The absolute rates of cosmic‐ray neutron production and neutron flux distribution on the ground were determined at sea level and mountain altitude at a geomagnetic latitude λ = 44°N in 1964. The thermal neutron flux was measured with a well‐calibrated BF3 counter; a Maxwellian energy distribution with a shifted neutron temperature was assumed. By using two differently moderated BF3 counters, the fast‐neutron flux was determined in the energy range 0.4 ev to 10 Mev. The neutron fluxes were also estimated from the measured production rate, and they are in good agreement with the measured fluxes; possible occurrence of air‐ground boundary effects on the neutron flux distribution was considered. Anisotropy of the thermal neutron flux on the ground was experimentally demonstrated, and the angular distribution was well filled by the first two terms of a spherical‐harmonics expansion. The air‐ground boundary effects are discussed on the basis of experimental results.

A compilation of gamma spectrometric data from soil samples collected at 27 fallout monitoring locations in the San Francisco Bay area reveals that natural terrestrial gamma-ray dose rates range from 3-13pr/hr with a modal value of 5.5pR/hr (a 10-yr background dose of 480 mr). There is good agreement between the natural dose rates determined by radioactivity measurements made in the field with a portable counter and those derived in the laboratory by pulse-height analyses of soil samples. Analysis of the measurements made with the portable NaI counter from March I958 through March 1968 yields an average fallout gamma-ray integrated dose for the 27 locations of 280 mr, assuming that all fallout was deposited evenly with respect to time. Roughly three times as much gammaemittine. fallout was devosited in the San Francisco Bay area between March 1958 and the end of 1960"as has been deposited since 1960.

A measurement of the average energy required to create an ion pair in nitrogen by 250 MeV/amu c 6 + ions is described. A value of 36.6±0.7 eV was obtained.

There is increasing interest in estimating the magnitudes of population exposure due to the operation of nuclear facilities. This paper discusses the population exposures that might result from high energy accelerators. External whole body radiation is the dominant type of radiation exposure, while exposure resulting from the leakage of radionuclides into the atmosphere or ground water is negligible. At large distances from operating accelerators, neutrons are the predominant form of radiation. The transport of high-energy neutrons through the atmosphere is not well understood at distances greater than about 1000 m from the radiation source. Consequently, estimates of population exposure out to distances of 80 km from high energy accelerators may be quite inaccurate. It is extremely important that health physicists and administrators be aware of the limitations of estimates of population exposure. A model is described which facilitates the estimation of population exposure due to accelerators and its limitations are discussed. Suaaestions are made for experimental investigations --which would improve the model.