AUGUST 1, 1939                PHYSICAL REVIEW          VOLUME 56
                         Printed in U.S. A.
 
          Neutron Production and Absorption in Uranium*
 
            H.L. Anderson, E. Fermi and Leo Szilard
            Columbia University, New York, New York
                    (Received July 3, 1939)
 
 
 
     IT has been found  that there is an abundant emission of neutrons from uranium under the
action of slow neutrons, and it is of interest to ascertain whether and to what extent the number of
neutrons emitted exceeds the number absorbed.
     This question can be investigated by placing a photo-neutron source in the center of a
large water tank and comparing, with and without uranium in the water, the number of thermal
neutrons present in the water. In the previous experiments of this type, it was attempted to have
as closely as possible a spherically symmetrical distribution of neutrons. The number of thermal
neutrons present in the water was determined by measuring along one radius the neutron density p
as a function of the distance r from the center, and then calculating  r2pdr. A difference in favor
of uranium of about five percent was reported by von Halban, Joliot and Kovarski.4
     Since one has to measure a small difference, slight deviations from a spherically
symmetrical distribution might give misleading results. The present experiments which are based
on the same general principle do not require such symmetry. In order to measure the number of
thermal neutrons in the water we filled the tank with a ten-percent solution of MnSO4. The
activity induced in manganese is proportional to the number of thermal neutrons present. A
physical averaging was performed by stirring the solution before measuring the activity of a
sample with an ionization chamber. To obtain an effect of sufficient magnitude, about 200 kg of
U3O8 was used.
     The experimental arrangement is shown in Fig. 1. A photo-neutron source, consisting of
about 2 g of radium and 250 g of beryllium was placed in the center of the tank. The geometry
was such that practically all neutrons emitted by the source and by the uranium oxide were slowed
down and absorbed within the tank. Each irradiation extended over several half-life periods of
radiomanganese and the observed activity of the solution was about four times the background of
the ionization chamber. Alternating measurements were taken with the cans filled with uranium
oxide, and with empty cans of the same dimensions. The activity proved to be about ten percent
higher with uranium oxide than without it. This result shows that in our arrangement more
neutrons are emitted by uranium than are absorbed by uranium.
     In order to find the average number of fast neutrons emitted by uranium for each thermal
neutron absorbed by uranium, we have to determine what fraction of the total number of neutrons
emitted by the photo-neutron source is, in our experiment, absorbed in the thermal region by
uranium. The number of photo-neutrons