Measurement of bremsstrahlung spectra from 25 MeV electrons on Ta as a function of radiator thickness and emission angle

Abstract

The effects of radiator thickness and angle of emission on the energy spectra of photons produced by 25 MeV electrons were studied. Bremsstrahlung energy spectra at four different angles relative to a beam of electrons incident on Ta radiators of three different thicknesses were measured by observing time of flight spectra of photoneutrons from a moveable liquid deuterium target. The radiators were 1 8 , 1 2 and 1 radiation length thick. Observations were made at 0°, 1.7°, 3.4° and 5.1°. The photon flux at 5.1° was about 10% of the flux in the forward direction, where the maximum flux occurred. The angular distribution of mean intensity of photons is nearly the same for all the radiators, and is a consequence of multiple scattering of the incident electrons combined with the intrinsic angular distribution of bremsstrahlung. The thicker radiators produced more intense but softer photon spectra, that is, having relatively fewer high energy photons. Spectra from a given radiator showed only slight softening with increasing angle of emission. This is consistent with multiple scattering of the electrons before they radiate. However, the observed angular width of the photon flux distribution is smaller than multiple scattering theory predicts for electrons. Agreement with the data is obtained only when the characteristics of individual bremsstrahlung events are taken into account. For the thinnest radiator the measured width of the angular distribution is within 7% of the value calculated in this way. Monte Carlo calculations by others agree well with our measurements of spectral hardness and of the angular distribution of average intensity.