A position--sensitive detector (the Muon Beam Profile Monitor μBPM) for nonrelativistic muon beams and to be used in high magnetic fields was developed. The μBPM consists of a grid of 10+10 scintillating fibers probing the muon beam intensity along the horizontal (X) and the vertical (Y) direction perpendicular to the beam direction (Z), i.e., at Xi and Yj coordinates of the fibers (i,j = 1...10). The readout of the fibers is realized by avalanche microchannel photodiodes (AMPDs). The usage of AMPDs results in a compact design of the device, its insensitivity to magnetic fields, and the ability to handle high event rates.

We report an observation of the formation of muonium [Mu=(mu(+)e(-)) bound state] with kinetic energies between 1 and 40 keV on 500-nm-thick solid argon, xenon, and nitrogen (N-2) layers. The thin films are deposited on a 250-mu m-thick aluminum target which is bombarded with a 3.6-MeV mu(+) beam. We measured the charge state of the muons emerging from the layers as a function of exit energy. The solid layers investigated are of potential interest for the efficient moderation of mu(+) down to energies of 10 eV for use as a source for a low-energy mu(+) beam. For the cryogenic insulators investigated, we find that the measured energy-dependent neutral fraction of the exiting muon beam is well interpreted by velocity scaling of known cross sections of protons for electron capture and electron loss down to muon energies of 1 keV. The experimental results are well reproduced by a Monte Carlo simulation. The total fraction of muonium in the exiting beam is found to be 2x10(-3) for argon and nitrogen as well as for the aluminum substrate without deposited layer, and about 20% less than this for xenon. [S1050-2947(98)07411-3].

At the Paul Scherrer Institute very slow, nearly 100% polarized, positive muons with an energy of \sim\mbox10 eV are produced by moderating a secondary beam of surface muons in a thin film of an appropriate condensed gases. These epithermal muons can be used as a source of a tertiary beam of tunable energy between \sim\mbox10 eV and \sim\mbox20 keV. Such a beam allows the μSR technique to be extended to the study of thin films and surfaces. In order to be able to perform time differential μSR experiments we have developed an ultra‐thin detector that registers the passage of keV muons and permits to trigger the experiment. The results achieved so far demonstrate that first investigations of thin film samples can be performed with the present set‐up.

Motivated by the possibility of using condensed gas moderators to produce very slow (epithermal) polarized muons we have studied the moderator properties of rare gas solids and solid N2 thin films as a function of the growing parameters (deposition temperature and rate, heat treatment) and of the film thickness. For Ar and Kr moderators the moderation efficiency is found to depend on the growth temperature and an annealing effect is observed. The dependence can be interpreted in terms of the changes in granularity and porosity of the condensed gas layers when the growth parameters are varied. From the thickness dependence of the moderation efficiency the escape depth of the very slow muons was determined. Its large value indicates a suppression of electronic energy loss mechanisms for the epithermal muons. The time dependent decay properties of the moderators were also investigated and found to depend solely on the rest gas pressure. At a pressure of 10−10 mbar the moderation efficiency remains stable over a period of days.

During the last few decades, a variety of methods has been developed which makes use of polarized positive muons as a microscopic probe of the magnetic properties of condensed matter (muon spin rotation, relaxation, resonance,μSR). Until now, available beams for μSR studies have delivered 100% polarized muons with energies in the MeV range, resulting in a deep penetration of the muons into the sample material under investigation. This presently limits the applications of theμSR technique to the study of the bulk characteristics of matter. To be able to control the implantation depth, a very low energy beam of polarized muons is being developed at the Paul Scherrer Institute. Very slow polarized muons (kinetic energy ∼ 10 eV, polarization ∼ 90%) are obtained from the moderation of a high energy muon beam in a thin film of an appropriate condensed gas. These muons can be used as a source for a beam of tunable energy between a few tens of eV and some tens of keV. Implantation depths in the range of few to a few hundreds of nanometers can thus be achieved by varying the energy.

A new experiment has been set up at the Paul Scherrer Institut to search for muonium to antimuonium conversion. No event was found to fulfil the requested signature which consists of the coincident detection of both constituents of the antiatom in its decay. Assuming an effective (V - A) x (V - A) type interaction an improved upper limit is established for the conversion probability of P() less than or equal to 8 x 10(-9) (90% C.L.), which is almost 2 orders of magnitude lower compared to previous results and provides a sensitive test for theoretical extensions of the standard model.

In the standard model lepton number violating processes are forbidden, but are allowed in many extensions to it. Muonium to antimuonium conversion has been explicitly allowed in the framework of left‐right‐symmetric models or in supersymmetric models. A new experiment at PSI has demonstrated its ability to improve the sensitivity to such a rare process significantly.

We have piloted mechanical compression of spinpolarized 3He by a titanium piston compressor. Questions of materials and design are discussed, followed by a thorough investigation of relaxation sources in the course of compression. The latter are traced mainly to regions with large surface to volume ratio, through which fast passage is demanded, therefore. We conclude from this feasibility study that polarized 3He may be compressed this way up to many bars without serious polarization losses.

MAMI was successful in producing a 855 MeV electron beam in August 1990.