by Gino Segrè
Given her husband’s distinction, Laura could have insisted on living on Bathtub Row, but she was content with a three-bedroom apartment in building T-186, a typical two-story four-apartment wooden structure. Admittedly, Laura was somewhat taken aback to see blankets and sheets with USED stamped on them, although relieved when she was informed the army supplies were not “used” but had been issued by the United States Engineer Detachment. It was still a far cry from the ironed linen sheets she had grown up with and that she continued to use in Chicago.
The apartment was satisfactory in spite of shoddy workmanship and minimal insulation from weather and noise, but the accommodations were made more palatable by Laura’s discovery that the apartment below was occupied by a couple she had known in Rome a decade earlier, the German-born physicist Rudolf Peierls and his ebullient Russian wife, Genia. They had been in Los Alamos for a while and were delighted to take Laura under their wings. It helped that they had a daughter a year younger than Nella and a boy a year older than Giulio.
The morning after Laura and the children arrived, one of the Boys of Via Panisperna appeared. Emilio Segrè had moved to Los Alamos in 1943 and announced, in the spirit of former times, “While the Pope is not here, I’ll bring your mail.” Perhaps it was America’s more secular atmosphere, but the title of the Pope had seldom been used once Fermi had crossed the Atlantic. Now, in such a strange and wondrous setting, it felt welcoming to Laura.
When she inquired why mail delivery by Emilio was necessary, he replied, “There is no home delivery. All the mail goes to the Tech Area and there it stops.” It did more than stop there; the mail was read and censored lest information be leaked. Since correspondents were not supposed to know that the mail they were receiving had been censored, outgoing letters were deposited unsealed. If they passed inspection, the censor would seal them and send them on. If not, they were returned to the sender.
Much to Laura’s amazement, Emilio, who had been called Basilisk by the Boys, conjuring up images of an ill-disposed reptile, became her close friend. The bond between them strengthened because they shared a common recent tragedy. Her father and his mother had each been taken by SS troops in Rome the previous October and sent to concentration camps in Germany. Because of the wartime embargo on mail, they had learned the news only a month earlier, in July. Mail had become a virtual lifeline now that three years of wartime silence had been broken. Finally they were able to receive letters from Italy and learn about the respective fates of their parents. Their spirits had been buoyed by the fall of Fascism the previous September, but their wishes for a brighter future in Italy were quickly dashed.
The Allies’ invasion of Sicily, ending victoriously in August 1943, led the Italians to conclude that the war was lost. The Fascist Grand Council had deposed Mussolini on July 24. Secret negotiations for an armistice were set in motion and concluded on the eighth of September. Announced publicly, this was the event that had inspired such hope in Laura and Emilio. But the result of the armistice was that the Germans, feeling betrayed, had acted quickly to take charge of as much of Italy as they could.
The country was effectively placed under German military command, an occupation marked by severe reprisals and massacres of civilians. With fierce fighting between the Germans and the Allies taking place in the fall and winter and heavy losses suffered by both sides, the Allies’ invasion was stymied. Rome and every part of Italy north of the city remained under German control.
The situation in Italy had initially not seemed menacing, but many Jews had already gone into hiding, either by procuring false documents or by finding a safe refuge. Laura’s sister Anna and her husband, Alberto Montel, fled with their two young children from Torino to Switzerland. After living in comfort in Italy, they fell on hard times: they were short of money. Laura and Enrico began to send them a monthly remittance of $100, quite a significant amount of money then.
In Rome, the state of affairs for Jews was rapidly deteriorating. The first sign of alarm came when the Germans seized the registration records of Jewish members from the city’s main synagogue. And then they struck. On the sixteenth of October 1943, at 5:30 in the morning, SS troops surrounded the old ghetto, where many of the poorer Jews still lived, and began going house to house arresting those they found. Loading them onto transport trucks, they brought them to the courtyard of the nearby Military College. The SS troops then set out to arrest the Jews living elsewhere in the city. A little more than a thousand Jews, some ten percent of those living in Rome, were swept up in the roundup on that October day.
Emilio Segrè’s parents had been warned of the house arrests, and while his father waited in their escape car, his wife—hurrying back to their home to collect some jewelry—was intercepted by SS troops and taken away. His father, who had witnessed the arrest, could do nothing. He was driven away by their chauffeur, taken to a monastery, and remained hidden until his death a year later.
The SS had also arrested Laura’s father, Admiral Capon, in the roundup. As the situation for Jews began to deteriorate, Laura’s three siblings and their families had already gone into hiding. Fermi’s sister, Maria, repeatedly offered to hide Laura’s father. Partially paralyzed after a stroke and still believing his rank in the Italian navy would protect him, the admiral had declined.
It took the Allies four attempts before they could finally break through the German lines. On the fifth of June 1944, they entered Rome, greeted there with jubilation by its citizens. And communication with the United States was finally restored. That is when the Fermis heard, both from Amaldi and from Fermi’s sister, Maria, about the arrest of Admiral Capon. Some embers of hope continued to burn, as in a November 15, 1944, letter from Laura’s brother-in-law that says the admiral “sent a card from Ferrara and we think he was sent to Theresienstadt. We have not had any news since then. We can still hope he’ll be exchanged for German prisoners by the Allies. A request has been presented to the Jüdische Flüchtlingshilfe [Jewish refugee agency] and it might be good for you to try and get some information from competent sources in Washington.”
Fermi’s first letter to Edoardo Amaldi after the liberation of Rome comments on Laura’s state of anxiety: “As you can imagine, Lalla [Laura’s nickname] has been very pained by the news of her father; the uncertainty of his fate has been much worse than knowing him dead.” The letter’s phrasing “Lalla has been very pained” rather than “we have been very pained” is a telling reminder of Fermi’s reserve in emotional matters. Equally striking is that Laura, in her engaging book, makes no mention of this.
The uncertainty about exactly what had happened to the deported Roman Jews in October 1943 would hover until the end of the war. Their ultimate destiny was feared, but not substantiated until a few survivors gave testimony about their gruesome deaths. Laura’s father was murdered in an Auschwitz gas chamber immediately after arriving at the camp. And Emilio’s mother never made it to Auschwitz, perishing in a rail car on the way there.
Laura and Emilio hardly spoke, not even to family members, about the deaths of their respective father and mother amid the horrors of the Holocaust. But when Laura and Emilio reconnected years after their youthful days in Rome and found themselves on top of an isolated mesa in the great American Southwest during the summer of 1944, they commiserated—and waited nervously for letters from Italy with hopes of positive news.
30
GÖTTERDÄMMERUNG
When Richard Wagner’s turbulent opera Götterdämmerung premiered in 1876, German audiences were undoubtedly shaken by the doomsday scenario of the last part of his famous Ring cycle. The world was engulfed in flames, its ultimate destruction sealed. It was a nightmarish scenario, and the Los Alamos scientists could readily imagine Hitler cast as the pivotal evil force. Fears that Germany would develop the bomb before the Allies succeeded in doing so were particularly pervasive among Los Alamos’s refugee physicists. Two factors came to bear: the first, a definite unknown, was how close the Germans were to having a bomb
and the second, more apparent, was the many obstacles Allied scientists needed to overcome before they could succeed.
The physicists on the mesa would have been shocked to learn the true state of affairs of nuclear research in 1944 Germany. It was nowhere close to developing a bomb. The German workforce of outstanding physicists, many of whom were Jewish or married to someone Jewish, had been seriously depleted by racial laws. Organizationally, there was nothing that approached the scope of the Manhattan Project. The Uranverein (Uranium Club), formed in 1939 to explore fission, had morphed into various structures that attempted to develop and produce nuclear weapons. However, they had never even achieved the basic precursor: a sustained critical nuclear reaction such as Fermi and his colleagues had accomplished with the pile in December 1942.
In regard to steps toward for making a bomb, the Germans failed to appreciate, as Szilard and Fermi had, that carbon obtained commercially was likely to contain hidden impurities that needed to be removed before it could be used as a moderator in a pile. In addition, the Allies were partially successful in preventing the Germans from having access to heavy water, the alternative moderator.
The leadership of the German atom bomb project was also wanting. In June 1942 it was essentially entrusted to Werner Heisenberg, a theorist who had little understanding of experimental trials. Paul Harteck, one of Germany’s leading experimenters working on the German bomb project, put it succinctly: “How can you be a leader in such technological matters when you have never run an experiment in your whole life?” In contrast, Oppenheimer—also a theorist—adeptly ran the Los Alamos project, aware of his limitations and ready to resolve them. Heisenberg had no such comparable qualities.
Nor did Heisenberg possess Fermi’s facility for rapidly estimating the order of magnitude of any physical phenomenon. Rudolf Peierls, who knew Heisenberg well from having worked as a student with him, observed, “Though a brilliant theoretician he was very casual about numbers.” This was a disastrous flaw in a project that depended so much on exactitude. Because of its own failed attempts, the Uranverein was confident that the Allies would not be able to develop the bomb. On the other hand, the Manhattan Project was very much driven by the belief that the German enemy could beat them to the bomb.
The vast enterprise of the Manhattan Project had been created with a mix of major fiscal and human resources, a large dose of optimism, and the commitment of many of the world’s top scientists. In June 1944, counting construction workers, machine operators, military personnel, and scientists, the project had altogether 129,000 employees, a figure dwarfing the effort the Germans mounted. Among the scientists in the Manhattan Project were leading physicists from England and Canada, known collectively as the British Mission, who had joined with their American counterparts in a spirit of cooperation. In Los Alamos, the British team was led by James Chadwick, the discoverer of the neutron, and it included Frisch and Peierls, the two physicists who had alerted their peers that building a fission bomb was within reach.
Nevertheless, Allied bomb development was in jeopardy during the summer of 1944. Those at Los Alamos were plagued by doubts of reaching their goal of having several bombs ready by the middle of 1945, the time estimated for them to still have an impact on the war. The strategy that had been developed was to proceed on two fronts, one using U-235 and the other plutonium, but it was beginning to look as if neither would be successful in time.
Difficulties had been encountered both in the production of sufficient fissile material for the bombs and in the development of a bomb detonation mechanism, two interrelated aspects of the problem. The separation of U-235 from natural uranium at Oak Ridge was proceeding so slowly that by mid-1945 there might be enough for only one uranium bomb, with no prospect of having more soon after. Production of plutonium was more promising, but the Hanford reactor was not scheduled to enter into operation until the end of September. If it did not function properly, there would not be enough plutonium available for even one bomb, much less for several of them.
Detonation mechanisms, the other major cause of concern, had already been discussed at the Los Alamos orientation meeting in April 1943. Two mechanisms had been seriously considered. The first, the so-called gun mechanism, was relatively straightforward. A piece of fissile material with a Fermi neutron reproduction factor k less than one would be shot at a second similar piece placed near the first. When united, the two subcritical pieces would become supercritical, with k closer to two—that is, two neutrons would be produced for every one that was absorbed. An explosion would occur at an exponential rate, in microseconds. The bomb needed to be small enough to be carried by a plane.
The second mechanism hinged on being able to surround with explosives a sphere of fissile material not dense enough to be critical. When simultaneously ignited, the explosives would cause the sphere to implode and quickly reach the density at which it became supercritical. But placing them so as to have uniform compression of the fissile material was extremely delicate. The implosion method was largely ignored in 1943 because it was intrinsically much more complicated than the gun mechanism and there was uncertainty as to whether it could be developed. In any case, it was felt at the time that the gun mechanism would work for plutonium as well as for U-235.
The detonation crisis in the summer of 1944 was precipitated by the realization that though the gun mechanism would function as anticipated for U-235, it would not do so for plutonium. The plutonium produced in reactors contained too large a percentage of the radioactive isotope Pu-240. It would vitiate the gun mechanism by prematurely detonating; the bomb would fizzle. There was also no hope of separating the desired isotope of plutonium, Pu-239, from Pu-240.
After more than two years of work by many brilliant scientists, an unprecedented mobilization of resources, and hundreds of millions of dollars having been spent, there was a grave threat that the Manhattan Project would have to be declared a failure on a monumental scale.
The circumstances were so dire that in mid-July 1944, Oppenheimer notified Conant, the head of the National Defense Research Council, and arranged a meeting in Chicago with him, Compton, Fermi, and Groves to discuss how they might salvage the project. Their conclusion was that Los Alamos needed to undertake a crash program to develop another detonation mechanism for the plutonium. There was only one real candidate: the implosion mechanism. If that could not be made to work, there was no substitute.
Oppenheimer had hoped to have Fermi come to Los Alamos full-time in August 1944 to help the laboratory overcome the crisis, but Fermi’s presence in Hanford had to be the Manhattan Project’s overall priority. Since its reactors were scaled-up versions of CP-1 and CP-2, he had been a vital guide for the building of Hanford’s reactors. If they did not operate properly, there was no backup. The whole project was poised on the edge of a ridge, with chasms on both sides. There would be success or utter failure. The Hanford team would understandably have been even more concerned had Fermi not been present in September when they would try for the first time to have the reactors reach criticality. As Compton asserted, “Enrico Fermi was our anchor man on such occasions.”
The reactor did go critical as scheduled, a few minutes after midnight on Wednesday, September 28. Everything was working as expected. But after operating for a few hours at a higher level than any previous reactor, trouble set in: the power output was inexplicably decreasing. The operators started moving the control rods out to keep the reactor steady, but that helped for only a little while. By morning the reactor was completely dead.
A distressed Crawford Greenewalt, directing the Hanford Project for DuPont, turned to a friend driving with him to the site the next morning and said that it could not be due to any problem with the properties of the material they were using “since this had clearly been well worked through by Fermi.” It had to be something else: water leaking into the reactor from one of the pipes, or perhaps something wrong with the water itself. The growing suspicion that the stopping was due instead to the mat
erial was confirmed in everyone’s mind early Thursday morning when, seemingly miraculously, the pile began operating again. It went critical at 7:00 a.m., dying once again twelve hours later.
The Princeton physicist John Wheeler was Hanford’s local nuclear physics expert in 1944. He had considered that this might occur but had thought the possibility was remote and had not found a way to pretest the reactor. When it did occur, Fermi and Wheeler agreed on what must be taking place: the reactor was being poisoned.
The sequence had to be that after absorbing a neutron, some of the U-238 nuclei in the reactor turned into U-239, which then decayed in two steps to plutonium 239, the desired end product. However, other U-238 nuclei underwent fission. In one of the decay chains that followed, a formidable absorber of neutrons was being produced, so powerful that it could absorb all the neutrons being produced, effectively shutting down the reactor. But then why did the reactor turn on again after a few hours? There was an answer for this as well: the absorber was unstable. Within a few hours, its nuclei decayed into other nuclei that did not absorb neutrons. The reactor would then start up and the cycle would repeat itself.
Fortunately there was a solution. Eugene Wigner had designed the reactors for maximum efficiency, but on Wheeler’s advice, DuPont had planned on having a margin of safety by installing additional material and equipment should they be needed. Time would be required to make the changes and connect the necessary extra water for cooling, but it could be done reasonably expeditiously. And this would fix the reactor because, as Fermi and Wheeler now calculated, making the changes would allow it to overcome the poisoning.