W. Schrieder

A study of the strong interaction effects on pionic 3d and 4f levels in 181Ta, natRe, natPt, 197Au, 208Pb, 209Bi and 237Np

Abstract The pionic X-ray spectra of 181 Ta, nat Re, nat Pt, 197 Au, 208 Pb, 209 Bi and 237 Np have been investigated to obtain data for a systematical study of the pionic 3d and 4f levels. In order to reduce background the experimental method has been improved by using an array of Compton suppression BGO-shields and neutron time-of-flight discrimination. The widths of the pionic 3d levels are observed to be narrower by a factor 1.5 than theoretical predictions obtained with standard optical potentials. A deviation is also found for the strong interaction quadrupole shift, ϵ 2 , for both the 4f and 3d orbits. A possible explanation for the smaller width, Γ 0 (3 d ), can be found by extending the parametrization of the optical potential with absorption terms.

Deeply bound 1s and 2p states in pionic atoms

Abstract New results were obtained on pionic 1s levels in 24Mg, 27Al and 28Si, on 2p states in 93Nb and natRu, as well as on 3d states in 93Nb, natRu, natAg and natCd. For the first time the predicted repulsive shifts for the pionic 2p level have been observed. The following values were obtained for the strong-interaction shifts and widths (all values in keV); 24Mg: e1s0 = −80.5 ± 0.6 and Γ1s0 = 24.3 ± 1.6; 27Al: e1s0 = −115.5 ± 1.4 and Γ1s0 = 28.8 ± 1.2, 28Si: e1s0 = −131.6 ± 2.0 and Γ1s0 = 41 ± 4;93Nb: e2p0 = −11 ± 3, Γ2p0 = 64 ± 8, e3d0 = 0.74 ± 0.02 and Γ3d0 = 0.402 ± 0.016; natRu: e2p0 = −48 ± 7, Γ2p0 = 77 ± 24, e3d0 = 1.39 ± 0.08 and Γ3d0 = 0.75 ± 0.08; natAg: e3d0 = 1.94 ± 0.07 and Γ3d0 = 1.44 ± 0.05 and natCd: e3d0 = 2.14 ± 0.09 and Γ3d0 = 1.65 ± 0.07. The results are in reasonable agreement with the predictions of the standard optical potentials.

Muon attachment in prompt fission of237Np

The probabilities of muon attachment to fission fragments in the prompt fission of237Np have been determined as a function of fragment mass and total kinetic energy release in a (μ−,f1f2e−)-coincidence measurement. A strong dependence of the attachment probability on the fragment mass is observed. The experimental results are compared with theoretical predictions.

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.

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.

Fission probabilities of 237U and 236Pa using the (d, tf) and the (d, alpha f) reactions

The fission probabilities of 237U and 236Pa have been determined over the excitation energy range from the respective first to above the second chance fission thresholds using the 238U(d, tf) and 238U(d, alpha f) pickup reactions for the deuteron energy Ed=24.85 MeV.

A Study of the Strong Interaction Effects of Deeply Bound Pionic Levels in Pionic Atoms

The pionic X-ray spectra of 181Ta, natRe, natPt, 197Au, 208Pb, 209Bi and 237Np have been investigated to obtain data which provide for a systematical investigation of the pionic 3d and 4f levels. Special attention was paid to experimental methods for obtaining clearly improved results as compared to earlier experiments, by using an array of Compton suppression BGO-shields and by using neutron time-of-flight discrimination to reduce background. Also to explore X-rays populating deeply bound pionic 1s-levels in 24Mg, 27A1, 28Si and 2p-orbits in 93Nb, natRu, respectively, coincidence techniques in combination with these high resolution Ge-BGO Compton suppression spectrometers were applied. Only deviations in the width of the pionic 3d-levels are observed. They are narrower by a factor 1.5 as compared to theoretical predictions. The same deviation as for Γ0(3d) is found for the strong interaction quadrupole shift, e2, for both the 4f- and 3d-orbits. An explanation for the difference in Γ0(3d) can be found by adding an isovector absorption term to the optical potential with a negative value for the parameters ImB1 or ImC1.

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.