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Beta Beams for Neutrino Production
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Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 227–231 www.elsevier.com/locate/npbps
Beta Beams for Neutrino Production E. Wildner CERN, Geneva, Switzerland
Abstract To use accelerated beta active radioactive ions to produce collimated high energy neutrino beams was proposed by P. Zuchelli in 2002. Since then, several ideas related to how to design Beta Beam facilities have been studied. Design studies of an accelerator complex, based on CERN accelerators, have been supported by research programs within the European Commisison FP6 and FP7 frameworks (EURISOL Design Study and EUROnu). In these studies 6 He and 18 Ne are used as beta emitters to produce antineutrinos and neutrinos respectively. Alternative isotopes for neutrino beam production, 8 Li and 8 B, are investigated in the now ongoing study, EUROnu. Due to the higher reaction Q-value of these ions, the resulting neutrino energies are higher. The isotopes we need for neutrino beams have to be produced in large quantities using non conventional methods. Latest research on production of isotopes that are presently considered for Beta Beams will be discussed. The work achieved gives a good ground to propose the Beta Beam, which is based on known technology, for neutrino production. In this status review we concentrate on technical issues related to a possible Beta Beam facility using the CERN infrastructure. Keywords: Beta Beams, neutrino factory, isotope production, EURISOL, EUROnu, neutrino oscillations
1. Introduction The aim of Beta Beams is to produce (anti-)neutrino beams from the decay of beta active ions circulating in a storage ring [1]. Beta Beams produce pure νe or ν¯ e beams depending on if the accelerated ion is a β+ or a β− emitter. The neutrino energy depends on the Qvalue of the beta decay and of the chosen relativistic γ boost of the stored ions. The neutrino spectrum is well known from the electron spectrum and the reaction energy value, Q, is in the order of 10 MeV for the isotopes investigated here. By providing intense and pure νe or ν¯ e beams, it would be possible to study the sub-leading νe → νμ and ν¯ e → ν¯ μ oscillation. The sensitivity limit of the Beta Beam depends on the number of ions that can be accelerated and of the energy of the stored ions in the Beta Beam decay ring. Energy and ion intensities have to be chosen according to the availability of a detector at a distance from the source suitable for the required 0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.037
physics reach. Sensitivity to the mass hierarchy depends on the distance the neutrino beam traverses the earth. Beta beams are technically optimal for accelerators with γ boosts less than 350. Beta Beams have a unique sensitivity to CPV due to the fact that the detector is not far and neutrino interaction with matter is limited. Considering the neutrino interaction cross-sections, and the beam divergence change with Q-values and with the γ-boost of the ions, we get a merit factor Q/γ. If we choose to increase the neutrino energy by increasing the Q-value of the chosen radioactive ions, the needed neutrino fluxes from the accelerators will increase approximately with Q. By choosing higher γ-boost, the beam divergence is smaller and the flux would be favorable; on the other hand the longer decay times require higher circulating currents in the decay ring or longer straight sections. The resulting length of the decay ring and the inclination angle with respect to ground influence cost and civil engineering. The baseline, the distance from
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the neutrino source to the detector, needs to be longer for higher neutrino energies to correspond to the first oscillation maximum, and we need a more inclined decay ring. For the two scenarii presently studied, the decay ring length is 2500 m and the inclination angle is 0.6◦ for a neutrino beam pointing at the Fr´ejus detector and 3◦ for a baseline from CERN to Gran Sasso or Canfranc.
the low-Q isotopes, for the same ion γ-boost. On the other hand, due to larger neutrino beam divergence from high-Q isotopes, the needed accelerated intensities need to be higher (increases approximately like the increase in Q-value). The high intensities impose important constraints on the impedances from the accelerator equipment seen by the accelerated beam, to avoid instabilities caused by collective effects.
2. Isotopes for Beta Beams Beta Beams are based on the acceleration of beta active isotopes. To achieve good efficiency of the acceleration scenario, and sufficient flux to get good physics reach, the isotopes have to be chosen carefully. The life time of the chosen isotopes should be such that we get enough decays at high relativistic γ, in the decay ring, but as few as possible at the beginning of the acceleration, where the relativistic γ is low, decays not useful and decay times shorter. The optimum life-time is given by the chosen acceleration scenario and is usually in the order of a second in the ion rest frame. The reaction Q-value influences the neutrino energy and the neutrino beam divergence. Extraction of the ions from the production target and their transport into the charge breeder (ion source) have to be fast to limit the decay losses. Isotopes generating hazardous waste products that cannot be safely handled either at the production phase or after decay would not be an acceptable choice. Isotope production giving a sufficient amount of radioactive ions for acceleration is one of the important challenges for Beta Beams. Noble gases are chemically stable and, for this reason, they are good candidates. The charge-to-mass ratio of the ions has to be large enough for efficient acceleration. Highly charged ions induce space charge phenomena that have unwanted effects on the beam properties. A specific accelerator can accelerate fully stripped ions up to Z/A times its maximum proton energy, where A is the total number of nucleons and Z is the atomic number of the ion. Two ion pairs with the required properties have been selected for studying a Beta Beam facility: 6 He/18 Ne and 8 Li/8 B for νe and ν¯ e production respectively. Nuclear physics has not so far shown considerable interest in these ions, therefore production methods have to be specifically developed for Beta Beams. 6 He and 18 Ne have Q-values of 3.5 MeV and 3.3 MeV, and decay times at rest of 0.8 s and 1.67 s respectively; in this context we refer to them as low-Q ions. The alternative high-Q ion pair, 8 Li and 8 B, has Q-values of 13.0 MeV and 13.9 MeV respectively. The high-Q ions give higher neutrino energy compared to
3. The EURISOL Scenario A Beta Beam facility, using the low-Q isotope pair He/18 Ne and a baseline from CERN to Fr´ejus in France has been studied within the EURISOL Design Study (2005-2009) [2]. The EURISOL scenario is based on CERN infrastructure and machines, and on existing technologies. This is of interest to keep the cost of the neutrino facility low. The ions are accelerated in an ion Linac after being collected in a charge breeding ECR source [3]. They then pass through a Rapid Cycling Synchrotron (RCS) [4], the CERN PS synchrotron and the last acceleration stage before the decay ring is the CERN SPS. The decay ring [5] would have a circumference of 6900 m and a straight section length of 2500 m. The main bending magnet field is 6 T. Consequently superconducting technology is necessary. The EURISOL scenario is shown in Figure 1. 6
Figure 1: The EURISOL Layout.
The CERN SPS allows a maximum γ-value of 150 (6 He) or 250 (18 Ne). The choice of energy, corresponding to a γ-value of 100, was made to optimize the physics reach at the chosen baseline 130 km from CERN: the proposed MEMPHIS detector in the Fr´ejus tunnel which is a Mton water Cherenkov detector. The required annual rates are 2.9·1018 anti-neutrinos from 6 He and 1.1·1018 neutrinos from 18 Ne. In 2009 experiments at the ISOLDE facility at CERN showed satisfactory results for the production of 6 He. 18 Ne was till recently not possible to produce in sufficient quantities.
E. Wildner / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 227–231
The ions start to decay from the moment of production and during acceleration. Only 50% of 6 He and 80% of 18 Ne eventually reach the decay ring. This is due to the present cycle time of the CERN accelerator complex. The advantage of using the CERN complex is that the infrastructure can be re-used. However the machines are not designed for Beta Beams and this means that some compromises need to be accepted. The result of studies of the radiation doses for maintenance is shown in Table 1. For doses to the population from the airborne activity released in the environment we get for the RCS 0.7, for the PS 0.6 and for the Decay Ring 5.6 μSv. The results are compatible with the CERN rules for radiation safety, and do not represent a show stopper, see [8]. The SPS has not yet been studied. The Decay Ring study does not include the collimation losses. hour day week
RCS, quad 15 3 2
PS, dipole 10 6 2
SPS × × ×
DR, arc 5.4 3.6 1.4
Table 1: Residual Ambient Dose Equivalent Rate at 1 m distance from the beam line (mSv h−1 ). Calculated for 6 He.
Irradiation damage of accelerator equipment requires development of extra shielding and handling methods. So far, only the impact of the radiation on the CERN PS magnet coils has be studied: 60 years of operation with a EURISOL-type beta beam seems possible [9]. The decay products have a different magnetic rigidity than the parent particles and are essentially impinging on the magnet mid-planes. This fact can be used for magnet protection, for example open mid-plane magnet design of the superconducting magnets. Feasibility studies on open midplane dipole and quadrupole magnets have been carried out based on energy deposition calculations, see [10], [11] and [12]. Injection of the beam into the decay ring in short bunches of 5·1012 ions, 5 ns long [13] necessitates momentum collimation of almost 50% of the ions, see Figure 2. The work on collimation is ongoing. Some 5·1012 6 He ions have to be collimated per cycle (6 s). The decay into 6 Li corresponds to 5·1012 ions per meter that has to be either dumped after the straight sections or taken care of inside the accelerator equipment like dipoles and quadrupoles in the arcs. The dipoles in the collimation sections would receive between 1 kW and 10 kW of power and shielding should be considered. The dump at the end of the Decay Ring straight sections would receive 30 kW of beam-power.
229
Figure 2: The distribution of losses in the decay ring.
Neutrino energies from ”low-Q” Beta Beams with a γ-boost of 100 are similar to the energies of the atmospheric neutrino background. To suppress this background, the ion bunches have to be very intense and very short: a small duty factor in the accelerator is needed. For this reason, the accelerating system in the decay ring would experience unprecedented heavy transient beam loading from the short and intense bunches. Since there is no net transfer to the beam, the problem might be resolved using a linear phase modulation in the absence of the beam, to reduce gap transients. A feasibility study of this proposal is presently ongoing [14]. A high-Q superconducting cavity would be a preferred choice. 4. The Beta Beam within EUROnu A more recent idea for Beta Beam ion choice and an innovative method to produce them has been made [15]. This proposal suggests using 8 Li and 8 B as antineutrino and neutrino emitters respectively. The higher Q-values of these ions would permit higher neutrino energies without increasing the energy of the accelerated ions. The higher neutrino energy would increase physics reach with respect to the 6 He and 18 Ne case, if we use the same accelerators, see Figure 3. In this case, the baseline would be around 700 km (CERN-Canfranc or CERN-LNGS). However, if we take into account the larger beam divergence and and increased cross-section the result is that we would need ∼ 5 times more ions in the decay ring. These high intensities are difficult to produce. The possible use of other detectors (Liquid Argon) for these energies would increase the needed flux by a factor 10 [16]. This proposal is being studied within EUROnu (European Framework Program FP7, design studies) [17] and the research on the low-Q option is continuing, in particular the production of 18 Ne, the low energy part of the complex and studies of the effects of the high intensities in the accelerators (collective effect instabilities) [18]. A circulating beam of 6 Li or 7 Li produces the Beta
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Figure 3: Physics reach for the Beta Beam: CPV. Beta Beams with higher γ compete well with a low energy neutrino factory. We also see that a combination of a Beta Beam and a super-beam to the shorter baseline Fr´ejus give good improvement. for the ”low-Q” ions.
Beam isotopes by repetitive traversals of a supersonic gas jet target, see Figure 4. The target serves as a stripper and will also be used for beam cooling. The 7 Li beam energy is 25 MeV and the energy loss over the target is 300 keV. A preliminary lattice design for the production ring is available [19], [20]. A first demonstration on the cooling efficiency is described in [21]. Preliminary studies of the supersonic gas jet target [15] with a jet velocity of 2200 m/s (volume 4.3 m3 /s) show that four orders of magnitude more gas flux than presently available would be needed. For this reason, the direct kinematics reaction with liquid or thin solid targets is investigated.
2010 show that the produced Li ions can be extracted from the catcher. Cross section and kinematics measurements of the reactions in the target are measured at INFN-LNL, Legnaro. Preliminary data for 8 Li production exist. Experiments to measure 8 B are planned for spring 2011. These measurement are necessary for the design of the collection device and for the internal target development. The 60 GHz ion source is developed by Laboratoire de Physique et de Cosmologie in Grenoble within the SEISM collaboration [3]. Magnetic tests of this source have been successful and tests with the gyrotron are planned for 2011. The RFQ and a stripper have to be designed for ions with charge state +1. Extensive studies of the physics reach with larger duty factors [22] show that the small duty factors cannot be relaxed beyond 1% for the present bunch structures and intensities, even if the neutrino energies are higher for the high-Q ions. Figure 5 shows the θ13 and the δcp sensitivity for different duty factors for intensities in the decay ring corresponding to production rates taken from [15]. With the existing merging system developed for the EURISOL Beta Beams, where we intrinsically lose half of the accelerated ions, [13], we may be able to relax somewhat the requirement for short bunches and/or inject more bunches to increase the flux.
Figure 5: The θ13 (left) and δcp (right) sensitivity for different duty factors. The 0.1 % duty factor (blue) gives similar sensitivity and with 1 % SF (red) the sensitivity has decreased slightly. These results are based on fluxes proposed in [15].
5. Summary of Isotope Production
Figure 4: A way to produce radioactive ions for Beta Beams using multiple traversals of a gas jet target, serving also as stripper and for ionization cooling of the beam.
Collection of the ions after production in the target is studied at CRC, Louvain-la-Neuve. Experiments during
The presently known production rates of Beta Beam ions is shown in Table 2. Very encouraging experimental results for 6 He have already been achieved [6]. During 2009, new proposals for production of 18 Ne [7] exist that will be experimentally verified at the ISOLDE facility at CERN, hopefully during 2011. Preliminary results indicate that production rates are compatible with the Beta Beam to Fr´ejus.
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Type
Accelerator
Beam
ISOL & n-converter ISOL & n-converter ISOL
SPL Saraf/GANIL Linac 4
p d p p p p 3 He p 7 Li/d 6 Li/3 He
ISOL
Cyclotron/Linac
ISOL
Linac
Production-Ring Production-Ring
Linac Linac
Ibeam mA 0.1 15 6
Ebeam MeV 2 · 103 40 160
Pbeam kW 200 600 700
10
70
700
> 170
21
3600
0.16 0.16
25 25
4 4
Target W/BeO C/BeO 19F (Molten salt loop) 19F (Molten salt loop) MgO (80 cm disk) d 3 He
Isotope 6
He He 18 Ne 6
Flux ions s−1 5 · 1013 5 · 1013 1 · 1013
18
Ne
2 · 1013
18
Ne
2 · 1013
8
Li B
no data no data
8
Table 2: Production rates, estimated (ions/second). In the case of He the stated rates are measured (projected to the needed target power).
6. Conclusion Studies within EUROnu show that the Beta Beam is a good option for neutrino production: the low-Q option with baseline from CERN to Fr´ejus seems now (2010) to be feasible with both 6 He and 18 Ne. Collective effects and RF are studied, to insure stable high intensity beams. The low energy part of the accelerators need consolidation once we know better what the ion source can deliver (charge state distribution and flux). The EUROnu high-Q Beta Beam is not yet an option. The newly proposed method for 18 Ne production, has to be experimentally verified. The acceleration of very high intensity beams in the CERN complex is challenging. Optimization of machines and beams for good physics is ongoing. Acknowledgments We acknowledge the financial support of the European Community Research Infrastructure Activity under the FP6 ”Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003506395) and the financial support of the European Community under the European Commission Framework Program 7 Design Study: EUROnu, Project Number 212372. The EC is not liable for any use that may be made of the information herein. References [1] P. Zucchelli, A novel concept for a anti-nu/e / nu/e neutrino factory: The beta beam, Phys. Lett. B532 (2002) 166–172. doi:10.1016/S0370-2693(02)01576-9. [2] Final report of the EURISOL design study (2005-2009), a Design study for a European Isotope-Separation-On-Line radioactive ion beam facility. [3] T. Thuillier, et al., High frequency ECR ion source (60-GHz) in preglow mode for bunching of beta-beam isotopes, PoS NUFACT08 (2008) 089.
[4] A. Lachaize, A. Tkatchenko, The Rapid Cycling Synchrotron of the Beta-Beam facility, EURISOL DS task note 12-25-20080012. [5] A. Chanc´e, Etude et conception de lanneau de dsintegration, PhD thesis, Universit´e Paris Sud. [6] T. Y. Hirsh, et al., High yield production of He-6 and Li-8 RIB for astrophysics and neutrino physics, PoS NUFACT08 (2008) 090. [7] T.Stora, Baseline ion source for beta-beam, Europeean Strategy for Future Neutrino Physics, http://indico.cern.ch/conferenceDisplay.py?confId=59378. [8] S. Trovati, M. Magistris, M. Silari, CERN-SC-2008-070-RPTN, CERN-DG-2009-051-RP-TN. [9] M. Kirk, 7th beta beam task meeting, CEA, Saclay. [10] J. Bruer, M.Sc. Thesis, Linkping University. [11] F. Borgnolutti, (private communication, documentation ongoing). [12] E. Y. Wildner, F. W. Jones, Simulation of Decays and Secondary Ion Losses in a Betabeam Decay RingParticle Accelerator Conference PAC07 25-29 Jun 2007, Albuquerque, New Mexico. [13] A. Chanc´e, S. Hancock, Stacking simulations in the beta-beam decay ringPrepared for European Particle Accelerator Conference (EPAC 06), Edinburgh, Scotland, 26-30 Jun 2006. [14] G. Burt, (private communication). [15] C. Rubbia, A. Ferrari, Y. Kadi, V. Vlachoudis, Beam cooling with ionisation losses, Nucl. Instrum. Meth. A568 (2006) 475– 487. arXiv:hep-ph/0602032, doi:10.1016/j.nima.2006.02.161. [16] M.Mezetto, (private communication). [17] EUROnu - A High Intensity Neutrino Oscillation Facility in Europe, FP7-INFRASTRUCTURES-2007-1. [18] C. Hansen, et al., Collective Effect Studies for a Beta Beam Decay Ring. To be Published in AIP Conf. for NUFACT10. [19] M. Schauman, Development and lattice design of an ion production ring for a beta-beam facility, BSc thesis, Technische Hochschule, Aachen. [20] J. Wehner, Monte Carlo Simulation of an ion production ring for a beta beam neutrino facility, BSc thesis, Technische Hochschule, Aachen. [21] E. Benedetto, Ionization Cooling in a Low-energy ion Ring with Internal Target for Beta-beams Production1st International Particle Accelerator Conference: IPAC’10, 23-28 May 2010, Kyoto, Japan. [22] C. Hansen, E. Wildner, E. Fernandez-Martinez, Limitations in the use of Barrier Buckets in the Beta Beam Decay Ring for the FP7 Scenario, PoS NUFACT09.
Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 227–231 www.elsevier.com/locate/npbps
Beta Beams for Neutrino Production E. Wildner CERN, Geneva, Switzerland
Abstract To use accelerated beta active radioactive ions to produce collimated high energy neutrino beams was proposed by P. Zuchelli in 2002. Since then, several ideas related to how to design Beta Beam facilities have been studied. Design studies of an accelerator complex, based on CERN accelerators, have been supported by research programs within the European Commisison FP6 and FP7 frameworks (EURISOL Design Study and EUROnu). In these studies 6 He and 18 Ne are used as beta emitters to produce antineutrinos and neutrinos respectively. Alternative isotopes for neutrino beam production, 8 Li and 8 B, are investigated in the now ongoing study, EUROnu. Due to the higher reaction Q-value of these ions, the resulting neutrino energies are higher. The isotopes we need for neutrino beams have to be produced in large quantities using non conventional methods. Latest research on production of isotopes that are presently considered for Beta Beams will be discussed. The work achieved gives a good ground to propose the Beta Beam, which is based on known technology, for neutrino production. In this status review we concentrate on technical issues related to a possible Beta Beam facility using the CERN infrastructure. Keywords: Beta Beams, neutrino factory, isotope production, EURISOL, EUROnu, neutrino oscillations
1. Introduction The aim of Beta Beams is to produce (anti-)neutrino beams from the decay of beta active ions circulating in a storage ring [1]. Beta Beams produce pure νe or ν¯ e beams depending on if the accelerated ion is a β+ or a β− emitter. The neutrino energy depends on the Qvalue of the beta decay and of the chosen relativistic γ boost of the stored ions. The neutrino spectrum is well known from the electron spectrum and the reaction energy value, Q, is in the order of 10 MeV for the isotopes investigated here. By providing intense and pure νe or ν¯ e beams, it would be possible to study the sub-leading νe → νμ and ν¯ e → ν¯ μ oscillation. The sensitivity limit of the Beta Beam depends on the number of ions that can be accelerated and of the energy of the stored ions in the Beta Beam decay ring. Energy and ion intensities have to be chosen according to the availability of a detector at a distance from the source suitable for the required 0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.09.037
physics reach. Sensitivity to the mass hierarchy depends on the distance the neutrino beam traverses the earth. Beta beams are technically optimal for accelerators with γ boosts less than 350. Beta Beams have a unique sensitivity to CPV due to the fact that the detector is not far and neutrino interaction with matter is limited. Considering the neutrino interaction cross-sections, and the beam divergence change with Q-values and with the γ-boost of the ions, we get a merit factor Q/γ. If we choose to increase the neutrino energy by increasing the Q-value of the chosen radioactive ions, the needed neutrino fluxes from the accelerators will increase approximately with Q. By choosing higher γ-boost, the beam divergence is smaller and the flux would be favorable; on the other hand the longer decay times require higher circulating currents in the decay ring or longer straight sections. The resulting length of the decay ring and the inclination angle with respect to ground influence cost and civil engineering. The baseline, the distance from
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E. Wildner / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 227–231
the neutrino source to the detector, needs to be longer for higher neutrino energies to correspond to the first oscillation maximum, and we need a more inclined decay ring. For the two scenarii presently studied, the decay ring length is 2500 m and the inclination angle is 0.6◦ for a neutrino beam pointing at the Fr´ejus detector and 3◦ for a baseline from CERN to Gran Sasso or Canfranc.
the low-Q isotopes, for the same ion γ-boost. On the other hand, due to larger neutrino beam divergence from high-Q isotopes, the needed accelerated intensities need to be higher (increases approximately like the increase in Q-value). The high intensities impose important constraints on the impedances from the accelerator equipment seen by the accelerated beam, to avoid instabilities caused by collective effects.
2. Isotopes for Beta Beams Beta Beams are based on the acceleration of beta active isotopes. To achieve good efficiency of the acceleration scenario, and sufficient flux to get good physics reach, the isotopes have to be chosen carefully. The life time of the chosen isotopes should be such that we get enough decays at high relativistic γ, in the decay ring, but as few as possible at the beginning of the acceleration, where the relativistic γ is low, decays not useful and decay times shorter. The optimum life-time is given by the chosen acceleration scenario and is usually in the order of a second in the ion rest frame. The reaction Q-value influences the neutrino energy and the neutrino beam divergence. Extraction of the ions from the production target and their transport into the charge breeder (ion source) have to be fast to limit the decay losses. Isotopes generating hazardous waste products that cannot be safely handled either at the production phase or after decay would not be an acceptable choice. Isotope production giving a sufficient amount of radioactive ions for acceleration is one of the important challenges for Beta Beams. Noble gases are chemically stable and, for this reason, they are good candidates. The charge-to-mass ratio of the ions has to be large enough for efficient acceleration. Highly charged ions induce space charge phenomena that have unwanted effects on the beam properties. A specific accelerator can accelerate fully stripped ions up to Z/A times its maximum proton energy, where A is the total number of nucleons and Z is the atomic number of the ion. Two ion pairs with the required properties have been selected for studying a Beta Beam facility: 6 He/18 Ne and 8 Li/8 B for νe and ν¯ e production respectively. Nuclear physics has not so far shown considerable interest in these ions, therefore production methods have to be specifically developed for Beta Beams. 6 He and 18 Ne have Q-values of 3.5 MeV and 3.3 MeV, and decay times at rest of 0.8 s and 1.67 s respectively; in this context we refer to them as low-Q ions. The alternative high-Q ion pair, 8 Li and 8 B, has Q-values of 13.0 MeV and 13.9 MeV respectively. The high-Q ions give higher neutrino energy compared to
3. The EURISOL Scenario A Beta Beam facility, using the low-Q isotope pair He/18 Ne and a baseline from CERN to Fr´ejus in France has been studied within the EURISOL Design Study (2005-2009) [2]. The EURISOL scenario is based on CERN infrastructure and machines, and on existing technologies. This is of interest to keep the cost of the neutrino facility low. The ions are accelerated in an ion Linac after being collected in a charge breeding ECR source [3]. They then pass through a Rapid Cycling Synchrotron (RCS) [4], the CERN PS synchrotron and the last acceleration stage before the decay ring is the CERN SPS. The decay ring [5] would have a circumference of 6900 m and a straight section length of 2500 m. The main bending magnet field is 6 T. Consequently superconducting technology is necessary. The EURISOL scenario is shown in Figure 1. 6
Figure 1: The EURISOL Layout.
The CERN SPS allows a maximum γ-value of 150 (6 He) or 250 (18 Ne). The choice of energy, corresponding to a γ-value of 100, was made to optimize the physics reach at the chosen baseline 130 km from CERN: the proposed MEMPHIS detector in the Fr´ejus tunnel which is a Mton water Cherenkov detector. The required annual rates are 2.9·1018 anti-neutrinos from 6 He and 1.1·1018 neutrinos from 18 Ne. In 2009 experiments at the ISOLDE facility at CERN showed satisfactory results for the production of 6 He. 18 Ne was till recently not possible to produce in sufficient quantities.
E. Wildner / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 227–231
The ions start to decay from the moment of production and during acceleration. Only 50% of 6 He and 80% of 18 Ne eventually reach the decay ring. This is due to the present cycle time of the CERN accelerator complex. The advantage of using the CERN complex is that the infrastructure can be re-used. However the machines are not designed for Beta Beams and this means that some compromises need to be accepted. The result of studies of the radiation doses for maintenance is shown in Table 1. For doses to the population from the airborne activity released in the environment we get for the RCS 0.7, for the PS 0.6 and for the Decay Ring 5.6 μSv. The results are compatible with the CERN rules for radiation safety, and do not represent a show stopper, see [8]. The SPS has not yet been studied. The Decay Ring study does not include the collimation losses. hour day week
RCS, quad 15 3 2
PS, dipole 10 6 2
SPS × × ×
DR, arc 5.4 3.6 1.4
Table 1: Residual Ambient Dose Equivalent Rate at 1 m distance from the beam line (mSv h−1 ). Calculated for 6 He.
Irradiation damage of accelerator equipment requires development of extra shielding and handling methods. So far, only the impact of the radiation on the CERN PS magnet coils has be studied: 60 years of operation with a EURISOL-type beta beam seems possible [9]. The decay products have a different magnetic rigidity than the parent particles and are essentially impinging on the magnet mid-planes. This fact can be used for magnet protection, for example open mid-plane magnet design of the superconducting magnets. Feasibility studies on open midplane dipole and quadrupole magnets have been carried out based on energy deposition calculations, see [10], [11] and [12]. Injection of the beam into the decay ring in short bunches of 5·1012 ions, 5 ns long [13] necessitates momentum collimation of almost 50% of the ions, see Figure 2. The work on collimation is ongoing. Some 5·1012 6 He ions have to be collimated per cycle (6 s). The decay into 6 Li corresponds to 5·1012 ions per meter that has to be either dumped after the straight sections or taken care of inside the accelerator equipment like dipoles and quadrupoles in the arcs. The dipoles in the collimation sections would receive between 1 kW and 10 kW of power and shielding should be considered. The dump at the end of the Decay Ring straight sections would receive 30 kW of beam-power.
229
Figure 2: The distribution of losses in the decay ring.
Neutrino energies from ”low-Q” Beta Beams with a γ-boost of 100 are similar to the energies of the atmospheric neutrino background. To suppress this background, the ion bunches have to be very intense and very short: a small duty factor in the accelerator is needed. For this reason, the accelerating system in the decay ring would experience unprecedented heavy transient beam loading from the short and intense bunches. Since there is no net transfer to the beam, the problem might be resolved using a linear phase modulation in the absence of the beam, to reduce gap transients. A feasibility study of this proposal is presently ongoing [14]. A high-Q superconducting cavity would be a preferred choice. 4. The Beta Beam within EUROnu A more recent idea for Beta Beam ion choice and an innovative method to produce them has been made [15]. This proposal suggests using 8 Li and 8 B as antineutrino and neutrino emitters respectively. The higher Q-values of these ions would permit higher neutrino energies without increasing the energy of the accelerated ions. The higher neutrino energy would increase physics reach with respect to the 6 He and 18 Ne case, if we use the same accelerators, see Figure 3. In this case, the baseline would be around 700 km (CERN-Canfranc or CERN-LNGS). However, if we take into account the larger beam divergence and and increased cross-section the result is that we would need ∼ 5 times more ions in the decay ring. These high intensities are difficult to produce. The possible use of other detectors (Liquid Argon) for these energies would increase the needed flux by a factor 10 [16]. This proposal is being studied within EUROnu (European Framework Program FP7, design studies) [17] and the research on the low-Q option is continuing, in particular the production of 18 Ne, the low energy part of the complex and studies of the effects of the high intensities in the accelerators (collective effect instabilities) [18]. A circulating beam of 6 Li or 7 Li produces the Beta
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E. Wildner / Nuclear Physics B (Proc. Suppl.) 229–232 (2012) 227–231
Figure 3: Physics reach for the Beta Beam: CPV. Beta Beams with higher γ compete well with a low energy neutrino factory. We also see that a combination of a Beta Beam and a super-beam to the shorter baseline Fr´ejus give good improvement. for the ”low-Q” ions.
Beam isotopes by repetitive traversals of a supersonic gas jet target, see Figure 4. The target serves as a stripper and will also be used for beam cooling. The 7 Li beam energy is 25 MeV and the energy loss over the target is 300 keV. A preliminary lattice design for the production ring is available [19], [20]. A first demonstration on the cooling efficiency is described in [21]. Preliminary studies of the supersonic gas jet target [15] with a jet velocity of 2200 m/s (volume 4.3 m3 /s) show that four orders of magnitude more gas flux than presently available would be needed. For this reason, the direct kinematics reaction with liquid or thin solid targets is investigated.
2010 show that the produced Li ions can be extracted from the catcher. Cross section and kinematics measurements of the reactions in the target are measured at INFN-LNL, Legnaro. Preliminary data for 8 Li production exist. Experiments to measure 8 B are planned for spring 2011. These measurement are necessary for the design of the collection device and for the internal target development. The 60 GHz ion source is developed by Laboratoire de Physique et de Cosmologie in Grenoble within the SEISM collaboration [3]. Magnetic tests of this source have been successful and tests with the gyrotron are planned for 2011. The RFQ and a stripper have to be designed for ions with charge state +1. Extensive studies of the physics reach with larger duty factors [22] show that the small duty factors cannot be relaxed beyond 1% for the present bunch structures and intensities, even if the neutrino energies are higher for the high-Q ions. Figure 5 shows the θ13 and the δcp sensitivity for different duty factors for intensities in the decay ring corresponding to production rates taken from [15]. With the existing merging system developed for the EURISOL Beta Beams, where we intrinsically lose half of the accelerated ions, [13], we may be able to relax somewhat the requirement for short bunches and/or inject more bunches to increase the flux.
Figure 5: The θ13 (left) and δcp (right) sensitivity for different duty factors. The 0.1 % duty factor (blue) gives similar sensitivity and with 1 % SF (red) the sensitivity has decreased slightly. These results are based on fluxes proposed in [15].
5. Summary of Isotope Production
Figure 4: A way to produce radioactive ions for Beta Beams using multiple traversals of a gas jet target, serving also as stripper and for ionization cooling of the beam.
Collection of the ions after production in the target is studied at CRC, Louvain-la-Neuve. Experiments during
The presently known production rates of Beta Beam ions is shown in Table 2. Very encouraging experimental results for 6 He have already been achieved [6]. During 2009, new proposals for production of 18 Ne [7] exist that will be experimentally verified at the ISOLDE facility at CERN, hopefully during 2011. Preliminary results indicate that production rates are compatible with the Beta Beam to Fr´ejus.
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Type
Accelerator
Beam
ISOL & n-converter ISOL & n-converter ISOL
SPL Saraf/GANIL Linac 4
p d p p p p 3 He p 7 Li/d 6 Li/3 He
ISOL
Cyclotron/Linac
ISOL
Linac
Production-Ring Production-Ring
Linac Linac
Ibeam mA 0.1 15 6
Ebeam MeV 2 · 103 40 160
Pbeam kW 200 600 700
10
70
700
> 170
21
3600
0.16 0.16
25 25
4 4
Target W/BeO C/BeO 19F (Molten salt loop) 19F (Molten salt loop) MgO (80 cm disk) d 3 He
Isotope 6
He He 18 Ne 6
Flux ions s−1 5 · 1013 5 · 1013 1 · 1013
18
Ne
2 · 1013
18
Ne
2 · 1013
8
Li B
no data no data
8
Table 2: Production rates, estimated (ions/second). In the case of He the stated rates are measured (projected to the needed target power).
6. Conclusion Studies within EUROnu show that the Beta Beam is a good option for neutrino production: the low-Q option with baseline from CERN to Fr´ejus seems now (2010) to be feasible with both 6 He and 18 Ne. Collective effects and RF are studied, to insure stable high intensity beams. The low energy part of the accelerators need consolidation once we know better what the ion source can deliver (charge state distribution and flux). The EUROnu high-Q Beta Beam is not yet an option. The newly proposed method for 18 Ne production, has to be experimentally verified. The acceleration of very high intensity beams in the CERN complex is challenging. Optimization of machines and beams for good physics is ongoing. Acknowledgments We acknowledge the financial support of the European Community Research Infrastructure Activity under the FP6 ”Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003506395) and the financial support of the European Community under the European Commission Framework Program 7 Design Study: EUROnu, Project Number 212372. The EC is not liable for any use that may be made of the information herein. References [1] P. Zucchelli, A novel concept for a anti-nu/e / nu/e neutrino factory: The beta beam, Phys. Lett. B532 (2002) 166–172. doi:10.1016/S0370-2693(02)01576-9. [2] Final report of the EURISOL design study (2005-2009), a Design study for a European Isotope-Separation-On-Line radioactive ion beam facility. [3] T. Thuillier, et al., High frequency ECR ion source (60-GHz) in preglow mode for bunching of beta-beam isotopes, PoS NUFACT08 (2008) 089.
[4] A. Lachaize, A. Tkatchenko, The Rapid Cycling Synchrotron of the Beta-Beam facility, EURISOL DS task note 12-25-20080012. [5] A. Chanc´e, Etude et conception de lanneau de dsintegration, PhD thesis, Universit´e Paris Sud. [6] T. Y. Hirsh, et al., High yield production of He-6 and Li-8 RIB for astrophysics and neutrino physics, PoS NUFACT08 (2008) 090. [7] T.Stora, Baseline ion source for beta-beam, Europeean Strategy for Future Neutrino Physics, http://indico.cern.ch/conferenceDisplay.py?confId=59378. [8] S. Trovati, M. Magistris, M. Silari, CERN-SC-2008-070-RPTN, CERN-DG-2009-051-RP-TN. [9] M. Kirk, 7th beta beam task meeting, CEA, Saclay. [10] J. Bruer, M.Sc. Thesis, Linkping University. [11] F. Borgnolutti, (private communication, documentation ongoing). [12] E. Y. Wildner, F. W. Jones, Simulation of Decays and Secondary Ion Losses in a Betabeam Decay RingParticle Accelerator Conference PAC07 25-29 Jun 2007, Albuquerque, New Mexico. [13] A. Chanc´e, S. Hancock, Stacking simulations in the beta-beam decay ringPrepared for European Particle Accelerator Conference (EPAC 06), Edinburgh, Scotland, 26-30 Jun 2006. [14] G. Burt, (private communication). [15] C. Rubbia, A. Ferrari, Y. Kadi, V. Vlachoudis, Beam cooling with ionisation losses, Nucl. Instrum. Meth. A568 (2006) 475– 487. arXiv:hep-ph/0602032, doi:10.1016/j.nima.2006.02.161. [16] M.Mezetto, (private communication). [17] EUROnu - A High Intensity Neutrino Oscillation Facility in Europe, FP7-INFRASTRUCTURES-2007-1. [18] C. Hansen, et al., Collective Effect Studies for a Beta Beam Decay Ring. To be Published in AIP Conf. for NUFACT10. [19] M. Schauman, Development and lattice design of an ion production ring for a beta-beam facility, BSc thesis, Technische Hochschule, Aachen. [20] J. Wehner, Monte Carlo Simulation of an ion production ring for a beta beam neutrino facility, BSc thesis, Technische Hochschule, Aachen. [21] E. Benedetto, Ionization Cooling in a Low-energy ion Ring with Internal Target for Beta-beams Production1st International Particle Accelerator Conference: IPAC’10, 23-28 May 2010, Kyoto, Japan. [22] C. Hansen, E. Wildner, E. Fernandez-Martinez, Limitations in the use of Barrier Buckets in the Beta Beam Decay Ring for the FP7 Scenario, PoS NUFACT09.