tuvestigation m a d e by Heisenberg, in December 1939, led t o the result that while ordinary water was unsuitable as a mode r a t o r , it should b e possible with heavy w a t e r (D 2 0) o r very pure carbon t o produce energy in positive a m o u n t , provided the · moderator a n d uranium were arr a n g e d in layers. T h i s arrangement, however, demanded the highest degree of purity o f the substances involved. A t the s a m e time, it was e v i d e n t t h a t a certain m i n i m u m size of a p p a r a t u s was necessary for t h e production of energy. Nevertheless, with a small s e t it is still possible to determine whether there would be a prod u c t i o n of energy if the apparatus were suitably enlarged. T h u s if we feed such a s m a l l plant with neutrons from an internal source, more n e u t r o n s must escape from the surface t h a n are supplied b y the source, if the layer a r r a n g e m e n t is favorable t o energy p r o d u c t i o n ; if unfavorable, then fewer neutrons escape t h a n are supplied by t h e source. These small model p l a n t s , which a r e continuously fed from a n e u t r o n source, a r e called "neutron-inj e c t e d piles." T h e ratio, k, of t h e number of n e u t r o n s escaping from the pile t o that fed i n by the source can b e used to characterize t h e pile. I f k ^ 1, t h e arrangem e n t is unsuitable for t h e production of e n e r g y ; if k > 1, energy will b e produced o n enlarging t h e pile. I n 1940, measurements of t h e most imp o r t a n t effective cross sections were carried out, especially b y Bothe a n d his collaborators and by Dôpel and Heisenberg. At t h e same time, investigations o n the m a s s e s a n d energies of t h e fission products were being p u r s u e d b y Jentschke and Pranlcl (7) and by Flammersfeld, P . Jensen, a n d G e n t n e r (£), and on t h e spectrum of t h e neutrons produced b y Kirchner and von D r o s t e a n d b y Bothe a n d Gentner (β). The theory of neutron absorption in the TJ238 resonance line w a s laid down by F l u g g e a n d Heisenberg. On t h e technical side the following results were the most i m p o r t a n t : t h e absorption cross section of h e a v y water proved t o be so low that this substance was certainly usable in the construction of a uranium pile (Dôpel and Heisenberg). T h e work of von Droste on large quantities of sodium u r a n a t e , and of H a r t e c k a n d t h e H a m b u r g group— G r o t h , H . Jensen, K n a u e r , a n d Suss—on U 3 0 8 in solid carbon dioxide, furnished the first criteria for the distribution of neutron d e n s i t y i n certain arrangements of uran i u m and moderator. I n the a u t u m n of 1940, the first pile, constructed of layers of U 3 0s and light paraffin, w a s built a t Berlin-Dahlem a n d its characteristics measured (Wirtz, Fischer, Bopp). This model pile gave, as expected, k
