Nuclear Instruments and Methods in Physics Research B40/41 (1989) 599-602 North-Holland, Amsterdam 599 zyxwvutsr SEM OBSERVATIONS OF HIGH ENERGY ALPHA-IRRADIATED METAL SURFACES zyxwvutsrqponmlk P. SEN and P.M.G. NAMBISSAN Saha Institute zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA of Nuclear Physics, Calcutta-7000009, India S.V. NAIDU * and S.C. SHARMA Department of Physics, The University of Texas at Arlington, Arlington, Texas 76019, USA zyxwvutsrqponmlkjihgfedcbaZYXWVUTS In this paper we present results from a study of the surface morphology of alpha-irradiated tantalum and tungsten using scanning electron microscopy (SEM). The surfaces of well polished polycrystalline Ta and W were irradiated with 32 MeV alpha-particles to a total dose of - 4.2 X 10” (u/ cm*. Although the irradiation conditions were same for both of these samples, the resulting surface deformations are strikingly different. A small number of unruptured blisters of irregular shape and - 20 to 40 pm in size is seen on the Ta surfaces. The W surfaces are characterized by a much higher number of square shaped flakes of different sizes varying from 5 to 50 pm. Possible reasons for the differences between the surface features seen on these samples are discussed. 1. Introduction The reductions of surface damage and erosion due to blistering are important for the operation of fusion reactors. Hence, an understanding of the behavior of helium and defects in metals is important. Simulation of first wall surface damage, blistering, void nucleation and growth, and helium embrittlement are among the important problems [l-4]. Several models have been proposed to explain the formation of surface blistering and flaking on helium bombarded metal surfaces [5-71. At relatively high ion energies the surface erosion pro- cess becomes complex as the production of point de- fects and their influence become more prominent [4,8-lo]. A correlation between the surface structures and the internal defect clusters has been found [ll-131. In this paper we study the surface morphology of high energy alpha-irradiated tantalum and tungsten using scanning electron microscopy (SEM). The results of this study may provide further information about the surface erosion of helium-bombarded metals. Energy Cyclotron Center, Calcutta. Prior to bombard- ment, the alpha-particle beam was passed through a 0.5 mil thin tantalum degrader foil to uniformly scatter the beam over target area. This avoided a hot spot in the beam. The specimens were in good thermal contact with a water-cooled copper target flange. The temperature of the specimens during irradiation was monitored with a temperature sensor and it was kept below 150 o C. The beam current was about 2 PA and the total dose was - 4.3 X 10” cy/cm2. The irradiation conditions for both the specimens were the same and these are summarized in table 1. After irradiation the specimens were ex- amined using the Philip PSEM 500 scanning electron microscope. Table 1 Summary of the irradiation conditions and the defect parame- ters for alpha-irradiated Ta and W. 2. Experimental procedure High purity tantalum and tungsten specimens of thickness - 1 mm were well polished and annealed at - 1200” C for several hours. These specimens were subjected to irradiation at normal incidence in high vacuum by 32 MeV alpha-particles at the Variable * Present address: Department of Physics, Grambling State University, Grambling. LA 71245, USA. 0168-583X/89/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) Sample Tantalum Tungsten a-particle energy (MeV) Average curent (PA) Total dose ( a/ cm2) Irradiation temperature (O C) Mean ion depth (urn) Displacement energy Ed (eV) Vacancy concentration Vacancy migration temperature (” C) 32 32 2 2 4.2 x 10” 4.3 x 10” <150 138 22 6.9~10~~ - 75 cl50 120 42 5.1 x10-3 - 575 IV. ION IMPLANTATION