120 TRANSURANIC ELEMENTS IN THE ENVIRONMENT 



glass microscope slide [Figs. 4(b), 4(c), and 4(d)]. The polycarbonate square is freed 

 from the needle by rotating it so that the corner of the square opposite that stuck by the 

 needle strikes the slide and causes the square to rotate and fall. 



The emulsion layers are then removed from the polycarbonate square by placing a 

 cover glass on top of the square. Water is introduced between the cover glass and slide 

 with a glass microbrush made from a 20-|Ul glass disposable pipet. (A 0.025-mm-diameter 

 tungsten wire doubled and threaded through the lumen forms a loop at one end. A small 

 amount of glass wool is placed through this loop and is then drawn into the end of the 

 pipet. The glass fibers are then cut off about 2 mm from the end of the pipet.) The 

 microbrush is dipped in water, and the glass fibers are touched to the edge of the cover 

 glass to allow the water to flow from the brush to between the slide and the cover glass. 



The emulsion is then removed by gently moving the cover glass a few millimeters 

 from side to side [Fig. 4(e)] ; this rolls the swollen emulsion from the surface of the film 

 but not from the fission-fragment tracks themselves. The cover glass is carefully lifted 

 from the glass microscope slide, taking care not to lose the polycarbonate square 

 containing the particle. 



Particle Mounting 



To mount a particle, the polycarbonate square is placed in a selected grid location on a 

 beryllium sample mounting block* [Fig. 4(f)] . These sample mounting blocks are 25 mm 

 in diameter and 13 mm thick and fit the standard electron-microprobe sample holders, 

 which grip the sides and provide the necessary electrical contact. The top surface of the 

 block is highly polished and contains a grid network of 1-mm squares inscribed on the 

 surface. The squares are numbered in mirror-image fashion both vertically and 

 horizontally through the center. 



With coaxial (reflected light) illumination and 15 X magnification under a stereo- 

 microscope, the polycarbonate squares are moved from the microscope slide to the 

 beryllium block with an electrolytically sharpened tungsten needle. 



The polycarbonate square is then dissolved and washed back from the particle with 

 dichloroethane, leaving the particle usually connected to the main body of polycarbonate 

 by a thin isthmus of plastic. This connection does not seriously affect the microprobe 

 analysis and aids in later locating the particles and holding them on the beryllium block. 

 A glass microbrush is rinsed in dichloroethane to remove any foreign material and is filled 

 by immersing the bristled end in a second beaker of dichloroethane. The magnification 

 was increased to 105 x. Dichloroethane from the brush is dispensed on the beryllium 

 block just in front of the polycarbonate square until the square is engulfed in the 

 solution. The microbrush is then used to push the solution back from the particle. Gelatin 

 replicas of the fission-fragment tracks remained with the particles. 



The beryllium block is returned to the photomicroscope where a second Polaroid 

 picture of each particle is made at a magnification of 556 x to identify the particles after 

 the gelatin has been removed. 



The gelatin with each particle is oxidized by exposure to an oxygen plasma for 3 hr in 

 a low-temperature asher.t In this asher a gas plasma is generated in oxygen with the 

 energy of electrons in the gas. Power is supplied to electrons at 13.56 MHz by a 



*Walter C. McCrone Associates, Inc., catalog number XIlI-403-3. 

 t Manufactured by international Plasma Corporation. 



