Superconducting Kaon Spectrometer Research Articles

Missing-Mass Measurement of the 12C(K-, K+) Reaction at 1.8 GeV/c with the Superconducting Kaon Spectrometer

Abstract We performed a measurement of the inclusive missing-mass spectrum of the $^{12}$C$(K^-, K^+)$ reaction at an incident beam momentum of 1.8 GeV/c. This measurement was carried out by using the Superconducting Kaon Spectrometer (SKS) and the K1.8 beamline spectrometer at the Hadron Experimental Facility in J-PARC. From the missing-mass of the $^{12}$C$(K^-, K^+)$ reaction, the binding energy of a $\Xi ^-$ hyperon in a core $^{11}$B nucleus, $B_{\Xi ^-}$, can be calculated. Our experimental setup yielded a good energy resolution of 8.2 MeV (full width at half maximum), which allowed us to observe significant enhancements in the proximity of the $^{12}_{\Xi }$Be production threshold region. In order to extract information from the missing-mass spectrum, we employed several fitting parameters assumptions. A good agreement with the spectrum shape was obtained by adding two Gaussian functions, with the constant experimental resolution for the $\Xi$-hypernuclear states, to the background distribution. The peak positions were obtained to be $B_{\Xi ^-} = 8.9 \pm 1.4$ (stat.) $^{+3.8}_{-3.1}$ (syst.) MeV and $B_{\Xi ^-} = -2.4 \pm 1.3$ (stat.) $^{+2.8}_{-1.2}$ (syst.) MeV. Another model assumption, one Breit–Wigner function with $B_{\Xi ^-} = -2.7 \pm 2.2$ (stat.) $^{+0.5}_{-0.7}$ (syst.) MeV and $\Gamma = 4.1 \pm 2.1$ (stat.) $^{+1.2}_{-0.7}$ (syst.) MeV, also yielded a similar $\chi ^2$ value.

Operation of the SKS Magnet at the K1.8 Beam Line in the J-PARC Hadron Hall

The Superconducting Kaon Spectrometer (SKS) magnet has been operated for 5 years in the hadron hall of the Japan Proton Accelerator Research Complex (J-PARC). The SKS had two long-term suspensions. One was due to damage by the 2011 off Pacific coast Tohoku Earthquake and the other was caused by the radioactive material leak accident at the hadron hall that occurred on May 23, 2013. The SKS was planned to be moved to another beam line to repair the damage caused by the earthquake. The planned move was postponed for processing after the accident.

Gamma-ray spectroscopy of 12C via the (π+,K+) reaction

Abstract A γ-ray spectroscopy experiment via the C 12 ( π + , K + ) reaction was carried out at KEK-PS in 2005. The K6 beam line and Superconducting Kaon Spectrometer (SKS) were employed to obtain a missing mass spectrum for 12ΛC. An upgraded germanium detector array, Hyperball2, was introduced to detect γ rays emitted from hypernuclei in coincidence with the ( π + , K + ) reaction. As a result of further analysis, a γ-ray peak at 6048 keV (preliminary) was newly observed and assigned as the M1 transition from the 6-MeV excited state to the ground state.

Beam and SKS spectrometers at the K1.8 beam line

High-resolution spectrometers for both incident beams and scattered particles have been constructed at the K1.8 beam line of the Hadron Experimental Facility at J-PARC. A point-to-point optics is realized between the entrance and exit of QQDQQ magnets for the beam spectrometer. Fine-pitch wire chamber trackers and hodoscope counters are installed in the beam spectrometer to accept a high rate beam up to 107 Hz. The superconducting kaon spectrometer for scattered particles was transferred from KEK with modifications to the cryogenic system and detectors. A missing-mass resolution of 1.9 ± 0.1 MeV/c2 (FWHM) was achieved for the ∑ peaks of (π±, K+) reactions on a proton target in the first physics run of E19 in 2010.

The first experiment at the J-PARC K1.8 beam line using the SKS spectrometer

The Superconducting Kaon Spectrometer (SKS) system has been constructed at the J-PARC K1.8 beam line. With all detectors being operational since October, 2009, a series of commissioning data have been taken successfully such that the SKS system performance could be assessed. In particular, the (π−, p) elastic scattering and the P(π−, K+)Σ− reaction were used for this purpose with the beam momentum of 0.5 GeV/c and 1.25 GeV/c, respectively. The measured resolution of Σ− missing mass was 1.7 MeV/c2(FWHM) corresponding to 3.3 MeV/c (FWHM) momentum resolution of the SKS system. Overall, the SKS system showed an expected performance which proved the readiness for the first physics experiment at the K1.8 beam line.

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.

Measurement of inclusive pion double charge exchange on 16O at T 0 ≥ 0.5 GeV with the SKS spectrometer at KEK

The inclusive cross section for pion double charge exchange on 16O at T0 = 0.5 and 0.75 GeV was measured with the superconducting kaon spectrometer (SKS) at KEK in a joint ITEP/KEK experiment. The result shows a relatively weak energy dependence of the measured cross section, which is in contradiction with its rapid drop predicted within the conventional model of two sequential single charge exchanges. The data of this experiment agree with the results that were obtained previously from similar measurements at ITEP and which are indicative of a significant contribution from the mechanism of inelastic Glauber rescatterings for T0 ≳ 0.6 GeV.

Sigma-nucleus potential in A=28.

We have studied the (pi(-),K+) reaction on a silicon target to investigate the sigma-nucleus potential. The inclusive spectrum was measured at a beam momentum of 1.2 GeV/c with an energy resolution of 3.3 MeV (FWHM) by employing the superconducting kaon spectrometer system. The spectrum was compared with theoretical calculations within the framework of the distorted-wave impulse approximation, which demonstrates that a strongly repulsive sigma-nucleus potential with a nonzero size of the imaginary part reproduces the observed spectrum.

Spectroscopic study of Lambda 10B, Lambda 12C, Lambda 28Si, Lambda 89Y, Lambda 139La, and Lambda 208Pb by the ( pi +,K+) reaction.

Hypernuclear mass spectra of ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{10}$B, ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{12}$C, ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{28}$Si, ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{89}$Y, ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{139}$La, and ${\mathrm{}}_{\mathrm{\ensuremath{\Lambda}}}^{208}$Pb were measured by the (${\mathrm{\ensuremath{\pi}}}^{+}$,${\mathit{K}}^{+}$) reaction with an energy resolution of 2 MeV (FWHM). The experiment was carried out with a 1.06 GeV/c ${\mathrm{\ensuremath{\pi}}}^{+}$ beam from the KEK 12-GeV PS and with the superconducting kaon spectrometer (SKS). For light hypernuclei, core-excited states were resolved for the first time. For heavy hypernuclei, peak structure reflecting \ensuremath{\Lambda} shell structure was observed. \textcopyright{} 1996 The American Physical Society.

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.

Electroproductions of light Λ and Σ hypernuclei

A spectroscopic study of Λ hypernuclei by the (π+, K+) reaction has been performed for the wide hypernuclear mass range using a new superconducting kaon spectrometer (INS-SKS) at the KEK 12 GeV PS. A Λ12C spectrum with 2 MeV (FWHM) energy resolution was measured and core excited states of Λ12C were clearly identified for the first time. It is discussed that the excitation energies and cross sections of these states will be closely related with the choice of ΛN interaction models as well as hypernuclear structure. Hypernuclear spectra of Λ10B, Λ28Si, Λ89Y, Λ139La and Λ208Pb were also obtained and binding energies of a Λ hyperon were deduced for each Λ orbital. The mass number dependence of the Λ binding energies is, within the present experimental errors, consistent with a calculation based on the Woods-Saxon potential.

The Λ hypernuclear spectroscopy with the SKS spectrometer at KEK 12 GeV PS

A spectroscopic study of Λ hypernuclei by the ( π +, K +) reaction has been performed for the wide hypernuclear mass range using a new superconducting kaon spectrometer (INS-SKS) at the KEK 12 GeV PS. A Λ 12C spectrum with 2 MeV (FWHM) energy resolution was measured and core excited states of Λ 12C were clearly identified for the first time. It is discussed that the excitation energies and cross sections of these states will be closely related with the choice of ΛN interaction models as well as hypernuclear structure. Hypernuclear spectra of Λ 10B, Λ 28Si, Λ 89Y, Λ 139La and Λ 208Pb were also obtained and binding energies of a Λ hyperon were deduced for each Λ orbital. The mass number dependence of the Λ binding energies is, within the present experimental errors, consistent with a calculation based on the Woods-Saxon potential.

Core-excited states of Lambda 12C hypernuclei formed in the ( pi +,K+) reaction.

A spectroscopic study of the $_{\ensuremath{\Lambda}}^{12}\mathrm{C}$ hypernucleus by the (${\ensuremath{\pi}}^{+}, {K}^{+}$) reaction has been performed using a new superconducting kaon spectrometer (INS-SKS) at the KEK 12 GeV Proton Synchrotron with an energy resolution of 2 MeV (FWHM). In addition to two prominent peaks which correspond to the $s$ and $p$ orbitals of a $\ensuremath{\Lambda}$ hyperon, for the first time two smaller peaks were clearly observed at excitation energies of 2.6 and 6.9 MeV. These two peaks are interpreted as states where the $^{11}\mathrm{C}$ excited core and a $\ensuremath{\Lambda}$ hyperon in the $s$ orbit are weakly coupled. The excitation energies and the cross sections of these peaks provide information on the $\ensuremath{\Lambda}N$ interaction.

A large silica aerogel Cherenkov counter for SKS

A large silica aerogel Cherenkov counter has been constructed as a kaon trigger counter to veto pions of 0.7 GeV/ c for experiments studying hypernuclei by the (π +, K +) reaction with the superconducting kaon spectrometer (SKS) at the KEK 12-GeV proton synchrotron. It features a large single diffusion box with a sensitive area of 140 cm×120 cm and a thickness of about 40 cm. Special attention was paid to the shape of a reflector in order to achieve uniform efficiency over the sensitive area. A mean photoelectron number of N e = 6−7 was obtained for 0.72 GeV/ c ( β = 0.984) pions with a refractive index of 1.06.

A superconducting spectrometer for the study of hypernuclei via (π, K+) reactions

A superconducting kaon spectrometer (INS-SKS) is under construction at Institute for Nuclear Study, University of Tokyo. The spectrometer, with 1/1000 momentum resolution and 100 msr acceptance, has been designed for intensive study of hypernuclei via (π, K+) reactions. It is to be installed in a new experimental hall of the KEK 12 GeV proton synchrotron and is scheduled to be ready for experiments by the end of 1990.