Photon Flux Monitor

In the BGO-OD experiment photons, generated by bremsstrahlung form a electron beam, are used for meson photo production. To measure the absolute cross section these photonic reactions the absolute photon flux is needed. For this the Photon Flux Monitor has been build. The detector consists of two parts: one that measures the absolute rate (GIM) and one that measures only a relative rate (FluMo). The detector is divided in these two parts because the expected photon rates of up to 50MHz is to much to be counted by one detector. The idea is to count the absolute number of photons at low rates with the GIM and determine the fraction of the rate measured by the FluMo. So that at high rates the real rate can be recalculated from the rate measured by FluMo.

Gamma Intensity Monitor (GIM)

The GIM is made of lead glass and uses the cherenkov effect to detect charged particles from the shower generated by an incoming photon. It has a photon detection efficiency of nearly 100%. The GIM is build out of just one single lead glass block with a height, width and depth of 14 × 14 × 28 cm³ and one 2“ phototube (Hamamatsu R2083). Since it suffers from radiation damage it can not be placed continuously in the photon beam. Instead FluMo is used which measures a constant fraction of the photon flux.

Flux Monitor (FluMo)

The FluMo detector is build out of five Saint-Gobain BC-404 scintillators, which are read out with ET-Enterprise 9111513 photomultipliers. A fixed percentage of photons from the photon beam is converted into electron-positron pairs while passing through the experiment. These charged particles will then be detected by the scintillators. The discriminator thresholds are set in order to assure the detection of both particles. Events where only one electron or positron impinged upon the detector are discarded.

To determine the exact fraction of measured photons the absolute flux is measured with the GIM in dedicated low intensity runs. For each event it is determined whether it was observed in FluMo.

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