 they may be in a data group which is already open. In the former case it is also possible to make use of a small library of cross sections which is incorporated in the module and which contains data for a number of neutron detectors in two standard group schemes. If these cross sections are used there is provision to adjust the values for the thermal group and the lowest epithermal group to allow for variations due to temperature. Results are printed out at the set of points at which the flux was given, and the contribution from each group may be included if desired.
GAMSOURCE (WRS module No. 38474) Calculates the source strength of gamma rays arising from neutron capture in a system represented by one-dimensional geometry.
The gamma ray sources are 'prompt' or 'delayed', 'prompt' including all gammas produced by neutron capture (but not from fission or inelastic scattering), 'delayed' comprising gammas released in the decay of irradiation products.
To calculate the prompt source, values of the average microscopic absorption cross section for each element present in a region are calculated for each neutron energy group from a cross section library obtained by processing cross sections from UKNDL. In this library the epithermal absorption cross section is represented by a 100 step histogram between 1 MeV and 0.07 eV.
Values for the thermal group are obtained by averaging over a Maxwellian spectrum.
The photons produced are assigned to prompt energy groups. Two standard prompt group schemes are available: one of 5, the other of 13 groups. For the 5 group scheme, the gamma spectra are those used by the COMPRASH code, while the 13 group spectra are those compiled by Side otham. The spectra may also be inputted if a different group scheme is used.
GAMSOURCE also allows independent gamma sources to be read in.
The method for delayed gamma sources is similar to that of the prompt gammas, but differs in that there are no standard group schemes. It is therefore necessary to supply values of the number of photons emitted in each group during the decay process. The module may generate a new mesh for the gamma sources.
GRAB (Gamma Ray Build-up) (WRS system module No. 60221) Calculates the gamma ray flux, dose or heating rate in a slab shield using the build-up factor solution.
Gamma ray sources of different energies may be distributed throughout the system. To summarize the method, an integration is performed over the source volume and source energy range of a kernel representing the contribution to the quantity of interest from uncollided photons, to which is applied a factor that takes into account scattered photons.
The module requires values of the absorption coefficient (i.e. total cross section) and the energy absorption coefficient in each energy group for each material. For five standard groups these may be calculated by the module from the material compositions using a stored set of values of the mass absorption coefficient and the mass energy absorption coefficient for a number of commonly used elements. The module uses the Taylor approximation to the build-up factor, and therefore requires values of the parameters which define the factor for each group. It should be noted that since only one set of values can be used, a choice of build-up factor must be made with regard to the quantity of primary interest and the materials present. For each region, results are produced at the points of a uniform mesh which may be specified by the user. A separate integration is performed for the flux at each output point, and the execution time for the module therefore depends on the total number of output points requested. Dose and heating rates are, however, calculated from the flux values, so that little extra time is required for these. The contributions from selected groups may be totalled according to specifications in the input data. For the first point of every region the module can also print out the contributions to the flux from the source in each region.
MIXGAM (WRS system module No. 1300) Generates macroscopic cross section 