DEO XGEM SHIELDING CERN / AD Bertrand LEFORT (TSO) 5/6/2012

Transcription

1 DEO XGEM SHIELDING CERN / AD Bertrand LEFORT (TSO) blefort@cern.ch 5/6/2012

2 Problematic XGEM The Antiproton Decelerator (AD) delivers anti-proton beams to five different experiments. The beam is extracted to one of the experiments in a spill of a few hundred nanoseconds, containing about 30 million antiprotons per spill. Gas electron multiplier (GEM) detectors are used for profile measurement in the experimental areas of the AD. The GEM is located inside a pendulum that can be moved into the beam, but at 5MeV kinetic energy the assembly acts almost as a beam stopper for antiprotons. XGEM on its pendulum The GEM-foil itself consists of a double sided 50 μm copper-clad polymer foil, perforated with a high density of chemically etched holes (typically ten thousand per square centimeter). On application of a potential difference between the two sides, the foil acts as a charge multiplier for electrons produced by ionization in the gas. The beam interacts with the material of the GEM. This destruction creates radiations all around the detector. DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 1

3 Radiation Problem The radiation generated by the annihilations are nearby walk-paths and experimental barracks. Applying the ALARA principle, we must reduced this amount of radiation and make them as low as reasonably achievable. DE0 XGEM XGEM location inside the DE0 transfer line. As shown on the drawing the DEO GEM is close to footbridges (green parts) where people can stand during beam time. For information, the distance between GEM and the nearest walk-path is about 2 meters. DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 2

4 Shielding DE0 GEM Beam line topology The following drawing represents the ejection line and the precise location of the DEO GEM. GEM DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 3

5 DEO area In order to understand the problematic, it is necessary to have a complete view of the DEO area where distances to walls and obstacles clearly appear. In this first illustration, the complete area is represented. The structures that may limit the size of the elements that can be craned into the area are also represented (Water pipes, foot-bridge, radiation detector, geodesic reference...). DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 4

6 The previous illustration shows a simplified view of the DEO ejection line. The vacuum chamber that come from the AD machine enter the area by the left, goes through a first magnet (in blue), the GEM that need to be shielded (in white) and finally goes through the bending magnet (red and metallic) that allow to send the beam either to an experiment or another. GEM body The GEM has an embedded ionic-pump (on the back) that is sustained by a metallic structure fixed on the back wall top. Therefore it is impossible to directly cover the GEM with a concrete block. Area components A geodesic reference on the back wall and a cable tray (in green) are to be considered in the final solution. They can t be removed. The following top view show clearly those two elements: DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 5

7 The Arrows painted on the floor shows paths used. They must stay available after the shielding installation: they are part of the patrol path and are needed for maintenance. Beam Line Dimensions The following layout shows the beam line devices and the GEM detector dimensions. DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 6

8 2050 mm 486 mm 1285 mm Shielding Blocks at CERN The following table gather the dimensions of the shielding blocks available at CERN: Wide Height Depth CERN standard shielding block size. DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 7

9 Shielding needs RP group has ran simulations. In order to obtain one order of magnitude of attenuation, 40 cm of concrete must be installed over the GEM. Distance GEM-Shielding GEM specialists had been contacted. The distance between the shielding and the GEM is limited by: The Ribbons cable located on the top of the GEM (position and height change with the GEM position) The Proximity electronic box The gas tubes The compressed air circuit and valves. Shielding Length The purpose of this shielding is mainly to contain the radiations inside the DE0 area. The distance between the GEM and the closest foot-bridge is 2 meters. It corresponds to the closest point when someone can stand during beam time. As shown on the 3D model, the only way to completely protect the footbridge would be to insert shielding blocks under it. In other words it would require to have the footbridge removed, which is unacceptable. The conclusion is that we have to go as far as we can under the footbridge. The other footbridge is farther, more than 5 m. The radiation decaying with the square root of the distance, the factor 10 attenuation we obtain with 40 cm of concrete is already achieved. Nevertheless we will try to reduce even more this amount installing additional shielding near the GEM. DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 8

10 Proposed Solution There are two way to setup a shielding over the DE0 XGEM, one using a 3/4 legs metallic structure that support the shielding and the other one using a 2 leg structure that is bolted to the back wall. The next illustration shows a view with the 3 leg structure that could be loaded with a concrete block. As It can be seen, this solution impose to be close to the vacuum chamber (aluminum must be used), has a quite important footprint and only allow to support one single concrete block. The next solution is more efficient in terms of loads and footprint. DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 9

11 As It can be seen, this solution is compatible with the requirement we defined in the beginning of this document: access paths stay unblocked. The other major improvement compared to the first solution is that it can support more than one block. In fact with those dimensions 3 blocks can be installed. This would imply that we need to use steel but with this configuration, we are far enough from the beam line so there is no need to use non magnetic compounds. DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 10

12 Another interesting point that reduces the radiation amount is that this configuration allows the blocks to go under the footbridge. As you can see, the solid angle is greatly reduced inserting part of the block under the footbridge. The insertion distance is not determined, it will depends on the technique used to crane the block into the area. With this solution, the needed blocks are: Width Height Depth Number of blocks The small footprint of this solution allows to add some more shielding in order to reduce the radiation that goes toward the other footbridge. A 1600x800x400 block is set on the floor, in front of the GEM. DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 11

13 This final configuration should achieve the RP specification and also respect the operational needs. The following view shows the whole area (the door has been removed). DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 12

14 Conclusion We found a valid solution for the shielding of the DE0 area that is simple to realize and to install. This solution fulfill the requirements we determined and do not interfere with daily operation needs. This solution is easily removable for magnets and vacuum chamber maintenance. In this document we voluntarily do not detailed the structure crossbeam section. The total structure load and tolerance must be computed by a mechanical engineer. Once computed he will determine the type and the section of the crossbeam we need and also the characteristic of the socket used to bolt this structure to the wall. DEO XGEM Shielding / Bertrand LEFORT (BE/OP). 13