war to use it in both homogeneous and heterogeneous uranium-oxide experiments.
Nuclear research in the Soviet Union during this period was limited to skillful laboratory work. Two associates of Soviet physicist Igor Kurchatov reported to the
Japanese studies toward an atomic bomb began first within the military. The director of the Aviation Technology Research Institute of the Imperial Japanese Army, Takeo Yasuda, a lieutenant general and an alert electrical engineer, conscientiously followed the international scientific literature that related to his field; in the course of his reading in 1938 and 1939 he noticed and tracked the discovery of nuclear fission. In April 1940, foreseeing fission's possible consequences, he ordered an aide who was scientifically trained, Lieutenant Colonel Tatsusaburo Suzuki, to prepare a full report. Suzuki went to work with a will.
Niels Bohr had returned from Princeton to Copenhagen at the beginning of May 1939, preoccupied with the gathering European apocalypse. His friends had urged him to send for his family and remain in the United States. He had not been tempted. Refugees still escaping from Germany and now fleeing Central Europe as well needed him; his institute needed him; Denmark needed him. Hitler proposed on May 31 to compromise the neutrality of the Scandinavian countries with nonagression pacts. The pragmatic Danes alone accepted, fully aware the pact was worthless and even demeaning but unwilling to invite invasion for a paper victory. By autumn, when the John Wheelers offered to shelter one of Bohr's sons in Princeton for the duration of the conflict, Bohr reserved the offer against future need. “We are aware that a catastrophe might come any day,” he wrote in the midst of Poland's agony.
Catastrophe for Denmark waited until April 1940 and came then with brutal efficiency. Bohr was lecturing in Norway. The British had announced their intention to mine Norwegian coastal waters against shipment of Norwegian iron ore to Nazi Germany. On the final evening of his lecture tour, April 8, Bohr dined with the King of Norway, Haakon VII, and found King and government lost in gloom at the prospect of a German attack. After dinner he boarded the night train for Copenhagen. A train ferry carried the cars across the Oresund at night to Helsing0r while the passengers slept. Danish police pounding on compartment doors woke them to the news: the Germans had invaded not only Norway but Denmark as well. Two thousand German troops hidden in coal freighters moored near Langelinie, the Copenhagen pier of Hans Christian Andersen's Little Mermaid, had stormed ashore in the early morning, so unexpected a sight that night-shift workers bicycling home thought a motion picture was being filmed. A major German force had marched north through Schleswig-Holstein onto the Danish peninsula as well, crossing the border before dawn. German aircraft marked with black crosses dominated the air. German warships commanded the Kattegat and Skagerrak passages that open Denmark and southern Norway to the North Sea.
The Norwegians fought back, determined that their King, court and parliament must escape to exile. The Danes, in their flat country where Panzers might roll, did not. Rifle fire crackled in the streets of Copenhagen in the early morning, but King Christian X ordered an immediate ceasefire, which took effect at 6:25 a.m. By the time Bohr's train arrived in the capital city what Churchill would call “this ruthless
The American Embassy quickly passed word that it could guarantee the Bohrs safe passage to the United States. Bohr again chose duty. His immediate concern was to burn the files of the refugee committee that had helped hundreds of emigres to escape to exile. “It was characteristic of Niels Bohr,” his collaborator, Stefan Rozental, writes, “that one of the first things he did was to contact the Chancellor of the University and other Danish authorities in order to protect those of the staff at the Institute whom the Germans might be expected to persecute.” Those were Poles first of all, but Bohr also sought out government leaders to argue for concerted Danish resistance to any German attempt to install anti-Semitic laws in Denmark.
He even found time on the day of the occupation to worry about the large gold Nobel Prize medals that Max von Laue and James Franck had given him for safekeeping. Exporting gold from Germany was a serious criminal offense and their names were engraved on the medals. George de Hevesy devised an effective solution — literally: he dissolved the medals separately in acid. As solutions of black liquid in unmarked jars they sat out the war innocently on a laboratory shelf. Afterward the Nobel Foundation recast them and returned them to their owners.
Norsk Hydro was a prime German objective and there was heavy fighting around Rjukan, which held out until May 3, the last town in southern Norway to surrender. Then a management under duress reported to Paul Harteck that its heavy-water facility, the Vemork High Concentration Plant, could be expanded to increase production of the ideal neutron moderator to as much as 1.5 tons per year.
“What I should like,” Henry Tizard wrote Mark Oliphant after he had studied the Frisch-Peierls memoranda, “would be to have quite a small committee to sit soon to advise what ought to be done, who should do it, and where it should be done, and I suggest that you, Thomson, and say Blackett, would form a sufficient nucleus for such a committee.” Thomson was G. P. Thomson, J.J.'s son, the Imperial College physicist who had ordered up a ton of uranium oxide the previous year to study and felt ashamed at the absurdity. He had concluded after neutron-bombardment experiments that a chain reaction in natural uranium was unlikely and a war project therefore impractical. Tizard, who had been skeptical to begin with and had taken Thomson's conclusions as support for his skepticism, appointed Thomson chairman of the small committee; James Chadwick, now at Liverpool, his assistant P. B. Moon and Rutherford protege John Douglas Cockcroft were added to the list. Blackett was busy with other war work, although he would join the committee later. The group met informally for the first time on April 10 in the Royal Society's quarters at Burlington House.
It probably met as much to hear a visitor, the ubiquitous Jacques Al-lier of the Banque de Paris and the French Ministry of Armament, as to discuss the Frisch-Peierls work. Allier warned the British physicists about the German interest in heavy-water production and bid for collaboration on nuclear research between Britain and France. Only then, Thomson notes in the minutes he kept, did they consider “the possibility of separating isotopes… and it was agreed that the prospects were sufficiently good to justify small-scale experiments on uranium hexafluoride [a gaseous uranium compound].” They proposed rather ungenerously to remind Frisch to avoid “any possible leakage of news in view of the interest shown by the Germans.” They were willing to inform him that his memorandum was being considered but not to supply details. (Peierls' name seems not yet to have made an impression on Thomson, and Tizard apparently retained the second Frisch-Peierls memorandum in his files.) “We entered the project with more scepticism than belief,” the committee would report later, “though we felt it was a matter which had to be investigated.” Thomson's minutes make that skepticism evident. Tizard for his part wrote Linde-mann's brother Charles, a science adviser to the British Embassy in Paris, that he considered the French “unnecessarily excited” about the perils of German nuclear research. “I still… think that [the] probability of anything of real military significance is very low,” he estimated in a note written the same week to the British War Cabinet staff.
It might have been as unpromising a start as the first meeting of the Briggs Uranium Committee had been, but the men on the Thomson committee were active, competent physicists, not military ordnance specialists, and whatever their initial skepticism they understood where the numbers Frisch and Peierls had used came from and what they might mean. At a second meeting on April 24 Thomson recorded laconically that “Dr. Frisch produced some notes to show that the uranium bomb was feasible.” Many years later Oliphant recalled a more expansive response: “The Committee generally was electrified by the possibility.” Chadwick's good opinion helped. He had just begun exploring fast-neutron fission himself with his new Liverpool cyclotron, the first in England, when he saw the Frisch-Peierls memorandum. At the April 24 meeting he awarded the emigres' work chagrined confirmation: he “was embarrassed,” says Oliphant, “confessing that he had reached similar conclusions, but did not feel justified in reporting them until more was known about the neutron cross sections from experiments. Peierls and Frisch had used calculated values. However, this confirmatory evidence led the Committee to pay great attention to the development of techniques for… separation.”
Chadwick agreed to undertake the necessary studies. For several more weeks, until their protests through Oliphant registered with Thomson, Frisch and Peierls would be walled off from their own secrets. But work toward a
