[en] We studied the performances of the LaBr₃:Ce scintillator when optically coupled to NaI:Tl and CsI:Na in a Phoswich detector for the R and D phase of the gamma ray calorimeter PARIS (Photon Array for the studies with Radioactive Ion and Stable beams). This detector has the purpose to measure γ-energies in a wide range (100 keV–40 MeV), and it will be used principally as a part of the SPIRAL2 instrumentation at GANIL. In this communication we report on the study of the light yield and energy resolution for gamma detection realized by coupling the phoswiches with various photomultiplier tubes, providing different characteristics. We were interested in investigating the possible degradation of the scintillation light produced by the LaBr₃:Ce due to the presence of NaI:Tl/CsI:Na crystals, before being detected on the photocathode. For this purpose we realized all the measurements employing a standard ADC and QDC read-out system leading the possibility to perform a gate-based event selection. In this study we measured an energy resolution of 4.6% with an uncollimated ¹³⁷Cs source for a 50.8×50.8×50.8 mm³ LaBr₃:Ce coupled to a 50.8×50.8×152.4 mm³ NaI:Tl. This value is 30% bigger than the energy resolution measured for a 50.8×50.8×101.6 mm³ stand-alone LaBr₃:Ce but still in the specifications for the PARIS collaboration physics list.

[en] Avalanche fluctuations set a limit to the energy and position resolutions that can be reached by gaseous detectors. This paper presents a method based on a laser test-bench to measure the absolute gain and the relative gain variance of a Micro-Pattern Gaseous Detector from its single-electron response. A Micromegas detector was operated with three binary gas mixtures, composed of 5% isobutane as a quencher, with argon, neon or helium, at atmospheric pressure. The anode signals were read out by low-noise, high-gain Cremat CR-110 charge preamplifiers to enable single-electron detection down to gain of 5× 10³ for the first time. The argon mixture shows the lowest gain at a given amplification field together with the lowest breakdown limit, which is at a gain of 2×10⁴ an order of magnitude lower than that of neon or helium. For each gas, the relative gain variance f is almost unchanged in the range of amplification field studied. It was found that f is twice higher (f∼0.6) in argon than in the two other mixtures. This hierarchy of gain and relative gain variance agrees with predictions of analytic models, based on gas ionisation yields, and a Monte-Carlo model included in the simulation software Magboltz version 10.1