Dear Raksha,
the issue is with your source routine. You left the default particle direction assignment as is:
TXFLK (NPFLKA) = UBEAM
TYFLK (NPFLKA) = VBEAM
TZFLK (NPFLKA) = WBEAM
This means the direction cosines UBEAM, VBEAM and WBEAM are taken from the BEAMPOS card (if it is not present, then the direction is along the positive z axis).
In the source routine the Isotropic angular divergence option set on the BEAM card is not taken into account. You need to specify it manually.
The easiest way is to replace the default 3 lines with the following one:
CALL RACO(TXFLK(NPFLKA),TYFLK(NPFLKA),TZFLK(NPFLKA))
With the RACO subroutine FLUKA will automatically sample a random direction from an isotropic distribution.
Cheers,
David