The amount of suitable PAC isotopes required for a measurement is between about 10 to 1000 billion atoms (10 -10). The right amount depends on the particular properties of the isotope. 10 billion atoms are a very small amount of substance. For comparison, one mol contains about 6.22x10 particles. 10 atoms in one cubic centimeter of beryllium give a concentration of about 8 nmol/L (nanomol=10 mol). The radioactive samples each have an activity of 0.1-5 MBq, which is in the order of the exemption limit for the respective isotope.