Present Status of Neutron Fundamental Physics at J-PARC

Prog. Theor. Exp. Phys. 2012, 02B007 (11 pages) DOI: 10.1093/ptep/pts075

Present status of neutron fundamental physics at J-PARC

Yasushi Arimoto1, Haruhiko Funahashi2, Nao Higashi3, Masahiro Hino4, Katsuya Hirota5, Shohei Imajo6, Takashi Ino1, Yoshihisa Iwashita7, Ryo Katayama3, Masaaki Kitaguchi4, Kenji Mishima8, Suguru Muto1, Hideyuki Oide3, Hidetoshi Otono9, Yoshichika Seki5, Tatsushi Shima10, Hirohiko M. Shimizu11,KaoruTaketani1, Takahiro Yamada3,Satoru Yamashita8, and Tamaki Yoshioka12 1High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0802, Japan 2

Institute for the Promotion of Excellence in Higher Education, Kyoto University, Yoshida, Kyoto 606-8501, Downloaded from Japan 3Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan 4Research Reactor Institute, Kyoto University, Kumatori, Osaka Kyoto 590-0494, Japan 5Nishina Center, RIKEN, Wako, Saitama 351-0198, Japan 6Department of Physics, Kyoto University, Kyoto 606-8502, Japan http://ptep.oxfordjournals.org/ 7Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan 8International Center for Elementary Particle Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan 9Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan 10Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan 11Department of Physics, Nagoya University, Nagoya 464-8602, Japan 12Department of Physics, Kyushu University, Fukuoka 812-8581, Japan

Received September 18, 2012; Accepted November 13, 2012; Published December 27, 2012 at CERN LIBRARY on September 27, 2013 ...... A neutron beamline for the study of fundamental physics has been constructed at the spallation neutron source of the Japan Proton Accelerator Research Complex (J-PARC). In-ﬂight measurement of the neutron lifetime and the development of the transport optics of ultracold neutrons are in progress. The present status of the beamline and these experiments is reported......

1. Introduction The neutron is a chargeless massive particle with a lifetime in the macroscopic range; consequently, it is suitable for the precision measurement of the small inﬂuence of new physics beyond the standard model of elementary particles. The instantaneously luminous cold neutrons from the Japan Proton Accelerator Research Complex (J-PARC) enable us to carry out new types of high precision measurements. A neutron beamline, neutron optics and physics (NOP), has been constructed at beam port BL05 of the materials and life-science research facility (MLF) at J-PARC [1]. The beamline is currently undergoing recommissioning after the shutdown of the J-PARC accelerator in 2011. We give an overview of the NOP beamline and the status of experimental activities.

2. NOP beamline at port BL05 of J-PARC/MLF The NOP beamline at port BL05 views the coupled moderator of the neutron source of J-PARC/MLF. A shutter and a biological shield are in the regions of L = 2.3–4.3 m and L = 4.3–7.2 m, respectively, where L is the distance from the moderator.

© The Author(s) 2012. Published by Oxford University Press on behalf of the Physical Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. PTEP 2012, 02B007 Y. Arimoto et al.

Fig. 1. Schematic view of the configuration of the NOP beamline for the study of neutron optics and fundamental physics at the beam port BL05. Supermirror guides and benders are intensively employed for the efficient extraction and transport of cold neutrons. A supermirror is a multilayer of two different materials deposited on a substrate. Neutron interference in the multilayer results in additional reflectivity beyond the critical velocity of the total reflection of monolayers, where the critical velocity is the maximum value of the velocity component normal to the mirror surface. The additional reflectivity is applied in the increase of the angular acceptance and the bending power of cold neutrons. The performance of the supermirrors is measured as the critical velocity of the total reflection. The m-value is commonly used to describe the neutron critical velocity of supermirrors. m is defined as m = vc/vc,Ni where vc is the critical velocity of a supermirror and vc,Ni 7 m/s is the critical velocity of the surface of nickel with natural abundance. Supermirror guides of m = 2 with a cross section of 100 mm × 110 mm are installed in the shutter and biological shields and they transport neutrons into the inlet of beam benders. A pre-position shield covers the region of L = 7.2–12 m. A triple-fold beam bender is installed in a void space 2 m high and 1 m wide inside the pre-position shield. Three benders with curvature radii of approximately 100 m distribute cold neutrons into three beam branches: a polarized beam branch, an unpolarized beam branch, and a low-divergence beam branch. The interiors of the beam benders are filled with helium gas for the suppression of beam attenuation. Fast neutrons are absorbed in the direct beam dump placed in the region of L = 12–16 m. The bent neutrons are transported downstream through beam holes penetrating the beam dump. The beam holes accommodate the transport optical components and the cross section of each beam hole is approximately 15 cm × 15 cm. The configuration of the NOP beamline is shown in Fig. 1. Polarized beam branch A multichannel magnetic supermirror bender with m = 2.8 is installed to provide polarized neutrons. The length of the bender is 4.5 m and the cross section is 10 cm × 4cm. The bent neutrons are designed to be transported by an m = 2 straight guide in the region of L = 12– 16 m, which has not yet been installed. After the completion of the installation and alignment of the additional guide, a flux of 4.0 × 108 cm−2 s−1 MW−1 and polarization of 99.8% can be obtained for a neutron energy larger than 1.5 meV [2,3]. This beam branch is currently used for the in-flight measurement of neutron lifetimes. Unpolarized beam branch A five-channel supermirror bender is installed to bend neutrons upward with a curvature radius of 100 m. The length of the bender is 4 m and the cross section is 5cm× 4cm. The bent neutrons

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are designed to be transported by an m = 2 straight guide in the region of L = 12–16 m, which is prepared for installation. A neutron ﬂux of 1.2 × 109 cm−2 s−1 MW−1 is expected after the completion of the additional guide installation and alignment. This beam branch is currently used for the R&D of the time-focusing transport optics of ultracold neutrons (UCNs). Low-divergence beam branch Two supermirrors of m = 3 are installed to transport neutrons slower than 1.2 × 103 m/s with a density of 1.8 × 106 cm−2 μsr−1 s−1 MW−1. This beam branch is currently used for the R&D of a multilayer interferometer of pulsed neutrons, following the successful demonstration of a multilayer interferometer with spatially separated paths for the steady beam [4].

3. In-ﬂight measurement of neutron lifetime

The neutron lifetime τn is one of the key parameters for big-bang nucleosynthesis. Improvement of the accuracy of τn has become important to refine the consistency with the primordial abundance of light elements following suppression of the uncertainty of the baryon-to-photon ratio of the universe by observation of the anisotropy of the cosmic microwave background [5]. The desired accuracy −3 −3 of τn/τn is at the level of 10 or better. 10 accuracy has been reported in a measurement of the decrease rate of UCNs gravitationally trapped in a bottle with almost perfect reflectivity [6]. The reflection loss due to the up-scattering of neutrons on the inner surface of the bottle was suppressed by lowering the bottle temperature. However, the improved value deviated remarkably from the world average τn = 885.7 ± 0.8 s[7] and the latest world average has been changed to τn = 880.1 ± 1.1s[8]. Thus, additional measurements based on different techniques are awaited to clarify the value and the experimental accuracy of the neutron lifetime. The systematic error of the confined neutron experiments was dominated by the uncertainty in the correction of the confinement imperfection. Gravitomagnetic confinement experiments are in progress to avoid the complexity of total reflection on the material surface, although proper treatment of the neutron leakage due to the spin flip is necessary [9]. The best accuracy of τn in the in-flight geometry was achieved by the measurement of Penning- trapped protons produced in a cold neutron beam path [10]. In this case, the experimental error was dominated by the uncertainty in the neutron intensity monitoring. Another type of τn measurement in the in-flight geometry was last reported in 1989 [11]. Decay electrons were detected by a time-projection chamber (TPC). Neutron intensity was monitored 3 by detecting protons produced by the (n, p) reaction by He diluted in the same chamber. τn was obtained from the ratio of the decay rate and reaction rate since both of them are inversely proportional to the neutron velocity as

/ τ = 1 × Sn n , n 3 (1) ρ( He)σ0v0 Sβ/β

3 3 3 where ρ( He) is the number density of He atoms, σ0 the absorption cross section via the He(n, p) reaction for neutrons with velocity v0, Sn and Sβ the counting rates of neutron-induced protons and neutron-decay electrons, and n and β their detection efﬁciencies, respectively. The large background via neutron-induced reactions was suppressed by introducing monochromatized neutron bunches into the TPC and selectively detecting decay electrons and reaction protons only when neutron bunches were traveling inside the sensitive volume and were not transmitting through chamber windows and other materials in the beam path. The monochromatized neutron

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Beam Shutter Beam Monitor TPC

Spin Flip- 6LiF Beam Cather Chopper

Cosmic Veto Counter Lead Shield 1m

Fig. 2. Schematic view of the experimental setup of the in-ﬂight neutron lifetime measurement installed at the polarized beam branch of the NOP beamline. Polarized neutrons from the polarized beam branch are chopped into bunches by the spin-ﬂip chopper and transported to the time projection chamber (TPC).

2350

(B) (C) (A) (D) (E) 340 0 20 40

(F) (H) (G)

Fig. 3. A drawing of the SFC installed at the polarized beam branch of the NOP beamline: (A) first rf flipper, (B) first and second magnetic supermirrors, (C) 6Li shutter, (D) second rf flipper, (E) third magnetic supermirror, (F) guide coil, (G) neutron absorber, and (H) lead γ -ray shields. bunches were obtained from a reactor-based CW neutron beam by a combination of mechanical chopping and crystal diffraction. Therefore, the incident neutron intensity was doubly suppressed. The reported value was τn = 878 ± 27(stat.) ± 14(syst.) s, corresponding to τn/τn = 3.5 × 10−2, which was limited by statistical error. An improved measurement using the TPC is in progress at the NOP beamline [12]. Figure 2 shows the experimental setup of the τn measurement at the NOP beamline. The neutron velocity is well defined at every neutron time-of-flight (TOF) for a pulsed neutron beam, which enables us to avoid intensity loss due to additional monochromators. The remaining requirement is the fast chopping of neutrons synchronized to the neutron TOF. Our choice is to apply a new type of beam switching optics using a combination of spin-selective mirrors and spin flippers, which is referred to as a spin-flip chopper [13].

3.1. Spin-ﬂip chopper Figure 3 shows the top view of the spin-ﬂip chopper (SFC) installed at the polarized beam branch of the NOP beamline.

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Fig. 4. Performance of the spin-ﬂip chopper.

The SFC uses spin-selective optics to switch the neutron beam using a combination of magnetic supermirrors and spin flippers. Polarized neutrons from the polarized beam branch of the NOP beamline are incident to the SFC. A constant magnetic field is applied on the beam path to guide the neutron polarization. A radio-frequency (rf) magnetic field, which resonates with the neutron spin precession, can be applied in the rf flippers. On transmission through the radio-frequency flipper, the polarization direction is conserved or reversed by turning it off and on, respectively. The magnetic supermirror is a multilayer of thin magnetic layers deposited on a substrate. The neutrons are selectively reflected according to the spin polarity about the magnetization axis. Consequently, only successively reflected neutrons are transported to the outlet of the SFC. Thus the outlet of the SFC is bunched by synchronizing the operation of the rf flipper to the neutron TOF. However, imperfections in the beam polarization and the spin-selectivity of the reflection cause unpreferred neutron output during the beam-off state of the SFC. The perfectness of the spin- selectivity can be improved by employing multiple spin-selective reflections in series. By employing the triple series reflection, the present version of the SFC chops the neutron beam with an intensity contrast of about 400, as shown in Fig. 4, which indicates that the beam intensity during the beam- off state is 1/400 of the beam intensity during the beam-on state. The transition time between the beam-on and beam-off states is 0.5 ms, which corresponds to a bunch length of about 5 cm. During the τn measurement, the bunch length is adjusted by about 50 cm, which is half of the length of the TPC sensitive region for maximizing the signal statistics. The cross section of the output bunches is 2 cm × 2 cm.

3.2. Time projection chamber The developed TPC has a sensitive region with a length of 96 cm and a 29 cm × 30 cm cross section, in which 99.9% of decay electrons are detectable, as shown in Fig. 5. It has 24 anode wires distributed along the beam axis and induced charges are drifted in a transverse direction. The detection gas is a 3 mixture of He and CO2. He at a level of a few ppm is diluted for monitoring the neutron intensity 3 by detection of protons produced through the He(n, p) reaction, and τn is to be measured according to Eq. (1).

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A 412.0 Top LiF Frame MWPC Hanger

Anode/Cathode Frame (t6)

Cathode Frame (t6) Field Cage Front Panel

Field Cage Frame (Top) Field Wire Land (t2) 418.0

Field Cage Frame (Bottom)

400.0 550.0

B Top LiF Frame 1070.0

1032.0

Anode/Cathode Frame Cathode Frame

Field Cage Frame (Top) Side Bar Corner Piller 289.0

MWPC Hanger

Field Cage Frame (Bottom)

Al Pedestal

980.0 1080.0

Fig. 5. Front and side views of the time projection chamber developed for the in-ﬂight measurement of neutron lifetime.

The TPC is designed to maximally suppress backgrounds to relevant signals. The TPC is covered with plastic scintillators to veto cosmic rays and with radiation shields against the room background. The inner surface of the chamber is covered with 6LiF containing polytetraﬂuoroethylene to suppress prompt γ -rays induced by the neutrons scattered by the detection gas. Polyetheretherketone is selected as the material for the wire frames since it contains negligibly small radioactivity. The background level of the present version of the TPC is almost comparable with the neutron decay rate. The background contribution will be subtracted by a trajectory analysis combined with information on the time evolution of neutron beam bunches. The systematic uncertainty in the background subtrac- tion is estimated to be suppressed at the level of 0.1% by combining measurements with different gas pressures and 3He partial pressures [12].

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3 Additional sources of systematic errors in ρ( He)σ0v0 in Eq. (1) arise from the uncertainties in the partial pressure of 3He, the reaction cross section of the 3He(n, p) reaction, the temperature gradient of the detection gas, and the N2 contamination, which causes irrelevant proton signals induced by the 14N(n, p) reaction. The uncertainty in the 3He partial pressure is estimated to deliver the largest systematic error of 0.5%. Suppression of these uncertainties is being studied in parallel. A statistical uncertainty of 1% is expected to be achieved within 150 days, assuming a primary proton beam power of 220 kW.The acceptance of the SFC can be improved by one order of magnitude by enlarging the magnetic mirrors and by adjusting the optical layout with better accuracy.

4. Neutron time focus The non-zero permanent electric dipole moment of neutrons (NEDM) unambiguously signals the breaking of time-reversal (T) invariance. The NEDM has not yet been observed, although there have been various attempts to discover a non-zero NEDM [14–18]. The best upper limit is |dn| < 2.9 × 10−26 ecm(90% C.L.) [18], which is very close to the predictions of some new physics models beyond the standard model of elementary particles. The present upper limit is achieved by mea- suring the electric-field dependence of the spin precession frequency of ultracold neutrons (UCNs) confined in a material bottle with a UCN density of about 1cm−3. Denser UCN confinement is desired for further improvement and accelerator-based UCN sources are being developed. An accelerator source allows the flexible arrangement of neutron moderators and consequently enables intense UCN sources. New UCN sources are expected to realize a UCN density of 103 cm−3 or greater and an experimental sensitivity of 10−27 ecmor better. An instantaneously dense UCN is possible at high-intensity pulsed proton beam facilities. In the case of the designed value of the J-PARC linac, a peak current of 50 mA and a beam energy of 400 MeV corresponds to a peak power of 20 MW, which is more than one order of magnitude more intense than CW proton machines to be used for ongoing and planned UCN sources. In addition, the average beam power can be small, which decreases the heat load to the cryogenics of the moderator and the UCN source, and enables stable operation in practice. The rebuncher is estimated to realize

neutron accelerator (1) (2) region (3) (4) accelerate decelerate

position neutron (1) accelerator region (4)

(2) (3) Density Velocity

position

Fig. 6. Rebuncher concept. (1) Neutrons are generated as a pulse. (2) Neutrons spread spatially during transport. (3) Neutrons are accelerated/decelerated appropriately. (4) Some neutrons are focused at the storage volume.

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Magnet

UCNs Ni Guide tube He Detector

Shutter RF resonance circuit

Amp Motor M Timing signal DAQ 180 cm 360 cm

Fig. 7. Experimental setup of the demonstration experiment at ILL. The neutron accelerator is installed in the middle of the guide tube. The rf and data acquisition (DAQ) systems are synchronized with the shutter operation.

Fig. 8. Schematic visualization of the time sequence of the chopping and rf-application as a function of neutron TOF. The rf was applied in a TOF range of 0.3–0.38 s and decelerated neutrons are designed to be focused in time at the detector position. a statistical sensitivity of 10−27 ecm within 5000 hours of measurement time [19]. An additional increase in the UCN density is under consideration to extend the sensitivity down to 10−28 ecm. A new type of UCN transport optics to recover the UCN density at spatially separated regions is being developed at the unpolarized beam branch. Pulsed UCNs spread spatially during transport to the spatially separated storage volume according to their velocity distribution. If faster UCNs are decelerated and slower ones are accelerated in the middle of the transport, spatial density can be recovered at the storage volume as shown in Fig. 6 [20–23]. We refer to this type of transport optics as a UCN rebuncher in this paper. The rebuncher requires a neutron accelerator/decelerator that can be operated while synchronized to the UCN TOF. We apply the neutron velocity change to the spin flip induced by an rf field under a static magnetic field. The neutron kinetic energy is increased or decreased by 2μB depending on the spin direction about the static field B,whereμ is the magnetic dipole moment of the neutron. The

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Fig. 9. Ratio of neutron counting rates for rf-on and rf-off cases as a function of TOF at the detector position. The line shows the simulation.

Fig. 10. Schematic side view of the Doppler shifter installed at the unpolarized beam branch of the NOP beamline. Very cold neutrons (VCNs) are reflected on the multilayer mirror downward into the Doppler shifter and converted to ultracold neutrons (UCNs) on reflection by the moving mirror. The reflectivity of the moving quadruple-stack multilayer mirror is also shown as a function of neutron velocity normal to the mirror.

9/11 PTEP 2012, 02B007 Y. Arimoto et al. spin is flipped when the rf frequency ω satisfies the resonance condition ω = 2μB.WhenUCNs are transported in a neutron guide through a static magnetic field with the gradient along the beam axis, the velocity change can be selected by adjusting the rf frequency. Consequently, the arrival time at a certain position can be focused by sweeping the rf frequency synchronized with the UCN TOF. UCN focusing with a rebuncher was demonstrated at the PF2 TES beamline in the high-flux reactor at the Institut Laue-Langevin (ILL) with the experimental setup shown in Fig. 7 [24]. The unpolarized CW neutron beam was mechanically chopped to emulate a pulsed UCN source and the rf was applied during the period of relevant neutrons passing through the rf coil. The time sequence of the chopping and rf-application is shown in Fig. 8 as a function of the neutron TOF. The rf was applied during TOF = 0.3–0.38 s and UCNs were partly accelerated and decelerated and focused in time at the detector position. The rebunching of decelerated UCNs was observed as the deviation from unity in the ratio of TOF spectra for rf-off and rf-on cases at TOF = 1–1.2 s, as shown in Fig. 9.The TOF dependence of the rebunched UCNs was consistently reproduced with the numerical simulation assuming that the transportation in the neutron guide was completely specular and the velocity mixing between longitudinal and transverse velocity components was negligibly small. The intensity enhancement at the focal point was limited by the time width of the mechanical chopper. Further study of the rebuncher with sharper UCN pulses is in progress at the unpolarized beam branch of the NOP beamline for application to the proposed measurement of the neutron electric dipole moment at J-PARC [19]. The very cold neutrons contained in the cold neutron beam are converted to UCN by a Doppler shift of the reflection by a moving mirror [25,26]. We employed a quadruple-stack multilayer mirror, which is capable of reflecting neutrons with a velocity around v ∼ 70 m/s, as shown in Fig. 10. Neutrons with an incident velocity of v ∼ 140 m/s are converted to UCNs by moving the mirror backward at a velocity of 70 m/s.

5. Interferometry The low-divergence beam branch is designed for multilayer neutron interferometry. Two supermirrors of m = 3 are installed to transport neutrons slower than 1.2 × 103 m/s with a density of 1.8 × 106 cm−2 μsr−1 s−1 MW−1. A multilayer interferometer with spatially separated paths was successfully demonstrated for a steady beam from the reactor neutron source [4]. Further development to allow acceptance of a pulsed beam is in progress. Additionally, an extension to the energy region of very cold neutrons (VCNs) is also in progress.

6. Summary A neutron beamline for the study of the fundamental physics has been constructed at the J-PARC spallation neutron source. After the recovery of the J-PARC accelerator, recommissioning of the beamline, neutron lifetime measurement, and the development of UCN transport optics are in progress. The physics run for the lifetime measurement is scheduled for this year. A variety of experiments is also planned. Higher-order terms of the angular correlations in neutron beta decay depend on the hadron structures and radiative corrections, and precise data for those terms are useful to develop theoretical methods to treat these effects. It is planned to measure the angular correlations in neutron beta decay with an accuracy better than 10−3 at the polarized beam branch. High-statistics measurements of neutron scattering of rare isotopes and noble gases at the unpolarized beam branch are proposed to accumulate basic data on neutron-induced nuclear reactions and to search for new fundamental interactions with the sub-micrometer range, respectively. Develop- ment of a neutron interferometer with TOF capability is planned at the low-divergence beam branch

10/11 PTEP 2012, 02B007 Y. Arimoto et al. in order to realize interferometry with wide energy range, which will be useful in achieving the sensitivity needed to observe new phenomena such as the Aharonov–Casher effect and the general relativistic effect in the gravitational ﬁeld of the Earth.

Acknowledgements This work was partially supported by a Grant-in-Aid for Creative Scientiﬁc Research 19GS0210 of the Ministry of Education of the Japanese Government and the Oversea Research Program of the Neutron Science Division of KEK. We thank Dr Geltenbort for his great help and the Institut Laue-Langevin for the quick decision to accommodate our UCN test experiment during the J-PARC shutdown in 2011.

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