The signals from heavy ions produced by galactic cosmic rays will be used for independent
monitoring of the stability of the energy scale at high energies: 500 MeV for carbon ions and 8 GeV
for iron.

The spectrum on Fig.~1a has been obtained by selecting crystal hits in low-multiplicity layers (to
get rid of the bulk of nuclear interactions) and by correcting hit energy from the track
path-length in the crystal. As can be seen, very narrow peaks (the carbon peak has a 5\% only
width) can be identified with good statistics.

Fig.~1b shows that the carbon peak position has daily variations due to the primary spectrum
dependence on the geomagnetic latitude. By averaging over longer timescales, the position is stable
within 1\% or less (for the whole calorimeter) after a few days of accumulated data.

In order to get a more accurate result in the future, one needs a tighter selection to better
reject nuclear interactions, which will decrease the event rate. Therefore, we will monitor groups
of crystals having the same layer and tower position (central, corner or side), instead of
individual crystals. Light elements will be used to monitor the high-energy scale calibration on
timescales of weeks or months, and heavier elements for longer timescales.

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Fig.~1a -- HeavyIons_new.gif Fig.~1b -- Cmpv.gif

Caption:

(a) Spectrum of crystal hit energy, obtained for all calorimeter crystals and 4 days of on-orbit
operations.

The hit energy is defined as a vertical equivalent energy, which is obtained as the hit raw energy
multiplied by the ratio of the crystal height to the measured trajectory path-length (as given by
the main track reconstruction). Individual species can be identified from left to right : Be and B
(secondary peaks), carbon (highest peak), nitrogen and oxygen, then neon, magnesium and silicon,
and iron on the far right. The statistiscs decreases with the atomic number because of the
abundances and of the decreasing efficiency of the onboard HIP filter.

(b) Carbon peak time history over several days. Black points are sampled every 90 min (approx. each
orbit), and red points are daily averages.