• PDF / 323,885 Bytes
• 8 Pages / 420.48 x 639 pts Page_size
• 14 Downloads / 190 Views

DOWNLOAD

REPORT

---

STRUCTURAL

EFFECTS OF RADIATION DAMAGE IN

A. MANARA, P.N. GIBSON Joint Research Centre, 21020 Ispra

SILICA BASED GLASSES

(Varese),

ITALY

and M. ANTONINI Istituto di Fisica and GNSM, Modena, ITALY

Universita'

di Modena,

41100

ABSTRACT The results of recent measurements of optical absorption, etching rate and transmission electron microscopy in pure silica and borosilicate glasses are reported and discussed. At dose saturation conditions, the dependence of the optical density associated with the production of single atomic defects from the mass of the impinging particle shows a marked saturation at masses > 20 amu. The corresponding etching rates increase by about 4 times with respect to unirradiated samples. In borosilicate glasses, the temperature dependence of the threshold dose rate of electrons to initiate the nucleation of bubbles shows a marked increase from about 300°to 6O0 K.

INTRODUCTION In previous papers, we reported and discussed results concerning the behaviour of amorphous silica and of silica-based glasses incorporating various oxides to simulate high-level nuclear waste /I - 3/. In those investigations, on one side the basic damage mechanisms were examined in terms of production, saturation and thermal annealing of single atomic defects; on the other side the more direct implications of 04-recoil damage on borosilicate and waste glasses were recorded in terms of density variation, stored energy and microstructural modifications. From the first class of experiments it was found that different kinds of particles introduce different types of atomic defects at dose saturation levels which depend upon the mass of the bombarding particle and upon the previous thermal and irradiation history of the sample. From the second class of experiments, heavily electron damaged glasses revealed large microstructural modifications consisting of bubble formation and phase segregation. These results have raised interest to correlate the damage produced at the basic level in pure silica with the larger modifications induced in more complex glasses. In particular, a connection can be looked for between the type and number of atomic defects present at dose saturation with the onset of nucleation effects. For this purpose a careful analysis of the mass dependence of saturation levels is necessary and a new particle of intermediate mass,i.e.

350 neon, has been therefore added to the previously adopted light and heavy ions as a bombarding external source. The choice of neon has been made both to fulfill this requirement and because neon seems to be among the first heavy ions which at high energy can produce clearly visible nuclear tracks in silicate glasses /4/. Further correlations might be found between the first stages of nucleation of defects and the enhancement of chemical activity of the damaged regions /5/. The connection might even be of more interest when considering that the deep penetrationsof high energy heavy ions adopted in our irradiations allow us to examine the etching rate not only near the surf