Plutonium ${ }^{239}Pu$ is an unstable nuclei and decays with a half-life   $\tau =2.4\times {10}^4$ years by producing smaller nuclei including an alpha particle. The alpha particle flux density (number of the alpha particles per unit area per unit time) near the surface of a large plate of ${ }^{239}Pu$ having thickness d= 1mm is $n\times {10}^{11}$. Find n.  Density of plutonium is $p=19800kg/m³$ and mass of an atom of ${ }^{239}Pu$ is $m_0=3.84x{10}^{-25}kg$.