TRIP OF THE USSR ACADEMY OF SCIENCES DELEGATION TO THE USA FOR THE GORDON CONFERENCE ON NUCLEAR CHEMISTRY

The Gordon Conferences are named after the scientist who, more than twenty years ago, proposed bringing together scientists every summer, chiefly young ones, for an unofficial exchange of opinions, new results, and research plans in various fields of knowledge. These conferences are organized by the American Association for the Advancement of Science (AAAS) and are held every summer in three small settlements in the state of New Hampshire. This summer, 36 Gordon Conferences were held on the most diverse branches of the natural sciences. Since 1952, among the other Gordon Conferences, conferences on nuclear chemistry have also been held. The term “nuclear chemistry” is not widely used in our country, and its meaning is not generally accepted. Apparently, the basic content of nuclear chemistry may be defined as the study of transformations of complex nuclei, primarily by chemical methods.

Having accepted the kind invitation to take part in the sixth Gordon Conference on Nuclear Chemistry, two Soviet scientists—Prof. N. A. Perfilov (Radium Institute, USSR Academy of Sciences) and I—visited the USA in June–July of this year, where we took part in the conference and then made a two-week trip to a number of American laboratories. On the return journey we spent four days in Paris and visited two French nuclear laboratories.

We flew out of Moscow on the morning of June 22 and six hours later found ourselves in Stockholm, or more precisely in the transit hall of the Stockholm airport, where we spent eight hours waiting for the plane to New York. Another seventeen hours—and our airplane, having made along the way only one brief stop in Copenhagen and leaving the Atlantic Ocean behind, landed in New York. On our watches, already set back seven hours, it was eight o’clock in the morning—the time of our departure from Moscow the previous day.

On the day of our arrival in New York we still had to travel to the site of the conference, which was opening the next day—a small settlement, Meriden, located 160 km northwest of Boston in the picturesque countryside of the state of New Hampshire. We covered the road from New York to Meriden (430 km) together with Prof. G. Friedlander and Prof. D. Miller, who met us in New York, in an automobile, thanks to which we had the opportunity to see the states of New England: Connecticut, Massachusetts, Vermont, and New Hampshire.

The sessions of the conference took place in the gymnasium of Kimball Union Academy—a school that prepares boys aged 14–18 for admission to colleges. The conference participants—about 120 people in all—lived in rooms intended, during the school term, for the pupils, and ate in the school dining hall.

The first session, on the morning of June 24, was opened with brief welcoming speeches by the director of Kimball Union Academy and the chairman of the conference—one of the leading radiochemists of the Radiation Laboratory in Berkeley, Dr. Earl K. Hyde. The first two sessions were devoted to the current evidence for the electronic structure of the actinide elements and were held under the chairmanship of Prof. K. A. Hutchison of the University of Chicago. M. Fred of Argonne National Laboratory reported his latest results on the study of the optical spectra of the actinide elements and their compounds. Fred’s conclusions that the first 5f electron appears in certain thorium compounds are consistent with the notions of the actinides as analogous to the rare-earth elements of the sixth period of D. I. Mendeleev’s system. The next report—by D. Gron of Argonne Laboratory—contained data on the absorption and fluorescent spectra of compounds of the actinide elements.

In the report by V. Zachariasen (University of Chicago) much data was presented on the radii of the atoms of elements from actinium to americium in various crystalline structures. Curves of the dependence of atomic radii on the atomic number of the elements

of the seventh and sixth periods of the Mendeleev system differ noticeably, so that, although Zachariasen asserted that his data in general did not contradict the conclusions of M. Fred, he at the same time tended to the conclusion that the first \(5f\)-electron appears in protactinium, while uranium has 5 \(f\)-electrons. The last report devoted to the study of the structure of atoms of actinide elements was an extensive report by K. Hutchison on the paramagnetism of compounds of transuranium elements, in many respects analogous to the paramagnetism of the rare earths.

The next two sessions were held under the chairmanship of the well-known radiochemist Prof. G. Friedlander, who works at the Brookhaven National Laboratory. The general theme of these sessions was the study of nuclear reactions at high energies.

Prof. A. Turkevich of the University of Chicago presented a report on calculations of the intranuclear nucleon cascade—the first stage of nuclear interaction at high energies—by the Monte Carlo method. In these calculations, performed on an electronic counting machine at Los Alamos and continued later in the computing bureau of the Argonne Laboratory, the so-called Goldberger model, subsequently tested by Bernardini and a number of other authors, was substantially developed. In the calculations a three-dimensional nucleus was considered and relativistic kinematic relations were used. In this connection various Fermi distributions of protons and neutrons were assumed and (taking into account the Pauli principle) data on nucleon–nucleon interaction cross sections (elastic and with meson production) and meson–nucleon cross sections were used. The calculation of one case of nuclear interaction (i.e. the number of secondary particles and their angular and energy distribution) takes about 5 seconds. Tables containing the results of calculations of the intranuclear nucleon cascade for six nuclei (\(\mathrm{Al}, \mathrm{Cu}, \mathrm{Ru}, \mathrm{Ce}, \mathrm{Bi}, \mathrm{U}\)) and energies of bombarding nucleons from 82 to 2000 MeV have been prepared for publication. The data obtained agree well with experiments.

At present, calculations by the Monte Carlo method are also being carried out for the second stage of high-energy nuclear reactions—the stage of evaporation of nucleons. Some results of such calculations were presented in the remarkable report by G. Friedlander on reactions of protons of energy 3 Bev with heavy nuclei. This report also discussed the question of different mechanisms of fission of heavy nuclei when they are bombarded with high-energy nucleons, and gave excitation functions for the yields of \(\mathrm{Ba}^{128}\), \(\mathrm{Ba}^{140}\), and \(\mathrm{Mo}^{99}\) from uranium and bismuth in the range of energies of the bombarding protons from 0.5 to 3 Bev.

The third report on problems of nuclear reactions at high energies was read by the Swedish guest G. Rudstam. This report was devoted to deep-spallation reactions for nuclei \(V\), \(Mn\), \(Co\), and \(As\) under the action of 50–170 MeV protons. Rudstam proposed a rather convenient empirical formula for describing the mass spectrum of products of deep-spallation reactions. L. Winsberg’s report contained extensive material on studies of nuclear reactions at the Bevatron in Berkeley. Reactions with a change of nuclear charge \(\Delta Z = -2, -1, 0, +1\), and \(+2\) were analyzed on the basis of data on elementary processes of nucleon scattering and production of single \(\pi\)-mesons and their pairs. Mass spectra of products of bombardment of the nuclei \(Cu\), \(Ta\), \(Pb\), \(Bi\), and \(U\) with protons of energy on the order of a Bev were presented. The results obtained are well described by Rudstam’s empirical formula. The next speaker, N. Porile (University of Chicago), described studies of the interaction of 450-MeV protons with bismuth and tantalum nuclei. In these experiments the probability of transfer to the named nuclei of various excitation energies was established, and the experimental results were compared with the data of calculations by the Monte Carlo method. Then I. Russell reported on radiochemical analysis of fission fragments of \(\mathrm{U}^{238}\) upon capture of \(\pi^{-}\)-mesons, which showed the similarity of the mechanism of such fission to fission under the action of 130-MeV protons. The last, third, session on high-energy nuclear reactions, held under the chairmanship of Prof. A. Turkevich, was devoted to reports by Soviet participants in the conference. Prof. N. A. Perfilov reported on the work of his laboratory on the study of fission and fragmentation of silver and uranium under the action of high-energy protons. These studies were carried out with the aid of special photographic emulsions developed in the laboratory of N. A. Perfilov and distinguished by a combination of fine grain size and high sensitivity. The report aroused great interest; many questions were asked, especially concerning the properties of the photographic emulsions.

The content of my report consisted of work carried out in 1950–1951 by A. E. Ignatenko, A. I. Mukhin, V. S. Penkina, V. A. Shkoda-Ulyanov, and myself on a synchrocyclotron that then belonged to the Joint Institute for Nuclear Research. In this work we investigated the emission of secondary neutrons (with energies up to \(\sim 15\text{–}20\) MeV) from various nuclei—from beryllium to lead—under the action of primary neutrons with average energies of about 120 and about 380 MeV.

My report also elicited a number of questions. In addition, R. Peil of the Radiation Laboratory of the University of California reported on measurements analogous

...by us, carried out there in 1954 by V. Crandall and J. Millbarn and yielding similar results.

Subsequently Perphilov and I repeated our reports at seminars of the Radiation Laboratory in Berkeley and the National Laboratory in Brookhaven. In addition, at these seminars I reported the results of work on the investigation of photoneutrons carried out by P. S. Baranov, V. S. Roganov, and me at the FIAN synchrotron. All our communications will be published in the journals “Phys. Rev.” or “Rev. Sci. Instr.”

Two sessions of the conference were devoted to papers on the fission of nuclei. The chairman was R. Leachman from Los Alamos. R. Stokes from Los Alamos reported a number of new data on the thresholds and cross sections of fission of heavy nuclei. In particular, he presented data on the fission cross sections of uranium-238 by neutrons from threshold to 24 MeV, on the fission cross sections of uranium-233, uranium-235, and plutonium-239 by neutrons with energies from 2 to 10 MeV. He also reported interesting results on the determination of the thresholds of deuteron-induced fission of nuclei in experiments in which nuclei are bombarded by deuterons and coincidences are observed between the fission events and the protons emitted in the breakup of the deuterons. In connection with the expenditure of energy to break the bond of the neutron and proton in the deuteron, the kinetic energy of the captured neutrons may turn out to be negative in this way—it was, for example, found that the threshold for fission of uranium-235 corresponds to a neutron kinetic energy of approximately minus 1.5 MeV.

P. Fong gave a communication on calculations of the excitation energies of fragments in the fission of various nuclei. The purpose of these calculations is to continue Fong’s previously published attempts to explain the two-humped mass spectrum of fission fragments.

I. Halpern of the University of Washington reported on the basic data on fission of nuclei obtained at that university. Among these results, of special interest is the establishment of three maxima in the mass spectrum of fragments in the fission of radium by protons with an energy of 11 MeV, as well as a detailed study of the angular distribution of fission fragments of radium, bismuth, thorium, uranium, and other nuclei by 43-MeV alpha particles, 22-MeV deuterons, and 11-MeV protons.

In the report of E. Schéberger from the Argonne Laboratory it was shown that the ratio of the yields of the fragments Sr^89, Ag^111, Cd^115, and Sb^127 remains the same in the three resonance maxima of uranium-235 fission by neutrons with energies of 1.1, 3.17, and 9 eV.

The report by R. Paine of the University of California (Berkeley) was devoted to determining the average number of secondary neutrons emitted during spontaneous fission of nuclei from uranium to fermium, and to data on the distribution of the number of fission events by the number of secondary neutrons.

At the present time experiments have been begun and the first results obtained on determining the number of emitted neutrons in various elementary fission events; in these experiments, the relation between the number of neutrons and the energy of the fission fragments is established.

The report by L. Glendenin of the Argonne Laboratory was devoted to the formation of isomers among fission fragments. The author also reported data on the chemical separation of isotopes that give delayed neutrons during fission.

The last report devoted to the fission of nuclei was a review report on the state of the theory of fission, made by the Swedish scientist W. Swiatecki, who is now working in Berkeley.

On June 27 elections were held for chairman and vice-chairman of the Gordon Conference on Nuclear Chemistry for the coming year, 1958. As has already been said, the chairman of the conference this year was Earl Hyde from Berkeley. In previous years the conference chairmen were G. Seaborg (1952), C. Coryell (1953), G. Friedlander (1954), A. Turkevich (1955), and L. Yaffe (1956). The vice-chairman of the present conference was Prof. N. Sugarman of the University of Chicago. By open ballot D. Hysing from the Argonne Laboratory was elected chairman of the 1958 conference, and M. Perlman from Brookhaven vice-chairman.

Somewhat apart from the main program of the conference was E. Martell’s report “The Present State of the Strontium-90 Problem,” read in connection with the widespread fears throughout the world of the harmful consequences of the testing of atomic and hydrogen bombs. The speaker gave data on measurements of the amount of Sr^90 in air and soil samples in North and South America, in Japan, and on the eastern coast of the Mediterranean Sea. The results of analyses of the Sr^90 content in the soil of the USA (depending on the annual amount of precipitation) and in the milk of Chicago farms indicate a noticeable increase in the concentration of Sr^90 in recent years, marked by a large number of atomic tests.

The last three reports of the conference, read at sessions chaired by M. Perlman, were devoted to the problems of beta decay. R. Davis from Brookhaven spoke about his experiments on studying the conversion of chlorine-37 into argon-37 under the action of neutrinos emitted by the reactor of the Savannah River plant. Having achieved a sharp reduction in the background count of his installation (from 1000 per day to 7 per day, while counting from the effect of the decay of A^37, equal to 17 per day), Davis determined with good accuracy the cross section of this reaction: \(\sigma = (1.1 \pm 0.2)\cdot 10^{-45}\ \text{cm}^2\).

Since in the case where the neutrino and antineutrino were different particles (in particular, if the two-component theory of the neutrino is correct), this reaction should not occur at all, the result of Davis’s experiment is fundamentally important for the theory of the neutrino and, in general, for the theory of weak interactions. If this result is correct, then the neutrino, like the gamma quantum, appears in the full sense of the word to be a neutral particle. Moreover, in this case the mass of the neutrino need not be identically equal to zero, and double beta decay turns out not to be forbidden. However, the results of Davis’s experiments cannot be regarded as entirely convincing, since the background experiments were performed by him with the installations located in quite a different place; simply turning off the reactor for a long period to check the background could not be carried out because of the special purpose of the reactor. S. Treiman of Princeton University gave a detailed survey of the state of the theory of beta decay that arose in connection with the “overthrow of parity.”

At present, however, it is difficult to speak of any definite state of the theory of beta decay, since new experimental facts, sometimes of extraordinary importance, are continually accumulating.

The state of “storm and stress” in the field of beta decay was very well described in the report of V. Telegdi (University of Chicago), who has distinguished himself in recent years by a number of interesting experimental and theoretical works.

V. Telegdi reported on an extensive program of experiments on beta decay being carried out in various American laboratories. This program includes the following experiments:

1. Passing beta-decay electrons through an electric (magnetic) field with their subsequent scattering and thereby studying their polarization.

2. Scattering of beta-decay electrons and positrons on polarized electrons of iron (experiments of H. Frauenfelder).

3. Studying the angular correlation of $\gamma$-quanta from the annihilation of $\beta$-decay positrons in polarized targets.

4. Studying the annihilation of $\beta$-decay positrons in a gas situated in a magnetic field (experiments of L. Page). In this case the singlet and triplet states of positronium correspond to different angular distributions, so that it is possible to determine the fraction of one and the other states, i.e., the polarization of the $\beta$-particles.

5. Observation of the circular polarization of bremsstrahlung gamma quanta of beta-decay electrons (experiments of M. Goldhaber).

6. Studying the circular polarization of photons emitted in $\beta\gamma$-decay, i.e., in other words, the polarization of nuclei after emission of $\beta$-particles (experiments of F. Boehm and A. Wapstra).

7. Studying the beta decay of $Li^8$ nuclei polarized by capture of polarized neutrons in $Li^7$.

8. Studying the beta decay of $B^{12}$ nuclei polarized by capture of polarized $\mu^-$ mesons in $C^{12}$.

9. Observation of the angular distribution of protons and of the angular correlation of electrons and protons in the $\beta$-decay of polarized neutrons.

This extensive program has already yielded a number of interesting results. Sometimes such results are far from final and even contradictory. For example, V. Telegdi reported that Frauenfelder had found a practical absence of polarization of positrons of the $\beta^+$ decay
${}_{31}Ga^{66} \to {}_{30}Zn^{66}$ $(P = 0.1 \pm 0.3)$.

Since, according to recently obtained Berkeley results, the magnetic moment of $Ga^{66}$ (as well as of $Zn^{66}$) is zero, in this case a purely scalar Fermi $0—0$ transition should take place. From Frauenfelder’s experiments it was possible to conclude that parity is not conserved only in the Gamow–Teller variants of $\beta$-decay, whereas for the Fermi interaction the law of conservation of parity is valid.

A little later, at the Massachusetts Institute of Technology (MIT), I saw M. Deutsch, who described his experiments on the study of the polarization of $\gamma$-rays emitted in the annihilation of positrons of $\beta^+$ decay. Deutsch studied the passage of such $\gamma$-rays through iron layers of various thicknesses in the absence and in the presence of a parallel direction of the $\gamma$-rays and of the external magnetic field. It turned out that both in the case of $Ga^{66}$ and in another example of a $0—0$ transition ($Cl^{34}$), positrons were completely polarized. Thus the results of the experiments of Deutsch and Frauenfelder contradicted one another. Only at the beginning of August in Moscow did Prof. Ya. A. Smorodinskii, who had returned from Yugoslavia, inform me that during the summer seminar of nuclear physicists taking place there a telegram had been received from the USA stating that Deutsch was right, whereas a mistake had been found in Frauenfelder’s experiments. Thus it became clear that in the Fermi variant of beta decay as well, parity is not conserved.

In the course of the Gordon Conference, in the corridors we were also told of some new results obtained in different laboratories in the USA. Thus, for example,

mer, D. Heizinga told of the establishment at the Argonne Laboratory of the fact of the formation, in the $\alpha$-decay of the isotope $\mathrm{Pu}^{239}$, of $\mathrm{U}^{235}$ with an excitation energy of less than 50 ev ($I=1/2$, $T_{1/2}=26.3$ min.); C. Amiel of Berkeley reported on the form of the excitation functions of reactions induced by accelerated nitrogen ions.

At noon on June 28 the conference ended, and we began a two-week trip through the laboratories of the USA. Together with E. Hyde and Prof. C. Coryell (MIT) we drove in an open car about 200 km, from Meriden to Cambridge. Again we saw picturesque New England landscapes, with which during the course of the conference we had become acquainted almost every day on automobile outings between the morning and evening sessions. Passing through the beautiful valley of the Connecticut River, which separates the states of New Hampshire and Vermont, we then turned east and crossed the state of Massachusetts. The city of Cambridge is situated on the northern bank of the Charles River. Opposite Cambridge, on the southern bank, is Boston, where for reasons incomprehensible to us Soviet citizens are not admitted. In Cambridge there are two large educational institutions: Harvard University and MIT. We became thoroughly acquainted with the MIT synchrotron and with the work being carried out there. Explanations were given by Prof. D. Frisch. At the MIT synchrotron they had recently begun to work with a new injector—the Van de Graaff generator, whose use ensured a high stability of the accelerator’s operation and made it possible to increase severalfold the intensity of the $\gamma$-quantum beam (which, however, is still lower than at the FIAN synchrotron). For about a year we had already known D. Frisch by correspondence, through letters in which we exchanged reports on studies of the Compton effect on the proton. Naturally, on meeting we discussed the state of this work in detail. It was pleasant for me to learn that the rise of the scattering cross sections at an angle of $45^\circ$ in the region of low $\gamma$-quantum energies, which had been observed at MIT and had not been noticed by us, turned out to be connected with instrumental effects. Prof. D. Frisch became interested in the results of new calculations of the Compton effect on the proton carried out by the FIAN theoreticians A. M. Baldin and A. V. Petrunkin. We also met other scientists working at MIT: A. Wattenberg (many, probably, know from Laura Fermi’s book the story of the bottle of Chianti, on whose label, at Wattenberg’s suggestion, after the start-up of the first reactor all those present, headed by E. Fermi, signed their names), D. Ritson, and R. Williams. Dr. Williams told us that in connection with Lattes’s communication from Chicago that there exists a form of $\pi$-mesons with spin not equal to zero, he was engaged in checking whether asymmetry of $\pi\mu$-decay is observed. The study of $\pi\mu$-decay of stopped $\pi$-mesons in a propane bubble chamber seems not to confirm Lattes’s report.

Several methodological innovations that were demonstrated to us at MIT should be noted. For recording the energy of relativistic electrons, a gaseous Cherenkov counter is being developed, filled with a fluorine-containing organic liquid with refractive index $n=1.28$. By heating a thick-walled vessel containing the liquid, one can approach critical conditions and thus vary the refractive index (down to $n\sim1$) and the threshold of operation of the counter. Searches are under way for optimal methods of spectrometry of $\gamma$-quanta of high energies. Thus, for example, experiments have been begun with an enormous crystal of sodium iodide weighing 31 kg (and costing 7500 dollars). A promising method being developed at MIT is the creation of a $\gamma$-quantum spectrometer counter in which a chip or rod of lead glass, densely filling a large vessel with a 20–30 liter volume, is flooded with a scintillating organic liquid of high refractive index, for example $\alpha$-fluoronaphthalene ($n=1.62$), with the addition of $\alpha$-NPO (15 g/l). In such a detector the advantages of shower Cherenkov counters made of lead glass (small radiation length) are combined with a relatively large light output, due not to Cherenkov radiation but to scintillations. We were shown new photoelectron multipliers, FEU-6810 (similar to our FEU-19) and FEU-RCA—C—7170. In the latter model the photocathode is made concave from inside, which made it possible to obtain a sufficiently short resolving time of the FEU ($\Delta t \approx 2\cdot10^{-9}$ sec.), despite the relatively large diameter of the photocathode (12.5 cm). Very attractive is the method used at MIT for working with liquid hydrogen and deuterium. Instead of preparing and transporting vessels for liquefied gases, they use a thin-walled metal cup connected through a coil to cylinders containing gaseous hydrogen and deuterium. The liquefaction of these gases takes place in the coil, which is continuously flowed around by freely evaporating liquefied helium. Such a method of liquefying hydrogen and deuterium is possible, of course, only thanks to the special cheapness, in the Boston area, of liquid helium, a liter of which costs there only two dollars.

On the same day (June 29), when we were at the MIT synchrotron, we also visited Prof. M. S. Livingston (Harvard University), who is now heading the construction of an electron synchrotron with strong focusing at 6–7.5 Bev, which he expects to put into operation in January 1960. Prof. Livingston told us and other guests in detail about the design of his accelerator.

After meeting with Prof. V. Weisskopf and inspecting the MIT reactor, we attended a luncheon given by the MIT club in honor of all the foreign participants in the Gordon Conference on Nuclear Chemistry. Invitation tickets to this meeting, whose organizers included C. Coryell, V. Weisskopf, and M. Deutsch, were handed to us while still in Meriden, and we were pleasantly surprised to find that an appendix to the tickets contained a bibliography of our published works.

The next morning, together with Earl K. Hyde from Cambridge, we drove across all of Boston to the airport and, after a ten-hour flight, found ourselves in San Francisco. From the airplane, the entire land of the United States gradually opened before us, from the Atlantic to the Pacific coast: first the states of New England, then, quite far off, the largest tributary of the Niagara Falls, Lake Erie with the cities of Buffalo and Detroit, Lake Michigan and Chicago. A brown snakelike ribbon below was the Mississippi River. Then came America’s granary—Iowa, Nebraska, Wyoming. In the state of Utah a large salt lake attracted special attention. Then came the bare land of Nevada, and beyond the Sierra Nevada mountains, in which our northern Lake Tahoe, which resembles Sevan, is located—the green California, and, finally, the scattering of houses of Oakland and San Francisco on the shore of the boundless Pacific Ocean.

Having crossed from the San Francisco airport over the bay of the same name by the well-known Bay Bridge, we soon reached Berkeley, where on the following two days—July 1 and 2—we inspected the Radiation Laboratory of the University of California.

On the morning of July 1 we were cordially received by Glenn Seaborg, already personally known to us from the Gordon Conference; he introduced us to the head of the Physics Division of the laboratory, E. McMillan, to the head of the Chemistry Division, I. Perlman, and later also to the director of the laboratory, E. Lawrence. The greater part of the time was taken up by visits to four accelerators of the Radiation Laboratory.

E. Lofgren demonstrated to us the bevatron, which, until the startup of the new synchrophasotron at Dubna, had been the largest accelerator in the world. Shortly before our arrival in Berkeley this machine had resumed operation after a four-month shutdown for preventive maintenance, repair of the cooling system, and installation of new targets allowing simultaneous operation with different beams. The maximum proton current is now about \(1.2 \cdot 10^{11}\) particles per pulse. It was very pleasant for me to meet again E. Segrè and O. Chamberlain, already known to me from last year’s Moscow conference. They told us about the experiments, then beginning, with a purified beam of antiprotons, obtained by passing a mixture of negative particles through the system magnet—lens—absorbers—second lens—second magnet. In August it was planned to carry out the first irradiation by such a beam of L. Alvarez’s hydrogen bubble chamber. Since this chamber had already come up, one cannot fail to mention the installation for automatic processing of the photographs obtained in the chamber, which A. Rosenfeld showed us. The operation of the installation is based on the use of photosensitive elements and the application of electronic computing technology. The operator’s task is reduced merely to selecting, on the photographs, the particle tracks to be analyzed, setting the “finder” at the beginning of a track, and stopping the movement of the “finder” at the end of the track. In less than 10 minutes, two “Frankenstein” machines [which select on the cards the coordinates of a series of points along the track, the angles between the direction of the track and the tangent at these points, and the radius of curvature of the track] and an IBM-650, which processes the “Frankenstein” data, provide all information on the masses and energies of the primary particle and of all particles—products of the nuclear interaction observed in the photograph. At present the staff members of the chamber group are working on increasing the speed of processing photographs to 30 tracks per hour, and also on solving the problem of the neutral particles that carry away part of the momentum: the machines must “learn” to count what the mass of such particles is (if it is assumed that in each case there is only one neutral particle).

The installation for automatic processing of observational data with a bubble chamber is undoubtedly a major methodological achievement. Among other methodological novelties one may also note the Cherenkov counter, about which E. Segrè told us, with sensitivity limited in energy not only from below but also from above, owing to the fact that for particles of high energy the condition for total internal reflection of Cherenkov light is violated. Very useful for work with various vacuum installations, gas-discharge counters, etc., seems to me the “Mylar” plastic foil widely used in the USA, possessing high mechanical strength: a sheet of foil 25 microns thick cannot be torn by hand.

The second accelerator of the Radiation Laboratory—the proton synchrocyclotron, shown to us by Thornton—had been thoroughly rebuilt and had only recently begun to operate in the new regime. At present, on this accelerator, protons reach

energy of 710 MeV, the gap between the poles of the magnet is 35 cm, and the magnetic field reaches 23 thousand gauss. In addition to protons (whose extracted current is \(3\cdot 10^{-8}\) A), deuterons, He\(^3\), and He\(^4\) will be accelerated; along with the two proton beams, neutron and \(\pi\)-meson beams will be used.

R. Kenney showed us the synchrotron of the Radiation Laboratory, on which work is being carried out with a beam of bremsstrahlung radiation up to 340 MeV (photoproduction of \(\pi^+\)-mesons on hydrogen and deuterium, scattering of \(\gamma\)-quanta up to 140 MeV on hydrogen, production of pairs on electrons).

In conclusion of our tour of the accelerators, E. Hubbard and G. Seaborg demonstrated to us a linear heavy-ion accelerator with a waveguide about 3 m in diameter, which began operating on April 11, 1957. The energy of the accelerated ions (C, N, O, Ne) is approximately 10 MeV per nucleon, and the peak current of the accelerated ions (over an area of only \(2\times 6\ \mathrm{mm}^2\)) is 3–4 μA. The principal task of work on the new accelerator is the production of transuranium elements. It should be noted, however, that, as Prof. Seaborg told us, the ratio of the yields of fission and deep spallation reactions rises sharply in going from lighter to heavier bombarding particles. Therefore Seaborg considers the bombardment of relatively light “projectiles” by as heavy targets as possible to be a more promising method for obtaining new transuranium elements. Seaborg informed us that, “according to rumors,” a new transuranium element—No. 102—had been obtained in Stockholm by irradiating Cm\(^ {244}\) nuclei with C\(^ {13}\) ions:

\[ {}_{96}\mathrm{Cm}^{244}+{}_{6}\mathrm{C}^{13}\to 4(6)n+102^{253(251)} . \]

Soon after this, on July 9, the synthesis of element No. 102 was officially announced in the press and over the radio*).

During our visit to the Chemical Division of the Radiation Laboratory we became acquainted with many of its workers who had made an active contribution to the synthesis of transuranium elements: S. Thompson, A. Ghiorso, and B. Cunningham. Among the equipment used in work with the new elements, special mention should be made of the ultraprecise balances used in experiments to determine paramagnetic susceptibility in microsamples (\(\sim 6\ \mu\mathrm{g}\)) of curium. These unique balances make it possible to measure forces down to \(10^{-9}\), and with an accuracy down to \(10^{-11}\) g. To measure the thermal effects of various chemical reactions of the actinides (for example, the heat evolved when a microdose of a substance dissolves in acids), a calorimeter of very small heat capacity and excellent thermal insulation is used, allowing temperature changes down to \(10^{-5}\) degrees to be registered. We also examined the hot laboratory, boxes for work with fairly high (although below the level of the hot laboratory) activities, and laboratory ion-exchange columns for separating microquantities of compounds of the transuranium elements.

The technique of alpha- and beta-spectroscopy, demonstrated to us by I. Perlman, D. Rasmussen, and D. Hollander, is at a high level in the Chemical Division of the Radiation Laboratory. In Perlman’s group, the presence of an isomer of Pu\(^ {237}\) with a half-life of 0.2 sec and a level energy of 145 keV was recently discovered. As we were told, the existence of another isomer recently discovered in the laboratory had been predicted by the Soviet physicist L. L. Goldin. Nuclear spectroscopists in Berkeley are thoroughly familiar with the work of S. A. Baranov and K. N. Shlyagin on the electron spectra of heavy nuclei.

On July 3 we visited Stanford University in Palo Alto (California) and inspected there a linear electron accelerator on which R. Hofstadter and V. Panofsky work.

Before the tour began, we were introduced to the head of the University Physics Department, F. Bloch, and to T. D. Lee, who had come to Palo Alto to deliver three lectures on the problem of nonconservation of parity. In the corridor of the Physics Department we saw an announcement for the next seminar: two papers by V. B. Berestetskii from Nuclear Physics were to be reported.

As in all the laboratories where we had been, in Stanford we were treated to lunch. This offering of lunch, during which the employees who have only just met them converse with the guests in an entirely informal and easy manner, is a kind of custom. Almost everywhere in the laboratories the dining rooms operate on a cafeteria system, and together with our hospitable hosts we took trays, moved them along the rails by the long counter, and chose our food, and after lunch collected everything ourselves and returned it to the kitchen.

We arrived at Stanford University from Berkeley only by noon, and the traditional lunch served here not as an interruption but as an introduction to the tour of the laboratory. V. Panofsky, already known to Soviet physicists from the 1956 Moscow Conference, showed us the accelerator, after which R. Hofstadter related

) See the note in the present issue of UFN*, p. 825.

on the latter and, it must be said, very interesting results of his studies of the scattering of high-energy electrons (500–600 Mev) by nuclei. Comparing the data on the total cross sections for the scattering of electrons by protons and deuterons, Hofstadter established that the radius of the neutron (if one assumes for it the same structure as for the proton) is \((0.50 \pm 0.10)\cdot 10^{-13}\) cm, whereas the radius of the proton is equal to \((0.77 \pm 0.05)\cdot 10^{-13}\) cm. It follows from this that either the neutron is a more compact formation than the proton, or the charge distribution in the neutron is characterized by a positive maximum closer to the center and a negative maximum near the edge. The study of electron scattering by nuclei in which the first \(p\)-shell is being filled (Li, Be, B) showed that these nuclei, in contrast to others, have a characteristic increase of charge density toward the edge of the nucleus, which agrees with the idea of their shell structure. After R. Hofstadter’s account of his work, we met again with V. Panofsky in order to hear about his new physical results.

One of Panofsky’s new works was devoted to the search for possible new weakly interacting particles with masses between 20 and 350 electron masses. In these experiments a counter with a large light aperture was used (thanks to a magnet having focusing in two directions and in energy). No new particles were found. At present the production of positrons at an angle of \(90^\circ\) under bombardment of protons by gamma quanta with energies up to 150 Mev is being investigated.

By comparing the experimentally measured photofactor for the proton, obtained from Hofstadter’s experiments, Panofsky hopes to obtain information about deviations of the experimental results from the formulas of quantum electrodynamics.

Methodologically close to this work is the study of the inelastic production of \(\pi^+\)-mesons by electrons on hydrogen. The spectra of secondary electrons for an angle of \(75^\circ\) are now being investigated, and the experimental data are being compared with Chu and Low’s theory of meson photoproduction.

At the Stanford accelerator the production of \(\pi^+\)-mesons on hydrogen by polarized gamma rays is also being studied (Mozley), as well as the influence, predicted by M. Ter-Mikaelian in the USSR, of atomic structure on the spectrum of bremsstrahlung arising in single crystals (copper).

The last of Panofsky’s present works is devoted to the study of the angular distribution of neutrons in the capture of \(\mu\)-mesons by deuterium atoms (it is assumed that capture occurs on an individual proton, while the nucleus plays the role of observer).

On the evening of July 3 we returned to Berkeley. Holidays were beginning (July 4 is celebrated in the USA as Independence Day), and therefore the next three days were free from visits to laboratories.

On the morning of July 4 we drove by car together with E. Hyde, J. Hollander, and V. Svyatetsky 400 km from Berkeley—to Yosemite National Park, situated near the border of the states of California and Nevada, in the foothills of the Sierra Nevada. In the program of our trip through the USA, sent to us back in May, the days of the trip to Yosemite were listed as an “inspection of the picturesque places of California.” And indeed, Yosemite Park is an extraordinarily picturesque place, reminiscent of our Caucasian beauty. At an elevation of about 1200 m above sea level lies the Yosemite Valley, surrounded on three sides by cliffs a kilometer high; in the center of it is Yosemite Village, with houses for visitors and a restaurant. Here we spent the first day of the trip. Rich vegetation, including the famous sequoias, the transparent and cool mountain Merced River, whimsical cliffs around, and the many animals inhabiting them, which are evidently used to the customs of the preserve and not afraid of people—bears, does, deer, squirrels—all this gives the locality a distinctive charm. On the second day of our stay in Yosemite we climbed another kilometer, to Glacier Point, and viewed from above Yosemite Valley and the mountains of the Sierra Nevada, whose peaks are covered with snow. On our last day in California, July 6, we spent time seeing San Francisco, perhaps the most distinctive of American cities, connected for us with the name of Jack London.

On the morning of July 7, Earl Hyde and his wife took us to the San Francisco airport, and by the middle of the day we arrived in Chicago. For the first time during the trip we began to rearrange our watches forward and changed from a ten-hour to an eight-hour difference with Moscow time.

On July 8 we were at the Enrico Fermi Institute for Nuclear Studies at the University of Chicago. Opposite the institute stands the historic building where, on December 2, 1942, Enrico Fermi and his collaborators started the world’s first nuclear reactor. To our surprise, we did not find on the building the memorial plaque, a photograph of which is given in Laura Fermi’s book Atoms in the Family. We were even more amazed when we learned that the plaque had been removed because the building itself is to be demolished in the near future.

The main part of our stay at the Fermi Institute we devoted to becoming acquainted with the work on elementary-particle physics being carried out on the institute’s synchrocyclotron by the groups of Prof. R. Hildebrand and Prof. V. Telegdi.

Among the works now being prepared by Prof. Hildebrand, the following should be mentioned:

1. Investigation of the capture of \(\mu^-\)-mesons in hydrogen and deuterium with the aim of determining the fraction of capture in deuterium that occurs with spin flip.

2. In this same work (Hildebrand’s experiments will be carried out in a hydrogen bubble chamber 25 cm in diameter), the angular distribution of electrons from the decay of \(\mu^-\)-mesons, especially in the region of low energies, will be studied.

3. Electrons and positrons from the internal conversion of \(\pi^0\)-mesons in the reaction

\[ \pi^0 \to \gamma + e^+ + e^- . \]

1. The planar distribution of electrons and positrons in the reaction

\[ \pi^0 \to 2e^- + 2e^+ \]

will be studied in order to determine the intrinsic parity of the \(\pi^0\)-meson.

1. The theorist I. Nambu has predicted the existence of a new neutral meson with a mass 2–3 times greater than that of the ordinary \(\pi^0\)-meson (the purpose of this prediction is to explain the differing results of Hofstadter’s experiments on the structure of protons and neutrons). An attempt will be made to discover this meson.

Among the works carried out by Prof. Telegdi, the following should be noted:

1. Study of the decay of \(\mu^-\)-mesons (into an electron and 2 neutrinos) in various substances (with different degrees of depolarization being observed).

2. Restoration of the polarization of \(\mu^+\)-mesons, which disappears because of the formation by \(\mu^+\)-mesons of mesonium, through splitting of the triplet state of mesonium by the action of an external magnetic field.

3. Measurement of the spectrum of electrons from \(\mu\)-meson decay for different orientations of the external magnetic field (different directions of precession of the spin of the \(\mu\)-mesons).

4. Study of the capture of \(\mu^-\)-mesons in 37 different elements and their compounds. Analysis of these results led Telegdi to the conclusion that the mechanism of the interaction of \(\mu\)-mesons with nucleons is the same as the mechanism of the interaction of beta particles with nucleons. Contrary to the Fermi and Teller theory, it was established that the fraction of \(\mu\)-meson capture in different atoms in mixtures of different elements is proportional only to the atomic concentration and does not depend on the atomic number.

Lack of time did not allow us to inspect in comparable detail the other laboratories of the Fermi Institute, in which major scientific forces are concentrated. We visited only briefly the director of the institute, Prof. S. Allison, who is engaged in the study of reactions under the action of accelerated heavy ions; the laboratory of Prof. K. Hutchison, who studies the paramagnetism of transuranium elements; the “offices” (including the office of the head of the laboratory and the room where the secretary is located—it is also the operator’s and stenographer’s room) of the radiochemical laboratories of Professors N. Sugarman, A. Turkevich, and E. Anders. Only at lunch were we able to talk with the theoretical physicists of the University of Chicago M. Gell-Mann and J. Mayer, G. Wentzel, M. Goldberger, and I. Nambu.

On July 9 we visited the Argonne National Laboratory in Lemont (near Chicago), where we were received by Russian-speaking visitors.

In this laboratory we were shown a heavy-water power reactor that supplies the laboratory with energy, and the CP-5 pile for experimental investigations. These installations were shown to us by M. Hamermesh. The laboratory carries out precision measurements of cross sections by the neutron time-of-flight method. The length of the neutron flight tube in these measurements is 120 m, so that the energy of the neutrons for which data are obtained reaches 10 keV.

At present, a 1000-channel neutron time-of-flight analyzer on ferrite rings has begun operating in the laboratory; it was built on the principle of the 256-channel analyzer demonstrated in Geneva in 1955 and is being put into mass production.

Another installation—for studying the spectrum of \(\gamma\)-rays from radiative capture of neutrons—contains a quartz crystal of size \(30 \times 30 \times 0.4\ \mathrm{cm}^3\) and makes it possible to measure the energy of gamma quanta in the region up to 7 MeV. The reflection angle of gamma quanta with an energy of 7 MeV is 2 min, whereas the angular resolving power is 10 sec.

Among the experiments now beginning at the Argonne Laboratory, of particular interest is the study of beta decay of polarized neutrons (this same experiment is now being done by Robson in Canada). At the Argonne Laboratory a completely polarized neutron beam has already been obtained by reflecting them from a large cobalt mirror.

Among the fission studies being carried out at the Argonne Laboratory, mention should be made of the observation of coincidences of prompt gamma rays with fission fragments of a definite energy. The low cost of liquid helium makes it possible to carry out a number of experiments on the measurement of cross sections either by activation of isotopes at ultralow temperature, when the activation cross section is far higher than under ordinary conditions.

In the Physical Laboratory a large mass spectrograph with a radius of curvature of 254 cm and double focusing, shown to us by V. Manning, has been built.

This instrument can be used for isotope separation both in preparing targets and in determining the nature of irradiation products (for effective isotope separation, samples of \(10^{-15}\)–\(10^{-16}\) g are sufficient).

It should be noted that isotope separators exist in all large laboratories, which substantially facilitates the task of studying various nuclear reactions and isolating new isotopes.

At Argonne Laboratory there is also a large computing center equipped with two electronic computers. Calculations of nuclear interactions by the Monte Carlo method, previously carried out at Los Alamos, are being transferred to this center. Prof. A. Turkevich explained to us in general terms the nature of these calculations.

Straight from Argonne Laboratory we went to the Chicago airport for our last flight in America, this time to New York. Of the four remaining days of our stay in the USA, we spent two days seeing New York, and two days visiting Brookhaven National Laboratory, where we went on July 11 together with Prof. G. Friedlander, who was serving as head of the Chemistry Division of this laboratory. Brookhaven is located on Long Island, approximately 110 km from New York.

Upon our arrival at Brookhaven we found stenographic transcripts, kindly sent to us by Prof. R. E. Marshak from Rochester, of the Seventh Annual Rochester Conference on High Energy Physics, which had taken place in April of that year.

We began our tour of Brookhaven Laboratory by visiting the buildings of the 25-Bev proton accelerator under construction, which were shown to us by one of the authors of the idea of accelerators with strong focusing, H. S. Snyder. The ring tunnel for the foundation of the accelerator, 800 m long, and the building above the tunnel are already completed. In January 1958 assembly of the 240 magnet sections is to begin; these will be assembled on iron piles independent of the building, driven into the ground to a depth of about 17 m. Alongside preparations for the start of magnet assembly, development of the high-frequency accelerating power-supply system is under way.

After the seminar, at which we gave talks, the head of the laboratory’s Physics Division, Professor S. Goudsmit, introduced to us a number of collaborators who wished to discuss particular questions. M. Barton of the University of Illinois told me about data obtained there on the photodisintegration of helium at energies of 140–280 Mev; M. Moravcsik reported his considerations concerning the photoproduction of \(\pi\)-mesons. Meanwhile J. Hornbostel and E. Salant demonstrated to N. A. Perfilov a series of photographs obtained with a spark chamber.

The following day we visited the cosmotron, for which explanations were given by Prof. Lyle Smith, known to our physicists from his visit to Moscow in the spring of 1956. For almost six months the cosmotron had not operated after damage to the winding and a fire. By the time of our arrival the repair had already been completed and resumption of regular operation of the accelerator was near. The cosmotron injector, a Van de Graaff generator, was already producing a current of 10 mA, while the number of accelerated protons per pulse reached \(10^{11}\); it was expected that this number would be increased by another order of magnitude. Work was under way to stretch the pulse duration from 0.1 to 1 sec.

A significant part of the work on the cosmotron will be carried out with extracted proton beams (the current in such a beam, 3 cm in diameter, is about 50% of the internal current), and also with \(\pi\)- and \(K\)-mesons. In addition to continuing the work interrupted by the breakdown of the accelerator (for example, experiments on the production of \(K^+\)-mesons in pp collisions, studies of various interactions of \(K\)-mesons in J. Steinberger’s bubble chamber, etc.), it was intended to begin new experiments at the cosmotron as well. Thus, V. Fitch proposed to study, by the anisotropy of \(\mu e\)-decay, the polarization of \(\mu\)-mesons formed in the decay of \(K\)-mesons.

At the Brookhaven cosmotron a group of researchers from Columbia University is now working, since the synchrocyclotron there has recently gone out of operation because of an accident. This group, headed by L. Lederman and C. Tinlotzky, determined the gyromagnetic ratio for the \(\mu\)-meson with high precision: \(g = 2(1.00112 \pm 0.0007)\), with a contribution of 0.0005 to the total error coming from the inaccuracy in the mass of the \(\mu\)-meson. The same group is studying the long-lived \(K^0\)-particles, has observed up to 150 cases of their decay, and has established that their lifetime lies between \(10^{-8}\) and \(10^{-6}\) sec.

On the evening of July 12 we returned from Brookhaven Laboratory to New York, and the next evening found us already over the Atlantic Ocean, in an airplane, on the way to Paris.

After the holidays of July 14 and 15, we availed ourselves of the opportunity, kindly provided to us by Prof. J. Teillac and Prof. M. Gaisiński, who had been guests in Moscow the previous year, to become acquainted with several French laboratories. On July 16 we were at the Institut du Radium in Paris, and on July 17 at the construction site of the new scientific research atomic accelerator center at Orsay (about 25 km from Paris).

At the Radium Institute in Paris, extensive work is being conducted on the chemistry of naturally occurring radioactive elements and radiation chemistry.

Among the physical studies of this institute, mention should be made of research on nuclear spectroscopy and experiments on the nuclear scattering of nucleons and gamma rays. A broad program of research has been planned in connection with the construction of the research center at Orsay, begun a year and a half ago. We were the first foreign visitors at Orsay. At present the construction of a 150 MeV synchrocyclotron is nearing completion there; the 700-ton magnet has already been assembled. In the same building a cyclotron of F. Joliot-Curie, which will be modified and will operate at different frequencies in order to accelerate protons, deuterons, and heavy ions to various energies, is being moved there. In this same building an isotope separator is being assembled for obtaining various targets and for isolating the products of bombardment of atomic nuclei (up to several milligrams of light-nucleus isotopes per day). The main building is beginning to assemble a linear electron accelerator of the type of the accelerator at Stanford University. At the end of 1958 this accelerator is expected to be put into operation at an energy of 250 MeV, at the end of 1959—at 500 MeV, and at the end of 1960 at 1 BeV. The construction is financed by the French government, and at present the cost of construction is estimated at 5 billion francs. The atomic center at Orsay is a branch of the Sorbonne University in Paris.

On July 18 of this year we flew from Paris and returned to Moscow.

In conclusion, it is necessary to dwell on several general features characteristic of the American scientific research laboratories we inspected.

First of all, it should be noted that work in the field of nuclear physics and high-energy particle physics is being carried out on a very broad front.

A characteristic feature of the large research centers is the concentration in these institutions not only of purely physical work, but also of various kinds of chemical, biological, and medical research.

It is necessary to emphasize the very close contact among the various scientific research institutes of the USA: physicists often visit Physical Society meetings in various parts of the country, conferences, mutual visits with lectures, correspondence, and exchange of preliminary reports. Completed work is published very quickly. As one of the merits, one should note the rapid industrial assimilation of the newest apparatus, the rapid transfer of laboratory novelties into mass production. Critical technology is widely used in laboratory practice.

A considerable part of the experimental research is carried out by senior students, so that the number of scientists with academic degrees working on the various accelerators is relatively small.

It should also be noted that experimental physicists have a high level of theoretical training.

It is characteristic that all experimental physicists, even those of the highest ranks and titles, continue themselves to participate directly in physical experiments.

Since American physicists draw a large part of their information from personal communication with various scientists, from oral reports, they read the scientific literature relatively little. Recently, however, their interest in the scientific literature has greatly increased in connection with the rapid development of Soviet science and the Americans’ desire to be aware of its outstanding achievements.

As is known, the principal Soviet physics journals are now translated into English and published in the USA. However, under such a system there is always a delay of several months in the receipt of information. Therefore there is now a strong desire to study the Russian language; circles for learning Russian exist at many universities and laboratories.

American scientists received us quite hospitably and benevolently. They warmly support the need to strengthen peaceful coexistence, to improve Soviet-American relations, and to develop cultural intercourse between our countries in every possible way. The opinion of the majority of American scientists undoubtedly coincides with the words of one prominent physicist, spoken by him when he showed us his installation, assembled on the rotating mount of a naval artillery gun: “This is the best and most correct way to use such devices.” Scientists in the USA expressed a sincere desire to see Soviet guests in their country in the future, to visit us, to attend scientific conferences, and to exchange the latest scientific results. The trip to the USA and personal contact with many American scientists working in the field of nuclear physics and radiochemistry strengthened our confidence that scientists of all countries, in their overwhelming majority, do not wish to contribute to war hysteria, want peace and progress, want their creative discoveries not to inflame enmity but, on the contrary, to pave the way toward friendship and cooperation among peoples.

V. I. Gol’danskii