Angular Distributions Of Alpha Particles Research Articles

Mechanisms of Modulation Formation in Elastic Diffraction Scattering of Charged Particles

In this work, the authors describe experimental data on the angular distributions of alpha particles on light alpha-cluster nuclei. The angular distributions of elastic scattering of alpha particles are analyzed under the assumption of complete absorption inside the sphere of the cluster structure and the sphere of the interaction core. These assumptions made it possible to decompose the scattering amplitude into components—scattering on the nucleus itself and on different cluster structures. The calculations result in the following. The amplitude for describing diffraction elastic scattering by 4n ± 1 light nuclei in the framework of the theory of diffraction scattering is a superposition of wave functions in the scattering approximation on a completely black core and on its absolutely black substructures. The nucleon association modes, as well as the scattering modes on the correlated motion of nucleons and spatially separated nucleons, make a significant contribution to the total scattering amplitude. This manifests itself as an increase in angular distributions due to the interference of modes of various cluster structures and nucleons. Owing to these effects, not only an uneven rise in the angular distributions of differential cross sections over the Rutherford section is possible, but also a rise at rear angles.

Relation of Experimental Features of P-Even, T-Odd Asymmetries in Ternary Nuclear Fission Induced by Cold Polarized Neutrons to Triple and Quintuple Scalar Correlations

In employing the concept of the isotropy of space, the coefficients D of P-even, T-odd asymmetry in the angular distributions of prescission alpha particles emitted as third particles in the ternary fission of nonoriented target nuclei that is induced by polarized cold neutrons are expressed in the lowest orders of perturbation theory in the neutron polarization vector in terms of two P-even scalar coefficients D3 and D5 associated with, respectively, triple and quintuple correlations depending on the unit vectors kα, kLF, and σn, which determine the features of the coefficients being studied. On the basis of the above representation, the experimental values of the coefficients D3 and D5 are determined by using the experimental values of D and the angular distributions of alpha particles emitted in the analogous reaction induced by unpolarized neutrons. The resulting coefficients D3 and D5 are compared with the analogous coefficients found by means of the classical method of trajectory calculations and by means of quantum-mechanical fission theory relying on the concept that it is the rotational mechanism that is responsible for the appearance of the asymmetries being studied. It turns out that the classical method, which disregards the interference between fission amplitudes for different S-wave neutron resonances, leads to an irremovable contradiction between the calculated coefficient D3 and the analogous experimental coefficient for the 233U target nucleus. In the case of employing the quantum-mechanical approach, it is concluded that three-body calculations of the Coriolis interaction—perturbed amplitude of the angular distributions of alpha particles moving in the Coulomb fields of fission fragments are required for the 233U target nucleus.

Inverse-kinematics study of78Kr +40Ca at 10 AMeV

The CHIMERA multi-detector array at LNS Catania has been used to study the inverse-kinematics reaction of 78 Kr + 40 Ca at a bombarding energy of 10 AMeV. Analysis of the experimental data focused on a class of selected events consistent with the complete fusion and subsequent binary split of the of the reacting system. This class of events features a broad A, Z distribution of fission fragments centered about symmetric fission while exhibiting relative velocities significantly higher than given by Viola systematics. The center-ofmass angular distribution (dσ/dΘ) of the fission fragments exhibit an unexpected anisotropy inconsistent with a compound-nucleus reaction and indicates a dynamic fusion-fission like process. The observed angular distribution features an asymmetric forward-backward peaking most prevalent for mass-asymmetric events. Furthermore, the more massive fragment of mass-asymmetric events appears to emerge preferentially in the forward direction, along the beam axis, in analogy to dynamic fragmentation of projectile-like fragments. Analysis of the angular distribution of alpha particles emitted from these fission fragments suggests the events are associated mostly with central collisions. The study of low-energy heavy-ion reaction dynamics in the energy domain moderately in excess of the Coulomb barrier offers a unique insight into the large-scale collective motion of the nucleons in the nucleus. Fusion-fission constitutes an important reaction channel for many such nuclear reactions and is an important probe into the nuclear structure of matter. As the bombarding energy increases, the role of dynamically-induced decay modes

Effects of rotation of fissioning nuclei in the angular distributions of prompt neutrons and gamma rays originating from the polarized-neutron-induced fission of 233U and 235U nuclei

The results of an experiment devoted to searches for effects of rotation of fissioning nuclei in the angular distributions of prompt neutrons and gamma rays originating from the polarized-neutron-induced fission of 233U nuclei are presented. The effects discovered in these angular distributions are opposite in sign to their counterparts in the polarized-neutron-induced fission of 235U nuclei. This is at odds with data on the relative signs of respective effects in the angular distribution of alpha particles from the ternary fission of the same nuclei and may be indicative of problems in the model currently used to describe the effect in question. The report on which this article is based was presented at the seminar held at the Institute of Theoretical and Experimental Physics and dedicated to the 90th anniversary of the birth of Yu.G. Abov, corresponding member of Russian Academy of Sciences, Editor in Chief of the journal Physics of Atomic Nuclei.

Fusion cross sections for 7Li + 12C system at near barrier energies

Elastic scattering and alpha particle angular distributions for the reaction 7Li + 12C have been measured at E lab = 7.5 , 9 , 12 , and 15 MeV. Statistical and direct reaction model calculations have been performed to deduce the fusion and direct reaction cross sections. The fusion cross sections measured in the present work are found to be consistent with those obtained by measurement of characteristic γ-rays, which are very different from those measured by direct detection of evaporation residues. The contribution from direct reactions to the alpha particle cross section has been quantified by exact finite range transfer calculations and found to be negligible ( < 6 % ) at all the bombarding energies studied. The measured fusion cross sections at all the energies studied are nearly equal to the reaction cross sections which imply that the direct reaction contributions are less and fusion is the dominant process.

P-even correlations in binary and ternary nuclear fission induced by polarized neutrons

The evolution of a fissile nucleus from transition fission states specified at the saddle point of the deformation potential to fission states associated with prescission configurations of this nucleus and characterized by a pearlike shape of the nucleus is studied within the quantum-mechanical theory of fission processes that is based on the time-independent formalism. The coefficients of P-even asymmetries in the angular distributions of a light fragment and a third particle are calculated on the basis of the idea of the one-step mechanism of the production of a third particle and two fragments from the ternary fission of nuclei that is induced by polarized thermal neutrons. In order to confirm the developed concepts, it is proposed to repeat, at a higher level of statistical accuracy, experiments devoted to observing left-right asymmetries in the angular distributions of alpha particles from the ternary fission of nuclei.

Alpha-Particle Angular Distributions of At and Rn Isotopes and Their Relation to Nuclear Structure.

We report on an extensive on-line nuclear orientation study of the angular distribution of $\ensuremath{\alpha}$ particles emitted in the favored decay of neutron deficient At and Rn nuclei near the $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}126$ shell closure. Surprisingly large anisotropies were observed, showing pronounced changes from one isotope to another. Comparing these data with several theoretical models shows that anisotropic $\ensuremath{\alpha}$ emission in favored decays from near-spherical nuclei can well be explained within the shell model, implying that it is mainly determined by the structure of the decaying nucleus.

Multifragmentation of24Mg+24Mg heavy-ion reactions leading to alpha-particles

Angular distributions of alpha-particles emitted from magnesium + magnesium reactions are calculated at different incident energies. The alpha-particles are sequentially emitted from long-lived states of primary fragments populated in direct reactions. The present reaction yields to multifragmentation with the result of twelve alpha-particles which are dealt with using the cluster model. The statistical scission model is used in developing the angular distributions of the fragments. The angular distributions and variances of this multifragmentation process have been numerically calculated. The obtained results are in good agreement with previous calculations.

Direct measurement of the branching ratio Gamma n/ Gamma f of 238U in inelastic alpha scattering in the giant resonance region.

We have directly measured, for the first time, the neutron-fission branching ratio, ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{n}}$/${\mathrm{\ensuremath{\Gamma}}}_{\mathit{f}}$, of an actinide nucleus ${(}^{238}$U) excited in inelastic scattering as a function of excitation energy. This was accomplished using a \ensuremath{\approxeq}4\ensuremath{\pi} array of fission counters to separate fission neutrons from decay neutrons in the simultaneous coincidence experiments (\ensuremath{\alpha},\ensuremath{\alpha}'f) and (\ensuremath{\alpha},\ensuremath{\alpha}'nf\ifmmode\bar\else\textasciimacron\fi{}). The measured branching ratio agrees well with previous photoneutron data. In a separate experiment, we have measured the angular distribution of alpha particles scattered from $^{238}\mathrm{U}$ in coincidence with fission decay (\ensuremath{\alpha},\ensuremath{\alpha}'f). This provides us with an estimate of the cross section (in the fission channel) for exciting the isosca- lar giant monopole and giant quadrupole resonances. This estimate, combined with the ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{n}}$/${\mathrm{\ensuremath{\Gamma}}}_{\mathit{f}}$ measurement, allows us to place bounds on the neutron-fission branching ratio for the isoscalar giant monopole and giant quadrupole resonances themselves.

Production and deexcitation of highly deformedGa67

The energy spectra and angular distributions of alpha particles and protons emitted from $^{67}\mathrm{Ga}$ compound nuclei at excitation energies of 110 to 116 MeV formed in four different reactions have been measured. The populated angular momentum ranges deduced from fusion cross section information correspond to 0--24 \ensuremath{\Elzxh} for $^{4}\mathrm{Cu}$, 0--38 \ensuremath{\Elzxh} for $^{12}\mathrm{Mn}$, 0--48 \ensuremath{\Elzxh} for $^{16}\mathrm{V}$, and 0--50 \ensuremath{\Elzxh} for $^{40}\mathrm{Al}$. The experimental data are analyzed using the angular-momentum-dependent statistical model and the rotating liquid drop model. Average multiplicities and barriers are extracted for different zones of angular momentum. Comparisons with the models indicate large deformations of the $^{67}\mathrm{Ga}$ nuclei at the higher angular momenta and suggest large deformation related increases in the nuclear level density and decreases of emission barriers. The p/\ensuremath{\alpha} ratio decreases with increasing angular momentum less than expected for spherical emitters at the higher angular momenta. The average \ensuremath{\alpha} emission barriers are found to be close to those which are calculated for emission from the tips of prolate deformed nuclei such as those predicted by the liquid drop model. The experimental proton barriers appear somewhat smaller than those calculated using the liquid drop model shapes.

Nuclear shapes from alpha-gamma-ray angular correlations.

The angular distribution of alpha particles with respect to the spin direction of the residual nuclei is used to probe the shapes of the emitting systems. Two compound systems have been studied: $^{170}\mathrm{Yb}$ and $^{110}\mathrm{Sn}$. Deformed emission shapes are required in order to understand the alpha-gamma correlations in the $^{170}\mathrm{Yb}$ system, but not for the $^{110}\mathrm{Sn}$ compound system. These conclusions are identical to those reached by the study of giant resonances built on excited states for very similar systems.

Assaying of targets for nuclear measurements with a gridded ionization chamber

An ionization chamber with a Frisch grid is used to determine both the energy ( E) of the charge particles emitted from the sample positioned coplanar with the cathode, and the cosine of the emission angle (ϑ) with respect to the normal of the cathode Using the combined information on cosϑ and E, problems in particle counting due to sample absoprtion and scattering effect can be circumvented and sample source strengths are readily determined to an accurary of 0.3%. However, it is emphasized that the source strength can be determined from the particles emitted in a large solid angle close to 2τ sr, which means a considerable higher efficiency than for the conventional low geometry counting techniques. Moreover the present method, within reasonable limits is insensitive to source shape and thickness homogeneity. The technique will be illustrated by measurements of alpha particles and fission fragments emitted from a set of four vacuum evaporated UF 4, three electrodeposited and one suspension-sprayed 235U 3O 8 layers. The energy and angular distributions of alpha particles and of the heavy alpha recoils emitted from a self transferred 224Ra source will be discussed. The low energetic alpha recoils might be useful as probes for the investigation of ultrathin ( < 400 Å) layers.

The12C(n,α) Reaction and the Kerma Factor for Carbon atEn= 14.1 MeV

Energy and angular distributions of alpha particles from the bombardment of carbon foils with 14.1-MeV neutrons were measured with a magnetic spectrometer. The observations included alpha particles...

Emission of alpha particles in statistical multistep compound reactions

Excitation functions and angular distributions of alpha particles emitted from the $^{27}\mathrm{Al}(^{3}\mathrm{He}, \ensuremath{\alpha})$ and $^{27}\mathrm{Al}(\mathrm{p}, \ensuremath{\alpha})$ reactions are measured at a composite nucleus excitation energy of about 30 MeV. The experimental results together with the widely varying correlation widths extracted from the fluctuation analysis are successfully interpreted on the basis of the statistical multistep compound emission mechanism. Confirmation is found of the interpretation of each experimental correlation width in terms of a coherent sum of the widths of the relevant stages of the reactions, previously proposed in the case of proton channels. Moreover, the $\ensuremath{\alpha}$-spectrum shape and the correlation width values are shown to be consistent with an $\ensuremath{\alpha}$ preformation picture of the target nuclei considered.NUCLEAR REACTIONS $^{27}\mathrm{Al}(^{3}\mathrm{He}, \ensuremath{\alpha})$ $E=8.2\ensuremath{-}13$ MeV; $^{27}\mathrm{Al}(\mathrm{p}, \ensuremath{\alpha})$, $E=18.5\ensuremath{-}25.6$ MeV; measured $\ensuremath{\sigma}(E;\ensuremath{\vartheta})$; statistical multistep compound emission, calculated $\ensuremath{\sigma}(E)$, $\ensuremath{\sigma}({E}_{\ensuremath{\alpha}})$; generalized autocorrelation analysis, extracted coherence widths; suggested $\ensuremath{\alpha}$ preformation.

Alpha-Particle Angular Distributions with Respect to Spin Direction

Angular distribution of alpha particles with respect to the spin direction of residual nuclei from fusion of 176-MeV $^{20}\mathrm{Ne}$ with $^{150}\mathrm{Nd}$ has been measured with the spin spectrometer. Below the Coulomb barrier, the ratio of the 90\ifmmode^\circ\else\textdegree\fi{} to 0\ifmmode^\circ\else\textdegree\fi{} yields with respect to spin direction increases with decreasing ${E}_{\ensuremath{\alpha}}$. This effect is not shown by a statistical-model calculation using penetrabilities for spherical potentials, which suggests that the $\ensuremath{\alpha}$-mitting nuclei are deformed with their longest axis perpendicular to the spin direction. If so, the deformation increases as the spin rises from \ensuremath{\sim} 34 to $64\ensuremath{\hbar}$.

Energy and angular distributions of particles emitted in (nucleon, ) reactions

Calculations, in the framework of the exciton model, of angle-integrated energy distributions and angular distributions of alpha particles emitted in (nucleon, α) reactions are presented. The reaction mechanism is assumed to be a knock-on process involving α particles pre-formed in the target nucleus. The results obtained indicate that a satisfactory reproduction of the (nucleon, α) process is achieved. The main contribution to the process derives from interactions involving pre-formed α particles of energy comparable with the Fermi one.

Observation of asymmetric electric field around laser-imploded target

Strong anisotropy on angular distribution of alpha particles in D-T reaction was observed in locally irradiated exploding pusher mode compression. This indicates asymmetric generation of an electric field localized around the laser irradiated spots whose scale length and shape were estimated to be about 300 μm and radially extended from the irradiated spots.

The130Te(p,α)127Sb reaction and the role of the collective neutron excitations

The (p, alpha ) reaction on 130Te has been investigated at a proton bombarding energy of 35.2 MeV. Angular distributions of alpha particles have been measured over a limited angular range for states up to 3.2 MeV excitation in 127Sb and have been compared with DWBA calculations. A microscopic formulation of the three-nucleon transfer reaction has been employed in the calculations, together with Sb wavefunctions from a particle-core coupling model. Good agreement between theory and experiment is obtained for several states; disagreement for others is most probably attributable to neglect of some components of the collective neutron excitations.

The reaction106Cd(3He,α)105Cd

The Harwell multi-angle magnetic spectrograph and tandem Van de Graaff electrostatic generator have been used to study the states of 105Cd populated in the single neutron pickup reaction 106Cd(3He, alpha )105Cd at an incident 3He energy of 18.0 MeV. Eleven states of the final nucleus were identified up to an excitation energy of 3 MeV. Orbital angular momentum ln values of the transferred neutron and single neutron spectroscopic factors of the states excited were extracted by comparison of the measured alpha-particle angular distributions with predictions based on the distorted wave Born approximation. The results are compared with the predictions from pairing theory.

The High Excited Levels of 27Al

The spectrum and angular distribution of alpha particles scattered by 27 Al at 22.2 MeV bombarding energy have been measured. Among the prominent peaks in the spectrum, those corresponding to excited states at 5.434, 6.547, 7.28 and 7.50 MeV were studied by measuring the gamma ray spectrum in coincidence with scattered alpha particle. From the measured gamma ray branching ratios, (5/2, 7/2, 9/2) + was assigned to the former two levels and 7/2 - was assigned to the latter two levels. In addition, from gamma ray transition rate consideration, 3/2 - , 9/2 + and (3/2, 5/2) + were found appropriate for 4.055, 4.508 and 5.491 MeV level respectively.