Quasi-monoenergetic neutron beam from Li target

Dear users,

I am trying to simulate the reaction 7Li(p,n)7Be on the Lithium target
with the protons in the energy range from 20-100 MeV. We are using
proton beam + Li target for the production of quasi-monoenergetic
neutron beam for cross-section measurements etc.
There is no quasi-monoenergetic peak at the energy of incident protons
in the results of the simulation. What am I doing wrong ?

Thank you,
Mitja

PS This is my inp file:
TITLE
Lithium target
GLOBAL 1.0 1.0
LOW-NEUT
DEFAULTS
PRECISIO
BEAM -.041 -1.0 -1.0 PROTON
BEAMPOS 0.0 0.0 0.0
GEOBEGIN
COMBNAME
     0 0 li_target
RPP concin -20. 20. -20. 20. -20. 20.
RPP det1 -1. 1. -1. 1. 5. 5.1
RCC target 0.0 0.0 1.0 0.0 0.0 0.7 5.0
END
BLKBODY 5 -concin
VOID 5 +concin -target -det1
TARGET 5 +target
DET 5 +det1
END
GEOEND
MATERIAL 3. 6.941 .534
LITHIUM
MAT-PROP 40.0 LITHIUM
MAT-PROP 10.0 LITHIUM
DPA-ENER
STERNHEI 3.1221 .1304 1.6397 .95136 2.4993
.14LITHIUM
ASSIGNMA BLCKHOLE BLKBODY
ASSIGNMA VACUUM VOID DET
ASSIGNMA LITHIUM TARGET
USRTRACK 1. NEUTRON -21. DET 1. 100.0f4_1
USRTRACK 0.05 1D-03 7.0&
RANDOMIZ 1.
START 1e6 0.0
STOP
Received on Fri Mar 06 2009 - 09:41:42 CET