ABSTRACT
Magnetic fields play such roles in star formation as the angular momentum transport in star-forming clouds, thereby controlling circumstellar disc formation and even binary star formation ...efficiency. The coupling between the magnetic field and gas is determined by the ionization degree in the gas. Here, we calculate the thermal and chemical evolution of the primordial gas by solving chemical reaction network where all the reactions are reversed. We find that at ∼1014–1018 cm−3, the ionization degree becomes 100–1000 times higher than the previous results due to the lithium ionization by thermal photons trapped in the cloud, which has been omitted so far. We construct the minimal chemical network which can reproduce correctly the ionization degree as well as the thermal evolution by extracting 36 reactions among 13 species. Using the obtained ionization degree, we evaluate the magnetic field diffusivity. We find that the field dissipation can be neglected for global fields coherent over ≳ a tenth of the cloud size as long as the field is not so strong as to prohibit the collapse. With magnetic fields strong enough for ambipolar diffusion heating to be significant, the magnetic pressure effects to slow down the collapse and to reduce the compressional heating become more important, and the temperature actually becomes lower than in the no-field case.
Eu-doped SrAl2O4 (Eu:SrAl2O4) crystals are prepared by a floating zone (FZ) method. In photoluminescence (PL), an intense broad emission peak around 510nm due to 5d-4f transition of Eu2+ appear under ...excitation at 250–450nm. The PL decay time constants are about 800ns due to the 5d-4f transitions of Eu2+. In the nondoped sample, emission peaks around 480 and 560nm are observed upon excitation at 316nm. The X-ray induced scintillation spectra also show an intense broad emission peak around 510nm, and the decay time constants are approximately 500ns. In the pulse height spectra of the 2.5% Eu:SrAl2O4 crystal, the absolute light yield is ~46,000ph/MeV. In thermally stimulated luminescence (TSL) after X-ray irradiation, strong TSL glow peaks are observed at 120 and 400°C. In optically stimulated luminescence (OSL) after X-ray irradiation, emission peaks are observed around 510nm while exposing the samples to the stimulation light at 800nm.
The Nd-doped LaMgAl11O19 single crystals were synthesized by the floating zone method, and the photoluminescence and scintillation properties were evaluated. Under X-ray irradiation, several sharp ...emission peaks due to the 4f–4f transitions of Nd3+ were observed at 900, 1060, and 1340 nm in the near-infrared range, and the decay curves show the typical decay time for Nd3+. The samples show good afterglow properties comparable with practical X-ray scintillators. The 1% and 3% Nd-doped LaMgAl11O19 samples show a good linearity in the dynamic range from 6–60,000 mGy/h.
Abstract
The recent discovery of high-redshift (
z
> 6) supermassive black holes (SMBH) favors the formation of massive seed BHs in protogalaxies. One possible scenario is the formation of massive ...stars
via runaway stellar collisions in a dense cluster, leaving behind massive BHs without significant mass loss. We study the pulsational instability of massive stars with the zero-age main-sequence (ZAMS) mass
and metallicity
, and discuss whether or not pulsation-driven mass loss prevents massive BH formation. In the MS phase, the pulsational instability excited by the
ϵ
-mechanism grows in
. As the stellar mass and metallicity increase, the mass-loss rate increases to
. In the red supergiant (RSG) phase, the instability is excited by the
κ
-mechanism operating in the hydrogen ionization zone and grows more rapidly in
. The RSG mass-loss rate is almost independent of metallicity and distributes in the range of
. Conducting stellar structure calculations including feedback due to pulsation-driven winds, we find that the stellar models of
can leave behind remnant BHs more massive than
. We conclude that massive merger products can seed monster SMBHs observed at
z
> 6.
Tm-doped MgAl2O4 transparent ceramics were synthesized by the Spark Plasma Sintering (SPS) method, and scintillation and thermally stimulated luminescence (TSL) properties were investigated. Under ...X-ray irradiation, the Tm-doped MgAl2O4 transparent ceramics clearly showed some emission peaks at 295, 360, 390, 455, 475, 500, 520, and 670 nm. Based on the observed scintillation decay curves, the origins of the emission peaks were attributed to 4f-4f transitions of Tm3+ ions. The emission peaks due to 4f-4f transitions of Tm3+ ions were also observed in a TSL spectrum. The TSL glow peaks of the Tm-doped MgAl2O4 transparent ceramics were detected at 68, 140, 200, 313, and 452 °C. The 1.0% Tm-doped MgAl2O4 transparent ceramics had a linear TSL response in a dose range from 0.01 to 100 mGy.
•Thermally stimulated luminescence (TSL) properties of Tm-doped MgAl2O4 transparent ceramics were investigated.•The TSL glow peaks of the Tm-doped MgAl2O4 transparent ceramics were detected at 68, 140, 200, 313, and 452 °C.•The Tm-doped MgAl2O4 transparent ceramic had a linear TSL response in a dose range from 0.01 to 100 mGy.
Translucent ceramics of undoped and Eu-doped NaMgF3 were synthesized by the spark plasma sintering method. Their optical, scintillation, thermally stimulated luminescence (TSL), and optically ...stimulated luminescence (OSL) properties were examined. In photoluminescence and scintillation spectra, Eu-doped NaMgF3 mainly showed the emission peaks at 360 and 375 nm, ascribed to 4f-4f and 5d-4f transitions of Eu2+, respectively. The TSL glow curve consists of the glow peaks at 94, 132, 166, 203, 249, 316, and 377 °C. The OSL signal could be detected by stimulation at 415 nm. The dominant luminescence center in TSL and OSL processes was Eu2+ ions, showing the emission peaks at 360 and 375 nm. The Eu-doped NaMgF3 had linear TSL and OSL responses from an irradiation dose of 0.01 mGy.
•Translucent ceramics of undoped and Eu-doped NaMgF3 were synthesized by the spark plasma sintering method.•The photoluminescence quantum yield, TSL, and OSL are largest for 0.5–1% Eu.•The TSL is linear from 0.01 to 100 mGy, and the OSL is linear from 0.01 to 1000 mGy.
Pr-doped GdTaO4 crystals with various Pr-concentrations were prepared using a floating zone furnace, and the photo- and radio-luminescence properties were evaluated. Pr:GdTaO4 exhibits photo- and ...radio-luminescence with several sharp emission peaks due to Pr3+. The decay time constants are in tens of μs order and monotonically decrease with dopant concentration, and the values are reasonable as that of the 4f–4f transitions of Pr3+. The pulse height distribution under 137Cs γ-ray irradiation demonstrates that the 0.5% Pr:GdTaO4 crystal shows the highest scintillation light yield of 5110 photons/MeV among the samples.
•Pr-doped GdTaO4 crystals were investigated as a notably high dense scintillator.•Samples exhibit luminescence with a several sharp emission peak due to Pr3+.•The light yields of 0.5% Pr:GdTaO4 was 5110 photons/MeV under 137Cs γ-rays.
ABSTRACT The existence of black holes (BHs) of mass at is a big puzzle in astrophysics because even optimistic estimates of the accretion time are insufficient for stellar-mass BHs of to grow into ...such supermassive BHs. A resolution of this puzzle might be the direct collapse of supermassive stars with mass into massive seed BHs. We find that if a jet is launched from the accretion disk around the central BH, the jet can break out of the star because of the structure of the radiation-pressure-dominated envelope. Such ultralong gamma-ray bursts with duration of -106 s and flux of 10−11-10-8 erg s-1 cm-2 could be detectable by Swift. We estimate an event rate of . The total explosion energy is 1055- . The resulting negative feedback delays the growth of the remnant BH by about or evacuates the host galaxy completely.
Dy-doped MgAl2O4 transparent ceramics were synthesized by the Spark Plasma Sintering (SPS) method, and photoluminescence (PL), scintillation, and dosimetric properties were investigated. Under ...excitations of 300, 360, and 390 nm, the Dy-doped sample clearly showed some emission peaks at 480, 580, and 660 nm. Based on the observed PL decay curves, the origins of the emission peaks were attributed to 4f-4f transitions of Dy3+ ions. The emission peaks due to 4f-4f transitions of Dy3+ ions were also observed in scintillation. As dosimetric properties, the Dy-doped sample showed thermally stimulated luminescence (TSL). The TSL glow peaks of the Dy-doped samples were detected at 53, 141, 193, 245, 304, 381, and 469 °C. The 0.1 % Dy-doped sample had a linear TSL response in a dose range from 0.1–100 mGy.
PDF
