A new particle detector design proposed at the U.S. Department of
Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) could
greatly broaden the search for dark matter – which makes up 85 percent
of the total mass of the universe yet we don’t know what it’s made of –
into an unexplored realm.
While several large physics experiments have been targeting theorized
dark matter particles called WIMPs, or weakly interacting massive
particles, the new detector design could scan for dark matter signals at
energies thousands of times lower than those measurable by more
conventional WIMP detectors.
The ultrasensitive detector technology incorporates crystals of
gallium arsenide that also include the elements silicon and boron. This
combination of elements causes the crystals to scintillate, or light up,
in particle interactions that knock away electrons.
This scintillation property of gallium arsenide has been largely
unexplored, said Stephen Derenzo, a senior physicist in the Molecular
Biophysics and Integrated Bioimaging Division at Berkeley Lab and lead
author of a study published March 20 in the Journal of Applied Physics that details the material’s properties.
“It’s hard to imagine a better material for searching in this
particular mass range,” Derenzo said, which is measured in MeV, or
millions of electron volts. “It ticks all of the boxes. We are always
worried about a ‘Gotcha!’ or showstopper. But I have tried to think of
some way this detector material can fail and I can’t.”
The breakthrough came from Edith Bourret, a senior staff scientist in
Berkeley Lab’s Materials Sciences Division who decades earlier had
researched gallium arsenide’s potential use in circuitry. She gave him a
sample of gallium arsenide from this previous work that featured added
concentrations, or “dopants,” of silicon and boron.
Derenzo had previously measured some lackluster performance in a
sample of commercial-grade gallium arsenide. But the sample that Bourret
handed him exhibited a scintillation luminosity that was five times
brighter than in the commercial material, owing to the silicon and boron
that imbued the material with new and enhanced properties. This
enhanced scintillation meant it was far more sensitive to electronic
excitations.
“If she hadn’t handed me this sample from more than 20 years ago, I
don’t think I would have pursued it,” Derenzo said. “When this material
is doped with silicon and boron, this turns out to be very important
and, accidentally, a very good choice of dopants.”
Derenzo noted that he has had a longstanding interest in
scintillators that are also semiconductors, as this class of materials
can produce ultrafast scintillation useful for medical imaging
applications such as PET (positron emission tomography) and CT (computed
tomography) scans, for example, as well as for high-energy physics
experiments and radiation detection.
The doped gallium arsenide crystals he studied appear well-suited for
high-sensitivity particle detectors because extremely pure crystals can
be grown commercially in large sizes, the crystals exhibit a high
luminosity in response to electrons booted away from atoms in the
crystals’ atomic structure, and they don’t appear to be hindered by
typical unwanted effects such as signal afterglow and dark current
signals.
Some of the larger WIMP-hunting detectors – such as that of the Berkeley Lab-led LUX-ZEPLIN project
now under construction in South Dakota, and its predecessor, the LUX
experiment – incorporate a liquid scintillation detector. A large tank
of liquid xenon is surrounded by sensors to measure any light and
electrical signals expected from a dark matter particle’s interaction
with the nucleus of a xenon atom. That type of interaction is known as a
nuclear recoil.
A crystal of gallium arsenide
Credit: Wikimedia Commons
In contrast, the crystal-based gallium arsenide detector is designed
to be sensitive to the slighter energies associated with electron
recoils – electrons ejected from atoms by their interaction with dark
matter particles. As with LUX and LUX-ZEPLIN, the gallium arsenide
detector would need to be placed deep underground to shield it from the
typical bath of particles raining down on Earth.
It would also need to be coupled to light sensors that could detect
the very few infrared photons (particles of light) expected from a
low-mass dark matter particle interaction, and the detector would need
to be chilled to cryogenic temperatures. The silicon and boron dopants
could also possibly be optimized to improve the overall sensitivity and
performance of the detectors.
Because dark matter’s makeup is still a mystery – it could be
composed of one or many particles of different masses, for example, or
may not be composed of particles at all – Derenzo noted that gallium
arsenide detectors provide just one window into dark matter particles’
possible hiding places.
While WIMPs were originally thought to inhabit a mass range measured
in billions of electron volts, or GeV, the gallium arsenide detector
technology is well-suited to detecting particles in the mass range
measured in millions of electron volts, or MeV.
Berkeley Lab physicists are also proposing other types of detectors
to expand the dark matter search, including a setup that uses an exotic
state of chilled helium known as superfluid helium to directly detect
low-mass dark matter particles.
“Superfluid helium is scientifically complementary to gallium
arsenide since helium is more sensitive to dark matter interactions with
atomic nuclei, while gallium arsenide is sensitive to dark matter
interacting with electrons,” said Dan McKinsey, a faculty senior
scientist at Berkeley Lab and physics professor at UC Berkeley who is a
part of the LZ Collaboration and is conducting R&D on dark matter
detection using superfluid helium.
“We don’t know whether dark matter interacts more strongly with
nuclei or electrons – this depends on the specific nature of the dark
matter, which is so far unknown,” he said.
Another effort would employ gallium arsenide crystals in a different
approach to the light dark matter search based on vibrations in the
atomic structure of the crystals, known as optical phonons. This setup
could target “light dark photons,” which are theorized low-mass
particles that would serve as the carrier of a force between dark matter
particles – analogous to the conventional photon that carries the
electromagnetic force.
Still another next-gen experiment, known as the Super Cryogenic Dark Matter Search experiment, or SuperCDMS SNOLAB, will use silicon and germanium crystals to hunt for low-mass WIMPs.
“These would be complementary experiments,” Derenzo said of the many
approaches. “We need to look at all of the possible mass ranges. You
don’t want to be fooled. You can’t exclude a mass range if you don’t
look there.”
Stephen Hanrahan, a staff scientist in Berkeley Lab’s Molecular
Biophysics and Integrated Bioimaging Division; and Gregory Bizarri, a
senior lecturer in manufacturing at Cranfield University in the U.K.,
also participated in this study. The work was supported by Advanced
Crystal Technologies Inc.
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Source: Berkeley Lab/News Center
