From the access point of view, EHN2 is divided in three areas: It should be noted that the M2 beam can be operated in two main modes:
  1. As a muon beam with a maximum authorised flux of 2 108 muons per SPS cycle. In this case it is not possible to dump the beam before the exit of the experimental hall. Therefore the outside area downstream of the hall (zone PPE231) is included in the same interlock chain as the experimental zone PPE221 itself. The ground level in this zone has been increased such that the muon beam is finally dumped underground over a length of 1100 metres.
  2. As a hadron beam with a maximum authorised flux of 108 hadrons per SPS cycle. As the target and detectors of the experiment have a total thickness of about 20% of an interaction length, the experiment has been shielded by a 1.6 metres thick concrete wall on the control room (Salève side) of the hall. On the back (Jura) side of the hall there is no such shielding, due to lack of space. Therefore the area outside on the Jura side of zone PPE221 has been added to the PPE231 zone and thus included in the interlocked area. The last part of the beam zone PPE211 is adequately shielded on both sides and therefore the area on the Jura side of PPE221 can be left accessible with beam.

Please find below a schematic view of the situation in and around EHN2.


Due to the special layout of the M2 line, the muon flux above the beam can be quite significant. Therefore it is forbidden to access the crane when beam is on. For that reason the five access points to the crane have all been blocked by lids that are interlocked with the PPE211 zone. An example of such an access ladder is shown in the picture below:

A contact makes sure that the PPE211 area cannot be closed when the lid is open. As the lid is normally closed with a lock, a special key is required to open the lid. This key can be taken when the PPE211 door, shown in the picture below, is in key access or free access mode.

When PPE211 is in one of these modes, the push-button for crane key release, indicated on the picture below, lights up.

When this button is illiminated, one may push it and take one of the four keys for crane access. These keys have the same function (though a different format) as the keys for normal access to PPE211. With this key one may obtain access to the crane. A little yellow light on door PPE211 flashes when one of the five lids is open (like in case of an emergency passage). The zone can only be put back to closed mode when all lids are closed and all keys are back in position.

Near all access points to the crane, as well as next to door PPE211, an instruction sheet shows the location of the access points to the crane, together with instructions. See the picture below for an example:


Last updated on 6 April 2000 by L.Gatignon