Publication 3

"On Controlled Fragmentation of Protons by Stimulated Means"

By
Eugene B. Pamfiloff

Session CE - Nuclear Reactions.
ORAL session, Friday morning, October 31
Canyon A, TM

[CE.001] Testing for incoherence in hadron quasifree scattering

Quasifree scattering of hadrons and electrons from nucleons within complex nuclei yields a broad peak, with its width due largely to internal nucleon motion. Correlations and coherence among projectile-nucleon collisions can also influence this peak. When a range of target nuclei has been studied, the exponent \alpha in the distribution of cross sections as a function of energy loss ømega can be found by fitting to A^\alpha. Very strongly absorbed particles will yield \alpha=1/3. This analysis has been carried out for quasifree scattering of \pi^-, K^+, and protons, and for \pi^- charge exchange scattering. The fitted exponents approach those expected from a Glauber model for values of the scaling variable y within about 150 MeV/c of y=0, but are larger for more negative y. It is concluded that hadron scattering cannot be used to measure nucleonic momentum distributions because of correlations and coherence spoiling the assumed quasifree nature of the scattering, but that responses near the y=0 peak can be used to measure the magnitudes of hadron-nucleon scattering within complex nuclei.

[CE.002] Axial Spin Observables in dp Breakup

We describe a measurement of the axial spin observables A_z, C_y,x-C_x,y and C_zz,z in the dp-breakup reaction using a 270 MeV deuteron beam. Axial spin observables are of interest because their operator structure links them to three-nucleon forces^1. In a two-body final state, these observables are forbidden by parity conservation. Faddeev calculations confirm an observed upper limit of 10^-3 for A_z at 9 MeV ^2 but predict a much larger effect at 270 MeV. The vector and tensor correlation coefficients C_y,x-C_x,y and C_zz,z have never been previously measured. The experiment was carried out at the Indiana University Cooler storage ring. Polarized deuteron beams of four different vector and tensor combinations with different signs, as well as an unpolarized deuteron beam was bombarded against a polarized proton target. The proton polarization direction was cycled every 2s between \pm x, \pm y and \pm z. Two charged particles were detected in coincidence by a detector covering approximately half of the phase space. The measurements will be compared to Faddeev calculations that correctly represent the experimental acceptance. Preliminary results will be shown.

1. L.D. Knutson, Phys. Rev. Lett. 73, 3062 (1994) 2. E.A. George et al., Phys. Rev. C54,1523 (1996)

[CE.003] Determining Neutron-Proton Density Differences from Medium-Energy Nucleon-Nucleus Scattering.

Recently there has been renewed interest in determining neutron density distributions at the surfaces of stable neutron-rich nuclei.\footnote C.J. Horowitz and J. Piekarewicz, Phys. Rev. Lett. 86, 5647 (2001) and Refs. therein. Since proton rms radii are now precisely known, \footnote B.A. Brown, Phys. Rev. Lett. 85, 5296 (2000) data on the difference in radius between the surfaces of the neutron and proton distributions are highly desirable. The symmetry term in the optical potential for nucleon-nucleus scattering may depend on this difference.\footnote G.R. Satchler, Nucl. Phys. A91, 75 (1967). Phenomenological analyses indicate that this term includes a surface imaginary part – provided that the imaginary diffuseness parameter is symmetry dependent\footnote R.E. Shamu et al., Bull. Am. Phys. Soc. 46, 87 (2001) and Refs. therein. – which is large in the nucleon energy range 15-30 MeV, where collective effects may be negligible. Microscopic calculations are needed to interpret these phenomenological results in terms of surface density distribution differences.

[CE.004] ^48Ti(n,xnyp\gamma) reaction cross section using spallation neutrons for E_n = 1 to 250 MeV

Excitation functions have been measured for the interaction of fast neutrons (up to a neutron energy of 250 MeV) on ^48Ti. The Los Alamos National Laboratory spallation neutron source, at the LANSCE/WNR facility, provided a ``white'' neutron beam. The prompt reaction \gamma rays were measured with the large-scale Compton-suppressed Ge spectrometer GEANIE. Excitation functions were converted to partial \gamma ray cross sections taking into account the dead-time correction, the target thickness, and the detector efficiency. Partial gamma-ray cross sections for transitions in ^47,48Ti and ^47,48Sc will be presented.

[CE.005] Cross sections for \gamma-ray production in the ^193Ir(n,xn\gamma) reactions^\ast

Discrete \gamma-ray spectra have been measured for nuclei populated in ^193Ir(n,xn\gamma) reactions, with x \leq 11, as a function of incident neutron energy using the ``white'' neutron source at the Los Alamos Neutron Science Center's (LANSCE) WNR facility. The energy of the incident neutrons was determined using the time-of-flight technique. The data were taken using the GEANIE spectrometer comprised of 26 high-purity Ge detectors with 20 BGO escape-suppression shields. The absolute cross sections for emission of more than 150 \gamma-rays from the ^183-193Ir isotopes were determined for neutron energies 1~MeV < E_n < 200~MeV. Comparison with model calculations, using the GNASH reaction model, was made up to E_n = 30~MeV incident neutron energies. The total ^193Ir(n,n^\prime)^193mIr cross section for the production of the 80-keV isomer, from threshold up to E_n = 20~MeV incident neutron energies, was determined using a combination of experimental data and nuclear reaction modeling.

^\astThis work was performed under the auspices of the U.S. Department of Energy by the University of California, Los Alamos National Laboratory under contract No. W-7405-ENG-36 and Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.

[CE.006] Structure of heavy helium isotopes via the isobaric analog states in Lithium.

Study of nuclei far removed from the valley of stability provide an opportunity to test the models of nuclear structure. In particular, heavy isotopes of helium attract much attention due to their extreme neutron to proton ratio. In this work we studied the T=5/2 states in ^9Li and T=3/2 states in ^7Li, isobaric analog states of ^9He and ^7He respectively.

Experiment on investigation of T=5/2 states in ^9Li was performed on ACCULINA spectrometer in Dubna, Russia. Excitation function for ^8He+p elastic scattering was measured in the center-of-momentum (cm) energy range from 1.6 to 5.8 MeV. Three T=5/2 states in ^9Li (isobaric analogs of ^9He) were observed. Restrictions on the spin-parity assignments to these states are provided according to R-matrix calculations, and conclusions regarding the structure of ^9He are given.

T=3/2 states in ^7Li was populated in interaction of ^6He with protons using the ^6He beam provided by TWINSOL RNB facility at the University of Notre Dame. Neutron decay channel of T=3/2 resonances in ^7Li (n+^6Li(T=1)) was observed in a combination of \gamma-ray and neutron detectors. Preliminary results of this experiment will be reported.

[CE.007] Proton Transfer Spectroscopy in the Tin Region

The pure neutron rich tin radioactive ion beams available at the Holifield Radioactive Ion Beam Facility provide a unique opportunity for a whole new class of nuclear structure studies using single nucleon transfer reactions. Such transfer reactions populate single particle and hole states inaccessible in decay studies, so far the exclusive source for nuclear structure information in the region around doubly-magic 132Sn, with the exception of neutron transfer and Coulomb excitation experiments using radioactive ion beams. As a prelude for proton transfer experiments using radioactive beams, we have investigated and will present preliminary results from reactions of 550-MeV 124Sn and 130Te beams with a 2.3 mg/cm2 7Li target. Of special interest is the inverse (7Li, 8Be) reaction channel, which populates proton hole states in 123In and 129Sb, respectively . This reaction channel can be clearly identified by the subsequent break-up of 8Be and the detection of the two correlated alpha particles in one or more crystals of the CsI based particle identification detector HyBall. The coincident, almost background-free gamma ray spectra of the CLARION clover array allow the identification of proton hole states in 123In and 129Sb. Extension of the experiments to the study of proton particle states and the prospects of doing such reactions with radioactive ion beams will be discussed.

[CE.008] Measurements of neutron-rich tin isotopes using (d,p) reactions in inverse kinematics at the HRIBF

Knowledge of the structure of the single-particle and single-hole nuclei neighboring double-shell closures away from the valley of stability is vital to our understanding of exotic nuclei. In particular, measuring the single-particle and states in ^133Sn is important to both nuclear structure physics and nuclear astrophysics. The recent advent of intense, purified neutron-rich beams at the HRIBF at ORNL has provided the opportunity to perform neutron transfer reactions in inverse kinematics to measure the l-values and spectroscopic factors for unstable neutron-rich nuclei. We will present results from a test experiment using the stable beam ^124Sn to illustrate the technique which we propose to utilize to measure states in ^133Sn and other n-rich tin isotopes.

[CE.009] Yield Measurements of Sn Nuclei Near the Proton Drip Line

The doubly-magic nucleus ^100Sn has previously been produced using 1 GeV/nucleon ^124Xe and ^112Sn beams on a Be target at GSI and using a 64 MeV/nucleon ^112Sn beam on a Ni target at GANIL. The reported cross sections varied immensely. Using the coupled cyclotrons at the NSCL and the A1900 fragment separator, the production yields of neutron-deficient nuclei, with A\sim100, were measured using an E/A=140MeV/nucleon ^124Xe beam. Be, Ni, and Nb targets were chosen to study the effects of different target material on the production of Sn. The yield dependence on the momentum for the light Sn isotopes using these targets will be discussed. This work was supported by the NSF under the grant PHY-01-10523.

[CE.010] Studies of N/Z equilibration via Heavy-Residue Isoscaling

A new experimental approach to study the process of N/Z equilibration in isospin asymmetric heavy-ion collisions has been developed at Texas Aamp;M. The apporach is based on the N/Z information contained in the yield ratios of heavy residues from two isospin-asymmetric deep-inelastic collisions. In this talk, the isotopic and isobaric scaling behavior of the yield ratios of heavy projectile residues from the collisions of 25 MeV/nucleon ^86Kr projectiles on ^124Sn and ^112Sn targets will be presented with respect to the neutron number N and the neutron excess N--Z, respectively [1]. The corresponding logarithmic slopes of the yield ratios with respect to N and N--Z are obtained as a function of Z and A, respectively, and residue excitation energies are deduced from velocities. The relations of the scaling parameters with the N/Z of the primary projectile fragments is employed to gain access to the degree of N/Z equilibration prior to fragmentation as a function of the excitation energy. This approach to N/Z equilibration is shown to provide important information on the N/Z degree of freedom in collisions between massive isospin-asymmetric nuclei around the Fermi energy and offer a rigorous testing ground of current transport models. Preliminary results of N/Z equilibration studies obtained with the newly commissioned Superconducting Solenoid Line at Texas Aamp;M will be presented and plans for the near future will be outlined. Finally, possibilities of further studies that will be offered by re-accelerated neutron-rich RIBs from the proposed upgrade of the Texas Aamp;M facility [2] will be mentioned.

[1] G.A. Souliotis et al., Phys. Lett. B submitted, nucl-ex/0305027.

[2]``A Proposed Facility Upgrade for the Texas Aamp;M Univ. Cycloton Institute'', accessible at: http://cyclotron.tamu.edu.

[CE.011] Two-proton and d-alpha correlation functions in Sn+Sn and Xe+Au collisions

Two-proton and d-alpha correlation functions have been studied in central Sn+Sn and Xe+Au collisions at E/A=50 MeV/u. A comparison between the properties of these observables will be presented.

Furthermore, the measured correlation functions are used to determine the volume and the density of the emitting sources. The effects of collective motion on the volume and on the overall shape of two-particle correlations will also be discussed.

This work was supported by the National Science Foundation under Grant No. PHY-011-0253.

[CE.012] Isospin Dynamics in Heavy Ion Fragmentation Reactions

The isovector part of the nuclear EOS is not well known for the densities different from saturation, and its density dependence is very differently predicted by various models.On the other hand it is important for the structure of exotic nuclei and for astrophysical processes.At lower densities it can be investigated by studying fractionation of the isospin in nuclear fragmentation reactions. We analyse these effects in simulations of heavy ion reactions including density fluctuations in order to realistically describe the fragmentation. We study the isospin dynamics and the distributions and asymmetry of the primary fragments. Qualitatively different effects arise in central collisions with bulk fragmentation and in peripheral collisions with neck fragmentation.The neck fragments appear systematically more neutron-rich due to a dynamical nucleon migration which is very sensitive to the symmetry term in regions just below normal density. In particular we discuss and compare to experiment the isoscaling parameters and isospin inbalance coefficients, which could give information on the driving forces in the isospin dynamics, and thus on the density dependence of the isovector forces.

[CE.013] On Controlled Fragmentation of Protons Through Stimulated Means

We propose a system to bring about the controlled fragmentation of solitary protons or groups of nucleons through stimulated means. Recent progress in high-energy and inertial confinement physics indicates that it is within our technical ability to stimulate confined single protons or groups of nucleons to individually fragment into a predetermined quantity of energy as well as providing an array of elementary and rudimentary particles. By adjusting the level and form of stimulation as described in the paper, the extent of proton fragmentation can be controlled and as a result, a quantity of energy (Ef) greater than 3.50 MeV per nucleon can be produced. Although the upper limits will be established through experiment, conservative limits for Ef are substantial as compared with the free-energy produced in a common d + t fusion event. We examined the results of countless nuclear collisions between accelerated particles and ions, other high-energy interactions along with low-energy nuclear events with particular emphasis to those areas where a mass deficit exists or where mass is converted into energy. For example, during a typical nuclear event, a 235U nucleus will fission and convert mass into energy at the rate of approximately .9 MeV/nucleon (235U + n -> 141Ba + 92Kr + 3n + Ef ). During a commonplace fusion event of two protons (p + p -> 2H + Ef) about .05The fusion of 2H + 3H -> 4He + n + Ef (d + t -> à + n + Ef) provides 3.40 MeV/nucleon of energy. When Pb or Au ions are accelerated and directed to collide at energies of 160 or 200 GeV/nucleon, a substantial portion of mass is converted.

[CE.014] Transfer to the Continuum and Breakup of Exotic Nuclei

The interest in studying breakup of dripline nuclei is ongoing, as measured observables become more detailed and precise. In parallel, the theorectical community has developed models with the aim of specifically handling the breakup of these exotic structures. Assuming a two-body projectile p=(c+x) impinging on a target t, the solution of the Faddeev equation (c+x+t) with the appropriate boundary conditions would provide the exact result for the 2 -> 3 scattering occuring.

Alternatively, the continuum discretization coupled channel method (CDCC) that explicitly introduces the breakup couplings of p=(c+x), treats the breakup reaction as t(p,p*)t, where p* corresponds to unbound solutions of (c+x). This is known to provide an equivalent solution to the Faddeev, as long as the CDCC model space is sufficiently large.

Less established is the use of the transfer to the continuum. The breakup reaction is then modelled as t(p,c)y*, where y* corresponds to the continuum of (t+x). In some cases, it is advantageous to describe the process as 'transfer to the continumm' however it is not clear if/when the approach is adequate. In this contribution we wish to discuss the validity of these two approaches for calculating breakup observables.

Part C of program listing

Eugene B. Pamfiloff
boris@2xtreme.net
 

Copyright  © 1999 - 2005 by Eugene B. Pamfiloff