EPJ Web o Con e ences 146, 01012 (2017) DOI: 10.1051/epjcon /201714601012
ND2016
7Be(n,α)and7Be(n,p) c oss-sec ion measu emen o he
cosmological li hium p oblem a he n TOF acili y a CERN
M. Ba bagallo1,a, N. Colonna1, O. Abe le2, J. And zejewski3, L. Audouin4,V.B
´
eca es5, M. Bacak6,J.Balib ea
5,
S. Ba os7,F.Be
ˇ
c ´
aˇ
8, C. Bein ucke 9, E. Be houmieux10, J. Billowes11, D. Bosna 12, M. B ugge 2, M. Caama˜
no13,
F. Cal i ˜
no14, M. Cal iani2, D. Cano-O 5, R. Ca della2, A. Casano as14, D.M. Cas elluccio15,16, F. Ce u i2, Y.H. Chen4,
E. Chia e i2,G.Co
´
es14,M.A.Co
´
es-Gi aldo17, L. Cosen ino18, L.A. Damone1,19, M. Diakaki10, C. Domingo-Pa do20,
R. D essle 21, E. Dupon 10,I.Du
´
an13,B.Fe n
´
andez-Dom´
ınguez13,A.Fe a i
2,P.Fe ei a
7, P. Finocchia o18,V.Fu man
22,
K. G¨
obel9,A.R.Ga c
´
ıa5,A.Gawlik
3, T. Gloda iu23,I.F.Gonc¸al es7,E.Gonz
´
alez5, A. Go e do ski24, E. G iesmaye 6,
C. Gue e o17, F. Gunsing10,2, H. Ha ada25, T. He ich9,S.Heini z
21,J.Heyse
26, D.G. Jenkins27, E. Je icha6,
F. K¨
appele 28, Y. Kadi2, T. Ka abuchi29,P.Ka igin
6,V.Ke le o
24, V. Kh yachko 24,A.Kimu a
25,N.Ki el
21,
M. Kokko is30,M.K i
ˇ
cka8, E. Leal-Cidoncha13, C. Lede e 31,H.Leeb
6, J. Le endegui-Ma co17,S.LoMeo
15,16,
S.J. Lonsdale31,R.Losi o
2, D. Macina2, J. Ma ganiec3,T.Ma
´
ınez5,C.Massimi
16,32, P. Mas inu33, M. Mas oma co1,
F. Ma eucci34,35, E.A. Mauge i21, E. Mendoza5, A. Mengoni15, P.M. Milazzo34, F. Ming one16,M.Mi ea
23,
S. Mon esano2, A. Musuma a18,36,R.Nol e
37, A. Op ea23, N. Pa onis38,A.Pa lik
39, J. Pe kowski3, J.I. Po as2,40,
J. P aena17,40, J.M. Quesada17,K.Rajee
41, T. Rausche 42,43,R.Rei a h
9, A. Riego-Pe ez14, P.C. Rou 41, C. Rubbia2,
J.A. Ryan11, M. Saba ´
e-Gila e2,17, A. Saxena41, P. Schillebeeckx26, S. Schmid 9, D. Schumann21, P. Sedyshe 22,
A.G. Smi h11, A. S ama opoulos30, G. Taglien e1,J.L.Tain
20,A.Ta i e
˜
no-Saldi ia20, L. Tassan-Go 4, A. Tsinganis30,
S. Valen a8, G. Vannini16,32,V.Va iale
1,P.Vaz
7, A. Ven u a16, V. Vlachoudis2,R.Vlas ou
30,A.Wallne
44,S.Wa en
11,
M. Weigand9,C.Weiss
2,6,C.Wol
9, P.J. Woods31,T.W igh
11,P. ˇ
Zugec12, and he n TOF Collabo a ion
1Is i u o Nazionale di Fisica Nuclea e, Sezione di Ba i, I aly
2Eu opean O ganiza ion o Nuclea Resea ch (CERN), Swi ze land
3Uni e si y o Lodz, Poland
4Ins i u de Physique Nucl´
eai e, CNRS-IN2P3, Uni . Pa is-Sud, Uni e si ´
e Pa is-Saclay, 91406 O say Cedex, F ance
5Cen o de In es igaciones Ene g´
e icas Medioambien ales y Tecnol´
ogicas (CIEMAT), Spain
6Technische Uni e si ¨
a Wien, Aus ia
7Ins i u o Supe io T´
ecnico, Lisbon, Po ugal
8Cha les Uni e si y, P ague, Czech Republic
9Goe he Uni e si y F ank u , Ge many
10 CEA Saclay, I u, Gi -su -Y e e, F ance
11 Uni e si y o Manches e , UK
12 Uni e si y o Zag eb, C oa ia
13 Uni e si y o San iago de Compos ela, Spain
14 Uni e si a Poli `
ecnica de Ca alunya, Spain
15 Agenzia nazionale pe le nuo e ecnologie (ENEA), Bologna, I aly
16 Is i u o Nazionale di Fisica Nuclea e, Sezione di Bologna, I aly
17 Uni e sidad de Se illa, Spain
18 INFN Labo a o i Nazionali del Sud, Ca ania, I aly
19 Dipa imen o di Fisica, Uni e si `
a degli S udi di Ba i, I aly
20 Ins i u o de F´
ısica Co puscula , Uni e sidad de Valencia, Spain
21 Paul Sche e Ins i u (PSI), Villingen, Swi ze land
22 Join Ins i u e o Nuclea Resea ch (JINR), Dubna, Russia
23 Ho ia Hulubei Na ional Ins i u e o Physics and Nuclea Enginee ing, Romania
24 Ins i u e o Physics and Powe Enginee ing (IPPE), Obninsk, Russia
25 Japan A omic Ene gy Agency (JAEA), Tokai-mu a, Japan
26 Eu opean Commission, Join Resea ch Cen e, Geel, Re ieseweg 111, 2440 Geel, Belgium
27 Uni e si y o Yo k, UK
28 Ka ls uhe Ins i u e o Technology, Campus No h, IKP, 76021 Ka ls uhe, Ge many
29 Tokyo Ins i u e o Technology, Japan
30 Na ional Technical Uni e si y o A hens, G eece
31 School o Physics and As onomy, Uni e si y o Edinbu gh, UK
ae-mail: [email p o ec ed]
c
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/).
EPJ Web o Con e ences 146, 01012 (2017) DOI: 10.1051/epjcon /201714601012
ND2016
32 Dipa imen o di Fisica e As onomia, Uni e si `
a di Bologna, I aly
33 Is i u o Nazionale di Fisica Nuclea e, Sezione di Legna o, I aly
34 Is i u o Nazionale di Fisica Nuclea e, Sezione di T ies e, I aly
35 Dipa imen o di As onomia, Uni e si `
a di T ies e, I aly
36 Dipa imen o di Fisica e As onomia, Uni e si `
a di Ca ania, I aly
37 Physikalisch-Technische Bundesans al (PTB), Bundesallee 100, 38116 B aunschweig, Ge many
38 Uni e si y o Ioannina, G eece
39 Uni e si y o Vienna, Facul y o Physics, Vienna, Aus ia
40 Uni e si y o G anada, Spain
41 Bhabha A omic Resea ch Cen e (BARC), India
42 Cen e o As ophysics Resea ch, Uni e si y o He o dshi e, UK
43 Depa men o Physics, Uni e si y o Basel, Swi ze land
44 Aus alian Na ional Uni e si y, Canbe a, Aus alia
Abs ac .The Cosmological Li hium P oblem e e s o he la ge disc epancy be ween he abundance o
p imo dial 7Li p edic ed by he s anda d heo y o Big Bang Nucleosyn hesis and he alue in e ed om
he so-called “Spi e pla eau” in halo s a s. A possible explana ion o his longs anding puzzle in Nuclea
As ophysics is ela ed o he inco ec es ima ion o he des uc ion a e o 7Be, which is esponsible o he
p oduc ion o 95% o p imo dial Li hium. While cha ged-pa icle induced eac ions ha e mos ly been uled
ou , da a on he 7Be(n,α)and7Be(n,p) eac ions a e sca ce o comple ely missing, so ha a la ge unce ain y
s ill a ec s he abundance o 7Li p edic ed by he s anda d heo y o Big Bang Nucleosyn hesis. Bo h eac ions
ha e been measu ed a he n TOF acili y a CERN, p o iding o he i s ime da a in a wide neu on ene gy
ange.
1. In oduc ion
The inno a i e neu on ime-o - ligh acili y n TOF based
on a neu on spalla ion sou ce, was buil a CERN wi h he
aim o add essing he eques o high accu acy nuclea da a
o Nuclea As ophysics and o ad anced nuclea ene gy
sys ems [1]. In pa icula Nuclea As ophysics p esen s
many cases ha equi e high p ecision neu on induced
eac ion da a, o example o imp o e modelling o s ella
Nucleosyn hesis and Big Bang Nucleosyn hesis.
One o he mos impo an un esol ed p oblems in
Nuclea As ophysics is he so-called “Cosmological
Li hium p oblem” (CLiP) [2]. I e e s o he la ge
disc epancy ( ac o 2–3) be ween he abundance o
p imo dial 7Li p edic ed by he s anda d heo y o Big
Bang Nucleosyn hesis (BBN) and he alue in e ed
om he so-called “Spi e pla eau” in halo s a s. In he
amewo k o S anda d Model, a possible explana ion
o his longs anding puzzle is ela ed o he inco ec
es ima ion o he des uc ion a e o 7Be. Indeed in
he s anda d heo y o BBN, 95% o p imo dial 7Li is
p oduced by he decay o 7Be ( 1/2=53.2 days), ela i ely
la e a e he Big Bang, when he empe a u e o he
Uni e se was low enough o allow o elec ons and nuclei
o combine in o a oms. The e o e, he abundance o 7Li is
essen ially de e mined by he p oduc ion and des uc ion
o 7Be a empe a u e alues be ween 0.2<T9<1.2,
co esponding o neu on ene gy be ween abou 20 keV
and 120 keV.
Cha ged-pa icle induced eac ions esponsible o
he des uc ion o 7Be a e known wi h sa is ac o y
accu acy. On he o he hand da a on neu on induced
eac ions, namely 7Be(n,α)4He and 7Be(n,p)7Li eac ions,
ha e been so a sca ce o comple ely missing, mainly
due o unsu moun able expe imen al di icul ies a ising
om 7Be speci ic ac i i y (∼13 GBq/µg). Conce ning
7Be(n,α) eac ion c oss-sec ion, only one measu emen
exis s, pe o med a he mal ene gy in he 60’s [3]. This
c oss sec ion was hen ex apola ed o he ene gy ange
o in e es o BBN in a compila ion by Wagone [4].
On he o he hand 7Be(n,p) eac ion, ha ep esen s he
leading channel among 7Be des uc ion p ocesses du ing
BBN [5], has been measu ed in he 80’s and c oss-sec ion
de e mined only up o 13.5 keV [6].
Taking ad an age o s a e-o -a echniques o he
p oduc ion o high-pu i y adioac i e samples, o high
pe o mance de ec ion sys ems and, especially, o he in-
no a i e ea u es o a new beam line and measu ing s a ion
(EAR2) pa icula ly sui ed o challenging measu emen s
on sho -li ed adioiso opes, bo h (n,α) and (n,p) eac ion
c oss-sec ions on 7Be ha e been ecen ly measu ed a
nTOF (CERN).
The wo measu emen s, pe o med wi h wo di e en
silicon de ec ion sys ems, p o ide o he i s ime nuclea
da a on 7Be(n,α) and 7Be(n,p) c oss-sec ion in a wide
neu on ene gy ange, allowing o cla i y he ole o hese
eac ions in Nuclea As ophysics.
2. The n TOF acili y
The acili y is based on he 7 ns wide, 20 GeV/c pulsed
p o on beam om CERN P o on Synch o on (PS) wi h
ypically 7 ×1012 p o ons pe pulse, impinging on a lead
spalla ion a ge . The p ocess yields abou 300 neu ons
pe inciden p o on, which a e subsequen ly collima ed and
guided ough wo beam lines o di e en expe imen al
a eas (EAR1 and EAR2, loca ed espec i ely a ∼180 m
and ∼20 m om he spalla ion a ge ) whe e he samples
unde in es iga ion and he de ec ion sys ems a e loca ed.
A laye o selec ed liquid (i.e demine alized wa e o
bo a ed wa e ) a ound he spalla ion a ge mode a es he
ini ially as neu ons down o a whi e ene gy spec um,
which spans om meV o GeV neu on ene gy [7]. In o de
o a oid he o e lap o consecu i e neu on bunches, he
ime dis ance be ween wo consecu i e p o on pulses is a
mul iple o 1.2 s, ela ed o PS wo king cycle.
Bo h 7Be(n,α) and 7Be(n,p) eac ion c oss-sec ions
ha e been measu ed a he newly buil second expe imen al
a ea, n TOF-EAR2, ha is cha ac e ized by an ex emely
2
EPJ Web o Con e ences 146, 01012 (2017) DOI: 10.1051/epjcon /201714601012
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Figu e 1. n TOF neu on lux a EAR1 wi h no mal (blue) and
bo a ed ( ed) wa e as mode a o compa ed wi h he neu on lux
a EAR2 (black) [8].
high in ensi y neu on lux (∼107neu ons/pulse), abou
30 imes mo e in ense han EAR1 [8], as shown in Fig. 1.
The a ailabili y o a pulsed and high luminosi y neu on
beam makes EAR2 ideal o measu emen s on iso opes
a ailable in e y small amoun s, wi h sho hal -li es, o
bo h, as indeed is he case o 7Be.
3. Measu emen and esul s o he
7Be(n,α) eac ion c oss-sec ion
7Be(n,α)4He eac ion is cha ac e ized by a Q- alue o
18.99 MeV. A he neu on ene gies unde in es iga ion,
he wo alpha-pa icles a e emi ed back- o-back wi h a
maximum ene gy o 9.5 MeV each, depending on he
8Be s a e popula ed in he eac ion ha p oceeds indeed
ough ene gy le els popula ed by γ- ay ansi ions. In
he p esen measu emen ew o hem we e expe imen ally
accessible, he pa ial c oss-sec ions o he co esponding
7Be(n,γα)4He eac ions we e he e o e measu ed and la e
on he o al c oss-sec ion was de e mined as desc ibed in
de ails in Re . [9].
The de ec ion sys em used in he measu emen
consis ed o wo de ec o sandwiches, each sandwich
composed by wo single pad silicon de ec o s 140 µm
hick and 3 ×3cm
2wide. Each sandwich hos ed a 1.4 µg
7Be sample, co esponding o abou 20 GBq ac i i y.
Bo h samples we e p epa ed a Paul Sche e Ins i u e
(PSI) [10], al hough wo di e en echniques we e used
o p oduce hem. The i s sample was elec opla ed on a
5µm hick Al oil and he second one was p epa ed by
d ople deposi ion on a 0.6 µm hick polye hylene oil.
Mo e de ails abou he expe imen al se up and samples
can be ound in Re . [11,12]. The s ack o de ec o s and
7Be samples was placed in ai inside a sca e ing chambe
shielded by 1 cm lead in o de o educe he ex e nal dose
due o he 478 keV γ- ays a ising om he 7Be decay and,
consequen ly, o acili a e anspo a ion and ope a ions in
p oximi y o he se up. The whole assembly was placed
in he neu on beam. P io o he 7Be(n,α) measu emen ,
he pe o mances o he de ec ion sys em, pa icula ly i s
adia ion ha dness and i s capabili y o de ec ing high
ene gy cha ged pa icles and ejec ing backg ound e en
i exposed o an ex emely ha sh en i onmen , we e ully
es ed, p o ing he easibili y o he measu emen [13].
Such a sys em allowed o de ec in coincidence he
alpha pa icles emi ed, p o iding a peculia signa u e
o he occu ence o 7Be(n,γα) eac ions and a s ong
Figu e 2. Sca e plo o signal ampli udes in all possible pai s o
de ec o s o he s ack. Top le and bo om igh plo e e o pai s
hos ing he 7Be sample, hey clea ly shows he expec ed back- o-
back coincidences no p esen in unco ela ed pai s o silicons.
ejec ion o any o he sou ce o backg ound. Fu he mo e,
he use o wo di e en sandwich de ec o s p o ided
edundancy and allowed on he one hand o compa e he
a e o ue e en s, and on he o he hand o es ima e he
backg ound ela ed o andom coincidences e alua ed by
analyzing unco ela ed pai s o de ec o s. The eal and
andom coincidences can be clea ly dis inguished in Fig. 2,
which shows he ampli ude co ela ion o all silicon pai s:
op-le and bo om- igh panels a e ela ed o pai s o
silicon hos ing he 7Be samples while all he o he s e e
o unco ela ed pai s o de ec o s.
Applying p ope h esholds allowed o selec coinci-
dence e en s as a unc ion o neu on ene gy and inally
o de e mine he c oss-sec ion o he 7Be(n,α) eac ion
in he ene gy ange 10 meV–10 keV [9]. The esul o
his measu emen a he mal ene gy was ound o be
consis en wi h he only p e ious exis ing measu emen .
In he ene gy ange o in e es o BBN he p esen da a
indica e ha Wagone ’s compila ion o e es ima es by a
ac o o 10 he eac ion a e. The esul s o he p esen
measu emen lea e un esol ed he Cosmological Li hium
P oblem, whose solu ion should hen be sea ched in o he
physical scena ios.
4. Measu emen o he 7Be(n,p) eac ion
c oss-sec ion
In o de o comple e he n TOF p og am on he CLiP,
he 7Be(n,p)7Li c oss-sec ion was also measu ed. The
measu emen was pe o med by de ec ing he eme ging
p o ons, emi ed ei he wi h ene gy equal o 1.44 MeV
(b. .∼99%) o wi h 1.02 MeV (b. .∼1%), depending on
he s a e o 8Li popula ed in he p ocess. In his case he
de ec ion sys em consis ed in a elescope made o 20 µm
and 300 µm silicon s ips de ec o , 5 ×5cm
2ac i e a ea.
The sys em was p e iously cha ac e ized a n TOF-EAR2
using he well-known 6Li(n, )4He eac ion, and he esul s
o his measu emen showed ha low ene gy cha ged
pa icles could be easily disc imina ed om backg ound
in he elescope.
Con a ily o ou coming α-pa icles emi ed in
7Be(n,α)4He eac ion, low ene gy p o ons emi ed he e
do no ep esen a s ong signa u e o he occu ence o
he 7Be(n,p)7Li eac ion, so in o de o a oid a p io i
some backg ound, an ex emely high pu i y sample had
o be used. I has been ealized s a ing om 200 GBq o
3
EPJ Web o Con e ences 146, 01012 (2017) DOI: 10.1051/epjcon /201714601012
ND2016
Figu e 3. E-E coincidences ma ix o e en s de ec ed in he
elescope. The ed a ow poin s o he egion o p o ons emi ed
in he 7Be(n,p)7Li eac ion.
7Be collec ed om he neu on spalla ion sou ce SINQ a
PSI. A e chemical pu i ica ion i has been made a ailable
in he o m o ni a e solu ion o he ISOLDE acili y
a CERN [14], whe e a 7Be beam has been p oduced,
accele a ed o 30 keV, iso opically sepa a ed a he GLM
beam line and implan ed on a 18 µm aluminum backing.
A sample o ∼1.1 GBq o al ac i i y was p epa ed and
successi ely ins alled a he EAR2 neu on beam line a
nTOF.
A n TOF he sample was il ed o 45◦wi h espec he
inciden di ec ion o he neu on beam and p o ons om
he eac ion de ec ed and iden i ied in he elescope, as
clea ly shown in Fig. 3. A p elimina y analysis indica es
ha i will be possible o de e mine he c oss sec ion o
he 7Be(n,p) eac ion om he mal neu on ene gy o a ew
hund ed keV, hus co e ing o he i s ime he egion o
in e es o BBN, om 20 o 200 keV, and sheding some
ligh on he longs anding CLiP p oblem.
5. Conclusions
In o de o in es iga e on Nuclea Physics solu ions o
he long s anding Cosmological Li hium P oblem, he
7Be(n,α) and 7Be(n,p) eac ion c oss-sec ions ha e been
measu ed in a wide ene gy ange a he n TOF acili y in
he new, high lux, expe imen al a ea (EAR2).
The 7Be(n,α)4He measu emen has e ealed ha he
eac ion a e used in BBN calcula ion has been so a
o e es ima ed, lea ing hen he p oblem unsol ed. On he
o he hand p elimina y esul s on 7Be(n,p)7Li eac ion
look ex emely p omising, al eady p o ing ha a inal
wo d on he ole o his eac ion in BBN can be p o ided
by his measu emen .
Re e ences
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(2016)
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(1963)
[4] R.V. Wagone , As ophys J. 148, 3 (1967)
[5] C. B oggini, Jou . Cosm. As . Phys 06, 030 (2012)
[6] P. Koehle e al., Phys. Re . C 37, 917 (1988)
[7] M. Ba bagallo e al., Eu . Phys. J. A 49, 156 (2013)
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e-Gila e e al., in p epa a ion
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4