From SURF: “LZ team installs detector in water tank”

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Sanford Underground levels

Sanford Underground Research facility

October 16, 2017
Constance Walter

Sally Shaw, a post-doc with the University of California Santa Barbara, poses next to the sodium iodide detector recently installed inside the water tank. Courtesy photo.

The huge water tank that for four years housed the Large Underground Xenon (LUX) dark matter detector now stands empty. A small sign over the opening that reads, “Danger! Confined space,” bars physical entry, but a solitary note sung by Michael Gaylor, a science professor from Dakota State University, once jumped that barrier and reverberated for 35.4 seconds.

Starting this week, the tank will be filled with the sounds of collaboration members installing a small detector that will be used to measure radioactivity in the cavern. It’s all part of the plan to build and install the much larger, second-generation dark matter detector, LUX-ZEPLIN (LZ).

LBNL Lux Zeplin project at SURF

“We need to pin down the background event rate to better shield our experiment,” said Sally Shaw, a post doc form from the University of California, Santa Barbara (UCSB).

The detector, a 5-inch by 5-inch cylinder of sodium iodide, will be placed inside the water tank and surrounded by 8 inches of lead bricks. The crystal will be covered on all sides except one, which will be left bare to measure the gamma rays that are produced when things like thorium, uranium and potassium decay. Over the next two weeks, the team will change the position of the detector five times to determine the directionality of the gamma rays.

Scott Haselschwardt, a graduate student at UCSB, said this is especially important because there is a rhyolite intrusion that runs below the tank and up the west wall of the cavern.

“This rock is more radioactive than other types of rock, so it can create more backgrounds,” he said. This wasn’t a problem for LUX, Haselschwardt said, but it was smaller than LZ and, therefore, surrounded by more ultra-pure water.

But LZ is 10 times larger and still must fit inside the same tank, potentially exposing it to more of the radiation that naturally occurs within the rock cavern. And while this radiation is harmless to humans, it can wreak havoc on highly sensitive experiments like LZ.

“Because it is so much closer to the edges of the water tank, there was a proposal to put in extra shielding—perhaps a lead ring at the bottom of the tank to shield the experiment,” Shaw said.

Like its much smaller cousin, LZ hopes to find WIMPs, weakly interacting massive particles. Every component must be tested to ensure it is free of any backgrounds, including more than 500 photomultiplier tubes, the titanium for the cryostat and the liquid scintillator that will surround the xenon container. But if the backgrounds emanating from the walls of the cavern are too high, it won’t matter.

“The whole point is to see whether the lead needs to be used in the design of the shield,” said Umit Utku, a graduate student at University College in London. “Maybe we will realize we don’t need it.”

Shaw, who created a design for lead shielding within the tank, said it’s critical to fully understand the backgrounds now.

“If we do need extra shielding, we must adjust the plans before installation of the experiment begins,” she said.

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About us.
The Sanford Underground Research Facility in Lead, South Dakota, advances our understanding of the universe by providing laboratory space deep underground, where sensitive physics experiments can be shielded from cosmic radiation. Researchers at the Sanford Lab explore some of the most challenging questions facing 21st century physics, such as the origin of matter, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines—including geology, biology and engineering.

The Sanford Lab is located at the former Homestake gold mine, which was a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis earned a share of the Nobel Prize for Physics in 2002 for a solar neutrino experiment he installed 4,850 feet underground in the mine.

Homestake closed in 2003, but the company donated the property to South Dakota in 2006 for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. The state Legislature has committed more than $40 million in state funds to the project, and South Dakota also obtained a $10 million Community Development Block Grant to help rehabilitate the facility.

In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.

In December 2010, the National Science Board decided not to fund further design of DUSEL. However, in 2011 the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments. The SDSTA, which owns Sanford Lab, continues to operate the facility under that agreement with Berkeley Lab.

The first two major physics experiments at the Sanford Lab are 4,850 feet underground in an area called the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) experiment is housed in the same cavern excavated for Ray Davis’s experiment in the 1960s.
LUX/Dark matter experiment at SURFLUX/Dark matter experiment at SURF

In October 2013, after an initial run of 80 days, LUX was determined to be the most sensitive detector yet to search for dark matter—a mysterious, yet-to-be-detected substance thought to be the most prevalent matter in the universe. The Majorana Demonstrator experiment, also on the 4850 Level, is searching for a rare phenomenon called “neutrinoless double-beta decay” that could reveal whether subatomic particles called neutrinos can be their own antiparticle. Detection of neutrinoless double-beta decay could help determine why matter prevailed over antimatter. The Majorana Demonstrator experiment is adjacent to the original Davis cavern.

Another major experiment, the Long Baseline Neutrino Experiment (LBNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab, is in the preliminary design stages. The project got a major boost last year when Congress approved and the president signed an Omnibus Appropriations bill that will fund LBNE operations through FY 2014. Called the “next frontier of particle physics,” LBNE will follow neutrinos as they travel 800 miles through the earth, from FermiLab in Batavia, Ill., to Sanford Lab.

Fermilab LBNE