Slow Luminescence Research Articles

Development of inorganic scintillators

A review is presented on different lines of scintillator research in relation to the different applications and requirements. Applications of inorganic scintillators are found in very different fields. Often the scintillator requirements are correspondingly different. In a number of cases there is a strong interest in scintillators with a fast response and a high light yield. E.g. for positron emission tomography the materials Lu 2SiO 5(Ce) and LuAlO 3(Ce), both with a relatively high density, are studied. For small gamma cameras with a good position resolution, e.g. for emission mammography, the less dense material YAlO 3(Ce) is of interest. For thermal-neutron detection neutron-gamma ray discrimination is occasionally very important. Here BaLiF 3, with both a very fast and a relatively slow luminescence component, and BaLiF 3(Ce) are interesting new materials. And finally, for high-energy physics the crucial combination is a fast response and a short radiation length. Here all attention is focussed on the dense, high-atomic-number material PbWO 4, a scintillator which has a very low light yield.

Fast luminescence of undoped PbWO 4 crystal

The results of an investigation of PbWO 4 luminescence properties obtained by time-resolved techniques under VUV, X-ray synchrotron radiation and γ-radiation excitation are presented. The fast luminescence in the 430 nm band with fwhm of 0.5 eV is characterized by a decay time of about 4–5 ns under X-ray excitation. A slow luminescence with decay time of more than 1 μs can be attributed to a green emission band. An intermediate decay time was also detected. This time changes from 20 ns (for 360 nm emission) to 50 ns (for 550 nm). The PbWO 4 fast band excitation spectrum and reflectivity were measured in the fundamental absorption region using VUV synchrotron radiation (4–150 eV). A comparison between these two spectra allowed us to suppose the excitonic type of the excitation mechanism of the fast emission band. The role of Pb 2+ ions in the PbWO 4 luminescence is discussed.

Slow luminescence from trapped charges in oxidized porous silicon

Porous silicon exhibits a fast (~ l ns) luminescence band in the blue-green (2.3–2.7 eV) spectral region when post-treated by rapid thermal oxidation. In addition we observe for the blue emission a time-delayed signal with a typical time constant of order l s that indicates carrier trapping. This component is present at low temperatures and has a sharp excitation onset at 4.3 eV. Quenching of the slow signal after additional thermal treatment of aged samples is accompanied by changes in the IR spectrum.

Modeling Methane Coot Flames and Ignitions

ABSTRACT Ignition diagram of methane has been simulated, in three different reaction vessels, by means of a model based on detailed kinetic scheme involving 21 reaction intermediates and 88 reactions. Comparison with literature experimental diagrams appears satisfactory, all the phenomena being well or fairly predicted. The mechanism of methane cool flames implemented in the model is analyzed. Self quenching ability depends on competition between accelerating and terminating steps involving methyl radical. No HCHO inhibiting effect seems to occur, on the contrary HCHO acts as a cool flame promoter. Two stage and single stage ignitions are also well predicted, as it is slow combustion. Slow reaction luminescence is also simulated.

Suppression of the slow component of BaF 2 crystal by introduction of SrF 2 and MgF 2 crystals

The emission spectrum, the time dependence of the luminescence, the light output, the excitation and the reflectance spectra have been measured for pure BaF 2 crystal, for BaF 2:Sr crystals with Sr 2+ concentrations of 2 and 5 mol% and for BaF 2:MgF 2 crystal with a mole mixing ratio of 1:2. The measurements have been made by using synchrotron orbital radiation and high-energy electrons produced by gamma-rays. It is shown that the BaF 2, BaF 2:Sr and BaF 2:MgF 2 crystals have a fast component luminescence with decay times around 0.90 ns. The slow and fast components luminescence of the BaF 2 crystal are suppressed by the introduction of Sr 2+ and Mg 2+ ions. It is, however, clearly seen that the suppression of the slow component is more effective than that of the fast component, especially in the BaF 2:MgF 2 crystal. The Sr 2+ and Mg 2+ ions act as electron trapping centers, preventing the formation of self-trapped excitons by electron-hole pairs, that is the slow component.

Time-resolved spectroscopy of gaseous xenon excited in the vicinity of the first resonance line

Time-resolved measurements of gas-phase xenon luminescence in the first and second continua by means of synchrotron radiation excitation in the vicinity of the first resonance line in the pressure range 50–1000 Torr were performed. By using a fast channelplate-detector (fwhm = 250 ps) the fast and slow rise and decay luminescence components were analysed. A strong collisional mixing of 0 u + and 1 u/0 u - states in the manifold of high vibrational levels is observed. From the measurements constants of collisional-induced predissociation of Xe 2 * (0 u +) υ ⪢ 0, collisional mixing in high υ, vibrational relaxation and three-body recombination from atomic 3P 1 and 3P 2 levels are derived. Additionally, lifetimes of vibrationally excited 0 u + state were extracted from the experimental data. A set of consistent rate constants measured in a most direct way is presented.

Auger-free luminescence of the BaF 2:Sr, BaF 2:MgF 2 and CsBr:LiBr crystals under excitation of VUV photons and high-energy electrons

The emission spectrum, the time dependence of the luminescence, the excitation spectrum and the reflectance spectrum have been measured for the pure BaF 2 crystal, the BaF 2:Sr crystals with Sr ++ concentrations of 2 and 5 mol%, the BaF 2:MgF 2 crystal with a mole mixing ratio of 1:2, and the pure CsBr crystal and the CsBr:LiBr crystal with a mole mixing ratio of 1:1. The measurements have been made by using synchrotron orbital radiation and high-energy electrons produced by gamma-rays. It is shown that the BaF 2, the BaF 2:Sr and the BaF 2:MgF 2 crystals have the Auger-free luminescence having decay times around 0.90 ns. The slow and fast luminescence components of the BaF 2 crystal are suppressed by introduction Sr ++ and Mg ++ ions. It is, however, clearly seen that the suppression of the slow component is more effectively occurring than that of the fast component, especially in the BaF 2:MgF 2 crystal. By analyzing the Auger-free luminescence spectrum, it is shown that the valence band widths are 2.7 ± 0.3 eV, 2.7 ± 0.3 eV and 4.2 ± 0.3 eV and the band-gap energies are 11.1 ± 0.3 eV, 11.1 ± 0.4 eV and 11.9 ± 0.4 eV for BaF 2, BaF 2:Sr and BaF 2:MgF 2 crystals, respectively. The introduction of LiBr into CsBr has resulted in a significant enhancement in the luminescence intensity, attaining 40 times larger intensity than that of CsBr crystal at room temperature, due to the suppression of the non-radiative relaxation of the outer-most core holes due to the absorption induced by the Urbach effect. The observations of the increases in the band gap-energy, the exciton energy and the decay time of the CsBr:LiBr crystal compared with those of the CsBr crystal support the argument given in the present study regarding the suppression of the Urbach effect in the CsBr:LiBr crystal. The decay times observed are 0.20 ± 0.05 ns and 0.90 ± 0.06 ns for CsBr and CsBr:LiBr crystals, respectively.

Fast and slow visible luminescence bands of oxidized porous Si

The visible luminescence of porous Si is known to contain at least two spectrally distinct emission bands with widely different response times. The orange-red luminescence component (1.5–1.9 eV) decays in times being of the order of 10 μs at room temperature. The blue-green band (2.3–2.6 eV) is very much faster with response time in the 10-ns range. It is shown that with increasing degree of oxidation the fraction of the fast luminescence intensity rises from ∼1% of the total in the as-prepared porous Si to become the dominant spectral component in strongly oxidized material. For the rapid-thermal-oxidized material excited with 337-nm radiation, the intensity of the fast luminescence is comparable to that in the as-prepared state.

Suppression of BaF2 Slow Component of X-RAY Luminescence in Non-Stoichiometric Bao.9R0.1F2.1 Crystals (R=Rare Earth Element)

ABSTRACTA series of BaF2 crystals doped with 10% mole fraction of each rare earth element has been grown, and the effect of these dopants on the slow luminescence (due to anionic self trapped excitons) and fast luminescence (due to core-valence transitions) has been studied. While significant suppression was observed for both components, the best dopants for suppressing the slow component of barium fluoride (up to 25-50%) while maintaining the luminosity of the fast component are La, Y, and Lu. The luminescence of Ba0.9Eu0.1F2.1 is almost entirely fast (>90%), but with low light output. For two rare earth dopants (La and Gd), the effect of slow and fast component suppression was studied as a function of concentration (Ba1-xRxF2+x with x≦0.5 for R=La and x≦0.3 for R=Gd). The suppression is non-linear with dopant concentration, with the relative degree of slow component suppression correlated with the melting point of these samples.

Decay kinetics of the slow component of Pb 2+ emission in KX (X = Cl, Br, I) crystals

Deviations from the exponential behaviour of the slow luminescence decay and the intensity ratio of the fast and the slow components, respectively, are inverstigated for the main ultraviolet Pb 2+ emission in KCl:Pb 2+, KBr:Pb 2+ and KI:Pb 2+ in the 4.2–200 K temperature range. An explanation of the non-exponential decays is proposed as a consequence of slow local lattice relaxation in the excited state, New values of the slow component lifetimes are identified and the correction of other parameters of a three-level model is discussed.

Decay time measurements on “pure” CsI scintillators prepared by different methods

The discovery of the fast decay time of pure CsI and the various results of the measured samples led us to investigate the decay time of CsI crystals prepared by different methods. Carefully grown or prepared pure CsI behaves as fast (ns range) scintillators with a strongly or totally suppressed slow (μs range) decay component. The presence or the absence of the slow component is related to the preparation method, and to the remaining built-in contamination of the samples. Pure CsI crystals prepared by the zone melting technique show a fast luminescence at 310 nm with an intensity of 5% relative to NaI(Tl) for electrons. The slow luminescence is strongy suppressed, practically absent. This fast luminescence consists f two components: an ultrafast component and a fast one with decay times of 1.8 ns and 12.9 ns respectively; the intensity of of the two components is 0.32 for gamma radiation. It means that the relative intensity of the ultrafast component is 1.2%, that of the fast one is 3.8% relative to NaI(Tl) for electrons. Pure CsI prepared by the cold press technique has an ultrafast component of 1.0 ns and a fast one of 9.6 ns with an intensity ratio of 0.65 for electrons; the slow component is totally absent. The relative light output of this luminescence is 4%; i.e., the relative intensity of the ultrafast component is 1.6%, that of the fast one is 2.4% relative to NaI(Tl) for electrons. The relative intensity of the ultrafast component of BaF 2 (220 nm) is 4%, that of the slow (310 nm) one is 16% relative to NaI(Tl) for electrons. Pure CsI crystals prepared by the conventional growing method have a slow luminescence (decay time: 3 μs) with a relative intensity of 26% to NaI(tl) for electrons. They have a fast luminescence of 8% relative to CsI(Tl) for alphas if the surface was polished by ethanol, but a strong slow luminescence of 58% relative to the CsI(Tl) if it was polished by water or ethylene glycol.

OXYGEN CONSUMPTION AND LUMINESCENCE OF ISOLATED Porichthys PHOTOPHORES IN RESPONSE TO ADRENERGIC STIMULATIONS

Abstract—Isolated photophores of Porichthys maintained in saline at 20oC, consume oxygen at a mean rate of 115 ± 5 ± 10 3 nmol O2 min1 (n= 165).In the presence of 0.1 and 1 mM adrenalin or noradrenaline the photophore produces a fast peak of light followed by a slow luminescence. A large increase in oxygen consumption occurs prior to the fast light response and it remains slightly above the resting level during the following slow light emission.When isolated photophores do not produce the fast peak of light(in response to isoprenalin or after treatment with phentolamine), there is only a slight transient increase of oxygen consumption. During the production of the slow luminescence the oxygen consumption remains close to the resting level.A relationship between the oxygen consumption and the light emission during the fast luminescence has been found: it is suggested that most of the oxygen consumed could be involved in a triggering mechanism of light production.

On the Characteristics of the Visible Chemiluminescence Following Free Radical Lipid Peroxidation

The characteristics of the visible luminescence that follows the lipid peroxidative process were investigated either in the autoxidation of rat brain homogenates or in the azo-bis-amidinopropane initiated lipid peroxidation of erythrocyte plasma membranes and liver microsomes. In these systems the luminescence decay observed after total inhibition of the lipid peroxidation is not an iron-catalyzed process, and follows a complex kinetics comprising fast and slow components. The slow component of the decay lasts for several hours at 27 degrees C and amounts to nearly half of the total intensity measured prior to the inhibition of the oxidative process by propyl gallate. The addition of thiols (diethyldithiocarbamate, penicillamine or dithiothreitol) to a lipid peroxidizing system inhibits the chain oxidation and catalyzes the dark decomposition of one (or several) of the luminescence precursors, following first order kinetics. The effect of temperature on the slow luminescence decay corresponds to an activation energy of 18.5 kcal/mol.

Slow luminescence emission from polymers with a hyperbolic decay law. A survey of commercial materials with an apparatus of wide aperture

A survey of 98 commercial polymers with an apparatus responding to a sample area of 20 cm2 revealed luminescence in about a third of them with a characteristic, hyperbolic decay extending over several minutes after brief irradiation with an incandescent source. The time dependence of the intensity was described well by the Debye—Edwards equation, of the form I = At−m, with m between 0.67 and 1.17 except for polyvinylcarbazole, with m = 1.76. In a few cases studied in more detail, the emission could be stimulated by wavelengths extending into the visible region, showed a negligible temperature dependence (6–40°C), and was little affected by removal of oxygen or water from the atmosphere around the samples. The source of the emission is ascribed to charge recombination of photoejected electrons from chromophores in the polymer structure or (in most cases) present as adventitious impurities which were not extractable in two cases. Nylon 6, which showed little emission alone, gave well-defined hyperbolic curves after heating in air or after addition of the fluorescent dye Benzoxanthene Yellow H2495.

Thermo-Optical Spectrophotometries in Analytical Chemistry

I. INTRODUCTION A. Background The incorporation of lasers as light sources in analytical spectroscopic instrumentation has resulted in spectacular detection limits. These detection limits arise from several unique properties of laser radiation. As an example, the spectral purity and high power of the laser facilitates detection of individual gas phase atoms with isotopic resolution by both fluorescence and ionization techniques.1−3 The temporal coherence of the laser has been used to discriminate signals generated by very fast scatter processes from relatively slower luminescence relaxation.4, 5 In addition, the spatial coherence of the laser allows measurement by thermo-optical techniques of very small absorbance, the subject of this review. A recent review in this journal considered the related technique of photoacoustic spectroscopy, another high-sensitivity absorbance method.6

Comparison of the decay of slow delayed luminescence in triazine-susceptible and -resistant biotypes of Erigeron canadensis L.

The decay of flash-induced delayed luminescence was compared in the seconds to minutes time region in triazine-susceptible and -resistant chloroplasts. The slow luminescence component assigned to S 2Q − B and S 3Q − B recombination decayed more rapidly in resistant as compared to susceptible chloroplasts ( t 1 2 ~ 8.5 and 44 s, respectively) indicating that the midpoint potential of q b is lowered in the resistant chloroplasts. The luminescence intensity depended on the number of exciting flashes. The oscillation pattern was opposite in the two biotypes and could be computer simulated with the assumption that the Q B/Q B − ratio was about 1 in sensitive and 1.5 in resistant chloroplasts. Thus atrazine resistance also induces a change in the oxidation-reduction state of the Q B pool.

The adrenergic control of the photocyte luminescence of the Porichthys photophore

1. 1. The isolated photophore or photocytes of Porichthys notatus developed fast and/or slow luminescence in response to stimulation by adrenergie agonists (norepinephrine, epinephrine, isoproterenol, phenylephrine; 1 nM to 1 mM). 2. 2. The isolated photophore and isolated photocytes showed different specific dose-response curves. 3. 3. The α-adrenergic antagonist phenotalamine blocks partially or completely (1) the fast luminescence of photophore and photocytes and (2) the slow luminescence of the photophore. 4. 4. The specificity of the agonist effects and action of phentolamine suggest that α-receptors are associated with neural structures present in the photophore and α- and β-receptors with photocytes. 5. 5. The amplitude of the luminescent response of the photophore is taken to be the sum of the light emitted by the photocytes directly and indirectly stimulated by endogenous and exogenous catecholamines.

Slow luminescence of acetophenone initiated by multiphoton vibrational excitation of triplet molecules

Slow luminescence of acetophenone initiated by multiphoton vibrational excitation of triplet molecules

Luminescence of Isolated Photocytes from Porichthys Photophores: Adrenergic Stimulation

ABSTRACT A fast transient peak of light, followed by a slow luminescence, was produced by isolated photophores of Porichthys notatus when exposed to 1 mm-epinephrine, norepinephrine or phenylephrine, an α-adrenergic agonist. Only a slow luminescence was induced by 1 mm-isoproterenol, the β-adrenergic agonist. After removal of the reflector and surrounding tissues, the isolated photogenic cells, i.e. the photocytes, responded to norepinephrine and phenylephrine by producing a fast transient peak of light, and to isoproterenol by a slow luminescence. Epinephrine evoked both types of luminescence. The emission latency time, the amplitude and the total duration of the light responses were significantly less than those of the photophore. The results suggest that both epinephrine and norepinephrine combine with sympathetic α or β receptors upon the photocytes to trigger light production.

Induction patterns of delayed luminescence from isolated chloroplasts. I. Response of delayed luminescence to changes in the prompt fluorescence yield

1. Using a phosphoroscope, delayed luminescence and prompt chlorophyll fluorescence from isolated chloroplasts have been compared during the induction period. 2. Two distinct decay components of delayed luminescence were measured a “fast” component (from ≈1 ms to ≈6 ms) and a “slow” component (at ≈6 ms). 3. The fast luminescence component often did not correlate with the fluorescence changes while the slow component significantly changed its intensity during the induction period in a manner which could usually be linearly correlated with variable portion of the fluorescence yield change. 4. This correlation was evident after preillumination with far-red light or after allowing a considerable time for dark relaxation. 5. The close relationship between the slow luminescence component and variable fluorescence yield was observed with a large range of light intensities and also in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea which considerably changes the fluorescence induction kinetics. 6. Valinomycin and other antibiotics reduced the amplitude of the 6 ms (slow) luminescence without affecting its relation with the fluorescence induction suggesting possibly that a constant electrical gradient exist in the dark or formed very rapidly in the light, which effects the emission intensity. 7. Changes in salt levels of suspending media equally affected the amplitude of both delayed luminescence and variable fluorescence under conditions when the reduction of Q is maximal and constant. 8. The results are discussed in terms of several models. It is concluded that the model of independent Photosystem II units together with photosynthetic back reaction concept is incompatible with the data. Other alternative models (the “lake” model and photosynthetic back reaction; recombination of charges in the antenna chlorophyll; the “W” hypothesis) were in closer agreement with the results.