Q:

Volume emitter that is more intense towards the outside of the object

How do I create a volume emitter shader that is more intense towards the outside of the object?  I would basically like to create tubes that look like the blue (not purple) tubes in this image: 

(source: colorado.edu)
(Figure is by Steve Burrows and the Kapteyn–Murnane Group, JILA.  From this article.) 
But, I cannot figure out how to get the "depth" coordinate of the image in order to feed this to the intensity of the emitter. 
Any ideas on how to do this in cycles? 

A:

You might be able to do something with the Ray length output of the lightpath node, however based on your example image I think you might be looking for the layer weight (or fresnel) node:

The above node setup mixes an emission shader with a transparent shader based on the angle of the surface to the camera.  In this case sharper angles emit light, while more direct angles are transparent.