Figure 1. — A chain 

 saw was used to out 

 vertical slots in 

 trees for film 

 placement. 



For purposes of comparison, two 15.2-cm areas of bark within 31 cm of breast 

 height were removed from the trees each time populations were radiographed. All insects, 

 mountain pine beetle egg galleries, and gallery starts in the samples were counted. 

 Plastic screen cages also were stapled on the trees in this area to collect insects 

 emerging from the bark. 



Moisture content of the trees was determined by means of an electrical resistance 

 meter. Radiographic expose time was adjusted according to moisture content. Exposure 

 curves for different slab thickness-moisture content combinations (fig. 2) were pre- 

 sented by Amman and Rasmussen (1969). 



A portable Picker® X-ray machine with up to 110-kV output was used. All radio- 

 graphs were taken at 25 kV, 5 mA, and at a 51-cm film distance. The 25 kV level gave 

 abetter contrast than higher kilovoltage levels in laboratory tests (Amman and Rasmussen 

 1969). A cord that allowed the operator to be at least 12.5 m from the tube head during 

 operation was used to protect him from possible radiation exposure. All legs of the 

 quadruped stand telescoped for rapid height adjustment; wheels, mounted on the rear 

 set, facilitated movement of the head from tree to tree. A 1,500-watt gasoline- 

 powered alternator provided electricity for the unit. All equipment was easily moved 

 by two men. 



Because of trunk curvature, thickness of the area radiographed varied from about 

 5 cm in the center to about 2 cm on the outer edge. To avoid overexposure of sample 

 portions thinner than 5 cm, an aluminum foil filter, that thickened laterally from the 

 center, was placed over the sample area. The number of sheets of foil ranged from two 

 near the center to six at the outer edge of the sample. 



3 



