A survey was performed to search for TeV gamma-ray sources in the northern sky. In the search for steadily emitting sources 3TeV air shower data obtained by the Tibet-HD array and Tibet-III(Phase 1) array are analyzed. Nineteen prominent directions, including the direction of Crab Nebula, were found to have ∗M.Amenomori and H.Nanjo Dept. of Phys., Hirosaki University, Hirosaki, Japan S.Ayabe and K.Mizutani Dept. of Phys., Saitama University, Saitama, Japan S.H.Cui, L.K.Ding, H.B.Hu, C.L.Lan, H. Lu, S.L.Lu, J.R.Ren,Y.H.Tan, B.S.Wang, H.Wang, H.M.Danzengloubu, X.H.Ding, H.W.Zhang and J.L.Zhang Laboratory of Cosmic Ray and High Energy Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100039, China Guo, Labaciren, X.R.Meng, A.F.Yuan and Zhaxisangzhu Department of Math. and Phys., Tibet University, Lhasa 850000, China C.F.Feng, M.He, J.Y.Li, Y.G.Wang, L.Xue, N.J.Zhang and X.Y.Zhang Dept. of Phys., Shangdong University, Jinan 250100, China Z.Y.Feng, Q.Huang, H.Y.Jia, G.C.Yu and X.X.Zhou Institute of Modern Physics, South West Jiaotong University, Chengdu 610031, China X.Y.Gao, Q.X.Geng, J.Mu and X.C.Yang Dept. of Phy., Yunnan University, Kunming 650091, China K.Hibino, T.Sasaki, T.Shirai, N.Tateyama, S.Torii and T.Utsugi Faculty of Engineering, Kanagawa University, Yokohama, Japan N.Hotta, J.Huang, I.Ohta and S.Ozawa Faculty of Education, Utsunomiya University, Utsunomiya 321-8505, Japan F.Kajino, K.Kawata, M.Sakata, M.Takashima and Y.Yamamoto Dept. of Phys., Konan University, Kobe, Japan K.Kasahara Faculty of Systems Engineering, Shibaura Insti. of Tech., Saitama, Japan Y.Katayose and M.Shibata Faculty of Engineering, Yokohama National University, Yokohama, Japan G.M.Le and Z.H.Ye Center of Space Science and Application Research, Chinese Academy of Sciences, Beijing 100080, China M. Nishizawa National Institute for Informatics, Tokyo,Japan M.Ohnishi, T.Ouchi, A.Shiomi, M.Takita and H.Tsuchiya Institute for Cosmic Ray Research, University of Tokyo, Kashiwa, Japan T.Saito Tokyo Metropolitan College of Aeronautical Engineering, Tokyo, Japan H.Sugimoto and K.Taira Shonan Institute of Technology, Fujisawa, Japan T.Yuda Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan pp. 1–?? c ©2002 by Universal Academy Press, Inc.

Data from the Tibet III air shower array (with energies around 3 TeV) and from the Tibet II array (with energies around 10 TeV) have been searched for diffuse gamma rays from the Galactic plane. These arrays have an angular resolution of about 0.degrees9. The sky regions searched are the inner Galaxy, 20degrees less than or equal to l less than or equal to 55degrees, and outer Galaxy, 140 degrees l less than or equal to 225degrees, and \b\ less than or equal to 2degrees or less than or equal to 5degrees. No significant Galactic-plane gamma-ray excess was observed. The 99% confidence level upper limits for gamma-ray intensity obtained are (for \b\ less than or equal to 2degrees) 1.1 x 10(-15) cm(-2) s(-1) sr(-1) MeV-1 at 3 TeV and 4.1 x 10(-17) cm(-2) s(-1) sr(-1) MeV-1 at 10 TeV for the inner Galaxy, and 3.6 x 10(-16) cm(-2) s(-1) sr(-1) MeV-1 at 3 TeV and 1.3 x 10(-17) cm(-2) s(-1) sr(-1) MeV-1 at 10 TeV for the outer Galaxy, assuming a differential spectral index of 2.4. The upper limits are significant in the multi-TeV region when compared to those from Cerenkov telescopes in the lower energy region and other air shower arrays in the higher energy region; however, the results are not sufficient to rule out the inverse Compton model with a source electron spectral index of 2.0.

This paper has investigated the effect of calcining temperature (600–1000°C) on the mesopore structure and surface fractal dimension of silica xerogels derived from two-step acid–base-catalyzed TEOS (tetraethyl orthosilicate) systems. Two base contents were adopted in the base step. FE–SEM (field emission gun-scanning electron microscopy) observation and XRD (X-ray diffraction) analysis indicate that gel coarsening, agglomeration and sintering occurred successively with increasing temperature, and those prepared with higher base content exhibit easier agglomeration, lower sintering temperature and weaker crystallizability. Percent mesoporosity and surface fractal dimensions evaluation via image analysis methods shows that the mesoporosity of those prepared with lower base content increases below 700°C and decline at higher temperatures, while the surface fractal dimension appears to decrease above 800°C.

Silica xerogels were prepared by thermal drying wet gels in an electric oven (70 degrees C) after certain duration of ambient drying, and the relevant effect is investigated on the mesopore structures and surface fractal dimensions of the resultant xerogels. The silica gels were derived from a hydrochloric acid-catalyzed TEOS (tetraethylorthaosilicate) system, and both magnetic stirring and ultrasonic vibration were adopted during sol preparation. The percentage mesoporosity and surface fractal dimensions are evaluated using image analysis methods, based on FE-SEM (field emission gun-scanning electron microscopy) images. The results show that the mesoporosity of the resultant xerogels decreases with the duration of ambient drying for samples prepared using magnetic stirring and low-intensity ultrasonic vibration, while samples subjected to high-intensity ultrasound show a somewhat reverse trend. Samples prepared with magnetic stirring have almost constant surface fractal dimensions (nearly 3), irrespective of the ambient drying before thermal drying. The surface fractal dimensions of samples prepared using ultrasound increase with the duration of ambient drying.

An acid-catalyzed TEOS system has gelated under four different solvent evaporation conditions, and the singularity spectra of the resultant xerogels have been determined with image analysis methods and discussed in combination with their mesoporosity values. Generally, larger scale of singularity strength and lower entropy dimension are observed for the xerogel prepared in slower evaporation condition, along with finer gel textures and higher mesoporosity. However, when the solvent evaporation is extremely rapid, inverse results emerge.

This investigation deals with the effect of calcining temperature on the mesopore structures and surface fractal dimensions of MgO-Al2O3-SiO2 xerogels prepared via a two-step acid-base-catalyzed sol–gel process with/without alcohol washing before drying. The mesoporosity and surface fractal dimensions are evaluated by image analysis methods, based on FE-SEM (field emission gun-scanning electron microscopy) images. The results show that samples with alcohol washing before drying exhibit finer textures and lower sintering temperature. The mesoporosity increase before 700°C might be attributed to the microporosity reduction during large extent of further condensation (detectable weight loss), and continuous further condensation and sintering resulted in the monotonic decrease of mesoporosity above that temperature. Smoothing and sintering effects also contributed to the general decreasing trend of surface fractal dimensions with increasing temperatures. Complication of surface textures caused by heterogeneous distributions of pores resulted in the abnormal high surface fractal dimensions.

The present work has investigated the influence of ultrasonic vibration during sol preparation on the mesoporosity and surface fractal dimensions of resultant silica xerogels, in comparison with magnetic stirrings. The xerogels are derived from hydrochloric acid-catalyzed TEOS (tetraethyl orthosilicate) systems (acetone as solvent). The sols were agitated under ultrasonic vibration (of high or low intensity) or by magnetic stirring in sealed containers for 30 minutes, and then gelled in the air with solvent evaporation. The resultant wet gels were dried in ambient conditions after 11 days of aging or not, and became xerogels. Image analyses based on the FE-SEM (field emission gun-scanning electron microscopy) images of these xerogels showed that high intensity of ultrasonic vibration caused a distinct mesoporosity decrease of the resultant xerogels, and that despite the different ultrasonic intensities, samples prepared under ultrasonic vibrations showed similar surface fractal dimensions which were different from those of classic samples. The influence of aging is also presented.

Silica xerogels were prepared under three different solvent evaporation conditions before gelation and two calcination procedures after aging and drying, using a hydrochloric acid-catalyzed tetraethyl orthosilicate system with acetone as the solvent. The influence of calcination procedures on the percentage porosity and surface fractal dimensions of the resultant xerogels is investigated. The evaluation of percentage porosity and surface fractal dimensions based on field emission gun-scanning electron microscopy indicates that calcination generally causes an increase in percentage porosity and surface fractal dimensions. Calcination at 700 degrees C without an intermediate stay at 600 degrees C leads to larger extent of further condensation that hinders the increase of porosity and surface fractal dimensions, in comparison with calcination with an intermediate stay. The difference in porosity and surface fractal dimensions derived from calcination procedures is also influenced by the solvent evaporation conditions before gelation in preparation of the samples. When the solvent evaporation rate is slower, the difference is slight because further condensation in the resultant finer gel textures proceeds more efficiently. Copyright 2000 Academic Press.