dose-rates.
For both radiations, the lower the dose-rate, the longer was the
life-span of the animals.
Lindop and Rotblat
|L10] reported on the changes in
effectiveness (expressed in weeks of life-shortening/100 rad) as a function of
the instantaneous dose-rate at the extremely high intensities of 77 to 158.000
rad/min.
They showed that the maximum effectiveness (5.7 to 6.2 weeks/100 rad)
was at around 1000 rad/min, but could not explain the loss of effectiveness at
still higher dose-rates.
Oxygen depletion induced by these high intensities
could not be responsible for such an effect, since it was also seen in mice
made artificially hypoxic.
155.
Upton, Randolph and Conklin [U7] and Upton, Randolph and Darden [U10]
used variable dose-rates of gamma rays (80-1 rad/day) or of fast neutrons
(11 to 0.004 rad/day) to induce life-shortening on RF/Un male and female mice
(total doses of 10.000 and 1.000 rad, respectively).
In females, a consistent
reduction of the gamma-ray effectiveness was seen at the lowest intensities,
amounting to about a factor of 3, by comparison with acutely-delivered (6.7
or 80 rad/min) doses.
A further reduction to a factor of 6 was observed if
continuous irradiation was carried out to the time of death of about 50 per cent
of the animals.
For neutrons, loss of efficiency of continuous against acute
administration was 0.9 and decreased further to 0.7 for exposures protracted
to death of 50 per cent of the mice.
In male animals, the loss of efficiency
of the gamma rays was even higher, amounting to 0.1 upon continuous administration ad to 0.04 for irradiation protracted up to 50 per cent survival.
156.
Spalding et al.
[S21] performed experiments on RF female mice irradiated
with gamma rays (2.5 to 250 rad/hour, 100 to 1200 rad total dose).
Within a
given total dose the mean after-survival was changed more or less randomly with
the dose-rate.
Biological and environmental factors such as individual varia-
tions in radiosensitivity and cage effects rather than any identifiable physical
parameter were held responsible for these observations.
Dose-rate studies (gam-
ma rays, 0.1 - 100 rad/min, 100 - 300 rad weekly exposure for a substantial duration of the life-span) were also reported by Willhoit and Wiggins
[W5].
Some
decreased effect at the lower dose-rates was noted, but it was impossible to
attribute the effect to any specific disease, owing to the lack of pathology.
157.
In the experiments of Ullrich and Storer |U81 and Storer et al.
[Sh4] on
RF and Balb/c female mice lowering the dose-rate of gamma rays led to a modi-