Hydrogen-like light muonic ions, in which one negative muon replaces all the electrons, are extremely sensitive probes of nuclear structure, because the large muon mass increases tremendously the ...wave function overlap with the nucleus. Using pulsed laser spectroscopy we have measured three 2S-2P transitions in the muonic helium-3 ion (\(\mu^3\)He\(^+\)), an ion formed by a negative muon and bare helium-3 nucleus. This allowed us to extract the Lamb shift \(E(2P_{1/2}-2S_{1/2})= 1258.598(48)^{\rm exp}(3)^{\rm theo}\) meV, the 2P fine structure splitting \(E_{\rm FS}^{\rm exp} = 144.958(114)\) meV, and the 2S-hyperfine splitting (HFS) \(E_{\rm HFS}^{\rm exp} = -166.495(104)^{\rm exp}(3)^{\rm theo}\) meV in \(\mu^3\)He\(^+\). Comparing these measurements to theory we determine the rms charge radius of the helion (\(^3\)He nucleus) to be \(r_h\) = 1.97007(94) fm. This radius represents a benchmark for few nucleon theories and opens the way for precision tests in \(^3\)He atoms and \(^3\)He-ions. This radius is in good agreement with the value from elastic electron scattering, but a factor 15 more accurate. Combining our Lamb shift measurement with our earlier one in \(\mu^4\)He\(^+\) we obtain \(r_h^2-r_\alpha^2 = 1.0636(6)^{\rm exp}(30)^{\rm theo}\) fm\(^2\) to be compared to results from the isotope shift measurements in regular He atoms, which are however affected by long-standing tensions. By comparing \(E_{\rm HFS}^{\rm exp}\) with theory we also obtain the two-photon-exchange contribution (including higher orders) which is another important benchmark for ab-initio few-nucleon theories aiming at understanding the magnetic and current structure of light nuclei.
Avalanche photodiodes are commonly used as detectors for low energy x-rays. In this work we report on a fitting technique used to account for different detector responses resulting from photo ...absorption in the various APD layers. The use of this technique results in an improvement of the energy resolution at 8.2 keV by up to a factor of 2, and corrects the timing information by up to 25 ns to account for space dependent electron drift time. In addition, this waveform analysis is used for particle identification, e.g. to distinguish between x-rays and MeV electrons in our experiment.
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