SOX: search for sh o rt b a selin e n e u tr in o o sc illa tio n s w ith B o r e x in o
M V iv ie r 4, M A g o s t in i24, K A lt e n m iille r 24, S A p p e l24, G B e llin i15, J B e n z ig e r 19, N B e r t o n 4, D B ic k 5, G B o n fin i14, D B r a v o 22,
B C a c c ia n ig a 15, F C a la p r ic e 18, A C a m in a ta 3, P C a v a lc a n te 14, A C h e p u r n o v 25, K C h o i30, M C r ib ie r 1,4, D D ’A n g e lo 15, S D a v in i26, A D e r b in 17, L D i N o t o 3, I D r a c h n e v 26, M D u r e r o 4, A E t e n k o 12, S F a r in o n 3, V F is c h e r 4, K F o m e n k o 2, D F r a n c o 1, F G a b r ie le 14, J G a ffio t4, C G a lb ia t i18, C G h ia n o 3, M G ia m m a r c h i15,
M G o e g e r -N e ff24, A G o r e t t i18, M G r o m o v 25, C H a g n e r 5, T H o u d y 1,4, E H u n g e r fo r d 27, A ld o I a n n i14, A n d r e a I a n n i18, N J o n q u e r e s 7,
K J e d r z e jc z a k 8, M K a is e r 5, V K o b y c h e v 9, D K o r a b le v 2, G K o r g a 14, V K o r n o u k h o v 10, D K r y n 1, T L a c h e n m a ie r 11, T L a s s e r r e 1,4,
M L a u b e n s te in 14, B L e h n e r t28, J L in k 22, E L itv in o v ic h 1213, F L o m b a r d i14, P L o m b a r d i15, L L u d h o v a 15, G L u k y a n c h e n k o 12,13, I M a c h u lin 1213, S M a n e c k i22, W M a n e s c h g 6, S M a r c o c c i26,
J M a r ic ic 30, G M e n tio n 4, E M e r o n i15, M M e y e r 5, L M ir a m o n ti15, M M is ia s z e k 814, M M o n tu s c h i23, P M o s t e ir o 18, V M u r a to v a 17,
R M u s e n ic h 3, B N e u m a ir 24, L O b e r a u e r 24, M O b o le n s k y 1, F O r t ic a 16, M P a lla v ic in i3, L P a p p 24, L P e r a s s o 3, A P o c a r 21, G R a n u c c i15,
A R a z e t o 14, A R e 15, A R o m a n i16, R R o n c in 141, N R o s s i14, S S c h o n e r t24, L S c o la 4, D S e m e n o v 17, H S im g e n 6,
M S k o r o k h v a to v 1213, O S m ir n o v 2, A S o tn ik o v 2, S S u k h o t in 12,
Y S u v o r o v 2912, R T a r ta g lia 14, G T e s te r a 3, J T h u r n 28, M T o r o p o v a 12, C V e y s s ie r e 4, E U n z h a k o v 17, R B V o g e la a r 22, F v o n F e ilitz s c h 24, H W a n g 29, S W e in z 31, J W in t e r 31, M W o jc ik 8, M W u r m 31,
Z Y o k le y 22, O Z a im id o r o g a 2, S Z a v a ta r e lli3, K Z u b e r 28 a n d G Z u z e l8 B o r e x in o c o lla b o r a tio n
1A stro P articu le et Cosmologie, U niversite P aris D iderot, C N R S /IN 2P 3, C E A /IR F U , O bservatoire de P aris, Sorbonne P aris C ite, 75205 P aris Cedex 13, France
J o i n t In stitu te for N uclear Research, 141980 D ubna, R ussia
3D ip artim en to di Fisica, U niversita degli S tudi e INFN , Genova 16146, Italy
4C om m issariat a l ’E nergie A tom ique et aux Energies A lternatives, C entre de Saclay, IR FU , 91191 G if-sur-Y vette, France
5In s titu t fur E xperim entalphysik, U niversitat, 22761 H am burg, G erm any
6M ax -P lan ck -In stitu t fur K ernphysik, 69117 Heidelberg, G erm any
7C om m issariat a l ’E nergie A tom ique et aux Energies A lternatives, C entre de Saclay, D E N /D M 2 S /S E M T /B C C R , 91191 G if-sur-Y vette, France
8M. Smoluchowski In stitu te of Physics, Jagiellonian University, 30059 K rakow , Poland 9K iev In stitu te for N uclear Research, 06380 Kiev, U kraine
10In stitu te for T heoretical and E xperim ental Physics, 117218 Moscow, R ussia
11K epler C enter for A stro and P article Physics, U niversitat T ubingen, 72076 T ubingen, Germany.
12NRC K urchatov In stitu te , 123182 Moscow, R ussia
13N ational R esearch N uclear U niversity M E P hI (Moscow E ngineering Physics In stitu te ), K ashirskoe highway 31, Moscow, 115409, Russia.
14IN FN L ab o rato ri Nazionali del G ran Sasso, 67010 Assergi (AQ), Italy 15D ipartim ento di Fisica, U niversita degli S tudi e IN FN , 20133 M ilano, Italy 16D ipartim ento di C him ica, U niversita e INFN , 06123 P erugia, Italy
17St. P etersb u rg N uclear Physics In stitu te N RC K urchatov In stitu te , 188350 G atchina, R ussia 18Physics D ep artm en t, P rin c eto n University, P rinceton, N J 08544, USA
19C hem ical Engineering D ep artm en t, P rin c eto n University, P rinceton, N J 08544, USA 20Physics D ep artm en t, Q ueen’s University, K ingston ON K7L 3N6, C an ad a
21A m herst C enter for F un d am en tal Interactio n s and Physics D ep artm en t, U niversity of M assachusetts, A m herst, MA 01003, USA
22Physics D ep artm en t, V irginia Polytechnic In s titu te and S ta te University, B lacksburg, VA 24061, USA
23D ipartim ento di Fisica e Scienze della T erra U niversita degli S tudi di F erra ra e INFN , Via S aragat 1-44122, F errara, Ita l
24P h ysik-D epartm ent and Excellence C luster Universe, Technische U niversitat M unchen, 85748 G arching, Germany.
25 Lomonosov Moscow S tate U niversity Skobeltsyn In s titu te of N uclear Physics, 119234 Moscow, Russia.
26 G ra n Sasso Science In stitu te (IN FN ), 67100 L ’A quila, Italy.
27D ep a rtm en t of Physics, U niversity of H ouston, H ouston, T X 77204, USA.
28D ep a rtm en t of Physics, Technische U niversitat D resden, 01062 D resden, Germany.
29Physics and A stronom y D ep a rtm en t, U niversity of California Los Angeles (UCLA), Los Angeles, California 90095, USA.
30D ep a rtm en t of Physics and A stronom y, U niversity of Hawaii, H onolulu, HI 96822, USA.
31 In stitu te of Physics and Excellence C luster PR ISM A , Johannes Gutenberg-Universitaat M ainz, 55099 M ainz, Germany.
E-m ail: m a tth ie u .v i v ie r @ c e a .f r
A b s t r a c t . T he Borexino detecto r has convincingly show n its o u tstan d in g perform ances in th e low energy regim e th ro u g h its accom plishm ents in th e observation and stu d y of th e solar and geo neutrinos. It is th e n an ideal tool to perform a s ta te of th e a rt source-based experim ent for testin g th e longstanding hypothesis of a fo u rth sterile neu trin o w ith ~ eV 2 mass, as suggested by several anom alies accum ulated over th e p a st th ree decades in source-, reactor-, and accelerator- based experim ents. T he SOX p roject aim s a t successively deploying two intense radioactive sources, m ade of C erium (antineutrino) and C hrom ium (neutrino), respectively, in a dedicated p it located b en e a th th e detector. T he existence of such an ~ eV 2 sterile n eutrino would th e n show up as an unam biguous sp a tia l and energy d isto rtio n in th e count ra te of neutrinos interacting w ithin th e active d etec to r volume. T his article rep o rts on th e la test developm ents ab o u t th e first phase of th e SOX experim ent, nam ely CeSOX, and gives a realistic p rojection of CeSOX sensitivity to light sterile neutrinos in a sim ple (3+1) model.
1. I n tr o d u c tio n
T he sta n d a rd th ree-n eu trin o oscillation paradigm , associated w ith small squared m ass splittings A m 2 ^ 0.1eV2, has been successfully built up over th e past decades w ith various techniques using solar, atm ospheric, long baseline and m edium baseline reacto r n eu trin o experim ents.
However, th is w ell-established p icture m ight suffer from anom alous results b o th rep o rted in t h e (z+ disapp earan ce a n d (v^ app earan ce channels [1]. Those anom alies could jo in tly be in terp reted as th e existence of at least an additional fo u rth sterile n eutrino, w ith mixing p aram eters A m j|ew » 0.1eV2 and sin2(20new) ~ 0.1.
T he goal of th e SOX experim ent [2] is to definitively address th e anom alous deficits reported in th e d etected neu trin o count rates a t short baselines in t h e (ve ^ v e d isapp earan ce channel by
th e G allium n e u trin o [3, 4, 5] and reacto r a n tin e u trin o experim ents [6], respectively. It consists in deploying an intense MeV n eu trin o source b e n e ath th e B orexino d etecto r, and searching for b o th m eter-scaled sp atial and energy oscillation p a tte rn s in th e d etected n eu trin o spectrum , pushing th e concept of n eu trin o ’’oscillom etry” a step fu rth e r com pared to any previous n eutrin o oscillation experim ents [7].
2. C eS O X : t h e first p h a s e o f t h e S O X e x p e r im e n t
T he first, phase of th e SOX experim ent will m ake use of an intense a n tin e u trin o generato r (CeAN G) deployed at th e B orexino d e te c to r a t th e end of 2016. As such, i7e will be d etected th ro u g h th e inverse b e ta decay (IBD) reaction, which has a 1.8 MeV energy threshold and is alm ost free of any backgrounds.
2.1. The Borexino detector
T he Borexino d e te c to r [8] is an u ltra low background n e u trin o d etector, initially designed for th e observation and stu d y of solar and geo neutrinos. It is located deep underground un d er a 3800 m eter w ater equivalent overburden a t th e L ab o rato ri N azionali del G ran Sasso (LNG S). As shown by figure 1, th e d e te c to r conceptual design relies on nested spherical concentric volumes, which p ro tect and gradu ally shield th e innerm ost detectio n volum e against rad io activ ity and cosmic ray particles. Going inward, th e first sub-volum e is contained in a stainless steel ta n k (R = 9 m) and is filled u p w ith u ltra p u re w ater acting b o th as a rad io activ ity shielding and a cosmic m uon veto. T he second sub-volum e is contained in a stainless steel sphere (R = 6.85 m ), equipped w ith 2214 P M T s viewing th e innerm ost cen tral part, of th e detecto r, and is filled up w ith non-scintillating and transparent, oil, also acting as a. shielding against, radioactivity.
Finally, th e th ird sub-volum e is th e inner detection volum e (R = 4.25 m) and is m ade of 300 tons of liquid scintillator contained in a. transparent, nylon vessel. T he d e te c to r energy and vertex recon stru ction perform ances are respectively 5% at 1 MeV and 15 cm.
F ig u r e 1. Schem atic of th e B orexino d etecto r. See text, for fu rth e r details.
2.2. CeAN G : the C erium a ntineutrino generator
A radioactive i7e source su itable for th e observation of short, baseline oscillations must. meet, several requirem ents. Am ong th e most, im portant, are a. 0 (1 0 0 kCi) activ ity to get. reasonable
statistics, an energy sp ectru m th a t extends well beyond th e IBD energy th resh old and a long half-life so th a t th e source can be easily produced and tra n sp o rte d up to th e d etecto r. T he last two requirem ents are opposing and n ecessitate to use a pair of b e ta decaying isotopes, th e fath er having a long half-life while th e d a u g h te r has a short half-life. Four of such radioactive isotopes have been identified in nuclear d atabases, nam ely: 42A r —42 K, 90Sr —90 Y, 106R u —106 R h and 144Ce —144 P r. H aving a half-life of 285 days and a b e ta end-point energy of 3 MeV, th e 144Ce —144 P r p air is th e m ost su itable for th e SOX science case. A detailed discussion ab o u t th e choice of th e best pair of radioactive isotopes for m aking a z/e gen erato r in a source-based experim ent can be found in [9].
3. S o u r c e r e la te d t e c h n ic a l c h a lle n g e s a n d r e q u ir e m e n ts
T he prod uction and tra n s p o rt of a 144Ce-based ue g en erato r up to LNGS in Ita ly is technically and ad m in istrativ ely challenging. This section sum m arizes th e m ost im p o rta n t technical m ilestones th a t have to be com pleted to successfully perform phase 1 of th e SOX experim ent.
3.1. C eA N G production
T he p ro d uctio n of th e CeANG can be realized by isolating and e x tra ctin g 144Ce from spent nuclear fuel (SN F). T he R ussian SNF reprocessing com pany ’’Federal S ta te U n itary E n terp rise M ayak P ro d u c tio n A ssociation” , hereafter PA M ayak, has been identified to be th e only facility able to deliver such a source w ith P B q scale activity. C erium sep aratio n and ex tractio n , to g eth er w ith final source packaging and certification, takes several m onths. T he first step of th e C erium e x tra ctio n is a sta n d a rd reprocessing of SNF, called th e P u r e x ® process, leading to e x tractio n of U ranium , P lu to n iu m and N ep tu niu m and form ation of a so-called a P u re x raffinate which consists of fission and corrosion p ro d u cts, as well as lanthanides and actinides. In a second step, th e P u re x raffinate is purified from Stro ntiu m , Cesium and chem ical elem ents after corrosion;
and a feed solution consisting of lanthan ides and actinides is form ed after a double oxalate p recip itatio n procedure. In a th ird step, Cerium fraction is sep arated from th e feed solution using a com plexing displacem ent chrom ato graphy m etho d (see [9] and references th erein ). T he final stage of th e source p ro d u ctio n consists in calcining, pressing and inserting into a stainless steel capsule th e C erium oxyde. T he final p ro d u ct will be m ade of a few kilogram s of C eO 2 containing a few ten s of gram s of 144Ce. T he source delivery will hap p en in D ecem ber 2016.
3.2. C eA N G shielding
T he identified 144Ce —144 P r b e ta decaying pair for m aking th e CeA NG also em its gam m a rays, th a t have to be shielded against b o th for biological p ro tection purposes and to avoid source- induced backgrounds in th e B orexino d etecto r. T he shielding m aterial and dim ensions are m ostly driven by th e 2.185 MeV g am m a ray em itted w ith 0.7% intensity. T ungsten heavy alloys offer th e best com prom ise am ong all th e high density m aterials. A cylindrical tu n g ste n heavy alloy shielding, w ith density larger th a n 18.0 g cm - 3 , 60 cm height and diam eter, has been designed and is cu rren tly being m an u factured at X iam en Honglu Inc., C hina. Such dim ensions ensure a 19-cm thick shielding and a > 1012 gam m a-ray a tte n u a tio n factor along any directions. T he corresponding rad iatio n dose a t co n tact for a 100 kCi (3.7 x 1015 Bq) source is less th a n 1 ^ S v /h , com pelling w ith th e safety regulations im posed by th e In te rn atio n a l Agency for A tom ic E nergy (IA EA ), French nuclear safety a u th o rity (ASN), as well as Ita lia n nuclear safety a u th o rity for radioactive m aterial tra n s p o rta tio n and handling. T he shielding will be delivered early 2016.
3.3. C eA N G transportation and deploym ent at L N G S
As for any radioactive m aterial tra n sp o rta tio n , a certified container has to be used to ship th e CeANG from PA M ayak up to LNGS. T he so-called T N -M T R cask, jo in tly developed by CEA
and th e AREVA com pany for th e tra n s p o rta tio n of spent nuclear fuel, has been identified and auth orized for th e tra n s p o rta tio n of th e CeANG. T he itin erary ro u te has been decided, w ith a tra in tra n s p o rta tio n solution from PA M ayak to S t P e rte rsb u rg harbor, a d edicated b o at tra n s p o rta tio n from S t P e rte rsb u rg to Le H avre and a final tru c k tra n s p o rta tio n solution from Le H avre up to LNGS. T he com plete tra n s p o rta tio n solution takes less th a n a m onth, and will lim it th e source activity loss below 5%. T he expected delivery d a te at LNGS is end of Decem ber 2016.
3.Ą. C eA N G characterization
A full and precise CeANG ch aracterizatio n is a fu nd am en tal requirem ent to achieve th e best sensitivity over th e light sterile neu trin o m ixing p a ra m ete r space preferred by th e short baseline anom alies. T his section only deals w ith th e CeANG activ ity m easurem ent and th e shape of th e b e ta /n e u trin o spectrum . For th e interested reader, reference [9] discusses o th er source specifications, such as source radioactive im pu rity content, which is especially im p o rta n t to control to avoid source-induced backgrounds in th e SOX experim ent.
S .Ą .l. A ctivity m easurem ent A significant gain in th e SOX sensitivity can be achieved if th e source activity is known w ithin O(1% ) precision (see section 4 for m ore details). Two calorim etric devices are cu rren tly being bu ilt and calib rated a t CEA -Saclay and T U M /G enova, respectively, for m easuring th e heat power released by th e CeANG. A lthough slightly differently designed, those two calorim eters work on th e sam e principle, which consists in m easuring th e te m p e ra tu re in and o ut of a w ater loop circulating aro un d th e source. T he te m p e ra tu re difference is related to th e h eat power released by th e source, and hence on its activ ity th ro u g h a pow er-to-activity conversion factor P = 215.6 ± 1.3 W /P B q (itself d epending on th e shape of th e 144Ce and 144P r b e ta sp ectra, see next subsection). E ach calorim eter device has been carefully designed to m inim ize leaks, by using supsension platform s + insulations m aterials to lim it conductive losses, a vaccum vessel to prevent any convection losses and a m ultilayer insulation + vaccum vessel th erm alizatio n system to avoid h eat rad iatio n . C h aracterizatio n and calib ratio n of these setups are cu rren tly on-going, using electrical sources and an A lum inium m ock-up shielding.
3.4.2. B eta spectrum m easurem ent T he precision of th e source activity m easurem ent and th e expected IB D in teractio n ra te w ithin th e B orexino d e te c to r strongly depends on th e shape of th e CeA NG b e ta spectru m . 144Ce and 144P r b e ta sp e ctra b o th present non-unique forbidden tran sitio n s, for which th e spectral shape th eoretical predictions are u n certain a t th e few % level. Furtherm ore, past b e ta sp ectru m m easurem ents show discrepancies a t th e 10-15% level, which tra n sla te s into a 10% u n certain ty on th e predicted IBD ra te and would th e n spoil th e calorim etric m easurem ent. New m easurem ents of th e 144Ce and 144P r b e ta sp ectru m shapes are th u s necessary. Two setups, which use different techniques, have been developed for th e 144Ce and 144P r b e ta sp ectra m easurem ent. A first setup uses a cylindrical plastic scintillator coupled to two high q u a n tu m efficiency photo -m ultip lier tu b es, w hereas a second setu p uses a m ultiw ire cham ber in coincidence w ith a p lastic scin tillator to m easure b e ta rays and suppress gam m a-ray background. T hese setups are cu rrently running and been calib rated at CEA-Saclay.
A 5% u n certain ty on th e shape of th e 144Ce and 144P r b e ta sp e ctra is expected, im proving th e prediction of th e IBD ra te w ithin th e desired specifications.
4. P r o j e c t e d s e n s it iv it y o f C e S O X
T he p ro jected sensitivity of th e CeSOX experim ent to sterile neutrinos in th e fram ew ork of a (3+ 1) m odel is shown on figure 2, in light of th e two source-related system atic uncertainties discussed in th e previous section. T he baseline scenario for th e sensitivity calculations is a 100
kCi CeANG g en erator placed 8.5 m away from th e B orexino d e te c to r center, assum ing a 1.5 year d a ta tak in g tim e. In th is scenario, a wide range of th e sterile neu trin o p a ra m ete r space prefered by t h e (- i ^ (- i disap pearan ce channel anom alies is covered a t 95% C.L. F igure 2 also illustrates th e gain in sensitivity by using th e ra te inform ation brought by an a c tiv ity m e a su re m e n t (solid curves on th e left panel of figure 2) addition ally to th e s h a p e inform ation tdashed curve on th e left panel of figure 2). T h e right panel of figure 2 shows th e influence of th e 144P r b e ta spectrum shape uncertainty, and confirms th a t a 5% precision for th e b e ta sp ectru m m easurem ent can be targ e te d w ith o ut significantly degrading th e sensitivity contours.
F ig u r e 2. Sensitivity at th e 95% C.L. of th e CeSOX experim ent to light sterile neutrinos in a sim ple (3+1) model. (Left) Im p act of th e source activ ity u n c e rtain ty on th e ra te + sh a p e contours (solid lines) w ith respect to th e shape-only contour (dashed line). (R ight) Im p act of th e 144P r sp ectru m shape u n certain ty on th e ra te + s h a p e sensitivity contours.
5. C o n c lu s io n s
T he SOX experim ent, which aim s at te stin g th e neu trin o count ra te anom alous deficits observed in sho rt baseline experim ents in th e (- i ^ (- i d isappearance channel, will soon s ta rt th e first phase of its science program in D ecem ber 2016. A P B q scale activity ve source, m ade of 144Ce, will be deployed u n d e rn e a th th e B orexino d e te c to r . In th is article, th e m ost im p o rta n t m ilestones of th e CeSOX p ro ject have been reviewed, w ith a special em phasis on th e technical challenges th a t have to be addressed for th e source p ro d uctio n and tra n s p o rta tio n . Last b u t no least, a full ch aracterizatio n of th e CeANG is sim ultaneously on-going in th e SOX collaboration, which aims a t m inim izing th e source-related system atic u ncertainties. In light of these studies, it has been shown th a t th e p rojected CeSOX sensitivity covers m ost of th e light sterile n eu trin o p aram eter space preferred by th e short baseline anom alies.
R e fe r e n c e s
[1] A bazajian K et al 2012 Light sterile neutrinos: a w hite paper, P reprint hep-ph/12045379 [2] Bellini G et al 2013 JH E P 08 038
[3] G iunti C and Laveder M 2007 Mod. Phys. Lett. A 22 2499 [4] G iunti C and Laveder M 2011 Phys. R ev. C 83 065504
[5] G iunti C, Laveder M, Li Y, Liu Q and Long H 2012 Phys. Rev. D 86 113014 [6] M ention G et al 2011 Phys. Rev. D 83 073006
[7] C ribier M et al 2011 Phys. Rev. Lett. 10 7 201801
[8] A lim onti G et al 2009 Nuc. Instr. Meth. Phys. Res. A 60 0 568-93 [9] Gaffiot J et al 2015 Phys. Rev. D 91 072005
