One can see in the following picture the particle probabilities in the positive or negative M2-Hadron-beam.
The probabilies are given by Lau Gatinon behind the T9 target. I simply added a decay of 1100m (@Na58-target) and assumed that all pions that decay from Kaons are kept in the beam. All other particles then protons Kaons and Pions are neglectible. That is why the sum of the three particle probabilites is 1. The peak in the Kaon probability gives the best beam energy for Kaon physics. To study physics with Kaons, one needs to "tag" the Kaons. This is done by two CEDAR detectors.
The two CEDAR detectors are placed about 50 meters ahead the NA 58 target and belong to the beam diagnostics of the secondary beams.
Below one can see the principle of the detector. It works very similar to a RICH detector.
The separation of the Cherenkov rings produced in he gas is done by a diaphragm in the focal plane
of the mirror. The separated photons are detected by 8 circulary positioned PMs behind the diaphragm.
One can adjust the opening of the diaphragm to set the optimum in separation and acceptance.
To oparate with different beammomenta one has also to adjust the pressure. The aluminum vessel
may work with up to 15 bar pressure to separate protons with 60 GeV/c. But usually one operates
next to 10 bar for K and Pi separation arround 200 GeV/c momentum.
Indeed the optical system is a bit more complicated. Two lenses are included to compensate some
optical effects. But if you want to know more about the details please feel free to enter the
download area with some sources to this topic.
For an optimal setting one has to perform a socalled pressurescan. One starts with high pressure and
reduces it step by step. During this reduction one counts the multiplicity events of the PMs.
Normalized to the beamflux. One gets a plot like the following.
There one can see the effects of different lid and pressure settings and find the optimal setting.
All this works only if some points are considered. All the PMs have to work uniformly.
Just in this case one can align the CEDAR in the beam. This is the second point. The beam must be
parallel to the optical line of the optical system. One can see this from the counting ratios of
the PMs (up to down and left to right PMs). The third point is the beam spread.
The more the beam is parallel the better the separation works. The entrance window of the
CEDARs is around 10cm wide. Usually no problem for a hadron beam. But we think that we have huge
problems with beam halo. This will be analized. The entrance window is for muon beams too small
and the CEDARs must be removed.
Currently the whole setup of the CEDARs up to the pressure scans is performed by CERN.
The correct diaphragm and pressure settings are choosen by us.
Since the CEDAR detectors are up to 25 years old, one had to check the PMs. The result one can
find in the PM test section.
To be able to record the informations in DAQ, we have to splitt the signals. More about the
electronics one can find here.
If you want to learn something about the quality of separation, please look at the
MC-page.
Last but not least, you may have a look into the CEDARs. One rendered movie shows the location of the CEDARs in the Building 888, where Na58 is located, and gives an impression of the dimensions of the experiment. (Many thanks to Stephane Platchkov for the AutoCAD files of the Na58 detektors). The other movie is a flight through the CEDAR itself. There you can see the inner optics of the detector. Both movies require the Xvid codec and are preliminary versions.