Results

Using the parameters from tab. 2, intensities are calculated and are compared (fig. 4) with measurements of the bremsstrahlung spectra for nickel and diamond radiators obtained at Mainz. The crystal angles were aligned such that the discontinuity of the $[02\bar 2]$lattice vector was located around 350 MeV. For the incoherent intensity (fig. 4a) the Hubbell cross section (eq. 7) was employed for nickel whereas the incoherent contribution for total crystal intensity in fig. 4b stems from a calculation for carbon. Comparison of data and calculations on basis of absolute intensity are a more stringent test of the modelling then in terms of relative intensities $I^{\text{rel}}=I^{\text{coh}}/I^{\text{Ni}}$, which are shown in fig. 5. A check of the polarisation prediction is provided by a measurement of the beam asymmetry of coherent $\pi^0$ photoproduction[2]. The prediction is compared with the deduced photon polarisation and is in good agreement with the data for both collimation angles (fig. 6).

  

Figure: Comparison of data and predictions for the absolut intensities: a) $I^{\text {Ni}}$ with (top) and without (bottom) collimation, b) $I^{\text {cry}}$, c) relative intensity $I^{\text {rel}}=\frac {I^{\text {coh}}}{I^{\text {Ni}}}$ and d) polarisation

  

Figure: Calculation of absolute crystal intensity $I^{\text {cry}}=I^{\text {coh}}+I^{\text {inc}}+I^e$ compared to measurements of photon spectra at MAMI and TagX/Tokio (tab. 2)