Monte Carlo: Launching collimated photons

Launching collimated photons involves setting the initial trajectory of photons in one direction. The following code would launch photons along the z-axis, entering the tissue at the origin.

/**** LAUNCH 
   Initialize photon position and trajectory.
   Implements a collimated source.
*****/
i_photon += 1;	/* increment photon count */
W = 1.0;                    /* set photon weight to one */
photon_status = ALIVE;      /* Launch an ALIVE photon */

x = 0;                      /* Set photon position to origin. */
y = 0;
z = 0;
ux = 0;   /* No x-axis trajectory */
uy = 0;   /* No y-axis trajectory */
uz = 1.0; /* All trajectory is along z-axis */