Eur. Phys. J. Special Topics 162, 161–164 (2008) c EDP Sciences, Springer-Verlag 2008
DOI: 10.1140/epjst/e2008-00790-y

THE EUROPEAN PHYSICAL JOURNAL SPECIAL TOPICS

Quest for the

10

He nucleus

V. Chudoba1,2 , A.S. Fomichev1 , M.S. Golovkov1 , A.V. Gorshkov1 , V.A. Gorshkov1 , L.V. Grigorenko1 , S.A. Krupko1 , Yu.Ts. Oganessian1 , A.M. Rodin1 , S.I. Sidorchuk1 , R.S. Slepnev1 , S.V. Stepantsov1 , G.M. Ter-Akopian1 , R. Wolski1,3 , A.A. Korsheninnikov4,5 , E.Yu. Nikolskii4,5 , V.A. Kuzmin5 , B.G. Novatskii5 , D.N. Stepanov5 , P. Roussel-Chomaz6 , W. Mittig6 , V. Bouchat7 , V. Kinnard7 , T. Materna7 , F. Hanappe7 , O. Dorvaux8 , and L. Stuttge8 1 2 3 4 5 6 7 8

JINR, Flerov Laboratory of Nuclear Reactions, 141980 Dubna, Russia Czech Technical University, Faculty of Nuclear Sciences and Physical Engineering, 115 19 Prague, Czech Republic The Henryk Niewodniczanski Institute of Nuclear Physics, Krakow, Poland RIKEN, Hirosawa 2-1, Wako, Saitama 351-0198, Japan RRC “The Kurtchatov Institute”, Kurchatov Sq. 1, 123182 Moscow, Russia GANIL, BP. 5027, 14076 Caen Cedex 5, France Universite Libre Bruxelles, PNTPM, Bruxelles, Belgium Institute de Recherches Subatomiques, IN2P3/Universit´e Louis Pasteur, Strasbourg, France

Abstract. The spectrum of 10 He was studied by means of the 3 H(8 He, p)10 He reaction at a laboratory energy of 25 MeV/A and small center-of-mass angles. Missing mass spectrum of 10 He was derived from the obtained p-8 He coincidence. A resolution of 0.7 MeV was achieved in this spectrum for the measured 10 He energy. Most likely, a well isolated group of 10 events detected between 2 and 5 MeV and showing a maximum at about 3 MeV in the spectrum of the present work exhibits the 10 He g.s. resonance.

1 Introduction Search for the heavy neutron-rich isotope 10 He has a 40-years history. 10 He has been one of the key nuclei in the studies of the shell model, because it has a maximal neutron to proton ratio (N : Z = 4 : 1) among the known nuclei and should have double closed shell structure (Z = 2; N = 8). At first, experiments were carried out in which one tried to search a nuclear stable 10 He among fragments emitted in the spontaneous fission of 252 Cf, among products of reactions with thermal neutrons, high-energy protons, deuterons and heavy ions, e.g. [1–3]. Obtained negative results of these experiments indicated that 10 He could not be nuclear stable. A few of the very precise experiments were carried out searching for a stable 10 He, e.g. in the reactions 232 Th+11 B [4], 11 Li+12 C [5], but all these attempts were unsuccessful and only showed that the 10 He production cross-sections were too small. These results excluded the stability of 10 He. In 80th it was experimentally demonstrated, that the 9 He isotope is less unbounded than expected. The ground state of 9 He and two excited states were known from the (π − , π + ) reaction measured in Ref. [6]. The ground state resonance was found at 1.27 MeV [7] above the 8 He + n threshold. This results made a basis for theory calculation of the resonance properties of 10 He. Unstable 10 He should decay into three particles, 8 He+n+n, because the subsystems 8 He+n and n+n have no bound states. To clarify this situation it is necessary to study the 10 He nucleus taking into account the three-body decay. Using the first approximation (energy evaluated as

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The European Physical Journal Special Topics

2Er (n+8 He)−Epairing ) energy less than 1 MeV was obtained for the 10 He resonance state. This estimation is in agreement with the strict three-body calculations performed by the authors of Ref. [8] who obtained resonance energy 0.7–0.9 MeV. Later on, the 10 He nucleus was searched as a resonance state which was found in 1994. At RIKEN (Japan) a beam of 11 Li (61 MeV/A) hitting a CD2 target was used [9]. Two reactions were investigated: 11 Li+CD2 → 10 He+X and 11 Li+C → 10 He+X. 10 He was revealed in the invariant mass spectrum. A maximum with energy (1.2 ± 0.3) MeV above the 10 He decay threshold was observed and it was accepted as a ground-state resonance of 10 He with width Γ ≤ 1.2 MeV. Excited states were not observed for 10 He in this work. The direct decay of the 10 He ground state with simultaneous emission of two neutrons was confirmed. The reaction 10 Be(14 C,14 O)10 He was studied at Hahn–Meitner Institute in Germany, where the radioactive beam 14 C (24 MeV/A) bombarded a beryllium target [10]. The ground state with energy (1.07±0.07) MeV was found for 10 He. Since the assumed ground state of 9 He (1.27 MeV) was higher than the 10 He ground state, the authors of Ref. [10] came to a conclusion that only simultaneous emission of two neutrons is possible. Two excited states E∗ = (3.24 ± 0.20) MeV and E∗ = (6.80 ± 0.07) MeV were also observed for 10 He. Reported results are shown in Tab. 1. Table 1. Reported data on the Reference Energy of state (MeV) Width (MeV)

[9] 1.2 1.2

10

He spectrum.

[10] 1.07 0.3

This work ≈3 ≈2

In agreement with the shell model, 10 He was considered like a nucleus with filled shells, it is presumed, p1/2 state is located in low-energy region in 10 He. However, later a s-wave virtual state was observed as a ground state in 9 He (8 He + n) [11]. The energy of this newly observed ground state was less than 0.2 MeV, more than 1 MeV lower than the reported p-state. The shell 2s1/2 in 10 He is located close to 1p1/2 or even lower. In this context it is supposed, that closed shell is not situated for N = 8, but for N = 10, i.e. in nucleus 12 He. Aoyama [12] predicted the ground state of 10 He. The ground-state solution which corresponds to the experimentally observed [9,10] resonance energy around Er = 1.2 MeV could not be found, maximum energy of the ground state calculated by Ayoama in his theory papers was 0.05 MeV. On contrary, the solution of the first excited state was predicted at Er = 1.68 MeV. The authors of [13] concluded, the ground state of 10 He has not yet been observed. Also, it should exist in the energy region of the 8 He + n + n decay threshold. Very recently the spectrum of 9 He was studied [14]. The lowest resonance state was found at energy 2.0 MeV with a wide of ≈2 MeV and was identified as 1/2− . The position and width of the 1/2− resonance reported in the present work for 9 He disagree with those published in [6] and [7]. Other obtained results were generally consistent with the existing 9 He data. According to such new information, our expectations could be quantitatively different.

2 Choice of reaction The double charge exchange reaction 10 Be(14 C,14 O)10 He used in Ref. [10] implies a quite complicated exchange between two colliding nuclei, where two protons are transferred from the 10 Be target to the projectile 14 C from which two neutrons are transferred back to the target nucleus. The cross-section is extremely low, in order of ≈ 0.1 µb/sr or less than this value and hence it is very difficult to predict the probability value of the ground state population and its cross-section value. Generally it is better to use single step reactions. In the reaction 11 Li+2 H used in Ref. [9] proton was transferred from 11 Li to the deuteron. But when the proton is suddenly removed from the core of 11 Li, it does not mean that immediately could be obtained the ground state of the new nucleus. The proton removal might have influence on the width of the resonance and even influence on the resonance energy.

Symmetries and Spin

163

Fig. 1. Geometry layout of the experiment.

In our experiment the reaction of radioactive beam 8 He hitting against a tritium target was used. It has been shown in many experiments that it is a one step reaction tending to populate single-particle states. Results obtained recently on the 5 H resonance states populated in the two-neutron transfer reaction 3 He(t, p)5 H [15] give a hope that a similar study made for 10 He is possible. The reaction 8 He + t → p +8 He + n + n could allow us to determine the spin and parity of the 10 He resonance state. For this reaction, having a quite small negative Q-value, one could anticipate a large cross-section value of about 500 µb/sr. We considered to study this reaction at small angles in the center-of-mass system.

3 Experimental setup Experiment was performed at the separator ACCULINNA [16] installed on a primary beam line of the U-400M cyclotron. Its separation scheme involves an achromatic ion-optical system with a production target and wedge degrader. The primary beam of 11 B with energy 34 MeV/A and intensity of 1.5–3 pµA was used in our experiment. The secondary beam of 8 He with energy 25 MeV/A and intensity of ≈104 s−1 was focused on the 4 mm thick target cell filled with tritium gas. The target cell was equipped with 12.7 µm stainless steel entrance and exit windows hermetically welded to the cell body. For the sake of safety this cell was embedded into a protective volume also supplied with 12.7 µm stainless steel windows. When the target was cooled to 25 K a gas pressure of 812–820 mbar was set in this volume. Experimental setup and the studied reaction diagram are shown in Fig. 1. Time-of-flight method was used to identify and to measure energy of the beam particles. A pair of multi-wire proportional chamber determining the position of the incident particle on the experimental target was used. In this work we used two silicon detector telescopes for charged reaction products. The first ∆E × E telescope located 10 cm upstream of the target was intended only for the detection of protons, as soon as they could be emitted back from the target in the laboratory system. This telescope consisted of a 300 µm thick annular double-sided detector and 1000 µm thick single-sided annular detector. Both detectors had 82 mm external and 32 mm inner diameters and covered angular range of 158◦ –171◦ . Another telescope installed 36.5 cm downstream of the target consisted of 5 identical layers of Y and X 1000 µm single-sided square detectors divided into 16 strips. Each detector had side of 60 mm and therefore products flying in angular interval of 0◦ –5◦ could be detected. The array of 36 scintillation modules of the time-of-flight neutron spectrometer DEMON [17] was installed at a distance of 310 cm behind the tritium target and covered angular range of 0 ◦ –12 ◦ , in horizontal and vertical directions.

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4 Preliminary results Quite a large cross-section value expected for the 3 H(8 He, p)10 He reaction allows one to anticipate that the 10 He spectrum could be observed in rather clear conditions being well above background. Conditions fulfilled in the present experiment allowed us to acquire clean spectra with a quite high experimental resolution. No event corresponding to the formation of the 10 He was observed in expected energy interval between 0 and 2 MeV above the 10 He decay threshold. Missing mass spectrum of 10 He was derived from the obtained p-8 He coincidence. A resolution of 0.7 MeV was achieved in this spectrum for the measured 10 He energy. It is noteworthy that one detected event in this spectrum corresponds to estimated cross section of 16 µb/sr for the 3 H(8 He, p)10 He reaction. So low achieved cross section limit leads one to conclusion about the lack of a narrow 10 He resonance in the energy region from 0 to 2 MeV. Most likely, a well isolated group of 10 events detected between 2 and 5 MeV and showing a maximum at about 3 MeV in the spectrum of the present work exhibits the 10 He g.s. resonance. The p-8 He correlations obtained for these 10 events show a pronounced tendency to a strong final state interaction between the two neutrons emitted in the 10 He decay. The low cross section limit achieved in the present project in the measurement of the 10 He missing mass spectrum implies that conclusion about a narrow 10 He g.s. resonance at 1.07 MeV drawn in [10] can’t be confirmed. One can reconcile the 10 He spectrum obtained in the present work with the spectrum reported for this nucleus in [9]. Indeed, the sudden removal of proton from the 9 Li core of 11 Li was employed in the work of [9] and the specific halo structure of 11 Li could be the cause that the low-energy side of the 10 He resonance was populated preferentially in this reaction. This could shift the observable maximum of the 10 He g.s. resonance from 3 MeV, as it is given in the present work, to the value of 1.2 MeV presented in [9]. This work was partially supported by the Russian Foundation for Basic Research grants 05-02-16404 and 05-02-17535, and by INTAS grants 03-51-4496 and 03-54-6545.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

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