Lamb Shift in Muonic Helium: X-ray Detection System

by Andrea Gouvêa

Institution: Universidade de Coimbra
Year: 2015
Keywords: Avalanche photodiodes; Lamb shift; X-ray detection system
Record ID: 1324893
Full text PDF: http://hdl.handle.net/10316/27085


An experiment with the aim to determine the Lamb shift in muonic helium has been carried out by the Charge Radius Experiments with Muonic Atoms (CREMA) Collaboration at the Paul Scherrer Institure (PSI) Switzerland. The goal of the experiment is to measure several transitions between the 2S and 2P energy levels in muonic helium ions (μ4He+ and μ3He+) with a precision of 50 ppm, and consequently to determine the α particle and helion nuclear charge radii (RMS value) with a relative accuracy of 3×10-4. This experiment comes after the good results attained in the muonic hydrogen Lamb shift experiment, where the muonic hydrogen Lamb shift was measured and the proton charge radius determined, which lead to the proton radius puzzle. The next proposal of the CREMA Collaboration was to extend the Lamb shift measurements to muonic helium in an attempt to help to solve the proton radius puzzle and provide additional information about the helium nucleus. This thesis is dedicated to the X-ray detection system to be used in the experiment, including the detectors and pre-amplifier system. The detectors chosen are avalanche photodiodes. A detailed study of two different types of avalanche photodiode (APD), reach-through avalanche photodiode (RT-APD) from Hamamatsu Photonics and large area avalanche photodiode (LAAPD) from Radiation Monitoring Devices Inc. (RMD) has been carried out. The two APD types have different depletion region thicknesses, and consequently different detection efficiencies for 8 keV X-rays emitted by muonic helium ions. Regarding RT-APDs two prototypes, with 5 × 5 mm2 and 3 × 5 mm2 active areas, have been investigated. The studies have shown that the energy resolution improves with decreasing temperature, reaching 9.5% at 0°C for the larger prototype, and the minimum detectable energy is about 0.9 keV for the optimal operation gain region. The gain variation with temperature increases with the bias voltage applied, varying between -1.0% per °C for a bias voltage of 200V and -1.7% per °C for a bias voltage of 400V. The gain non-linearity between X-rays and visible light pulses has been investigated for different temperatures, showing a non-linearity of 25% at -20°C whereas it is only 10% at 20°C for a bias voltage of 350V. The overall performance of this type of APD is inferior to the prototypes from RMD with the additional drawback of smaller active areas available. Concerning LAAPD from RMD, detection efficiencies between 53 and 65% for 8 keV X-rays have been measured for homogeneous irradiation of the whole efficient surface area of 13.5 × 13.5 mm2 and considering the effect of the X-ray incident angle. Energy resolution values below 20% (at FWHM have been measured for 8 keV X-rays, at -30°C and one prototype has even shown a much lower value of 9%. The energy resolution behaviour at low temperatures has been investigated. No significant dependence on temperature was observed between -30°C and -20°C since the dark current difference is not so significant for the degradation of the energy resolution. APDs…