ASACUSA XGEM SHIELDING

Transcription

1 ASACUSA 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. 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. ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 1

3 ASACUSA XGEM XGEM location inside the ASACUSA experimental area. As seen on the drawing the ASACUSA GEM is close to many points where people can stand during beam time. For information, the distance between GEM and the nearest walk-path is about 3 meters. ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 2

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

5 Beam Line Dimensions The following layout shows the beam line and the GEM detector dimensions. 2050mm 1285mm 486mm Distance to walls and path-way The vacuum chamber is centered in the ASACUSA area and distance to walls is 2850mm (TBC). On one side a laser hut occupy one meter wide (TBC). On both side a 800 mm path-way must be left for circulation and evacuation purpose. The GEM is 1285mm wide. Using a security margin we can considerate that the overall GEM dimension is 1700mm wide. The space left for shielding is then 1300mm ( = 1300). In other words, 650mm on each side. Shielding Blocks at CERN The following table gather the dimensions of the shielding blocks available at CERN: ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 4

6 Wide Height Depth CERN standard shielding block size. Shielding needs RP group has ran simulations. In order to obtain one order of magnitude of radiation attenuation, 40 cm of concrete must be installed around the GEM. Distance GEM-Shielding GEM specialists has 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. All those components should fit in the 415mm security margin we already have added to the GEM wideness dimension. Even if the power dissipation should be closely checked, the tunnel we want to build should create natural convection. ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 5

7 Shielding basic components According to GEM dimensions and shielding requirements the basic shielding component are the followings. Shielding basics blocks The T shape in the center represents the GEM contour (the vacuum chamber is also represented). On each sides, at 200 mm, the walls are made of two blocks. The top shielding is made using a 800 mm thick blocks. The small blocks position (below or above medium sized block) is still TBC. Blocks list with dimensions sorted in function of the spatial orientation: Width Height Depth Number of blocks ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 6

8 Shielding Length The purpose of this shielding is mainly to contain the radiations inside the experimental area. The distance between the GEM and the ASACUSA Door is 18 meters. The door is 3m height and the vacuum chamber center is located at 1200 mm from the ground. 600 mm α 3 m 18 m ASACUSA DOOR Shielding length needs. Based on those figures, the theoretical shielding length should be 6m. This result is unacceptable due to the fact that we can not block the access to the RFQD device. A good compromise would be that the shield cover the two next quadrupoles and the 2 horizontal correctors (2.5m). This could be achieve using 3 walls of 800mm each. On the other side (toward ATRAP laser hut), the problem is the same. The radiation must be contained inside the experimental area. Distance to ASACUSA back wall. The maths stay the same and the needed shielding length is 833 mm. ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 7

9 Primary Solution Using all the previous results. The following shielding should be implemented. Primary solution. With this configuration, RP requirements are fully achieved and the GEM access is still possible. The small block position (below or above big block) is still TBC. The union of the two last blocks should be aligned with the GEM center axis. Blocks list with dimensions sorted in function of the spatial orientation: Width Height Depth Number of blocks ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 8

10 Artistic view Artistic view of the primary shielding configuration in ASACUSA area. Secondary Solution Using all the previous results and considering that the opposite walls are far from the GEM (radiation-wise). The following shielding could be implemented. This solution has a lower footprint therefore it benefit to ASACUSA experimental area users. ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 9

11 Secondary solution. With this configuration, RP requirements are also achieved and the GEM access is still possible. The small block position (below or above big block) is still TBC. The union of the two vertical blocks should be aligned with the GEM center axis. Blocks list with dimensions sorted in function of the spatial orientation: lajos ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 10

12 Artistic view Artistic view of the secondary shielding configuration in ASACUSA area. Possible Milestone As shown in this diagram. This solution could be implemented in less than two weeks. ASACUSA XGEM Shielding / Bertrand LEFORT (BE/OP). 11