H. Hotchi

γ-ray spectroscopy in Λ hypernuclei

Abstract The present status of hypernuclear γ -ray spectroscopy with Hyperball is summarized. We observed two γ transitions of 16 Λ O( 1 − → 1 − , 0 − ) and obtained the strength of the ΛN tensor force. In 10 B( K − , π − γ ) data, we did not observe the spin-flip M1 transition of 10 Λ B( 2 − → 1 − ), but γ rays from hyperfragments such as 7 Λ Li( 7 / 2 + → 5 / 2 + ) and 9 Λ Be( 3 / 2 + → 1 / 2 + ) were observed. In 11 B( π + , K + γ ) data, we observed six γ transitions of 11 Λ B. We also attempted an inclusive γ -ray measurement with a stopped K − beam.

Mesonic and nonmesonic weak decay widths of medium-heavy {lambda} hypernuclei

We have measured the energy spectra of pions and protons emitted in the weak decay of {sup {lambda}}{sub 12}C, {sup {lambda}}{sub 28}Si, and hypernuclei produced via the ({pi}{sup +},K{sup +}) reaction. The decay widths of the {pi}{sup -} mesonic decay ({lambda}{yields}p{pi}{sup -}) and the nonmesonic decay ({lambda}N{yields}NN) were extracted. The present results demonstrate an increase of the mesonic decay width due to a distortion of the pion wave function in nuclear medium for the first time. The ratios of the neutron- to proton-induced nonmesonic decay widths, {gamma}{sub n}({lambda}n{yields}nn)/{gamma}{sub p}({lambda}p{yields}np), were evaluated by a direct comparison of the measured proton energy spectra with the calculated ones. No theoretical calculation which has been proposed so far can simultaneously account for both the nonmesonic decay widths and the {gamma}{sub n}/{gamma}{sub p} ratios in the present data.

Σ-nucleus potential studied with the (π-, K+) reaction on medium-to-heavy nuclear targets

In order to study the Sigma-nucleus optical potential, we measured inclusive (pi^-,K^+) spectra on medium-to-heavy nuclear targets: CH_2, Si, Ni, In and Bi. The CH_2 target was used to calibrate the excitation energy scale by using the elementary process p + pi^- -> K^+ + Sigma^-, where the C spectrum was also extracted. The calibration was done with +-0.1 MeV precision. The angular distribution of the elementary cross section was measured, and agreed well with the previous bubble chamber data, but with better statistics, and the magnitudes of the cross sections of the measured inclusive (pi^-,K^+) spectra were also well calibrated. All of the inclusive spectra were found to be similar in shape at a region near to the Sigma^- binding energy threshold, showing a weak mass-number dependence on the magnitude of the cross section. The measured spectra were compared with a theoretical calculation performed within the framework of the Distorted Wave Impulse Approximation (DWIA). It has been demonstrated that a strongly repulsive \sig-nucleus potential with a non-zero size of the imaginary part is required to reproduce the shape of the measured spectra.

Realizing a high-intensity low-emittance beam in the J-PARC 3-GeV RCS

The J-PARC 3-GeV rapid cycling synchrotron is now developing beam studies to realize a high-intensity lowemittance beam with less beam halo. This paper presents the recent experimental results while discussing emittance growth and its mitigation mechanisms.

Recent Progress of 1-MW Beam Tuning in the J-PARC 3-GeV RCS

This paper presents the recent progress of 1-MW beam tuning in the J-PARC 3-GeV RCS, especially focusing on our approaches to beam loss issues.

The Path to 1 MW: Beam Loss Control in the J-PARC 3-GeV RCS

The J-PARC 3-GeV RCS started a 1-MW beam test in October 2014, and successfully achieved a 1-MW beam acceleration in January 2015. Since then, a large fraction of our effort has been concentrated on reducing and managing beam losses. In this paper, recent progresses of 1-MW beam tuning are presented with particular emphasis on our approaches to beam loss issues.

Design Study on the 9 GeV Proton Linac at KEKB Tunnel for the Next Generation Neutrino Experiment

Toward to the next generation experimental investigation of neutrino physics, the J-PARC accelerator group is now considering several candidates to extend the proton beam power to multi-MW region. One of the major candidates is the adoption of a superconducting proton linac. A neutrino is generated by bombarding a proton beam to a target. In Monte-Carlo simulations with GCALOR or FLUKA2011 package, the generated neutrino flux per proton beam energy monotonously increases with the energy reaching to around 9 GeV, and then saturation is seen in higher energy [1]. Therefore, if we only focus on the efficiency of neutrino generation, it is efficient to improve the average beam current with staying the beam energy to 9 GeV. It motivated us to consider the 9 GeV proton linac for neutrino beam generation. For the study on the design, we assume that the linac is constructed in the KEKB tunnel. As shown in Fig.1, the tunnel is fourfold symmetric structure and its circumference is 3 km. The accelerator cavities are placed only in the straight sections. In the each straight section, beam energy reaches 1.2, 3.8, 6.4 and 9 GeV, respectively. From 1.2 GeV to 9 GeV, ILC type superconducting cavities with two kinds of geometrical beta (βg) are placed. The 2 straight is βg = 0.93, and the 3 to 4 straights are βg = 1.0. In this presentation, we introduce the present accelerator design under the term of construction in the KEKB tunnel.

The Evaluation of the Residual Dose Caused by the Large-Angle Foil Scattering Beam Loss for the High Intensity Beam Operation in the J-PARC RCS

The Japan Proton Accelerator Research Complex 3-GeV rapid cycling synchrotron (RCS) has adopted the multi-turn charge-exchange injection scheme that uses H beams. During injection, both the injected and circulating beams scatter from the charge-exchange foil. Therefore, the beam loss caused by the large-angle scattering from the foil occurs downstream of the injection point. For countermeasure against the uncontrolled beam loss, a new collimation system was developed and installed in the summer shutdown period in 2011. During beam commissioning, this uncontrolled beam loss was successfully localized for a 300 kW beam. Since the present target power of the RCS is 1 MW, the accurate simulation model to reproduce experimental results has been constructed in order to evaluate residual dose at higher power operation.

Design of Injection and Extraction at an 8-GeV Booster Ring and the J-PARC Main Ring for Multi-MW Output Beam Power

The 240-kW output beam power in the MR has been achieved for a 30-GeV user operation with the repetition cycle of 2.48 sec and injecting proton beams of 380-kW equivalent intensity from RCS. The MR aims to realize 750-kW beam operation with faster repetition cycle of ~1 sec and injecting proton beams of 600-kW equivalent intensity from RCS. The MR is developing the new type power supplies of MR main magnets and high-impedance core of MR RF cavities toward faster repetition cycle. We are now exploring the further beam power upgrade scenario of the J-PARC accelerators. As one possible scenario toward a multi-MW output beam power from MR, a new 8-GeV booster ring (BR) between RCS and MR is also designed. The injection energy of the MR increases from present 3-GeV to 8-GeV. The higher injection energy of the MR would be able to mitigate a space charge force in MR injection energy region and secure the acceptance clearance of beam from MR physical aperture. In this paper, the designed injection and extraction system of BR are described. Additionally, a new concept of BR extraction and MR injection toward multi-MW output beam power are described.

The Realignment of the Beamline for J-PARC 3 GeV RCS

J-PARC 3GeV RCS suffered from the misalignments of several millimeters of the magnets in both horizontal and vertical directions caused by the Tohoku Region Pacific Coast Earthquake on March 11, 2011. As the result of the orbit calculation showed that the beam loss was acceptable for beam operation at 300kW, beam operation with the current placement was implemented until May, 2013. However according to the simulation of beam loss at 1MW operation, it was found out that the beam loss increased and the horizontal emittance expanded. Therefore it was understood that 1MW operation was difficult without the realignment of the beamline [1]. The realignment of the beamline was carried out from July to November, 2013 in conjunction with the upgrade of Linac. During the realignment, the adjustment of the magnets and the ceramics chambers was mainly performed. The magnets were adjusted to within ±0.2mm. The ceramics chambers were aimed to be adjusted within ±0.5mm. Beam commissioning started on January 30, 2014. RCS succeeded in injection of 400MeV beam from the upgraded Linac and extraction of 3GeV beam to MLF. In this paper, the alignment result of the magnets and the ceramics chambers that constitute the beamline of 3GeV RCS is reported.