Search

A measurement of the 2SLamb shift in muonic hydrogen (μ−p) is being prepared at the Paul Scherrer Institute (PSI). The goal of the experiment is to measure the energy difference ΔE(25P3/2−23S1/2) by laser spectroscopy (λ≈6μm) to a precision of 30 ppm and to deduce the root mean square (rms) proton charge radius with 10−3relative accuracy, 20 times more precise than presently known. An important prerequisite to this experiment is the availability of long-lived μp2S-atoms. A 2S-lifetime of ∼1 μs – sufficiently long to perform the laser experiment – at H2gas pressures of 1–2 hPa was deduced from recent measurements of the collisional 2S-quenching rate. A new low-energy negative muon beam yields an order of magnitude more muon stops in a small low-density gas volume than a conventional cloud muon beam. A stack of ultra-thin carbon foils is the key element of a fast detector for keV-muons. The development of a 2 keV X-ray detector and a 3-stage laser system providing 0.5 mJ laser pulses at 6 μm is on the way.

We present the study of μpatom scattering in solid hydrogen. Anomalously large emission of Epμ≤1.9 meV μp's from a solid H2layer was observed for the first time. This three times greater μpatom yield is due to non-elastic phonon scattering. As a result, it becomes possible to generate an ultracold flux of μpatoms. The recent calculations of the total and differential cross sections agree with all experimental results of μpatom scattering in solid H2.

Since the discovery that muonic deuterium at energies near a few eV could travel distances of the order of 1 mm in condensed hydrogen, and in particular that muonic tritium and muonic deuterium could emerge from the surface of a solid hydrogen layer, the advantages of solid targets have enabled the study of several processes important in muon catalyzed fusion. A review of the results is presented, emphasizing the strengths and limitations of the use of solid hydrogen layer targets.

Resonant scattering of muonic hydrogen atoms via back decay of the molecular complex, a key process in the understanding of epithermal muonic molecular formation, is analyzed. The limitations of the effective rate approximation are discussed and the importance of the explicit treatment of the back decay is stressed. An expression of the energy distribution for the back-decayed atoms is given.

The vast majority of muon catalyzed fusion research has been concerned with muonic molecules of hydrogen isotopes only, since the dynamics of higher-Z muonic atoms in general preclude the formation of molecular systems. In the specific case of hydrogen–helium mixtures, bound muonic molecular states can exist, and thus it is possible to search for the reaction μd 3He $$\xrightarrow{{\tilde \lambda f}}$$ μ+α(3.66 MeV)+p(14.64 MeV). Until recently, the theoretical predictions for the nuclear fusion rate in the μd 3He molecule, $${\tilde \lambda }$$ f, ranged over one order of magnitude, from 105to 106per second. An experimental upper limit has been measured for $${\tilde \lambda }$$ fin HD + 3He giving a value (<6×104s−1[1]). We report on the analysis of an experiment in D2+ 3He which has shown a signal coming either from the muon catalyzed reaction, or from the fusion in flight of 3He's formed from dμd fusion.

The TRIUMF E742 experiment has measured the energy dependence of the scattering cross-sections of muonic deuterium and tritium on hydrogen molecules for collisions in the energy range 0.1–45 eV. The experimental setup permits the creation of muonic atom (μdor μt) beams. The multilayered target system gives the possibility to choose the type of interactions to study and to isolate a particular interaction. The scattering of μdor μtbeams on H2is analyzed via the muon transfer reaction to neon. The time-of-flight method is used to measure the scattering cross section as a function of the energy of the muonic atom beam. The results are compared, using Monte Carlo simulations, with theoretical calculations which have been recently performed with high accuracy.

Our measurement compares the energy dependence of the scattering cross-sections of muonic deuterium and tritium on hydrogen molecules for collisions in the energy range 0.1–45 eV. A time-of-flight method was used to measure the scattering cross-section as a function of the muonic atom beam energy and shows clearly the Ramsauer–Townsend effect. The results are compared with theoretical calculations by using Monte Carlo simulations. The molecular pdµ and ptµ formation creates background processes. We measure the formation rates in solid hydrogen by detecting the 5.5 MeV (pdµ) and 19.8 MeV (ptµ) γ-rays emitted during the subsequent nuclear fusion processes.

Strong solid state effects in low energy scattering of pµ atoms in solid hydrogen are reported and analyzed. Such effects have been observed in TRIUMF experiment E742 where muons are stopped in thin frozen (3 K) layers of hydrogen. Emission of low energy pµ atoms from the hydrogen layer into adjacent vacuum was much higher than expected, based on calculations which ignored the solid nature of hydrogen. Monte Carlo simulations, performed using the scattering cross-sections with solid state effects taken into account, show the important role of the coherent elastic Bragg scattering in the diffusion of pµ atoms. For pµ energies lower than the Bragg cut-off limit (∼2 meV) the total scattering cross-section falls by several orders of magnitude, the hydrogen target becomes transparent and the emission of cold pµ atoms takes place.

Solid hydrogen in the form of an inhomogeneous layered target offers several experimental advantages when compared with liquid or gas. Beams of non-thermalized muonic hydrogen atoms allow us to explore resonant molecular ion formation processes near eV kinetic energies. Isotopically specific layers make it possible to separate competing and confusing interactions and to employ the time of flight for comparison with predictions based on theoretical energy dependences. Unambiguous charged fusion product detection simplifies absolute intensity measurements. The systematic uncertainties encountered in resonant molecular ion formation measurements, using solid hydrogen target layers, are being investigated with simulations which use the many calculated energy-dependent rates and cross-sections which are now available. The importance of the rates for processes such as muon transfer and elastic scattering are discussed, and results of some recent analyses are presented.

The DEAR (DAΦNE Exotic Atom Research) experiment at the new φ-factory DAΦNE of Laboratori Nazionali di Frascati aims for a 1% measurement of the line shift of the K-complex due to strong interaction in kaonic hydrogen. This presentation is meant to prove that the high statistics and good resolution obtainable with DEAR will be able to disentangle the kaonic hydrogen K-complex lines (a cascade unconstrained fit), obtaining in this way constraints for what concerns the cascade parameters and, consequently, information regarding the physical processes involved in the kaonic hydrogen atomic cascade.

The muon transfer rate $$(\tilde \lambda _{d{\text{ }}^{\text{3}} {\text{He}}} )$$ from deuterium to 3at temperatures of 39.5 K and 50.0 K, respectively, has been determined as a by-product of the experiment aimed to search for muon catalyzed d3He-fusion. The measurements were performed in the muon beam at PSI with a multi-anode high pressure ionization chamber as a target and detector of the incoming muons and the charged reaction products. The chamber was filled with the HD + 3He(5.6%) gas mixture of 9.21% LHD density. The analysis of time distributions of dd-fusion events was able to determine the transfer rates: $$\tilde \lambda _{d{\text{ }}^{\text{3}} {\text{He}}} $$ (50 K)=(2.32± 0.09)× 108s-1, $$\tilde \lambda _{d{\text{ }}^{\text{3}} {\text{He}}} $$ (39.5 K)=(2.33± 0.16)·108s-1. These values, normalized to liquid hydrogen density (LHD, 4.25· 1022atoms/cm3), significantly exceed the transfer rates at room temperature, in agreement with theoretical predictions.

The scientific program and the experimental setup of the DEAR (DAΦNE Exotic Atom Research) experiment at the new φ-factory DAΦNE of Laboratori Nazionali di Frascati are described. The objective of DEAR is to perform a 1% measurement of the Kαline shift due to the strong interaction in kaonic hydrogen. A measurement will also be performed on kaonic deuterium for the first time. The aim is to investigate low-energy $$\overline K N$$ physics and to understand SU(3) chiral symmetry breaking. The setup takes advantage of the unique features of the “kaon beam” from the φ decay in DAΦNE; of a low-temperature pressurized gaseous target; and of a detector for soft X-rays – the Charge-Coupled Device (CCD) – characterized by a very good energy (and spatial) resolution and by an unprecedented background rejection capability. The DEAR experiment represents a major effort in the study of low energy $$\overline K N$$ interactions and has the potential to produce a breakthrough in the field.

The energy and time distributions of the decay X-rays of excited, metastable, molecular (dµHe)*-resonances were measured. The comparison of the observed energy spectra with calculated ones suggests that decay from the rotational state J= 1 dominates at the investigated conditions. The muon transfer rates from ground state deuterium to the helium isotopes 3He and 4He at low temperatures were determined from the time distributions of these spectra. Additionally, the temperature dependence of the muon transfer rate was clearly established in deuterium / 4He mixtures.

The transfer reaction of negative muons from muonic protium to 3and 4in binary and triple gas mixtures was studied. In the binary mixtures the transfer rates to the two helium isotopes were determined from the time distribution of the 7-keV X-rays of the intermediate muonic molecule (pµHe)*. The experimental transfer rate to 4is in good agreement with theoretical predictions, whereas the rate to 3is a factor 2 to 3 smaller than the predicted ones. Radiative branching ratios of the (pµHe)8 molecular decay were obtained. Muon transfer from excited states of muonic protium gives the main contribution to the total intensity of the µHe Lyman series in the binary mixtures. Values of q1sHeare determined.

We report on the results of an experiment aimed at observing muon-catalyzed d3He-fusion with a setup previously used for studies of the muon-catalyzed dd-fusion. The basic element of the setup is a high pressure ionization chamber operating as an active target. In this experiment the chamber was filled with an HD + 3He (5.6%) gas mixture at 13.2 bar pressure and 50 K temperature. These conditions were chosen as optimal for formation of the 3Heμd-molecules with a low level of background from the d-μ-dfusion. The chamber was exposed to the negative muon beam at PSI. During a 3-week data-taking period, 9.7 × 108muon stops have been selected. The analysis of the data was able to determine a new upper limit for the d3He-fusion rate in the 3Heμd-molecule (λf≤ 6× 104s-1), which is more than three orders of magnitude lower than the previously existed limit.

Muon-catalyzed fusion was first observed in the pdμsystem, and remains a good test of our understanding of the underlying molecular and nuclear processes. In contrast to the ddμand dtμsystems, no resonant behavior is expected, which considerably simplifies the dynamics. We will discuss data taken with solid H-D mixtures of 0.05%, 2%, 15% and 75% D2. In these measurements we observed simultaneously muons from pdμ→μ+3He and γs from pdμ→μ3He + γ. A simulation code incorporating the relevant physics processes has been developed for the analysis of this data. Preliminary results are presented for the fusion and molecular formation rates. This new data stringently tests the currently-accepted fusion scheme. A new value of the astrophysical S-factor is derived.

Emission of muonic tritium from a solid hydrogen layer has been studied via imaging of the muon-decay electrons and the time-of-flight distributions have been compared with detailed Monte Carlo calculations. Results are consistent at the 10% level with the theoretical prediction of a Ramsauer-Townsend minimum cross-section energy.

Many atomic and molecular processes may occur after a muon has been stopped in a mixture of hydrogen and helium isotopes. In particular, the muonic molecule μd3He can be formed. This molecule either decays or undergoes muon catalyzed fusion. Theory predicts a fusion rate much lower than the decay rate; moreover, the various theoretical predictions differ from each other by several orders of magnitude. With the experiment presented here we intend to measure the effective fusion rate via the detection of the 14.7 MeV fusion proton. A new target and gas mixing system, designed at JINR, were used in a ten-day test run at PSI. The fusion protons and products from other transfer and background reactions were measured with several different detectors (plastic scintillators, BGO, Germanium, neutron, and Silicon detectors). The formation rate of the μd3He molecule and an upper limit for the effective fusion rate have been measured and are presented here.

The muonic hydrogen atom in the 2s state provides the possibility of achieving high precision laser spectroscopy experiments from which a high precision value of the proton radius can be deduced. This will ultimately allow an increased precision in the test of QED in bound systems. Important progress has been made in recent years in the ability to stop muons in a low pressure gas target and in the understanding of the 2s-metastability in muonic hydrogen. As a consequence the 2s–2p laser spectroscopy experiment is now feasible and we present here the basic experimental concept considered by our collaboration.

Abstract: Muon transfer from hydrogen isotopes to helium is of importance in muon-catalyzed fusion and serves as a unique tool for the direct observation of the mesomolecular ion (pµHe)*. During two data-taking runs in 1994 and 1995 at PSI, measurements of the transfer from protium to helium were carried out using CCD's and Ge-diodes as independent X-ray detectors to obtain information on the time distribution and intensity of the 7 keV decay X-ray of the mesomolecular ion. Dual1H2 +4 He and triple1H2 + Ne +3,4He gas mixtures at low temperatures were investigated to extract transfer rates. Protium of high purity was available. For the first time a transfer rate from muonic protium to3He was measured. A general survey of our measurements and preliminary analysis is presented.

Abstract: Muon-catalyzed fusion in deuterium has traditionally been studied in gaseous and liquid targets. The TRIUMF solid hydrogen layer target system has been used to study the fusion reaction rates in the solid phase at a target temperature of 3 K. Both branches of the cycle were observed; neutrons by a liquid organic scintillator, and protons by a silicon detector located inside the target system. The effective molecular formation rate from the upper hyperfine state and the spin exchange rate have been measured, and information on the branching ratio parameters has been extracted.

Abstract: We propose a direct measurement of muon sticking to alpha particles in muon-catalyzed dt fusion at a high density. Exploiting the features of a multilayer thin-film target developed at TRIUMF, the sticking is determined directly by detection of charged fusion products. Experimental separation of initial sticking and stripping may become possible for the first time. Monte Carlo simulations, as well as preliminary results of test measurements are described.

Abstract: A technique of neutral muonic atom beams is proposed in the TRIUMF E742 experiment for measuring the scattering cross sections of muonic hydrogen isotopes in solid hydrogen. We present the results of Monte Carlo modeling of pµ and dµ atoms transport under the conditions of this experiment, taking into account the main physical as well as the geometrical aspects. The optimization of set-up parameters is performed in order to choose the most sensitive experimental conditions.

Abstract: Preliminary results are reported for an experiment at TRIUMF where a time-of-flight technique was tested for measuring the energy dependence of the rate for muon-catalyzed dt fusion. Muonic tritium atoms were created following transfer of negative muons from muonic protium in a layer of solid hydrogen (protium) containing a small fraction of tritium. The atoms escaped from the solid layer via the Ramsauer-Townsend mechanism, traversed a drift region of 18 mm, and then struck an adjacent layer of deuterium, where the muonic atom could form a molecular system. The time of detection of a fusion product (neutron or alpha) following muon arrival is dependent upon the energy of the muonic tritium atom as it traverses the drift region. By comparison of the time distribution of fusion events with a prediction based on the theoretical energy dependence of the rate, the strength of resonant formation can in principle be determined. The results extracted so far are discussed and the limitations of the method are examined.

Abstract: Several experiments performed by our group in recent years have put into evidence the complex structure of the time distributions of the muonic X-rays following transfer from muonic hydrogen isotopes to heavier elements. Simulations have shown that a substantial fraction of the µp atoms in the ground state have epithermal energies. Therefore, an energy dependence of the transfer rate seems a reasonable assumption for the explanation of the complex time structure.

Abstract: Knowledge of the cross sections for scattering of µp, µd and µt on molecules of hydrogen isotopes is necessary not only for checking the algorithmic solution of the Coulomb three-body problem but also for a general and correct description of the kinetics of muonic and molecular processes in mixtures of hydrogen isotopes. We plan to measure the scattering cross-section energy dependence of the reactions µx+ H2rarrµx+H2 (x=d, t) in the energy collision range from 0.1 to 45 eV, using a multilayered target system recently developed at TRIUMF.

Abstract: To investigate the energy dependence of muon transfer to oxygen, we performed measurements in gaseous mixtures of hydrogen and oxygen. The time distributions of the muonic oxygen X-rays showed the same structure as the one observed earlier in H2+SO2 mixtures. In the delayed part of these distributions, one can distinguish a short-time and a long-time component. From the latter, we deduced the transfer rates from thermalized muonic protium, respectively deuterium, to oxygen. The short-time component can be interpreted as being due to muon transfer from epithermal muonic hydrogen atoms. The time parameters are characteristic for the deceleration process as well as for energy-dependent transfer rates. With results of recent research on the formation and the thermalization of muonic hydrogen, we performed Monte Carlo simulations in order to test the hypothesis of an energy-dependent transfer rate.

Abstract: In an experiment at TRIUMF to study muon-catalyzed fusion and associated atomic and molecular effects, negative muons were stopped in a solid protium hydrogen layer containing a small amount of deuterium. Most of the resulting µp atoms disappeared by formation of ppµ molecules or by muon transfer to a deuteron. The µd can drift almost freely through the hydrogen layer due to the Ramsauer-Townsend effect and may even leave the layer. If a thin neon layer is frozen atop the hydrogen, the exiting muonic atoms will very rapidly release their muon to a neon atom. The analysis of the time structure of the neon X-rays is used to determine the rates of the slower processes involved in the evolution of the µp. This analysis has been performed with the help of Monte Carlo calculations, which simulate the kinetics of both µp and µd atoms in the hydrogen mixtures.

Abstract: In experiments using the TRIUMF solid hydrogen target system, the knowledge of the target thickness and uniformity is often essential in order to extract physical parameters from the data. We have characterized the thickness and uniformity of frozen targets using the energy loss of alpha particles. An accuracy of sim5% was achieved, a limit imposed by the uncertainty in the stopping powers: The details of the method are described, and the thickness calibration of the target is presented.

Abstract: The knowledge of muon transfer from protium to tritium is essentially theoretical and the different theoretical values disagree partially. Using solid hydrogen-tritium targets, with different tritium concentrations, we obtained precise experimental results for the transfer rate to tritium and the ppµ molecular formation rate. The time spectra of neutrons and alpha particles produced after dtµ fusion are used to determine the transfer rate lambdapt and the molecule formation rate lambdappµ.

Abstract: Negative muons were trapped in gaseous mixtures of hydrogen and oxygen. The time distributions of the muonic oxygen X-rays showed a clear deviation from a single exponential structure. From these time distributions we evaluated the transfer rate from thermalized muonic hydrogen atoms to oxygen. In order to test the hypothesis of an energy-dependent transfer rate to oxygen we performed Monte Carlo simulations of the thermalization and the transfer process. We report on the present status of our investigations.

Abstract:: The present status of research of muon transfer from the ground state of muonic protium to 4He is reviewed. The analysis of a recent measurement in a triple gas mixture of H2+4He+Ne at 15 bar and room temperature is presented and the result is compared to the existing experimental and theoretical rates. The average muon transfer rate from protium to 4He determined from all lifetime measurements is .

The yields of muonic x rays of the Lyman series of nitrogen, neon and argon have been analysed to determine capture ratios A(Z1,Z2) in about thirty gaseous mixtures under different experimental conditions. In addition, capture ratios A(Z,H) have been determined in hydrogen mixtures by taking into account transfer processes from muonic hydrogen to elements Z. The influence of the spectral flux density of the muons on capture ratios has been analysed. Our per-atom ratios form a coherent set and confirm the value A(Ar,Ne) = 1. 262(10).

The charge exchange reaction of negative muons from the $$(\mu {\text{p}})_{1{\text{s}}} $$ atom to oxygen has been measured in gaseous mixtures of H2+ O2. The measurements were performed at three different relative oxygen concentrations ranging from 0.2% to 0.8% and total pressures 3.5–15 bar. A mean transfer rate of $$\lambda _{{\text{pO}}}^{\text{t}} = 0.85(2) \times 10^{11} {\text{s}}^{{\text{ - 1}}} $$ , describing the transfer from the ground state of thermalized $$\mu {\text{p}} $$ atoms to oxygen, was determined. In order to investigate the energy dependence of the transfer rate, Monte Carlo simulations of the $$(\mu {\text{p}})_{1{\text{s}}} $$ thermalization and the muon transfer were carried out. The comparison of measured and simulated time spectra yielded an epithermal transfer rate $$\lambda _{{\text{pO}}}^{\text{e}} $$ =3.9 $$\times $$ 1011s-1in the energy interval 0.12–0.22 eV. The analysis with the model of “Two components” shows that all measured time spectra can be reproduced with the same set of parameters.

The emission of muonic tritium atoms from a thin film of hydrogen isotopes into vacuum was observed. The time and position of the muon decays were measured by tracking the decay electron trajectory. The observations are useful both for testing the theoretical cross sections for muonic atomic interactions, and producing an atomic beam of slow μ-t with a controllable energy.

An experimental setup, including a cryogenic gas target with temperature control and gas filling systems, has been developed to study muon catalyzed fusion in gas mixtures of hydrogen and helium isotopes.

In recent TRIUMF experiments, a μ-beam is stopped in a solid hydrogen film with a small fraction of T2. The Ramsauer-Townsend (RT) mechanism allows μtto escape into vacuum with a few eV of energy. To study the emission process, an imaging system was used to determine the position of muon decays. Experimental histograms are in good agreement with a Monte Carlo simulation.

The objective of the DEAR experiment is a precision measurement of the strong interaction shifts and widths of the K-series lines in kaonic hydrogen and the first observation of the same quantities in kaonic deuterium. These results will allow a precise determination of the isospin dependent kaon-nucleon scattering lengths and will help to determine the kaon-nucleon sigma terms. With the present performance of the DAFNE collider a kaonic hydrogen measurement with a precision better than the best existing experiment seems feasible now. In a preparing experiment kaonic nitrogen X rays were measured.