Two-proton Knockout Reactions Research Articles

In-beam γ -ray spectroscopy of Mg32 via direct reactions

Background: The nucleus $^{32}$Mg ($N=20$ and $Z=12$) plays a central role in the so-called "island of inversion" where in the ground states $sd$-shell neutrons are promoted to the $fp$-shell orbitals across the shell gap, resulting in the disappearance of the canonical neutron magic number $N=20$. Purpose: The primary goals of this work are to extend the level scheme of $^{32}$Mg, provide spin-parity assignments to excited states, and discuss the microscopic structure of each state through comparisons with theoretical calculations. Method: In-beam $\gamma$-ray spectroscopy of $^{32}$Mg was performed using two direct-reaction probes, one-neutron (two-proton) knockout reactions on $^{33}$Mg ($^{34}$Si). Final-state exclusive cross sections and parallel momentum distributions were extracted from the experimental data and compared with eikonal-based reaction model calculations combined with shell-model overlap functions. Results: Owing to the remarkable selectivity of the one-neutron and two-proton knockout reactions, a significantly updated level scheme for $^{32}$Mg, which exhibits negative-parity intruder and positive-parity normal states, was constructed. The experimental results were confronted with four different nuclear structure models. Conclusions: In some of these models, different aspects of $^{32}$Mg and the transition into the island of inversion are well described. However, unexplained discrepancies remain, and even with the help of these state-of-the-art theoretical approaches, the structure of this key nucleus is not yet fully captured.

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.

Towards a consistent understanding of the exotic nucleus 4214Si28

The issue of magicity of has been a contentious one. Fridmann et al., through studies of two-proton knockout reaction , presented strong evidence in support of magicity and sphericity of . However in complete conflict with this, Bastin et al., gave equally strong empirical evidence, to show that the N = 28 magicity had completely collapsed in to make it well deformed. The consensus at present though is in favour of the validity of the latter experiment. However, our QCD based theoretical model supports the result of Fridmann experiment. They had explored the amazing persistence of the unique exotic nucleus as a stable structure within the nucleus; even after stripping off six-protons through the isotonic chain:. Thus it is the novel and unexpected stability of proton shell closure at Z=14 in, which is playing such a dominant role in ensuring its magicity, while the neutron magic number N = 28, goes into hiding or actually disappears. Recently, SAA has shown that the fusion experiment of a beam of halo nucleus 6He with the target nucleus 238U , actually provided strong evidence that the “core” of the halo nucleus has the structure of a tennis-ball (bubble) like nucleus, with a “hole” at the centre of its density distribution. This provides us with clear-cut support for our Quantum Chromodynamics based model of clusters of tritons in neutron-rich nuclei. Here we show that our QCD based model, provides support to Fridmann et al. , showing that, has a spherically magic structure of 14 (14-tritons) with a tennis-ball (bubble) like structure with “hidden” N=28 neutrons.

Spectroscopy of strongly deformed Ne32 by proton knockout reactions

Low lying states of neutron-rich 32Ne were populated by means of one- and two-proton knock- out reactions at the RIKEN Radioactive Isotope Beam Factory. A new transition is observed at 1410(15) keV and assigned to the 4+ 1 → 2+ 1 decay. With this energy the R4/2 ratio is calculated to be 2.99(6), lying close to the rigid rotor limit and suggests a high degree of collectivity and strongest deformation among neutron-rich Neon isotopes. Comparisons of experimental inclusive and exclu- sive reaction cross sections with shell model and eikonal reaction dynamical calculations reveals considerable quenching for this highly asymmetric system and contributes to systematic trends.

Kinematical Analysis of the Neutrino-induced Two-proton Knock-out Reactions

Kinematical Analysis of the Neutrino-induced Two-proton Knock-out Reactions

Search for the15Be ground state

A two-proton knockout reaction from a ${}^{17}$C beam at 55 MeV/u was used to produce the unbound nucleus ${}^{15}$Be. No significant number of events were observed for the decay of ${}^{15}$Be into a neutron and a ${}^{14}$Be fragment. An upper limit for the ${}^{14}$Be production cross section of 0.079$\phantom{\rule{0.16em}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{0.16em}{0ex}}$0.038 mb was extracted, which is more than an order of magnitude smaller than the predicted two-proton knockout reaction cross section. Based on these results we conclude that any populated states in ${}^{15}$Be decay through three sequential neutron decays into ${}^{12}$Be, via the unbound first excited state of ${}^{14}$Be. Therefore, these states in ${}^{15}$Be must be neutron unbound by more than 1.54 MeV, which is the location of the first excited 2${}^{+}$ state in ${}^{14}$Be.

Triple configuration coexistence inS44

The neutron-rich $N=28$ nucleus $^{44}\mathrm{S}$ was studied using the two-proton knockout reaction from $^{46}\mathrm{Ar}$ at intermediate beam energy. We report the observation of four new excited states, one of which is a strongly prolate deformed 4${}^{+}$ state, as indicated by a shell-model calculation. Its deformation originates in a neutron configuration which is fundamentally different from the ``intruder'' configuration responsible for the ground-state deformation. Consequently, we do not have three coexisting shapes in $^{44}\mathrm{S}$, but three coexisting configurations, corresponding to zero-, one-, and two-neutron particle-hole excitations.

Two-proton knockout fromMg32: Intruder amplitudes inNe30and implications for the binding ofF29,31

gamma-ray decays from excited states in {sup 30}Ne and inclusive and exclusive cross sections were measured in the {sup 9}Be({sup 32}Mg,{sup 30}Ne+gamma)X two-proton knockout reaction at incident beam energies of 99.7 and 86.7 MeV/nucleon. The measured cross section is suppressed compared to calculations and is indicative of a reduced overlap of initial and final state wave functions in {sup 32}Mg and {sup 30}Ne. We interpret this reduction as due to large 4p4h amplitudes present in the {sup 30}Ne ground state wave function, but not {sup 32}Mg. Such large intruder components are predicted to help stabilize the heavier fluorine isotopes against neutron emission.

Density dependence of single-proton and two-proton knockout reactions under quasifree conditions

We consider high-energy quasifree single- and two-proton knockout reactions induced by electrons and protons and address the question of what target-nucleus densities can be effectively probed after correcting for nuclear attenuation (initial- and final-state interactions). Our calculations refer to ejected proton kinetic energies of 1.5 GeV, the reactions $(e,{e}^{'}p)$, $(\ensuremath{\gamma},\mathit{pp})$, and $(p,2p)$, and a carbon target. It is shown that each of the three reactions is characterized by a distinctive sensitivity to the density of the target nucleus. The bulk of the $(\ensuremath{\gamma},\mathit{pp})$ strength stems from the high-density regions in the deep nuclear interior. Despite the strong attenuation, sizable densities can be probed by $(p,2p)$ reactions provided that the energy resolution allows one to pick nucleons from $s$ orbits. The effective mean densities that can be probed in high-energy $(e,{e}^{'}p)$ reactions is of the order of 30%--50% of the nuclear saturation density.

Neutron decay spectroscopy of neutron-rich oxygen isotopes

Neutron decay spectroscopy of neutron-rich oxygen isotopes has been performed using the two-proton knock-out reaction 9Be( 26Ne, X) 24,23,22O. A combination of the knock-out of valence and core protons can explain the three observed spectra. These knock-out processes are selective and preferentially populate hole states. The observed narrow resonance state in 23O at an excitation energy of 2.8(1) MeV was assigned to the 5 / 2 + state.

In-beam γ-ray spectroscopy and inclusive two-proton knockout cross section measurements atN≈40

In-beam gamma-ray spectroscopy of Fe-66,Fe-68 following intermediate-energy one- and two-proton knockout from cobalt and nickel secondary beams has been performed at the National Superconducting Cyclotron Laboratory. New transitions have been observed in Fe-66 and Fe-68. This is the first observation of gamma-ray transitions in Fe-68. In addition, Cr-64 was produced using the Be-9(Fe-66,Cr-64)X two-proton knockout reaction. An unexpectedly low inclusive cross section is observed for this reaction, an order of magnitude smaller than for the Be-9(Ni-68,Fe-66)X reaction. This observation is discussed in terms of a significant structural difference between Fe-66 and Cr-64 and considerable admixtures of nu(pf)(n-2) (g(9/2))(+2) configurations in the ground and excited states of Cr-64 at N = 40.

Spectroscopy ofMg36: Interplay of Normal and Intruder Configurations at the Neutron-Rich Boundary of the “Island of Inversion”

We report on the first spectroscopy study of the very neutron-rich nucleus (36)(12)Mg24 using the direct two-proton knockout reaction 9Be(38Si,36Mg+gamma)X at 83 MeV/nucleon. The energy of the first excited 2+ state of 36Mg, E(2+(1)=660(6) keV, was measured. The magnitude of the partial cross sections to the ground state and the 2+(1) state is indicative of strong intruder admixtures in the lowest-lying states as suggested by Monte Carlo shell-model calculations.

Center-of-mass effects in electromagnetic two-proton knockout reactions

The role of center-of mass (CM) effects in the one-body nuclear current in the description of electromagnetically induced two-nucleon knockout reactions is discussed in connection with the problem of the lack of orthogonality between initial bound states and final scattering states obtained by the use of an energy-dependent optical model potential. Results for the cross sections of the exclusive $^{16}$O(e,e$'$pp)$^{14}$C and $^{16}$O($\gamma$,pp)$^{14}$C knockout reactions in different kinematics are presented and discussed. In super-parallel kinematics CM effects produce a strong enhancement of the $^{16}$O(e,e$'$pp)$^{14}$C$_{\rm g.s.}$ cross section which strongly reduces the destructive interference between the one-body and $\Delta$-current and the sensitivity to the treatment of the $\Delta$-current found in previous work.

Shell structure atN=28near the dripline: Spectroscopy ofSi42,P43, andS44

Measurements of the $N=28$ isotones $^{42}\mathrm{Si}$, $^{43}\mathrm{P}$, and $^{44}\mathrm{S}$ using one- and two-proton knockout reactions from the radioactive beam nuclei $^{44}\mathrm{S}$ and $^{46}\mathrm{Ar}$ are reported. The knockout reaction cross sections for populating $^{42}\mathrm{Si}$ and $^{43}\mathrm{P}$ and a 184 keV \ensuremath{\gamma}-ray observed in $^{43}\mathrm{P}$ establish that the ${d}_{3/2}$ and ${s}_{1/2}$ proton orbits are nearly degenerate in these nuclei and that there is a substantial $Z=14$ subshell closure separating these two orbits from the ${d}_{5/2}$ orbit. The increase in the inclusive two-proton knockout cross section from $^{42}\mathrm{Si}$ to $^{44}\mathrm{S}$ demonstrates the importance of the availability of valence protons for determining the cross section. New calculations of the two-proton knockout reactions that include diffractive effects are presented. In addition, it is proposed that a search for the ${d}_{5/2}$ proton strength in $^{43}\mathrm{P}$ via a higher statistics one-proton knockout experiment could help determine the size of the $Z=14$ closure.

Cross-shell excitation in two-proton knockout: Structure ofCa52

The two-proton knockout reaction $^{9}\mathrm{Be}$($^{54}\mathrm{Ti}$,$^{52}\mathrm{Ca}+\ensuremath{\gamma}$) has been studied at 72 MeV/nucleon. Besides the strong feeding of the $^{52}\mathrm{Ca}$ ground state, the only other sizeable cross section proceeds to a ${3}^{\ensuremath{-}}$ level at 3.9 MeV. There is no measurable direct yield to the first excited ${2}^{+}$ state at 2.6 MeV. The results illustrate the potential of such direct reactions for exploring cross-shell proton excitations in neutron-rich nuclei and confirms the doubly-magic nature of $^{52}\mathrm{Ca}$.

Two-proton knockout on neutron-rich nuclei

Two-proton knockout reactions on neutron-rich nuclei [Phys. Rev. Lett. 91 (2003) 012501] have been studied in inverse kinematics at intermediate energy. Strong evidence that the two-proton removal from a neutron-rich system proceeds as a direct reaction is presented, together with a preliminary theoretical discussion of the partial cross sections based on eikonal reaction theory and the many-body shell model. They show that this reaction can be used to characterize the wave functions of the projectiles and holds great promise for the study of neutron-rich nuclei.