8He Beam Research Articles

Study of proton and deuteron pickup reactions (d,3He), (d,4He) with 8He and 10Be radioactive beams at ACCULINNA-2 fragment separator

The extremely neutron-rich systems 7H, 6H were studied in the 2H(8He, 3He)7H and 2H(8He, 4He)6H proton and deuteron pickup reactions with a 26 AMeV secondary 8He beam produced at the new ACCULINNA-2 fragment separator. In addition, the same proton and deuteron pickup reactions were generated using the 42 AMeV 10Be beam, and the population of low-lying 9Li and 8Li states was measured in reactions 2H(10Be,3He)9Li and 2H(10Be,4He)8Li, respectively. The latter were used as reference measurements in order to check the setup calibration over the excitation energy of 7,6H and to determine the real experimental energy resolution which was compared with Monte Carlo calculations. The corresponding results obtained for the superheavy hydrogen systems 7H, 6H are presented and discussed. Typical excitation spectra of the 9Li and 8Li nuclei are also shown.

Observation of a correlated free four-neutron system

A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. To our knowledge, only neutron stars represent near-pure neutron systems, where neutrons are squeezed together by the gravitational force to very high densities. The experimental search for isolated multi-neutron systems has been an ongoing quest for several decades1, with a particular focus on the four-neutron system called the tetraneutron, resulting in only a few indications of its existence so far2–4, leaving the tetraneutron an elusive nuclear system for six decades. Here we report on the observation of a resonance-like structure near threshold in the four-neutron system that is consistent with a quasi-bound tetraneutron state existing for a very short time. The measured energy and width of this state provide a key benchmark for our understanding of the nuclear force. The use of an experimental approach based on a knockout reaction at large momentum transfer with a radioactive high-energy 8He beam was key.

Elastic scattering of 8He + 4He and two-neutron transfer and the influence of resonances in 12Be

Scattering of 8He from 4He has been performed through the coincident detection of the scattered particles using a silicon array within a helium-4 gas volume with a 8He beam. This permitted the reconstruction of the interaction probability as a function of position within the gas and hence the excitation function to be calculated together with the angular distribution of the products. These measurements also enabled the reconstruction of the two-neutron transfer to 6He + 6He. The measurements provide the first characterisation of the scattering of the α+4n, 8He, halo nucleus from 4He and the 2n-transfer to populate the symmetric final state. The results indicate the presence of a L=4 component at excitation energies of 14–15 MeV, consistent with a broad (∼1 MeV) Jπ=4+ resonance. These measurements would agree with generalized two-centre cluster model (GTCM) calculations indicating a cluster, or molecular, structure predicted at this energy.

New excited states in the halo nucleus 6He

The low-lying spectroscopy of 6He was investigated via the 2-neutron transfer reaction p(He8,t) with the 8He beam delivered by the SPIRAL facility at 15.4 AMeV. The light charged particles produced by the direct reactions were measured using the MUST2 Si-strip telescope array. Above the known 2+ state, two new resonances were observed: at E⁎=2.6±0.3 MeV (width Γ=1.6±0.4 MeV) and at 5.3±0.3 MeV with Γ=2±1 MeV. Through the analysis of the angular distributions, they correspond to a 2+ state and to an L=1 state, respectively. These new states, challenging the nuclear theories, could be used as benchmarks for checking the microscopic inputs of the newly improved structure models, and should trigger development of models including the treatments of both core excitation and continuum coupling effects.

Recoiled Proton Tagged Knockout Reaction for 8He

An experiment for knockout reaction induced by 8He beam at 82.5 MeV/nucleon on CH2 and C targets was carried out. The 6He and 4He core fragments at forward angles and the recoiled protons at large angles were detected coincidently. From this exclusive measurement the valence nucleon knockout mechanism and the core knockout mechanism are separated, which can be applied to the exclusive spectroscopic study on the structure of exotic nuclei.

Experimental Study of Two-Neutron Correlation in 8He+

A neutron detector array was used in a breakup reaction experiment at RIKEN with an 82.5 MeV/u 8He beam impinging on the CH2 and C targets. The array was calibrated using the cosmic ray, the γ ray from the 6He+Pb reaction and the mono-energetic neutrons from the 7Li(p, n)7Be(g.s.+0.43 MeV) reaction. The position resolution, timing resolution and neutron detection efficiency were obtained accordingly. Cross-talk rejection conditions were developed based on analysis of the data taken from the 7Li(p, n)7Be(g.s.+0.43 MeV) test experiment, and finally a preliminary two-neutron correlation function for the 8He breakup reaction was investigated.

Recoil proton tagged knockout reaction for 8He

We report for the first time the discrimination of the core fragment knockout and valence nucleon knockout reaction mechanisms at medium energy range, by the use of the recoil proton tagging technique. Intense 8He beams at 82.3 MeV/u were supplied by the RIPS beam line at RIKEN, and impinged on both hydrogen and carbon targets. Recoil protons were detected in coincidence with the forward moving core fragments and neutrons. The core fragment knockout mechanism is identified through the polar angle correlation and checked by various kinematics relations. This mechanism may be used to extract the cluster structure information of unstable nuclei. On the other hand, with the selection of the tagged valence nucleon knockout mechanism, a narrower peak of 7He ground state is obtained. The extracted neutron spectroscopic factor Sn=0.512(18) is relatively smaller than the no-tagged one, and is in good agreement with the prediction of ab initio Greenʼs function Monte Carlo calculations.

The COMBAS fragment separator

The basic ion-optical characteristics of the COMBAS fragment separator are analyzed. The momentum distributions of radioactive 6He, 8He, and 9Li nuclei obtained in the reaction 11B (33 A MeV, where A is the mass number of a particle) + 9Be (332.6 mg/cm2) have been investigated in forward-angle measurements on the COMBAS fragment separator. The momentum and angular (horizontal) acceptances of the COMBAS separator have been measured using the 6He, 8He, and 9Li beams. It has been ascertained that the images of the 6He, 8He, and 9Li nuclear beams in final achromatic focus of the separator Fa approximately twofold exceed the size of the beam on a producing target (input focus F0), at which the primary beam has a diameter of 6 mm. The intensities of 6He, 8He, and 9Li beams obtained at a 5-μA intensity of the primary 11B beam are 6.9 × 105, 2 × 104, and 4.7 × 105 particles/s, respectively. These values are sufficient for use in spectroscopic measurements. It is proposed that time-of-flight analysis of nuclear reaction products at the exit from the COMBAS separator will be used not only to measure the energy of transported particles over the whole operating range of the momentum acceptance, but also to identify them by mass A and charge Z without loss of these particles. The problem of reducing the count rates of detectors and further improvement of their energy resolution for detected particles can be solved by placing a high-resolution magnetic spectrometer past the second target accepting the secondary radioactive nuclear beams.

Properties of the 7He ground state from 8He neutron knockout

The unbound nucleus 7He, produced in neutron-knockout reactions with a 240 MeV/u 8He beam in a liquid-hydrogen target, has been studied in an experiment at the ALADIN-LAND setup at GSI. From an R-matrix analysis the resonance parameters for 7He as well as the spectroscopic factor for the 6He(0+)+n configuration in its ground-state have been obtained. The spectroscopic factor is 0.61 confirming that 7He is not a pure single-particle state. An analysis of 5He data from neutron-knockout reactions of 6He in a carbon target reveals the presence of an s-wave component at low energies in the α+n relative energy spectrum.A possible low-lying exited state in 7He observed in neutron knockout data from 8He in a carbon target and tentatively interpreted as a Iπ=1/2− state, could not be observed in the present experiment. Possible explanations of the shape difference between the 7He resonance obtained in the two knockout reactions are discussed in terms of target-dependence or different reaction mechanisms at relativistic energies.

Unbound states studied by direct reactions

The t(6He,8He)p t(8He,10He)p reactions were studied in experiments with 25A MeV 6He beam and 27.4A MeV 8He beams and an unique tritium gas target. Results on the 8He and 10He excitation energy spectra are presented. The 8He spectrum exhibits 3 states, apart of the ground state and known (2+) state at 3.6 MeV, the third state is seen at 5.4 MeV. A tentative assignment 1+ is proposed for this structure. The 10He spectrum shows no event at the excitation energy below 3 MeV what is in contradiction with the earlier reports on this nucleus. A high statistics of α-n coincidence events collected for the 8He beam run has been subjected to analysis in order search for the bound tetraneutron. The search produced a negative result.

Structure of drip-line nuclei 7,8He via direct reactions

The direct reactions of the 8He beam on proton, (p,p′) and (p,d), were measured at 15.7 A.MeV. The excitation spectra for the weakly-bound 8He and the unbound 7He were extracted. The spectroscopy of these drip-line nuclei is compared to the predictions of microscopic models.

Spectroscopy of 212Po and 213At using a 8He radioactive beam and EXOGAM

A beam of 28 MeV 8He particles with an average intensity 2 × 105 ions s−1, produced by the SPIRAL facility at GANIL, was used to initiate fusion–evaporation reactions in thick targets of 208Pb and 209Bi. Excited states of 212Po and 213At were populated in (8He,4n) reactions. The emitted γ-rays were detected using the EXOGAM array composed of four Compton suppressed germanium Clover detectors in a close geometry configuration. Preliminary results show that 212Po has been populated to high spin (I ≃ 14) with a significantly increased yield compared with our previous 8He beam experiment. An apparently increased relative population of states up to 12+ was observed compared with published results from a 208Pb(9Be,αn)212Po reaction at 48 MeV. Evidence is presented for a previously unreported de-excitation pathway: 69 keV (13−→ 12+). States up to Iπ = (39/2−) have also been observed in 213At.

Reactions induced beyond the dripline at low energy by secondary beams

Reactions induced on protons at low incident energy (3.5 MeV/n) were measured with a 8He beam accelerated by Spiral at Ganil. The particles were detected in the active target Maya, filled with C4H10 gas. The beam was stopped in the detector, so energies from incident beam energy down to detector threshold were covered. Proton elastic scattering, one neutron pick-up (p; d) and (p; t) reactions were observed. In the (p; d) reaction very high cross-sections of the order of 1barn were observed, that could be reproduced using a direct reaction formalism. This is the first time that this strong increase of transfer reaction cross-sections at very low energy predicted for loosely bound systems was observed. Spectroscopic factors are in agreement with a simple shell model configuration. No evidence for a low lying excited state in 7He was found.

Quasi-free scattering with 6,8He beams

This paper presents the results of experimental investigations of ( p , p n ) , ( p , p α ) and ( p , p 6He) reactions with relativistic 6He and 8He beams impinging on a hydrogen target. Information on the structure of extremely neutron-rich nuclei was obtained by using the unique characteristic of quasi-free scattering processes and inverse kinematics. For the first time, momentum distributions of complex clusters inside 6He and 8He were measured. Spectroscopic factors for valence neutrons, α- and 6He clusters were deduced from the experimental data. They demonstrate that the filling of the p 1 2 shell is the essential feature of the 8He structure. It is shown that the 8He cluster structure can be considered as a 6He core plus two valence neutrons.

Estimates of the 7H width and lower decay energy limit

The 7H nucleus most likely possesses an unique decay dynamics: the four-neutron emission (or 5-body decay). We estimated theoretically the 7H width as a function of its decay energy. Uncertainty inherent to the estimated decay energy leaves a room for a possible very small width of 7H. In an attempt to observe a long living quasi stable 7H nucleus, produced in the reaction 2H(8He,7H)3He, we set an upper limit of 3 nb/sr for the reaction cross section. We utilized a 20.5 MeV/amu 8He beam bombarding a 5.6 cm thick liquid deuterium target, and searched for 7H within 0°–50° CM angles. The obtained cross section limit corresponds to a 7H lifetime less than 1 ns, which allows us to estimate a lower limit of 50–100 keV for the 7H energy above the 3H+4n breakup threshold.

Experimental evidence for the existence of 7H and for a specific structure of 8He.

Experimental search for the superheavy 7H isotope was performed in the reaction p(8He,pp)7H with the 8He beam at 61.3A MeV. The evidence for existence of the 7H state near the t+4n threshold was obtained. In the same experiment, the p(8He,t) reaction populating the ground and excited 2(+) state of 6He was investigated. The obtained results argue on a specific structure of the 8He ground state containing the 6He subsystem in the excited 2(+) state with a large weight.

Evidence for a new low-lying resonance state in 7He.

Low-lying resonance states in 7He(6He+n), formed after fragmentation reactions of a 227 MeV/nucleon 8He beam on a carbon target, have been studied. Coincidences between 6He nuclei and neutrons, corresponding to the one-neutron knockout channel in 8He, were selected. The relative energy spectrum in the 6He+n system shows a structure, which is interpreted as the 7He ( Ipi = 3/2(-)) ground state, unbound with 0.43(2) MeV relative to the 6He+n system and a width of Gamma = 0.15(8) MeV overlapping with an excited ( Ipi = 1/2(-)) state observed at 1.0(1) MeV with a width of Gamma = 0.75(8) MeV.

8He– 6He: a comparative study of electromagnetic fragmentation reactions

Dissociation of a 227 MeV/u 8He beam in a lead target has been studied in a kinematically complete experiment. The results are compared with similar data from the well-known halo nucleus 6He. Coulomb–nuclear interference in the inelastic channel gives evidence for a I π =1 − resonance in 8He at E x=4.15 MeV. The deduced B(E1) value of 0.38±0.07 e 2 fm 2 indicates a uniform distribution of the four neutrons around the α-particle core.

High resolution beam line of the U400M cyclotron and a possibility of RIB accumulation and cooling in the K4 storage ring

The high resolution beam line ACCULINNA put into operation on a primary beam line of the JINR U400M cyclotron is discussed in the framework of the TREBLe project [1]. The capability of the beam line to produce radioactive ion beams is demonstrated by means of nuclear fragmentation of a primary 14N beam, with an energy of 51 MeV/u, on a 170 mg/cm 2 carbon target. Characteristics of the obtained 6He, 8He and 8B radioactive beams are presented. A scheme of accumulation and cooling on the orbit of the storage ring K4 is proposed for a low intensity radioactive beam obtained from this beam line.