The drawings and other information were the culmination of a stunning intelligence coup: the secrets of the Manhattan Project delivered directly to the Soviet Union's own bomb-makers.
Educated in Leningrad and at Cambridge, Dr Khariton belonged to a tiny group of physicists who would pioneer the Soviet bomb, working first at Laboratory No 2, a research facility in Moscow, and then at Arzamas-16, Russia's first nuclear weapons design laboratory.
Now 89, he is one of the most honoured and privileged men of Soviet science, a three-time winner of the title of Hero of Socialist Labour, the highest civilian award, with a personal railway carriage at his disposal that still carries him between Moscow and Arzamas-16, 200 miles to the east.
The sketches and other intelligence he saw in the summer of 1945 were the work of perhaps the most successful spy ring ever. The key figure was Klaus Fuchs, a German-born physicist who fled to Britain to escape Hitler, joined the American bomb project and then systematically passed what he learned to Moscow.
The story of Fuchs, arrested and jailed in Britain in 1950, and other atomic spies is well known, though some names and their exact roles are still murky. The story of their Russian customers, the scientists who received their information - and argued bitterly over what to do with it - is not.
Without the help of stolen US plans, says Dr Khariton, it would have taken him and his colleagues many more months to build a bomb, the first of which was tested in August 1949. 'It brought forward our explosion by two or three years. It accelerated our whole programme.'
In his apartment in southern Moscow, he describes a particularly helpful bit of intelligence. It was a cross-section of America's prototype bomb. He slices a trembling hand through the air to illustrate the angle of the cut. 'The information was very helpful but also very difficult. We had sketches, but we did not have all the calculations. We had to repeat all the work done by the Americans, to do all the sums ourselves, to make sure it was not disinformation.'
Dr Khariton tells how he and other physicists with access to stolen secrets in 1945 argued over how to use them. They were already working on a bomb of their own design, and were reluctant to shift their energies into copying an American blueprint. The mushroom cloud over New Mexico, followed by the destruction of Hiroshima and Nagasaki, put an end to all that. The mere fact of these blasts revealed the most important secret of all: atom bombs worked.
Stalin had initiated a modest nuclear programme in 1943, making Igor Kurchatov scientific director of a new Moscow institute, Laboratory 2. Stunned by the US test, he stepped up the effort dramatically. In August 1945 he put his secret police chief, Lavrenti Beria, in overall charge of the bomb project, and ordered that, no matter the cost, America's feat be matched forthwith in Russia.
'It was decided we should copy the Americans. We had a copy of their bomb and relations with them were getting worse,' Dr Khariton recalls. 'We had been working on our own design, but here was a plan that had already been tested. It worked. It was decided we should first build a bomb we already knew would work.'
The decision split Soviet scientists, accelerated the onset of the Cold War and sent Dr Khariton off to a small town of around 3,000 people called Sarova, east of Moscow, to help set up Construction Bureau 11 - the design laboratory now known as Arzamas-16. Dr Khariton arrived there on 2 April 1946, preceded by a detachment of NKVD - the forerunner of the KGB - who ringed the area with barbed wire and staked out a patch of forest for what would become Russia's most secretive facility. Dr Khariton, the former Cambridge student of the great Lord Rutherford, was named scientific director and as such was the man who built Russia's first bomb.
There was little in Sarova aside from a disused monastery, converted into an orphanage and then a prison camp, and a small armaments factory. Once Dr Khariton arrived, though, frantic building began. This was to become the centre of a a vast nuclear archipelego stretching from uranium mines in Eastern Europe to processing plants in Siberia. The town vanished: it was never again shown on maps and was declared off-limits to all visitors.
While Dr Khariton set to work at Arzamas trying to copy the American bomb, Peter Kapitsa, perhaps the greatest Russian physicist of all, was being dismissed from the directorship of his research institute in Moscow. Kapitsa, the most eminent of Soviet physicists, was also the most outspoken opponent of trying to mimic Americans. Like Dr Khariton, he had studied at Cambridge's Cavendish Laboratory. He died in 1984 but, thanks to a collection of letters recently published in Moscow, we now know of his role in the great nuclear debate that convulsed Soviet science and politics.
Kapitsa did not oppose nuclear weapons, as was believed in the West for many years, but in November 1945, he wrote to Stalin to argue against copying: 'What we are doing is merely trying everything the Americans did, instead of finding our own way. We forget that the American way is very long and expensive.'
He complained bitterly of the way Stalin's secret police chief was running the bomb programme: 'Comrade Beria's shortcoming is that a conductor should not only wave a baton but should also know how to read the score. He is poor at this.' Soon after sending the letter, he got a present from Beria: an inlaid double-barrelled shotgun. He was one of only a few, however, who declined Beria's invitation to suicide.
Dr Khariton, by contrast, supported the decision to copy America, but also had trouble - although not quite so dramatic - with the NKVD. He remembers an investigating commission sent to Arzamas-16 to arrest a laboratory supervisor. 'I phoned Beria and told him they wanted to take away a useful man. He asked whether he was very useful. I said yes. He ordered the commission to leave him alone.
'It is strange to say, but this terrible man was an excellent organiser. I met him several times. He knew nothing about physics, but he understood very quickly what was really necessary and helped get it.' But was he not terrified by Beria's reputation as a psychopath? 'I found him quite helpful, actually.'
The most worrying moment was the first test of their work. 'If our first test was not successful, you can imagine what pleasures would have awaited us. We all lived in our own secret world and thought there must be other people kept secret from us and who would replace us if we failed.'
But he never found out if there were. The test was a success. Russia exploded its first bomb - using 6.5kg of plutonium, says Dr Khariton - on 29 August 1949 near Semipalatinsk in Kazakhstan. Dr Khariton was there to watch, along with Beria and Igor Kurchatov, the legendary Russian scientist who, as head of Laboratory 2 (later called the Laboratory for Measuring Instruments and today the Kurchatov Institute) had run the administrative side of the bomb project since 1943.
'When the bomb went off, Beria kissed Kurchatov and me on the forehead. I've never seen him so happy.' If Beria was delighted, the West was appalled. President Harry Truman refused at first to believe the Soviet Union had developed the bomb so quickly, and even years later he still suspected that the test had been faked.
Two years later, Soviet scientists exploded a bomb made according to their own design. It was much smaller than the American one and twice as powerful, Dr Khariton said. He suggested that a shortage of plutonium meant Moscow had no bombs at all between the two blasts.
He gladly concedes the role played by the nuclear spies. He was never told where the information came from, and had never heard of Fuchs until his arrest in 1950. But he regrets never getting a chance to thank him personally before he died in East Berlin in 1988. 'We asked them to pass on our thanks, but this could not be done. I don't know why. I heard that after his death his widow was sent some medals. Without any doubt he helped us at a very dangerous time of the Cold War. He was a great man.'
One of his motives for talking today, though, is anger at boasts by former KGB officers that spies, not scientists, were the fathers of Russia's hydrogen bomb too. Ideas and calculations by the US H-bomb pioneer, Edward Teller, did find their way along the spy network to Moscow, Dr Khariton concedes. 'They gave us information but they did not realise it was all wrong. We developed the hydrogen bomb absolutely independently.' The US exploded a primitive thermonuclear device in November 1952, but it was a massive two-storey hulk. The first real H-bomb belonged to the Soviets, and was exploded in October 1953.
Dr Khariton is deeply proud of Soviet bomb-making and can recite at length why it helped preserve, not endanger, peace. But it is the science that really excites him: hand-drawn diagrams of chain reactions, arguments he had more than half a century ago over his first published paper, the pioneering calculations done by Yakov Zeldovich in 1939, the ingenious trick discovered by Andrei Sakharov to make a hydrogen bomb possible.
The study of his Moscow flat has large potted plants and bookshelves crammed with scientific works. There are photographs of Einstein, Sakharov and other former colleagues. All are dead. Dr Khariton is the last of the pioneering physicists from the heart of the nuclear archipelago.
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