The isotopic composition of the enhanced low energy nitrogenand oxygen cosmic rays can provide information regarding the sourceof these particles. Using the Caltech Electron/Isotope Spectrometeraboard the IMP-7 satellite, a measurement of this isotopic compositionwas made. To determine the isotope response of the instrument, acalibration was performed, and it was determined that the standardrange-energy tables were inadequate to calculate the isotope response.From the calibration, corrections to the standard range-energy tableswere obtained which can be used to calculate the isotope response ofthis and similar instruments.
The low energy nitrogen and oxygen cosmic rays were determinedto be primarily ^(14)N and ^(16)O. Upper limits were obtained for theabundances of the other stable nitrogen and oxygen isotopes. To the84% confidence level the isotopic abundances are: ^(15)N/N ≤ 0.26(5.6- 12.7 MeV/nucleon), ^(17)0/0 ≤ 0.13 (7.0- 11.8 MeV/nucleon),(18)0/0 ≤ 0.12 (7.0 - 11.2 MeV/nucleon). The nitrogen compositiondiffers from higher energy measurements which indicate that ^(15)N, whichis thought to be secondary, is the dominant isotope. This implies thatthe low energy enhanced cosmic rays are not part of the same populationas the higher energy cosmic rays and that they have not passed throughenough material to produce a large fraction of ^(15)N. The isotopic composition of the low energy enhanced nitrogen and oxygen is consistentwith the local acceleration theory of Fisk, Kozlovsky, andRamaty, in which interstellar material is accelerated to severalMeV/nucleon. If, on the other hand, the low energy nitrogen andoxygen result from nucleosynthesis in a galactic source, then thenucleosynthesis processes which produce an enhancement of nitrogenand oxygen and a depletion of carbon are restricted to producingpredominantly ^(14)N and ^(16)O.
