„That was the basic radiobiology that was.“

“On the Nature of Gene Mutation and Gene Structure” – this was the title of a paper published in June 1935 that was to have a great impact on early molecular genetics. Long known as the “Green Pamphlet” or the “three-man-paper”, it has now attained cult status.² The paper originated mainly in Berlin-Buch. The authors of this paper were the geneticist Nikolai Vladimirovich Timoféeff-Ressovsky (1900-1981), the radiation physicist Karl Günther Zimmer (1911-1988) and the theoretical physicist Max Delbrück (1906-1981). Each author was singly responsible for one of the first three parts of the paper (Timoféeff-Ressovsky: Some Facts from Mutation Research), (Zimmer: The Target Theory and Its Relation to the Triggering of Mutations) and (Delbrück: Atomic-Physics Model of Gene Mutation). The fourth part, the conclusion (Theory of Gene Mutation and Gene Structure), was written jointly. In the conclusion the authors stated: “We view the gene as an assemblage of atoms within which a mutation can proceed as a rearrangement of atoms or a dissociation of bonds (triggered by thermal fluctuations or external infusion of energy) and which is largely autonomous in its operations and in relation to other genes.”³ The remarkable reception the paper received was mainly due to the detailed presentation of it by Erwin Schrödinger (1887-1961) in his 1944 book What is Life? ⁴

Given the prominence of the Green Pamphlet, it is surprising how little we know about the life and work of the youngest of the authors – Karl Günther Zimmer. When the paper was published he had not yet turned 24 years of age. By contrast, the two other authors, Timoféeff and Delbrück, have received much recognition.⁵ In differing measures, both were legendary for their scientific achievements, charismatic personalities and gifted self-presentation already during their lifetimes. Zimmer, on the other hand, did not have an extroverted personality, but rather was more reserved. But besides his collaboration on the three-man-paper, his other research achievements were also outstanding. The hundredth anniversary of his birth on July 12, 2011 is a welcome occasion to report more about him.⁶

Early years

Karl Günther (he also used the spelling Günter) Arthur Zimmer was born in Breslau, now Wroclaw, as the son of the ministerial official Arthur Zimmer and his wife Elsa, née Geipel. A few months after he started school in Breslau, the family moved to Berlin in 1917. He attended the Helmholtz Secondary School in Berlin-Schöneberg, where he completed his final school examinations (Abitur), graduating “with distinction” at Easter 1929. A few weeks later his father died. From May 1929 to May 1933 Zimmer studied physics, chemistry and philosophy at Friedrich Wilhelm University in Berlin. During several semesters he was also a guest student at the School of General Studies and Resource Management at Technical College Berlin-Charlottenburg, (now Technical University Berlin). He wrote his dissertation in photochemistry⁷ at the Institute of Radiation Research of the Berlin School of Medicine under the supervision of the physicist Walter Friedrich (1883-1968). His dissertation received the grade valde laudabile (very good), and his first reviewer was the physicist-chemist Max Bodenstein (1871-1942). On July 27, 1934 he was conferred his PhD degree with the predicate cum laude. A short time later Zimmer went to the University of Aberdeen (Scotland) to work for several months as a research fellow in the Natural Philosophy Department. Radiation dosimetry problems already played a role in his first publications.

Back in Berlin, Zimmer accepted a low-paying part-time job as consultant in the radiation department of the Cecilienhaus Berlin-Charlottenburg. He was the only physicist in what was then the largest radiological clinic in Berlin, which was headed by Artur Pickhan (1887-1969). The beautiful Art Nouveau façade of the Cecilienhaus, built between 1907 and 1909, can still be seen today. However, it is no longer a hospital.

Contacts to the Auer Company evolved out of Zimmer’s work at Cecilienhaus.⁸ One of the fields in which this company was engaged was the production of preparations for medical radiation purposes based on naturally occurring radium isotopes. At that time the physicist Nikolaus Riehl (1901-1990) worked in the radiology department of the Auer Company.⁹ Zimmer and Riehl soon became friends. Zimmer later also became an employee of the Auer Company, but only part-time. In the future, Riehl and Zimmer were to have a lot to do with each other and would have to endure hard times together.

Sustainable research in Berlin-Buch

The borderline area between physics and medicine had attracted Zimmer’s interest already early in his studies. He was particularly fascinated by the problem of primary physicochemical processes that can be induced in simple biological objects by ionizing radiation. He wanted to find out as much as possible about this and therefore looked for opportunities to work in interdisciplinary research. Hence, in 1933 he began his collaboration with the geneticist Timoféeff-Ressovsky at the Kaiser Wilhelm Institute for Brain Research in Berlin-Buch. In the same year the American geneticist and later Nobel Prize laureate Hermann Joseph Muller (1890-1967) worked in this institute as visiting researcher.¹⁰ Muller had recognized the potential of modern physics for the solution of biological problems early on and therefore urged biologists to work with physicists. Timoféeff gladly followed this advice. Moreover, the collaboration with Zimmer as guest researcher did not cost anything. In the spring of 1933 Gerhard Fischer, the mechanic on Timoféeff’s team responsible for dosimetry, had to leave Germany for political reasons. Zimmer’s collaboration must have been very fortuitous for Timoféeff, especially because Zimmer was already a specialist in radiation dosimetry. Already after only one-year’s collaboration between Timoféeff and Zimmer, Max Delbrück, who joined the group in 1934, was so impressed by the results that years later he stated that at that time Timoféeff and Zimmer performed “by far the best work in the area of quantitative mutation research”.¹¹

Dr. Karl G. Zimmer. Around 1935 in Berlin

© Collection Professor Heinz Bielka, with kind consent

Apparently, the young Zimmer successfully met the demanding intellectual requirements of Timoféeff very quickly. Already in 1934 he published a paper as single author on the relationships between dose and mutation rate and thus brought a new aspect into the research of the Timoféeff group.¹² When in the same year the legendary discussion rounds between physicists and biologists began, which Delbrück had initiated (and in which also Riehl occasionally participated), Zimmer was one of the leading participants in the discussion and also a mediator between the two disciplines with which he was already familiar. “Two or three times a week we met, mostly in Timoféeff-Ressovsky’s home in Berlin [-Buch], where we talked, usually for ten hours or more without any break, taking some food during the session. There is no way of judging who learned most by this exchange of ideas, knowledge and experience,” he recalled about this time.¹³ One of the outcomes of the discussions was the preparatory work for and drafting of the Green Pamphlet, another was Delbrück’s ultimate shift of research focus to genetics.

Zimmer is reported to have been very career-oriented, but was considered “apolitical”. He never became a member of the NSDAP, unlike many scientists in those years. His workload must have been very heavy. In 1936, as a research assistant at Cecilienhaus, he published a monograph on radium dosimetry¹⁴ that received very positive reviews in the U.S. He also focused on dose measurements during X-ray diagnostics in humans. Until 1936, besides the Green Pamphlet, alone six other high-level, original papers emerged from his unpaid work in Berlin-Buch regarding the mutation-inducing effects of various types of radiation in the fruit fly Drosophila¹⁵. Zimmer had thus been involved in the research work of the Buch group as “second man” for quite some time. So it was only logical that he became an official (and paid) member of Timoféeff’s research group in the Department of Genetics, starting in January 1937. At the same time, with Timoféeff’s consent, he maintained his association with Cecilienhaus and with the Auer Company. Henceforth Zimmer, along with Timoféeff, was responsible for the acquisition of research funds for the Department of Genetics. Timoféeff was one of the most highly funded scientists in zoology in Germany between 1934 and1945, ranking 4th place with 84,040 Reichsmark. Zimmer ranked 7th with 38,100 Reichsmark.¹⁶

In 1937 Zimmer published another monograph on the nature, generation and biological effect of radiation.¹⁷ While in previous studies in Berlin-Buch, X-rays of different wavelengths as well as alpha, beta and gamma rays had been investigated in their effect on simple biological objects, now the effect of neutrons, which had been discovered in 1932, was the main research interest. Neutrons generate very high ionization densities in tissue and therefore appeared to Timoféeff and Zimmer to be particularly suited to determine whether a single radiation hit is sufficient to induce a mutation. Due to lacking experimental opportunities, experiments with neutrons could not be carried out in Germany at that time. During lengthy research stays in Holland (Philips Laboratories in Eindhoven), England and Sweden, Zimmer was able to perform the respective experiments and gain first experience in the difficult and completely new field of neutron dosimetry. From summer 1939 on, the Department of Genetics had access to its own neutron generator in Buch. With this new technology, the experimental possibilities of the Department were expanded considerably. At the same time the profile shifted from genetics towards biophysics. Due to his expertise in this field, Zimmer recognized very early that by using fast neutrons, problems of radiation genetics, theoretical radiation biology and cancer radiation therapy could be elucidated. Moreover, the production and use of radioactive isotopes opened up new research fields, which particularly interested the Auer Company management from a market-strategic perspective.¹⁸

In biological material, neutrons induce so-called recoil protons through energy transfer to hydrogen nuclei. After extensive experiments in which Zimmer achieved a sufficiently reliable dosimetry of the recoil protons, a direct comparison of neutrons with other types of radiation was possible. This revealed some interesting results. In general, the biological effect of neutrons was greater than that of equal doses of X-rays. After neutron-induced damage, the recovery phase was significantly longer than after damage caused by X-rays. On the other hand, the mutation rates in fruit flies generated by neutrons in the classic CIB test were much lower than the mutation rates of all other types of radiation studied. The induced mutations were directly proportional to the radiation dose up to a saturation level. Threshold values were also not observed for neutrons, showing that ionizations were hit events here too. Chromosome mutations were observed in rats after neutron irradiation, and their mechanism was studied in more detail. Thus, in a very short time numerous new insights were gained.¹⁹

Timoféeff and Zimmer summarized the findings of the experiments they had made in radiation genetics over many years in the book “The Hit Principle in Biology”. However, the manuscript they completed in 1944 could only be published after the war in 1947. In the Soviet Union and the Soviet-occupied zones the book was immediately listed on the index, because it did not coincide with the doctrine prevailing there. The British physicist Douglas E. Lea (1910-1947), who visited Zimmer in Berlin in 1936, wrote a similar book independent of his Berlin colleagues, but focused not so much on the hit principle, but rather on the hit area – the “target” theory. Both books complemented each other quite well.²⁰ In 1969 Zimmer commented on the fate of the book he co-authored with Timoféeff: “This [...] book, appearing in German, had a difficult start in those years. However, it was considered by the military government of the eastern part of Germany to be of such interest as to make them officially collect and pulp as many copies as they could get hold of.” ²¹

Even if today no great importance is attached to neutrons for cancer therapy, and protons and heavy ions are primarily used in radiation therapy instead of neutrons, the concepts of neutron therapy Zimmer developed were completely novel for his time.

With the provision of artificial radioactive isotopes, two objectives were pursued. First, production and purification methods for a large number of radioactive elements needed to be developed: chlorine, bromine, iodine, phosphorus, arsenic, manganese, copper, thallium, silicon, cerium and strontium.²² Many of these radionuclides were first produced in Germany in Berlin-Buch. Some of the nuclides were then used in animal experiments as radioactive indicators to study their chemical and physiological effects. The indicator method was developed in the 1930s by György von Hevesy (1885-1966, Nobel Prize 1943). Besides Zimmer, other scientists were also involved in these experiments in Buch (Elena A. Timoféeff-Ressovsky, Hans-Joachim Born, Joachim Gerlach, Paul Max Wolf). One particularly promising result of the Buch group was the labeling of the tobacco mosaic virus (TMV) by biological means through radio phosphorus, which was achieved in collaboration with Gerhard Schramm (1910-1969) of the virus research unit.²³

In his years at Buch, Zimmer also made important theoretical contributions apart from his basic experimental research. One example is the so-called statistical ultramicrometry with ionizing radiation; he also systematically applied hit theory to interpret radiobiological dose response curves. With several publications, Zimmer also contributed to the elucidation of the mechanism of the effect of ionizing radiation on solutions.²⁴

Timoféeff’s research group was always open to researchers of various disciplines, some of whom participated continually, some more sporadically in the work and discussions. This ensured that new aspects continually came up for consideration. For instance, the theoretical physicist Pascual Jordan (1902-1980) influenced the discussions in Buch considerably with his ideas. He interpreted mutations to be microphysical reactions in the sense of quantum mechanics (of which he was one of the founders).²⁵ Riehl, already mentioned above, drew attention to energy migration and energy propagation phenomena, whose importance for biological processes was then investigated jointly in Buch.²⁶ And it was always Zimmer who propelled the implementation of new topics and significantly influenced the results.

When in 1945 this phase of Zimmer’s scientific career came to a close, he had already made his mark in science, although he was only 34 years old. “Beautiful years of fruitful work” – that was his personal summary.²⁷

Unavoidable contract research

The changes in research funding which took place in Germany from 1937 on (especially the establishment of the Reich Research Council) were also felt in the Department of Genetics of the Institute for Brain Research. In the course of the reorganization of the Institute during the change in directors in spring 1937, the Department became an independent unit with an own budget.²⁸ To compensate for budget cuts on the part of the General Administration of the Kaiser Wilhelm Society (KWG), new funding possibilities had to be tapped. This led to increased cooperation with the Auer Company, which had supported the KWG already when it was founded in 1911 and had contributed part of the foundation’s assets. Existing personal relationships between Timoféeff, Riehl and Zimmer are likely to have promoted greater cooperation. After the Auer Company had agreed to assume the costs for a Philips neutron generator needed in the Buch institute that went into operation in the summer of 1939, the company in return was allowed to establish a biophysical branch in Buch, which the radio-chemist Hans-Joachim Born (1909-1987) subsequently headed.²⁹ From then on, many studies were carried out together with scientists who were either employees of the Kaiser Wilhelm Society or of the Auer Company, or – not infrequently – who were employed by both. The physician Alexander Catsch (1913-1976) also worked for the Auer Company in Buch before transferring to the Department of Genetics in 1941.³⁰

From 1939 on, not only the Auer Company, but also other clients turned increasingly to the Department of Genetics for help in solving special problems. In the search for new funding, Department employees sometimes took the initiative. The staff of the Department, and in particular Zimmer, was considerably burdened by the various orders. Since historical research has already investigated this aspect very thoroughly³¹, here only the most important research contracts shall be listed:

• Check of the sealing of the filters in gas masks using radioactive indicators without use of chemical warfare agents (order made by the Auer Company, which had long been Germany’s largest gas mask manufacturer).
• The production and application of artificial radioactive isotopes as described above (commissioned by the Auer Company, which for decades had been a purveyor of naturally occurring radiation preparations and now wanted to expand its offerings with artificial radiation sources).
• Development of an efficient oxygen level indicator for pilots when flying at high altitudes and analysis of possible risks to the pilots by cosmic radiation on high-altitude flights (commissioned by the Ordnance Office of the German Army).
• Development of radiation protection measures, especially against X-rays and neutrons (commissioned by the Uranium Society).
• Studies on the biological effects of fast neutrons. Following the irradiation of rats, changes in blood cell count were analyzed (together with the research institute of the German Reichspost).

The aim of all of these studies was ultimately to save lives, regardless of whether it was a matter of civilians, firemen, pilots, soldiers or skilled uranium workers.

Zimmer was introduced by his friend Riehl into the so-called Uranium Society. There he was soon considered an expert – for the increasingly urgent – radiation protection. In a memorandum written in October 1941 and entitled “The possibilities of radiation injury and how to protect against it when working with uranium products”, he warned research groups involved in the project especially about health hazards caused by neutrons.³² Riehl occasionally let Zimmer substitute for him in matters of uranium production or accompany him at important meetings. It is not known whether Zimmer did this simply as a favor for Riehl as a friend or whether Zimmer worked on a large scale for the uranium industry. He was not, however, Riehl’s deputy. If this had been the case the Russians would have treated him differently after 1945. For example, because it was not allowed, he never visited the (see below) Oranienburg plant.

Some historians have alleged that Zimmer (and his boss Timoféeff), took part in war-related, strategically vital projects on behalf of the military services.³³ They did not mention, however, the directive of the Führer [Hitler] from June 9, 1942 which was valid for all researchers, regarding the Reich Research Council.³⁴ Thus, scientists in the research institutions were clearly obligated to give priority to investigations for the war effort. To conduct research at all in the last years of the war and to keep the researchers from being sent to the front, apparently many ongoing projects were given a new designation and declared “as important for the war effort”. They were thus inevitably classified as “secret”. At least in part, this also applies to the “Nuclear Physics Research Reports”, an internal, top-secret publication series of the German Uranium Society.³⁵ It was confiscated by the Americans in 1945 and returned to the FRG only at the beginning of the 1970s. Today the reports are kept in the German Museum. Zimmer and Riehl were among the authors of research reports in this series. Zimmer’s contribution was entitled “Report on studies of the relative effectiveness of X-rays and fast neutrons with regard to the generation of chromosomal mutations.”³⁶ The example shows that it was first this “top-secret” classification that gave regular research a seemingly conspiratorial nature. “Studies on the biological effect of radiation” was the title of another research paper classified at that time as secret, which Zimmer delivered on order of the Reich Research Council.³⁷

A report written in February 1989 by a group of the Berlin Academy of Sciences which investigated allegations in this regard against the Department of Genetics and Timoféeff came to the conclusion:

“The implementation of research projects with military relevance was ordered by ‘organs of the Reich Research Council’ [...] ‚ and at least from 1939 on it was not possible for a scientific institute or an institute department in Germany to evade such an order. [...] [Moreover] an evaluation of the research projects that were actually carried out and in particular the findings that resulted [...] and the significance could not primarily be made according to the formal administrative classification.’ [...] In the Department of Genetics‚ research was almost exclusively limited to basic research and the classification [as important for the war effort] must have had a purely formal character.”³⁸

The occasional assertion that Zimmer carried out war contracts without Timoféeff’s knowledge is hardly credible. During the war, the orders may have had to be carried out under Zimmer’s name, since probably only a German citizen (thus not Timoféeff) could function as contractor. There are no indications that the trusting relationship of Timoféeff to Zimmer could have been impaired or that Zimmer acted in a disloyal manner toward Timoféeff. On the contrary, Timoféeff took considerable risks (for that time, during the war) to employ two half-Jews (Catsch, Peter Welt). Zimmer was the only person Timoféeff confided in about this. Zimmer must also have known of or supported other risky activities of Timoféeff towards the end of the war (e.g. support of prisoners of war).

Hostage of the occupying force³⁹

In the spring of 1945 the Allies advanced onto German territory from different sides. Immediately behind the lines of the fighting troops, teams of the various secret services were operating. Their task was to track down German specialists – primarily scientists and technicians – to interrogate or detain them and to seize documents and diverse equipment for the Allies to use. In this way, the Americans discovered that the main company producing pure and thus reactor-suitable uranium was the Auer Company in Oranienburg. Consequently, the Auer plant was completely destroyed by U.S. air strikes on March 15, 1945 to prevent that a future Soviet nuclear program could benefit from the uranium production.

But the Auer plant and Nikolaus Riehl, the manager for uranium production, were long in the cross-hairs of Soviet intelligence. Like many other members of the German Uranium Society, Riehl had known the Austrian physicist and science publisher Paul Rosbaud (1898-1963) for many years. Rosbaud had earned his doctorate in the 1920s at what is now the TU Berlin under the supervision of Hermann Mark (1895-1992) and later worked for the Julius Springer publishing house, inter alia as editor of the journal “Naturwissenschaften” [Natural Sciences]. Riehl and other German researchers had no idea that prior to and during the war Rosbaud informed the British intelligence service MI6 about militarily interesting developments in Germany regarding nuclear and missile research. As was later revealed, the British secret intelligence service was infiltrated in those years by Soviet intelligence. It is therefore assumed that the information from Rosbaud also reached the Russians. In any case, in the spring of 1945 Riehl and also Rosbaud were quite high on the list of persons wanted by the Soviet secret service NKVD (Rosbaud managed to escape to the Americans).⁴⁰

K.G. Zimmer was apparently the first employee of the Auer Company to be captured by the NKVD. He was forced to reveal the whereabouts of Riehl, who had evacuated to Rheinsberg in Brandenburg with members of the staff and with the remains of the equipment after the destruction of the Oranienburg factory. Two officers who were temporarily in the service of the NKVD, the later renowned physicist Lev A. Artsimovich (1909-1973) and Georgi N. Flerov (1913-1990)⁴¹, went to Rheinsberg in mid-May 1945 with their hostage Zimmer in tow and immediately arrested Riehl there. They took him to the NKVD headquarters in Berlin-Friedrichshagen, where Avraami P. Zavenyagin (1901-1956), the deputy of the NKVD head Lawrentij P. Berija (1899-1953) resided with his staff. Zavenyagin was responsible for Soviet nuclear research. Riehl and his family were flown out to Moscow already in the beginning of June 1945. It was to be ten years before they were allowed to return to Germany. Zimmer was at first released from being held hostage.

From the Soviet perspective, the arrest of Riehl was a major success in two respects. First, in the destroyed Auer plants the NKVD had captured considerable quantities of uranium oxide (about 100 tons), which were invaluable for the Soviet nuclear program and which they wanted to utilize in the best way possible with the help of Riehl. Second, as a person Riehl almost ideally fulfilled the requirements of the Soviets for the planned tasks: he had experience in diverse areas – physics, chemistry, metallurgy, research, technology and management – and more than any of the others had expertise in uranium production. He also spoke fluent Russian. He landed in a Russian prison, but he was one of the best-treated prisoners. He was “in the golden cage” as he later described his situation.⁴² He utilized the relative freedom that was given him to improve the situation of other captive scientists, among them Zimmer and colleagues from Berlin-Buch.

In September 1945, Timoféeff and Sergei R. Zarapkin (1892-1960), the Russian geneticist who had worked for many years in Buch, were arrested by the NKVD and deported to Russia. In October 1945 the same happened to Zimmer, Catsch and Born. The arrest was followed by weeks of extensive interrogations by the NKVD and stays in various prisons.⁴³ Then the three Germans were detained in a POW camp for German specialists in Krasnogorsk near Moscow while Timoféeff and Zarapkin as Soviet citizens were later tried in court. Each was sentenced to ten years in the GULAG.

Research at secret locations

The Soviet leadership responded to the American atomic bombing of Hiroshima and Nagasaki on August 6th and 9th, with Decree 9877 on August 20, 1945. The aim was to achieve a nuclear standoff with the U.S. as soon as possible. The decree gave Berija and Zavenyagin far-reaching powers, but they also in turn came under massive pressure. The measures following the decree included the ruthless recruitment of specialists from Germany (a group to which Timoféeff, Zimmer and others belonged) and the brutally accelerated production of atomic bombs. In this context, new “sharashkas” – secret research and development laboratories in the Soviet GULAG system – were created. People imprisoned there had no rights.⁴⁴

Elektrostal: A few months after Riehl’s arrival in Russia, it was decided to relocate the Soviet uranium production to Elektrostal, 70 km east of Moscow. Riehl was charged with managing the construction of the factory and production. Initially, operations were slow to get started, but from the beginning of 1946 they stabilized somewhat. At this time Riehl learned that Zimmer, Born and Catsch were in the Krasnogorsk camp. He succeeded in getting all three assigned to his group. However, it was difficult to find appropriate tasks for them in the uranium production process. But somehow this succeeded until in December 1947 a far better solution for Zimmer and colleagues opened up in Sungul.

In Elektrostal the Zimmers suffered a cruel stroke of fate. In July 1940 Zimmer and Elisabeth Charlotte Cron (1917-2003) had married. In 1947 she was allowed to follow her husband to Russia. Soon after her arrival she contracted polio and was treated in Moscow, but a permanent disability could not be averted. The marriage remained childless.

Riehl remained in Elektrostal until 1950. At this time the plant produced about one ton of uranium per day. On August 29, 1949 the Soviet Union had detonated its first atomic bomb at Semipalatinsk, thereby ending the American nuclear monopoly. “The work of Nikolaus Riehl and his colleagues at Elektrostal, especially in 1945 and 1946 [in 1946 with the involuntary aid of Zimmer], had a direct impact on the production of the first [Soviet] plutonium bomb.”⁴⁵ However, contrary to some reports, the proportion of German researchers working on the bomb was small. “Only Nikolaus Riehl’s research group contributed to the building of the bomb, whose method for the production of plates made of ‘pure’ metallic uranium was able to produce larger quantities faster than the rival method of Sinaida Jerschowa [1904-1995, known as “the Russian Madame Curie”]. Through this method, the path to the plutonium bomb is said to have been shortened by one year.”⁴⁶

Sungul: The founding of the scientific detention center in Sungul, which existed from 1946 to 1955, dates back to a proposal of Valentin A. Kravcenko, a major general of the Soviet Secret Service (NKVD). The proposal was implemented into action by Zavenyagin. Perhaps inspired by interrogations of Zimmer, Kravcenko had written the following in a report in November 1945: “When we consider that we have scientists who have worked in this field (Krebs, Timoféeff-Ressovsky, Zimmer, Born) and also have the laboratory facilities in Oberschlema [a branch of the KWI for Biophysics, which was headed by Adolf Krebs (1902-1977), who after brief Russian captivity lived in the U.S. from 1947 on] and in Vacha [a modern research hall near Staßfurt, which was equipped for a 3 million Volt facility], then it is possible to establish a similar research institution in the USSR, which exactly like the laboratory in [Berlin-Buch] can thoroughly investigate the special influences and effects [of radiation] on living organisms.“⁴⁷

Sungul (now Snezhinsk) is located in West Siberia, approximately at the 55th parallel north in the middle between Yekaterinburg and Cheljabinsk, as the cities are now called. Here the camp known as “Laboratory B” was established.⁴⁸ The Russian Federal Nuclear Center now on the same site describes itself as follows in its promotional material:

„The settlement of Sungul is situated in one of the most beautiful places of the Middle Urals on the wooded peninsular with an area of about 6.5 km², which is connected with the ‚continent’ by a narrow istmuth 800 m wide. On the north the peninsular is washed by the lake Silach and the adjacent small lake Ergaldy, and on the south  — by the lake Sungul. Viewed from the neighboring Cherry Hills these lakes, as to  their sizes, shore contours and the existence of numerous small bays, islands and peninsulas present the beautiful piece of Nature.“⁴⁹

Zimmer and his colleagues would not have found it that idyllic, but nevertheless better than Elektrostal, when they were transported there in December 1947. Timoféeff and Zarapkin had been transported to Sungul before them. Both had just barely survived a lengthy period in the GULAG.⁵⁰ In Sungul two departments were established, one for radiobiophysics under Timoféeff and one for radiochemistry under Voznessensky. Zimmer, Born and Catsch worked on Timoféeff’s team, as they had done in Berlin-Buch. Zimmer later wrote:

“I was prohibited from working in the field of radiation biology because I did not adhere to the then prevailing Soviet doctrine. But I had the opportunity to gain in-depth experience in the production, handling and measurement of very highly radioactive radiation sources and in the field of radiation protection and to read the scientific literature of all countries.”⁵¹

The availability of very strong sources of radiation was due to the fact that the (still existing) Mayak Plutonium Factory was located near Sungul.

In September 1950 Riehl also arrived with his family in Sungul, after having been named director of “Laboratory B”. From his description, we know that research activities there focused on the treatment, effect and use of radioactive fission products from reactors. In addition to dosimetric problems, Zimmer focused on the incorporation of radionuclides in the organs of the rat and rabbit as well as the determination of the maximum permissible doses of radiation. Catsch began to develop methods to remove incorporated radionuclides from the body. The separation of plutonium from reactor fission products was investigated by Born.

In collaboration with Riehl, Zimmer wrote a book in 1948 in Russia about energy migration processes in animate and inanimate matter, in particular taking into account research carried out in Berlin-Buch. Riehl translated the German text into Russian, and Zimmer translated it into English. The manuscript was reviewed by high-ranking Russian scientists, approved and released for printing. Riehl describes that he had already seen an advance copy of the book. At the instigation of proponents of Trofim D. Lyssenko (1898-1976) the printing of the book was stopped, because the geneticists Nikolaj K. Koltsov (1872-1940) and Timoféeff were mentioned in the text. Riehl’s book could eventually be published in modified form without Zimmer’s contribution. Both authors were forbidden to keep a copy of the original manuscript. For Zimmer, as Riehl reports, that was a very frustrating experience.⁵²

Agudseri: Since the German nuclear specialists were considered bearers of secrets, it was customary to make them spend two or three years in quarantine before returning to their homeland. For the Riehl group, Agudseri was chosen, a place near Suchumi on the Black Sea. After almost five years in Sungul, Zimmer was taken to Agudseri in October 1952. “There I had the opportunity to work with problems of dosimetry using organic luminescent materials and to submit the findings for publication,” he reported.⁵³ From the spring of 1952 on, Riehl was also quarantined there, and in the following years he and Riehl published jointly.

When the return of the German specialists to their homeland became imminent, the Soviets, in coordination with the East German government, created a list of 18 Germans who had worked on the Soviet atomic project. Those on this list were only supposed to be allowed to return to the GDR. All members of the Riehl group, among them Riehl himself, Zimmer, Born and Catsch were on the list. The Soviets feared that these specialists, in case they were released to the Federal Republic, could still pass on their existing knowledge of secret research projects and secret places to Western intelligence agencies. The East German leadership wanted to have the experts in order to build up its own capacity for utilizing nuclear energy.⁵⁴ They particularly intensively tried to attract Riehl with generous offers in the GDR, but even Walter Ulbricht did not succeed. Riehl and also Zimmer and Catsch were determined to settle in the Federal Republic.

Karl G. Zimmer (left) and Nikolaus Riehl, around the end of the 1950s.
The private photo was kindly provided by Dr. I. Fiedler.

Zimmer and his wife arrived in West Berlin on April 1, 1955. The Catsch family absconded to the West at the end of April 1955. Riehl and his family followed in June 1955, after four weeks in East Berlin. Born initially remained at the Academy of Sciences in Berlin-Buch, where according to a directive dated June 9, 1955, the research area “Applied Isotope Research” was founded under his leadership. Before he accepted an appointment in October 1957 to what is now Technische Universität München, he had completed his Habilitation at the TU Dresden.⁵⁵ As could hardly be expected otherwise, Riehl and Zimmer were interrogated after their arrival in the West in particular by the British Secret Service. “The atomic returnees were the most valuable collection of information; among them, the best sources were former members of the Riehl group.”⁵⁶ The other leading German specialists in the Soviet atomic project, who were equal in rank to Riehl, remained in East Germany: Max Volmer (1885-1965), Gustav Hertz (1887-1975) and Manfred von Ardenne (1907-1997).

On August 25, 1989, without mentioning them by name, the Soviet official party newspaper “Pravda” lauded the achievements of the scientists taken to the Soviet Union from the Kaiser Wilhelm Institute for Brain Research in Berlin-Buch (Zimmer, Catsch, Born) for their studies on the effects and damage that radioactive radiation induces in living organisms. ⁵⁷ A posthumous recognition in the wake of glasnost.

A New Beginning in Germany: Karlsruhe and Heidelberg

Once in the Federal Republic, the new beginning in his profession at first proved difficult for Zimmer. The Max Planck Society (MPS) felt obligated to help him, but had no real use for him. Therefore Zimmer first of all accepted an invitation to be a guest lecturer in Stockholm. When in November 1955 there was still no change at the MPS and it became clear that Zimmer would accept good offers in other countries, Adolf Butenandt, Alfred Kühn and Hans Friedrich-Freksa wrote a joint letter to the president of the MPS, Otto Hahn, and asked him to make use of his influence for Zimmer. Also Georg Melchers, Boris Rajewsky and Gerhard Schramm expressed their support for Zimmer. Werner Heisenberg wrote the following to the MPS general administration:

“Dr. Zimmer is an excellent specialist in the field of radiation biology, and it would not be at all responsible to allow the few specialists we have in this field to emigrate from here to other countries. Objectively, I think it would be best if Dr. Zimmer [...] could work at or near a reactor station.”⁵⁸

In the MPS, agreement was ultimately reached that Zimmer would receive a monthly subsidy amounting to DM 1500.00 from the Max Planck Society, effective January 1, 1956 and limited to two years. At first, he was to continue to work in Sweden. It was assumed that within this time period the problem would solve itself, which subsequently happened.

For the continuation of his academic career it was important for Zimmer to be able to present a Habilitation. Probably through the intercession of Pascual Jordan, who was full professor there, the University of Hamburg recognized four volumes of Zimmer’s printed works as equivalent to a Habilitation. On May 17, 1956 Zimmer received Habilitation certification in Hamburg and – because a career in Germany was still uncertain – on October 15, 1956 was also conferred Habilitation certification by the University of Stockholm.⁵⁹

With financial support from the Max Planck Society and the Wallenberg Foundation, Zimmer was able to renew old connections again. The plant geneticist Professor Åke Gustafsson (1908-1988) enabled him to have a long visiting researcher stay at the Forest Research Institute in Stockholm. Through Gustafsson he came into contact with the radiobiologist and cancer researcher Lars Ehrenberg (1921-2005). In subsequent years Zimmer was to publish important papers in collaboration with him.

As soon as he was able to publish again, Zimmer made good use of this opportunity. In just two years – in 1956 and 1957 – he published 18 papers, including five in Russian journals. He marked his return to the scientific community with reviews and commentaries on the developments of the last ten years.⁶⁰ Furthermore, he was invited to lectures, especially in Scandinavia.

The efforts to find a suitable position for Zimmer took place in an exciting period. Due to the ratification of the Treaties of Paris in May 1955, the Federal Republic of Germany had attained sovereignty to a large extent. The restrictions that the Allies had hitherto imposed on nuclear physics research and its applications were thus lifted. The Federal Government decided to establish the first German nuclear reactor in Karlsruhe and not, as Heisenberg had wished, in Munich. The reactor station was founded in Karlsruhe in 1956, and the reactor began operations five years later. The Technische Hochschule München (now Technische Universität München) received a smaller reactor that began operations sooner. It was located at the institute of Heinz Maier-Leibnitz (1911-2000), who collaborated closely with Nikolaus Riehl in constructing the reactor. A “German nuclear program” was conceived in April 1956.⁶¹ In the course of these developments, it was planned to found new institutes for radiation biology in Karlsruhe and Munich. The director of the Karlsruhe Institute was also supposed to represent radiation biology in Heidelberg. Zimmer received offers from Munich and Heidelberg. He chose the University of Heidelberg.⁶² From September 1, 1957 he was associate professor and from 1958 professor for Radiation Biology in Heidelberg. Already on July 1, 1957 he had assumed his position of institute director at the reactor station in Karlsruhe (later Nuclear Research Center Karlsruhe and/or Research Center Karlsruhe, today: Karlsruhe Institute of Technology). After the Technical University of Karlsruhe established a chair for radiation biology, Zimmer’s colleague Catsch was appointed to this position.

Upon the initiative of Pascual Jordan and with the support of the physicist Eduard Justi (1904-1986), who at that time was president of the Academy, as well as the human geneticist Otmar von Verschuer (1896-1969), Zimmer was elected to be a regular member of the Academy of Sciences and Literature, Mainz on October 28, 1958. As justification the Academy stated: “The Academy [gains] in the person of Professor Zimmer for the interconnected field of biophysics a researcher of first rank and high international prestige.”⁶³ In August 1959 Zimmer presented his treatise “Studies on Quantitative Radiation Biology” to the Academy. H.D. Griffith, who 1934 had been his host in Aberdeen, soon translated the book into English. No less a figure than Alexander Hollaender (1898-1986) wrote a review for the journal Science. Referring to the Green Pamphlet, Hollaender wrote “Zimmer is the only one of the three men [Timoféeff, Zimmer, Delbrück] who has continued to work in this field, and he is now one of the important investigators in radiation biology.”⁶⁴ On the initiative of Hollaender, nearly all publications of Zimmer were deposited at the University of Tennessee (Zimmer Papers).⁶⁵

Less known is the fact that Zimmer played a role in the founding phase of the German Cancer Research Center (DKFZ) in Heidelberg. At the suggestion of Adolf Butenandt, the DKFZ was also to have a Department of Radiation Biology under Zimmer or Catsch. After Zimmer accepted an appointment to the University of Heidelberg in September 1957, his chair was introduced as one of the contributions of the university to the DKFZ. Zimmer, who meanwhile was very well equipped in Karlsruhe, was supposed to “be available” in Heidelberg for problems of radiation biology.⁶⁶

Professor Karl Günther Zimmer 1971 in Karlsruhe.

© mit freundlicher Genehmigung des ehemaligen Kernforschungszentrum Karlsruhe, dem jetzigen Karlsruher Institut für Technologie, KIT

Already in the Green Pamphlet the question was raised about the processes that are initiated by the localized supply of energy in biological material, which is associated with the absorption of radiation. Zimmer had barely arrived back in “normal” research operations, when he succeeded in 1957 together with the brothers Lars and Anders Ehrenberg in making a spectacular finding: Using electron spin resonance spectroscopy, the researchers for the first time detected the formation of free organic radicals on living material following irradiation.⁶⁷ Thus, a new field of research was opened, which Zimmer and his team worked on intensively in the following years in Karlsruhe. First and foremost, bacteriophages were used as biological models. The numerous further studies at the Karlsruhe Institute cannot be discussed further here. Among the approximately 550 publications that have emerged from the Institute of Radiation Biology under Zimmer’s directorship until his retirement in 1979, many were on the effects of radiation on enzymes (using the example of ribonuclease), on the characterization of primary chemical reaction products in irradiated DNA and on the effect of radiation-damage on the matrix function of DNA. Research was also pursued on radiation-induced energy transfer through elastic nuclear collisions. Zimmer rarely appeared as co-author of the original papers of his staff. Instead he wrote many overview articles that are still worth reading today.⁶⁸

“What characterized Zimmer as a researcher personality – and what he expected from his staff – was diligent and systematic research work on a problem that had been recognized as relevant, a critical, open attitude towards new developments and – last not least – curiosity.“⁶⁹ A number of future university professors and chair holders have worked for Zimmer, including Ulrich Hagen (1925- 2007), first as department head in the Karlsruhe Institute, later in Munich, Hermann Dertinger (*1939) and Gerhart Hotz (*1925) in Karlsruhe, Horst Jung (*1937) in Hamburg as well as Horst Traut (*1932) in Münster. By promoting the advancement of high-performing scientists, Zimmer made an important contribution to radiation biology in Germany. Dertinger and Jung published the book “Molecular Radiation Biology” during their years with Zimmer and with his support. The text was based on lectures that Zimmer held alone from 1957 to 1968 and later with the participation of the authors.⁷⁰

Zimmer was asked to hold a Failla Memorial Lecture, the highest scientific distinction in the field of radiation research, in Coronado, California in February 1966. The lecture is held in commemoration of one of the pioneers in the field of biophysics and radiation biology, the American Gioacchino (Gino) Failla (1891-1961). Another honor followed in 1969 with the request to hold a Douglas Lea Memorial Lecture in commemoration of the British radiation physicist Lea, who died at an early age. As was already mentioned previously, Zimmer had known Lea personally.⁷¹

Zimmer viewed the institute he headed as an institution of basic research and quantitative radiation biology, as a branch of molecular biological basic research. His motto was: “Truly serious, enduring and systematic work at a scientific problem solely for the sake of knowledge and without any applied (practical) goal yields often automatically and at the side results of great practical value which work oriented explicitly toward a specific practical goal often misses.”⁷²

A remarkable life in research

In his obituary of Zimmer, Peter Herrlich wrote “Professor Zimmer already is a textbook name”.⁷³ In his modest way, Zimmer would have probably modified this so correct statement.

The life and work of Karl Günther Zimmer is remarkable for a number of reasons.

1. Zimmer belonged to the first generation of physicists who sustainably changed biology. The geneticist Guido Pontecorvo (1907-1999) described this as follows: “In the years immediately preceding World War II, something new happened: the introduction of ideas (not techniques) from the realm of physics into the realm of genetics, particularly to the problems of size, mutability, and self-replication of genes. The names of Jordan, Frank-Kamenetski, Friedrich-Freksa, Zimmer, and Delbrück, with Muller and [sic] Timofeef-Ressovsky as their biological interpreters, are linked to this development. [...] The debt of genetics to physics, and to physical chemistry, for ideas began to be substantial then, and it has been growing steadily all the time.”⁷⁴ Five of the seven scientists mentioned above worked for a time in Berlin-Buch, a sixth (Friedrich-Freksa) worked in Berlin-Dahlem.
2. As co-author of the Green Pamphlet Zimmer’s name is inseparably associated with the early history of molecular biology.
3. Zimmer was one of the founders and protagonists of quantitative radiation biology. This was based on his pioneering solutions of dosimetric problems.
4. As one of the first scientists, Zimmer persistently warned of the dangers that are associated with the effects of high-energy radiation on people and advocated effective radiation protection.
5. Zimmer is one of the representatives of that generation of scientists whose biographies reflect the upheavals and dramatic events of the 20th century in a special way. For about half of his 46 professional years, Zimmer had to live and work in totalitarian states, six years during the war and another ten years in Russian detention. Although he experienced much hardship, he never let himself be discouraged.
6. Zimmer had to spend the years in his lifetime that are usually considered to be the most productive ones for a scientist in detention camps. Nevertheless he achieved outstanding scientific work both beforehand (between 24 and 34 years old) and also afterwards (between 45 and approximately 60years old). Despite this time-related discontinuity, Zimmer’s career path was characterized by continuity in content, as Hollaender has quite rightly pointed out.

In particular at the research site Berlin-Buch, it is not possible to commemorate Zimmer without also thinking of Timoféeff. It can be viewed as a stroke of fortune that Timoféeff and Zimmer met each other and were able to work together for many years. The meeting must have been on equal footing and characterized by mutual respect. Timoféeff , who was 11 years older, was however not the “teacher” or “mentor” of Zimmer, and Zimmer was certainly not Timoféeff’s “in-house physicist.” Zimmer was attracted by the “the brilliant personality of Timoféeff, which made team-work an exciting adventure“.⁷⁵ In 1934 he himself was already an independent researcher. Moreover, part of the research that was conducted in Timoféeffs research groups, in particular using neutrons, would hardly have been conceivable without Zimmer. When Timoféeff decided in the spring of 1945 to remain in Berlin-Buch and not go to the West, Zimmer and also Born and Catsch stayed with him. We do not know any details about the cooperation that took place later in Sungul, but discussions from this time may have been incorporated in Zimmer’s and Timoféeff’s work after 1955. Zimmer hinted at this in 1966 when he referred to his work with Timoféeff as “a cooperation that was to last for nearly 20 years and in various places as determined by the most inconvenient political events overshadowing the life of many of us.”⁷⁶

The last known publication by Zimmer was a tribute to his longtime colleague Timoféeff.⁷⁷ Shortly before that, Zimmer had published his reminiscences about a field to whose development he had contributed extensively: “That was the basic radiobiology that was“⁷⁸ – “only yesterday an avant-garde but today a workaday field.”⁷⁹

On February 29, 1988, Karl Günther Zimmer died in Karlsruhe of a heart attack.

1. For the title of one of his last works Zimmer used the aphorism adapted by Gunther Stent (1924-2008), with his consent. Zimmer, Karl Günther (1981) That was the basic radiobiology that was: a selected bibliography and some comments. Advances in Radiation Biology 9: 411-467; Stent, Gunther S. (1968) That was the molecular biology that was. Science 160: 390-395.
2. Timoféeff-Ressovsky, Nikolai V., Karl G. Zimmer., Max Delbrück (1935) Über die Natur der Genmutation und der Genstruktur. [On the Nature of Gene Mutation and Gene Structure] Nachrichten von der Gesellschaft der Wissenschaften zu Göttingen, Mathematisch-physikalische Klasse, Fachgruppe VI, Biologie, Neue Folge, 1,13: 189-245. http://www.jnorman.com/cgi-bin/hss/38569
3. Timoféeff-Ressovsky et al. (as note 2), p.238. Italics as in the original text. In 1957 Zimmer referred to this paper with the words: “The main idea was to imagine the gene as a well-organized and stable, molecule-like structure that can be transferred into another about equally stable state either artificially by radiation or spontaneously by accidental accumulation of sufficient thermal energy in one degree of freedom. ” Zimmer, Karl Günther (1957) A physicist’s comments on some recent papers on radiation genetics. Hereditas 43: 201-210, here p. 201.
4. Schrödinger, Erwin (1944) What is Life? The physical aspects of the living cell. Cambridge: Cambridge University Press. See also: Perutz, Max F. (1987) Physics and the riddle of life. Nature 326: 555-558.
5. For example in: Max Delbrück Center for Moleculare Medicine (publ.) (2008) Geneticists in Berlin-Buch. With contributions by Fritz Melchers, Manfred F. Rajewsky, Jens Reich and Volker Wunderlich. Foreword by Walter Birchmeier. Berlin; Ratner, Vadim A. (2001) Nikolay Vladimirovich Timofeeff-Ressovsky (1900-1981): twin of the century of genetics. Genetics 158: 933-939; Fischer, Ernst Peter (2007) Max Delbrück, Genetics 177: 673-76.
6. Earlier treatments: Catsch, Alexander (1971) On the 60th Birthday of K.G. Zimmer. Strahlentherapie 142: 124-125; Herrlich, Peter (1988) In memoriam Karl Günther Zimmer (1911-1988). Radiation Research 116: 178-80; Anonymous (1988) In Commemoration of Karl Günther Zimmer. Hausmitteilungen des Kernforschungszentrums Karlsruhe, No. 1, p. 10; Pasternak, Luise; Günter Pasternak (2004) Zimmer, Karl Günter, p. 38-40. In: Luise Pasternak (ed.) Wissenschaftler im biomedizinischen Forschungszentrum. Berlin-Buch 1930-2004 [Scientists in the Biomedical Research Center Berlin-Buch 1930-1934]. Frankfurt am Main etc.: Peter Lang; http://en.wikipedia.org/wiki/Karl_Zimmer and http://dictionary.sensagent.com/karl+zimmer/en-en.
7. Zimmer, Karl Günther (1933) Der Reaktionsmechanismus der photochemischen Umwandlung von o-Nitrobenzaldehyd zu o-Nitrosobenzoesäure im ultraviolettem Licht [The reaction mechanism of the photochemical transformation of o-nitrobenzaldehyde to o-nitrobenzoic acid in ultraviolet light]. Zeitschrift für physikalische Chemie 23: 239-255. In the dissertation, details of his curriculum vitae.
8. For information on the Auer Company see http://de.wikipedia.org/wiki/Auergesellschaft. There is a web link to a detailed article on the history of the Auer Company (PDF).
9. Riehl was the son of a German engineer and a Russian mother. He grew up in St. Petersburg and lived in Berlin from 1918 on. There he studied physics and earned his doctorate in 1927 under the supervision of Lise Meitner (1878-1968). Otto Hahn (1879-1968) was a friend of his. From 1927-1945 Riehl worked for the Auer Company in Berlin, from 1939 on as director of the newly created scientific division. In 1938 he completed his Habilitation at TH Berlin (now TU Berlin). His main areas of work were physics of luminescence and applied nuclear physics. Riehl is considered the father of the fluorescent lamp.
10. See the contribution by Wunderlich in “Geneticists in Berlin-Buch” (as note 5).
11. Delbrück, Max (1970) A physicist’s renewed look at biology: twenty years later. Science 168: 1312-1315.
12. Zimmer, Karl Günther (1934) Ein Beitrag zur Frage nach der Beziehung zwischen Röntgenstrahlendosis und dadurch ausgelöster Mutationsrate [A contribution to the question of the relation between X-ray dose and the thus induced mutation rate]. Strahlentherapie 51: 179-184.
13. Zimmer, Karl Günther (1966) The Target Theory, pp. 33-42. In: John Cairns, Gunther S. Stent, James D. Watson (Eds.), Phage and the Origins of Molecular Biology. Cold Spring Harbor: Cold Spring Harbor Laboratory Press, here p. 37.
14. Zimmer, Karl Günther (1936) Radiumdosimetrie. Verfahren und bisherige Ergebnisse [Radium Dosimetry. Methods and Results to Date]. Leipzig: Georg Thieme.
15. Zimmer with Timoféeff, and in part with A. Pickhan, E. Wilhelmy (1935/36) Strahlentherapie [Radiation Therapy] 53: 134-138; 54: 265-278; 55: 77-84; 56: 488-496; 57: 521-531, with H.D. Griffith (1935) British Journal of Radiology 8: 40-47.
16. Deichmann, Ute (1992) Biologen unter Hitler: Vertreibung, Karrieren, Forschung [Biologists under Hitler: Expulsion, Careers, Research]. Frankfurt am Main: Campus.
17. Zimmer, Karl Günther (1937) Strahlungen. Wesen, Erzeugung und Mechanismus der biologischen Wirkung [Radiation. Nature, Generation and Mechanism of the Biological Effect] Leipzig: Georg Thieme
18. More details at: Gausemeier, Bernd (2005) Natürliche Ordnungen und politische Allianzen. Biologische und biochemische Forschung an Kaiser Wilhelm-Instituten 1933-45 [Natural systems and political alliances. Biological and biochemical research at Kaiser Wilhelm Institutes 1933-45]. Göttingen: Wallstein, here pp. 170-186 and Bielka, Heinz (2002) Geschichte der Medizinisch-Biologischen Institute Berlin-Buch [History of the Medical-Biological Research Institutes in Berlin-Buch]. 2nd edition. Berlin, Heidelberg: Springer.
19. To mention here: Zimmer, Karl Günther (1938) Dosimetrische und strahlenbiologische Versuche mit schnellen Neutronen I [Dosimetric and radiobiological experiments with fast neutrons I]. Strahlentherapie 63: 517-527, Zimmer, Karl Günther, Nikolai V. Timoféeff-Ressovsky (1939) Note on the biological effects of densely ionizing radiation. Physical Review 55: 411, Zimmer, Karl Günther (1940) Dosimetrische und strahlenbiologische Versuche mit schnellen Neutronen III [Dosimetric and radiobiological experiments with fast neutrons III]. Strahlentherapie 68: 74-78, Catsch, Alexander, Otto Peter and Karl Günther Zimmer (1947) Strahlenbiologische Untersuchungen mit schnellen Neutronen [Radiobiological investigations with fast neutrons], Zeitschrift für Naturforschung 2b: 1-5.
20. Timoféeff-Ressovsky, Nikolai V.; Zimmer, Karl Günther (1947) Das Trefferprinzip in der Biologie [The Target Principle in Biology]. Leipzig: Hirzel; Lea, Douglas E. (1946) Action of radiations on living cells. Cambridge: University Press. Zimmer repeatedly focused on these problems: Zimmer, Karl Günther (1960) Studien zur quantitativen Strahlenbiologie [Studies on Quantitative Radiation Biology]. Wiesbaden: Franz Steiner; Zimmer (1966) Target Theory, as note 13; Zimmer (1969) The 11th Douglas Lea Memorial Lecture: From target theory to molecular radiobiology. Physics in Medicine and Biology 14: 545-553. Timoféeff also devoted himself to this topic again: N. V. Timoféeff-Ressovsky, Vladimir I. Ivanov and Vladimir J. Korogodin (1972) Die Anwendung des Trefferprinzips in der Strahlenbiologie [The application of the hit principle in radiation biology]. Jena: Fischer [in Russian 1968].
21. Zimmer, Lea Memorial Lecture (as note 20), here p. 546.
22. Archives of the Max Planck Society, Dept. I, Rep 34, No.100.
23. Born, Hans-Joachim, A. Lang, Gerhard Schramm, Karl Günther Zimmer (1941) Versuche zur Markierung von Tabakmosaikvirus mit Radiophosphor [Attempts to label tobacco mosaic virus with radio phosphorus]. Die Naturwissenschaften 29: 22-223.
24. Zimmer, Karl Günther (1943) Statistische Ultramikrometrie mit Röntgen-, Alpha- und Neutronenstrahlung [Statistical Ultramicrometry with X-rays, alpha and neutron radiation]. Physikalische Zeitschrift 44, 233-243; Zimmer (1943) Ergebnisse und Grenzen der treffertheoretischen Deutung von strahlenbiologischen Dosis-Effekt-Kurven [Results and limitations of the target theory interpretation of radiobiological dose-effect curves]. Biologisches Zentralblatt 63: 72-107; Zimmer (1944) Mechanismus der Wirkung ionisierender Strahlen auf Lösungen I [Mechanism of the effect of ionizing radiation on solutions I]. Physikalische Zeitschrift 45: 265-267.
25. Jordan, Pascual, Karl Günther Zimmer, Nikolai V. Timoféeff-Ressovsky (1948) Über einige physikalische Vorgänge bei der Auslösung von Genmutationen durch Strahlung II [On some physical processes involved in the induction of gene mutations by radiation II]. Zeitschrift für Vererbungslehre 82: 67-73.
26. Riehl, Nikolaus, Robert Rompe, Nikolai V. Timoféeff-Ressovsky, Karl Günther Zimmer (1943) Über Energiewanderungsvorgänge und ihre Bedeutung für einige biologische Prozesse [On some energy migration processes and their importance for some biological processes]. Protoplasma 38: 105-126.
27. Zimmer on September 24, 1957 to President Otto Hahn. Archives of the Max Planck Society, Dept. II, Rep. 1A, personnel file Zimmer.
28. Bielka, History (see note 18), Schmuhl, Hans-Walter (2002) Hirnforschung und Krankenmord. Das Kaiser-Wilhelm-Institut für Hirnforschung 1937-1945 [Brain Research and Patient Murder. The Kaiser Wilhelm Institute for Brain Research1937-1945] Vierteljahreshefte für Zeitgeschichte 50: 538-609.
29. http://en.wikipedia.org/wiki/Hans-Joachim_Born
30. http://en.wikipedia.org/wiki/Alexander_Catsch
31. Gausemeier, Systems (as note 18); Schmaltz, Florian (2005) Kampfstoff-Forschung im Nationalsozialismus: zur Kooperation von Kaiser-Wilhelm-Instituten, Militär und Industrie [Research into warfare agents during National Socialism: On the cooperation between the Kaiser Wilhelm Institutes, the military and industry] Göttingen: Wallstein, here pp. 246-291; Maier, Helmut (ed.) (2007) Gemeinschaftsforschung, Bevollmächtigte und der Wissenstransfer: die Rolle der Kaiser-Wilhelm-Gesellschaft im System kriegsrelevanter Forschung des Nationalsozialismus [Joint research, agents and knowledge transfer: the role of the Kaiser Wilhelm Society in the system of war-relevant research of National Socialism]. Göttingen: Wallstein, here pp. 434-444; Nagel, Günter (2002) Atomversuche in Deutschland. Geheime Uranarbeiten in Gottow, Oranienburg und Stadtilm [Nuclear tests in Germany. Secret uranium work in Gottow, Oranienburg und Stadtilm]. Zella-Mehlis, Meiningen: Heinrich Jung; Stange, Thomas (1998) Die kernphysikalischen Ambitionen des Reichspostministers Ohnesorge [The nuclear research ambitions of the Minister of the Reichspost, Wilhelm Ohnesorge]. Berichte zur Wissenschaftsgeschichte 21: 159-174.
32. Gausemeier, Systems (see note 18), here p. 184.
33. Gausemeier, Systems (see note 18), Schmaltz, Kampfstoff (see Note 31).
34. Hentschel, Klaus; Ann M. Hentschel (1996) Physics and National Socialism: An Anthology of Primary Sources. Basel, Berlin: Birkhäuser, here p. 303.
35. http://de.wikipedia.org/wiki/Uranprojekt [uranium project] and http://de.wikipedia.org/wiki/Kernphysikalische_Forschungsberichte [Nuclear Physics Research Reports] The authors were not allowed to keep copies of their reports.
36. Walker, Mark (1990) Die Uranmaschine: Mythos und Wirklichkeit der deutschen Atombombe [The uranium machine: myth and reality of the German atomic bomb]. Berlin: Siedler, here p. 313 ff. The author thanked Zimmer, among others, for his critical reading of a draft version.
37. Hentschel, Physics (see Note 34), here p. 324.
38. Schmaltz, Kampfstoff (see Note 31cites this passage on p. 282f. The quotation is taken over here.
39. For more information on the events presented in this and the following section, please refer to: Albrecht, Ulrich, Andreas Heinemann-Grüder, Arend Wellmann (1992) Die Spezialisten: Deutsche Naturwissenschaftler und Techniker in der Sowjetunion nach 1945 [The specialists:German scientists and engineers in the Soviet Union after 1945]. Berlin: Dietz, here pp. 48-82; Oleynikov, Pavel V (2000) German Scientists in the Soviet Atomic Project. The Nonproliferation Review 7: 1-30; Mick, Christoph (2000) Forschen für Stalin: Deutsche Fachleute in der sowjetischen Rüstungsindustrie 1945-1958 [Research for Stalin: German experts in the Soviet defense industry 1945-1958]. Munich, Vienna: Oldenbourg.
40. The physicist and author Arnold Kramish (1923-2010) recounted this story as a thriller that is largely based on fact,. Kramish, Arnold (1987) Der Greif. Paul Rosbaud – der Mann, der Hitlers Atompläne scheitern ließ. Munich: Kindler. American original (1986) The Griffin - the greatest untold espionage story of World War II, Boston: Houghton Mifflin Co. See also http://de.wikipedia.org/wiki/Paul_Rosbaud
41. Flerov noted in 1942 that after the discovery of nuclear fission, no further publications appeared in American and German journals on this topic. From that he correctly concluded that research must be going on there in a secret war effort. He communicated this observation in a letter to Stalin. After that the Soviet Union began its own efforts to develop nuclear weapons.
42. The following book translated and extensively annotated by the American chemist Frederick Seitz (1911-2008) is also very informative: Riehl, Nikolaus; Seitz, Frederick (1996) Stalin’s Captive: Nikolaus Riehl and the Soviet Race for the Bomb. American Chemical Society and the Chemical Heritage Foundation. Original German version: Riehl, Nikolaus (1988) Zehn Jahre im goldenen Käfig. Erlebnisse beim Aufbau der sowjetischen Uran-Industrie [Ten years in the golden cage. Experiences in building the Soviet uranium industry]. Stuttgart: Riederer. Cf. http://de.wikipedia.org/wiki/Nikolaus_Riehl (entry needs revision) and Horst Kant (2003) Riehl, Nikolaus. Neue Deutsche Biographie [New German Biography] 21: 587-88.
43. The interrogation records are partially accessible at: The Federal Commissioner for the records of the State Security Service of the former German Democratic Republic, e.g. under HA IX/11 and HA IX/ 20699, Archives of the Max Planck Society, Dept. I, Rep. 34, No.100. However, too far-reaching conclusions should not be drawn from the records.
44. http://en.wikipedia.org/wiki/Sharashka By going on strike, the physicist Dr. Karl-Heinrich Riewe attempted to make the Soviets comply with a contract they had concluded with him. He was sentenced to 25 years imprisonment for sabotage and was then summarily shot. Timoféeffs longtime employee Zarapkin refused to give up his work on theoretical genetics. He was therefore taken back to the GULAG where he died soon after. See Oleynikov, German Scientists (as note 39), here p. 14 and Riehl, Stalin’s Captive (as note 42), here p. 129.
45. Oleynikov, German Scientists (as note 39), here p. 21.
46. Mick, Research (as note 39), here p. 151.
47. The quotation was taken from Maier, Joint research (as note 31), here p. 426.
48. http://en.wikipedia.org/wiki/Laboratory_B_in_Sungul%E2%80%99
49. Legakov, B.G. et al. (2003) Pulse nuclear reactors at RFNC-VNIITF. Snezhinsk: Russian Federal Nuclear Center – All-Russia Scientific Research Institute of Technical Physics. Chapter 1: History of pulse nuclear reactors at RFNC-VNIITF.
50. See the contribution of M. Rajewsky in “Geneticists in Berlin-Buch” (as note 5).
51. Archives of the Max Planck Society, Dept. II, Rep. 1A, personnel file Zimmer.
52. Riehl, Stalin’s Captive (as note 42), here p. 200.
53. Archives of the Max Planck Society, Dept. II, Rep. 1A, personnel file Zimmer.
54. According to Nagel, Nuclear tests (as note 31), here p. 211, the physicist Robert Rompe (1905-1993) traveled on behalf of the GDR Ministry for State Security to Suchumi, to bring the specialists offers from the GDR. Because Rompe had worked with the Timoféeff group prior to 1945, Zimmer and the other former Buch scientists knew him personally. Despite this (or because of this?) Rompe did not succeed in his mission.
55. The four professors Born, Catsch, Riehl and Zimmer remained friends throughout their entire lives.
56. Maddrell, Paul (2006) Spying on Science: Western Intelligence in Divided Germany 1945-1961. Oxford: Oxford University Press, here p. 209.
57. Albrecht, The Specialists (as note 39), here p. 61.
58. W. Heisenberg to the general administration of the MPS, December 12, 1955. MPS Archives, Dept. II, Rep. 1A, personnel file Zimmer
59. Academy of Sciences and Literature, Mainz, file Zimmer
60. Zimmer, A physicist‘s comments (as note 3); Zimmer, Karl Günther (1956) The development of quantum biology during the last decade. Acta radiologica 46: 595-602.
61. For the details of this development see: Eckert, Michael (1988) Neutrons and politics: Maier-Leibnitz and the emergence of pile neutron research in the FRG. Historical Studies in the Physical and Biological Sciences 19: 81-113.
62. The institute in Munich was then headed by Otto Hug (1913-1978).
63. Academy of Sciences and Literature, Mainz, file Zimmer
64. Zimmer, Karl Günther (1960) Studien zur quantitativen Strahlenbiologie. Wiesbaden: Franz Steiner; Zimmer, Karl Günther (1961) Studies on Quantitative Radiation Biology. Edinburgh, London: Oliver & Boyd (also Russian translation of the book, Atomisdat Moskva 1962). Review by Hollaender, Alexander (1961) Hit and target theories. Science 134:1233.Already in 1939 Hollaender had obtained clear evidence from radiobiological experiments that genes are made of nucleic acids. “…[He] fired one of the first shots in the molecular biological revolution” (von Borstel, R.C.; Charles M. Steinberg (1996) Alexander Hollaender: Myth and Mensch. Genetics 143: 1051-1056, here p. 1051).
65. Zimmer Papers: http://www.lib.utk.edu/spcoll/manuscripts/0839.html
66. Wagner, Gustav; Andrea Mauerberger (1989) Krebsforschung in Deutschland. Vorgeschichte und Geschichte des Deutschen Krebsforschungszentrums [Cancer research in Germany. Background and history of the German Cancer Research Center]. Berlin, Heidelberg: Springer. pp.58, 71, 76, 78.
67. Zimmer, Karl Günther, Lars Ehrenberg, Anders Ehrenberg (1957) Nachweis langlebiger magnetischer Zentren in bestrahlten biologischen Medien und deren Bedeutung für die Strahlenbiologie [Evidence of long-lived magnetic centers in irradiated biological media and their significance for radiobiology]. Strahlentherapie 103: 3-15; Zimmer, Karl Günther (1959) Evidence for free-radical production in living cells exposed to ionizing radiation. Radiation research, Supplement 1: 519-529.
68. Zimmer, Karl Günther (1960) The development and prospects of quantitative radiobiology. International Journal of Radiation Biology, Supplement 1: 1-8; Zimmer, Karl Günther; A. Müller (1965) New light on radiation biology from electron spin resonance studies. Current Topics in Radiation Research 1: 3-48; Jung, Horst, Karl Günther Zimmer (1966) Some chemical and biological effects of elastic nuclear collisions. Current Topics in Radiation Research 2: 69-128; Zimmer, Karl Günther (1967) Entwicklung und gegenwärtige Probleme der molekularen Strahlenbiologie [Development and current problems of molecular radiation biology. Strahlentherapie 134: 161-174; Zimmer (1969) Some recent studies in molecular radiobiology. Current Topics in Radiation Research 5: 1-38; Zimmer (1973) Perspectives in molecular radiation biology: The biophysical approach. Current Topics in Radiation Research Quarterly 9: 2-6.
69. Catsch, 60th Birthday (as note 6), here p. 125.
70. Dertinger, Hermann, Horst Jung (1969) Molekulare Strahlenbiologie [Molecular Radiation Biology]. With a foreword by K.G. Zimmer. Berlin, Heidelberg, New York: Springer.
71. Zimmer, Karl Günther (1966) The Fourth Failla Memorial Lecture: Some unusual topics in radiation biology. Radiation Research 28: 830-843; Zimmer, Lea Memorial Lecture (as note 20).
72. Cited after Herrlich, In memoriam (as note 6), p. 179.
73. Herrlich, In memoriam (as note 6), p. 180.
74. Pontecorvo, Guido (1958) Trends in Genetic Analysis. New York: Columbia University Press, here p. 2 f.
75. Zimmer, Target theory (as note 13), here p. 33.
76. Zimmer, Failla Lecture (as note 71), here p. 830.
77. Zimmer, Karl Günther (1982), N.W. Timoféeff-Ressovsky 1900-1981. Mutation Research 106: 191-3.
78. Zimmer, Basic radiobiology (as note 1).
79. Stent, Molecular biology (as note 1).

I would like to express my sincere gratitude to the daughters of Professor Nikolaus Riehl, Dr. Ingeborg Hahne (Leonberg-Warmbronn) and Dr. Irene Fiedler (Baldham), for their valuable comments and Carol Oberschmidt (Berlin) for her excellent translation.

July 8, 2011
Prof. em. Dr. rer. nat. Volker Wunderlich
Max Delbrück Center for Molecular Medicine, 13125 Berlin, Germany. Email: vwunder@mdc-berlin.de

Note added in January 2012: For further information on the work of Zimmer see the following publications:

80. Sloan, Phillip R., Brandon Fogel (Eds.) (2011) Creating a Physical Biology: The Three-Man Paper and Early Molecular Biology. Chicago: University Chicago Press. Published on December 14, 2011. Contains a first English translation of the “Green Pamphlet” from 1935. See also: Michael A. Goldman (2011) Molecular Biology: Seed of Revolution. Nature 480, 317.
81. Hagen, Ulrich, J.T. Lett (1988) In memoriam Karl Günter Zimmer, 1911-1988. Radiation and Environmental Biophysics 27: 245-246.
82. Schwerin, Alexander von (2010) Medical Physicists, Biology, and the Physiology of the Cell (1920–1940) In: Luis Campos, Alexander von Schwerin (Eds.) Making Mutations: Objects, Practices, Contexts. Preprint 393. Max Planck Institute for the History of Science, Berlin.