The ISIS pulsed muon facility: Past,present and future

The pulsed muon facility at ISIS of the Rutherford Appleton Laboratory has been operational since March 1987. It is now fully scheduled for condensed matter research using polarised surface muons, atomic physics with sub-surface muons, and μCF experiments requiring negative cloud muons. The design and performance of the present beam are briefly discussed and recent improvements to the facility are described. Essential future upgrades have recently received international support and funding, which will lead to a complete facility comparable in extent to those of the continuous meson factories at PSI and TRIUMF, but with the unique advantages of the pulsed nature of the source. Such an upgraded facility will provide unprecedented opportunities for muon science at ISIS, unmatched by any other facility until the end of the decade.     

Proposal for muon and white neutron sources at CSNS

The China Spallation Neutron Source (CSNS) is a large scientific facility with the main purpose of serving multidisciplinary research on material characterization using neutron scattering techniques. The accelerator system is to provide a proton beam of 120 kW with a repetition rate of 25 Hz initially (CSNSⅠ), progressively upgradeable to 240 kW (CSNS-Ⅱ) and 500 kW (CSNS-Ⅱ'). In addition to serving as a driving source for the spallation target, the proton beam can be exploited for serving additional functions both in fundamental and applied research. The expanded scientific application based on pulsed muons and fast neutrons is especially attractive in the overall consideration of CSNS upgrade options. A second target station that houses a muon-generating target and a fast-neutron-generating target in tandem, intercepting and removing a small part of the proton beam for the spallation target, is proposed. The muon and white neutron sources are operated principally in parasitic mode, leaving the main part of the beam directed to the spallation target. However, it is also possible to deliver the proton beam to the second target station in a dedicated mode for some special applications. Within the dual target configuration, the thin muon target placed upstream of the fast-neutron target will consume only about 5% of the beam traversed; the majority of the beam is used for fast-neutron production. A proton beam with a beam power of about 60 kW, an energy of 1.6 GeV and a repetition rate of 12.5 Hz will make the muon source and the white neutron source very attractive to multidisciplinary researchers.     

Statistical study of the muon catalyzed d-t fusion cycle

In this paper we discuss the statistics of the main branch species of the muon catalyzed d-t fusion. From a master equation we derive and numerically solve kinetic equations for the average density and the covariances of a system composed of muons, muonic deuterium and muonic tritium atoms, muon molecular d-t ions, muonic helium, helium and neutrons. The system consists of an initial fixed amount of muons in a 50–50% D₂ + T₂ mixture without any external muon source. It is known that the probability distribution function of the population species with the exception of the neutron and helium follow a multinomial distribution function.     

The intense neutron generator of 14 MeV neutrons

An innovative intense neutron generator of 14 MeV neutrons for the irradiation of future reactor materials is presented. Negative pions are produced inside a 5–10 T magnetic field by an intense deuteron beam interacting with a carbon target. The pions and the muons from pion decay in flight are collected in the backward direction and stopped in a deuterium-tritium-hydrogen target of high density. Using an 18 MW deuteron beam at 1.5 GeV (12 mA=7.5 × 10¹⁶d/s), circa 10¹⁶gt^– /s can be generated, decaying to muons of which up to 10¹⁵ µ^–/s stop in the D/T/H mixture. Assuming X_c=100 fusions per muon, the µCF source produces 14 MeV neutrons with a source strength of up to 10¹⁷ n/s, i.e. a neutron power of 200 kW. The environment of the second target, the neutron source itself, can be made to resemble part of the Tokamak ring to be simulated for irradiation test samples.     

Neutrons from muons underground

Results of many-year experimental investigations of neutrons originating from cosmic-ray muons at depths of 25, 316, 570, 3650, and 5200 mwe are presented. Mechanisms of neutron generation by muons underground are outlined qualitatively. Methods of relevant measurements are described in detail, and the resulting properties of neutrons, including their total yield, flux, energy spectrum, multiplicity, and spatial distribution, are reported.     

Accelerator plans at TRIUMF

A recently completed Project Definition Study has proposed a network of accelerators to take the existing 500 MeV 150 μA proton beam at TRIUMF to 30 GeV. This facility would be capable of providing beams of kaons, antiprotons and other hadrons of intensities 100 times greater than those presently available. In addition, large numbers of low energy muons should be available and this facility is potentially the most powerful muon source planned for the future. The proposed facilities are described and the potential for future muon beams reported.     

The competition between kondo and RKKY interaction in CeTSn compounds

This article reviews our investigations of the series CeTSn where T stands for a transition element. Our research was stimulated by the unusual behavior of CeNiSn. These compounds are close to the border between magnetic and nonmagnetic Kondo systems. For the understanding of their physical properties a detailed knowledge of the magnetic properties is essential. μSR experiments allow one to reveal even weak magnetic correlations. Special attention is given to the explanation of experimental details. Pulsed (ISIS) and continuous (TRIUMF) muon beams were used as complementary techniques. Experiments in zero and longitudinal (with respect to the muon spin) applied field allowed the determination of the static width of the magnetic field distribution seen by the muons due to ionic moments as well as the fluctuation rate of the internal field. As a part of our experiments, the muon site has been determined and the procedure for this will be described.     

Single and multiple muons and the generation of neutrons in the LVD experiment

The large geometric factor and good spatial resolution of the Large Volume Detector (LVD) ensures statistically significant and highly accurate measurements of muon trajectories and determination of the multiplicity of muon groups. The developed algorithm allows us to reconstruct 2 × 10⁶ muon events (single muons and muon groups). Characteristics of muon groups are obtained and the specific yield of neutrons produced by single muons, muon groups, and showers is determined.     

Time‐differential radio‐frequency muon spin resonance (TD‐RFμSR) technique at a pulsed muon beam

Longitudinal‐field μSR methods, e.g., radio‐frequency μ⁺ spin resonance (RFμSR), are well suited to investigate dynamic processes that destroy the phase coherence of the muon spin ensemble. Additional information on relaxation processes of the muon species under investigation is obtained from time‐differential (TD) data acquisition. In this paper we describe the set‐up of a TD‐RFμSR spectrometer installed at the ISIS pulsed muon facility at the Rutherford Appleton Laboratory (RAL, Chilton, UK). As an example, results of TD‐RFμSR measurements on muons in diamagnetic environment μ^d in a boron‐doped silicon sample under illumination at 55 K are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

Dosimetry and spectrometry at accelerator based neutron source for boron neutron capture therapy

An innovative accelerator-based neutron source for boron neutron capture therapy has started operation at the Budker Institute of Nuclear Physics, Novosibirsk. This facility is based on a compact vacuum insulation tandem accelerator designed to produce proton current up to 10 mA. Epithermal neutrons are proposed to be generated by 1.915 MeV protons bombarding a lithium target using ⁷Li(p,n)⁷Be threshold reaction.In the article, techniques to detect neutron and gamma-rays at the facility are described. Gamma radiation is measured with NaI and BGO gamma-spectrometers. The total yield of neutrons is determined by measuring the 477 keV γ-quanta from beryllium decay. For the rough analysis of the generated neutron spectrum we used bubble detectors. As the epithermal neutrons are of interest for neutron capture therapy the NaI detector is used as activation detector. We plan to use a time-of-flight technique for neutron spectra measurement. To realize this technique a new solution of short time neutron generation is proposed.     

Production of pulsed ultraslow muons and first μSR experiments on thin metallic and magnetic films

Standard muon spin rotation (μSR) spectroscopy implants 4 MeV spin-polarized positive muons to investigate the bulk properties of matter. Success in producing epithermal muons opens interesting possibilities for studying ultrathin films, interfaces, and even surfaces. At the ISIS Facility, Rutherford Appleton Laboratory (Chilton, UK), we have produced a pulsed ultraslow muon beam (E< 20 eV) and have performed the first μSR experiments. Due to the pulsed feature, the implantation time is automatically determined and, by adjusting the final muon energy between about 8 and 20 eV, depth slicing experiments are possible down to monolayers distances. We describe slicing experiments across a 20 nm copper film on quartz substrate with evidence for a 2 nm copper oxide surface layer. A preliminary experiment on a hexagonal cobalt film suggests the existence of muon precession in the local magnetic field. The results are discussed in relation to the morphological features of the film.     

Ultra slow muon microscopy by laser resonant ionization at J-PARC, MUSE

As one of the principal muon beam line at the J-PARC muon facility (MUSE), we are now constructing a Muon beam line (U-Line), which consists of a large acceptance solenoid made of mineral insulation cables (MIC), a superconducting curved transport solenoid and superconducting axial focusing magnets. There, we can extract 2 × 10⁸/s surface muons towards a hot tungsten target. At the U-Line, we are now establishing a new type of muon microscopy; a new technique with use of the intense ultra-slow muon source generated by resonant ionization of thermal Muonium (designated as Mu; consisting of a μ ^?+? and an e^???) atoms generated from the surface of the tungsten target. In this contribution, the latest status of the Ultra Slow Muon Microscopy project, fully funded, is reported.     

μCF based 14 MeV intense neutron source

Results of a design study for an advanced scheme of a μCF based 14 MeV intense neutron source for test material irradiation including the liquid lithium primary target and a low temperature liquid deuterium-tritium (D–T) mixture as a secondary target are presented. According to this scheme negative pions are produced inside a 150-cm-long 0.75-cm-radius lithium target. Pions and muons resulting from the pion decay in flight are collected in the backward direction and stopped in the D–T mixture. The fusion chamber has the shape of a 10-cm-radius sphere surrounded by two 0.03-cm-thickness titanium shells. Assuming 100 fusions per muon in this scheme one can produce 14-MeV neutrons with a source strength up to 10¹⁷ n/s. A neutron flux of up to 10¹⁴ n/cm²/s can be achieved in a test volume of about 2.5 l and on the surface of about 350 cm². The results of the thermophysical and thermomechanical analysis show that the technological limits are not exceeded. This source has the advantage of producing the original 14 MeV fusion spectrum without tails, isotropically into a 4π solid angle, contrary to the d-Li stripping neutron source. This revised version was published online in August 2006 with corrections to the Cover Date.     

The O–H stretching band in ice Ih derived via eV neutron spectroscopy on VESUVIO using the new very low angle detector bank

Strong demand exists for an experimental facility enabling new experimental investigations on condensed matter systems based on epithermal neutron scattering at high energy and low momentum transfers. This need will be met by the very low angle detector (VLAD) bank, to be installed on the VESUVIO spectrometer at the ISIS spallation neutron source. The equipment will operate in the scattering angular range 1°<2θ<5°. Scattering measurements from a polycrystalline ice sample using a VLAD prototype demonstrates the effectiveness of the detection technique adopted for the construction of the full detector array. The resulting density of states in ice is 9±2 atoms/cell, in agreement with previous measurements. PACS 61.12.Ex; 63.20.Dj; 63.50.+x     

A 300 µA Upgrade for the ISIS Accelerator

The Rutherford Appleton Laboratory (RAL) is home to the world's leading spallation neutron source ISIS [1]. The ISIS neutron producing target is driven by a 50 Hz, 800 MeV, 200 _A proton beam from a rapid cycling synchrotron, which is fed by a 70 MeV H_ drift tube linac (DTL) which in turn accepts beam from an H_ 665 keV Cockcroft-Walton preinjector. The ever increasing international demand for neutrons has motivated a bid to build a second target station at ISIS, for which £100 million funding has recently been approved by the U.K. government [2]. The second target station, operating at 10 Hz, will provide new scientific opportunities in soft condensed matter, biology and advanced materials.     

Delayed formation of muon-containing species in organic liquids observed by spin resonance technique

The resonance technique has been applied to observe diamagnetic muons and, for the first time by the resonance, Mu-substituted radicals in organic liquids under strong decoupling magnetic fields. In benzoquinone solutions in benzene the relaxation of Mu-cyclohexadienyl radicals through reaction with quinone was directly observed by the radical resonance technique. The product of this reaction was then observed by the diamagnetic muon resonance as a slow formation. Similar slow formation was observed for diamagnetic muons in neat CS₂ and for Mu-radicals in benzene and styrene. Such slow formation can never be observed by the rotation technique due to dephasing problem, and thus the previous method is expected to provide new source of information on slow reaction dynamics of muon containing species.     

The Muon Science Facility at the JKJ Project

The muon science facility is one of the experimental arenas of the JKJ project, which was recently approved for construction in a period from 2001 to 2006, as well as neutron science, particle and nuclear physics, neutrino physics and nuclear transmutation science. The muon science experimental area is planned to be located in the integrated building of the facility for the materials and life science study. One muon target will be installed upstream of the neutron target in a period of phase 1. The beam line and facility are designed to allow the later installation of a 2nd muon target in a more upstream location. The detailed design for electricity, cooling water, primary proton beam line, one muon target and secondary beam lines (a superconducting solenoid decay muon channel, a dedicated surface muon channel, and an ultra slow muon channel) is underway. In the symposium, a latest status of the muon science facility at JKJ project will be reported. This revised version was published online in August 2006 with corrections to the Cover Date.     

Frictional accumulation of negative muons and its application

A novel method is proposed for the efficient conversion of intermediate energy negative muons into a low-energy muon beam. It is based on using an electric field to eject muons from a moderator consisting of a large number of thin carbon foils placed perpendicularly to the axis of a high-field solenoid. High-energy muons are made to slow down within the moderator to an energy where further slowing down is inhibited by the electric field acceleration between the foils. The muons accumulate at low energy within the moderator hopping from one foil to the next until they come out as a low-energy muon beam. The resulting phase compression factor exceeds 1000. Efficient initial injection of the muons into the moderator is obtained either by letting the muons enter it in a direction opposite to the acceleration force or by producing the muons within a magnetic trap containing the moderator. A practical configuration based on the second scheme is presented. By implementing the method into the most intense muon production configurations a new pathway is opened that may ultimately compete with other schemes in the selection of the optimal source for high-energy muon colliders.     

Neutron events in the underground monitor of the Tien Shan high-altitude station

The neutron multiplicity M spectrum was measured at the neutron monitor installed in the underground room of the Tien Shan high-altitude station (3340 m above sea level) of the Lebedev Physical Institute under a ground layer 20 m of water equivalent thick. To a first approximation, the differential multiplicity spectrum is power-law: dN/dM = 0.3 · M^−3.7m⁻²s⁻¹. The spectrum intensity is lower than the intensity of events in the ground-based NM64 supermonitor by a factor of 350–450. The spectrum slope exponent γ + 1 = 3.7 ± 0.1 is identical to the exponent of the energy spectrum of bremsstrahlung gamma-rays of energetic muons (above 1 TeV) generated in a lead absorber of the monitor. In this case, the experimental intensity is hundred times higher than the expected intensity of events from muon bremsstrahlung. The spatial distribution of neutrons in the monitor suggests that they are produced by single particles. The temporal distribution of neutrons in the monitor is exponential with lifetime constant τ = 360 − 390 μs. Difficulties are indicated in the interpretation of the multiplicity spectrum by electromagnetic and nuclear interactions of muons in the monitor without involving new penetrating particles. Original Russian Text ? A.P. Chubenko, A.L. Shchepetov, L.I. Vildanova, M.I. Vildanova, P.A. Chubenko, 2007, published in Kratkie Soobshcheniya po Fizike, 2007, Vol. 34, No. 4, pp. 21–31.     

Development of the pulsed muon facility at ISIS

The ISIS pulsed surface muon facility at RAL is presently undergoing a major expansion to provide three experimental ports with simultaneous single muon pulses at 50 Hz. This upgrade, funded by the European Community (EC), is described together with recent development results which are relevant to its future scientific programme. These new beam lines are expected to be available for experiments in June 1993.