Basic Physics of Nuclear Medicine/Print version

Atomic Mass Unit

The conventional unit of mass, the kilogram, is rather large for use in describing characteristics of nuclei. For this reason, a special unit called the Atomic Mass Unit (amu) is often used. This unit is sometimes defined as 1/12th of the mass of the stable most commonly occurring isotope of carbon, i.e. ¹²C. In terms of grams, 1 amu is equal to 1.66 x 10^-24 g, that is, just over one million, million, million millionth of a gram.

The masses of the proton, m_p and neutron, m_n on this basis are:

and

while that of the electron is just 0.00055 amu.