Proceedings o f the Third Eurasian Conference “Nuclear Science and its Application”, October 5 - 8 , 2004.
EXPERIMENTAL QUEST FOR NEUTRON MATTER
Wolski R.
Joint Intitute for Nuclear Researches, Dubna, Russia
Henryk Niewodniczahski Institute o f Nuclear Physics, Cracow, Poland
Recent experimental results on 4H, 5H, and 4n nuclear systems are presented in this paper. Investigations of nuclear systems which are highly excessive of neutrons are important to understand nuclear interactions at large isospin. At present, only light nuclei close to neutron drip line are experimentally accessible. The advanced, ab initio theoretical methods for with realistic nucleon-nucleon interactions successfully describe lower energy levels for nuclei up to A=10 [1], Nevertheless, extremely neutron rich, often unbound, systems are still a challenge for the theory. The 4H, 5H, and 4n (tetraneutron) are examples of such neutron rich objects. Our knowledge of these systems is quite differentiated. The 4H has a single particle nature and its energy states seem to be predictable. The ground state (g.s.) of the 5H is expected to be of halo type with a strong correlation of two valence neutrons outside the tritium core. As for the tetraneutron nothing is known about its structure. The interest on the energy position of the 4H g.s. has been recently revived due to its importance for study of the 5H since the former is a subsystem of the later. A low laying resonance of 4H g.s makes 5H g.s a short lived one, because the later, instead of a direct 3- body decay, would decay sequentially into 2n+t channel through the 4H g.s as an intermediated step. The 4H resonance has been extensively investigated. A recognized estimation for the 4H was obtained from R-matrix analysis of p+3He scattering and, assuming the charge symmetry, furnished two lowest overlapping resonances at 3.19 MeV and 3.50 MeV [2] for ground and possible excited states. However, the resonance energy of the ground state measured in different reactions varies from 1.7 to 8 MeV [3], There were several experimental attempts to search for unbound 5H. Earlier works are compiled in Ref. [4], In an experiment with a stable beam, using 7Li(6Li 8B)5H reaction a resonance for the 5H at the energy of 5.2 MeV and with a width of 4 MeV has been claimed [5], In reactions induced by the absorption of slow pions in 6Li and 9Be targets (see e.g. Ref. [6]) the measured missing-mass spectra were reproduced by a phase space with broad resonances at energies of 7.4 and 11 MeV.
Direct reactions with secondary radioactive beams offer a serious advantage for studies of neutron rich exotic systems. One can expect that with neutron rich projectiles the production mechanisms will be more transparent due to relative simplicity of processes like one-step transfer of nucleon or cluster which are mainly involved.
Contradictive results on the 4H and 5H have been recently reported [7-10], Both these nuclei have been investigated at GSI by the invariant mass method. The 4H and 5H were produced simultaneously in the fragmentation of a 6He beam at 240 MeV/n on a carbon target [7], The double t+n coincidences were treated as originated from decay of the 4H system, whereas the t+n+n events were attributed to the 5H. The 4H resonance was found at rather low energy of 2.67 MeV, and a broad structure of the 5H was observed only [7], see Fig. 1. On the other side, earlier at FLNR, Dubna, Korsheninnikov et al. utilized the 1-proton transfer reaction p(6He,2p)5H, and by detecting the triple 2p+t coincidences, found a well defined peak which is consistent with a 1.7 MeV 5H resonance [8], see Fig. 2.
54
Proceedings o f the Third Eurasian Conference “Nuclear Science and its Application”, October 5 - 8 , 2004. Fig. 1. The invariant mass measurement at
GSI for the t+n+n system obtained in the fragmentation of the 6He beam at 240MeV/n on a
carbon target. The symbols are for the
experimental data. The solid, dotted, and dashed lines are a result of 3-body microscopic calculations of the 5H system assuming l/2+, 3/2+ and 5/2+ states respectively, from Ref. [7],
Fig. 2. The missing mass spectrum of 5H from
the reaction p(6He,2p)5H with Dubna 6He beam at 36 MeV/n. The histogram is for the experimental data. The curves 1-3 represent calculations of various backgrounds forming t+n+n continuum. The curve 4 is a fit assuming a Breit-Wigner resonance and background folded with the experimental conditions, from Ref [8],
In order to continue study of the 4H and 5H systems, the transfer reactions in t+d and t+t collisions respectively, have been applied at FLNR, Dubna. The 58~MeV triton beam, safe cryogenic deuterium or tritium targets and European Neutron Array "DEMON", for neutron detection, have been used there in kinematic complete measurements. For the 4H system the g.s. resonance was obtained at a higher energy than that of GSI [9], The parameters of the resonance are consistent with the established ones. The results for the 5H obtained in the t(t,p)5H reaction occurred to be not straightforwardly comparable with other data. A DWBA estimation of the 2-n transfer
process in this reaction indicates a large “ angular momentum ” effect. The reaction yield of
transferred angular momentum L=0 feeding the 5H g.s., is much reduced respect to those of higher L. Therefore the energy spectrum obtained corresponds mainly to 5H populated by L=2 transfer. Fig. 3(c) shows a possible affect of the g.s. resonance at around of 2 MeV as a distortion of the 5H decay angular correlation [10],
Fig. 3. The angular correlations of triton decaying
in CM of 5H obtained in the t+t reaction at FLNR. (a): solid line is for pure L=2 decay, dotted and dashed ones for 5H decay of 3/2+ and 5/2+ states respectively,
(b): symbols are the experimental data for the 5H
excitation energy bin of 3.5-5.5 MeV, the line is a model prediction for the interfering 3/2+ and 5/2+ resonances, shadow area is for the model folded with experimental conditions, (c): the same as (b) but for the energy bin 0-2.5 MeV. Deviation indicates a presence of the g.s. at ~ 2 MeV, [10],
The 1-proton transfer reaction d(6He,5H)3He has been later studied at FLNR. This projectile- target system was chosen because it gives opportunity to excite simultaneously also isobaric analogue states T=3/2 of 5He nucleus by the analogues reaction d(6He,5He)3H.
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Proceedings o f the Third Eurasian Conference “Nuclear Science and its Application”, October 5 - 8 , 2004.
Fig. 4. The 5H energy spectrum obtained in
the d(6He,5H)3He reaction at Dubna. Symbols are for the 3He+t coincidences, the arrow is t+n+n threshold, curves 1 and 2 are for an isolated g.s. resonance and the phase-space shape (PS)
respectively. PS includes final state n-n
interaction, from Ref. fill.
The experimental conditions for the investigation T=3/2 states in 5H and 5He by the reactions in question are practically the same. The result is shown in Fig. 4. One can see that without inclusion of a prominent and rather narrow 5H g.s. resonance one is unable to describe the spectrum. It is worth to mention that a similar spectrum have been obtained for the neutron transfer reaction leading to high 5He excitation. The similarity of the 5H and 5He excitation energy spectra is the first experimental evidence for the T=3/2 analogue state in 5He [11],
The question of existence of bound or quasi-bound tetraneutron system has attracted much interest, and several attempts to observe such object were done in the past, e.g. compilation [12], No manifestation of bound or resonant 4-n system has been found. The subject became much discussed after a suggestion that a bound tetraneutron could be formed in a break-up of 14Be projectiles [13], Theoretical estimations of 4-n g.s. are generally in accord that the system is unbound, but do not exclude existence of low laying resonance. Beams of 8He ions offer a new opportunity for study of
tetraneutron. 4n system can be produced from the 8He by the alpha transfer reactions on light
targets. Such experiment has been done at GANIL and preliminary results are shown in Fig. 5, [14],
Coincidence with neutrons sH e(d,bU )4 n results EtotfU): 24-50 /VSeV . 5BPS calculation + C background LM> AJig/e f U ) ; 7-2Q __ nn_nn calculation + C background Boundn \ »bound 4n L * c in (4 n ) in Alev
Fig. 5. The tetraneutron spectrum obtained
from d(8He,6Li)4n reaction with a 8He beam of 15.4 MeV/n at GANIL. The black histogram is for the inclusive yield of 6Li, the grey one for 6Li in coincidence with neutrons, dotted line is for 5 body (6Li +4n) phase-space distribution (PS), solid one is for the 5 body PS modified by the inclusion of 2 pairs of n-n final state interaction, from Ref. [13],
The obtained spectrum seems to follow phase-space shape with a 2o confidence level enhancement around 2 MeV. A weaker enhancement one could see also at -1.5 MeV. It is the region of bound 4n. However, even if true, the enhancement could be most likely explained as isospin forbidden transition leading to the particle stable 6Li(0+) state at 3.56 MeV, not as a bound 4n. More statistics is obviously needed to make conclusions.
The partial support by the in2p3-Poland accord No 02-106 is acknowledged. 56
Proceedings o f the Third Eurasian Conference “Nuclear Science and its Application”, October 5 - 8 , 2004. REFERENCES
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