ings, individual preirradiation measure- 
ments are advisable. 
Fading Corrections 
We have demonstrated that fading 
occurs after irradiation, whether the 
glass is heated or not (although sub- 
sequent fading is reduced appreciably 
by heat treatment). The possibility of 
applying corrections to the glass 
measurements to compensate for fading 
is evident if a relationship between the 
rate of fading and the time after 
irradiation can be developed. 
The nomogram in Fig. 9 provides a 
simple method for utilizing Eq. 1. The 
left-hand scale is a logarithmic time 
scale extending from 1 hour to 1 year. 
Electron-beam Dosimetry 
The change of optical density (fading 
change) between 0 and 8 days after 
irradiation corresponds to the area 
between the curve in Fig. 4 for readings 
after 10 min of heating and the curve 
for readings after 8 days of storage. 
The right-hand scale is graduated in 
linear units with unity corresponding 
to the 8-day period. 
Work by this laboratory (reported in the article (3) by 
Schulman, Klick, and Rabin) has shown the feasibility of 
using silver-activated phosphate glass for dosimetry 
3-Mev electron beams. Since the intensity of ionization 
in matter falls off much more rapidly for electrons than for 
gamma rays” it is of importance to determine the effect of 
placement of the glass when it is used as a dosimeter for 
electron beams, and the possible effect of variation of ion- 
ization density in the glass itself because of its appreciable 
thickness compared with the maximum range of the elec- 
tron beam. 
A previous calibration of the silver-activated phosphate 
glass dosimeter is needed for interpretation of the changes 
in absorption after receiving doses of high-energy elec- 
trons. The Co® gamma-ray calibration of the glass, Fig. 
5, can be used for this purpose. However, the relative 
duration of irradiation must be taken into account. For 
example, the Van de Graaff accelerator used in this work, 
delivers a 10°-rep dose in about 10 sec whereas the Co? 
source, used for calibration, delivers the same dose of 
gamma rays in about 12 hours. The difference in fading 
during the two irradiations must be taken into account. 
This has been done in these experiments by exposing more 
than one set of glasses to the same dose of electrons and 
measuring them at two different time intervals after 
irradiation. 
Two stacks were assembled, each consisting of seven 
glasses of approximately 0.12-cm thickness one on top of 
the other. The stacks were secured with Scotch tape. 
* The effective range for electrons is about 0.5 cm. for each 
Mev of energy in a substance of unit density. 
3-Mev electrons - average dose rate 
3.6 X10° rep/min 
B 
Calibration= Co®°, 
1.5 X 103 rep/min 
B= Glass (measured | hour 
after irradiation) 
C= Glass (measured | day 
after irradiation) 
as 
04 O06 O8 1.0 12 1.4 16 
Absorber Thickness (gm/cm?) 
ie} 02 
DEPTH-DOSE CURVES in glass, as determined by self- 
dosimetry, compared with that in aluminum (11) 
They were then irradiated with the surtaces of the glasses 
perpendicular to the direction of the beam. The energy 
of the electrons was 3 Mev and the beam current was ad- 
justed to deliver a dose of 10° rep in the region of maximum 
ionization density (at an average dose rate of 3.6 X 108 
rep/min). After irradiation, the glasses were given heat 
treatment of 10 min at 130° C, group “‘B”’ receiving it one 
hour after irradiation and group “‘C” receiving it one day 
after irradiation. Measurements were taken of both 
groups of glasses immediately following heat treatment. 
Doses were determined from the calibration chart con- 
structed from Co® exposures (Fig. 5), by using the values 
obtained immediately after heat treatment applied one 
hour after irradiation. Depth-dose curves were drawn 
from apparent doses received by the top six glasses in each 
group. The seventh and bottom glass in each group ap- 
parently was beyond the range of the beam, as it received 
no detectable dose. 
The abscissae in the figure are in terms of absorber thick- 
ness, obtained by multiplying the distance of penetration 
by the density of the material, so that materials of different 
density may be compared. The ionization density curves 
for the glass show the characteristic rise to a maximum. 
After the maximum is reached, the curves fall off rapidly to 
a value close to zero. 
The ‘‘true’’ curve for glass should lie between curves B 
and C. This is because the 1-hour and 1-day waiting 
periods for curves B and C are shorter and longer respec- 
tively than the irradiation times for corresponding doses 
involved in the Co® calibrations. 
The figure above also shows a 3-Mev-electron ionization- 
density curve for aluminum (11), with the maximum at 
10° rep. The three curves are similar in shape, and the 
maxima appear at the same absorber thickness. The 
maximum range in the aluminum is about 0.2 gm/cm? 
greater than in the glass. Furthermore, the ionization 
density in glass as determined by both groups of glasses 
remains appreciably lower than that in the aluminum 
shortly after the maximum values are exceeded. The ex- 
planation for these differences is not known, but the 
differences are likely caused by a difference in the scatter- 
ing properties of the aluminum and glass. 
The dose rates of the Van de Graaff and Co® sources 
were in a ratio of ~2,000:1. The Van de Graaff results 
thus support the results of Schulman, Klick, and Rabin (3) 
who found no significant dependence of glass sensitivity on 
dose rate. 
These experiments illustrate the applicability of this 
glass for electron-beam dosimetry and stress the impor- 
tance of placement and geometry of the dosimeter system 
in interpreting correctly the dose received. 
