Spring plate 
> 
/ 
FIG. 2. Carbide tip (left) and tool shaft 
tron flux of 10!% n/cm?/sec for 1 hr. 
The isotopes Co®, Ti®! (3), W185, W187 
and Ta'®? were formed. 
Choice of isotopes. The following 
points have to be considered: 
1. The isotope must be formed in 
sufficient amount during irradiation. 
2. The isotope must emit radiation 
suitable for detection. 
3. The isotope must have a half-life 
long enough to allow for transportation 
from the pile and machining tests. 
4. The half-life should, on the other 
hand, be short enough to minimize the 
need for handling precautions. 
5. If possible, the isotope should be a 
pure beta emitter ‘to avoid the health 
hazards involved in gamma emission. 
Only the tungsten isotopes are 
formed in sufficient amount during a 
short irradiation (a few days) to require 
consideration. The nuclear data are 
given in Table 2 (4). 
Both isotopes fulfill requirements 
1-3. Requirement 5 is better satisfied 
by W185. However, 4 greatly favors 
W'*7 because its 24.1-hr half-life neces- 
sitates precautions against health haz- 
ards for only one week or so. Further- 
more, the shorter half-life and higher 
neutron-capture cross section of W187 
require a shorter irradiation time. 
Thus, W'*’ is preferred for the present 
work. 
FIG, 3. 
Tungsten-187 is formed with a tar- 
get-element cross section of 10.2 barns. 
Thus, only very short irradiations in a 
high flux of thermal neutrons are neces- 
sary for production of high activities. 
Mechanical effects of irradiation. 
Prolonged bombardment in a high flux 
of fast neutrons may be supposed to 
cause certain changes in the crystal 
structure (6) and thus in the mechanical 
properties of carbide tips. However, 
if the irradiation takes place in the 
thermal shield of the pile, where the 
flux of fast neutrons is low, the risk of 
structural changes may be supposed to 
besmall. In this case, mechanical prop- 
erties are considered unchanged (9). 
Desired Specific Activity 
Preliminary experiments had shown 
that transfer of at least 5 mg of tip 
material could be expected from the 
rake (see Fig. 1) during a period of 
20 min. This corresponds to a total 
of 2,000 meters of chips. 
The G-M measurements were 
planned to be made with a 10% geome- 
try (as calibrated with UX.) and with 
a mosaic of small pieces of chip with a 
total length of 100 mm. Thus, the 
specific activity of the carbide tips 
necessary for a counting rate of 1,000 
cpm is 20 mc/gm. 
Now each tip weighed about 35 
TABLE 2—Tungsten Isotope Data 
Isotope Half-life 
W-185 76d 
W-187 24.1h 
B-energy (Mev) 
0.68, 0.48 
1.33 (30%), 0.63 (70%) 
y-energy (Mev) 
— (6) 
0.696, etc. 
152 
Lathe with radiation shield and dust extractor in front 
gm; thus, a total of 1.4 curies was 
necessary for two tips. This neces- 
sitates rather rigorous precautions as to 
health hazards, so an activity of only 
10-mc/gm tip material was chosen, 
giving a counting rate of 500 cpm. 
Machining Procedure 
To reduce irradiation costs, the tips 
were irradiated before they were 
fastened to the shafts. Complications 
would be expected if the activated tips 
were attached to the shaft with the 
usual soldering and polishing method. 
Therefore a clamping method was used. 
Figure 2 shows one of the cemented 
carbide tips ground to shape and ready 
for mounting on the tool shaft. The 
edge of the spring plate is pressed 
against the groove cut across the top 
surface of the tip. The pin holds the 
tip in position during machining. 
Turning is carried out in the normal 
manner except that the tips are 
handled by remote control. Operators 
are protected by a shield and a fan to 
remove radioactive dust (Fig. 3). A 
filter in the fan, checked after the work 
is finished, exhibits no contamination. 
The feed is engaged for 30 sec. The 
tool is then unfastened and mounted in 
position for photographing the top sur- 
face of the tip. The last chips to be 
formed during each 30-sec operation 
are collected and measured with the 
G-M counter. The tip of quality no. 1 
«was used for a total of 35 runs of 30 sec 
each, and no. 2, for 43 runs. 
Preparation of Chips 
The turning chips, almost flat to 
start with, become more and more 
