NOTES FROM THE BARTOL RESEARCH FOUNDATION.

ON THE INTERPRETATION OF NEUTRON MEASUREMENTS IN COSMIC RADIATION.*

BY

H. A. BETHE, t S. A. KORFF AND G. PLACZEK.t

The measurements of cosmic-ray neutrons are essentially determined by the energy distribution of neutrons in the atmosphere. The various factors influencing this distribution are investigated. It is found that diffusion equilibrium exists in the atmosphere except for about one meter of water at the top and one meter above ground. It is probable that the energy distribution below xoo key can be represented by a simple diffusion formula; it does not contain an appreciable number of thermal neutrons. The calculation of the number of neutrons originally produced from the present experimental data is uncertain because of capture of fast neutrons. As an example of the influence of the ground, the neutron distribu- tion is calculated in the air above a water surface. Measure- ments in the water may be most suitable for a quanti tat ive determination of the neutron intensity.

The various experimental arrangements are discussed. A I/v-detector such as a BF3 counter measures essentially the density of neutrons if care is taken to minimize the effect of recoil nuclei by using a high bias. The evaluation of experi- ments using hydrogenic material to slow down the neutrons is discussed. From these two types of experiments, pro- visional results for the neutron production are obtained which are in agreement with each other. From the altitude distri- bution of neutron intensity it can be concluded that the production of neutrons of very long range is not very im-

* An abst ract of an article published in full in the Physical Review 57, 573 (x94o).

t Cornell University, Ithaca, New York.

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Dec., I94o.1 T H E BARTOL RESEARCH FOUNDATION. 777

portant. The role of neutrons for the energy budget of cosmic radiation is discussed.

ON THE CONTRIBUTION TO THE IONIZATION AT SEA-LEVEL PRO- DUCED BY THE NEUTRONS IN THE COSMIC RADIATION.*

BY

S. A. KORFF.

Since neutrons constitute a numerically important com- ponent of the cosmic radiation, 1 the question arises as to the amount of ionization in the atmosphere at sea-level which they produce. This ionization will be produced in two ways: (a) By recoil-nuclei produced by the neutrons in collisions; and (b) by the ionizing radiation ejected from nuclei as a result of neutron-induced disintegrations. We may set limits to each of these contributions.

It is believed that most of the neutrons in the atmosphere at sea-level are evaporated out of nuclei by the hlgh-energy cosmic radiation, and have initial energies of IO to 3o Mev. It has been pointed out by Bethe, Korff, and Placzek 2 that these neutrons will first be inelastically scattered by nitrogen nuclei. In this process, the neutron enters the nitrogen nucleus, causes the ejection of one or more nuclear particles, and usually emerges with about four Mev. energy. The bulk of the energy of the neutron is used in breaking up the binding of the nucleus, and hence is not transformed into ionization. The ionizing particles which are emitted will in general have energies which are small compared to the initial energy of the neutron, and it may be estimated 2 that of the order of five Mev. per neutron will appear as ionization.

The measurements of the Montgomerys 3 show that at sea- level the rate of production of neutrons in air is q = Io -7 neutrons per cc. per second. Hence the ionization produced

* Condensed from an article published in full in Terrestrial Magnetism and Electricity, page I33, June, I94o.

x S. A. Korff, Rev. Mod. Phys., xx, 2I I -2I 9 (I939). 2 H. A. Bethe, S. A. Korff and G. Placzek, Phys. Rev., 57, 573-587 (I94O).

C. G. and D. D. Montgomery, Phys. Rev., 56, IO-I2 (I939).