The XENON dark matter research project is an interesting long-running project that strives to detect dark matter. I understand that the basic principle of the project design is that WIMPs are expected to "flash" and "ripple" when they interact with an atom's nucleus, as detailed in this Physics post.

My question is what makes liquid xenon the best medium for this? I understand that xenon is extremely low reactivity and has a really long half-life, but it seems to me you should want a highly reactive medium if you are hoping to find weakly interactive particles. Low reactivity seems like the wrong tool. What am I missing here?

  • $\begingroup$ I could be wrong, but I think part of it has to do with experimental control: Xenon is a relatively stable medium, so that you know precisely how reactive it is, that is, it is easy to control and easier to distinguish various nuclear processes. If one had a more reactive medium, then there could potentially be other interactions that may be difficult to parse from a dark matter detection. $\endgroup$ – N. Steinle Apr 25 '19 at 21:02
  • $\begingroup$ I think when xenon is described as "low reactivity" that should be read as "low chemical reactivity". Once you make it, it doesn't disappear through unwanted reactions with your container or impurities. $\endgroup$ – BowlOfRed Apr 25 '19 at 21:02
  • $\begingroup$ @BowlOfRed Yes, but the XENON project is trying to find nuclear force reactions. That's heavily in the "chemical reaction" camp, no? $\endgroup$ – 21380 Apr 25 '19 at 21:04
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    $\begingroup$ @fredsbend No, I wouldn't say so. Chemical reactions are those that interact with electron and rearrange electron shells, not nuclei. Here's an interesting document with some comments about using noble liquids on page 30. taup2009.lngs.infn.it/slides/jul1/mckinsey.pdf $\endgroup$ – BowlOfRed Apr 25 '19 at 21:18
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    $\begingroup$ @BowlOfRed Ah, yes, that makes more sense. Sorry, my chemistry and physics education stopped in highschool, many years ago. $\endgroup$ – 21380 Apr 25 '19 at 21:23

The liquid xenon is serving a dual purpose.

First, the xenon nuclei serve as a target for the WIMPs to interact with. There are a number of hand-wavy reasons why heavy nuclei would be more likely to interact with most kinds of WIMPs than low-mass nuclei: more mass density, more charge on the nucleus, more electron density in case the WIMP interaction is with the electrons rather than the nucleus, and probably some other reasons. The details, and the theoretical models which justify them, occupy a chapter in most PhD dissertations on WIMP searches.

Second, the liquid xenon acts as a scintillator, transforming deposited energy into light which can be carried to a photodetector. This is true regardless of the source of the ionizing radiation: the xenon also scintillates in response to cosmic ray muons and intrinsic radioactivity.

Noble liquids are nice to work with experimentally because they are self-purifying: any chemical contaminant which is not xenon will freeze out of the liquid. All of the noble liquids have similar chemistries and behave as scintillators. (There are some neutron experiments which use liquid helium as the same kind of target-plus-detector combination.) But if increasing the nuclear mass increases the cross-section for interacting with WIMPs, you want your target to be made of the heaviest available nucleus. And the heaviest non-radioactive noble gas is xenon.

  • $\begingroup$ Thanks, this clears up some things. Could they not make a noble gas cocktail, covering heavy and light weights at the same time? Also, if density is wanted, wouldn't a solid scintillator be better, maybe frozen xenon, since that's only a few degrees cooler? $\endgroup$ – 21380 Apr 25 '19 at 22:05
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    $\begingroup$ liquid xenon is transparent, and denser than aluminum per (youtube.com/watch?v=rs3o3xDYgZ0). $\endgroup$ – JEB Apr 26 '19 at 0:45

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