results of 14 determinations of back- 
ground (100-sec counts) made during 
the course of a single day’s operation 
during which time 65 subjects were 
measured. The last three determina- 
tions, while apparently self-consistent, 
are significantly higher than the previ- 
ous sequence. The average of the first 
11 measurements, extending over a 
period of six hours, is 443.8 eps. As 
can be seen from the last column of the 
table, the scatter of the individual 
measurements around this average ap- 
pears to be statistical. 
A more detailed description of the 
mechanical features of the counter 
design will be found in the report LA- 
1717 (6). 
Body Potassium Content 
The Human Counter has been used 
to make a preliminary study of the 
variation of the potassium content of 
people as a function of their body 
weight and age. Potassium is natu- 
rally radioactive, the isotope re- 
sponsible for the activity being K*°, 
abundance 0.01%, half-life 1.25 « 10° 
yr (6). The decay is to Ca‘? by emis- 
sion of a 1.4-Mev beta particle or to 
A‘ by electron capture. In the latter 
branch of the decay scheme, a 1.45- 
Mev gamma ray is emitted. It is this 
gamma ray which is used to measure 
the potassium. The gamma emission 
rate of ordinary potassium is 2.96 dps/ 
gm (6). Since the average person 
contains about 150 gm of potassium 
(7), the gamma emission rate from this 
source is expected to be about 450 dps. 
Body Activity (cps) 
(e) 50 100 
Body Weight (Ib) 
FIG. 4. Gamma 
At 10% efficiency, one would expect 
45 cps from the potassium in the 
average human body. 
Since potassium is largely con- 
centrated in muscle, very little being 
found in bone and fat, potassium con- 
tent might be expected to vary ap- 
preciably with body type. Measure- 
ments were therefore made on a group 
of people of both sexes whose ages 
varied from 4 years to 75 years. Most 
of the group had no_ occupational 
connection with radioactive materials. 
The group was “‘random”’ to the extent 
that it represented visitors to the 
Health Research Laboratory during the 
July, 1955 open house at Los Alamos 
Scientific Laboratory. 
Two 100-sec counts were made on 
each subject and backgrounds were 
measured periodically throughout the 
day. The ratemeter trace was used 
as a monitor against short-term back- 
ground drift. The gross counting rate 
observed from the subjects was usually 
slightly below the background with the 
detector empty because of soft radia- 
tion entering the open end of the shield 
and absorbed by the subject. (This 
end will be closed with a 2-in. lead door 
to eliminate this effect.) Correction 
was made for background depression 
using the results of the measurement of 
a series of sugar phantoms of various 
weights. The precision of the deter- 
minations is believed to be +6 eps or 
better in all cases. 
The results of this study are shown 
in Fig. 4. In spite of the consider- 
able scatter of the data, certain trends 
150 200 
activity of human subjects as function of body weight 
+HV 
CO 
Signal | 
FIG. 3. Schematic of photomultiplier circuit 
are evident. The group of points in 
the lower left portion of the graph 
represent children. The scatter here is 
probably within the statistical precision 
of the measurement, and may reflect 
a certain uniformity of body build 
among these subjects. There is no 
significant difference between girls and 
boys. The adult points seem to scatter 
downwards from the extrapolated 
line passing through the points for 
children, presumably representing the 
addition of fat to the basic frame. 
Note that the average activity of the 
women is significantly below that of the 
men and that the range of variation 
among adults (more than a factor of 2) 
Assaying Reactor Accidents 
The Human Counter could be 
used to determine the body dose 
resulting from overexposure to 
thermal neutrons. The neutrons 
activate body sodium to an 
extent proportional to the dose 
received. It is estimated that 
an acute exposure to thermal 
neutrons, resulting in a dose of 
0.3 rep, would produce enough 
Na*™ activity to be measured 
with this detector. 
This application, which could 
be of interest in nuclear power 
plants, was pointed out to us by 
Dr. Austin Brues. 
