Reactions near the neutron drip-line

Nuclear Physics A 752 (2005) 279c–288c

Reactions near the neutron drip-line Yorick Blumenfeld Institut de Physique Nucléaire, IN2P3-CNRS, 91406 Orsay Cedex, FRANCE Abstract : The advent of radioactive ion beams has opened the possibility of studying nuclear reactions induced by neutron-rich nuclei. In particular, light nuclei near the drip-line, or even resonances beyond the drip line can be populated and investigated. In this paper techniques developed to study direct reactions in inverse kinematics are described. The potentialities of such studies are illustrated by examples chosen from experiments performed at the GANIL facility. Recent investigations of the structure of neutron rich Helium isotopes using elastic, inelastic and transfer reactions are described, followed by a search for the ultimate exotic nucleus, the tetra-neutron, and a study of the behavior of neutron distributions when approaching the drip-line for Oxygen isotopes. I. Introduction The detailed study of the properties of unstable nuclei has been at the forefront of nuclear physics research in recent years. In particular, light neutron-rich nuclei have revealed novel structures such as neutron halos and skins, and brought new insight into the understanding of how nucleons assemble and correlate. The role of the continuum is exacerbated when approaching or crossing the drip-line, and will generate new and unexpected phenomena, uniquely characteristic of these species. The halo phenomenon[1] was discovered two decades ago through the simple measurement of total reaction cross sections. Since then, the sophistication of reaction experiments with radioactive beams has steadily increased, and we will attempt to show that today, extensive angular distributions for precisely identified states can be measured with very low beam intensities. Direct reactions to study unstable nuclei are performed in inverse kinematics, where the radioactive beam strikes a target containing the light probe. The kinematics of the reaction are generally reconstructed from the measurement of the energy and angle of the recoiling particles. Details on the experiments will be given in section 2. Section 3 will present studies on bound and unbound helium isotopes. In section 4, a search for tetra-neutron systems through the (d,6Li) reaction will be described while section 5 will present results on the heavier neutron-rich oxygen isotopes. Conclusions and an outlook will be given in section 6. II. Experimental methods In the experiments described in the following, the radioactive beam is ether produced by fragmentation of a stable primary beam on a thin production target, and analyzed by a fragment separator, or, in the case of 8He beams, delivered by the new SPIRAL facility at GANIL. In the latter case, a 13C primary beam from the GANIL cyclotrons impinges on a thick C target, and the 8He are mainly produced by projectile fragmentation. After ionization to the 2+ charge state, they are accelerated to 15 MeV/A by the CIME cyclotron. The typical 8 He beam intensity is 15000 pps. The unstable nuclei of interest impinge on a solid CH2 or CD2 target. The kinematics of the recoiling light particles are shown on fig. 1 in the case of (p,p'), (p,d) and (d,p) 0375-9474/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2005.02.146

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reactions induced by a hypothetical 30 MeV/A 32Mg beam. It can be seen that a precise measurement of the energy and angle of the recoiling light particle will furnish the excitation energy imparted to the nucleus and the scattering angle in the center of mass frame. The MUST detector[2] has been designed to fulfill the requirements of such experiments. It consists of 8 large area Si-strip, Si(Li), CsI telescopes with associated electronics and data acquisition system. The first stage of the telescopes consists in a 300µm thick 60 X 60 mm2 Si-strip detector with a strip pitch of 1 mm in both the horizontal and vertical directions. Each strip detector is backed by a lithium drifted silicon diode (Si(Li)) of approximately 3mm compensated thickness and a 15mm thick CsI crystal. The detector is modular and the set up is adapted to each experimental situation (elastic, pick-up or stripping) in order to cover with maximum efficiency the angular range of interest. Particles stopping in the first stage (protons of less than 6 MeV) are identified by energy and time of flight measurements, while higher energy particles traversing the strip detector are identified with the standard ∆E-E technique.

fig. 1 : kinematics of the recoiling particle for various direct reactions induced by a 30 MeV/A 32Mg beam : a) (p,p') b) (d,p) c) (p,d). Solid line is for the ground state, dashed line for a hypothetical 1 MeV excited state Because of the large emittance of the secondary beams, it is necessary to perform event by event ray tracing of the incoming nuclei. This is commonly done at GANIL using two low pressure multi-wire proportional counters (CATS)[3] located upstream from the target. These detectors yield a position resolution of approximately 0.3mm in X and Y and also furnish a start signal for time of flight measurements with a resolution of 400 ps. These counters function reliably for counting rates up to 5X105pps. In order to select the reaction channel of interest and thus strongly reduce the background in the particle telescopes, the scattered nuclei are detected and identified, either in the focal plane of the SPEG spectrometer[4], through an energy loss measurement in a Bragg chamber and a time of flight measurement between a fast plastic scintillator and one of the tracking detectors, or in a plastic scintillator placed behind the target. As can be seen from fig.1, the excitation energy resolution depends both on the energy and the angular resolution of the light particle measurement. A typical value of the excitation energy resolution obtained in such experiments is 500 keV, and will vary slightly according to kinematical conditions. III. Helium Isotopes Neutron-rich Helium isotopes are particularly intriguing objects. The even isotopes He are borromean and are known to present extended neutron wave functions. Odd 7,9He are unbound, and rather little is known about their spectroscopy. A comprehensive program has thus been implemented at GANIL, aiming to study the various He isotopes using elastic, inelastic and transfer reactions. In a first step, in order to further probe the halo structure of 6,8

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He, elastic and inelastic scattering on protons was measured over a large angular range. The angular distributions for elastic scattering and inelastic scattering to the first 2+ state of 6He located at 1.87 MeV are presented on figs. 2a) and 2b) respectively[5]. The data are compared to theoretical calculations performed using a fully microscopic, complex, non local potential[6] based on large basis shell model calculations of 6He[7]. Two shell model densities are investigated. In the first, labeled no halo, complete 6
ω wave functions are used. However, in this case, the binding energy of the last neutron is larger than the experimental separation energy. In the second case, labeled halo, the bound state potential is changed so that the 1p-shell binding energy approaches the experimental single neutron separation energy. The neutron density then exhibits an extended halo-like tail. In the case of elastic scattering, the halo (full line) and non halo (dashed line) calculations are very similar up to 60o and differ notably at larger angles. The few data beyond this angle are better reproduced by the halo description but clearly data at larger momentum transfers are required to probe the halo structure through elastic scattering. The inelastic scattering distribution is remarkably sensitive to the halo structure over the entire angular domain. The inelastic data are better accounted for by the halo calculation.

fig. 2 : Angular distributions for elastic a) and inelastic b) scattering of 6He + p. The dashed line corresponds to "no halo" calculations and the solid line to "halo" calculations [5]. While the main structure of 6He can be represented as an α+2n configuration, the question arises whether some t+t clustering could also be present. In the case of the 6Li nucleus, both α+d and 3He+t clustering were shown to be possible, and the importance of both configurations was studied by analyzing the angular distributions of the 6Li(p,3He)4He

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reaction[8]. In order to perform a similar study for 6He, angular distributions for the 6 He(p,t)4He reaction were measured at GANIL using a 25 MeV/A secondary 6He beam and detecting the reaction products with the SPEG spectrometer and the MUST array[9]. The α+2n configuration will lead to 2n transfer and thus mainly forward angle 4He, while the t+t configuration will populate backward scattering angles. The experimental angular distribution measured is displayed on fig. 3.

fig.3 : Angular distribution measured for the 6He(p,t)4He reaction [9]. Dashed line is calculation with a pure a+2n configuration; crosses with a pure t+t configuration and solid line with spectroscopic factors of 1 and 0.09 for a+2n and t+t respectively. DWBA calculations including both 2n and t transfer were performed. The coupling to the continuum was taken into account through a dynamical potential described in ref.[10], which was used as the entrance channel 6He + p potential. As no data exist for α+t elastic scattering near the present energy, elastic scattering data for α +3He [11] was used to obtain the potential for the exit channel. The potential derived in ref [12] gave a good simultaneous description of α + 3He elastic scattering and the 6He(p,t)4He reaction. The calculations are compared to the data on fig. 3. The dashed line corresponds to the DWBA calculation where only the 2n transfer is taken into account, with a spectroscopic amplitude equal to 1. The crosses correspond to the triton transfer, with a spectroscopic amplitude of 0.25. The solid line represents the coherent sum of these 2 processes, with the above values of spectroscopic amplitudes. The best fit to the backward angles leads to a spectroscopic factor for the t+t configuration between 0.06 and 0.09, which is small. Its inclusion is important however, to reproduce the angular distribution in both the forward and backward directions. Very recently, further experiments have been performed making use of the 8He beam from the SPIRAL facility. In one experiment, using a CH2 target, and the MUST detectors placed in the forward hemisphere, angular distributions for 8He(p,p'), 8He(p,d) and 8He(p,t) reactions were measured simultaneously. Confrontation with theoretical calculations is ongoing, in order to extract information on the density and transition density distributions, and on spectroscopic factors, and will be presented in forthcoming publications [13]. By using a CD2 target and placing the MUST telescopes in the backward hemisphere, resonances in 9He were investigated through the 8He(d,p) reaction[14]. Unfortunately the lack of statistics does

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not allow to conclude on the existence of an eventual low-lying s-wave resonance as the ground state of 9He. IV Search for the tetra-neutron Even more exotic that the study of neutron-rich Helium isotopes, the debate about the possible existence of neutral nuclei has a long history that may be traced back to the early sixties [15]. However, forty years later there is an overall consensus that no conclusive evidence of a bound or resonant multi-neutron has yet to be found. The availability of intense neutron-rich radioactive beams provides the opportunity to reinvestigate this long-standing problem in a highly selective way, since some of these nuclei may exhibit a structure of a core + multi-neutron cluster. The existence of a bound 4-neutron system has been recently suggested by experimental results on 14Be break-up. Neutrons were detected in liquid scintillator cells in coincidence with 10Be fragments. Six neutron events exhibited a deposited energy greater than the energy deduced from the time-of-flight measurement, and their characteristics were compatible with an interpretation in terms of a bound tetra-neutron state [16]. If confirmed, the observation of « Tetraneutron » would require an important revision of the existing models of nuclear forces. Several theoretical studies have been conducted, and the most recent published by S.Pieper [17] stresses the huge impact that a bound tetraneutron would have on our present knowledge on nuclear 2- and 3-body forces. However, despite their sophistication such calculations cannot give any definite conclusion regarding the existence of the 4n as a resonance. Pieper does suggest that a resonance some 2 MeV above threshold would not be incompatible with his calculations. In order to approach the problem from another experimental viewpoint, a search for that tetra-neutron via the α-particle transfer reaction 8He(d,6Li)4n in inverse kinematics was undertaken[18]. The predominant structure of the 8He ground state is usually viewed as an almost inert α-particle core, surrounded by a skin of four neutrons . The 4-neutron system can thus be released by transferring the α-particle core from the 8He projectile to the deuteron target nucleus. The c.m. energy spectrum of the 4n system was directly determined by measuring kinetic energies and emission angles of 6Li ejectiles, using the MUST light charged particle detection array. Such a missing mass measurement has the potential to provide evidence for either a bound or a resonant tetra-neutron. Additionally, neutrons were detected in coincidence with the 6Li using a set of four plastic detectors with thickness 90mm placed behind the MUST telescopes. The excitation energy spectrum of the 4-neutron system in the center of mass, deduced from the 6Li total energy and scattering angle, without any condition on the neutron detectors, is shown on fig.4a). Negative energies correspond to an eventual bound tetraneutron, and positive energies to the continuum with eventual resonant states. This spectrum corresponds to data obtained with 4.7.109 8He incident on the 1.1 mg/cm2 CD2 target. In order to estimate the contribution of background due to the presence of 12C nuclei in the CD2 target, data were also accumulated using a pure carbon target. Within the limited statistics, no structure was observed in the spectrum with the 12C target. The carbon background is shown in fig.4a) (dashed line), with appropriate normalization. The experimental spectrum was compared with phase space calculations, shown in fig.4a). The dotted curve represents the result of a five body phase space calculation (i.e 4 independent neutrons + 1 6Li in the exit channel), Results corresponding to two correlated pairs of neutrons (nn-nn) in the exit channel are shown as a solid line in fig.4a). Both curves are normalized to the high energy part of the experimental spectrum, where the decreasing slope results only from kinematical cuts. Note that background due to the contribution of 12C in the target is added to these curves. One observes that the 4n spectrum above 5 MeV is

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nicely reproduced by the (6Li-nn-nn) simulation. This result might emphasize the importance of nn-nn correlations in the structure of 8He ground state. Another interesting feature of this spectrum is the presence of a resonant-like structure at about 2.5 MeV above threshold and a second one at about 1 MeV below threshold, in the “bound tetraneutron” region. However, large statistical uncertainties preclude any firm conclusion. Moreover, it must be noted that the counts observed in the spectrum below the 4n threshold, which conceivably could correspond to a bound tetra-neutron, may be attributed to the background due to the carbon of the target, in the limit of statistics.

C O U N T S

b)

a) C O U N T S

E*cm(4n) (MeV)

E*cm(4n) (MeV)

fig 4. a): Center of mass energy spectrum of the 4n system from the 8He(d,6Li)4n reaction. Dashed line is the contribution from the C in the target. Dotted and solid lines are phase space calculations (see text). B): Same in coincidence with neutrons.

Additional information should be provided by examining the data in coincidence with neutrons, which could help to select the reaction channel of interest. Fig. 4b) displays the 4n missing mass spectrum in coincidence with at least one neutron triggering the plastic detectors. As in fig.4a), plain and dotted lines represent the results of (6Li-nn-nn) and (6Lin-n-n-n) phase space calculations in case of coincidence with one or more neutrons. Note the good agreement of (6Li-nn-nn) phase space predictions with the coincident spectrum above 5 MeV. On the other hand, no counts were observed in the coincident spectrum, corresponding to the peak observed with ~20 counts at ~1 MeV below threshold. If such a structure would originate from the formation a bound tetra-neutron, a coincident/singles ratio of ~30% would have been expected, according to simulations. The total absence of counts in the coincident spectrum precludes this interpretation, and, as discussed above, the likely origin of these counts in the negative c.m. energy region is 6Li emitted in reactions on 12C nuclei in the CD2 target. Corresponding to the resonant-like structure with ~30 counts observed at 2.5 MeV in the singles spectrum, about ten counts are observed in the coincident spectrum of fig.4b) below E(4n)=5 MeV, which are not reproduced by phase-space calculations. Here the coincident/single counting ratio is consistent with the predictions of the Monte Carlo simulations quoted above. In the limits of the present statistics, these results could be consistent with the existence of a resonance in the four-neutron system located around 2.5 MeV above the 4n threshold. Very recently, this experiment has been performed anew, and a factor 4 larger statistics were accumulated. A definite conclusion on the significance of the resonance should thus be forthcoming.

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V. Neutron-rich oxygen isotopes The modification of magic numbers far from stability is one of the major topics of radioactive beam research. Departures from standard shell model behavior have been observed for several regions near the neutron drip-line. In particular, evidence for quenching of the N=8, 20 and 28 shell gaps has been given around 12Be, 32Mg and 44S respectively. Several reasons can be invoked to interpret modifications of shell structure for from stability. The most straightforward is the onset of deformation, which certainly plays a role in the island of inversion around 32Mg. The importance of the n-p monopole interaction between spin-orbit or spin-flip partners is a possible explanation for shell modifications in several regions[19]. Perhaps the most tantalizing interpretation is the weakening of the spin-orbit potential far from stability due to the diffuse density distribution which accompanies neutron halos or skins. However definite experimental evidence of such an effect has yet to be shown. The oxygen isotopic chain plays a particular role among light nuclei. The last bound oxygen isotope was found to be 24O, while the last bound fluorine isotope was found to be 31 F. This suggests rapid structural changes for the oxygen isotopes as a function of neutron number. Recent studies of 22O nuclei[20,21] have provided unexpected results. They measured the energy of the first 2+ excited state to be E(2+)=3.17 MeV, and the quadrupole transition probability B(E2)=21 ± 8e2fm4. The systematics of the 2+ state energy for the oxygen isotopes shows that the energy decreases starting from 16O to 20O and increases for 22 O which suggests a subshell closure at N=14. This subshell closure is also supported by the B(E2) value which is lower for 22O than for 20O. It should be noted, that this increase of E(2+) energy for N=14 is not observed for the neighboring isotopic chains of Carbon, Neon and Silicon. The value of B(E2), which is measured through electromagnetic processes, is directly related to the proton excitations in the nucleus. Therefore, the neutron distributions are viewed through the n-p interaction, the behavior of which may not be well known far from stability. It thus appears important to also investigate the excitations using a probe directly sensitive to the neutrons in the nucleus, hence the use of proton elastic and inelastic scattering. Indeed, at energies around 50 MeV, the interaction strength of protons with the neutrons in the nucleus is 3 times greater than the interaction strength between protons. Proton elastic and inelastic scattering was measured in inverse kinematics at GANIL with 20O[22] and 22O[23] secondary beams at 43 and 46.6 MeV/A respectively. Angular distributions for elastic scattering and inelastic scattering to the first 2+ state were obtained, despite a beam intensity limited to 1000 pps in the case of 22O. A phenomenological analysis was used as a first approach to the angular distributions. Two optical potential recipes were tested, Bechetti and Greenlees[24] and CH89[25] and CCBA calculations were performed with the code ECIS. The elastic angular distributions are reasonably reproduced with both potentials, CH89 doing a slightly better job at large angles in the case of 22O. For the inelastic scattering, the normalization of the calculation to the data yields the deformation parameters βp,p' = 0.55 ± 0.06 for 20O and βp,p' = 0.23 ± 0.04 for 22O. This sharp decrease of the (p,p') deformation parameter for 22O reflects a smaller contribution of the neutrons to the 2+ excitation, which is a direct indication of a strong N=14 subshell closure.

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fig. 5 : Angular distributions for elastic (top) and inelastic (bottom) scattering to the 2+ state in 22O. Lines are microscopic calculations (see text). For the inelastic scattering, the dashed line corresponds to the transition density provided by the QRPA calculation, while the solid line is calculated with a neutron transition density renormalized so as to best fit the data. Additional information can be provided by a microscopic analysis using nucleon nucleus elastic and transition potentials obtained by folding[26] the nuclear ground state[27] and transition densities[28] obtained with self-consistent HFB and QRPA calculations respectively, with the DDM3Y interaction. The imaginary optical potential was parametrized in Woods-Saxon form using the recent phenomenological optical model potentials[29]. As discussed above, neutrons and protons may contribute in different ways to the 2+ excitation. This is naturally expressed by the Mn/Mp ratio where Mn(p) is the transition matrix element for the neutrons (protons). The experimental Mp value is deduced from the measurement of B(E2)

using the relation B(E2)= M p / e 2. The microscopic values of Mp obtained from the proton transition density calculated in the continuum QRPA approach are found in very good agreement with the experimental data. The Mn values are then deduced by renormalizing the neutron transition density in order to reproduce the experimental data for the inelastic angular distribution. The resulting calculations in the case of 22O are compared to the data in fig.5. In this way, we have obtained the values of Mn/Mp = 3.25 ± 0.8 and 2.50 ± 1.0 for 20O and 22O respectively. A decrease of the Mn/Mp ratio is observed going from 20O to 22O, while in the case of a simple picture, in which neutrons and protons play the same role, an increase according to Mn/Mp = N/Z is expected. In single closed shell nuclei, values of Mn/Mp > N/Z are generally observed. Therefore, 20O exhibits a typical behavior of a single closed shell nucleus, contrarily to 22O. This emphasizes the strong N=14 shell closure in 22O. 2

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VI Conclusions A broad program of direct reaction studies using light neutron-rich radioactive beams is being carried out at GANIL, allowing us to probe their structure in unprecedented detail. It has led in particular to new insight into the properties of neutron-rich helium and oxygen isotopes and a promising search for multi-neutron systems has been initiated. This program should be intensified in coming years taking advantage of new detector systems named TIARA and MUST II [30]. Due to its fully integrated chip electronics, the latter will provide a solid angle coverage three times greater than the current MUST detector, with a total volume six times smaller. In addition to increasing the efficiency for the study of unbound states, these two systems will open the possibility of γ-particle coincidence experiments, which will greatly increase the energy resolution capability for the study of bound states, and lead to precise spectroscopy for heavier nuclei and fission fragments to be delivered by the SPIRAL and SPIRAL II facilities. Acknowledgements I wish to thank my colleagues from IPN_Orsay, SPhN Saclay, and GANIL for allowing me to present some of our results prior to publication. I particularly acknowledge our students Emilia Bechava, Lydie Giot, Emilie Rich, and Flore Skaza, as well as Erik Tryggestad, for the huge amount of work they have put into the analysis of these experiments, and for the enlightening discussions that we have together. References [1] P.G. Hansen, A.S. Jensen and B. Jonson, Ann. Rev. Nucl. Part. Sci. 45 (1995) 591. [2] Y. Blumenfeld et al., Nucl. Inst. Meth. A421 (1999) 471. [3] S. Ottini-Hustache et al., Nucl. Inst. Meth. A431 (1999) 476. [4] L. Bianchi et al., Nucl. Inst. Meth. A276 (1989) 509. [5] A. Lagoyannis et al., Phys. Lett. B518 (2001) 27. [6] P.J. Dortmans and K. Amos, Phys. Rev. C49 (1994). [7] P. Navratil and B.R. Barrett, Phys. Rev. C57 (1998) 3119. [8] M.F.Werby et al., Phys. Rev. C8 (1973) 106. [9] L. Giot et al., Nucl. Phys. A738 (2004) 426. [10] R.S. Mackintosh et al., Phys. Rev. C67 (2003) 034607 [11] O.F. Nemets et al., Yad. Phys. 42 (1985) 809. [12] K. Rusek et al., Phys. Rev. C64 (2001) 044602. [13] F. Skaza, PhD thesis, University Paris XI (2004), and F. Skaza et al., submitted. [14] E. Tryggestad, private communication. [15] D.R.Tilley, H.R.Weller, and G.M.Hale, Nucl. Phys. A541 (1992) 1 [16] F.M.Marques et al., Phys. Rev. C65 (2002) 044006. [17] S.Pieper, Phys.Rev.Lett. 90 (2003) 252501. [18] E. Rich et al., Proceedings of the EXON conference, St Petersburg, Russia, July 2004. [19] T.Otsuka et al., Phys. Rev. Lett. 87 (2001) 082502. [20] P. Thirolf et al., Phys. Lett. B485 (2000) 16. [21] M. Belleguic et al., Nucl. Phys. A682 (2001) 136c. [22] E. Khan et al., Phys. Lett. B490 (2000) 45. [23] E. Becheva et al., Proceedings of the XLII Int. Wint. Meeting on Nucl. Phys, Bormio, Italy, Jan 2004, page 322. [24] F.D. Bechetti, Jr and G.W. Greenlees, Phys. Rev. 182 (1969) 1190.

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