If the material is composed of N different nuclei, the interaction occurs on nucleus k if

k -1 n k

ST <£ ST < ST. (5.14)

i = 1 i = 1 i = 1

Figure 5.2 shows an illustration of this selection method.

(1) Capture

There are two ways to handle capture:

• Analogue capture, which is a ‘true’ capture: the neutron is absorbed with probability oa/o-T where absorption denotes all reactions like

((n, f), (n, a),…) except (n, n,…).

• Implicit capture: the neutron has a weight Wn which is decreased to W = (1 -(oa/oT))Wn by the capture. This way of handling captures is useful to obtain better statistics (the neutron is still alive after a capture, but its contribution is smaller).