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1. Introduction. Nuclei with a Neutron Halo

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1. Introduction. Nuclei with a Neutron Halo PRODUCTION AND IDENTIFICATION OF THE SUPERHEAVY HYDROGEN HYPERNUCLEUS ~H 1 2 1 1 1 3 3 L. Majling • t, Yu. Batusov , J. Lukstins , A. Parfenov , M. Solar and B. Sopko (1) Joint Institute for Nuclear Research, Dubna, Russia (2) Nuclear Physics Institute, GAS, Rei, Czech Republic (3) Czech Technical University, Prague, Czech Republic t E-mail: [email protected] Abstract The hypernucleus ~H, existence of which has been suggested recently, could be identified unambiguously in experiments with light relativistic hypernuclei prepared for Nuclotron. Key-words: relativistic hypernuclei, exotic nuclei 1. Introduction. Nuclei with a Neutron Halo In the last twenty years a new branch of nuclear physics, namely physics of nuclei in the vicinity of the neutron drip line has been constituted [l]. The dripline is the limit of the nuclear landscape, where additional neutrons can no longer be kept in the nucleus. This dripline has been really reached for light nuclei (Z :::; 6), see Fig. 1. One can see irregularities at the border of neutron stability. There is considerable interest in unbound nuclear systems close to the driplines, both in themselves and as subsystem of Borromean halo nuclei. The term Borromean was coined in ref. [2] to denote a bound three-body system (core + n + n) for which no binary subsystem is bound. In Fig. 1, the /3-stable isotopes are marked by ( E9) and the type of neutron halo is also given. The short range of the nuclear force and the low separation energy of the valence nucleons results, in some cases, in considerable tunneling into the classical forbidden region and more or less pronounced halo may be formed. As a result the valence and the core subsystems are to a large extent separable. Therefore, halo nuclei may be viewed as an inert core surrounded by a low density halo of valence nucleons and described in few-body or cluster models. There have been used different experiments which have contributed to the present picture of nuclei with neutron halo. The development of techniques for the production of exotic radioactive nuclei and making beams of them, has been of key importance for the progress of the field. The ground-state properties such as mass, spin and moments are mainly measured with stopped low-energy beams at ISOL facilities. The energetic radioactive beams obtained with the in-flight technique are the main tool for studies of nuclei at the driplines. In this contribution, we discuss the potential of hypernuclear physics. 282 2. Stabilizing role of the J\ hyperon It is well known that A hyperon makes the nuclear system more stable. In the lower part of Fig. 1, the solid line determines hyperfragments observed in emulsion [3]. The stabilizing influence of the A hyperon is obvious: the 'lake of instability' (8 Be, 9 B) is filled as well as irregularities at the border (5He, 7He ). Large scale systematic studies of hypernuclei began with the advent of separated K- beams, which permitted the use of counter technique and confirmed the brilliant suggestion of Podgoretsky [4]: instead of hunting down decays of random fragments, to study hypernuclear production in strangeness exchange reaction Az (K-, 7r-) ~Z. In one-step direct reactions such as (K-, 7r-) or (7r+, K+) [5, 6] and (K-, 7r 0 ) or (e, e' K+) [7, 8] the level structure ofhypernuclei can be experimentally studied and information on structure can be obtained. 19 nuclei ~EJ~l 12¥ll 13¥IEJf~EJEJI isc 11 1? 112°c1 13 172 I 8~P I I 10~ll 11~IEJI B IE]~ I ~ I ~ I 7Bel 19~1110Be1111~e1112Bel 114~el 6 7 1 ~ 11 ~ l~l 9Li I I 112~i1 f3HJ f4HJ f6Hel f8Hel LJ~ L:::J ~ [l]Li]EJ C!iJ hypernuclei Notation Production 4H 6H 7H • n -t A (K-,n-) or (n+,K+) ~H Ao A* A* o p-t A (K-, n°) or (e, e' K+) 4 He 5 He 6 He 7 He 9 He * pp-t nA (K-,n+) or (n-,K+) A• A• Ao Ao ~He A * ~Li X~i ix Li llLi ~~i * * ~Be 9 Be 10 Be 11 Be 12Be 13Be A • A 0 A 0 A* A* JOB 11 B 11B L5B XB Ae A• A* A* 12 0 isc 16c A• Ao A* Figure 1: N /Z diagram of light nuclei and hypernuclei. Hypernuclei are shown together with their production reaction. See text for detail Recently, Tamura solved the huge technical problems and constructed "Hyperball", a large acceptance Ge detector array dedicated to hypernuclear 1-ray spectroscopy [9]. An impressive result has been obtained: the observed 1-rays from ~Li hypernucleus indicate a significant (:::::: 20 %) contraction of the 6 Li a+ d) core in ~Li. So, HYPERNU- 283 CLEAR PHYSICS COULD BE USED IN THE STUDIES OF THE LOOSELY BOUND NUCLEAR SYSTEMS such are nuclei with neutron halo. However, there is only one hypernucleus with neutron halo, 1He, which can be produced in one-step direct reaction, p-> A. The announcement of the plan for the new hypernuclear facility FINUDA at the ¢­ factory DA<I>NE (Frascati) [11], initiated great expectations. It opens unprecedented possibilities: very low energy of kaons from the ¢ decay allow the use of very thin solid stopping targets. The detector is designed to register both the JT- from the formation re­ action and charge products from the hypernuclear decay. We suggested [12] to study there STRANGENESS AND DOUBLE CHARGE EXCHANGE (S&DCX) reaction (K-,7r+), which opens way to the production of neutron-rich hypernuclei. There are two paths how to arrive at the An p-2 states: either by the pion charge exchange: K- p-> A7r0 , JTO p-> nJT+ or through the L,N -> AN conversion: K- P __, JT+ L,-, L,-p-> An. Hypernuclei produced in such a reaction are marked by * in Fig. 1. As we can see no A hypernucleus produced in S&DCX reaction has been observed until now. Unfortunately, both protons have to be in the same target nucleus, so cross sections are inevitably low. Nevertheless, the search for neutron-rich A hypernuclei is one of the items on the FINUDA's list [13]. The current status of experimental efforts to produce neutron-rich A hypernuclei is given in Table 1. We have not only upper limits for reactions with stopped kaons, [14], [15], but even first positive results [16] for another type of S&DCX reaction, namely (7r-, K+). In the experiment KEK-PS-E521, about 40 events were registered in the bound region of 1gu. The results confirmed qualitatively the calculations given by Tretyakova and Lanskoy [17, 18, 19]: The cross sections of the (7r-, K+) reaction are smaller by three orders of magnitude than those of (JT+, K+) reaction; the two-step mechanism dominates over the single-step mechanism [17] (via virtual ;:,- admixture in the hypernuclear state appearing due to ;:,- p +-+ An coupling [20]); the cross section is larger for targets with neutron orbit vacancy. Table 1: Status of the hypernuclei production in S&DCX reactions Reaction Ref. ~H lH ~He igu 1KBe ix_c (K:;t, JT+) [14] (KEK) < 23 < 6.1 < 6.2 [15] (FINUDA) < 3.5 < 4.9 < 2.6 (7r-,K+) [16] (KEK) 12±2 7 [19] (calculation) 22 2.5 The results of a recent experiment [21], revived our interest in lightest neutron-rich hypernuclei. There, the primary beam of the radioactive ion 6 He was used for the pro­ duction of the resonance state in proton knock-out reaction 6 He (1 H, 2He) 5 H. The two protons from the decay of 2 He from the reaction were detected as a peak at an energy 1.7 MeV above the 3 H+n+n threshold. In the next section we will show that ~H [22], can be identified unambiguously through its pionic decay ~H -> JT- + 6 He. 284 3. Experiment At the LHE JINR, an original approach of hypernuclear experiments was elaborated on the basis of beam nuclei excitation to produce high energy hypernuclei (up to 3.5 A·GeV the experiments, carried out at the Synchrophasotron and 6 A-GeV planned at the Nu­ clotron). Data on the hypernuclear lifetimes and the production cross sections were ob­ tained [23] using the streamer chamber in the Synchrophasotron beams. The Nuclotron beams offer new possibilities of carrying out hypernuclear experiments under condition of the significantly increased data collection rate. It was suggested [24, 25, 26] to investigate the properties of the lightest hypernuclei by using the SPHERE spectrometer. We hope that in a short period the spectrometer based on the proportional chamber trackers will be commissioned. In addition, the extracted Nuclotron beam energy is expected to be increased soon up to 4-6 A-GeV - the value necessary for hypernuclear experiments. In this context we see that the hypernuclear research program for the first year could be extended. The measurement of the binding energy of the lightest ~H, ~He hypernuclei and the lifetimes of light hypernuclei are planned among the first, with the ultimate task [27] of the scheduled experiments to investigate the AN weak interaction matrix in nonmesonic decays of \0 B. Recently, it was proposed to search for the neutron halo hypernucleus ~Hin the very beginning of the research program. The suggestion is based on physical interest and the advantages of the experimental approach. PC, PC, PC, \ 8 He M Figure 2: SPHERE spectrometer adapted for the first hypernuclear experiments, for example, for AH production (particularly, ~H) in the 7Li beam. The stripping proton 7 from Li->~H +pis not shown. T - target; S, C1,2 - trigger counters; V - vacuum decay volume; M - magnet; PC1_ 4 - proportional chambers Let us note some advantages of the research method and analyze how they can be achieved at the Nuclotron experiments.
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