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A high intensity 6He beam for the β-beam neutrino oscillation facility

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2012 EPL 98 32001

(http://iopscience.iop.org/0295-5075/98/3/32001)

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May 2012

EPL, 98 (2012) 32001 www.epljournal.org

doi: 10.1209/0295-5075/98/32001

A high intensity 6He beam for the β-beam neutrino oscillationfacility

Thierry Stora1(a), Etam Noah

1(b), Rastislav Hodak

1,5, Tsviki Y. Hirsh

2,3, Michael Hass

2,

Vivek Kumar2,6, Kuljeet Singh

2, Sergey Vaintraub

2,3, Pierre Delahaye

4,

Hanna Franberg-Delahaye4, Marie-Genevieve Saint-Laurent

4 and Gerard Lhersonneau4

1 CERN - CH-1211 Geneva 23, Switzerland2Department of Particle and Astrophysics, Weizmann Institute of Science - Rehovot, Israel3 Soreq Nuclear Research Center - Yavne 81800, Israel4Grand Accelerateur National d’Ions Lourds (GANIL), CEA/DSM-CNRS/IN2P3BP 55027, F-14076 Caen Cedex 5, France, EU5Department of Nuclear Physics and Biophysics, Comenius University - SK-842 48 Bratislava, Slovakia6 Centre for Medical Physics, UIEAST, Panjab University - Chandigarh 160014, India

received 9 January 2012; accepted in final form 4 April 2012published online 11 May 2012

PACS 29.38.Gj – Reaccelerated radioactive beamsPACS 14.60.St – Non-standard-model neutrinos, right-handed neutrinos, etc.PACS 25.40.-h – Nucleon-induced reactions

Abstract – This work presents the production and extraction of the short-lived radionuclide 6Hein yet unmatched yields from the ISOLDE facility at CERN. It is the first report of 6He productionusing spallation neutrons via the 9Be(n, α)6He reaction. These neutrons are produced from the1.4GeV proton beam of the Proton Synchrotron Booster (PSB) striking a tungsten converter, andare impinging on a porous BeO material. The central position of 6He in future experiments is due toits role as a necessary radioactive nucleus to realize the β-beam at CERN, a next-generation facilityto study neutrino oscillation parameters, and hence neutrino masses. In the β-beam scenario, anintense beam of radioactive 6He nuclei will be produced, accelerated to multi-GeV energies andstored in a dedicated storage ring. The resulting virtually mono-directional anti-neutrino beamfrom the decay of the stored 6He nuclei will be directed towards a remote underground neutrinodetector. A similar beam of, e.g., 18Ne will provide neutrinos, an ideal concept to test CP violationin the neutrino sector. The results of the present experiment demonstrate for the first time thatthe necessary conditions for the realization of the proposed β-beam scheme with anti-neutrinoscan be fulfilled.

editor’s choice Copyright c© EPLA, 2012

Introduction. – The results of solar, atmospheric,accelerator and reactor neutrino experiments [1] accumu-lated over the last decades demonstrate unequivocally thephenomenon of neutrino flavor oscillations and, hence, thenon-vanishing of neutrino masses. As such, it is definitively“physics beyond the Standard Model”. This developmenthas triggered vast interest on new physics phenomena thatmay be explored in the leptonic sector, culminating inseveral scenarios of large facilities that will provide thenecessary additional information on the neutrino mixing

(a)E-mail: thierry.stora@cern.ch(b)Present address: University of Geneva, Section de Physique,DPNC - CH-1211 Geneva 4, Switzerland.

angles, shedding unique light on the issue of CP viola-tion and other related subjects. The T2K collaboration,one of the present long baseline neutrino oscillation facil-ities, has recently reported new bounds on the θ13 mixingangle, with important implications for the design para-meters of the main three next-generation facilities [2].Neutrinos are respectively produced from µ decays in the“neutrino factory”, from π decays in the “superbeams”(high intensity beams) and from ultra-relativistic radioac-tive ion decays in the “β-beam” [3]. Briefly, the β-beamconcept consists of β-decaying radioactive nuclei at ultra-relativistic energy with a γ factor beyond one hundred,that are stored in a decay ring, thus providing a mono-source neutrino beam, collimated within a narrow cone

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Thierry Stora et al.

by the relativistic boost and directed towards a “far-side”neutrino detector. The original proposal [4], taking advan-tage of the existing facilities at CERN, Geneva (the ProtonSynchrotron (PS) and the Super Proton Synchrotron(SPS) accelerators) was based on 6He and 18Ne as elec-tron anti-neutrino and neutrino sources, respectively. Sucha facility provides pure sources of anti-neutrinos in thecase of β− decays or, in the occurrence of β+ decays,of neutrinos at GeV energies. This is therefore an idealtool to increase the sensitivity for CP-violation studiesin the neutrino sector which offers unique advantagesover other possibilities. This paper presents results froma test experiment at ISOLDE, CERN for efficient produc-tion and extraction of 6He in yet unmatched yields, thusproviding a firm experimental support for the feasibil-ity of the β-beam facility. It provides the first report of6He release efficiency, a parameter required to computethe precise production rates of anti-neutrino. This factor,which cannot be obtained by other methods such as abinitio or Monte Carlo computer simulations, was thereforea critical missing information to decide upon the technicalfeasiblity of the β-beam.

Experimental set-up. – The production of radioac-tive ion beams at ISOLDE, CERN follows and extends theoriginal scheme of the ISOL (Isotope Separation On-Line)method, introduced sixty years ago at the Niels BohrInstitute, and now applied in numerous other radioactivebeam facilities [5]. A primary particle beam is interceptedby a target of thickness from one to several interactionlengths. The nuclear products are thermalized beforethey diffuse towards an ion source to form a secondaryradioactive ion beam. A subsequent separation step alongpredefined mass-over-charge ratios is provided by a massspectrometer. In most cases, the ISOL method is foundto achieve higher intensities and better beam quality thanthe complementary method of in-flight fragmentation.These features are particularly relevant for β-beam pro-duction, where the required rates extend by severalorders of magnitude the intensities presently deliveredto the nuclear physics and astrophysics communityin operating facilities [6]. In order to deliver 2.9 · 1018anti-neutrinos/year, the β-beam production target mustextract and deliver to the ion source several 1013 6Heatoms per second. In order to achieve this yet unprece-dented rate a highly efficient production scheme mustfirst be identified, in conjunction also with the choiceof a suitable target material and appropriate geometryfor fast extraction. 6He, an anti-neutrino emitter wasselected because of its appropriate half-life of 807ms andbecause its chemical properties are well suited for theISOL methodology [7]. In addition, 6He production via9Be(n, α)6He reaction, benefits from high cross-sectionover a wide neutron energy spectrum [8].

Production and extraction experiment. A criticalelement in the production scheme was the identificationand acquisition of an appropriate BeO ISOL target

Fig. 1: (Colour on-line) Scanning electron microscopy pictureof the porous, small-grained BeO target material after 24h at1450 ◦C under vacuum, scale: 10µm. Picture of a standardneutron spallation source and target oven used at ISOLDE,CERN.

material. While inorganic fibers arranged into feltshave been proposed [9], we have directed our choiceto sintered pellets of porous, micrometric BeO of 71%nominal density (Brush Ceramic Products Inc., Tucson,USA), following our recent findings which confirm thatthis class of materials meets the requirements for ISOLproduction [10]. The material was found to remain stableat temperatures up to 1400 ◦C in contact with rhenium,whereas higher temperatures and contacts with someother refractory metals had resulted into fast ageingtogether with unwanted chemical reactions [11]. A thinrhenium foil was therefore inserted into the standardtantalum target oven used at ISOLDE in order to insurechemical compatibility between the target and the oven.The entire irradiaton setup is based on the standardISOLDE target and spallation source configuration, asshown in fig. 1 [12]. It consists of a tungsten neutronconverter located next to a cylindrical target oven inwhich the BeO pellets are inserted. This is a simplifiedversion of an optimized geometry developed for theβ-beam in which the target surrounds the converter,covering a wider solid angle for better interception of theemitted neutrons [13].The ISOLDE facility receives pulsed 1.4GeV protons

from the PSB accelerator. During the experimental beamtime, the 6He beam intensity produced from spallationneutrons was assessed at the ISOLDE tape station, wherethe quality and intensity of the delivered beams of thefacility is being normally monitored, with particular focuson the evaluation of the production and extraction of6He atoms from the hot target. This was achieved usinga water-cooled transfer line between the target and acompact electron beam impact ion source —Versatile ArcDischarge Ion Source (VADIS)— suited for the productionof singly charged noble gas ions, in a so-called VD7configuration [14]. The efficiencies required to compute theatom production from a given measured beam intensity

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Record yield of 6He from a porous BeO target at ISOLDE and future possibilities

Table 1: Decay data of the activation products used in the measurements.

Reaction Abundance (%) Half-life (h) Range (MeV) γ-ray intensity E (keV) (BR (%))115In(n,n′)115In 95.70 4.49 0.5–12 336(45.9)27Al(n, α)24Na 100.00 15.0 8–20 1368(100), 2754(100)58Ni(n, 2n)57Ni 68.27 36.0 16–34 1378(82), 1920(12)209Bi(n, 4n)206Bi 100.00 149 26–38 516(41), 803(99), 881(66), 1718(32)209Bi(n, 6n)204Bi 100.00 11.2 48–70 374(82), 899(98), 984(59)209Bi(n, 7n)203Bi 100.00 11.8 57–87 820(30), 825(15)209Bi(n, 8n)202Bi 100.00 1.72 70–100 422(84), 657(61), 960(99)

were determined with stable tracers; this is classically doneby injecting known fluxes of stable He in the target unitthrough a calibrated leak and recording the ion currentto determine the ion source efficiency; the transmission ofthe beam from the ion source to the tape station throughthe mass spectrometer was computed from the ratio ofthe sum of all separated ion beam currents over the totalbeam current from the ion source. For the β-beam, theabove efficiencies will be somewhat different, since therequired ion sources, transfer lines and mass spectrometerare specifically developed to meet the needs of the facility.This aspect will be further addressed in the last part ofthis paper.

Estimation of the experimental in-target 6He produc-tion using the Activation Foils technique. The 6Herelease efficiency may be determined by simply compar-ing the extracted 6He beam experimental yields with thecomputed in-target 6He production. The efficiencies recon-structed from the isotope experimental release profiles asshown in the following sections, can in turn be confrontedwith this reference figure. The calculation of the in-target6He production inevitably requires a detailed knowledge ofthe spallation neutron spectrum generated by the CERN1.4GeV protons impinging on the ISOLDE converter.The ISOLDE spallation neutron source is characterizedby a wide energy range between 0.1 and 1400MeV, apeak located in the MeV region and a tail that extendstowards high energies. Considering the 9Be(n, α)6He reac-tion, only the neutrons in the 1–15MeV range directlycontribute. However, (n, xn) reactions inside the targetand surrounding materials induced by the higher-energyneutrons may also have a significant contribution throughthe generation of secondary neutrons. Therefore, a directmeasurement of the spallation neutron spectrum in theentire energy range was implemented by the “ActivationFoils” technique which is widely used in neutron spec-tra measurements. In this technique, the activation foilsare selected so that the activation cross-sections coverevenly the energy range under consideration. Evaluationof the produced activities from each reaction channelgive an indication on the intensity and composition ofthe neutron spectrum for the different energy intervals.The full irradiation spectrum is then reconstructed onthe basis of activity measurements and the knowledgeof theoretical energy contributions from each reaction

0 20 40 60 80 100

En [MeV ]

0.0

0.5

1.0

1.5

2.0

σ[m

b]

×103

115In(n, n )115mIn27Al(n, α)24Na58Ni(n, 2n)57Ni209Bi(n, 4n)206Bi209Bi(n, 6n)204Bi209Bi(n, 7n)203Bi209Bi(n, 8n)202Bi

Fig. 2: (Colour on-line) Cross-sections of the fast neutron-induced reactions.

channel. Four metallic foils of aluminum, nickel, indiumand bismuth were irradiated at three different positionswith respect to the ISOLDE neutron converter. Two foilstacks were positioned at 90◦ with respect to the converteraxis at distances of 8 and 18 cm, respectively. A third foilstack was placed at 45◦ vertically in the forward direc-tion, at 12 cm from the center. The materials were chosenfor reasons of pure isotopic abundance (apart from Ni),a wide coverage of energies by several (n, xn) or (n,X)reactions and most importantly, production of long-livedgamma emitter isotopes. The reaction channels used inthis analysis are listed in table 1 and the correspondingcross-sections are shown in fig. 2.

Results and discussions. –

Neutron spectrum unfolding. The neutron spectrum isunfolded on the basis of activity measurements by solvingan integral equation of the form

Ai ∝∫σi (E) ·φ (E) dE, (1)

where Ai is the activity measurement of isotope i,σi is the cross-section and φ is the unknown neutronspectrum. The integral equation is written in a matrixform Ai =

∑j σij ·φj and for known cross-sections, the

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Thierry Stora et al.

corresponding φ can be determined by minimizing theχ2 function

χ2 =∑i

(Ami −

∑σij ·φgj

)2(ρmi )

2 , (2)

where φg is an initial guess of the neutron spectrum andρm stands for the standard deviation error of the measuredactivities. This approach converges to a unique solution incases in which the number of activity measurements (7 inthe present case) is larger than the number of the unknownparameters —the number of energy points needed to definethe neutron spectrum. More complex unfolding methodsuse non-linear corrections for the guessed spectra togetherwith an iterative procedure [15]. This type of problems isusually addressed by introducing additional data pointsfor the neutron spectrum from Monte Carlo simulations,with the assumption that the unfolded and simulativespectra do not differ much. An additional term is addedto the χ2 function

χ2 =∑i

(Ami −

∑σij ·φgj

)2(ρmi )

2 +∑j

(φsj −φgj

)2(ρφ)

2 , (3)

where φs is the neutron spectrum from simulations whileρφ is a weight that determines the level of expecteddeviations from the simulative spectrum and the χ2

minimization is performed by standard techniques. Thisiterative technique has been applied using initial “guessspectra” obtained by FLUKA [16] and GEANT4 [17]simulations.Using this method, the resultant spectrum converges to

a unique solution, regardless of the initial guess spectra,as shown in fig. 3. The unfolding procedure converges andprovides estimated activity values close to the measuredones, demonstrated by a low χ2. A peak located around60MeV, probably due to (p, xn) reactions, which waspredicted by FLUKA, is more pronounced after unfolding.Nevertheless, the overall shape of the spectrum was notdrastically modified. Note that the neutron spectra arepresented by isolethargic plots, with the neutron fluxmultiplied by the energy value in each bin, allowing bettervisualization of the detailed spectrum structure.

Release properties. The extraction efficiency fromthe hot BeO target was measured in the temperaturerange from 700 to 1400 ◦C by monitoring the shape ofthe beam intensity after the proton beam impact on theneutron converter, which we define as a release curve. Thephysico-chemical processes inherent to the release of 6Hewere analyzed as well. The residence time in the compactVADIS ion source is short compared to the measured timeconstants, so that the time constants obtained from thetheoretical model can be attributed solely to the targetmaterial and the containment volume. A representative setof data providing the release of 6He at 1400 ◦C is shown infig. 4. The curve is deduced from a sample of the activitycollected after different delay times t following the proton

10−1 100 101 102 103

E [MeV ]

0

1

2

3

4

φ(E

)·E[n

/p/c

m2]

×10−3

FLUKA

Unfolded

10−1 100 101 102 103

E [MeV ]

0

1

2

3

4

φ(E

)·E[ n

/p/c

m2]

×10−3

GEANT4Unfolded

Fig. 3: (Colour on-line) Unfolded spectra starting from aninitial guess of the FLUKA and GEANT simulations, respec-tively. Note that the apparently prominent peak at 70MeV isan artifact of the logarithmic scale of the y-axis in units of φE.

beam impact by beta decay at the ISOLDE tape station,corrected for the transport and ion source efficiencies, asexplained in [18]. 6He is released from the productionunit, following a first rising part and a subsequent decayafter the short proton beam impact. The characteristicshape of the release curve originates from the diffusionand effusion processes in the production unit, from theporous BeO matrix up to the ion source. Diffusion refersto transport phenomena of the isotopes in the BeO solidmatrix and effusion refers to processes associated withthe transport of the atoms in the gas phase. The timeconstants related to diffusion and effusion phenomena arededuced from a theoretical fit of the data. Here a refinedmodel is elaborated from the simple model proposedin [19] and accounts for the presence in the ISOLDEproduction unit of a transfer line connecting the targetand the ion source, as described in [20]. In short, themodel introduces an additional time constant to describethe effusion phenomena. The theoretical function takes theform of a convolution product

p(t)∝ peff (t) pdiff (t), (4)

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Record yield of 6He from a porous BeO target at ISOLDE and future possibilities

Table 2: Parameters of the fit functions, release efficiencies and6He in-target production for different target temperatures.

Temperature teff1 teff2 tdiff Release Fitted 6He(◦C) (ms) (ms) (ms) efficiency in-target

(%) production(N0)

700 5.5 32 320 59 2.7 · 1010800 5.6 28 150 71 2.6 · 10101000 4.7 28 1600 51 4.1 · 10101130 3.3 27 190 79 3.1 · 10101400 1.8 24 270 82 2.9 · 1010

Fig. 4: (Colour on-line) Release curve obtained at 1400 ◦C; theblack squares are the datapoints of the time evolution of 6Heisotope/s released from the target unit in function of time aftertheir production at the proton beam impact, at t= 0; the redline is the theoretical fit to the data. The residuals of the fitfunction are shown in the insert.

where pdiff (t) is the time-dependent diffusion out ofa spherical BeO grain of 1µm, and takes the classicalform which follows from solving Fick’s diffusion equationwith a diffusion time constant tdiff , and peff (t)∝ (1−exp(−t/teff1)) exp(−t/teff2) was introduced in [20] inorder to account for effusion processes in an ISOLDEproduction unit. A fit function is shown in fig. 4. The6He ion yield (the 6He beam intensity normalised to 1p ·µA proton beam incident on the neutron converter)is deduced from the integral of the theoretical fits. Thefit could reproduce the experimental release curves fordifferent temperatures throughout the duration of thetests with a similar or better match than the exampledisplayed in fig. 4, using teff1, teff2, tdiff , and N0 thenumber of 6He atoms produced in the target as the fourfree fitting parameters. The parameters, together with theoverall release efficiencies, are displayed in table 2 and infig. 5 for different target temperatures.

Fig. 5: (Colour on-line) Empty squares: experimentallymeasured yields reported for different target temperatures. Fullcircles: released fraction determined from the release curves.

Discussion. Using unfolding results, the neutron spec-trum inside the BeO target can be modified respectively,and consequently used to correct the 6He production.According to calculations, the total 6He in-target produc-tion in the BeO target through the entire run was2.8 · 1010 6He/µC and 2.4 · 1010 6He/µC with FLUKAand GEANT4, respectively, and 2.7 · 1010 6He/µC afterconsidering the unfolding corrections. With the measuredexperimental release curves and deduced yields performedat different target temperatures over several days, 6Hein-target production ranged from 2.6 · 1010 to 4.1 · 10106He/µC of incident protons, well in agreement with thedata reported via the neutron spectrum unfolding. Morethan 80% of the produced isotopes were therefore releasedat a target temperature of 1400 ◦C. The present yieldsof 5.4 · 107 6He+/µC improve the figures ranging from4.0 · 105 to 4.7 · 107 6He+/µC documented at ISOLDE,the best previous figure corresponding to proton beamdirectly impacting onto uranium carbide targets. Theeffusion time constants agree with previous data collectedat ISOLDE on Ne isotope release properties with calciumoxide targets in the same target geometry [20]. Thediffusion time constants deduced from the fits have aweak temperature dependence, and taking a diffusionpath of 1µm, corresponds to a diffusion constant of 10−13

to 10−14m2 s−1, some orders of magnitude higher thanthe published figures [21]. This points towards ill-definedinitial conditions assumed in the fit function, assuminga homogeneous distribution of the produced 6He in theBeO grains. However, the 9Be(n, α)6He reaction providesthe 6He nuclei with considerable energy, thus enabling thenuclei to traverse a given grain and carry out a random-walk–like motion which terminates at sites randomlydistributed in the BeO material. It is beyond the scope ofthe present study to investigate in details the impact ofthe recoil energy distribution on the fitted diffusion timeconstants. It should be noted, though, that in the β-beam

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configuration, a similar neutron converter BeO targetlayout is envisaged, with a water-cooled converter to dissi-pate the deposited beam power of an incoming 200kWproton beam, and a larger target which is capable of inter-cepting a larger fraction of the emitted neutrons. As aninitial approach towards predicting release efficiency in theβ-beam configuration, we compute the release curveassuming the same diffusion and effusion parameters,scaled by the thick target volume. For a production unitoperated at 1400 ◦C this translates to a release efficiencyof 57%, a figure supporting current numbers presentlyused to compute anti-neutrino fluxes for the β-beamfacility [22].

Summary. – The present work was devoted to theexperimental realization of the 9Be(n, α)6He reaction withspallation neutrons as an anti-neutrino source for β-beamat CERN. The central role of 6He in numerous futureschemes is due to its suitability as an intense source ofneutrinos that can be subsequently used to study, e.g.,neutrino oscillations and neutrino masses [23]. This canbe achieved by using the proposed concept of a β-beamat CERN, where 6He will be produced in a radioactiveion beam facility, accelerated to GeV-per-nucleon energiesand stored in a storage ring. Our present experiment showsthat about 82% of the 6He has been diffused out of thetarget, which translates into more than 50% for the 6Herelease efficiency at the foreseen β-beam facility.

∗ ∗ ∗We would like to acknowledge the help of E. Piselli

for proton beam monitoring during the experiments.We acknowledge the financial support of the Euro-pean Commission under the FP6 Research Infrastruc-ture Action-Structuring the European Research AreaEURISOL DS Project Contract No. 515768 RIDS.

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