Direct Breakup Research Articles

Coulomb breakup of the neutron-rich isotopes 15C and 17C

Coulomb breakup of unstable neutron-rich carbon isotopes 15,17C has been studied at energies around ∼500–600 MeV/nucleon. Non-resonant low-lying dipole strength is observed in these isotopes which can be explained by a direct breakup mechanism. In addition to the decay neutron from excited projectile, γ-rays emitted from excited fragments after Coulomb breakup are measured in coincidence, giving access to quantitative spectroscopic information. The spectroscopic factor deduced for a valence neutron occupying the s1/2 level in the 15C ground state is consistent with that obtained earlier from (d,p) transfer reactions. The analysis for Coulomb breakup of 17C shows that most of the cross section yields the 16C core in excited states. The predominant ground-state configuration of 17C is found to be 16C(2+)⊗νs,d.

Development of Methods for Calculation of Deuteron-Lithium and Neutron-Lithium Cross Sections for Energies up to 50 MeV

For the generation of evaluated nuclear data sets required for the International Fusion Materials Irradiation Facility project, the basic features of the deuteron-lithium and neutron-lithium nuclear interactions are examined. Factors complicating the evaluation of deuteron-lithium reaction characteristics and weak points of previous calculations and evaluations are discussed. A new method to obtain double differential cross sections of particles emitted in d+Li reactions is described. The method is based on the diffraction approach, a modified intranuclear cascade model, and the usual evaluation techniques. The cross sections predicted by this method are in good agreement with existing experimental data for deuteron interactions in thick lithium targets. The study of neutron-lithium interactions is performed on the basis of different approaches: coupled channels, diffraction scattering, and direct breakup models.

Multiple fragmentation dynamics after laser excitation: A simple theoretical approach

A model to describe molecular fragmentation into several fragments is presented. The case where dissociation is induced by the absorption of a single or several photons from ultrashort pulses is considered. Time-dependent perturbation theory is employed to describe the excitation process and the subsequent fragmentation dynamics is treated using various theories treating direct and indirect break-up processes. The multiple fragmentation of the Fe(CO)5 molecule serves as a numerical example. Different experiments are simulated and, regarding the simplicity of the model, an overall excellent agreement between theory and experiment is found, thus providing an insight into the excitation and fragmentation dynamics under various conditions.

Measurement of 42 MeV 7Li projectile breakup on 208Pb target near grazing incidence

Breakup of 42 MeV 7Li projectiles in the field of a 208Pb target are studied near the grazing angle. In inclusive breakup measurements, angular distribution of the α and t fragments are found to be rather flat in nature. The α–t coincidence measurements are carried out with wide angular separations between the detected fragments. The choice of detection angles enables measurement in the same range of fragment relative energies but very different 7Li∗ scattering angles. The direct breakup contribution is found to dominate over the sequential breakup for the θα>θt set, while the reverse is true when θα<θt. Calculations performed in a full quantum mechanical framework reproduce the overall shape of the direct breakup part of the present data and several existing data on 197Au and 208Pb targets at nearby energies. The sequential breakup is observed only from the decay of the 4.63 MeV (7/2−) resonant excited state of 7Li and its angular distribution does not fully agree with the existing theoretical predictions. Unlike the direct part, the sequential breakup cross section does not decrease with increasing scattering angles. Similar measurements with halo nuclei would be helpful to study their unknown resonant states.

Low energy deuteron-induced reactions on27Aland56Fe

Measurements of the elastic and inelastic scattering cross sections for deuterons on ${}^{27}\mathrm{Al}$ and ${}^{56}\mathrm{Fe}$ have been completed at 5 and 7 MeV. Additional measurements of the $(d,p),$ $(d,\ensuremath{\alpha}),$ and $(d,n)$ spectra on these targets at the same energies have been made. An optical model analysis has yielded optical parameters for these nuclei. These are not consistent with some global models based on fits to data above 9 MeV. A Hauser-Feshbach calculation for each target at the appropriate bombarding energy has been compared with the various particle spectra. Evidence for both direct and compound nuclear reaction contributions is found. A small contribution is found from direct three-body breakup $(d+\stackrel{\ensuremath{\rightarrow}}{T}n+p+T),$ but $(d,n)$ and $(d,p)$ direct reactions to bound states are more common. Approximately 75% of the absorption cross section corresponds to compound nuclear processes.

Halo excitations in fragmentation of [formula omitted]He at 240 MeV/u on carbon and lead targets

Dissociation of a 240 MeV/u beam of 6He, incident on carbon and lead targets, has been studied in kinematically complete experiments to investigate low-lying excitation modes in the halo nucleus 6He. It is shown that alignment effects characterize the inelastic scattering and allow an unambiguous assignment of the spin of a narrow resonance observed in the excitation energy spectrum. The differential cross sections for the 6He inelastic scattering on carbon and lead targets were deduced from the measured momenta of the two neutrons and the α-particle. An analysis of these distributions shows that quadrupole and, possibly, monopole excitations characterize the hadronic interaction, while the dipole mode is dominating in Coulomb dissociation. Neither theoretically predicted new resonance states in 6He nor nuclear excitation of a dipole mode were found. Direct evidence has been obtained for strong suppression of Coulomb post-acceleration in direct Coulomb breakup in a lead target.

Measurement of 42 MeV 7Li projectile breakup on 58Ni target beyond grazing incidence

Angular distributions of the elastic and breakup cross sections of 42 MeV 7Li-projectile on 58Ni target are measured at wide angles beyond the grazing angle. Coincidence measurements are done at unequal angle pairs. In this angular region, the direct breakup is found to be very small and strongly dominated by the nuclear breakup process, while the sequential breakup contributions from the decay of the 7 2 − , 4.63 MeV resonant state of 7Li remain significantly large within its corresponding sequential cone. Some indications of contributions from the higher sequential states ( 5 2 − 6.68 MeV, 5 2 − 7.46 MeV) are found.

Fragment angular correlation in the breakup of 16O ions at 95 MeV/ A

Fragment angular correlations in projectile breakup reactions are very sensitive to interference between different multipolarities entering in the excitation-dissociation process of the projectile. In particular, it was proposed to disentangle L = 1 and L = 2 contributions in direct breakup reactions of 16O with low relative energy between the α and 12C fragment, which are of astrophysical interest. We studied the experimental aspects of extracting those angular correlations in extreme kinematical conditions usually encountered in breakup experiments of astrophysical interest. The breakup of 95 MeV/ A 16O projectiles induced by a 208Pb target was measured using the high-resolution spectrometer SPEG at Ganil for the coincident detection of the fragments. Sequential breakup via the 12.53 MeV level of 16O is analyzed in this framework and it favors an one-step M2 excitation of this level.

Continuum-discretized coupled-channels analysis of6Li→+4Hescattering atEc.m.=11.1MeV

Data for the elastic scattering of 27.8 MeV polarized {sup 6}Li from {sup 4}He and new data for the {sup 6}Li{r_arrow}{alpha}+d sequential breakup have been analyzed using the coupled-channels (CC) method with potentials derived via the cluster-folding (CF) procedure. The three low-lying T=0 resonant excited states of {sup 6}Li as well as nonresonant continuum states were taken into account in the calculations by means of the continuum-discretized coupled-channels (CDCC) method. No adjustable parameters were used. The analysis shows a dominant role of the projectile breakup into {alpha}+d in the {sup 6}Li+{sup 4}He elastic scattering. The deuteron transfer process is found to be of less importance. The calculations reproduce the values of the differential cross section for the sequential breakup channels at forward scattering angles. Predictions for the direct breakup near the {sup 6}Li{r_arrow}{alpha}+d breakup threshold are made. {copyright} {ital 1997} {ital The American Physical Society}

The 12C + α reaction rate from the elastic 16O breakup

Evidence for direct elastic breakup of 16O into the α- 12C continuum with relative energies ranging from 900 to 1800 keV has been obtained in the scattering of 1527 MeV 16O projectiles off 208Pb. An interpretation of E2 breakup including nuclear and Coulomb contributions leads to reduced electromagnetic transitions probabilities and astrophysical S e2 factors in reasonable agreement with direct measurements, showing that the method can be applied to extract the E2 part of the 12C(α, γ) reaction rate.

Direct reaction calculation of the 11Li Coulomb dissociation spectrum.

Analyzing the recent experimental data of Shimoura et al., we show that the dissociation energy spectrum of an intermediate energy $^{11}\mathrm{Li}$ nucleus, interacting with a very heavy target, is well reproduced by a direct Coulomb breakup mechanism, provided that in the data the n-n relative energy is subtracted from the dissociation energy. \textcopyright{} 1996 The American Physical Society.

Studies of light nuclei beyond the particle driplines: the two-proton emitter 12O

The decay dynamics of the ground state two proton emitter 12O, which lies beyond the proton dripline, were investigated following the population of 12O via single-neutron stripping of an 13O projectile. The decay products, two protons and 10C, were detected in coincidence and correlations among these particles were studied. No evidence was seen for 2He emission in the decay. This appears consistent with direct breakup or possibly sequential decay through 11N.

Coulomb dissociation of a halo nucleus 11Be and direct breakup mechanism

We have made a kinematically complete measurement of the Coulomb dissociation of 11 Be on Pb target at 72 A MeV. The low-lying dipole strength distribution indicated the importance of the direct breakup mechanism, which was also supported by the observed effect of the Coulomb post acceleration. The E1 dominance of the reaction was confirmed by the angular distribution of 10 Be in the rest frame of 11 Be.

Coulomb dissociation reaction and correlations of two halo neutrons in 11Li

We have performed an exclusive measurement for the Coulomb dissociation reaction, Pb( 11Li, 9Li+2n) X at an incident energy of 43 A MeV. The deduced excitation energy spectrum of the 11Li nucleus shows a prominent peak at 1 MeV with a large tail towards higher excitation energies. The relative energy spectra for the n-n and 9Li−2n sub-systems show that the excitation is dominated by the relative motion between 9Li and the center-of-mass of the two neutrons. The above characteristics are explained by a direct Coulomb breakup mechanism with an extended ‘di-neutron’ model.

Excitation functions of proton and deuteron induced reactions on iron and alpha-particle induced reactions on manganese in the energy region up to 25 MeV.

Excitation functions were measured by the stacked-foil technique for $^{\mathrm{nat}}\mathrm{Fe}$(p,xn${)}^{56}$Co, $^{\mathrm{nat}}\mathrm{Fe}$(p,xn${)}^{57}$Co, $^{58}\mathrm{Fe}$(p,n${)}^{58}$${\mathrm{Co}}^{\mathit{m}+\mathit{g}}$, $^{57}\mathrm{Fe}$(p,\ensuremath{\alpha}${)}^{54}$Mn, $^{\mathrm{nat}}\mathrm{Fe}$(d,xn${)}^{58}$${\mathrm{Co}}^{\mathit{m}+\mathit{g}}$, $^{56}\mathrm{Fe}$(d,2n${)}^{56}$Co, $^{56}\mathrm{Fe}$(d,\ensuremath{\alpha}${)}^{54}$Mn, $^{55}\mathrm{Mn}$(\ensuremath{\alpha},n${)}^{58}$${\mathrm{Co}}^{\mathit{m}+\mathit{g}}$, and $^{55}\mathrm{Mn}$(\ensuremath{\alpha},2n${)}^{57}$Co reactions from the respective thresholds to 14.12 MeV in work with protons, 12.97 MeV with deuterons, and 25.52 MeV with alpha particles. The radioactivity of the activation products was determined by high resolution \ensuremath{\gamma}-ray spectroscopy. Statistical model calculations taking into account the precompound effects were performed for all the reactions, using a consistent set of model parameters. The experimental excitation functions of the proton and alpha-particle induced reactions are described within 10 to 15 percent by the calculation over the whole investigated energy range. The shapes of the excitation functions of the deuteron induced reactions are also reproduced well by the model calculation. However, a reduction factor (R=0.56) is needed to take account of the direct breakup process and, thereby, to obtain good agreement between the experimental data and model calculations.

Coulomb dissociation of a halo nucleus 11Be at 72 A MeV

We have measured the excitation energy spectrum of the 10Be+n system in the Coulomb dissociation of the one-neutron-halo nucleus 11Be on a Pb target at 72 A MeV. We have determined the dipole strength distribution dB ( E1) / dEx which shows a strong peak at about 800 keV excitation energy. The spectral shape is consistent with a direct breakup model. The Coulomb post accelaration effect has been observed also to support the direct breakup mechanism. The E1 dominance of the Coulomb dissociation is confirmed by observed angular distributions of 10Be in the 11Be rest frame.

Fingerprints of a possible low-lying resonance in 11Li

Radioactive beams of Borromean halo-like nuclei like 11Li have provided possibilities for studying the exotic structure of such objects. Since the information is obtained in collision studies, in particular from break-up channels, one has to disentangle the structure information from the reaction mechanism. A satisfactory treatment within a three-body model for the projectile requires a four-body continuum calculation. As an exploratory step towards this end, we give a phenomenological discussion based on data for 6He and 11Li projectiles for high and low energies from 1 GeV A-1 down to 30 MeV A-1. Data for both transverse and longitudinal momentum distributions for the three-body constituents of these projectiles are used. We argue that the data can be consistently understood in a picture where the high-energy data mainly correspond to direct break-up of the projectile ground state in a Serber model, while the low-energy data exhibit the fingerprint of a sequential break-up process via a narrow low-lying resonance, observed in 6He but hitherto not identified in 11Li.

Electromagnetic excitation of 11Li.

We have performed a kinematically complete measurement of the Coulomb dissociation of 28 MeV/nucleon $^{11}\mathrm{Li}$ into $^{9}\mathrm{Li}$ and two neutrons by a Pb target. From the energies and angles of the emitted neutrons and of $^{9}\mathrm{Li}$, the excitation energy E of $^{11}\mathrm{Li}$ was determined on an event-by-event basis, and the Coulomb dissociation cross section as a function of excitation energy was constructed. The photonuclear cross section ${\mathrm{\ensuremath{\sigma}}}_{\mathit{E}1}$(E) and the dipole strength function dB(E1)/dE were determined from the Coulomb dissociation cross section. ${\mathrm{\ensuremath{\sigma}}}_{\mathit{E}1}$(E) has a peak at E=1.0 MeV and a width \ensuremath{\Gamma}=0.8 MeV. These parameters are consistent with the picture of a soft dipole mode. However, a significant post-breakup Coulomb acceleration of $^{9}\mathrm{Li}$ suggests instead a direct breakup. The complete kinematical measurement also allowed neutron and $^{9}\mathrm{Li}$ momentum distributions to be constructed in the rest frame of the $^{11}\mathrm{Li}$. The momentum distributions were fitted with Gaussian functions, yielding width parameters ${\mathrm{\ensuremath{\sigma}}}_{9}$=18\ifmmode\pm\else\textpm\fi{}4 MeV/c and ${\mathrm{\ensuremath{\sigma}}}_{\mathit{n}}$=13\ifmmode\pm\else\textpm\fi{}3 MeV/c. A more general feature of the breakup mechanism of $^{11}\mathrm{Li}$ could be deduced from these measurements. It was found that the $^{9}\mathrm{Li}$ and neutron momentum distributions and the neutron-neutron relative momentum distribution could be reproduced if the $^{11}\mathrm{Li}$ excitation energy was partitioned between the $^{9}\mathrm{Li}$ and the neutrons by a three-body phase space distribution. This indicates there is no directional correlation between the halo neutrons, and shows that the halo neutrons do not exist as a dineutron bound to a $^{9}\mathrm{Li}$ core.

Transfer and break-up reactions in the 30 [formula omitted

Peripheral interactions of the 22 Ne + 93 Nb system at 30 A· MeV incident energy have been investigated using the forward 48 CsI scintillators of the AMPHORA multidetector array for the detection of light charged particles at forward angles. In addition fragments have been identified near the grazing angle in 4 Si-CsI telescopes. It is found that the transfer probability remains large at this intermediate energy. The stripping of one or two nucleons from the projectile to the target and the pick-up of one nucleon have been clearly observed. Binary quasi-projectile break-up events involving emission of one α are dominant. Dissociation of the projectile into up to five products has also been observed. The production of fragments is deduced to result mainly from sequential decay of the primary fragments rather than from a direct break-up process. The yields of fragments, except for the lightest ones, have been reproduced in the framework of a random-walk model.

Projectile break-up with two outgoing intermediate-mass fragments in 26 AMeV32S+197Au

Correlations between two intermediate-mass fragments resulting from 840 MeV32S projectiles interacting with197Au were measured. The angular correlations and absolute two-fragment cross sections agree with predictions of a semi-microscopic model for direct dynamic projectile break-up. This suggests a predominantly direct mechanism for the production of intermediate-mass fragment pairs. It is argued that the intensity of such a channel which, compared to decays yieldingα particles, is much larger than observed for the sequential decay of corresponding primary excited nuclei, supports the direct-break-up interpretation. The model mechanism yields also strongly damped projectile fragments of intermediate mass.