SPACAL Detector

The SPACAl calorimeter measures energy of scattered e+- . The energy distribution at the kinematic region covered by SPACAL has a prominent maximum ("kinematic peak") close to the incident electron beam energy of 27.5 GeV. Position of this peak is (nearly) the same for the whole SPACAL region. This fact can be used to homogenize SPACAL response. To reach measurement precision better than 1%, kinematic peak method has been developed enabling:

to monitor the stability of SPACAL cells' response during data taking
to perform the equalisation of cells' response after each run period
to provide energy calibration taking into account possible time dependence of SPACAL response during run periods.

The method uses selected events with scattered e+- having cluster energy in the kinematic peak region and compares the shape of their energy distribution with that of simulated events to obtain a correction factor. Since the ep cross section at a given SPACAL impact point falls off rapidly with the increasing distance from beam pipe, only the inner SPACAL part which includes about 50% of the cells can be calibrated in this way.
Prague group ( M.Nozicka, P.Reimer, J.Zalesak, J.Zacek; at various times) has been responsible for SPACAL calibration and monitoring for several years. An illustration of the method is shown in Fig. 1 . It can be seen that when calibration is applied the (yellow) kinematic peak is moved closer to the expected maximum at 27.5 GeV and becomes narrower than the original distribution.