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 should have a photon detection efficiency of nearly 100%. The detector is made of lead glass and uses the cherenkov effect to detect charged particles from the shower generated by an incoming photon. The GIM is build out of just one single lead glass block and one 2“ phototube (Hamamatsu R2083). That way a booster base with separate HV connections for the last 3 dynodes can be used as voltage divider for the photomultiplier. This ensures better countrate stability as active or passive bases, because at high rates capacitors (in a passive base) or transistors (in a active base) can’t deliver enough current and so the voltage at the dynodes will drop. When the voltage at the dynodes drops, the amplification drops as well and the signal at the output will be smaller.

Flux Monitor (FluMo)

The FluMo detector is build out of three Saint-Gobain BC-404 scintillator and copper foil between the first and second scintillator, the scintillator have a high efficency for electrons. A fixed percentage of incoming photons will convert into electron-positron pairs within the copper foil. These charged particles will then be detected by the two following scintillators. To not count charged particles not comming from the converter, there is a veto scintillator in front of it. Also only electrons from pair production, and not from compton scattering, should be counted. Therefor only events with an energy deposit of two electrons should be counted.

Current Status

Both parts of the Photon Flux Monitor are installed in the experiment and were used during several beam times. The detector is working but some improvements are still needed.

The FluMo detector needs improved photomultiplier bases, because with the current ones the detector saturates at 10 MHz. These bases are currently produced and will be installed until end of 2012.

The GIM detector currently suffers from radiation damage, therefore a new detector material for this detector is investigated.