The experiment will be set up at the E5 channel. A sketch of the set-up is given in Figure 5. The different parts of the experimental set-up and their role in the experiment will be described below.
|Figure 5: Set-up of the experiment at the channel at PSI. A very similar set-up will also be used outside the pion area for the tuning of the spectrometer with X-rays from an ECR-source.|
A proton beam current of 1.5 mA and the full thickness of target E are required and are taken as basis of our rate estimates. The high beam current can also be fully exploited as the experiment is not rate limited. The beam momentum is 110 MeV/c . The cyclotron trap will be tuned to a momentum of accepted particles of about 82 MeV/c. The particles will be stopped in a thin walled (Kapton) cylindrical gas target (diameter 60 , length 200 ) with the cylinder axis coinciding with the axis of the magnetic field. The particle density of the target gas corresponds to a pressure range from 1 bar to 40 bar at room temperature. The required particle densities can be reached by cooling the target. In order to optimize the stops in the target gas as compared to the ones in the walls the thickness of the cylinder wall is restricted to a thickness of 50 m of Kapton. The X-ray window at the front plane of the target cylinder will be made out of 5 Mylar supported by a stainless steel grid.
The magnetic field configuration and the deceleration scheme of the cyclotron trap has been optimized with tracking calculations. These calculations predicted a rate of about pion stops for hydrogen at a pressure of 1 . In preparation of the present proposal a stop rate for pions has been determined in deuterium gas at a pressure of measuring the pionic transition (see chapter 3.3 and Figure 6). It can be safely assumed that the pion stop rate is proportional to the particle density in the pressure region considered and that there is no difference in stopping power between the two hydrogen iosotopes. The measured number of about stopped pions agrees well with the value given above if one scales the pressures.
Muons will be produced by decays of pions orbiting inside the cyclotron trap. This results in a muon stop rate which is one order of magnitude smaller than the pion stop rate. This reduction is compensated, however, by a higher X-ray yields in the atoms.
It is foreseen to shim the so-called form iron of the cyclotron trap in order to increase the injection efficiency by about a factor of two with a corresponding gain in stopping power.