# What Does "sr" Stand For in Cosmic Ray Flux Units?

In summary, the unit of cosmic ray flux is expressed as flux /(m^2.sr.s.GeV) and the steradian (sr) is the unit of solid angle. A sphere has 4π steradians. The reference point for calculating steradians is the origin, with the area being invariant. When considering the geometry factor of a detector in outer space, the reference point would be the detector itself, with its aperture covering a fixed solid-angular resolution in the sky along its line of sight.
the unit of cosmic ray flux is expressed as flux /(m^2.sr.s.GeV), so what does sr stand for?
Thanks.

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

humanino said:

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

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

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.

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

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?

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

Thanks humanino, I definitely appreciate your support.

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?

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??

## 1. What is the meaning of "sr" in cosmic ray flux units?

The abbreviation "sr" stands for "steradian", which is a unit of solid angle. In the context of cosmic ray flux units, it refers to the total area of the sky covered by the detector used to measure the flux.

## 2. How is "sr" related to the flux of cosmic rays?

In cosmic ray flux units, "sr" is used as a measure of the solid angle covered by the detector. Since the flux of cosmic rays is measured by counting the number of particles passing through a given area, the solid angle of the detector plays a crucial role in determining the accuracy of the measurement.

## 3. Why is "sr" used instead of other units of solid angle?

"Sr" is a commonly used unit of solid angle in the field of physics, and it is especially useful when dealing with large areas of the sky. It is also a dimensionless unit, making it easier to manipulate in calculations and comparisons.

## 4. Can "sr" be converted to other units of solid angle?

Yes, "sr" can be converted to other units of solid angle such as square degrees or square arcminutes. The conversion factor depends on the size of the detector and the distance between the detector and the cosmic ray source.

## 5. Is "sr" a standard unit of measurement for cosmic ray flux?

Yes, "sr" is a widely accepted unit of measurement for cosmic ray flux. However, some studies may use other units such as square degrees or square arcminutes, so it is important to check the units being used in a particular study or analysis.

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