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China's PandaX Project Looks For Dark Matter In the Heart of a Marble Mountain
the_newsbeagle writes "Chinese engineers love their superlatives: Biggest dam, fastest train, etc etc. Now they've constructed the deepest underground dark matter detector beneath a mountain in Sichuan province. Such dark matter seekers have to be buried deep to shield them from cosmic rays, because that radiation would be picked up by the detector and could be confused for radiation generated by dark matter. Other dark matter detectors are similarly subterranean: LUX, in the United States, is at the bottom of an abandoned mine in South Dakota, and a European effort called XENON lies below the Gran Sasso mountain. The Chinese researchers hope their PandaX detector will finally reveal the much-hypothesized, never-seen dark matter particles known as WIMPs."
Actually, XENON isn't a European project, it's an international collaboration with leadership in the United States and members in Europe and China. The device is in Europe, but that's sort of incidental. Here's the membership: XENON-100
I predict (On nothing more than an opinion) it won't be found because the current models have simply not accounted for enough normal matter. We're only talking one decimal place, right?
Yeah, those Chinese engineers.
Like Superbowl.
Hah!
We just had this last Friday, except that was for the search for WIMPs in an Italian mountain. Not much changed in the search for WIMPs in the last week.
Ah, so less than one decimal place then. Even closer than I thought.
I can't help to feel a bit of both schadenfreude and melancholy. Being a westerner myself, I wonder if Neil Stephenson were more of a prophet than a futurist.
for some Dark Matter today
In case anyone was wondering, WIMPs are Weakly Interacting Massive Particles.
It seems likely that DM is akin to foam or a positive warpage of regions of spacetime. Such positive warpage of spacetime would appear to increase the gravitation of subsumed/circumscribed regions in the same way that negative warpage/depression normally results in gravitation.
As to the exact mechanism that would give rise to such positive warpage, one might speculate that matter-antimatter collisions are not nullifications as is normally assumed, but produce some sort of quasi-particles that are retained in spacetime and may give rise to baryonic matter in minute quantities. Most of it will never recombine into baryons, at least not for some time, and will be retained, causing the positive curvature.
Indeed, this foam may be responsible for the existence of normal matter altogether, as it may have protected the matter from collisions during/after the BB. The extra antimatter was able to collide with enough recombined matter from the froth to cancel itself out leaving our matter behind.
The gap is large than that, as the 5:1 ratio given there is from things like the CMB, and not direct observation of matter. Direct observation of matter only finds about a quarter or less of the matter that the such a model predicts, so saying there is 5 times as much dark matter as matter already assumes there is a huge chunk of normal matter we haven't yet seen. No one is assuming we are omniscient and have seen all there is to be seen, although there are some quantitative upper bounds on specific kinds of visible matter in narrow contexts (e.g. microlensing searches).
A marble mountain? Here I've been under that impression that both granite and marble had a detectable amount of radioactivity of their own, so even given 20 miles of the stuff, there would still be a background count contaminating the data.. Can someone fact check me on that?
Cheers, Gene
> Chinese engineers love their superlatives
I wonder where they learned *that* from?
Max.
[Never let anyone see the undersides of your family.] AKA "Put your best foot forward" A chinese philosophy that dooms them to fail. So goes the "Biggest dam, fastest train" myth. It's all shiny until it fails. Cracks in a newly built dam. High speed trains held together with duct-tape. I'm surprised they didn't use duct-tape to hold together their mission on the moon. Or did they?
The biggest worry for a dark matter experiment is neutrons. Alpha radiation and beta radiation are charged particles, and are stopped by a modest amount of material because the electromagnetic force is strong. Gamma radiation is photons, but because there are so many charged nuclei and electrons in matter, a gamma ray will eventually interact in enough shielding. Typically that's something like a a meter of lead. But neutrons aren't charged, so they don't experience the electromagnetic force, and so they can pass through shielding more easily.
What's worse is neutrons look similar to the signal they're looking for. A WIMP will recoil off a nucleus in the detector, leaving behind some energy. Charged particles are more likely to recoil off he electrons in the atoms. But neutrons also recoil off of nuclei, and so a lot of neutron radiation would make it difficult to pick out a WIMP signal.
The radiation from uranium, thorium, and potassium in the rock is mostly alpha, beta, and gamma radiation, and so they can shield it. But one of the biggest sources of neutron radiation is cosmic rays. A cosmic ray muon (which is like a heavy, unstable version of the electron) is heavy enough that it can knock neutrons loose from their nuclei. Muons are at almost a perfect charge to mass ratio that they can pass through matter without losing too much energy. It takes several thousand meters of water to shield from all the muons being created in the upper atmosphere. At the earth's surface, there's about one muon per square centimeter per second. Deep enough underground, you can cut that to one per square meter per day or better. That's something like a billion-found reduction in muons, which also means a large reduction in neutrons.
Just about every kind of rock has background radiation of its own, which must be dealt with (some more than others). However, radiation from rocks is typically easier to deal with than cosmic rays from space --- it's lower energy stuff that can be blocked by a few extra layers of extra lead/copper shielding (carefully screened for even lower radioactivity), instead of energetic particles that go through hundreds of meters of material unhindered. You have to worry about things like radon (radioactive gas) seeping out of the rocks and getting into the equipment; but, these are known effects to watch out for deal with by proper ventilation/sealing.
Don't worry, there is clearly lots of marble left to be mined, your future countertops will continue to be as cheap as your disposable electronics.
...a cosmology news story so a bunch of Slashdot armchair physicists without a lick of physics education past high school can tell the science community they've been doing it wrong for the last 70 years.
Generally speaking, the deeper the mine is the less likely it is to have to deal with cosmogenically activated radiation sources or direct cosmic radiation. Same reason that these kinds of experiments often use "ancient lead" that has been buried under the seas for thousands of years: the deeper you go. Stuff that has been underground for thousands or millions of years is vastly less likely to have been made radiative from the sun or other cosmic sources.
"None can love freedom heartily, but good men; the rest love not freedom, but license." --John Milton
Same reason that these kinds of experiments often use "ancient lead" [aspera-eu.org] that has been buried under the seas for thousands of years: Stuff that has been underground for thousands or millions of years is vastly less likely to have been made radiative from the sun or other cosmic sources.
Newly-mined lead is not radioactive due to the sun or cosmogenic sources, but because of isotopes of lead which are produced from decays of uranium and thorium (which already existed from the supernova remnants that formed our current solar system) --- see here. So, you don't want lead that you've just mined from millions of years deep underground --- that stuff is still pretty hot. What you want is stuff that was mined by the Romans two millenia ago, separated from the underground crud full of uranium/thorium, and left to cool off since then.
Go ahead and see how well you do making a mountain of marbles.
(hint: explore "angle of repose")
On a serious note, that radiation is fairly well known, and can be filtered out I assume.(I could be wrong)
Also, I am assuming that cosmic radiation may have different characteristics than underground(known?) radiation.
At least that is what I gathered from TFA.
Disclaimer: While astrophysics is a hobby interest, I am by no means an astrophysicist. If this comment lowers your IQ, I apologize. :-)
Down With Slashdot BETA!!! I've been around the corner and seen the oliphant; you can only abuse me from your perspecti
Have the Chinese created the black hole that will swallow the Earth yet? What's taking them so long? Destroy the Earth already!
Everyone knows the best place to find dark matter is on Uranus
If you mod me down the terrorists will have won
Hey, I know a dark subterranean place where you can find lots of WIMPS ...
I know that the detector tank in the bottom of Homestake is lead shielded, but that lead is very old, no newly mined lead in it. It had to be at least 100 years old to even be considered for recycling into that shielding. I used to live in Rapid City in the '60's, even have a wife I still miss buried there, but in those years, Homestake, 50 miles away in Lead, SD was an actively producing gold mine. And environmental disaster as it struggled to remain profitable, it eventually had to close, and I am glad that another use has been found for its extended underground.
The Lead/Deadwood area tried to survive on tourists, but I imagine much of that allure has faded after the state raided and closed the Pink Lady in the '70's, the countries oldest continuously operated whorehouse. The girls were clean, checked daily to keep them that way, and they contributed 5 to 7 million a year to the local charities. When they had the liquidation sale, somebody wanting a piece of history had to bid $50,000 just to get the front door. End of an era as it had been there, a fully functioning, locally respected member of the community for over 140 years. I felt a little sad at the passing of a legend.
Cheers, Gene
That's a far better explanation than mine, thanks. My wet ram, at nearly 80, doesn't always recall the scale of the age, just that it was old and they paid a high premium for it because it was old & well "cooled".
Cheers, Gene
He was mocking you.
Why is there so much interest in searching for Dark Matter?