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Here, we report on the present state of development of a liquid xenon detector composed by multiwire ionisation chambers, which has been developed for positron emission tomography (PET). It uses the fast scintillation signal for coincidence and subsequent trigger of the event processing. The charge signals induced on the wires are used for position determination along the radial and transaxial directions of the tomograph, as well as for energy information. The following detector characteristics have been achieved: (i) coincidence time resolution better than 1.5 ns (fwhm); (ii) position precision along the radial direction equal to 5 mm; (iii) position resolution in the transaxial direction of 0.8 mm (fwhm); (iv) energy resolution of 17% (fwhm) for 511 keV /spl gamma/-rays; (v) efficiency of trigger or about 100%; (vi) overall efficiency of detection of 60%. The prospects for the development of this detector in view of its application in PET are discussed.

Some basic features of detectors operated in the self-quenching streamer mode (SQS, also referred to as limited streamer mode), for example their pulse height spectra, are not easy to interpret, requiring the development and check of models of the complex process involved. Further comparison is made between experimental data and the behavior foreseen by different models. In particular, the dependence of the total charge on the applied voltage and the reconstruction of the pulse form are considered. The tests presented tend to reject some models and to favor the one presented by I. Gallimberti (1979) for the long streamers. >

Spectroscopic measurements of the emission spectra or argon/ethane and methane/ethane spectra are presented. In addition, time-resolved measurements were performed in argon/ethane, argon/methane, and methane/ethane in the vacuum ultraviolet region of the spectrum. In argon/ethane mixtures the energy spectra present a complex pattern, not only as a result of the many dissociation processes that may take place but also due to the high sensitivity to the presence of impurity molecules. In argon/methane mixtures, the presence of impurities is no longer detected, probably because of the high cross section for the quenching of atomic excited states of argon by methane molecules, for high methane concentrations. In methane/ethane and in argon/methane the number of photons emitted per electron is of the same order of magnitude for the proportional mode, while in argon/ethane this number is much lower, even when operated in the SQS mode. >

The prototype of a recently proposed liquid xenon drift chamber for PET with simultaneous detection of scintillation and ionization signal has been built. First tests of the chamber under irradiation by 511 keV /spl gamma/-photons are reported. >

The variation with the charge gain of the total number of photons emitted per electron, in the visible and NIR regions (0.4