THE SPECTRUM AND ANGULAR-DISTRIBUTION OF X-RAYS SCATTERED FROM A WATER PHANTOM

To calculate the response of an image receptor to the x rays emerging from a scattering medium, it is necessary to know the x-ray spectrum a nd intensity as a function of the angle of incidence on the receptor. To permit this calculation for any x-ray spectrum incident on a medium , these functions must be known for monoenergetic x rays. For monoener getic x rays in the range 20-70 keV we have measured with a high-purit y germanium detector the spectrum,and intensity of x rays emitted from a water phantom at angles of 0 degrees-50 degrees to the direction Of the primary beam. The spectrum and intensity of emitted x rays have a lso been calculated by the Monte Carlo method. At small exit angles, m ost of the x rays have energies close to the incident energy. As the e xit angle increases, the fraction of multiply scattered x rays increas es. At very large exit angles, the dominant feature of the spectrum is the peak due to these multiply scattered x rays. For small scattering angles the Monte Carlo calculations are in good agreement with the me asurements over the range of energies. For large scattering angles the scattered photon fluence predicted by Monte Carlo modeling is consist ently lower than the measurement in the region just below the full ene rgy peak. The cause of the discrepancies is not fully understood, but cannot be accounted for by Compton broadening alone. An alternate appr oach to model incoherent scattering is proposed.