Differential cross sections and analyzing powers for the pp ! dp 1 reaction have been measured to within T cm p 85 keV of threshold. The S-wave strength is extracted and found to be ϳ13% larger than that determined from recent measurements of np ! dp 0 . Both the analyzing power and the anisotropy in the angular distribution show the expected energy dependence and these data are used to obtain values for the two contributing P-wave amplitudes. [S0031-9007(96)00564-9] PACS numbers: 13.75. Cs, 24.70. + s, 25.40.Ve Threshold pion production has received renewed attention in recent years both because technological developments have made high resolution experiments possible and because of the realization of the pivotal role played by this fundamental process. These threshold reactions provide direct information on the nuclear axial charge and indirect information regarding the pNN form factor and certain weak nuclear transitions [1-3]. Additionally, precision measurements of the pp ! dp 1 cross section and spin observables could potentially distinguish between competing production mechanisms such as mesonexchange currents (MEC) and off-shell rescattering.Considering that the NN ! ͑NN͒p reaction has been extensively studied for 50 years, it may seem surprising that there could be anything new to learn about it. However, the vast majority of the work is concentrated at energies where contributions from resonant production mechanisms dominate. Only a handful of experiments examine the threshold region where it is expected that nonresonant mechanisms are important [4][5][6][7][8].We present in this Letter a measurement of the cross section and analyzing power for the pp ! dp 1 reaction at nine energies between 287.7 and 294.1 MeV (E thresh 287.511). The lowest energy corresponds to T cm p ϳ 85 keV; the lowest energy at which analyzing powers were previously measured is more than 1 MeV above threshold [6].The measurements were carried out at the electroncooled proton storage ring (the Cooler Ring) at the Indiana University Cyclotron Facility (IUCF) using an internal H 2 gas jet target. The proton beam was 70% polarized and had an energy spread of approximately 60 keV FWHM.Sufficiently close to threshold, the pions, which are directly detected, are confined to a narrow cone about the beam direction with an opening angle given by sinu max h͞b cm g cm , where b cm and g cm are the boost parameters of the center-of-mass system and h p cm p ͞m p c. This energy dependence of the pion is used to determine the absolute beam energy to within 620 keV. To our knowledge, no other investigation of threshold pion production has obtained this level of energy precision which becomes essential at the lower end of the covered energy range. The detector (Fig. 1) contained left-right symmetric plastic scintillators which measured DE and E of the forward-going particles. Particles not stopping in the DE and E counters (E1 and E2, respectively) were vetoed by a double layer of plastic scintillators. Pions with energies less than 24 MeV and p...

Cross sections for pp→pn ϩ have been measured at incident proton energies of 294.3, 299.3, 306.3, 314.1, and 319.2 MeV (0.11ϽϽ0.42) to investigate hadronic ϩ production near threshold. Pion angular distributions are presented for 294.3, 299.3, and 319.2 MeV. At 294.3 and 299.3 MeV they are consistent with isotropy, indicating s-wave dominance for the angular momentum of the ϩ pn system. The shapes of the pn, n ϩ , and p ϩ invariant mass distributions provide further evidence for dominance of s-wave mechanisms close to threshold. At 319.2 MeV significant anisotropy indicates contributions from higher partial waves. The total cross sections show an energy dependence consistent with an early partial conservation of axial current based calculation and with a recent calculation that considers only s-wave contributions. Tentative values for the strongest contributing partial wave amplitudes are presented. ͓S0556-2813͑97͒01307-1͔

The inelastic scattering of 35 MeV protons has been used to excite seven states below 2.1 MeV in Nd. A comparison between a coupled-channels calculation that assumes an octupole coupled structure for the 5 state at 1.517 MeV and the observed cross section suggests that the wave function of this state includes only a small two-quasiparticle component. In addition, the present results indicate that the level at 2.073 MeV, previously assigned J =4, actually has J =3 For some time, sequences of negative parity states (3 5, 7, etc. ) observed in spectra of even-even lanthanide nuclei have been interpreted in terms of the coupling of an octupole phonon to the sequence of states (2&, 4&, 6+&, etc. ) built on the ground state [1]. In vibrational and rotational nuclei, these negative-parity sequences can be called "octupole bands. " The octupole band interpretation is motivated by the similarity of the spacings between the states in the negative-parity band to corresponding spacings in the ground-state band. However, an octupole band interpretation can be tested via the collection of several different kinds of spectroscopic information. For example, the transition quadrupole moments for E2 transitions in an octupole band should be equal to those for the corresponding transitions in the ground-state band. One example of such a test is given in a recent study ['2 of high-spin states in 74Se.In another test [3 of the octupole band interpretation of negative-parity states, the inelastic scattering of protons by Nd was used to probe the 5 member of the negative-parity band. If a 5 state is indeed a member of an octupole band, then it should be populated in a (p, p') reaction via a two-step process involving successive F2 and E3 excitations. The cross section for such a process can be computed by performing a coupled-channels calculation using P2 and Ps parameters extracted from an analysis of the differential cross sections of the 2+& and 3& states in the nucleus. In the Nd(p, p') experiment reported in Ref. [3], it was found that such a calculation underpredicted the observed cross section for the 5 state by an order of magnitude, implying that a simple octupole coupled interpretation for this 5 state is inappropriate.Predictions of strong octupole correlations and even static octupole deformation have been made in the N=86 -90 region (for example, see Ref.[4]). Therefore, tests similar to the ones described above are necessary to provide a more detailed understanding of the negativeparity states of nuclei in this region. In the present work, we report on a study of Nd using proton inelastic scattering. Results for seven states having J = 2 -5 are reported. In this nucleus, a negative-parity band of states was observed previously in a p-ray study and was interpreted as an octupole band [4].The data reported here were obtained using 35 MeV protons accelerated by the Princeton University AVF Cyclotron. The targets used for this experiment consisted of NdqOs (enriched to 97.5% in~~s Nd) evaporated onto 20-pg/cm2 carbon foils. Th...