Energy and angular momentum transfers in equilibrium and pre-equilibrium 158Gd(α, xn) reactions

Neutron time-of-flight spectra and γ-ray multiplicities were measured in coincidence with discrete γ-rays for specific exit channels in the ¹⁵⁸Gd(α, xn) reactions at E_α = 70 MeV. The neutron energy spectra and angular distributions were analysed in terms of equilibrium and non-equilibrium decay processes. Significant emission of non-equilibrated neutrons was found, amounting to about 40 %, 25 % and 15 % of the total neutron emission in the 4n, 5n and 6n exit channels, respectively. The corresponding average kinetic energies in this precompound phase were around 12, 8 and 4.5 MeV. The angular momentum carried away by the neutrons was found to be rather constant for all exit channels and on the average about 3? units. It is found that the total energy and angular momentum removed by the neutrons and γ-rays agrees within the experimental errors with the calculated values for the compound nucleus.     

Excitation and neutron decay of xenon studied by forward-angle scattering of high-energy 17O ions

Excitation energy spectra and neutron decay of xenon have been measured in extreme forward angle scattering of 250A and 400A MeV ¹⁷O ions, using one quadrant of the CELSIUS storage ring as a magnetic spectrometer. The observed excitation energy spectrum, ranging from 12 to 36 MeV, has been compared with Coulomb cross section calculations. Neutron and missing energy spectra are compared with statistical decay calculations using the code CASCADE. In addition to statistical decay from giant resonances and other continuum excitations, a large fraction of forward peaked fast neutrons was observed throughout the entire excitation energy region. A possible reaction mechanism behind these fast neutrons is discussed.     

Giant resonance decays in 20Ne, 22Ne and interference effects with quasifree scattering processes

The charged particle (c) decay from excited states up to the giant quadrupole resonance (GQR) in ²⁰Ne has been studied in a kinematically complete ²⁰Ne(α, α′ c) coincident experiment at E_α = 155 MeV. Angular correlation functions and branching ratios are extracted for the α₀, α₁ and p₀ decay channels. The (α, α′ α₀) angular correlation functions are analysed in PWBA in terms of coherent interference with the quasifree scattering process leading to the same final states. Good fits to the data are achieved over a large range of excitation energies. Branching ratios have been extracted and compared to results of Hauser-Feshbach calculations. Above E_x = 12.5 MeV excitation energy a discrepancy was found between the experimentally observed α₀ branching ratios and the HF predictions. These results yield evidence for a direct α₀ decay mechanism of the split isoscalar giant quadrupole resonance in ²⁰Ne. Some results are presented also for a ²²Ne(α, α′ c) coincidence experiment. Qualitative comparison has been made between the general decay behaviour of the two Ne isotopes.     

Emission spectra of the cations of some fluoro-substituted phenols in the gaseous phase

Emission spectra of the cations of 2,5- and 3,5-difluorophenol, of 2,3,4- and 2,4,5-trifluorophenol, of 2,3,5,6-tetrafluorophenol and of 2,3,4,5,6-pentafluorophenol have been obtained in the gas phase using low-energy electron beam excitation. The band systems are assigned to the B?(π^?1) → X?(π^?1) electronic transitions of these cations by reference to photoelectron spectroscopic data. The He(Iα) photoelectron spectra and the ionisation energies of ten fluoro-substituted phenols are reported. The symmetries of the four lowest electronic states of these cations are inferred from the radiative decay studies. The lifetimes of the lowest vibrational levels of the B?(π^?1) state of the six fluoro-substituted phenol cations above have also been measured.     

164Ta,identification of a new isotope

The new isotope¹⁶⁴Ta (T _1/2=13.6(2)s) was produced in the¹²⁷I(⁴⁰Ca, 3n) reaction and transported to a measuring station by helium jet. Decay properties were observed withα, γ, andγ-γ spectroscopy. TheZ assignment of the new isotope was based ongg-X-ray coincidence measurements. The mass assignment was deduced from measured excitation functions. The twoγ rays assigned to the new activity most likely originate from the 4⁺→2⁺→0⁺ decay sequence in¹⁶⁴Hf. A smallα branch, ≦0.016(5)%, with anα decay energy ofE _α=4,625(15) keV was found and assigned to the decay of¹⁶⁴Ta. The deduced upper limit of theα width for this branch,W _α≦0.6(2), is in accordance with those known for neighbouring nuclei.     

Charged-particle decay of 28Si induced by quasi-monochromatic annihilation photons in the GDR

Spectra of protons and alpha-particles, emitted from the giant electric dipole resonance of ²⁸Si excited by quasi-monochromatic photons from in-flight annihilation of accelerated positrons, were measured. Cross sections for six charged-particle decay channels were extracted in the energy region from 16.8 to 25.7 MeV. While the (γ, α₀) cross section is completely statistical in nature, the (γ, p) process was found to have a 65% semidirect component, mainly in the (γ, p₀) and (γ, p₁₊₂) channels.     

Decay of the new isotope167W

Theα- and β-decay of the new isotope¹⁶⁷W were observed in the reaction of 191 MeV³⁶Ar with¹³⁶Ba using a helium-jet and a fast tape-transport system. The identification is based on coincidences with tantalum X-rays and excitation function measurements. The measured half-life is 19.9(5) s. A decay scheme comprising 12 levels was constructed from γγ-coincidences. The β-decay is discussed within experimental and calculated level systematics. Alpha-rays with energy 4.55(2) MeV were assigned to¹⁶⁷W.     

Alpha induced reactions

A comprehensive model forα particle induced reactions, recently proposed by two of the authors, has been improved and utilised in the analysis of proton andα particle spectra, angular distributions, excitation functions, spectra of neutrons emitted in the decay of continuum states on nuclei with 50<A<230 atα energies varying from 50 to 160 MeV. A unique set of average parameters has been used in all the calculations. Most of them were the same utilised in previous analyses of nucleon andπ ^? induced reactions. The energy and mass dependence (which turns out to be quite weak) of the additional parameters which are needed to describe theα particle induced reactions has been determined. The comparison with experimental data shows that the model discussed allows one to reproduce successfully the cross sections ofα particle induced reactions with an accuracy comparable to the one already gained in the analysis of nucleon andπ ^? reactions with the Exciton Model.     

Investigation of α-induced reactions on niobium and tantalum

Excitation functions of (α,xn yp) reactions on⁹³Nb withy≦6 andx≦10 have been measured in four different irradiations from 16 to 171 MeV bombarding energy by Ge(Li) spectroscopy of the residual activity of stacked foils. Similarly, previous measurements of¹⁸¹Ta(α,x n y p) reactions from 15 to 103 MeV were extended up to 170 MeVα-energy for residual nuclei withy≦4 andx≦12. Including isomeric states a total number of 37 residual activities could be identified in⁹³Nb+α. From the rather complexγ-decay spectra altogether 28 different excitation functions of sizable length could be established. For181Ta+α 10 different residual activities were analyzed so that together with the former data a set of 15 excitation functions is available. The whole set of excitation functions provides a large data basis for probing the validity of combined equilibrium and preequilibrium reaction models in a wide energy range.     

Interference effects in alignment and orientation processes of atoms excited by polarized radiation

The dynamics of the processes of alignment and orientation of atoms subjected to the action of polarized radiation has been studied theoretically in the energy range of excitation of autoionization resonances. The alignment parameters A ₂₀ for the 4p ⁴5p states of Kr II populated through the excitation and autoionization decay of Kr I 3d ⁹ np resonances depend on energy due to the interference between different resonance channels and the channel of direct photoionization. It is predicted that the orientation parameter O ₁₀ of some Kr II 4p ⁴5p states and the parameter of the angular distribution of photoelectrons β_e1 strongly depend on energy. The absolute photoionization cross sections are calculated for the population of the Kr II 4p ⁴5p states in the case where the energy of excitation photons corresponds to the first four 3d ⁹ np resonances. A good agreement between the calculated and measured photoionization cross sections proves that it is important to take into account the interference between different resonance channels in order to appreciate the dynamics of the Auger decay of Kr I 3d ⁹ np resonances.     

Production and decay of269110

In an experiment carried out to identify element 110, we have observed anα-decay chain, that can be unambiguously assigned to²⁶⁹110. In a scries of preexperiments the excitation functions of the fusion reactions⁵⁰Ti +²⁰⁸Pb→²⁵⁸104^* and⁵⁸Fe +²⁰⁸Pb→²⁶⁶108^* were measured with high precision in order to get the optimum projectile energies for the production of these heavy elements. The cross-section maxima of the 1n evaporation channels were observed at excitation energies of 15.6 MeV and 13.4 MeV, respectively. These data result in an optimum excitation energy of 12.3 MeV of the compound nucleus for the production of²⁶⁹110 in the reaction⁶²Ni +²⁰⁸Pb→²⁶⁹110 + 1n. In irradiations at the corresponding beam energy of 311 MeV we have observed a decay chain of 4 subsequent a decays. This can be assigned to the isotope with the mass number 269 of the element 110 on the basis of delayed α-α coincidences. The accurately measured decay data of the daughter isotopes of the elements 108 to 102, obtained in the previous experiments, were used. The isotope²⁶⁹110 decays with a hair-life of (270 _-120 ⁺¹³⁰⁰ ) μs by emission of (11.132±0.020) MeV alpha particles. The production cross-section is (3.3 _-2.7 ^+6.2 ) pb.     

The nature of activation centers in Y2SiO5:Pr3+, Gd2SiO5:Pr3+, and Lu2SiO5:Pr3+ crystals

A complex study of the energy spectra and relaxation channels for the excitation energy of activation centers in Y₂SiO₅:Pr³⁺, Lu₂SiO₅:Pr³⁺, and Gd₂SiO₅:Pr³⁺ was performed. An analysis of the low-temperature optical spectra showed that the energy parameters and the character of field splitting of the ¹ D ₂ and ³ H ₄ activator ion terms were substantially different in crystals of different crystallographic types. The pseudosymmetry effect was observed in splitting of the ¹ D ₂ and ³ H ₄ terms of Pr³⁺ ions situated in nonequivalent crystal lattice cation sites of Y₂SiO₅ and Lu₂SiO₅. Activator ions nonuniformly populated nonequivalent cation sites of the Y₂SiO₅ crystal lattice. At high activator ion concentrations (>1 at %), luminescence decay in Y₂SiO₅ could not be described by a simple exponential time dependence. The complex luminescence decay law was caused by activator ion excitation energy migration and capture by acceptors. The role of energy acceptors was played by activator ion dimers.     

Alpha-decay study of188Pb and180,182Hg

Theα decay of mass-separated¹⁸⁸Pb and^180,182Hg has been studied at the GSI on-line mass separator. Alpha singles spectra as well asα-X-t andα-e-t coincidence events were collected. Fine structure in theα decay of¹⁸⁸Pb was measured for the first time, feeding the 2₁ ⁺ and 0₂ ⁺ states in¹⁸⁴Hg. Theα decay of^180,182Hg feeds low-lying states in^176?178Pt. One of these states could be identified as a 0⁺ state. The deexcitation pattern could be established. As a by-product an alpha-decay scheme of¹⁸⁰Au towards levels in¹⁷⁶Ir is given.     

Pre-equilibrium alpha emission accompanying deep-inelastic16O+58Ni collisions

α particles were measured in coincidence with projectile-like reaction products (oxygen and carbon) produced in deep-inelastic¹⁶O+⁵⁸Ni collisions at about 6 MeV/N bombarding energy. The kinematic analysis of the HI andα energies measured as a function ofΘ _α gives strong evidence for a sequential process: the target-like fragments are excited by the deep-inelastic collision and undergo subsequentα decay. In contrast, the angular correlations show a pronounced forward peak, indicative of direct or pre-equilibriumα emission. The emission time for the latter is estimated to be of the order of 2×10^?21 s. To resolve this conflict of co-existing statistical and direct features of the pre-equilibrium emission, the concept of a hot spot is proposed. From the angular correlation and from theα multiplicities, a local temperature ofT?3.5 MeV is deduced which agrees well with the temperature derived from the shape of theα spectra. The spot size is estimated to be 1/5 of the sphere.     

The mechanism of the 7Li(d, 2α)n reaction from Ed = 3 to 15 MeV

The ⁷Li(d, 2α)n reaction proceeds almost entirely through excitation and sequential decay of the 16.63 and 16.92 MeV levels in ⁸Be, for incident energies 1 to 13 MeV above their threshold. The energy dependence for forming these levels with the neutron emitted at 0° is approximately that predicted assuming the neutron is a spectator from the incident deuteron. None of the individual spectra, the angular dependence of the cross section at fixed E_d, or the bombarding-energy-dependence of the cross section for forming the levels is consistent with the involvement of a spectator neutron from the ⁷Li target.     

Untersuchung von57Co mit der Reaktion57Fe(p,n)57Co

A neutron time-of-flight investigation of the⁵⁷Fe(p, n)⁵⁷Co reaction performed at bombarding energies of 4.9, 5.6 and 6.2 MeV results in new energy levels of⁵⁷Co. In addition, gamma spectra andn-γ-coincidence spectra were taken to complete the decay scheme of⁵⁷Co up to an excitation energy of 4 MeV.     

The 56Fe(α, γ)60Ni reaction at excitation energies in the region of the giant dipole resonance

Excitation functions for the ⁵⁶Fe(α, γ₀)⁶⁰Ni and ⁵⁶Fe(α, γ₁)⁶⁰Ni reactions have been measured at 90° to the beam direction over the bombarding energy range 8.0–17.6 MeV. Gamma-ray angular distributions were measured at ten bombarding energies. Excitation functions for the ⁵⁹Co(p, γ₀)⁶⁰Ni and ⁵⁹Co(p, γ₁)⁶⁰Ni reactions were measured over the range E_x = 16.58–16.92 MeV and compared with the (α, γ) data. The angular distribution data indicate that the (α, γ₀) and (α,γ₁) reactions proceed through 1^?, and 1^? and 3^? states, respectively. The (α, γ) excitation functions are discussed with respect to isospin splitting of the ⁶⁰Ni giant dipole resonance. The fine structure observed in the excitation functions is shown to be most probably due to Ericson fluctuations. The gross (α, γ) cross sections are shown to be in reasonable agreement with the results of calculations made using the theory of Hauser and Feshbach assuming excitation of the giant dipole resonance.     

Evidence for boson-mode excitations in the16O(α,n) and20Ne(α,n) excitation functions

Excitation functions for¹⁶O(α,n) and²⁰Ne(α,n) have been measured from threshold to 26 and 31 MeV compound excitation energy, respectively. The dominating compound states are interpreted as boson excitation modes of nuclei with α-particle structure.