Scattering of 6He at energies around the Coulomb barrier

Sca e ing o 6He a ene gies a ound he Coulomb ba ie A.M. S´anchez-Ben´ı ez†, D. Esc igk, M.A.G. ´ Al a ez‡, M.V. And ´es‡, C. Angulo+, M.J.G. Bo gek, J. Cab e a+, S. Che ubini§, J.M. Espino‡, P. Figue a§, M. F ee ], J.E. Ga c´ıa-Ramos†, J. G´omez-Camacho‡, M. Gulino§, O.R. Kakuee¶, I. Ma el†, C. Me elco], A.M. Mo o‡, J. Rahighi¶, K. Rusek†, D. Smi no *, O. Tengbladk, P. Van Duppen*, V. Ziman] †Depa amen o de F´ısica Aplicada, Uni e sidad de Huel a, E-21071 Huel a, Spain ‡Depa amen o de F´ısica A ´omica, Molecula y Nuclea , Uni e sidad de Se illa, E-41080 Se illa, Spain kIns i u o de Es uc u a de la Ma e ia, CSIC, E-28006 Mad id, Spain ¶Van de G aa Labo a o y, Nuclea Resea ch Cen e, AEOI, PO Box 14155-1339, Teh an, I an +Cen e de Reche ches du Cyclo on, Uni e si ´e ca holique de Lou ain, B-1348 Lou ain-la-Neu e, Belgium * Ins i uu oo Ke n-en S alings ysica, Uni e si y o Leu en, B-3001 Leu en, Belgium ]School o Physics and Space Resea ch, Uni e si y o Bi mingham, B15 2TT B imingham, England §INFN Labo a o i Nazionali del Sud and Sezione di Ca ania, I-95123 Ca ania, I aly E-mail: [email p o ec ed] Abs ac . We ha e measu ed elas ic c oss sec ions o he sca e ing o 6He a ELab = 14,16,17,18,22 MeV on 208Pb in he angula anges o 20◦−64◦and 135◦−170◦. A signi ican amoun o 4He e en s is ound a ene gies well below he Coulomb ba ie , ha becomes dominan abo e i . Op ical Model calcula ions ha e been pe o med including a dynamic pola iza ion po en ial. Ve y la ge imagina y di - useness pa ame e is needed in o de o desc ibe he expe imen al dis ibu ions. PACS numbe s: 01.30.Cc; 13.75.-n; 24.10.H ; 25.10.+s; 25.60.Dz; 25.60.Gc; 25.60.Bx; 21.10.G ; 27.20.+n Submi ed o: J. Phys. G: Nucl. Phys. 208Pb(6He,6He)208Pb@14,16,17,18,22 MeV 2 1. In oduc ion The in e es in s udying he elas ic sca e ing o 6He a ises om i s weakly bound na u e, which signi ican ly a ec s he dynamics o he collision. This nucleus is known o ha e a well de eloped 3-body s uc u e consis ing on an alpha pa icle and wo alence neu ons weakly bound by less han 1 MeV [1, 2]. Coupling o he s a es om he con inuum may play an impo an ole in he dynamics o he eac ion p ocess by means o long ange eac ion mechanisms. In pa icula , we a e in e es ed in con inuum s a es coupled wi h he g ound s a e o 6He by he dipole componen o he Coulomb ield o he a ge . Tha is wha Coulomb dipole pola izabili y accoun s o , and i has been ound ha i gi es ise o a signi ican educ ion o he elas ic c oss sec ions o o he sys ems [3, 4, 5]. In his pape we epo on measu emen s o elas ic sca e ing and alpha p oduc ion c oss sec ions o 6He+208Pb a Coulomb ba ie ene gies. Due o he high Z alue o a ge and he ela i ely low sca e ing ene gy, we expec impo an e ec s om he Coulomb dipole coupling. 2. Expe imen The expe imen was pe o med a he Cen e de Reche ches du Cyclo on a Lou ain- la-Neu e, Belgium [6]. The bomba ding ene gies o he seconda y 6He beam we e 14, 16, 17, 18 and 22 MeV, wi h a cu en in ensi y on a ge o abou 5 ×106pps (106 pps o he 22 MeV beam). Accele a ed 6He ions we e sca e ed on a sel suppo ed a ge wi h he ollowing iso opic composi ion: 208Pb 87(2)%, 207Pb 1.20(2)%, 206Pb 11.4(2)%. A a ge o 0.950 mg/cm2 hickness was used o each ene gy excep o 22 MeV, whe e beam in ensi y was lowe and a hickness o 2.080 mg/cm2was chosen. Elas ic sca e ing c oss sec ions o a 12 MeV 4He beam om 208Pb was also measu ed o no malize da a and o cancel sys ema ic e o s in he 6He+208Pb elas ic c oss sec ions. Two 5 mm collima o s we e used o p o ide good beam de ini ion and o educe ame sca e ing e en s. The ou going pa icles we e de ec ed by h ee a ays o de ec o s, LEDA (used in s anda d and LAMP con igu a ions) and DINEX. Figu e 3 shows a schema ic lay-ou o he expe imen al se up. LEDA silicon s ip de ec o s (SSD) a e desc ibed in [7]. DINEX de ec o a ay (see igu e 1) consis s o ou 90◦ci cula silicon elescopes, o hickness 40µm ( on ) and 500µm (back). The on de ec o is a single sided SSD de ec o , a so called CD- ype s ip de ec o [8]. DINEX de ec o a ay was able o achie e a good iden i ica ion o 4He and 6He e en s wi h an adequa e angula esolu ion (see igu e 2). LEDA, LAMP and DINEX de ec o a ays co e h ee di e en angula anges (see igu e 3): LEDA 5◦−12◦wi h an angula esolu ion o 0.5◦, LAMP 20◦−64◦wi h an angula esolu ion o 3◦, and DINEX 135◦−170◦wi h an angula esolu ion o 5◦. The solid angle co e ed by LEDA+LAMP+DINEX a ays was 33% o 4π. Co ec ions o beam alignmen and e iciency o he elec onic chain we e also e alua ed and included in he inal analysis. As an example he elas ic c oss sec ion di ided by he Ru he o d c oss sec ion ob ained 208Pb(6He,6He)208Pb@14,16,17,18,22 MeV 3 a 18 and 22 MeV a e shown in igu e 4, and he alpha p oduc ion c oss sec ion a he ene gy o 22 MeV is shown in igu e 5. The LAMP egion includes he onse o Ru he o d sca e ing egime a lowe angles, p o iding us he needed e e ence expe imen al poin s o a global no maliza ion. The e o e he analysis o he da a om LEDA egion is edundan , whe eas implies addi ional non i ial co ec ions due o he high a e o de ec ed pa icles. 3. Analysis We ca ied ou an op ical model (OM) analysis o he expe imen al da a using he code ECIS [9]. In he calcula ions he e ec s o he dipole couplings o he s a es in he con inuum we e included by means o a dynamic dipole pola iza ion po en ial (DDPP) [10]. I s Coulomb pa was calcula ed explici ly om he heo e ically p edic ed B(E1) ene gy dis ibu ion [11]. In he analysis we used a Coulomb adius o RC= 9.29 m. Fo he nuclea po en ial we ha e chosen Woods-Saxon adial o m ac o s o bo h eal and imagina y pa s wi h a adius R =Ri= 10.07 m [12]. We made a i o he 22 MeV dis ibu ion by means o ou -pa ame e sea ch. The dep hs and di usenesses o he nuclea po en ial we e a ied in he sea ch, wi h he s a ing alues aken om [12]. The ene gy o 22 MeV is abo e he Coulomb ba ie so he calcula ions a e expec ed o be sensi i e o he pa ame e s o he nuclea po en ial. The alue o he imagina y di useness ob ained a 22 MeV (ai=2(1) m) i s well he da a also a lowe ene gies. This e y la ge alue sugges s he p esence o he s ong nuclea componen o he DDPP. The Coulomb componen was explici ly included in he calcula ions. This esul is in acco dance wi h p e ious expe imen al wo ks [12, 13, 14]. Solid cu es in igu e 4 a e OM calcula ions using ai=2.21 m. Only he dep hs we e a ied in he sea ch o he ene gy o 18 MeV. In o de o see he e ec o he la ge imagina y di useness and he sensi i i y o he esul , OM calcula ions we e also pe o med wi h a ixed and mo e s anda d alue o ai=0.819 m. This alue was used in [15] o he sca e ing o 6Li on 208Pb. The esul o his calcula ion is shown in igu e 4 (do ed cu es). The la ge imagina y di usseness ob ained o 6He dumps oscilla ions in he egion o he Coulomb ainbow. P elimina y coupled channels calcula ions we e pe o med o he ene gy o 22 MeV, assuming a simple di-neu on model o 6He. The couplings wi h he s a es om he con inuum we e included by means o con inuum - disc e ized coupled - channels (CDCC) echnique.The OM calcula ions wi h ai=2.21 m (solid cu e) and CDCC calcula ions (dashed cu e) i he expe imen al da a equally well, al hough a in e media e sca e ing angles hey lead o qui e di e en esul s. This calls o new expe imen al da a in his angula ange. In o de o ackle he p ocess o neu on ans e o he a ge , ano he app oach was adop ed assuming ha he de ec ed alpha pa icles a backwa d angles we e p oduced ia he ans e o a ully co ela ed di-neu on clus e o ei he he a ge bound s a es o con inuum s a es, he la e accoun ing o he b eakup con ibu ion. Bo h p ocesses 208Pb(6He,6He)208Pb@14,16,17,18,22 MeV 4 we e ea ed using he p io o m o he DWBA ampli ude. The calcula ed angula dis ibu ions o he alpha pa icles a e compa ed wi h he expe imen al da a in igu e 5. 4. Summa y and ou look We ha e in es iga ed he elas ic sca e ing o 6He on 208Pb a bomba ding ene gies ELab = 14,16,17,18,22 MeV in a wide ange o angles. A signi ican amoun o 4He e en s is ound a ene gies well below he Coulomb ba ie , ha becomes dominan abo e i . Op ical model calcula ions including a dynamic dipole pola iza ion po en ial can ep oduce p ope ly he da a, p o ided ha a e y la ge imagina y di useness is used. Thus, Coulomb dipole pola izabili y accoun s only o pa o he abso p ion om elas ic channel. Da a a 22 MeV can be equally well desc ibed by he CDCC calcula ions. The ans e o 2n o he a ge and di ec b eak-up we e included simul aneously in a di e en app oach, achie ing pa ially o ep oduce alpha p oduc ion c oss sec ion. New sca e ing da a would be needed o ully unde s and he mechanism o 6He in e ac ion wi h 208Pb in he icini y o he Coulomb ba ie . Acknowledgmen s The au ho s would like o hank he s a a he CYCLONE RIB acili y a he Cyclo on Resea ch Cen e in Lou ain-la-Neu e, Belgium, o p o iding us wi h an in ense and good quali y adioac i e beam. This wo k has been pa ially suppo ed by he Spanish Di ecci´on Gene al de In es igaci´on, Ciencia y Tecnolog´ıa unde p ojec s numbe FPA2000-1592-C03-02, FPA2002-04181-C04-02/04, he Belgian p og am P5/07 on in e uni e si y a ac ion poles o he Belgian-s a e Fede al Se ices o Scien i ic, Technical and Cul u al A ai s, and he Eu opean Communi y-Access o Resea ch In as uc u e ac ion o he Imp o ing Human Po en ial P og amme, con ac No ¯ HPRI-CT-1999-00110. A. M. S. B. acknowledges a esea ch g an om he Spanish MCyT. Re e ences [1] Taniha a I, 1996 J. Phys. G: Nucl. Pa . Phys. 22 157-8 [2] Hansen P G and Jonson B, 1987 Eu ophysics Le e s 4409-14 [3] And ´es M V, G´omez-Camacho J and Naga ajan M A, 1995 Nucl. Phys. 583A 817-20 [4] Mo o A M and G´omez-Camacho J, 1999 Nucl. Phys. 648A 141-56 [5] Ma el I, G´omez-Camacho J, Rusek K e al ., 1998 Nucl. Phys. 641A 188-202 [6] Ve ie J, 1997 Nucl. Phys. 616A 97c-106c [7] Da inson T, B and ield-Smi h W, Che ubini S, e al ., 2000 Nucl. Ins um. Me hods 454A 350-8 [8] Os owski A N, Che ubini S, Da inson T, e al ., 2002 Nucl. Ins um. Me hods 480A 448-55 [9] Raynal J, 1981 Phys. Re . 23C 2571-85 [10] And ´es M V, G´omez-Camacho J and Naga ajan M A, 1994 Nucl. Phys. 579A 273-84 208Pb(6He,6He)208Pb@14,16,17,18,22 MeV 5 [11] Thompson I J, Danilin B V, E os V D, e al ., 2000 Phys. Re . 61C 24318 [12] Kakuee O R, Rahighi J, S´anchez-Ben´ı ez A M, e al ., 2003 Nucl. Phys. 728A 339-49 [13] Aguile a E F, Kola a J J, Nunes FM, e al ., 2000 Phys. Re . Le . 84 5058-61 [14] Aguile a E F, Kola a J J, Becche i F D, e al ., 2001 Phys. Re . 63C 061603-6 [15] Keeley N , Benne S J, Cla ke N M, e al ., 1994 Nucl. Phys. 571A 326-36 208Pb(6He,6He)208Pb@14,16,17,18,22 MeV 6 Figu e 1. DINEX de ec o a ay. Figu e 2. Mass spec um a E= 14 MeV, θlab = 161◦ob ained wi h DINEX de ec o a ay. Figu e 3. Ske ch o he expe imen al se -up. 208Pb(6He,6He)208Pb@14,16,17,18,22 MeV 7 6He+208Pb dσ / dσR E = 18.0 MeV 0 50 100 150 0.0 0.5 1.0 1.5 θc.m. (deg) E = 22.0 MeV 0 50 100 150 0.0 0.5 1.0 1.5 Figu e 4. Angula dis ibu ions o elas ic c oss sec ion di ided by he Ru he o d c oss sec ion compa ed wi h OM and CDCC calcula ions. 100 120 140 160 θc.m. (deg) 1 10 100 1000 dσ/dΩ (mb/s ) He + Pb, 22 MeV 6208 Figu e 5. Angula dis- ibu ion o alpha p oduc- ion c oss sec ion com- pa ed wi h DWBA calcu- la ion.