Micro- and Nano-Scales Three-Dimensional Characterisation of Softwood

Understanding the mechanical response of cellular biological materials to environmental stimuli is of fundamental importance from an engineering perspective in composites. To provide a deep understanding of their behaviour, an exhaustive analytical and experimental protocol is required. Attention is focused on softwood but the approach can be applied to a range of cellular materials. This work presents a new non-invasive multi-scale approach for the investigation of the hygro-mechanical behaviour of softwood. At the TOMCAT beamline of the Paul Scherrer Institute, in Switzerland, the swelling behaviour of softwood was probed at the cellular and sub-cellular scales by means of 3D high-resolution phase-contrast X-ray imaging. At the cellular scale, new findings in the anisotropic and reversible swelling behaviour of softwood and in the origin of swelling hysteresis of porous materials are explained from a mechanical perspective. However, the mechanical and moisture properties of wood highly depend on sub-cellular features of the wood cell wall, such as bordered pits, yielding local deformations during a full hygroscopic loading protocol.

Towards a New μ→eγ Search with the MEG II Experiment: From Design to Commissioning

The MEG experiment represents the state of the art in the search for the Charged Lepton Flavour Violating μ+→e+γ decay. With its first phase of operations at the Paul Scherrer Institut (PSI), MEG set the most stringent upper limit on the BR (μ+→e+γ)≤4.2×10−13 at 90% confidence level, imposing one of the tightest constraints on models predicting LFV-enhancements through new physics beyond the Standard Model. An upgrade of the MEG experiment, MEG II, was designed and it is presently in the commissioning phase, aiming at a sensitivity level of 6×10−14. The MEG II experiment relies on a series of upgrades, which include an improvement of the photon detector resolutions, brand new detectors on the positron side with better acceptance, efficiency and performances and new and optimized trigger and DAQ electronics to exploit a muon beam intensity twice as high as that of MEG (7×107 μ+/s). This paper presents a complete overview of the MEG II experimental apparatus and the current status of the detector commissioning in view of the physics data taking in the upcoming three years.

Searching for the Muon Decay to Three Electrons with the Mu3e Experiment

Mu3e is a dedicated experiment designed to find or exclude the charged lepton flavor violating μ→ eee decay at branching fractions above 10−16. The search is pursued in two operational phases: Phase I uses an existing beamline at the Paul Scherrer Institute (PSI), targeting a single event sensitivity of 2·10−15, while the ultimate sensitivity is reached in Phase II using a high intensity muon beamline under study at PSI. As the μ→ eee decay is heavily suppressed in the Standard Model of particle physics, the observation of such a signal would be an unambiguous indication of the existence of new physics. Achieving the desired sensitivity requires a high rate of muons (108 stopped muons per second) along with a detector with large kinematic acceptance and efficiency, able to reconstruct the low momentum of the decay electrons and positrons. To achieve this goal, the Mu3e experiment is mounted with an ultra thin tracking detector based on monolithic active pixel sensors for excellent momentum and vertex resolution, combined with scintillating fibers and tiles for precise timing measurements.

Efficient Production of High Specific Activity Thulium-167 at Paul Scherrer Institute and CERN-MEDICIS

Thulium-167 is a promising radionuclide for nuclear medicine applications with potential use for both diagnosis and therapy (“theragnostics”) in disseminated tumor cells and small metastases, due to suitable gamma-line as well as conversion/Auger electron energies. However, adequate delivery methods are yet to be developed and accompanying radiobiological effects to be investigated, demanding the availability of 167Tm in appropriate activities and quality. We report herein on the production of radionuclidically pure 167Tm from proton-irradiated natural erbium oxide targets at a cyclotron and subsequent ion beam mass separation at the CERN-MEDICIS facility, with a particular focus on the process efficiency. Development of the mass separation process with studies on stable 169Tm yielded 65 and 60% for pure and erbium-excess samples. An enhancement factor of thulium ion beam over that of erbium of up to several 104 was shown by utilizing laser resonance ionization and exploiting differences in their vapor pressures. Three 167Tm samples produced at the IP2 irradiation station, receiving 22.8 MeV protons from Injector II at Paul Scherrer Institute (PSI), were mass separated with collected radionuclide efficiencies between 11 and 20%. Ion beam sputtering from the collection foils was identified as a limiting factor. In-situ gamma-measurements showed that up to 45% separation efficiency could be fully collected if these limits are overcome. Comparative analyses show possible neighboring mass suppression factors of more than 1,000, and overall 167Tm/Er purity increase in the same range. Both the actual achieved collection and separation efficiencies present the highest values for the mass separation of external radionuclide sources at MEDICIS to date.

Development of a cold atomic muonium beam for next generation atomic physics and gravity experiments

A high-intensity, low-emittance atomic muonium (M =\mu^+ + e^-=μ++e−) beam is being developed, which would enable improving the precision of M spectroscopy measurements, and may allow a direct observation of the M gravitational interaction. Measuring the free fall of M atoms would be the first test of the weak equivalence principle using elementary antimatter (\mu^+μ+) and a purely leptonic system. Such an experiment relies on the high intensity, continuous muon beams available at the Paul Scherrer Institute (PSI, Switzerland), and a proposed novel M source. In this paper, the theoretical motivation and principles of this experiment are described.

The search for the neutron electric dipole moment at PSI

The existence of a nonzero permanent electric dipole moment (EDM) of the neutron would reveal a new source of CP violation and shed light on the origin of the matter–antimatter asymmetry of the Universe. The sensitivity of current experiments using stored ultracold neutrons (UCN) probe new physics beyond the TeV scale. Using the UCN source at the Paul Scherrer Institut, the nEDM collaboration has performed the most sensitive measurement of the neutron EDM to date, still compatible with zero (|d_n|<1.8\times 10^{-26} \, e {cm}|dn|<1.8×10−26ecm, C.L.,90%). A new experiment designed to improve the sensitivity by an order of magnitude, n2EDM, is currently under construction.

Mulan: a part-per-million measurement of the muon lifetime and determination of the Fermi constant

The part-per-million measurement of the positive muon lifetime and determination of the Fermi constant by the MuLan experiment at the Paul Scherrer Institute is reviewed. The experiment used an innovative, time-structured, surface muon beam and a near-4\piπ, finely-segmented, plastic scintillator positron detector. Two in-vacuum muon stopping targets were used: a ferromagnetic foil with a large internal magnetic field, and a quartz crystal in a moderate external magnetic field. The experiment acquired a dataset of 1.6 \times 10^{12}1.6×1012 positive muon decays and obtained a muon lifetime \tau_{\mu} = 2\, 196\, 980.3(2.2)τμ=2196980.3(2.2)~ps (1.0~ppm) and Fermi constant G_F = 1.166\, 378\, 7(6) \times 10^{-5}F=1.1663787(6)×10−5 GeV^{-2}−2 (0.5~ppm). The thirty-fold improvement in \tau_{\mu}τμ has proven valuable for precision measurements in nuclear muon capture and the commensurate improvement in G_FF has proven valuable for precision tests of the standard model.

The Meson Production Targets in the high energy beamline of HIPA at PSI

Two target stations in the 590 MeV proton beamline of the High Intensity Proton Accelerator (HIPA) at the Paul Scherrer Institut (PSI) produce pions and muons for seven secondary beamlines, leading to several experimental stations. The two target stations are 18 m apart. Target M is a graphite target with an effective thickness of 5 mm, Target E is a graphite wheel with a thickness of 40 mm or 60 mm. Due to the spreading of the beam in the thick target, a high power collimator system is needed to shape the beam for further transport. The beam is then transported to either the SINQ target, a neutron spallation source, or stopped in the beam dump, where about 450 kW beam power is dissipated. Targets, collimators and beam dumps are described.

The Mu3e experiment

The experiment aims for a single event sensitivity of 2\cdot 10^{-15}2⋅10−15 on the charged lepton flavour violating \mu^+\rightarrow e^+ e^+ e^-μ+→e+e+e− decay. The experimental apparatus, a light-weight tracker based on custom High-Voltage Monolithic Active Pixel Sensors placed in a 1 T magnetic field is currently under construction at the Paul Scherrer Institute, where it will fully use the intense 10^88\mu^+μ+/s beam available. A final sensitivity of 1 \cdot 10^{-16}1⋅10−16 is envisioned for a phase II experiment, driving the development of a new high-intensity continuous muon source which will deliver >10^99\mu^+μ+/s to the experiment.