Other Gamma-Ray Dosimeters 
At present there is no gamma-ray dosimeter that has all the desired features 
for dosimetry work in mixed radiations. The most frequenily used laboratory 
device is the graphite-wall CO, flow chamber. This is primarily a dose-rate, 
gamma-detecting system, but it has an undesirable energy-dependent neutron 
response (3). The exact value of its neutron response is not known. However, 
this dosimeter does help obtain order-of-magnitude information. 
The photographic-film dosimeter system of Ehrlich and Fitch (4) is another 
common method of gamma-ray detection. The fast-neutron and thermal- 
neutron response of the film is only vaguely known, and consequently this system 
has limited applications for mixed dosimetry work until exact neutron-response 
data are obtained. 
Almost every laboratory concerned with X- and gamma-ray dosimetry uses 
the Victoreen condenser-type ionization chamber. The work of Aebersold (6) 
reveals that the Victoreen chamber responds to neutrons but is not suitable for 
use as a neutron dosimeter. Variations in the neutron response between one 
Victoreen chamber and another may be as great as 20% (6). Since the 
Victoreen responds to X- and gamma radiation more efficiently than it does to 
neutron radiation, its accuracy for gamma-ray measurements in the presence of 
neutrons will depend on the relative proportion of the neutrons and gamma rays. 
these dosimeter systems with the C-CO, 
flow chamber at the tower shielding 
facility provided much valuable infor- 
mation. The C-CO, chamber meas- 
urements were made at low power and 
extrapolated to the actual power set- 
tings used. The chemicals indicated a 
gamma dose somewhat higher than the 
C-CO, flow chamber extrapolation; re- 
sults were highly reproducible. 
Calibrations at Los Alamos indicate 
that the thermal-neutron sensitivities 
of the anhydrous chloroform and 
tetrachloroethylene systems are 6.7 X 
107°. and 1.71 X 10-!° rep/n/cm? 
respectively. An experiment  con- 
ducted during Operation Teapot has 
indicated that the maximum value 
for the fast-neutron response of the 
anhydrous chloroform system is about 
2%. 
APPENDIX 
We wish to evaluate P, the ratio of 
energy absorbed in 1 gm of tetrachloro- 
ethylene from 1 rep of fast neutrons to 
the energy absorbed in 1 gm of tissue 
from 1 r of gamma rays. Now 
E.», due to 1 rep ny in chemicals 
P= ne 
E.», due to 1 rep of y in tissue 
Ex, due to 1 rep ny in chemicals 
~ Ey, due to 1 rep of neutrons in tissue 
= FD rem(E)/FD,(E) 
= Dehem(E)/D,(E) 
where F is the flux in n/cm? needed to 
make a rep, and D-rem(#) and D,(E£) are 
the dose or energy absorbed per gram 
per n/cm? for chemical and _ tissue 
respectively. Since 
Derem(Mev/gram) = [EZ,0;f.Qi) 
TABLE 5—Computation of Energy-Absorption Ratios 
E oe oC1 
(Mev) (barns) 103acQcfe (barns) 10°sc. Qeifcr 10°Z,0;:f:Q; 103 ExSi0sf. Qs 10°D,(E) Iz 
0.1 4.5 4.55 2.0 1.54 
0.2 4.1 4.1 3.0 2.3 
0.5 3.4 3.4 3.0 2.3 
1.0 2.6 2.6 2.0 1.54 
220) 1a Lil 3.0 2.3 
4.0 1.9 19 3.0 2.3 
6.0 1.0 1.0 PSH | 2.1 
8.0 1.5 1.5 2.5 9) 
LOZO; 1-0 Teal 2.2 Very 
6.09 0.6 40 0.015 
5.4 RR 70 = =0.015 
5.7 2.8 100 0.028 
4.1 4.1 150 0.027 
4.0 8.0 190 0.042 
4.2 16.8 275 0.061 
3.1 18.6 270 =0.069 
3.4 27.2 325 0.083 
2.8 28.0 350 0.080 
* Personal communication, G. 8S. Hurst, Oak Ridge National Laboratory. 
then 
P = [EX,0;f:Q; chemical|/D,(E) 
where Dehem = dose (Mev/gm), E = 
neutron energy (Mev), 2; = summa- 
tion over-all types of atoms present, 
o; = cross section of the ith type of 
atom (cm?) f; = average fraction of 
energy lost by the neutron during its 
collision with the ith kind of atom, 
(assuming isotropic center of mass scat- 
tering, f; = 2M/(m + M)? where m is 
the neutron mass and M is the mass of 
the recoil atom), Q; = number of atoms 
of the element 7 in 1 gm of medium 
From the above, since tetrachloro- 
ethylene is C2Cl, 
ts DAB) 
where oc, fc, Qc are appropriate values 
for carbon and oq, fa, Qc are the 
appropriate values for chlorine. We 
used Qc = Ps >< 1021, fe = 0.14, Qcfe 
= 1.01 X 107, Qa = 14.5 XK 107, far = 
0.053, Qafar = 0.77 X 1074; DE) 
values are from Hurst (2). 
Table 5 shows values for P that are 
obtained by inserting the values for oc 
and gq (8). Note that the upper limit 
for P is 8.3% at 8 Mev. 
* * * 
The author wishes to express appreciation 
for the interest shown and guidance given by 
G. S. Hurst, John A. Auzier, and Karl Z. 
Morgan of ORNL. The author is also 
indebted to W. H. Langham and P. Harris 
of Los Alamos for their calibration and 
evaluation of these systems. 
Also, without the unlimited cooperation of 
the physics and engineering department of the 
Radtobiological Laborctory of The University 
of Texas and the United States Air Force, 
Austin, Texas, none of these studies could 
have been completed. 
BIBLIOGRAPHY 
1. G. S. Hurst, R. H. Ritchie, H. N. Wilson. A 
count-rate method of measuring fast neutron 
tissue dose, Rev. Sci. Instr. 22, 981 (1951) 
2. G.S. Hurst. An absolute tissue dosimeter for 
fast neutrons, Brit. J. Radiol. 27, 353 (1954) 
3. G. S. Hurst, W. A. Mills, F. P. Conte, A. C. 
Gupton. Principles and techniques of mixed 
radiation dosimetry—application to acute 
lethality studies of mice with the cyclotron, 
Radiation Research 4, No. 1, (1956) 
4. M. Ebrlich, S. H. Fitch. Photographic x- 
and gamma-ray dosimetry, NucLEONIcs 9, 
No. 9, (1951) 
5. L. Ebrinberg, A. Saeland. Chemical dosim- 
etry of radiations giving different ion densities. 
An experimental determination of G values of 
Fet++ oxidation, JENER 8 (1954) 
6. P. C. Aebersold, G. A. Anslow. Fast neutron 
energy absorption in gases, walls, and tissue, 
Phys. Rev. 69, 1 (1946) 
7. H. H. Rossi cited by J. W. Boag. The relative 
efficiency of different ionizing radiations. 
NBS Report #2946 (National Bureau of 
Standards, U. S. Department of Commerce, 
Washington, D. C., 1953) 
8. D. J. Hughes, J. A. Harvey. 
sections, BNL-325 (1955) 
Neutron cross 
21 
