The tagging detector

The tagging detector is one of the most delicate parts of this experiment.
At ESRF the only practical possibility to tag the produced high energy photon by measuring the position of the electron that has undergone the Compton scattering process is by means of an internal tagger. This implies that the device has to be extremely compact, reside inside the vacuum chamber of the storage ring, as close as possible to the main electron orbit, and must work in the environment of the storage ring, where high fluxes of x-rays are produced in the dipoles and in the ondulators (synchrotron radiation).
The detector consist of 10 plastic scintillators for triggering purposes and 128 silicon microstrips (300 microns width) that will provide the position measurement of the scattered electron. The expected energy resolution of the tagger is influenced by the geometrical properties of the primary beam and by the machine optics. In the final configuration en energy resolution of 16 MeV (FWHM) for the produced photon is expected.

the above figure shows (left) a a typical energy spectrum with maximum energy o 1.1 GeV as measured by the silicon microstrips. The high energy tail is due to bremsstrahlung on the residual gas of ESRF. The green spectrum (right) shows the resolution obtained corresponding to the design goal of 16 MeV (FWHM)