#include <stdlib.h>
#include <stdio.h>
#include <math.h>

int
findCAngle (float eg, float ee, float *thc)
{

  /* eg: incoming gamma ray energy */
  /* ee: interaction energy (energy deposited in crystal) */

  /* declarations */

  float egp, cosa;

  /* calc compton angle */

  egp = eg - ee;
  cosa = 1. + 0.511 / eg - 0.511 / egp;

  if (cosa > 1)
    cosa = 1;
  if (cosa < -1)
    cosa = -1;

  *thc = (float) acos (cosa);

#if(0)
  printf ("findCAngle: eg %f, ee %f, *thc %f\n", eg, ee, *thc);
#endif

  /* done */

  return (0);

}