Angular Distributions Of Α-particles Research Articles

Angular Distributions ofαParticles Emitted by Deformed Oriented Nuclei

Angular Distributions of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">α</mml:mi></mml:math>Particles Emitted by Deformed Oriented Nuclei

The angular distributions of α-particles emitted by oriented nuclei and their relation to nuclear structure and deformation

α-particle angular distribution data from an extensive on-line nuclear orientation study of the favoured decay of nearly spherical, neutron deficient At and Rn nuclei near N=126, as well as of deformed Fr and Pa isotopes are presented. From the comparison of these data with various theoretical models it is found that for nearly spherical nuclei, anisotropic α-emission in favoured decays can be well explained within the shell model but not by a tunnelling model. On the other hand, for the deformed nuclei studied here, a description on the basis of α-particle tunelling through the Coulomb barrier is quite appropriate.

Reinvestigation of proton threshold states in26Al

The27Al(3He,α)26Al reaction has been used to determine 1-transfers for states in26Al between 6.3- and 6.5- MeV excitation. Angular distributions of α-particles between 5° and 15°, when added to our previously published results confirm a strong 1=0 component for a state at 6363 keV but indicate pure 1=2 for states at 6410 and 6498 keV. We discuss implications for nucleosynthesis.

THE MONTE-CARLO SIMULATION OF THE EFFECT OF THE SURFACE OXIDE LAYER ON THE LIFETIME MEASUREMENTS IN RESONANCE REACTIONS

The binary collision Monte-Carlo method is used to calculate the blocking angular distri-bution of α-particles of about 2 MeV around〈110〉axis in Al single crystal, in which a surface Al2O3 amorphous layer of different thickness is considered. The thermal vibrations of lattice atoms and electronic multiple scattering are taken into account. The resulting angular distribution is closer to the experimental measurements than that with perfect crystal.

Decay of the ground state and the [formula omitted] isomer in 217Ac and g-factor measurements from perturbed α-particle angular distributions

The nucleus 217Ac was studied using the 205Tl( 16O, 4n), 206Pb( 15N, 4n) and 209Bi( 12C, 4n) reactions. The mesurements included αγ, γγ, α-ce and ce-ce coincidence experiments as well as γ-ray and α-particle perturbed angular distribution studies. Results are g( 217g Ac) = + 0.85(1) and T 1 2 ( 217 g Ac) = 69(4) ns , the shortest known α-decay of a ground state. 217mAc decays mainly by a γ-cascade, but also by α-emission to the single-particle states π h 9 2 , f 7 2 , i f 13 2 of 213Fr. Spins and parities of levels to the 29 2 + isomer at 2013 keV with T 1 2 = 740(40) ns and g = + 0.347(5) are determined. The level scheme of 217Ac and the α-decay of N = 128 isotones are discussed.

Investigation of the149Sm (n, α)146Nd reaction induced by fast neutrons

The energy and angular distributions of α-particles emitted in the149Sm(n, α)146Nd reaction have been measured at neutron energies of about 12, 14 and 18 MeV. The results have been analysed in terms of statistical, pre-equilibrium and knock-on models.

The ( 3He, α) strength functions in rare-earth nuclei

The 156Gd( 3He, α) 155Gd, 157Gd( 3He, α) 156Gd and 171Yb( 3He, α) 170Yb reactions have been studied over a wide region in excitation energy. The bombarding energy was 26 MeV, and the reaction products were analyzed by means of a particle telescope. Strength functions and angular distributions have been measured. All strength functions contain a strong low-energy peak which is shown to be the result of the Coriolis coupling. Additional αγ coincidence experiments were performed in order to study the population of the yrast levels as a function of excitation energy. These population patterns, as well as the α-particle angular distributions, are consistent with a dominating large- l transfer in the excitation region up to the neutron binding energy. Comparisons are made with calculated strength functions based on the particle-rotor model. Only a part of the total yield observed at high excitation energy can be accounted for by the model.

Study of the Reactions 9Be(n,α)6He and 9Be(n,αα)nn at 14.1 MeV

The differential cross sections for the reactions 9 Be(n, α 0 ) 6 He(g.s.) and 9 Be(n, α 1 ) 6 He * and the α-α particle-correlation spectrum from the 9 Be(n, αα)nn reaction have been measured using 14.1 MeV neutrons. The measured angular distributions of α-particles were in good agreement with those of Perroud and Sellem at 13.99 MeV, and were compared with DWBA calculations. The spectroscopic factors for the cluster transfer reactions on 9 Be have been deduced. The observed correlation spectrum implies that this four-body breakup reaction is dominated by a sequential decay process in the measured α-particle energy region. The second and third excited states in 6 He have been confirmed to be located at around 3.4 MeV and 6.0 MeV, respectively.

Research highlights with the spin spectrometer

The excitation energy and angular momentum dependence of the entry states in fusion reactions measured with the spin spectrometer is discussed. A new decay mode involving the onset of localized stretched dipole radiation at half the accompanying stretched E2 collective radiation is found in 157–161Yb. The appearance of this mode correlates smoothly with neutron number and spin. Possible interpretations are presented in terms of the evolution of the nuclear shapes from prolate to aligned-quasiparticle oblate to collective oblate and then to triaxial. Evidence for nuclear deformation that increases with spin at very high excitation is presented based on α-particle angular distributions measured relative to the spin direction, using a new method for deriving the spin alignment.

The15N(p,? 0)12C reaction at stellar energies

The15N(ρ, α0)12C reaction has been investigated in the energy range ofE p (lab)=78-810keV. The measurement of the excitation functions and α-particle angular distributions involved solid targets as well as a quasi-point supersonic jet gas target. The determination of absolute cross sections has been carried out with the gas target. The observed energy dependence of the total cross sections can be described in terms of two-level Breit-Wigner shapes including the resonances atE p (J π)=335(1−) and 1,028(1−)keV. The data lead to a zero-energy intercept of the astrophysicalS(E) factor ofS(0)= 65±4MeV-b. The angular distributions are asymmetric around 90° and require an additional amplitude in the reaction mechanism, which interferes predominantly with the 335 keV resonance. The origin of this background amplitude is discussed.

The modified SOD method related to a simple model of the scattering process

The angular distributions of α-particles scattered from Cu at 13 and 14 MeV have been measured in the angular range (10÷170)°. The total reaction cross-sections have been extracted by means of the modified SOD method. The interference between the Coulomb and nuclear amplitudes is analysed and related to a simple model of the scattering process.

The [formula omitted] proton orbit size in 209Bi

Angular distributions of α-particles from the reaction 209Bi(t, α) 208Pb (ground state) have been measured at bombarding energies E t = 8.5 MeV and E t = 9.0 MeV. Zero-range (ZR) DWBA analyses of the data are used to determine the radial extent of the wave function of the lh 9 2 orbit in which the transferred proton is bound in 209Bi. The applicability of the sub-Coulomb ZR DWBA treatment is investigated by comparison of ZR and exact finite-range DWBA calculations. A value of the (t, α) normalization constant of (20.8 ± 3.1) × 10 4 MeV 2 · fm 3, based on forward dispersion relations, has been obtained from this comparison. The asymptotic amplitude of the wave function is measured directly and the rms radius is extracted via a Woods-Saxon model. A value of √〈 r 2〉 = 6.10 −0.08 +0.12 fm, corresponding to a point proton and to a local Woods-Saxon potential, is obtained. The rms radius and radial wave function of the 1 h 9 2 proton are compared with the results of electron scattering and muonic atom data on targets of 209Bi and 208Pb and with the results of Hartree-Fock calculations.

A detailed investigation of the thermal neutron induced ternary fission of 235U

The energy and angular distributions of the long-range α-particles released in the thermal neutron induced fission of 235U have been measured as a function of the mass and kinetic energy of the fission fragments. Only one of the fission fragments was detected, its mass being determined by kinetic energy and time-of-flight measurements. The dependence of the light particle kinetic energy spectra on its final direction is examined as a function of mass and kinetic energy of the fission fragments. The angular distribution of all α-particles with respect to the light fragment direction has a most probable value of 81.3°±0.5° with a dispersion (FWHM) of 18.5°±0.8° which compares very well to the angular spread measured in the spontaneous fission of 252Cf. Other striking similarities between the ternary fission of 236U and 252Cf are pointed out. A positive correlation between the mean emission angle and the mass of the light fragment resolves the controversy which remained on this point up to now. The influence of the total fragment kinetic energy on the width of the angular distribution is investigated in detail. A preliminary interpretation of our data shows that α-accompanied fission should no longer be considered as contradictory to the semi-statistical models of fission.

The 18O(p, α)15N reaction at stellar energies

Abstract The 18O(p, α)15N reaction has been investigated in the energy range Ep = 72–935 keV. The three known resonances above Ep = 620 keV have been confirmed and four new resonances have been found below Ep = 340 keV. All observed resonances correspond to known compound states in 19F. Information on resonance energies, total widths and ωγ values is reported. The low-energy resonances are superimposed on a non-resonant reaction yield, which varies smoothly with beam energy and which exhibits pronounced α-particle angular distributions asymmetric around 90°. The explanation of these data requires either interferring amplitudes of broad resonances with differing parities or a direct (p, α) reaction mechanism. The investigated energy range corresponds to the important temperature range of T = (0.05–2.5) × 109 K. The energy averaged astrophysical reaction rates are compared with predictions.

The (t, α) reaction on 121Sb and 123Sb

Angular distributions of α-particles from the (t, α) reaction on 121Sb and 123Sb have been measured for incident tritons of 12 MeV. Levels up to excitation energies of 5.2 and 4.9 MeV in 120Sn and 122Sn, respectively, have been identified and analysed by the DWBA. Values of orbital angular momenta of the transferred protons have been assigned and spectroscopic factors deduced for all strongly excited levels. The extracted ground-state wave functions of the target nuclei have been compared with the calculated ones. A mixture of collective degrees of freedom is present in the low-lying states, weakly excited by the above very selective proton pick-up reaction. These states are populated by the pick-up of external protons (outer shells). At higher excitation energy (between 4 and 5 MeV) there are many strongly excited states populated by the proton pick-up from the Z = 50 proton core (inner shells); a predominantly 1p-1h character has been assigned to these states.

Evidence for quasi-static scission configurations obtained from long range particle accompanied fission distributions

New experimental results on light-particle-accompanied neutron-induced fission of 236U are presented. The widths of the α-particle angular distributions are the same for 236U (nth) and 252Cf(s.f). This angular width appears to be a decreasing function of the total fission fragment kinetic energy. These results suggest that the characteristics of this mode of fission are not inconsistent with a semi-equilibrium model for scission.

Single pole dominance in ?-elastic and inelastic scattering from calcium isotopes

The presence of a “zero-pole” in nuclear diagonalS-matrix was found to be dominant in both elastic and inelastic scattering angular distributions of α-particles (E=24 and 29 MeV) on Ca40,42,44,48 nuclei. The inelastic scattering amplitudes were evaluated by making use of the derivative relation of Austern and Blair for one and two phonon excitation mechanism. Reduced matrix elementsC1(I) and deformation lengthsBIR are derived. Consistent vibrational model description for Ca40,42,44,48 energy levels is presented.

Study of the 9Be(n, α)6He reaction in the neutron energy range from 12.2 to 18.0 MeV

Angular distributions of α-particles emitted in the 9Be(n, α0)6He reactions have been measured at neutron energies of 12.2, 14.1 and 18.0 MeV. Excitation functions for forward emission of the reaction products have been determined in the neutron energy range 12.2–18.0 MeV. The results indicate the dominance of heavy particle stripping in the mechanism of the 9Be(n, α0)6He reaction in the investigated neutron energy range.

A study of the ( n, α) reactions induced by 18 MeV neutrons in 151Eu, 151Eu, 159Tb, 16SHo and 169Tm

The energy spectra of α-particles emitted in the 151 Eu(n, α) 148 Pm, 153 Eu(n, α) 150 Pm, 159 Tb(n, α) 156 Eu, 165 Ho(n, a) 162 Tb and 169 Tm(n, α) 166 Ho reactions have been measured at a neutron energy of 18.15 MeV. For the 165 Ho(n, α) 162 Tb reaction the α-particle angular distribution has been measured as well. The results have been analysed in terms of statistical, pre-equilibrium and knock-on models.

Alpha-particle and proton scattering from even mass molybdenum isotopes

Abstract Angular distributions of α-particles and protons elastically and inelastically scattered from 92,94,96,98,100 Mo have been measured for 32 MeV incident α-particles and for 12 MeV incident protons. Levels up to the first 3 − state in each nucleus were studied in the proton spectra with a resolution of 50 keV (FWHM). A total of 62 states were analysed in the α-particle spectra which were recorded up to 12 MeV excitation energy with a resolution (FWHM) of 100 keV. The highest energy up to which states were resolved was approximately 6 MeV in 92 Mo, decreasing to about 2.5 MeV in 100 Mo. Optical-model and single-step DWBA analyses were made. Spin and parity assignments as well as deformation lengths extracted from the present results are presented and compared with those obtained by other experiments. Transition rates inferred from the deduced deformation lengths are compared to theoretical estimates made for the octupole states. Evidence was found of states that may possibly be described as mixtures of one- and two-phonon excitations.