Theodore Alvin Holtzberg (Theodore Hall), the son of Barnett Holtzberg and Rose Moskowitz, was born in Far Rockaway, New York City, on 20th October, 1925. His father had a successful fur business until the Great Depression: "When he was still very young the Depression struck and the Holtzbergs suddenly exchanged prosperity for hardship, an experience that left its mark. Young Ted, as he was known, was none the less a spoiled child, adored in particular by a mother who had believed before he arrived that she could have no more children." (1)
Theodore Hall came under the influence of his brother Edward Hall, who was 11 years older and a scientist. According to Joan Hall: "Ed was another very bright boy, whose interests took a more practical, inventive turn. I think he started doing the family's electrical repairs at the age of six or seven, and he later became an exceptionally gifted engineer.... When Ted was four, I was told, Ed announced to his parents that he was taking over his brother's education. This activity was deeply enjoyed by both of them, and the pupil's rate of progress was remarkable. I think he began learning algebra when he was seven. Ted lapped everything up with ease and delight. Partly through Ed and partly through his innate curiosity, he learned a lot about the world in which he was growing up - a world in which the Great Depression was at the front of everyone's mind. Ed, having graduated with two engineering degrees from City College, had great difficulty in finding a job. With his usual unsentimental practicality, he changed his name from Holtzberg to Hall to evade anti-Semitic prejudice - and thought it wise to change Ted's name as well, though Ted was destined to join a profession loaded with Jews." (2)
Edward helped develop a mathematical talent in the boy that soon displayed an astonishing talent for mathematics and physics. "In 1940, aged 14, he had achieved some of the highest marks ever recorded in the entrance examination at Columbia University, but was then told he was too young to enroll. He went instead to Queen's College, New York, transferring two years later to Harvard, where he was allowed to skip the first year of his course." (3) While at Harvard University he became a member of the John Reed Society, an organization that was dedicated to the ideas of the author of Ten Days That Shook The World.
Hall graduated from Harvard at the age of eighteen. One of his Harvard tutors, John Van Vleck, arranged for him to work on the Manhattan Project under the leadership of Robert Oppenheimer. He later claimed that he became concerned about the consequences of an American monopoly of atomic weapons after the war. He was especially worried about the possibility of the emergence of a fascist government in the United States. Could such a government use nuclear weapons to dominate the world? A friend, Saville Sax, a member of the Communist Party of the United States, introduced him to Sergei Kurnakov, a NKVD agent. (4)
Kurnakov reported in November 1944: "Rather tall, slender, brown-haired, pale and a bit pimply-faced, dressed carelessly, boots appear not cleaned for a long time, slipped socks. His hair is parted and often falls on his forehead. His English is highly cultured and rich. He answers quickly and very fluently, especially to scientific questions. Eyes are set closely together; evidently, neurasthenic. Perhaps because of premature mental development, he is witty and somewhat sarcastic but without a shadow of undue familiarity and cynicism. His main trait - a highly sensitive brain and quick responsiveness. In conversation, he is sharp and flexible as a sword.... He comes from a Jewish family, though doesn't look like a Jew. His father is a furrier; his mother is dead.... He is not in the army because, until now, young physicists in government jobs at a military installation were not being drafted. Now, he is to be drafted but has no doubts that he will be kept at the same place, only dressed in a military uniform and with a correspondingly lower salary."
Kurnakov asked Theodore Hall if the United States were developing flying bombs similar those being used by Nazi Germany (V-I and V-2). Hall replied "No, it's much worse." Hall told Kurnakov that he was part of a team working on an "atomic bomb" that would have a "colossally destructive impact". Kurnakov claimed that he warned Hall: "Do you understand what you are doing? Why do you think it is necessary to disclose U.S. secrets for the sake of the Soviet Union?" Hall answered: "There is no country except for the Soviet Union that could be entrusted with such a terrible thing. But, since we can't take it away from other countries, we should let the USSR know about its existence and be aware of the progress of experiments and construction. Then, at a peace conference, the USSR - on which my generation's fate depends - won't find itself in the position of a blackmailed power.... All the outstanding physicists of the U.S., England, Italy and Germany (immigrants), and Denmark are working on this thing.... We know that both Germany and the USSR are working on a bomb whose action is based on fission of the atom of some elements (uranium and another element, which we call plutonium and which is the 94th element. However, there are few doubts that the U.S. is ahead of the rest, since all of Europe's brains, except the Russians, are concentrated in this country, and billions are being spent. Besides, we have four (or more) cyclotrons, and in other countries, according to data we have, there are no more than two in any country." (5)
Over the next few months Hall had several meetings with Kurnakov where he explained the main principles of the atomic bomb and provided him with a large number of documents. (6) "He proposed organizing meetings... to inform us about the progress of experiments, for he considers this most important: i.e., the principle itself is not important, since it is known to everyone, but the stage at which practical experiments on explosion and its control, the bomb shell's construction, etc., are important." (7) It is claimed that Hall revealed important information about the more sophisticated of the two bomb designs being developed, the plutonium weapon eventually dropped on Nagasaki. It seems that Hall supplied his first secrets from Los Alamos before Klaus Fuchs did. (8)
Hall's wife later explained his motivation for his actions: "By October, 1944, Ted had formed the decision to pass information about the project to the Soviet Union. When I asked him why, in 1947, he explained that he had feared the United States 'might become a very reactionary power after the war', and if it had a nuclear monopoly it might use this overwhelming advantage to dictate to the rest of the world. This motive.... raises two questions that now have to be
addressed: why did he think that danger existed? And why did he choose to inform the Soviets? The first question is relatively easy, as he later explained in detail what his thinking had been. He had watched the rise of Nazism and Fascism in European countries devastated by the worldwide economic depression, and had seen the ugly emergence of the racist, anti-Semitic American far right. He realized that the depression had been relieved only by the war (which created a boom in the US economy), and he feared that after the war it would return - as I believe was predicted by some analysts at the time - and bring with it the danger of a Nazi-style takeover. The horrors of the war and the holocaust were so manifest, and the aims of the allies (including the USSR) so clearly noble and united, that like millions of other Americans he expected the end of the war to bring an epoch of peace, cooperation and rationality to the world. But then again, he thought, what if it didn't happen that way? He argued this problem back and forth in his mind for some time, conscious also that to avoid a decision was in effect to make one. He felt, he told me, that it would be better to make a mistake - even a big mistake that would ruin the rest of his life - than to fail to act through timidity or inertia." (9)
At first the NKVD was concerned that Hall was a double agent working for the FBI. However, when the material was examined by Leonid Kvasnikov he confirmed that the information would be extremely helpful to Soviet scientists working on the development of an atom bomb. Kvasnikov had also recruited Harry Gold, Julius Rosenberg, David Greenglass and Ruth Greenglass to help with this work. On 8th January, 1945, Kvasnikov sent a message to Pavel Fitin about the progress he was making. "(David Greenglass) has arrived in New York City on leave... In addition to the information passed to us through (Ruth Greenglass), he has given us a hand-written plan of the layout of Camp-2 and facts known to him about the work and the personnel. The basic task of the camp is to make the mechanism which is to serve as the detonator. Experimental work is being carried out on the construction of a tube of this kind and experiments are being tried with explosive." (10)
Theodore Hall suggested that it would look more natural if he had meetings with a woman. It was arranged for Lona Cohen to become his courier. Hall wrote out the information with milk on a newspaper (a variant of invisible ink). Soviet spymaster, Anatoli Yatskov, complained: "With our workload, this method of conveying material is extremely undesirable. We couldn't discern several words in the report, but there were not many such words; the material generally is highly valuable." (11) On 26th May, 1945 Leonid Kvasnikov was able to report to Pavel Fitin: "Theodore Hall's material contains: (a) A list of eight places where work on Enormoz is being carried out... (b) A brief description of the four methods of production of 25 - the diffusion, thermal diffusion, electromagnetic, and spectrographic methods. The material has not been fully worked over. We shall let you know the contents of the rest later." (12)
The Soviet government was devastated when the atom bombs were dropped on Hiroshima and Nagasaki on 6th and 9th August, 1945. Allen Weinstein, the author of The Hunted Wood: Soviet Espionage in America (1999): "On August 25, Kvasnikov responded that the station had not yet received agent reports on the explosions in Japan. As for Fuchs and Greenglass, their next meetings with Gold were scheduled for mid-September. Moscow found Kvasnikov's excuses unacceptable and reminded him on August 28 of the even greater future importance of information on atomic research, now that the Americans had produced the most destructive weapon known to humankind." (13)
Kvasnikov's boss, Pavel Fitin, wrote to Vsevolod Merkulov: "Practical use of the atomic bomb by the Americans means the completion of the first stage of enormous scientific-research work on the problem of releasing intra-atomic energy. The fact opens a new epoch in science and technology and will undoubtedly result in rapid development of the entire problem of Enormoz - using intra-atomic energy not only for military purposes but in the entire modern economy. All this gives the problem of Enormoz a leading place in our intelligence work and demands immediate measures to strengthen our technical intelligence." (14)
In 1946, Hall moved to the University of Chicago, where he finished out his Master's and Doctoral degrees in Physics. An analysis of the Venona decrypts showed that Hall had passed atomic secrets to the Soviet Union. He was interviewed by Alan H. Belmont in March 1951. "Although FBI agents put pressure on him to confess he gave nothing away, and they could find no other evidence against him beyond the Venona documents. Since Venona was still yielding fresh secrets at that time and promised to be a counter-intelligence gold mine for many years to come, the US security authorities believed they could not afford to let Moscow know they were cracking the code... So it was that, in the expectation that they might catch other fish in future, the FBI let Theodore Hall swim free." (15) In fact, the Soviets had known about the breakthrough because of information provided by Soviet spies, Kim Philby and William Weisband.
In 1962 Theodore Hall was invited to join the Cavendish Laboratory at Cambridge University. (16) Brian Cathcart has argued that it was surprising that the authorities allowed him to go: "In the 1960s he moved to Cambridge to pursue ground-breaking work in biology which earned him an international reputation. Did the US authorities, knowing as they did, beyond doubt, of his past role, really allow him to go without extracting some price?... His move from the United States in 1962 required US government approval, and that this was forthcoming remains curious. Why should the FBI, knowing what it did, have done Hall a favour? There is no evidence that he paid any price in terms of giving information, and he firmly denied this himself, but still it seems odd." (17)
Hall was interviewed by Special Branch in England and they made it clear they knew he had been a Soviet spy. However, he was given a work permit and he was never questioned again. He switched disciplines from nuclear physics to the emerging field of biophysics, where over the coming 20 years he was to make several important contributions to knowledge. Chief among these was the work he completed at Cambridge developing what became known as the Hall Method for mapping and measuring minute concentrations of chemicals in biological specimens. He remained at Cambridge University until his retirement in 1984.
The Venona papers were declassified in 1996 and Hall was denounced as a traitor and there were calls for him to be prosecuted, but the British authorities took no action against him. Joan Hall later recalled: "When the whole story came out, I was glad. For us personally the danger was clearly past, and Ted now had a chance to tie together the two ends of his life in a coherent way. This gave him more confidence that in the end he had done something worthwhile in addition to his scientific work - something he had paid for with long years of anxiety and constraint. Ted's efforts. and those of Fuchs, and of others who tried in legal ways to contain the nuclear threat, probably helped to stave off a violent crisis for several decades during which the 'socialist' nations tried to control the arms race. There is good reason to believe that the delay gave humanity an extended time of hope - a time during which our children and grandchildren, with millions of others, grew up to take the reins of struggle into their hands."
Suffering from terminal cancer Theodore Hall told his story to two American journalists, Joseph Albright and Marcia Kunstel: "In 1944 I was 19 years old - immature, inexperienced and far too sure of myself. I recognise that I could easily have been wrong in my judgement of what was necessary, and that I was indeed mistaken about some things, in particular my view of the nature of the Soviet state. The world has moved on a lot since then, and certainly so have I. But in essence, from the perspective of my 71 years, I still think that brash youth had the right end of the stick. I am no longer that person, but I am by no means ashamed of him." (18) The book, Bombshell: the Secret Story of America's Unknown Atomic Spy Conspiracy, was published in 1997.
Hall explained his motivation in a memorandum he wrote in 1997: "I believed that the socialist economic system had achieved great successes in economic production and in education, literacy, health care, social security, etc. etc.... I saw the world during the Second World War, so far as politics was concerned... as containing, simplistically, the good guys and the bad guys. And the good guys were pretty much a continuum encompassing Rooseveltians at one end and some mixture of Communists and Trotskyists at the other end. The bad guys were fascists, Nazis and enemies of the people.... I wasn't acting against the intentions of the US people. I felt myself to be part of a broad democratic front. These actions were undertaken at a time before the beginning of the Cold War, and I saw myself as part of the political front insisting on peaceful and harmonious relations between the peoples of these states.... There was no big change in deciding to do it. I thought it would be a good thing, something that was called for, that the situation called for it. It was just something that fell to my lot, so to speak. I thought that it was not a particularly dangerous thing to do - there was some danger connected with it - but that it would close off one avenue of possible postwar development that should be sealed off, prevented. The bad guys might prevail ... so I just saw what I was doing myself as helping to seal off a danger which was there but probably wouldn't happen anyhow. I thought that by taking this step I could still further reduce the possibility of the bad guys taking over, and with very little risk to myself." (19)
Theodore Hall died in Cambridge on 1st November, 1999.
Rather tall, slender, brown-haired, pale and a bit pimply-faced, dressed carelessly, boots appear not cleaned for a long time, slipped socks. His hair is parted and often falls on his forehead. His English is highly cultured and rich. He answers quickly and very fluently, especially to scientific questions. Eyes are set closely together; evidently, neurasthenic. Perhaps because of premature mental development, he is witty and somewhat sarcastic but without a shadow of undue familiarity and cynicism. His main trait - a highly sensitive brain and quick responsiveness. In conversation, he is sharp and flexible as a sword.... He comes from a Jewish family, though doesn't look like a Jew. His father is a furrier; his mother is dead.... He is not in the army because, until now, young physicists in government jobs at a military installation were not being drafted. Now, he is to be drafted but has no doubts that he will be kept at the same place, only dressed in a military uniform and with a correspondingly lower salary...
As the conversation went on, Hall's nervousness grew... he began biting his nails. I took out a clipping from the New York Times with a story about the U.S. preparing "flying bombs" similar to the German ones, and said, "Perhaps you are disturbed by that?" Then he gave a deep sigh and said: "No, it's much worse."
He told me that a new secret weapon represents an "atomic bomb" with a colossally destructive impact. I interrupted: "Do you understand what you are doing? Why do you think it is necessary to disclose U.S. secrets for the sake of the Soviet Union?" He answered: "There is no country except for the Soviet Union that could be entrusted with such a terrible thing. But, since we can't take it away from other countries, we should let the USSR know about its existence and be aware of the progress of experirnents and construction. Then, at a peace conference, the USSR - on which my generation's fate depends - won't find itself in the position of a blackmailed power.... All the outstanding physicists of the U.S., England, Italy and Germany (immigrants), and Denmark are working on this thing." The list of physicists is on the last page of (Hall's) report."We know that both Germany and the USSR are working on a bomb whose action is based on fission of the atom of some elements (uranium and another element, which we call plutonium and which is the 94th element. However, there are few doubts that the U.S. is ahead of the rest, since all of Europe's brains, except the Russians, are concentrated in this country, and billions are being spent. Besides, we have four (or more) cyclotrons, and in other countries, according to data we have, there are no more than two in any country."
Theodore Hall was a spy who got away with it, and he was a spy of the first rank. Of all the scientists, diplomats and others who passed atomic secrets to Moscow - Fuchs, Maclean, Nunn May, Pontecorvo, Cairncross, Greenglass and the rest - it is likely that only Fuchs was more valuable to the Soviet bomb programme, and even on that point there may be some dispute. Hall's role, however, remained unknown to the public for 41 years after the Second World War, and though he lived all his life in the West - the latter part of it in England - he was never punished.
His story is extraordinary in many ways. Both as a scientist and as a spy he was a prodigy: he was just 19 years old and one of the youngest members of the staff at the Los Alamos bomb laboratory when he passed his most important information to Soviet agents. Yet he seems to have had clear, well-defined political motives for his actions, and he never in later life disowned or regretted what he had done.
Perhaps more remarkably, although he was identified by the US security services as an agent as early as 1950, they were never able to prosecute him for fear of compromising the source of their information. That same source - the so-called Venona decrypts - also pointed the finger of suspicion at Fuchs, who confessed, and at Maclean, who fled, but the young Hall stood his ground and remained a free man.
If there is a mystery about him, it relates to his later life. In the 1960s he moved to Cambridge to pursue ground-breaking work in biology which earned him an international reputation. Did the US authorities, knowing as they did, beyond doubt, of his past role, really allow him to go without extracting some price?
Hall was born in 1925, the youngest child of a Jewish family called Holtzberg who were in the fur business in New York. When he was still very young the Depression struck and the Holtzbergs suddenly exchanged prosperity for hardship, an experience that left its mark. Young Ted, as he was known, was none the less a spoiled child, adored in particular by a mother who had believed before he arrived that she could have no more children.
As he grew up he came under the influence of his brother Ed, who was 11 years older and a scientist. Ed helped develop a mathematical talent in the boy that soon proved outstanding: in school Ted was promoted into a class three years ahead of his age group and still was able to help other children to follow the lessons. This, and some wartime fast- streaming for promising scientists, helps explain how he came to be a Harvard graduate when he was only 18 and was instantly headhunted for the Manhattan Project.
He was not only precocious as a scientist. The Depression and the rise of Fascism in Europe, with its stong anti-Semitic element, encouraged the boy in radical ideas and here again his brother Ed, who brought home left-wing literature, was a strong influence. At Harvard Ted mixed with young men sympathetic to Communism and became a member of the John Reed Society, dedicated to the ideas of the author of Ten Days That Shook The World.
At Los Alamos in mid-1944 Hall found work on the atomic bomb proceeding apace under the direction of Robert Oppenheimer, and he himself was soon drawn into important, cutting-edge research. He became concerned, he said later, about the consequences of an American monopoly of atomic weapons after the war. Say there was another depression, could a Fascist government emerge in the United States? Could such a government use the bomb to dominate the world? And as for the Soviet Union, which unlike Britain was being excluded from the atom project, was it not an ally, and was it not bearing the brunt of the fight against Hitler?
With a single-mindedness and determination that few of his age could muster, Hall decided to let Moscow into the secret himself. During a short holiday in New York in late 1944, and with the help of a left-wing friend, he made contact with Soviet officials and presented them with a first bundle of documents. Over the ensuing months other bundles followed.
Although Moscow was at first sceptical, the information proved to be extremely valuable, notably for what it revealed about the more sophisticated of the two bomb designs being developed, the plutonium weapon eventually dropped on Nagasaki. It was, moreover, timely, for it seems that Hall supplied his first secrets from Los Alamos before Fuchs did.
In 1950 Fuchs was arrested and convicted as a spy in Britain amid enormous international scandal, and he became known as the man who gave Stalin the atom bomb. The story of the Soviet weapon is of course more complex than that, partly because they had their own scientists who had their own ideas and partly because, as we now know, Fuchs was far from being alone in passing on secrets. Indeed, at least two spies in the Manhattan Project have never been identified.
We know this because of Venona. This was coded telegram traffic between Moscow and its missions abroad during the 1940s, which was intercepted by the US and later partially decoded. Much of the traffic related to intelligence and one of the messages in particular pointed to Hall. Dated 12 November 1944, it stated unequivocally that Theodore Hall, aged 19, a furrier's son and Harvard graduate working at Los Alamos, had met a Soviet agent and handed over a report about the Manhattan Project.
By the time the codebreakers had uncovered this gem, however, it was 1950; Hall had long since left Los Alamos, had severed his connections with Soviet agents and was working quietly in academic research in Chicago. Although FBI agents put pressure on him to confess he gave nothing away, and they could find no other evidence against him beyond the Venona documents.
Since Venona was still yielding fresh secrets at that time and promised to be a counter-intelligence gold mine for many years to come, the US security authorities believed they could not afford to let Moscow know they were cracking the code. (In fact Moscow knew anyway, thanks to Kim Philby, among others.) So it was that, in the expectation that they might catch other fish in future, the FBI let Theodore Hall swim free.
It was not until 1996, by which time he had retired and was already suffering from cancer and Parkinson's disease, that Hall's past caught up with him. The Venona papers had been declassified and opened to the public and (although it took some months before anyone noticed) his espionage record was there to be seen.
Exposed, eventually, in the press, he was denounced as a traitor and there were calls for him to be prosecuted, but the storm soon passed. He himself was absolutely unrepentant. In a statement reproduced in Bombshell, he wrote:
In 1944 I was 19 years old - immature, inexperienced and far too sure of myself. I recognise that I could easily have been wrong in my judgement of what was necessary, and that I was indeed mistaken about some things, in particular my view of the nature of the Soviet state. The world has moved on a lot since then, and certainly so have I. But in essence, from the perspective of my 71 years, I still think that brash youth had the right end of the stick. I am no longer that person, but I am by no means ashamed of him.
Ted Hall is survived by his wife Joan, who has known of his espionage record since before their marriage 52 years ago, and by two daughters, Ruth and Sara. A third daughter, Deborah, was killed in a road accident in 1992.
Theodore Alvin Holtzberg (Theodore Hall), physicist and spy: born New York 20 October 1925; married 1947 Joan Krakover (two daughters, and one daughter deceased); died Cambridge 1 November 1999.
Theodore Hall, who has died at the age of 74, was the American atomic scientist discovered by the United States authorities to have been a wartime Soviet spy - but who was never prosecuted.
The information he gave Moscow was at least as sensitive as that which sent Julius and Ethel Rosenberg to the electric chair. But the Americans decided not to charge Hall because of the security and legal difficulties of disclosing that they had penetrated some of the Soviet Union's most secure diplomatic codes. Subsequently, and with the tacit consent of the British security authorities, Hall spent more than 30 productive years as a respected researcher at Cambridge University until he retired in 1984 aged 59.
His espionage was uncovered through the highly-secret Venona project, Washington's long and painstaking effort to decrypt 35,000 pages of Soviet diplomatic traffic intercepted between 1942 and 1946. In the end only about 3,000 of the original texts were ever recovered, mostly long after their transmission.
But it was through the gradual decoding of these double-encrypted signals - a triumph only made public by the US National Security Agency in 1994, some 40 years after the start of the project - that the Rosenbergs and their associates were charged with spying, and the espionage activities of the British agents Klaus Fuchs, Donald Maclean, Guy Burgess and Kim Philby were unmasked.
So great was the pressure to maintain the secrecy of the Venona breakthrough that Fuchs would probably never have been prosecuted had he not confessed. Philby's senior position in MI5 had given him full knowledge of the project - although this was not realised until after his flight from Beirut in 1963 - enabling him to secure his fellow agents' escape to Russia.
The first clear reference to Hall had come in a message transmitted on November 12 1944 by the KGB station in New York but not eventually deciphered until April 1961. "BEK visited Teodor KhOLL, 19 years old, the son of a furrier. He is a graduate of Harvard university. As a talented physicist, he was taken on for government work... At the present time H is in charge of a group at CAMP-2. He handed over to Bek a report about the camp and named the key personnel employed on ENORMOUS."
"Bek" was Sergei Kurnakov, a Soviet journalist working in New York; "Camp-2" was the US scientific research centre at Los Alamos, and "Enormous" was Moscow's cryptonym for the Manhattan Project, America's top-secret programme to develop the atomic bomb.
This was the only message that ever mentioned Hall, and his fellow spy Saville Sax, by name. All other references used their code names - MLAD (Young) for Hall and STAR (Old) for Sax - which the Venona team had unravelled much earlier but could not identify. The message had been sent shortly after Hall had started work at Los Alamos.
From an early age he had displayed an astonishing talent for mathematics and physics. In 1940, aged 14, he had achieved some of the highest marks ever recorded in the entrance examination at Columbia University, but was then told he was too young to enrol. He went instead to Queen's College, New York, transferring two years later to Harvard, where he was allowed to skip the first year of his course.
There he immersed himself in relativity and quantum mechanics and was well-enough regarded to be awarded a special scholarship. As the Nazis went from one military success to another in Europe, he also became increasingly involved in leftwing politics, along with his room-mate Sax.
Hall had become a star pupil of Professor John Van Vleck, one of America's leading experts in quantum theory. The professor had been secretly recruited by the leading US scientist Robert Oppenheimer to help design the atomic bomb - and he, in turn, recommended Hall for work at Los Alamos.
Hall's government interviewer had said no more than that the proposed job was very secret and important to the war effort. The first idea of passing information to the Russians apparently emerged when Hall mentioned the project to Sax.
In June 1944, after Hall had been working at Los Alamos for some months, he was awarded a first-class degree in absentia by Harvard. This brought him promotion, at the age of 18, to head a team involved in designing the implosion trigger for one of the experimental bombs - the one which was eventually detonated successfully at the Trinity site in New Mexico on July 16 1945.
After preliminary tests of the mechanism's practicality, Hall took annual leave to celebrate his 19th birthday with his parents in New York. But - as he explained to two American journalists, Joseph Albright and Marcia Kunstel, 50 years later - he had by then decided to tell the Russians about his work.
"It seemed to me that an American monopoly was dangerous and should be prevented," he told the reporters. "I was not the only scientist to take that view: for example, both Einstein and Nils Bohr both felt keenly that the best political policy was to reach an understanding (with the Russians) - the opposite of the cold war."
In New York, the Russians treated Hall and Sax with great suspicion, apparently believing they were an FBI plant, but the two were eventually steered towards Kurnakov. The journalist had sufficient scientific knowledge to realise that Hall's information was highly significant, particularly after the American had handed him a list of eminent scientists working at Los Alamos.
The KGB's November 22 message certainly aroused Moscow's interest, which was then conveyed to the new recruits. Initially, Sax arranged to meet Hall in Albuquerque, the nearest city to Los Alamos, so that Hall could hand over important details of what eventually became the Fat Man bomb dropped on Nagasaki. Sax, in turn, passed the notes to the Russians in New York.
Not surprisingly, Moscow's interest in the technology soared after two bombs had been dropped on Japan and a fulltime KGB agent was assigned to maintain contact with Hall. She was Lona Cohen, who resurfaced in Britain in 1961 as Helen Kroger, and was given a 20-year jail sentence for her part in the Portland spy ring.
The risks the two young Americans were running were highlighted after Hall had met Cohen in Albuquerque. Their encounter came just after President Truman had announced that the Hiroshima and Nagasaki bombs had been developed at Los Alamos. Arriving at the railway station to return to New York, Cohen ran into a massive military security check. Under the pretext of searching for her mislaid ticket, she handed her box of tissues to one of the security men searching her luggage. As she boarded the train, he reminded her she had forgotten to take it back. Tucked inside were the latest atomic secrets from Hall.
Hall maintained this contact for some years after the war but severed the relationship late in 1949 out of growing distaste for the military application of nuclear physics. In a radio interview years later, he said he had had no interest in assisting the Soviet Union; his sole motive had been to help the world. From then on, his skills were applied wholly to biological science.
But, literally as the Russians exploded their first nuclear device in September 1949, Meredith Gardner, the brilliant cryptanalyst struggling to read the Venona intercepts, successfully decrypted a message which was clearly based on a document written by Klaus Fuchs - and known to very few others. Though Fuchs was tipped off by Philby and tried to resign as chief scientist at the Harwell nuclear establishment, he was interrogated by MI5 and eventually confessed.
Suspicion also fell on Hall, and he was put under surveillance by the FBI. But the bureau's energies were by then principally devoted to rounding-up those associated with the Rosenbergs. Hall and Sax were questioned, but both denied being involved in espionage.
Hall, having by then broken his Soviet connection, was openly involved in left-wing politics, hardly the norm for a covert Soviet agent. The FBI remained suspicious but had no basis on which to mount a prosecution, except for the classified and often ambiguous Venona transcripts. Its inquiries were eventually shelved in 1951.
Hall moved into medical research at the Sloan-Kettering institute in New York and then, in 1962, was invited to join the Cavendish laboratory in Cambridge. A year later, when he applied for renewal of his British work permit, he was questioned by the Special Branch, who made it clear they had a great deal of information about his past. A few weeks later, however, his permit was renewed without explanation, and he was never questioned again.
As the years went by, the waters gradually closed over his past, even in America, and he was able to visit friends and relations there. Hall continued his scientific work in Cambridge after retirement, producing a lifetime total of 166 scientific papers. He and his wife Joan, whom he married in 1947, had three daughters, two of whom survive him.
Jonathan Steele writes: Ted Hall always wanted to be remembered as a scientist, rather than for his role as a youthful atom spy.
He arrived in Britain in 1962 for what was meant to be just one year at the Cavendish laboratory, but ended up living and working in Cambridge for the rest of his life. A soft-spoken and diffident man, he treasured a festschrift produced after his retirement by colleagues from around the world who called him "the great historical figure of biological microanalysis".
He described the main achievement of his career as the development of equations which enabled researchers to look at X-ray data through electron microscopes and deduce the concentrations of various elements. He helped to set up the biological microprobe laboratory in Cambridge, which started to use cryo-techniques to analyse frozen-hydrated sections and gave a huge impetus to the study of the state of ion and water in cells.
Hall was also praised by his colleagues for his generosity to beginners in the field as someone who was always ready to share his time and knowledge. But although he saw this work as more important than his role in passing secrets to the Russians, Hall was enough of a realist to understand that espionage has always aroused public interest, and that this aspect of his life would be seized upon in due time. Initially, he refused to confirm his role - though he made it clear that he was keeping quiet for fear of prosecution, not because he felt ashamed. When the news broke, he already had Parkinson's disease and inoperable kidney cancer.
Eventually, in an interview for the Jeremy Isaacs/CNN television series on the cold war broadcast earlier this year, he explained his role more fully: "I decided to give atomic secrets to the Russians because it seemed to me that it was important that there should be no monopoly, which could turn one nation into a menace and turn it loose on the world... as Nazi Germany [had] developed. There seemed to be only one answer to what one should do. The right thing to do was to act to break the American monopoly".
Hall had earlier told the authors of the book Bombshell: "Maybe the course of history, if unchanged, could have led to atomic war in the past 50 years; for example, the bomb might have been dropped on China in 1949 or the early 1950s. Well, if I helped to prevent that, I accept the charge."
Hall and his wife Joan, who was a teacher of Russian and Italian, were members of the Campaign for Nuclear Disarmament, though they did not go on marches. They took a keen interest in leftwing politics but kept a low public profile, in line with his modest, but caring, manner.
Theodore Alvin Hall, scientist and spy, born October 20 1925; died November 1 1999