Building an innovative cross-border experiment on deep dark matter – during a pandemic


To get to the SNOLAB cleanroom, where the SENSEI experiment is to be hosted, a trip over a kilometer underground inside a working mine, pictured here, is required. Credit: SNOLAB

In the search for dark matter particles, a tabletop experiment at the heart of a Canadian mine could do the trick. The SENSEI collaboration uses Skipper Loaded Torque, or CCD, devices which are the most sensitive sensors of their kind, imagined decades ago and only recently made.

The collaboration recently proven that the experiment has a sensitive dark matter detector and that it can reduce background rates in an underground experimental area at the Department of Energy’s Fermilab. Now the collaboration is running a larger, exponentially quieter, and more responsive version of the experience more than a mile underground at SNOLAB in Canada.

With the COVID-19[feminine pandémie et la fermeture de la frontière canado-américaine, l’expérience aurait pu facilement prendre du retard en 2020. Au lieu de cela, elle est dans sa phase de mise en service – test avec environ 20% du matériau cible que l’expérience utilisera lorsque les couches externes de blindage seront en place. Un travail d’équipe extraordinaire entre les physiciens des deux côtés de la frontière a permis d’avancer dans les délais.

Rester en sécurité tout en faisant progresser la science

Alors que la pandémie s’installait pour son troisième ou quatrième mois, il est apparu à Javier Tiffenberg, scientifique associé au Fermilab et collaborateur de SENSEI, que l’installation expérimentale prévue par l’équipe du Fermilab, qui devait commencer en 2020, devait être repensée.

Sans moyen de se rendre sur le site, l’équipe SENSEI a contacté SNOLAB pour voir si le personnel était disposé à installer lui-même l’expérience hypersensible avec le guidage à distance du Fermilab. Le personnel de SNOLAB est déjà familiarisé avec les défis uniques de l’installation d’expériences dans un laboratoire propre situé dans une mine en activité. Cette fois, ils effectueraient une installation d’une semaine pour une expérience à laquelle ils n’allaient pas participer à l’origine.

SNOLAB était un jeu.

“Nous serons leurs mains car ils ne peuvent pas être ici”, a déclaré Silvia Scorza, chercheuse au SNOLAB, à propos de la perspective adoptée pour le projet. Elle fait partie des employés de SNOLAB qui ont aidé à installer des projets à distance pendant la pandémie.

“Lorsque les gens de SNOLAB ont dit qu’ils étaient intéressés à contribuer à cela, puis de notre côté, les ingénieurs et les techniciens ont dit:” Oui, nous pouvons le faire “, j’étais super excité parce que je pensais vraiment que cela se préparait”, Tiffenberg mentionné.

Greg Derylo, engineer in the particle physics division of Fermilab, designed the layout of SENSEI, worked with the drawing group to make drawings of all mechanical parts and sourced parts from workshops on-site and off-site machining. Due to the effects of COVID-19 restrictions on access to the Fermilab campus, he also performed most of the physical assembly of the experiment.

Derylo said dismantling at Fermilab and reassembling at SNOLAB was still part of the plan. But the remote installation presented a new problem.

“The real stuff comes in terms of who is doing this assembly underground,” he said. The main concern was the handling of the skipper’s fragile (and expensive) CCDs, which are “very sensitive to electrostatic damage”. Less than what a person can feel in their hand after rubbing their feet on a carpet and touching a doorknob could destroy the sensors. Thus, physicists and technicians from SNOLAB followed a special course in electronic manipulation.

Tests, tests and a very pandemic party

Before the experiment could be put into SNOLAB’s hands, it had to be tested and documented.

“We tested everything at Fermilab. We put everything together the same way they would there, ”Derylo said.

The first test was mechanical – assembling the outer shell of the experiment to confirm it would hold a vacuum – and a thermal performance test. SENSEI relies on cryogenics to “cool”. To do this, the Fermilab team has set up additional instrumentation to monitor temperatures and perform diagnostics. Both worked as expected.

In early fall 2020, the Fermilab team installed a set of test modules in the experiment, turned it on, cooled it down, cooled it down and operated the modules. The reading went off without a hitch. The team celebrated – each member from their own location via Zoom – with champagne.

Documentation and hand modeling

Usually, the documentation is more of a series of reminders than step-by-step instructions, and team members may have a hunch of the process or largely rely on their memory.

Creating instructions for a team unfamiliar with the experience required higher level communication. This mainly meant creating documentation with much more detail.

SENSEI labels

The labels of Fermilab engineer Greg Derylo are visible on SENSEI. Derylo added the labels for ease of documentation when he realized that different team members had different definitions of front and back for the experiment. Credit: SNOLAB

Because the team knew the installation would be remote, they took advantage of their own assembly during testing.

“We took pictures of everything,” Tiffenberg said. “Having this documentation was essential. “

But achieving this has not been without challenges. With multiple people documenting at different times due to the pandemic, communication within the group has also become more important than usual. Different technicians had different perspectives, literally.

“In fact, we actually had opposing definitions of what the front of the setup was,” Derylo said. The cure? SENSEI’s newest and one of its most important components: self-adhesive labels.

The team also incorporated breakpoints into the documentation. Once a SNOLAB pair hit one of these points, they could determine if they had time to take the next step of a shift or if they needed clarification from the team. of Fermilab.

The first draft was ready around the beginning of 2021. Derylo said the documentation was divided into different sections and was around 70 pages long. The document resembles a sketch “heavily spiced with photos”. Drawings of the vacuum system, cooling system and electrical wiring were also provided, but were not part of the booklet.

“Then we took it apart but tried to keep as many parts together as possible and then shipped it to SNOLAB in January,” Tiffenberg said.

An ultra-clean clean room in a working mine

Pandemic issues aside, the process of setting up experiments in the SNOLAB underground facility has always been complex. After all, this is a clean lab located more than a kilometer underground, inside a working mine.

“Careful planning and preparation starts on the surface,” said Scorza.

There, at the start of the day, physicists, engineers and technicians dressed as miners (plus masks and pandemic contact tracing badges) await “the cage”, a mining elevator to take them deep underground. .

SNOLAB underground entrance

Entrance to the underground lab, view from the dirty side of the car wash: Anyone entering the lab from the other side must shower and change their clothes. Every item that enters the lab must go through the car wash. Credit: SNOLAB

The descent takes less than five minutes. But then there’s a nearly mile-long journey through a mining tunnel with a railroad track running alongside it.

“So watch your steps,” Scorza said.

At the end of this trek, a clean lab. Before entering, the boots are washed in a special station. Clothing and boots are left on one side. People entering the clean lab undress, shower, and dress in new, clean clothes from across the shower to avoid contamination. A “responsible laboratory coordinator” verifies that the laboratory is allowed to work – by checking oxygen levels, among other parameters. It takes about an hour between the surface and the start of the work, and the same – minus the shower – for the return. In a 10 hour day, that leaves about eight hours per team to put the experiment together.

A team of four people, made up of scientists from SNOLAB and a technician, take turns and work in pairs for the installation. (No one can be alone underground, and there are restrictions on the maximum number of people to maintain physical distance.)

“The packaging is very special,” Tiffenberg said.

SNOLAB own installation

Opposite the mine, on the other side of the car wash is the lab itself, a super clean facility. Credit: SNOLAB

Every part received has to go through “the underground car wash,” Scorza said. Each item is triple wrapped and on a double pallet to streamline the complex process of unpacking and wiping materials before they can enter the cleanroom lab. On the other side of the car wash, the clean lab. For SENSEI, the parts included the pipe, cable, tank, copper parts for the interior side, cryogenics, lead and copper shielding layers and the bell.

“When these things were designed, of course shipping everything that is assembled was not on the list of requirements,” Tiffenberg said. “For the vast majority of the things we shipped, everything was going perfectly fine.”

Only a few small pieces of plastic – easily replaceable – came loose and broke.

Once everything was opened up and “looked good” Tiffenberg said it was a “big, big relief”.

The installation began on April 19 and was completed at the end of the summer.

Steve Linden SENSEI

On August 6, SNOLAB researcher Steve Linden continues to assemble SENSEI in the underground clean room. Credit: SNOLAB

Although the SNOLAB cleanroom is equipped with telephones and Wi-Fi, communication outside of documentation is mainly done during weekly meetings. The plan changed from week to week, depending on whether the team was able to go underground due to COVID availability and restrictions.

Some days, that involved extreme care: touching those skipper’s CCDs. In others, it involved using a crane to move lead shielding. Scorza said the mix of work SNOLAB enables scientists to undertake – practical and analytical – gives them a more comprehensive experience in experimental physics. “And, it’s a lot of fun. At least for me. “

“I think that the fact that a team, a team of SNOLAB scientists, not very involved in the experiment originally, is able to progress with the underground facility shows that, first of all, physicists are very flexible, ”said Scorza. “And (it’s) a testament to the robustness of the plan for this experiment.” Hats off to the Fermilab team.

Tiffenberg said he was grateful the installation went without a hitch or surprise.

“It took a long time to get to this situation where there are no surprises. At first, everything was like, “Okay, we spend a lot of time adjusting, coordinating, reviewing things. And it took a long time. But now that things are moving, this time that we took there, we appreciate, because now everything is no surprises. “

Although now that the experiment is taking data, a surprise in the form of a scientific discovery would be a nice reward.


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