If the blender is on while you’re watching TV, its whirring may make it difficult for you to hear your show. Physicists have a name for that.
“In this case, the noise of the blender is a background because it's interfering with what you're trying to receive,” says Yusuf Ahmed, a research student at SNOLAB, a science facility in Ontario.
A background signal is anything that mimics, and thus obstructs, the signal an experiment is searching for. Ahmed recently studied these pesky signals in SNOLAB’s low-background-counting group, which identifies, measures and characterizes different types of backgrounds.
One source of background is the shower of cosmic rays that constantly bombard the Earth. While the space particles don’t affect most experiments, they could completely drown out the whisper of a faint, rare signal coming from a particle of dark matter or a neutrino.
Background is a huge problem for sensitive particle detectors—so much so that SNOLAB is located 2 kilometers underground to protect them. The rocks above SNOLAB block the cosmic rays: It’s like turning off the blender.
But other “noisy appliances” are on, even underground. They produce radiation, vibration, light and heat. SNOLAB’s low-background-counting group takes the best possible measurements of these backgrounds to help mitigate their effects on delicate experiments attempting to figure out our universe.
Coming from within
An experiment’s background can originate in the experiment itself. Manufacturing a particle detector uses natural resources, like water, soil and metals, that contain slightly radioactive elements. The radiation the elements emit isn’t enough to affect human health, but it is enough to disrupt an experiment.
Researchers in the low-background-counting group measure the amount of radiation emitted by every material used to build an experiment, a process known as material screening. These measurements help the scientists that send in these samples, from SNOLAB and other labs, decide which materials to use.
“The better we are at material screening, the better measurements these experiments can make,” says Steffon Luoma, a staff scientist in SNOLAB’s low-background-counting group.
Different materials emit different amounts and types of radiation and must be measured by different types of detectors. Scientists count one type of radiation, known as gamma radiation, using high-purity germanium detectors. They are able to take very precise measurements of the number of gamma rays emitted by a material because the background detectors do their work underground.
“To sensitively measure background, the low-background detectors themselves have to have a low background,” says Nasim Fatemighomi, a staff scientist in SNOLAB’s low-background-counting group.
When the low-background-counting group screened samples of acrylic used to build the DEAP experiment, which searched for dark matter at SNOLAB, they found it emitted more background radiation than they had previously thought. These measurements allowed the DEAP collaboration to work with manufacturing companies to produce purer, less radioactive acrylic.
Low background counting has also found that steel produced before World War II—and the United States’ first test of atomic weapons—emits less radiation than steel produced after. So does the lead from ancient shipwrecks, which has been shielded from cosmic rays for thousands of years by the ocean.
In addition to helping scientists with materials selection, measuring background helps scientists determine how much shielding an experiment needs. If scientists cannot block a source of background, measuring the amount helps them to subtract it from the experiment’s data during analysis.
“Low-background-counting techniques play a vital role in both current and future rare-event experiments,” Fatemighomi says.
How low can you go
Anything in the vicinity of an experiment can produce background. So scientists have to be careful. For example, any food brought into SNOLAB’s underground lunchroom must be double bagged to prevent it from spreading radioactive dust to the experiments.
These precautions can't stop all radiation that affects experiments underground. The rock walls of the mine emit radioactive radon gas. Radon gas can enter small pores of experiments. As it decays, it mimics the signals of dark matter and neutrinos. Scientists must find ways to deal with it.
To keep radon gas out, the neck of the SNO+ experiment, which is studying neutrinos, is filled with nitrogen gas, but this gatekeeping is not infallible. That’s why Fatemighomi, SNOLAB’s resident radon expert, developed a system that can count how much radon makes its way into the nitrogen at the atomic level. During this lengthy procedure, Fatemighomi passes samples of the nitrogen through radon traps.
“I'm so impressed that we can partially isolate and count as low as ten radon atoms out of a few liters of nitrogen gas, which is made up of trillions upon trillions of atoms,” says Ahmed, who worked under Fatemighomi’s supervision on these techniques. “That's amazingly low and not something achievable by current commercial radon detectors.”
Luoma specializes in measuring any and all backgrounds coming from the environment. This includes radioactive particles coming from the walls of the mine and its equipment, as well as non-radioactive phenomena in the environment that can overshadow signals in SNOLAB’s sensitive experiments. Many experiments sit on spring-supported seismic platforms to isolate them when heavy equipment and vehicles pass by. Some experiments must be kept very cold to decrease thermal noise. Even the electromagnetic waves coming off lights and equipment must be measured so that their signals can be isolated and removed from experimental data.
The work to keep out backgrounds, or reduce their effects, is varied—and never finished. As particle physics and astrophysics experiments become more and more sensitive, scientists must also push the boundaries of low-background-counting techniques.
“Our work is continuous and we have to be very meticulous,” Luoma says. “We're always improving techniques and looking for new sources of backgrounds. The littlest things can make differences that you wouldn't anticipate.”