The circular power revolutionizing physics
© 2001-CERN; The area under which the tunnel for CERNs LHC can be found is shown near to Geneva and lac Leman
A ring 27km long of superconducting magnets with a number of accelerating structures, currently resides 100 meters underground in the Franco-Swiss border near Geneva. Known for being the world’s largest and most powerful particle accelerator, the Large Hadron Collider (LHC) remains the latest addition to CERN’s accelerator complex.
Since 2010, when the accelerator became operational, inside it you will find two high-energy particle beams that travel in opposite directions at very close to the speed of light before they are made to collide. This powerful experiment has been the driving force behind several major discoveries in physics including the discovery of the god particle: the Higgs boson in 2012. This major breakthrough has brought the scientific world one step closer to understanding our world and the origin of the universe.
Hamamatsu Photonics, an established player in terms of photonics-led innovation, has been working with CERN on this project, and its planned upgrade, which aims to amplify the performance of the LHC for more potential discoveries after 2029. The upgrade named High Luminosity Large Hadron Collider (HL-LHC) has the objective to increase the integrated luminosity by a factor between 5 and 7 beyond the LHC’s design value. Hence, the technology used in those experiments has to meet even more severe conditions, as well as push the limits of what currently exists.
©2008 CERN; CMS Courtesy of Maximilien Brice
Hamamatsu 8 inch pixel array detector
The upgrades of the HL-LHC and its related experiments are currently underway to achieve an even higher frequency of proton-to-proton collisions, in order to measure the Higgs boson properties more precisely and to search for dark matter of which little is known.
Although more data can be obtained by increasing the frequency of these collisions, this will generate higher radiation and impact the photodiode (PD) array. The PD array is used in the calorimeter of the CMS experiment, which measures the energy of the particles.
The challenge to solve is the fact that the PD arrays gradually lose their sensitivity while being exposed to radiation. One simple solution is to apply a higher voltage to the PD array to maintain high sensitivity even when exposed to radiation, however, this is only possible to a certain degree.
Additionally, CERN required an even larger-area PD array to reduce both the costs and the dead space of the entire detector system.
The final main challenge was the manufacturing of an even larger-area PD array – never done before – with quantities of circa 27,000 pieces in a short timeframe. Therefore, Hamamatsu was faced with overcoming this while finding the right solution for higher sensitivity in its PD array.
To meet the high-tech needs of the HL-LHC experiments, Hamamatsu Photonics designed and developed the world’s largest photodiode (PD) with the highest radiation resistance among PD detectors used in high-energy physics applications. Ideal for particle and radiation detection through the measurement of ionization energy deposits, it has both high resistance to radiation and a large area required for the HL-LHC project.
At first, Hamamatsu focused on the high resistance to radiation aspect. They successfully developed a large-area PD array prototype that can be made from a single 6-inch diameter wafer, is highly resistant to radiation, and operates at voltages as high as 800 V.
To utilize larger-diameter wafers as the material for a larger-area PD array, Hamamatsu installed new manufacturing equipment for an 8-inch diameter. The manufacturing process conditions were reviewed from scratch in order to improve the uniformity of the thin film thickness and impurity concentration formed on the wafers.
By taking these steps, Hamamatsu managed to fabricate a PD array about twice the area of the previous one while maintaining the same high level of radiation resistance and performance characteristics.
Many other technological solutions have been and are currently being designed to support the demanding expansion of the world’s most powerful particle accelerator. Each year as we get closer to its operational date, we also get a bit closer to finding out the true nature of our universe.
© 2018 CERN Ordan Julien Marius; Civil engineering for the High-Luminosity LHC (HL-LHC)
© 2021 CERN Hertzog Samuel Joseph; Civil engineering is completed for HL-LHC (HiLumi) project at Point 1
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