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Results from a complete set of polarization-transfer observables for quasifree (prf) scattering at 495 MeV are reported. Measurements were carried out on CD2, carbon, and calcium targets at a laboratory scattering angle of 18° using the new neutron time-of-flight facility at LAMPF. The ratio of spinlongitudinal to spin-transverse responses at a momentum transfer of approximately 1.72 fm _1 is extracted from the data and compared to distorted-wave calculations incorporating a random-phase approximation description of the nuclear response. No enhancement is observed in the experimental ratio.PACS numbers: 25.40.Lw, 24.70.+SThe investigation of collective spin-isospin excitations in nuclei has been a rich field for observation of new phenomena [1]. At low excitation energies (co < 20 MeV) and small momentum transfers (q ^ 0) the spin-isospin particle-hole interaction is strongly repulsive, leading to collective excitations such as the giant Gamow-Teller resonance. The quenching of measured Gamow-Teller strengths relative to a model-independent sum rule has led to extensive theoretical studies of possible mechanisms, ranging from conventional configuration mixing to underlying subnucleonic degrees of freedom.At higher excitations (co^ 30-160 MeV) and momentum transfers (q~\-2 fm -1 ), the spin-longitudinal interaction, taken to be driven by one-pion exchange, is expected to become mildly attractive, while the spin-transverse interaction mediated by rho exchange continues to remain repulsive due to the larger mass of the rho compared to the pion. Although signatures of shell structure, such as discrete states and giant resonances, disappear in this sector, the nucleus should continue to respond collectively through the action of the residual particle-hole interaction. The very dissimilar momentum dependence of the spin-longitudinal and -transverse interactions should give rise to different phenomena in the corresponding nuclear spin-isospin responses. For example, relative to a noninteracting Fermi gas, the spin-transverse response (RT) is predicted to be quenched and pushed to higher energy loss, while the spin-longitudinal response (R L ) is predicted to be enhanced and shifted to lower energy loss due to an enhanced pion field within the nucleus [2]. The very (non)existence of such collectivity is directly connected to the form and magnitude of the spinisospin-dependent parts of the residual interaction, and hence to the basic underlying degrees of freedom in the nuclear system.Quasifree scattering can provide detailed information on the nuclear response. At intermediate energies (incident particle energies of several hundred MeV) and moderate momentum transfers (q > 0.5 fm _1 ), quasifree scattering is a dominant part of the nuclear excitation spectrum. A broad structure is seen with a peak near co=q 2 /2m (m being the nucleon mass) and a width determined by the Fermi momentum. At momentum transfers above about 1 fm" 1 , this region of excitation is well separated from the low-lying discrete states and resonance r...

The empirical distributions of Gamow-Teller strengths in 98 Tc and 115 Sn have been obtained via the (p,n) reaction at £^=120 MeV and E p = 200 MeV on 98 Mo and 115 In targets. This information is used to calculate the cross sections for absorption of solar neutrinos in 98 Mo and 115 In, nuclides which are being considered as neutrino detectors to accomplish solar-neutrino spectroscopy.PACS numbers: 25.40.Ep, 96.60.Kx There is a long-standing discrepancy 1 between the solar-neutrino capture rate measured in the 37 C1 experiment 2 and the value predicted from a standard solar model and calculated neutrino-absorption cross sections. 3 This discrepancy may be caused by a lack of understanding of the physics of the solar interior 4 or, if the sun produces the expected spectrum of neutrinos, then their character is altered before they reach the earth (by oscillation or decay). 5 In order to distinguish between these two classes of solutions, a number of neutrino detectors have been proposed to accomplish what may be regarded as solar-neutrino spectroscopy. Much experimental and theoretical work has been devoted to proposed radiochemical experiments 6,7 employing 71 Ga and 81 Br targets, a geochemical experiment using the production 8 of 98 Tc from 98 Mo, and a counter experiment 9 using 115 In.Quantitative estimates of neutrino capture rates can be made by combining the Gamow-Teller (GT) and Fermi (F) strengths for each level in a given target with the calculated solar-neutrino spectra. For the gallium detector, most of the capture rate is expected to arise from the experimentally determined ground-state to ground-state transition. However, for the other three detectors mentioned above, there are large or dominant transitions occurring between states that are not connected by experimentally accessible beta-decay transitions. The Fermi strength can be calculated, but for all the above nuclei there is no complete deter-

The C(n, p) B reaction was studied using the white neutron source at the Los Alamos Meson Physics Facility/Weapons Neutron Research center with a continuous incident neutron energy from 60 to 260 MeV. Double differential cross sections were measured in the angular range 11 ( Ol b & 37'. Using the neutron time-of-Right facility at the Indiana University Cyclotron Facility, we also studied the C(p, n) N reaction at R"= 186 MeV and the C(p, n) N reaction at E~= 160 and 186 MeV. Double differential cross sections were measured between Ol b --0 and Ol b --50' in 5' steps. Spin observables D&&, A&, and P were measured at Hi b --5', 9', 13' with Ep = 160 MeV and Hl b --15, 20' with Ep = 186 MeV. Angular distributions of differential cross section and spin observables for low-lying transitions in the residual nuclei are compared with distorted-wave impulse approximation (DWIA) calculations. A multipole decomposition analysis was performed to study the giant dipole and giant spin-dipole resonances. The contributions of the quasifree reaction in the giant resonance region were subtracted. The empirical results of energy distributions for dipole (AL = 1) transition are compared with DWIA calculations using nuclear structure information obtained with a conventional shell model and also with a random phase approximation. PACS number(s): 25.40.Hs, 27.40.+z

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The polarization transfer observable Dr, r, (0 ) has been measured for H(p, n)2p, C(p, n) N, and C(p, n) N reactions with beam energies of 318 and 494 MeV. The deuterium results are in close agreement with free NN scattering constrained to the AJ = 1+ channel. The results for the carbon targets, however, are systematically more negative than those for deuterium, which suggests a di8'erence between the free and effective nucleon-nucleon interactions. Such a medium modification of the spin-dependent interaction may have important implications for interpretation of other intermediate-energy scattering experiments.PACS numbers: 25.40.Kv, 21.30, +y, 24.70.+s Measurements of the nuclear spin response require an accurate knowledge of the characteristics of the probe responsible for the transition. For example, in the analysis of recent quasifree (p, n) measurements it was necessary to assume that the probe characteristics are those of free nucleon-nucleon (NN) scattering [1,2]. It is likely that the NN interaction is changed when the reaction takes place in the nuclear medium, but the effects of distortions, inaccurate knowledge of the free interaction itself, and incomplete knowledge of the nuclear wave functions present obstacles against unambiguous evidence for such changes. Recent strategies have involved the study of transitions such as those to "stretched" nuclear configurations at bombarding energies where the optical potential and relevant components of the effective (free) interaction appear to be well determined [3,4]. Such studies have been used to explore the effective interaction in the momentum transfer range of approximately q = 1 -3 fm-'In this Letter we report evidence for a modification of the effective interaction near momentum transfer q = 0. Our result is based on the comparison of polarization transfer for several 4J = 1+ transitions at a scattering angle of 0~For this type of transition at this angle, angular momentum transfer of AL = 0 dominates and the effects of details of the nuclear wave functions and spinorbit distortions are minimized. Polarization transfer for 1+ transitions at this angle is sensitive to the relative strength of the central and tensor-exchange interactions.We have measured the longitudinal polarization transfer observable DL, I. (0') for the zH(p, n) and C(J7, n) reactions at 318 MeV and 494 MeV, and for the~4C(p, n) reaction at 494 MeV. The final-state interaction in the 2H(p, n)2p reaction restricts the two residual protons to a So state for small energy loss at O'. This reaction is therefore well described as 6J = 1+ and, under the assumption that nuclear medium effects are negligible for the deuteron, serves to characterize the free NN interaction amplitudes relevant to 1+ transitions at O'. The H(J7, n) results reported here agree well with the recent lower-resolution measurements of McNaughton et al [5].. Furthermore, both sets of zH measurements agree well with free NN scattering constrained to the LJ = 1+ channel. The r2C(p, n) and~4C(P, n) results, however, dif...