The Defector. The incredible life of nuclear physicist Bruno Pontecorvo.

They say that the 1950 brought a warm autumn to Rome. And I imagine that the concierge at the hotel was taken by surprise when a plainclothes Italian police officer, accompanied by some other figures talking with an Anglo-Saxon accent, inquired whether a family staying at the hotel had already departed. Yes, they had — just the week before — abruptly cutting short their planned vacation. The family had made an unusual exit: a tall, distinguished Italian man, his beautiful, young wife with a Scandinavian passport, and three young boys. Though they had booked a longer stay, they left earlier than planned, departing in the evening, an odd choice that at first did not raise any suspicions.

As I recount this story, I cannot confirm every detail, but I imagine that a Soviet agent, or perhaps a member of the Italian Communist Party, accompanied Bruno Pontecorvo and his family — either to the airport, where an unmarked plane awaited them, or to the Stazione Termini, where they might have boarded a train bound for Stockholm.

[Note: after posting a first version of this article, I found a seminar given by Professor Frank Close at Oxford University, showing evidence that the Pontecorvo family did board a SAS flight from Rome to Stockholm, via Munich, along with two other people probably flying under false names. Copy of the ticket is in the appendix to this article.]

A man on the run

Twelve years earlier, Bruno’s former colleague and friend, Enrico Fermi, had taken a similar route, fleeing with his family. The destinies of Pontecorvo and Fermi appear to be somehow intertwined, their paths crossing repeatedly during an era of persecution and war. Being a nuclear physicist —particularly a Jewish nuclear physicist — was perilous in the late 1930s. Now the war was over, though.

In 1934, Pontecorvo made a crucial contribution to Fermi's groundbreaking experiment on the properties of slow neutrons, paving the way for the discovery of nuclear fission. However, as the shadow of fascism grew darker and the Italian regime aligned itself more closely with Hitler, Bruno Pontecorvo left Italy in February 1936. He relocated to Paris, joining the laboratory of Irène and Frédéric Joliot-Curie at the Collège de France (there is always a Curie in all nuclear stories!)

This marked the first of many times Pontecorvo and his family were compelled — or chose — to move from one country to another, caught in an intricate web of personal choices, political convictions, and scientific ambition.

From that initial escape in 1936, Pontecorvo’s journey took him across borders: from France to the United States, then Canada, England, and finally, back to Italy—where, in 1950, he vanished under mysterious circumstances, leaving for an unknown destination.

Italian nuclear physicists had a tendency to disappear

Italian nuclear physicists, it is well known, had a peculiar tendency to disappear—but not all shared the same fate. Enrico Fermi resurfaced in New York after vanishing from the Nobel ceremony in Stockholm in 1938, his escape orchestrated to flee Mussolini’s racial laws. Ettore Majorana, another important figure in Fermi's group, famously vanished that same year after purchasing a ticket for a ship journey from Naples to Palermo. Majorana’s disappearance remains an enigma to this day (yet another interesting story to write about).

Bruno Pontecorvo, however, left the world in suspense for five long years. During this period, the intelligence services of the United States and the United Kingdom looked everywhere but failed to locate him. His last known traces could be found in Stockholm — the city where his Swedish wife Marianne was born -- and Helsinki. And only speculations about his destiny!

(There is something fascinating, at least for me, about the way nuclear history weaves its threads through familiar places like Rome, Stockholm, Los Alamos, Chicago, Dubna, and even my hometown, Trieste. In fact, Trieste itself played a unique role as a gateway for physicists defecting from the Soviet Union to the West)

Did you watch the movie Oppenheimer?

Did you watch the recent Hollywood film about Oppenheimer? You may recall the atmosphere of suspicion that surrounded Robert Oppenheimer and other physicists, questioning their true allegiances and political believes. And you might remember about Klaus Fuchs, the German physicist working on the Manhattan Project, who was arrested in February 1950 for passing critical information from the American, British, and Canadian nuclear weapons program to the Soviet Union — just months before Pontecorvo’s disappearance later that year.

Though Bruno Pontecorvo had been excluded from the Manhattan Project due to his known sympathies for the Communist Party, his work granted him access to laboratories, classified information, and a network of key physicists. This made him a person of interest for counterintelligence agencies. Pontecorvo’s expertise in neutron physics and his knowledge of reactor design, particularly from his time at the Chalk River Laboratories in Canada, where he worked on heavy water reactors, added to the growing concerns (I got an offer for a job at Chalk River, some years ago. I declined after checking on a map where it is: the most remote, isolated place you can imagine!)

For five years no one knew what happened. Where had Pontecorvo gone? What secrets might he have carried with him? Was he still alive? The fear grew darker with each passing year.

Comrade Bruno Maksimovich Pontekorvo

In 1955, the uncertainty shattered like glass. Bruno Pontecorvo reemerged—not in Italy, England, or Canada, but in Dubna, a remote Soviet city that had become the nucleus of the USSR’s nuclear science program. Now known as Бру?но Макси?мович Понтеко?рво (Bruno Maksimovich Pontekorvo), he appeared as a Soviet scientist, a visible symbol of Cold War espionage and scientific defection.

When he disappeared from Rome, on September 1st, 1950, Pontecorvo and his family were first taken to Stockholm, in Sweden, where they could count on the complicity of the family of his wife Marianne Pontecorvo, neé Nordblom. After spending a night in Stockholm, they did take a flight to Helsinki in Finland, and eventually they illegally crossed the border with the Soviet Union escorted by Soviet agent.

All the roads take to Dubna

Pontecorvo remained in Dubna for the rest of his life, working and living there with his wife and children, who also built their lives within the confines of the Soviet system. During his Soviet career, he was awarded the Stalin Prize in 1954 and the Lenin Prize in 1963, alongside numerous state honors. Yet, despite these recognitions, he was never allowed to leave the Soviet Union again. Later in life, as he reflected on his choices, Pontecorvo seemed to express— if not regret — a sense of disillusionment.

When I visited Dubna many years after Pontecorvo's passing in 1993, I tried to locate the Pontecorvo family’s home, but my efforts were in vain. Bruno Pontecorvo passed away on September 24, 1993, in Dubna. In accordance with his will, his ashes were divided between two places: one part buried in Dubna’s Bolshevolzhskoye Cemetery, and the other in Rome, Italy, at the Testaccio Cemetery.

Read more

• A beautiful book on the life of Bruno Pontecorvo, written by Frank CLose: Half-Life: The Divided Life of Bruno Pontecorvo, Physicist or Spy.
• An account on the Pontecorvo affair, published on Nature: Nuclear physics: New light on a cold war conundrum | Nature
• An article in Swedish, with a lot of interesting material on this story, and original documents from the time: Marianne och Bruno – Gefle Dagblad
• In Russia, Bruno Pontecorvo is considered a prominent figure and his memory is still honoured: Exhibition dedicated to Bruno Pontecorvo opened in Dubna – Joint Institute for Nuclear Research

About me

I’m passionate about radiation and radiation safety, and I lead these efforts at a top MedTech company. My experience includes working with the European Commission and international physics laboratories, where I developed my expertise in nuclear physics (without causing any explosions!). With a PhD in applied nuclear physics, I’ve published research in peer-reviewed journals and enjoy crafting content that makes complex topics in science, safety, and security accessible and engaging—because everyone loves a good science story!

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Documents and Photos

Bruno and Marianne, in 1970

Fermi's group: the Via Panisperna Boys

Travel documents to escape from France to the United States

Before moving from France to the US, Bruno applied for an immigration visa to Sweden - where Marianne returned to live with her parents after delivering their first child Gil (Bruno and Marianne had Gil before being married). However, Sweden declined the visa to Bruno, hence Marianne moved back to France - where they first met - and they moved together to the United States.

The defection

A document proving that the Swedish police was involved in the investigation about Bruno Pontecorvo's disappearance, and some articles on newspapers discussing it -- and already suspecting he could have defected to the URSS.

The airplane ticket

This document comes from a presentation by Professor Frank Close at Oxford University, who had access to MI5 and FBI declassified documents. "Mr. Wittha" was most likely a KGB Soviet agent, and "Mr. Allegrini" a member of the Italian Communist Party.

Life in the Soviet Union

This document is a certificate awarded to Bruno Maksimovich Pontekorvo by the Committee for Lenin Prizes in Science and Technology under the Council of Ministers of the USSR. Dated 21 April 1963, it officially recognizes Pontecorvo as a recipient of the (prestigious) Lenin Prize, one of the highest honors in the Soviet Union for exceptional achievements in science, technology, arts, or literature. The certificate, numbered 1202, includes Pontecorvo's photograph and bears official seals and signatures confirming the award.

The Swedish Population Register (1930)

On the Swedish State Archive, it is possible to get a copy of the Population Register from 1930, where the name of 13 year old Helena Marianne Nordblom is recorded, along with her family.

The Resident Alien ID cards of Gil and Tito Nils Pontecorvo

The Resident Alien's Border Crossing Identification Card of Gil and Tito Nils Pontecorvo Gil was born in France, and the document states that he was an Italian citizen. Tito Nils was born in Canada, and the document states that he was a Canadian citizen. In the airplane tickets from the MI5 archives, they are both registered as British citizens. Also Bruno Pontecorvo was registered as British citizen. What is more interesting for me is the handwritten text and signature by Marianne Pontecorvo, here using also her full name "Helena Marianne" as reported in the extract from the Swedish Population Register from 1930.