Scintillation properties of ceramics based on zinc oxide
V. A. Demidenko, E. I. Gorokhova, I. V. Khodyuk, O. A. Khristich, S. B. Mikhrin, P. A. Rodnyi
Scintillation properties of ceramics based on zinc oxide
Vladimir A. Demidenko a , Elena I. Gorokhova a , Ivan V. Khodyuk b , ∗ , Ol’ga A. Khristich a ,Sergey B. Mikhrin b , Piotr A. Rodnyi b a Federal State Unitary Organization “Research and Technological Institute of Optical Materials All-Russia Scientific Center”, S.I. Vavilov State OpticalInstitute, Babushkina 36-1, 192171 Saint-Petersburg, Russia b St. Petersburg State Polytechnic University, Polytekhnicheskaya 29, 195251 Saint-Petersburg, Russia
Received 17 December 2006; accepted 31 January 2007
Abstract
Ceramics ZnO:Zn of 20 mm diameter and 1.6 mm thickness with an optical transparency up to 0.33 in the visible region have been preparedby hot pressing technique. Scintillating and luminescent characteristics such as emission spectra, decay time, yield, and TSL glow curve havebeen measured under X-ray excitation. Two emission bands peaking at 500 and 380 nm were detected, the light output was about 80% ofthat for standard BGO scintillator, main decay constant was 10 . ± . Keywords:
ZnO; Optical ceramics; Fast scintillators
1. Introduction
The well known blue-green phosphor ZnO:Zn, that is zincoxide containing excess zinc, has been under investigation fora few decades (Shionoya and Yen, 1999), because of its usein vacuum fluorescent and field-emission displays. Lumines-cent characteristics of the ZnO:Zn are studied in detail thoughphysical mechanisms responsible for its emission are barelyunderstood yet (Shionoya and Yen, 1999; Van Dijken et al.,2000). Zinc oxide is non-hygroscopic, stable over a wide rangeof temperatures, mechanically robust, and sufficiently dense,5 .
61 g / cm . Currently some compounds based on ZnO are con-sidered as promising fast scintillators (Moses, 2002; Simpsonet al., 2003; Kubota et al., 2004). Efficient ultrafast ( (cid:2) < ) scintillations have been observed in ZnO:Ga and ZnO:Inphosphors (Moses, 2002; Simpson et al., 2003). The ZnO:Znscintillators containing Li have been developed for high-counting-rate neutron imaging (Kubota et al., 2004; Katagiri etal., 2004). The scintillators based on ZnO show a high radiationhardness and appropriate stopping power. Such scintillating ∗ Corresponding author. Tel.: +7 812 555 3739; fax: +7 812 552 7574.
E-mail address: [email protected] (I.V. Khodyuk).1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.radmeas.2007.01.050 materials could hold a central position among fast scintillators,however, for many purposes the large-volume scintillators arerequired. Zinc oxide is available mostly in powder or thin filmform and only recently a small single crystal ZnO has beenproduced (Simpson et al., 2003).In this work, we have studied some scintillating andthermo-stimulate luminescent (TSL) properties of the ZnO:Znceramics.
2. Experimental
A hot pressing technique was used to transform the ZnO:Znpowder into a transparent ceramic scintillator. A problem whensintering ZnO ceramics is the hexagonal symmetry of its crys-tal structure. Anisotropic grains in the ceramics cause consid-erable light scattering in the grain boundaries resulting in lightabsorption in the bulk. Nevertheless, the ZnO:Zn ceramics withreasonable optical transparency (Fig. 1) have been obtained.To create a high quality optical ZnO:Zn ceramics, we haveused our preceding experience in the ceramics sintering fromnon-cubic materials (see e.g., Gorokhova et al., 2005). Insetof Fig.1 illustrates a good optical transparency of the ZnO:Znceramics. Vladimir A. Demidenko et al. / Radiation Measurements 42 (2007) 549–552 O p t i c a l t r an s pa r en cy ( % ) Fig. 1. Optical transparency of the ZnO:Zn ceramics of 1.6 mm thickness.Inset: a photo of a fragment of the ZnO:Zn.
The specimens of a 20 mm diameter and 1.6 mm thicknesswere used in the measurements. The concentration of excessZn in ZnO was within 10 –10 cm − . All specimens werechecked by X-ray diffraction. The density of the obtainedZnO:Zn ceramics was more than 99% of that for single crys-tal. The microstructure of the ceramic specimens was studiedusing an optical microscope.The emission spectra were measured at a steady state X-rayexcitation (40 kV, 10 mA) with a photomulplier tube FEU-106and a MDR-2 grating monochromator with 1200 lines/mm. Thesame set-up was used for measurement of TSL in the rangefrom 80 to 500 K. The kinetic curves were recorded at pulsed (< ) X-ray excitation using standard START-STOP detec-tion system described elsewhere (Rodnyi et al., 2001). Theemission spectra were measured in the reflection mode: the an-gle between a X-ray beam and a photomulplier was 90 ◦ . Decaytime curves of the ceramics were measured in the transmissionmode: a photomulplier tube was placed behind the sample inthe line with a X-ray beam. The scintillation characteristics ofthe ZnO:Zn ceramics were compared with those of Bi Ge O (BGO) scintillator of the same thickness using it as a standard.The studied sample and BGO were placed as far as possible insimilar conditions.
3. Results
The emission spectrum of the ZnO:Zn ceramics is shown inFig. 2 (curve 1). The spectrum displays an intense broad bandpeaking near 500 nm (2.5 eV) and a weak narrow band peak-ing at 380 nm (3.2 eV). In pure polycrystalline ZnO the similaremission bands have almost equal intensities (curve 2, Fig. 2).Emission spectrum of the BGO scintillator is presented inFig. 2, curve 3, for comparison.The light output, which was determined as area under emis-sion curve, of the ZnO:Zn ceramics was 80% of that of BGO.The light output of the ZnO:Zn ceramics increases by factor 2at cooling of the specimen to 80 K. This increase occurs owingto the growth of the blue-green emission, the intensity of the380 nm band does not change with temperature. I n t en s i t y ( a . u . ) Fig. 2. X-ray induced emission spectra of the ZnO:Zn ceramics (1), the purepolycrystalline ZnO (2) and BGO (3), T =
295 K. µ s) C oun t s C oun t s
30 60 90Time (ns)2 1
Fig. 3. Luminescence decay time curve of the ZnO:Zn ceramics (1) and BGO(2) under X-ray excitation at room temperature. Inset: the beginning parts ofthe curves.
The ZnO powder shows a proportional growth of both emis-sion bands with decreasing the temperature. Besides, at 80 Kthe 3.2 eV band shows a structure consisting of four additionalmaxima with a step of about 0.1 eV between them. This valueis closed to the energy of longitudinal optical phonons in ZnO,0.073 eV (Park et al., 1966). So, we can suppose that additionalmaxima appear from phonon replications.The luminescence kinetic characteristics of the ZnO:Zn incomparison with that of BGO are presented in Fig. 3. Onecan see that the ZnO:Zn ceramics shows better decay timecharacteristics than BGO. The decay curve of the ZnO:Zn canbe approximated by two exponential components: fast onewith constant (cid:2) = . ± . (cid:2) = . ± . (cid:2) s. One can see that contribution of the fastcomponent in the total yield is a considerable one. We anticipatethat the ZnO:Zn possesses a low level of afterglow, because the ladimir A. Demidenko et al. / Radiation Measurements 42 (2007) 549–552 I n t en s i t y ( a . u . ) Fig. 4. TSL glow curve of the ZnO:Zn ceramics after X-ray irradiation at80 K. scintillation intensity at 50 (cid:2) s is 0.1% of the maximumintensity.The TSL glow curve of the ZnO:Zn ceramics irradiated dur-ing 100 s by X-rays at 80 K is shown in Fig. 4.
4. Discussion
The observed optical transparency (33% at 1.6 mm thick-ness) of the ZnO:Zn ceramics fabricated by the hot pressingtechnique is not a bad result for a non-cubic material. The ce-ramics show two emission bands like ZnO:Zn and ZnO pow-ders. A relatively week and sharp UV band (380 nm) (belowabsorption edge) is well studied in pure ZnO and assigned toa radiative recombination of excitons (Van Dijken et al., 2000;Wilkinson et al., 2001). The origin of a more intense and broadband (500 nm) in the visible part of the spectrum was unknownfor a long time. Recently Van Dijken et al., (2000) have shownthat this band arises from a recombination of electrons on oxy-gen vacancies. Excess interstitial Zn increases the number ofoxygen vacancies in ZnO (Lott et al., 2004) and, as a result,the intensity of the visible emission grows.The shapes of the emission spectra of the ZnO:Zn ceram-ics and the ZnO:Zn powder from which this ceramics has beenprepared are very similar. Note, that the ZnO:Zn ceramics fab-ricated by spark plasma sintering (SPS) technique emits only aUV band (Kubota et al., 2004). These authors assigned the dis-appearance of the 500 nm emission band to a strongly reducingatmosphere of the SPS technique.We have not carried out time-resolved spectrum measure-ments, but an experiment with the corresponding filters showsthat the fast ( ∼
10 ns) and slow ( ∼ (cid:2) s) components of the X-ray stimulated emission are connected with the UV and visiblebands, respectively. The radiative lifetime of excitons in pureZnO lies in the subnanosecond range (Wilkinson et al., 2001),while in the ZnO:Zn ceramics the observed decay of the ex-citonic band is about 10 ns. The increase in the decay time ofthe ZnO:Zn may be related to the presence of shallow traps, which are formed due to the boundaries between grains of ce-ramics. Thus, in the ZnO:Zn ceramics obtained from the stan-dard ZnO:Zn phosphor the excitonic emission (380 nm band)produces fast scintillations, while the slow component (500 nmemission band) is responsible for a high light output. The con-centration of excess Zn in the ZnO:Zn phosphor was chosenfrom the consideration of maximum light yield. So, we supposethat if scintillation response time were more important thanits light output, one could increase the contribution of the fastcomponent of the ZnO:Zn ceramics via the Zn concentrationdecrease.The result of TSL measurement shows that the depth of maintraps or defects created at irradiation of the ZnO:Zn ceramics atliquid nitrogen temperature is 0.21 eV. According to Lott et al.(2004) the shallow traps in ZnO are associated with the residualhydrogen impurity. It is worth noting that the irradiation ofthe ZnO:Zn ceramics at room temperature does not produce anoticeable TSL which is an evidence of high radiation hardnessof the studied specimen.
5. Conclusions
The optical ceramics ZnO:Zn of 20 mm diameter and 1.6 mmthickness have been prepared by hot pressing technique. A mainwide emission band peaking at 500 nm is presumably associatedwith the recombination of electrons on oxygen vacancies. Thisband provides the high light output of the ZnO:Zn ceramics. The380 nm narrow band, which is caused by excitonic emission, isresponsible for fast response of the scintillator. In general, theobtained ZnO:Zn ceramics offer rather good characteristics: • optical transparency of up to 33% in the visible region; • high intensity of the X-ray stimulated luminescence, whichis comparable with that for standard BGO scintillator; • main decay constant of about 10 ns; • low level of afterglow; • predominant blue-green emission; • high radiation hardness.The elaborated hot pressing method can be used for formingother ZnO-based ceramic scintillators from powder compounds.Finally, we expect that ZnO-based ceramics can become po-tential candidates for new fast scintillators. References
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