The race to the conquest of the space was not the only field of science in which Americans and Soviets had been rivalling each other between the ‘50s and the ‘70s, the age (in)famously known as the Cold War. Weapons race was the obvious contender for the first place, with the largest investment of money and manpower. Far behind weapons and space exploration in terms of funding, but not less relevant in the chase of prestigious results, was the construction of particle accelerators, the most expensive tools for fundamental research ever conceived by scientists. When I think of particle accelerators, in terms of their size, the potential impact on the economy and the environment necessary for their construction, the amount of time investment from the early design stages to the operation, the manpower for the building and running, I am always (positively) surprised that national governments could still be in favour of funding such purely intellectual enterprises. The sheer amount of energy consumption of the LHC is in itself impressive: about 600 GWh per year, with a figure of 750 GWh estimated for the new 6.8-TeV peak energy. This new record was first reached by LHC on last April 25, 2022, after a long shutdown of about 4 years. Overall, the current CERN energy consumption is estimated at about 1,5 TWh per year, which amounts to 0,3% of the yearly energy production in France, that is roughly the energy consumption of the city of Lille.

Such issues were not a concern in the Cold War times, when it was felt that conceding an advantage to the other party in any field, especially in science and engineering, would have marked a parallel defeat on social and economic grounds, even surpassing the impending risk of a real, “hot” war that was always looming at the door. Therefore, the construction of bigger and bigger accelerating machines was inevitable, after Ernest Orlando Lawrence deposited the copyright of the first cyclotron in 1932. His first proton accelerator could be held in a hand, was made of two electromagnets recycled from old arc converters and some copper wire, and costed about 300 dollars of the time, about 5,000 of today’s dollars. Now, if you want to accelerate a charged particle in a circular path, the Lorentz force must equilibrate the centripetal force, and the final equation reads BR=Cv, where B is the magnetic field, R is the path radius, v is the particle velocity, and C=m/q is its mass/charge ratio. However, with this you get just a particle spinning in circles at constant velocity. To accelerate the particle to higher and higher speeds, energy must be supplied by an external field, typically an AC current with frequency 𝜔 synchronised to the particle magnetic circling. Therefore, for a particle of given C, you can choose values for B and 𝜔, and your particle will move by increasing R in a spiral path, spanning the whole machine volume, as its energy increases. The problem is that for very fast particles approaching relativistic speeds, the mass starts increasing as m=𝛾m0. Hence, either the magnetic field, or the AC frequency must be progressively adjusted, so that you would have built respectively an isocyclotron, or a synchrocyclotron. This was the case, until in 1944 the Soviet scientist Vladimir Veksler invented the synchrotron concept: a machine in which the radius of the particle path is fixed, and variable modules of B and AC fields are placed all along the trajectory, to accelerate the particles along the fixed path.

The first GeV-machine with synchrotron design in the US was the 3 GeV Cosmotron at Brookhaven, first operated at 1 Gev in 1952, with the New York Times headlining the “first Billion Volt shot”. However, it was topped only two years later by the 6-GeV Bevatron in Berkeley. Brookhaven was planning a major update of its machine to reach 10 GeV, and entered in a friendly competition with the CERN teams that were already designing the 25-GeV proto-synchrotron. Such a challenge between American and European scientists resulted in a general slowing down of the projects, which stumbled against a number of design problems until the mid-60s. In the meantime, in the Soviet Union the first synchrocyclotron, called “Phasotron”, was timed to be online on Stalin’s 70th birthday, December 6, 1948. This was followed by two proton synchrotrons whose energy and switch-on dates were also nicely adjusted to bridge the gap left in the West. Dubna’s 10 GeV “Synchro-phasotron” surpassed the 6 GeV American Bevatron and, from 1957 until 1967, when the 25 GeV proto-synchrotron at CERN was finished, it offered the highest energy in the world. The same was true of the innovative “U-70” proton synchrotron built in Protvino, which used alternating gradient focusing, and held the world record with 76-GeV maximum proton energy from 1967 until 1972, when the Fermilab machine in Batavia, Illinois, started up. However, by 1976 the Main Ring at Fermilab attained 500 GeV, thus ushering the new era of TeV-machines, which would evolve into the proton-antiproton storage rings, and eventually to the hugely successful Super-proton-synchrotron (SPS) and Large-Hadron-Collider (LHC) projects at CERN. The race was considered over on the Soviet side, and the Protvino machine would remain the fastest accelerator in Russia up to date.

One day in 1978 Anatoli Bugorsky, a 36-year-old researcher at the Institute for High Energy Physics in Protvino, where the U-70 machine was installed, was checking a piece of accelerator equipment that had some kind of malfunctioning. According to the official reports, while leaning over the piece of equipment Bugorsky stuck his head in the “space through which the beam passes on its way from one part of the accelerator tube to the next”, and saw “a flash brighter than a thousand suns”. (I presume the Russian expression must have been different, the translators likely had in mind Oppenheimer’s quote from Baghavad-Gita “If the radiance of a thousand suns were to burst at once into the sky…”, which he famously recited after the first nuclear explosion in Alamogordo.) The proton beam entered near his nose and exited from the back of the head above his left ear. Anatoli felt no pain. The left side of his face swollen beyond recognition, he was taken to a clinic in Moscow so that doctors could observe his death over the following two to three weeks. Over the next few days, skin on the back of his head and on his face just next to his left nostril peeled away to reveal the path the beam had burned through the skin, the skull, and the brain tissue. The inside of his head continued to burn away: all the nerves on the left were gone in two years, partly paralyzing that side of his face. Still, not only did Bugorsky not die, but he remained normally functioning, capable of finishing his PhD in physics and having a normal life, with a job, a wife, and children. For the first few years, the only real evidence that something had gone neurologically awry were occasional seizures, and rare “grand mal” seizures.

Bugorsky worked at the same IHEP institute in Protvino until retirement. He applied for disability status, so as to get free epilepsy medication, but his request was rejected by the medical board. For years, he was a “poster boy” for Soviet, and later Russian, radiation medicine, which was entirely happy to take the credit for his good fortune. Because everything connected with nuclear energy was kept secret in the Soviet Union, Bugorski could not talk about his accident. About twice a year he went to the Moscow radiation clinic to be examined, where he met other members of the silent “brotherhood of nuclear-accident victims”. Recently, he expressed his availability to travel to the West for research and study purposes, but the Russian government never let him, probably afraid that his one-of-a-kind case represents precious human material for studying the effects of high-energy protons. Maybe something to do with lethality of futuristic, proton-beam based weapons? We cannot know. But then, let’s try at least to estimate the damage Bugorsky received from the proton beam.

In conventional radiation physics, a human body receiving a dose of about 1 Gray (1 Joule of energy per kg of matter) in a short time of few seconds, would develop acute radiation syndrome; at 10 Gray, bone marrow starts to be attacked; at about 50 Gray, neurovascular syndrome is almost fatal; and 100 Gray is largely enough to kill a person. However, such figures are estimated on the rare global nuclear events (Hiroshima and Nagasaki bombing, and Chernobyl accident survivors), plus a – fortunately very small – statistics of nuclear accidents, and are relative to the biophysics of high energy photons (gamma rays), of medium-low energy neutrons, and contamination from radionuclides. We have very scarce information about the radiation effects of exposure to radiation in the form of protons moving at near the speed of light. Only some data have been accumulated from astronauts exposed to high-energy cosmic radiation when flying beyond the low-Earth orbit.

The field of proton therapy, to treat cancer and other diseases, is a blooming area both for the design of novel, more compact and economic particle accelerators to be installed near hospitals, and for the study of such radiation effects in the human body. However, the typical energies of proton beams for radiotherapy are limited in the maximum range of 200-250 MeV. This is due to the well-known Bragg peak effect. Because of their electric charge, protons traversing dense matter make an enormous number of collisions per unit pathlength, as compared to the very sparse interactions of gamma rays. Therefore, their energy loss dE can be described as practically continuous, in small packets dE/dx over an infinitesimal length dx. The energy loss, or “stopping power” curve is described by the Bethe-Bloch equation, firstly derived in semiclassical form by Bohr in 1913, and extended in 1930 to non-relativistic, and later to fully relativistic quantum treatment, by Bethe. A few years later, Felix Bloch provided the link between the two formulations, hence the formula naming. The peculiar feature of the formula is that the stopping power has an inverse dependence on the particle energy, that is dE/dx ∝ 1/E. This means that, the slower the particle, the more intense its energy loss. It is like braking in your car: unless you see some obstacle, you don’t want to stop suddenly, rather you start braking gently and increase the braking strength as soon as the car slows down to a full stop.

When looking at the energy loss as a function of penetration depth, a fast charged particle enters the matter and loses energy at a nearly constant rate for most of its path, until it is slowed down to enter this 1/E regime, in which its energy-loss rate rapidly increases until the particle is stopped. Eventually, the largest part of the particle energy is delivered in a small spot at the end of its trajectory: this is the Bragg peak. The depth at which this occurs is linked to its initial energy. For protons, if we want to deliver energy to a cancer in the human body, at typical depths of 10-20 cm, the initial energy must be in the range 200-300 MeV, otherwise the particle would exit from the tissue and deposit the largest part of its energy somewhere else. However, for the 76-GeV protons that entered the head of Anatoli Bugorsky, the radiology knows its limits. By looking at the energy loss curves at very high energies, one sees that after the so-called “minimum of ionization”, which occurs for any charged particle around a value of the particle momentum in units of mcp/mc~3 (for protons this corresponds to a kinetic energy around 2.5 GeV) their energy loss rate turns roughly constant, at about 2-3 MeV/cm in a material equivalent to human tissues, before starting to rise again thanks to bremsstrahlung. Therefore, protons of 76 GeV traversing about 25 cm of matter, partly bone and partly brain, could have deposited something between 50-100 MeV in the tissues. The peak luminosity of the U-70 proton beam is 1,7 x 1013 protons per pulse, at a rate of 0.11 Hz. If we imagine that Anatoli could have held his head in front of the pulse for like, a couple of seconds, before jumping back terrified, it means he must have got no more than one shot, which makes for about 200 Joules. We can estimate an ionization volume of 25 cm length by about 1 cm wide, hence an irradiated mass of about 80 cc, or~80 grams. This turns into an equivalent dose of about 2500 Gray… Of course, this humongous amount of energy is in this case localised to a very well defined region, and is not a total-body irradiation. But still, surviving a dose of 25 times the lethal threshold in his brain… How lucky Anatoli was!

Looking into a thousand suns

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