Shiro Sakuragi

A new type of luminescence mechanism in large band-gap insulators: Proposal for fast scintillation materials

Abstract Luminescence components of BaF 2 (180–230 nm), CsF (220–500 nm), CsCl (200–300 nm), CsBr (220–280 nm), and RbF (200–450 nm) have been investigated at room temperature by using synchrotron radiation as a light source. These luminescence components have excitation thresholds at the energy differences from the outermost Ba 2+ 5p, or M + m p ( m = 4 and 5 for M = Rb and Cs, respectively) core state to the conduction band, and are attributeted to the radiative decay of electrons in the X − n p halogen valence bands ( n = 2, 3, and 4 for X = F, Cl and Br, respectively) to the outermost-core hole states. Single bunch operation of synchrotron radiation has been used to determine the lifetime of the above luminescence. The measured lifetimes in ns are: BaF 2 : 0.88±0.02; CsF: 2.9±0.1; CsCl: 0.88±0.07; CsBr: 0.07±0.03; and RbF: 1.3±0.1 From the above results, it is proposed that insulators having large band-gap energy compared to the energy difference between the valence band and the outermost-core state are candidates for fast scintillation materials, for example, CsCl, CsBr, RbF, KF, BaCl 2 , BaBr 2 , and BaI 2 .

Infrared image guide with bundled As–S glass fibers

An infrared image guide made of bundled As-S glass fiber cores has been developed. The transmission range of this fiber was 2-6 microm. At the 3.3- and 4.8-microm wavelengths the minimum optical loss was 0.6 dB/m. With this image guide efficient transmission of the thermal image was experimentally established.

Radiation damage of CsI(Tl) crystals above 103 rad

Abstract CsI(Tl) crystals of 1×1×10 cm3 were exposed to 60Co γ rays with various doses in the range from 103 to 106 rad. The pulse height reduction, measured with a 137Cs isotope placed at the small end of the crystal opposite to a photomultiplier, varied from 28% at 1.58×103 rad to more than 85% at 1.83×106 rad. The radiation damage continued to be significantly saturated between 103 and 105 rad. Irradiation caused absorption of light at short wavelengths, mainly below 600 nm, making strongly irradiated crystals reddish. No sizable spontaneous recovery was observed. It was found that the damage quickly (within hours) recovers to a considerable extent by exposure to sunlight.

Auger-free luminescence due to interatomic transitions of valence electrons into core holes in BaF2

Abstract The nature of BaF 2 emission is investigated by measuring excitation spectra using synchrotron radiation for the energy range up to 40 eV. Intense emission bands, appearing at 5.6 and 6.4 eV in the Auger-transparent region, are ascribed to a radiative decay of valence electrons into Ba 2+ 5 p core holes. This luminescence process of a new type is discussed.

Polycrystalline KRS-5 Infrared Fibers For Power Transmission

Power transmitting capabilities of polycrystalline (PC) KRS-5 fibers have remarkably increased since the first report by researchers at Hughes Research Laboratories. Maximally transmitted power of CW CO2 laser beam through PC KRS-5 fiber has reached up to about 100W. The power density at the fiber input end is 36 KW/cm2 and remarkable damage is not observed. Mechanical properties of PC KRS-5 fiber is superior to any other PC fiber, therefore PC KRS-5 fiber is a hopeful waveguide for the practical use of CO2 laser power transmission. And also it is applicable for IR sensor systems in a short range. When the power transmission capabilities of PC KRS-5 fiber will be attained up to several hundreds watt, it will be widely used in the field of the CO2 laser machining.

KRS-5 optical fibers capable of transmitting high-power CO 2 laser beam

Polycrystalline KRS-5 optical fibers were experimentally demonstrated to have high-power transmission capabilities for a cw CO(2) laser beam. A typical sample of fibers provided by extrusion, 1 mm in diameter and 0.87 m in length, carried a laser power of 68 W to its output end and was found to remain free from damage at an incoming laser intensity up to 30 kW/cm(2).

Synthesis and Properties of Semiconducting Iron Disilicide β-FeSi2

Beta-iron disilicide β-FeSi2 bulk crystals were prepared by a horizontal gradient freeze method. The grown β-FeSi2 bulk crystals were characterized by structural, electrical and optical measurements. The β-FeSi2 single phase was determined by X-ray diffraction measurements. The electrical resistivity of β-FeSi2 increased exponentially with decreasing temperature. From optical absorption spectra that were obtained from transmittance measurements, the energy band gap was determined to be 0.85 eV. The refractive index, extinction coefficient and dielectric constants were calculated using optical reflectance spectra by Kramers-Kronig analysis. In addition to β-FeSi2 bulk materials, β-FeSi2 films were prepared on Si(100) substrates by a laser ablation method using the above β-FeSi2 bulk crystals as target materials. Strong Raman signals from the β-FeSi2 films were observed at 171 cm-1, 190 cm-1, 199 cm-1 and 247 cm-1, indicating deposition of well-oriented high-quality films. The optical absorption coefficient at 1.0 eV and energy band gap were determined to be 1.99× 105 cm-1 and 0.85 eV, respectively.

Spectral profile of the two-photon absorption coefficients in CaF2 and BaF2

Two-photon absorption spectra due to the valence excitons in CaF2 and BaF2 have been measured using the second harmonic of the output of a XeCl-excimer-laser pumped dye laser as a light source. The band gap energies of CaF2 and BaF2 are obtained as 11.8 and 10.6 eV at 298 K, respectively. The two-photon absorption coefficient β in the spectral region below the 2P exciton is expressed by an exponential function of the photon energy in both crystals. The values of β at the 2P exciton peak are about 2×10−9 cm/W for CaF2 and 3×10−9 cm/W for BaF2.

The new scintillation material CsI and its application to position sensitive detectors

Abstract The new scintillation material CsI has been investigated for a position sensitive detector which is used together with CsI(Tl) coupled to a common PMT. Spatial resolution has been achieved with a simple pulse shape discriminator. Combinations of CsIBGOCsI(T1) and CsIBaF 2 CsI(T1) are also shown to be useful as position sensitive detectors.

Transfer of Excitation Energy from Pr3+ to Gd3+ in YF3:Pr3+,Gd3+

Luminescence and excitation spectra for YF3:Gd3+, YF3:Pr3+, and YF3:Pr3+,Gd3+ have been studied in the vacuum ultraviolet (VUV) spectral region at room temperature. In YF3:Gd3+, Gd3+ ions absorb VUV light ranging from 150 to 200 nm due to 4 f–4 f transitions, yielding an ultraviolet (UV) luminescence line at 311 nm originating from the 4 f–4 f transition (6P7/2→8S7/2 state). In YF3:Pr3+,Gd3+, Pr3+ ions absorb the VUV light (150–200 nm) due to 4 f–5d transitions, and the absorption gives rise to the UV luminescence of Gd3+ ions that is much stronger than that of YF3:Gd3+. In this paper, we discuss the energy transfer process from Pr3+ to Gd3+ ions in YF3:Pr3+,Gd3+ excited by VUV light.