UK delivers super-cool kit to USA for next-generation dark matter experiment

UK delivers super-cool kit to USA for next-generation dark matter experiment
A worker inspects the titanium cryostat for the LUX-ZEPLIN experiment in a clean room at the Sanford Underground Research Facility in South Dakota. (© SURF)

A UK built titanium chamber designed to keep its contents at -100°C has been shipped to the USA, where it will soon become part of a next-generation dark matter detector to hunt for the dark matter particle know as a WIMP (Weakly Interacting Massive Particle).

The search for the dark matter particle is important because the nature of dark matter, which physicists describe as the invisible component of the Universe, has eluded scientists since its existence was deduced in 1933. The search to find out what dark matter is made of, or whether it can be explained by tweaking the known laws of physics, is considered one of the most pressing questions in particle physics, on a par with the previous hunt for the Higgs boson, hence the need for the next-generation dark matter experiment.

The cryostat chamber

The cryostat chamber was built by a team of engineers at the UK’s Science and Technology Facilities Council (STFC) and has travelled around the world to the LUX-Zeplin (LZ) experiment, located 1,400m underground at the Sanford Underground Research Facility (SURF) in South Dakota, USA.

After arriving at the surface facility at SURF, the Outer Cryostat Vessel of the cryostat chamber spent five weeks being fully assembled and leak checked in the SURF Assembly Lab (SAL) clean room. It has now been disassembled for transportation to the underground location at SURF. Meanwhile, the Inner Cryostat Vessel is now in the SAL clean room getting prepared for the leak tests.

STFC’s Dr Pawel Majewski, technical lead for the cryostat, said: “The cryostat was a feat of engineering with some very stringent and challenging requirements to meet. Because of the huge mass of the cryostat – 2,000kgs – we had to make sure it was made of ultra radio-pure titanium. It took nearly two years to find a pure enough sample to work with. Eventually, we got it from one of the world’s leading titanium suppliers in the US, where Electron Beam Cold Heart technology was used to melt the titanium.”

The cryostat is a vital part of LZ, as it keeps the detector at freezing temperatures. This is crucial because the detector uses xenon. However, for the experiment to work, the xenon, which itself has low background radiation, must be kept in a liquid state, which is only achievable at around -100°C.

The dark matter experiment

LZ is the latest experiment to search for the dark matter particle called a WIMP (Weakly Interacting Massive Particle). Many scientists believe finding WIMPs will provide the answer to one of the most pressing questions in physics – what is dark matter?

WIMPs are thought to make up the most of dark matter – the as-yet-unknown substance which is thought to make up about 85% of the universe. Yet, because WIMPs are thought not to interact with normal matter, they are practically invisible using traditional detection methods.

Liquid xenon emits a flash of light when struck by a particle, and this light can be detected by very sensitive photon detectors called photomultiplier tubes. If a WIMP collides with a xenon nucleus it is expected to produce a burst of light.

LZ will be at least 100 times more sensitive to finding signals from dark matter particles than its predecessor. The new experiment will use 10 metric tons of ultra-purified liquid xenon, to tease out possible dark matter signals. Xenon, in its gas form, is one of the rarest elements in the Earth’s atmosphere

Although this is a major milestone for the experiment, there are still many components yet to be assembled and tested. It is currently expected that the experiment will start taking data in 2020.

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