Longitudinal Momentum Distributions Research Articles

One-neutron knockout from 51–55 Sc

Results are presented from a one-neutron knockout experiment at relativistic energies of $ \approx 420 A$ MeV on 51-55Sc using the GSI Fragment Separator as a two-stage magnetic spectrometer and the MINIBALL array for gamma-ray detection. Inclusive longitudinal momentum distributions and cross-sections were measured enabling the determination of the contributions corresponding to knockout from the $ \nu p_{1/2}$ , $ \nu p_{3/2}$ , (L = 1 and $ \nu f_{7/2}$ , $ \nu f_{5/2}$ (L = 3 neutron orbitals. The observed L = 1 and L = 3 contributions are compared with theoretical cross-sections using eikonal knockout theory and spectroscopic factors from shell model calculations using the GXPF1A interaction. The measured inclusive knockout cross-sections generally follow the trends expected theoretically and given by the spectroscopic strength predicted from the shell model calculations. However, the deduced L = 1 cross-sections are generally 30-40% higher while the L = 3 contributions are about a factor of two smaller than predicted. This points to a promotion of neutrons from the $ \nu f_{7/2}$ to the $ \nu p_{3/2}$ orbital indicating a weakening of the N = 28 shell gap in these nuclei. While this is not predicted for the phenomenological GXPF1A interaction such a weakening is predicted by recent calculations using realistic low-momentum interactions $ V_{low k}$ obtained by evolving a chiral N3LO nucleon-nucleon potential.

Characteristic Spectrum of Very Low-Energy Photoelectron from Above-Threshold Ionization in the Tunneling Regime

We report an experimental and theoretical study of very low-energy photoelectrons in tunneling ionization process from noble gas atoms interacting with ultrashort intense infrared laser pulses. A universal peak structure with electron energy well below 1 eV in the photoelectron spectrum, corresponding to the double-hump structure in the longitudinal momentum distribution, is identified experimentally for all atomic species. Our quantum and semiclassical analysis reveal the role of long-range Coulomb potential in the production of this very low-energy peak structure.

One-neutron removal reactions on Al isotopes around theN=20shell closure

The one-neutron removal cross sections of neutron-rich Al isotopes and longitudinal momentum distributions of the residues have been measured for A=33 to 36 at relativistic energies (≈900 MeV/u). The inclusive data have been interpreted within the eikonal approximation. The evolution of the single-particle occupancy in the ground state of 33,34,35Al has been studied and compared with shell model predictions. The inferred 2s1/2 neutron occupancy in the 33Al ground-state wave function is 20% to 40% lower than the predicted one. The inclusive data do not exclude the presence of intruder states. Some intruder l=1 occupancy is found in 34Al, similarly to 33Mg. The single-particle 1f7/2 occupancy shows a gradual increase at N=22. Correspondingly, a decrease of the 1d3/2 strength has been observed.2 MoreReceived 4 October 2011DOI:https://doi.org/10.1103/PhysRevC.85.044312©2012 American Physical Society

Publisher's Note: Guided-atom laser: Transverse mode quality and longitudinal momentum distribution [Phys. Rev. A84, 043618 (2011)

Publisher's Note: Guided-atom laser: Transverse mode quality and longitudinal momentum distribution [Phys. Rev. A<b>84</b>, 043618 (2011)

Dispersion of longitudinal momentum distributions induced in fragmentation reactions

On the basis of systematic measurements of fragmentation reactions, which provide a detailed overview on the velocity distributions of residual nuclei, an improved description of the kinematical properties of the fragmentation residues is established. This work is dedicated to the fluctuations of their momentum distributions. In contrast to previous investigations, limited to close-to-projectile fragments, we extended our study to the entire production range, down to the lightest observed fragments. In this context, beside the contribution of abrasion and evaporation processes, we considered the effect of the thermal break-up on the width of the momentum distributions. Using approximated theoretical descriptions of the different reaction stages, a new analytical formula for the variance of the momentum distribution is derived, which is well adapted to technical applications.

Low-Energy Photoelectron Angular Distributions of Above-Threshold Ionization of Atoms and Molecules in Strong Laser Fields

We present high-resolution fully differential experimental data on low-energy photoelectron angular distributions (PADs) of above-threshold ionization of atoms (Xe), and diatomic (H2, O2) and triatomic (CO2) molecules at the same Keldysh parameter (~1.4) in near-infrared strong laser fields (25 fs, 795 nm). We observe that the electron zero longitudinal momentum distributions (along laser-polarization direction) show a feature of pronounced dip or peak for those targets. However, the low-energy PADs are similar, even though the binding energy and electronic core structure are very different. The PADs show that the dominant angular momentum of zero-order above-threshold ionization (E <; 0.5 eV) is L = 4 (ng Rydberg states). The experimental observations challenge the current qualitative models that are now only available for atoms. In the end, we discuss the role of Coulomb focusing and the effect of the electron longitudinal momentum on the transverse-momentum distribution.

Measurement of the longitudinal momentum distribution of30S after one-proton removal from31Cl

The breakup reactions of Cl-31 + C-12 at 44A MeV are investigated experimentally. The longitudinal momentum distribution of S-30 after one-proton removal from Cl-31 has been obtained. By assuming core plus a valence proton structure, the momentum distribution is studied using the few-body Glauber model. The calculation shows that the width of momentum distribution for a valence proton with l = 2 is consistent with the experimental data, which indicates a dominant d-wave component for the valence proton in Cl-31.

Structure of23Al from the one-proton breakup reaction and astrophysical implications

The ground state of the proton-rich nucleus 23Al has been studied by one-proton removal on a carbon target at about 50 MeV/nucleon using the EXOGAM + SPEG experimental setup at GANIL. Longitudinal momentum distributions of the 22Mg breakup fragments, inclusive and in coincidence with gamma rays de-exciting the residues, were measured. The ground-state structure of 23Al is found to be a configuration mixing of a d-orbital valence proton coupled to four core states - 0$^{+}_{gs}$, 2$^{+}_{1}$, 4$^{+}_{1}$, 4$^{+}_{2}$. We confirm the ground state spin and parity of 23Al as $J^{\pi} = 5/2^{+}$. The measured exclusive momentum distributions are compared with extended Glauber model calculations to extract spectroscopic factors and asymptotic normalization coefficients (ANCs). The spectroscopic factors are presented in comparison with those obtained from large-scale shell model calculations. We determined the asymptotic normalization coefficient of the nuclear system $^{23}$Al$_{gs}$ $\rightarrow$ $^{22}$Mg(0$^{+}$) + p to be $C^{2}_{d_{5/2}}$($^{23}Al_{gs}$) = (3.90 $\pm$ 0.44) $\times$ 10$^{3}$ fm$^{-1}$, and used it to infer the stellar reaction rate of the direct radiative proton capture $^{22}$Mg(p,$\gamma$)$^{23}$Al. Astrophysical implications related to $^{22}$Na nucleosynthesis in ONe novae and the use of one-nucleon breakup at intermediate energies as an indirect method in nuclear astrophysics are discussed.

Structure ofN22and theN=14subshell

One-neutron knockout data for N18−22 are analyzed in the eikonal approximation of the Glauber model. The role of the s-d shell and the crossing of the N=14 neutron subshell are discussed. Of particular interest is the nucleus N22, where the knockout data provide a sensitive experimental test for a possible halo structure of its ground state. The observation of a narrow momentum distribution of the N21 fragments, together with an essential 1s1/2 contribution needed to describe the observed longitudinal-momentum distribution, allow the firm conclusion that the ground state of N22 is a well-developed nuclear halo. The results also show that the N=14 subshell in N22 is somewhat reduced as compared to that of O23.Received 8 February 2011DOI:https://doi.org/10.1103/PhysRevC.83.064313©2011 American Physical Society

Inverse-kinematics one-neutron pickup with fast rare-isotope beams

Measurements and reaction model calculations are reported for single-neutron pickup reactions onto a fast $^{22}\mathrm{Mg}$ secondary beam at 84 MeV per nucleon. Measurements made on both carbon and beryllium targets, having very different structures, were used to investigate the likely nature of the pickup reaction mechanism. The measurements involve thick reaction targets and $\ensuremath{\gamma}$-ray spectroscopy of the projectile-like reaction residue for final-state resolution, which permit experiments with low incident beam rates compared to traditional low-energy transfer reactions. From measured longitudinal momentum distributions we show that the ${}^{12}\mathrm{C}({}^{22}\mathrm{Mg},{}^{23}\mathrm{Mg}+\ensuremath{\gamma})X$ reaction largely proceeds as a direct two-body reaction, with the neutron transfer producing bound $^{11}\mathrm{C}$ target residues. The corresponding reaction on the $^{9}\mathrm{Be}$ target seems to largely leave the $^{8}\mathrm{Be}$ residual nucleus unbound at excitation energies high in the continuum. We discuss the possible use of such fast-beam one-neutron pickup reactions to track single-particle strength in exotic nuclei and also their expected sensitivity to neutron high-$\ensuremath{\ell}$ (intruder) states, which are often direct indicators of shell evolution and the disappearance of magic numbers in the exotic regime.

Fully differential measurement on above-threshold ionization of CO and CO_2 molecules in strong laser fields

We present a fully differential measurement on above-threshold ionization of diatomic (CO) and linear triatomic (CO2) molecules in strong femtosecond laser fields (25fs, 795nm) at a laser intensity of ∼8×1013W/cm2. The photoelectron longitudinal momentum distribution of CO in the laser polarization direction shows a broader distribution near zero momentum in the laser polarization plane with a shoulder around 0.38a.u., whereas it shows a clear maximum for CO2. The relative photoelectron yields near zero longitudinal momentum are less dependent on the laser intensity. The difference may result from the molecular structure. Even though molecular targets have different binding energy and outer orbitals, the near-threshold photoelectrons have similar angular distribution, and the dominant angular momentum of the near-threshold photoelectron is L=4 (ng Rydberg state) for both targets.

Quantitative rescattering theory of correlated two-electron momentum spectra for strong-field nonsequential double ionization of helium

We study the correlated two-electron momentum spectra of the nonsequential double ionization (NSDI) of an atom in an intense laser field based on the recently developed quantitative rescattering (QRS) theory. According to the rescattering model, an electron that was released earlier in the laser pulse may return to collide again with the target ion. NSDI can occur directly if this electron knocks out another electron in a process similar to the ($e,2e$) process or indirectly by first exciting the target ion to an excited state which is later ionized by the laser field. Using QRS, we obtain the returning electron wave packet. By multiplying this wave packet with standard field-free ($e,2e$) differential cross sections, or with inelastic electron-impact excitation cross sections and the subsequent tunneling ionization, we obtain the correlated two-electron momentum spectra. The calculated spectra agree mostly with the experimental data. However, experimental data show additional features that cannot be accounted for by these two mechanisms only, and other mechanisms for NSDI are suggested. The contributions of these mechanisms to the longitudinal ion momentum distributions are also analyzed. The present quantum mechanical QRS calculation verifies the validity of rescattering model for the NSDI processes at the most fundamental and detailed level.

Fragmentation of 7Li relativistic nuclei on a proton into the 3H + 4He channel

In a track nuclear photoemulsion exposed to a beamof $^7$Li nuclei accelerated to a momentum of 3 GeV/$c$ per nucleon at the synchrophasotron of the Joint Institute for Nuclear Research (JINR, Dubna), 13 events in which $^7$Li nuclei interacting with protons break up into $^3$H and $^4$He fragments were detected among 3730 inelastic-interaction events. For this fragmentation channel, the cross section was found to be $8\pm2$ mb. The average value of the fragment total transverse momentum was $214\pm5$ MeV/$c$. This value exceedsmarkedly the average value of the transverse-momentumtransfer in the coherent dissociation of $^7$Li nuclei on track-emulsion nuclei ($166\pm5$ MeV/$c$). The recoil-proton transverse momentum was on average 98% of the total proton momentum. The longitudinal-momentum distribution of protons was characterized by a variance of 16 MeV/$c$ and a mean value of $37\pm2$ MeV/$c$.

A new approach to measure momentum distributions and production cross-sections of neutron rich nuclei using fragment separators

The longitudinal momentum distributions of 34 neutron-rich isotopes of elements produced by fragmentation of a 76Ge beam at 132 MeV/ u with 13 ≤ Z ≤ 27 were scanned using a novel experimental approach where a variety of targets with different thicknesses was used with the fragment separator at constant magnetic rigidity. In comparison to models that describe the shape and centroid of fragment momentum distributions, a parametrization based on the measured data was derived. Details of the transmission calculations, the analysis of their uncertainties, as well as the general analysis of momentum distributions and deduced cross-sections obtained with this approach are presented.

One-neutron knockout from 24–28Ne isotopes

One-neutron knockout reactions of 24–28Ne in a beryllium target have been studied in the Fragment Separator (FRS), at GSI. The results include inclusive one-neutron knockout cross-sections as well as longitudinal-momentum distributions of the knockout fragments. The ground-state structure of the neutron-rich neon isotopes was obtained from an analysis of the measured momentum distributions. The results indicate that the two heaviest isotopes, 27Ne and 28Ne, are dominated by a configuration in which a s1/2 neutron is coupled to an excited state of the 26Ne and 27Ne core, respectively.

Structure of 33Mg sheds new light on the [formula omitted] island of inversion

The first reaction spectroscopy on the ground state structure of 33Mg through the measurement of the longitudinal momentum distribution from the one-neutron removal reaction using a C target at 898A MeV is reported. The experiment was performed at the FRS, GSI. The distribution has a relatively narrow width (150±3 MeV/c (FWHM)) and the one-neutron removal cross-section is 74±4 mb. An increased contribution from the 2p3/2 orbital is required to explain the observation showing its lowering compared to existing model predictions. This provides new information regarding the configuration of 33Mg and the island of inversion.

Light-front quark distributions in the nucleon and nucleon electromagnetic form factors

Longitudinal and transverse quark momentum distributions in the nucleon are calculated from a phenomenological quark-nucleon vertex function obtained through an investigation of the nucleon electromagnetic form factors within a light-front framework.

Longitudinal and transverse parton momentum distributions for pion and nucleon within relativistic constituent quark models

Longitudinal and transverse quark momentum distributions for the pion and the nucleon are calculated from quark-hadron vertexes obtained by an investigat ion of electromagnetic form factors within relativistic constituent quark models. The no-helicity flip generalized parton distributions for the pion are evaluated within three different models. The relevance for the pion of the one-gluon-exchange dominance at short range for the behavior of the form factor at large momentum transfer and of the parton distributions at the end points is shown.

Longitudinal Recoil Ion momentum distributions for proton-argon ionizing collisions at intermediate energies

Proton-argon ionizing collisions at intermediate energies are studied by means of the classical trajectory Monte Carlo method (CTMC). In particular, we focus on the longitudinal and transverse recoil momentum distributions for the process. We compare the profile of the longitudinal momentum distribution at the abrupt rise region corresponding to the electron capture to the continuum (ECC) processes, with those obtained for simpler targets like H and He. Selected subsets of the data show that the recoiling multielectronic ion plays a major role in the low energy electron emission, influencing the angular asymmetry of the electronic emission.

Production of very neutron-rich nuclei with aGe76beam

Production cross sections for neutron-rich nuclei from the fragmentation of a $^{76}\mathrm{Ge}$ beam at 132 MeV/u were measured. The longitudinal momentum distributions of 34 neutron-rich isotopes of elements $13\ensuremath{\leqslant}Z\ensuremath{\leqslant}27$ were scanned using a novel experimental approach of varying the target thickness. Production cross sections with beryllium and tungsten targets were determined for a large number of nuclei, including 15 isotopes first observed in this work. These are the most neutron-rich nuclides of the elements $17\ensuremath{\leqslant}Z\ensuremath{\leqslant}25$ ($^{50}\mathrm{Cl}$, $^{53}\mathrm{Ar}$, $^{55,56}\mathrm{K}$, $^{57,58}\mathrm{Ca}$, $^{59,60,61}\mathrm{Sc}$, $^{62,63}\mathrm{Ti}$, $^{65,66}\mathrm{V}$, $^{68}\mathrm{Cr}$, and $^{70}\mathrm{Mn}$). A one-body ${Q}_{g}$ systematics is used to describe the production cross sections based on thermal evaporation from excited prefragments. Some of the fragments near $^{58}\mathrm{Ca}$ show anomalously large production cross sections.