L. Schellenberg

Strong interaction effects in pionic 208Pb

Abstract 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 quadrupole moment and strong interaction parameters from muonic and pionic X-ray studies of 237Np

Abstract The X-ray spectrum of muonic and pionic 237Np has been investigated with muons and pions stopped in a NpO2 target. The nuclear spectroscopic quadrupole moment was determined to be Q=3.886±0.006 b from the splittings of the muonic 5g→4f hyperfine complexes. The B(E2)↓-values for the first and second excited states were evaluated as 3.17±0.08 and 2.77±0.10 e2b 2, respectively. A comparison between the muonic and pionic 5g→4f hyperfine complexes yields the strong interaction parameters for the pionic 4f state. For the first time a change of sign as function of Z for the strong interaction quadrupole shift ϵ2(4f) has been observed. The standard optical model predictions agree reasonably well with the measured strong interaction monopole optical shift, ϵ0(4f), and width, Г 0 (4f) , while they disagree with the experimental value for ϵ2. A stronger s-wave repulsion in the potential could explain this effect.

The probability of prompt and delayed fission of muonic237Np

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:Pt=(54±17)%,Pf(3d→1s)=(41±21)%,Pf(2p→1s,E=6.218 MeV)=(61±19)%, andPf(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 probability of non-radiative decay of the 3d level in muonic 237Np

Abstract 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.

Experimental evidence for muonicX-rays from fission fragments

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 intensityIs of this structure relative to the strengthIpf observed for prompt fission has been determined to beIs/Ipf=(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.

Radiationless transition probabilities in muonic208Pb,232Th, and238U

The probabilities for non-radiative (n.r.) excitationsPn.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 ofPn.r. (3p→1s), measured for the first time, is about 90% for the actinides232Th and238U, but only about 8% for208Pb. The value ofPn.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.

Nuclear excitation and prompt fission in muonic238U

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.

Muon capture rates in233U,234U,235U,236U,238U, and237Np

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.