Cool write up! I had to remind myself these things
- nitrogen has an atomic number of 7 (and 7 protons)
- Nitrogen-9 means nitrogen with an atomic mass of 9
Therefore N-9 means a nucleus with 7 protons and 2 neutrons!
If I read the article correctly, N-9 is actually a helium nucleus (or alpha particle) with some protons that happen to stick around near to it for fractions of a second that are too short to comprehend - but apparently they detected that a single proton decides to leave first (-> C-8), then one pair (-> Be-6) and finally another (-> He-4).
And I had to look this up. The most stable form of Nitrogen is N14, so I guess it normally has 7 neutrons and N9 is produced when N14 sheds 5 neutrons. Other isotopes are N13 (rare, short lived) and N15 (0.4% of all nitrogen, stable).