Lights out, windows darkened, doors closed. It’s not after hours at the Surface Assembly Lab (SAL), it’s just time for the LUX-ZEPLIN (LZ) dark matter detector’s first on-site eye exam.
LZ’s “eyes” are two massive arrays of photomultiplier tubes (PMTs), powerful light sensors that will detect any faint signals produced by dark matter particles when the experiment begins in 2020. The first of these arrays, which holds 241 PMTs, arrived at Sanford Underground Research Facility (Sanford Lab) in December. Now, researchers are testing the PMTs for the bottom array to make sure they are still in working condition after being transported from Brown University, where they were assembled.
“These PMTs have already undergone rigorous testing, down to their individual components, and this will be their final test,” said Will Taylor, a graduate student at Brown University who has been working with the LZ collaboration since 2014.
Once testing is completed, the bottom PMT array will be placed in the inner cryostat. The same process will be followed for the top array. The inner cryostat will be filled with xenon, both gaseous and liquid, and placed in the outer cryostat. Then, the entire detector will be submerged in the 72,000-gallon water tank in the Davis Campus on the 4850 Level of Sanford Lab.
“As you can imagine,” Taylor said, "it will be impossible to change out a faulty PMT after the experiment is completely assembled. This is our last chance to ensure each PMT is working perfectly.”
While researchers do expect a few PMTs to “blink out” over LZ’s five to six year lifetime, only the best of the best will make it into the detector. So, just how do researchers transform the SAL into an optometrist’s office?
First, the array is placed in a special enclosure called the PALACE (PMT Array Lifting And Cleanliness Enclosure). There, the PMTs are shielded from light and dust. This enclosure also allows researchers access to the PMTs through a rotating window and to connect data collection systems to different sections of PMTs at a time.
“We test by section, collecting data from 30 PMTs per day,” said Taylor. “Each individual PMT has a serial number and is tagged to its own data, so we know exactly what each PMT is ‘seeing.’”
For the first test, researchers look at what is called the “dark rate” of each PMT. To perform this test, researchers seal up the PALACE, turn off the lights in the cleanroom and black out the windows. In this utter darkness, PMTs are monitored for “thermal noise.”
“At a normal temperature, particles vibrate around inside the PMTs. When this happens, it is possible for electrons to ‘jump off’ and produce a signal that PMTs will detect,” Taylor explained. While most of this “thermal noise” will vanish once the experiment is cooled to liquid xenon temperature (-148 °F), researchers want to ensure the PMT's dark rate is at the lowest threshold possible before being installed in LZ.
"Typically, these false signals come from a single photoelectron,” Taylor said. “With the dark test, we can measure how many photoelectrons signals occur every second.”
So how much is too much noise? While a bit of noise (100-1000 events per second) is tolerable; rates closer to 10,000 events per second would be far too high, resulting in too many random signals that could overshadow WIMP signals during the experiment.
“That’s why it is incredibly important to make sure each PMT has a low dark rate,” said Taylor.
Lighting it up
For the second test, called an “after-pulsing” test, researchers will flash a light, imperceptible to the human eye, at the PMTs. This test determines the health of each PMT’s internal vacuum. Why is this important?
When light from a reaction inside the detector hits a photocathode of a PMT, an electron will be emitted. This single electron will be pulled through the PMT, hitting dynodes. Each time the electron hits an dynode, more electrons are emitted. This process continues, amplifying the original signal, turning the original electron into many, many, many electrons.
“That’s how we get an electron signal strong enough to read out,” Taylor said. “For that to work, however, those electrons have to be able to bounce between those dynodes without interruption.”
To decrease particle “traffic,” each PMT has a vacuum. The vacuum ensures there are no gas particles present to interfere with the amplification process. If a vacuum is faulty, gas particles may get in the way and hit an electron. This would cause the gas particle to bounce away and set off a second pulse of electrons, amplifying a signal of its own.
“This is called an ‘after-pulse,’” Taylor said. “The after-pulse is indicative of how good the vacuum, and thus the PMT, really is.”
Rather than depriving the PMTs of light as they did during the dark test, researchers now create a signal of their own to measure the after-pulse. To do this, an LED is affixed to the inside of the PALACE.
“We flash the LED at a rate of 1 kilohertz for 30 seconds. That’s a total of 30,000 flashes of the LED,” Taylor said. While that might sound like a lot of light, it’s actually not even perceptible to the human eye. “Each flash lasts 10 nanoseconds and produces only 50-100 photons—so the human eye can’t detect it.”
It is enough, however, for the PMT to detect it with a sizable initial pulse. Because researchers know exactly when the initial pulse was created, they can align their data to see when after-pulses occur and measure their strength.
“This helps us see how healthy the vacuum is and determine if the PMT is fit for LZ,” Taylor said.
After a week of testing, researchers have announced the bottom array has 20/20 vision.
“Accepting the first of the two PMT arrays onsite, is one of many milestones toward the assembly and installation of the LZ experiment," said Markus Horn, research support scientist at Sanford Lab and a member of the LZ collaboration. "While the detector assembly progresses at the Surface Lab, underground the installation of the xenon gas and liquid nitrogen cooling system begins. That would be the heart and the lung of LZ. But that’s another story!”