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.
© XENON Collaboration; PMT bottom put in place
© XENON Collaboration; People in PMT-scaled
© XENON Collaboration; PMT arrays
The experiment is located at the Gran Sasso National Laboratory (LNGS), about 120 km northeast of Rome in Italy, which is one of the largest underground laboratories in the world. To increase the chances of observing dark matter particle interactions, 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 a small layer of 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 photodetectors. Ideal for the observation of rare particle interactions, the temperatures in the TPC are extremely low (165 K or -108°C), making testing of PMTs by reproducing this exact environment 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.
© XENON Collaboration; XENONnT PMT assembly
It was through collaboration that Hamamatsu managed to 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 across Europe 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 was continuously improved over time. This was achieved by selecting components with the lowest radioactivity, thoroughly testing them before they were shared 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 on performance evaluation and screening tests, the dedicated PMT was considerably enhanced step-by-step and pushed to its technical limits.
Today, the XENON collaboration has published many papers dealing with its concepts, theoretical and observed backgrounds, and technical setups. The next - even bigger experiment - is already in discussion and the actions to generate an even greater performance are well 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 mysteries behind our universe...
“I have been collaborating with Hamamatsu from the start of the XENON program and I am very pleased with our current sensors in XENONnT which allow us to observe single photons with high efficiency and very low levels of intrinsic radioactivity”. Laura Baudis, Prof. Dr. Laura Baudis (currently working for XENON and the future DARWIN)
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