Gammas from Neutron Capture

Interaction Codes
Code	Description
ttt327ccc	Gammas from neutron capture on ttt
	(see below for explanation of ccc)

The routine MCG_SET_GAMMA_CASCADE places the gammas that result from a neutron capture. Captures on H, D, $^{23}$Na, $^{24,25,26}$Mg, $^{27}$Al, $^{28,29,30}$Si, $^{32,34}$S, $^{35}$Cl, $^{39,40,41}$K, $^{40,42,44,46,48}$Ca, $^{50,52,53,54}$Cr, $^{55}$Mn, $^{54,56,57,58}$Fe, $^{58,60,62}$Ni and $^{207}$Pb are supported, and this can be extended if required.

The captures are dealt with in three ways, corresponding to the three types of MCDS banks describing neutron capture.

The captures on hydrogen and deuterium are simple; producing only a 2.225 and a 6.25 MeV gamma repectively. The code is correspondingly simple.

Captures on $^{35}$Cl are considerably more complicated. The neutron captures to an excited state of $^{36}$Cl at 8.58 MeV. The decay is modelled by a 75 level structure, the 75 levels are known to account for 99% of the total transition strength. The code then cascades down the levels (transition probablities stored in the upper triangle of a 75$\times$75 matrix) producing the gammas. Angular correlations are modelled for 6 decay routes (accounting for 53.8% of transition strength).

The remaining captures decay schemes are all calculated before inclusion into SNOMAN, the MCDS banks holding a lookup table of decay paths. This has the benefits of simpler code and better performance, but unfortunately makes checking the numbers non-trivial.

Interaction codes for (n,$\gamma$) events all have the form tttpppccc, as usual. The target code, ttt, is assigned the id of the particle that captured the neutron. The process code, ppp, is always 327 (neutron capture with gamma emission). The channel code, ccc, has information about the decay path. For $^{35}$Cl captures it holds the index of the energy level the capture state decayed to initially. (Energy levels can be found in the MCDS bank, the ground level having an index of 1 and the capture level an index of 75). For the other captures the channel code holds the number of the decay path taken.

The gamma decay routines, which can be used to generate the spectra for specific isotopes or natural elements, set the energies and directions (either isotropic or correlated) of the gammas. The user is then only free to set the time and position of the decay. These gamma decays are also automatically produced as part of the neutron transport code.