Nuclea Ins . and Me hods in Physics Resea ch, A 969 (2020) 163981
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A compac ission de ec o o ission- agging neu on cap u e expe imen s
wi h adioac i e issile iso opes
M. Bacak1,2,3,∗, M. Aïche4, G. Bélie 5, E. Be houmieux3, M. Diakaki 3, E. Dupon 3,
F. Gunsing3,1, J. Heyse6, S. Kopecky6, B. Lau en 5, H. Leeb2, L. Ma hieu4, A. Moens 6,
S. Rich e 6, P. Schillebeeckx6, O. Se o 7, G. Sibbens6, J. Taieb 5, D. Vanleeuw 6, V. Vlachoudis 1,
O. Abe le1, S. Amaducci8,9, J. And zejewski10, L. Audouin11, J. Balib ea12, M. Ba bagallo13,
F. Beč ář14, J. Billowes15, D. Bosna 16, A. B own17, M. Caamaño18, F. Cal iño19, M. Cal iani1,
D. Cano-O 12, R. Ca della1, A. Casano as19, F. Ce u i1, Y.H. Chen11, E. Chia e i1,15,20,
N. Colonna13, G. Co és19, M.A. Co és-Gi aldo20, L. Cosen ino21, L.A. Damone13,22,
C. Domingo-Pa do23, R. D essle 24, I. Du án 18, B. Fe nández-Domínguez 18, A. Fe a i 1,
P. Fe ei a25, P. Finocchia o 21, V. Fu man 26, K. Göbel 27, A.R. Ga cía 12, A. Gawlik 10,
S. Gila doni1, T. Gloda iu28,a, I.F. Gonçal es 25, E. González-Rome o 12, E. G iesmaye 2,
C. Gue e o20, H. Ha ada 29, S. Heini z 24, D.G. Jenkins 17, E. Je icha 2, F. Käppele 30, Y. Kadi 1,
A. Kalama a31, P. Ka igin 2, A. Kimu a29, N. Ki el 24, I. Knapo a 14, M. Kokko is 31, M. K ička 14,
D. Ku ulgil27, E. Leal-Cidoncha 18, C. Lede e 32, J. Le endegui-Ma co 20, S. Lo Meo 33,8,
S.J. Lonsdale32, D. Macina 1, A. Manna 8,9, J. Ma ganiec10,34, T. Ma ínez 12, A. Masi 1,
C. Massimi8,9, P. Mas inu35, M. Mas oma co 13, E.A. Mauge i 24, A. Mazzone 13,36, E. Mendoza 12,
A. Mengoni33, P.M. Milazzo 37, F. Ming one 1, A. Musuma a 21,38, A. Neg e 28, R. Nol e 34,
A. Op ea28, N. Pa onis 39, A. Pa lik 40, J. Pe kowski 10, I. Po as 41, J. P aena 41, J.M. Quesada20,
D. Radeck34, T. Rausche 42,43, R. Rei a h 27, C. Rubbia 1, J.A. Ryan 15, M. Saba é-Gila e 1,20,
A. Saxena44, D. Schumann 24, P. Sedyshe 26, A.G. Smi h 15, N.V. Sosnin 15, A. S ama opoulos 31,
G. Taglien e13, J.L. Tain 23, A. Ta i eño-Saldi ia 19, L. Tassan-Go 11, S. Valen a14, G. Vannini8,9,
V. Va iale13, P. Vaz 25, A. Ven u a 8, R. Vlas ou 31, A. Wallne 45, S. Wa en 15, C. Weiss 2,
P.J. Woods32, T. W igh 15, P. Žugec 16,1
1Eu opean O ganiza ion o Nuclea Resea ch (CERN), Swi ze land
2Technische Uni e si ä Wien, Aus ia
3CEA I u, Uni e si é Pa is-Saclay, F-91191 Gi -su -Y e e, F ance
4CENBG, CNRS/IN2P3-Uni e si é de Bo deaux, G adignan, F ance
5CEA, DAM, DIF, F-91297 A pajon, F ance
6Eu opean Commission, Join Resea ch Cen e, Geel, Re ieseweg 111, B-2440 Geel, Belgium
7CEA, DEN, Cada ache, F ance
8Is i u o Nazionale di Fisica Nuclea e, Sezione di Bologna, I aly
9Dipa imen o di Fisica e As onomia, Uni e si à di Bologna, I aly
10 Uni e si y o Lodz, Poland
11 Ins i u de Physique Nucléai e, CNRS-IN2P3, Uni . Pa is-Sud, Uni e si é Pa is-Saclay, F-91406 O say Cedex, F ance
12 Cen o de In es igaciones Ene gé icas Medioambien ales y Tecnológicas (CIEMAT), Spain
13 Is i u o Nazionale di Fisica Nuclea e, Sezione di Ba i, I aly
14 Cha les Uni e si y, P ague, Czech Republic
15 Uni e si y o Manches e , Uni ed Kingdom
16 Depa men o Physics, Facul y o Science, Uni e si y o Zag eb, Zag eb, C oa ia
17 Uni e si y o Yo k, Uni ed Kingdom
18 Uni e si y o San iago de Compos ela, Spain
∗Co esponding au ho a : Eu opean O ganiza ion o Nuclea Resea ch (CERN), Swi ze land.
E-mail add ess: [email p o ec ed] (M. Bacak).
aDeceased.
h ps://doi.o g/10.1016/j.nima.2020.163981
Recei ed 27 Feb ua y 2020; Recei ed in e ised o m 15 Ap il 2020; Accep ed 16 Ap il 2020
A ailable online 21 Ap il 2020
0168-9002/©2020 The Au ho s. Published by Else ie B.V. This is an open access a icle unde he CC BY license
(h p://c ea i ecommons.o g/licenses/by/4.0/).
M. Bacak, M. Aïche, G. Bélie e al. Nuclea Ins . and Me hods in Physics Resea ch, A 969 (2020) 163981
19 Uni e si a Poli ècnica de Ca alunya, Spain
20 Uni e sidad de Se illa, Spain
21 INFN Labo a o i Nazionali del Sud, Ca ania, I aly
22 Dipa imen o di Fisica, Uni e si à degli S udi di Ba i, I aly
23 Ins i u o de Física Co puscula , CSIC - Uni e sidad de Valencia, Spain
24 Paul Sche e Ins i u (PSI), Villingen, Swi ze land
25 Ins i u o Supe io Técnico, Lisbon, Po ugal
26 Join Ins i u e o Nuclea Resea ch (JINR), Dubna, Russia
27 Goe he Uni e si y F ank u , Ge many
28 Ho ia Hulubei Na ional Ins i u e o Physics and Nuclea Enginee ing, Romania
29 Japan A omic Ene gy Agency (JAEA), Tokai-mu a, Japan
30 Ka ls uhe Ins i u e o Technology, Campus No h, IKP, 76021 Ka ls uhe, Ge many
31 Na ional Technical Uni e si y o A hens, G eece
32 School o Physics and As onomy, Uni e si y o Edinbu gh, Uni ed Kingdom
33 Agenzia nazionale pe le nuo e ecnologie (ENEA), Bologna, I aly
34 Physikalisch-Technische Bundesans al (PTB), Bundesallee 100, 38116 B aunschweig, Ge many
35 Is i u o Nazionale di Fisica Nuclea e, Sezione di Legna o, I aly
36 Consiglio Nazionale delle Rice che, Ba i, I aly
37 Is i u o Nazionale di Fisica Nuclea e, Sezione di T ies e, I aly
38 Dipa imen o di Fisica e As onomia, Uni e si à di Ca ania, I aly
39 Uni e si y o Ioannina, G eece
40 Uni e si y o Vienna, Facul y o Physics, Vienna, Aus ia
41 Uni e si y o G anada, Spain
42 Depa men o Physics, Uni e si y o Basel, Swi ze land
43 Cen e o As ophysics Resea ch, Uni e si y o He o dshi e, Uni ed Kingdom
44 Bhabha A omic Resea ch Cen e (BARC), India
45 Aus alian Na ional Uni e si y, Canbe a, Aus alia
ARTICLE INFO
Keywo ds:
Fission de ec o
233U
n_TOF
Time-o - ligh
ABSTRACT
In he measu emen o neu on cap u e c oss-sec ions o issile iso opes, he ission channel is a sou ce o
backg ound which can be emo ed e icien ly using he so-called ission- agging o ission- e o echnique. Fo
his pu pose a new compac and as ission chambe has been de eloped. The design c i e ia and echnical
desc ip ion o he chambe a e gi en wi hin he con ex o a measu emen o he 233U(n, 𝛾) c oss-sec ion a he
n_TOF acili y a CERN, whe e i was coupled o he n_TOF To al Abso p ion Calo ime e . Fo his measu emen
he ission de ec o was op imized o ime esolu ion, minimiza ion o ma e ial in he neu on beam and o
alpha- ission disc imina ion. The pe o mance o he ission chambe and i s applica ion as a ission agging
de ec o a e discussed.
1. In oduc ion
The neu on cap u e c oss-sec ions o issile iso opes a e o in e es
in nuclea eac o as hey in luence he neu on economy o he eac o .
Howe e , he knowledge o hose c oss-sec ions is limi ed due o di i-
cul ies associa ed o he backg ound om he ission eac ion channel.
Fo he issile iso opes 233U, 235U and 239Pu he ission c oss-sec ion
is on a e age a ac o 2 o 10 la ge han hei espec i e cap u e
c oss-sec ion, depending on he iso ope and ene gy ange, which is
shown in Fig. 1. This implies ha in a measu emen o he cap u e
c oss-sec ion he 𝛾- ays coming om he ission channel a e a majo
sou ce o backg ound, which has o be aken ca e o in he analysis.
In he pas [1] a me hod o e icien ly dealing wi h his sou ce o
backg ound has been de eloped, he so-called ission- agging o ission-
e o echnique. In addi ion o he 𝛾-de ec o his echnique employs
a ission de ec o o measu e he ission agmen s. The 𝛾- ays om
he ission eac ion can hen be iden i ied o agged by ope a ing
he wo de ec o s in coincidence. In ecen yea s new e o s ha e
been made o measu e he cap u e c oss-sec ions o he issile iso opes
using he ission- agging echnique a di e en acili ies [2–5]. Despi e
ission- agging echnique’s e ec i eness in dealing wi h he ission
backg ound, i has he d awback o in oducing ano he componen o
he backg ound, namely he sample subs a es and he de ec o i sel .
In he ecen measu emen s pe o med a n_TOF a Mic omegas based
de ec o was used as a ission de ec o [2,4]. The so-called mic o-mesh
o such a de ec o is made ou o coppe , which is a signi ican sou ce o
backg ound due o i s la ge sca e ing and cap u e c oss-sec ions. Wi h
he goal o measu ing he 233U(n, 𝛾) c oss-sec ion a new ission de ec o
was designed aiming a educing he backg ound om he de ec o and
p o iding he necessa y pe o mance o eliably iden i y he ission 𝛾-
ays. The design o he desc ibed ission chambe (FICH) is adap ed o
he measu emen o he 233U(n, 𝛾) u ilizing he ission chambe coupled
o he n_TOF To al Abso p ion Calo ime e (TAC) [6] in expe imen al
a ea 1 (EAR1) o he n_TOF acili y [7].
2. The mul i-pla e ission chambe
Due o he expe ience ob ained om he measu emen wi h he
ission- agging mic omegas de ec o s [2,4], a simple ioniza ion cell
geome y is chosen as he basic de ec o design o minimize he ma-
e ial in beam. The de elopmen o he ission chambe is ocused
on di e en , pa ially con adic o y, c i e ia: excellen ime esolu ion
o he coincidence and ime-o - ligh measu emen ; low quan i ies o
s uc u al ma e ial o a oid addi ional backg ound o he cap u e mea-
su emen ; easonable amoun o 233U o ob ain a su icien coun a e
o high s a is ics measu emen s; compac design as he ission chambe
(FICH) mus i inside he TAC o n_TOF. All hese equi emen s a e
de ailed in he ollowing sec ions.
2.1. Technical desc ip ion
As a esul he ission chambe (FICH) is designed as a mul i-
pla e ioniza ion chambe con aining wo s acks o axial ioniza ion cells.
Figs. 2 and 3show CAD d awings and pic u es o he chambe . The
housing is made o a 1.5 mm hick aluminium ube wi h an ou e
diame e o 66 mm and a leng h o 78 mm. Wi h a maximum ou e
diame e o 90 mm and a o al leng h o 120 mm including he langes
wi h he gas connec ions and windows, i i s nicely in he 𝛾-calo ime e
lea ing su icien space o i s abso be (explained in Sec ion 3.1.2)
and he connec ing beam pipes. Two s acks o se en ioniza ion cells
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M. Bacak, M. Aïche, G. Bélie e al. Nuclea Ins . and Me hods in Physics Resea ch, A 969 (2020) 163981
Fig. 1. C oss-sec ion a ios o he ission 𝜎𝑓and cap u e 𝜎𝛾channels o he issile
iso opes 233U, 235U and 239Pu.
each a e moun ed di ec ly on hei espec i e mo he boa ds and a e
inse ed om each end o he chambe . The s acks ha e a minimum
inne diame e o 50 mm lea ing enough space o he n_TOF neu on
beam wi h a FWHM o oughly 16 mm and a o al wid h o less han
40 mm. In o al 8 anodes a e collec ing signals om 14 233U a ge s
deposi ed on he ca hodes. The a angemen o he ca hodes, anodes
and deposi s is illus a ed in Fig. 4. To a oid c oss- alk om alpha-
pa icles, he ioniza ion cells a e sepa a ed by 20 μ aluminium, ei he
one 20 μ anode oil o wo 10 μ ca hode oils, esul ing in a o al o 300 μ
aluminium in he neu on beam which is a negligible neu on beam
pe u ba ion. The chambe is closed wi h aluminized 25 μ hick Kap on
windows o p o ide a Fa aday cage.
2.2. Choice o gas and gas sys em
The gas is o high impo ance and has o exhibi a high d i eloci y
and p o ide he bes possible alpha- ission sepa a ion. High pu i y
e a luo ome hane CF4is a as gas and is o en used whe e high
coun a es a e expec ed [8] bu has he d awback o being elec o-
nega i e, wo sening he ene gy esolu ion and hence he alpha- ission
disc imina ion. Ne e heless, he ad an age i o e s due o i s highe
d i eloci y compa ed o o he gases ou weighs he disad an ages.
Fission agmen s (FF) and 𝛼-pa icles deposi hei ene gy in he gap
be ween he elec odes illed wi h he gas. Simula ions [8] ha e shown
ha a gap dis ance o abou 1.5 mm is su icien o achie e a easonable
alpha- ission sepa a ion o 252C . Due o mechanical conside a ions he
gap is chosen o be 3 mm. To achie e a d i eloci y o abou 11 cm∕μs
an elec ic ield o 1400 V∕cm is applied a a mosphe ic p essu e. This
d i eloci y co esponds o a o al elec on d i ime o 27 ns in he
3 mm gap, leading o a sui able in insic ime esolu ion. In o de o
gua an ee s able condi ions h oughou he measu emen pe iod o ou
weeks a gas p essu e and low egula ion sys em was employed and is
schema ically shown in Fig. 5. The ission chambe was ope a ed wi h
a cons an gas low o 0.1 l∕min and a an absolu e p essu e o 1100 mba
o allow o he use o hin windows o he ission chambe , hence o
educe he backg ound in he 𝛾-calo ime e .
Fig. 2. CAD d awings o he ission chambe and a sec ional iew. The g een blocks a ound he chambe ep esen he p eampli ie s.
Fig. 3. Pic u es o he ission chambe in he lab (le ) and embedded in one hal o he TAC abso be (whi e) wi h elec onics and gas supply connec ed ( igh ).
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M. Bacak, M. Aïche, G. Bélie e al. Nuclea Ins . and Me hods in Physics Resea ch, A 969 (2020) 163981
Fig. 4. Pic u e o one o he wo s acks o ioniza ion cells moun ed on he mo he boa d (le ). A angemen o he ca hodes (C) and anodes (A) o one s ack ( igh ), 233U deposi s
a e indica ed in ed. The e is one anode ha eads only om 1 deposi while he o he s ead signals om wo deposi s.
Fig. 5. Schema ic illus a ion o he ull FICH se -up including he chambe , a schema ic d awing o he gas sys em, and pic u es o one p eampli ie - il e p in ed ci cui boa d
(PCB) and o he gas egula ion sys em wi h he gas il e s (THE).
2.3. Dedica ed elec onics
To ensu e a good ime esolu ion and educe po en ial 𝛼-pa icle
pile-up, as elec onics adap ed o he geome y o he ioniza ion cells
and he elec on d i eloci y ha e been de eloped. Cha ge p eam-
pli ie s wi h a sho RC decay ime cons an ha e been de eloped o
ensu e good ampli ude disc imina ion, a oid sa u a ion due o e y
high alpha ac i i y and o p ese e he good iming esponse o he
chambe . A dedica ed ca d combining he p eampli ie and a as
iming il e ampli ie was di ec ly moun ed on he ission chambe .
This educes he inpu capaci ance and imp o es he signal- o-noise
a io. A pic u e o hose ca ds can be seen in he lowe le pa o Fig. 5.
The signals eco ded by he da a acquisi ion sys em we e digi ized,
s o ed and p ocessed o line using he pulse shape analysis ou ine
de eloped by he n_TOF collabo a ion [9]. An example o a ypical
signal o a ission agmen is shown in Fig. 6 wi h a ull wid h a hal
maximum (FWHM) o 34 ns and a ise ime (10-90 %) o 16 ns.
2.4. Fissile deposi s
Thin u anium oxide laye s, wi h 99.9361 % en ichmen in 233U, had
a diame e o 40.00 ± 0.02 mm and we e molecula pla ed on 10 μ hick
aluminium oils a JRC-Geel. The impu i ies in he sample ha e a
negligible e ec on ission. Ne e heless, a small e ec on he 233U𝛼-
a io is expec ed due o he i s cap u e esonance o 234U a 5.15 eV.
Fig. 6. A e age shape o a ission agmen signal om he FICH.
This con ibu ion can be aken in o accoun du ing he esonance
analysis. The ac i i y o each o he 14 samples hos ed in he chambe
has been de e mined by well-de ined solid angle 𝛼-pa icle coun ing
and amoun s o an a e age 𝛼-ac i i y o abou 1.16 MBq o an a e age
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M. Bacak, M. Aïche, G. Bélie e al. Nuclea Ins . and Me hods in Physics Resea ch, A 969 (2020) 163981
Fig. 7. Neu on luence a n_TOF EAR1 185 m om he sou ce.
a eal densi y o 264.5 μg∕cm2pe sample (wi h a s anda d de ia ion o
30.9 μg∕cm2among he 14 samples), which pe mi s ission agmen s o
escape he deposi s, esul ing in a o al mass o 46.5(3) mg o 233U.
3. Fission- agging expe imen a n_TOF
233U is a p ime example o he applica ion o ission agging as i
exhibi s a ission c oss-sec ion which is on a e age a ac o 10 la ge
han he co esponding cap u e c oss-sec ion. Thus, he ission eac ion
will in oduce a backg ound in o he measu emen ha comp ises o
wo componen s: he p omp componen caused by he de-exci a ion
o he highly exci ed ission p oduc s and he delayed componen
caused by ei he ission neu ons being cap u ed in he expe imen al
se -up o decays o uns able ission agmen s wi h hal -li es la ge
han a ew nanoseconds up o mic oseconds. The p omp componen
causes a much la ge backg ound and appea s quasi-ins an aneous
wi h he ission eac ion and can be easily quan i ied and emo ed
using ission- agging. The delayed componen can also be s udied wi h
ission- agging bu depends on he expe imen al se -up’s sensi i i y o
neu ons and shall no be he ocus o his wo k.
3.1. Expe imen al se -up
3.1.1. The n_TOF acili y a CERN
The n_TOF expe imen al a ea 1 (EAR1) acili y [7] is de o ed o
he measu emen o ene gy dependen neu on c oss-sec ions in an
ene gy ange om he mal up o GeV. Neu ons a e p oduced by
a high-in ensi y 20 GeV∕c p o on beam impinging on a lead a ge
and mode a ed in a bo a ed wa e -laye down o he mal ene gies.
The p o on beam is deli e ed by CERN’s P o on Synch o on wi h an
a e age p o on beam in ensi y o 7⋅1012 o 4⋅1012 p o ons pe bunch
o dedica ed o pa asi ic bunches espec i ely. The neu on luence as
a unc ion o he a i al ime in EAR1 loca ed app oxima ely 185 m om
he lead a ge is shown in Fig. 7.
The n_TOF acili y p o ides a ully digi al Da a Acquisi ion sys em
(DAQ) [10] and a la ge s o age space, namely he CERN Ad anced
STORage manage (CASTOR) [11]. The wa e o ms o all signals a e dig-
i ized wi h high pe o mance digi ize s, ADQ412 o ADQ414 [12], wi h
12 o 14 bi esolu ion espec i ely which a e ope a ed a
500 MSamples∕s. This allows an o line analysis o be pe o med wi h
dedica ed pulse shape analysis ou ines [9]. The digi ize s a e igge ed
wi h a common ex e nal clock o a oid ime d i s be ween he di e en
channels.
3.1.2. The n_TOF To al Abso p ion Calo ime e
The n_TOF To al Abso p ion Calo ime e TAC [6] is designed o de-
ec in coincidence he 𝛾- ays o he elec o-magne ic cascade ollowing
a neu on cap u e e en . The TAC is a segmen ed 4𝜋scin illa o a ay
consis ing o 40 BaF2c ys als moun ed in a honeycomb s uc u e which
holds he ull sphe ical de ec o shell as shown in Fig. 8. The sphe ical
BaF2shell has a 20 cm and 50 cm inne and ou e diame e espec i ely,
co e ing 95 % solid angle esul ing in an e iciency o de ec ing a
leas one 𝛾- ay om a cascade close o 100 %. To educe he neu on
sensi i i y, namely he p obabili y o de ec ing neu ons o he beam
sca e ed om he in-beam ma e ials, a so-called abso be is placed
be ween he c ys als and he sample o be measu ed. The abso be is
made ou o polye hylene loaded wi h 7.56 w% na u al li hium o abso b
sca e ed neu ons and consis s o wo sphe ical shell hal es in which
he ission chambe was embedded as shown in he igh panel o Fig. 3.
TAC e en s a e cha ac e ized by h ee pa ame e s: he ime-o - ligh ,
he numbe o hi c ys als e e ed o as c ys al mul iplici y 𝑚𝑐𝑟 and
he sum o he deposi ed ene gy in all 40 c ys als 𝐸𝑆𝑢𝑚 wi hin a ime
coincidence window o 𝑇𝑇 𝐴𝐶
𝑐𝑜𝑖𝑛𝑐 = 12 ns.
3.2. FICH Pe o mance
3.2.1. Pulse heigh spec um and alpha- ission disc imina ion
Fig. 9 shows he pulse heigh spec um o he ission chambe o
neu ons o less han 10 keV ene gy and wi hou neu on beam (beam
o ). Small pulse heigh s a e domina ed by he 𝛼-pa icle backg ound
and a e se e al o de s o magni ude la ge han he con ibu ion o he
Fig. 8. Pic u e o he ully assembled (le ) and open ( igh ) TAC.
5

M. Bacak, M. Aïche, G. Bélie e al. Nuclea Ins . and Me hods in Physics Resea ch, A 969 (2020) 163981
Fig. 9. Pulse heigh spec a o 233U wi h and wi hou neu on beam o e en s wi h
𝐸𝑛<10 keV om a single FICH channel.
Fig. 10. Pulse heigh spec a o di e en 𝐸𝑛 egions co esponding o di e en ission
agmen o 𝛼-pa icle a ios om all FICH channels.
ission e en s. The blue line in Fig. 9 is a scaled e sion ( o isual-
iza ion pu poses) o he pulse heigh spec um o he ission chambe
wi hou neu on beam and shows he 𝛼-peak which co esponds o
𝛼-pa icles ha deposi hei ull ene gy in he gas.
The ela i ely poo sepa a ion o ission agmen s and 𝛼-pa icle
backg ound a a ound 0.09 V is no su p ising conside ing he high 𝛼-
pa icle coun a e. Choosing app op ia e condi ions allows o s udy he
esponse o he FICH o ission agmen s wi h a much be e sepa a ion
as shown in Fig. 10 whe e he pulse-heigh spec a o di e en alpha
o ission a ios a e displayed. Fo example, ga ing on he i s and
la ges esonance a 1.6 eV< 𝐸𝑛<1.9 eV, co esponding o he TOF
egion o 10.6-9.7 ms, imp o es he sepa a ion o 𝛼-pa icles and ission
agmen s. Fu he mo e, he cha ac e is ics o he neu on luence a
n_TOF EAR1 can also be exploi ed by choosing he neu on ene gy
egion a ound o co esponding o a TOF ange o 5-15 μs (0.8-7 MeV),
see Fig. 7. Due o he highe luence in his TOF egion he alpha- ission
sepa a ion is u he imp o ed.
3.2.2. Gain moni o ing
The gain o he FICH has been moni o ed h oughou he measu e-
men by coun ing he numbe o ission agmen (FF) e en s (>0.1 V)
pe nominal (7⋅1012 p o ons) pulse. Fig. 11 shows he gain luc ua ion
o one o he ioniza ion cells o e ime, indica ed in RunNumbe . No
Fig. 11. Va ia ion o he gain o e ime: numbe o e en s wi h 𝑎𝑚𝑝 > 0.1 V and
𝐸𝑛<0.8 MeV pe p o ons pe un — ypically 4 h pe un. The ed line co esponds
o he weigh ed a e age o 8.36 FF coun s pe pulse wi h an unce ain y o he i o
0.1 %. The blue dashed lines indica e he s anda d de ia ion o he da a poin s which
is 1 %.
Fig. 12. Ra io o he ission coun a es o dedica ed (D) and pa asi ic (P) beam pulse
ypes.
d i o he gain can be obse ed, p o ing a good de ec o s abili y
h oughou he whole measu emen ime o abou ou weeks.
3.2.3. Dead ime and alida ion
Wi h high coun a es dead ime and pile-up can become se e e.
Due o i s design as a as ission chambe , coun a es o se e al MBq
should be sus ainable wi hou he need o co ec o pile-up e ec s in
ission agmen de ec ion. Fig. 12 shows he a io o he coun a es
o dedica ed (D) and pa asi ic (P) beam pulse ypes o ission e en s
(𝑎𝑚𝑝 > 0.1 V). A good ag eemen wi h 1 % is eached up o 1 MeV,
indica ing ha he e a e no pile-up o dead ime issues. The ou lie
a ound 55 keV mos likely co esponds o dips in he neu on lux due
o aluminium esonances, hence e y low s a is ics.
To e i y he sa is ac o y beha iou o he ission de ec o he shape
o he 233U(n, ) c oss-sec ion has been calcula ed om he FICH e en s
and he shape o he neu on lux. The esul ing shape o he 233U(n, )
c oss-sec ion has hen been no malized o e alua ed lib a ies in he
neu on ene gy ange om 8.1 eV o 17.6 eV because his egion is well
sepa a ed a oiding in e e ence om neighbou ing esonances, as has
been sugges ed in [13]. Fig. 13 shows he a io o he scaled 233U(n, )
6
M. Bacak, M. Aïche, G. Bélie e al. Nuclea Ins . and Me hods in Physics Resea ch, A 969 (2020) 163981
Fig. 13. Ra io be ween he expe imen ally de e mined c oss-sec ion, scaled o he
e alua ed lib a ies in he neu on ene gy ange om 8.1 eV o 17.6 eV, and he
co esponding e alua ed lib a y.
c oss-sec ion ob ained om his wo k and he e alua ed lib a ies,
ENDF/B-VII.1 [14], ENDF/B-VIII.0 [15], JEFF-3.3 [16] and JENDL-
4.0u2 [17], om 0.1 eV up o 10 keV. The de ia ions a e wi hin he
e alua ions’ unce ain ies in he esol ed esonance egion (<600 eV)
while he e alua ions a e disc epan in he un esol ed egion (>600 eV).
Thus, aking only he esol ed esonance egion in o accoun i can
be concluded ha he ission chambe is wo king sa is ac o ily in he
neu on ene gy ange o his measu emen (<10 keV). An accu a e
p omp ission backg ound sub ac ion o he measu emen o he
233U(n, 𝛾) can hus be assu ed.
3.3. Fission agging
E en s ha p oduce signals in bo h de ec o s (FICH & TAC) in
coincidence a e ela ed o ission e en s. The ime co ela ion is gi en
by he ime di e ence be ween he de ec ion o he e en in he wo de-
ec o s. P omp ission e en s (small ime di e ence) a e cha ac e ized
by high 𝛾-mul iplici y [18], as was obse ed and sugges ed in p e ious
wo ks [2,3].
3.3.1. E en econs uc ion
The coincidence algo i hm is based on he use o a coincidence
window 𝑇𝑇 𝐴𝐶−𝐹 𝐼𝐶𝐻
𝑐𝑜𝑖𝑛𝑐 be ween TAC and FICH and allows posi i e and
nega i e ime di e ences. Fig. 14 shows he dis ibu ion o ime di e -
ences 𝛿𝑇 =𝑇 𝑂𝐹𝑇 𝐴𝐶 −𝑇 𝑂𝐹𝐹 𝐼𝐶𝐻 o all ound coincidences and can be
explained as ollows:
•E en s wi h 𝛿𝑇 < −200 ns show a la dis ibu ion and co espond
o andom coincidences.
•The shape o −200 ns < 𝛿𝑇 < −20 ns can be desc ibed by an expo-
nen ial si ing on op o he cons an backg ound. The exponen ial
inc ease co esponds o e en s whe e a 𝛾- ay is emi ed be o e he
nucleus issions. These e en s can be explained by he exis ence
o he (n, 𝛾 ) p ocess ( ission isome s) [19–22].
•A main peak o −10 ns < 𝛿𝑇 < 10 ns co esponding o he p omp
ission e en s as sugges ed by he cha ac e is ics o hose e en s
wi h high 𝐸𝑠𝑢𝑚 and 𝑚𝑐𝑟, indica ed by he blue line.
•Ano he sha p s uc u e o side peak o 10 ns < 𝛿𝑇 < 20 ns is an
a e ac o he e en econs uc ion p ocess. The ime di e ence
be ween he main peak and his side peak co esponds exac ly
o he TAC coincidence window o 12 ns which is he minimum
ime di e ence be ween wo TAC e en s due o how he TAC
coincidence econs uc ion algo i hm wo ks. The posi ion o he
side peak will shi wi h he TAC coincidence window.
Fig. 14. Time di e ence dis ibu ion be ween TAC and FICH e en s. The selec ion
e e s o he algo i hm ha selec s he co esponding p omp e en , as desc ibed in
he ex . The ea u es o he dis ibu ion a e explained in he ex .
Fig. 15. Compa ison o he di e en e en selec ion algo i hms using ei he la ges
𝐸𝑠𝑢𝑚, la ges 𝑚𝑐𝑟 o smalles 𝛿𝑇 as p ima y c i e ia. In he zoom in he egion o
𝐸𝑆𝑢𝑚 <6 MeV he supp ession o he 480 keV 𝛾- ay emi ed in he10B(n, 𝛼) eac ion
and he 1435 keV 𝛾- ay om inelas ic sca e ing o ission neu ons on138Ba can be
obse ed.
•E en s wi h 𝛿𝑇 > 20 ns o m an exponen ial ail and co espond
o delayed e en s. Such e en s can be induced by ission neu ons
which a e subsequen ly cap u ed in he expe imen al se -up hus
emi ing 𝛾- ay cascades o isome ic s a es o he ission p oduc s
ha de-exci e ia 𝛾- ay cascades wi h a delay co esponding o he
hal -li e o he isome ic s a e. These e en s a e ela ed o ission
bu a e no p omp ission 𝛾- ays.
Fo easons o causali y he TAC-FICH coincidence window may no
be smalle han he TAC coincidence window, o he wise he e is he
possibili y o losing coincidences a i icially. The op imal ime window
is a comp omise be ween pile-up and e icien agging. 𝑇𝑇 𝐴𝐶−𝐹 𝐼𝐶𝐻
𝑐𝑜𝑖𝑛𝑐 >
𝑇𝑇 𝐴𝐶
𝑐𝑜𝑖𝑛𝑐 can lead o mul iple coincidences ound o a single FICH e en .
The di e en coincidences will be cha ac e ized by di e en TAC
e en s, hence di e en 𝐸𝑠𝑢𝑚,𝑚𝑐𝑟 and 𝛿𝑇 . I wo o mo e TAC e en s a e
assigned o he same FICH e en , he TAC e en wi h he highes c ys al
mul iplici y 𝑚𝑐𝑟 is selec ed as he co esponding p omp ission e en .
I he TAC e en s happen o ha e he same c ys al mul iplici y hen he
e en wi h highe 𝐸𝑠𝑢𝑚 is selec ed as he co esponding p omp ission
e en . In p inciple hese c i e ia a e a bi a y and he pe o mance o
he di e en e en selec ion algo i hms is illus a ed in Fig. 15. I shows
7
M. Bacak, M. Aïche, G. Bélie e al. Nuclea Ins . and Me hods in Physics Resea ch, A 969 (2020) 163981
Fig. 16. F ac ion o ound coincidences wi h espec o he o al numbe o ission
e en s in he FICH (black do s; le axis). F ac ion o e en s whe e exac ly one TAC
e en is ound o one FICH e en wi h espec o all ound coincidences ( ed squa es;
igh axis). Bo h o e en s wi h 𝑎𝑚𝑝 > 0.09 V and 1.6 eV< 𝐸𝑛<1.9 eV. No ice he
di e en scales on he wo y-axes.
he 𝐸𝑠𝑢𝑚 spec a o he co esponding p omp ission e en s selec ed
wi h di e en algo i hms. I is e iden ha no ma e which algo i hm
is chosen he di e ence is negligible.
Fig. 16 shows he e ec o di e en coincidence windows
𝑇𝑇 𝐴𝐶−𝐹 𝐼𝐶𝐻
𝑐𝑜𝑖𝑛𝑐 on he o al numbe o ound coincidences no malized o
he o al numbe o ission e en s de ec ed by he FICH (black do s;
le axis)). A s eady inc ease can be seen wi h inc easing 𝑇𝑇 𝐴𝐶−𝐹 𝐼𝐶𝐻
𝑐𝑜𝑖𝑛𝑐
which is unde s andable, al hough hose addi ionally ound TAC e en s
in coincidence a e no necessa ily ela ed o he p omp ission e en
bu migh co espond o andom o delayed e en s. On he o he hand
Fig. 16 shows he numbe o coincidences whe e exac ly one TAC e en
is ound o one FICH e en ( ed squa es; igh axis). Wi h inc easing
coincidence window 𝑇𝑇 𝐴𝐶−𝐹 𝐼𝐶𝐻
𝑐𝑜𝑖𝑛𝑐 he numbe o one o one coincidences
d as ically dec eases, as he p obabili y o mul iple agging s a s o
inc ease. A coincidence window 𝑇𝑇 𝐴𝐶−𝐹 𝐼𝐶𝐻
𝑐𝑜𝑖𝑛𝑐 sligh ly la ge han he
𝑇𝑇 𝐴𝐶
𝑐𝑜𝑖𝑛𝑐 is al eady su icien o ag close o 99 % o he FICH e en s while
a window oo la ge migh esul in an unce ain assignmen o mul iple
TAC e en s o a FICH e en . To educe his unce ain y he 𝑇𝑇 𝐴𝐶−𝐹 𝐼𝐶𝐻
𝑐𝑜𝑖𝑛𝑐
coincidence window is se o 14 ns.
FICH-TAC coincidence agging also allows o a be e alpha- ission
sepa a ion as he p obabili y o agging an 𝛼-pa icle is negligible
compa ed o a ission agmen . In Fig. 17 he agged ission ampli-
ude spec a o di e en 𝐸𝑛 egions a e compa ed o he ampli ude
spec a o he bes achie able sepa a ion solely using he FICH. The
imp o emen is ob ious and allows he in es iga ion o he shape o
he ission agmen ene gy deposi ion in he ission chambe below
wha was possible wi h he FICH alone. I shall be no ed ha he TAC
da a o 0.8 MeV< 𝐸𝑛<7 MeV is usually no used in he analysis o
c oss-sec ions due o he so-called 𝛾- lashe ec [6,7,23] which blinds
he de ec o .
3.3.2. Tagging and FICH e iciency
In analogy o p e ious wo ks [2,4], he agging e iciency 𝜀𝑇 𝑎𝑔𝑔(𝐴𝑡ℎ;
𝐸𝑆𝑢𝑚, 𝑚𝑐𝑟)desc ibes he p obabili y o de ec ing a ission e en iden-
i ied as such by he FICH in he TAC and depends on he applied
ampli ude h eshold 𝐴𝑡ℎ. I is de ined as he a io be ween he agged
ission e en s 𝑐𝑇 𝑎𝑔𝑔(𝐴𝑡ℎ;𝐸𝑆𝑢𝑚, 𝑚𝑐𝑟)and he o al ission coun s de ec ed
by he TAC 𝑐𝐹 𝑖𝑠𝑠−𝑇 𝐴𝐶 (𝐸𝑆𝑢𝑚, 𝑚𝑐𝑟)(dependencies on 𝐸𝑆𝑢𝑚 and 𝑚𝑐𝑟 a e
implici o eadabili y):
𝜀𝑇 𝑎𝑔𝑔(𝐴𝑡ℎ) = 𝑐𝑇 𝑎𝑔𝑔(𝐴𝑡ℎ)
𝑐𝐹 𝑖𝑠𝑠−𝑇 𝐴𝐶
.(1)
Fig. 17. Pulse heigh spec a o he e en s in he ission chambe o 0.8 MeV<
𝐸𝑛<7 MeV compa ed o agged e en s o di e en 𝐸𝑛 egions om all FICH channels.
The neu on lux o n_TOF can be seen in Fig. 7.
The ission de ec ion e iciency 𝜀𝐹 𝐼𝐶𝐻 (𝐴𝑡ℎ)is he p obabili y o
de ec ing a ission eac ion by he FICH de ec o and depends only on
he ampli ude h eshold 𝐴𝑡ℎ applied o he FICH e en s.
Unde he assump ion ha he p obabili y o de ec ing a ission
e en in one o he de ec o s does no depend on whe he i was
de ec ed in he o he one, he agging e iciency 𝜀𝑇 𝑎𝑔𝑔 and he ission
de ec ion e iciency 𝜀𝐹 𝐼𝐶𝐻 a e he same quan i y and he agging
e iciency depends only on he applied ampli ude h eshold 𝐴𝑡ℎ.
Following equa ion (1)𝑐𝑇 𝑎𝑔𝑔 and 𝑐𝐹 𝑖𝑠𝑠−𝑇 𝐴𝐶 ha e o be de e mined
o calcula e he agging e iciency. While he agged coun s 𝑐𝑇 𝑎𝑔𝑔 can
be aken di ec ly om he coincidence algo i hm, he TAC e en s
co esponding o ission eac ions 𝑐𝐹 𝑖𝑠𝑠−𝑇 𝐴𝐶 ha e o be cleaned om he
backg ound i s . The backg ound consis s o he ambien , he neu on
beam induced and he sample induced backg ound. The 233U(n, 𝛾)
eac ion se s he lowe h eshold o he sum ene gy as he calcula ion
will be biased i sum ene gies below he neu on sepa a ion ene gy o
234U𝑆𝑛(233𝑈+𝑛) = 6.85 MeV a e conside ed. Thus as a gene al ule he
e iciency will only be calcula ed o 𝐸𝑆𝑢𝑚 >8 MeV o a oid his com-
ponen o he backg ound comple ely. Dedica ed measu emen s o he
ambien and neu on beam induced backg ound ha e been pe o med
o es ima e hei con ibu ion o he o e all backg ound. Fu he mo e,
he backg ound sub ac ion is less p one o unce ain ies and s a is-
ical luc ua ions o high c ys al mul iplici y and la ge sum ene gies
because he e is li le backg ound o such condi ions bu a comp omise
be ween sys ema ic and s a is ical unce ain ies has o be made. Ne e -
heless, he sensi i i y wi h espec o he applied condi ions in c ys al
mul iplici y and sum ene gy has o be in es iga ed and is shown o wo
di e en ampli ude h esholds 𝐴𝑡ℎ in Fig. 18. E en hough he esidual
backg ound is sub ac ed a a ia ion o lowe mul iplici ies can be
obse ed ha dec eases wi h inc easing mul iplici ies. Fo 𝑚𝑐𝑟 >6
he a ia ion o he calcula ed e iciency becomes smalle han he
s a is ical unce ain y, indica ing ha only ission e en s a e le in he
calcula ion. One po en ial explana ion o he sys ema ic end could be
addi ional backg ound componen s i.e. eac ions induced by sca e ed
( om he samples) o ission neu ons. Indeed, neu ons emi ed in
he ission p ocess can be cap u ed, p e e ably in he BaF2c ys als
hemsel es leading o TAC e en s wi h la ge deposi ed ene gies. This
migh also explain why he calcula ed e iciency in Fig. 18 shows a
s onge dependence on he mul iplici y o 𝐸𝑆𝑢𝑚 >8 MeV compa ed
o he mo e es ic i e condi ion 𝐸𝑆𝑢𝑚 >10 MeV, as he ission neu on
induced backg ound should no exceed 10 MeV sum ene gy acco ding
o he neu on sepa a ion ene gies o ba ium iso opes, i.e. 𝑆𝑛(135𝐵𝑎 +
𝑛) = 9.1 MeV.
8
M. Bacak, M. Aïche, G. Bélie e al. Nuclea Ins . and Me hods in Physics Resea ch, A 969 (2020) 163981
Fig. 18. Fission agging e iciency 𝜀𝑇 𝑎𝑔𝑔 as a unc ion o he c ys al mul iplici y and
sum ene gy o wo di e en ampli ude h esholds 𝐴𝑡ℎ in he neu on ene gy in e al
om 1.6 eV o 1.9 eV.
Fig. 19. Fission agging e iciency calcula ed in se e al neu on esonances o
𝐴𝑡ℎ = 0.076 V,𝑚𝑐𝑟 >6and 𝐸𝑆𝑢𝑚 >10 MeV, hei weigh ed a e age ( ed line) and
he s anda d de ia ion o he da a poin s (blue dashed lines). The unce ain ies a e
calcula ed om he wo highly co ela ed quan i ies in Eq. (1).
Fig. 20. Tagging e iciency 𝜀𝑇 𝑎𝑔𝑔 as a unc ion o he FICH ampli ude h eshold (black
ci cles). A scaled FICH ampli ude spec um in coincidence wi h he TAC o e en s
wi h 𝑚𝑐𝑟 >6and 𝐸𝑆𝑢𝑚 >10 MeV is shown oo (g ey line) as i is di ec ly ela ed o
he e iciency.
Fig. 18 shows ha o 𝑚𝑐𝑟 >6 he sensi i i y o he backg ound is
educed wi hin e o ba s as bo h condi ions in 𝐸𝑆𝑢𝑚 coincide.
Using only e en s wi h 𝑚𝑐𝑟 >6and 𝐸𝑆𝑢𝑚 >10 MeV he e i-
ciency was calcula ed in se e al neu on esonances in o de o e i y
a possible a ia ion. The alues o he e iciency o he used am-
pli ude h eshold 𝐴𝑡ℎ = 0.076 V we e all in ag eemen wi hin hei
unce ain ies, as shown in Fig. 19. Thus he a e age agging e i-
ciency o e all neu on ene gy in e als is calcula ed o 𝜀𝑇 𝑎𝑔𝑔(𝐴𝑡ℎ =
0.076 𝑉) = 89.6(1) % and shown as a unc ion o he ission ampli ude
h eshold in Fig. 20. The la e allows o calcula e he agging e iciency
o any gi en ampli ude cu and shows he s abili y o he alue o
𝜀𝑇 𝑎𝑔𝑔 (𝐴𝑡ℎ = 0.076 𝑉)wi h espec o small gain luc ua ions, which a e
equi alen o small a ia ions in he ampli ude h eshold. This gi es
u he con idence in he accu acy o he agging e iciency, which is
c ucial o assess he cap u e c oss sec ion.
In he measu emen o he 233U𝛼- a io he cap u e esponse is
ob ained by sub ac ing he e iciency co ec ed agged coun s om he
o al coun s in he calo ime e . Wi hou gi ing a de ailed calcula ion,
om he 233U cap u e and ission c oss-sec ions i can be expec ed ha
an unce ain y in he agging e iciency o 0.1 % ansla es in o a 1 %
unce ain y in he 233U𝛼- a io on a e age. The esul s show ha his
de ec o is well sui ed o ob ain an accu a e alpha- a io.
4. Conclusions
A new compac ission chambe was de eloped and op imized o
he use in ission agging expe imen s o measu e cap u e c oss-sec ions
o issile iso opes. The de elopmen aimed a he use o he de ec ion
sys em a he n_TOF acili y (CERN), coupled o he To al Abso p ion
Calo ime e o EAR1, bu can be gene alized o o he se -ups. The
ission chambe was op imized o iming pe o mance wi h an a e age
signal ise ime o abou 16 ns and a FWHM o 34 ns which is op imal
o he high speci ic 𝛼-pa icle coun a es om 233U as well as he
alpha- ission disc imina ion and allows using a na ow coincidence
window be ween he calo ime e and he FICH acili a ing low pile-up
in he coincidence econs uc ion. I s compac ness hos ing a o al o 14
samples as well as he minimal amoun o s uc u al ma e ial in beam
p o ide excellen condi ions o low backg ound and high s a is ics
measu emen s. The whole expe imen al se -up was u he designed
o achie e good pe o mance, especially s abili y o e ime as well as
e ec i ely agging he ission e en s wi h an e iciency close o 90 %.
An expe imen aiming a measu ing he 233U(n, 𝛾) c oss-sec ion
was pe o med and he esul s ha e shown ha he de eloped ission
chambe is well sui ed o ag he p omp ission 𝛾- ays, hence o ha e a
good con ol o e he ission backg ound in he cap u e measu emen .
Resul s o his measu emen will be p esen ed in a sepa a e publica ion.
Decla a ion o compe ing in e es
The au ho s decla e ha hey ha e no known compe ing inan-
cial in e es s o pe sonal ela ionships ha could ha e appea ed o
in luence he wo k epo ed in his pape .
CRediT au ho ship con ibu ion s a emen
M. Bacak: So wa e, Valida ion, Visualiza ion, In es iga ion, Fo -
mal analysis, W i ing - o iginal d a , Da a cu a ion. M. Aïche: Me hod-
ology. G. Bélie : Concep ualiza ion, In es iga ion, Me hodology, Re-
sou ces, W i ing - e iew & edi ing. E. Be houmieux: So wa e, Vali-
da ion, In es iga ion, Concep ualiza ion, Me hodology, P ojec admin-
is a ion, W i ing - e iew & edi ing, Da a cu a ion, Supe ision. M. Di-
akaki: In es iga ion, Concep ualiza ion, Me hodology, W i ing - e iew
& edi ing. E. Dupon : In es iga ion, Concep ualiza ion, Me hodology,
P ojec adminis a ion, W i ing - e iew & edi ing, Funding acquisi ion,
Da a cu a ion, Supe ision. F. Gunsing: In es iga ion, Concep ualiza-
ion, Me hodology, P ojec adminis a ion, W i ing - e iew & edi ing,
9