J.C. Jourdain

Study of the Δ isobar excitation in nuclei with the (3He,t) reaction

Abstract Data for the Δ excitation by the (3He,t) reaction at 2.0 GeV on various nuclei are presented together with results on 12C at 1.5 and 2.3 GeV. The Δ peak position and width are found to be target mass independent. Comparison with the p(3He,t) Δ++ reaction, however, shows a shift of the position in energy of the Δ peak. Implications of these results are discussed.

The (3He,t) reaction at intermediate energies: Spin-isospin transitions to states in 12N and 13N

Abstract The (3He, t) reaction has been studied at intermediate energies, 600 MeV–2.3 GeV. The reaction is a single-step direct reaction, and the angular distributions are well described by DWIA calculations. At these energies the σρ component of the interaction dominates. The ratio of the isovector strengths ¦J σρ / ρ ¦ derived from the data on 13C appears to remain roughly constant in the energy range 300–700 MeV per nucleon. The triton spectra exhibit a prominent quasi-free peak. The data at momentum transfers q ⪢ 1.7 fm−1 show a shift of the peak towards smaller energy loss than observed in (e, e) experiments at corresponding four-momentum transfers. The shift becomes progressively larger with increasing q and is about 45 MeV at q = 2.5 fm−1. The shift may be caused by particle-hole correlations in the spin-longitudinal channel.

First observation of the Δ resonance in relativistic heavy-ion charge-exchange reactions☆

Abstract Excitation of the Δ resonance is observed for the first time with heavy ions. It is very strong in the two mirror charge-exchange reactions ( 20 Ne , 20 F ) and ( 20 Ne , 20 Na ) at 950 MeV per nucleon. With similar incident energies per nucleon, the Δ is weakly excited with a 14 N beam and nearly absent with a 12 C beam. The role of the ejectile bound states is discussed.

Study of decay and absorption of the Δ resonance in nuclei with a 4π detector

Charge exchange reactions have been investigated at Saturne with various projectiles (3He, (vec d), 12C,16O, 20Ne, 40Ar) and incident energies between.5 and 1.1 GeV/nucleon1. These experiments have given clear indication of the selectivity of charge exchange reactions for spin isospin excitations. In particular, a strong excitation of the Δ resonance shows up for energy transfers higher than the pion mass. The most striking common feature of all these charge exchange reactions is a shift of about 70 MeV between the positions of the Δ bump on nuclei and on hydrogen targets (fig.1). This result has been interpreted by theoreticians as a collective effect in the spin longitudinal channel2 and has been quantitatively reproduced by recent calculations3. An alternative qualitative explanation is the projectile excit ation4.

Coherent pions in charge-exchange reactions☆

Abstract Data are presented on the 12 C ( 3 He , t π + ) reaction at 2 GeV bombarding energy. In coincidence with forward scattered tritons, pions are momentum-analysed in a large acceptance detector. For events where the target is left in its g.s. or low-lying states, the angular correlation between the momentum transfer vector and the momentum of the pion is sharply peaked at small relative angles.

The (3He, t) reaction at intermediate energies: Spin-isospin multipole transitions

Data for the (3He, t) reaction at 900 MeV and 2 GeV on the targets 26Mg, 40Ca, 48Ca, 54Fe, 90Zr and 208Pb are presented. A multipole decomposition for the data at 900 MeV has been made and the different distributions have been analysed. From the L = 0 cross section the Gamow-Teller strength distributions are extracted and compared with (p, n) data. The L = 1 and 2 distributions are analysed in a schematic model which describes the general systematics fairly well. The spectra at 2 GeV and Θ = 4° show for all targets a well-developed quasi-elastic peak. The A-dependence of the cross section is analysed in a simple model.

Exchange effect in the 166 MeV α-particle elastic scattering on 6Li

Abstract The angular distribution of the elastic scattering of 166 MeV α-particles on 6Li nuclei has been measured from 3° to 175° (c.m.). The backward rise of the cross section is explained in terms of two-nucleon pick-up. The symmetry of the entrance and exit channels makes the measurement a good test of the applicability of finite-range DWBA calculations for this exchange effect. This effect is shown to become negligible for heavier nuclei like 40Ca.

Pion photoproduction on 3He at and above the 3-3 resonance

Abstract The differential cross section for the 3 He (γ, π + ) 3 H reaction has been measured at fixed values of q2 in the region of the first resonance, and at fixed θπ versus q2 up to 13 fm−2. The observed shift of the resonance towards lower energies, and the absence of a minimum near 11.6 fm−2 might be explained by recent theoretical calculations.

Charge symmetry in the mirror reactions 3H(d, 3He)2n and 3He(d, t)2p at 83 MeV

Abstract The nn and pp final-state interaction spectra were measured at 2.5° and 7.5° in the mirror reactions 3 H(d, 3 He)2n and 3 He(d, t)2p at 83 MeV deuteron energy. The Watson-Migdal treatment does not give a good description of these spectra. Application of the comparison method of Slobodrian et al. gives a mean value for a nn of −15.5 fm, and very large values for r on : the variation of the production form factor with the relative energy of the two nucleons seems also very large. The coherent addition of a constant amplitude to the Watson-Migdal amplitude introduces only a limited number of parameters: this method gives for r on a mean value of 3.3 fm, but the values found for a nn (around −12 fm) are in general smaller than the theoretical predictions. A more rigorous theoretical treatment of these deuteron induced reactions seems necessary. Measurements have also been performed on the mirror reactions 3 H(d, t)pn, 3 He(d, 3 He)pn and 3 H(d, t) 2 H, 3 He(d, 3 He) 2 H.

Excitation of the Δ resonance in relativistic heavy ion charge exchange reactions

Abstract Excitation of the Δ resonance has been observed for the first time in a heavy ion reaction. The strength of the Δ signal depends on the projectile nuclear structure. In the same incident energy range, about 900 MeV per nucleon, we observe — a strong Δ excitation in 27 Al( 20 Ne, 20 F) and 27 Al( 20 Ne, 20 Na), — a weak Δ in 12 C( 14 N, 14 C) and — nearly no Δ excitation in ( 12 C, 12 B) and ( 12 C, 12 N) whatever the target is among a sample of nuclei from 12 C to 89 Y.