A radioactive material decays for an interval of time equal to its average life. What fraction of the material has remained undecayed?

A radioactive material decays for an interval of time equal to its average life. What fraction has decayed?

How do $\beta$-particles differ from the electrons obtained by thermionic emission and by photoelectric emission?

Show graphically the variation in the mass of a radioactive substance with time. Mark the values of time on the X-axis in terms of half-life ($T, 2T, 3T$ etc.) and the corresponding values of mass on the Y-axis.

Define decay constant for a radioactive substance. Give its unit.

Obtain a relation between half-life of a radioactive substance and decay constant ($\lambda$).

Define average (mean) life of a radioactive substance. How are half-life and average life-related?

Define two units of radioactivity. How are they related?