A ZERO-DEGREE SPECTROMETER IN CELSIUS AND THE D(D,2-PI)HE-4 REACTION CLOSE TO THRESHOLD

In near-threshold reactions the heavy reaction products are emitted wi thin a narrow cone in the forward direction. To detect these a small-s ize charged-particle telescope has been built that is operated inside the vacuum chamber within the dipole magnets of the CELSIUS storage ri ng. The first aim has been to study two-pion production close to thres hold. By measuring the d(d,2 pi)He-4 reaction at a beam energy of 569 MeV, corresponding to 29 MeV above the production threshold for two ne utral pions in the c.m., we add a low-energy end point to the previous experiments concerning this reaction. The two pions are produced in a T=0 channel which is the signature of the so called ABC-effect.