The present manuscript summarizes novel studies on the application of large area avalanche photodiodes (LAAPDs) to the detection of X-rays and vacuum ultraviolet (VUV) light. The operational characteristics of LAAPDs manufactured by Advanced Photonix Inc. were investigated for X-ray detection at room temperature. The optimum energy resolution obtained in four investigated LAAPDs with active areas between 80 and 200 mm 2 was found to be in the range 10-18% for 5.9 keV X-rays. The observed variations are associated to the dark current differences between the several LAAPDs. Moreover, the LAAPD simplicity, compact structure, absence of entrance window and high counting rate capability (up to about 10 5 /s) turn it out to be useful in diverse applications, mainly lowenergy X-ray detection, where LAAPDs selected for low dark current are able to achieve better performance than proportional counters. LAAPDs have been also investigated as VUV photosensors, where they present advantages compared to photomultiplier tubes. Since X-rays are used as reference in light measurements, the gain non-linearity between X-rays and VUV-light pulses has been investigated. The ratio between 5.9 keV X-rays and VUV light gains decreases with gain. Variations of 10 and 6% were observed for VUV light produced in argon (~128 nm) and xenon (~172 nm) for a gain 200, while for visible light (~635 nm) a variation lower than 1% was measured. The effect of temperature on the LAAPD performance was investigated for X-ray and VUV-light detection. Relative gain variations of about -5% per ºC were observed for the highest gains. The excess noise factor was found to be independent on temperature, presenting values of 1.8 and 2.3 for gains of 50 and 300, respectively. The energy resolution variation with temperature is not related to the excess noise factor, being mainly associated to the dark current. LAAPDs have been tested under intense magnetic fields up to 5 T. Their response in X-ray and visible-light detection practically does not vary with the magnetic field intensity while for 172 nm VUV light a significant amplitude reduction of more than 20% was observed.

An experiment measuring the 2S Lamb shift in muonic hydrogen (m À p) was performed at the Paul Scherrer Institute, Switzerland. It required small and compact detectors for 1.9 keV X-rays (2P-1S transition) with an energy resolution around 25% at 2 keV, a time resolution better than 100 ns, a large solid angle coverage, and insensitivity to a 5 T magnetic field. We chose Large Area Avalanche Photodiodes (LAAPDs) from Radiation Monitoring Devices as X-ray detectors, and they were used during the last data taking period in 2003. For X-ray spectroscopy applications, these LAAPDs have to be cooled in order to suppress the dark current noise; hence, a series of tests were performed to choose the optimal operation temperature. Specifically, the temperature dependence of gain, energy resolution, dark current,

The behaviour of large-area avalanche photodiodes for X-rays, visible and vacuum-ultra-violet (VUV) light detection in magnetic fields up to 5 T is described. For X-rays and visible light detection, the photodiode pulse amplitude and energy resolution were unaffected from 0 to 5 T, demonstrating the insensitivity of this type of detector to strong magnetic fields. For VUV light detection, however, the photodiode relative pulse amplitude decreases with increasing magnetic field intensity reaching a reduction of about 24% at 5 T, and the energy resolution degrades noticeably with increasing magnetic field. r

The charge radius of the proton, the simplest nucleus, is known from electron-scattering experiments only with a surprisingly low precision of about 2%. The poor knowledge of the proton charge radius restricts tests of bound-state quantum electrodynamics (QED) to the precision level of about 6 × 10–6, although the experimental data themselves (1S Lamb shift in hydrogen) have reached a precision of 2 × 10–6. The determination of the proton charge radius with an accuracy of 10–3 is the main goal of our experiment, opening a way to check bound-state QED predictions to a level of 10–7. The principle is to measure the 2S–2P energy difference in muonic hydrogen (µ–p) by infrared laser spectroscopy. The first data were taken in the second half of 2003. Muons from our unique very-low-energy muon beam are stopped at a rate of ~100 s–1 in 0.6 mbar H2 gas where the lifetime of the formed µp(2S) atoms is about 1.3 µs. An incoming muon triggers a pulsed multistage laser system that delivers ~0.2 mJ at λ ≈ 6 µm. Following the laser excitation µp(2S) → µp(2P) we observe the 1.9 keV X-rays from 2P–1S transitions using large area avalanche photodiodes. The resonance frequency, and, hence, the Lamb shift and the proton radius, is determined by measuring the intensity of these X-rays as a function of the laser wavelength. A broad range of laser frequencies was scanned in 2003 and the analysis is currently under way. PACS Nos.: 36.10.Dr, 14.20.Dh, 42.62.Fi

The paper describes an experiment aimed at studying muon capture by 3