Se up o he measu emen o he 235U(n, ) c oss sec ion ela i e o n-p sca -
e ing up o 1 GeV
A. Manna1,2,∗,O. Abe le3,V. Alcayne4,S. Amaducci5,6,J. And zejewski7,L. Audouin8,V. Babiano-Sua ez9,
M. Bacak3,10,11,M. Ba bagallo3,12,S. Benne 13,E. Be houmieux11,D. Bosna 14,A. S. B own15,M. Busso16,17,
M. Caamaño18,L. Caballe o9,M. Cal iani3,F. Cal iño19,D. Cano-O 4,A. Casano as19,F. Ce u i3,E. Chia e i13,20,3,
N. Colonna12,G . P. Co és19,M. A. Co és-Gi aldo20,L. Cosen ino5,S. C is allo16,21,L. A. Damone12,22,P. J. Da ies13,
M. Diakaki23,M. Die z24,C. Domingo-Pa do9,R. D essle 25,Q. Ducasse26,E. Dupon 11,I. Du án18,Z. Eleme27,
B. Fe nández-Domíngez18,A. Fe a i3,I. Fe o-Gonçal es28,P. Finocchia o5,V. Fu man29,R. Ga g24,A. Gawlik7,
S. Gila doni3,K. Göbel30,E. González-Rome o4,C. Gue e o20,F. Gunsing11,S. Heini z25,J. Heyse31,D. G. Jenkins15,
E. Je icha10,U. Ji i25,A. Junghans32,Y. Kadi3,F. Käppele 33,A. Kimu a34,I. Knapo á35,M. Kokko is23,Y. Kopa ch29,
M. K iˇ
cka35,D. Ku ulgil30,I. Lada escu9,C. Lede e -Woods24,J. Le endegui-Ma co20,S.-J. Lonsdale24,D. Macina3,
T. Ma ínez4,A. Masi3,C. Massimi1,2,P. F. Mas inu36,M. Mas oma co3,13,E. Mauge i25,A. Mazzone12,37,E. Mendoza4,
A. Mengoni38,1,V. Michalopoulou3,23,P. M . Milazzo39,M. A. Millán-Callado20,F. Ming one3,J. Mo eno-So o11,
A. Musuma a5,6,A. Neg e 40,R. Nol e26,F. Ogálla 41,A. Op ea40,N. Pa onis27,A. Pa lik42,J. Pe kowski7,C. Pe one40,
L. Pie san i16,21,E. Pi o ano26,I. Po as41,J. P aena41,J. M. Quesada20,D. Ramos Do al8,R. Rei a h30,D. Rochman25,
C. Rubbia3,M. Saba é-Gila e20,3,A. Saxena43,P. Schillebeeckx31,D. Schumann25,A. Sekha 13,A. G. Smi h13,
N. Sosnin13,P. Sp ung25,A. S ama opoulos23,G. Taglien e12,J. L. Tain9,A. E. Ta i eño-Saldi ia19,L. Tassan-Go 3,23,8,
B. Thomas30,P. To es-Sánchez41,A. Tsinganis3,S. U lass3,32,S. Valen a35,G. Vannini1,2,V. Va iale12,P. Vaz28,
A. Ven u a1,D. Vesco i16,44,V. Vlachoudis3,R. Vlas ou23,A. Wallne 45,P. J. Woods24,T. J. W igh 13, and P. Žugec14
1Is i u o Nazionale di Fisica Nuclea e, Sezione di Bologna, I aly
2Dipa imen o di Fisica e As onomia, Uni e si à di Bologna, I aly
3Eu opean O ganiza ion o Nuclea Resea ch (CERN), Swi ze land
4Cen o de In es igaciones Ene gé icas Medioambien ales y Tecnológicas (CIEMAT),Spain
5INFN Labo a o i Nazionali del Sud, Ca ania, I aly
6Dipa imen o di Fisica e As onomia, Uni e si à di Ca ania, I aly
7Uni e si y o Lodz, Poland
8IPN, CNRS-IN2P3, Uni . Pa is-Sud, Uni e si é Pa is-Saclay, F-91406 O say Cedex,F ance
9Ins i u o de Física Co puscula , CSIC - Uni e sidad de Valencia, Spain
10Technische Uni e si ä Wien, Aus ia
11CEA Saclay, I u, Uni e si é Pa is-Saclay, Gi -su -Y e e, F ance
12Is i u o Nazionale di Fisica Nuclea e, Ba i, I aly
13Uni e si y o Manches e , Uni ed Kingdom
14Depa men o Physics, Facul y o Science, Uni e si y o Zag eb, C oa ia
15Uni e si y o Yo k, Uni ed Kingdom
16Is i u o Nazionale di Fisica Nazionale, Pe ugia, I aly
17Dipa imen o di Fisica e Geologia, Uni e si à di Pe ugia, I aly
18Uni e si y o San iago de Compos ela, Spain
19Uni e si a Poli ècnica de Ca alunya, Spain
20Uni e sidad de Se illa, Spain
21Is i u o Nazionale di As o isica - Osse a o io As onomico d’Ab uzzo, I aly
22Dipa imen o di Fisica, Uni e si à degli S udi di Ba i, I aly
23Na ional Technical Uni e si y o A hens, G eece
24School o Physics and As onomy, Uni e si y o Edinbu gh, Uni ed Kingdom
25Paul Sche e Ins i u (PSI), Villigen, Swi ze land
26Physikalisch-Technische Bundesans al (PTB), Bundesallee 100, 38116 B aunschweig, Ge many
27Uni e si y o Ioannina, G eece
28Ins i u o Supe io Técnico, Lisbon, Po ugal
29Join Ins i u e o Nuclea Resea ch (JINR), Dubna, Russia
30Goe he Uni e si y F ank u , Ge many
31Eu opean Commission, Join Resea ch Cen e, Geel, Re ieseweg 111, B-2440 Geel,Belgium
32Helmhol z-Zen um D esden-Rossendo , Ge many
33Ka ls uhe Ins i u e o Technology, Campus No h, IKP, 76021 Ka ls uhe, Ge many
34Japan A omic Ene gy Agency (JAEA), Tokai-mu a, Japan
EPJ Web o Con e ences 239, 01008 (2020) h ps://doi.o g/10.1051/epjcon /202023901008
ND2019
© The Au ho s, published by EDP Sciences. 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 4.0 (h p://c ea i ecommons.o g/licenses/by/4.0/).
35Cha les Uni e si y, P ague, Czech Republic
36Is i u o Nazionale di Fisica Nuclea e, Sezione di Legna o, I aly
37Consiglio Nazionale delle Rice che, Ba i, I aly
38Agenzia nazionale pe le nuo e ecnologie, l’ene gia e lo s iluppo economico sos enibile (ENEA), Bologna, I aly
39Is i u o Nazionale di Fisica Nazionale, T ies e, I aly
40Ho ia Hulubei Na ional Ins i u e o Physics and Nuclea Enginee ing (IFIN-HH),Bucha es
41Uni e si y o G anada, Spain
42Uni e si y o Vienna, Facul y o Physics, Vienna, Aus ia
43Bhabha A omic Resea ch Cen e (BARC), India
44G an Sasso Science Ins i u e (GSSI), L’Aquila, I aly
45Aus alian Na ional Uni e si y, Canbe a, Aus alia
Abs ac . The neu on induced ission o 235U is ex ensi ely used as a e e ence o neu on luence mea-
su emen s in a ious applica ions, anging om he in es iga ion o he biological e ec i eness o high ene gy
neu ons, o he measu emen o high ene gy neu on c oss sec ions o ele ance o accele a o d i en nuclea
sys ems. Despi e i s widesp ead use, no da a exis on neu on induced ission o 235U abo e 200 MeV. The
neu on acili y n_TOF o e s he possibili y o imp o e he si ua ion. The measu emen o 235U(n, ) ela i e
o he di e en ial n-p sca e ing c oss-sec ion, was ca ied ou in Sep embe 2018 wi h he aim o p o iding
accu a e and p ecise c oss sec ion da a in he ene gy ange om 10 MeV up o 1 GeV. In such measu emen s,
Recoil P o on Telescopes (RPTs) a e used o measu e he neu on lux while he ission e en s a e de ec ed
and coun ed wi h dedica ed de ec o s. In his pape he measu emen campaign and he expe imen al se -up a e
illus a ed.
1 In oduc ion
Neu on c oss sec ion s anda ds a e undamen al ing edi-
en s o bo h measu emen s and e alua ions o neu on-
induced eac ion c oss sec ions. Howe e , no c oss sec ion
s anda d exis s o neu on ene gies abo e 200 MeV. This
led he In e na ional A omic Ene gy Agency o issue a e-
ques o a new absolu e measu emen o neu on induced
ission c oss sec ions, ela i e o n-p sca e ing, o es ab-
lish a ission c oss sec ion s anda d abo e 200 MeV [1].
An e ec i e choice o he eac ion o be s udied is he
235U(n, ) eac ion, al eady one o he mos impo an s an-
da d c oss sec ion a he mal neu on ene gy and be ween
0.15 MeV and 200 MeV [2]. Despi e i s impo ance in un-
damen al nuclea physics and i s widesp ead use as e e -
ence o neu on luence measu emen s, only wo da a se s
a e a ailable in he ene gy ange be ween 20 and 200 MeV
and wo absolu e expe imen al measu emen s ha e been
pe o med in he high ene gy egion, abo e 200 MeV [3, 4]
( igu e 1). Mo eo e he ission p ocess o U anium a
high exci a ion ene gy is an impo an opic as i is ela ed
o undamen al quan i ies o exci ed nuclea ma e , like
he iscosi y and ansien e ec s [10, 11].
Thanks o he in ense neu on beam, wi h a wide ene gy
spec um anging om he mal o 1 GeV, he n_TOF a-
cili y a CERN can answe o he eques o he IAEA o
new measu emen s. A dedica ed measu emen campaign
was ca ied ou wi h he aim o p o iding accu a e and p e-
cise c oss sec ion da a o he 235U(n, ) eac ion in he high
ene gy egion.
2 Expe imen al se -up
Neu ons a n_TOF (see [13] o an ex ensi e desc ip ion)
a e p oduced by spalla ion eac ion: bunches o p o ons o
20 GeV/c momen um a e sho e e y ew seconds by he
∗e-mail: [email p o ec ed]
Figu e 1. The 235U(n, ) c oss sec ion om he JENDL/HE [5]
and IAEA [1] e alu a ions, he expe imen al da a [6, 7] measu ed
ela i e o he n-p c oss sec ion and a new heo e ical calcula-
ion [8] based on he in anuclea cascade model INCL++ [9]
coupled o he deexci a ion model GEMINI++ [12].
CERN P o on Synch o on on a massi e lead a ge ; abou
300 neu ons a e p oduced pe inciden p o on. The a-
cili y ea u es wo beam lines wi h a whi e neu ons spec-
um, and wo expe imen al a eas a he nominal dis ance
o 185 m o EAR-1 [14] and 20 m o EAR-2 [15] om
he neu on-p oducing a ge .
The measu emen o he 235U(n, ) c oss sec ion was pe -
o med in Sep embe 2018 in he i s expe imen al a ea,
whe e he neu on spec um ex ends up o 1 GeV, and also
o p o i om he good ene gy esolu ion in he high en-
e gy egion, hanks o he longe ligh pa h.
The expe imen al se up included de ec o s o coun he is-
sion e en s om he U anium a ge and, a he same ime,
2
EPJ Web o Con e ences 239, 01008 (2020) h ps://doi.o g/10.1051/epjcon /202023901008
ND2019
35Cha les Uni e si y, P ague, Czech Republic
36Is i u o Nazionale di Fisica Nuclea e, Sezione di Legna o, I aly
37Consiglio Nazionale delle Rice che, Ba i, I aly
38Agenzia nazionale pe le nuo e ecnologie, l’ene gia e lo s iluppo economico sos enibile (ENEA), Bologna, I aly
39Is i u o Nazionale di Fisica Nazionale, T ies e, I aly
40Ho ia Hulubei Na ional Ins i u e o Physics and Nuclea Enginee ing (IFIN-HH),Bucha es
41Uni e si y o G anada, Spain
42Uni e si y o Vienna, Facul y o Physics, Vienna, Aus ia
43Bhabha A omic Resea ch Cen e (BARC), India
44G an Sasso Science Ins i u e (GSSI), L’Aquila, I aly
45Aus alian Na ional Uni e si y, Canbe a, Aus alia
Abs ac . The neu on induced ission o 235U is ex ensi ely used as a e e ence o neu on luence mea-
su emen s in a ious applica ions, anging om he in es iga ion o he biological e ec i eness o high ene gy
neu ons, o he measu emen o high ene gy neu on c oss sec ions o ele ance o accele a o d i en nuclea
sys ems. Despi e i s widesp ead use, no da a exis on neu on induced ission o 235U abo e 200 MeV. The
neu on acili y n_TOF o e s he possibili y o imp o e he si ua ion. The measu emen o 235U(n, ) ela i e
o he di e en ial n-p sca e ing c oss-sec ion, was ca ied ou in Sep embe 2018 wi h he aim o p o iding
accu a e and p ecise c oss sec ion da a in he ene gy ange om 10 MeV up o 1 GeV. In such measu emen s,
Recoil P o on Telescopes (RPTs) a e used o measu e he neu on lux while he ission e en s a e de ec ed
and coun ed wi h dedica ed de ec o s. In his pape he measu emen campaign and he expe imen al se -up a e
illus a ed.
1 In oduc ion
Neu on c oss sec ion s anda ds a e undamen al ing edi-
en s o bo h measu emen s and e alua ions o neu on-
induced eac ion c oss sec ions. Howe e , no c oss sec ion
s anda d exis s o neu on ene gies abo e 200 MeV. This
led he In e na ional A omic Ene gy Agency o issue a e-
ques o a new absolu e measu emen o neu on induced
ission c oss sec ions, ela i e o n-p sca e ing, o es ab-
lish a ission c oss sec ion s anda d abo e 200 MeV [1].
An e ec i e choice o he eac ion o be s udied is he
235U(n, ) eac ion, al eady one o he mos impo an s an-
da d c oss sec ion a he mal neu on ene gy and be ween
0.15 MeV and 200 MeV [2]. Despi e i s impo ance in un-
damen al nuclea physics and i s widesp ead use as e e -
ence o neu on luence measu emen s, only wo da a se s
a e a ailable in he ene gy ange be ween 20 and 200 MeV
and wo absolu e expe imen al measu emen s ha e been
pe o med in he high ene gy egion, abo e 200 MeV [3, 4]
( igu e 1). Mo eo e he ission p ocess o U anium a
high exci a ion ene gy is an impo an opic as i is ela ed
o undamen al quan i ies o exci ed nuclea ma e , like
he iscosi y and ansien e ec s [10, 11].
Thanks o he in ense neu on beam, wi h a wide ene gy
spec um anging om he mal o 1 GeV, he n_TOF a-
cili y a CERN can answe o he eques o he IAEA o
new measu emen s. A dedica ed measu emen campaign
was ca ied ou wi h he aim o p o iding accu a e and p e-
cise c oss sec ion da a o he 235U(n, ) eac ion in he high
ene gy egion.
2 Expe imen al se -up
Neu ons a n_TOF (see [13] o an ex ensi e desc ip ion)
a e p oduced by spalla ion eac ion: bunches o p o ons o
20 GeV/c momen um a e sho e e y ew seconds by he
∗e-mail: [email p o ec ed]
Figu e 1. The 235U(n, ) c oss sec ion om he JENDL/HE [5]
and IAEA [1] e alu a ions, he expe imen al da a [6, 7] measu ed
ela i e o he n-p c oss sec ion and a new heo e ical calcula-
ion [8] based on he in anuclea cascade model INCL++ [9]
coupled o he deexci a ion model GEMINI++ [12].
CERN P o on Synch o on on a massi e lead a ge ; abou
300 neu ons a e p oduced pe inciden p o on. The a-
cili y ea u es wo beam lines wi h a whi e neu ons spec-
um, and wo expe imen al a eas a he nominal dis ance
o 185 m o EAR-1 [14] and 20 m o EAR-2 [15] om
he neu on-p oducing a ge .
The measu emen o he 235U(n, ) c oss sec ion was pe -
o med in Sep embe 2018 in he i s expe imen al a ea,
whe e he neu on spec um ex ends up o 1 GeV, and also
o p o i om he good ene gy esolu ion in he high en-
e gy egion, hanks o he longe ligh pa h.
The expe imen al se up included de ec o s o coun he is-
sion e en s om he U anium a ge and, a he same ime,
de ec o s o measu e he neu on luence ela i e o he
neu on-p o on elas ic sca e ing eac ion, he p ima y e -
e ence o neu on c oss sec ions. Fission e en s we e de-
ec ed using an ionisa ion chambe om he Physikalisch-
Technische Bundesans al [16] and a eac ion chambe
based on Pa allel Pla e A alanche Coun e s (PPACs) de-
eloped a Ins i u de Physique Nucléai e d’O say [17, 18].
Th ee ecoil p o on elescopes (RPTs), de eloped by he
Na ional Ins i u e o Nuclea Physics (INFN, I aly) and he
Physikalisch-Technische Bundesans al (PTB, Ge many),
a e used o measu e he neu on luence. Figu e 2 shows,
he scheme o he expe imen al se up: he wo ission
chambe s wi h a o al o en issile deposi s placed in he
neu on beam, downs eam wo Polye hylene (C2H4) a -
ge s and he h ee elescopes placed a a small angle wi h
espec o he neu on beam.
Figu e 2. Layou o he 235U(n, ) ela i e o 1H(n,n)1H c oss
sec ion measu emen se -up ca ied ou in EAR-1 a n_TOF. Fis-
sion eac ions a e measu ed wi h a ission chambe and a se o
Pa allel Pla e A alanche Coun e s. Th ee elescopes poin ing a
wo di e en C2H4 adia o s we e used o measu e he neu on
luence.
2.1 Fission eac ion de ec ion
The Pa allel Pla e A alanche Coun e s consis o 3 elec-
odes, a cen al anode su onded by wo ca hodes; he 3
mm gaps be ween elec odes a e illed wi h o ced low
o C3F8main ained a low-p essu e (4 mba ). The elec-
odes a e made o 1.7 µm hick myla oils, coa ed wi h
aluminium o gold o make hem conduc i e. The coa ing
in he ca hodes is di ided in o 2 mm wide s ips o allow
he localisa ion o he a alanche by using a delay line; he
ime di e ence be ween delay line ou pu s p o ides a one-
dimensional posi ion. By combining he signals om he
wo o hogonal ca hodes s ips, he ission agmen a-
jec o y can be econs uc ed. The ission eac ion cham-
be includes 3 PPACs wi h wo 235U samples in be ween.
Each sample consis ed in a 8 cm diame e , 300 µg·cm−2
hick laye o 235U (pu i y o 92.7%) ha was placed on a
600 µg·cm−2aluminum backing by elec odeposi ion.
The ission e en s a e iden i ied by coinciden anode sig-
nals o wo consecu i e PPACs. This ea u e, combined
wi h he as iming p ope ies o he de ice (9 ns FWHM),
gua an ees a e y high signal- o-backg ound a io. In ad-
di ion, since hese de ec o s a e qui e insensi i e o he γ-
lash p oduced by high ene gy eac ions a he spalla ion
a ge , i is possbile o ecognize he e en s p oduced by
1 GeV neu ons. Howe e , he PPACs can de ec only is-
sion agmen s emi ed in o a o wa d cone wi h an open-
ing angle o abou 60◦wi h a de ec ion e iciency ha is
no easily e alua ed.
The second sys em used o measu e he ission eac ion
is an ioniza ion chambe (PPFC) which con ains a pa allel
pla e elec ode assembly wi h a hickness o 100 µm each.
The chambe is equipped wi h 235U (pu i y o 99.93%) de-
posi s o 42 mm diame e and abou 300 µg·cm−2a eal
densi y on bo h sides o ou aluminum elec odes. In o-
al 32.660(3) mg o 235U a e used. In addi ion, o s udy he
backg ound eac ions due o he aluminium, he e a e wo
a ge s wi h only he backing. The chambe is ope a ed a
a mosphe ic p essu e wi h a con inuous gas low o 90%
a gon and 10% me hane mix u e.
The de ec o is e y well cha ac e ized in e ms o de ec-
ion uni o mi y o he issile deposi s and o agmen de-
ec ion e iciency (abou 95%). The use o he PPFC is
howe e limi ed o up o a ew hund eds MeV o neu on
ene gy because o he backg ound p oduced by neu on-
induced eac ions on he aluminium oils. I is pe ec ly
sui ed o he measu emen o he 235U(n, ) c oss sec ion
in he ene gy ange om he mal o ca. 150 MeV; he e-
o e, i is used o s udy and calib a e he PPAC agmen -
de ec ion e iciency in he ene gy ange below 100 MeV.
2.2 Flux measu emen se -up
The numbe o neu ons impinging on he 235U samples
is measu ed simul aneously o ission by de ec ing ecoil
p o ons emi ed om he wo polye hylene a ge s placed
downs eam o he ission chambe s ( igu e 2). Due o he
p esence o ca bon in he samples, i is necessa y o use a
elescope o disc imina e he p o ons om o he cha ged
pa icles exploi ing he ∆E-E me hod. The ene gy de-
posi ed in he di e en laye s o he elescope is di e en
o di e en pa icles, due o he di e en s opping powe .
In non- ela i is ic app oxima ion, he p oduc be ween he
ene gy loss in he ansmission de ec o s (∆E) and he e-
maining kine ic ene gy (E) in he s op de ec o is p opo -
ional o he cha ge (z) and he mass (M) o he in e ac ing
pa icle. G aphing ∆E agains E yield a amily o hype -
bolas co esponding o he di e en alues o z2M.
Two RPTs we e buil wi h he same design, he hi d ol-
lows a di e en concep .
The wo de ec o s, indica ed in igu e 2 as RPT-INFN
(shown in de ail in igu e 3) ha e a compac design. The
wo elescopes consis o a apezoidal s uc u e o ou
independen BC408 plas ic scin illa o s wi h inc easing
hickness: 0.5, 3.0, 6.0 and 6.0 cm. Each scin illa o
is coupled o a 1" Hamama su R1924A Pho omul iplie
(PMT), excep o he i s scin illa o , ha , due o he low
hickness, is coupled o wo PMTs (one a each side). One
3
EPJ Web o Con e ences 239, 01008 (2020) h ps://doi.o g/10.1051/epjcon /202023901008
ND2019
Figu e 3. Le : s uc u e o one o he wo RPT-INFN: ou plas-
ic scin illa o s and wo Silicon de ec o s. Righ : de ail o he
case wi h he Silicon de ec o s inside.
o he wo RPT-INFN has wo silicon de ec o s placed be-
o e he i s hin scin illa o . The silicon de ec o s, 300 µm
hick inside a aluminium case (in igu e 3, igh ), allow o
educe he minimum de ec able neu on ene gy om 30
MeV o 10 MeV.
The coincidence be ween he di e en de ec o s gua an-
ees he supp ession o he γbackg oung and he e en s
ha do no come om he C2H4 a ge . Mo eo e , hanks
o he mul i-s age s uc u e i is possible o selec , o each
neu on ene gy, he bes con igu a ion in e ms o num-
be o de ec o s in coincidence, o educe he backg ound
coming om n+12C eac ions. The wo elescopes we e
ins alled a di e en angles: he ups eam de ec o was
placed a θ=25◦ o he neu on beam di ec ion, and he
dows eam one a α=20◦. This allowed o inc ease he
s a is ics, as well as o minimise sys ema ic unce ain ies
ela ed o he angula posi ion.
Conside ing he e y as esponse o he scin illa o s, he
elescope is able o iden i y e en s gene a ed by neu ons
a i ing as close as 80 ns a e he γ- lash, co esponding
o an ene gy o ∼1 GeV.
The hi d elescope, indica ed in igu e 2 as RPT-PTB
(shown in de ail in igu e 4), consis s o h ee disc e e de-
ec o s.
This iple-s age RPT has wo ansmission de ec o s
ha consis o a squa e EJ204 plas ic scin illa o , 45×45
mm2(∆E1) and 38×38 mm2(∆E2) in size, espec i ely.
The hicknesses o he de ec o s a e op imised o he neu-
on ene gy ange o be co e ed and a y be ween 0.5 mm
and 5 mm. Mo eo e he dimension o he ∆E2 de ec o ,
which has he smalles ans e se dimensions o all he
h ee de ec o s, de ines he solid angle o he RPT-PTB.
The s op de ec o (E) is a cylind ic EJ204 scin illa o o
80 mm diame e . Also his de ec o hickness is op imized
wi h espec o he ene gy ange: 50 mm o neu on en-
e gy om 25 MeV o 100 MeV, 100 mm om 50 MeV
o 150 MeV. The ∆E scin illa o de ec o s a e inse ed in
a oo like housings wi h 0.1 mm aluminium walls co e ed
wi h a di use whi e e lec o on he inside and coupled o a
XP2020Q PMTs ( igu e 4, igh ). Due o he di use e lec-
Figu e 4. Le : s uc u e o he iple-s age RPT-PTB: h ee plas-
ic scin illa o s. Righ : de ail o he in e io o he co e o he
∆E de ec o s wi h a di use whi e e lec o o educe he inhomo-
genei y o he ligh collec ion.
o he inhomogenei y o he ligh collec ion is less han
10%. The en elope and he on side o he cylind ical
E de ec o s a e also co e ed wi h a whi e di use e lec o
and coupled o XP2020 PMTs.
The main design goal o his ins umen is o achie e di-
ec ional sensi i i y o backg ound supp ession and e-
duced in luence o angula s aggling o an accu a e de i-
ni ion o he solid angle, equi ing, du ing he da a analy-
sis, a iple coincidence.
3 The measu emen campaign
The 235U(n, ) measu emen a n_TOF equi ed a i e-week
beam ime in o de o collec enough s a is ics. I s a ed
on Sep embe 17 h and ended on Oc obe 29 h, o a o al
o 3.8·1018 p o ons on a ge .
Du ing he measu emen campaign di e en C2H4 a ge s
we e used; hei hicknesses we e op imized o ma ch he
neu on-ene gy ange o in e es . In pa icula o he
low neu on ene gy ange ( om 10 o 200 MeV) a ge s
wi h 1 and 2 mm o hickness we e used, whe eas he
co esponding ca bon samples o sub ac he backg ound
eac ion we e o 0.5 and 1 mm hickness. Fo highe
neu on ene gies, up o 1 GeV, polye hylene and ca bon
samples wi h 5 and 2.5 mm hickness we e used.
The RTP-PTB and he RPT-INFN wi h he Silicon de-
ec o s we e a he same angle wi h espec o he neu on
beam, 25◦, and poin ing a he same sample. While he
hi d elescope, a 20◦, was poin ing always a he hicke
a ge , o inc ease he s a is ics o he high ene gy pa .
In he able 1 he p o ons on a ge o each de ec o and
con igu a ion a e summa ized.
4 Conclusion
The 235U(n, ) eac ion is one o he mos impo an s an-
da d c oss sec ion bo h in nuclea physics and o i s
4
EPJ Web o Con e ences 239, 01008 (2020) h ps://doi.o g/10.1051/epjcon /202023901008
ND2019
Figu e 3. Le : s uc u e o one o he wo RPT-INFN: ou plas-
ic scin illa o s and wo Silicon de ec o s. Righ : de ail o he
case wi h he Silicon de ec o s inside.
o he wo RPT-INFN has wo silicon de ec o s placed be-
o e he i s hin scin illa o . The silicon de ec o s, 300 µm
hick inside a aluminium case (in igu e 3, igh ), allow o
educe he minimum de ec able neu on ene gy om 30
MeV o 10 MeV.
The coincidence be ween he di e en de ec o s gua an-
ees he supp ession o he γbackg oung and he e en s
ha do no come om he C2H4 a ge . Mo eo e , hanks
o he mul i-s age s uc u e i is possible o selec , o each
neu on ene gy, he bes con igu a ion in e ms o num-
be o de ec o s in coincidence, o educe he backg ound
coming om n+12C eac ions. The wo elescopes we e
ins alled a di e en angles: he ups eam de ec o was
placed a θ=25◦ o he neu on beam di ec ion, and he
dows eam one a α=20◦. This allowed o inc ease he
s a is ics, as well as o minimise sys ema ic unce ain ies
ela ed o he angula posi ion.
Conside ing he e y as esponse o he scin illa o s, he
elescope is able o iden i y e en s gene a ed by neu ons
a i ing as close as 80 ns a e he γ- lash, co esponding
o an ene gy o ∼1 GeV.
The hi d elescope, indica ed in igu e 2 as RPT-PTB
(shown in de ail in igu e 4), consis s o h ee disc e e de-
ec o s.
This iple-s age RPT has wo ansmission de ec o s
ha consis o a squa e EJ204 plas ic scin illa o , 45×45
mm2(∆E1) and 38×38 mm2(∆E2) in size, espec i ely.
The hicknesses o he de ec o s a e op imised o he neu-
on ene gy ange o be co e ed and a y be ween 0.5 mm
and 5 mm. Mo eo e he dimension o he ∆E2 de ec o ,
which has he smalles ans e se dimensions o all he
h ee de ec o s, de ines he solid angle o he RPT-PTB.
The s op de ec o (E) is a cylind ic EJ204 scin illa o o
80 mm diame e . Also his de ec o hickness is op imized
wi h espec o he ene gy ange: 50 mm o neu on en-
e gy om 25 MeV o 100 MeV, 100 mm om 50 MeV
o 150 MeV. The ∆E scin illa o de ec o s a e inse ed in
a oo like housings wi h 0.1 mm aluminium walls co e ed
wi h a di use whi e e lec o on he inside and coupled o a
XP2020Q PMTs ( igu e 4, igh ). Due o he di use e lec-
Figu e 4. Le : s uc u e o he iple-s age RPT-PTB: h ee plas-
ic scin illa o s. Righ : de ail o he in e io o he co e o he
∆E de ec o s wi h a di use whi e e lec o o educe he inhomo-
genei y o he ligh collec ion.
o he inhomogenei y o he ligh collec ion is less han
10%. The en elope and he on side o he cylind ical
E de ec o s a e also co e ed wi h a whi e di use e lec o
and coupled o XP2020 PMTs.
The main design goal o his ins umen is o achie e di-
ec ional sensi i i y o backg ound supp ession and e-
duced in luence o angula s aggling o an accu a e de i-
ni ion o he solid angle, equi ing, du ing he da a analy-
sis, a iple coincidence.
3 The measu emen campaign
The 235U(n, ) measu emen a n_TOF equi ed a i e-week
beam ime in o de o collec enough s a is ics. I s a ed
on Sep embe 17 h and ended on Oc obe 29 h, o a o al
o 3.8·1018 p o ons on a ge .
Du ing he measu emen campaign di e en C2H4 a ge s
we e used; hei hicknesses we e op imized o ma ch he
neu on-ene gy ange o in e es . In pa icula o he
low neu on ene gy ange ( om 10 o 200 MeV) a ge s
wi h 1 and 2 mm o hickness we e used, whe eas he
co esponding ca bon samples o sub ac he backg ound
eac ion we e o 0.5 and 1 mm hickness. Fo highe
neu on ene gies, up o 1 GeV, polye hylene and ca bon
samples wi h 5 and 2.5 mm hickness we e used.
The RTP-PTB and he RPT-INFN wi h he Silicon de-
ec o s we e a he same angle wi h espec o he neu on
beam, 25◦, and poin ing a he same sample. While he
hi d elescope, a 20◦, was poin ing always a he hicke
a ge , o inc ease he s a is ics o he high ene gy pa .
In he able 1 he p o ons on a ge o each de ec o and
con igu a ion a e summa ized.
4 Conclusion
The 235U(n, ) eac ion is one o he mos impo an s an-
da d c oss sec ion bo h in nuclea physics and o i s
Table 1. Summa y o s a is ics (p o ons on a ge , "po ")
collec ed o each con gu a ion used du ing he measu emen
campaign a n_TOF.
Sample Thickness RPTs a 25◦RPT a 20◦
(po ) (po )
C2H41 mm 7.08·1017 -
C2H42 mm 8.41·1017 -
C2H45 mm 7.44·1017 2.06·1018
C 0.5 mm 3.27·1017 -
C 1 mm 3.86·1017 -
C 2.5 mm 3.14·1017 1.55·1018
widesp ead use as e e ence o neu on lux measu e-
men s. Ne e heless ew measu emen s a e a ailable in
he neu on ene gy ange be ween 20 and 200 MeV and
none abo e 200 MeV. To ul il his lack o expe imen al
da a a campaign o measu e he 235U ission eac ion, ela-
i e o he 1H(n,n)1H eac ion, which is conside ed he p i-
ma y e e ence in his ene gy egion, has been ca ied on
a he n_TOF acili y a CERN in 2018. The se up was de-
signed o measu e simul aneously he ission eac ions by
means o wo ission agmen de ec o s and he neu on
lux using h ee di e en ecoil p o on elescopes. Du ing
he expe imen al campaign da a ha e been acqui ed using
di e en C2H4and C a ge s o he e alua ion o he neu-
on lux and sys ema ics. The da a analysis is in p og ess
oge he wi h an in ense MC simula ion campaign o e al-
ua e he de ec o s e iciencies using Gean -4 oolki [19].
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published
5
EPJ Web o Con e ences 239, 01008 (2020) h ps://doi.o g/10.1051/epjcon /202023901008
ND2019