The half lives of two radioactive material $A$ and $B$ are respectively $T$ and $2 T$. If the ratio of the initial masses of the materials $A$ and $B$ is $8: 1$. Then, the time after which the ratio of the masses of the materials $A$ and $B$ becomes $4: 1$ is

The energy released by the fission of one uranium nucleus is 200 MeV . The number of fissions per second required to produce 128 W power is

At room temperature, gaseous hydrogen is bombarded with a beam of electrons of 13.6 eV energy. The series to which the emitted spectral line belongs to

The half-life of a radioactive substance is 12 min . The time gap between $28 \%$ decay and $82 \%$ decay of the radioactive substance is