Which of the following physical quantities is measured using the unit $rad$?

The atomic mass number of the element thorium is $232$ and its atomic number is $90$. The end product of this radioactive element is an isotope of lead (atomic mass $208$ and atomic number $82$). The number of alpha and beta particles emitted is:

$A$ nucleus decays by ${\beta ^ + }$ emission followed by a gamma emission. If the atomic and mass numbers of the parent nucleus are $Z$ and $A$ respectively, the corresponding numbers for the daughter nucleus are respectively.

If $_{92}U^{238}$ undergoes successively $8 \alpha$-decays and $6 \beta$-decays,then the resulting nucleus is

Consider the decay of a free neutron at rest: $n \rightarrow p + e^-$. Show that the two-body decay of this type must necessarily give an electron of fixed energy and,therefore,cannot account for the observed continuous energy distribution in the $\beta$-decay of a neutron or a nucleus.

$A$ radioactive nucleus decays as follows:
$X \xrightarrow{\alpha} X_1 \xrightarrow{\beta} X_2 \xrightarrow{\alpha} X_3 \xrightarrow{\gamma} X_4$
If the mass number and atomic number of $X_4$ are $172$ and $69$ respectively, then the atomic number and mass number of $X$ are: