Nuclear Probabilities
A big nucleus is on the order of 10-14 m, or an area of 10-28 m2 . This unit of area has its own name: the barn: 1 barn = 10-28 m2.
Probability of a specific happening is simply:
where ? is the area measured in barns.
Included in the ?i is the chance that the neutron did not hit anything (escape)!