Calibration of the STAR Forward Time Projection Chamber with Krypton-83m

The principles of the calibration of a time projection chamber with radioactive Krypton83 are explained. The calculation of gain correction factors and the methods of obtaining a precise energy calibration are illustrated. The properties and advantages of Kr are summarized and compared to other radioactive calibration sources. It was shown that the Krypton calibration for the STAR FTPC is feasible and recommendable although the pad geometry of the FTPC causes a considerable deterioration of the measured spectrum due to only partially detected charge clusters. The STAR experiment (Solenoidal Tracker at RHIC) is one of four experiments operating at the Relativistic Heavy Ion Collider (RHIC) in Brookhaven. STAR searches for hadronic signatures of the quark gluon plasma formation and investigates the behavior of strongly interacting matter at high energy density in collisions of heavy nuclei [1,2,3]. While the tracking in the central region is provided by a large TPC, the two Forward Time Projection Chambers (FTPCs) are required to give position, charge, and momentum information of particle tracks in the forward rapidity regions of the STAR experiment [4,5]. The calibration method using Krypton-83m has been developed by the ALEPH collaboration [6] and is being used by the DELPHI experiment [7]. It is also applied very successfully by the NA 49 experiment where the achieved measured accuracy (i. e. the relative error of the mean value ∆E/E) is better than 0.5 % [8].