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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).

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.

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.

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.

The experimental muon transfer rates from muonic protium and deuterium to heavier elements show a complexZ dependence. Instead of the expected monotonicZ dependence considerable variations between neighboring elements are observed. Transfer to neon has a specially small rate. Furthermore, the time distribution of the muonic X-rays shows in a number of molecules a complex structure, and depends in the case of a H2 + CH4 gas mixture also on the total pressure.

A set of experiments is reviewed which makes use of the emission of muonic deuterium from the surface of a layer of solid hydrogen. The behaviour of muons in a solid target system has been studied via detection of muon decay electrons, muonic X-rays, and fusion products (neutrons and charged particles). The emission of muonic deuterium is understood to result from the Ramsauer-Townsend scattering minimum. The energy distribution of the emitted atoms ranges from tenths of eV to about 10 eV, and can be controlled to some extent. A proposal is described to use muonic tritium emission to measure the energy dependence of muonic molecular formation.

The ratios of prompt to delayed fission yields for the isotopes U-233, U-234, U-235, U-236, U-238, Np-237, Pu-242, and Pu-244 and the fission probabilities relative to each other have been investigated experimentally. Using the value of the total fission probability for Np-237 the absolute probabilities for prompt and delayed fission have been determined. The fission probabilities per muon capture P(fc) have been derived for all the isotopes and compared with an evaluation based on excitation functions from theory.

After the feasibility of vacuum isolated μ−d production was demonstrated at TRIUMF in 1989, development was begun on a target system that would take advantage of the process to aid in the understanding of the muon catalyzed fusion cycle. Minimal neutron backgrounds, the ability to use silicon detectors, and compatibility with tritium were considered important for a very versatile target system. The advantages which the target gives in isolating μCF process will be outlined.

A study of muonic238U has been performed in a combined (μ−,γ f) and (μ−,γγ) coincidence experiment to investigate the role of non-radiative transitions and their fission probabilities. An augmentation of the outer fission barrier ofΔEb=(0.6±0.1) MeV due to the presence of the muon is deduced. A significant contribution to the prompt fission yield not only results from the (2p→1s) and (3d→1s) non-radiative transitions, but also from other radiationless transitions. Specifically, the measured fission probabilities of the transitions (2p→1s), (3d→1s), and (3p→1s) are (1.5±0.4)%, (5.7±1.7)%, and (5.3±1.9)%, respectively.

The present work extends the systematics of nuclear charge radii obtained by the method of muonic atoms to nuclei with 8\ensuremath{\le}Z,N\ensuremath{\le}20. The accuracy of the measured muonic Lyman transition energies of generally \ensuremath{\le}10 eV leads to a precision in the model-independent nuclear charge radii differences of 2--3 am for the isotope shifts and 4--9 am for isotone shifts. Both isotope and isotone shifts within the s-d shell behave ``anomalously'' with respect to the systematics of heavier nuclei. However, such behavior is predicted on theoretical grounds, if mixing in the s-d shells and the strong deformation of some of the nuclei in this region are considered. We compare charge radii from elastic electron scattering data with muonic atom rms radii and suggest that observed deviations are possibly due to incomplete dispersion corrections.

Fission fragments from the reaction237Np(μ−,γ,f) have been measured in coincidence with muonic X-rays. The efficiency of the fission fragment detector is determined from (μ−,γ,f)-data of the same experiment. The total fission probability perμ-stopPt has been measured as well as the fission probabilities Pf of the non-radiative muonic (3d→1s)- and (2p→1s)-transitions; the latter has been divided into two parts leading to different mean excitation energiesE:P t =(54±17)%,P f (3d→1s)=(41±21)%,P f (2p→1s,E=6.218 MeV)=(61±19)%, andP f (2p→1s,E=6.525 MeV)=(57±18)%. The influence of the muon on the fission barrier is discussed. The fission probability after muon capture is compared with a calculated value using a distribution of nuclear excitation energies following muon capture and the fission probability as measured in a238U(3He,αf)-reaction.

The probabilities for non-radiative (n.r.) excitationsP n.r. in the muonic nuclides208Pb,232Th, and238U have been determined from (μ−,γγ)-measurements by comparing the intensities of muonic X-ray transitions in single and coincidence spectra. The value ofP n.r. (3p→1s), measured for the first time, is about 90% for the actinides232Th and238U, but only about 8% for208Pb. The value ofP n.r. (3d→1s) is found to be 10 % for233Th, 13% for238U, and about 4% for208Pb. For208Pb a vanishing strength of the n.r. decay of the 2p-level is found, while for232Th and for238U n.r. strengths of about 20% and 26%, respectively, are observed. By regarding two subcomplexes of the 2p→1s transitions leading to different mean excitation energies the n.r. transition probabilities were found to be different for238U only, 21.6% and 31.1.%, respectively.

The time spectra for muon induced fission of233U,234U,235U,236U,238U, and237Np were measured simultaneously, detecting both fragments in coincidence in a fast multi parallel plate avalanche detector. The observed mean lifetimes τ are 68.9 ± 0.3 ns, 70.6 ± 0.2 ns, 72.2 ±0.2 ns, 74.3 ± 0.3 ns, 77.0 ±0.4 ns, and 69.8 ±0.2 ns for the above isotopes, respectively. No second time component, tentatively ascribed to a possible fission isomer, was observed. The deduced total capture rates are compared with theoretical predictions.

The muonic atom 2${\mathit{p}}_{1/2}$-1${\mathit{s}}_{1/2}$ and 2${\mathit{p}}_{3/2}$-1${\mathit{s}}_{1/2}$ transition energies were measured with an experimental accuracy of better than 20 ppm for the isotope chain $^{112,114,116,117,118,119,120,122,124}\mathrm{Sn}$. Precise values for the Barrett equivalent nuclear radii ${\mathit{R}}_{\mathit{k}\mathrm{\ensuremath{\alpha}}}$ and their differences as well as root-mean-square radii were deduced. The \ensuremath{\Delta}N=2 isotope shifts between the even Sn isotopes show a subshell effect at the neutron number N=64. Otherwise, there is a nearly linear decrease with increasing N, in accordance with the general systematics of nuclear charge radii. Our muonic atom results are in a good agreement with recent optical data, including odd-even staggering. Hartree-Fock calculations reproduce the general trend but not the subshell effect. Regarding the nuclear polarization corrections, the problem in the 2p splitting found earlier in ${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$-Zr and ${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$-Pb seems also to persist in ${\mathrm{\ensuremath{\mu}}}^{\mathrm{\ensuremath{-}}}$-Sn.

In the spectrum of muonic X-rays of238U measured in coincidence with prompt fission events a structure has been found which is attributed to 2p→1s transitions of the muon attached to heavy fragments. The intensityI s of this structure relative to the strengthI pf observed for prompt fission has been determined to beI s/I pf=(6.0± 2.1) %. Although the experimental significance for the appeareance of this phenomenon is weak, this is the first experimental indication for its occuxence.

The Tables summarize experimental results from muonic atom transition energies, nuclear charge parameters from elastic electron scattering, and K x-ray isotope shifts in so far as they provide information on nuclear ground-state charge radii. Numerous experimental results for optical isotope shifts have been published elsewhere; for eight elements the relevant information is condensed ("projected") here to one optical line per element. A model-independent analysis which combines data from all three experimental methods is applied to these elements and is presented as an illustration of the improved accuracy for the rms radii and Barrett radii which result from this analysis.

The X-ray spectrum of muonic 237 Np has been investigated with stopped muons in a NpO 2 target, containing about 10 g of 237 Np. The probability of the radiationless muonic 3d→1s transition in 237 Np , (9±4)% , was obtained by comparing the relative intensities of the main muonic X-ray transitions in singles and coincidence spectra. The coincidences were gated by the 2p→1s transitions.

Energies and intensities of 53 nuclear γ-lines in muonic atoms of Sm, Gd, Eu, Tm, Yb, Ta, W, Re, Os, Ir, Pt, Hg, Tl, and Bi have been measured with Ge(Li) detectors. By comparison with γ-lines from radioactive sources the isomer shifts for transitions from nuclear rotational levels, quadrupole and octupole vibrational levels, and single-particle levels have been deduced. The effect limiting the accuracy of the extracted isomer shifts is the correction for the magnetic hyperfine splitting of the nuclear levels.

Muonic X-ray energies of the K-series of the light natural elements Be, B, C and N have been determined with a precision of 5 to 10 eV. Equivalent nuclear charge radii have been deduced. The results, expressed in terms of rms radii, are of similar or better accuracy than recent electron scattering data. Of particular interest is the rms radius of carbon as a reference value for elastic electron scattering cross sections. A model-independent evaluation yields 〈r 2 〉 1 2 = 2.467 (17) fm .

We have determined the magnetic hyperfine structure (HFS) in four nuclear transitions of muonic 209Bi. The results yielded an improved value of the magnetic HFS constant of the ground state. A 1 ( 9 2 − ) = 2.00 (7) keV . New experimental values were established for three excited states, namely A 1 ( 13 2 + ) = 2.9 (2) keV , A 1 ( 9 2 + ) = 2.1 (2) keV and A 1 ( 15 2 + ) = 2.4 (2) keV . In addition, the isomer shifts (IS) of the above-mentioned transitions were determined making use of the HFS constants. We find the following IS's of excited states: 13 2 + , 3.77 (28) keV ; 9 2 + , 6.56 (32) keV ; 15 2 + , 6.29 (18) keV .

The energies of muonic x-ray transitions sensitive to the nuclear charge parameters have been measured with a precision up to 9 ppm in muonic /sup 90/Zr. Combining our muonic-atom results for the equivalent nuclear charge radii R/sub k/,..cap alpha.. with recent elastic electron scattering data, the rms radius of /sup 90/Zr could be determined as /sup 1/2/ = 4.2726(9) fm. In addition to the nuclear monopole charge distribution, nuclear polarization correlations between the muonic 1s and 2s and the 1s and the two 2p states could be determined. With additional constraints from elastic electron scattering and theory, the nuclear polarization corrections of the four lowest-lying states could be evaluated with precisions of the order of 15%. Of particular significance is the result for the two 2p states, because the experimentally determined 2p/sub 1/2/ correction is smaller than the nuclear polarization correction in the 2p/sub 3/2/ state, in contrast to current theoretical models.

Energies of muonic x rays of the Lyman series of the even sulphur isotopes /sup 32/S, /sup 34/S, and /sup 36/S and of natural phosphorus have been determined with absolute precisions up to 23 parts/10/sup 6/. Equivalent Barrett charge radii R/sub k/,..cap alpha.. have been deduced. Their differences between the sulphur isotopes amount to ..delta..R/sub k/,..cap alpha..( /sup 34/S- /sup 32/ S) = 29.7(1.4) am and ..delta..R/sub k/,..cap alpha..( /sup 36/S- /sup 34/ S) = 18.7(1.5) am. Combining these results with recent elastic electron scattering data, we obtain in a model-independent way ..delta../sup 1/2/( /sup 34/S- /sup 3/ 2S) = 23.1(1.2) am and ..delta../sup 1/2/( /sup 36/S- /sup 3/ 4S) = 13.0(1.2) am.

In order to test the QED corrections, and in particular the vacuum polarization, muonic 4f-3d transitions in Ba and Ce and 5g-4f transitions in Tl and Pb have been measured using Ge(Li) detectors. An absolute energy accuracy of ± 8 eV corresponding to a relative error of 17–19 ppm has been obtained. The experimental energies agree with theory within the total uncertainty of ± 10 eV. This corresponds to a test of the QED vacuum polarization calculations of between 0.35 % and 0.50 %.

Energies of muonic X-rays of the Lyman series of seven light natural elements (F, Na, Al, Si, P, S and K) have been determined with an accuracy of between 15 and 30 eV. Equivalent charge radii have been deduced. For μ−-K, transitions to the 2s state allowed an additional constraint on the Fermi c-t diagram. The results are in good agreement with, and generally of an order of magnitude higher precision than, comparable electron scattering data.

The results of precise measurements of the energies of the 2p32−1s12and 2p12−1s12 muonic X-ray transitions of 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, 98Mo and 100Mo are reported. The data were analyzed in terms of the Barrett moments 〈rke−αr〉 of the nuclear charge distributions from which the equivalent nuclear radii Rk and the differences ΔRk between neighboring isotopes were computed. Systematic shell effects have been observed at the neutron numbers N = 50 and N = 56.

Muonic transition energies between low-lying states in ${\ensuremath{\mu}}^{\mathrm{\ensuremath{-}}}$${\mathrm{\ensuremath{-}}}^{208}$Pb have been measured in a precision experiment using a Compton suppression spectrometer. Nuclear charge parameters and nuclear polarization corrections have been adjusted to the data. In particular, experimental correlations between the nuclear polarization corrections of the lower muonic levels have been established. A set of nuclear charge parameters reproducing all the experimental transition energies can only be obtained by assuming an inversion in magnitude of the nuclear polarization corrections not only for the 2p states, but also for the two 3p states. Such a result is unexplained by present theoretical models, but corroborates the findings of a recent experiment concerning the 2p states in ${\ensuremath{\mu}}^{\mathrm{\ensuremath{-}}}$${\mathrm{\ensuremath{-}}}^{90}$Zr. Using our best-fit parameters, a $^{208}\mathrm{Pb}$ rms radius of 〈${r}^{2}$${〉}^{1/2}$=5.5031(11) fm has been obtained.

Energies and intensities of muonic X-rays, nuclear γ-rays and μ-capture γ-rays were measured in natural muonic thallium with Ge(Li) detectors. The absolute intensities of higher μ X-rays were reproduced by a cascade calculation starting with a statistical population at n = 20 including K-, L- and M- conversion. The electron screening effect was deduced from energies of higher μ X-rays. Eight prompt nuclear μ-rays were found, due to an excitation of (4.1 ±1.0) % and (14.6 ± 3.0) % of the first 3 2 + level in μ 203 T1 and μ, 205 T1, respectively, and of (8.8 ± 0.9) % of the 5 2 − level at 2.63 MeV in 205 T1. This excitation explains the anomalous intensity ratios of the 2p-ls and 3d-2p fine structure components. From the nuclear y-rays of the first excited states were deduced: the magnetic h.f. splittings e 203 ( 1 2 ) = 2.57 ± 0.15 keV , e 205 ( 1 2 ) = 2.30 ± 0.02 keV, muonic isomer shifts ΔE is (203) = 0.26 ± 0.15 keV, ΔE is (205) = −(0.31 ± 0.11) keV; E2/M1 mixing ratios δ μ 2 (203) = 0.89 ± 0.27, δ μ 2 (205) = 1.00 ± 0.05, and the half-life in the presence of the muon T μ ( 3 2 ) = 1.4 ± 0.1 nsec in 205 T1. The values of e( 1 2 ) and ΔE ls were compared with shell-model calculations. From δ μ 2 and T gm ( 3 2 ) evidence for a magnetic nuclear polarization was found. From the 2p-ls, 3d-2p, 3p-2s and 2s-2p transitions the size and shape of the nuclear charge distribution and a nuclear polarization due to the 1s 1 2 muon of ΔE np = 8.3 ± 1.2 keV and 9.5 ± 1.2 keV for 203 Tl and 205 T1 respectively were determined in agreement with theoretical estimates. An isotope shift of ΔE = 10.35 ± 0.25 keV was measured for the 1s 1 2 state which is compared with data from optical spectroscopy. From an analysis of the time distribution of delayed γ-rays from μ-capture, an isotopic effect of (5.4 ± 2.5) % was found for the capture rates, in fair agreement with predictions from Primakoff's μ-capture formula.

NaI(T1) and Ge(L1) detectors have been used singly and in coincidence to study the gamma rays associated with the decay of 67 h 99 Mo Twenty-four of the twenty-eight observed transitions have been located by energy fit and by coincidence data, the remainder by energy fit alone The resulting decay scheme consists of levels at 140 5 ( 7 2 + ) , 142 6 ( 1 2 − ) , 181 1 ( 5 2 + ) 509 1 ( 3 2 − ) , 534 3, 671 5 ( 5 2 − ) , 761 6 ( 5 2 + ) , 920 5 ( 3 2 + , 1 2 + ) , 1004 0 ( 3 2 − ) , 1072 9 ( 5 2 − ) , 1129 3 ( 1 2 − , 3 2 − ) , 1142 0 3 2 − ) , 1199 ( 1 2 − , 3 2 − )

The following conversion ratios of the 191 keV transition in 197Au have been measured with a double focussing β-spectrometer: K/L = 5.9±0.2, LI/LII = 10.7±0.5, LI/LIII = 74±15. The LI/LII value is consistent witg a practically pure M1 transition (E0(LI)/M1 ≦0.9% orE2/M1 ≦ 0.6%). which leads to a calculated αK = 1.01±0.03. The LI/lIII value corresponds to a mixture E2M1 = (0.9−0.5+0.7)% giving αK = 0.983±0.005. The two derived αK agree with each other. Their values exclude reported αK which give strong E0 admixtures and spin 1 2 + of the 269 keV level. Some other αK measurements and the theoretical value (core-particle model) are in good agreement with our result and give a strong preference for spin 3 2 + of the 197Au 269 keV level.

Electric quadrupole X-ray transitions (5g → 3d, 4f → 2p, and 3d → 1s) have been observed in muonic Tl, Pb and Bi. From the 3 → 1 transitions, energy splittings of the n = 3 levels were deduced. From a comparison of the relative intensities of E1 and E2 transitions, the population ratios 5g 5f , 4f 4d , and 3d 3p were deduced. These ratios are well reproduced by a cascade calculation assuming a statistical initial population at n = 20, including K, L and M shell conversion. In the case of 205 T1 discrepancies between the experimental and the calculated 3d-1s/3p-1s intensity ratio can be explained by nuclear excitation. From the 3 p 3 2 → 1 s 1 2 intensity in 209 Bi we can deduce the ratio of the radiationless to the X-ray transition width and give limits for prompt neutron emission from the 3d level.

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 Kα line 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.

In muonic 190,192Os the magnetic hf-spliting of the 2+ → 0+ nuclear transition was measured. The hf-splittings ΔE( 190 Os ) = 665 ± 80 eV and ΔE( 192 Os ) = 800 ± 80 eV of the 2+ state are reduced at 59% compared to a point-like magnetic moment in agreement with the assumption of a magnetization density of a magnetization density distribution proportional to R2 ϱcharge(R) as calculated from a rigid rotator. The influence of these results on the analysis of muonic isomer shifts is pointed out.

Energies of the 5g-4f muonic X-rays have been measured in nat Hg and 203 Tl with an accuracy of about 25 eV. They are found to be smaller than theoretical energies including recently revised calculations of higher-order vacuum polarization, by about two standard deviations.

The X-ray spectrum of pionic 208 Pb has been measured. The deduced values of the strong interaction monopole shift with respect to the point Coulomb energy are ϵ 0 (4f) = 1.49±0.02 keV and ϵ 0 (3d) = 19.4±1.2 keV. The observed s strong interaction absorption widths of this spherical nucleus are found to be Γ 0 (4f) = 1.25±0.02 keV and Γ 0 (3d) = 47.0±3.6 keV. Important in the analysis of the pionic 4f level is the intensity balance for this level, giving an additional check on the measured absorption width of the pionic 4f level. For the more peripheral 4f state the measured strong interaction shifts and widths are well explained by standard optical model calculations. The values for the 3d state, however, are not in agreement with these calculations.

The mean life times of negative muons bound to actinide nuclei have been measured by detecting the time difference between a stopped muon and the arrival of fragments from delayed fission after muon capture. The deduced capture rates c are 1.392(4)·107/s for237Np, 1.290(7)·107/s for242Pu and 1.240(7)·107/s for244Pu. The results are compared with published data for the fission and the neutron decay channels and for the electron decay of the bound muon. Including a former measurement of c for239Pu, an isotopic dependence of the muon capture rates in the Pu isotopes is clearly observed.

Muon transfer from the ground state of muonic deuterium to a helium atom proceeds mainly via the formation of a muonic molecule in an excited state. A large number of decay X rays (∼ 6.8 keV) from these (dµHe)* molecules were observed for the4He as well as for the3He case. The time distributions of these X rays allow the determination of the ground state transfer rate. The simultaneous employment of Ge/Si-detectors and CCDs for the same target conditions allows the determination of the branching ratio of radiative to nonradiative decay for the first time.

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.