[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/

[2] 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.

[jloughry]

Also, you don't want to breathe it; xenon is an anesthetic gas and will put you to sleep.