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

2. 2 Orientation – Existing CEDAR The CEDAR_West nose has 8 PMTs surrounding a Nitrogen-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 environment with minimal temperature fluctuations.

3. Design Considerations NA62 will use a CEDAR Cerenkov counter filled with hydrogen at 3.6bar. Electronics, and the regions between the CEDAR and the vacuum beampipes, will be enclosed in a nitrogen atmosphere. High beam flux (50 MHz K + ) requires existing PMTs to be replaced with faster photodetectors and electronics. Conceptual design locates the new photo-detectors at a radius of 30 cm, and associated electronics within an envelope of radius 50 cm. This is consistent with simulation of background radiation. Modest heat-flow is required to maintain a constant temperature. The existing protective and thermally-insulated metal cover around the nose section of the CEDAR will be replaced. Thermal modelling is necessary to minimise temperature gradients. CEDAR Mechanics Conceptual Design 3

4. Photodetector Support Structure The eight sets of photo-detectors & electronics will each be located in pods within a latticework support structure The support structure will be built in two halves for ease of assembly around the beampipe and dowelled to a rigid cylinder permanently attached to the CEDAR flange Cerenkov light is reflected radially outwards by 8 ellipsoidal mirrors and channelled to the active surfaces of the 8 sets of PMTs using light-collection cones Provision will be made for fine adjustments to the orientation of the mirrors CEDAR Mechanics Conceptual Design 4

5. Figure 2: Conceptual design model for CEDAR Mechanics. The support structure for the 8 sets of mirrors, light-collection cones and photodetectors is an openwork structure built in two halves and enclosed by a thermally insulated vessel flushed with Nitrogen gas and maintained at constant temperature. Optical performance will be monitored with UV LEDs situated near the quartz windows.

6. Nitrogen-flushed gas enclosure The section of hydrogen-filled beampipe attached to the CEDAR will be shortened and fitted with a thinner aluminium window. The support structure will be enclosed by an insulated cylinder flushed with gaseous nitrogen. This stretches from the CEDAR flange to enclose the space separating the CEDAR from the upstream vacuum beampipe. [Is this acceptable to CERN Safety?] A second, smaller enclosure will surround the space separating the CEDAR from the downstream vacuum beampipe. Nitrogen will be fed into the enclosure through the 8 panels external to the electronics pods and be at a slight over-pressure. Temperature stability will be maintained by heat transfer to/from a heat exchanger through which the nitrogen flows. CEDAR Mechanics Conceptual Design 6

7. Fig 2: Dimensioned Cross-sectioned Line Drawing of Conceptual Design Model

8. Safety considerations and Monitoring All PMTs and front-end electronics will be situated inside the insulated cylinder within a nitrogen atmosphere Cabling will be attached to the outer surface of the cylinder with patch panels at either end - the 8 pods and on the floor Power will be cut if the nitrogen pressure drops, or if hydrogen is detected within either enclosure above a concentration of 0.5% [Is this reasonable?] Temperature stability of the Nitrogen enclosure will be maintained to ± 0.5 o C [Is this reasonable?] A set of UV LEDs will be situated on the nose symmetrically about the quartz windows and flashed to monitor the long-term optical performance of the mirrors and light-collection cones CEDAR Mechanics Conceptual Design 8

9. Summary The conceptual design has addressed: – Mechanical challenges involved in redesigning the optical interface to the photodetectors – Safety requirements concerning flammable materials – Temperature stability and monitoring – Preliminary ideas on installation Further design progress requires: – Choice of photodetectors – Approval of conceptual design including Nitrogen enclosure CEDAR Mechanics Conceptual Design 9