A zero-degree spectrometer in CELSIUS and the d(d,2π)4He reaction close to threshold

Abstract In near-threshold reactions the heavy reaction products are emitted within a narrow cone in the forward direction. To detect these a small-size charged-particle telescope has been built that is operated inside the vacuum chamber within the dipole magnets of the CELSIUS storage ring. The first aim has been to study two-pion production close to threshold. By measuring the d(d,2π) 4 He reaction at a beam energy of 569 MeV, corresponding to 29 MeV above the production threshold for two neutral 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.