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1 Example GB05: splitting by cross-section
2 ----------------------------------------
3
4 This example illustrates a technique that uses physics cross-sections to
5 determine the splitting [killing] rate in a shielding problem. This technique
6 is supposed to be an invention, and this example here is not optimized. The
7 technique is applied here to neutrons.
8
9 In the classical treatment of the shielding problem, the shield is divided
10 in slices at the boundaries of which particles are splitted[killed] if moving
11 forward[backward]. In the present technique, we collect the cross-section of
12 "absorbing/destroying" processes : decay, capture, inelastic. We then use the
13 generic biasing facilities to create an equivalent of a spitting process, that
14 has a "cross-section" which is the sum of the previous ones. This process is
15 competing with other processes, as a regular one. The occurence of this process
16 is hence the same than the "absorbing/destroying" processes together. When this
17 process wins the competition, it splits the track, with a splitting factor 2 (ie
18 the original track is kept and a copy of it is created). This splitting is hence
19 occuring at the same rate than the absorption, resulting in an expected
20 maintained (unweighted) flux.
21
22
23 The geometry is made of a single block of concrete it. Behind it (in the +z
24 direction) a thin empty volume is placed to print out the particles which are
25 exiting the shield.
26
27 As in any generic biasing use, a biasing operator (taking decisions on what
28 biasing to apply) and a biasing operation (applying these decisions) are defined.
29 These are:
30 GB05BOptrSplitAndKillByCrossSection for the operator,
31 GB05BOptnSplitAndKillByCrossSection for the operation.
32
33 The operator is created in the detector construction, and receives here the
34 names of the absorbing/destroying processes to counterbalance for.
35 At tracking time, it collects the up to date cross-section of these processes
36 in the ProposeNonPhysicsBiasingOperation(...) method, and passes the sum to the
37 GB05BOptnSplitAndKillByCrossSection operation.
38
39 The operation uses the cross-section (interaction length) to sample the
40 distance to "interaction" with a classical exponential. If it wins the race
41 (ie it proposes the smallest of the interaction distances among all processes)
42 its GenerateBiasingFinalState(...) method is called, and it applies splitting
43 or killing (Russian roulette) if the track moves forward or backward.
44
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47
