12C1 and 8Be production in 12C + 208Pb collisions

The mechanisms involved in the production of fast α-particles in ¹²C-induced reactions have been studied for the ¹²C + ²⁰⁸Pb system at the bombarding energies of E_12c = 132, 187 and 230 MeV. Absolute cross sections for the reactions ²⁰⁸Pb(¹²C. ¹²C¹→α + ⁸Be), ²⁰⁸Pb(¹²C, ⁸Be(g.s.)) and ²⁰⁸Pb(¹²C, ⁸Be(2.94 MeV)) have been determined by coincidence measurement of two or three correlated α-particles. Inclusive α-particle production cross sections were also measured at E_12c = 187 MeV. It is found that the inelastic process (¹²C, ¹²C¹→α + ⁸Be) does not contribute significantly to fast α-particle production but that the production of ⁸Be by projectile fragmentation is an important source of α-particles. At the highest bombarding energy (230 MeV) it appears that the ¹²C → 3α fragmentation reaction becomes more prominent at the expense of the ¹²C→α + ⁸Be fragmentation channel.     

Emission of α-particles in deep inelastic reactions induced by 148 MeV 14N

Energy and angular correlations between α-particles and heavy ejectiles have been measured for 148 MeV ¹⁴N incident on ¹²C, ²⁷Al and ⁵⁸Ni targets. The coincidence cross sections for ejectiles with Z < 6 may be parametrised as a product of singles cross sections for the detection of α-particles and heavy ions. For ²⁷Al and ⁵⁸Ni targets, this indicates that the α-particles are emitted at an early stage of the reaction prior to the formation of the deep inelastic fragments. For the ¹²C target, however, kinematic effects are found to dominate the observed correlations. For $\text{Z ≧ 6}$ ejectiles the observed correlations appear consistent with the emission of α-particles from the recoiling nucleus produed in the reaction.     

Coulomb excitation of levels in 143Nd and 145Nd

The low-lying states of ¹⁴³Nd and ¹⁵⁴Nd have been studied by means of Coulomb excitation with ¹⁶O and α-particles. Angular distribution measurements were carried out for some transitions in ¹⁴⁵Nd with 11.2 MeV α-particles. Level energy decay schemes and B(E2)↑ values were measured for two states in ¹⁴³Nd and for six states in ¹⁴⁵Nd. Some spin assignments have been established for the ¹⁴⁵Nd nucleus. ¹⁴³Nd and ¹⁴⁵Nd have been theoretically described by coupling one and three particles, respectively, to quadrupole vibrations, and rather good agreement with experiment was achieved.     

Large-angle elastic scattering of α-particles from 20Ne and 22Ne

Resonances observed in the 177° (lab) excitation function of α-particles scattered elastically from ²⁰Ne are investigated in terms of a single Regge pole. The position of the pole in the complex angular momentum plane is determined from angular distributions measured at incident α-particle energies of 25.8 and 27.0 MeV, at which energy values prominent maxima were observed in the excitation function and the pole contribution to these angular distributions is therefore expected to be a maximum. The complete trajectory of the pole over the energy region investigated (24.6 to 31.7 MeV) is obtained by means of extrapolation. The pole trajectory across the surface absorption region of the target nucleus clearly shows the reason for the occurrence of only three large resonances in this energy region, the remaining resonances being attenuated considerably. An angular distribution obtained for ²²Ne(α, α)²²Ne at an incident energy of 27.0 MeV also indicates the presence of the Regge pole found for ²⁰Ne at the same energy, even though its strength is severely reduced due to the isotopic dependence effect.     

Light-particle emission in the reactions of 16O with Ti

Inclusive energy spectra and angular distributions for heavy ions $\text{(Z ≧ 3)}$ produced in the reactions of 227 MeV and 310 MeV ¹⁶O with Ti were measured. Also measured at the projectile energy of 310 MeV were energy and angular correlations between light charged particles (Z ≦ 2) and heavy ions. From comparisons with statistical model calculations upper limits to the complete fusion cross sections of 647 mb and 265 mb were derived for projectile energies of 227 MeV and 310 MeV, respectively. At 310 MeV the cross section of incomplete fusion processes was estimated to be over 505 mb. Emission of fast, high-energy α-particles and protons was observed to be a characteristic feature of quasi-elastic, deep-inelastic and fusion-like reactions. Average multiplicities of fast light particles in coincidence with heavy ions at +20° and +40° were estimated to be of the order of 1. The prompt emission of light appears to be the principal mechanism which limits complete fusion. A second component of α-particles observed in coincidence with deep-inelastic projectile-like fragments and having an energy comparable to Coulomb energies of particles emitted from target-like and projectile-like fragments appears as an excess yield in the direction of the recoiling target-like fragments. This component cannot be accounted for in terms of sequential emission processes and may result from a mechanism other than the one which leads to fast particle emission.     

The breakup of 16O into He: An event-by-event study of nucleus-nucleus collisions at 75, 175 and 2000 MeV/A

We present an event-by-event study of the breakup of the ¹⁶O in ¹⁶O + emulsion nucleus interactions at 75, 175 and 2000 MeV/A. The events are categorized according to their multiplicity of projectile He nuclei. The multiplicity depends on the degree of target destruction. Although the fragmentation model describes the gross features of inclusive He spectra, an event-by-event study reveals deviations from the model. The momenta of the He nuclei, emitted from the projectile, depend on helium multiplicity and the breakup properties of the target nucleus. The probability that the ¹⁶O projectile breaks up into multiple He fragments is larger at 75 MeV/A than at 2000 MeV/A. At 75 MeV/A the mean velocity of projectile He is on the average 0.06c below the projectile velocity. This recoil velocity depends on the target nucleus destruction also for the most peripheral collisions.     

Multiparameter study of 1H, 3H and 4He in fast neutron induced fission of 235U

The emission probabilities per fission of α-particles, tritons and protons have been measured in fast neutron induced fission of ²³⁵U. The measurements were carried out at neutron energies of 120, 180, 230 and 550 keV. AΔE-E semiconductor detector telescope was used to identify different light charged particles and the fission fragments were detected with an ionization chamber. The three-parameter data corresponding to the pulse heights from the ΔE-E detectors and the ion-chamber were recorded event by event on magnetic tape and were analyzed off-line by computer. No significant variation in the most probable energy ($\text{E}$) and the standard deviation (σ_E) of the energy spectra of different light charged particles with incident neutron energy was observed, although $\text{E}{}_{\mathrm{\alpha }}$ was seen to have a slightly higher value beyond E_n = 230 keV. The yield of α-particles in fission induced by neutrons of E_n ～ 200 keV was found to be higher by about 20 % than that in thermal neutron induced fission. The yields of tritons and protons were found to increase significantly with neutron energy.     

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.     

The half-life of 207Bi and decays of 211At and 211Po

The half-life of ²⁰⁷Bi was obtained from the genetic relation between ²⁰⁷Po and ²⁰⁷Bi, and between ²¹¹At and ²⁰⁷Bi. The half-life was found to be 33.4 ± 0.8 y. The half-life of ²⁰⁷Po was determined to be 5.81 ± 0.04 h by following the decay of the characteristic γ-rays from ²⁰⁷Po. The half-life of ²¹¹ At was determined tobe 7.23 ± 0.02 h by following the decay of γ-rays and α-particles from ²¹¹At and ²¹¹Po. The half-lives determined in the present work for ²⁰⁷Po and ²¹¹At agree with the literature although the half-life of ²⁰⁷Bi differs considerably from the currently accepted value of 38 y. The branching ratio of ²¹¹ At decaying through EC and α-decay modes was determined together with the branching ratios of the three α-particles emitted from ²¹¹Po.     

Break-up of 6Li and 7Li on tin and nickel nuclei

The break-up of the stable lithium nuclei on ⁵⁸Ni and ¹¹⁸Sn has been investigated in the bombarding energy range 12–24 MeV. The emerging α-particles have been analysed showing that the cross sections peak at the angle of grazing collisions. Inspection of the present data suggests the conclusion that several different reaction mechanisms contribute to the continuous spectrum of the α-particles.     

The 12C nucleus as a 3α system with forbidden states

We have calculated the 0₁⁺ ground state and the 0₂⁺ and 0₃⁺ first excited states of ¹²C in the model of three α-particles whose interaction is described using a local attractive potential with forbidden states, which well reproduces the α-α scattering phases. The methods of projection of the states forbidden by the Pauli exclusion principle in a system of three composite particles are discussed. The results of the calculation show that the condition of orthogonality to the forbidden states is sufficient to prevent the system from collapse. The binding energy value obtained (15.06 MeV) is somewhat overestimated. This discrepancy seems to come from neglecting triple exchanges of nucleons of different α-particles and from the fact that the ¹²C real nucleus differs from the 3α system.     

Investigation of α-particle emission from the interactions of fast neutrons with 161Dy and 163Dy nuclei

The energy spectra of α-particles emitted in the ¹⁶¹Dy(n, α)¹⁶¹Gd and ¹⁶³Dy(n,α)¹⁶⁰Gd reactions have been measured at neutron energies equal to 14.1 and 18.2 MeV. The results have been analysed in terms of Hauser-Feshbach, pre-equilibrium and knock-on models. The experimental data can be described assuming the existence of preformed α-clusters in target nuclei and the processes involving only few degrees of freedom.     

Light-particle emission in the reaction of 93Nb + 14N at 132, 159 and 208 MeV

Nonequilibrium light-particle emissions have been investigated in the reaction ⁹³Nb + ¹⁴N at 132, 159 and 208 MeV by measuring inclusive differential cross sections of p, d, t, ³He and α. The experimental data were analyzed in terms of three models: (i) an extended exciton model, (ii) a coalescence model, and (iii) a moving thermal source model. The angle-integrated energy spectra of the protons were well described by the extended exciton model in which projectile nucleons were assumed to be transferred to the target one by one, but those of composite particles were not. On the other hand, the composite particle spectra (except for α at forward angles) were successfully described by the coalescence model using spectra consistent with those for the protons. Extracted coalescence radii P₀ were about 140 MeV/c for d and t, and about 220 MeV/c for α. The light-particle spectra were also fitted by the moving-source model assuming isotropic emission from a source moving with approximately half of the beam velocity and with much higher temperature than that of the compound nucleus. Extracted temperatures followed the systematics of a recent compilation for the various reactions. A discussion of these analyses is given.     

Time-of-flight measurement of the evaporation residues from fusion of 5.9 mev/u 84kr and 27al

Mass and Z-distributions of the evaporation residues from compound-nucleus formation in the reaction 5.9 MeV/u ⁸⁴Kr on ²⁷Al were measured using a time-of-flight ΔE ? E telescope, which is described in detail. The high recoil velocity attained by choosing the heavy reaction partner as projectile made it possible to resolve the reaction products by mass and atomic number. Data were taken in the angular range from 1.5 to 6°. The residue distributions are compared to evaporation calculations assuming the statistical decay by fission and particle evaporation of the compound nucleus ¹¹¹Mn formed at an excitation energy of 108 MeV with angular momenta up to $\text{L}{}_{\mathrm{<mtext>CN</mtext>}}\text{≈ 69}\text{h}\text{?}$. The data are consistent with the assumption of statistical equilibrium. Details of the de-excitation process, in particular the fission competition and the influence of nuclear deformations at high angular momenta, are discussed.     

Energy levels of 56Co studied by the 58Ni(d, α)56Co reaction near 0°

Tensor analyzing powers, T₂₀, of outgoing α-particles in the ⁵⁸Ni($\text{d}$, α)⁵⁶Co reaction at detection angles near 0° have been measured for excited states in ⁵⁶Co for beam energies of 6.75, 7.0, 7.5, 9.0 and 9.5 MeV. Thirty-seven spin-parity combinations for ⁵⁶Co excited states have been deduced. Previous J^π ambiguities for 11 of these states have been eliminated, and results in conflict with existing assignments for the levels at 3.235 and 3.378 MeV have been obtained. A search for 0⁺ states was carried out from angular distribution measurements at forward angles of the unpolarized (d, α) reaction. The combined results from this and previous experiments were found to be in reasonable agreement with calculated level schemes.     

Energy dependence of fusion and reaction cross sections in the 9Be + 12C system

Angular distributions of protons, deuterons. tritons and α-particles were measured for the system ⁹Be + ¹²C at lab energies between 12 and 27 MeV. The compound nucleus model with level densities calculated according to the Gilbert-Cameron formula describes satisfactorily the measured proton, deuteron and triton data. In the α-particle spectra contributions from other processes seem to be present. In the analysis the fusion cut-off angular momentum was adjusted at each energy in order to reproduce correctly the proton, deuteron and triton channels. From this analysis the fusion cross section was determined as a function of the energy. The results were compared with fusion and total reaction cross section values calculated from a potential model with the real part of the interaction potential obtained from the double folding procedure of Satchler.     

A study of fluctuations in the large-angle excitation functions of the reaction 21Ne(3He, α)20Ne(g.s.) and the elastic scattering of α-particles from 20Ne

The results of optical-model calculations for the elastic scattering of α-particles from ²⁰Ne as well as of a DWBA calculation for ²¹Ne(³He, α)²⁰Ne(g.s.) suggest that the fluctuations observed in the excitation functions of these reactions at large angles are related. The agreement between calculated and measured excitation functions is reasonable — a result which is very encouraging considering the lack of any a priori knowledge regarding the potentials. The results support the view that the fluctuations in the excitation function of the reaction ²¹Ne(³He, α)²⁰Ne(g.s.) are primarily due to a direct-reaction process and are associated with the exit-channel configuration.     

Spin-parity assignments of excited states in 12C

The reaction ¹²C(α, α')¹²C¹(3α) is observed by using 90 MeV a-particles in ionographic matter. The energies of the three α-particles from the break-up of ¹²C¹ are plotted in Dalitz diagrams. The density distributions for the three α-particles in the diagrams are calculated by assuming the spins and parities for the states in ¹²C¹(3α). The spin-parities for the states are assigned by comparing the observed density distributions with those calculated.     

Determination of angular momenta in the incomplete-fusion channels of the reactions Ta+22Ne and Ir+12C

By using the γ-ray-multiplicity technique, experiments were carried out to determine the input angular momenta lⁱⁿ corresponding to the heavy-ion induced incomplete-fusion reactions involving the emission of fast charged particles. The α-particle and Li-nuclei emission channels were investigated in the reaction ¹⁸¹Ta + ²²Ne at the average Ne-ion energy of 155 MeV and for the α-particle emission channel in the reaction ^natIr+¹²C at E = 100 MeV. The separation of the reaction channels was carried out using an X-ray spectrometer. The angular distributions of the fission fragments were measured in the reaction ¹⁸¹Ta(²²Ne, αf). The data obtained indicate that input angular momenta are equal to about 60 ? and 50 ? for the channels of Li emission and α-emission, respectively, being practically independent of the particle energy. For the α-emission channel the lⁱⁿ values are the same for both reactions within experimental errors. The lifetime of the system of interacting target and projectile nuclei, prior to the emission of an α-particle, has been estimated to be equal to 10^?20s.