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This keyword should be used only for calculations where small quantities of impurities in an iron matrix are to be irradiated. In a run without this keyword the activity of the impurities would probably be masked by the activity of the iron. In order to remove the “background” this keyword causes the iron matrix to be replaced by a matrix of a fictitious stable nuclide with no induced reactions so that the printed inventories and dose rates refer only to the impurities.

An example use, where a 1 ppm silver impurity is present in an iron sample, is given below. The output data will reflect only the silver and reactions on silver, however the gamma dose rate will reflect decays of silver isotopes in an iron matrix rather than in solid silver:

< -- Control phase -- >
* Title of the simulation
< -- Initial phase -- > 
MASS 1.0 2
Fe 99.9999
Ag 1.0E-4
< -- Inventory phase -- >