/ / 2 TRANS URANIC ELEMENTS IN THE ENVIRONMENT 



measured before it is liandled in the laboratory. Each fiUer is then dissolved in a 40% 

 (vol. /vol.) solution of 1 ,2-dichloroethane in dichloromethane. The filters are folded, and 

 each is placed in a 1-ml volumetric flask. A second clean, unused filter is placed in the 

 same flask to give sufficient polycarbonate to form a 50-mm^ film. Volume of the 

 dichloroethane solution in the flask is adjusted to about % ml. This mixture is stirred 

 until the polycarbonate filters dissolve. The flasks are stoppered and allowed to stand for 

 30 min to allow trapped air bubbles to rise to the surface. 



The clear polycarbonate solution containing the particles is poured onto a clean, 

 50-mm (2-in.) square glass plate (see Fig. 2). One edge of a second 50-mm^ glass plate is 

 used to spread the solution evenly over the surface of the first plate. The solution is 

 stirred continuously with the second plate for about half a minute while the solution 

 thickens. A 50-mm^ 1.6-mm-thick acrylic support with a 45-mm-diameter hole is placed 

 on top of the wet film. The support and plate combinafions are placed in covered petri 

 dishes for 16 hr while the films continue to dry. 



The glass plates are then removed by dipping the support and plate combinations in 

 distilled water and prying the supports from the glass with tweezers. 



Film Irradiation 



The cast film is irradiated in a thermal neutron fluence of about 9 X 10'^ neutrons/ciu^ 

 to produce fission -fragment tracks in the polycarbonate film by which particles 

 containing fissionable material can be identified. Films are arranged for irradiation by 

 stacking the supports on top of each other, thus sandwiching each film between two 

 supports. Included in the stack are blank films that are prepared in the same way as the 

 sample films from clean unused filters. The assembled stack is wrapped with cellophane 

 tape. Wrapped with each stack are preweighed 25.4-mm-diameter 0.25-mm-thick type 

 302 stainless-steel disks. The induced radioactivity from 27-day ^ ' Cr in these disks is 

 later measured to determine the thermal neutron tluence to which the particles are 

 exposed. 



The packaged stacks are irradiated in a 3-in.-diameter hole in a light-water-cooled 

 enriched-uranium-fueled standard pile with graphite reflectors (Axtmann et al., 1953). 

 Following irradiation, the induced radioactivity of the stacks is allowed to decay several 

 days before the packaged stacks are returned to the laboratory. 



Film Etching 



The polycarbonate film is etched for 10 min in 6A'NaOH at 52 to 55°C to make the 

 fission -fragment tracks visible with an optical microscope. During this etching process, a 

 portion of all polycarbonate surfaces is dissolved: the outer surface of the cast film, the 

 surface around the particle, and especially that along the fission-fragment tracks. 



Emulsion Coating 



For the identificafion of the fissionable material in each particle, the alpha-particle 

 emission rate is measured by coating the polycarbonate film with a photographic 

 emulsion, which is developed after a predetermined exposure time. 



Kodak type NTB nuclear track emulsion is used to coat irradiated films. Under 

 darkroom lighting (No. 2 Wratten-filtered) a 4-oz. jar of emulsion is partly immersed in a 

 water bath that is maintained at 40°C until the emulsion melts (between 15 and 20 min). 

 Slightly over half the molten emulsion is carefully poured into a narrow polyethylene 



