A fast gas ionization calorimeter filled with C3F8 for operation at high counting rates and hard radiation environment

The performance of a gas ionization EM calorimeter with planar electrodes and steel absorbers has been studied with a 26.6 GeV/c electron beam at the 70 GeV IHEP accelerator. The design of the calorimeter is optimized for the operation at high counting rates by minimizing the coupling inductance and by choosing rather fast and heavy perfluoroalkane C3F8 (vdr=0.07 mm/ns at a reduced field E/N=1.0×10−16 V cm2). This gas has been used for the first time in calorimetry applications. The total calorimeter thickness is ≈21X0. The signal readout has been done by remote 25 Ω low-noise preamplifiers coupled to towers via 25 Ω cable of 3 m length. The choice of a 25 Ω input impedance provides a complete matching between preamplifier, cable and tower. The studies of the calorimeter consisted in measuring the signal and noise spectra at different values of HV, ADC gate width and gas pressure. The electron attachment rate in C3F8 with a stated purity of 99.99% is quite low (at a given E/N the mean free path of electrons is λ=2.2 cm at 1 atm). The intrinsic energy resolution of the calorimeter after noise subtraction is found to be independent of the gas pressure and equal to ≈7% at E=26.6 GeV/c.