# Quick question

1. Dec 5, 2009

### toadehep

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

2. Dec 5, 2009

steradian

3. Dec 5, 2009

### blechman

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

4. Dec 5, 2009

### toadehep

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

5. Dec 5, 2009

### 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.

6. Dec 5, 2009

### humanino

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

7. Dec 5, 2009

### 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?

8. Dec 5, 2009

### blechman

Thanks humanino, I definitely appreciate your support.

9. Dec 6, 2009

### 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?

10. Dec 6, 2009

### 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??

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