12C Target Research Articles

Few-body systems with [formula omitted] and −2

The structure of the T = 1 iso-triplet hypernuclei, He Λ 7 , Li Λ 7 and Be Λ 7 within the framework of an α + Λ + N + N four-body cluster model is studied. A phenomenological Λ N charge symmetry breaking (CSB) interaction is introduced. The Λ binding energy of the ground state in He Λ 7 is predicted to be 5.16 ( 5.36 ) MeV with(without) the CSB interaction. The five-body cluster model calculation of α α n Λ Λ for Be Λ Λ 11 , which has been observed as Hida event, is performed. The calculated B Λ Λ is 18.23 MeV , which is less bound by about 2 MeV than the experimental data. The structure calculation in Li Ξ − 7 and Li Ξ − 10 are performed within the framework of α Ξ − n n and α α Ξ − n four-body model. Li Ξ − 7 and Li Ξ − 10 are predicted to have bound states. We propose to observe the bound states in future ( K − , K + ) experiments using 7Li and 10B targets in addition to the standard 12C target.

Minor actinide fission induced by multi-nucleon transfer reaction in inverse kinematics

In the framework of nuclear waste incineration and design of new generation nuclear reactors, experimental data on fission probabilities and on fission fragment yields of minor actinides are crucial to design prototypes. Transfer-induced fission has proven to be an efficient method to study fission probabilities of actinides which cannot be investigated with standard techniques due to their high radioactivity. We report on the preliminary results of an experiment performed at GANIL that investigates fission probabilities with multi-nucleon transfer reactions in inverse kinematics between a 238U beam on a 12C target. Actinides from U to Cm were produced with an excitation energy range from 0 to 30 MeV. In addition, inverse kinematics allowed to characterize the fission fragments in mass and charge. A key point of the analysis resides in the identification of the actinides produced in the different transfer channels. The new annular telescope SPIDER was used to tag the target-like recoil nucleus of the transfer reaction and to determine the excitation energy of the actinide. The fission probability for each transfer channel is accessible and the preliminary results for 238U are promising.

Systematic behavior in the isospin dependence of projectile fragmentation of mirror nuclei with A = 20–60

The cross sections of the fragments that are produced by the 80 A MeV |Tz| = 2 mirror nuclei 20Mg/20O, 24Si/24Ni, 36Ca/36S, 40Ti/40Ar, 44Ca/44Cr and 60Ni/60Ge colliding on the 12C target are calculated using the statistical abrasion–ablation model. The isospin dependence of the projectile fragmentation (or the isospin effect) in the fragment cross section distributions of the A = 20–60 mirror projectile nuclei is studied systematically. The isospin effect in the fragment cross section distributions is found to decrease and even disappear with increasing violence of the collisions. The isospin effect is found strongly related to the different proton and neutron density distributions. It is suggested that the isospin effect in the fragment cross section distributions may serve as an observation to extract the density distributions of asymmetric nucleus.

Quasi-Elastic Scattering of 16C from 12C at 47.5 MeV/Nucleon

Differential cross sections for the quasi-elastic scattering of16 C at 47.5 MeV/nucleon from 12C target are measured. Coupled-channels calculations are carried out and the optical potential parameters are obtained by fitting the experimental angular distribution.

Particle identification with time-of-flight and pulse-shape discrimination in neutron-transmutation-doped silicon detectors

A method for the identification of energetic charged particles has been investigated based on the employment of pulse-shape discrimination (PSD) in a silicon detector in addition to conventional time-of-flight (ToF) techniques. The method makes use of the fact that, at fixed energy, the particle's velocity, or ToF, is a measure of the particle's mass A while the time structure of the current pulse in a silicon energy detector, used as the ToF stop, permits identification of nuclear charges Z. In the measurements presented here, ToF and PSD methods were applied simultaneously. We used micro-channel plate (MCP) detectors as fast time pick-offs and surface-barrier (SB) n-type Si detectors made from homogeneously neutron-transmutation-doped (n-TD) silicon. As the particles, products from the reactions of a 400 MeV 20Ne beam impinging on 12C, 27Al and 208Pb targets were employed. With using fast current-sensitive pre-amplifiers for the 250 and 800 μ m SB detectors a major progress in particle identification with respect to both, mass A and charge Z was achieved. In addition, using a stack of two closely mounted n-TD SB detectors, a timing measurement between the two detectors permitted to study PSD in the rear detector with the time trigger derived from the foremost one. Finally, the internal PSD method was implemented which determines pulse rise-time in single detectors from analyzing differentiated preamplifier timing signals.

Elastic scattering of 6He from 12C at 38.3 MeV/nucleon

The microscopic optical potential of nucleus-nucleus interaction is presented via a folding method with the isospin dependent complex nucleon-nuclear potential, which is first calculated in the framework of the Dirac–Bruecker–Hartree–Fock approach. The elastic scattering data of 6He at 229.8 MeV on 12C target are analyzed within the standard optical model. To take account of the breakup effect of 6He in the reaction an enhancing factor 3 on the imaginary potential is introduced. The calculated 6He+12C elastic scattering differential cross section is in good agreement with the experimental data. Comparisons with results in the double-folded model based on the M3Y nucleon-nucleon effective interaction and the few the body Glauber-model calculations are discussed. Our parameter free model should be of value in the description of nucleus-nucleus scattering, especially unstable nucleus-nucleus systems.

Study of the structure of borromean nucleus 17Ne

The 17Ne nucleus is a possible candidate with a two-proton borromean halo structure. Since the theoretical model is difficult to handle the three-body system, it is difficult to determine the two-proton halo structure in 17Ne. In the present research, we try to study the breakup reaction of 17Ne. For the Borromean nuclei, one-proton knockout results in an unstable nucleus which is decaying further by proton emission. This process will result in an angular correlation between the direction of the aligned recoiling unstable nucleus and its decay products. The angular correlations can give us information about the configuration of the valence proton in the 17Ne. Furthermore, theoretical calculations indicate that the momentum distributions of 16F c.m are sensitive to the structure of the halo in 17Ne. Thus the measurement of momentum distributions of 16F c.m may also give us conclusive information on the structure of 17Ne. The present experiment has been done at IMP in Lanzhou using the radioactive beam 17Ne at 30.8 MeV/u on a 43mg/cm2 12C target. The data analysis is under procedure and the primary results are provided.

Λ-HYPERNUCLEUS PRODUCTION IN PROTON-INDUCED REACTION

The Λ-hypernucleus (LHN) production in the proton-induced reaction is studied in the distorted wave impulse approximation(DWIA). The cross sections for the LHN production in the reactions where the proton bombards the 6Li, 12C and 16O targets, respectively, are calculated. It is shown that the reaction cross sections are of the order of μb, and the distortion effects tend to reduce the cross sections by a factor of 3~10. For the sΛ–LHN production, the differential cross section is decreased with the increasing mass of the target nucleus. The pΛ–LHN production cross section is normally higher than that for the sΛ–LHN production. The double differential cross sections (DDXS) with respect to the momenta of the outgoing proton and kaon are also demonstrated. The missing mass spectra of the inclusive reaction p+A → p+K++X for the 6Li, 12C and 16O targets, an alternative way to study hypernuclear physics, are proposed. From these spectra, the masses of LHN can accurately be extracted. Moreover, the exotic LHN production in the same type of reaction is also studied . The same physical quantities are calculated. It is shown that the magnitude of the cross section is also in the order of μb. The halo effect of the core nucleus that locates at a place far away from the stable line would make the wave function broader, and consequently reduces the production cross section.

Signals for Strange Quark Contributions to the Neutrino (Antineutrino) Scattering in Quasi-Elastic Region

Strange quark contributions to the neutrino (antineutrino) scattering are investigated on the elastic neutrino-nucleon scattering and the neutrino-nucleus scattering for 12C target in the quasi-elastic region on the incident energy of 500 MeV, within the framework of a relativistic single particle model. For the neutrino-nucleus scattering, the effects of final state interaction for the knocked-out nucleon are included by a relativistic optical potential. In the cross sections we found some cancellations of the strange quark contributions between the knocked-out protons and neutrons. Consequently, the asymmetries between the incident neutrino and antineutrino which is the ratio of neutral current to charged current, and the difference between the asymmetries are shown to be able to yield more feasible quantities for the strangeness effects. In order to explicitly display importance of the cancellations, results of the exclusive reaction 16O(\nu, \nu' p) are additionally presented for detecting the strangeness effects.

Configuration of the valence neutrons of 17B**Supported by National Natural Science Foundation of China (10475098, 10605033, 10221003), Chinese Academy of Sciences (CXTD-J2005-1, KJCX2-SW-N18) and Hundred Talent Project of CAS (0501080BR0)

The reaction cross section of 17B on 12C target at (43.7±2.4) MeV/u has been measured at the Radioactive Ion Beam Line in Lanzhou (RIBLL). The root-mean-square matter radius (Rrms) was deduced to be (2.92±0.10) fm, while the Rrms of the core and the valence neutron distribution are 2.28 fm and 5.98 fm respectively. Assuming a ``core plus 2n'' structure in 17B, the mixed configuration of (2s1/2) and (1d5/2) of the valence neutrons is studied and the s-wave spectroscopic factor is found to be (80±21)%.

Cluster structures of 18O and 20O up to 20 MeV excitation energy from the (7Li, p)-reaction

We studied the band structure of 18O and 20O using the (7Li, p)-reaction at an incident energy of 44 MeV on 12C and 14C targets. Spectra have been measured from the ground state up to 20 MeV excitation energy. We found 27 and 38 new states for 18O and 20O, respectively. The even-parity bands have been analysed up to now, i.e., some bands were extended by further members. The 0+ band head of the molecular band in 18O at 7.796(5) MeV was identified for the first time.

Microscopic optical potential of nucleus–nucleus elastic scattering

Microscopic optical potential of nucleus–nucleus interaction is presented by a folding method with the isospin-dependent complex nucleon–nucleus optical potential, which is calculated in the framework of the Dirac–Bruecker–Hartree–Fock approach. A local density and improved local density approximations are adopted to make the potentials in nuclear matter relevant to finite nuclei. The 4He scattering off 12C at various energies E< 200 MeV are studied first. Then the elastic scattering data of 6He at 229.8 MeV on 12C target are analyzed within the standard optical model. To take account of the breakup effect of 6He in the reaction a phenomenological, large enhancing factor on the imaginary potential is introduced by hand. The calculated 6He+12C elastic scattering differential cross section is in reasonable agreement with the experimental data. Meanwhile comparisons with results in the double-folded model based on M3Y nucleon–nucleon effective interaction and the few body Glauber-model calculations are discussed. Our model should be of value in the description of the nucleus–nucleus scattering, especially unstable nucleus–nucleus systems.

Improvement of thickness and uniformity of isotopically enriched 12C targets on backings by the HIVIPP method

We have made enriched 12C targets to accurately measure the cross-section of the 12C( α, γ) 16O reaction, which is very important in nuclear astrophysics. Isotopically enriched 12C targets for studying this small cross-section, especially for use with an intense pulsed α beam was desired to meet the following requirements: (1) use of impurity-free enriched 12C, (2) stability for a long time measurement and (3) uniform thickness in the range 200–300 μg/cm 2. To meet these experimental requirements, isotopically enriched amorphous 12C powder was converted into graphite powder in an electric furnace at a temperature of 3000 K and subsequently the graphite powder was deposited on a thick Au backing via the HIVIPP method. Targets thus prepared could be made thicker than 200 μg/cm 2. They had a good uniformity and a very high stability against irradiation with high intensity ion beams.

Hypernuclear spectroscopy program at JLab Hall C

Hypernuclear production by the (e,e′K+) reaction has unique advantages in hypernuclear spectroscopy of the S=−1 regime. The second-generation spectroscopy experiment on 12C, 7Li and 28Si targets has been recently carried out at JLab Hall C with a new experimental configuration (Tilt method) and also using a new high-resolution kaon spectrometer (HKS). The experiment is described and preliminary results are presented together with the empasis of significance of the (e,e′K+) reaction for Λ hypernuclear spectroscopy and its future prospects.

Measurement of Nuclear Transparency for theA(e,e′π+)Reaction

We have measured the nuclear transparency of the A(e,e'pi+) process in 2H, 12C, 27Al, 63Cu, and 197Au targets. These measurements were performed at the Jefferson Laboratory over a four momentum transfer squared range Q2=1.1 to 4.7 (GeV/c)2. The nuclear transparency was extracted as the super-ratio of (sigmaA/sigmaH) from data to a model of pion-electroproduction from nuclei without pi-N final-state interactions. The Q2 and atomic number dependence of the nuclear transparency both show deviations from traditional nuclear physics expectations and are consistent with calculations that include the quantum chromodynamical phenomenon of color transparency.

The neutral-current neutrino-nucleus scattering in the quasielastic region

The neutral-current neutrino-nucleus scattering is calculated through the neutrino-induced knocked-out nucleon process in the quasielastic region by using a relativistic single particle model for the bound and continuum states. The incident energy range between 500 MeV and 1.0 GeV is used for the neutrino (antineutrino) scattering on the 12C target nucleus. The effects of the final state interaction of the knocked-out nucleon are studied not only on the cross section but also on the asymmetry due to the difference between neutrinos and antineutrinos, within a relativistic optical potential. We also investigate the sensitivity of the strange quark contents in the nucleon on the asymmetry.

The first 2+ state of 14Be

The first excited state of the most neutron-rich beryllium isotope, 14Be, has been studied by inelastic scattering with a 12C target at 68 MeV/nucleon. The state has been observed at Ex=1.54(13) MeV, and its spin-parity has been determined as Jπ=2+ from the measurement of the angular distribution. This provides a complete set of energies of the first 2+ states in the beryllium isotopic chain. The extracted deformation length of 14Be has been found to be substantially smaller than that of the neighboring nucleus 12Be, in which the N=8 shell closure is quenched, indicating hindered collectivity in the 14Be nucleus.

High Resolution Spectroscopy ofBΛ12by Electroproduction

An experiment measuring electroproduction of hypernuclei has been performed in hall A at Jefferson Lab on a 12C target. In order to increase counting rates and provide unambiguous kaon identification two superconducting septum magnets and a ring imaging Cherenkov detector were added to the hall A standard equipment. An unprecedented energy resolution of less than 700 keV FWHM has been achieved. Thus, the observed (Lambda)(12)B spectrum shows for the first time identifiable strength in the core-excited region between the ground-state s-wave Lambda peak and the 11 MeV p-wave Lambda peak.

Microscopic interpretation of the (t, 3He) reaction at 130 MeV on 90Zr and isovector monopole strength

The 130-MeV primary tritium beam of the AGOR facility with an intensity of up to 108 pps and the Big Bite Spectrometer experimental setup have been used to study the (t, 3He) reaction between 0° and 5° lab angles on 12C and 90Zr targets. The standard ray-tracing procedure has allowed us to obtain excitation-energy spectra up to 30 MeV in six angular bins for each residual nucleus, with an average energy resolution of 350 keV. We have used the DWBA reaction mechanism model to reproduce those spectra and their angular distributions. In this approximation, the form factor was treated as a folding of an effective projectile-nucleon interaction with a transition density. The effective projectile-nucleon interaction has been adjusted to reproduce the 0° cross section of the 1+ ground state of 12B populated in the 12C(t, 3He) reaction. We have employed RPA wave functions of excited states to construct the form factors. This DWBA+RPA analysis is used to compare calculated and experimental cross sections directly and to discuss the giant resonance excitations in the 90Y nucleus. In this talk, we give some details on this analysis. We show that there are important contributions of L = 2 transitions in the observed cross sections for the 1+ final states that explain the previous difficulties in clearly identifying the monopole strength distributions. We then have a better indication of where the L = 0 part is located with this reaction and its microscopic analysis.

Spin and parity determinations of excited 15N based on polarized and unpolarized 12C(7Li, α)15N reaction data at Elab = 34 MeV

From an experiment conducted at the Florida State University Accelerator Laboratory with a 34MeV polarized 7Li beam bombarding a 12C target, we have obtained angular distributions and analyzing powers for states of 15N up to 20MeV in excitation energy. This study not only offers the possibility to assign spin and parity to several states in 15N, but also serves to obtain nuclear potential parameters used in Distorted Wave Born (DWBA) and Coupled Channel Born (CCBA) Approximations to generate theoretical angular distributions and vector analyzing powers that give the best description of the experimental data. Under the assumption that the reaction mechanism is a three nucleon transfer, the determination of shell model nucleonic configurations and spectroscopic factors is possible for the 15N states studied.