Which of the following has the maximum penetrating power?

In the reaction $_{93}Np^{239} \xrightarrow{} _{94}Pu^{239} + (?)$,the missing particle is

An element with atomic number $84$ and mass number $218$ loses one $\alpha$-particle and two $\beta$-particles in three successive stages. The resulting element will have:

What is the symbol for the nucleus remaining after $_{20}Ca^{42}$ undergoes $\beta$-emission?

Which of the following statements is correct?

$A$ device used for the measurement of radioactivity is