The halo structure is expected to influence the reaction mechanisms in nuclear collisions. The aim of the experiment discussed in the present paper is to compare the fusion excitation functions for the systems 6He + 64Zn and 4He + 64Zn, in order to investigate the effects of the 6He two-neutron halo structure on the fusion reaction mechanism at energies around the Coulomb barrier. In particular, new

fusion cross-sections for the 4He + 64Zn systems at sub-barrier energies were measured to cover the same energy region of our previous measurements for 6He + 64Zn. The fusion cross-section was measured by using an activation technique. We have observed an enhancement of the fusion cross-section in the reaction induced by 6He when compared to the one induced by 4He on the same 64Zn target

Fisichella, M.

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DOI 10.1393/ncc/i2011-10927-5Communications: SIF Congress 2010IL NUOVO CIMENTO Vol. 34 C, N. 5 Settembre-Ottobre 2011Fusion cross-section in the 4,6He+ 64Zn collisions aroundthe Coulomb barrierM. Fisichella(∗)Dipartimento di Fisica, Universita` di Messina - Messina, ItalyINFN, Laboratori Nazionali del Sud - Catania, Italy(ricevuto il 31 Dicembre 2010; revisionato il 5 Marzo 2011; approvato il 5 Marzo 2011; pubblicatoonline il 2 Agosto 2011)Summary. — The halo structure is expected to influence the reaction mechanismsin nuclear collisions. The aim of the experiment discussed in the present paper is tocompare the fusion excitation functions for the systems 6He + 64Zn and 4He + 64Zn,in order to investigate the effects of the 6He two-neutron halo structure on the fu-sion reaction mechanism at energies around the Coulomb barrier. In particular, newfusion cross-sections for the 4He + 64Zn systems at sub-barrier energies were mea-sured to cover the same energy region of our previous measurements for 6He + 64Zn.The fusion cross-section was measured by using an activation technique. We haveobserved an enhancement of the fusion cross-section in the reaction induced by 6Hewhen compared to the one induced by 4He on the same 64Zn target.PACS 25.60.-t – Reactions induced by unstable nuclei.PACS 25.60.Pj – Fusion reactions.1. – IntroductionIn the last years, many studies concerning nuclear collisions at energies around andbelow the Coulomb barrier have been performed to understand the role of the nuclearstructure on the reaction mechanisms. The interest on this topic arose with the develop-ment of radioactive ion beams (RIBs) which allowed the study of the reactions inducedby unstable nuclei and, in particular, by halo nuclei, like 6He, 11Li, 11Be, 19C or 8B.These nuclei are characterized by a well-bound core and by a low binding energy ofthe outer nucleons which occupy single-particle states having low angular momentum(s or p). The corresponding single-particle wave function of these valence nucleons ex-tends mostly outside the potential well. A lot of experimental and theoretical studieshave been performed in order to investigate on the effects of the projectile halo structure(∗) E-mail: fisichella@lns.infn.itc© Societa` Italiana di Fisica 4748 M. FISICHELLAon reaction mechanisms around the barrier (see, e.g., [1]). Most authors agree that directprocesses are favoured because of the low binding energies of the valence nucleons andtheir extended tails, however, contradictory effects on the fusion cross-section have beenpredicted. A possible way to approach the problem is to consider the combination of twokinds of effects due to the presence of a halo projectile on the fusion mechanism: staticand dynamic effects. The first ones are due to the long range of the radial wave functionassociated with the valence orbitals; the second ones are due to the coupling of the rela-tive motion of projectile and target to their intrinsic excitations or to the other reactionschannels including the breakup. Static effects generate a reduction of the Coulomb bar-rier and therefore by taking only them into account one should expect an enhancementof the fusion cross-section around and below the barrier. The role of dynamic effects hasbeen investigated using Coupled Channel (CC) calculations including the coupling notonly to the bound but also to the continuum states (break-up states). These Continuum-Discretised Coupled Channel (CDCC) calculations [2, 3] predict an enhancement of thesub-barrier complete fusion cross-section and a suppression above it with respect to sin-gle barrier calculations. However, the calculated cross-sections and hence the amount ofsub-barrier enhancement, depend upon the adopted approximations and the phase spacein the continuum considered.Different theoretical approaches, based on a time-dependent wave packet formalism,which uses a three-body model (core, halo and target) [4], lead to opposite results: thefusion cross-section in the neutron halo case is expected to be slightly suppressed withrespect to the non-halo case.From the experimental point of view low-energy reaction studies with halo nuclei aredifficult for the low beam currents and for the low cross-sections of some reaction channelespecially below the barrier. 6He beams are available in several radioactive beam facilitieswith good intensities (up to 107 pps) and for this reason fusion reactions induced by thishalo nucleus are the most studied of any involving light radioactive beams.The 6He + 209Bi fusion excitation function has been measured at the University ofNotre Dame [5, 6] using an activation technique by detecting the α particles emitted inthe decay of the evaporation residues. An enhancement of the sub-barrier fusion withrespect to a single barrier penetration [5, 6] and also to fusion induced by 4He onto thesame 209Bi target [7] is observed.The fusion reactions 4,6He + 238U, were measured in Louvain-la-Neuve [8], by detect-ing the two fission fragments in coincidence. An enhancement of the fission cross-sectionfor the 6He case with respect to the 4He one has been observed. However the analy-sis of the light particles, emitted in coincidence with the fission fragments, allowed toconclude that the observed fission enhancement is due to transfer-fission events whereasno suppression/enhancement effects are observed in the comparison of the fusion-fissionexcitation functions for the two systems.The fusion excitation functions of 6He and 8He on 197Au were measured in Dubna [9]and in Ganil [10], respectively, by using activation techniques. An enhancement of thesub-barrier fusion cross-section for the 6He- and 8He-induced reactions is observed withrespect to the one induced by 4He on the same target [10]. Both experiments have shownthat the cross-section for neutron transfer is much larger than the fusion one below andaround the barrier.Fusion for the system 11Be + 209Bi has been measured at RIKEN [11] by detectingthe characteristic alpha particles emitted in the evaporation residue decay, as well asfission fragments. The fusion cross-section data, for the 11Be-induced collision werecompared with those for the well-bound 10Be nucleus and for the weakly bound 9BeFUSION CROSS-SECTION IN THE 4,6He + 64Zn COLLISIONS ETC. 49Fig. 1. – Comparison of the experimental cross-sections of the evaporation residues (white his-togram) with the prediction of CASCADE calculations (black histogram).nucleus impinging on the same target. The fusion excitation functions reported in [11]for the three reactions 9,10,11Be + 209Bi show no difference within the errors.In summary, apparently contradictory conclusions on the presence of enhancement/suppression effects on fusion excitation functions around the barrier in collisions inducedby halo nuclei, have been reached by various authors and new experimental data, espe-cially below the barrier, are needed to further investigate this topic.2. – The 6He+ 64Zn experimentIn a previous study, we measured elastic scattering, direct reactions and fusion forthe systems 4,6He + 64Zn at the radioactive beam facility of Louvain-la-Neuve [12, 13]aiming to investigate on the effects of the 6He halo structure on the reaction mechanismsat energies around the Coulomb barrier. To measure the fusion cross-sections we usedan activation technique based on the off-line measurement of the atomic X-ray emis-sion following the electron capture decay of the evaporation residues produced in thereaction. This technique will be discussed more in detail in the next paragraph. Theexperimental set-up is extensively described in [12]. Stacks of four 64Zn targets, eachfollowed by a 93Nb catcher, were irradiated with 6He and 4He beams in order to mea-sure different points in the fusion excitation functions with a single beam energy. Thefusion cross-sections were obtained by summing up the cross-sections of all evaporationchannels. By comparing the excitation functions for heavy residue production of the twosystems an enhancement of the cross-section for the reaction induced by the halo nucleus6He was observed when compared to the collision induced by 4He. Since the same heavyresidues can be produced by fusion evaporation but also by other reaction processes, weperformed a comparison of the measured cross-section for the different channels with sta-tistical model calculations [12]. Figure 1 shows the comparison between the experimentalcross-section for each heavy residue produced in the 6He + 64Zn reaction and statistical-model predictions for evaporation residues formed in the decay of the compound nucleus70Ge obtained with the CASCADE code at a center-of-mass energy of about 10.5MeV.A good agreement for all the products is observed, with the exception of 65Zn where alarge enhancement with respect to the calculation was observed. A possible explanationfor the much larger experimental yield of 65Zn with respect to the calculated one couldbe that other reaction mechanisms are contributing to this particular channel. Indeed,as explained in detail in [12], the 65Zn is formed not only in the fusion-evaporation50 M. FISICHELLA11010 210 38 10 12 14 16 186He+64Zn4He+64ZnEcm(MeV)σfus(mb)Fig. 2. – 4He + 64Zn (triangles) and 6He + 64Zn (open circles) fusion excitation functions [12].process but also in one-neutron transfer or two-neutron transfer reactions followed byneutron evaporation. In [12,13] we have subtracted the contribution due to the transferby replacing the measured value for 65Zn with the one calculated using the statisticalmodel. After this correction, the fusion excitation functions for 6,4H+ 64Zn, shown infig. 2, are rather similar within the errors and within the measured energy range. Itseems that at this energies the presence of the halo structure does not imply particulareffects on the fusion mechanism. We cannot say anything about the possible effects thatthe 6He valence neutrons could generate at energies below the Coulomb barrier, sincethe 4He data in [12, 13] do not cover the lower part of the energy range investigated forthe 6He + 64Zn system.3. – New 4He+ 64Zn experiment3.1. Activation technique. – To investigate the role of the halo structure at energieslower than the Coulomb barrier we measured additional points in the fusion excitationfunction of 4He + 64Zn to cover the same energy range of the 6He data previously mea-sured in [12,13] and shown in fig. 2. This new experiment was performed at the Tandemaccelerator of the Ruder Boskovic Institute in Zagreb with an average 4He beam currentof about 1011 pps. The reaction was measured at Elab = 8.1, 9.2 and 10.1MeV. As inour previous experiments [12, 13] the fusion excitation function was measured using theoff-line activation technique. In this experiment, for each beam energy it was irradiatedone 64Zn target (540μg/cm2) followed by a Nb catcher (1000μg/cm2). We note that, asin the present experiment, where a fusion reaction is induced by a low-energy light pro-jectile onto a medium-mass target, a large fraction of the produced evaporation residuesdoes not have enough kinetic energy to exit the target and their direct detection is notpossible. However, by choosing a suitable target, as in the present case, it is possible toobtain radioactive ER unstable against EC decay. We “detect” the ER by looking atthe atomic X-rays emitted in their decay. The small fraction of residues emerging fromthe target is stopped in the 93Nb catcher placed behind the target. At the end of theirradiation time the activated target and catcher are placed close to a Si(Li) detector tomeasure the activity. The targets irradiated at the Zagreb Tandem were measured at theLaboratori Nazionali del Sud in Catania for about twenty days. One of the advantages ofFUSION CROSS-SECTION IN THE 4,6He + 64Zn COLLISIONS ETC. 51Fig. 3. – Typical X-ray spectrum measured off line for the reaction 4He + 64Zn. It is possible todistinguish two peaks which correspond to the Kα and Kβ X-ray emission of Zn.this technique is the 100% intrinsic detection efficiency of the Si(Li) detector in the X-rayenergy region of interest (7–11KeV). Moreover, the atomic X-ray in this energy regioncan be detected with extremely low background. We note that X-rays due to the decayof heavy residues produced in the reaction of 4He with the Nb catcher contribute to anenergy region of the X-ray spectrum different than the one of our interest and thereforedo not represent a problem.3.2. Experimental results. – In fig. 3 a typical X-ray off-line spectrum for the reaction4He + 64Zn is shown. It corresponds to the target activated at the highest energy, Elab =10.1MeV. From the X-ray energy it is possible to identify the Kα and Kβ contributiondue to the decay of the Ga into Zn. As one can see, the background level is very smallwhen compared to the signal.Characteristic X-ray energies only identify different elements but not different iso-topes. However it is possible to discriminate isotopes by following the X-ray activity ofthe element as a function of time. In fig. 4, the activity curve for the Ga peak is shown.10 30 50 100 150 200 250 300 350 400Time (h)Activity (decays/hour)Fig. 4. – Activity curve for the 67Ga isotope extracted in the activation run.52 M. FISICHELLA11010 210 38 10 12 14 16 186He+64Zn4He+64Zn4He+64Zn New DataEcm(MeV)σfus(mb)Fig. 5. – 6He + 64Zn (circles) and 4He + 64Zn (triangles) fusion excitation functions. Invertedtriangles represent the present 4He data.As one can see from the slope of the activity curve, only the contribution of the 67Gaisotope, having a half-life of 3.26 days, is present.This is in agreement with the predictions of statistical-model calculations. In fact theonly evaporation residues produced in the 4He + 64Zn fusion reaction at these energiesare predicted to be 67Ge and 67Ga. The first one is a short-lived nucleus (t1/2 = 18minutes) and it decays by EC 100% into 67Ga. For this reason, after few hours, allproduced 67Ge is transformed into 67Ga, therefore, we observe only this contribution.For the above reason, the total fusion cross-section corresponds to the one measuredfor the longer-lived 67Ga (T1/2 = 3.26 d) whose production cross-section can be extractedby using the following formula:σ =A0expNiNtλ0KαεT,(1)where A0exp is the experimentally measured activity at the end of the irradiation asobtained from the fit of the activity curve (fig. 4). This term is then corrected for theknown Kα fluorescence probability and for the detector efficiency (εT ); λ0 is the decayconstant; Nt is the number of target atoms per cm2 and Ni is the integrated beam current(i.e. the number of incident particles during irradiation). In fig. 5 the excitation functionsfor the present 4He + 64Zn data and for the previous data of 4,6He + 64Zn [12, 13] areshown. The consistency of the present 4He data with the ones of [12,13] is confirmed bythe good agreement of the two points at around 9.5MeV.As one can see, an enhancement of the fusion excitation function at energies belowthe Coulomb barrier in the case of 6He with respect the 4He one is observed. Theenhancement effect for 6He-induced fusion shown in fig. 5, includes both the effect dueto the larger 6He radius and the possible effects due to the coupling to other channelsincluding break-up. However, from this comparison, it is not possible to know if the roleof one of the two effects is more important than the other.FUSION CROSS-SECTION IN THE 4,6He + 64Zn COLLISIONS ETC. 534. – ConclusionsIn the present paper, our previous comparison of fusion excitation functions for thesystems 6,4He + 64Zn around the Coulomb barrier [12, 13] has been extended to lowerenergies by measuring a new set of 4He + 64Zn fusion cross-sections. The results showan enhancement of the 6He-induced fusion cross-sections with respect to the one inducedby 4He on the same target in the energy region around and below the barrier.From the experimental data published so far, apparently contradictory conclusions onthe presence of enhancement/suppression effects on fusion excitation functions aroundthe barrier in collisions induced by halo nuclei have been reached by different authors.One has to notice, however, two important points: most of existing data do not reallyexplore the region below the barrier with reasonable errors; different authors reachedtheir conclusions by performing a different kind of analysis and this may affect the finalresult as discussed in [7]. Therefore, for a better understanding of the discussed topicnew experimental data in the low energy region are surely needed as well as coherentanalysis procedures of existing experimental data.REFERENCES[1] Canto L. F. et al., Phys. Rep., 424 (2006) 1.[2] Hagino K. et al., Phys. Rev. C, 61 (2002) 037602.[3] Diaz-Torres A. and Thompson J. J., Phys. Rev. C, 65 (2002) 024606.[4] Ito M. et al., Phys. Lett. B, 37 (2006) 53.[5] Kolata J. J. et al., Phys. Rev. Lett., 81 (1998) 4580.[6] De Young P. A. et al., Phys. Rev. C, 58 (1998) 3442.[7] Canto L. F. et al., Nucl. Phys. A, 821 (2009) 51.[8] Raabe R. et al., Nature, 431 (2004) 823.[9] Penionzhkevich Yu. E. et al., Eur. Phys. J. A, 31 (2007) 185.[10] Lemasson A. et al., Phys. Rev. Lett., 103 (2009) 232701.[11] Signorini C. et al., Nucl. Phys. A, 735 (2004) 329.[12] Di Pietro A. et al., Phys. Rev. C, 69 (2004) 044613.[13] Di Pietro A. et al., Eur. Phys. J. ST, 150 (2007) 15.

Fusion cross-section in the 4,6He + 64Zn collisions around

the Coulomb barrier

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Fusion cross-section in the 4,6He + 64Zn collisions around the Coulomb barrier

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