The neutron flux is a scalar quantity used in nuclear physics and nuclear reactor physics. It is the total distance travelled by all free neutrons per unit time and volume. Equivalently, it can be defined as the number of neutrons travelling through a small sphere of radius R in a time interval, divided by a maximal cross section of the sphere (the great disk area, π π R 2 ) and by the duration of the time interval. The dimension of neutron flux is L − − 2 T − − 1 and the usual unit is cm s (reciprocal square centimetre times reciprocal second).