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Underwater telescope to study neutrinos
Darksky wrote to us with information about the proposed 'Antares' telescope. The proposal would be to put a telescope 2.4 kilometers underwater, in an attempt to study neutroino/cosmic rays. The telescope would use the the Earth as shield from cosmic rays, and hopefully study the muons liberated by the neutrinos.
It's a pain in the ass, I'm telling you.
We can produce massive amounts of neutrinos. But it also takes massive amounts of work. I work on a project called MINOS--Main Injector Neutrino Oscillation Search. We will be shooting a beam of neutrinos (mainly muon type) from Fermilab outside of Chicago to northern Minnesota. At each end, we will look at the flux of each type of neutrino and compare the ratio of types. If the ratio is significantly different than 1, but the overall number of neutrino ratio between detectors is around 1, then this will indicate that neutrinos do indeed have mass. Cool project, but a little beside the point.
Anyway, I've been touring the facilities the last few days, and I can tell you that the accelerator beamline is over 1 mile long, just to reach northern Minnesota. The two detectors, one on each side, will require abou 2,700 tons of steel each (no exaggeration!). So while a neutrino antenna may be able to use different (lower) energies than we are using, I can't imagine that the required equipment would be much different than this. More information can be found at U of Minnesota.
Right, I assume that's what's meant by "if you can produce and detect neutrinos." On top of the concern you've written, there's also the problem that there's an enormous background of neutrinos whizzing around from stars, supernovae, and the matter-antimatter annihilation after the big bang.
Separating your signal from this noise would be difficult, I would think, even if you could detect neutrinos readily.
Are you on drugs? The best line up there is 'Yet the fact is that we cannot directly measure or observe gravity.' Please tell me your entire post is some sort of joke?!?
- Hey
- , we {really|actually|probably|...} should be looking for gravitons. They would have useful and practical value in society. Imagine {such things as|having|having such things as} graviton space engines, graviton suspensor fields, and the like... yum! But anyway...
Sorry to be picky, but usingHemos, it's "neutrino." Man, you need a spelling checker.... ;)
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"'Is not a quine' is not a quine" is a quine.
"'Is not a quine' is not a quine" is a quine.
Quine "quine?
I was under the impression that it's still an open question whether neutrino oscillations occur.
Like what if it's Schrödinger's cat? As far as you know the barfed up gravitons are both there and not there...
Yes, the Sudbury Neutrino Observatory is located
in an old mine in Northern Ontario. Check out:
http://www.sno.phy.queensu.ca/
some novel idea. http://amanda.berkeley.edu http://www.pico.unl.edu amanda has been around for a few years.. not as easy as dropping a few pmt's - photo multiplier tubes overboard and pointing them at the ocean floor..
Maybe I'm missing something but since we don't know where cosmic rays come from (according to the article), and since the earth's mass is only blocking half of the spatial sphere around the Antares, isn't there going to be some kind of problem with cosmic rays coming from the half of the spatial sphere around the Antares that isn't blocked by earth's bulk?
"Destroy science and religion. Science would re-emerge exactly the same; but not religion." - Penn Jillette, paraphrased
Don't believe me? Check out the Autodynamics founders' response on the last neutrino "evidence".(Super-K) Have an open mind; think for yourself before you judge.
I don't know what became of DUMAND; it may have fallen prey to Congress in a budget cycle, because it was too small to have a constituency to defend it. Kind of like NASA's science programs.
I'm working in one of the labs that collobrated for DUMAND and I believe that it died due to budget cuts. Some of the detector strings are being used in NESTOR in Greece though.
"When you sit with a nice girl for two hours, it seems like two minutes. When you sit on a hot stove for two minutes, it
You sort of got the lead part backwards :) Lead stops gammas perfectly well. You have to be careful when using lead to shield betas, because you get bremstrahllung (sp?) X-rays. Compact neutron shielding is the hardest one. You first have to thermalize any fast neutrons (with water or some kind of plastic with lots of hydrogen, for the reasons you described), then surround that with a neutron absorber (e.g. boron or cadmium), but _that_ produces gammas (and alphas also in the case of boron), which have to further be shielded by, e.g. lead.
But they're looking at neutrinos, not neutrons anyway.
The problem with undersea detectors is that you can't control the water quality as well as in a tank. Also there is a limitation on the effective size of the tank due to the attenuation of the cherenkov radiation by water. Pure 18megaohm water has an attenuation length of about 300 meters(the intensity drops by ~67% after 300m) so the volume you're sampling is limited by the number and location of the detectors. True you can add more detectors but there are pratical problems with that.
The shielding in the mines are pretty good considering its about 1500m underground so you have about 1500m of earth and rock shielding you. However the ocean detectors can have more shielding if it's placed at a sufficient depth.
"When you sit with a nice girl for two hours, it seems like two minutes. When you sit on a hot stove for two minutes, it
Okay, what are the side effects of being exposed to a constant bombardment of this type of energy, could this be lethal? Will the bird building its nest on the detectors be fried? I don't know enough about dangerous forms of energy, but the xray post about communications made me think about using these things for communication, hence the thought of danger, if I recall correctly x-rays aren't the best to have shot on you ;)
SNO should be able to measure mass of the neutrinos, because it is able to measure the three flavors of neutrino. The various other detectors (not sure about Super Kamiokande) only cover some part of the energy spectrum, and thus can't simultaneously see the three flavors (electron, muon, and tao meson). SNO provides both good timing resolution and signal level information, which may show promise. I think it either is just about to go online, or just has.
Sure. But given that they largely pass through ANYTHING due to their small size, or whatever, how do you plan to receive the transmission?
The idea is actually quite old. There is a small detector in lake baikal, and since some years the detector at the southpole. To find out more, go to this link: www.ifh.de/nuastro/index_e.html Using lakes or ice instead of watertanks has the sideeffet that you can make your detector larger when you want to. AMANDA at southpole is planned to be a cubic kilometer!
uh-huh, is that why we can't use radio waves for transmission? Because there's too many natural sources? It is too difficult to distinguish your signal from the static? Or are you saying that there are NO natural sources of radiation in the "radio bands?"
The more general form of that equation is E^2 = p^2c^2 + m^2c^4, where p is the momentum and c is the speed of light (299792458 metres per second).
If you see an electron flying past with momentum p, you can speed up until you're travelling with the same velocity, at which point it will look (to you) as if it has no momentum, p=0. Then you will measure it to have energy E=mc^2. Now, if neutrinos had zero mass (it looks like it's close to, but not quite, zero), they would travel at the speed of light - in which case you could *never* speed up until you were travelling with the same velocity, since you have mass and therefore can't get to light speed. Hence, if you measure the energy of a neutrino, it will always come mostly from the momentum, not the mass. In fact, this is true for any particles travelling fast enough (the physics jargon term for "fast enough" is "highly relativistic" - meaning that the speed is so high that Newtonian ideas go out the window).
If you're interested, find a decent text on relativity:- the relevant chapters of Halliday, Resnick and Walker, "Fundamentals of physics" are quite good at explaining the basics without needing anything more than high-school maths.
The rate of interaction (cross-section) between normal mater and neutrinos is vanishingly small. However, if you make a lot of them, it begins to be a problem. But if peoples guesses have any meaning, we would need several orders of magnitude more of them for them to start doing the kind of damage cosmic rays do.
It was a simple question dumbass..I thought I had read, somewhere, that they had used an old mine because of the shielding it provided. I'm was inquiring if anyone knew anything further. That's all. Sooorryy, I guess I'm not as worldly as yourself.
Yes there have been quite a few underground neutrino detectors over the last ten years or so. However all of them keep returning anomalous results. They were intended to measure the level of neutrinos streaming out of the sun from fusion reactions. However, the measurement from the detectors has uniformly been below the predicted level of neutrinos by a factor of three. Originally it was merely thought to be an error in measurement or faulty detectors. But as more facilities and more sensitive detectors were built, the measurement of 1/3 of the neutrinos that calculations predict has gotten increasingly solid.
:)
Perhaps this one will finally shed some light on the 'puzzle'.
The current models of the Sun indicate great quanities of neutrinos should be produced, but observation has said otherwise. This project might be able to figure out where the lost neutrinos have gone.
anyway, the ramblings from an ex-particle physics geek.
--
Gonzo Granzeau
Gonzo Granzeau
"Nothing the god of biomechanics wouldn't let you into heaven for.." -Roy Batty
actually, someone has already hung a network of photomultipliers off of Hawaii to do the same thing.
As far as the people who put big tanks of stuff with photomultipliers in them, they usually use dry-cleaning fluid in them, not H20. They have the PM tubes to detect the flashes, but actually count the amount of argon gas "cooked" off by the neutrino collisions for the aggregate rate.
Oh, but there _must_ be, if current quantum theory is correct.
Each force has an associated particle that carries it. Too bad I can only remeber that photons carry electomagnetic, and gravitons are _supposed_ to carry gravity.
Of course, the theory could be wrong...
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making neutrinos will be much easier than detecting them.
Check out the capture rate of the neutrino detectors... it's pretty sad.
"Its" for "it's" (et vice-versa) really grinds my ass, as does sloppy punctuation.
But it mentioned Canada, and a lot of Americans mindlessly flame Canada, thinking that it's funny. Actually, it often is, but sometimes it gets on my nerves. Anyhow, I was criticizing not about your post, but about anticipated replies to your post (which, incidentally, haven't materialized -- oops). See the difference? You posted your post, and that's all you did; nobody can blame you for the replies to it, nor did I try to do so.
In a nutshell, I wasn't flaming you.
It's much more likely that my cat ate them and them threw them up on the kitchen counter. He likes to do that.
"Prejudice is wrong; you should hate everyone the same."
I was under the impression that it's still an open question whether neutrino oscillations occur.
The SuperK experiment did detect the neutrino oscillations so the fact that it happens is somewhat well established. The findings also provide strong evidence for the existence of neutrino mass since other explanations have been eliminated.
"When you sit with a nice girl for two hours, it seems like two minutes. When you sit on a hot stove for two minutes, it
It's much more likely that my cat ate them and them threw them up on the kitchen counter. He likes to do that.
Mine's too lazy and out of shape to jump up that high. He'd barf 'em up on the rug, or throw 'em behind a bookcase.
You rule, though (or should I say, your cat rules) if you can get him to throw up on a slick, easily cleaned surface. My damn cat goes for the rug like an arrow. He probably thinks he's doing me a favor by making the barf (or more often, hairballs) less noticeable. He's very thoughtful that way. Unfortunately, I usually notice it anyway, at least when I step in it.
I thought it was interesting because scientists theorize that the "missing" mass associated with current models of the universe could possibly be accounted for in the neutrinos - if it turns out the neutrinos actually have mass.
SuperK has provided good experimental evidence for the existence of neutrino mass. However, whether this accounts for the missing mass or not is still up to debate.
"When you sit with a nice girl for two hours, it seems like two minutes. When you sit on a hot stove for two minutes, it
I'd be happy to answer.... NO JOKE!! We humans do not enjoy a level of technology to directly observe gravity. Newton can watch, and feel the apple hit his head. But Newton could never, and we can never look at gravity in a microscope. WE cann't touch gravity, weigh gravity, or look at gravity. Have you ever seen gravity?? Scientist get around this problem by observing the effects of gravity on its environment. The apple falling, or the stars expanding away, or the turning of a spiral galaxie. One of the things the Super conducting super collider would have given us humans is the advanded way of observing an ever smaller arena of sub atomic particles. The people responsible for thinking up those type of devices decided that they would make a particle accelerator big enough to send two nutrinos in opposite directions at the speed of light, and actually force them to collide on a closed loop, and then (hopefully) observe very small particles breakdown into smaller particles. The Big thinkers are trying to show the existence of particles like gravitons, but that won't happen in an underwater observatory type deal. IT might take several hundred years to get that one chance collision. AND at that, they might not find what they think they will find.
It isn't a lie if you belive it.
- Larry Niven's "deep radar".
- The scanning technology used to detect the first "bobbles" in The Peace War (Vernor Vinge).
I also recall a relatively recent short story in Analog which got the hazards of being close to (as in, on a planet around) a supernova right: unless you have a neutrino shield, the number of neutrino-collision events in your body will kill you almost instantly from radiation effects. Dumping a solar mass of high-energy neutrinos overwhelms the tiny odds of any one of them deigning to notice that your atoms exist.Time is Nature's way of keeping everything from happening at once... the bitch.
In this example, gravity is a force of nature that is thought of in two ways: Strong nuclear force, and Weak nuclear force. These two forces combine to be what we call gravity.
The strong nuclear and weak nuclear force are different from gravitational forces. The strong nuclear force is mediated by gluons or mesons depending on whether it is affecting quarks or hadrons. The weak nuclear force is mediated by W-,W+, and Z0 bosons. The two forces do not combine to form gravity. Gravitons are postulated to mediate gravitational attractions but has not been detected yet.
It is a rare event to actually stop a nutrino, and the chances are slim that you can force it to happen in a lab. So some smart scientist decided one day to use an old abandoned mine under mount fuji in Japan. There is several giga-tons of mountain for nutrinos to go thru untill the poor particle finds itself in a big nutrino trap. hehehehe The equipment is so sensitive that nothing must ever move in its presence, and people are advised to not go near them.
That is totally untrue. I know for a fact that SuperK is located in Toyama near Mizumi. Also its in a aluminum mine. Last I heard Mt. Fuji wasn't being mined. Also the equipment is not that sensitive. People routinely go to the top of the tank to adjust stuff. In addition, the mining company is doing some blasting a couple hundred meters away the control center and tank and this is not affecting the instruments.
"When you sit with a nice girl for two hours, it seems like two minutes. When you sit on a hot stove for two minutes, it
That about wraps it up. I haven't had time to check out the details of this proposal. I'll be interested to see how they plan to get the water sufficiently clear that they can detect the scintillations from the muons. If the water is too murky, then the tiny Cherenkov flashes get absorbed, and you don't get any signal. Perhaps the sea water clears up if you go down deep enough?
It's surprising but some locations in the ocean have water clear enough that the attenuation length is about 200m. For example, the DUMAND site off Hawai'i had water that was about that clear. For comparision 18megaohm (100% pure) water has an attenuation length of about 300m so its not that bad.
"When you sit with a nice girl for two hours, it seems like two minutes. When you sit on a hot stove for two minutes, it
How are they going to filter out the light produced by bioluminescent marine organisms? I thought they were common in the deep ocean.
I believe that the bioluminesence occurs at a wavelength quite different from the cherenkov radiation that the neutrino interactions produce. The neutrino interactions produce radiation at about ~400-500 nm which is right at or above the upper edge of human vision. So you can probably filter out bioluminescence by constructing the appropriate filter or ignoring lower energy photons.
"When you sit with a nice girl for two hours, it seems like two minutes. When you sit on a hot stove for two minutes, it
n0w th4t all the LAME m0dEr4t0rEzZZZzZ arent fucking looking... fuck shit damn bitch piss ass
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Gravitons do exist, even if only as a theoretical construct in the realm of quantum gravity, which is the attempt to unify the theory of relativity with quantum mechanics, by trying to explain the behaviour of the universe at planck lenghts (1 x 10 to the -35th power).
A mechanism called perturbative quantum field theory has been successfully used to express and explain electrodynamics. Perturbative quantum field theory is essentially the sum of many method, summing all (or as many as possible) potential pathways which are renormalized after the subtraction of infinities from the corresponding Feynman diagrams. In the realm of quantum gravity, this same approach is being used by some physicists, and one of the theoretical constructs used to aid in this expression are gravitons, described as zero order approximations to quantized gravitational waves in flat space-time. There are already a set of expected characteristics of these particles, such that they are spin two massless particles.
It might not be immediately obvious what, if any, connection a neutrino observatory could have in regard to gravitons, but I would argue that the attempt to understand both the sources and behaviours of neutrinos, especially at high energies, would translate to a potential ability to examine events at the planck length, which to do so in an accelerator would require an accelerator 10 to the 15th times more powerful than any currently existing.
Nunc Tutus Exitus Computarus.
I believe that they tried to get the US Navy to initially fund the project. Why the Navy? Well, suppose I have this big metal tube that is travelling underwater. Suppose it is being driven by a nuclear power plant. As it is sort of difficult to shield, neutrinos get released. Now if you could detect neutrinos?
Actually, the studies of the Sun's vibrations ("helioseismology") have done a really good job of confirming that the sun is indeed put together the way astrophysicists have always claimed. Thus, the sun is indeed fusing properly.
This evidence supports the proposals that the deficit of observed neutrinos from the sun has to do with something weird happening to the neutrinos themselves rather than something weird happening to the sun.
I'm saying that the neutrino background is a lot higher than the radio background, because radio waves are easily absorbed, and because there's an enormous neutrino background from the epoch of matter-antimatter recombination. The closest parallel to that in the radio is the cosmic microwave background, which has a much smaller energy density and is not in a band we use for communication. I think you have no sense whatsoever of the differences involved. Just because there are natural sources for both doesn't mean that they are equal in magnitude.
The probability of detecting any one nuetrino is incredibly small. But the physics works out pretty well with our beam size going into 2700 tons of steel.
I don't have any numbers on me, but we are already being bombarded by tons of neutrinos. The things are so penetrating that they can go right through the earth and keep going through you and back into space. Naturally, there will be no bird nests on the detector (or anything else). In fact, the primary detector in Minnesota will be about 2/3 mile below ground. Thankfully, there is a substantial difference between photons and neutrinos: photons interact very strongly with electrons, and neutrinos barely interact with matter at all. And xrays are, of course, a type of photon. No, its not good to have xrays shot at you, because they tend to excite electrons, which effect can then cascade into all kinds of things, including genetic mutation-->cancer.
havnet they tired this before? maybe not a telescope at the middleof the earth, but a big tank of pure H20, lite hydrogen(1 proton, no neutron) some where at the bottom of a long series of tunnels under the earth?
hoping for somethign new...its an idea...
Gorfin
Oh... so that's what happened to Captain Nemo. Seriously though, that's a decent engineering accomplishment. Working under that much water could be considered more difficult than working in space.
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Am I the only one who thinks Microsoft is a misnomer? Perhaps Macrosoft would be a better fit?
Hey, just stick a Logitech web cam underwater. Heck, if we wait long enough, that guy in Florida might already have it done for us. :)
* Better get clear of this post. It's gonna get BLASTED! *
So much for the comm satellite market...
--The basis of all love is respect
Hey we should be looking for gravitons. They would have useful and prictical value in society. Imagine graviton space engines, graviton suspensor fields...yum. Anyways....
;)
The proposal would be to put a telescope 2.4 kilometers underwater, in an attempt to study neutroino/cosmic rays.
Hemos its 'neutrino'. Man, you need a spell checker...
--
Don't lead me into temptation... I can find it myself.
/me rushes to Foresight Exchange to check the ticker for activity on claim Neut....
spawn_of_yog_sothoth
Will it be able to peak into womens bedrooms???
Here are a couple of links to the antares site, with some more in depth information about the project.
/ intro_texte.html
m l
http://antares.in2p3.fr/antares/antares.html
http://antares.in2p3.fr/antares/booklet/english
Here's a site with links to most of the other research involving neutrinos.
http://www.phys.washington.edu/~superk/links.ht
Nunc Tutus Exitus Computarus.
The Sudbury Neutrino Observatory is a similar project - a sphere of heavy-water buried 2km underground.
--The more you know, the less you know.
Yes, but the whole point in going undersea is the vast size of the site. The mines in the mountains are rather limited in size and access - and still they dont shield well enough and produce too many false hits/events. Undersea you can install an arbitrary number of detectors, and the shielding is perfect.
--Coke
. . . and hopefully study the morons liberated by the nutrinos.
You can't liberate morons, by nutrition or any other means. They have to liberate themselves.
This is too, but on a different level.
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The Internet is the Suppository of All Knowledge. You get it in the end.
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Hey now... Work in Cambridge... Live in Arlington... To be fair, the sphere is a *little* bigger than that =)
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Joseph Foley
InCert Software Corp.
Some nights, when it is really quiet and no one is around, I sit really still and I can feel them shooting through me. Cosmic... ;)
Large print giveth, and the small print taketh away
There was an article in Scientific American about detecting neutrinos/etc. a couple issues back.
Aaron J. Shaver > adrec@internetcds.com
Hey now... Work in Cambridge... Live in Arlington... To be fair, the sphere is a *little* bigger than that =)
Actually, I work in Burlington, but don't tell anybody.
Come to think of it, Arlington's cool. My vet's there. I should extend the Death Line out to 128.
How are they going to filter out the light produced by bioluminescent marine organisms? I thought they were common in the deep ocean.
200m seems a stretch. The clearest SK has been is 80m - 100m, depending on how you measure it. I can't imagine ocean water being clearer than SK water. If you are correct, then it is indeed surprising.
BTW, who are you?
I could be wrong, but I think this project has everything to do with neutrinos and nothing to do with neutrons (pretty big difference)... although you are right about having to shield the detector... since neutrinos routinely fly right through the planet (and trillions through every square inch of you) every day it is a rare occurence that one gets stopped and detected. Hence the use of giant spheres of heavy water buried deep in the ground.
-r
I read somewhere that not only was neutrino flux low, but the vibrational frequency of the sun was wrong.
The sun, as everyone knows, is fluid -- it's generally considered to have a solid core, though. Because it's fluid, it oscillates somewhat, and that oscillation can be measured. The problem, according to this article, is that the period of the oscillations is wrong for a solid core -- OTOH, it's almost perfect for a constant-density fluid sphere. The first and second harmonics also match this model (I haven't seen anything about the third harmonics, presumably they don't match or are too small to measure).
This adds up to a possible explanation for the lack of neutrinos: the sun isn't fusing. This is also supported by the shrinkage we've measured and deduced.
The energy output we recieve would then be provided mainly by gravitational collapse.
I don't know what this will do to the future life of the sun -- possibly reduce it to millions of years rather than billions. Presumably our theories of stellar lifecycles need a little adjusting.
I wonder when the sun flamed out? And will it start up again?
Oh, BTW, the study which originally reported the discrepency was russian; it was repeated with new data in the UK. I don't remember anything else about it.
-Billy
my cat smokes mad amounts of weed.
I want to be able to create a singularity in my basement!
Come on, please dont moderate stuff like this as funny. At least make sure it is worth more than a chuckle, and the humor is related to the topic at hand. May the Meta Moderator have his way with you.
if it turns out the neutrinos actually have mass.
hmm, if E=mc2, and they are moving (thus energy in some form or another, to lose to the muons) wouldn't they have to posess mass? If not, could somebody explain why?
+&x
Blek-blek . . . heh heh . . . blek-blek, blek-blek . . . heh heh heh. Arf. Arf.
Goobblegobble!
a sphere of heavy-water buried 2km underground.
"Lime and limpid green the sound surrounds the icy waters underground . .
I don't know what became of DUMAND; it may have fallen prey to Congress in a budget cycle, because it was too small to have a constituency to defend it. Kind of like NASA's science programs. <sigh>
Time is Nature's way of keeping everything from happening at once... the bitch.
The reason why water is used in radiation shielding is that it contains two Hydrogen atoms per water molecule. Since an atom of Hydrogen has more or less the same mass than a neutron, it acts like a billiard ball: the neutron tends to stop, and the proton is transferred all the kinetic energy. And since the proton is a charged particle, it stops quickly.
Anyway; that means it's good shielding against neutrons. You still have neutrons coming from the environment ("thermal neutrons"), and if the telescope is exposed directly to water, this solves the problem. But you don't need to put it so far down underwater.
Finally, gamma radiation: that's photons. how do you stop these? With difficulty, most of the times. Lead does it, but then lead also emits thermal neutrons! So you have to choose a way to stop both, which you can't.
Except that at this depth, most of the gamma radiation coming from the Sun is absorbed already.
It's a bit of an anti-Hubble: Hubble needs to be in space to be exposed to as much "noise" as possible, whereas this telescope needs to be shielded from almost everything.
"There is no surer way to ruin a good discussion than to contaminate it with the facts."
Does not Canada have an advance research lab, located in an old mine, in Northern Ontario, which studies this very phenomenom?
'Way cool. Wish I'd'a thunk of it, damn.
I think that's what he meant.
If only it worked that way. The figure I've seen is that a neutrino can go through a trillion miles of lead without interacting with anything, so your chances of detecting an individual neutrino are vanishingly small. Unless you can generate enormous quantities (with enormous energy requirements), you won't get enough of a signal at your detector to be able to communicate. Fortunately for Comsat, their technology is unlikely to be supplanted by neutrino systems any time soon.
Time is Nature's way of keeping everything from happening at once... the bitch.
"There is no surer way to ruin a good discussion than to contaminate it with the facts."
Bear with me here a sec... it has been a few years since any of my Nuke Eng classes.
... back to water and neutrinos. There are other people who can speak more accurately on this than I, but I believe they are looking for a muon interaction, which comes out in light (maybe cherinkov (sp?) radiation?). So you need something that is translucent, shielded (from pesky cosmic rays), and huge. It is feasible to have a large, dark, water chamber. It isn't for most other elements.
Now yes, you do get neutrons from the environment. But very very few. Neutron decay is not a common mode of decay for most isotopes. You tend to get more alpha, beta, and gamma decays. A 'thermal' neutron is a neutron who's velocity is predominately determined by the temperature of the medium it is traveling in. Some neutrons are released as 'thermal', others are released as 'fast' (like from a fission reaction).
Water is good in reactors and for shielding of neutrons because it contains hydrogen, and hydrogen is good for three reasons. One, as you mentioned, it is approximately the same mass as a neutron. Think of billiard balls. If you have one very fast ball, and a lot of slow balls, after very few collisions you have a lot of moderate speed balls. In cases of heavier elements, think of bowling balls with the billiard ball... It takes a lot of collisions to reduce the speed.
The second reason hydrogen is good for reactors is that it doesn't absorb neutrons very easily. (if you wanted just shielding from neutrons, you would maybe use boron, which is an excellent and cheap neutron absorber.) And the third reason is that it tends to reflect neutrons very readily. Other elements work better (like heavy water), but water is the cheapest.
Lead tends to be stable. But it isn't completely stable unless you have old lead, because you have trace amounts of unstable forms of lead.
So
. . . the problem is that my underwear is entirely too tight, as you were kind enough to point out in another discussion.
. . . as a U.S. citizen and resident, speaking on behalf of my entire nation (or at least the civilized part, anyway, which basically extends from Mem. Drive to Somerville, with some scattered outposts across the river) I would like very much to apologize to our Northern neighbors for the stupid jokes which will damn near inevitably be attracted by the above post.
That's where they usually turn up.