We present the first detection of beryllium in two turnoff stars of the old , metal-poor globular cluster NGC 6397 .
The beryllium lines are clearly detected and we determine a mean beryllium abundance of \log ( Be/H ) = -12.35 \pm 0.2 .
The beryllium abundance is very similar to that of field stars of similar Fe content .
We interpret the beryllium abundance observed as the result of primary spallation of cosmic rays acting on a Galactic scale , showing that beryllium can be used as a powerful cosmochronometer for the first stellar generations .
With this method , we estimate that the cluster formed 0.2–0.3 Gyr after the onset of star formation in the Galaxy , in excellent agreement with the age derived from main sequence fitting .
From the same spectra we also find low O ( noticeably different for the two stars ) and high N abundances , suggesting that the original gas was enriched in CNO processed material .
Our beryllium results , together with the N , O , and Li abundances , provide insights on the formation of this globular cluster , showing that any CNO processing of the gas must have occurred in the protocluster cloud before the formation of the stars we observe now .
We encounter , however , difficulties in giving a fully consistent picture of the cluster formation , able to explain the complex overall abundance pattern .