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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

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?

qzw 8y ago
Dredging up memories from high school chem: not actually impossible odds. Half-life means the time it takes for half of the atoms out of a given quantity to decay. There are 6.02E23 atoms per mole, and about 8 moles per kg for xenon. So with a large enough quality of xenon-124, some decay event should happen within a reasonable timeframe. Of course it’s still a tremendous technical feat to actually setup and detect such an event.
Benjamin_Dobell 8y ago
I think the article just did a poor job of portraying the rarity of the event. This article links to a Gizmodo article[1] which does a better job of explaining that they had 3.2 tonnes xenon-124 and many occurrences: > After 214 days of observing (177 days of usable data), the researchers’ analysis revealed approximately 126 two-neutrino double electron capture events. [1] https://gizmodo.com/dark-matter-detector-makes-incredible-neutrino-observat-1834270148 https://gizmodo.com/dark-matter-detector-makes-incredible-ne...
- ianai 8y ago
  Indeed, larger samples will observe higher rarity events/outcomes.
iamgopal 8y ago
It's probability, not rule I think.
gus_massa 8y ago
(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.
- deleted 8y ago
  [deleted]
- the8472 8y ago
  The decay in question is from xenon-124, so they're probably using a natural isotope mix which will also contain those quasi-stable ones.
  - gus_massa 8y ago
    Wikipedia says that the natural abundance of Xenon-124 is 0.095% = 1/1000. Adding that correction I get a few decays per day that is more similar to the data reported in other commit that is 126 events in 177 days. Edit: I think I'm missing a ln_e(2) in the calculation, but it's only a 30% more or 30% less. It's not a huge change in the result.
    - tibyat 8y ago
      article says “1,300 kilograms (2,866 pounds) of xenon isotope” not sure if that was intentional..

qzw 8y ago

Dredging up memories from high school chem: not actually impossible odds. Half-life means the time it takes for half of the atoms out of a given quantity to decay. There are 6.02E23 atoms per mole, and about 8 moles per kg for xenon. So with a large enough quality of xenon-124, some decay event should happen within a reasonable timeframe. Of course it’s still a tremendous technical feat to actually setup and detect such an event.

Benjamin_Dobell 8y ago

I think the article just did a poor job of portraying the rarity of the event. This article links to a Gizmodo article[1] which does a better job of explaining that they had 3.2 tonnes xenon-124 and many occurrences: > After 214 days of observing (177 days of usable data), the researchers’ analysis revealed approximately 126 two-neutrino double electron capture events. [1] https://gizmodo.com/dark-matter-detector-makes-incredible-neutrino-observat-1834270148 https://gizmodo.com/dark-matter-detector-makes-incredible-ne...

ianai 8y ago
Indeed, larger samples will observe higher rarity events/outcomes.

iamgopal 8y ago

It's probability, not rule I think.

gus_massa 8y ago

(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.

deleted 8y ago
[deleted]
the8472 8y ago
The decay in question is from xenon-124, so they're probably using a natural isotope mix which will also contain those quasi-stable ones.
- gus_massa 8y ago
  Wikipedia says that the natural abundance of Xenon-124 is 0.095% = 1/1000. Adding that correction I get a few decays per day that is more similar to the data reported in other commit that is 126 events in 177 days. Edit: I think I'm missing a ln_e(2) in the calculation, but it's only a 30% more or 30% less. It's not a huge change in the result.
  - tibyat 8y ago
    article says “1,300 kilograms (2,866 pounds) of xenon isotope” not sure if that was intentional..