1. CEDAR MECHANICS Liverpool Conceptual Design (Subject to funding from STFC) 1 CEDAR Mechanics Conceptual Design

2. Design Considerations - Mechanical NA62 will use the West CEDAR filled with hydrogen at 4 bar. High beam flux (50 MHz K + ) requires existing PMTs to be replaced with faster photo-detectors. A high-intensity muon halo, accompanied by low-energy neutrons, surrounds the beam. Simulations indicate that a radius of 20 – 50 cm minimises background. The radius may vary with azimuth. Conceptual design locates the photo-detectors at a radius of 35 cm and associated electronics within an envelope of radius 50 cm. The existing protective and thermally-insulated metal cover around the nose section of the CEDAR cannot be used. Thermal isolation to minimise temperature gradients is vital. Serious design effort is needed if cooled (200 K) PMs are used. CEDAR Mechanics Conceptual Design 2

3. Design Considerations - Safety Safety considerations require a nitrogen blanket around the optical-readout electronics and HV to eliminate any possibility of an explosion in the event of a hydrogen leak from the CEDAR. The vacuum beampipe needs to be separated from any source of hydrogen by a nitrogen-filled enclosure and all potential hydrogen leaks monitored and connected to beam and power cut-outs. CERN flammable safety rules require all welds are 100% X-rayed. Care must be taken to avoid enclosures large enough to pose a safety risk when filled with gases other than air. The concept presented here has been discussed with Jonathan Gulley, the CERN flammable safety expert. CEDAR Mechanics Conceptual Design 3

4. 4 Orientation – Existing CEDAR The CEDAR nose has 8 PMTs surrounding a hydrogen-filled section of beampipe. Each PMT is rigidly mounted in close proximity to one of the eight quartz windows. The whole nose is surrounded by a thermally insulated, light-tight, metal box to give a well- protected, robust enclosure

5. Overview – Adapting the CEDAR Nose Remove current PMTs and location fixtures Re-use the nose: all internal optical components are optimised, and the seals on the quartz windows are perfectly good for hydrogen at 4 bar. The 8 new sets of photo-detectors are separated from the quartz windows in the nose and mirrors and lenses are required to focus the Cerenkov light onto them. The mechanical challenge is to preserve the current optical stability and ensure that all the Cerenkov light reaches the photo-detectors. This requires the precision mounting of the photo-detectors and optical components in a rigid, light-weight structure that can be precisely located relative to the quartz windows of the existing nose. Since the insulated nose cover cannot be used, sufficient insulation must be applied to the nose and surrounding region to minimise temperature gradients and fluctuations in the hydrogen filling of the CEDAR. CEDAR Mechanics Conceptual Design 5

6. Overview - Beampipe The section of hydrogen-filled beampipe attached to the CEDAR requires a new, thinner aluminium window. We suggest reducing the length of this pipe, since the upstream hydrogen does not contribute to the identification of K +. This will reduce (by a small amount) the beam scattering. A small, nitrogen-filled enclosure will separate this shortened length of pipe from the CERN vacuum beampipe. The lightweight structure housing the PMs will be mounted around the beampipe, but not supported off it. CEDAR Mechanics Conceptual Design 6

7. Photo-detector Support Structure The eight systems of photo-detectors and electronics will each be located at the end of a radial, tubular ‘spider’ arm The spider arms will be supported by a light-weight aluminium flange surrounding the beampipe A mirror to reflect the Cerenkov light radially outwards towards the photo-detectors will be located at the knee of each arm A lens (or mirror-cones) at the outer end of each arm will focus the light onto the photo-detectors Provision will be made for fine adjustments to the orientation of the mirror We have the technology in Liverpool to fabricate the spider from CFC or aluminium CEDAR Mechanics Conceptual Design 7

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10. Support Structure – Rigidity and Safety Thermally insulating material will fill the space between each of the 8 spider arms. A layer of insulation, enclosed by a tough CFC membrane, will cover the front and back surfaces to give a rigid, lightweight disc structure. This will ensure dimensional stability and compliance with flammable safety codes Installation: We propose to build the structure in two halves, which can be ‘clam-shelled’ around the beampipe Cables from the photo-detector electronics will be routed to the floor around the upstream wall of the support structure CEDAR Mechanics Conceptual Design 10

11. Support Structure –Design Flexibility The simple concept presented here has assumed radial symmetry to give a disc-structure, but the detailed design will be sensitive to the 3-dimensional radiation map around the beam pipe. Whilst the support structure is likely to be elliptical rather than circular, it will be possible to engineer a structure that accommodates each of the 8 sets of photo-detectors at or close to the optimum radial distance and azimuthal position. Complications will arise if the structure is non-planar and these should be avoided if possible. Provision will be made in the design to adjust the position of the support structure along the beam pipe so that light gathering is as efficient as possible while minimising background radiation. CEDAR Mechanics Conceptual Design 11

12. Alignment of Support Structure to CEDAR The disc structure will be supported from the floor, with provision to move it along the beampipe to its design location Small adjustments to its position vertically and horizontally relative to the beampipe will enable precise location It will be positioned as close to the CEDAR nose as is reasonably practicable, and precisely located by means of dowels. A lightweight, thermally-insulated, light-tight cylinder will enclose the region between the CEDAR nose and the spider knee-joints. This will maintain a dust-free, nitrogen atmosphere. Thermal insulation will be applied to the nose and surrounding region of the CEDAR to minimise temperature fluctuations of the hydrogen gas. CEDAR Mechanics Conceptual Design 12

13. Cooling of Photo-detectors and Electronics Provision will be made for stabilizing the temperature of the photo-detectors and for thermally isolating the photo-detectors and readout electronics. If liquid nitrogen cooling is required, current practice suggests a system of dewars, with an automatic filling system, mounted on a separate structure supported off the floor, with a cold finger from each dewar cooling the photo-detectors at the end of each spider arm. This is a significant complication. Liquid-nitrogen cooling will need substantial design input, as will the enclosures for the readout electronics. Particular attention will need to be paid to modelling the thermal environment and minimising any temperature gradients and thermal instability in the surrounding air. CEDAR Mechanics Conceptual Design 13

14. Safety considerations and Monitoring Monitoring, connected to an emergency cut-out system, is necessary for the pressure of hydrogen in the CEDAR and to test for leakage of hydrogen from the window seals into the light-tight cylindrical enclosure. The temperature of each readout package must be monitored. A supply of gaseous nitrogen is required to flow through the compartments housing the electronics and HV to ensure an atmosphere free of oxygen. This will eliminate any risk of explosion associated with a potential hydrogen leak. A small overpressure will ensure a nitrogen atmosphere surrounding the optical components in the spider arms and the light-tight cylinder and prevent any possibility of moisture condensing from the atmosphere. The light-tight cylinder will be fitted with a release valve to prevent build up of nitrogen pressure. The cylinder will incorporate lightweight panels spring- loaded onto a frame so that in the unlikely event of a CEDAR window seal failing catastrophically they will be ejected and the hydrogen gas will be able to escape rapidly into the atmosphere. It is suggested that the main body of the CEDAR is instrumented with photocouples so that a continuous readout of hydrogen temperature as a function of position is available for diagnostic purposes. CEDAR Mechanics Conceptual Design 14

15. Critical Information for Design Progress Choice of Photodetectors – Number, Size, Packing, – Power, Requirements for Cooling and Temperature Stabilisation Cerenkov Photons – Spatial and Angular envelope as a function of upstream position – Criteria for choice of 45 o mirror – Spatial and Angular envelope of photons after reflection – Criteria for choice of optical reflectors or lens Radiation Map – Determination of optimum location of photodetectors CEDAR Mechanics Conceptual Design 15

16. Summary The conceptual design has addressed the mechanical challenges involved in redesigning the optical interface to the PMs Preliminary ideas on installation and tooling have been addressed Safety requirements concerning flammable materials have been discussed with CERN Safety and addressed Detailed design work is needed on materials and the geometry of the structure, loading and supports, electronics housing, cooling and temperature stability, and cable loads and routing. Further design progress requires: – Choice of photodetectors – Spatial and angular envelope of upstream and reflected photons – Radiation map to determine optimum location of photodetectors CEDAR Mechanics Conceptual Design 16