Decay times- phosphors

Long decay time phosphor for use in radar. Orange emission at 575 nm... 

Luminescent Pigments. Luminescence is the abihty of matter to emit light after it absorbs energy (see Luminescent materials). Materials that have luminescent properties are known as phosphors, or luminescent pigments. If the light emission ceases shortly after the excitation source is removed (<10 s), the process is fluorescence. The process with longer decay times is referred to as phosphorescence. 

Yellow-emitting (526 nm) phosphor decay time in nanoseconds... 

Organic scintillation phosphors include naphthalene, stilbene, and anthracene. The decay time of this type of phosphor is approximately 10 nanoseconds. This type of crystal is frequently used in the detection of beta particles. 

The most important mineral example is natural scheelite. ScheeUte emits a bright blue emission in a broad band centered at 425 nm (Fig. 4.9) with a decay time of several ps. Calcium tungstate CaW04 has long been known as a practical phosphor, and has been carefully studied. The intrinsic blue luminescence center is the complex ion in which the central W metal ion is... 

The results of their decay-time measurements are summarized in Table IX. The measurements were made using either cathode-ray or ultraviolet excitation. For the emissions excited by cathode rays the technique of Bril and Klasens (750) was employed. The ultraviolet excitation was accomplished with a cathode-ray tube equipped with a fast ultraviolet-emitting phosphor. 

When fired in a reducing atmosphere, Y3A15Oi2 exhibits a pronounced afterglow due to traps formed by oxygen vacancies [5.356]. Subsequent annealing in air diminishes this effect and leads to decay times of 200 300 ns therefore, Y3Al5Ol2 Ce3+ is used in flying-spot scanner tubes. The emission maximum is at 550 nm. This phosphor is classified under P46 (TEPAC) and KG (WTDS) (see Section 5.5.4.3). 

ZnO Zn is a typical example of a self-activated phosphor. In the case of zinc oxide, it is an excess of zinc which enables the phosphor to luminesce. The production is carried out by thermal oxidation of crystallized zinc sulfide in air at ca. 400 °C. The green luminescence, with a broad maximum at 505 nm, has a very short decay time of 10-6 s. As a phosphor for cathode-ray tubes, ZnO.Zn is classified in the TEPAC list as P 24 and in the WTDS system as GE. 

Because of the high resolution required for monitor tubes, phosphors with smaller particle sizes (4-6 pm) than for entertainment tubes are often needed. For monitor tubes which reproduce slow movement only, phosphor mixtures with longer decay times are used to diminish image flickering. For the reproduction of faster movement, phosphors with shorter decay times are used. For monochrome monitors with amber as the image color Cd5(P04)3Cl Mn2 + or a blend of Y202S Eu3+ and (Zn, Cd)S Cu+ is used. 

Decay. The decay time requirements must be adhered to very precisely for cathode-ray tube phosphors. The measuring devices consist of fast excitation sources (flash lamps, lasers), photomultipliers with very low time constants, and an oscilloscope [5.440]. 

The rate of emission of photons from a scintillator is XNoeh, where N0 is the total number of photons emitted and X is the decay constant of the phosphor. For Nal(Tl) the decay time x (=A I) 220ns, whereas for fast plastic x 2ns (i.e., in the first ns 40% of the light from a plastic scintillator is typically emitted, whereas only 0.5% is emitted from Nal(Tl)). 

Production of good phosphors requires rigorous attention to synthesis or activation conditions. Careful control of activator concentrations, usually measured in parts per million, is essential. Multiple activators may be necessary to achieve the desired color, brightness, response or decay times. Virtually umneasurable, trace amounts of unwanted substances may have such severe deleterious effects on phosphor performance that the materials are usually produced under clean room conditions rivaling those used in the production of silicon based integrated circuits. 

Often temperature measurements are made using thermocouples or optical pyrometry. However, in situations where rapid motion or reciprocating equipment is present at high temperatures, it is best to use other techniques. For many phosphors, the prompt fluorescence decay time (t) varies as a function of temperature and is defined by ... 

The time needed to reduce the light intensity to e (36.8%) of its original value is defined as the prompt fluorescence decay time. An example of this quantity for several thermographic phosphors is shown in Fig. 1. Notice the fluorescence decay time decreases by four orders of magnitude when the temperature increases from 600 to 1100°C. 

Fig. 1 Prompt fluorescence decay time for a selection of phosphors.

Phosphor paint Excitation laser wavelength (nm) YSZ thickness (mm) Observed fluorescence (YSZ side) Measured fluorescence decay time (t)... 

A quantitative determination of fluorescence intensity as a function of cycling temperature is more complex. It was decided to use a ratio of 0.2 (20%) of the maximum emission intensity as the criteria to determine the viability of fluorescence for a given TSP sample. If the fluorescence emission is small, it will be difficult to measure the decay time and obtain a corresponding surface temperature. There will come a point in intensity where a phosphor system cannot be used to measure temperature. The decision ratio of 0.2 was completely arbitrary and was based on the observation that the apparent fluorescence measurement uncertainty was about 10% (intensity fraction of 0.1), which was two times the measured error for the 611 nm line for Y2O3 Eu. 

Recent research has shown that phosphor-based TSPs could be used for high temperature thermometry applications. Fluorescence from YAGiEu and YAG Ce could be detected through several thin YSZ samples. The average prompt fluorescence decay time for YAGiCe was measured to be 62.7 zb 2.9 ns, which is close to the accepted value of 65 ns. This result can be used directly to develop an operational high temperature heat flux gauge. 

The time response of the IPDA is usually determined by the decay time of the phosphoresence of the phosphor layer. For P-20 phosphor, it usually takes about 2 ms for the output to decay to 10% of the initial value. A typical decay characteristic for an MCP intensifier is shown in Figure 6. The decay is not exponential. Faster decays can be obtained using other phosphors, but phosphor efficiency and optimal matching to the PDA response may then be compromised. 

TABLE 16.16 Phosphor Properties of the Decay-Time Thermometers [64]... 

Phosphor Emission line, nm Absorption bands, nm Excitation device Decay time Tamb, ps Sensitivity, ps/°C Temperature range, °C... 

We have already briefly discussed luminescence decay times. Reiterating, the decay time of a phosphor has been defined as the time for the steady state luminescence intensity to decay to 1/e, or 0.368, of its original Intensity. It has been found that the intensity of photon emission builds up in the order of microseconds, i.e.- 10 sec. to a specific value, i.e.- the excitation process takes only a few microseconds. Since the intensity also decays in microseconds (if the excitation source is removed), there is an equilibrium value attained in the presence of the excitation source, which is a combination of both excitation time and decay time. This so-called steady state is called Iq, and is promulgated by the population of emitting... 

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