Measurement of gas bremsstrahlung from electron storage rings

Bremsstrahlung is produced in electron storage rings when the particle beam interacts with the storage ring components or with the residual gas molecules in the storage ring vacuum. The interaction of the particle beam with the gas molecules occurs continuously during storage ring operation. At straight sections of a storage ring, the gas bremsstrahlung contribution from each interaction adds up to produce a narrow monodirectional photon beam that travels down the beamlines. At the Advanced Photon Source (APS), where the stored electron beam energy is 7 GeV, gas bremsstrahlung generated in the straight sections of the insertion device beamlines, which is a total of 15.38 m in effective length, can be significant. This paper presents the results of the bremsstrahlung spectrum and energy measurements from the insertion device beamlines of the APS using a hermetic lead glass calorimeter. The calorimeter consists of 25 lead glass blocks, each 6.3 cm × 6.3 cm × 35 cm, stacked into a 5 × 5 array. Phototubes connected to the lead glass blocks collected the signals. The calorimeter is calibrated using the maximum energy of the observed bremsstrahlung spectrum. The total energy radiated and the bremsstrahlung spectrum were measured as a function of the storage ring beam current for both electrons and positrons of 7 GeV energy. Vacuum in the straight section was continuously monitored, and measurements to determine the nature of the residual gas inside the storage ring were performed. The measured bremsstrahlung spectra are fitted to a function that shows an approximate 1E behavior. The gas bremsstrahlung rate from the APS undulator beamline straight path of 15.38 m is measured as 60.0 ± 2.0 GeV/s/nT/mA. Significant contribution of bremsstrahlung from sources other than the residual gas molecules is observed. The maximum bremsstrahlung energy measured in a beamline during the experiment is 369.0 ± 22.0 GeV/s/nT/mA.