This project involves the study of nuclear structure for very light nuclei lying nearby and beyond the neutron drip-line: 5H, 7He, 9He and 10He. Very little is known for these exotic species, and the experimental situation is not always clear.
The experiments will be performed at JINR Dubna. The exotic nuclei will be produced by transfer reaction induced by radioactive beams. The high intensity beams of 3H, 6He and 8He will be accelerated or produced in-flight by fragmentation with U400M cyclotron, and conducted to the target with the ACCULINNA separator. Cryogenic targets filled with deuterium or tritium will be used to induce (d,p) or (t,p) reactions, in order to reach the nuclei of interest. We will apply to these transfer reactions the missing mass method, which has been used successfully to obtain structure information on unbound nuclei not being accessible in direct observation. It is useful to detect the remnant of the ejectile after its decay, in order to decrease the background and to improve the resolution. New information on the linear momentum distributions inherent to the a cores bound in the halo (skin) of 6He and 8He nuclei will be obtained by means of quasi-free scattering of these cores occurring when 25-30 AMeV 6He and 8He bombard He targets. This will yield new information on the energy states of the groups of halo (skin) neutrons remaining as free remnants.
The reactions considered in the present project are the following: 3H(t,p)5H, 6He(d,p)7He, 8He(d,p)9He, 8He(t,p)10He. Their kinematics presents the same characteristics: the heavy unbound residue is emitted at forward angle in the laboratory with high velocity, whereas the light recoil fragment (proton) is emitted at backward angles with very low energy. The detection system being similar in all the proposed experiments involves multiwire proportional chambers for beam tracking, a telescope consisting of very thin silicon detectors and annular Si strip detectors for the detection of the low energy backward protons, and a time-of-flight spectrometer for the detection of the remnant of the heavy residue emitted at forward angles. The DEMON array will be used for the detection of neutrons emitted in the decay of the unbound nuclei. The two quasi-free reactions 4He(6He,2a) and 4He(8He,2a) result in two high-energy a particles both emitted symmetrically in forward direction. The structure of the two telescopes will be optimised for these characteristics. Five experimental campaigns are planned in the next three years for each of the above-mentioned nuclei. Calculations will be performed in order to compare with theoretical predictions.