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TO THE MECHANISM OF PARTICLE RELEASE IN NUCLEAR REACTIONS

TO THE MECHANISM OF PARTICLE RELEASE IN NUCLEAR REACTIONS. Joint Institute for Nuclear Research, Dubna, Moscow region, Russian Federation. I. Elementary microscopic states of complex nuclei are manifested: • in radioactive decay processes;

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TO THE MECHANISM OF PARTICLE RELEASE IN NUCLEAR REACTIONS

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  1. TO THE MECHANISM OF PARTICLE RELEASE IN NUCLEAR REACTIONS Joint Institute for Nuclear Research, Dubna, Moscow region, Russian Federation I. Elementary microscopic states of complex nuclei are manifested: • in radioactive decay processes; •• in specific nuclear reactions as: a) Scattering; b) Coulomb excitation; c) Stripping reactions. S. A. Karamian

  2. II. Bulk reactions proceed via continuum of excited levels being treated within statistical and macroscopic approaches. Among them could belisted the most abundant processes as: • Compound nucleus formation and decay; •• Fission; ••• Nucleon emission from highly excited nuclei, and so on. III.In statistical model,the nucleus is characterized by temperature, entropy and total angular momentum. All nucleons are assumed identical and their individual quantum numbers make no significance. IV. It would be yet interestingto deduce the status of nucleons inside a nuclear volume from the data reached in reaction experiments. V. Some examples of intrinsic structure manifestation are given below.

  3. Observation of low probability for (, α)reactions that requires the pre-formation factor. Conclusion: α-clusters and multi-quark objects may be present in nucleus but with low probability ~10–2; In reactions with heavy ions, the probability of α emission is oppositely very high. Conclusion: alphas are formed through the special mechanism of internal coalescence; Observation of an excess in the Tl values for emission of neutrons with l ≥3 at INNA experiment. Internal single nucleon orbits possess a high momentum, but centrifugal barrier suppresses their emission. The re-arrangement of orbits is needed. The enhanced yield of INNA means an effect of internal states; MANIFESTATION OF INTERNAL STATES IN REACTIONS OF STATISTICAL MECHANISM

  4. Observation of the preferential population of high-spin states in (, n) and (, p) reactions with isomeric targets. Survival of the structure selectivity indicates incomplete mixing of specific states even despite excitation energy of E* ≈ 7-15 MeV; There-arrangement of internal states in advance of particle emission suppresses the absolute reaction rate as compared to the standard statistical estimates. Some major details are given below.

  5. REFERENCES TO THE ORIGINAL WORKS I. Reactions induced by 23 MeV bremsstrahlung Ref. [1]: S.A. Karamian, “Threshold and spin factors in the yield of bremsstrahlung-induced reactions”. Preprint JINR, E15-2012-65, Dubna, to be published in Phys. of Atomic Nuclei. Ref. [2]: S.A. Karamian, “Yield of bremsstrahlung-induced reactions as a probe of nucleon-nucleon correlations in heavy nuclei”. In: Proc. of 4-th Intern. Conf. NPAE-2012, p. 141, Kiev, Ukraine. Ref. [3]: S.A. Karamian, “To the mechanism of alpha particle emission induced by photons”. Submitted to Phys. Lett. B (2013).

  6. II. Inelastic acceleration of thermal neutrons by isomers Ref. [4]: S.A. Karamian and J.J. Carroll, “Cross section for inelastic neutron “acceleration” by 178m2Hf”. Phys. Rev. C (2011) v. 83, p. 024604. Ref. [5]: O. Roig, G. Belier, et al., “Evidence for inelastic neutron acceleration by the 177Lu isomer”. Phys. Rev. C (2006) v. 74, p. 054604. Ref. [6]. S.A. Karamian, A.G. Belov, et al., “Upper limit for 180mTa depletion by neutrons”. In: Book of Abstracts of 63d Conf. on Nucl. Spectroscopy, Science, St-Petersburg, 2013.

  7. THRESHOLD DEPENDENCE OF THE (,n) AND (,p) REACTION YIELDS

  8. SPIN-DEPENDENCE: ISOMER YIELDS • To systematize the spin dependence, the yields are plotted as a function of a new parameter: • [Im(Im +1) – It(It + 1)], • where Im and It are the spin values for the product isomer and the target nucleus. Choice of this parameter is very natural, despite somewhat new and original. • The process probability in thermodynamics approach must be proportional to a number of microstates at definite thermal energy. Let’s remind the nuclear level density anzatz: • This equation practically includes the subtraction of the rotational energy Erot ~ I(I + 1) from total excitation E* in order to get the thermal energy Etherm = E* - Erot. The rotational energy could be considered as a form of kinetic energy.

  9. SYSTEMATIC OF YIELDS VERSUS “SPIN PARAMETER” 179Hf (,p)178mLu 9/2 9 178m2Hf (,n)177m2Hf 16 37/2 180mTa (,p)179m2Hf 9 25/2

  10. ENCOUNTER DATA FOR THE (, α) EXPERIMENT

  11. EXPERIMENTAL RESULTS FOR THE YIELD OF (, ) REACTIONS

  12. MEASURED YIELDS OF THE (, p) REACTIONS

  13. RELATIVE YIELDS OF (, p) AND (, ) IN RATIO TO (, n) REACTIONS AT Ee=23 MEV VERSUS THRESHOLD PARAMETER VALUE.

  14. Z–DEPENDENCE OF THE (, ) - REACTION YIELD. SOLID CURVE GIVES THE GUIDE FOR EYES.

  15. REACTION MECHANISM PATTERN1 • At low energy, (, ) yield is suppressed, probably, due to the pre-formation factor same as in  decay; • With 100 MeV protons, electrons, and photons, the pre-equilibrium exiton model is applicable: • J.R.Wu and C.C.Chang, Phys.Rev., C17, 1540 (1978) – theory. • Formation factor for  of about (10-2 – 10-3) is deduced from experiments. • W.R.Dodge, et.al., Phys.Rev., C32, 781 (1985): - yield by 20 times lower the proton emission; • This model is hardly applicable to the case of 23 MeV bremsstrahlung. • Free energy of about 5-7 MeV above  threshold does not allow generation of 4 excitons by photons; • Conclusion: preformation factor as in  decay is preferable.

  16. HI  REACTION MECHANISM PATTERN2 • Alpha-emission must be suppressed when no quasi-free α is available in a nucleus, but at the same time nucleons are ready for ejection; • Nucleons within the unexcited target nucleus are located and paired at definite orbits. They manifest themselves as non-interacting particles due to the Pauli principle; • In reactions with charged particles (HI), a strong impact of the projectile generates immediately a directed flow of perturbed nucleons and they could easily be joined together forming an α-cluster due to“Internal coalescence”.

  17. SCHEME OF INNA PROCESS WITH THERMAL NEUTRONS

  18. INNA TRANSITIONS WITH 178m2Hf AND 180mTa

  19. CROSS SECTION OF THE INNA PROCESS (S0 is ht S wave strength function)

  20. TRANSMISSION COEFFICIENTS Tℓj FOR NEUTRONS WITH ORBITAL MOMENTUM ℓ (178Lu NUCLEUS)

  21. N=107 (180Ta) SINGLE PARTICLE LEVELS OF SHELL Internal orbital momentum of nucleons is great, like 5,6,7.

  22. SUMMARY: MODIFICATION OF MECHANISM Centrifugal barrier allows emission of neutrons with minimum orbital momentum ℓ=0;1;2. Neutrons sitting at ℓ=3-7 orbits must proceed through the re-arrangement of orbital moments. So that, emission rate is suppressed and statistical decay widths are reduced. Possible process is virtual tunneling of a neutron pair with ℓ=0 and consequent pair break outside of the nucleus. One of neutrons remains inside nucleus and another one is emitted with high ℓ.

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