High Energy Physics
Scaling up chances for discovery
Courtesy of INFN Gran Sasso National Laboratory (LNGS)
Over 20 years ago, a small group of scientists came together to lead an experiment to unmask the exact nature of dark matter. What started as a relatively small experiment has now grown to be one of the most impressive in the world.
The scientists developed a prototype detector XENON10, which consisted of a dual-phase time projection chamber (TPC) filled with liquid xenon. The aim is to detect dark matter candidates in the form of weakly interacting massive particles (WIMPs) by looking for rare interactions via nuclear recoils in the liquid xenon target chamber. Barely using 15 kg of xenon at the time, the size of the experiment increased gradually to 160 kg (2008-2016) and more recently to 3.2 tons (2015-2018).
Today, XENONnT, the name of the latest experiment to date, uses about 8.6 tons of xenon! This increase is the result of an incremental collaboration of like-minded institutes joining forces, determined to elucidate one of the biggest mysteries of our universe. To our knowledge so far, dark matter was not perceived via any interaction except gravity.
Hamamatsu Photonics, an established player in terms of photonics-led innovation, supported the collaboration from the start by providing the first photomultiplier tubes (PMTs) needed to detect these mysterious interactions.
This impressive growth came with many challenges in terms of performance and quality. The technology had to match the mounting expectations of this world-leading experiment now stretching to almost 10 tons of liquid xenon.
How did the technology evolve and what kind of challenges did this experiment face in order to function correctly?
© XENON Collaboration; PMT bottom put in place
© XENON Collaboration; People in PMT-scaled
© XENON Collaboration; PMT arrays
Located at the Gran Sasso National Laboratory (LNGS), about 120 km northeast of Rome in Italy, it is one of the largest underground laboratories in the world. To increase the chances of observing particle interaction, cosmic radiation must be drastically reduced. For this reason, the laboratory is under 1,400 m of rock.
Underground, the cylindrical-shaped TPC is filled with mostly liquid and gas xenon on top, and two arrays of PMTs, one at the top of the experiment in the gaseous phase and one at the bottom of the liquid layer.
These PMTs are no ordinary models. The incredibly harsh environment requires extreme sensitivity and low intrinsic radioactivity from these detectors. Ideal for the observation of these particles, the temperatures in the TPC are extremely low (165K or -108°C), and therefore reproducing the exact environment and testing the PMTs is quite challenging.
Although Hamamatsu had already provided cryogenic-type PMTs for similar experiments in the past, this challenge required the company to find a solution to provide the ideal sensitivity in harsh conditions without being able to test locally.
How did the Hamamatsu engineers provide this technology without being able to replicate such harsh environments?
© XENON Collaboration; XENONnT PMT assembly
It was a collaboration that shed light on how Hamamatsu should advance its technology. The company transformed its technology to fit the exact requirements by engaging in high-levels of communication with XENONnT researchers. From researchers in Italy to engineers in Japan, together, they built and tested the ideal PMT from one end of the world to another.
At first, some PMTs did not function as required due to the incredibly challenging environment. In fact, the experiment was so sensitive that even the tiniest amount of micro-light emission coming from the very material of the PMT was found to create noise. To suppress it, Hamamatsu engineers had to replace the materials and add mechanical barriers within the PMTs to ensure the removal of sparks.
The intrinsic radiation of PMTs were continuously improved over time. This was achieved by selecting components with the lowest radioactivity, thoroughly testing before sharing them with the XENON team for approval and, finally incorporating them into the PMT base design.
For many years, both sides exchanged and improved, all while the experiment increased in size and in demand. Thanks to this continuous customer feedback of performance evaluation and screening tests, the dedicated PMT was considerably enhanced step-by-step and pushed to its technical limits.
Today, XENONnt has published many papers on dealing with its concepts, theoretical background and technical setup. The next - even bigger experiment - is already in discussion and the actions to generate an even greater performance is underway. As each of these experiments grow, so does the technology to support it. This will continue until we are able to uncover together the mystery behind our universe...
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