Fast-Neutron-Insensitive 
Chemical Gamma-Ray Dosimeter 
Acid formation in stabilized chloroform or tetrachloroethylene serves as a 
measure of gamma-ray dose that is rate- and temperature-independent and 
linear to over 200,000 r. 
By SANFORD C. SIGOLOFF 
Adaptation to personnel monitoring is described 
Radiobiological Laboratory, University of Texas, Austin, Texas 
INCREASED AVAILABILITY of nuclear 
machines for radiobiological research 
has created many dosimetry problems. 
The ideal technique for mixed-radiation 
dosimetry would incorporate two do- 
simeters, one completely insensitive to 
gamma rays and the other completely 
insensitive to neutrons. 
A useful technique for measurement 
of fast neutrons exists in the two-pro- 
portional-counter method (1-3). How 
chemical dosimetry can be used for 
gamma-ray measurements in the pres- 
ence of neutrons is discussed here. 
Chemicals + Film— 
Over-All Properties 
Both chloroform and_ tetrachloro- 
ethylene respond to X- and gamma 
radiation, producing stable, water- 
soluble acids. If a dosimeter contain- 
ing these reagents* is given a known 
dose of radiation, the radiation-gener- 
ated acids can be extracted with an 
acidimetric dyef, titrated with 107% 
normal sodium hydroxide, and a ecali- 
bration value for the number of milli- 
equivalents of acid per roentgen can be 
obtained. Evaluation of unknown-dose 
Something New in Personnel Dosimetry 
A new approach to personnel monitoring for persons unlikely to receive a 
significant dose (e.g. secretarial help) is afforded by chemical dosimeters sensi- 
tive below 1 r. 
dosimeter. 
It parallels the usual film badge with a self-indicating chemical 
The film badge ts developed only when the chemical dosimeter indi- 
cates the accumulation of a dose > 1 r or after the passage of a fairly long 
period, say 3 months. 
Weekly processing of the film badges of personnel who 
consistently get zero dose is thus avoided. 
The necessary dosimeter is at hand in the form of alcohol-and-thiorea-stabi- 
lized tetrachloroethylene plus an acidimetric dye. 
acids that results in a color change in the acidimetric dye. 
can be read with an accuracy of 20% in the range 0.5-3 r. 
Incident radiation produces 
This color change 
Complete color 
change from red to amber results from a 8-r exposure tn a system having a 
hydrocarbon-dye ratio of 6:1. 
pendent; it is linear with dose up to 10,000 r. 
The dosimeter is rate- and temperature-inde- 
Energy dependence is like that 
of the high-range dosimeter described in this article. 
For details see American Industrial Hygiene Association Quarterly for 
December, 1956. 
exposures is accomplished by titrating 
thé exposed dosimeter, determining the 
number of milliequivalents of acid 
generated, and comparing witha known 
calibration value. 
Titration techniques afford accuracy 
to +10%. Spectrophotometric analy- 
sis can increase the accuracy to +38%. 
A liter of solution is enough for 700 or 
more dose determinations. 
Shelf life of these systems is excellent. 
Solutions have been stored at room 
temperature for periods greater than 
ten months with no decrease in their 
radiation sensitivity. An undesirable 
property of the anhydrous chloroform 
dosimeter is its volatility. This prop- 
erty precludes its use except for short- 
term experimentation. For long-term 
exposures the less volatile tetrachloro- 
ethylene is used. Both of these sys- 
tems have been flame-sealed in siliconed 
Neutraglas ampules and used in cooper- 
ative dosimetry efforts with other 
laboratories. 
Dosimetry Characteristics 
The chemical systems described here 
have many properties desirable in a 
*Reagent-grade chloroform (CHCl) or 
tetrachloroethylene (C2Cl,) is fractionated 
in a high-efficiency still to a constant refrac- 
tive index. The distillate is collected in a 
thoroughly cleansed Pyrex container, and 
stabilizing agents, such as hexylresorcinol 
or 2,6-di-tert butyl 4 methylphenol, are 
added. The concentration of the stabili- 
zing agent controls radiation sensitivity. 
We usually use 0.2% by weight of the sta- 
bilizing agent dissolved in the chlorinated 
hydrocarbon. 
+Such as chlorophenol red (3’, 3/-di- 
chlorophenol sulfonephthalein). 
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