Double ionization of He by 3-keV electron impact is explored in a kinematically complete experiment. The 12-dimensional, 4-particle final-state momentum space is mapped for two slowly ejected electrons ͑k b,c , 1.2 a.u.͒, the He 21 ion ͑k He , 5 a.u.͒ and the fast electron for all relative emission angles and energy partitions. Different reaction mechanisms are identified by examining the angular correlation between the slow electrons as a function of the momentum transfer. A fivefold differential cross section is presented for asymmetric energy sharing in coplanar geometry.[S0031-9007(99)08576-2] PACS numbers: 34.80.Dp Double ionization (DI) of helium, i.e., the breakup of the most simple atomic system containing more than one electron, is of basic importance for the understanding of correlated many-particle quantum systems under the action of time-dependent perturbations. It is for this reason that numerous studies have been devoted to He double ionization induced by projectiles as different as photons from threshold to tens of keV (see, e.g., [1]), ions ranging from keV protons (see, e.g., [2]) to GeV͞u U 921 [3], for electron [4], positron [5], and antiproton [6] impact. Recently, He 21 production in intense femtosecond laser fields was explored [7].The most detailed information and the most stringent test for theory are provided by kinematically complete experiments in which the energies and emission angles, i.e., the momenta of all participating particles, are determined in the final state. Such experiments, however, are extremely difficult to perform due to the small double ionization cross sections and the small phase space of the final continuum state which is mapped using conventional electron spectroscopy. For photoabsorption, which is the most simple among the above mentioned situations with only three particles in the final state, experiments have become possible since 1993 (see, e.g., [8]). For ion impact the first and only kinematically complete measurement was reported in 1996 [9] for a high incident charge state of 281. For electron impact, where the cross sections are small, the kinematically complete, so-called ͑e, 3e͒ experiments seemed to be beyond experimental capabilities for the helium target until recently when, besides the present study, Taouil and co-workers succeeded in observing ͑e, 3e͒ coincidence events from helium [10] (pioneering studies on argon were performed in 1989 [11] followed by a few other measurements for Kr, Ar, and Ne [12]).In this Letter we report on a kinematically complete experiment for double ionization of helium by 3-keV electron impact. A novel technique for studies of electron impact ionization, an electron-electron-recoil-ion coincidence [13], was used which has been successfully applied before to study ion induced single and multiple target ionization (for a recent review, see [14]). In the present experiment the entire momentum vectors k b , k c of two slowly outgoing electrons e b and e c ͑E b,c , 20 eV͒ emitted in a single ͑e, 3e͒ reaction have been detecte...
