HiLumi LHC Project (3): What Nikhef Is Contributing to the More Powerful CERN Accelerator

In one of the assembly halls at Nikhef’s Mechanical Technology department, there is a prototype of the most important Dutch projects related to the high-luminosity LHC accelerator at CERN.

It is a cylindrical frame, one and a half meters high and two meters in diameter, consisting of six wheel-shaped discs arranged around a central tube. Hundreds of clips and clamps hold the thin metal pipes of a cooling system in place. The structure looks impressive, but thanks to the carbon fiber it’s made of, it weighs less than ten kilograms.

This prototype is not part of the upgraded HiLumi LHC accelerator, which is scheduled to go into operation in 2030. Nikhef has traditionally focused on the experiments at the accelerator. If the accelerator changes, the detectors must be tailored to it.

The project in the hall is part of the ATLAS detector: the inner tracker, or ITk for short. To be precise, it concerns the endcap—the terminal section—of the innermost detector layer of the ATLAS detector. A tiny component of the largest scientific instrument on Earth, which is as big as the Royal Palace on Dam Square.

ATLAS is also undergoing a massive overhaul during the years that CERN is working on the upgraded accelerator. The detector is being prepared in numerous ways for the increase in proton collisions within the accelerator.

One of the projects is a completely new inner detector, six meters long, with even finer-mesh silicon pixel sensors that record the trajectories of particles released near the beam more accurately and more quickly. This is necessary to keep up with the estimated 5 billion collisions per second.

Nikhef is designing and building the endcap structures at both ends of the inner tracker, which will ultimately hold hundreds of sensor panels precisely in place. These electronic panels are being built by other institutes around the world. The assembly and testing of one of the two ITk endcaps is taking place at Nikhef.

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Frame for the ATLAS ITk subdetector in the Nikhef workshop in Amsterdam.

The prototype in the assembly hall was built over the past few years to test and optimize the design. Ultimately, two endcaps were built. One of them is being further outfitted in Amsterdam and will eventually be fully equipped with sensor panels. The other is now at DESY in Hamburg, which will assemble this endcap.

A cleanroom has been set up in the Amsterdam workshop for this work, ready for the arrival of the sensor panels, which will be inserted into the structure like flower petals and are therefore called petals. Eventually, the completed endcaps will go to CERN for further testing and integration with the rest of the inner tracker, deep within the heart of the underground ATLAS detector.

Thanks in particular to new sensor technologies and a new cooling system, the ITk endcaps are paving the way for the HiLumi LHC era. Due to the enormous particle fluxes and rapid processing on chips within the sensor panels, on-site cooling is crucial. Nikhef designed, tested, and constructed a special CO₂ cooling system for the ITk. A key requirement for this component is that the detector must be able to operate for years under intense and sometimes extreme conditions, including heat and radiation near the proton beams.

A smaller ATLAS project in the run-up to the HiLumi LHC is focused on improvements to the detector’s muon chambers. Muons are heavier versions of electrons, which are produced in large numbers during proton collisions in the CERN accelerator. Because of their mass, they travel relatively far through the detector, which is why the muon chambers are arranged in a shell around the outside of ATLAS to record their trajectories.

Muon chambers are gas-filled detectors in which the charged muons cause ionization, generating an electrical signal between a central high-voltage wire and the wall. To achieve greater precision, the existing muon chambers in ATLAS—some of which were built by Nikhef—will be replaced in the coming years with versions featuring thinner tubes. In addition, the timing of the readout electronics will be improved and the high-voltage system replaced. Nikhef is involved in all these modifications through testing and coordination; the hardware is supplied by other institutes.

For the other two Nikhef experiments at CERN—ALICE and LHCb—the advent of high luminosity also represents a significant change. In fact, these two experiments were already prepared for the higher intensities of the HiLumi LHC during the previous long shutdown, with new components and systems.

The most important adaptation to the upcoming particle flux at LHCb was the switch to a data system based on graphics processing units (GPUs). Such processors were developed for the gaming industry, but have proven highly suitable for particle physics applications that involve handling large amounts of data simultaneously. Nikhef contributed a significant amount of expertise to the transition to GPUs at LHCb. In principle, LHCb can now evaluate all measurement data from the millions of sensors in the detector in real time and store it for further offline study.

The ALICE experiment, too, was already prepared for the arrival of larger particle beams in an earlier phase. ALICE was specifically developed to study collisions of heavy ions such as lead, in order to investigate the quark-gluon plasma that must have existed in the very early universe shortly after the Big Bang.

In the massive underground ALICE detector, the inner tracker system (ITS)—the innermost detector layer closest to the beam—is entirely composed of fast silicon sensors with built-in electronics mounted in a specially developed ultra-light support structure.

Currently, the ITS2 is undergoing another upgrade with large, strip-shaped, curved silicon sensors in the innermost layer, with contributions from Nikhef and Utrecht University. Such sensors provide a more seamless image of the quark-gluon plasmas created when atomic nuclei collide, even at the higher intensities that the HiLumi LHC will deliver.

The intense beams in the upgraded accelerator produce more collisions, which generate far more signals in the detectors. More on this tsunami of signals in Episode 4 of this series.

Next episode: Preparing for a deluge of signals.

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