[ExI] Neutrino interaction web

Richard Loosemore rpwl at lightlink.com
Fri Mar 4 18:25:47 UTC 2011

Adrian Tymes wrote:
> On Fri, Mar 4, 2011 at 5:54 AM, Richard Loosemore <rpwl at lightlink.com> wrote:
>> I wouldn't go that far :-), but the collision cross section for neutrinos is
>> estimated to be such that if you want to catch a significant fraction of
>> them you need *roughly* a 1 light-year thick sheet of lead.
> Ah, but that's with atoms.  All that empty space.

Gee, Adrian:  you got a one-light-year-thick wall of lead there, which 
has been compressed into a nice convenient-sized sheet! 8-)  I reckon 
each neutrino will hit, what? 10^^17 nuclei on its way through???  I 
don't think the empty space is your problem.  :-)

>> So, I don't see any problem with Adrian's ideas except that you need to take
>> a 1 light-year thick sheet of lead then compress it to make a thin sandwich
>> that we can install in orbit, to collect the sun's neutrino output.
>> Shouldn't be too difficult.  Bit of engineering, is all.
> Neutron stars are compressed matter of that sort.  Except they're balls, not
> sheets, and unless you've got a way to get them to spread out despite their
> own gravity pulling them into a ball is the tricky part.

Okay lets see.  Average density of a neutron star is 5^^17 kg/m3.

Average density of lead is about 1^^4 kg/m3.

So to make the light year thick sheet of lead have the same cross 
sectional density as a neutron star you would have to compress it in one 
dimension by a factor of about 1^^14.

One light year is about 10^^16 meters.

So that is pretty good!  You would only need about 100 meters of neutronium.

Comparable to the size of a neutron star, in fact.  So if you want to 
capture neutrinos, a neutron star would do quite nicely.

So, next step.  You've got your neutron star, and it is definitely 
collecting a lot of the neutrinos from the sun:  now what do you do with 
it?  :-)

(Meanwhile, better hurry, because I think your neutron star is eating 
our sun!).

Richard Loosemore

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