Abstract In order to recover the charge lost by electron-ion recombination, we doped pure liquid argon with a photosensitive hydrocarbon, tetra-methyl-germanium (TMG). in the 3 ton ICARUS TPC. A charge increase of 25% to 220% was observed for different electric fields and for energy densities ranging from I .6 to 32 MeV/cm. The 3 ton liquid argon TPC has been in operation since May 1991 with pure liquid argon (LAr). In these three years we measured the basic parameters of the detector responses, such as the spatial resolution, the electron diffusion coefficient, the correlation of collected charge with electric field and energy density, by cosmic rays and external gamma ray sources [I]. In addition we have been continuously monitoring the stability of the liquid argon purity, the effectiveness of the recirculation system. and the reliabiIity of the electronics read-out. So far no degradation of any part of this detector has been found. As is well known, an ionizing particle in liquid argon will produce electron-ion pairs and excitons along the track. Depending on the ionization density and electric field, some of the pairs recombine and emit vacuum ultraviolet (VUV) photons with energy distribution peaked at 128 nm (9.7 eV). On the other hand, photon emission from excitons exhibits energy distribution peaking at the same energy (9.7 eV). With our data by minimum ionizing muons (mip), stopping muons and stopping protons, we have measured the coliected charge as a function of energy density 11.6 to 32 MeV/cm) and electric field (100 to 500 V/cm) in pure liquid argon. We found that the electron escape probability depends heavily on these two parameters. The percentage of free electron yield can vary from 70% to 14% at different energy densities and electric fields. This nonlinear detector response may degrade the particle identification capability of the liquid argon TPC. A possible solution to improve the linearity of the detector response is to introduce photosensitive dopants able to convert part of the scintillation light, either from electronion recombination or by direct excitation, into additional free electron-ion pairs, thus enhancing the linearity as a function of the deposited energy density and electric field. We chose TMG as photosensitive dopant because of the following advantages
