200.
Sixteen-weeks-old female CF1 mice received 100 rad/week of fission neu-
trons for 4 weeks at dose-rates of 1,
3,
6 and 35 rad/min;
6045 gamma rays at the same dose-rates [V3, Vij.
or 200 rad/week of
In all groups thymic lymphoma
was the main cause of death and its incidence was not significantly affected by
the dose-rate.
On the basis of mean survival time or of mortality curves, life-
time was reduced by about 65 per cent by neutrons and by about 50 per cent by
gamma rays.
The neutron efficiency
gamma by more than a factor of 2.
was therefore higher than that of the
The mean survival times of both neutron- and
gamma-irradiated animals were significantly shorter with 35 than with 1 rad/min.
201.
Vogel and Jordan [V5, V6] compared the lethality of fission neutrons pro-
duced by a CP-5 reactor to that of 605, gamma rays, both radiations being delivered at about 1 rad/min.
CF1 female mice were exposed according to a complex
pattern of fractionation in which 13 weekly doses of 300 rad of gamma rays were
delivered into 1, 3 or 6 equal fractions per week.
Since the arbitrary RBE va-
lue chosen was 5, the corresponding total weekly dose of neutrons was 60 rad,
delivered into 1, 3 or 6 fractions per week.
The data showed that the postu-
lated RBE of 5 was too high.
202.
Upton, Randolph and Darden [U10! reported that with fast neutrons the
life expectancy of RF female mice was shortened by 0.8 days/rad, irrespective
of dose-rate, whereas the life-shortening efficiency of the gamma rays decreased
from about 0.3 days/rad at 7 rad/min to about 0.15 days/rad at 5 rad/day.
The
RBE increased therefore with decreasing dose-rate from about 2.7 to 5.4.
203.
In the experiments of Upton, Randolph and Conklin [U7] variable dose-
rates of x and gamma rays (80 - 1 rad/day) and of fast neutrons (11 to 0.004
rad/day) were administered to RF mice.
100 rad/min of both radiations.
There were also groups treated at about
Gamma rays at the low dose-rates were invari-
ably and consistently less effective than at the high dose-rates.
the contrary, showed less dependence on the intensity.
Neutrons, on
The RBE evaluated on
the basis of the average life-shortening effect (days/rad) was about 3 at high
dose-rates and it increased to about 8 for terminated chronic irradiation. When
exposure continued until mortality reached about 50 per cent of the mice, a
further increase of the effectiveness to about 14 took place.
Without knowledge
of the dose-rate-RBE relationships it was impossible to foresee whether higher
RBE's might be found at even lower dose-rates.
predict any trend at different neutron energies.
Similarly, it was impossible to
However, since 5-MeV neutrons