Two-neutron Removal Cross Sections Research Articles

Two-neutron removal cross sections from C15,16 at around 240 MeV/nucleon

The cross sections for two-neutron removal from $^{15,16}\mathrm{C}$ on a carbon target have been measured at around 240 MeV/nucleon. The measured cross section for $^{15}\mathrm{C}$ is smaller than for $^{16}\mathrm{C}$. The trends of the cross sections for $^{15,16}\mathrm{C}$ are inconsistent with the previously reported experimental data, but in agreement with the theoretical predictions based on eikonal-model calculations. Based on the present results of $^{15,16}\mathrm{C}$ combined with the available experimental data on $^{17--20}\mathrm{C}$, an odd-even staggering of the two-neutron removal cross sections along the neutron-rich carbon isotopic chain from $^{15}\mathrm{C}$ to $^{20}\mathrm{C}$ is observed, and this feature is well reproduced by the theoretical calculations.

One- and two-neutron removal cross sections of O24

Cross sections of one- and two-neutron removal reactions of 24O, leading to the 23O(½+) ground state and to bound final states of 22O, have been measured at the National Superconducting Cyclotron Laboratory. The experiment was conducted using the S800 spectrograph and a 24O beam energy of 92.3 MeV/u. The measured 23O ground state and 22O inclusive cross section values, of 74(11) mb and 146(33) mb, respectively, are in good agreement with calculations using eikonal reaction dynamics and shell-model nuclear structure overlaps. The widths at half maximum of the associated parallel momentum distributions of these cross sections, deduced from Gaussian fits, are 115(13) MeV/c for 23O and 309(36) MeV/c for 22O in the projectile rest frame. The data and calculations strongly support the shell-model description of 24O as a spherical, doubly-magic structure.

Spectroscopy of nuclei around Sn100 populated via two-neutron knockout reactions

We report on the in-beam gamma spectroscopy of $^{102}$Sn and $^{100}$Cd produced via two-neutron removal from carbon and CH$_2$ targets at about 150 MeV/nucleon beam energy. New transitions assigned to the decay of a second 2$^+$ excited state at 2470(60) keV in $^{102}$Sn were observed. Two-neutron removal cross sections from $^{104}$Sn and $^{102}$Cd have been extracted. The enhanced cross section to the 2$^+_2$ in $^{102}$Sn populated via the $(p,p2n)$ reaction is traced back to an increase of shell-model structure overlaps, consistent with the hypothesis that the proton-induced two-deeply-bound-nucleon removal mechanism is of direct nature.

Total reaction cross sections and neutron-removal cross sections of neutron-rich light nuclei measured by the COMBAS fragment-separator

Preliminary results of measurements of the total reaction cross sections σR and neutron removal cross section σ-xn for weakly bound 6He, 8Li, 9Be and 10Be nuclei at energy range (20-35) A MeV with 28Si target is presented. The secondary beams of light nuclei were produced by bombardment of the 22Ne (35 A MeV) primary beam on Be target and separated by COMBAS fragment-separator. In dispersive focal plane a horizontal slit defined the momentum acceptance as 1% and a wedge degrader of 200 μm Al was installed. The Bρ of the second section of the fragment-separator was adjusted for measurements in energy range (20-35) A MeV. Two-neutron removal cross sections for 6He and 10Be and one –neutron removal cross sections 8Li and 9Be were measured.

Eikonal reaction theory for two-neutron removal reactions

The eikonal reaction theory (ERT) proposed lately is a method of calculating one-neutron removal reactions at intermediate incident energies in which Coulomb breakup is treated accurately with the continuum discretized coupled-channels method. ERT is extended to two-neutron removal reactions. ERT reproduces measured one- and two-neutron removal cross sections for 6He scattering on 12C and 208Pb targets at 240 MeV/nucleon and also on a 28Si target at 52 MeV/nucleon. For the heavier target in which Coulomb breakup is important, ERT yields much better agreement with the measured cross sections than the Glauber model.

One- and two-neutron removal from the neutron-rich carbon isotopes

Reactions that involve the fast removal (knockout) of one and two neutrons from the neutron-rich carbon isotopes, $^{15\ensuremath{-}19}\mathrm{C}$, by light nuclear targets are studied within an eikonal reaction model. Shell model calculations are used to describe the excitation energy spectra and the structures of the carbon isotopes. The calculated one-neutron knockout cross sections from the $^{A}\mathrm{C}$ isotopes, to particle-bound configurations of the $^{A\ensuremath{-}1}\mathrm{C}$ residues, are in agreement with the available experimental data. The two-neutron removal cross sections, producing $^{A\ensuremath{-}2}\mathrm{C}$ residues, receive contributions from both the direct, single-step two-neutron knockout and the indirect mechanism, involving single-neutron removal strength to neutron-unbound excited states in the $^{A\ensuremath{-}1}\mathrm{C}$ system followed by neutron emission. The latter two-step reaction mechanism is shown to be dominant. The empirical odd-even staggering of the single-neutron separation energies along the carbon isotopic chain is reflected in the two-neutron removal data. This staggering and the magnitudes of the two-neutron removal cross sections are reproduced qualitatively by the theoretical calculations.

Neutron removal studies on 19C

The measurement of one- and two-neutron removal cross section and longitudinal momentum distribution of 19C after reaction with a Be target at 64 A MeV are reported. This is the first study of the two-neutron removal for the nucleus. Besides that, the present one-neutron removal study helps to confirm negligible energy-dependent effects. This comes from an agreement between data at high and low energy in a wide momentum range. A comparison of the two-neutron removal momentum widths presented for Carbon isotopes shows interesting feature for 19C.

Momentum distributions of 14C and 15C fragments from 16C breakup

The longitudinal momentum distributions of 14C and 15C fragments from 16C breakup have been measured at 83 A MeV by means of recently-developed direct time-of-flight technique. The distributions are characterized by FWHM of 142±14 MeV/ c and 90±9 MeV/ c for 14C and 15C, respectively. A Glauber-type analysis performed for the 15C distribution gives the relative s-wave spectroscopic factor to be around 30%, which is smaller than that for the halo nucleus 15C where the s-wave is dominant. The two-neutron removal cross section is found to be larger than the one-neutron cross section.

Momentum distribution of 15B fragments from the breakup of 17B.

The two-neutron removal cross section (sigma(-2n)) and the longitudinal momentum distribution of 15B fragments from the breakup of 17B on 9Be were measured at 70A MeV. The distribution in the projectile rest frame is characterized by a FWHM of 80+/-10 MeV/c for 15B. The sigma(-2n) is found to be 0.22+/-0.05 b. A Glauber-type analysis of the data provides clear evidence of a two-neutron halo structure in 17B.

Halo structure of (14)Be.

The two-neutron halo nucleus (14)Be has been investigated in a kinematically complete measurement of the fragments ((12)Be and neutrons) produced in dissociation at 35 MeV/nucleon on C and Pb targets. Two-neutron removal cross sections, neutron angular distributions, and invariant mass spectra were measured, and the contributions from electromagnetic dissociation (EMD) were deduced. Comparison with three-body model calculations suggests that the halo wave function contains a large nu(2s(1/2))(2) admixture. The EMD invariant mass spectrum exhibited enhanced strength near threshold consistent with a nonresonant soft-dipole excitation.

Total reaction and 2n-removal cross sections of 20-60A MeV 4,6,8He, 6-9,11Li, and 10Be on Si.

Total reaction cross sections, ${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$, of 20--60A MeV $^{4,6,8}\mathrm{He}$, $^{6--9,11}\mathrm{Li}$, and $^{10}\mathrm{Be}$ were measured by injecting magnetically separated, focused, monoenergetic, identified secondary beams of those projectiles into a Si detector telescope and measuring their energy-deposition spectra. These ${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$'s, accurate to about 3%, were compared with predictions of optical, strong absorption, and microscopic models. The latter gave the best overall fit to the data, providing long-tailed matter densities were assumed. The best available optical potentials generally overpredicted the data by about 10%. Strong absorption calculations, in which the isospin-dependent term is quite important, were often unsuccessful, especially for projectiles with large neutron excess. Two-neutron removal cross sections were measured for $^{6}\mathrm{He}$ and $^{11}\mathrm{Li}$; the $^{11}\mathrm{Li}$ data were slightly overpredicted by a microscopic model which includes correlation effects for the $^{11}\mathrm{Li}$ valence neutrons. Both 2n and 4n removal from $^{8}\mathrm{He}$ were observed, in about a 2:1 ratio. Subtraction analysis of the data indicates that $^{4}\mathrm{He}$ is a good core within $^{6}\mathrm{He}$ and $^{8}\mathrm{He}$, as is $^{9}\mathrm{Li}$ within $^{11}\mathrm{Li}$. \textcopyright{} 1996 The American Physical Society.

6He and neutron momentum distributions from 8He in nuclear break-up reactions at 240 MeV/u

Neutron and He-6 momentum distributions from He-8 break-up reactions in a C target have been measured at 240 MeV/u. The two-neutron removal cross section was found to be sigma(-2n) = 0.27 +/- 0.03 b. The nature of the momentum distributions is interpreted in some simple reaction scenarios.

Longitudinal and transverse momentum distributions of 9Li fragments from break-up of 11Li

Transverse and longitudinal momentum distributions of 9Li fragments from 11Li break-up reactions in C, Al and Pb targets have been measured at 280 MeV/u. The two-neutron removal cross-section was measured to be σ −2 n = 0.26 ± 0.02 b for the carbon target, σ −2 n = 0.47 ± 0.08 b for the aluminum target and σ −2 n = 1.9 ± 0.4 b for the lead target. No significant difference is observed between the narrow widths (FWHM ≈ 47 MeV/c) of the transverse and longitudinal momentum distributions of the 9Li fragments. The physical implications of this are discussed.

Closed-form expressions for cross sections of exotic nuclei

We study p+11Li cross sections, in the intermediate-energy region, with the nucleus characterized by a halo structure. The eikonal approximation and “tϱ” optical potential are used to get an intuitive picture of the reaction mechanism. By using gaussian nucleon densities for 11Li, 9Li and the halo, we obtain similar expressions for the total nuclear reaction and two-neutron removal cross section in the Glauber approximation. Closed-form analytical expressions for those cross sections are derived. By comparing the results with the exact ones, we can conclude that we have derived a fair approximation to them. We also estimate the “in-medium” Pauli-blocking effect on the cross sections.

Coulomb and nuclear effects in the fragmentation of exotic nuclei

The role of the nuclear and the Coulomb interactions in the dissociation of the neutron-rich exotic nuclei 11Li, 11Be and 14Be incident on a number of targets has been studied. The two-neutron removal cross sections of 11Li and 14Be and one-neutron removal cross section of 11Be have been calculated. The theory of diffraction dissociation and diffraction stripping has been used to account for the nuclear effects. The Coulomb-dissociation cross sections have been calculated within the framework of a direct fragmentation model formulated earlier. The Coulomb dissociation along with the nuclear cross sections explain satisfactorily the target-mass dependence of the experimental total two-neutron removal cross sections of 11Li and 14Be around 30 MeV/ A incident energy. The beam-energy dependence of this cross section is also well reproduced. The stripping contributions dominate the nuclear cross sections. The dependence of the reaction mechanism underlying the two-neutron removal process on the separation energy of the two-neutron system in the projectile is discussed. The role of the “post-acceleration effect” on the energy spectra of the produced fragments is investigated.

Quadrupole contribution to two-neutron removal in heavy ion collisions.

In this Brief Report, electric quadrupole corrections to the two-neutron removal cross section measured in heavy ion collisions are estimated for {sup 197}Au and {sup 59}Co targets. The quadrupole process is assumed to proceed primarily through excitation of the giant isovector quadrupole resonance, which then decays by neutron emission. For {sup 59}Co, the contribution from {ital E}2 radiation is found to be small, while for {sup 197}Au we find the quadrupole contribution resolves the discrepancy between experiment and the simple predictions of the Weissacker-Williams virtual photon method.

Glauber model analysis of the fragmentation reaction cross sections of 11Li

A theory to evaluate the reaction, interaction, and two-neutron removal cross sections of 11Li is presented in the framework of the Glauber theory in order to elucidate the role of the halo-neutron density distribution. The nuclear parts of the cross sections can be calculated with the neutron densities as input. Two kinds of the densities are compared with experiment at 800 MeV/nucleon. A good agreement with experiment is obtained for light targets. It is emphasized that the optical-limit approximation is not good for calculating the reaction cross section of a nucleus with halo structure. The reaction mechanism underlying the two-neutron removal process is discussed in some detail. A relation of meson-production cross sections with the property of the profile function is also discussed.

Reaction mechanism of 11Li at intermediate energy

The reaction of 11Li, a nucleus characterized by a halo structure, is analyzed by the ( 9Li + n + n)-target four-body model in the intermediate energy region. The eikonal approximation and adiabatic approximation are employed not only to make the microscopic calculation feasible but to get intuitive pictures for the reaction mechanism. To test the accuracy of the eikonal approximation in the intermediate energy region, the deuteroon reaction is also analyzed by the (p + n)-target three-body model and compared with the coupled discretized continuum-channels analysis. It is found that the eikonal approximation works quite successfully for various reaction mechanisms including the break-up process. The total reaction cross section and the two-neutron removal cross section of the 11Li + 12C reaction are analyzed. The measured interaction cross section is nicely reproduced in our calculation. It is found that the elastic break-up process plays an important role in the intermediate energy region.

Two-neutron removal cross sections of 11Li projectiles

We investigate the interplay of the nuclear and Coulomb interaction in the fragmentation of relativistic 11Li projectiles incident on several targets. The 11Li nucleus is assumed to have a cluster-like structure, with a (bound) di-neutron system coupled to a 9Li core in an s-state. The obtained nuclear cross sections show marked differences with the “experimental” data. Taking the calculated nuclear contribution at face value results in a Coulomb dissociation cross section, σ C, which is well reproduced by recent modified-RPA calculation. The pure cluster model then overestimates σ C by about 20%.

Target fragment mass distribution for the reaction of 13.3 GeV/nucleonSi28withAu197

We have measured the production cross sections for 59 target fragments from the interaction of 13.3 GeV/nucleon $^{28}\mathrm{Si}$ with $^{197}\mathrm{Au}$. We have deduced fragment mass-yield distributions from these data. The cross sections are consistent with limiting fragmentation (within \ifmmode\pm\else\textpm\fi{}20%). The electromagnetic dissociation cross section for the one-neutron removal process is in agreement with expectations although a large two-neutron removal cross section was observed.