
Dark Matter Detector Records One of the Rarest Events Known to Science - elorant
https://www.sciencealert.com/a-dark-matter-detector-just-detected-one-of-the-rarest-events-ever-in-science
======
lisper
Arstechnica has a much better summary:

[https://arstechnica.com/science/2019/04/dark-matter-
detector...](https://arstechnica.com/science/2019/04/dark-matter-detector-
identifies-extraordinarily-rare-radioactive-decay/)

But the best source is the original paper, which is quite readable even for a
non-expert:

[https://arxiv.org/pdf/1904.11002.pdf](https://arxiv.org/pdf/1904.11002.pdf)

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zw123456
This is, I am sure, a dumb question, but I read this
[https://en.wikipedia.org/wiki/XENON](https://en.wikipedia.org/wiki/XENON) and
I just don't know enough to completely understand the concept, but my dumb
question is how do they know the event they saw was a spontaneous decay and
not caused by a WIMP or whatever Dark Matter is? If it is such a rare event,
how do we know that it wasn't a dark matter particle that caused it but the
reaction was different than anticipated ? Can anyone take on a ELI5 here ?

~~~
lisper
It's not quite as rare as the article makes it out to be. It's not like they
by chance happened to detect a single event that only happens every few
trillion years. They had a TON (actually three tons) of xenon. Decay events
happened every few days, and they collected over 100 of them. Those events
have a particular "signature" that allows them to be distinguished from other
kinds of events. Even then, they needed to apply statistical methods to
compute the odds that the data they were seeing was real and not from random
chance, and the result was that the odds were not quite high enough to qualify
as a discovery by the usual standards.

Details are in the original paper, which is pretty accessible:

[https://arxiv.org/pdf/1904.11002.pdf](https://arxiv.org/pdf/1904.11002.pdf)

~~~
KeenFox
I'm always amazed at how much good science happens based on inferring things
from what's known or guessed with high confidence.

~~~
lisper
Why is that amazing? How would you expect it to work?

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madengr
Wow, 2800 pounds of Xenon, at $120/gram, is about $150M. Hope they don’t have
leaks. I thought these detectors used dry cleaning fluid.

~~~
Herrin
I worked on a different xenon-based experiment[1] for my PhD. We were looking
for similar rare decays to learn about neutrinos. Our observation of double
beta decay of xenon 136 used to be the record for rarest decay.

Xenon has some very nice properties as a particle detector for rare events:

1\. For one thing, it's a noble element, and so it can be purified very well
to reduce background decays of other elements.

2\. It's also a very heavy atom, and so it's self-shielding from external
radiation. The core of the detector is shielded very well from radiation
coming from the outside, so any signal you see there is most likely from
decays of xenon or from things like dark matter WIMPs that don't interact much
with matter.

3\. It is a natural scintillator[2]. It gives off light when an interaction or
decay ionizes the xenon atoms. That lets you actually detect the event, and by
collecting the scintillation light, and the electrons from the ionization, you
can get a decent measurement of the energy of the event.

4\. It's recyclable. The XENON1T experiment follows the XENON100 experiment.
The 100 kg from XENON100 were reused in XENON1T, and the tonne from XENON1T
will be reused in future experiments. So the cost gets amortized.

$120/g sounds on the expensive side. The price is always changing based on
supply and demand. One manufacturer deciding to use xenon in some process, or
finding a way to replace xenon with argon, can swing the price by an order of
magnitude.

As for leaks, I can say on our experiment we took the possibility very
seriously. The entire xenon gas system was made of ultra-high-vacuum plumbing,
and we helium leak checked every connection. When the xenon was outside the
experiment in bottles, we had sniffers around the bottles to make sure they
weren't leaking. We also had emergency systems in place if we needed to
recover the xenon, including a "balloon of last resort" that would've captured
the xenon in the event of a catastrophic failure.

[1] [https://www-project.slac.stanford.edu/exo/](https://www-
project.slac.stanford.edu/exo/)

[2]
[https://en.wikipedia.org/wiki/Scintillator](https://en.wikipedia.org/wiki/Scintillator)

~~~
joncrane
Can you talk more about the "baloon of last resort" and the process to
recapture the gas from it?

Has a "baloon of last resort" ever been needed in any similar experiments?

~~~
Herrin
We never had to use it, thankfully.

Our experiment (like the XENON experiments) used liquid xenon. The boiling
point is roughly -110 °C, so it requires cooling to stay liquid. If that
cooling had failed, the xenon would have started to boil and turn to gas.
Gaseous xenon takes up something like 300x more volume than liquid.

So we had a few things to deal with this. We had some pretty giant UPSes to
provide backup power. Imagine a shipping container filled with lead acid
batteries. That was enough to keep the cooling running for about a day. During
that time, we would start recovering the xenon. That would involve running
compressors to stuff it back into bottles before power ran out. We also had a
limited ability to cryopump the xenon out. Cryopumping involves cooling a gas
cylinder (usually with liquid nitrogen) so the gas condenses inside. But that
was always limited by the amount of LN we had on hand, which wasn't much.

But suppose we couldn't get all the xenon back into the cylinders before power
ran out, or if our compressors failed. First, our detector would have failed.
It was made of thin copper to reduce radioactivity. The xenon would have mixed
with the HFE (Novec) fluid we were using for coiling. After that, as the xenon
continued to boil, it would have burst some burst disks built into the system.
And those were connected to the balloon.

The "balloon" was some plastic material, about 10x10x20 meters that we had
stashed in an alcove off to the side of our experiment. It would have
hopefully contained the xenon.

Since we never had to use it, I'm not completely sure what the process would
have been. We would have shipped it off to some industrial gas facility, and
they have the equipment to distill it out. And then we'd probably have to
spend more time purifying it ourselves.

I doubt such a thing has ever been used. Most experiments, even underground
ones, weren't dealing with the constraints we were. The mine we were in wasn't
dedicated to science, and a dedicated facility would have had better support.
For example, we couldn't run generators to deal with a power failure because
there were limits to how much diesel equipment could be running underground
with only natural ventilation, and the ventilation fans didn't have backups.
Likewise, a dedicated facility would have had a better supply of liquid
nitrogen. We only had one portable dewar, rather than a large tank.

------
le-mark
_Thanks to the XENON1T dark matter detector lodged under the Gran Sasso
mountains of Italy, scientists have recorded one of the rarest events to ever
be detected: a special type of radioactive decay in xenon-124.

It's an amazing feat, because the decay of this isotope is extremely,
extremely slow. In fact, xenon-124 has a half-life of 1.8 x 10 to the power 22
years – roughly one trillion times longer than the age of the Universe._

Does that qualify as impossible odds?

~~~
gus_massa
(I'll repost one of my comments in a similar thread, with a few minor
changes.)

If you have only one xenon atom in a box, to have a 50% chance that to see
that it decayed you should wait a "1.8 x 10^22 years—or about a trillion times
the age of the Universe".

If you have two atoms in a box, you must wait approximately half of the time
to have a 50% chance to see the decay. (It's not exactly one half, there are
some technical details here, but one half is a good approximation.)

They have 2 tons of xenon. I'm not sure if 1 ton = 1000kg or 907 kg, but it
doesn't change the result too much. I'm also not sure about the isotope of
Xenon they are using, so I'll use 131 as the atom mass, but it doesn't change
the result too much. With 2 tons you have 2 x 907 * 1000 / 131 x 6.02x10^23 =
8E28 xenon atoms, so you expect to have a few decays per hour (if I did the
calculations correctly).

I'm not sure if they can detect the 100% of the decays and the noise level, so
it probably takes a longer time to detect a good signal.

~~~
contravariant
>If you have two atoms in a box, you must wait approximately half of the time
to have a 50% chance to see the decay. (It's not exactly one half, there are
some technical details here, but one half is a good approximation.)

Really? As far as I know radioactive decay is mostly believed to be a Poisson
process (or rather the time it takes for a single atom is believed to be
exponentially distributed), in which case the time it will take is _exactly_
one half, on average.

~~~
a1369209993
Are the two of you using the same answer to the question "Does a X% chance of
2 events (in given interval) contribute X% or 2X% to the chance of seeing one
event?"? That is, do you mean [time for a 50% chance of seeing ≥1 decay] or
[time to see, on average, 0.5 decays].

~~~
contravariant
In my case I'm talking about the (average) time until the first decay.

------
eps
For something this exceedingly, impossibly rare I’d expect an in-depth
discussion of how this is NOT an instrument mistake or a mis-interpretation of
the measurements. There appears to be none of that.

~~~
SiempreViernes
It's a 4.4 sigma detection, so not a discovery by particle physics standards.
[https://www.nature.com/articles/s41586-019-1124-4](https://www.nature.com/articles/s41586-019-1124-4)

I've heard suggestions that the result is more of a curiosity than of
fundamental importance, and that this is why they get to publish a uncertain
result in nature.

~~~
zaroth
_4.4 sigma ... uncertain result_. You partical physicists are a funny bunch!

~~~
SiempreViernes
The famous 750 GeV bump of 2016 reached a maximum significance of 4.4 before
dropping down as more data was added.

Particle physics has a _lot_ of events, so locally improbable things get
likely when considering the totality of the data sets.

