reaction was taken as 15.4 uM Fett+/ 
liter/1,000 rep. 
A Van de Graaff accelerator was 
used as a source of 3-Meyv electrons in 
some experiments.* Doses were calcu- 
lated from meter readings of current 
and voltage in accordance with a pre- 
vious method (11). 
Irradiation. When the Co® source 
was used, three to six replicate glasses 
were used in each determination, the 
glasses being stacked horizontally one 
on top of another in a cylindrical glass 
ampule (diameter, 49 in.; height, 144 
in.) with stopper. All ampules were 
filled with water and irradiated in fixed 
positions in special sample holders, to 
match geometry and absorber charac- 
teristics to those used in source 
calibration. 
Measurements. Optical density 
was measured in a Beckman DU spec- 
trophotometer with photomultiplier at- 
tachment. A special metal sample 
holder with four matched apertures 
to accommodate three glasses and 
a “blank” was used. Aperture di- 
mensions were 0.80 X 0.80 cm. The 
“blank” aperture is used as the 100 %- 
transmission reference; the absorbancy 
indext is calculated by dividing the 
* We wish to express our appreciation to 
J. G. Trump and K. A. Wright of the De- 
partment of Electrical Engineering, Mass. 
Inst. of Technology for their continuing 
cooperation in making available the facili- 
ties of the Van de Graaff accelerator and for 
calculation of the electron doses. 
+ The absorbancy index (AI) is related to 
the absorption coefficient (AC) by the fac- 
tor log. 10 = 2.3. Thus, AC = 2.3 X AI. 
observed optical density by the meas- 
ured thickness of the glass. Slit 
widths used were 0.4—0.5 mm, and each 
glass was measured at four wave- 
lengths: 3,500, 4,000, 4,500, and 5,000 
A. [Where data are shown only for 
4,000 A see NYO-3345 (13) for more 
complete results.] 
The absorbancy index reported here 
is the AI of the irradiated glass minus 
the AI of the nonirradiated glass. 
Figure 1 shows the arrangement for 
erasing glasses. A Coors porcelain pi- 
pette rest (glazed except bottom) with 
thermocouple attached holds the glasses 
while in the oven and while cooling. 
Experimental Results 
The characteristics that follow were 
studied with a view toward practical 
utilization. The nature of these stud- 
ies was also indicated by earlier results 
obtained by Schulman e¢ al. (1, 3). 
More complete data are contained in 
NYO-3345 (18). 
1. Effect of storage temperature on 
irradiated glasses not given heat 
treatment. Experiments were con- 
ducted to determine the effect of stor- 
age temperature on fading character- 
istics of glasses irradiated with a 
gamma-ray dose of 1.30 X 10° rep. Six 
glasses were stored at each of five 
temperatures in °C: room temperature 
(23-30°), refrigerator temperature (2- 
4°), 20°, 37°, and 55°. No differences 
are evident between 20°, 37° and room 
temperature for about the first week of 
storage with the exception of the 
Calibration: Co®° | 9.0 X 10% rep/hr 
2 3:4 618 
° 1 Hour after irradiation 
e After 10 min at 130°C 
a After 8 days at room temp. 
oo 2 3 4 6 846 234 6 87 
Dose (rep) 
FIG. 4. Calibration of silver-activated phosphate glass irradiated with Co? 
gamma rays: measured before heating, after heat treatment for 10 min at 
130° C, and after 8-day storage at room temperature 
Probable Percentage Limits of Error* 
Wave- 
length One glass Mean of three glasses 
(A) (%) (%) 
3,500 +3.3 +1.9 
4,000 +4.2 +2.4 
4,500 +4.7 +2.7 
5,000 +5.4 +3.1 
* For Corning 9761 glass on the basis of 
48 irradiations at ~1.30 X 105 rep. Prob- 
ability level is 99%. 
4,000 A measurements where a small 
difference was noted for the 37° tem- 
perature as compared to 20° and room 
temperature (see Fig. 2). 
Appreciable differences are apparent 
for the refrigerator and 55° tempera- 
tures as compared with the other three 
temperatures. Thus extremes in stor- 
age temperature should be avoided. 
Cold and hot storage temperature re- 
tard and accelerate fading rates, re- 
spectively, of glasses not given a 
stabilizing heat treatment. 
2. Effect of heat treatment after ir- 
radiation. Glasses irradiated with a 
gamma-ray dose of 3.70 X 105 rep were 
heated, 60 min after irradiation, to 
temperatures of 130° and 150° C. 
(Temperatures were achieved with a 
small precision furnace and measured 
by a thermocouple placed close to the 
glasses.) Glasses were also heated to 
100° C in boiling water. Figure 3 pre- 
sents the absorbancy indexes observed 
(1) immediately after heating for 5, 10, 
15, and 20 min at the different room 
temperatures and (2) after 1, 2, 4, and 
8 days of storage at room temperature. 
The data at each temperature indicate 
that: 
eThe light absorption decreases with 
time and temperature of heating. 
©The curves begin to flatten out after 
ten min of heating. 
eThere is little additional decrease 
in light absorption after about 15 
or 20 min of heating at the three 
temperatures. 
The fading at room temperature is 
reduced considerably after the heating 
process, decreasing from about 8% 
after eight days in the 100°-C experi- 
ment to about 4% after eight days in 
the 150°-C experiment (4,000 A). 
These experiments indicate the use- 
fulness of heating the glass to reduce 
the rate of fading after irradiation, al- 
though heating does not eliminate all 
