Search Results (64 total)

The excitation of the double-phonon giant dipole resonance was observed in heavy projectile nuclei impinging on targets of high nuclear charge with energies of 500–700 MeV/nucleon. New experimental data are presented for ¹³⁶Xe and ²³⁸U together with further analysis of earlier data on ²⁰⁸Pb. Differential cross sections dσ/dE^* and dσ/dθ for electromagnetic excitations were deduced. Depending on the isotope, cross sections appear to be enhanced in comparison to those expected from a purely harmonic nuclear dipole response. The cumulative effect of excitations of two-phonon states composed of one dipole and one quadrupole phonon, of predicted anharmonicities in the double-phonon dipole response, and of damping of the dipole resonance during the collision may account for the discrepancy. In addition, decay properties of two-phonon resonances were studied and compared to that of a statistical decay.

Angular distributions of Xe fragments produced in peripheral collisions of a ¹³⁶Xe beam (700 MeV/nucleon) with ²⁰⁸Pb and ^natSn targets were measured. Equivalent sharp-cutoff minimum impact parameters were derived on the basis of a semiclassical description for the electromagnetic excitation of one- and two-phonon giant resonances. The results are compared with current standard parametrizations of minimum impact parameters and with the soft-spheres model using realistic mass density distributions for projectile and targets.

Pion-nucleus elastic scattering at energies above the Δ(1232) resonance is studied using both π⁺ and π^- beams on ¹²C, ⁴⁰Ca, ⁹⁰Zr, and ²⁰⁸Pb. The present data provide an opportunity to study the interaction of pions with nuclei at energies where second-order corrections to impulse approximation calculations should be small. The results are compared with other data sets at similar energies and with four different first-order impulse approximation calculations. Significant disagreement exists between the calculations and the data from this experiment.

Pion-induced double charge exchange (π⁺,π^-) on ^76,78,80,82Se, leading to the double isobaric analog states (DIAS) and the ground states of ^76,78,80,82Kr, has been studied at a laboratory angle of 5⁰ and incident pion kinetic energy of 293.2 MeV. Cross sections for these transitions have been extracted, and those for the DIAS are compared to two simple models of pion double charge exchange.

We have measured pion inelastic scattering from ²⁰Ne with 180 MeV π⁺ and π^- at 12° and from 15° to 90° in 5° steps and with 120 MeV π⁺ from 15° to 90° in 5° steps. We report here on states in the first four K^π=0⁺ bands, including the 6.73MeV 0⁺ and the 8.78-MeV ⁶⁺ levels. Coupled-channels processes are shown to be important for many states.

Measurements were performed using pion double charge exchange to observe the double isobaric analog state (DIAS) in the odd-A nuclei ⁵⁴V, ⁵⁹Co, ¹¹⁵In, and ¹⁹⁷Au. These results are combined with data for other odd-A targets and used to investigate the N, Z, and A dependence of DIAS cross sections.

Differential cross sections for the double-isobaric analog-state transition in the ⁵⁸Ni(π⁺,π^-) reaction were measured at 5° for eight incident energies between 120 and 292 MeV, inclusive. The results are compared with theoretical calculations and with previous data for T=1 targets, and mass dependence is discussed.

The energy dependence of the giant dipole resonance built on the isobaric analog state was measured in pion-induced double charge exchange on ¹³C at incident pion energies of 140–295 MeV. The cross section increases with energy. The excitation function has a typical non-spin-flip signature, expected from simple considerations and verified by sequential model calculations. Three narrower previously reported low-lying states in ¹³O (E_x=3.1, 4.5, and 6.1 MeV) exhibit a spin-flip signature and may correspond to the giant Gamow-Teller strength built on the isobaric analog state.

Resonances were observed in the continuum in pion-induced double charge exchange (at T_π=295 MeV) on ⁵¹V at Q=-14.0, -32.4, and -48.0 MeV, on ¹¹⁵In at Q=-25.0, -37.3, and -49.6 MeV, and on ¹⁹⁷Au at Q=-35.0, -44.3, and -54.9 MeV. The three resonances have different characteristic angular distributions. Partial angular distributions were measured on ⁵¹V and ¹¹⁵In and observed to have J^π=0⁺, 1^-, and 2⁺ shapes, respectively. Based on their centroid energies, angular distributions, and observed cross sections, we identify the resonances as the double isobaric analog state, the giant dipole built on the isobaric analog, and the double giant dipole resonance. The new observations are in close agreement with the gross features reported recently for the double giant resonances, and provide some of the best signals obtained to date for the double resonance states in nuclei.

Double charge exchange cross sections for 50 MeV pions to double isobaric analog states in ⁴⁴Ca, ⁵⁶Fe, and ⁹³Nb were measured. The results indicate that the effective double charge exchange operator at this energy has a strong short-range component. In addition, evidence is presented tending to implicate isospin mixing as the mechanism for excitation of the analog of the antianalog in ⁹³Nb.

Pion double charge exchange on ¹³⁸Na (T=13), at a bombarding energy of 292 MeV, and a scattering angle of 5°, has been used to populate the double isobaric analog state in ¹³⁸Ce. The measurements provide determinations of Q value, width, and center-of-mass cross section. Results are compared with systematics and with the isobaric-multiplet-mass equation.

Measurements were made of pion double charge exchange on ^128,130Te at θ_lab=5° and T_π=292 MeV. Observations were made of the double isobaric analog, the giant dipole resonance built upon the isobaric analog, and the giant dipole resonance built upon itself. Cross sections for these states were found and compared with all previously known cross sections at T_π=292 MeV, and new mass dependences were fitted to these data. A new state below the double isobaric analog state in ¹²⁸Te was observed and its cross section and width were determined.

Inclusive and exclusive spectra of the He⁴(π,π′p) reaction were measured with π⁺ and π^- at T_π=140 MeV and θ_π=40°, and at 180 MeV and 30°, 40°, 60°, and 80° using the EPICS system at LAMPF in coincidence with eight plastic scintillators. The inclusive spectra yield cross section ratios R_π=σ(π⁺)/σ(π^-)=1.1±0.1 at both T_π and all θ_π. Exclusive spectra were obtained between He^4* excitation energies E_x=21.5 and 45.0 MeV. Angular correlation functions for (π,π′p) were extracted at T_π=180 MeV at the four pion angles for selected regions of E_x. The (π⁺,π^+′p) angular correlations show a distinct peak near the free proton knockout angle; however, the (π^-,π^-′p) data show either a flat distribution or a minimum near that angle. Calculations, using a distorted wave impulse approximation (DWIA) code which models the (π,π′p) reaction as a pion-included proton knockout, predict peaks in the angular correlation functions in the quasifree knockout direction and reproduce quite well the (π⁺,π^+′p) data above E_x=30 MeV. However, these calculations do not resemble the (π^-,π^-′p) data at any excitation energy. Above E_x=30 MeV, the ratios R_πp=σ(π⁺,π^+′p)/σ(π^-,π^-′p) were found to be unexpectedly large (up to R_πp=50) near the free proton knockout angle and very small (R_πp=0.3) in the opposite direction in contrast to DWIA predictions of 8 and 5, respectively. These discrepancies are evidence that strong interference occurs between the quasifree proton knockout and another process, especially in the (π^-,π^-′p) reaction. Near the He⁴→p+t breakup threshold, where the 2^-, T=0 state in He⁴ (E_x=21.8 MeV) is known to exist, the ratio R_πp was found to be between 1 and 2 at all pion angles which is in reasonable agreement with the expected value of 1 for the excitation and decay of a state of good isospin.

The double isovector giant dipole state has been observed in the (π^-,π⁺)ΔT_x=+2 double-charge-exchange reaction on ¹³C, ²⁷Al, ⁴⁰Ca, ⁵⁶Fe, ⁵⁹Co, and ⁹³Nb. The resonances observed in the (π^-,π⁺) reaction are closely related via Coulomb displacement energy and isospin symmetry to the resonances measured in the inverse (π⁺,π^-) reaction on the same nuclei. The new observations provide a direct determination of the upper isospin component of the double giant dipole state, which is generally very weak in the (π⁺,π^-) reaction. The comparison between the double dipole in the two double-charge-exchange modes gives valuable information on the isospin splitting and the Pauli-blocking effects for isotensor transitions.

New data for elastic and inelastic π⁺ and π^- scattering by ²⁰⁸Pb at 180 MeV are presented. The elastic scattering data were analyzed with a standard Kisslinger potential in a coordinate space (r space) calculation using the free π-N amplitudes to calculate the potential coefficients. Momentum-space (p-space) calculations were also performed using the free π-N amplitudes to calculate on-shell matrix elements and the Landau-Tabakin model for the off-shell. The same ground-state proton and neutron densities were used in these two spaces and the fits are good. The distorted-wave impulse approximation calculations in both spaces, with the same collective model transition densities, reproduce the inelastic differential cross sections quite well. The proton and neutron transition multipole matrix elements and their ratios were calculated from the deformation parameters extracted from fits to inelastic scattering data. It is found that the ratios calculated in these two spaces are quite close to each other and close to those found in a comparison of (e,e’) and (p,p’) experiments, while the absolute value of matrix elements found in the p-space calculations are ∼15–20 % smaller than those from the r-space calculations. The r-space proton matrix elements are generally in good agreement with those obtained from electron scattering. The effects of the pion-nucleon interactions and form factors used in these calculations were investigated.

A generalization of the continuum open-shell r-space linear response random-phase approximation method to finite temperatures (i.e., TDRPA method) is applied in a study of the systematics of the electric double giant dipole resonance (DGDR) in magic and open-shell nuclei covering a wide range of nuclear masses. Viewing the DGDR as a single-phonon vibration in a hot nucleus, whose temperature corresponds to the centroid energy of the ground-state (zero-temperature) GDR, the TDRPA equation has been solved for the eleven nuclei whose DGDR state has recently been investigated in pion double-charge-exchange (DCX) experiments. The expected general features of a collective vibration in hot nuclei, particularly the broadening and the downshift of the GDR excitation energy due to increase in temperature, were found to be small, yet quite distinguishable. Except for the lighter nuclei, a reliable estimate for the DGDR energy would be E^DGDR≊2E^GDR and the estimate for its width would be Γ^DGDR≊ √2 Γ^GDR. However, the changes in the integrated cross sections of the DGDR due to finite temperature are found to be much more significant. An exact A^-1/6 power law for the DGDR energy is suggested by the TDRPA solutions. A gratifying agreement between the TDRPA predictions and experiment has been established for the Q values of the relevant pion DCX reactions after taking into account the proper Coulomb and symmetry energy corrections.

The double isobaric analog state ‖IAS⊗⊗IAS〉 and a new resonance below the ‖IAS⊗IAS〉 have been observed in pion-induced double-charge-exchange reactions on ⁹³Nb. Angular distributions have been measured for both transitions. The cross section observed in the T_< resonance is 21% of that for the ‖IAS⊗IAS〉 in ⁹³Tc. We mention various possibilities for the nature of the observed resonance but have no unique explanation for its large cross section.

In a series of experiments, differential cross sections for elastic scattering of π⁺ and π^- from ⁴He were measured at the Clinton P. Anderson Meson Physics Facility (LAMPF). Data were taken at incident energies between T_π=90 and 240 MeV using different experimental setups which covered the angular range from far forward (≊10°) to far backward angles (≊170°). A phase-shift analysis of the data was carried out. The experimental phase shifts are compared with values predicted by a first-order optical model program.

The double isovector giant-dipolar state has been observed in the (π^-,π⁺) ΔT_z=+2 and (π⁺,π^-) ΔT_z=-2 double-charge-exchange reactions on ⁴⁰Ca and ²⁷Al. The resonances observed in the (π^-,π⁺) reaction are closely related via Coulomb energy and isospin symmetry to the resonances measured in the inverse (π⁺,π^-) reaction on the same nuclei. The new observations provide strong support for the identification of the double giant-dipole state in nuclei. The differences between the self-conjugate and non-self-conjugate cases are outlined.

Angular distributions for pion-induced double-charge-exchange (DCX) reaction have been measured at 164 MeV on the T=0 target nuclei ¹²C, ²⁴Mg, ³²S, and ⁴⁰Ca. The data are compared to calculations using the Δ-nucleon interaction model with realistic wave functions and πNN and ρNN form factors deduced from fits to previously published DCX data on ¹⁶O. The calculations reproduce the general shape of the angular distributions and the absolute magnitude of the cross sections to within a factor of two.

Giant resonances were observed in (π⁺,π^-) double charge exchange on ¹³C, ²⁷Al, and ⁵⁹Co at excitation energies of 8.8, 14.2, and 27.9 MeV, respectively, where the giant dipoles built on the isobaric analog states are expected to appear. Partial angular distributions were measured for both ¹³C and ²⁷Al and observed to have a dipole shape. The resonance on the T=1/2 targets has a single isospin value (T=3/2) and is the analog of the T_> giant dipole state in the above nuclei. The results are in good agreement with photonuclear reactions and indicate a new general method for an accurate determination of the T_>=T+1 giant dipole state in T=1/2 nuclei.

We present cross sections for the reactions ⁴⁴Ca,⁵⁶Fe(π⁺,π^-) leading to the residual ground states and double-isobaric-analog states (DIAS) for incident pion energies from 100 to 300 MeV. Data include both angular distributions and 5° excitation functions. Ground-state cross sections were also obtained from the ¹²C and ¹⁶O contaminants in the ⁴⁴Ca target. The DIAS data are compared to known systematics and to theoretical predictions. Some features of the data are reproduced by the theoretical predictions, but major discrepancies exist.

Exclusive ⁴He(π^±,π^±’p)³H spectra in the excitation energy range between 21.5 and 44 MeV in ⁴He were measured at T_π=180 MeV at several pion and proton angles. The cross section ratio R_πp=σ(π⁺,π⁺’p)/σ(π^-,π^-’p) was found to be between 1 and 2 near 22 MeV which is in the region of the J^π=2^-, T=0 state. Between 25 and 40 MeV in the continuum, which comprises the giant dipole resonance and broad 2^- and 2⁺ states, R_πp ranges from 0.22 to 45 in sharp contrast to the predicted values of ≊1 from a model assuming the entire yield is due to the sequential decay of a state of good isospin, and 6 to 9 from a distorted-wave impulse approximation calculation assuming quasifree proton knockout.

We report the observation of double isovector giant dipole resonances in (π⁺,π^-) and, for the first time, also in (π^-,π⁺) double-charge-exchange reactions at T_π=292 MeV. The resonances have been observed on nine targets covering a wide range of mass at Q values around ≊-50 MeV in (π⁺,π^-), but at significantly lower energy (Q=-33.8 MeV) in the inverse reaction on ⁵⁶Fe. The centroid energies in both reactions are very close to the energies at which the ΔT_z=±2 double giant dipole (J^π=0⁺,2⁺,ΔT=2) states are expected to appear. Angular distributions have been measured for the (π⁺,π^-) resonances on ⁴⁰Ca,⁵⁶, Fe, and ⁹³Nb and each is observed to have a quadrupole shape. Based on their energies, angular distributions, cross sections, and the comparison between the (π⁺,π^-) and (π^-,π⁺) data on ⁵⁶Fe, we identify the resonances as double giant dipole states (i.e. states arising from a charge-exchange dipole operator acting twice on the ground-state wave function). The data from the present and recent observations indicate that the newly discovered resonance is a general collective feature of all nuclei.

Cross sections for the ¹²C(π^±,π^±’p)¹¹B reaction have been measured and compared with the results of distorted-wave impulse-approximation calculations that employ the vactirozation approximation for quasifree scattering. Near the giant dipole resonance of ¹²C the calculations underestimate the cross sections for (π^-,π^-’p). The angular distributions for (π⁺,π⁺’p) and (π^-,π^-’p) are different and indicate that in this region there is interference between direct decay and semidirect decay.