Who developed the study of radioactivity?

Tritium undergoes radioactive decay giving

The number of $\beta$-particles emitted in the radioactive decay process $_{92}U^{238} \to _{82}Pb^{206} + x(_{2}He^{4}) + y(_{-1}e^{0})$ is:

The radionuclide $_{90}^{234}Th$ undergoes two successive $\beta$-decays followed by one $\alpha$-decay. The atomic number and the mass number respectively of the resulting radionuclide are

Which metal is used in radiation shielding?

The element with atomic number $84$ and mass number $218$ changes to another element with atomic number $84$ and mass number $214$. The number of $\alpha$ and $\beta$-particles emitted are respectively: