Tagging Efficiency Measurements at High Intensities

The Swedish National Laboratory MAX-lab contains a facility for production of bremsstrahlung photons for photonuclear experiments. A so called "tagging technique" allows for a determination of the energy of the individual photons via a spectrometric measurement of the energy of the corresponding electron. Some of these photons are lost in a collimation process. The fraction of photons in the collimated beam divided by the number of particles that are detected in the spectrometer is called the tagging efficiency and is used to determine the photon flux that impinges on the target, which is a parameter that is needed in order to determine absolute cross sections. The tagging efficiency depends on various factors, such as radiator thickness, steering of the beam and collimator size, and is measured approximately once a day to make sure that the experimental setup does not change in time. The measurements are usually made at an intensity that is several orders of magnitude lower than during production runs, due to the fact that a detector has to be placed directly in the photon beam. It is assumed that the measured tagging efficiency is the same as the tagging efficiency at high intensities, but this has not been verified before for the upgraded facility that was built in 2005. This thesis presents measurements of the tagging efficiency at various intensities at the new facility, showing that the tagging efficiency is independent of intensity and that there is no systematic error in measuring it at much lower intensities than under normal experimental conditions. The measurements at high intensities were done using lead absorbers that attenuated the photon beam. The experimental setup and the analysis of the data is presented. The tagging efficiency was found to be, within error bars, independent of the intensity over three orders of magnitude, ranging up to approximately 4% of full intensity.