutrino for ${\beta }^{-}$) alongside the electron, giving (T).

**(D) $\gamma$-ray emission $\to$ (R).** The proton-proton chain emits $\gamma$ rays - at deuteron formation and at positron annihilation - so $\gamma$ emission accompanies the conversion of hydrogen to helium in stars. ${}_{27}^{60}\mathrm{Co}$ is of course also a familiar $\gamma$ source (its $\beta$ decay leaves ${}^{60}\mathrm{Ni}$ excited, which then emits two $\gamma$ rays), and the Resonance booklet darkens (Q) and (T) here as well; the official key gives (R) only, and that is what is stored.