Dark Matter remains one of the more vexing
questions in science.
We’ve devised all sorts of ways of looking
for it but still have found nothing.
In April of 2019 though, a team analyzing
data from a dark matter detector called XENON1T
announced it had witnessed something extraordinarily
rare.
XENON1T was an experiment that’s pretty
much what it sounds like:
an absolutely enormous tank filled mostly
with the element xenon
cooled to -96 degrees Celsius.
While the vat contained 3.2 metric tons of
liquid xenon, the experiment had a targeted
exposure rate of 1 metric ton per year, hence
the 1T in the name.
What were they trying to expose the xenon
to?
Dark matter, obviously, but specifically one
favorite candidate for what dark matter could
be called Weakly Interacting Massive Particles,
or WIMPs.
WIMPs are thought to be heavy, slow-moving
particles.
As the name would suggest, these hypothetical
particles don’t interact with normal matter
much,
even though about a billion of them are predicted
to pass through each square meter on Earth
every second.
So the hope is that while watching a gigantic
tank of xenon very closely,
a WIMP will collide with an atom, transfer
some energy to the atom’s nucleus, and in
turn will excite other xenon atoms.
The process will release faint signals of
ultraviolet light and trace amounts of electrical
charge
which can be detected by sensors at the top
and bottom of the tank.
To make sure the experiment was isolated from
sources that could cause false signals like
cosmic rays, the xenon was about 1,400 meters
beneath a mountain in Italy.
Then just to be safe, the detector was shielded
inside a tank of water nearly three stories
tall.
Once the experiment was set up, it was allowed
to run “blind,”
meaning the scientists couldn’t access the
data of interest until the analysis was done.
Now the results are in.
If you couldn’t tell by the fact that I’m
not wearing my “We Detected Dark Matter”
party hat, we didn’t detect any dark matter.
XENON1T collected data from 2016 to December
of 2018 without a whiff of a WIMP.
But this experiment wasn’t a failure.
In fact, it saw something that had never been
seen before.
There are nine isotopes of xenon, and one
in every thousand is xenon-124.
Xenon-124 was thought to be relatively stable,
but would still decay into Tellurium-124 by
a rare process called two-neutrino double
electron capture.
This occurs when two protons in the nucleus
simultaneously grab two electrons from the
nearest shell, turning into neutrons and releasing
two neutrinos.
As electrons in higher shells cascade down
to fill the holes that have been created,
they give off X-rays and also free up other
surrounding electrons.
However, these telltale signs are very hard
to detect as they can be masked by background
radiation.
So before we could measure it, the half-life
of xenon-124—that is, the time it takes
for half the xenon-124 in a sample to decay
to tellurium—was thought to be about 160
trillion years.
XENON1T was designed to be extremely sensitive
and isolated from background sources,
and after pouring over the data, the scientists
noticed 126 instances where the detectors
picked up signals that matched those expected
by xenon-124’s double electron capture.
These instances allowed them to calculate
how long xenon-124’s half life actually
is.
Are you ready?
Because it’s a big number.
Xenon-124’s half life is actually 18 sextillion
years.
Thats 18 with 21 zeros after it, over a trillion
times longer than the current age of the universe.
That’s the longest half life we’ve ever
directly measured, over now second place bismuth
209’s 19 quintillion years.
While XENON1T didn’t spot a WIMP, the team
is excited by the discovery of their record-setting
half life.
It’s not only a feather in their cap, but
a demonstration of just how sensitive their
instrument is.
They’re not giving up either: an even bigger
tank of about 8 metric tons of xenon is being
built.
While another consolation prize, like the
even rarer decay of the isotope xenon-136
would be nice, I’m sure the team is really
holding out for the sign of a Dark Matter
interaction, and you can bet I’ll have my
party hat ready if it happens.
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Experiments with Xenon aren’t the only way
we’re looking for the missing mass of the
universe.
Check out this video on How Close We Are to
Finding Dark Matter.
Make sure you subscribe to Seeker to know
when we peer even further down into the details
of the universe, and as always, thanks for
watching.
