Noboru Sasao

Observables in neutrino mass spectroscopy using atoms

The process of collective de-excitation of atoms in a metastable level into emission mode of a single photon plus a neutrino pair, called radiative emission of neutrino pair (RENP), is sensitive to the absolute neutrino mass scale, to the neutrino mass hierarchy and to the nature (Dirac or Majorana) of massive neutrinos. We investigate how the indicated neutrino mass and mixing observables can be determined from the measurement of the corresponding continuous photon spectrum taking the example of a transition between specific levels of the Yb atom. The possibility of determining the nature of massive neutrinos and, if neutrinos are Majorana fermions, of obtaining information about the Majorana phases in the neutrino mixing matrix, is analyzed in the cases of normal hierarchical, inverted hierarchical and quasi-degenerate types of neutrino mass spectrum. We find, in particular, that the sensitivity to the nature of massive neutrinos depends critically on the atomic level energy difference relevant in the RENP.

Observation of pulsed x-ray trains produced by laser-electron Compton scatterings

X-ray generation based on laser-electron Compton scattering is one attractive method to achieve a compact laboratory-sized high-brightness x-ray source. We have designed, built, and tested such a source; it combines a 50 MeV multibunch electron linac with a mode-locked 1064 nm laser stored and amplified in a Fabry-Perot optical cavity. We directly observed trains of pulsed x rays using a microchannel plate detector; the resultant yield was found to be 1.2x10(5) Hz in good agreement with prediction. We believe that the result has demonstrated good feasibility of linac-based compact x-ray sources via laser-electron Compton scatterings.

Measurements of the electron drift velocity and positive-ion mobility for gases containing CF4

Abstract Measurements of the electron drift velocity and positive-ion mobility for gases containing CF 4 have been performed. The results are presented for four types of gas mixtures containing CF 4 and hydrocarbons: CF 4 +CH 4 , CF 4 +C 2 H 6 , CF 4 +C 3 H 8 and CF 4 +iso-C 4 H 10 . To make a comparison, also positive-ion mobilities for Ar+C 2 H 6 and Ar+C 3 H 8 gas mixtures have been measured. The drift velocities of electrons in a few mixtures are found to exceed 10 cm/μs, twice that in a conventional drift chamber gas such as Ar(50%)+C 2 H 6 (50%). In particular, the electron drift velocity in CF 4 (10%)+CH 4 (90%) is larger than 10 cm/μs, even at a reduced electric field E / P of more than 1 V/(cm Torr). Results on the measurements of the positive ion mobility are consistent with the interpretation that the hydrocarbon ions are the positive ions eventually drifting toward the cathode in the studied gas mixtures. Their mobilities in CF 4 are found to be smaller than those in Ar.

Dynamics of two-photon paired superradiance

We develop for dipole-forbidden transition a dynamical theory of two-photon paired superradiance (PSR). This is a cooperative process characterized by two photons emitted back to back with equal energies. By irradiating the trigger laser from two target ends, with its frequency tuned at the half energy between two levels, a macroscopically coherent state of medium and fields dynamically emerges as time evolves, and a large signal of amplified output occurs with a time delay. The basic semiclassical equations in 1 + 1 space-time dimensions are derived for the field-plus-medium system to describe the space-time evolution of the entire system and are numerically solved to demonstrate the existence of both explosive and weak PSR phenomena in the presence of relaxation terms. The explosive PSR event terminates accompanying a sudden release of most of the energy stored in the target. Our numerical simulations are performed using the vibrational transition X-1 Sigma(+)(g)upsilon = 1 -> 0 of a para-H-2 molecule and taking many different excited atom number densities and different initial coherences between the metastable and the ground states. In an example with a number density close to O(10(21) cm(-3)) and a high initial coherence, the explosive event terminates several nanoseconds after the trigger irradiation, when the phase relaxation time larger than O(10 ns) is taken. After PSR events the system is expected to follow a steady-state solution which is obtained by analytic means and is made of many objects of field condensates endowed with a topological stability.

Test of BaF2 scintillators coupled to a photomultiplier with a CsTe photocathode

Basic properties of BaF2 scintillators coupled to a photomultiplier with a CsTe photocathode have been investigated. The signal from the photomultiplier shows a drastic reduction of the slow component of the BaF2 scintillation light while preserving the good properties associated with the fast component.

A test of 150 cm × 20 cm wide time-of-flight scintillation counters

Abstract We have tested 150 cm × 20 cm wide, 2 cm and 3 cm thick time-of-flight counters using the newly produced scintillator SCSN-38. Some combinations of light guides and 2″ photomultipliers were examined. With the 3 cm thick scintallator the best rms time resolution of 150 ps has been obtained for minimum-ionizing particles. If the contribution from the TOF start counter is subtracted, the rms time resolution becomes about 140 ps.

Observation of coherent two-photon emission from the first vibrationally excited state of hydrogen molecules

Yuki Miyamoto1, Hideaki Hara1, Susumu Kuma1,†, Takahiko Masuda1, Itsuo Nakano1, Chiaki Ohae2,‡, Noboru Sasao1,∗, Minoru Tanaka3, Satoshi Uetake4,∗, Akihiro Yoshimi1, Koji Yoshimura1, and Motohiko Yoshimura4 1Research Core for Extreme Quantum World, Okayama University, Okayama 700-8530, Japan 2Graduate School of Natural Science and Technology, Okayama University, Okayama 700-8530, Japan 3Department of Physics, Osaka University, Toyonaka, Osaka 560-0043, Japan 4Research Center of Quantum Universe, Okayama University, Okayama 700-8530, Japan ∗E-mail: sasao@okayama-u.ac.jp, uetake@okayama-u.ac.jp

Externally triggered coherent two-photon emission from hydrogen molecules

We report coherent enhancement of two-photon emission from the excited vibrational state of molecular hydrogen triggered by irradiating mid-infrared pulses externally. We previously observed the two-photon emission triggered by the internally generated fourth Stokes photons. By injecting independent mid-infrared pulses externally, it is possible to control experimental parameters and investigate the mechanism in more detail. In this article, we describe the two-photon emission using the external trigger pulses. Its spectrum and dependence on the energy and timing of the trigger pulse are presented along with numerical simulations based on the Maxwell-Bloch equations. The measured number of emitted photons is 6 10^11 photons/pulse and the resulting enhancement factor from the spontaneous emission is more than 10^18. This value is three orders of magnitude higher than that of the previous experiment. External control of emission process is expected to be essential for observation of weaker process of radiative emission of neutrino pair.

Radiative emission of neutrino pair from nucleus and inner core electrons in heavy atoms

Radiative emission of neutrino pair from atomic states is a new tool to experimentally investigate undetermined neutrino parameters such as the smallest neutrino mass, the nature of neutrino masses (Majorana vs Dirac), and their C P properties. We study effects of neutrino pair emission either from inner core electrons or from the nucleus in which the zeroth component of quark or electron vector current gives rise to large coupling. Both the overall rate and the spectral shape of photon energy are given for a few cases of interesting target atoms. Calculated rates exceed those of previously considered target atoms by many orders of magnitude.

Time-of-Flight Counters for VENUS Detector

A time-of-flight counter system has been installed in the VENUS detector of the TRISTAN experiment at KEK. A time resolution of about 200 ps was obtained for a long scintillator of 466 cm. The system is being successfully operated in experiments.