How to build a dark matter detector - Jenna Saffin
More than two kilometers below
the surface of northern Ontario,
suspended in 345,000 liters
of ultra-pure water,
there’s a perfect sphere.
It contains 3600 kilograms
of liquid argon,
cooled to -180 degrees Celsius.
Scientists continuously monitor
this chamber from above ground,
looking for a glimmer
of light in the darkness.
Because down here,
deep beneath the Earth’s surface
and cocooned in a watery shield,
that light would indicate the presence of
one of the universe’s greatest mysteries:
dark matter.
All the matter we can see,
planets, stars and galaxies,
doesn’t create enough gravitational pull
to explain
the universe’s larger structure.
It’s dark matter, which is estimated
to make up 25% of the known universe.
But despite its prevalence,
so far we haven’t been able
to detect it directly.
It’s no small challenge.
Dark matter was so named because it
doesn’t interact with any type of light,
visible or otherwise,
which means our usual observation tools
simply don’t work
when trying to observe it.
But while dark matter may not be visible
in the electromagnetic spectrum,
it’s still matter,
so we should be able to measure
its interactions with other matter.
And if our current model
of physics is correct,
billions of sub-atomic
dark matter particles
are passing through
the Earth every second.
Despite the prevalence of dark matter,
its interactions are predicted to be rare
and extremely weak.
To detect these interactions,
dark matter experiments need to be
incredibly sensitive.
With such sensitive equipment,
the ever-present background radiation
on Earth’s surface
would create so much noise in the data
that any dark matter particles
would be completely overwhelmed.
It would be like trying to hear
a pin drop on a busy city street.
To solve this problem,
scientists have had
to dig deep into the Earth.
Dark matter experiments are set up
in specialized underground labs,
either in mines or inside mountains.
The rock that makes up
the Earth’s crust works like a filter,
absorbing radiation
and stopping disruptive particles.
The ultra-pure water
in which the detector is suspended
adds an additional layer
of radiation filtering.
This shielding ensures that only
the particles scientists are looking for
can make their way into the detectors.
Once these particles reach
an experiment’s inner vessel,
scientists have a chance
of detecting them.
The detector media are chosen because
they’re exquisitely sensitive detectors
that can be purified extremely well.
These could be a liquid noble gas,
germanium
and silicon crystals,
a refrigerant,
or other materials.
When radiation interacts,
it leaves tell-tale signs,
such as light or bubbles,
which can be picked up by the sensors
inside the detector.
The detector media are held
in a central chamber made of glass
or a special type of acrylic.
These chambers have to be able
to hold the substance inside
without interacting with it
while withstanding incredible pressure
from the water outside.
The inner vessel is surrounded
by powerful sensors
designed to detect even the
tiniest blips of light,
or the sound vibrations
caused by a single bubble.
Each sensor records data 24/7,
and experiments run for months
and years at a time,
generating terabytes of data every day.
Building dark matter detectors
is as much a feat of engineering
as it is a feat of physics.
By the time an experiment
is ready to start collecting data,
years or decades of work and investment
have already gone into it,
to the tune of tens
of millions of dollars.
As of 2017, no dark matter particles
have been directly detected.
That’s not entirely surprising.
Physicists expect these interactions to be
incredibly rare and difficult to detect.
In the meantime,
scientists continue
to develop new technologies
and increase detector sensitivity,
closing in on where dark matter is hiding.
And when they find it,
we’ll finally be able to bring the
universe’s darkest secrets into the light.
