When a radioactive substance is subjected to vacuum,the rate of disintegration per second

During the emission of a positron from a nucleus, the mass number of the daughter element remains the same but the atomic number

Who observed that when the nucleus of a uranium atom was bombarded with fast-moving neutrons,it became so unstable that it immediately broke into two nuclei of nearly equal mass,besides other fragments?

Which among the following minerals contains a radioactive element?

The isotopic mass of ${}_{3}^{7}Li$ is $7.016005 \ u$ and those of the $H$ atom and neutron are respectively $1.007825 \ u$ and $1.008665 \ u$. Then,the binding energy of the ${}_{3}^{7}Li$ nucleus is: (in $MeV$)

Which one of the following statements is correct?