Neutron beam imaging with micromegas detectors in combination with neutron time-of-flight at the n_tof facility at CERN

366 NEUTRON BEAM IMAGING WITH MICROMEGAS DETECTORS IN COMBINATION WITH NEUTRON TIME-OF-FLIGHT AT THE n_TOF FACILITY AT CERN F. Belloni1, S. And iamonje2, E. Be houmieux1, M. Cal iani2, E. Chia e i2, N. Colonna3, Y. Gioma a is1, C. Gue e o2, F. Gunsing1, F. J. Iguaz1, M. Kebbi i1, J. Pancin1, T. Papae angelou1, A. Tsinganis2,4, V. Vlachoudis2, S. Al s ad 5, J. And zejewski6, L. Audouin7, M. Ba bagallo3, V. Béca es8, F. Beč ář9, J. Billowes10, V. Boccone2, D. Bosna 11, M. B ugge 2, F. Cal iño12, D. Cano-O 8, C. Ca apiço13, F. Ce u i2, E. Chia e i1,2, M. Chin2, G. Co és12, M. A. Co é-Gi aldo14, M. Diakaki4, C. Domingo-Pa do15, I. Du an16, N. Dzysiuk17, C. Ele he iadis18, A. Fe a i2, K. F a al1, S. Ganesan19, A. R. Ga cía8, G. Giub one15, M. B. Gómez-Ho nillos12, I. F. Gonçal es13, E. González-Rome o8, E. G iesmaye 20, P. Gu usamy19, D. G. Jenkins21, E. Je icha20, Y. Kadi2, F. Käppele 22, D. Ka adimos4, P. Koehle 23, M. Kokko is4, M. K ička9, J. K oll9, C. Lange 5, C. Lede e 5,24, H. Leeb20, L. S. Leong7, R. Losi o2, A. Manousos18, J. Ma ganiec6, T. Ma í nez8, C. Massimi25, P. F. Mas inu17, M. Mas oma co3, M. Meaze3, E. Mendoza8, A. Mengoni26, P. M. Milazzo27, F. Ming one25, M. Mi ea28, W. Mondalae s29, C. Pa adela16, A. Pa lik24, J. Pe kowski6, A. Plompen29, J. P aena14, J. M. Quesada14, T. Rausche 30, R. Rei a h5, A. Riego12, F. Roman2,28, C. Rubbia2,31, R. Sa men o13, P. Schillebeeckx29, S. Schmid 5, G. Taglien e3, J. L. Tain15, D. Ta ío16, L. Tassan-Go 7, S. Valen a9, G. Vannini25, V. Va iale3, P. Vaz13, A. Ven u a26, R. Ve saci2, M. J. Ve meulen21, V. Vlachoudis2, R. Vlas ou4, A. Wallne 24, T. Wa e10, M. Weigand5, C. Weiss20, T. J. W igh 10, P. Žugec11 1 CEA Saclay, DSM/IRFU/SPhN, Gi -su -Y e e, F ance 2 Eu opean O ganiza ion o Nuclea Resea ch (CERN), Gene a, Swi ze land 3 Is i u o Nazionale di Fisica Nuclea e, Ba i, I alia 4 Na ional Technical Uni e si y o A hens, A hens, G eece 5 Goe he Uni e si ä , F ank u , Ge many 6 Uniwe sy e Łódzki, Lodz, Poland 7 Cen e Na ional de la Reche che Scien i ique/IN2P3, IPN, O say, F ance 8 Cen o de In es igaciones Ene ge icas Medioambien ales y Tecnólgicas, Mad id, Spain 9 Cha les Uni e si y, P ague, Czech Republic 10 Uni e si y o Manches e , Ox o d Road, Manches e , Uni ed Kingdom 11 Depa men o Physics, Facul y o Science, Uni e si y o Zag eb, C oa ia 12 Uni e si a Poli ecnica de Ca alunya, Ba celona, Spain 13 Ins i u o Tecnológico e Nuclea , Ins i u o Supe io Técnico, Uni e sidade Técnica de Lisboa, Lisboa, Po ugal 14 Uni e sidad de Se illa, Se illa, Spain 15 Ins i u o de Fisica Co puscula , CSIC-Uni e sidad de Valencia, Spain 16 Uni e sidad de San iago de Compos ela, San iago de Compos ela, Spain 17 Is i u o Nazionale di Fisica Nuclea e, Labo a o i Nazionali di Legna o, Legna o, I aly 18 A is o le Uni e si y o Thessaloniki, Thessaloniki, G eece 19 Bhabha A omic Resea ch Cen e, Mumbai, India 20 A omins i u , Technische Uni e si ä Wien, Aus ia 21 Uni e si y o Yo k, Hesling on, Yo k, Uni ed Kingdom 22 Ka ls uhe Ins i u e o Technology, Campus No d, Ins i u ü Ke nphysik, Ka ls uhe, Ge many 23 Oak Ridge Na ional Labo a o y, Oak Ridge, TN, USA 24 Uni e si y o Vienna, Facul y o Physics, Aus ia 25 Dipa imen o di Fisica, Uni e si à di Bologna, and Sezione INFN di Bologna, I aly 26 Agenzia nazionale pe le nuo e ecnologie, l'ene gia e lo s iluppo economico sos enibile, Bologna, I aly 27 Is i u o Nazionale di Fisica Nuclea e, T ies e, I aly 28 Ho ia Hulubei Na ional Ins i u e o Physics and Nuclea Enginee ing, Bucha es -Magu ele, Romania 29 Eu opean Commission JRC, Ins i u e o Re e ence Ma e ials and Measu emen s, Depa men o Physics and As onomy, Geel, Belgium 30 Uni e si y o Basel, Basel, Swi ze land 31 Labo a o io Nazionali del G an Sasso, Is i u o Nazionale di Fisica Nuclea e, Asse gi (AQ), I aly A bulk mic omegas de ec o wi h he anode segmen ed in 2 o hogonal di ec ions and equipped wi h a neu on/cha ged pa icle con e e is employed a he neu on ime-o - ligh (nTOF) acili y a CERN o de e mine he inciden neu on beam p o ile and beam in e cep ion ac o as a unc ion o he neu on ene gy de e mined by he ime o ligh . Disc epancies be ween expe imen al esul s and simula ions in he alues o he beam in e cep ion ac o ange up o 12 % and a e o be asc ibed o a de ec in he mesh o he bulk. Ne e heless he de ec o p o ed o be eally use ul o checking he alignmen o he neu on beam op ics o he acili y. Measu emen s wi h a new pixelized bulk de ec o o he de e mina ion o he beam in e cep ion ac o a e o seen be o e he end o 2012. 367 1. In oduc ion The neu on ime-o - ligh (n_TOF) acili y a CERN is a whi e neu on sou ce based on spalla ion o 20 GeV/c p o on beam wi h pulses 6 ns .m.s. hi ing a Pb a ge wi h a minimum epe i ion a e o 2.4 seconds. A 185 m long e acua ed ube leads neu ons o an expe imen al a ea (EAR o EAR-1) equipped wi h se e al de ec o s o neu on- induced cap u e, ission and (n, α) c oss sec ions. A de ailed desc ip ion o he acili y can be ound in [1]. Neu on-induced eac ion measu emen s need an accu a e knowledge o he inciden lux. A n_TOF his is measu ed wi h a silicon moni o [2] and a Mic o-mesh gaseous s uc u e (mic omegas) de ec o [3] o mic obulk ype [4], as well as a e y accu a e es ima ion o he spa ial p o ile, de e mined wi h a mic omegas de ec o o bulk ype [5] in o de o calcula e he beam in e cep ion ac o , i.e. he co ec ion o be applied o he ex ac ed neu on induced c oss sec ions when he sample is smalle han he beam diame e . Mic omegas de ec o s a e a s ack o one ioniza ion chambe and one p opo ional chambe . A hin mesh sepa a es he wo communica ing egion, whe e wo di e en elec ic ields a e applied o ob ain espec i ely he ioniza ion and mul iplica ion cha ge egimes. When he anode and he mesh o he de ec o a e cons uc ed in o a single piece, his is called bulk. The main di e ence be ween bulks and mic obulks is he hickness o he mesh, hinne in he la e hanks o he ab ica ion p ocedu e. 2. De ec o The n_TOF beam p o ile is a ci cula bulk o 4 cm adius wi h a ypical bulk hickness o 128 μm. The eadou is smalle han he anode i sel and i is ob ained by segmen ing a squa e po ion o 6 cm side o he anode in wo o hogonal di ec ions, called X and Y in he e e ence ame o he de ec o , by 106 s ips. The de ec o allows he e o e he de e mina ion o he beam image wi h a spa ial esolu ion o 0.5 mm as a unc ion o he ime-o - ligh . A 2 μm hick laye o BC4 en iched in 10B is deposi ed on he d i and ac s as a neu on/cha ged pa icle con e e h ough he 10B(n,α)7Li eac ion. The d i o mesh dis ance is 4 mm, and he de ec o is ope a ed wi h and admix u e o A (90 %), CF4 (8 %) and C4H10 (2 %). In o de o educe he numbe o channels necessa y o collec he ou pu o he s ips and o he mesh, wo gassiplex ca ds a e employed. 3. Analysis The analysis p ocedu e consis s in iden i ying signals caused by α pa icles o 7Li pa icles p oduced by he in e ac ion o neu ons wi h he con e e and, o e e y in e ac ion, associa ing o each s ip o he de ec o i s signal ampli ude. The baseline le el in each s ip is de e mined h ough a measu emen wi hou beam, called pedes al un. The ollowing s ep consis s in sub ac ing, o each s ip, he baseline le el o he egis e ed signals and iden i ying he s ips wi h a le o e highe han a ce ain h eshold. The se o hese s ips is called clus e and an algo i hm o ind he cen e o cha ge o each clus e is applied. By exploi ing he ime in o ma ion o he mesh signal, a bi-dimensional his og am is illed wi h pai s o cen e s o cha ges calcula ed in he wo o hogonal di ec ions in ime coincidence and he beam image is ob ained as a unc ion o ime o ligh . Since he axis o he de ec o a e il ed o 135° wi h espec o he loo and walls o he expe imen al a ea, a o a ion has o be applied o he beam image in o de o isualize i in he e e ence sys em o he labo a o y. A bi-dimensionnal Gaussian i o he beam p o ile allows he s udy o i s wid hs. In he end, he beam in e cep ion ac o is ex ac ed by calcula ing he a io o he numbe o neu ons hi ing he a ea co e ed by a sample o he numbe o neu ons in he whole beam. 4. Resul s Da a collec ed in he yea s 2009 - 2012 ha e been analyzed. In 2009 he s udy o he in luence o he beam op ics alignmen on he beam p o ile was s udied, allowing o de ec a misalignmen o he second collima o (Fig. 1). A s udy o he gain o he de ec o showed ha he dis ance be ween he mesh and he anode was no cons an because o a sligh ly cu ed mesh plane, p o oking di e ences in gain be ween di e en s ips. This issue was pa ially sol ed by i ing expe imen al da a o exclude he non ope a i e s ips. Ne e heless, gi en he ac ha he p oblem conce ns a la ge po ion o he de ec o , i was no possible o o ally co ec o his. Fig. 2 shows a compa ison o he beam in e cep ion ac o o a sample o 1 cm diame e ob ained om simula ions, om he measu ed da a and om he Gaussian i o he la e . Disc epancies be ween he esul s gi en by he i o ou da a and simula ions each 12 %, bu hei ends a e simila and in ag eemen wi h he expec a ions. Since he low ene gy neu ons a e p oduced a e se e al sca e ings in he mode a o o he spalla ion a ge (made o H20), he neu on beam should be wide a low neu on ene gies han a high neu on ene gies, wi h he consequence ha he beam in e cep ion ac o inc eases wi h he neu on ene gy. 368 Fig. 1. Beam p o ile p ojec ed along he e ical axis in he e e ence sys em o he labo a o y. Fig 2. Beam in e cep ion ac o o a sample o 1 cm diame e . 5. Conclusions A s udy o he beam p o ile was pe o med a he n_TOF acili y wi h a mic omegas de ec o o bulk ype. Thanks o hese s udies a misalignmen o he second collima o was de ec ed and co ec ed o . The end o he ex ac ed beam in e cep ion ac o as a unc ion o he inciden neu on ene gy is in ag eemen wi h expec a ions, bu always highe and disc epancies each 12%. This is asc ibed o a de ec in he mesh o he de ec o and a measu emen wi h a new pixelized mic omegas de ec o o bulk ype is al eady scheduled be o e he end o 2012. REFERENCES 1. The n_TOF Collabo a ion, CERN n_TOF Facili y: Pe o mance Repo // CERN/INTC-O-011, INTC-2002-037, CERN-SL-2002-053 ECT. - 2001. 2. Ma one S. e al. // Nuclea Ins umen s and Me hods in Physics Resea ch. - 2004. - Vol. A517. - P. 389 - 398. 3. Gioma a is Y., Rebou gea d Ph., Robe J.P., Cha pak G. Mic omegas: a highg anula i y posi ion-sensi i e gaseous de ec o o high pa icle- lux en i onmen s // Nucl. Ins . Me h. - 1996. - Vol. A376. - P. 29 - 35. 4. Gioma a is Y., De Oli ei a R., And iamonje S. e al. Mic omegas in a bulk // Nucl. Ins . Me h. - 2005. - Vol. A560. - P. 405 - 408. 5. And iamonje S., A ie D., Be houmieux E. e al. De elopmen and pe o mance o mic obulk mic omegas de ec o s // J. Ins um. - 2010. - Vol. 5. - P. 02001 - 02013.