A. Cecilia

Radiation induced formation of color centers in PbWO4 single crystals

The changes in the absorption spectra induced by Co60 irradiation and high temperature annealing was studied for selected PbWO4 crystals. Based on radiation and annealings induced absorption spectra, four color centers are proposed, which could be responsible for shaping the absorption spectrum of PbWO4 crystals below 3.6 eV, namely Pb3+, O−, F, and F+ centers. Thermoluminescence (TSL) glow curves above room temperature were also studied on these samples to determine further the trap levels resulting from irradiation. Correlation between the results obtained by TSL and Co60 irradiation induced absorption changes is discussed.

Improvement of several properties of lead tungstate crystals with different doping ions

Abstract A very good radiation resistance of lead tungstate crystals is mandatory for their use in the high-precision electromagnetic calorimeter of the CMS experiment at LHC. Since the beginning of 1996 we have organised systematic investigations of the parameters influencing the radiation hardness of this crystal. Two classes of parameters have been particularly studied, the first one related to the control of the stoichiometry and structure-associated defects, the second one connected with the suppression and the charge compensation of existing defects with different kinds of doping ions. This paper reports about the second part of this study and complements a first paper where the role of the stoichiometry was already discussed. Results of tests are given on a significant statistical sample of full size crystals (23 cm) which show a considerable improvement in the optical properties and the radiation resistance of appropriately doped crystals.

SIGNIFICANT IMPROVEMENT OF PBWO4 SCINTILLATING CRYSTALS BY DOPING WITH TRIVALENT IONS

Abstract Doping by different trivalent ions (La3+, Lu3+, Gd3+, Y3+, Sc3+) was systematically studied in PbWO4 crystals in the same way, using raw material from the same source. The result was compared with an undoped sample (actually a Pb-rich one) and samples doped with divalent (Cd2+) and pentavalent (Nb5+) ions. All the trivalent ions but Sc3+ gave a significant improvement in transmittance in the short wavelength region (330–450 nm) and in radiation hardness. Among all the tested dopants, Y3+ and Gd3+ showed the best result; these dopants may be worth further studies, since they have another advantage of the segregation coefficient close to unity.

Improvement in radiation hardness of PbWO4 scintillating crystals by La-doping

Abstract La-doping significantly improves the radiation hardness of PbWO 4 against γ-ray irradiation. The induced absorption coefficient of La-doped samples is as small as 2 m −1 at 420 nm even at 10 8 rad (10 6 Gy). The radiation-induced phosphorescence is more than three orders of magnitude less in La-doped PbWO 4 crystals than in undoped ones.

Influence of La3+-Doping on Radiation Hardness and Thermoluminescence Characteristics of PbWO4

(a) ENEA-INN/TEC, Via Anguillarese 301, S.Maria di Galeria, 00060 Roma, Italy (b) Institute of Physics, Cukrovarnicka 10, 16200 Prague, Czech Republic (c) INFN, Dept. of Physics, University of Roma 1, P. le A. Moro 2, Roma, Italy (d) SIT, Shonan Institute of Technology, Fujisawa 251, Japan (e) CRYTUR Preciosa a.s., Palackeho 175, 51119 Turnov, Czech Republic (f) KEK, National Laboratory for High Energy Physics, Tsukuba 305, Japan (g) INFM and Dept. of Physics, University of Milano, Via Celoria 16, 20133 Milano, Italy (h) Furukawa Co., Yoshima, Iwaki 970-11, Japan

Scintillation characteristics of PbWO4 single crystals doped with Th, Zr, Ce, Sb and Mn ions

Abstract In our previous papers we reported that La 3+ , Gd 3+ , Y 3+ , and Lu 3+ dopings significantly improve the optical and scintillation characteristics, including radiation hardness, of PbWO 4 single crystals. The hypothesis of Pb 2+ deficiency in the grown crystals and its charge compensation by the mentioned trivalent ions occupying the Pb 2+ sites has been proposed to explain the observed improvement. Under such assumption, tetravalent-ion doping may be as efficient as or even more efficient than trivalent-ion doping in improving PbWO 4 scintillator. Recently we extended the dopant ions to stable tetravalent ions (Th 4+ , Zr 4+ ) and some other ions (Ce 3+,4+ , Sb 3+,5+ , Mn 2+ ). We have found that the tetravalent-ion (Th 4+ ) doping gives similar improvement compared with the trivalent-ion doping in consistency with the expectation mentioned above. The ions with two different possible charge states (Ce 3+,4+ , Sb 3+,5+ ) gave finite but much less improvement than the trivalent and tetravalent ions. Ce 3+,4+ doping reduced the light yield significantly. Annealing of undoped PWO in PbO vapour was also studied to examine the possibility of reducing the Pb 2+ deficiency

γ irradiation of poly(vinyl chloride) for medical applications

Abstract In this work we have studied the effect of γ radiation on poly (vinyl chloride). Radicals produced by irradiation were investigated by EPR measurements at room temperature and the formation of conjugated double bonds was probed by UV–VIS absorbance measurements; samples were also submitted to infrared measurements to be analysed in terms of oxidation products.

Influence of Si-codoping on YAG:Ce scintillation characteristics

A set of undoped, Ce/sup 3+/-doped and Si/sup 4+/-codoped Y/sub 3/Al/sub 5/O/sub 12/ (YAG) single crystals were grown by the Czochralski method and submitted to /spl gamma/ irradiation in the dose range 1-236 Gy at RT. Their irradiation induced absorption coefficient was calculated and analyzed. Crystals also underwent light yield (LY) and scintillation decay measurements using the /sup 241/Am and /sup 22/Na radioactive sources, respectively, in 1 /spl mu/s time gate. The induced absorption at 550 nm (at the Ce/sup 3+/ luminescence peak) decreases with Ce concentration and further decrease is obtained in the Si/sup 4+/-codoped samples. LY values are independent on the Ce/sup 3+/ concentration within 0.12% - 0.3% and on the moderate Si/sup 4+/ codoping as well.

Refractive index and absorption length of YAP: Ce scintillation crystal and reflectance of the coating used in YAP : Ce single-crystal matrix

Abstract Recent works show growing interest in optically isolated YAP : Ce single-crystal matrices as high spatial resolution detectors for imaging applications in Nuclear Medicine. Such matrices are manufactured by Preciosa Crytur (Czech Republic) by means of a special sticking procedure of YAP : Ce pillars about 10 mm in length and with cross section ranging between 0.6 mm × 0.6 mm and 3 mm × 3 mm. In this work, we carried out an intensive study on both the complex refractive index of YAP : Ce crystal and the reflectance, versus, the incidence angle, of the coating used as optical insulator in matrix pillar made by Preciosa Crytur.

Ultraviolet transparency and activator oxidation state of Ce3+-doped phosphate glasses

Abstract A set of Ce3+ activated phosphate glasses is studied. Dependence of the absorption band-edge position on the activator content and glass composition is discussed with respect to variation of the so-called optical basicity. XPS analysis of the samples is performed to get information about the oxidation state of the Ce activator cations in the glasses.