Quasifree Knockout Reactions Research Articles

Quenching of single-particle strengths of carbon isotopes 9-12,14-20C with knockout reactions for incident energies 43–2100 MeV/nucleon * *Supported by National Natural Science Foundation of China (U2067205,11775013,11775316), Fundamental Research Funds for the Central Universities (2020MS033) and Natural Science Foundation of Shanxi Province, China (202103021223047)

To study the quenching of single-particle strengths of carbon isotopes, a systematic analysis is performed for 9-12,14-20C, with single neutron knockout reactions on Be/C targets, within an energy range from approximately 43 to 2100 MeV/nucleon, using the Glauber model. Incident energies do not show any obvious effect on the resulting values across this wide energy range. The extracted quenching factors are found to be strongly dependent on the proton-neutron asymmetry, which is consistent with the recent analysis of knockout reactions but is inconsistent with the systematics of transfer and quasi-free knockout reactions.

Unveiling the two-proton halo character of 17Ne: Exclusive measurement of quasi-free proton-knockout reactions

The proton drip-line nucleus 17Ne is investigated experimentally in order to determine its two-proton halo character. A fully exclusive measurement of the 17Ne(p,2p)16F→15⁎O+p quasi-free one-proton knockout reaction has been performed at GSI at around 500 MeV/nucleon beam energy. All particles resulting from the scattering process have been detected. The relevant reconstructed quantities are the angles of the two protons scattered in quasi-elastic kinematics, the decay of 16F into 15O (including γ decays from excited states) and a proton, as well as the 15O+p relative-energy spectrum and the 16F momentum distributions. The latter two quantities allow an independent and consistent determination of the fractions of l=0 and l=2 motion of the valence protons in 17Ne. With a resulting relatively small l=0 component of only around 35(3)%, it is concluded that 17Ne exhibits a rather modest halo character only. The quantitative agreement of the two values deduced from the energy spectrum and the momentum distributions supports the theoretical treatment of the calculation of momentum distributions after quasi-free knockout reactions at high energies by taking into account distortions based on the Glauber theory. Moreover, the experimental data allow the separation of valence-proton knockout and knockout from the 15O core. The latter process contributes with 11.8(3.1) mb around 40% to the total proton-knockout cross section of 30.3(2.3) mb, which explains previously reported contradicting conclusions derived from inclusive cross sections.

Quasifree ( p,pN ) scattering of light neutron-rich nuclei near N=14

Background: For many years, quasifree scattering reactions in direct kinematics have been extensively used to study the structure of stable nuclei, demonstrating the potential of this approach. The (RB)-B-3 collaboration has performed a pilot experiment to study quasifree scattering reactions in inverse kinematics for a stable C-12 beam. The results from that experiment constitute the first quasifree scattering results in inverse and complete kinematics. This technique has lately been extended to exotic beams to investigate the evolution of shell structure, which has attracted much interest due to changes in shell structure if the number of protons or neutrons is varied. Purpose: In this work we investigate for the first time the quasifree scattering reactions (p, pn) and (p, 2p) simultaneously for the same projectile in inverse and complete kinematics for radioactive beams with the aim to study the evolution of single-particle properties from N = 14 to N = 15. Method: The structure of the projectiles O-23, O-22, and N-21 has been studied simultaneously via (p, pn) and (p, 2p) quasifree knockout reactions in complete inverse kinematics, allowing the investigation of proton and neutron structure at the same time. The experimental data were collected at the (RB)-B-3-LAND setup at GSI at beam energies of around 400 MeV/u. Two key observables have been studied to shed light on the structure of those nuclei: the inclusive cross sections and the corresponding momentum distributions. Conclusions: The knockout reactions (p, pn) and (p, 2p) with radioactive beams in inverse kinematics have provided important and complementary information for the study of shell evolution and structure. For the (p, pn) channels, indications of a change in the structure of these nuclei moving from N = 14 to N = 15 have been observed, i.e., from the 0d(5/2) shell to the 1s(1/2). This supports previous observations of a subshell closure at N = 14 for neutron-rich oxygen isotopes and its weakening for the nitrogen isotopes.

Quasifree (p, 2p) Reactions on Oxygen Isotopes: Observation of Isospin Independence of the Reduced Single-Particle Strength.

Quasifree one-proton knockout reactions have been employed in inverse kinematics for a systematic study of the structure of stable and exotic oxygen isotopes at the R^{3}B/LAND setup with incident beam energies in the range of 300-450 MeV/u. The oxygen isotopic chain offers a large variation of separation energies that allows for a quantitative understanding of single-particle strength with changing isospin asymmetry. Quasifree knockout reactions provide a complementary approach to intermediate-energy one-nucleon removal reactions. Inclusive cross sections for quasifree knockout reactions of the type ^{A}O(p,2p)^{A-1}N have been determined and compared to calculations based on the eikonal reaction theory. The reduction factors for the single-particle strength with respect to the independent-particle model were obtained and compared to state-of-the-art abinitio predictions. The results do not show any significant dependence on proton-neutron asymmetry.

Determination of a dineutron correlation in Borromean nuclei via a quasi-free knockout (p,pn) reaction

To discuss the dineutron correlation in the ground state, the quasi-free neutron knockout reaction on $^6$He is investigated.In the present work, the momentum distribution of the two emitted neutrons is calculated with the $\alpha$~+~$n$~+~$n$ three-body model and a simple reaction model to assess the effects of the knockout process via the $^5$He resonance and the target dependence in the momentum distribution. A clear signature of the dineutron correlation can be obtained by choosing the kinematical condition so the process via the $^5$He resonance is excluded because the $^5$He resonance drastically changes the momentum distribution. Using the proton target is important in the quantitative discussion on the dineutron correlation by the knockout reaction. In addition to theoretical arguments, a possible experimental setup to determine the dineutron correlation via the quasi-free knockout reaction is considered.

Uncovering the properties of exotic nuclei via reactions with relativistic radioactive beams

Measurements of reactions with relativistic radioactive beams provide powerful tools to extract properties of short-lived nuclei and nuclear matter. A brief overview is given by selecting a few examples with emphasis on knockout reactions and heavy-ion induced electromagnetic excitation. The latter is being utilized to extract the collective dipole response of exotic nuclei, e.g., to study the Pygmy dipole resonance which is related to the neutron skin in heavy neutron-rich nuclei. Knockout reactions are being used to study the shell structure of exotic nuclei. They give also access to investigate the role of nucleon-nucleon correlations in nuclei and nuclear matter as a function of neutron-proton asymmetry. A new technique based on a fully exclusive measurement of quasi-free knockout reactions in inverse kinematics is discussed.

Study of the 6He structure in the reaction of quasifree scattering 4He( 6He, 2 α)

In our experiment we studied three body momentum correlations of 6He breakup products emitted in the quasifree knockout reaction 4He( 6He, 2 α) at 6He beam energy 25 A MeV . For the first time a detailed study of nuclear structure was done in the reaction of quasifree scattering with an unbound system as a spectator. In the experiment we used a detection system of large angular acceptance allowing us to investigate the process in a wide range of kinematical parameters. It was shown that the data obtained in the experiment are well consistent with the predictions obtained with the use of the Plane Wave Impulse Approximation allowing for the final state interaction between the two spectator neutrons.

Continuum protons from 58Ni(p,p') at incident energies between 100 and 200 MeV.

The inclusive reaction $^{58}\mathrm{Ni}$(p,p') was studied at incident energies of 100, 120, 150, 175, and 200 MeV for scattering angles between 15\ifmmode^\circ\else\textdegree\fi{} and 120\ifmmode^\circ\else\textdegree\fi{}. Angular distributions at various emission energies are analyzed in terms of a quasifree knockout reaction mechanism, combined with parametrized systematics which should represent multistep contributions. Reasonable agreement between the experimental data and calculated distributions is obtained, with a consistent incident-energy dependence. The proportion of the quasifree component amounts to \ensuremath{\sim}30% of the preequilibrium yield. The quasifree contribution appears to be in agreement with the first-step yield of the statistical multistep-direct emission theory.

Quasifree knockout of charged particles from 4He with 100 MeV protons.

Exclusive measurements have been made of $^{4}\mathrm{He}$(p,2p), (p,pd), (p,pt), and (p,ph) at 100 MeV. The primary protons were measured at two angles, 45\ifmmode^\circ\else\textdegree\fi{} and 60\ifmmode^\circ\else\textdegree\fi{}, in coincidence with secondary protons, deuterons, tritons, or helions covering a wide range of angles on the opposite side of the beam, from -15\ifmmode^\circ\else\textdegree\fi{} to -90\ifmmode^\circ\else\textdegree\fi{} in plane, and from 0\ifmmode^\circ\else\textdegree\fi{} to 30\ifmmode^\circ\else\textdegree\fi{} out of plane. The satisfactory agreement in shape between the measured energy-sharing distributions and results of distorted-wave impulse approximation calculations suggests that this formalism is adequate for modeling the quasifree knockout reactions induced by 100 MeV protons on $^{4}\mathrm{He}$. It is found that (70\ifmmode\pm\else\textpm\fi{}10)% of the inclusive continuum yields in $^{4}\mathrm{He}$(p,p') is attributable to quasifree scattering, with \ensuremath{\sim}30% arising from multiple scattering. At low energies of the scattered proton, the knockout of nucleons constitutes over 90% of the quasifree component, whereas at high energies the knockout of deuterons, tritons, and helions dominates the yield. Consequently the cluster knockout contributions fill in the yield at higher energies in the inclusive spectra, and quasifree peaks are not discernible.

The 2H( 3He, 3He p)n reaction with 33-MeV polarized 3He ions

Triple-differential cross sections and vector analyzing powers have been measured for the 2H( 3He, 3He p)n reaction at E He = 32.5 MeV using polarized 3He ions. Data for 3He lab angles in the range 15°–30° and proton lab angles in the range 20°–60° are presented. The results are compared to both PWIA and DWIA calculations for a quasi-free knockout reaction. Energy-sharing distributions obtained for several combinations of θ He and θ p where quasi-free scattering (QFS) predominates are fairly-well reproduced by both types of calculation, but the DWIA calculations are better able to reproduce both the shapes and magnitudes of the quasi-elastic scattering maxima. The vector analyzing powers are generally small (¦ A y¦ <0.3) near the QFS peaks, and are in agreement with the values predicted by both PWIA and DWIA calculations where QFS predominates. At angle pairs where quasi-free scattering is less pronounced, the PWIA calculations are still in fair agreement with the data in some cases, whereas the DWIA predictions reproduce neither the cross section nor the analyzing-power data. For θ He = 30°, θ p = 45°, the cross section is dominated by the np final-state-interaction peak corresponding to singlet deuterons. The relatively structureless analyzing power at these angles is consistent with a two-stage reaction process.

Three-particle impulse approximation and d(N, 2N)N reactions

The three-particle impulse approximation with truncation (TIAT) has been applied to the investigation of d(p, 2p)n and d(p, pn)p quasi-free knock-out reactions (QFR) in the nonrelativistic scattering region (E<160 MeV). It is stated that the truncation procedure leads to the approximate restoration of the three-particle unitary relationships which are violated explicitly within the usual formulation of the single-collision mechanism. It is shown that the TIAT explains sufficiently well all the available experimental data for the value of the truncation parameter used by the authors in their earlier papers for the investigation of other QFR.

Spin orbit effects in quasifree knockout reactions

An efficient calculational method for distorted wave impulse approximation analyses of quasifree knockout reactions is described. Since the usual Racah algebra is eliminated, it is straightforward to include spin-orbit distortions. Results are presented for 150 MeV (p,2p) and (p,pn) reactions. In some cases the effects of the spin-orbit terms are large.

Distortion effects in (π, πp) quasifree knockout reactions

Distorted wave impulse approximation calculations for the ($\ensuremath{\pi}, \ensuremath{\pi}p$) quasifree knockout reaction on $^{16}\mathrm{O}$ and $^{40}\mathrm{Ca}$ have been carried out for a variety of bombarding energies and angle pairs. In general, the distortion effects arising from the interaction of the pions and protons with the target core are strong and vary with bombarding energy. The results emphasize the need to include these interactions properly in any analysis of experimental data.NUCLEAR REACTIONS DWIA reaction theory of proton knockout from nuclei $^{16}\mathrm{O}$, $^{40}\mathrm{Ca}({\ensuremath{\pi}}^{+}, {\ensuremath{\pi}}^{+}p)$, ${T}_{\ensuremath{\pi}}=100\ensuremath{-}300$ MeV, calculate $\ensuremath{\sigma}({\ensuremath{\theta}}_{1}, {\ensuremath{\theta}}_{2}, {E}_{1})$.

Distorted-Wave Analysis of the ReactionC12(π+, π+p)B11

A comparison of distorted-wave impulse-approximation calculations with recent $^{12}\mathrm{C}({\ensuremath{\pi}}^{+}, {\ensuremath{\pi}}^{+}p)^{11}\mathrm{B}$ data shows excellent agreement and demonstrates the applicability of the quasifree knockout reaction model. Both the shape and the magnitude of the energy-sharing distributions are well described by the calculations.

Quasi-free collisions of 400 MeV protons in deuterium and carbon

Abstract The first results from a program to compare quasi-free proton and neutron knockout from nuclei are presented. The binding energy of the s and p neutron hole states in 12 C have been obtained. Relative cross sections for proton and neutron knockout from 2 H and 12 C were determined for two very different kinematical conditions. The ratio, R of σ(p, pn) to σ(p, 2p) is in agreement with estimates based on the impulse approximation for 2 H but there is a deviation for 12 C. These observations suggest that refinements will have to be made to the theoretical ideas used to analyze quasi-free knockout reactions.