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204
NEUTRON EXPOSURE TO LUNAR ASTRONAUTS
Jacob Kasiner, B. G. Oltman, Yehuda Feige,* and Raymond Goldt
The flux and energy distribution of neutrons generated by
cosmic rays in lunar material has been inferred from a knowledge of the situation at the earth’s surface. It is suggested that
lunar astronauts may suffer significant exposure to thermal
neutrons so that their tissue sodium could well be activated
to sodium-24. It would be very informative for the future
assessment of risks to personnel on the moon to subject
lunar astronauts to whole-body counting.
The flux and energy distribution of neutrons generated by cosmic rays in lunar material can be inferred from knowledge of neutrons generated by
cosmic rays in the earth’s atmosphere andat the airground interface. ‘1-+)
A number of factors will enhance the neutron flux
at the moon’s surface. The total flux will be greater
because of the vastly higher cosmic-ray intensity resulting from the lack of a shielding atmosphere and
magnetic field. Also, the higher average atomic mass
of lunar materials will result in a greater neutron
yield and lower slowing down so more of the neutrons
can leak out into space.
The number of thermal or slow neutrons will be relatively even greater because of the lack of nitrogen to
act as a sink (for the production of 4#C®)). Further-
more, the moon’s gravity will trap most thermal neu-
trons (especially on the night side, where the moon’s
escape velocity of 2.4 km/sec is certainly greater than
the mean neutron speed). Finally, the neutron decay
(r = 18 min) will restrict the slow neutrons to within
about one or two moon radii of the lunar surface.
We have derived the theoretical distributions at an
air-SiO, interface for various energy groups of neu-
trons using a modification of an Argonne reactor code.
Figures 161 and 162 are a presentation of the situa-
tion for two widely different energy groups. At the
moon’s surface, (vacuum-matter interface) the flux
would, of course, be considerably less than half of the
maximum intensity within the lunar crust. This factor
_could be as much as 10, thus reducing the derived
fluxes by a factorof5,
From flights at 20,000 ft over Florida, we have confirmed the mean free path for cosmic ray protons in
our atmosphere to be 145 g/em*. Thus, our atmosphere (1 kg/cm?) reduces the incident cosmic-ray
proton flux by a factor of exp (—1000/145) or about
* Present address: Health Physics Division, Israel Atomic
Energy Commission, Soreq,Israel.
+ Reactor Physics Division.
10-3. One would, therefore, expect the thermal neu-
tron flux at the moon’s surface to be greater than on
the earth by the factors, 1000 (lack of atmosphere) x
2 (lack of magnetism) X 2 (atomic mass yield) xX 2
{gravity turn around), i., greater by a factor of
8000. If the hydrogen content of the lunar surface is
significant, the leakage of thermal neutrons will be
even greater.)
The thermal neutron flux at an air-land interface
(Argonne) has been measured by its @N(n,p)#4C
reaction to be 3.7 X 1078 n/cm?/sec.©) Thus, at the
moon’s suriace we estimate between 20 and 30 n/cm?/
sec for thermal neutrons.
Our measurement of the fast neutron flux between
1 and 10 MeV gives a numerically similar value of
4 x 10-3 n/cem?/sec at the land-air interface. The
gravity and decay effect are, of course, irrelevant;
thus the flux will be relatively constant at 10-15 n/
cm*/sec to a distance of about 2-3 moon radii before
geometry begins to play a part.
As far as we can tell, none of the neutron leakage
due to galactic protons striking the lunar surface is
severe enough to produce significant dosage problems,
at least as compared with the risk of solar flares,
However, the thermal neutron cross section (varying
as 1/v) will be very great at or near the altitude of
low-fiying astronauts, e.g., 100-mile-high orbits. On
the cold night side, for example, the astronaut tissue
sodium could well be activated to *4Na. It would be
very informative for the future assessment of risk to
personnel on the moon to count lunar astronauts in a
whole-body counter. Resonant foils should accompany the expeditions; or at the very least, gold plat-
ing on electronic components can serve as thermal
neutron detectors.
For future lunar projects, however, thermal neutrons are so easily shielded by a few millimeters of
*LiF or #°B that we see no reason at all to expose
personnel to slow neutrons.
REFERENCES
1. Lingenfelter, R. E., Canfield, E. H., and Hess, W. N. J.
Geophys. Res. 66, 2665 (1961).
2. Kastner, J., Oltman, B. G., Marinelli, L. D., and Klems, J.
Argonne National Laboratory Radiological Physics Division Annual Report, July 1963-June 1964. ANL-6938, p. 71.
3. Gold, R. Phys. Rev, 166, 1406 (1968).
4. Spergel, M. Health Phys. 13, 845 (1967).
5, Gold, R. Phys. Rev. 165, 1411 (1968).