Kamil Sedlak

Direct spectroscopic observation of a shallow hydrogenlike donor state in insulating SrTiO3.

We present a direct spectroscopic observation of a shallow hydrogenlike muonium state in SrTiO(3) which confirms the theoretical prediction that interstitial hydrogen may act as a shallow donor in this material. The formation of this muonium state is temperature dependent and appears below ∼ 70K. From the temperature dependence we estimate an activation energy of ∼ 50 meV in the bulk and ∼ 23 meV near the free surface. The field and directional dependence of the muonium precession frequencies further supports the shallow impurity state with a rare example of a fully anisotropic hyperfine tensor. From these measurements we determine the strength of the hyperfine interaction and propose that the muon occupies an interstitial site near the face of the oxygen octahedron in SrTiO(3). The observed shallow donor state provides new insight for tailoring the electronic and optical properties of SrTiO(3)-based oxide interface systems.

High-Field μSR Instrument at PSI: Detector Solutions

The High-Field μSR instrument is a highly challenging project under realization at the Swiss Muon Source [1] of the Paul Scherrer Institut (PSI, Switzerland). The detector system of the new spectrometer has to satisfy strict requirements on the time resolution and compactness. Muon-spin precession signals with frequencies of up to 1.3GHz in magnetic fields up to 9.5T have to be detected, the reduction of the signal amplitude should not exceed 50%. This requires an accuracy of better than 140 ps (sigma) in measuring the muon-positron time correlations – a time resolution unprecedented for such high fields. The small spiraling radius of the decay positrons in high fields (∼ 1 cm in 9.5T) sets restrictions on the maximal radial dimension of the detector. Preservation of the 10 ppm uniformity of the magnetic field at the sample position requires all detector components located in the vicinity of the sample to be non-magnetic. R&D work on the detector development for the High-Field project at PSI has started in 2004. It was realized that the required time resolution can hardly be achieved within the “standard” detector technology using photomultiplier tubes (PMTs), the limiting factors being attenuation and broadening of the light pulses in the indispensable light guides. Other potentially promising photosensors have been evaluated and the choice was made in favor of Geiger-mode Avalanche Photodiodes (G-APDs) [2]. These novel solid-state photodetectors deliver performance similar to that of PMTs, being at the same time insensitive to magnetic fields, compact, and non-magnetic (when choosing an appropriate packaging). The potential of the G-APD based detector technology for μSR and its reliability have been proven in [3, 4, 5]. The found technical solutions and the gained experience constituted an essential ground for working out the concept of the detector system of the High-Field μSR instrument.

Geant4 Simulation of the New ALC

Monte Carlo simulation programs based on the Geant4 package have become indispensable tools in modern particle physics. Following the advances in muon-spin rotation (μSR) instrumentation we have written a simulation code, MUSRSIM, currently in use in the development and optimisation of contemporary μSR spectrometers. MUSRSIM was employed for optimising the design of a new type of avoided level crossing (ALC) instrument, which is characterised by the exclusive use of Geiger-mode avalanche photodiodes instead of the standard photomultiplier tubes. The simulation code allowed us to study in detail the influence of the detector geometry, of the initial muon beam properties, and (to a limited extent) of the electronic signal processing on the predicted spectra. The good agreement between the simulation results and the measured data, validates MUSRSIM as a reliable software tool also for future instrument development work.

{\mu }{\rm SR}

The series production of Nb3Sn and NbTi cable-inconduit conductors (CICC) for the winding of various coils in the International Thermonuclear Experimental Reactor (ITER) magnet system is being completed in 2015. The tests of the ITER conductor samples are running in the SULTAN test facility in Villigen, Switzerland, in the scope of a framework contract, flanked by six bilateral agreements between IO and the Domestic Agencies (DAs) of China, EU, Korea, Japan, Russian Federation, and USA. The key acceptance parameter of the tests for the conductor samples made from the series production ITER CICCs is the current sharing temperature Tcs at the nominal operating field and current after applied electromagnetic cyclic loadings at the nominal field and sinusoidal swing of conductor current from zero to nominal one. The Tcs is defined as the maximum temperature at which the conductors operate (nominal current and field) before developing an electric field of 10 μV/m. This paper gathers the latest test results of the ITER series production CICC. The qualified conductor units will be used in the winding of the toroidal field and poloidal field coils, as well as the central solenoid. The test results for the series production conductors are compared with the results of the conductors tested at the early stages of their development. The performance variation among the suppliers and the performance evolution since the start of the conductor production are discussed as well.

Spectrometer

In this work we present the design, the construction and the measurements of the magnet for a so-called spin-rotator (Wien filter), a beam line device used to rotate the spin direction (and the associated magnetic moment) of muons in a beam used for condensed matter research at the Swiss Muon Source at the Paul Scherrer Institute. The design parameters-originating both from the properties of the preferred particle beam as well as the technological constraints for the high-voltage components generating the necessary electric field-were optimized for device compactness, cost and high beam transmission.