Slow Luminescence Component Research Articles

Light outputs of yttrium doped BaF_2 crystals irradiated with neutrons

The fast luminescence component of barium fluoride (BaF_2) crystals with a subnanosecond decay time can find wide application in particle physics and nuclear physics. However, the slow luminescence component with the 630 ns decay time could cause pile-up signals at a high rate environment. Doping of BaF2 crystals with rare earth elements suppresses the slow emission component, but at the same time the radiation hardness of the crystals deteriorates. This work presents the results of studying crystal samples, both pure BaF2 and those doped with yttrium in a proportion of 1 at.%Y, 3 at.%Y and 5 at.%Y, irradiated with a fast neutron fluence of about 2.3×1014 n/cm2. Their light output and decay kinetics were measured before and after irradiation. It is found that the light output loss of a pure BaF2 crystal after irradiation is about 7%, and the light output loss of yttrium doped samples after irradiation is about two times higher. The measurement results demonstrate that after irradiation the fast component of each sample has a relative light output loss 2–3% larger than the slow one.

Рентгенолюминесцентные свойства пленок оксида цинка на M- и A-плоскостях сапфира

The results of comparative studies of the processes of high-temperature synthesis, luminescence and scintillation characteristics of ZnO films on M(100)- and A(110)-orientation sapphire substrates are presented. It is shown that the use of the magnetron deposition method makes it possible to form, against the background of a continuous film, ensembles of individual [001] ZnO microcrystals with pronounced X-ray luminescent properties. The X-ray luminescence kinetics is characterized by two components: a fast component with a decay time in the order of a nanosecond and a slow luminescence component. The study of the films by photoluminescence spectroscopy revealed the features of the near-band-edge luminescence spectra of the samples, in particular, the presence of various excitonic emission channels. Differences in the spectral parameters of the near-band-edge luminescence band in the case of optical and X-ray excitation are found and interpreted.

X-ray Luminescent Properties of Zinc Oxide Films on the Sapphire M and A-Plane

The results of comparative studies of the processes of high-temperature synthesis, luminescence and scintillation characteristics of ZnO films on M(100)- and A(110)-orientation sapphire substrates are presented. It is shown that the use of the magnetron deposition method makes it possible to form, against the background of a continuous film, ensembles of individual [001] ZnO microcrystals with pronounced X-ray luminescent properties. The X-ray luminescence kinetics is characterized by two components: a fast component with a decay time in the order of a nanosecond and a slow luminescence component. The study of the films by photoluminescence spectroscopy revealed the features of the near-band-edge luminescence spectra of the samples, in particular, the presence of various excitonic emission channels. Differences in the spectral parameters of the near-band-edge luminescence band in the case of optical and X-ray excitation are found and interpreted. Keywords: Films, microcrystals, zinc oxide, X-ray luminescence, photoluminescence, excitonic emission.

Study on fast luminescence component induced by gamma-rays in Ce doped LiCaAlF6 scintillators

We discuss the origin of the fast luminescence component induced by fast electrons generated in gamma-ray interactions in Ce doped LiCaAlF6 scintillators. Although the slow luminescence component induced by Ce3+ emissions depends on the Ce concentration in the LiCaAlF6 scintillator, the fast component is independent. The fast component is suggested to be generated in the host matrix of the LiCaAlF6 crystal. From quantitative considerations based on Frank–Tamm equation, which shows the light yield of the Cherenkov radiation, the Cherenkov radiation was determined as the origin of the fast component. We, additionally, found that the slow rise time of main Ce3+ emissions in the Ce:LiCaAlF6 scintillator plays an important role to perform the pulse shape discrimination.

Energy Transfer from CdSe Quantum Dots to Porphyrin via Two-Photon Excitation

CdSe quantum dots have been used as energy donors to activate meso-tetra (4-sulfonatophenyl) porphine dihydrochloride. Pulses of 130 fs duration at a wavelength of 800 nm are used as the two-photon excitation light source. After excitation, steady-state and time-resolved fluorescence spectra are collected for samples with different ratio between the amount of porphyrins and quantum dots. Decay kinetic curves of CdSe quantum dots with and without porphyrins are well fitted by the biexponential decay curve, which indicates a combination of two components (excitonic and trapping state) in the luminescence behavior of CdSe quantum dots. Relative intensity weights of the excitonic and trapping state, total and nonradiative energy transfer efficiency, average luminescence lifetimes of donors are calculated. The nonradiative transfer mechanism becomes the leading factor as the concentration of acceptors gets higher. It is considered to take place through the channel of trapping states of CdSe quantum dots, which is expressed by the weight change between the fast and slow luminescence components. This deduction presents a new way of raising the energy transfer efficiency by increasing the trapping state proportion of the quantum dots, which can be easily realized by surface modification.

Decay course of slow emission component for thallium and lead centers in some alkali halide crystals

AbstractWe discuss some of our results on KCl and KBr crystals doped with very low concentration of Pb2+ and Tl+ ions. The aim is to understand validity of two theoretical models developed to explain an anomaly of some slow luminescence components. Low doping concentration follows an effort to suppress any parasitic effects on the measured decay curve. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence properties and afterglow in spinel crystals doped with trivalent Tb ions

Luminescence properties of Tb3+-doped spinel crystals have been investigated at various temperatures between 8 and 300K under excitation in the Tb3+ absorption bands and in the intrinsic absorption region. Two Tb3+ absorption bands are observed at 5.3 and 6.3eV in the intrinsic absorption edge region. With excitation in these bands, two series of sharp luminescence bands appear in the region between 1.8 and 3.5eV, which are to be well established as those due to the series transitions from the 5D3 and 5D4 excited states to the 7DJ (J=6, 5, 4, 3,2) ground states, with radiative lifetimes of 1.5 and 2.0ms, respectively. The same luminescence spectrum is produced with excitation in the intrinsic absorption region above 7eV. In this case, prominent slow rise and afterglow components of luminescence are observed which last for up to an hour depending on the temperature. The origins of the slow components of luminescence are connected to the presence of oxygen vacancies as electron trapping centers and Tb4+ ions as hole trapping centers. Optically created electrons and holes would be trapped at the above-mentioned centers and released thermally to activate Tb3+ ions giving rise to the slow luminescence components. Discussion will be made on the dynamical processes of electrons and holes in the excited states.

Ion-induced Luminescence of Alumina with Time-resolved Spectroscopy

Ion-induced luminescence of alumina was analyzed with a time-resolved spectroscopic method. By using a pulsed ion beam, we selectively observed the prompt and slow components in the total luminescence. The luminescence spectrum showed some broad peaks at 330 nm (F+ center), 410 nm (F center), 510 nm (F2 center), 560 nm (F2+ 2 center). By observing prompt luminescence in detail, new emission bands have been identified in the wavelength range from 400 to 450 nm. Furthermore, the lifetime of each luminescence was successfully evaluated with single photon counting. The emission band at 400 nm was found to have a short lifetime of 1.9 ns. From the analysis of a slow luminescence component, it was observed that the emission band with a long lifetime appeared around the wavelength of 330 nm.

Anomalous decay of the slow emission component in doped alkali halides

We provide a wide class of experimental fits in support of a model previously proposed to explain anomalous non-exponential decay of the slow luminescence component of Pb 2+ centers in alkali halide crystals. The model was based on the assumption that the lattice relaxes on the same time scale as the slow-component decay time. This relaxation is a response to the Jahn–Teller local distortion of the Pb 2+ luminescence centers. The original work studied KBr : Pb 2+ centers (at LHe temperature). We now apply the method to fit experimental data for the slow-component decay of KX : Pb 2+ and KX : Tl + (XCl, Br, I). An essential feature of the model is that lattice relaxation causes a time-dependent coupling of the Jahn–Teller levels. In the first place, the present successful fits provide evidence for the mechanism earlier proposed. Moreover, the dependence of the fitting parameters on lattice and impurity features allow systematic physical interpretation.

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.

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.

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.