Their names are in the history of the Institute

Memories of Professor Todor Ruskov

Rusko Ruskov (Hristov),
University of Maryland, USA

Two years ago, on 17 November 2020, after a short illness, my father Todor Ruskov (Hristov), an experimental physicist, known both in Bulgaria and in Russia, died and I decided to write about him in the newspaper "Dubna", for people here knew him and would be able to remember him. Besides, I am from Dubna: I was born, I have lived and worked in Dubna. Below I have tried to formulate the main milestones of the biography of Todor Ruskov, supplemented by my personal impressions, in a short story.

Todor Ruskov was born on 20 August in 1933, in the city of Ruse (on the Danube, Bulgaria) in the family of a lawyer and a teacher. His father, Rusko pop Hristov, became Mayor of Ruse in 1921 during the first "Ruse Commune" and did a lot for the citizens, especially for the poor. His mother, Todora Stancheva, comes from the famous Bulgarian revolutionary Mamarchev-Rakovski family that stood at the origins of the resistance movement against the Ottomans (19th century), for the liberation of Bulgaria from the Ottoman yoke.

Todor Ruskov left to study in the Soviet Union in 1952 - first at the Kyiv Polytechnic Institute and when the nuclear topic was declassified, he was transferred to Leningrad University, from which he graduated with honors in 1957. The happy time of starting a family with my mother Margarita and the birth of my sisters is associated with Leningrad. In the same year he entered the Physics Institute at the Bulgarian Academy of Sciences, where since November 1962 he had worked in the laboratory of nuclear physics.

In 1960-1963 Todor Ruskov took experiential courses at the Laboratory of Neutron Physics of the Joint Institute for Nuclear Research in the group of Fyodor Lvovich Shapiro. In the first year, he was sent to Moscow, to ITEP, to carry out experiments with polarized neutrons. In Dubna, he worked in the group of Shapiro, together with V.P.Alfimenkov, A.V.Strelkov, Yu.M.Ostanevich and other employees. Their group, according to his stories, was very friendly, but Sasha Strelkov was and remained one of his closest friends. I remember many of their heated disputes on their favorite physics, vivid discussions of experiments, both by Sasha and my father, father's stories about their comic dispute competitions, such as, who could ride the bike slower, who would eat his lunch last, and others. And Todor was a very good storyteller, with a subtle sense of humor. Perhaps, thanks to his stories, I also became a physicist (albeit a theorist)?

Returning to that era in Dubna, filled with youthful fervor and enthusiasm, after Moscow, Todor participated in the discovery of the Mossbauer isotope ¹⁴⁹Sm with the group, as well as in pioneering work on measuring the Mossbauer spectrum of ⁶⁷Zn (the most sensitive in energy, with a natural linewidth of 5·10^-11 eV) using Doppler modulation. The further goal of the second experiment was to measure the gravitational redshift of lines under terrestrial conditions predicted by Einstein, a phenomenon that follows from the principle of equivalence. The implementation of this idea was the dream of the whole group that was driven by Shapiro. My father vividly spoke about this several years ago in a book of memoirs of students dedicated to F.L.Shapiro.

***

I first met Fyodor Lvovich at the beginning of 1960. By that time, two and a half years had passed since I graduated from the university. I came to work at the Laboratory of Neutron Physics, where I met him. Even before coming to JINR, I heard about him as a talented physicist that supervised all the experimental work in the Laboratory, so I was a little worried before meeting him. That winter morning there was a seminar in the Laboratory and right before it started, it seemed, in the corridor, someone told Fyodor Lvovich that I had arrived. He came up to me and introduced himself. I was immediately impressed by his open look, in which there was no cunning, no arrogance or condescension. The only thing was, I would say, working goodwill. Without haste, but still quite quickly, he asked me about what I did and what I would like to do. Then he invited me to a seminar and offered to tour the Laboratory afterwards. Zhora Samosvat spoke at the seminar. I remember him smiling all the time during the report...

Fyodor Lvovich was a very democratic person. He was attentive to young people. One could address him with any question. Once, on a train from Dubna to Moscow, I tortured him with my questions on quantum mechanics. He answered, although he might be tired, but he did not show it. He never said that my question was inappropriate or like this. Sometimes in a conversation, if he did not know what to answer, he admitted that he did not know, but then almost always he would come and say the answer. He allowed me to study in his room and even sleep there on the couch during night measurements ... All this increased the feeling of respect for him even more.

People in their activities are different: some, although hardworking and with enough will and constancy, somehow cannot go beyond the limited contour of ideas and deeds, Fyodor Lvovich, on the contrary, broke the chains of such restrictions. He always had bold ideas. He was an optimist, although some reproached that his optimism was not real. In fact, his optimism was dictated by a strong desire to fulfil a certain idea and he achieved it.

T.Ruskov, researcher, INRAE, Sofia, Bulgaria, 1985
(From the book "F.L.Shapiro: a scientist and a person". Dubna: JINR, 1998.)

***

Returning to Bulgaria, in 1965, Todor Ruskov established his own group and discovered a new Mossbauer isotope, ¹⁶⁵Ho, using the capabilities of the nuclear reactor constructed with the participation of the Soviet researchers at the Institute for Nuclear Research and Nuclear Energy (INRNE) in Sofia. It should be noted that there are not so many Mossbauer isotopes in nature and the discovery of another one is a scientific success (and for Bulgaria, as well).

After specializing in Grenoble (1969-1970, France), Professor Ruskov developed the subject of observation of magnetic and structural phase transitions. To observe phase transitions both inside the volume and on the surface of the crystal, a signal from gamma quanta and conversion electrons is used. Examples include phase transitions in hematite and in crystals with a spin structure. In 1975, Todor Ruskov's monograph "The Mossbauer Effect" (in Bulgarian) was published that quickly became a bibliographic rarity.

Among the many applications of the Mossbauer effect with the ⁵⁷Fe isotope, Todor Ruskov, together with his group, established a laboratory at the Kremikovtsi metallurgical plant near Sofia for express analysis of iron ore roasting. Later, he studied the valence state of Fe in chromites from Tibet (together with geologists from California, USA, 2006), as well as the phase composition of carbon nanotubes filled with Fe (with scientists from Dresden, Germany, 2004-2006).

Dubna group on the Mossbauer effect at the desk of F.L.Shapiro (from left to right):
Alexander Strelkov, Viktor Alfimenkov (group leader), Yan Wu-Guang, Todor Ruskov and Yury Ostanevich.

It should be emphasized that in all his experiments, my father designed his own unique equipment (cryostats, furnaces, counters of gamma quanta, electrons and neutrons, electronic data storage devices, others). Therefore, strong and independent specialists grew up in his group - experimental physicists, electronic engineers - improving themselves on tasks. He treated Western achievements, both in his field and in general, with respect, understood what their power was, yet he believed that we should rely on our own power and win the competition due to greater ingenuity, the will to win, originality of implementing physical experiments.

Among the most significant, essentially pioneering and fundamental experiments carried out in Bulgaria in the 1970s-1980s, there was the first direct observation of the mechanism of diffusion of ⁵⁷Fe nuclei in a copper single crystal, as well as the measurement of the electric dipole moment of a photon using the ultrasensitive ⁶⁷Zn isotope.

The essence of the first experiment is that the dependence of the gamma resonance linewidth on the observation angle is different for two types of diffusion: vacancy, when an atom jumps to a neighboring free lattice node and internode, when the direction of diffusion is not so strictly limited. The very occurrence of the angular dependence is nontrivial and was predicted by the Soviet theorist M.A.Krivoglaz, based on the semiclassical theory of the Mossbauer effect (similar to the semiclassical theory proposed earlier by F.L.Shapiro). The experiment was exceptionally difficult, as it was carried out at temperatures only 30 degrees below the melting point of copper. For instance, by that time a group from Denmark had not managed to complete a similar experiment and a group from Germany was far behind in time. Professor Todor Ruskov and his group were internationally recognized.

The second experiment was conceived in Bulgaria and all the equipment required was constructed mainly there, but the experiment itself was carried out in Dubna during the "second Dubna period" (1980-1988) by Todor Ruskov. Dubna (and the Soviet Union in general) allowed to use accelerators (to produce a source of excited ⁶⁷Zn nuclei) and to work at liquid helium temperatures (4.2 K). All this, of course, was not available in Bulgaria, but that was what our powerful "brothers" were for.

The accelerator of SINP MSU became suitable, where Professor A.F.Tulinov (the same famous nuclear scientist known for the "nuclear shadow effect") provided Todor and his group with time on the beam. Old friends from FLNP, Sasha Strelkov and Slava Lushchikov, provided an absorber enriched with the ⁶⁷Zn isotope and another famous Dubna experimenter Boris Stepanovich Neganov (the same one that, independently of the West, invented such a key instrument of modern quantum science, like a dilution refrigerator) provided a premise, liquid helium and "his own wing". In addition, other friends from Moscow State University provided ZnO single crystals.

I remember when the talented physicist Ivan Spirov, my father's most selfless and devoted employer, drove by car to Moscow State University and irradiated the source with deuterons and then returned to Dubna at the same speed, since the half-life of the irradiated source was only 78 hours... Sasha Strelkov, my father's closest friend in Dubna and moreover, one of the greatest experimental physicists in Dubna, at that time already famous for his experiments with ultracold neutrons helped him.

I remember when, as a result of severe overwork, my father got to the Dubna hospital (I was a third-year student at the Physics Department of Moscow State University in that time and immediately came to him in Dubna). Soon he was discharged from the hospital and we were already sitting in his one-room apartment - Sasha, Ivan and me, and my father lying on the bed encouraged by us. Sasha enthusiastically was telling me (everyone was listening and smiling at Sasha's enthusiasm) why ultracold neutrons were reflected from the vessel wall, although each individually would like to be absorbed by the nearby nucleus... "It's all about the neutron wavelength! The neutron does not know which nucleus to join due to the uncertainty of the position!" Sasha said. And I was starting to get involved in providing theoretical support for my father's experiment.

The photon dipole moment experiment was my father's dream. If it had been discovered, he would have been awarded a Nobel Prize. After all, this would be a very non-standard confirmation of nonconservation of the combined parity, according to Landau's theory.

Todor Ruskov's idea was semi-classical - an electric dipole should accelerate or decelerate in the gradient of the electric field of nuclei of an irradiator and absorber that can result in an energy shift of the gamma-resonance line in ⁶⁷Zn. Details can be found in the original article. I remember that this idea was discussed with theorists, including V.A.Rubakov (INR), M.B.Voloshin (ITEP), S.B.Gerasimov (BLTP) that expressed support, but thought about quantum effects. Once, when meeting, Sergo Borisovich Gerasimov smiled at my father and said: "Do not listen to the theorists, but do your own experiment!"

Neganov first allocated a smaller room to my father's group, but when he saw that the first resonant spectrum was produced, he determined for them a larger room with a two-story ceiling and a huge wall-window. When someone was passing along the Institute's pedestrian alley, he could see everything that was happening inside.

After the next session of obtaining the source, busy times started, about 10-12 days. It was necessary to anneal the source, to withstand a certain temperature mode so as to restore the ideal crystal structure. They worked until late at night every day.

From the side of the pedestrian alley inside the room, a huge rubber canister filled with helium gas was seen flying up to the ceiling. In the middle of a large room, a small room two by two meters went down that resembled a partisan dugout. My father, along with his coworkers, often went down to this room, a miracle of the experiment took place and the "dugout" shone in the evening. And the people passing by our hall said: "Well, we can actually see that these people are working ..."

Only a few understand that a successful experiment is a miracle. Many tiny details have to come together to get the result. Todor Ruskov did it. As soon as he left the group for a while, everything started to fall apart. That was also due to the fact that the predecessors had passed some path, but described their experiment sparingly. You need to be an experimenter "from God", like Todor Ruskov, to understand all the hidden details...

After many attempts, a narrow (natural) line width was obtained that testified to the quality of the experiment. The line shift effect was gained and the dipole moment was estimated. Alternative line shift mechanisms were rejected as they resulted in the opposite effect. However, it was necessary to acquire more such successful experimental runs in order to improve the statistics of the result. A competitive Finnish group "simply" measured Einstein's gravitational redshift (on a 1 meter base). If they had several dozen successful sessions a year, then our group did not have even a dozen...

Professor Todor Ruskov dreamed of improving his experiment by using capillary tubes - waveguides to focus gamma rays (using total internal reflection). He was on the way of implementing this new technology, when perestroika broke out ... and the pogrom began both in science and in our states as a whole.

In addition to purely scientific activities, Professor Todor Ruskov extensively participated in public life. A worthy heir to his father, lawyer Rusko pop Khristov, he was an idealist, but he knew how to speak with both left and right representatives of the political spectrum. It is no coincidence that after the democratic changes that came, despite the fact that he was known for his leftist convictions (that he did not refuse), he was elected Chairman of the Scientific Council of INRNE BAS. People believed him.

In parallel, in the same years (1990-1993), he became Chairman of the Committee for the development of a new Charter of JINR and actively participated in the selection of candidates for a new director and in the reorganization of JINR. Subsequently, in Bulgaria, he was awarded the Order of Saints Cyril and Methodius 2nd degree for merits in the development of science and education.

In recent years, my father has carried out further work in the field of Mossbauer spectroscopy in the area of magnetic phase transitions in perovskites. In addition, his focus and efforts were aimed at the development of nuclear devices for the needs of nuclear power in Bulgaria in cooperation with JINR, FLNP, together with A.V.Strelkov. So, in 2016 he designed and constructed a radiometer that simultaneously measured gamma radiation (with good energy resolution) and neutron radiation for the needs of the Permanent Storage of Radioactive Waste in Bulgaria. The device could connect to a mobile phone and transmit data at a distance, so that operators were not threatened with radiation exposure. My father became the author of three patents and in 2020 was awarded a special award from INRNE for inventive activity.

***

Several episodes come to mind. My father came to Dubna on business, but he was also able to visit me. At that time I was preparing my dissertation on quarks and gluons in the group of A.V.Radyushkin at BLTP and Serezha Mikhailov and Sasha Bakulev were my close friends. The four of us were sitting on the veranda of BLTP, drinking coffee. The competition started, as I told my father that Seryozha knew poetry well. My father started reciting Eugene Onegin, got to about the third page, stopped a little. Immediately Seryozha picked up and continued ... I could only admire, since my memory for poetry is useless.

My father loved to sing - as did his whole family! Since childhood, his elder sister Pravda taught him Russian songs. Where? But Bulgaria did not declare war on the Soviet Union and in pre-war times, Soviet films were shown in Bulgaria, for example, "Volga-Volga", "Merry Fellows" and others. One of the favorite songs of recent years has been to Lermontov's poems "The Lonely Sail Is Showing White" that we sang together...

The Lonely sail is showing white
Through azure mists afar at sea.
What does it seek in far-off distance?
What has it lost in native realm?
.............
Below the sail are sky-blue currents,
Above are gold rays of the sun...
The rebel sail pleads for a tempest,
As though tempests bring the calm.