ANGULAR-DISTRIBUTION OF BREMSSTRAHLUNG FROM 15-MEV ELECTRONS INCIDENT ON THICK TARGETS OF BE, AL, AND PB

Bremsstrahlung spectra from thick cylindrical targets of Be, Al, and Pb have been measured at angles of 0-degrees, 1-degrees, 2-degrees, 4-degrees, 10-degrees, 30-degrees, 60-degrees, and 90-degrees relative to the beam axis for electrons of 15-MeV incident energy. The spectra are absolute (photons per incident electron) and have a 145-keV lower-energy cutoff. The target thicknesses were nominally 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20-cm-diam by 25-cm-long cylindrical NaI detector. Measured spectra were corrected for pile-up, background, detector response, detector efficiency, attenuation in materials between the target and detector and collimator effects. Spectra were also calculated using the EGS4 Monte Carlo system for simulating the radiation transport. There was excellent agreement between the measured and calculated spectral shapes. The measured yield of photons per incident electron was 9% and 7% greater than the calculated yield for Be and Al, respectively, and 2% less for Pb, all with an uncertainty of +/- 5%. There was no significant angular variation in the ratio of the measured and calculated yields. The angular distributions of bremsstrahlung calculated using available analytical theories dropped off more quickly with angle than the measured distributions. The predictions of the theories would be improved by including target-scattered photons.