Search

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 of which working gas contains diluted $^3$He. The measured neutron lifetime was $898\,\pm\,10\,_{\rm stat}\,^{+15}_{-18}\,_{\rm sys}\,$s., 28 pages, 20 figures, will be submitted to PTEP

In a neutron lifetime measurement at the Japan Proton Accelerator Complex, the neutron lifetime is calculated from the neutron decay rate and the incident neutron flux. The flux is obtained by counting the protons emitted from the neutron absorption reaction of ${}^{3}{\rm He}$ gas, which is diluted in a mixture of working gas in a detector. Hence, it is crucial to determine the amount of ${}^{3}{\rm He}$ in the mixture. In order to improve the accuracy of the number density of the ${}^{3}{\rm He}$ nuclei, we have suggested using the ${}^{14}{\rm N}({\rm n},{\rm p}){}^{14}{\rm C}$ reaction as a reference because this reaction involves similar kinetic energy to the $^3$He(n,p)$^3$H reaction and a smaller reaction cross section to introduce reasonable large partial pressure. The uncertainty of the recommended value of the cross section, however, is not satisfied with our requirement. In this paper we report the most accurate experimental value of the cross section of the $^{14}$N(n,p)$^{14}$C reaction at a neutron velocity of 2200 m s$^{-1}$, measured relative to the $^3$He(n,p)$^3$H reaction. The result was 1.868 $\pm$ 0.003 (stat.) $\pm$ 0.006 (sys.) b. Additionally, the cross section of the $^{17}$O(n,$\alpha$)$^{14}$C reaction at the neutron velocity is also redetermined as 249 $\pm$ 6 mb.

The decay lifetime of free neutrons (∼880 s) is an important parameter of the weak interaction and for Big Bang Nucleosynthesis. However, results of measurements currently show discrepancies depending on the method used. As most experiments nowadays employ ultra cold neutrons, we have developed a new cold-beam experiment which we perform at the Japan Proton Accelerator Research Complex. As a special feature, a polarized neutron beam is bunched by a spin flip chopper. A time projection chamber operated with He and CO2gas, including a well-controlled amount of3He, is used for detection of the beta-decays and simultaneous determination of the beam intensity. Using the data between 2014 and 2016, we evaluated our first, preliminary result of the neutron lifetime as 896 ± 10(stat.)−10+14(sys.) s. We plan several upgrades to achieve our precision goal of 1 s.

"Neutron Optics and Physics (NOP/ BL05)" at MLF in J-PARC is a beamline for studies of fundamental physics. The beamline is divided into three branches so that different experiments can be performed in parallel. These beam branches are being used to develop a variety of new projects. We are developing an experimental project to measure the neutron lifetime with total uncertainty of 1 s (0.1%). The neutron lifetime is an important parameter in elementary particle and astrophysics. Thus far, the neutron lifetime has been measured by several groups; however, different values are obtained from different measurement methods. This experiment is using a method with different sources of systematic uncertainty than measurements conducted to date. We are also developing a source of pulsed ultra-cold neutrons (UCNs) produced from a Doppler shifter are available at the unpolarized beam branch. We are developing a time focusing device for UCNs, a so called "rebuncher", which can increase UCN density from a pulsed UCN source. At the low divergence beam branch, an experiment to search an unknown intermediate force with nanometer range is performed by measuring the angular dependence of neutron scattering by noble gases. Finally the beamline is also used for the research and development of optical elements and detectors. For example, a position sensitive neutron detector that uses emulsion to achieve sub-micrometer resolution is currently under development. We have succeeded in detecting cold and ultra-cold neutrons using the emulsion detector., 9 pages, 5 figures, Proceedings of International Conference on Neutron Optics (NOP2017)

A possibility of new type experimental searches for new physics beyond the standard model is being introduced in the symmetry tests in neutron-induced compound states according to the successful operation of intense pulsed neutron sources. The basis of experiment design is discussed.

The MTV (Mott Polarimetry for T-Violation) experiment is running at TRIUMF-ISAC (Isotope Separator and ACcelerator), searching for a large T violation in polarized Li-8 beta decay via measurements of the triple vector correlation, R, in the beta decay rate function. The left/right backward scattering asymmetry of Mott scattering from a thin metal foil is measured using an electron tracking detector including a cylindrical drift chamber (CDC). To achieve 10-ppm precision in the Mott scattering asymmetry, we performed multiple studies on the expected systematic effects. The sources of the systematics have been identified and calibration systems have been developed to evaluate the fake effects. The first physics data was collected in 2016 and significantly improved on the result of our previous measurement, which achieved 100-ppm precision in 2010 using the first generation detector (planer drift chamber) at TRIUMF. The data measurement status, together with the results of the systematics studies, is described here. In addition to the T violation, we are preparing to test the Lorentz invariance in the weak sector via our Mott analyzer system. Unexplored Lorentz violating correlations can be tested using the MTV experimental setup. The testing principle and preparation status are also described here.

The MTV experiment (Mott Polarimetry for T-Violation Experiment) is running at TRIUMF, to search for a large T-violating transverse electron-polarization in polarized 8Li β-decay. We aim at reaching precision of 10−4 for the R-correlation, which is defined as a T-violating triple vector correlation in the β-decay rate function. A Mott polarimeter system using a CDC (Cylindrical Drift Chamber) is used to measure the left-right scattering asymmetry in the Mott scattering from a thin metal foil. In the present study, we aim to discuss systematic effects in Mott polarimetry using the CDC.