Celestia Forums

More of a physics questions than an astronomy one here...

OK, I know Neutrinos come in three basic flavours, and I know they don't interact very much with matter at all... but why not?

Are the neutrinos so small that they are much more likely to pass "between" atoms or through the spaces in the atoms? Or would they literally pass right through an atomic nucleus that was directly in their path as if it wasn't there?

The reason I ask is that I'm wondering if neutrinos could penetrate superdense objects like neutron stars or white dwarfs, or if the superdense material (having no gaps between the atoms/nuclei) would block them more effectively.

AFAIK they're going right through a nucleus. Only a direct hit on a neutron (or Proton?) leads to a possible detection.

maxim

In a nutshell, neutrino's lack charge, they lack color and they also lack subparticles having either. Thus they do not have the ability to directly interact with matter through either the strong or electro-magnetic forces.

Thus they do not have the ability to directly interact with matter through either the strong or electro-magnetic forces.

Interesting... what about weak and gravitational forces? Could the path of neutrinos be bent by a strong enough gravitational field (ie black hole, neutron star)?

And presumably that means they'd pass through superdense matter as easily as any other kind, right?

So when they DO interact with matter, what's going on? Maxim said that they only interact when they score a direct hit on a nucleon, but why wouldn't they pass through that too?

Neutrinos can interact with conventional matter only by the weak force, by inverse beta decay. Since they have mass, they're presumably also deflected by gravitational fields.
It's a statistical thing ... any given neutrino is very unlikely to interact with any given nucleon, so that it takes 45 light-years of water to attentuate a neutrino beam by 50%. But if you have a lot of neutrinos, you have a reasonable chance of observe an interaction with matter as they pass through some large volume. Hence the various neutrino telescopes, where incoming neutrinos interact with neutrons to convert chlorine to argon.
Since neutronium would present many more neutrons for a given path length, I'd guess it would stop many more neutrinos than an equivalent volume of conventional matter.

Grant

granthutchison wrote:Neutrinos can interact with conventional matter only by the weak force, by inverse beta decay.

Hrm. That's VERY interesting. Tell me more of this "inverse deta decay". What I found online suggests that this is where you get:

proton + electron ----> neutron + neutrino

But that can't be happening here can it, else you'd just be making more neutrinos! Or is it the other way around - the neutrino hits a neutron and becomes a proton + election? in which case, are the detectors detecting the electrons produced? Or photons produced in the reaction?

Or do you mean that it actually causes whatever it hits to decay into something else via the weak force?

It's a statistical thing ... any given neutrino is very unlikely to interact with any given nucleon, so that it takes 45 light-years of water to attentuate a neutrino beam by 50%.

Whoa

Since neutronium would present many more neutrons for a given path length, I'd guess it would stop many more neutrinos than an equivalent volume of conventional matter.

But presumably you'll need several lightyears thickness of neutronium to actually stop a neutrino beam completely, right?

Can neutron + neutrinos form elements like proton + eletron?
Just curious.

I found this:
http://hyperphysics.phy-astr.gsu.edu/hb ... o3.html#c2

which is a bit intriguing... Assuming that neutronium has a density of about 10^12 kg/m3, you'd need 166,000 km of it to block a neutrino beam (I presume that's what they mean by 'mean free path'), which is better than a few dozen lightyears but still somewhat awkward .

What I figure from other pages on that website is that neutrinos don't directly interact with nucleons, but there is a chance that they could encourage one to decay via the weak force into something else - in which case they get absorbed by the particle, and another particle (and electron and/or a photon?) is emitted.

What I'm now confused about is that apparently what is actually being detected in a neutrino detector is the Cerenkov radiation as the neutrino slows down to the speed of light in the water, instead of anything emitted by an interaction between the neutrino and a nucleon?

Yes they interact through the weak interaction and through gravity. The latter interaction isn't terribly important. It is the weak interaction which allows neutrino's to interact with matter. Basically the neutrino weak coupling has many things going against it: the weak interaction is weak (five orders less than the strong interaction), the weak interaction has a tiny range (two orders less than the strong interaction), the neutrino is small (it has no sub particles), the reaction isn't favorable (i.e. neutrons decay). These combine to make the neutrino-nucleon cross section twenty orders of magnitude smaller than for nucleon-nucleon reactions.

However super dense matter is opaque to neutrinos, such as neutron stars or the cores of collapsing stars just before they go bang. This is primarily due to the extreme nucleon densities in such super dense states of matter. Oh, there is one other situation where matter has been opaque to neutrinos, during the big bang. Presumably once the density of the early universe dropped below a critical threshold, the universe became transparent to neutrinos and hence there should be a background neutrino radiation similar to the background electromagnetic radiation. If this is ever detected it will be a beautiful proof of the big bang.

wcomer wrote:However super dense matter is opaque to neutrinos, such as neutron stars or the cores of collapsing stars just before they go bang. This is primarily due to the extreme nucleon densities in such super dense states of matter.

Not according to the equation in the hyperphysics link, unless the densities I'm using are too low. To block neutrinos, a neutron star (diameter about 16km?) would have to need a density of at least 10^16 kg/m3. Can it get that high? (how dense can matter get before it collapses in on itself anyway? Can you get densities up to 10^22 or 10^25 kg/m3?)

Oh Evil One,

The Cerenkov radiation isn't coming from the neutrino. It is coming from the electron or muon reaction product of the original electron or muon neutrino. As you have eluded, it is only charged particles which emit Cerenkov radiation.

Oh Evil One,

The neutrino cross sections are energy sensitive. The equation you use isn't relevant for higher energies that are present in the cores of neutron stars. IIRC, the diff is about 4 orders of magnitude. Regardless, opaque is a relative thing. My understanding is that within collapsing cores and neutron stars the "optical depth" of the neutrinos is large but smaller than the radius of the object. There is a neutrinosphere within the neutron star outside of which the neutron star is transparent to neutrinos. The boundary is certainly affected by both nucleon density and energy.

Two references:
http://www.ps.uci.edu/~silvestri/thesis ... ode17.html
http://cupp.oulu.fi/neutrino/nd-cross.html

wcomer wrote:Oh Evil One,

The Cerenkov radiation isn't coming from the neutrino. It is coming from the electron or muon reaction product of the original electron or muon neutrino. As you have eluded, it is only charged particles which emit Cerenkov radiation.

Oh, right. So either way, you're not actually detecting the neutrino directly at all, just the effects of its passage through the water.

Evil Dr Ganymede wrote:Tell me more of this "inverse beta decay"

You can mess the equation around in various ways:

antineutrino+proton => neutron+positron

neutrino+neutron => proton+electron

The salient point is that the (anti)neutrino is absorbed, rather than being one of the products as it is in beta decay.

Grant

Ok. So neutrinos pass through the water, a few of them react with the nucleons and are absorbed, changing the atom from element to another (increasing/decreasing the number of protons or neutron), which causes a positron or electron to be emitted, and the Cerenkov radiation from that passing through the water is what is actually detected?

That about right?

Evil Dr Ganymede wrote:That about right?

Yep. Or, in the case of the original Homestake Mine neutrino detector, you have a big tank of perchloroethylene, and neutrino interactions create argon gas from the chlorine. You detect the argon.

Grant

perchloroethylene

That wouldn't basically be "bleach" would it? I seem to remember something about these neutrino detectors being full of bleach...

Nice reminder. I've almost forgotten half of the things mentioned in this thread.

maxim.

Evil Dr Ganymede wrote:That wouldn't basically be "bleach" would it? I seem to remember something about these neutrino detectors being full of bleach...

Dry-cleaning fluid, I think. The story goes (not true, but it deserves to be) that the guy who built the Homestake Mine detector mysteriously started getting cold calls and junk mail from wire-coathanger manufacturers, shortly after he'd placed his order for X thousand gallons of perchloroethylene.

Grant

granthutchison wrote:

Evil Dr Ganymede wrote:Tell me more of this "inverse beta decay"

You can mess the equation around in various ways:

antineutrino+proton => neutron+positron

neutrino+neutron => proton+electron

The salient point is that the (anti)neutrino is absorbed, rather than being one of the products as it is in beta decay.

Grant

...and more of course.

Since the carriers of the weak force, the W and Z bosons, are about 100 times as heavy as the Proton, weak interactions probe very short distances ~ hbar/mass[W,Z] << Proton size. So the 3 neutrino species [and the charged leptons (electrons, muons and taus) ] weakly interact with any of the 6 quarks [u,d,s,c,b,t] by exchange of a W or Z boson.

The proton contains mainly u and d quarks but also small fractions of other quark flavours [s +anti-s, c + anti-c, b +anti-b, t + anti-t]. These other quark flavours can be produced in accelerators on earth or in cosmic ones, e.g. via Hawking radiation of black holes...

Neutrinos can also annihilate with anti-neutrinos into other particles!
via


This amazing process has been recently considered as a possibly most exciting probe of the primordial cosmic neutrino background that could tell us about the universe, when it was only 1 sec old!!!
That's when the neutrinos stopped to interact with the cosmic plasma and decoupled to become so-callled relic neutrinos. Note, the universe was quite small 1 sec after the Big Bang;-)
The density of these cosmic relic neutrinos is precisely known: 56 neutrinos/cm^3. The cosmic backround radiation due to relic neutrinos is in complete analogy to the CMB, the Cosmic Microwave Background radiation that told us recently so many exciting things about the universe, when it was 300000 years old (WMAP, BOOMERANG,...balloon expts).

The neutrinos decoupled much earlier than the microwave photons! Their density equals simply 3/22 of the known microwave photon density.

If there happen to exist cosmic sources of very high-energetic neutrinos these could interact with the low-energetic cosmic relic neutrinos and produce via the Z-boson intermediate resonance (see process above) characteristic ultra-high energy cosmic rays that may be detected on earth...

Bye Fridger