M_x and M_y denote the atomic masses of the parent and the daughter nuclei respectively in radioactive decay. The Q-value for a ${\mathrm{\beta }}^{-}$ decay is Q₁ and that for a ${\mathrm{\beta }}^{+}$ decay is Q₂. If ${\mathrm{m}}_{\mathrm{e}}$ denotes the mass of an electron, then which of the following statements is correct?

1. ${\mathrm{Q}}_1=({\mathrm{M}}_{\mathrm{x}}-{\mathrm{M}}_{\mathrm{y}}) {\mathrm{c}}^2 \mathrm{and} {\mathrm{Q}}_2=[{\mathrm{M}}_{\mathrm{x}}-{\mathrm{M}}_{\mathrm{y}}-2{\mathrm{m}}_{\mathrm{e}}]{\mathrm{c}}^2$

2. ${\mathrm{Q}}_1=({\mathrm{M}}_{\mathrm{x}}-{\mathrm{M}}_{\mathrm{y}}) {\mathrm{c}}^2 \mathrm{and} {\mathrm{Q}}_2=({\mathrm{M}}_{\mathrm{x}}-{\mathrm{M}}_{\mathrm{y}}) {\mathrm{c}}^2$

3. ${\mathrm{Q}}_1=({\mathrm{M}}_{\mathrm{x}}-{\mathrm{M}}_{\mathrm{y}}-2{\mathrm{m}}_{\mathrm{e}}) {\mathrm{c}}^2 \mathrm{and} {\mathrm{Q}}_2=({\mathrm{M}}_{\mathrm{x}}-{\mathrm{M}}_{\mathrm{y}}+2{\mathrm{m}}_{\mathrm{e}}) {\mathrm{c}}^2$

4. ${\mathrm{Q}}_1=({\mathrm{M}}_{\mathrm{x}}-{\mathrm{M}}_{\mathrm{y}}+2{\mathrm{m}}_{\mathrm{e}}) {\mathrm{c}}^2 \mathrm{and} {\mathrm{Q}}_2=({\mathrm{M}}_{\mathrm{x}}-{\mathrm{M}}_{\mathrm{y}}+2{\mathrm{m}}_{\mathrm{e}}){\mathrm{c}}^2$

In a nuclear reactor, moderators slow down the neutrons which come out in a fission process. The moderator used have light nuclei. Heavy nuclei will not serve the purpose, because: