In LS2 the LHCb detector will go through a metamorphosis. To facilitate running at five times larger interaction rate than in run 2 all readout electronics will be replaced. For LHCb’s vertex detector, the so-called ‘vertex locator’ or VELO, the upgrade is more than sophisticated electronics. To cope with the larger track density and improve track resolution the silicon strip detector — in its characteristic r-φ geometry that served so well in Run-1 and 2 – is replaced by a hybrid pixel detector. Its sensitive elements will be just over 5 mm from the LHCb beams, the closest of any vertex detector at the LHC.

To build a detector that can survive the harsh radiation environment and cope with the unprecedentedly large data rate has been a tour the force by a large international collaboration. The new VELO’s 55 µm square pixel sensors are bump-bonded to the VeloPix ASIC. The chip is developed by CERN and Nikhef engineers within the Medipix collaboration. The main difference with its close cousin ‘Timepix3’ is that it can read out much more data per second.

These state-of-the art detectors are cooled by silicon micro-channel plates, a technology pioneered by NA62. Engineers and physicists from CERN and Oxford have further developed this technique to allow the use of bi-phase CO₂ as a refrigerant to cool the detectors stably down to −30 ^oC. The CO₂ runs through minuscule channels etched inside  a 0.5 mm thick plate that must withstand CO₂ pressures well above 65 bar, the boiling pressure of CO₂ at room temperature. As the plate serves also as support for the sensors and the PCBs that connect them to the control and DAQ systems, the Nikhef design of a structure that minimizes movements under large changes in temperature has been a critical step as well.

Prototype vertex locator (VELO) pixel modules were developed and tested in a beam in the North Area last year ahead of the upgrade (Image: Julien Marius Ordan/CERN).

The construction of the 52 detector modules takes place in workshops at Nikhef and the University of Manchester. Last October the first three pre-production modules were tested in a test beam at CERN (see picture). The tests have led to some small modifications in the design. Mass production in Amsterdam and Manchester will start any time now and is expected to finish by the end of the summer. Finished modules will be shipped to yet another participating partner institute, the University of Liverpool, where they will be mounted on    the two detector halves that make up the final VELO.

Another critical and unique element of the LHCb detector is the intriguingly shaped vacuum envelope, also known as ‘the RF boxes’. (See picture). These separate the LHC vacuum from the detector volume. In the other LHC detectors the LHC beam runs through the experiment in a cylindrical beam-pipe. The beam-pipe serves as the barrier of the LHC vacuum and guides the beam mirror currents. The LHCb VELO detectors are positioned so close to the proton beams that a cylindrical beam-pipe would not give sufficient space for the LHC beams during injection. Therefore, the VELO and its vacuum envelope consist of two halves that are only moved into their nominal position when stable beams are declared. The complicated shape of the foil is explained by the requirement to minimize multiple scattering and retain sufficient rigidity to withstand small pressure differences between the LHC and VELO volumes.

Berend Munneke and Tjeerd Ketel inspect the alignment of the `left’ and `right’ RF boxes at Nikhef. Once installed at CERN the LHC beam will pass through the 7 mm diameter hole in the centre. (Image: Marco Kraan /Nikhef).

The construction of the vacuum envelopes is as intriguing as their shape: They are machined out of solid blocks of aluminium. A five axis CNC machine at Nikhef carves through the 300 kg block until nothing is left but the flange and a foil that is just about 250 µm thick. 

As ambitious LHCb physicists find that still too thick, technicians at CERN have developed a method to carve away another layer using ordinary chemistry: by placing the foil for a short time in a bath of NaOH, the thickness of the foil in the area that faces the beams can be reduced to a mere 150 µm.

Pierre Maurin (left) and Florent Fesquet pour an etching solution into a prototype RF box in order to remove another 100 micro-meter of aluminium. The green paint protects parts of the box that should not be thinned further. (Image: Maximilien Brice /CERN).

The construction of the new Velo and RF boxes is expected to be finalized by the end of the year. Installation will be done early 2020 after which integration with the new CO₂ cooling system and the rest of the detector can begin. Thanks to these detector upgrades the LHCb detector will be ready for the challenges posed by the new operating conditions after Long Shutdown 2, allowing LHCb physicists to continue with their ambitious and captivating physics program until at least 2029.