|
STRUCTURE
ACTIVITIES
ACHIEVEMENTS
PUBLICATIONS
EVENTS
HISTORY
USEFUL INFORMATION
SOCIAL LIFE
|
|
 |
 |
N u c l e a r P h y s i c s
Main directions of scientific activity (continuation - page7)
|
Study of Nuclear Structure with Quasielastic Proton Scattering at 1 GeV.
PNPI Group Leader: Y. Dotsenko
The shell structure of nuclei was studied at PNPI by (p, 2p) and (p, np)
reactions using the 1 GeV proton beam of the PNPI syncrocyclotron . Both reactions were studied in identical kinematical conditions. The scattered proton was detected with a magnetic spectrometer, while the knocked out nucleon (p or n) was detected with a time-of-flight spectrometer. A number of nuclei (more than 20, from 6Li to 208Pb) were studied, and detailed information on the proton- and neutron-shell energies was obtained. Prior these measurements, the neutron shell energies were in most cases unknown. An important result of these studies was the observation of the 1s1/2 and 1p1/2 proton and neutron shells in heavy nuclei including 90Zr and 208Pb. The shell splitting was observed in many of the studied nuclei. This shell splitting was explained in terms of the spatial deformation of the self-consistent nuclear field. It has been shown that the data on the proton and neutron shell energies allow one to study the deformation parameters of the nuclear density distributions both for protons and neutrons. Also, the measured spectra show high sensitivity to the differences between the proton and neutron root-mean square radii. The obtained data can serve as a solid basis for testing the existing nuclear models and for developing new theoretical approaches to microscopic descriptions of the nuclear structure.
(For more detail review see article "Study of nuclear structure with quasielastic proton scattering at 1 GeV" in PNPI report of
the High Energy Physics Division "Main Scientific Activities 1971-1996").
Ternary Fission.
PNPI Group Leader: A.A. Vorobyov, D.M. Seliverstov
A series of experiments on ternary nuclear fission were performed at the
Gatchina WWR-M neutron reactor. Two alternative approaches in description of the nuclear fission process were discussed in the 60s. Within the framework of the statistical fission model, descending from the saddle point was considered to be adiabatically slow, so that formation of the fragments occurs just before the scission point. On the contrary, in the dynamical model of fission the formation takes place at the barrier, and further separation of the fragments goes so rapidly that no significant redistribution of the fragment masses occurs. The ternary fission has occurred to be useful to tell between these two fission models.
The inclusive spectra of light nuclei (2,3H, 4,6,8,He, 7,8,9Li, 9,10,11Be, 11,12,13,14B, 14,15,16C, 20O) formed in the ternary fission were measured with a magnetic time-of-flight mass-spectrometer. The experiments were carried out with the 233U, 235U, 239Pu, and 242mAm targets exposed to thermal neutron fluxes. The next stage was the correlation experiment with a special setup which enabled simultaneous detection of the heavy fragments and the light nuclei 3H, 4He, 6He, 10Be. Detailed studies of the 235U and 239Pu thermal neutron induced fission, as well as spontaneous fission of 252Cf, were performed. As a result of these studies, the experimental data were obtained which created the basis for a detailed kinematics analysis aimed at determination of the initial conditions of the fragment separation. Such an analysis resulted in a rather large value (»
30 MeV) of the fragments initial energy. This value excludes the statistical model of nuclear fission.
(For more detail review see article "Ternary fission" in PNPI report of
the High Energy Physics Division "Main Scientific Activities 1971-1996").
Investigation of muon catalysis of nuclear dd- and dt- fusion.
PNPI Group Leader: A.A. Vorobyov, E.M. Maev, G.G. Semenchuk.
Muon catalysis is the combination of mesoatomic, mesomolecular and nuclear processes occurring when negative muons stop in mixtures of hydrogen isotopes,
and at last dd and dt-fusion reactions take place. The interest in the study of muon catalysis is associated with theoretical predictions, and then
with experimental confirmation of the presence of weakly bound levels in ddµ and dtµ molecules with quantum numbers J = ν = 1 and energies ~ 1.96 eV and ~ 0.66 eV,
respectively for ddµ and dtµ -molecules. This particular combination of these parameters led to a high rate of formation of mesomolecules and, as a consequence,
to a high yield of neutrons in dd and dt-fusion. The mechanism of formation of these mesomolecules is called resonant formation.
To study mesomolecular processes in HEPD of PNPI, a new effective experimental method was proposed using a high-pressure ionization chamber (P ~ 100 atm) as an active
target and detector of charged fusion products. Experiments using this method were first performed at PNPI and then continued at the meson factory of Paul Scherrer Institute
in Switzerland. As a result, all the main parameters of mesocatalysis of dd synthesis at T = 300 K were determined with high precision: the rate of formation
of ddµ molecules: λddµ = (2.76 ± 0.08)·106 ñ-1, the ratio of the yields of two dd fusion channels:
R=Y(3He+n)/Y(3H+p)=1.39 ± 0.04, the coefficient
of muon sticking to helium-3 nuclei: ωdd =0.122 ± 0.003. As one can see, the ionization chamber method turned out to be extremely effective
in the study of ddμ catalysis in comparison with the method of registration of fusion neutrons.
Subsequent measurements made it possible to determine the energy of the weakly bound level E11= 1,9651 eV, which coincided with the calculated
value to an accuracy of 2,5õ10-4. The obtained agreement between theory and experiment unambiguously confirms the resonant mechanism of the formation
of ddµ-molecules. A result of practical importance was provided by the measurement of the probability of muon sticking to a 4He nucleus
in the reaction dtµ -> 4Heµ + n. This determined the maximum possible number of dt-fusion cycles catalyzed by one muon, or the number
of neutrons per muon. This number turned out to be equal to 178 ± 13. This work in 2003 was awarded the Prize of A.F.Ioffe.
Another area of research associated with the use of ionization chambers was devoted to the capture of muons by light nuclei.
The aim of the experiment was to accurately measure the rate of capture of muons by the 3He. nucleus. This was achieved by spotting
the muon stops in the sensitive volume of the chamber with an accuracy of~10-4 and registration of charged particles with
the efficiency of 100%. The evaluqated rate of muon capture by 3He turned out to be in agreement with theoretical predictions based
on the hypothesis of partial conservation of the axial current.
A further development of the program of measurements of the rates of muon capture by light nuclei was the experiment on the capture of muons in hydrogen - MuCap.
The main measured parameter was the pseudoscalar form factor of the proton gp, the value of which is a sensitive test of the symmetries in the Standard Model.
To implement the goals of the experiment, a time-projection camera (TPC) was developed at the HEPD, operating at 10 atm of deuterium-depleted ultrapure hydrogen.
The capture rate was evaluated from the difference of the lifetimes of positive and negative muons, measured with the accuracies of 1%, which, in turn,
were measured with an extremely high accuracy of 10 ppm, which was provided by the statistics at a level of ~ 1010 muon stops. The selection of events
in the TPC was carried out under the condition that the signals from the stopping of muons in the gas and the signals from the decay electrons counters
coincide. As a result, the accuracy declared in the experiment was achieved, which made it possible for the first time to determine the value of gp,
which turned out to be close to the estimates of the Standard Model.
The results of the experiments had formed the basis of the world data
on the physics of mesocatalysis and allowed PNPI to take a leading place among scientific centers studying this phenomenon.
(These issues are discussed in more detail in the article
"Muon catalized dd- and dt-fusion." - (pages 210-221)
in the report of the High Energy Physics Department of PNPI
"Main directions of scientific activity in 1971-1996")
and in the file
"CHRONOLOGY OF STUDIES OF THE PROCESS OF MUON CATALYSIS OF DD and DT SYNTHESIS at PNPI"
of the article
"RESEARCH OF THE PROCESS OF MUON CATALYSIS OF NUCLEAR dd and dt SYNTHESIS AT PNPI".
|
|
|
|
