3.74 xX 10° rep 
Time interval between 
end of irradiation and 
10 minutes heating at 
130°C 
20.354 6 8100 234 6 8iD00 2 
Time After End of Irradiation (hours) 
oe 4,000 A 
pe 13.0 
=> 3.55 x 10°rep 
iE 16.0 ao? 
£140 1.23 x 10°rep eo 
x > —__,_,, E 
@ — 
v2 90 x 
i ®@ 110 |— 
> 80 2 
© 70 3.66 x0°rep 3 
e 
S 3. B 80 
+ 34 st 
30 1.21 x10°rep 
2612 me e 70 
‘10 min heat at 130 C = 
ae nt. 
I 234 6810 2030 60 100 200 6.0 
Time ofter End of Irradiation (hours) 
j 234 6 8ig 
FIG. 5. Fading characteristics of silver-activated phos- FIG. 6. 
phate glass given different doses, heated for 10 min at 
130° C, and stored at roomt emperature 
fading in glass of this particular 
composition. 
3. Effect of storage temperature on 
glasses given heat treatment. A 10- 
min post-irradiation heating at 130° C 
has been used to minimize the effect of 
storage temperature such as observed 
in Fig. 2. 
Three groups of three glasses each 
were irradiated with a gamma-ray dose 
of 3.66 < 105 rep. One hour after ir- 
radiation the glasses were measured 
and then heated for 10 min at 130° C. 
They were then measured again. One 
group was stored in a refrigerator (2— 
4° C), one at room temperature (23- 
26° C), and the third in an oven main- 
tained at 43° C. Measurements were 
taken during an 8-day period on the 
three groups. 
The data show no significant differ- 
ence with storage temperature (except 
at the eighth-day measurement when a 
small lowering in readings for the 
glasses held at 43° C was detected). 
Thus an additional virtue of heat 
treatment after irradiation is the con- 
siderable decrease of the dependence 
upon storage temperature of readings 
made long after irradiation. 
4. Calibration curves. Figure 4 
shows calibration curves at four wave- 
lengths from data obtained by ir- 
radiating glasses (six at each dose level) 
with gamma-ray doses ranging from 
about 1.3 X 10° rep to 7 X 10° rep. 
Sixty min after irradiation, these 
glasses were heated for 10 min at 
10° rep 
130° C. An extra minute was allowed 
for the glasses to reach the desired 
temperature after insertion into the 
oven. Following irradiation, measure- 
ments were taken before heating, 
within 30 min after heating, and after 
8 days of storage at room temperature. 
Each point shown in Fig. 4 is the 
average of six measurements. 
It is apparent that the slopes of the 
curves decrease appreciably above 
~3 X 10° rep. Consequently, sensi- 
tivity of the glass in this region is 
reduced. Precise experimental tech- 
niques and reproducibility of glass 
characteristics become increasingly im- 
portant to obtain accurate results. 
The percentage of fading, both dur- 
ing the heating period and during the 
8-day storage period, decreases sig- 
nificantly with lower doses. In fact, 
when the dose is below 4 X 104 rep, 
fading becomes almost negligible during 
the 8-day period after heat treatment. 
5. Fading of glasses given heat 
treatment after different doses. Fig- 
ure 5 shows measurements of the 
absorbancy index at 4,000 A observed 
after four different doses. The meas- 
urements were taken 1 hour after ir- 
radiation, after 10 min of heating at 
130° C, and at various times up to 
the eighth day of room-temperature 
storage. 
A fairly good approximation to the 
fading for the first 8 days after irradia- 
tion is given by the equation shown 
above 
Effect of heating glasses for 10 min at 130° C, after dif- 
ferent time intervals at room temperature following dose of 3.74 X 
Al = alnt+6 (1) 
where AI is the absorbancy index ob- 
served at a time ¢ after irradiation, and 
a and b are functions of the dose and 
wavelength, but are constant for a 
given dose and wavelength.* 
6. Effect of heating glasses after 
different postirradiation waiting pe- 
riods. In experiments described thus 
far heat treatment was used 1-114 
hours after irradiation. In practice 
heat treatment may not be convenient 
if it is necessary to apply it so soon 
after irradiation. A series of experi- 
ments was, therefore, conducted to 
study the behavior of the glass when it 
was heated after different time inter- 
vals following irradiation. 
Twenty-one glasses were irradiated 
with a dose of 3.74 X 10° rep. The 
glasses were separated into seven 
groups of three each, and all were 
measured one hour after the irradia- 
tion. Over a period of 8 days, each of 
six groups was given a 10-min heat 
treatment at 130° C, at different times 
after irradiation, including 8.5 hours, 
1, 2, 4, and 8 days, and about 2 months. 
The seventh group was allowed to fade 
at room temperature without heat 
treatment. All the glasses were meas- 
ured at various times after heat treat- 
ment up to the eighth day following 
irradiation, including measurements im- 
* Measurements taken at 3,500, 4,500, 
and 5,000 A show the same linearity (13). 
