The ABC effect-a puzzling low-mass enhancement in the pipi invariant mass spectrum, first observed by Abashian, Booth, and Crowe-is well known from inclusive measurements of two-pion production in nuclear fusion reactions. Here we report on the first exclusive and kinematically complete measurements of the most basic double-pionic fusion reaction pn-->dpi;{0}pi;{0} at beam energies of 1.03 and 1.35 GeV. The measurements, which have been carried out at CELSIUS-WASA, reveal the ABC effect to be a (pipi)_{I=L=0} channel phenomenon associated with both a resonancelike energy dependence in the integral cross section and the formation of a DeltaDelta system in the intermediate state. A corresponding simple s-channel resonance ansatz provides a surprisingly good description of the data.

The radiation emitted by 855 MeV electrons via planar channeling and volume reflection in a 30.5-μm-thick bent Si crystal has been investigated at the MAMI (Mainzer Mikrotron) accelerator. The spectral intensity was much more intense than for an equivalent amorphous material, and peaked in the MeV range in the case of channeling radiation. Differently from a straight crystal, also for an incidence angle larger than the Lindhard angle, the spectral intensity remains nearly as high as for channeling. This is due to volume reflection, for which the intensity remains high at a large incidence angle over the whole angular acceptance, which is equal to the bending angle of the crystal. Monte Carlo simulations demonstrated that incoherent scattering significantly influences both the radiation spectrum and intensity, either for channeling or volume reflection. In the latter case, it has been shown that incoherent scattering increases the radiation intensity due to the contribution of volume-captured particles. As a consequence, the experimental spectrum becomes a mixture of channeling and pure volume reflection radiations. These results allow a better understanding of the radiation emitted by electrons subjected to coherent interactions in bent crystals within a still-unexplored energy range, which is relevant for possible applications for innovative and compact x-ray or γ-ray sources

The two-pion production in pp-collisions has been investigated at CELSIUS in exclusive measurements from threshold up to Tp = 1.36 GeV. Total and differential cross sections have been obtained for the channels pnπ + π 0 , ppπ + π − , ppπ 0 π 0 and also nnπ + π + . For intermediate incident energies Tp > 1 GeV, i.e. in the region which is beyond the Roper excitation but at the onset of ∆∆ excitation, the total ppπ 0 π 0 cross section falls behind theoretical predictions by as much as an order of magnitude near 1.2 GeV, whereas the nnπ + π + cross section is a factor of five larger than predicted. An isospin decompostion of the total cross sections exhibits a s-channel-like energy dependence in the region of the Roper excitation as well as a significant contribution of an isospin 3/2 resonance other than the ∆(1232). As possible candidates the ∆(1600) and the ∆(1700) are discussed.Two-pion production in nucleon-nucleon collisions is an outstanding subject, since it connects ππ dynamics with baryon and baryon-baryon degrees of freedom. There is increasing evidence that the puzzling ABC effect observed in doublepionic fusion reactions may possibly be traced back to an isoscalar resonance phenomenon as source for the peculiar pion pair production in the ππ scalar-isoscalar state [1,2,3,4]. By contrast the isovector ππ channel in double-pionic fusion be-haves regularily, i.e. shows no ABC effect and follows the expectations from conventional t-channel ∆∆ calculations [5].In view of the challenging explanation [2,3,4] offered for the ABC effect it is interesting to study for comparison the behavior of ππ production in isoscalar, isovector and isotensor ππ channels in those cases, where the two actively participating nucleons do not fuse into a final nuclear bound system. From previous work it is known that the 1

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We measured a considerable increase of the emitted radiation by 120  GeV/c electrons in an axially oriented lead tungstate scintillator crystal, if compared to the case in which the sample was not aligned with the beam direction. This enhancement resulted from the interaction of particles with the strong crystalline electromagnetic field. The data collected at the external lines of the CERN Super Proton Synchrotron were critically compared to Monte Carlo simulations based on the Baier-Katkov quasiclassical method, highlighting a reduction of the scintillator radiation length by a factor of 5 in the case of beam alignment with the [001] crystal axes. The observed effect opens the way to the realization of compact electromagnetic calorimeters or detectors based on oriented scintillator crystals in which the amount of material can be strongly reduced with respect to the state of the art. These devices could have relevant applications in fixed-target experiments, as well as in satellite-borne γ telescopes.