Alpha rays emitted from a radioactive substance are:

The intensity of gamma radiation from a given source is $I$. On passing through $36 \, mm$ of lead,it is reduced to $\frac{I}{8}$. The thickness of lead which will reduce the intensity to $\frac{I}{2}$ will be......$mm$

$A$ radioactive nucleus emits a beta particle. The parent and daughter nuclei are

Alpha particles are

The modern treatment method $P.E.T.$ is based on:

$A$ free neutron decays spontaneously into: