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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 detectedand we determine a mean beryllium abundance of log(Be/H)=-12.35 +/- 0.2. Theberyllium abundance is very similar to that of field stars of similar Fecontent. We interpret the beryllium abundance observed as the result of primaryspallation of cosmic rays acting on a Galactic scale, showing that berylliumcan 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 theonset of star formation in the Galaxy, in excellent agreement with the agederived from main sequence fitting. From the same spectra we also find low O(noticeably different for the two stars) and high N abundances, suggesting thatthe original gas was enriched in CNO processed material. Our beryllium results,together with the N, O, and Li abundances, provide insights on the formation ofthis globular cluster, showing that any CNO processing of the gas must haveoccurred in the protocluster cloud before the formation of the stars we observenow. We encounter, however, difficulties in giving a fully consistent pictureof the cluster formation, able to explain the complex overall abundancepattern.Comment: to appear in A&

Beryllium in turnoff stars of NGC 6397: Early Galaxy spallation, cosmochronology and cluster formation