Reaction rates of astrophysical interest obtained with radioactive beams

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Nuclear Physics A583 (1995) 717-724

Reaction rates of astrophysical interest obtained with radioactive beams J. Vervier Institut de Physique Nucl~aire, UCL, B-1348 Louvain-la-Neuve (Belgium) ABSTRACT The reasons and the methods used for obtaining information interesting for nuclear astrophysics with radioactive beams are reviewed. Specific examples of experiments carried out with low ( < 1 MeV/A)- and high (tens of MeV/A)energy radioactive beams are presented, dealing with direct and indirect determinations of reaction cross sections, and measurements of spectroscopic data. The perspectives of the field are outlined. 1. INTRODUCTION During the past 5 years [1], Radioactive Nuclear Beams (RNB) have become an i m p o r t a n t tool for obtaining i n f o r m a t i o n i n t e r e s t i n g for Nuclear Astrophysics (NA), both at low energy (< 1 MeV/A) and at high energy (tens of MeV/A). In the present contribution, we first present the reasons why RNB are useful in NA, for measuring reaction cross sections with direct and indirect methods, and for obtaining spectroscopic data. The methods used to produce RNB in the two energy domains are briefly reviewed. Specific examples of experiments carried out to m e a s u r e directly reaction cross sections of astrophysical interest are then discussed, and a few samples of spectroscopic data obtained with low-energy RNB are summarized. Indirect determinations of reaction cross sections by the Coulomb dissociation of high-energy RNB are then presented, together with measurements of spectroscopic properties of nuclei far from stability using the latter RNB. Some perspectives of this field are finally outlined. 2. WHY ARE RADIOACTIVE N U C L E A R BEAMS I N T E R E S T I N G IN N U C T , ~ , R ASTROPHYSICS ? It is by now well known [2] that the production of energy and the synthesis of elements in stars result from nuclear reactions. In "quiet" stars, like our sun, which are in equilibrium conditions, the reaction rates are much lower t h a n the radioactive decay rates, i.e. the average time between two successive nuclear reactions involving the same nucleus is much higher t h a n the average decay time. As a consequence, the radioactive nuclei which are often produced in the nuclear reactions which occur in stars have ample time to decay before 0375-9474/95/$09.50 © 1995ElsevierScienceB.V. All rightsreserved. SSD1 0375-9474(94)00750-0

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being involved in other nuclear reactions : only the cross sections for nuclear reactions between stable nuclei have to be measured to understand these stars. In "violent" stellar events, the reverse situation often prevails : the reaction times are comparable or lower than the decay times. It is then necessary to determine the cross sections for nuclear reactions involving radioactive nuclei, in order to be able to propose "scenarios" for such stellar events [3]. Example of the latter situation can be found [2] : in the rapid neutron capture process (r-process), which leads to the synthesis of the so-called r-elements, examples of which being the heaviest known elements present in nature, i.e. the Th and U isotopes ; in type-II supernovae, i.e. the final explosive stage of a massive star, which lasts a few seconds ; in the primordial nucleosynthesis, which occured during the first three m i n u t e s of the Universe and led to the production of helium and of a few light elements. Similar conditions prevail in novae and X-ray bursts [2], events which occur in binary stellar systems, i.e. two close-lying stars (about 50% of the stars in the sky belong to such systems). Matter from one of the 2 stars is attracted by the other, which is either a white d w a r f (for novae) or a neutron star (for X-ray bursts), i.e. stellar "objects" comparable to giant atoms (for a white dwarf) or giant nuclei (for a neutron star). The impact of this matter on the companion star induces some kind of explosion near its surface, leading to the observable properties of these objects, i.e. a spectacular and temporary increase of its apparent magnitude (for novae) and a (more or less) periodic emission of X-rays (for X-ray bursts). In these two kinds of objects, nuclear reactions involving radioactive nuclei are important ingredients in the "scenarios" proposed to explain their properties. The determination of the cross sections for the latter nuclear reactions, until about 5 years, have mostly been based on theoretical calculations, with large associated uncertainties. Methods have since been developed to measure these cross sections, either directly, using low-energy RNB, or indirectly, through various m e a s u r e m e n t s with low- and high-energy RNB whose results have allowed to calculate these cross sections with a fair degree of confidence. The detailed understanding of the "violent" stellar events refered to above often requires the knowledge of the detailed spectroscopic properties of nuclei very far from the line of stability, such as their masses, lifetimes, decay schemes, level densities, fission barriers ... This need is well known for the r-process, which deals with very neutron-rich nuclei above 56Fe, sometimes very close to the neutron drip line. The so-called rp-process [4] leads to the synthesis of elements up to mass 65 to 75 in stellar situations such as present in novae and X-ray bursts, by the rapid capture of protons by elements close to neon, leading to very proton-rich nuclei above 19Ne often very close to the proton drip line. The calculated "paths" for the r- and rp-processes strongly depend on the above-mentioned spectroscopic properties : they can be measured with RNB, since the use of the latter beams allows to synthetize and study very exotic nuclei, both proton-rich and neutron-rich. RNB are thus very useful for NA, especially in two energy regimes. Lowenergy RNB (i.e. up to 1 MeV/A), with high intensities, allow to measure directly the cross sections for nuclear reactions between radioactive nuclei and (very often) protons and alpha particles ; it can indeed be shown [2] that, for the short lifetimes of interest in NA, i.e. up to about 1 hour, the Radioactive Beam technique (i.e. bombarding a stable target with a radioactive beam) is more

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efficient t h a n the Radioactive Target technique (i.e. bombarding a radioactive target with a stable beam). High-energy RNB (i.e. a few tens of MeV/A), on the other hand, can be used for indirect determination of the above-mentioned cross sections, and for obtaining spectroscopic informations on the very exotic nuclei interesting for NA. 3. HOW TO PRODUCE RNB SUITABLE FOR NA STUDIES ? Two general methods can be used to produce RNB [5] : the Isotope Separator On Line (ISOL) or two-accelerator method ; and the fragmentation method. For the first one, large quantities of radioactive nuclei are produced by bombarding a thick (pimary) target with high intensity primary (stable) beams produced by a first accelerator. These nuclei are extracted from the target as atoms or molecules, transformed into ions by a suitable ion source, and accelerated by a second accelerator. The radioactive secondary beams thereby obtained are sent on a (secondary) target, to study problems of interest in nuclear physics and nuclear astrophysics. Various schemes have been or are being constructed - or are at the planning stage - to obtain RNB by the ISOL method. They differ by the choice of : the primary beam, protons of low (tens of MeV) or high (hundreds of MeV) energy, or heavy ions ; the ion source, of the surface ionization-, Electron Cyclotron Resonance (ECR)- or Electron Beam Ion Source (EBIS)-types ; the second accelerator, cyclotron, linac or tandem. The only working facility of this type is presently at Louvain-la-Neuve, and is described in detail elsewhere [6] ; it includes a 30-MeV 500 ~tA proton cyclotron as first accelerator, an ECR ion source, and a K = 110 MeV cyclotron as second accelerator. By using the second cyclotron in the 6th-harmonic mode, RNB with energies above 0.5 MeV/A and intensities in the 108 to 109 particles per second range can be produced, allowing the direct m e a s u r e m e n t of the cross sections for astrophysically interesting nuclear reactions, examples of which will be given in Section 4. Other projects based on the same technique exist in Europe [7] (at GANIL, Caen, Catania, ISOLDE/CERN and Grenoble), in the United States (Oak Ridge National Laboratory), in J a p a n (Institute for Nuclear Study, Tokyo and in Canada (TRIUMF) [5]). For the fragmentation method [5], a high-energy (tens of MeV/A) primary stable beam is sent on a thin (primary) target, where it is fragmented into m a n y nuclei, some of which are radioactive, which are mostly emitted in the forward direction. The desired RNB, with energies comparable to the one of the p r i m a r y beam, are separated from the primary beam and from the other f r a g m e n t s by a suitable method, and sent on a (secondary) t a r g e t for experimental studies. Here again, various schemes exist - or are planned depending on the energy of the primary beam (tens or hundreds of MeV/A), on the separation method used (various magnetic dipoles, velocity filters, wedged absorbers, achromatic systems, or combination of these techniques), and, sometimes, on the use of a cooler ring, to accumulate and cool the RNB. Examples of such facilities are described elsewhere during the present Conference, i.e. at RIKEN (M. Ishihara) and at MSU (B. Sherrill) ; other exist in Europe (at GANIL, Caen, GSI, Darmstadt) or are in the construction or planning stage (at Dubna and Legnaro) [5,7]. Indirect determinations of the

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cross sections for astrophysically interesting nuclear reactions have been performed with these facilities, example of which will be given in Section 5. 4. LOW-ENERGY RNB FOR NA

The direct measurement of the cross sections for astrophysically important nuclear reactions involving radioactive nuclei has so far been performed in only a few cases, owing to the relatively low intensities of the available RNB and the smallness of these cross sections. The first reaction of this kind is 13N(p,T)140, which has been extensively studied at Louvain-la-Neuve. Its astrophysical interest [2,3] lies in the fact that it determines the transition from the cold CNO cycle of hydrogen burning : 12C(p,T)13N(~+) 13C(p,~) 140(~+) 14N(p,~/)150(~+)15N(p,a)12C to the hot CNO cycle : 12C(p,~) 13N(p,7)140(~+)14N(p,7)lSO(~+)lSN(p,a)12C The former cycle represents the dominant mechanism for the production of energy in main-sequence stars somewhat heavier than the sun, whereas the latter cycle is believed to play a dominant role in the novae phenomenon described in Section 2. The 13N(p,T)140 reaction cross section is dominated by a resonance, at 0.445 MeV center-of-mass energy in the 13N + p system, which corresponds to the first excited level of 140 at 5.17 MeV. Three measurements have been performed at Louvain-la-Neuve on this reaction : the direct determination of its cross section integrated over the 0.445-MeV resonance energy range, which yielded the radiative width F~ of this resonance ; the direct m e a s u r e m e n t of the energy ER and total width F of this resonance, through the study of the 13N + p elastic scattering in the resonance region ; the determination of the spectroscopic factor for the 13N(d,n)14Og.s. transition, which has allowed a (more reliable) calculation of the direct-capture contribution to the 13N(p,~)140 reaction cross section. The results of these experiments [8] have allowed to calculate the astrophysical S-factor for this reaction, and the region of the "phase diagramme" of the stellar densityversus-temperature plot where the cold and hot CNO cycles dominate. These results have also confirmed the importance of the 13N(p,~)140 reaction in the novae phenomenon, allowed to predict with a better degree of confidence some observables of the novae, and restricted the range of nuclei which can be produced by the slow-neutron capture s-process in some types of stellar events [9]. Another capture reaction of considerable interest is 19Ne(p,T)20Na [4]. It could lead to an "escape" from the hot CNO cycle through the reaction sequence : 150(a,~/) 19Ne(p,7)20Na(~+)20Ne

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It would thereby become the starting point of the rp-process in which, through successive proton captures on elements around A = 20, nuclei up to mass 65 to 75 could be synthetized, in a "hot" hydrogen-burning process which could occur in novae or X-ray bursts. The 19Ne(p,T)20Na reaction has been the subject of intense work in several laboratories during the past few years. In Louvainla-Neuve, measurements have been undertaken to study it directly, using - 109 particles per sec 19Ne beams which have been developed meanwhile [10]. Several resonances in the 19Ne + p system could play a role in this reaction. Their radiative widths F,t, or upper limits for them, have been determined by measuring the radioactivity of the 20Na nuclei produced, through their a- and 13+ decays. The energy ER and total width F of some of these resonances have been measured, as well as the cross section for the 19Ne(d,n)20Na (bound states) reaction. The results of these experiments are now being analyzed, and their impact on the above-mentioned astrophysical processes will be evaluated. Nuclear reactions involving 8Li, and in particular the 8Li(cc,n)llB reaction, have also been investigated ; their astrophysical relevance is related to inhomogeneous big-bang models and their implications on the primordial nucleosynthesis [11]. Interesting spectroscopic informations for NA have been obtained with (very) low-energy RNB, around on-line isotope separators which produce RNB with 50-100 keV energies. Examples are studies of "waiting points" nuclei in the r-process, for instance 79Cu and 130Cd at ISOLDE-CERN [12]. More recently, the 13 decay of 16N to unbound 160 levels decaying into 12C + 4He has been investigated at 2 laboratories, TRIUMF and Yale [13,14]. These data have yielded interesting information on the cross section for the 12C(a,T)160 reaction at very low energies, which is highly relevant for the production of 12C and 160 in the red giant phase of the stars. 5. HIGH-ENERGY RNB FOR NA

Indirect determinations of the cross section for astrophysically important nuclear reactions can be performed by Coulomb dissociation experiments [15]. This method is based on the principle of detailed balance, i.e. on the relation between the cross section for the capture reaction A(b,T)C and the one for the photodesintegration reaction C(T,b)A. The source of photons for the latter reaction is the virtual photon spectrum resulting from the Coulomb interaction of high-energy (tens of MeV/A) RNB with a high-Z target, for example 2OSpb. The advantages of this indirect method over the direct m e a s u r e m e n t of the cross section for the capture reaction are : much higher cross sections (mb versus ~tb), and hence higher counting rates or need of lower intensity RNB ; the possibility of studying resonances at lower energies, where the direct capture cross section becomes prohibitively low. The limitations of the method are : the reaction mechanism, which is often assumed to be a pure Coulomb interaction between the RNB and the target, but to which nuclear interaction and postacceleration corrections may have to be applied ; the fact that only ground-state to ground-state transitions (i.e. between Ag.s. and Cg.s.) can be measured, whereas the direct capture reaction may involve transitions to

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particle-bound excited levels of C ; the need for a good energy resolution, to exclude from the m e a s u r e m e n t transitions, in the photodesintegration reaction, to other particle-unbound levels of C than the one under investigation, and to particle-bound excited levels of A. This method has first been applied in RIKEN and GANIL to the 13N(p,7)140 reaction [16,17], where the above-mentioned limitations do not present strong problems. The results obtained are in very good agreement with those of the direct measurement of F~ described in Section 4. More recently, the llC(p,7)12N reaction has been investigated at GANIL by sending a 70.9 MeV/A 12N beam, with an intensity of 106 particles per sec, on a 120-mg/cm2 208pb target, and by registering, in coincidence, the llC nuclei in a spectrometer and the protons by twelve CsI(T1) detectors [18]. The 7-widths F~ for the resonances in the l lC(p,7)12N reaction corresponding to the levels of 12N at 1.19 and 1.8 MeV have been determined to be 6(4) and 70(20) meV, respectively. The astrophysical interest of this reaction lies in the possible production of 12C in hot hydrogen burning, through the reaction sequence : 7Be(a,7) 12C(p,~)12N(~+)12C the 7Be nucleus being produced by the "normal" p-p chain : H(H,~+)D(p,7)3He(a,7)7Be From the fact that the above-mentioned results for F~ are much lower than those calculated previously [19], it can be tentatively concluded that the production of 12C by the reaction sequence just described is not an important source of 12C in the Universe. In a more recent experiment performed at RIKEN, the Coulomb dissociation of 8B into 7Be + p has been investigated [20]. This reaction has to do with the solar neutrino problem, since one of the sources of the solar neutrinos lies in the decay of 8B produced by the 7Be(p,7)SB reaction which follows the above-mentioned "normal" p-p chain. The astrophysical S-factor for the latter reaction, obtained by this method, somewhat differs from previous data resulting from various direct determinations of the capture reaction cross section. The implications of these data for the solar neutrino problem still have to be worked out in detail. Interesting spectroscopic data for NA have also been obtained with highenergy RNB, examples of which are as follows. The nucleus 65As is almost proton bound, as results from the measurement of its half-life using the fragmentation of 75 MeV/A 78Kr beams performed at MSU [21] ; this implies that it is not the end point of the rp-process, which accordingly extends beyond A = 65. The half-lifes and decay properties of some neutron-rich exotic S and C1 nuclei have been studied at GANIL using 60 MeV/A 48Ca beams fragmented by a 64Ni target ; the results have allowed to understand the low 46Ca/48Ca abundance ratio in the Universe [22]. The detailed decay scheme of 20Mg to levels in 20Na which are proton unbound has been investigated by several groups at RIKEN, MSU and GANIL [23-25] ; these results have yielded

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information on the levels of 20Na which m a y be of importance for the 19Ne(p,T)20Na reaction discussed in Section 4. The discovery of bound-state p-decay at GSI, Darmstadt [26] confirms the existence of a new decay mode of the nuclei which may be of importance in some astrophysical scenarios. 6. CONCLUSIONS AND PERSPECTIVES In the present contribution, we have shown that RNB, both at low and at high energies, may yield very useful information for NA. The astrophysical problems which have been mentioned in connection with RNB range from primordial nucleosynthesis to type II supernovae, and include the solar neutrino problem, the r- and rp-processes, the cold a n d hot CNO cycles, novae and X-ray bursts, the nucleosynthesis of C and 0, the Ca-isotopes abundances in the Universe .... i.e. central issues - and not details - in NA. It can thus be anticipated that the field of NA as studied with RNB is likely to extend in the future. This statement is strengthened by the development of new facilities where such studies can be persued. In the low-energy regime, one m a y mention : the construction of a new cyclotron in Louvain-la-Neuve, specifically designed for the acceleration of RNB in the astrophysically interesting region i.e. between 0.1 and 0.8 MeV/A (ARENAS 3 programme) [27] ; the progresses in the construction of RNB facilities at INS, J a p a n and ORNL, USA [5], the 3 projects (ARENAS 3, INS, ORNL) being scheduled to work within about 3 years. At high energies, increases of the performances are planned at various existing facilities (RIKEN, MSU, GANIL, GSI), and new possibilities will be opened at other laboratories (the COMBAS projectile fragment-separator at Dubna [28] and the ADRIA project at Legnaro [29]). It is thus likely that the topics covered in the present contribution will be an important item for the next International Conference on Nucleus-Nucleus Collisions. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

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