Neutron-induced cross sections of actinides via the surrogate-reaction method

Neu on-induced c oss sec ions o ac inides ia he su oga e- eac ion me hod Q. Ducasse1,6, B. Ju ado1, M. Aiche1, L. Ma hieu1, T. To nyi2, A. Gö gen2, J. N. Wilson3, G. Ba eau1, I. Companis1, S. Czajkowski1, F. Giacoppo2, F. Gunsing4, M. Gu o msen2, A. C. La sen2, M. Lebois3, J. Ma a anz1, T. Rens øm2, S. Rose2, S. Siem2, I. Tsekhano ich1, G. M. T e en2, T. W. Hagen2, M. Wiedeking5, O. Se o 6, G. Bou oux7, P. Chau7, V. Méo 7, O. Roig7 1 CENBG, Chemin du Sola ium B.P. 120, 33175 G adignan, F ance 2 Uni e si y o Oslo, Depa men o Physics, P.O. Box 1048, Blinde n 0316 Oslo, No way 3 IPN O say, 15 ue G. Clémenceau, 91406 O say cedex, F ance 4 CEA Saclay, DSM/I u, 91191 Gi -su -Y e e cedex, F ance 5 iThemba LABS, P.O. Box 722, 7129 Some se Wes , Sou h A ica 6 CEA Cada ache, DEN/DER/SPRC/LEPh, 13108 Sain Paul lez Du ance, F ance 7 CEA DAM DIF, 91297 A pajon, F ance Abs ac . The su oga e- eac ion me hod is an indi ec way o de e mining c oss sec ions o eac ions ha p oceed h ough a compound nucleus. This echnique may enable neu on-induced c oss sec ions o be ex ac ed o sho -li ed nuclei ha o he wise canno be measu ed. Howe e , he alidi y o he su oga e me hod o ex ac ing cap u e c oss sec ions has o be in es iga ed. In his wo k we s udy he eac ions 238U(d,p)239U, 238U(3He, )238Np, 238U(3He,4He)237U as su oga es o neu on- induced eac ions on 238U, 237Np and 236U, espec i ely, o which good quali y da a exis . The expe imen al se -up enabled he measu emen o ission and gamma-decay p obabili ies. Fi s esul s a e p esen ed and discussed. 1 In oduc ion Neu on-induced c oss sec ions o sho -li ed nuclei a e c ucial o undamen al nuclea physics, as ophysics and also o applica ions such as eac o physics. In pa icula , (n, γ) c oss sec ions on mino ac inides a e one o he la ges sou ces o unce ain y in modelling new eac o s o nuclea was e ansmu a ion using as neu ons. Howe e , e y o en he high adioac i i y o he ac inides samples makes he di ec measu emen o hese c oss sec ions ex emely di icul . The su oga e- eac ion me hod is an indi ec way o de e mining c oss sec ions o compound-nuclea eac ions. This me hod was i s p oposed by J. D. C ame and H. C. B i [1] in he se en ies. I consis s o using a ans e eac ion o p oduce he same decaying nucleus as he one o med in he desi ed neu on-induced eac ion. The ans e eac ion leads o he hea y ecoil nucleus o in e es and o an ejec ile. The iden i ica ion o he ejec ile pe mi s o de e mine he mass A and cha ge Z o he decaying nucleus. In addi ion, one can deduce he exci a ion ene gy E* o he hea y nucleus by measu ing he kine ic ene gy and he emission angle o he ejec ile. The numbe o coincidences be ween he ejec iles and he decay p oduc s no malized o he o al numbe o de ec ed ejec iles EPJ Web o Con e ences DOI: 10.1051/ C Owned by he au ho s, published by EDP Sciences, 2013 , epjcon 201/ 01003 (2013) 42 34201003 This is an Open Access a icle dis ibu ed unde he e ms o he C ea i e Commons A ibu ion License 2.0, which pe mi s un es ic ed use, dis ibu ion, and ep oduc ion in any medium, p o ided he o iginal wo k is p ope ly ci ed. A icle a ailable a h p://www.epj-con e ences.o g o h p://dx.doi.o g/10.1051/epjcon /20134201003 EPJ Web o Con e ences allows o he ex ac ion o he decay p obabili y o he co esponding decay channel ,exp (*) A decay P E. Acco ding o he su oga e- eac ion me hod, he neu on-induced c oss sec ion o he nucleus A-1 is hen gi en by he equa ion: 1,exp () () () AAA decay n CN n decay n EEPE   (1) whe e () A CN n E  is he c oss sec ion o he o ma ion o he compound nucleus A a e abso p ion o a neu on wi h ene gy En, i is usually ob ained using he op ical model. The inciden neu on ene gy En and he exci a ion ene gy E* o he compound nucleus A a e ela ed by he exp ession E*=Sn+En·(A-1)/A, whe e Sn is he one-neu on sepa a ion ene gy in he nucleus A. The bene i o he su oga e me hod is ha in some cases he a ge needed is s able o less adioac i e han he a ge o he co esponding neu on-induced eac ion. The e o e, he su oga e- eac ion me hod may enable neu on-induced c oss sec ions o be ex ac ed o nuclea eac ions on sho -li ed nuclei ha o he wise canno be measu ed. Fo eq. (1) o apply wo condi ions ha e o be ul illed: he decaying nucleus has o be a compound nucleus and he decay p obabili y has o be independen o he spin J and pa i y π o he decaying nucleus. Howe e , a low exci a ion ene gies he decay p obabili y s ongly depends on Jπ. In addi ion, he e may be impo an di e ences be ween he Jπ dis ibu ions popula ed in he neu on and ans e eac ions. The e o e, conside able de ia ions be ween he neu on-induced esul s and he ones ob ained wi h he su oga e me hod may exis . While i is a he well es ablished ha he su oga e me hod wo ks well o ission a su icien ly high E* (see e.g. [2]), se e al ecen expe imen s ha e shown ha gamma decay is e y sensi i e o he di e ences in he popula ed Jπ dis ibu ions [3-5], which leads o signi ican disc epancies be ween he su oga e esul s and he neu on-induced da a a he lowes exci a ion ene gies. This is p obably due o he spin-pa i y selec i i y o neu on emission [5]. This selec i i y dec eases s ongly as he le el densi y o he esidual nucleus a e neu on emission inc eases. The e o e, he disc epancies be ween he su oga e esul s and he neu on-induced da a a e expec ed o dec ease wi h inc easing mass o he decaying nucleus and wi h inc easing exci a ion ene gy. In his wo k we s udy he alidi y o he su oga e me hod in he ac inide egion using an imp o ed expe imen al se -up ha enables he measu emen o ission and gamma-decay p obabili ies. Tha allows us o in es iga e he wo main issues de e mining he alidi y o he su oga e me hod: he “compound” cha ac e o he decaying nucleus and he Jπ dependence o he decay p obabili ies. 2 Expe imen The expe imen was pe o med in June 2012 a he Oslo Cyclo on Labo a o y (OCL). We used a 238U a ge wi h 99.7% iso opic pu i y p oduced a GSI. Two di e en beams we e used, a deu e on beam o 15 MeV and a 3He beam o 24 MeV. The expe imen al se -up is ep esen ed in Fig. 1. The ejec iles we e de ec ed a backwa d angles wi h he SiRi mul i-s ip silicon ΔE/E de ec o [6]. SiRi p o ided he iden i ica ion o he ejec iles, as well as hei kine ic ene gy and angle. Fission agmen s we e de ec ed in coincidence wi h he ejec iles. The ission de ec o was loca ed a o wa d angles and consis ed o 4 PPACs co e ing a solid angle o 40% ou o 4. The eac ion chambe housing SiRi, he PPACs and he 238U a ge was su ounded by he CACTUS a ay wi h 26 high-e iciency NaI de ec o s. CACTUS was used o de ec gamma ays wi h ene gies anging om abou 200 keV o 12 MeV in coincidence wi h he ejec iles. The di e en ans e eac ions on he 238U a ge lead o he p oduc ion o a ious hea y esidues. Table 1 lis s he ans e channels conside ed in he p esen wo k, and he co esponding neu on- induced eac ions o which he su oga e esul s can be compa ed. 01003-p.2 Fig. 1. Schema ic iew o he se -up o decay-p obabili y measu emen s wi h he SIRI elescope and he ission de ec o inside he CACTUS NaI a ay. Table 1. T ans e eac ions in es iga ed in his wo k and he co esponding neu on-induced ission eac ions. T ans e eac ions Neu on-induced eac ions 238U(d,p)239U* 238U(n, ) and 238U(n,γ) 238U(3He, )238Np* 237Np(n, ) and 237Np(n,γ) 238U(3He,4He)237U* 236U(n, ) and 236U(n,γ) Fission o gamma-decay p obabili ies a e ob ained om he ollowing he exp ession: exp coin decay singles decay (*) (*) (*) (*) NE PE NE E   (2) whe e Ncoin is he numbe o coincidences be ween he ejec iles and he ission o he gamma de ec o , Nsingles is he o al numbe o de ec ed ejec iles and  decay is he e iciency o he ission o he gamma de ec o . 3 P elimina y esul s The esul s p esen ed in his sec ion ha e been ob ained om he da a analysis o one elescope s ip ou o 64. The selec ed s ip was loca ed a 126 deg ees ela i e o he beam. Because o he ela i ely low s a is ics, hese esul s mus be conside ed as p elimina y. The le pa o Fig. 2 shows he ission p obabili y o 239U* as a unc ion o exci a ion ene gy ob ained om he 238U(d,p) eac ion in compa ison wi h he neu on-induced ission p obabili y o 239U* as gi en by TALYS [7]. Fo E* > 6 MeV he ag eemen be ween he wo cu es is e y good. Howe e , abo e 6.5 MeV he esul s om he su oga e eac ion a e clea ly below he neu on-induced esul s. The di e ence amoun s o 25% a mos . The eason o his disc epancy could be he b eakup o he deu e on, which means ha he neu on ha is ans e ed in he s ipping (d,p) eac ion o he 238U a ge 01003-p.3 WONDER-2012 EPJ Web o Con e ences nucleus escapes in o he b eakup phase space be o e he nucleus 239U becomes a compound nucleus. This leads o a backg ound o “s e ile” p o ons ha con amina es he singles p o on spec um. This phenomenon was al eady obse ed by B i and C ame [8] and a heo e ical in es iga ion was ini ia ed [9]. I is in e es ing o no e ha he ission p obabili y ob ained by B i and C ame o he 238U(d,p) eac ion is abou wo imes smalle han he one ob ained in his wo k. This could be due o he highe ene gy o he deu e on beam, 18 MeV, used in hei expe imen . To unde s and hese esul s model calcula ions based on he heo y o pa ial usion a e necessa y. Fig. 2. P elimina y esul s o he ission p obabili ies o 239U* (le ) and 237U* ( igh ) ob ained in he 238U(d,p) and in he 238U(3He,4He) eac ions, espec i ely, compa ed o TALYS [7] and ENDF/B-VII.1. On he igh o Fig. 2, he esul s o he ission p obabili y o 237U* induced in he 238U(3He,4He) eac ion a e p esen ed. The la ge luc ua ions o he su oga e esul a e pu ely s a is ical due o he limi ed numbe o e en s analysed. Aside om he s a is ical luc ua ions, he e is an o e all good ag eemen be ween he su oga e and he neu on-induced da a ep esen ed by he ENDF/B-VII.1 e alua ion. Since he a ge and he expe imen al se -up used o measu e he ission p obabili y o 239U* and 237U* we e he same, he e is no expe imen al sys ema ic e o ha explains he lowe ing o he ission p obabili y o 239U* o E*>6.5 MeV. Fig. 3 ep esen s he a io N-coin/Nsingles, i.e. he numbe o coincidences be ween a p o on and 1 NaI di ided by he o al numbe o de ec ed p o ons, o he 238U(d,p) eac ion. This quan i y di ided by he cascade-de ec ion e iciency o CACTUS gi es he gamma-decay p obabili y, see eq. (2). As expec ed, we can see ha he a io dec eases e y apidly a an E* o abou 4.9 MeV, in e y good ag eemen wi h he Sn o 239U*. This dec ease is due o he onse o neu on e apo a ion ha compe es wi h gamma emission and becomes he dominan de-exci a ion mode se e al hund eds o keV a e Sn. Gamma- ays emi ed ollowing neu on e apo a ion we e emo ed om N-coin by applying a h eshold o he ene gy o he NaI de ec o s. As can be seen on he le o Fig. 2, ission se s in a abou 5.7 MeV. The e o e, up o his exci a ion ene gy, he gamma-decay p obabili y o 239U* is no a ec ed by he backg ound coming om gamma emission by he ission agmen s. 4 Conclusions and pe spec i es We pe o med an expe imen a he OCL o u he in es iga e he alidi y o he su oga e me hod in he ac inide egion. The i s esul s o he ission p obabili y o 239U* induced in he 238U(d,p ) eac ion show a sys ema ic lowe ing ha could be due o deu e on b eakup. These esul s will p o ide e y aluable in o ma ion on he p ocess o o ma ion o a compound nucleus a e he s ipping (d,p) eac ion. No e ha he (d,p) eac ion is pa icula ly in e es ing o simula e neu on- induced eac ions in in e se kinema ics wi h adioac i e beams. 01003-p.4 Fig. 3. P elimina y esul s o he numbe o coincidences be ween p o ons and 1 NaI di ided by he numbe o de ec ed p o ons o he eac ion 238U(d,p) 239U*. In spi e o he la ge s a is ical luc ua ions, we obse e ha he ission p obabili y o 237U* induced in he 236U(3He,4He) eac ion is in o e all good ag eemen wi h he neu on-induced esul s. This means ha he lowe ing o he ission p obabili y ound o he 238U(d,p ) eac ion is no due o a sys ema ic e o in he expe imen al se up. The a io N-coin/Nsingles was ex ac ed o he 238U(d,p) eac ion. I p esen s he expec ed s ep dec ease a he neu on sepa a ion ene gy o 239U* ha e lec s he compe i ion be ween gamma emission and neu on e apo a ion. To ob ain he gamma-decay p obabili y, his a io has o be co ec ed o he cascade-de ec ion e iciency o CACTUS. The la e will be ob ained om a de ailed analysis o he expe imen al da a. The comple e analysis o he da a will allow us o de e mine also he ission and gamma-decay p obabili ies associa ed o he eac ions 238U(3He,4He), 238U(3He, ). The ensemble o he esul s om his wo k will p o ide an impo an s ep o wa d o es ablishing o which ex en he su oga e me hod can be applied o ex ac neu on- induced c oss sec ions in he ac inide egion. Acknowledgemen s We would like o hank he s a o he OCL o he suppo du ing he expe imen , Be ina Lommel and he GSI Ta ge Labo a o y o he p oduc ion o high-quali y 238U a ge s and Pascal Romain o pe o ming he TALYS calcula ion. This wo k was suppo ed by he Eu opean Commission wi hin he 7 h F amewo k P og amme h ough Fission-2010-ERINDA (P ojec No 269499), by he Eu opean A omic Ene gy Communi y's 7 h F amewo k P og amme unde g an ag eemen n° FP7- 249671 (ANDES) and by he F ench na ional esea ch p og amme GEDEPEON. MW acknowledges suppo om he Na ional Resea ch Founda ion o Sou h A ica. Re e ences 1. J. D. C ame and H. C. B i , Nucl. Sci. Eng. 41, 177 (1970) 2. G. Kessedjian e al., Phys. Le . B 692, 297 (2010) 3. N. D. Scielzo e al., Phys. Re . C 81, 034608 (2010) 4. J. N. Wilson e al., Phys. Re . C 85, 034607 (2012) 5. G. Bou oux e al., Phys. Le . B 712, 319 (2012) 6. M. Gu o msen e al., Nucl. Is um. Me h. A 648, 168 (2011) 7. A. J. Koning e al., Use Manual TALYS, www. alys.eu 8. H. C. B i and J. D. C ame , Phys. Re . C 2, 1758 (1970) 9. J. E. Esche , Re . Mod. Phys. 84, 353 (2012) 01003-p.5 WONDER-2012