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A new type of cold/ultracold neutron detector that can realize a spatial resolution of less than 100 nm was developed using nuclear emulsion. The detector consists of a fine-grained nuclear emulsion coating and a 50-nm thick \\[^{10}\\hbox {B}_4 \\hbox {C}\\] layer for the neutron conversion. The detector was exposed to cold and ultracold neutrons (UCNs) at the J-PARC. Detection efficiencies were measured as (0.163 ± 0.015 (stat) ± 0.013 (sys))% and (10.3 ± 1.3 (stat) ± 1.1 (sys))% for cold and ultracold neutrons consistently with the \\[^{10}\\]B content in the converter. Positions of individual neutrons can be determined by observing secondary particle tracks recorded in the nuclear emulsion. The spatial resolution of incident neutrons were found to be in the range of 11–99 nm in the angle region of tan\\[\\theta \\le 1.9\\], where \\[\\theta \\] is the angle between a recorded track and the normal direction of the converter layer. The achieved spatial resolution corresponds to the improvement of one or two orders of magnitude compared with conventional techniques and it is comparable with the wavelength of UCNs.

Measurements of the muonic helium atom hyperfine structure (HFS) are a sensitive tool to test the theory of three-body atomic systems and bound-state quantum electrodynamics (QED) and to determine fundamental constants of the negative muon magnetic moment and mass. The world’s most intense pulsed negative muon beam at J-PARC MUSE brings an opportunity to improve previous measurements and test further CPT invariance by comparing the magnetic moments and masses of positive and negative muons. Test measurements at D-line are now in progress utilizing MuSEUM apparatus at zero field. The first results already have better accuracy than previous measurements in the 1980s. Also, the investigation of a new experimental approach to improve HFS measurements by repolarizing muonic helium atoms using a spin-exchange optical pumping (SEOP) technique was started. If successful, this would drastically improve the measurement accuracy.

Abstract The hadron spectrum at finite density is an important observable for exploring the origin of hadron masses. In the KEK-PS E325 experiment, the di-electron decays of $\\phi$ mesons inside and outside nuclei were measured using $12 \\,\\mathrm{G}\\mathrm{e\\mathrm{V}}$ pA reactions. In the previous analysis, a significant excess was observed on the low-mass side of the $\\phi$ meson peak in the data for slow-moving $\\phi$ mesons ($\\beta \\gamma =p_{\\phi }/m_{\\phi }<1.25$) with the copper target, and in-medium vector meson spectral modification was verified. We used, for the first time, the PHSD transport approach to take into account the time evolution of the spatial density distribution of the target nuclei. Consistent with the previous analysis, a significant excess was observed in the present analysis as well. It was found that incorporating momentum dependence into the spectral modification leads to better agreement with the experimental results. For the slow-moving $\\phi$ mesons with the copper target, the newly obtained modification parameters are consistent with those from the previous analysis within the uncertainties.

Abstract The neutron lifetime has been measured by comparing the decay rate with the reaction rate of $^3$He nuclei of a pulsed neutron beam from the spallation neutron source at the Japan Proton Accelerator Research Complex (J-PARC). The decay rate and the reaction rate were determined by simultaneously detecting electrons from the neutron decay and protons from the $^3$He(n,p)$^3$H reaction using a gas chamber, the working gas of which contains diluted $^3$He. The measured neutron lifetime was $898\\,\\pm\\,10\\,_{\\rm stat}\\,^{+15}_{-18}\\,_{\\rm sys}\\,$s.

To solve the “neutron lifetime puzzle,” where measured neutron lifetimes differ depending on the measurement methods, an experiment with a pulsed neutron beam at J-PARC is in progress. In this experiment, neutrons are bunched into 40-cm lengths using a spin flip chopper (SFC), where the statistical sensitivity was limited by the aperture size of the SFC. The SFC comprises three sets of magnetic supermirrors and two resonant spin flippers. In this paper, we discuss an upgrade to enlarge the apertures of the SFC. With this upgrade, the statistics per unit time of the neutron lifetime experiment increased by a factor of 2.8, while maintaining a signal-to-noise ratio of 250–400, which is comparable to the previous one. Consequently, the time required to reach a precision of 1 s in the neutron lifetime experiment was reduced from 590 to 170 days, which is a significant reduction in time. This improvement in the statistics will also contribute to the reduction of systematic uncertainties, such as background evaluation, fostering further advancements in the neutron lifetime experiments at J-PARC.

The main purpose of this study was to compare three different treatment plans for locally advanced cervical cancer: (i) the inverse-planning simulated annealing (IPSA) plan for combination brachytherapy (BT) of interstitial and intracavitary brachytherapy, (ii) manual optimization based on the Manchester system for combination-BT, and (iii) the conventional Manchester system using only tandem and ovoids. This was a retrospective study of 25 consecutive implants. The high-risk clinical target volume (HR-CTV) and organs at risk were defined according to the GEC-ESTRO Working Group definitions. A dose of 6 Gy was prescribed. The uniform cost function for dose constraints was applied to all IPSA-generated plans. The coverage of the HR-CTV by IPSA for combination-BT was equivalent to that of manual optimization, and was better than that of the Manchester system using only tandem and ovoids. The mean V100 achieved by IPSA for combination-BT, manual optimization and Manchester was 96 ± 3.7%, 95 ± 5.5% and 80 ± 13.4%, respectively. The mean D100 was 483 ± 80, 487 ± 97 and 335 ± 119 cGy, respectively. The mean D90 was 677 ± 61, 681 ± 88 and 513 ± 150 cGy, respectively. IPSA resulted in significant reductions of the doses to the rectum (IPSA D2cm3: 408 ± 71 cGy vs manual optimization D2cm3: 485 ± 105 cGy; P = 0.03) and the bladder (IPSA D2cm3: 452 ± 60 cGy vs manual optimization D2cm3: 583 ± 113 cGy; P < 0.0001). In conclusion, combination-BT achieved better tumor coverage, and plans using IPSA provided significant sparing of normal tissues without compromising CTV coverage.

For flow visualization study of quantum turbulence in superfluid 4 He, He 2 ∗ excimers are unique tracers which follow only normal-fluid component flow above 1 K. To generate detectable small He 2 ∗ clouds (clusters) required for full-space velocity field measurements, we have adopted a new method based on neutron absorption reaction of 3 He impurities in 4 He and conducted proof-of-principle experiments. Generation of the He 2 ∗ excimers was detected by laser-induced fluorescence using photomultiplier tubes. The fluorescence was observed to increase proportionally to the neutron flux, suggesting that a sufficient amount of He 2 ∗ excimers were generated by neutrons. We also estimated the number of He 2 ∗ excimers possibly generated by γ -rays and found that the relevant contribution was less than 40%. Thus, the majority of the He 2 ∗ excimers was confirmed to be generated via n- 3 He absorption reactions.

High precision measurements of the ground state hyperfine structure (HFS) of muonium is a stringent tool for testing bound-state quantum electrodynamics (QED) theory, determining fundamental constants of the muon magnetic moment and mass, and searches for new physics. Muonium is the most suitable system to test QED because both theoretical and experimental values can be precisely determined. Previous measurements were performed decades ago at LAMPF with uncertainties mostly dominated by statistical errors. At the J-PARC Muon Science Facility (MUSE), the MuSEUM collaboration is planning complementary measurements of muonium HFS both at zero and high magnetic field. The new high-intensity muon beam that will soon be available at H-Line will provide an opportunity to improve the precision of these measurements by one order of magnitude. An overview of the different aspects of these new muonium HFS measurements, the current status of the preparation for high-field measurements, and the latest results at zero field are presented.

During radiotherapy for gastric lymphoma, it is difficult to protect the liver and kidneys in cases where there is considerable overlap between these organs and the target volume. This study was conducted to compare the three radiotherapy planning techniques of four-fields 3D conformal radiotherapy (3DCRT), half-field radiotherapy (the half-beam method) and intensity-modulated radiotherapy (IMRT) used to treat primary gastric lymphoma in which the planning target volume (PTV) had a large overlap with the left kidney. A total of 17 patients with gastric diffuse large B-cell lymphoma (DLBCL) were included. In DLBCL, immunochemotherapy (Rituximab + CHOP) was followed by radiotherapy of 40 Gy to the whole stomach and peri-gastric lymph nodes. 3DCRT, the half-field method, and IMRT were compared with respect to the dose–volume histogram (DVH) parameters and generalized equivalent uniform dose (gEUD) to the kidneys, liver and PTV. The mean dose and gEUD for 3DCRT was higher than for IMRT and the half-beam method in the left kidney and both kidneys. The mean dose and gEUD of the left kidney was 2117 cGy and 2224 cGy for 3DCRT, 1520 cGy and 1637 cGy for IMRT, and 1100 cGy and 1357 cGy for the half-beam method, respectively. The mean dose and gEUD of both kidneys was 1335 cGy and 1559 cGy for 3DCRT, 1184 cGy and 1311 cGy for IMRT, and 700 cGy and 937 cGy for the half-beam method, respectively. Dose–volume histograms (DVHs) of the liver revealed a larger volume was irradiated in the dose range <25 Gy with 3DCRT, while the half-beam method irradiated a larger volume of liver with the higher dose range (>25 Gy). IMRT and the half-beam method had the advantages of dose reduction for the kidneys and liver.