All relevant low-energy transition amplitudes for the D(d,n)'He and D(4p)'H reactions were determined from a fit to Legendre expansion coefficients of the available experimental data. A simple barrier penetrability model was used. Quintet S-wave transitions are found to contribute strongly thus obliterating the idea of neutron-lean "polarized" fusion energy production. The D + D interaction radius was determined with good accuracy for both reactions individudly. The astrophysical S functions show a small S-wave enhancement and Fwave suppression of the D (~f , p )~I f branch. Apart from possible applications the investigation of the D+D reactionswhich have received special interest from the beginning of nuclear reaction studiesis of interest in itself: the four-nucleon system is the lightest one exhibiting excited states. With the advent of faster computers and with the successes of rigorous theoretical calculations in the three-nucleon system e.g. 111 using modern meson-exchange potentials [2, 3, 4, 51 there is hope for realistic microscopic calculations (such as those based on Faddeev theory) also for this system, see e.g. [6]. The observed differences for the two branches of this system with identical particles in the entrance channel lend themselves to studies of the role of Coulomb interaction in the exit channel and possible charge-symmetry breaking effects. Comprehensive surveys of the four-nucleon system have been published recently [7, 81.On the other hand these fusion reactions play a role in nuclear astrophysics and furthermore they accompany the main reactions in planned light-particle fusion reactors. The economically advantageous concept of "neutron-lean" fusion energy production by using the 3He(d,p)4He reaction would only be realizable if the accompanying neutrons from the D(d,n)3He reaction could be suppressed. For this purpose the use of polarized "fuel" has been suggested where the neutron production via the entrancechannel spin-quintet states was assumed to be strongly suppressed [9, 101. This concept has been contradicted by some authors [11, 121 and will also be studied in this work.
