Angular Distributions Of Α-particles Research Articles

Scission-point distributions in LRA fission

Trajectory calculations were performed using the three-point-charge model. The input parameters took values distributed over a wide range. Using experi mental distributions, each trajectory was assigned a ‘weight’; and the trajectories and their statistical ‘weights’ were used in obtaining initial distributions which in turn reproduced other experimental distributions, such as the angular distribution ofα-particles, very well. Some information about the initial distributions has been obtained. The effect of the anticorrelation, between the finalα and fragment energies, on the initial distributions has been examined.

Exchange effects in the 7Li(3He, α)6Li reaction

Abstract Angular distributions of α-particles from the 7Li(3He, α)6Li reaction at 3He bombarding energies of 16 and 18 MeV are analyzed with an exact finite-range two-mode DWBA formalism using the computer code FANLU2. It is demonstrated that within the formalism used it is essential to include the exchange mode of the reaction. The overall features of the angular distributions are described remarkably well, but a strong dependence on optical model parameter sets makes it essential to adhere to a characteristic class of such sets. Spectroscopic factors for direct and exchange modes are extracted and discussed.

Investigation of the reaction at Eτ = 15 MeV

The 35Cl(τ, α)34C reaction has been used to study the properties of 34Cl levels up to an excitation energy of 5 MeV. Angular distributions of 37 levels were measured with a split-pole magnetic spectrograph, at a bombarding energy of 15 MeV. New levels have been found at 3847, 3964, 4206, 4321 and 4715 keV, all ± 10 keV. There is a strongly excited multiplet at Ex = 5.0 MeV with components at 4939 ± 11, 4958 ± 11, 4971 ± 11, 4998 ± 12 and 5010 ± 13 keV. A DWBA analysis of the α-particle angular distributions yielded ln values and spectroscopic factors. New spin and parity assignments were obtained. The T = 1 character of the levels at Ex = 4.21 and 4.72 MeV has been determined. Experimental spectroscopic factors are compared with results from recent many-particle shell-model calculations.

Alpha particles from 197Au irradiated by 500 MeV bremsstrahlung

The energy and angular distributions of α-particles in the reaction 197Au(γ, α) have been measured at nine different angles for bremsstrahlung with peak energy 500 MeV. The measurements were made using a telescope consisting of three Si surface-barrier detectors. The experimental results are compared with cascade-evaporation calculations and the angular distribution indicates the presence of directly emitted α-particles in competition with evaporated ones.

The excitation of 64Ni by the 65Cu(t, α) reaction

Angular distributions of α-particles from the (t, α) reaction on 65Cu have been measured for 12 MeV incident tritons. Levels up to an excitation energy of 7 MeV in 64Ni were studied with a resolution (FWHM) of 10 keV. Distorted-wave Born-approximation calculations were carried out and used to assign values to the angular momentum of the transferred protons as well as to determine relative spectroscopic factors for the outer shell-model orbits. A possible connection between collective states of the 64Ni core and the proton in the 2p 3 2 orbit was investigated by comparison between these data, other proton pick-up reactions on 65Cu and the inelastic scattering of 3He and α particles from 64Ni. Evidence is found for a two-step process in the proton pick-up reaction leading to known collective states of the core.

Mechanism of (α, 2α) reactions on 12C and 16O at intermediate energies

From physical considerations, in the scheme of the dispersion, approach with Feynman graphs to nuclear reactions, we choose a triangular graph to study (α, 2α) reactions with 12C and 16O. We discuss the dependance of the α-particle angular distribution upon the quasi-free α-α scattering at various energies; we determine in which cases it is justified to keep constant the α-α cross section.

Angular and energy distributions of α-particles in LRA fission

Detailed experimental studies have been made on the angular and energy distributions of the long-range α-particles (LRA) emitted from 252Cf during spontaneous fission. They show that the α-particle angular distribution is peaked at an angle (made by the trajectory of the α-particle with that of either of the main fragments) of about 90° and has a FWHM of 20–30. The α-particle energy spectrum is peaked at about 15 MeV and has a FWHM of 13 MeV. We have carried out computer calculations of the trajectories of the three particles produced in LRA fission. Such calculations lead to predictions of angular and energy distributions which may be compared with the experimental ones. The calculations were carried out using a set of initial conditions determined by the statistical theory of nuclear fission. The comparison with experimental results therefore provides a test of the statistical theory. The calculated results show that the statistical theory leads to an energy spectrum which is in good agreement with the experimental one, and that reasonable agreement between the calculated and the experimental angular distribution may be obtained if it is assumed that the α-particle is emitted within a short period of time, of the order of 3 × 10 −22 s after the instant of scission.

Spectroscopy of 22Na with the 20Ne( 3He, p) and 20Ne( 6Li, α) reactions

The 22Na nucleus has been studied by means of the 20Ne( 3He, p) reaction at 15 MeV incident energy and the 20Ne( 6Li, α) reaction at 18 MeV bombarding energy. Levels up to 10 MeV of excitation were measured with energy resolutions of 25 keV in the ( 3He, p) case and of 40 keV in the 6Li induced reaction using the University of Pennsylvania tandem Van de Graaff and multi-angle spectrograph. The systematics of the proton and α-particle angular distribution shapes allow the assignment of values of the transferred angular momentum for a number of transitions, resulting in several new spin-parity assignments. Comparison of the proton and α-particle spectra reveals that T = 1 levels are extremely weakly excited in the ( 6Li, α) reaction and this fact is used for assigning several T = 1 states. A distorted wave calculation of relative intensities was performed based on a description of the 22Na levels in terms of pure two-particle Nilsson states with associated rotational bands. This analysis is consistent with the ground state K = 3 band and the K = 0, T = 1 band built on the first 0 + state having simple two-Nilsson-particle character whereas the properties of the second (1.93 MeV) 1 + state are inconsistent with such a description. It is concluded that the combination of ( 3He, p) and ( 6Li, α) reactions employed in the present study provides a powerful spectroscopic tool.

(n, α) reactions on heavy nuclei induced by 15.1 MeV neutrons

The energy spectra of α-particles from the (n, α) reactions on 141Pr, 165Ho, 175Lu, 181Ta and 197Au induced by 14 MeV neutrons at 0° have been measured with a n-type silicon surface barrier semiconductor. They cannot be accounted for by the statistical model, with the inclusion of the shell effect for the level density, which is derived from the free gas model for a two-fermion system. The angular distribution of α-particles from 141Pr is strongly forward-peaked. It seems that there is an appreciable contribution of direct processes to these reactions.

The dependence of the spectroscopic factors on parameters of distorting potentials for the reaction 26Mg(τ, α) 25Mg

Angular distributions of α-particles from the reaction 26Mg(τ, α) 25Mg studied at E τ = 10.2 MeV have been analysed in terms of DWBA. To avoid the necessity of introducing a lower cut-off radius in the calculation of radial integrals, deep potentials have been used. Changes in the potential of the incident particles are found to have only a small influence on the calculated distributions. The fits to the experimental data can be optimized by readjusting one parameter only, namely the depth of the real part of the optical-model potential for α.-particles. The present analysis indicates that the dependence of the ratio of the spectroscopic factors S( l, j)/ S g.s.( l, j) on the exit channel parameters is weaker (by a factor of from 0.5 to 0.2) than the dependence of the absolute value of the calculated cross sections on the same parameters. The experimental value of the normalization coefficient D 0 2, obtained here for the (τ, α.) reactions is 22 ± 6.

Proton pick-up from the even calcium isotopes

The 42, 44, 46, 48Ca(t, α) proton pick-up reactions have been investigated at about 13 MeV incident energy, transition Q-values and α-particle angular distributions were measured for the stronger transitions. A common intensity scale was established by means of (t, α) reactions from a target consisting of known amounts of each of the even Ca isotopes. Assignments of l-values were made on the basis of the α-particle angular distributions. Relative spectroscopic factors were obtained by means of a DW analysis for the strongest 1 f 7 2 , 1 d 3 2 and 2 s 1 2 transitions. Some 1 f 7 2 transitions were observed for the targets of 40, 42, 44, 46Ca but not for 48Ca.

Proton-hole states in 47, 45K

Proton-hole states in 45,47K were excited by the 46,48Ca(t, α) reactions. The α-particle angular distributions for the transitions to 45K(0) and 47K(1) are similar and those to 45K(0) are similar. It is concluded that the 47K ground state has J π = 1 2 + rather than 3 2 + .

Level structure of48Sc from the50Ti(d, α)48Sc reaction

The50Ti(d, α)48Sc reaction was investigated using deuterons with the energy of 12.9 MeV. Energy spectra and angular distributions of α-particles emitted in this reaction have been measured. Relatively large cross-sections were found for transitions leading to the 0.14, 1.06 and 2.03 MeV excited states of the residual48Sc nucleus. The spin and parity values suggested for the 0.14 MeV state and the previously unobserved 1.06 MeV state areJπ=5+ andJπ=7+ respectively.

Angular distributions of α-particles from the reaction 11B( 3He, α) 10B

The angular distributions of α-particles from the reaction 11B( 3He, α) 10B leading to the four lowest states in 10B were measured for 3He energies of 1.0, 1.8 and 2.15 MeV. The distributions were compared with predictions made from the plane-wave, Butler pick-up theory which was extended to the case of ( 3He, α) reactions. The comparison indicates the presence of the direct interaction mechanism for the excitation of the three lowest states in 10B. The α-particles associated with the formation of the 2.15 MeV state in 10B were symmetric about 90° in the centre-of-mass system. The excitation functions for the formation of the five lowest states in 10B by the 11B( 3He, α) 10B reaction were measured for 3He energies from 1.0 MeV to 2.15 MeV.

Energy and angular distributions of α-particles in the [formula omitted] reaction at 14 MeV

Energy and angular distributions for α-particles in the 20 Ne(n, α) 17 O reaction have been obtained at E = 14.2 MeV, by using a fast-cycle cloud chamber. Departures from the typical evaporation distribution are observed in the low and high energy parts of the α-particle spectrum; the low energy part, however, is not especially significant owing to the distortion introduced by the (n, nα) reaction. For the high energy α-particles the effect can be traced back to a direct interaction. The c.m. differential cross section peaks at backward angles, an effect which becomes more pronounced for the more energetic α-particles. This feature, which is analogous with the reported results for the 12 C(n, α) 9 Be and 16 O(n, α) 13 C reactions, suggests that heavy particles stripping plays an important role in the 20 Ne(n, α) 17 O reaction.

(d, α) reaktionen an leichten kernen

The analysed beam from the Heidelberg cyclotron has been used to investigate (d, α) reactions on 24Mg, 25Mg and 28Si targets at incident beam energies between 11.45 und 12.1 MeV. Angular distributions of α-particles corresponding to the excitation of the lowest six to ten levels of the residual nuclei have been measured in the range between 15° and 165°. The data suggest a mixed direct and compound nucleus interaction mechanism. The latter seems to be predominant in the 25Mg(d, α) reaction. The angular distributions have been analysed by double pick-up theory where feasible. The (2 I+1) rule has been observed to hold for 25Mg(d, α) and to a lower degree of accuracy for 28Si(d, α). From the viewpoint of this rule it is suggested that the well-known 2.07 MeV level in 26Al is a doublet state as proposed earlier. The integrated cross-sections of various α-particle groups from the (d, α) reactions on 9Be, 14N, 16O, 31P and 32S have also been measured. A pronounced odd-even effect has been observed not only for the transitions to the ground states but also for the transitions to the excited states up to about 3 MeV.

(3He, α) Reactions on25Mg and26Mg at 5.50 MeV

Angular distributions of α-particles from the25Mg(3He, α)24Mg and26Mg(3He, α)25Mg reactions, leading to the ground and the low excited states of24Mg and25Mg, were measured at a bombarding energy of 5.50 MeV. Relative total cross-sections were obtained from the experimental results for transitions to the ground, 1.37, 4.12, 4.23, 5.22 and 6 MeV states of24Mg and for transitions to the ground, 0.58,0.98, 1.61 and 1.96 MeV states of25Mg. The ratios of the total cross-sections were compared with the predictions of the theory of Hauser and Feshbach and a satisfactory agreement has been found. In addition, the transition to the ground state of25Mg in the26Mg(3He, α)25Mg reaction seems predominantly due to a direct interaction mechanism.

The (d, α) reaction on N 15 and O 16 in the deuteron energy range 1–2.5 MeV

Excitation functions and angular distribution for the first two α-groups from the N 15 (d, α)C 13 reaction and those for the α 0-group from the O 16(d, α)N 14 reaction have been observed. For the first reaction a broad maximum in the excitation curve has been observed for both α-groups at the deuteron energy 1.6–1.8 MeV. Slight irregularities were observed in the excitation function for α-particles of the second reaction at E d=1.2, 1.9, 2.1 and 2.4 MeV. Angular distributions of α-particles from the N 15(d, α)C 13 reaction show a mixing between compound nucleus and direct interaction mechanism. Angular distributions of the α 0-group from the O 16(d, α)N 14 reaction are typical of the compound nucleus mechanism. Differential cross-sections for all α-groups are given in a wide energy range.

Investigation of the reaction Li 6(d, α)He 4 at deuteron energy 1–2.5 MeV

The relative yield of the reaction d+Li 6→2 He 4+22.3 MeV in the deuteron energy interval of 1–2.5 MeV and the angular distribution of α-particles at E and = 1, 1.25, 1.5, 1.75, 2.00, 2.25 and 2.50 MeV has been measured with the aid of semiconductor detectors. The experimental angular distributions agree well with the calculated ones in the assumption of pick-up interaction for the reaction Li 6(d, α)He 4.

The Angular Distribution of α-Particles from the Reaction 19F(pα)16O*(π)16O

The angular distribution of α-particles from the reaction 19 F ( p α) 16 O * (π) 16 O has been measured at fourteen energies between 800 kev and 1400 kev. The α -particles were detected and identified by coincidence with nuclear pair-electrons. The data were analyzed to give coefficients in the expansion formula σ(θ)= a 0 + a 1 cos θ+ a 2 cos 2 θ+ a 3 cos 3 θ+ a 4 cos 4 θ. The spins of the levels of the compound nucleus 20 Ne were assigned from the evidence of anomalies in the coefficients. For the resonances at 843-, 1126-, 1355-kev J =2 and for that at 1250 kev J =1.