KEK 12-GeV Proton Synchrotron Research Articles

Study of theKL0→π0π0νν¯decay

The rare decay K0(L) --> pi0 pi0 nu nu-bar was studied with the E391a detector at the KEK 12-GeV proton synchrotron. Based on 9.4 x 10^9 K0L decays, an upper limit of 8.1 x 10^{-7} was obtained for the branching fraction at 90% confidence level. We also set a limit on the K0(L) --> pi0 pi0 X (X --> invisible particles) process; the limit on the branching fraction varied from 7.0 x 10^{-7} to 4.0 x 10^{-5} for the mass of X ranging from 50 MeV/c^2 to 200 MeV/c^2.

Experimental study of the decayKL0→π0νν¯

The first dedicated search for the rare neutral-kaon decay $K_L^0\to\pi^0\nu\bar{\nu}$ has been carried out in the E391a experiment at the KEK 12-GeV proton synchrotron. The final upper limit of 2.6 $\times 10^{-8}$ at the 90% confidence level was set on the branching ratio for the decay.

Modification and Reconstruction of the SKS Superconducting Magnet

The Superconducting Kaon Spectrometer (SKS) magnet, which was in operation at the K6 beamline in the North Counter Hall of the KEK 12-GeV proton synchrotron from 1991 to 2005, was modified and moved to the newly constructed K1.8 beamline in the Hadron Hall at the Japan Proton Accelerator Research Complex (J-PARC). We reused the original coil assembly which includes coils and vessels, with a Gifford-MacMahon (GM) cryocooler for shield cooling. We used three Gifford-MacMahon/Joule-Thomson (GM/JT) cryocoolers instead of the previous 300-W helium refrigerator. The cooling power of a GM/JT cryocooler is 3.5 W at 4.5 K and 50 Hz. We also installed a new GM cryocooler to cool the GM/JT insert ports. Conventional copper current leads were replaced with Bi2223 high-temperature-superconductor (HTC) leads. We also required a new GM cryocooler to cool the HTC current leads. In total, we installed three GM/JT and three GM cryocoolers. We cooled the reconstructed SKS superconducting magnet at the K1.8 beamline from room temperature to 4.3 K over 1.5 months, applying LN2 and LHe directly. We confirmed that the three GM/JT cryocooles maintained the LHe level in the steady state. Helium gas pressure within the helium vessel was 113 kPa absolute and the condenser temperatures of the three GM/JT cryocoolers were 4.35 K, 4.46 K, and 4.35 K, respectively. We also measured the rise in temperature of the HTC leads when the magnet was continuously excited at a current of 400 A. The temperature of the HTC leads rose to 41 K and attained saturation after 5 hours. This was sufficiently safe for the Bi2223 HTC leads. Furthermore, we confirmed that two GM/JT cryocoolers could maintain the steady state; however, for safety reasons, we decided to use three GM/JT cryocoolers in the case of continuous excitation at more than 350 A.

Depth Profile of Radioactivity Induced in the Thick Concrete Shield in EP1 Beam Line at the KEK 12-GeV Proton Synchrotron Facility

The evaluation of radioactivity induced in the concrete shield is important for the decommissioning of accelerator facilities. Specific activities of gamma-ray emitters of nuclear spallation products and thermal neutron capture products and beta-ray emitters such as tritium and 14C, and 36Cl in the concrete shield along the 12-GeV proton beam line (EP1 beam line, High Energy Accelerator Research Organization) were determined. The depth profiles of the radioactivity of each nuclide in the 6-m-thick concrete shield of the beam lines were compared, and the secondary particles and induced nuclear reactions were discussed.

Search for the DecayKL0→π0νν¯

We performed a search for the K L0-->pi0nu nu[over] decay at the KEK 12-GeV proton synchrotron. No candidate events were observed. An upper limit on the branching ratio for the decay was set to be 6.7 x 10(-8) at the 90% confidence level.

Barrel photon detector of the KEK [formula omitted] experiment

Large lead/scintillator sandwich-type photon detectors, sensitive to visible energy depositions of 1MeV, have been built for the KL0→π0νν¯ experiment at the KEK 12-GeV proton synchrotron. The front barrel (FB) is composed of 16 modules with a length of 2.75m, forming the 1.5m-diameter cylinder. The main barrel (MB) is composed of 32 modules with a length of 5.5m, forming a 3.5m-diameter cylinder. Scintillation light is read out through wavelength-shifter (WLS) fibers. In this paper, the design, construction and performance of these detectors together with some R&D results are reported.

First search forKL→π0π0νν¯

The first search for the rare kaon decay KL→π0π0νν¯ has been performed by the E391a collaboration at the KEK 12-GeV proton synchrotron. An upper limit of 4.7×10−5 at the 90% confidence level was set for the branching ratio of the decay KL→π0π0νν¯ using about 10% of the data collected during the first period of data taking. First limits for the decay mode KL→π0π0P, where P is a pseudoscalar particle, were also set.Received 31 January 2007DOI:https://doi.org/10.1103/PhysRevD.76.011101©2007 American Physical Society

New limit on theKL0→π0νν¯decay rate

The first dedicated experiment for the rare kaon decay K{sub L}{sup 0}{yields}{pi}{sup 0}{nu}{nu} has been performed by the E391a Collaboration at the KEK 12-GeV proton synchrotron. A new upper limit of 2.1x10{sup -7} at the 90% confidence level was set for the branching ratio of the decay K{sub L}{sup 0}{yields}{pi}{sup 0}{nu}{nu} using about 10% of the data collected during the first period of data taking.

Development of Pulsed Power Modulator for Induction Synchrotron

A pulsed power modulator for the POP experiment of an induction synchrotron has been developed. Various difficulties in the development of the modulator, such as enormous power dissipation at a MOSFET, the resonant ringing in the output waveform, the isolation from the ground potential, and the incorrect action of a gate driving circuit, have been discussed and solved. The developed power modulator is installed into the existing accelerator, KEK 12GeV proton synchrotron. The POP experiment of the induction synchrotron has been successfully conducted. A single RF bunch injected from the 500 MeV booster ring was accelerated to the flat-top energy of 8 GeV.

Undoped CsI calorimeter for the [formula omitted] experiment at KEK-PS

An electromagnetic calorimeter consisting of 576 undoped CsI crystals has been used to search for the K L 0 → π 0 ν ν ¯ decay at the KEK 12-GeV proton synchrotron (E391a). The calorimeter is placed in a vacuum of lower than 10 −1 Pa. Special energy and timing calibrations have been performed. The energy calibration started from a check of the linearity for nine sample crystals using an electron beam. Then, after mounting all crystals in the E391a detector, we made an in situ calibration of the energy by using cosmic ray and punch-through muons. The gain constants obtained from both muons agree with each other with an accuracy of 2% ( σ ) . They were further refined by using γγ samples from π 0's produced off an aluminum plate (5-mm thick) hit by neutral beam particles. Using the final gain constants, we obtained a K L 0 mass resolution of 4.3 MeV/ c 2 ( σ ) for the K L 0 → π 0 π 0 π 0 decay. Also, using the timing constants measured for cosmic rays, we obtained a resolution of 0.51 ns ( σ ) for the timing difference among six γ's in the K L 0 → π 0 π 0 π 0 decay.

Observation of gaseous nitric acid production at a high-energy proton accelerator facility

High-energy protons and neutrons produce a variety of radionuclides as well as noxious and oxidative gases, such as ozone and nitric acid, in the air mainly through the nuclear spallation of atmospheric elements. Samples were collected from the surfaces of magnets, walls, and floors in the neutrino beamline tunnel and the target station of the KEK 12-GeV proton synchrotron facility by wiping surfaces with filter paper. Considerably good correlations were found between the amounts of nitrate and tritium and between those of nitrate and 7Be. This finding gives evidence that at high-energy proton facilities, nitric acid is produced in the radiolysis of air in beam-loss regions. Also, the nitric acid on the surfaces was found to be desorbed and tended to be more uniform throughout the tunnel due to air circulation. The magnitude of diminishing from the surfaces was in the order of tritium>nitrate> 7Be.

Scintillating Track Image Camera-SCITIC

A new type of track detector, scintillating track image camera (SCITIC) has been developed. Scintillating track images of particles in a scintillator are focused by an optical lens system on a photocathode on image intesifier tube (IIT). The image signals are amplified by an IIT-cascade and stored by a CCD camera. The performance of the detector has been tested with cosmic-ray muons and with pion- and proton-beams from the KEK 12-GeV proton synchrotron. Data of the test experiments have shown promising features of SCITIC as a triggerable track detector with a variety of possibilities.

Beam-dynamic effects of a droop in an induction accelerating voltage

Proof-of-principle experiments on the induction synchrotron concept are scheduled using the KEK 12-GeV proton synchrotron, in which rf bunches and a superbunch will be accelerated with a long step voltage generated in the induction accelerating gaps. An unavoidable droop in the induction voltage gives an additional focusing or defocusing force in the longitudinal direction. It largely deforms the barrier bucket confining the superbunch, leading to a nonuniform particle distribution. The effects are serious in an induction synchrotron with a transition energy. Longitudinal emittance blowup beyond the transition energy is not acceptable. The necessity of compensating for the droop is discussed.

Kaon physics at KEK

Kaon decay experiments have been a major research program of the KEK 12GeV proton synchrotron. The E246 experiment to search for T violation in K μ3 + decay has recently finished data taking and it is now in the analysis phase. After the completion of analysis of the 1998 data, its result is consistent with no T violation. Several byproduct physics are also analyzed using the E246 data. Now a new rare decay experiment, K L → π 0ν ν , is in preparation aiming for the data taking in 2003 and 2004.

Scintillator–Lucite sandwich detector for n/γ separation in the GeV energy region

A Scintillator–Lucite Sandwich Detector (SLSD) has been developed for n/γ separation in the GeV energy region. An efficient n/γ separation is achieved by measuring a correlation between the scintillation and Cherenkov light yields. The basic performance of the detector has been tested with e, π and p beams with momenta between 0.5 and 2.0 GeV/c ; the results were compared with a Monte-Carlo simulation. The n/γ separation capability of this detector has been studied by simulations in the energy range from 5 MeV to 12 GeV . The SLSD detector was successfully used for a beam survey of the new K L 0 beam line built for a K L 0→ π 0 ν ν ̄ experiment at the KEK 12-GeV proton synchrotron.

Spectroscopy of medium-heavy Λ hypernuclei via the(π+,K+)reaction

Excitation energy spectra of Λ89Y, Λ51V, and Λ12C have been measured via the (π+,K+) reaction by using the SKS spectrometer at the K6 beam line in the KEK 12-GeV Proton Synchrotron. In the Λ89Y spectrum, obtained with an energy resolution of 1.65 MeV (FWHM) and in the highest statistics so far, we have succeeded in clearly observing a characteristic fine structure in heavy Λ hypernuclear systems and precisely obtaining a series of Λ single-particle energies in the very wide excitation energy range of more than 20 MeV, for the first time. Also in the Λ51V spectrum, a similar structure to that of Λ89Y was observed. In the Λ12C spectrum, new core-excited states were clearly resolved thanks to the best energy resolution of 1.45 MeV so far achieved by using the SKS spectrometer.Received 23 April 2001DOI:https://doi.org/10.1103/PhysRevC.64.044302©2001 American Physical Society

Large horn magnets at the KEK neutrino beam line. II

For pt.I see Yamanoi et al., Proc. on Magnet Technology, p.711 (1997) We report on the status of the latest operation of the horn magnets. Our two types of large horn magnets were installed in the neutrino beam line at the KEK-12 GeV Proton Synchrotron (KEK-PS) and have been operated since March 1999. These two focusing magnets were designed to be excited at pulsed-high currents of up to 250 kA. One of the special characters of our horn magnet is a built-in pion-production target at the most upstream part of the inner conductor. This configuration enable us to increase the neutrino flux as high as possible with low-energy primary protons from the KEK-PS. Our horns have a coaxial-shape structure with a large diameter, i.e. a large volume of the magnetic field, in order to collect as many pions as possible. We estimate that the neutrino flux is enhanced by a factor of 14 by using this horn system. Both horns have been excited over 10/sup 6/ times at pulsed currents of 175 kA-250 kA. Some beginning problems were found in the peripheral apparatus of the horn magnets, and the built-in production target was found have a fatal mechanical damage. In the spring run, the performance of our horn magnet system was almost sufficient as a pion focusing device for a long-baseline neutrino oscillation experiment.

Optical design of beam lines at the KEK-PS new experimental hall

A new counter experimental hall [K.H. Tanaka et al., IEEE Trans. Magn. 28 (1992) 697] was designed and constructed at the KEK 12-GeV Proton Synchrotron (KEK-PS). The extracted proton beam from the KEK-PS is introduced to the new hall through the newly-prepared primary beam line, EP1, and hits two production targets in cascade. The upstream target provides secondary particles to the low momentum ( 0.4–0.6 GeV c ) separated beam line, K5, and the downstream target is connected to the medium momentum (0.6–2.0 GeV/ c) separated beam line, K6. Several new ideas were employed in the beam optical designs of EP1, K5 and K6 in order to increase the number and the purity of the short-lived secondary particles, such as kaons and pions, under the limited energy and intensity of the primary protons provided by the KEK-PS. These new ideas are described in this paper as well as the first commissioning results.

The superconducting kaon spectrometer — SKS

A superconducting kaon spectrometer has been installed in the north experimental hall of the KEK 12-GeV proton synchrotron. The spectrometer was designed to serve for nuclear physics experiments with meson beams in the 1 GeV/ c region, particular emphasis being laid on study of Λ-hypernuclei via ( π +, K +) reactions. In order to obtain Λ-hypernuclear data with better statistics and energy resolution, it was designed to have a good momentum resolution of 0.1% FWHM and a large acceptance of 100 msr. It consists of a large superconducting dipole magnet, tracking chambers, and trigger counters that can efficiently select kaons from large background of pions and protons. The overall energy resolution for scattering is realized together with a beam-line spectrometer in the K6 beam line, the momentum resolution of which was also designed to be better than 0.1% FWHM. A good energy resolution of better than 2 MeV FWHM has been confirmed in π −- 12C elastic scattering and in the ( π +,K +) reaction on 12C.

Experimental study of the rare decays KL0--> microe, KL0-->ee, KL0--> micro micro, and KL0-->eeee.

An experiment to search for the ${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}\ensuremath{\mu}e and ${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}ee decays and to measure the branching ratio of the ${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\mu} decay was performed at the KEK 12-GeV Proton Synchrotron. The single-event sensitivity of the present experiment for these channels was around 4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$. No event was observed in the fiducial region for the ${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}\ensuremath{\mu}e decay and one event, which could not be distinguished from the background, was observed in the fiducial region for the ${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}ee decay. The obtained upper limits on the branching ratios at 90% confidence level were B(${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}\ensuremath{\mu}e)9.4\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ and B(${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}ee)1.6\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$. The branching ratio of the ${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\mu} decay was obtained to be [7.9\ifmmode\pm\else\textpm\fi{}0.6 (stat \ifmmode\pm\else\textpm\fi{}0.2 (syst)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}9}$ based on 179 events in the fiducial region. Using 18 events with the effective mass ${\mathit{M}}_{\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$>470 MeV /${\mathrm{c}}^{2}$ in the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ sample for the ${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}ee decay search, the branching ratio of the ${\mathit{K}}_{\mathit{L}}^{0}$\ensuremath{\rightarrow}${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ decay was estimated to be [6\ifmmode\pm\else\textpm\fi{}2 (stat) \ifmmode\pm\else\textpm\fi{}1 (syst)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ for ${\mathit{M}}_{\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$>470 MeV/${\mathit{c}}^{2}$ and [7\ifmmode\pm\else\textpm\fi{}3 (stat)\ifmmode\pm\else\textpm\fi{}2(syst)]\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ for ${\mathit{M}}_{\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$>480 MeV/${\mathit{c}}^{2}$.