Pion Charge Exchange Reactions Research Articles

Microscopic calculation of double-dipole excitations

In the last decade experimental and theoretical research has led to the discovery of double giant resonances—a giant resonance built on top of another giant resonance. The double-dipole resonance was first identified in a pion charge exchange reaction @1# and was predicted earlier @2#. Later the double dipole was detected in Coulomb excitation in heavyion reactions @3–5#. Properties of double dipole modes and other types of double resonances were studied and are reviewed in several review articles @2–4,6#. Theoretical studies have been mostly of the ‘‘macroscopic’’ type, introducing collective coordinates in the description of double giant resonances @2–4,6#. Some papers used semimicroscopic models in which the random phase approximation ~RPA! was employed to define the collective phonon states. Some extensions of the RPA have also been suggested @7,8#. Truly microscopic calculations are very difficult. A shell-model calculation requires very large spaces involving configurations made up of particles excited to several major shells. One must therefore truncate the space and limit the calculation to one-particle–one-hole (1p-1h) and two-particle– two-hole (2p-2h) configurations involving particles and holes in several major shells. Studies of this type were performed for some nuclei a few years ago @9#. In this work we present shell-model calculations of the double giant dipole state in O and Ca. By choosing light nuclei we are able to include a relatively large space of 1p-1h and 2p-2h configurations and therefore are able to study in detail the distribution of strength and the splitting of strength into the various allowed spin and isospin components. In the case of O we are able to account for the coupling of J50 and 2, 1p-1h states to the corresponding 2p-2h configurations. We also discuss energy weighted sum rules ~EWSR! and the relationship between these sum rules for the single and double giant resonances ~see also Ref. @10#!. The sum rules are evaluated in the shell model basis ~numerically! and in a boson model.

Isovector Electric Monopole Resonance in60Ni

The isovector electric monopole resonance (IVMR) was studied by the ${}^{60}\mathrm{Ni}{(}^{7}\mathrm{Li}{,}^{7}\mathrm{Be}{)}^{60}\mathrm{Co}$ reaction at 65A MeV and at forward scattering angles including zero degrees. We confirmed the existence of the IVMR at an excitation energy of $20\ifmmode\pm\else\textpm\fi{}2\mathrm{MeV}$ with a width of $10\ifmmode\pm\else\textpm\fi{}2\mathrm{MeV}$ in ${}^{60}\mathrm{Co}$, which is an analog of the ${T}_{0}+1$ isospin component of the IVMR estimated at ${E}_{x}\ensuremath{\approx}31\mathrm{MeV}$ in ${}^{60}\mathrm{Ni}$. The result presently obtained is consistent with previous results using the pion charge-exchange reaction.

Isospin splitting in analog giant resonances excited by single pion charge exchange reactions

Giant isovector analog E1 resonances have been observed by means of the pion single-charge-exchange reactions ( π ±, π 0). The giant T = 1 analog E1 resonances in 12B and 12N, built on the ground state of 12C, were observed in the 12C( π ±, π 0) reactions. The giant T = 3 2 resonances in 13B and 9Li, built on the ground states of 13C and 9Be, were studied with the 13C, 9Be( π −, π 0) reactions, respectively. Analogs of these resonances were found in the 13C, 9Be( π +, π 0) reactions along with lower lying T = 1 2 resonances. These resonances together with the higher T = 3 2 resonances constitute isospin-splitting in the nuclei, 13N and 9B. These isospin assignments were made by comparing results from the ( π ±, π 0) reactions and their measured angular distributions. The assignments were confirmed by determining the decay properties of the resonances with coincident ( π 0p) measurements in the ( π +, π 0p) reactions. The proton angular distributions measured relative to the momentum transfer direction were consistent with a dipole process.

Pion capture and hydrogen bonds in deuterated methanol.

The probability of pion capture by hydrogen was measured for ${\mathrm{CH}}_{3}$OH, ${\mathrm{CD}}_{3}$OH, and ${\mathrm{CH}}_{3}$OD as a function of temperature from -150 to +250 \ifmmode^\circ\else\textdegree\fi{}C. The nuclear capture of a pion by a proton was identified using the pion charge-exchange reaction. The coincident photons from the ${\mathrm{\ensuremath{\pi}}}^{0}$ decay were detected by TRIUMF's large NaI spectrometers. We have found a relative difference of 1.79\ifmmode\pm\else\textpm\fi{}0.11 in the pion capture probability in solid and supercritical ${\mathrm{CD}}_{3}$OH that is interpreted in terms of the hydrogen-bond-breaking model.

Isobaric-analog-state transitions in pion charge-exchange reactions above the Δ(1232) resonance

Forward-angle differential cross sections for the ( π +, π 0) reaction to isobaric analog states were measured at 300, 425, 500, and 550 MeV on targets ranging in mass from 7Li to 208Pb. The extrapolated 0° cross sections are found to rise markedly between 300 and 425 MeV, contrary to the behavior of the free πN process. The mass and energy dependence of the cross sections at the higher energies are discussed within the framework of the Glauber model.

Isovector resonances in pion single-charge-exchange reactions

I report on the experimental study of L = 0, 1, and 2 isovector resonances in pion charge-exchange reactions.

Interpretation of new data on the pion charge-exchange reaction on bound hydrogen.

New data on the pionic charge exchange reaction on bound hydrogen in selected organic molecules have been analyzed using two alternative models. It appears that the model which describes both the charge exchange reaction and the atomic capture probabilities with the same set of parameters is capable of giving the most consistent representation of the data. Transfer of pions initially captured in hydrogen to a neighboring heavy atom is an important feature of this model. The transfer parameter is very accurately determined from the pion charge exchange data.

Importance of quadrupole coupling for low energy pion charge exchange reactions.

Pion single and double charge exchange reactions at 50 MeV on $^{14}\mathrm{C}$ are studied using a conventional coupled channels approach. Inclusion of quadrupole transitions through the ${2}^{+}$ T=1 state in $^{14}\mathrm{N}$ is essential for understanding these reactions. Fitting the single charge exchange data and including quadrupole contributions lead directly to a reasonable description of the double charge exchange data. It is the deep forward minimum of the single charge exchange data, not the forward peaking of the double charge exchange data, that is more likely to have new implications for the pion-nucleus interactions. Investigation of the $^{18}\mathrm{O}$ double charge exchange data supports these conclusions.

Precise measurement of the pion charge-exchange forward differential cross section at 522 MeV/c

A precise measurement of the differential cross section at zero degrees for the pion charge-exchange reaction ..pi../sup -/p ..-->.. ..pi../sup 0/n at p/sub ..pi../ = 522 MeV/c has been made. The result is dsigma/d/OMEGA(0/sup 0/) = 4.32 +- 0.11 mb/sr. 12 references.

EXCITATION OF GIANT RESONANCES IN PION CHARGE-EXCHANGE REACTIONS

The theory for excitation of electric giant isovector resonances in pion charge-exchange reactions is discussed. A comparison between the theoretical and the recent experimental results is made. 18 references.

Excitation of giant electric isovector resonances in pion charge exchange reactions

Pion single charge-exchange reactions exciting giant electric isovector resonances are studied for a series of nuclei. The reaction mechanism is treated in the distorted-wave impulse approximation. Emphasis is put on the nuclear structure aspects probed in these reactions. Transition densities for charge-exchange excitations are calculated using the sum-rule method and microscopically, in the Hartree-Fock---random-phase-approximation framework. The resulting cross sections are sensitive to the nuclear structure input. Double differential cross sections as functions of the nuclear excitation energy are calculated.NUCLEAR REACTIONS Pion single charge exchange exciting giant isovector resonances.

Widths of isovector monopole resonances

The widths of the isovector monopole ΔT z =1 component in 208Tl, 140La, 120In and 90Y are evaluated within the self-consistent Hartree-Fock+charge-exchange RPA using the Skyrme interaction. The coupling of 1p-1h states to 2p-2h configurations is microscopically calculated. The results are compared with those of recent pion charge-exchange reactions.

Comparison of models for pion capture on bound hydrogen

Different models of the pion charge-exchange reaction on bound hydrogen are compared in order to identify similarities and differences between the models.

Study of isovector resonances with pion charge exchange

Studies with the pion charge exchange reactions ( π ±, π 0) at 164 MeV using the π 0 spectrometer are yielding new results on the existence and systematic features of isovector resonances in nuclei. These experiments possess an unusually high signal/background ratio for isovector resonances of low-multipolarity. Results obtained to date are: (1) Observation and angular distribution measurement of the giant dipole resonances in nuclei 12C, 40Ca, 90Zr and 120Sn; (2) Observation and angular distribution measurements in the (π −, π 0) reaction on 90Zr and 120Sn of large signals possessing the expected angular distribution shapes and magnitudes for the isovector monopole resonance. Excitation energies are near the hydrodynamical model values 170 A − 1 3 MeV . Differential cross sections are approximately 0.7 J 2(qR) mb/sr. An overview of this experimental program, with emphasis on new results, and how they correlate with existing knowledge on the isovector resonances, is given.

Pion charge exchange reactions on bound hydrogen

An analysis is made of data on the pion charge exchange reaction in molecules containing hydrogen, using a molecular model which includes the possibility of transfer of the pion to a neighbouring heavier atom. Values obtained for the transfer parameter are consistent with values previously found in analyses of pionic X-ray emission from organic molecules. It is concluded that transfer is the dominant process involving bound hydrogen.

Angular distributions of single pion charge exchange reactions to isobaric analog states in light nuclei

Measurements of the angular distributions of the single charge exchange reactions to isobaric analog states for $^{13}\mathrm{C}({\ensuremath{\pi}}^{+}, {\ensuremath{\pi}}^{0})^{13}\mathrm{N}$ and $^{15}\mathrm{N}({\ensuremath{\pi}}^{+}, {\ensuremath{\pi}}^{0})^{15}\mathrm{O}$ at 165 MeV are described. The two angular distributions are very similar. The shapes are reproduced by semiphenomeno-logical isobar-doorway calculations and by second-order coupled channels calculations. The calculated magnitudes are low by about a factor of 2. The measurements are consistent with older angle-integrated cross sections for $^{13}\mathrm{C}$.NUCLEAR REACTIONS $^{13}\mathrm{C}$, $^{15}\mathrm{N}({\ensuremath{\pi}}^{+}, {\ensuremath{\pi}}^{0})$, ${T}_{\ensuremath{\pi}}=164$ MeV; measured $\ensuremath{\sigma}(\ensuremath{\theta})$; IAS transitions; enriched targets; test of reaction models.

Pion charge-exchange reaction and transition densities to isobaric analog states

Distorted wave impulse approximation calculations were performed for the (${\ensuremath{\pi}}^{+}$,${\ensuremath{\pi}}^{0}$) reaction to isobaric analog resonances in a series of nuclei employing two types of transition densities. The calculations are sensitive to the types of transition densities used, giving substantially different cross sections in the two cases.NUCLEAR STRUCTURE Pion charge exchange to isobaric analog states.

Pion Charge-Exchange Reaction as a Probe of Isovector Monopole Resonances

We argue that the nuclear giant isovector monopole resonance should be observable with pion charge-exchange reactions (${\ensuremath{\pi}}^{\ensuremath{-}}$,${\ensuremath{\pi}}^{0}$) in the energy region close to the (3, 3) resonance.

Application of the isobar-doorway model to pion charge exchange reactions

The pion-nucleus optical potential in the isobar-doorway model is extended to isospin nonzero nuclei. Many-body effects in isobar propagation are parametrized in terms of an energy shift and width and an isobar nonlocality. Isospin dependence of these parameters is found to play an important role in determining pion charge exchange cross sections.

A survey of pion charge-exchange reactions with nuclei

A survey study of the elastic charge-exchange reactions is presented. The charge-exchange cross sections are evaluated in a coupled-channel framework with optical potentials obtained from the Kisslinger and Laplacian models for the pi-nucleon off-shell scattering amplitude. Considerable differences between these two modles are obtained for pion kinetic energies below the pi-nucleon (3, 3) resonance. The effect of two-nucleon correlations is included by calculating the second-order pi-nucleus optical potential. We find that this effect dominates the charge-exchange reactions at energies of about 100 MeV.