# What is “A” in AGeV?

AGeV means GeV per nucleon. But why A letter is used for such a short cut? Why not NGeV, for example?

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This is an artifact of a long established coding for nuclei

• $Z$ -- the "atomic number" is the number of protons in the nucleus(or occasionally the "proton number")
• $A$ -- the "mass number" the number of nucleons in the nucleus

There is also the not often used

• $N$ -- the "neutron number"

The per-nucleon energy of course applies to both the protons and the neutrons, so you want $A$.

And yeah, the use of these letters doesn't make much sense, but it has been that way for a long, long time.

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And I was wondering what Ampere-GeV should be... –  Tobias Kienzler Aug 19 '12 at 17:05

The $A$ is a variable; it's actually $A\text{ GeV}$, where $A$ is the mass number.

For example, RHIC has run Au-Au collisions at a beam energy of $100A\text{ GeV}$ (among other values). You would interpret this as $100A$ being the number of GeV. With gold nuclei, $A = 197$, and so that is $(100\times 197)\text{ GeV} = 19700\text{ GeV}$ total energy per nucleus.

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