Triggering hadrons

Two trigger conditions have been realized to detect protons and pions, assuring the minimum background contamination.

According to simulation the most important source of background is due to electrons coming from pair production and Compton scattering processes occurring in the experimental target.

Therefore the double lead sandwich, whose total thickness corresponds to $\simeq$3 radiation lengths, has been designed to absorb most of the background.

On the contrary, almost all the pions and high energy protons are completely transmitted, giving a signal from all detectors. Therefore a good trigger for high energy hadrons has been obtained by the coincidence of the 3 quadrants detectors Q$_1$, Q$_2$, Q$_3$.

The low energy protons stopped inside MIDAS are triggered by the Q$_1$-Q$_2$ coincidence. These particles have a large energy loss rate, so the detection threshold can be set high enough to cut most of the electromagnetic background.

Simulations show that the above triggers allow to detect protons and pions with energy T$>$70 MeV and T$>$50 MeV respectively, while the electron background is suppressed with an efficiency of about 99%.

The remaining background mainly comes from pair production processes. In most of the cases only one of the two electrons enters MIDAS, while the other is emitted at a very low angle and then detected by the Cerenkov counter. Therefore this background can be eliminated requiring an anticoincidence between the Cerenkov signal and the MIDAS trigger.

With the previous configuration the experimental trigger rate due to background is estimated to be only $\simeq$0.5 times the pion count rate and mainly associated to Compton electrons.

These events can be discriminated off-line by the pulse height analysis.