quick question

[toadehep]

the unit of cosmic ray flux is expressed as flux /(m^2.sr.s.GeV), so what does sr stand for?
Thanks.

[humanino]

steradian

[blechman]

steradian = unit of solid angle. 4\pi steradians in a sphere.

[toadehep]

steradian

thanks.
suppose there is a piece of board in outer space, how to calculate its steradian?
what is the reference point?

[blechman]

Area(S)=R^2 \int_S\sin\theta~d\theta d\phi

So it depends on your origin (the value of R, since the area is obviously invariant). The same goes for arc length in one less dimension.

[humanino]

I voted for blechman in the 2009 PF member physics award. :approve:

[blechman]

as to your flux calculation you mentioned: R is the distance from you to the source, so YOU are the reference point. Is that what you meant?

[blechman]

I voted for blechman in the 2009 PF member physics award. :approve:

Thanks humanino, I definitely appreciate your support. :biggrin:

[toadehep]

many experiments are designed to measure the cosmic ray spectrum, including space probes. what confues me most is that, when considering the geometry factor of the
detector, how can we decide the steradan and what is the reference point in outerspace?

[blechman]

I'm not sure I understand the phrase, "reference point in outerspace". The detector has an aperture that covers a fixed solid-angular resolution in the sky along its line of sight. Maybe I misunderstand your question??