Glow peak

Fig. 21.15 Total glow peak plot ofln = ln(/(T)//- dT) versus 1/T. The points are experimental data and the solid line represents the least-squares fit.

Because the escape probability of carriers from trapping sites is proportional to exp(-fi/ D, the location of a glow peak on the temperature scale provides encoded information on the value of thermal activation energy E. Hence, a glow curve represents a spectrum of energies that are required to free carriers from the various species of traps in the material. 

Analytic solntions for a(T) were reported by Simmons and Taylor [12] for the case that retrapping can be neglected in a thin sample at high electric fields. They considered the presence of several trap levels of density A and demonstrated the snperposi-tion of the individnal glow peaks when the thermal ionization energies of these levels are very close to each other (Fig. 1.3). 

Figure 1.3 Examples of glow spectra resulting from several different discrete trap levels that are (a) widely separated, resulting in clearly resolved glow peaks (b) not sufficiently separated to be clearly resolved in the glow spectrum and (c) overlapping to produce a single, unresolved glow peak [12].

DTA of inorganic fire retardants run in oxygen may shift the peak position temperature or the amount of heat released. Sodium tetraborate reduced the volatile products exotherm considerably, increased the glowing exotherm, and stimulated the appearance of a second glowing peak around 510 °C, as seen in Figure 14. Sodium chloride also reduced the first exotherm, increased the size of the second, but did not produce a second glowing exotherm as did the... 

PV Sane, TS Desai, VG Tatake and Govindjee (1977) On the origin of glow peaks in Euglena cells, spinach chloroplasts and subchloroplast fragments enriched in system I and II. Photochem Photobiol 26 33-39... 

AW Rutherford, AR Crofts and Y Inoue (1982) Thermoluminescence as a probe of photosystem II photochemistry The origin of the flash-induced glow peaks. Biochim Biophys Acta 682 457-465... 

This phenomenon was observed with polymers some years ago [177—183]. The more recent investigations are due to Buben and Nikolskii [183]. These workers measured the emission from many amorphous and crystalline polymers and observed correlations between the temperature corresponding to the glow peak maxima and the structural transition temperatures of the materials. A detailed study of polyethylene thermoluminescence was made by Charlesby and Partridge [184]. The glow curve obtained after irradiation in vacuo possesses three peaks, a, j3 and y, whose luminescence intensities are proportional to the irradiation dose for doses below 5 x 104 rads. The maxima occurs, respectively, at —110, —65 and —27°C, when the total... 

An Arrhenius-type plot of the probability constant p (= 0.693/tl/2) shows a break between —65 and —40°C. The activation energies of the a, 0 and y peaks are identical at temperatures below the structural transition of polyethylene. These activation energies and the related pre-exponential factors agree closely with those for molecular motion in polyethylene (Table 10). If oxygen is present during irradiation, the 3 and y glow peaks are completely removed at low doses and replaced by a new peak, e, at lower temperature. The thermoluminescence emission spectra of the a, y, e and probably the j3, glow peaks are identical and correspond to the emission of aliphatic aldehydes. 

The technique of radiophotoluminescence excitation spectra is used to study the displacement of electrons from physical traps to solute molecules and vice-versa in solutions of biphenyl in methylcyclohexane. Bleaching in the infrared leads to the disappearance of trapped electrons and of the first peak of radiothermoluminescence. If a bleached sample is illuminated in one of the two biphenyl anion absorption bands, both infrared-stimulated RPL and a weak first glow peak reappear. It is concluded that of the two RTL peaks observed at 90° and 95° K. in methylcyclohexane glass, the first is caused by recombination of electrons with solute cations and the second by diffusion and recombination of solute anions and cations. 

In a sample with a restored infrared response, the first glow peak reappears but seems weaker than would be expected from the photoresponse. 

In a sample with a decreased response in the anion absorption bands, the second glow peak also is reduced. 

Buben and co-workers (32, 41,42) investigated RPL excitation spectra, and they observed in aromatic matrices a similar concomitant decrease of two types of bands (42). They attributed one of these to anions and the other to trapped holes. In our case at least, this hypothesis does not apply because it would be incompatible with the observed restoration of the infrared response and of the first glow peak which can be fully explained by the photodetachment of electrons from biphenyl anions and their retrapping in the matrix. 

The observed link between infrared response of a sample and the first glow peak proves conclusively that electrons liberated from the matrix are responsible for the first peak. 

The second glow peak is reduced when the number of trapped anions is reduced. Therefore, the second glow peak must be ascribed at least partially to the diffusion of biphenyl cations and anions and their mutual neutralization ... 

It is probable that at doses sufficient to produce a fair number of solvent radicals (e.g., about 1 Mrad), these also act as traps for the electrons and holes, as suggested by Buben and co-workers (32, 41, 42), and thus contribute to the second glow peak. 

In conclusion, the first glow peak (90°K.) in MCH glass containing biphenyl can be described by Reactions 1 and 3 and the second peak (95°K.) by Reaction 5. We cannot exclude at present a participation of solvent cations, as follows ... 

Besides radiation dosimetry, TL can be used for UV dosimetry by making use of what is termed as PTTL—Phototransfer thermoluminescence. If an irradiated material is heated past highest temperature glow peak, cooled to room temperature, and then subjected to UV light, some of the traps are refilled and TL is observed. This is termed as PTTL the intensity of emission depends upon the integrated flux of the UV photons to which the material has been exposed. Thus, from the intensity of PTTL one may back calculate the intensity of UV source. 

It is revealed that the intensity of the TL peak increased with the concentration of Cu" and arrived at the maximum value at 0.2 mol%. Above this concentration, TL intensity decreased, which may be due to concentration qnenching. It is observed that as the exposnre dose increases, the TL intensity also increases. The increase in the intensities of the glow peaks with increase of irradiation dose can be understood by the fact that more and more traps responsible for these glow peaks were filled with the increase of irradiation dose and subsequently these traps release the charge carriers on thermal stimulation to finally recombine with their counterparts, thus giving rise to different glow peaks. 

Order of kinetics Order of kinetics (b) was determined by calculating symmetry factor ( Xj) of the glow peak from the known values of shape parameters ... 

This parameter (y) for 160°C glow peak of LiNaS04 Cu,Mg was found to be 1.59, which suggests that this peak obeys second-order kinetics. 

Trap Parameters of 160°C Glow Peak of LiNaS04 Cu,Mg Phosphor... 

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