S. Bjørnholm

Dynamics of the fusion process

Abstract The binary reaction products from the interaction of a 208Pb beam with targets of 26Mg, 27Al, 48Ca, 50Ti, 52Cr, 58Fe and 64Ni have been studied with the aid of a large position-sensitive ring counter, operated in a two-particle coincidence mode. The intensity of γ-rays and X-rays per event was also recorded. Within a broad range around mass symmetry, the center of mass angular distributions, γ-ray multiplicities, total kinetic energy distributions, and absolute mass yields have been determined as a function of the bombarding energy, ranging from 1.0 to 1.8 times the interaction barrier. When analysed on the basis of fusion models, the cross section for the mass equilibration reaction demonstrates that deformations induced at contact influence the fusion of these heavy systems significantly and characteristically. The γ-ray multiplicities appear to be strongly influenced by statistical angular momentum components that remain with the fragments after separation. Their magnitudes indicate that during the reaction, the collision complex becomes at least as compact as the liquid-drop saddle shape appropriate to a non-rotating nucleus with the same total mass and charge. Finally, some angular distributions show that the entire process of coalescence and reseparation can occur before the system has made one-half revolution; i.e. within a time of 5–10 × 10−21 s. No X-ray emission from the combined system 208Pb + 48Ca is observed.

Spontaneously fissioning isomers in U, Np, Pu and Am isotopes

Abstract Spontaneously fissioning isomers have been discovered and assigned to 236U, 239Np, 236, 237, 240, 241, 242, 243Pu and 239, 241Am. They were produced by irradiation of thin targets with 9–14 MeV protons or deuterons. Fission-in-flight of recoiling reaction products was observed with plastic foils as fission track detectors. The half-lives range from 9 ns to 1.5 μs and the formation cross sections from 12 μb to 0.13 μb. The proton excitation functions for the 239mAm, 241mAm and 236mPu isomeric states show threshold behaviour typical of a (p, 2n) reaction. The differences EII in thresholds for the ground state and the isomeric state, respectively, are: E II ( 239 m Am ) = 2.9 ± 0.1 MeV , E II ( 241 m Am ) = 2.5 ± 0.1 MeV , E II ( 236 m Pu ) = 4.1 ± 0.2 MeV . An isomer ratio of 2.2 × 10−5 for 236mPu was obtained from the measured yield of the isomer and of the 326Pu ground state. Other isomer ratios were deduced using calculated ground-state yields. The variations in half-lives and excitation energies with Z and N are discussed and compared to theoretical calculations of the barrier shapes.

Intermediate states in fission

Abstract Shell effects give rise to modulations in the liquid-drop fission barriers. For heavy nuclei with neutron number in the vicinity of 146–148, a two-humped barrier results, and the states of the secondary minimum act as intermediate states in the fission process. The existence of such states is reflected in subbarrier fission induced by neutrons in the resonance region, in the occurrence of broad resonance structures in the fission excitation functions, in anomalous fragment angular distributions and in the observed fission isomerism. Theoretical consequences of a two-humped fission barrier are developed and compared with experiment.

Isomers in odd-mass Lu isotopes, ascribed to the [541] 12− Nilsson configuration

Abstract Irradiation of Yb isotopes with pulsed beams of 8–13 MeV protons and deuterons has led to the discovery of the following new isometric transitions in the Lu isotopes: a 29 keV, 160 sec, E3 transition in 169Lu; a 70 keV, 76 sec, E3 transition in 171Lu; a 345 keV, 1.3 μsec, E1 transition in 175Lu; a 0.67 sec level in 170Lu decaying by a 48 keV, M2 transition and a 44 keV, E2 transition; and a 64 keV, 430 μsec, E1 transition in 172Lu. The isomerism in the odd-mass isotopes is ascribed to the [541] 1 2 − Nilsson state, in analogy to the well-studied case in 173Lu. The results are discussed in relation to the parameters of the Nilsson model.

Excitation energy of the spontaneously fissioning isomeric state in 240Am

Abstract The excitation function for the reaction 241 Pu(p, 2n) 240m Am leading to the spontaneously fissioning isomeric state in 240 Am has been measured with proton energies 9.6–13.6 MeV. The excitation function shows a typical threshold behaviour, corresponding to a threshold of 10.7 ± 0.1 MeV. This is 3.15±0.25 MeV higher than the estimated threshold for the ground state reaction, and it is interpreted as the excitation energy of the isomeric state. Possible reasons for the unusual stability towards γ-emission of such a high lying level are discussed.

The decay of the 1.14 min isomer of Pa234 (UX2)

Abstract Sources of 24 d Th234(UX1) in equilibrium with its daughters, 1.14 min Pa234(UX2) and 6.75 h Pa234(UZ) were studied with a six-gap β-spectrometer, scintillation spectrometers and with e-γ and β-γ coincidence techniques. Quantitative chemical separation of the three nucleides was performed and the individual γ-spectra and disintegration rates were measured. The short lived Pa234 isomer was found to populate three intrinsically excited states in U234, a state with (K, I, π) = (0, 1−) at (790±5) keV, a (0, 0+) state at 811 keV and another (0, 0+) state at 1045 keV. Detailed results on β-, γ- and electric monopole transitions are presented and discussed. By applying three independent methods the relative disintegration rates of UX2 and UZ were found to be 100 to 0.13±0.03. A 70–73 keV isomeric transition of corresponding intensity was found in the conversion line spectrum. The possible assignments of the Pa234 levels are discussed.

Isomerism in 176Hf and some neighbouring even nuclei

Abstract Isomeric states in some doubly even nuclei are investigated with the use of a pulsed beam technique. In 176 Hf a level at 1562 has a half-life of 10.3 μs and a level at 1335 keV has a half-life of 13.0 μs. In 174 Yb an isomeric state at 1520 keV with a half-life of 820 μs is studied. In 176 Yb, an isomeric level with a half-life of 12 s is lying at 1041 keV, and in 180 W an isomeric level with a half-life of 5.2 ms and an energy of 1530 keV above the ground state is studied. All the isomers are ascribed to two-quasiparticle states which decay to a lower lying rotational band by K -forbidden γ-transitions. The level at 1562 keV in 176 Hf is ascribed to the coupling of the two proton orbitals 7 2 + [404] and 9 2 − [514] to a spin and parity 8 − . The other isomer in 176 Hf, at 1335 keV, and the one in 174 Yb, are ascribed to the coupling of the neutron orbitals 5 2 − [512] and 7 2 − [514] to give K π = 6 + . The two isomeric states in 176 Yb and 180 W are due to the coupling of the neutron orbitals 7 2 − [514] and 9 2 + [624] to a spin and parity 8 − . The branching ratios of the decays of the isomeric states are analysed and compared to theoretical predictions. The energies of the two-quasiparticle states are compared to calculations of the energy gap based on the Nilsson model. Comparisons are also made with pairing energies calculated on the basis of empirical neutron and proton separation energies.

Energy levels in 234U populated in the β-decay of 234Pa(UZ)

Abstract Intense sources of the 6.7 h 234 Pa isomer UZ have been investigated with Ge(Li) counters, a high-resolution 180° beta-ray spectrograph and with coincidence techniques. The Ge(Li) γ-spectrum of the 33 μs isomer of 234 U was recorded by means of delayed coincidences. A total of 130 transitions in 234 U was observed. The decay scheme includes the following levels, denoted by energy in keV and quantum number KI π for the rotational band heads only: 787(01 − ), 810(00 + ), 927(22 + ), 990(22 − ), 1127(22 + ), 1421(66 − ), 1496(33 + ), 1552(55 + ), (1693)((55 − )), 1723(33 − ) and 1723(44 + ). The majority of the levels has been identified in stripping and pick-up experiments (see the preceding paper). The intrinsic states correspond to theoretically predicted two-quasiparticle states and collective quadrupole and octupole excitations. Appreciable mixing of some of the two-quasiparticle configurations occurs and influences the decay pattern in a major way. Unexpectedly, there exists a K π = 2 + configuration with the energy 1127 keV in addition to the 2 + collective state at 927 keV. This level decays by strong transitions to the 2 + state at 927 keV and the 0 + state at 810 keV and only by weak transitions to the 0 + ground state.

Energy levels in 234U populated by the 233U(d, p) and 235U(d, t) processes

Abstract Enriched targets of 233 U and 235 U were irradiated with 13 MeV deuterons and proton and triton spectra from the neutron-transfer processes were recorded by a broad-range single-gap magnetic spectrograph with an over-all energy resolution of 0.1%. The results were compared with level schemes obtained in decay studies (see the following paper) and analysed in terms of the Satchler stripping theory and Nilsson wave functions. The (d, t) reactions on the 235 U target allowed the identification of two-quasiparticle components involving the 7 2 − [743] neutron plus a neutron-hole state. The following assignments and band head energies were obtained: K π = 0 + , 0 keV; K π = 2 − , 990 keV; K π = 6 − 1421 keV; K π = 1 − , 1434 keV; K π = 5 − , 1693 keV; K π = 4 + , 1884 keV and Similarly, the (d, p) reactions on the 233 U target identified two-quasiparticle components involving the 5 2 + [633] neutron plus a neutron particle state: K π = 0 + , 0 keV; K π = 2 + , 927 keV; K π = 2 + , 1127 keV; K π = 3 + , 1496 keV and K π = 5 + , 1552 keV.

THE MOMENT OF INERTIA AND THE ENERGY GAP OF FISSION ISOMERS

Abstract The moments of inertia and the energy gaps of protons and neutrons have been calculated within the pairing formalism with Nilsson wave functions for different prolate shapes of doubly even nucleides from radium to curium. Two different assumptions — a constant pairing strength G = const, and one that increases proportionally to the surface area G ∝ S — are investigated. The results are compared to available data for the ordinary ground-state shapes and for the shapes of the second minimum associated with fission isomers. The calculations account for the experimental results in the second minimum as well or even better than for the ground-state minimum. They support the theoretical description of the isomers as prolate shapes with a ratio of axes of about 1:2. The experimental data are not sufficiently accurate or numerous to decide which of the two assumptions about the pairing strength is the most realistic one.