Strontium-90 ($^{90}_{38}\mathrm{Sr}$) decays by $\beta^-$ emission to an isotope of yttrium (Y).
(a) Write the complete equation for this decay. [2]
(b) Explain why the existence of the antineutrino was proposed. [1]
(c) State the change in quark character that occurs in $\beta^-$ decay. [1]
(a) $^{90}_{38}\mathrm{Sr} \rightarrow {}^{90}_{39}\mathrm{Y} + {}^{\;\,0}_{-1}\mathrm{e}$ with correct nucleon and proton numbers on Y and the electron (accept $^{\;\,0}_{-1}\beta$, β$^-$ or e$^-$ for the electron)(1)
(a) $+\ \bar{\nu}_e$ included (accept $\bar{\nu}$ without subscript; reject $\nu_e$ / neutrino)(1)
(b) the beta particles are emitted with a range of (kinetic) energies / a continuous energy spectrum, so another (undetected) particle must carry away the missing energy (and momentum) (allow 'to conserve energy (in beta decay)' / 'otherwise energy would not be conserved'; ignore reference to momentum; reject 'to conserve charge' alone)(1)
(c) a down quark changes to an up quark (d → u) (accept udd → uud; reject 'neutron changes to proton' with no reference to quarks)(1)
Answer: (a) $^{90}_{38}\mathrm{Sr} \rightarrow {}^{90}_{39}\mathrm{Y} + \beta^- + \bar{\nu}_e$ (b) beta energies vary so energy not conserved without it (c) d → u
Examiner tip: An electron-antineutrino, not a neutrino, accompanies β⁻ decay — check lepton number: 0 = +1 + (−1).