Knockout Reactions Research Articles

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.

Two-proton decay from α -cluster states in C10 and N11

Two-proton decay from excited states in $^{10}\mathrm{C}$ and $^{11}\mathrm{N}$ has been characterized from an analysis of the $2p+2\ensuremath{\alpha}$ and $3p+2\ensuremath{\alpha}$ exit channels. Data from four previously published invariant-mass studies associated with inelastic excitation, multinucleon knockout, and neutron pickup reactions have been considered. A rotational band in $^{10}\mathrm{C}$ was identified built on the second ${0}^{+}$ state which has strong $\ensuremath{\alpha}$-cluster molecular structure. The members of this band were tentatively identified up to ${J}^{\ensuremath{\pi}}={4}^{+}$ and all identified states have a prompt $2p$ decay branch. The spins of the ${0}^{+}$ and ${2}^{+}$ members were deduced from comparisons of the momentum correlations in their $2p$ branches to those from known $2p$ emitters. The ${2}^{+}$ and the tentative ${4}^{+}$ members are both above the $\ensuremath{\alpha}$ threshold and exhibit large reduced decay widths for $\ensuremath{\alpha}$ emission. This band exhibits a similar moment of inertia to its analog in the mirror nucleus $^{10}\mathrm{Be}$. Evidence is also presented for a molecular band in $^{11}\mathrm{N}$ built on the second $3/{2}^{\ensuremath{-}}$ state which $2p$ decays. Tentative members of this band have been assigned up to ${J}^{\ensuremath{\pi}}=9/{2}^{\ensuremath{-}}$ and their excitation energies also match their likely analogs in the mirror nucleus $^{11}\mathrm{Be}$.

Short-range correlations in the extended quantum molecular dynamics model

Short-range potential has been added into an extended quantum molecular dynamics (EQMD) model. The rms radius, binding energy, and momentum distribution of $^{12}\mathrm{C}$ with different initial structures and short-range potential parameters have been checked. The separation energy of $^{12}\mathrm{C}(p,2p)^{11}\mathrm{B}$ reaction is calculated and compared with the experimental data, which indicates that our modified EQMD model can be taken as a reliable tool for studying proton pair knock-out reaction. Furthermore, the short-range correlation effects on emission time and momentum spectrum of two protons are discussed. Finally, the momentum correlation function of the emitted proton pair is calculated by Lednicky and Lyuboshitz's analytical method. The result explains that short-range repulsion causes high momentum tail and weakens the momentum correlation function.

Gene knock-out chain reaction enables high disruption efficiency of HPV18 E6/E7 genes in cervical cancer cells

A genome editing tool targeting the high-risk human papillomavirus (HPV) oncogene is a promising therapeutic strategy to treat HPV-related cervical cancer. To improve gene knockout efficiency, we developed a gene knockout chain reaction (GKCR) method for continually generating mutagenic disruptions and used this method to disrupt the HPV18 E6 and E7 genes. We verified that the GKCR Cas9/guide RNA (gRNA) cassettes could integrated into the targeted loci via homology-independent targeted insertion (HITI). The qPCR results revealed that the GKCR method enabled a relatively higher Cas9/gRNA cassette insertion rate than a control method (the common CRISPR-Cas9 strategy). Tracking of Indels by DEcomposition (TIDE) assay results showed that the GKCR method produced a significantly higher percentage of insertions or deletions (indels) in the HPV18 E6 and E7 genes. Furthermore, by targeting the HPV18 E6/E7 oncogenes, we found that the GKCR method significantly upregulated the P53/RB proteins and inhibited the proliferation and motility of HeLa cells. The GKCR method significantly improved the gene knockout efficiency of the HPV18 E6/E7 oncogenes, which might provide new insights into treatment of HPV infection and related cervical cancer.

“Piston” mechanism in a time-dependent two-level model

We analyze a recent study of the one-proton knockout reaction on $^{19}\mathrm{N}$ in the framework of a time-dependent two-level model. Focusing on the specific case of $^{18}\mathrm{C}$, we quantify the validity of the so-called ``piston'' mechanism in terms of the timescales involved in the process, namely, the reaction time, the period of oscillation between the mixed states, and the lifetime of the excited unbound state.

Examination of the sensitivity of quasifree reactions to details of the bound-state overlap functions

It is often stated that heavy-ion nucleon knockout reactions are mostly sensitive to the tails of the bound-state wavefunctions. In contrast, (p,2p) and (p,pn) reactions are known to access information on the full overlap functions within the nucleus. We analyze the oxygen isotopic chain and explore the differences between single-particle wave functions generated with potential models, used in the experimental analysis of knockout reactions, and ab initio computations from self-consistent Green's function theory. Contrary to the common belief, we find that not only the tail of the overlap functions, but also their internal part are assessed in both reaction mechanisms, which are crucial to yield accurately determined spectroscopic information.

Reaching into the N = 40 Island of Inversion with Nucleon Removal Reactions

One ambitious goal of nuclear physics is a predictive model of all nuclei, including the ones at the fringes of the nuclear chart which may remain out of experimental reach. Certain regions of the chart are providing formidable testing grounds for nuclear models in this quest as they display rapid structural evolution from one nucleus to another or phenomena such as shape coexistence. Observables measured for such nuclei can confirm or refute our understanding of the driving forces of the evolution of nuclear structure away from stability where textbook nuclear physics has been proven to not apply anymore. This paper briefly reviews the emerging picture for the very neutron-rich Fe, Cr, and Ti isotopes within the so-called N=40 island of inversion as obtained with nucleon knockout reactions. These have provided some of the most detailed nuclear spectroscopy in very neutron-rich nuclei produced at rare-isotope facilities. The results indicate that our current understanding, as encoded in large-scale shell-model calculations, appears correct with exciting predictions for the N=40 island of inversion left to be proven in the experiment. A bright future emerges with predictions of continued shell evolution and shape coexistence out to neutron number N=50, below 78Ni on the chart of nuclei.

Spectroscopic factors in dripline nuclei

Single-nucleon knockout reaction studies of the proton-dripline nuclei $^{9}\mathrm{C}$ and $^{13}\mathrm{O}$ suggest an appreciable suppression of spectroscopic factors. In this work, we calculate the one-neutron and one-proton spectroscopic factors for the mirror pair $^{9}\mathrm{C}\text{\ensuremath{-}}^{9}\mathrm{Li}$ and $^{13}\mathrm{O}$ using two variants of the continuum shell model: the complex-energy Gamow shell model and the real-energy shell model embedded in the continuum. Our results indicate that the continuum effects strongly suppress the spectroscopic factors of well-bound orbits in the dripline systems but have less impact on the spectroscopic factors of weakly bound states.

Spectroscopy of the T = 2 mirror nuclei 48Fe/48Ti using mirrored knockout reactions

A sequence of excited states has been established for the first time in the proton-rich nucleus 48Fe (Z=26, N=22). The technique of mirrored (i.e. analogue) one-nucleon knockout reactions was applied, in which the Tz= ±2 mirror pair, 48Fe/48Ti were populated via one-neutron/one-proton knockout from the secondary beams 49Fe/49V, respectively. The analogue properties of the reactions were used to help establish the new level scheme of 48Fe. The inclusive and exclusive cross sections were determined for the populated states. Large differences between the cross sections for the two mirrored reactions were observed and have been interpreted in terms of different degrees of binding of the mirror nuclei and in the context of the recent observations of suppression of spectroscopic strength as a function of nuclear binding, for knockout reactions on light solid targets. Mirror energy differences (MED) have been determined between the analogue T=2 states and compared with the shell model predictions. MED for this mirror pair, due to their location in the shell, are especially sensitive to excitations out of the f7/2 shell, and present a stringent test of the shell-model prescription.

Investigation of the ground-state spin inversion in the neutron-rich Cl47,49 isotopes

A first gamma-ray study of 47,49Cl spectroscopy was performed at the Radioactive Isotope Beam Factory with 50Ar projectiles at 217 MeV/nucleon, impinging on the liquid hydrogen target of the MINOS device. Prompt de-excitation gamma-rays were measured with the NaI(Tl) array DALI2+. Through the one-proton knockout reaction 50Ar(p,2p), a spin assignment could be determined for the low-lying states of 49Cl from the momentum distribution obtained with the SAMURAI spectrometer. A spin-parity J = 3/2+ is deduced for the ground state of 49Cl, similar to the recently studied N = 32 isotope 51K.

Coexisting normal and intruder configurations in 32Mg

Situated in the so-called “island of inversion,” the nucleus 32Mg is considered as an archetypal example of the disappearance of magicity at N=20. We report on high statistics in-beam spectroscopy of 32Mg with a unique approach, in that two direct reaction probes with different sensitivities to the underlying nuclear structure are employed at the same time. More specifically, states in 32Mg were populated by knockout reactions starting from 33Mg and 34Si, lying inside and outside the island of inversion, respectively. The momentum distributions of the reaction residues and the cross sections leading to the individual final states were confronted with eikonal-based reaction calculations, yielding a significantly updated level scheme for 32Mg and spin-parity assignments. By fully exploiting observables obtained in this measurement, a variety of structures coexisting in 32Mg was unraveled. Comparisons with theoretical predictions based on shell-model overlaps allowed for clear discrimination between different structural models, revealing that the complete theoretical description of this key nucleus is yet to be achieved.

Considering nonlocality in the optical potentials within eikonal models

Background: For its simplicity, the eikonal method is the tool of choice to analyze nuclear reactions at high energies ($E>100$ MeV/nucleon), including knockout reactions. However, so far, the effective interactions used in this method are assumed to be fully local.Purpose: Given the recent studies on nonlocal optical potentials, in this work we assess whether nonlocality in the optical potentials is expected to impact reactions at high energies and then explore different avenues for extending the eikonal method to include nonlocal interactions.Method: We compare angular distributions obtained for nonlocal interactions (using the exact R-matrix approach for elastic scattering and the adiabatic distorted wave approximation for transfer) with those obtained using their local-equivalent interactions.Results: Our results show that transfer observables are significantly impacted by nonlocality in the high-energy regime. Because knockout reactions are dominated by stripping (transfer to inelastic channels), nonlocality is expected to have a large effect on knockout observables too. Three approaches are explored for extending the eikonal method to nonlocal interactions, including an iterative method and a perturbation theory.Conclusions: None of the derived extensions of the eikonal model provide a good description of elastic scattering. This paper suggests that nonlocality removes the formal simplicity associated with the eikonal model.

Direct 16O-cluster knockout from 24Mg[formula omitted

The O(g.s.)16+8Be(g.s.) structure of the ground state of 24Mg nucleus has been probed experimentally using direct knockout reaction Mg24(O16,216O)8Be in the quasi free scattering kinematics domain. The direct 16O-cluster knockout finite range prediction indicates a fair amount of O(g.s.)16+8Be(g.s.) component in the Mg(g.s.)24.

Neutron-unbound states in Ne31

Background: The island of inversion near the $N=20$ shell gap is home to nuclei with reordered single-particle energy levels compared with the spherical shell model. Studies of $^{31}\mathrm{Ne}$ have revealed that its ground state has a halo component characterized by a valence neutron orbiting a deformed $^{30}\mathrm{Ne}$ core. This lightly bound nucleus with a separation energy of only ${S}_{n}=170$ keV is expected to have excited states that are neutron unbound.Purpose: The purpose of this experiment was to investigate the low-lying excited states in $^{31}\mathrm{Ne}$ that decay by the emission of a single neutron.Methods: An 89 MeV/nucleon $^{33}\mathrm{Mg}$ beam impinged on a segmented Be reaction target. Neutron-unbound states in $^{31}\mathrm{Ne}$ were populated via a two-proton knockout reaction. The $^{30}\mathrm{Ne}$ fragment and associated neutron from the decay of $^{31}\mathrm{Ne}$ were detected by the MoNA-LISA-Sweeper experimental setup at the National Superconducting Cyclotron Laboratory. Invariant-mass spectroscopy was used to reconstruct the two-body decay energy ($^{30}\mathrm{Ne}+n$).Results: The two-body decay energy spectrum exhibits two features: a low-lying peak at $0.30\ifmmode\pm\else\textpm\fi{}0.17$ MeV and a broad enhancement at $1.50\ifmmode\pm\else\textpm\fi{}0.33$ MeV, each fit with an energy-dependent asymmetric Breit-Wigner lineshape representing a resonance in the continuum. Accompanying shell-model calculations using the FSU interaction within NuShellX, combined with cross-section calculations using the eikonal reaction theory, indicate that these peaks in the decay energy spectrum are caused by multiple resonant states in $^{31}\mathrm{Ne}$.Conclusions: Excited states in $^{31}\mathrm{Ne}$ were observed for the first time. Transitions from calculated shell-model final states in $^{31}\mathrm{Ne}$ to bound states in $^{30}\mathrm{Ne}$ are in good agreement with the measured decay energy spectrum. Cross-section calculations for the two-proton knockout populating $^{31}\mathrm{Ne}$ states as well as spectroscopic factors pertaining to the decay of $^{31}\mathrm{Ne}$ into $^{30}\mathrm{Ne}$ are used to examine the results within the context of the shell-model expectations.

Isoscalar pairing correlations by the tensor force in the ground states of C12, O16, Ne20 , and S32 nuclei

We investigate roles of the tensor force (TF) on the pairing correlations in the ground-state structure of $^{12}\mathrm{C}, ^{16}\mathrm{O}, ^{20}\mathrm{Ne}$, and $^{32}\mathrm{S}$ nuclei within a deformed BCS model. For pairing matrix elements, we exploit the Brueckner $G$ matrix derived from the charge-dependent Bonn potential. In particular, the isoscalar (IS) neutron-proton ($np$) pairing is studied in detail in relation to spin-triplet even TF in the nucleon-nucleon interaction. Detailed analyses of the $np$ pairing and possible enhancement of the IS channel are performed by focusing on roles of attractive spin-triplet even and repulsive spin-triplet odd channels in the TF for the ground states of the nuclei with some deformation. It is also shown that the TF may play a crucial role of properly interpreting the experimental data of a two-nucleon knockout reaction from $^{12}\mathrm{C}$.

Opening angle and dineutron correlations in knockout reactions with Borromean two-neutron halo nuclei

Background: Knockout reactions with proton targets provide an invaluable tool to access the properties of two-neutron halo nuclei. Recently, experimental results for the average opening angle as a function of the intrinsic neutron momentum in $^{11}\mathrm{Li}$ have shown a localization of dineutron correlations on the nucleus surface.Purpose: Study the model dependence and the effect of distortion and absorption on the opening angle distributions to assess the reliability of this observable to extract properties of Borromean two-neutron halo nuclei.Method: A quasifree sudden model is used to describe the knockout process, where absorption effects are modeled by the eikonal $S$ matrix between the proton target and the core of the Borromean nucleus. Final states in momentum space are built within a three-body model for the projectile, which enables the description of momenta and opening angle distributions.Results: A strong dependence on absorption effects is found for the opening angle at large intrinsic momenta, while the region of lower momenta is mostly insensitive to them. Reasonable agreement with the available data is obtained for $^{11}\mathrm{Li}$ at low momenta with weights for $s$ and $p$ waves different from those previously reported, showing a model dependence in their extraction. For $^{19}\mathrm{B}$, test calculations show marked sensitivity to small $p$-wave components.Conclusions: The opening angle for $(p,pn)$ knockout reactions on Borromean nuclei at small intrinsic momenta is a reliable observable mostly sensitive to the structure of the Borromean nucleus. For larger momenta, the reaction mechanism leads to a larger distortion of the distribution. In the case of nuclei with small components of opposite parity to the dominant ones, this observable can be used to explore them. The relation between dineutron in coordinate space and opening angle in momentum space is found to be model dependent.

Halo effective field theory analysis of one-neutron knockout reactions of Be11 and C15

Background: One-nucleon knockout reactions provide insightful information on the single-particle structure of nuclei. When applied to one-neutron halo nuclei, they are purely peripheral, suggesting that they could be properly modeled by describing the projectile within a Halo Effective Field Theory (Halo-EFT). Purpose: We reanalyze the one-neutron knockout measurements of $^{11}$Be and $^{15}$C-both one-neutron halo nuclei-on beryllium at about 60MeV/nucleon. We consider Halo-EFT descriptions of these nuclei which already provide excellent agreement with breakup and transfer data. Method: We include a Halo-EFT description of the projectile within an eikonal-based model of the reaction and compare its outcome to existing data. Results: Excellent agreement with experiment is found for both nuclei. The asymptotic normalization coefficients inferred from this comparison confirm predictions from \emph{ab initio} nuclear-structure calculations and values deduced from transfer data. Conclusions: Halo-EFT can be reliably used to analyze one-neutron knockout reactions measured for halo nuclei and test predictions from state-of-the-art nuclear structure models on these experimental data.

Quenching of single-particle strengths in direct reactions

A discrepancy in the asymmetry dependence of spectroscopic factors extracted with different reaction probes calls into question whether the corresponding reaction models are properly understood. In this work, we present extracted spectroscopic factors from the Ar46,34(p,d)Ar45,33 transfer reactions in inverse kinematics at a beam energy of 70 MeV/nucleon. The results are consistent with previous measurements of these reactions at a lower beam energy [Lee et al., Phys. Rev. Lett. 104, 112701 (2010)], indicating that the transfer reaction is a reliable probe for the nuclear structure of exotic nuclei across a wide energy range. Results from a large body of transfer reaction measurements, (p,pN) measurements, and theoretical nuclear structure studies make a compelling case for much weaker asymmetry dependence than what is observed with single-nucleon knockout reactions on beryllium or carbon targets.Received 19 June 2020Revised 9 November 2020Accepted 8 June 2021DOI:https://doi.org/10.1103/PhysRevC.104.024608©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasDirect reactionsModels & methods for nuclear reactionsNuclear reactionsSpectroscopic factors & electromagnetic momentsTransfer reactionsNuclear Physics

In-beam γ -ray spectroscopy of Fe68 from charge exchange on Co68 projectiles

Excited states in the neutron-rich nucleus $^{68}\mathrm{Fe}$ were populated using a $^{9}\mathrm{Be}$($^{68}\mathrm{Co}, {}^{68}\mathrm{Fe}+\ensuremath{\gamma})\mathrm{X}$ charge-exchange reaction at 95 MeV/u. The new $\ensuremath{\gamma}$-ray transitions reported here for the first time complement data from $\ensuremath{\beta}$-decay studies and nucleon knockout reactions. In comparison to shell-model calculations with the LNPS effective interaction, two candidate states for the ${6}_{1}^{+}$ level emerge. The distinct population pattern of excited states and the magnitude of the cross section, ${\ensuremath{\sigma}}_{\mathrm{inc}}=0.51(6)$ mb, make this reaction a promising one for future in-beam $\ensuremath{\gamma}$-ray spectroscopy. Reaction calculations with nuclear structure input from a new, locally optimized Hamiltonian, $f7j4a$, together with general considerations for heavy-ion-induced charge-exchange reactions appear consistent with most of the observations, although challenges remain.

Contribution to inclusive (p,α) reactions from (p,pα) knockout at incident energies near 100 MeV

Background: The extent to which knockout, as opposed to a pickup reaction mechanism, contributes in pre-equilibrium $(p,\ensuremath{\alpha})$ reactions is still not clear. Even with exclusive $(p,\ensuremath{\alpha})$ reactions at conveniently low incident energies to well-defined final states, the issue often appears to be ambiguous. Recently Uozumi et al. [Y. Uozumi, Y. Fukuda, Y. Yamaguchi, G. Watanabe, and M. Nakano, Phys. Rev. C 102, 014604 (2020)] used the intranuclear cascade (INC) theory to study pre-equilibrium $(p,\ensuremath{\alpha})$ reactions in a range of incident energies from 42 to 300 MeV. They conclude that, below a projectile energy of about 120 MeV, knockout contributes almost nothing to the upper half of the pre-equilibrium $(p,\ensuremath{\alpha})$ yield. This result is in disagreement with several other studies.Purpose: We investigate whether existing $(p,p\ensuremath{\alpha})$ quasifree knockout results support the conclusion from the INC study.Method: The distorted-wave impulse approximation (DWIA) is used. The theory is known to give a good reproduction of experimental energy distributions and angular distributions, as well as absolute cross sections of $(p,p\ensuremath{\alpha})$ reactions at incident energies at and above 100 MeV. The DWIA is simply exploited as a convenient way of interpolation and extrapolation of the available experimental distributions.Results: The experimental cross-section distributions of the coincidence knockout reactions predict contributions to the pre-equilibrium yield much higher than those estimated by the INC study. At small scattering angles of only ${30}^{\ensuremath{\circ}}$ to ${40}^{\ensuremath{\circ}}$ the knockout yield is already quite substantial. At ${60}^{\ensuremath{\circ}}$ it accounts for approximately all of the pre-equilibrium yield.Conclusions: Even a lower-limit estimate of the coincident contribution at forward angles to the cross section of pre-equilibrium reactions is already sizable at low excitation. The cross section is expected to increase very rapidly at higher excitation energies (equivalently, lower $\ensuremath{\alpha}$-particle energy).