The $S.I.$ unit of radioactivity is

The half-life of radium is about $1600$ years. Of $100\, g$ of radium existing now, $25\, g$ will remain unchanged after ………. $years$

Radioactive element decays to form a stable nuclide, then the rate of decay of reactant $\left( {\frac{{dN}}{{dt}}} \right)$ will vary with time $(t) $ as shown in figure

If the radioactive decay constant of radium is $1.07 \times {10^{ – 4}}$ per year, then its half life period is approximately equal to ………$years$

The mean life of a radioactive material for alpha decay and beta decay are, respectively, $1620$ years and $520$ years. What is the half life of the sample (in years) ?