H. Bichsel

In the collisions of ultra-relativistic heavy ions, the trajectory ('track') of thousands of emerging particles are measured in time-projection-chambers (TPC). An important datum for this purpose is the localization of segments of the particle tracks in small volumes. The localization in the plane perpendicular to the particle velocity is given by the extent of the cloud of ionization produced by the particles, typically averaged over segments of a few centimeters of track. For particles with charge ±1, for about 80% of the segments ('pads' in current slang) the ionization cloud in gas at 1 atm has a diameter of less than 0.1 mm. For the others secondary electrons ('-rays') with energies exceeding 5 keV will produce ionization further from the track, and the position of such segments will be known with a larger uncertainty. In order to determine these uncertainties, the spatial distribution of the ionization by the -rays must be known. Also, we must know the spectrum of -ray energies. A fairly good approximation to this spectrum can be obtained with the Fermi-virtual-photon method, as outlined in earlier reports. This method is also known as the Weizsäcker-Williams or PAI method.¹ The spatial distribution of the energy deposited or, more appropriately, the ionization has been measured for electrons with energies up to about 5 keV in some gases, but not in Ar. A Monte Carlo program for the calculation of these distributions has been obtained from B. Grosswendt (at Physikalisch-Technische Bundesanstalt in Braunschweig), and is being investigated at present. Only preliminary results have been obtained so far, and need confirmation. Corresponding experiments would be very desirable.