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## Homework Statement

Calculate the minimum energy of a cosmic ray that it needs to have in order to "point" back to its origin. Let's define this as being deflected by the Galactic magnetic field of B = 1μG by less than 1°, as it traverses a distance of d = 1 kpc within the Galaxy. Calculate this for a proton and an iron nucleus.

## Homework Equations

r

_{L}= E/qB

## The Attempt at a Solution

so it's a pretty straightforward question, but where I've hit trouble is trying to figure out the larmor radius.

The original vector of the particle is a line tangent to a circle with radius r

_{l}. Would the 1 kpc be the arc length along that circle that the particle travels along? If so, then I should be able to use some geometry to relate the angle that the particle is deflected to the larmor radius, right?