The replacement of the previous VErtex LOcator (VELO) detector with a new high-speed hybrid pixel detector was one of the key milestones for the LHCb collaboration in 2022 (see also previous EP newsletter article). The upgrade was part of an R&D programme that allows the LHCb detector to cope with instantaneous luminosities of L = 2 × 10³³ cm⁻²s⁻¹ during Run 3. 

The construction of the new VELO detector was an intense collaborative effort including the Universities of Bristol, Glasgow, Liverpool, Oxford and Warwick, Moscow State University, University of Manchester, Universidade de Santiago de Compostela, Instituto Galego de Física de Altas Enerxías, AGH University (Poland), Nikhef, UFRJ (Brazil) and CERN. Module construction started in November 2021 at the University of Manchester (UK) and NIKHEF (NL).  The modules were collected at the University of Liverpool together with precisely machined bases from the University of Oxford, where they were integrated together with the complete cooling system and electronics umbilicals onto the vacuum compatible mechanical supports, forming two complete detector halves.  Following functionality and safety testing, the two halves were shipped to CERN where they underwent final checks.  In April-May 2022, just before the start of the 2022 run, they were then both inserted into the RF boxes in the primary vacuum of the LHC.

The VELO is a mobile detector, with the ability to retract the two halves from the beams to a safe position while the protons enter the LHC.  However, once the beams are in stable collisions, the VELO can be brought much closer, enabling it to reconstruct secondary decay vertices with unprecedented precision.  Even though all the calculations predicted that this could be done safely, the first time this procedure was done it was carried out gradually over a period of weeks during the 2022 run.  In October 2022, the VELO reached its design position of just 5.1 mm from the beams, after which the closing could be exercised routinely, and the commissioning of LHCb started in earnest.   

During the 2022-23 LHC shut down period, LHCb had to carry out work around the beam pipe. When this is done, the VELO vacuum volumes are filled with ultra pure neon, to respect safety standards. It was during this period that a very unfortunate incident occurred.  During a warm-up of the VELO, certain equipment failed, and the actions of the pumps around the VELO resulted in a build up of differential pressure between the boxes containing the VELO and the primary LHC vacuum.  The final pressure exceeded the specifications by a factor of 10, resulting in a plastic deformation of the thin box surfaces facing the LHC beams.  Luckily, the sense of the deformation was in a direction away from the delicate sensor modules, and investigations quickly showed that the VELO detector was not damaged in any way.  However, in order to respect the clearances around the beam, it was no longer possible to fully close the VELO boxes around the beams, and hence the data acquired by LHCb in 2023 was of reduced quality.  The decision was taken to plan a complete replacement of the RF boxes during the 2023-2024 shutdown period.

The replacement is a complex task which requires a complete dismantling of all the beam pipe, vacuum, and detector equipment around the LHCb interaction region to be able to access the boxes inside the vacuum tank.  

In a first step, the two spare VELO RF boxes were retrieved from storage, visually inspected (fig. 1) and prepared for installation by the CERN vacuum group (TE-VSC) in collaboration with the EP department and LHCb. One of the two boxes was yet to be coated with the standard LHC low-temperature non-evaporable getter compound (so-called NEG), composed of the Ti, Zr and V elements. This coating is applied to surfaces in the LHC that are exposed to electron, photon, and ion bombardment induced by the intense bunched beams of the machine. It protects against beam instabilities by guaranteeing a sufficiently low secondary electron yield. The beam vacuum side of the fragile, 250 µm thick, corrugated aluminum box was carefully cleaned (fig. 2) and successfully coated (fig. 3) with a layer of approximately 200 to 500 nm NEG by the CERN TE-VSC group. The two boxes were then ready to be transported to and installed at LHC Interaction Point 8.

At the same time, intense preparations were going on underground to prepare for the RF box replacement. The vacuum equipment and beampipe sections, many subdetectors and much of the equipment close to the VELO had to be completely dismantled and removed to give enough room for the RF boxes to pass out of the front of the tank. Teams worked around the clock to perform the decabling, dressed in special suits to avoid radioactive contamination. After all the preparation, the vacuum flanges could finally be removed, and the detector module halves extracted and placed in sealed baths for safe storage. The team from TE-VSC also performed an additional maintenance task, replacing the 200 kg ion pumps which are normally inaccessible under the VELO and were nearing the end of their lifetime.  The old RF foils were then pushed tightly together to allow them to be pulled out of the front of the VELO tank with specialised equipment. After the motion system was inspected and refurbished, the new foils could finally be installed, and the motion system could be used to close them completely for the first time. This allowed the work of recommissioning the vacuum system and rebuilding the beam pipe to start. This will be the main task of the coming weeks, after which the subsystems will be reinstalled and commissioned.

Figure 1: The vacuum chamber containing the spare VELO “C1 box” is being handled in building 113 to extract and visually inspect the box. Left picture, from left to right: Josef Sestak, Raphael Dumps. Right picture, from left to right: Yorick Delaup, Josef Sestak, Raphael Dumps.

Figure 2: The spare VELO “A3 box” being carefully cleaned in building 107. Top picture: Pierre Maurin.

Figure 3: The spare VELO “A3 box” after successfully applying the NEG coating in building 181. Middle picture: Raphael Dumps.

Simultaneously, there is a separate but equally important task which is being carried out 1000 km away. Due to the RF box replacement, the VELO no longer has spares of these delicate and crucial pieces. The Nikhef group in Amsterdam has stepped in and is in the process of manufacturing a spare pair of boxes. This work requires the expertise of the group as well as the large 5-axis simultaneous milling machine which was originally installed at Nikhef to support this engineering work. Each box takes approximately 8 months to manufacture, and the C side is currently in the advanced stages of completion [see Figure 4]. The inside is finished and is currently on a mould for support. The next step is to finish the outside, with the aim to go to a thickness of 250 micrometers in the thinnest region. “The original technicians are now retired”, says Freek Sanders, head of the Nikhef Mechanical Technology Department, “but we have taken the opportunity for a new generation to grow into these complex techniques”.

Figure 4: Stan Heijen (left) and Espen de Wit cleaning the inside of the completed box, in preparation for metrology.

Proposed almost 17 years ago, with a technical design report published in 2013 and full approval the following year, the VELO upgrade reflects the dedication and work of more than 150 people at 13 institutes. The described efforts reflect the dedication and commitment of the collaboration that has worked for the design and construction of this detector. With the timely progress of the repair work, everyone in the LHCb collaboration is therefore looking forward to seeing the first data from the new detectors and continuing the success of the LHC’s world-leading flavour-physics programme.

Note: The authors would like to thank their colleagues Raphael Dumps, Pierre Maurin, Josef Sestak and Yorick Delaup for their thoughtful comments and for kindly providing the pictures copyright.