range in the measured dose rates in each case (+10 per
cent about the mean for most field readings) meant that
relatively few locations were required for this purpose.
Most of the measurements were made in the larger towns,
as these contained the bulk of the population.
The mean y-dose rates over open ground derived from
the spectrometer readings are given in Table | for each of
the eight regions chosen for examination. The main towns
and the corresponding bedrock formations are indicated.
The spectrometric procedures allow determinations of the
component dose rates from potassium-40, the uranium
and thorium series, and the main fall-out y-emitters
(8Zr- Nb, Ru, Rh, 47Cs).
Estimates of mean soil
concentrations of the natural radioisotopes can be calculated from the component dose rates, assuming uniform
depth distribution in the ground?*", Such concentration
values are representative of the true soil contents of
radium, uranium and thorium only when these isotopes
aro in radioactive equilibrium with their y-emitting
daughters. These results are included in Table 1, and the
em for determ
eo
ination
necessar
lation tural
i
and fall-out
Yaadion: my,
at on detector
s for survey
s ofthe
oor measurem
ent location
as
r measurements
otail elsewhere11-1s neSur vey tech-
T cent (S.D.) for th
106 an over.
‘approximately + 19easured total
mean values for the natural emitters arc combined to
give a mean ‘equivalent uranium’ (eU) concentration for
the upper layers of the ground, that is, the amount of
uranium in equilibriam with its daughtors that would
yield the same y-dose rate as the potassium and the
uranium and thorium series in the actual situation. Also
listed for comparison purposes are the moan equivalent
uranium contents of the various bedrocks as estimated by
Billings®. It is noteworthy that the rangeof soil activities
is much narrower than that inferred for the bedrock
formations.
The average fall-out levels indicated in Table L apply
specifically to July and August 1962, whon almost all
readings fell between 2 and 3 ur.jh. When some of these
sites were re-checked in 1963, the fall-out dose rates were
nearly twice as great, ranging from 3 to 5 ur./h. In all
cases, *Zr—*Nb was the dominant contributor. By May
1964, levels of 1 ur./h were typical, the °Zr—*®Nb and other
short-lived emitters having nearly disappeared, leaving
137Cs as the most important fall-out y-omitter. These
resnlts are quite consistent with the more detailed information available for the New York City arca during this
period's,
In addition to the combined ionization chamber and
spectromoter readings summarized in Table 1 (5-10 in each
region, except for 16 at Conway), separate ionizationchamber readings were made at many other sites along
with a number of independent portable scintillometer
surveys. The uniformity of the outdoor radiation levels in
each area was remarkable.
2
In general, streets and side-
walks did not significantly alter the obsorved profile.
Any attempt to estimate population exposure to
environmental radiation must take into account the
3