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RICHARD BROOKE ROBERTS 333 PREWAR YEARS AT THE DEPARTMENT OF TERRESTRIAL MAGNETISM OF THE CARNEGIE INSTITUTION OF WASHINGTON The thesis was finished in late 1936, and on a visit to his wife Adeline's family in Washington he drove up to see Merle Tuve and told him of his plan of measurements for a scheduled year at Cavendish; Merle said, "Why not do them here? We have better equipment." Dick accepted the temporary arrangement, and later a fellowship was squeezed out of the Carnegie Institution of Washington (CIW) administration by Merle. So easily began a lifetime. The early work was on scattering experiments (protons, deuterons, and helium), which was hard fundamental work with N. P. Heydenburg. There followed some lithium work with Rumbaugh, and "the main event scientifically of 1938 was pinning down the Be7. This was really very satisfying as isotopes were being discovered right and left and this was my first (and last)" (AB). The Department of Terrestrial Magnetism (DTM) pressurized Van de Graaf generator split its first atoms on December 23, 1938. A quote from a January 1939 letter to Dick's father is included in AB: We have had a very exciting week in Physics. The annual theoretical conference started Thursday with an announcement by Bohr that Hahn in Germany had discovered a radioactive isotope of Barium as a product of bombarding uranium with neutrons.... Fermi also discussed the reaction and described an obvious experiment to test the theory. The remarkable thing is that this reaction results in 200 million volts of energy liberated and brings back the possibility of atomic power. Hafstad and I left the meeting as soon as Fermi finished to go to the lab to try the reaction. We had some trouble with a leak in the tube so it wasn't till Saturday afternoon that Meyers and I finally made the test. We had Uranium in our ionization chamber and bombarded with neutrons. We soon observed tremendous pulses corresponding to very large energy release. [There follows a drawing.] I told Tuve after supper and he immediately called Bohr and Fermi
RICHARD BROOKE ROBERTS 334 and they came out Saturday night and we ran the test again for them and they were immediately convinced. What we did of course is of no particular credit to us but it is nice to be the first to observe the actual splitting of a uranium atom. Another quote from AB to keep the history exactly straight: "We later found out that the Columbia group had done the same on Wednesday and Johns Hopkins on Thursday. Frisch was two weeks ahead." One witness that Saturday evening, Enrico Fermi, had done the same experiment in 1936, using a radium beryllium source of neutrons and had (for a good technical reason) placed a very thin aluminum foil between the uranium and the ionization chamber, which stopped all of the fission fragments. But for that foil the Italians, the Germans, and possibly the world would have known about atomic power and explosions much longer before the Second World War. Continuing to quote valuable history from AB: The following weeks were also hectic. The key to atomic power was the neutron emission that might accompany fission. Since it was technically difficult to observe the relatively few additional neutrons released during bombardment we looked for (and found) neutrons emitted after the beam was turned off.... In March Adeline and I went to Florida for a few days on the beach with my brother Walter . . . he recorded all our long discussions about uranium fission, how to make a pile, and particularly arrangements to control the pile as the delay in the neutron emission gave ample time for control. On his return to New York in May he presented his usual stack of applications for circuit patents. He then said here are a few along a different line and handed over the applications for the pile and its control. His superiors (?) decided RCA was not interested. I had no interest in the patent side as CIW had a policy of no patents .... After Florida I continued work with Salant on neutron scattering but my main efforts went into measuring cross-section for fission for neutrons of various energies. These were essential in calculating whether a chain reaction would run. By summer I wrote a long paper on the possibility of a
RICHARD BROOKE ROBERTS 335 chain reaction based on these cross sections but it was never published because of an agreement to keep such data out of the journals. I did however write one article which I believe is the first serious discussion of the possibility of atomic power for the Journal of Applied Physics. It concluded "The day of free atomic power is probably not in sight." One other historical note on fission. Tuve was scheduled to be present at the first meeting between the scientists and the government following the letters from Einstein and Sachs to Roosevelt. For some reason he could not attend and sent me along in his place. The meeting decided to appropriate $6000âan enormous sum in those daysâfor graphite so Fermi could estimate the possibilities of a graphite moderated pile. Also I remember an Army colonel saying a 20 KT bomb wouldn't do muchâhe had survived such a explosion (at Halifax?). In short he was not impressed. The next phase is 1939 to 1940, when Dick mostly helped in building the cyclotron at DTM, which was supposed to be a supplier of isotopes for half the days and for nuclear physics the other half. He designed the RF system and much else. In this period, many contacts were made that set up the future biophysics group at DTM, including Philip Abelson and Dean Cowie. The biophysics work had actually started much earlier, including a study with Louis Flexner on the transfer of 24Na from rat mother to embryo. The cyclotron did useful work during the war but, of course, it was short lived as a supplier of isotopes. From AB: "At the end of the war the cyc supplied isotopes around the world for the period before the AEC was ready to sell them.... Abelson and I used it for my one post-war physics experiment which showed that the neutron (s) from D plus light elements were mainly forward." It is worth noting that this was the first evidence that the Bohr compound nucleus did not apply everywhere and it motivated the theory of stripping reactions. AB continues: "In retrospect the cyclotron was a fine machine that came at the wrong time." The permanent benefit was the cyclotron building which housed the biophysics group for nearly thirty years.
