Atomic Energy, Vol. 86. No. 6. 1999

H I G H - E N E R G Y N U C L E A R P H Y S I C S AND N U C L E A R

A S T R O P H Y S I C S AT T H E R A D I U M I N S T I T U T E

O. V. Lozhkin

Research into high-energy nuclear physics and nuclear astrophysics at the Radium Institute is briefly

reviewed. The history of cosmic-ray research is outlined, from the early years of the Institute, as well as the

history of research on high-energy physics. The basic work on nuclear astrophysics, cosmochronology, and

astrochemistry is described. Research at the Institute on direct nuclear reactions, nuclear fragmentation and

multifragmentation, deeply inelastic nuclear processes, nuclear fission at high exciting energies, nuclide

formation at the limit of stability, multiple particle formation in relativistic internuclear reactions, and other

topics is considered Scientific and methodological accomplishments are noted.

At the beginning of the 1920s, the Radium Institute was one of the few scientific establishments investigating the atom-

ic nucleus and the nature of ionizing radiation in the atmosphere. In these years, nuclear physics was in its infancy, and the exis-

tence of cosmic rays of extraterrestrial origin was still doubted by some scientists. In physics, only very rudimentary ideas had

been formulated regarding the structure of the atomic nucleus, which was discovered by Rutherford in 1919, and it was still 10

years before the discovery of the neutron (Chadwick, 1932) and the proton-neutron model of the nucleus (Ivanenko, 1932).

At this period, research in the fields now known as high-energy nuclear physics and nuclear astrophysics (including

cosmic rays, astrochemistry, and cosmochronology) was under development. In the early 1920s, under the leadership of

L. V. Mysovskii, experimental research into ionizing radiation in the atmosphere began. The latitude dependence of the radia-

tion intensity was studied, the attenuation factors of radiation in water (experiments at the Lake Onega at a depth of up to 10 m),

in ice, and in lead were measured. The directionality of radiation was investigated (experiments with the water tower at the

Leningrad Polytechnic Institute as a filter), and the barometric effect was first observed (L. V. Mysovskii and L. R. Tuvim, 1926).

The data obtained confirmed that extraterrestrial cosmic rays existed and that their absorption in matter differed greatly from

7-ray absorption. In 1928, Mysovskii published the first Soviet monograph on cosmic rays, in which these results were outlined.

Measurements at great height by Institute researchers were of great importance. S. N. Vernov developed and implemented a new

method for observing cosmic rays in the upper layers of the atmosphere, using radioprobes at different latitudes, including the

tropics (1934). A. B. Verigo conducted celebrated observations of cosmic rays during the flight of the "SSSR 1-bis" stratospheric

balloon (1935), and measured the latitude effect at high latitudes during an icebreaker expedition to Franz Josef Land.

During the Institute's first decade, G. A. Gamow explored the quantum-mechanical theory of (z decay as a tunnel effect

(1928), which was of great importance for nuclear physics and nuclear astrophysics. This work, like the contemporary research

of R. Gurney and E. Condon, permitted the hypothesis that low-energy nuclear processes in stars were the source of their lumi-

nescence. In 1932, Gamow published the first Soviet monograph on the structure of the atomic nucleus and radioactivity. In

1935, L. V. Mysovskii published a monograph on new ideas in nuclear physics. A new method developed at the Radium Institute was important for research on the interaction of cosmic rays with

atomic nuclei: the method of thick-layer photoplates, which was proposed by Mysovskii for (x radiation in 1925. A manufac-

turing technology for photoemulsion with a high silver-bromide concentration in a layer of thickness 50 I.tm was developed, and

(x particles were recorded (L. V. Mysovskii and P. I. Chizhov, 1927). This marked the beginning of the wide use of the nucle-

ar-photoemulsion method, which is still an effective research tool in the physics of nuclei and elementary particles. Beginning

in the 1930s, photoemulsion was successfully used to investigate the interaction of cosmic rays with atomic nuclei. From the

V. G. Khlopin Radium Institute. Translated from Atomnaya l~nergiya, Vol. 86, No. 6, pp. 419--426, June, 1999.

392 1063-4258/99/8606-0392522.00 �9 1999 Kluwer Academic/Plenum Publishers

very first, interesting characteristics of nuclear fission under the action of cosmic rays were noted: the great multiplicity of sec-

ondary particles and the presence of secondary particles with a charge greater than that of helium (I. I. Gurevich, A. R Zhdanov,

and A. N. Filippov, 1938). Unusual cascades consisting of a large number of charged particles in photoemulsion were investi-

gated (A. R Zhdanov, 1939), as well as the complete fission of silver nuclei (A. R Zhdanov, N. A. Perfilov, and M. Yu. Deizen-

rot, 1944). In the 1940s, researchers at the Radium Institute looked for nuclear fission with the capture of negative mesons (dis-

covered by Anderson and Neddermeier in 1938) by nuclei in photoemulsion, in order to estimate the meson mass from the ener-

gy balance in the momentless fission observed. A mass of 140--200 m e was obtained, where m e is the electron mass

(R I. Lukirskii and N. A. Perfilov, 1945), even before the discovery of the pions in 1947. The first observation of nuclear fission

in emulsion on absorption of a heavy meson (mass around 1220 m e ) was reported, along with the existence of a neutral meson

of mass 400 m e . In recent years, research into the nuclear reactions of cosmic radiation has significantly expanded. Interesting

results have been obtained on the multiplicity of particle creation, the formation of helium isotopes in cosmic rays, and the angu-

lar distributions of particles in proton and c~-nuclear cascades.

Original work on nuclear interactions at relativistic energies was formulated at the Radium Institute, until cosmic-ray

research was unexpectedly shut down in 1964. However, with the onset of the space race, the predicted influence of the influ-

ence of cosmic rays on humans and equipment in space was confirmed. With this, cosmic-ray research resumed at the Institute.

The new research conducted at the Institute in the late 1960s, in collaboration with the Institute of Medicobiological Problems,

covered many aspects of the interaction of cosmic rays with matter and the dosimetry of radiation in spacecraft and in open space

(V. E. Dudkin, Z. S. Khokhlova, N. S. Shimanskaya, N. R. Novikova, V. I. Zakharov, N. P. Kocherov, R. M. Yakovlev, and oth-

ers). In 1968, this work was reviewed in the world's first monograph on nuclear reactions and spacecraft safety (E. E. Kovalev,

V. E. Dudkin, O. V. Lozhkin, V. I. Ostroumov, et al.). On the basis of research on the background, a monograph was published

on the natural neutron background of the atmosphere and the Earth's crust (G. V. Gorshkov, V. A. Zyabkin, N. M. Lyatkovskaya,

and O. S. Tsvetkov, 1966). Neutron formation under the action of cosmic rays and at the Earth's surface and at depths of up to

I000 m water (equivalent) was investigated. A method of measuring the cosmic radiation that causes sparks in astronauts' eyes

was developed on the basis of AgCI single crystals produced at the Institute (N. P. Kocherov and A. V. Voronov); instruments

for measuring the neutron background of planets in the solar system during spacecraft flights were developed (R. M. Yakovlev,

V. G. Lyapin, and I. O. Tsvetkov).

Cosmic-ray research is not the Institute's only contribution to nuclear astrophysics. Fundamental research on nuclear

geochronology and the development of nuclear methods of determining the age of geological formations, organized by

V. I. Vernadskii and V. G. Khlopin, led to the establishment of both cosmochronology and isotopic astrochemistry, which plays

an enormous role in modem cosmology. At the Radium Institute, new nuclear methods - the argon and xenon methods - of

determining the age of geological formations were developed. Since 1935, the laboratory on radioactive determination of the

age of rock has been in operation, under the supervision of I. E. Starik. For many years, the Radium Institute was the leading

Soviet center determining the age of rock by radioactive and isotopic methods. In 1961, Starik published a monograph on

nuclear geochronology.

Since 1953, a project has been underway at the Institute to determine the age of meteorites. The distribution and iso-

topic composition of uranium and lead in various classes of meteorites have been measured (I. E. Starik, K. A. Petrzhak,

M. M. Shats, t~. V. Sobotovich, M. A. Bak, A. V. Lovtsyus, G. P. Lovtsyus, I. N. Semenyushkin, and others). Data have been

obtained on the age of a large number of rock and iron meteorites, and agreement with the probable age of the Earth has been

established. In 1964, a model of the evolution of meteorite material was proposed, taking account of its origin in both solar

and presolar systems.

Experiments on nuclear reactions over a wide energy range are of great importance for nuclear astrophysics and astro-

chemistry, since they give an idea of the evolution of the isotopic composition of interstellar matter as a result of cosmic rays

and the formation of some elements whose origin in nucleosyntheisis is unlikely (the bypassed nuclides). At the Radium Insti-

tute, the isotopic composition of xenon in targets bombarded by high-energy protons was studied experimentally, for compar-

ison with meteorite data (Yu. A. Shukolyukov), and the mechanism of nuclear reactions that might participate in nucleosyn-

thesis, including neutrino-stimulated reactions, was analyzed; (Re-Os) cosmochronology and the systematics of nuclear-frag-

mentation cross sections under the action of cosmic rays were considered. Ya. N. Kramarovskii and V. P. Chechev have pub-

lished important monographs on radioactivity and the evolution of the Universe (1978) and the synthesis of elements in the

Universe (1987).

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Cosmic-ray experiments are fundamental to high-energy nuclear physics. At the Radium Institute, the study of

high-energy particles on an accelerator began immediately after the introduction of the 660-MeV synchrocyclotron at Dubna,

which was the world's largest accelerator at the time. The development of such research was greatly hastened by the decision of

the Presidium of the Soviet Academy of Sciences in 1950 to establish a new laboratory for the investigation of high-energy

nuclear reactions in the Radium Institute, under the leadership of N. A. Perfilov. For half a century, research on high-energy

accelerators at the Institute has steadily expanded our understanding of such reactions and permitted improvements in experi-

mental techniques. Scientists from other branches of the Institute have participated in such research at various times.

The first research into high-energy reactions was based on two methods, the photographic method and radiochemistry,

both of which were well developed at the Institute by the beginning of the 1950s. Radiochemical investigation of deep fission

under the action of high-energy particles was initiated in 1951 by the director of the Institute, B. A. Nikitin. The patenting of

uttrafine-grain nuclear photoemulsions by N. A. Perfilov in 1948 and the development of new methods of producing emulsions

with different properties and new methods of introducing the selected elements in the photolayer significantly expanded the

scope for photographic nuclear research (A. P. Zhdanov, N. A. Perfilov, N. R. Novikova, E. I. Prokof'eva, L. I. Shur, E. V. Fad-

ina, I. M. Kuks, N. P. Kocherov, V. I. Zakharov, V. S. Bychenkov, and others).

The photographic method was first used at Dubna in experiments on low-intensity pion beams in the synchrocyclotron,

to study the capture of slow negative pions by heavy nuclei (N. A. Perfilov, N. S. Ivanova, O. V. Lozhkin, and V. P. Shamov).

N. A. Perfilov and N. S. Ivanova first observed the fission of uranium nuclei in pion capture (1950), very quickly followed by

I. M. Frank and G. V. Belovitskii. In the first studies, the fission probabilities of nuclei from uranium to tungsten, the path-length

distribution of the fission fragments, and the multiplicity of the particles accompanying fission were determined. This research

demonstrated the large role of the symmetric fission mode of heavily excited nuclei and the lack of a dependence of the fis-

sion-fragment kinetic energy on the exciting energy. The mechanism of nuclear excitation by pions was elucidated.

In the first studies of reactions under the action of 480- and 660-MeV protons, using the nuclear-emulsion method and

the radiochemistry method, the fission probabilities of nuclei from tantalum to uranium were determined, and the first estimate

of the fissionability of silver nuclei was given. In photoemulsion studies, the first estimates of the nuclear excitation energies in

high-energy inelastic reactions of protons with nuclei were obtained (V. I. Ostroumov, 1953). In radiochemical research, a large

number of deep-fission products of La, Ta, Pb, Bi, and U nuclei were measured; targets made of the fission-product isotopes

63Cu and 65Cu were used for the first time in such research. Radiochemical research permitted the determination of the mean

charge and mass losses in deep fission, once chemical methods for the fast isolation of small quantities of short-lived isotopes

had been developed (B. A. Nikitin, A. N. Murin, B. K. Preobrazhenskii, I. A. Yutlandov, A. V. Kalyamin, M. A. Yakimov,

L. N. Moskvin, V. I. Baranovskii, and others).

Having obtained the first data on nuclear fission under the action of pions and protons, Institute scientists initiated a

broad program of research on fission with a high excitational energy, which continues to this day. The fission of nuclei from

uranium to silver was studied at proton energies in the range from 70 to 9000 MeV and under the action of multiply charged

C, O, Ne, and Ar ions of energy around 10 MeV/nucleon. Various methods are used: mass spectrometry in line with an accel-

erator, nuclear photoemulsions, CR-39 track detectors, and breakdown counters. A universal dependence of the fission proba-

bilities of the nuclei on the fissionability parameter at high energy protons was established (N. A. Perfilov, 1961); the mass and

angular distributions of the fission fragments and their energy spectra were studied; the excitation functions of fission in a wide

proton-energy range were obtained; effects associated with large angular momenta and exciting energy and the influence of

the nuclear structure on fission were analyzed; and the fission of relativistic nuclei was studied (N. A. Perfilov, V. P. Shamov,

A. I. Obukhov, B. N. Belyaev, V. D. Dmitriev, O. E. Shigaev, V. S. Bychenkov, V. D. Domkin, A. N. Smirnov, V. P. Eismont,

V. A. Plyushchev, and others).

Experiments on high-energy accelerators showed that the inelastic interaction of high-energy particles with complex

nuclei, in the case where the particle wavelength is less than the internucleon distance in the nuclei, may be understood within

the context of successive quasi-free incident-particle scattering at individual nucleons of the nucleus. Discussion of such con-

cepts at the Radium Institute was initiated by P. I. Lukirskii in 1951. A phenomenological approach incorporating the ran-

dom-test method was developed by M. Goldberger in the early 1950s to calculate the first stage of the reaction in this model:

the intranuclear cascade. The intranuclear-cascade model played a great role in the analysis of high-energy reactions. At the

Radium Institute, the first calculations of the cascade process in 1956-1958 were undertaken in analyzing the results of three

different experiments: the fission of uranium nuclei with negative-pion capture (I. I. P'yanov); the reaction of fast deuterons with

12C nuclei (F. G. Lepekhin); and the fission of 12C nuclei by 660-MeV protons (P. I. Fedotov). In subsequent years, calculations

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by the cascade-evaporative model were widely used in the analysis of experimental data. In the 1980s, a model of the intranu-

clear cascade was developed on the basis of a consistent theoretical approach: the kinetic-equation method (G. V. Matveev,

V. E. Bunakov, S. G. Yavshits, and V. A. Rubchenya). In the 1990s, further development of theoretical models of nuclear pro-

cesses within the framework of baryon-meson nuclear dynamics was based on quantum molecular dynamics (A. B. Fokin), the

hydrodynamic model (A. T. D'yachenko), and the statistical approximation (M. D. Zubkov).

Beginning in the mid-1950s, a major cycle of research on the fragmentation and multifragmentation of atomic nuclei

has been undertaken at the Radium Institute. The characteristics of a new nuclear phenomenon in the inelastic interaction of

high-energy particles with nuclei - the formation of multiply charged (Z > 3) fragments (nuclear fragmentation) - were deter-

mined for the first time in 1954 (O. V. Lozhkin and N. A. Perfilov). The first observations with 660-MeV protons revealed some

characteristic features of nuclear fragmentation: the broad charge distribution of the fragments, the presence of multiple frag-

ment formation, the features of the excitation functions and the A dependence of the fra~mentation cross sections. Research on

fragmentation and fission in the interaction of high-energy particles with nuclei was first reviewed in 1960 by N. A. Perfilov,

O. V. Lozhkin, and V. N. Shamov (in the journal Uspekhi Fizcheskoi Nauky) and the world's first monograph on nuclear reac-

tions under the action of high-energy particles was published by N. A. Perfilov, O. V. Lozhkin, and V. I. Ostroumov in 1962.

When the 10-GeV synchrophasotron went into operation at Dubna in 1957, nuclear fragmentation and other problems

could be studied at higher proton and meson energy. At the same time, experiments were conducted on nuclear fragmentation

at a proton energy of 930 MeV, when special emulsions made at the Radium Institute were irradiated by J. Fremlin at the Birm-

ingham proton synchrotron. This was the first collaboration of the Institute with a non-Soviet scientific center and yielded more

accurate information on the energy dependence of the fragmentation cross sections and the charge distribution of the fragments

(O. V. Lozhkin, N. A. Perfilov, J. Fremlin, and A. A. Rimskii-Korsakov, 1960).

Important information on the mechanism of nuclear fragmentation was obtained in measurements of the double dif-

ferential cross sections of 8Li fragment formation from nuclei with a large number of mass numbers (Be, C, Al, V, Au, and Th)

using a vacuum chamber with photoemulsion as the detectors. The basic parameters of the fragment sources (temperature,

velocity, Coulomb barrier) were determined within the evaporative approximation, and the role of nonstatic processes of frag-

ment tbrmation from heavy nuclei was evaluated (V. V. Avdeichikov, V. I. Bogatin, O. V. Lozhkin, N. A. Perfilov, and

Yu. R Yakovlev, 1964).

The multifragmentation of nuclei with the formation of several fragments with Z > 3 in the elastic nuclear reaction is

of particular interest. Research at relativistic proton and pion energies first gave an idea of the excitation functions of multiple

fragment formation, the charge distribution of the fragments, and the correlation of the characteristics in multifragmentation.

Considerable successes in investigating nuclear fragmentation were obtained in the 1970s and 1980s in experiments by

a new technique. In this period, electronic methods of studying the formation of intermediate-mass fragments from target nuclei

directly in the high-energy particle beams were developed and put to use: at the external beam of 660-MeV protons at the LYaP

synchrocyclotron and at internal proton, deuteron, and c~-particle beams (energy 2-15 GeV) at the LVE synchrophasotron in

Dubna. Thanks to the successful use of the dE-E detector method to identify the Z and A fragments, a new manufacturing tech-

nology tbr thin (up to 6 lam) surface-barrier silicon semiconductor detectors with high uniformity over the thickness was devel-

oped at the Radium Institute (V. V. Avdeichikov and O. V. Lozhkin, 1970). Measurement of the dispersion of the energy losses

of heavy ions from helium to argon on passing through dE detectors showed that silicon detectors have the limiting possible

energy resolution.

The use of d E E detectors to study nuclear fragmentation provided information on the isotopic composition of the frag-

ments in high-energy reactions. The broad isotope distributions showed that nuclear fragmentation is a good means of studying

the limits of nucleon stability. The boundary nuclei provide a unique opportunity for testing nuclear models developed for ordi-

nary nuclei. Their study is at the cutting edge of nuclear physics and is also of great importance for nuclear astrophysics. At the

Radium Institute, there has been continuing interest in nuclides close to the limit of stability. As long ago as 1937, A. E. Polesit-

skii identified 6He in studying the short-lived nuclides formed in the neutron bombardment of a beryllium target. In 1946, efforts

were made to find 5He in natural helium in experiments at the Institute's cyclotron (M. G. Meshcheryakov and T. I. Khrenina).

The results indicated the complete absence of 5He in helium of any origin (<10-14); this was later attributed to its nucleonic

instability. In 1961, the very heavy helium isotope 8He was discovered at the Radium Institute among the fragmentation prod-

ucts of AgBr nuclei in an emulsion bombarded by 930-MeV protons (O. V. Lozhkin and A. A. Rimskii-Korsakov). The double

[3 decay of 8He was later confirmed at Dubna (V. A. Yarba, Yu. A. Batusov, S. A. Bunyatov, and V. M. Sidorov, 1965). As later

became clear, 8He, an isotope with NIZ = 3, was the nuclide with the greatest neutron excess in nature, whose discovery had

395

been predicted in 1960 by Ya. B. Zel'dovich and V. I. Gol'danskii. In 1972, this discovery was recorded as item 119 in the State

register. Thus, of the four helium isotopes existing in nature, two were discovered at the Radium Institute.

After the discovery of 8He, researchers began to wonder about the nuclear stability of 10He, the search for which had

been undertaken at many laboratories. Institute researchers participated in the search for l~ on the basis of several experi-

ments: in multinucleon transfer when heavy ions interact with nuclei; among the fragmentation products of 232Th nuclei under

the action of 9-GeV protons; and in the analysis of the cross sections of fragment isotopes with a.neutron excess. The results of

all three studies indicated the nuclear instability of l~ as confirmed by subsequent work.

The use of a new experimental method for separate target-nucleus isotopes led to the discovery of isotope effects in the

fragmentation cross sections: specific isospin correlations of the complete fragment-formation cross sections in the isotope pairs 15~ 112"124Sm, 58"64Ni, and 10.11B in reactions at 660 MeV (V. I. Bogatin, V. K. Bondarev, V. E Litvin, O. V. Lozhkin,

N. A. Perfilov, and Yu. P. Yakovlev, 1973).

The detection of isotope effects in the fragmentation cross sections permitted the creation of the first phenomenologi-

cal model of nuclear fragmentation and the formulation of a new method for the thermometry of highly excited nuclear systems.

Isotope effects were also investigated at relativistic proton, deuteron, and a-particle energies. The study of isotope effects was

extended to charged-pith formation in the same reactions. Experiments at 58"64Ni and 112,124Sn targets with 660-MeV protons

led to the discovery of strong isotope effects in the cross sections of negative-pion formation and in the yield ratios of pions of

different signs, which required refinement of the nuclear model in pion-formation calculations.

Since the mid-1970s, two more methods have been used to study nuclear fragmentation to the Radium institute: preci-

sion mass spectrometry in line with the accelerator, when the ion source of the spectrometer is a proton-bombarded target; and

y spectrometry with the identification of radioactive fragments in targets bombarded by high-energy particles. Mass-spectro-

scopic measurements in line with the PIYaF synchrocyclotron for a 1-GeV proton beam gave unique information on the form

and fine structure of isotopic distributions of various secondary products (Li, K, Rb, Sr, Cs) over a broad lifetime interval; this

revealed the influence of the nuclear structure and permitted the determination of even--odd effects in the cross section

(B. N. Belyaev, V. D. Domkin, Yu. G. Korobulin, and others). The use of precision y spectrometry to identify the fission prod-

ucts in the irradiated target permitted the measurement of the complete cross sections of the formation of some radionuclides in the intermediate-mass range (7Be, 18F, 22Na, 24Na, 28Mg) over a wide range of target-nucleus mass in order to refine the exci-

tation functions of fragmentation, the A dependence of the cross sections, and the isotope effects (A. A. Rimskii-Korsakov,

A. A. Nosov, E. N. Izosimova, and Yu. P. Yakovlev). At the Radium Institute, the photoemulsion method was improved in the 1970s by means of semiautomatic precision

microscopy and automated analysis of the tracks in the emulsion. The nuclear photoemulsions at relativistic particle energies

provided a unique opportunity to study internuclear interactions with discrimination of the fission products from the projectile

nucleus and the target nucleus in 4~ geometry. The cooperation of different groups from various institutes at JINR and the

EMU-1 international collaboration played a ~eat role in this research, along with the adoption of consistent measurement meth-

ods. The characteristics of inelastic interactions of various relativistic nuclei (p, d, He, Li, C, O, Ne, Si, S, Au, Pb, U) over a

wide energy range (1-200 GeV/nucleon) with photoemulsion nuclei were studied in detail, with comparative analysis of the

complete cross sections, the topological functions, the distribution of the multiple secondary particles, and their kinetic charac-

teristics. This work, which continued until the 1990s, revealed many significant details of inelastic nuclear reactions at super-

relativistic energy and facilitated the development of superior new theoretical models based on quark-gluon nuclear dynamics.

In the 1980s, an important research goal was to obtain detailed experimental information on the complete fission of sil-

ver and bromium nuclei in emulsion under the action of protons and 2H, 4He, and 12C ions with a momentum of 4.5 GeV/sec

per nucleon. Numerous secondary-particle characteristics in the limiting fission of silver and bromium nuclei were studied with-

in a single methodological approach for the first time (N. A. Perfilov, V. A. Plyushchev, and Z. I. Solov'eva, 1981), and conclu-

sions were drawn regarding the nature of limiting nuclear fission. Also in this period, researchers searched for shock-wave

effects within the nuclear material under the action of relativistic nuclei. Experiments were conducted using nuclear emulsion

and new AgCI single-crystal detectors (N. P. Kocherov, A. V. Voronov, A. I. Bogdanov, and N. A. Perfilov).

In the 1980s, there was a surge of interest in deeply inelastic proton-nuclear and internuclear reactions leading to the

emission of high-energy particles and light nuclei in the cumulative region of the kinematic variables. According to current con-

cepts, these secondary particles give information on the high-momentum component of the target-nucleus wave function and the

local properties of the nuclear material and are related to the quark degrees of freedom. At the Radium Institute, research focused

on the measurement of complex particles (the nuclides 1'2'3H, 3'4'6He, and 6"7Li) in the variable range kinematically forbidden

396

for free scattering, in reactions at C, Cu, and I L2.L24Sn nuclei under the action of 6.6-GeV protons. To describe the characteris-

tics of the fragments in the cumulative region, the approximation of few-nucleon correlation in the nuclei was employed

(M. A. Braun, V. V. Vechernin, O. V. Lozhkin, and Yu. A. Murin, 1982). The measurement of the spectra of cumulative fragments

under the action of relativistic c~ particles and protons revealed the existence of scale invariance and factorization of the cross

sections in fragment formation, as in the creation of elementary particles.

The study of quasi-elastic nucleon and cluster ejection in the interaction of high-energy particles with nuclei has been

an important aspect of the research at the Radium Institute. The problem of nucleon associations in nuclei is still of great rele-

vance in nuclear theory. Since the 1950s, Institute scientists have been studying various facets of simple reactions and the clus-

ter structure of nuclei: quasi-elastic scattering of incident particles at few-nucleon clusters in light nuclei (R I. Fedotov,

Yu. I. Serebrennikov, V. I. Bogatin, O. V. Lozhkin, V. N. Kuz'min, R. M. Yakovlev, and others); and quasi-elastic o~-particle ejec-

tion from the periphery of Ag and Br nuclei by cascade nucleons (V. I. Ostroumov and R. A. Filov). The first intranuclear reac-

tions ar clusters were considered, in connection with the measurement of large momentum transfer in simple reactions, and a

model of the pole mechanism of fragment formation was developed. Reactions of (p, 2p) type at energies of 180-9000 MeV

were considered, and the general characteristics of three-particle processes were explored (E. L. Grigor'ev, O. V. Lozhkin,

Z. Marich, V. K. Suslenko, V. I. Kochkin, R. V. Afanas'eva, Yu. R Yakovlev, and V. M. Mal'tsev).

At the end of the 1980s, liquid-gas phase transition with high excitation of the nuclear material was investigated at the

Radium Institute. The dependence of the form of the fragment charge distribution on the energy of the incident particles was

studied at the LVE synchrophasotron. Using the telescope ofdE-E detectors of thickness 6 and 24 p,m, the cross sections for the

formation of fragments with Z = 5-14 from gold nuclei under the action of protons of energy 2.5-7.5 GeV and tx particles of

energy 1.3-I3.5 GeV were measured. In this experiment, there were indications of the existence of a minimum in the depen-

dence of the charge-distribution parameter of the fragments on the energy of the incident a particles, which may serve as a sig-

nal of a phase transition (V. V. Avdeichikov, A. I. Bogdanov, K. G. Denisenko, E. A. Ganza, O. V. Lozhkin, Yu. A. Murin,

V. A. Nikitin, and others).

Successes in high-energy physics at the Radium Institute in the 1990s have been associated with active participation in

international research programs. At the end of the 1990s, there is a growing interest in nuclear multifragmentation, which is asso-

ciated with the fundamental properties of the baryon material and its equation of state. Institute scientists are actively involved

in the creation of an electronic 4~ device for studying nuclear multifragmentation at the CELSIUS accelerator at Uppsala, with-

in the framework of the SNIS international collaboration; are studying near-threshold meson formation in internuclear reactions;

and are investigating the parameters of particle sources in intefferometric measurements, isotopic effects in reactions at heavy

ions, and the mass distribution of the residual nuclei in internuclear reactions (V. V. Avdeichikov, O. V. Lozhkin, A. I. Bogdanov,

Yu. A. Murin, A. B. Fokin, M. D. Zubkov, A. T. D'yachenko, A. V. Kuznetsov, I. D. Alkhazov, and V. G. Lyapin).

Work continues on nuclear astrophysics: neutrino-induced reactions in primary nucleosynthesis are analyzed

(Ya. M. Kramarovskii and V. R Chechev); and the reaction of antinuclei in cosmic rays with matter is studied (A. T. D'yachenko

and O. V. Lozhkin). Efforts are being made to determine the nature of the nuclear-active component of cosmic radiation, which

is responsible for multiple neutron formation in the detector located in the underground laboratory of the Radium Institute. This

research is envisioned as part of an international scientific collaboration.

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