Optical absorption measurements were performed on LiF:Mg and LiF:Mg, Ti crystals after irradiation and various thermal treatments. From the close correlation between the 4.0 and 5.5 eV absorption bands introduced by the Mg dopant a model for the trapping process in thermoluminescent LiF is developed basing on Z centers. Traps initiating main glow peak 5 are assumed to be Z2 centers being produced by irradiation via Z0 and Z3 centers in a two step process. The model explains radiation sensitization as well as the necessity for pre‐irradiation annealing in thermoluminescence dosimetric application.
The dependence of optical absorption and thermoluminescence of LiF crystals on F band bleaching time at different temperatures is used to study the mechanism of “restored” thermoluminescence as applied in thermoluminescence dosimetry. F centers turn out to act as electron sources for refilling of traps. Simultaneously, additional Z2 centers responsible for the TL main peak five are created by the presence of F centers during F band bleaching. The results confirm the close relationship between the 3.3 eV absorption band and the TL pre‐peaks.
I n order to clear the trap structure of the so-called T L peak 6 in thermoluminescent magnesium a,nd titanium doped lit,hium fluoride, an appropriate irradiation and post-annealing treatment is used. The comparison of the TL and optical measurements shows with increasing dose an increase a t the same rate of the height of TL peak 6 and the absorption band a t E = 5.5 eV. Because this band is due t.o Z, centers (Mg2+-F pairs), it is deduced that the same centers are responsible for TL peak 6. The result of the investigations is in agreement with a recently published model for the trapping process in Lip.Um die Frage nach der Struktnr der Haftstelle des sogenannten TL-Peak 6 im therrnolumineszenten magnesium und titandotierten Lithiumfluorid zu klaren, wird eine geeignete Bestrahlungsund Temperbehandlung angewendet. Der Vergleich von TL und optischen Messungen zeigt, daI3 die Hohe von TL-Peak 6 und die Hohe der Absorptionsbande bei E = 5,5 el7 in gleicher Weise mit der Dosis ansteigt. Da diese Absorptionsbande auf Z,-Zentren (AIg2+-F Paare) zuruckzufahren ist, kann gefolgert werden, da13 dieselben Zentren fur den TL-Peak 6 verantwortlich sind. Das Ergebnis der Untersuchungen ist in 'iibereinstimmung mit einem kiirzlich veroffentlichten Modell, welches den Haftstellen-ProzeB in LiF beschreibt.
The effect of sensitization is well known in LiF:Mg /l to 5/. Recently a new concept for the formation of thermoluminescence centers in LiF was developed which is based on Z centers /6/. The main idea is that the luminescent centers are Z2 centers which at about 200 OC by releasing of electrons transform into Z centers. After the sensitization procedure (by X-irradiation at high dose followed by anneal at about 300 OC) a high concentration of Z3 centers is present /7/. A s a consequence of this model it was then postulated that the 2 sensitization mechanism should be mainly due to a conversion of Z into Z centers by trapping of electrons thus leading to a higher concentration of Z2 centers in sensitized material. The aim of the present report is to indicate that such a backward reaction is really observed in sensitized LiF in contrast to the negative discussion reported by Jain and Kathuria /5/. 3Crystal growth, sample preparation, and analysis of the optical absorption spectra with the use of a computer a r e described in detail in /6/. Since there is no difference in the behaviour of the optical absorption bands of LiF:Mg, Ti and LiF:Mg concerning a series of Z bands peaking at 3.28 eV (Z?), 4.00 eV (Z2), and 5.50 eV ( Z 3 ) and F band peaking at 4.96 eV (see Table 1 of /6/), we have confined our studies to LiF:Mg because of the simpler analysis due to the missing Ti absorption bands which appear at the UV wavelength region.It should be stressed that the validity of this computer analysis has been ascertained by analyzing the continuous growth of those Z and F bands against increasing radiation doses keeping their peak positions and halfwidths constant.The sample was pre-annealed 1 h at 480 OC, cooled within 5 min to room temperature, and was then annealed for 24 h at 80 OC. The sample was then sensitized with a dose of about 1 K/kg and annealed 1 h at 280 OC. The sen-1) Kodatsuno 2-40-20, Kanazawa-920, Japan.2) Physikalisch-Technische Bundesanstalt, Institut Berlin (West).
Introduction Z bands a r e well known in alkali halide crystals with divalent impurities. The energetic positions of these optical absorption bands are nearby the positions of the specific F bands. Z centres a r e formed either directly during high energetic irradiation or afterwards during optical bleaching of F centres at room temperature (RT) due to photochemical processes /l/. As we have shown recently /2/ Z2 and Z3 absorption centres a r e of special interest in LiF:Mg, Ti as used for thermoluminescence dosimeter material.The aim of investigations presented here was to study the formation of Z1 centres due to F-band bleaching in X-irradiated Mg doped LiF crystals with the help of optical absorption measurements and computer analysis and to obtain data on the energetic position and halfwidth of the Z1 band. The Z1 centre is a complex of an F centre, a divalent cation and a cation vacancy /3/.3 Experimental Self-grown crystals with 200 at ppm Mg and a size of 8 x 8~2 mm were used for the study. The crystals were given a heat treatment at 480 C for 1 h and then cooled in a i r to RT within 5 min followed by annealing at 80 O C for 24 h. This thermal procedure a s recommended for dosimetric application helped here to avoid thermal changement during the bleaching procedure. The X-irradiated crystal (40 kV , 30 min, 0.3 kC kg-l) was bleached at RT with the radiation of an unfiltered -2 low-pressure W lamp (90% of spectral radiant flu at 254 nm, irradiance 2.0 mW cm ) for various bleaching times. Each bleaching step was followed by an absorption measurement and the computer analysis. The details of the resolution of the absorption spectra into a set of Gaussian shaped bands have been described earlier /2/. 0 Results and discussion Fig. 1 shows the effect of F-bleaching on the absorption spectra of an X-irradiajed LiF:Mg crystal. The lowest curve is the absorption spectrum of the unirradiated crystal and the topmost curve the absorption spectrum of the crystal after X-irradiation. The bleaching times a r e indicated as parameters. K158physica status solidi (a) 41
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