FM and MT cards have different meanings:

For example when hydrogen is molecularly bound in water, either pure or as a constituent in some other material, the binding affects energy loss in collisions experienced by slow neutrons. For this reason, special cross-section data treatments are provided that take binding effects into account. To use this special treatment, an additional MT card is required, as shown below.

c ---------------------------------------------------------

c WATER for neutron transport (by mass fraction)

c (ignore H-2, H-3, O-17, and O-18)

c Specify S(alpha,beta) treatment for binding effects

c ---------------------------------------------------------

M1 1001.50c -0.11190 $ H-1 and mass fraction

8016.50c -0.88810 $ O-16 and mass fraction

MT1 lwtr.01

FM card is tally multiplier and has several applications:

Application (1): A simple multiplication normally used for source activity consideration:

c ---------------------------------------------------------

c 1.0 Ci neutron source (track-length estimation inside cell 2)

c ---------------------------------------------------------

F4:n 2

FM4 3.7E10

Application (2): Reaction rate calcualtion: FMn (C M R)

c ---------------------------------------------------------

c C= material density

c M= material number

c R= reaction number

c ---------------------------------------------------------

F4:n 2

FM4 (1.10 33 107) $ 107 for (n,alpha) reaction, 1.10 g/cc, material #33

Hope the above answer will help.

Thank you very much Nima Ghal-Eh. 

Thank you very much Nima Ghal-Eh. 

I think there is a confusion here.

MT is used by the ENDF format to indicate a reaction number, whereas MCNP call that number FM.

In the manual you can read :

"The nomenclature between MCNP and ENDF/B is inconsistent in that MCNP often refers to the number of the reaction type as R whereas ENDF/B uses MT, but they are the same. The problem arises because MCNP has an MT input card used for the S(α,β) thermal treatment."

In practice, in your FM card (the tally multiplier), you can use either an MT number (the ENDF reaction code) or an FM number (the MCNP reaction code).

You understand now why you were confused ?

For calculation the reaction rate of the 6Li(n,α)T and 7Li(n,n՜α)T, I have to use FM4 with reaction number of 205. The cell that F4 is calculated in it, is filled with natural Li or LiPB or Li2O , …

What is c=material density in this example?