Two-neutron Removal Reactions Research Articles

Two-neutron knockout as a probe of the composition of states in Mg22,Al23 , and Si24

Simpson and Tostevin proposed that the width and shape of exclusive parallel momentum distributions of the A-2 residue in direct two-nucleon knockout reactions carry a measurable sensitivity to the nucleon single-particle configurations and their couplings within the wave functions of exotic nuclei. We report here on the first benchmarks and use of this new spectroscopic tool. Exclusive parallel momentum distributions for states in the neutron-deficient nuclei $^{22}$Mg, $^{23}$Al, and $^{24}$Si populated in such direct two-neutron removal reactions were extracted and compared to predictions combining eikonal reaction theory and shell-model calculations. For the well-known $^{22}$Mg and $^{23}$Al nuclei, measurements and calculations were found to agree, supporting the dependence of the parallel momentum distribution width on the angular momentum composition of the shell-model two-neutron amplitudes. In $^{24}$Si, a level at 3439(9) keV, of relevance for the important $^{23}$Al(p,$\gamma$)$^{24}$Si astrophysical reaction rate, was confirmed to be the $2^+_2$ state, while the $4^+_1$ state, expected to be strongly populated in two-neutron knockout, was not observed. This puzzle is resolved by theoretical considerations of the Thomas-Ehrman shift, which also suggest that a previously reported 3471-keV state in $^{24}$Si is in fact the ($0^+_2$) level with one of the largest experimental mirror-energy shifts ever observed.

Pairing properties of the double- β emitter Cd116

The pairing properties of the neutrinoless double-$\beta$ decay candidate $^{116}$Cd have been investigated. Measurements of the two-neutron removal reactions on isotopes of $^{114,116}$Cd have been made in order to identify 0$^+$ strength in the residual nuclei up to $\approx$3 MeV. No significant $L=0$ strength has been found in excited states indicating that the Bardeen-Cooper-Schrieffer (BCS) approximation is a reasonable basis to describe the neutrons in the ground state. This approximation avoids complications in calculations of double-$\beta$ decay matrix elements that use the quasiparticle random-phase approximation (QRPA) techniques. However this is not the case for the protons, where pair vibrations are prevalent and the BCS approximation is no longer valid, complicating the use of traditional QRPA techniques for this system as a whole.

Shape coexistence and isospin symmetry in A = 70 nuclei: Spectroscopy of the Tz = −1 nucleus 70Kr

Excited states in the Tz=−1 nucleus 70Kr have been populated using inelastic scattering of a radioactive 70Kr beam as well as one- and two-neutron removal reactions from 71,72Kr at intermediate beam energies. The level scheme of 70Kr was constructed from the observed γ-ray transitions and coincidences. Tentative spin and parity assignments were made based on comparison with the mirror nucleus 70Se. A second 2+ state and a candidate for the corresponding 42+ state suggest shape coexistence in 70Kr.

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.

Low- Z shore of the “island of inversion” and the reduced neutron magicity toward O28

The two odd-even fluorine isotopes $^{27,29}\mathrm{F}$ were studied via in-beam $\ensuremath{\gamma}$-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. A secondary beam of $^{30}\mathrm{Ne}$ was used to induce one-proton and one-proton--two-neutron removal reactions on carbon and polyethylene targets at midtarget energies of 228 MeV/$u$. Excited states were observed at 915(12) keV for $^{27}\mathrm{F}$ and at 1080(18) keV for $^{29}\mathrm{F}$. Both were assigned a $1/{2}_{1}^{+}$ spin and parity. The low transition energy for $^{29}\mathrm{F}$ largely disagrees with shell model predictions restricted to the $sd$ model space. Calculations using effective interactions that include the neutron $pf$ shell indicate that the $N=20$ gap is quenched for $^{29}\mathrm{F}$, thus extending the ``island of inversion'' to isotopes with proton number $Z=9$. Variations of the $N=20$ gap further reveal a strong correlation to the $1/{2}_{1}^{+}$ level energy in $^{29}\mathrm{F}$ and suggest a persistent reduced neutron gap for $^{28}\mathrm{O}$.

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.

Eikonal Reaction Theory for One- and Two-Neutron Removal Reactions

We present an accurate method of treating neutron removal reactions and it’s applications. According to the method, the nuclear and Coulomb breakup processes are consistently treated by the method of the continuum discretized coupled channels. This method is referred to as the eikonal reaction theory (ERT). We analyze the two types of removal reactions of 31Ne and 6He with ERT.

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

The structure of 19,20,22C has been investigated using high-energy (around 240 MeV/nucleon) one- and two-neutron removal reactions on a carbon target. Measurements were made of the inclusive cross sections and momentum distributions for the charged residues. Narrow momentum distributions were observed for one-neutron removal from 19C and 20C and two-neutron removal from 22C. Two-neutron removal from 20C resulted in a relatively broad momentum distribution. The results are compared with eikonal-model calculations combined with shell-model structure information. The neutron removal cross sections and associated momentum distributions are calculated for transitions to both the particle-bound and particle-unbound final states. The calculations take into account the population of the mass A−1 reaction residues A−1C and, following one-neutron emission after one-neutron removal, the mass A−2 two-neutron removal residues A−2C. The smaller contributions of direct two-neutron removal, that populate the A−2C residues in a single step, are also computed. The data and calculations are shown to be in good overall agreement and consistent with the predicted shell-model ground-state configurations and one-neutron overlaps with low-lying states in 18−21C. These suggest significant νs1/22 valence neutron configurations in both 20C and 22C. The results for 22C strongly support the picture of 22C as a two-neutron halo nucleus with a dominant νs1/22 ground-state configuration.Received 30 November 2011DOI:https://doi.org/10.1103/PhysRevC.86.054604©2012 American Physical Society

One- and two-neutron removal reactions from19,20C with a proton target

One- and two-neutron removal reactions from 19C and 20C have been studied using a liquid-hydrogen target at 40A MeV. A small cross section has been observed in the one-neutron removal reaction from 20C. The observed inclusive removal cross sections are compared with theoretical removal cross sections calculated by using shell model spectroscopic factors and Glauber-model single-particle cross sections. The observed momentum distributions are also compared with those calculated by using continuum-discretized coupled-channel methods. Good consistency between theory and experiment is shown in the one-neutron removal reaction from 19C. However, our theoretical calculation fails to reproduce the neutron removal reactions from 20C, which suggests that further improvements of the theoretical descriptions are necessary.Received 11 June 2011DOI:https://doi.org/10.1103/PhysRevC.84.064315©2011 American Physical Society

Measurement of the reaction cross section ofC18and observations of fragments fromC17andC18at80AMeV

The one- and two-neutron removal reactions from $^{17}\mathrm{C}$ and $^{18}\mathrm{C}$ as well as the reaction cross section of $^{18}\mathrm{C}$ have been studied using a carbon target at $80A$ MeV. The longitudinal momentum distributions of $^{15,16}\mathrm{C}$ fragments from $^{17}\mathrm{C}$ and $^{16,17}\mathrm{C}$ fragments from $^{18}\mathrm{C}$ were measured by a direct time-of-flight method. The width of $^{15}\mathrm{C}$ fragments from $^{17}\mathrm{C}$ is fairly smaller than that from other C isotopes. The experimental data are discussed within the framework of the Glauber model.

Diffraction dissociation contributions to two-nucleon knockout reactions and the suppression of shell-model strength

The contributions to the cross sections of intermediate energy two-nucleon knockout reactions from events in which one nucleon is removed by the stripping (inelastic breakup) mechanism and a second by the diffraction (elastic breakup) mechanism are discussed. The small additional contributions from two-nucleon diffraction events are also estimated. The approach used combines the eikonal reaction and shell model structure theory frameworks. For reactions involving the removal of two well-bound like nucleons, at incident energies of order 100 MeV per nucleon, the additional cross sections are shown to be of approximately the same size as those from events in which both nucleons are stripped in inelastic interactions. These more complete dynamical calculations now permit a quantitative comparison of the theoretical cross sections with recent partial cross-section measurements of the two-neutron (two-proton) removal reactions from neutron-deficient (neutron-rich) nuclei. As has been observed in both nuclear- and electron-induced single-nucleon knockout reaction analyses, the theoretical two-nucleon knockout cross sections overestimate the measured values, requiring a suppression of the two nucleon shell-model transition strengths. The deduced two-nucleon suppression factors, ${R}_{s}(2N)$, are consistent with a value of 0.5 for each of the five reactions considered.

Two-neutron knockout from neutron-deficientAr34,S30, andSi26

Two-neutron knockout reactions from nuclei in the proximity of the proton dripline have been studied using intermediate-energy beams of neutron-deficient $^{34}\mathrm{Ar}$, $^{30}\mathrm{S}$, and $^{26}\mathrm{Si}$. The inclusive cross sections, and also the partial cross sections for the population of individual bound final states of the $^{32}\mathrm{Ar}$, $^{28}\mathrm{S}$ and $^{24}\mathrm{Si}$ knockout residues, have been determined using the combination of particle and \ensuremath{\gamma}-ray spectroscopy. Similar to the two-proton knockout mechanism on the neutron-rich side of the nuclear chart, these two-neutron removal reactions from already neutron-deficient nuclei are also shown to be consistent with a direct reaction mechanism.

One-neutron halo structure inC15

The one- or two-neutron removal reactions as well as reaction cross sections for C-14,C-15 on carbon target have been studied by using 110A MeV Ne-22 primary beam on Riken Projectile Fragment Separator in RIKEN. The longitudinal momentum distributions of C-13,C-14 fragments from C-15 and C-13 fragments from C-14 breakup have been measured at 83A MeV by means of direct time-of-flight method. Full width at half maximum (FWHM) of the distributions have been determined to be 71+/-9 MeV/c and 223+/-28 MeV/c for C-14 and C-13 from C-15, and 195+/-21 MeV/c for C-13 from C-14. The FWHM for C-13 fragments from C-15 and C-14 breakup are consistent with the Goldhaber model's prediction. While the FWHM of C-14 fragments from C-15 is much smaller, which confirms the experimental results from MSU and GANIL, an anomalous enhancement from its neighbors has been observed in the measured reaction cross section of C-15. The experimental data are discussed in the framework of the Glauber model. The analysis of both the fragment momentum distributions and reaction cross sections indicates a dominant s-wave component in the ground state of C-15.

Two-neutron removal reactions for very neutron-rich nuclei

The two-neutron removal reactions of beams of 11Li, 14Be and 8He upon Be, Ni and Au targets were studied at 30 MeV/u. The cross sections and the neutron forward angular distributions were measured; they correlate strongly with the two-neutron separation energy of the projectile. Even though the coverage of the neutron detectors was limited, a rough neutron-neutron distribution could be extracted. A simplified interpretation of the data is presented.

Coulomb Dissociation of the Weakly-Bound Nucleus 11Li

Dissociation cross sections are calculated for the reaction llLi+Tar~et (with charge ZTe)->9Li + X by using the Glauber approximation, modified suitably for two-neutron removal reactions. It is found that the effect of finite qJ., the transverse component of momentum transfer, alters target· charge dependence of the Coulomb dissociation cross section as OC rx ZTl.74, while· the normal depen­ dence OC rx ZT2 is obtained if no such effect is included. Using a technique of secondary radioactive beams, Kobayashi et al.