The processes of fragmentation of oxygen nuclei in interactions with protons at 3.25 a ge v / c and mechanisms of formation of protons in π– , p, α, C (C) and p(16O, 20Nе) collisions at 3–300 GeV

Topicality and demand of the theme of dissertation. The establishment of regularities of fragmentation of relativistic nuclei in interactions with hadrons and nuclei is one of the fundamental problems of relativistic nuclear physics. Herewith, the experimental and phenomenological investigations of the processes of fragmentation of relativistic nuclei in hadron- and nucleus-nucleus collisions, allowing one to obtain the important information about cluster structure of the fragmenting nuclei (especially the light ones), its influence on cross-section of formation and content of fragments, as well as about contributions of different processes in their yield, play an important role. At collision energies of several GeV/nucleon, the fragmentation of nuclei is a dominant process accounting for about 80% of the reaction total cross-section. It is of special interest to study experimentally the processes of fragmentation of light relativistic even-even nuclei, such as l2C, l6O, and 20 Ne, because we expect the dominant manifestation of an а-cluster structure in these nuclei.
We still do not have the answer to the question whether the a-cluster structure is stationary or dynamic one manifesting itself at some definite levels of excitation of the fragmenting light nucleus, especially of the even-even nucleus. The effective method of testing the а-cluster structure of the fragmenting nucleus is the study of formation of a-particles at different levels of excitation of the projectile nucleus.
Also, it is of some definite interest to discover the strongly correlated few nucleon associations and multibaryon clusters in nuclei, existence of which was predicted by several models based on the quark-parton structure of hadrons. The solution of this problem is directly related with the experimental investigation of the processes of formation of particles in the kinematical region, forbidden for the particles formed in collisions of free nucleons, in hadron- and nucleus-nucleus collisions at high energies.
Due to above, it is of topical interest to conduct the systematic and complex study of nucleus fragmentation with identification of all possible fragments and secondary produced particles, and to measure precisely and determine their kinematical characteristics in wide ranges of collision energies and mass numbers of colliding nuclei on statistically rich experimental material to allow one to extract the new regularities of nucleus fragmentation.
It is of demand to obtain the precise data on cross-sections of formation of all possible isotopes of fragmenting nuclei, necessary for working out and testing the theoretical models and approaches for description of the phenomenon of nucleus fragmentation, and also for modeling the interaction of the primary cosmic radiation with nuclei of atmosphere. Besides, the experimental data on the mean multiplicity (inclusive cross-sections) of light fragments - protons, deuterons, tritium and helium-3 nuclei, formed in interactions of relativistic nuclei with hydrogen are of extreme demand for solution of one of the important problems of astrophysics - the passage of flux of particles through the interstellar medium.
The aim of research is to obtain a qualitatively new and quantitative information about the process of fragmentation of oxygen nuclei in interactions with protons at 3.25 A GeV/с and the establishment of the basic mechanisms of formation of protons, including cumulative in n’,p,a,C(C) and p(16O,2(>Ne) collisions in the range of primary energies 3-300 GeV.
Scientific novelty of the dissertational research is as following results done for the first time:
for the first time, stable and non-stable isotopes formation cross-sections were precisely measured for isotopes with Z = 1-8 in I6Op-collisions at 3.25 A GeV/c, and it was shown that mirror nuclei formation cross-sections coincide within statistical errors; 
in л C-, p20Ne- 16Op-collisions at 40 GeV/c, 300 GeV/с and 3.25 A GeV/c, respectively, contribution from major proton production mechanisms was estimated by means of semi-empirical method. It was demonstrated that these contributions do not depend on the energies and mass numbers of fragmenting light nucleus and are sensitive to the type of incident particle (pion or proton);
it was established that the mechanisms of formation of fast protons (p > 0.25 GeV/c) in l6Op, p12C, л l2C, p20Ne collisions at 3.25 A GeV/c, 9.9 GeV/c, 40 GeV/c, and 300 GeV/c, respectively, do not depend on the primary energy, type of the impinging particle or the light fragmenting nucleus, as well as the degree of excitation of the fragmenting nuclei (nuclear scaling);
the main mechanisms of deuteron formation, connected with the fusion of the cascade nucleons, Fermi breakup of the excited remnant nucleus, breakups of the comparatively fast light fragments and direct quasi-elastic knocking out the deuterons from oxygen nuclei by protons, in I6Op collisions at 3.25 A GeV/c were established;
the cross-sections of formation of the intermediate states of (sBe, 9B, l2C*) nuclei in l6Op collisions at 3.25 A GeV/c and their contributions along with 5He, ’Li nuclei to the inclusive cross-section of formation of a-particles were determined. The contribution of the breakup of the excited l2C* nuclei to the channel of formation of three a-particles was determined to be (38 ± 3)% of the cross-section of this channel;
it was proved that the formation of cumulative protons in l6Op, p'2C, л-12С, p20Ne, a12C, and 12CI2C collisions at 3.25 A GeV/c, 4.2 GeV/c, 9.9 GeV/c, 40 GeV/c, 300 GeV/c, and 4.2 A GeV/c, respectively, occurs through the collision of the impinging hadron with “fluctons”, formed as a result of fluctuation of the nucleon density of a nucleus in its ground state.
CONCLUSION
1. For the first time, under 4л acceptance conditions, a systematic analysis for the formation of proton fragments was carried out in l6Op collisions at 3.25 A GeV/c and it was shown that:
• shape of momentum spectrum of protons at p>0.25 GeV/c (except «evaporated» ones), emitted into the front hemisphere in the system of rest of the fragmenting nucleus, does not depend on the degree of the nucleus excitation;
• shape of momentum spectrum of protons, moving into the back hemisphere in the system of rest of the fragmenting nucleus, is strongly correlated with its degree of excitation;
• an irregularity observed in the spectrum of kinetic energy of fast protons in the region of T=70~90 MeV in the oxygen rest frame is shown to be due to the decay of two nucleon system after absorbing the slow pion;
• the basic mechanisms for the proton formation (“evaporation” mechanism, mechanism of Fermi decay of the excited remnant nucleus, and mechanism of direct knock out of protons at the process of cascading) were identified and their statistical weights (contributions) estimated to be (27.4±0.5)%, (32.0±0.6)%, and (40.6±0.7)%, respectively.
2. Based on an analysis of the normalized invariant inclusive differential cross-section of the proton formation in l6Op interactions at 3.25 A GeV/c, pl2C collisions at 9,9 GeV/c, n-12C interactions at 40 GeV/c, and p2<)Ne collisions at 300 GeV/c the conclusion was made that the mechanism of fast proton formation, especially those moving forward, have an universal character, expressed in its independence on the primary energy and type of impinging particle or the light fragmenting nucleus.
3. For the first time, a three stage phenomenological model for the analytical description of distribution of proton multiplicities in n C collisions at 40 GeV/c, p2()Ne collisions at 300 GeV/c and l6Op collisions at 3.25 A GeV/c was developed and the contributions for the main mechanisms of proton formation -«evaporation», Fermi decay and direct knocking out of by the primary proton and secondary particles were determined. It was established that these contributions are independent from the energy and mass number of light fragmentation nucleus, but are sensitive to the type of projectile particle (pion or proton), i.e. to the number of its valent quarks. It is necessary to point out that the values of these contributions (fractions) for p20Ne-collisions at 300 GeV/c and 16Op collisions at 3.25 A GeV/c coincide within uncertainties with the values determined from an analysis of the invariant structure function of protons, formed in l6Op interactions at 3.25 A GeV/c.
4. In the momentum spectrum of “forward deuterons” in the system of rest of the oxygen nucleus in the region of 0.40 < p < 0.55 GeV/c a shoulder was observed, which could be due to the fusion mechanism of fast cascade nucleons, decays of the relatively fast light fragments, and also due to the direct quasi elastic knocking out of deuteron from the oxygen nucleus by the primary proton. Average multiplicities of fragments correlate with availability of a deuteron in an event, but are independent from mechanism of deuteron formation. Coalescence model describes successfully the momentum spectrum of fast deuterons and a-particles.
5. A systematic comparison of experimental data on oxygen nucleus fragmentation in 16Op collisions at 3.25 A GeV/c with the CFEM model predictions was made. It was shown that for the realistic description of a nucleus fragmentation processes in hadron-nucleus collisions at high energies it is necessary to take into account the contribution of “evaporation” mechanism (even for such light nuclei as l6O), mechanism of fusion of fast cascade nucleons, and also an а-cluster structure of light nuclei in the CFEM model. The necessity for taking into account the small angular moment acquired by the fragmenting nucleus was proved.
6. Formation of a-particles in l6Op reactions and in different topological channels of oxygen nucleus disintegration was investigated, and their momentum and angular spectra were analyzed. Phenomenological models of formation of a-particles through breakup of intermediate nuclei xBe, 9B and excited nucleus 12C* were worked out and tested. In particular, it was established that:
• dominant channel of multifragmentation was a disintegration of oxygen nucleus with emission of helium nuclei, part of them being the products of decay of short - lived 5Li, 5He, xBe, 9B, and l2C* nuclei;
• in the channels of formation of 3 and 4 a-particles, a collinearity caused by decays of the instable xBe and 9B nuclei was revealed;
• for the first time, the contributions of the channels of decays of xBe and 9B nuclei and that of the excited 12C* nucleus into the inclusive channel of formation of a-particles were established quantitatively;
• contribution of the excited nucleus I2C* decays into the channel of formation of 3 a-particles was proved to be about 38% of the cross-section of this channel, and the remaining part of the cross-section of this channel was realized through the direct Fermi decay or quasi elastic knocking out of a single а-cluster from the weakly bound remnant nucleus, containing three a-particles;
• from the analysis of the angular spectrum of two-, three-, and four-nucleon fragments, the hint on the existence of an angular moment of the fragmenting remnant nucleus was obtained; the effect became more intensive at the transition from the lighter to the heavier fragments.
7. For the first time, under the conditions of 4л: acceptance, cross-sections of formation of stable and instable isotopes with charges Z=l-8 in l6Op collisions at 3.25 A GeV/c were measured. It was shown that cros sections for the formation of “mirror” nuclei with mass numbers, differing on AA = ± 1 from the basic mass number defined as A = 2Z, coincided within statistical errors. The cross-sections for the yields of instable isotope 9B and its “mirror” stable isotope 9Be also coincided within statistical uncerainties. A regularity observed can also be extended to the mirror nuclei (l5N, l5O), formed as a result of a loss of one nucleon by initial l6O nucleus in the peripheral collisions with target protons.
8. For the first time, the formation of cumulative protons in I6Op interactions at 3.25 A GeV/c, pl2C collisions at 4.2 GeV/c and 9,9 GeV/c, n-l2C- interactions at 40 GeV/c, p20Ne collisions at 300 GeV/c, and in a12C and 12C12C collisions at 4.2 A GeV/c was studied in detail and it was shown that:
• the values of a slope parameter for the distributions of invariant inclusive cross-sections of formation of the cumulative protons depending on the cumulative number [3 showed the universal character («nuclear scaling »), expressed by their independence from the type of projectile or target and the primary energy;
• an independence of the mean multiplicity of the cumulative protons in cumulative events from the same target nucleus on the primary energy and type of a projectile was established. It was shown that formation of cumulative protons proceeds predominantly through the «cold» model scenario. The argument for this version of scenario is based on the independence of the fraction of cumulative events on the mass number of a projectile nucleus or particle.
• an absence of correlations between mechanisms of formation of cumulative protons and secondary particles and fragments was revealed;
9. An independence of mechanisms of formation of light fragments was established. The correlation phenomena observed in experiment are the consequences of conservation laws for energy - momentum, electrical and baryon charges in the processes of fragmentation of nuclei.
The main framework of the researches conducted in the dissertation is the systematic and complex study of the processes of fragmentation of light nuclei in a wide range of primary energies with the use of a single methodology. The overwhelming majority of the results, included in this thesis, were obtained for the first time, and these data allow for a deeper and more detailed understanding of characteristics and properties of nuclei, fragmenting under an action of high energy particles and relativistic nuclei.
The problems, set in the dissertation, on complex experimental and phenomenological study of fragmentation processes and establishement of the dominant processes in formation of protons and light fragments in the л", p, a, C (C) and p (16O, 20Ne) collisions in a range of primary energies 3-300 GeV were fully solved.
Results of the dissertation work give a wide practical material for the test of theoretical models and approaches at investigation of problems of fragmentation of nuclei in hadron- and nucleus-nucleus collisions at high energies and can be used for the development of the theory of strong interactions, and can also be used for planning and carriying out the new experiments on the ion accelerators.
Data on cross-sections of formation of stable and unstable isotopes produced in interactions of oxygen nuclei with protons at high energies, obtained in the present thesis, can find their application in space physics and cosmochemical studies for determination of characteristics of the processes of cosmogenic nucleosynthesis.