Assertion : Cobalt $-60$ is useful in cancer therapy.

Reason : Cobalt $-60$ is a source of $\gamma  - $ radiations capable of killing cancerous cells

Assertion : Radioactive nuclei emit ${\beta ^ - }$ particles.

Reason : Electrons exist inside the nucleus

Beta rays emitted by a radioactive material are

The isotope ${ }_5^{12} \mathrm{~B}$ having a mass $12.014 \mathrm{u}$ undergoes $\beta$-decay to ${ }_6^{12} \mathrm{C} .{ }_6^{12 .}$ has an excited state of the nucleus $\left({ }_6^{12} \mathrm{C}^*\right)$ at $4.041 \mathrm{MeV}$ above its ground state. If ${ }_5^{12} \mathrm{~F}$ decays to ${ }_6^{12} \mathrm{C}^*$, the maximum kinetic energy of the $\beta$-particle in units of $\mathrm{MeV}$ is ( $1 \mathrm{u}=931.5 \mathrm{MeV} / c^2$, where $c$ is the speed of light in vacuum).

Assertion: ${}_Z{X^A}$ undergoes a $2\alpha  -$ decays, $2\beta  -$ decays and $2\gamma  - $ decays and the daughter product is ${}_{Z - 2}{X^{A - 8}}$

Reason : In $\alpha  - $decays the mass number decreases by $4$ and atomic number decreases by $2$. In $2\beta  - $ decays the mass number remains unchanged, but atomic number increases by $1$ only.

In a material medium, when a positron meets an electron both the particles annihilate leading to the emission of two gamma ray photons. This process forms the basis of an important diagnostic procedure called