arXiv subjects
Publications and source records attributed to M. I. Lopes.
The scintillation efficiency of liquid xenon for nuclear recoils has been measured to be nearly constant in the recoil energy range from 140 keV down to 5 keV. The average ratio of the efficiency for recoils to that for gamma-rays is found to be 0.19+-0.02.
The design and performance of a 1.2 liter liquid xenon chamber equipped with 7 two-inch photomultiplier tubes, with the purpose of studying the scintillation response of xenon to gamma-rays and neutrons, is described. Measurements with gamma-rays indicate a high VUV light collection efficiency resulting in ~5.5 photoelectrons per 1 keV of deposited energy. The energy resolution (FWHM) is 18% and 22%, for 122 keV and 511 keV gamma-rays, respectively. An algorithm for the reconstruction of the scintillation coordinates in (x,y) plane was developed and tested. The position resolution is estimated to be 6.9 mm (sigma) for 122 keV gamma-rays.
The attenuation length and refractive index of liquid xenon for intrinsic scintillation light (178nm) have been measured in a single experiment. The value obtained for attenuation length is 364 +- 18 mm. The refractive index is found to be 1.69 +- 0.02. Both values were measured at a temperature of 170 +- 1 K.
In the present paper, we report on our developments of liquid xenon detectors for medical imaging, positron emission tomography and single photon imaging, in particular. The results of the studies of several photon detectors (photomultiplier tubes and large area avalanche photodiode) suitable for detection of xenon scintillation are also briefly described.
A Large Area Avalanche Photodiode (LAAPD) was studied, aiming to access its performance as light detector at low temperatures, down to -80 C. The excess noise factor, F, was measured and found to be approximately independent of the temperature. A linear dependence of F on the APD gain with a slope of 0.00239 +/- 0.00008 was observed for gains above 100. The detection of low intensity light pulses, producing only a few primary electron-hole pairs in the photodiode, is reported.
In this study we address the feasibility and main properties of a positron emission tomograph (PET) based on RPCs. The concept, making use of the converter-plate principle, takes advantage of the intrinsic layered structure of RPCs and its simple and economic construction. The extremely good time and position resolutions of RPCs also allow the TOF-PET imaging technique to be considered. Monte-Carlo simulations, supported by experimental data, are presented and the main advantages and drawbacks for applications of potential interest are discussed.
First observation of liquid xenon scintillation due to alpha-particles and gamma-rays with a large area avalanche photodiode immersed in the liquid is reported. An energy resolution of 10% (FWHM) and a coincidence time resolution of less then 1 ns (FWHM) were measured with 5.5 MeV alpha-particles and 511 keV gamma-rays, respectively. The quantum efficiency of the photodiode for xenon scintillation light (wavelength 178 nm) is estimated to be ~100%.