Cross Sections Of Heavy Nuclei Research Articles

Phenomenological Description and Parameterization of Nucleon-Induced Fission Cross Sections of Heavy Nuclei in Intermediate and High Energy Region

Phenomenological Description and Parameterization of Nucleon-Induced Fission Cross Sections of Heavy Nuclei in Intermediate and High Energy Region

A systematics for negative pion induced nuclear fission cross-sections: Experimental data fitting and cascade exciton model validation

The developed fission systematics depicts the projectile energy and the target mass dependence of fission cross-sections of heavy nuclei. The developed model is fitted using experimental fission cross-section data of 500–1665 MeV negative pion induced fission in [Formula: see text]Sn, [Formula: see text]Au and [Formula: see text]Bi. The same systematics is further fitted using the cascade exciton model (CEM) calculations of the above-mentioned fission reactions. Parameterization of the model is performed by calculating its parameters and uncertainties in them. A comparison of the systematics predictions with experimental data and CEM calculations is made. Validation of the systematics using new target nuclei [Formula: see text]Eu, [Formula: see text]Ta, [Formula: see text]Hg and [Formula: see text]At (not included in the systematics development) over an extended range of pion energies, from 80 MeV to 2500 MeV, builds trust in it. The systematics is not valid for [Formula: see text]Th and [Formula: see text]U. The systematics is found to be valid for target nuclei from [Formula: see text]Sn to [Formula: see text]At and for incident negative pion energies from 80 MeV to 2500 MeV. Results are discussed in detail. Merits and demerits of the developed systematics have been concisely described. Valid conclusions and future perspectives are also given.

Cascade exciton model analysis of energetic pion-induced fission of heavy nuclei: Target mass and projectile energy dependence

A data bank of negative and positive (80–1665[Formula: see text]MeV) pion-induced experimental fission cross-sections of heavy nuclei from 119Sn to 238U is compiled using present results and published data from the literature. Corresponding calculations of fission cross-sections, using the cascade exciton model (CEM) are also included in the compilation. Fission cross-sections compiled in the data bank are examined critically. Mass and energy dependences of fission cross-sections are analyzed. Fission cross-sections of 238U targets are the highest and scale down approximately, for other targets studied, with fissility, [Formula: see text]. In the fissility expression, [Formula: see text] and [Formula: see text] are atomic and mass numbers of the target while [Formula: see text] refer to positive and negative pion projectiles. The presented data bank is of interest for students and researchers involved in the investigation of energetic light particle-induced fission of heavy nuclei. Nuclear fission of heavy nuclei has been classified into three regimes. Phenomenological discussion of the fission process is also given.

Fission cross sections of heavy nuclei as a probe of nuclear dissipation

Nuclear fission is hindered by dissipation. Using the stochastic Langevin model, we calculate the drop of fission cross sections caused by friction with respect to its standard statistical-model values, ${\ensuremath{\sigma}}_{f}^{\mathrm{drop}}$, as a function of the presadde dissipation strength $(\ensuremath{\beta})$ for $^{226}\mathrm{U}$, $^{234}\mathrm{U}$, and $^{242}\mathrm{U}$. It is shown that the sensitivity of ${\ensuremath{\sigma}}_{f}^{\mathrm{drop}}$ to $\ensuremath{\beta}$ is substantially enhanced with increasing the isospin of the U fissioning system. Furthermore, we find that under typical conditions of excitation energy and angular momentum populating $^{240}\mathrm{U}$ (via radioactive beams) and $^{200}\mathrm{Tl}$ (via stable beams), the fission cross section of the high-isospin heavy $^{240}\mathrm{U}$ demonstrates a greater sensitivity to friction than that of the light $^{200}\mathrm{Tl}$. Our findings suggest that on the experimental side, to accurately probe presaddle dissipation with fission cross sections of heavy nuclei, it is useful to choose radioactive nuclear beams induced reactions as a way to populate excited heavy fissioning systems with high isospin.

The interaction and fusion of arbitrarily oriented deformed nuclei

The fusion/capture cross sections between various deformed nuclei are calculated. It is shown that quadrupole and hexadecapole deformations of heavy nuclei are important for evaluating the barrier height, capture well depth, and fusion/capture cross sections of heavy nuclei. It is found that calculations of the capture cross section of two heavy deformed nuclei must be performed with averaging over all possible mutual orientations of the colliding nuclei.

Energy dependence of proton induced fission cross sections for heavy nuclei in the energy range 200–1000 MeV

Total cross sections for proton induced fission of ${}^{\mathrm{nat}}\mathrm{Pb}$, $^{209}\mathrm{Bi}$, $^{232}\mathrm{Th}$, $^{233}\mathrm{U}$, $^{235}\mathrm{U}$, $^{238}\mathrm{U}$, $^{237}\mathrm{Np}$, and $^{239}\mathrm{Pu}$ nuclei were measured in the range 200--1000 MeV with an energy step of 100 MeV. The experiment was carried out at 1 GeV in a PNPI synchrocyclotron. Complementary fragments of the binary fission from a thin target were detected in coincidence by two parallel plate avalanche counters (PPAC) located close to the target. An assembly of the two PPACs and with the target in between them was placed directly into the beam, which provided a large solid angle acceptance. The beam monitoring system employed a scintillation counter telescope that was used for direct proton counting as well as for registration of the elastic $\mathit{pp}$-scattering events from the auxiliary (${\mathrm{CH}}_{2}){}_{n}$ target. The measured energy dependence of the total fission cross sections is presented. The results are compared with other available experimental data as well as with calculations in the frame of the cascade-evaporation model.

A-dependence of the γ- and p-induced production of the Λ(1520) from nuclei

Using results of a recent calculation of the $\Lambda(1520)$ in the nuclear medium, which show that the medium width is about five times the free width, we study the A dependence of the $\Lambda(1520)$ production cross section in the reactions $\gamma ~A \to K^+ \Lambda(1520) A^\prime$ and $p~ A \to p~ K^+ \Lambda(1520) A^\prime$. We find a sizable A dependence in the ratio of the nuclear cross sections for heavy nuclei with respect to a light one due to the large value of the $\Lambda(1520)$ width in the medium, showing that devoted experiments, easily within reach in present facilities, can provide good information on that magnitude by measuring the cross sections studied here.

Role of neutron flow in the enhancement of heavy ion fusion at energies below the Coulomb barrier and the systematics of threshold barriers.

By integrating the classical formula for fusion of two charged spheres with a flat distribution of barriers extending down to a threshold barrier the fusion cross sections of heavy nuclei at energies near and below the Coulomb barrier have been successfully parametrized by P. H. Stelson. The systematics of the threshold barriers derived from the data are examined in terms of a simple picture in which the onset of free neutron flow between the collision partners promotes fusion at a large distance. A consistent correlation between observed barrier shifts and the conditions necessary to maintain free neutron flow has emerged.

Fields of ionizing radiations on the Tokamak-10 fusion unit

I. WRENDA 83/84. World request list for nuclear data. Nuclear data section. Vienna: IAEA (1983). 2. A. A. Goverdovskii, A. K. Gordyushin, B. D. Kuz'minov, et al., "Measurement of the fission cross sections of heavy nuclei by the method of pulse synchronization," in: Neutron Physics, Part 4 [in Russian], TsNIIatominform, Moscow (1984), p. 115. 3. A. A. Goverdovsky, A. K. Gordjushin, B. D. Kuzminov, et al., "The 236U and 2SSU to 235U fission cross section ratios in the neutron energy range 5-11 MeV," in: Proc. Int. Spec. Meeting on Transactinium Isotopes Nuclear Data, Uppsala (1984). 4. A. A. Goverdovskii, A. K. Gordyushin, B. D. Kuz'minov, et al., "Measurement of the fission cross section ratios of 236U and 2SSU by the method of isotopic admixtures," Problems of Atomic Science and Engineering, Ser. Nuclear Constnats 3 (57) [in Russian], (1984), pp. 13-15. 5. A. A. Goverdovskii, A. K. Gordyushin, B. D. Kuz'minov, et al., "Measurement of the ratio of the neutron-induced fission cross sections of 2STNp and 2~SU in the energy range 4-11 MeV," At. ~nerg., 58, 137-139 (1985). 6. J. Behrens and G. Carlson, "Measurements of the neutron-induced cross sections of 2S~U, 236U, and 2ssU to 2SSU from 0.i to 30 MeV," Nucl. Sci. Eng., 63, 250-267 (1977). 7. J. Meadows, "Neutron-induced fission cross section ratios of 2a4U, 2S6U and 23SU," Nucl. Sci. Eng., 65, 171 (1978). 8. C. Nordborg et al., "Fission cross section ratios of ~S2Th, 226U, and 2S5U," in: Proc. Int. Conf. on Nuclear Data, Vol. I, Harwell (1979), p. 910. 9. H. Knitter and C. Budtz-J~rgensen, "Barrier heights of plutonium isotopes from (n, n'f)," in: Proc. Int. Conf. on Nuclear Data for Science and Technology, Vol. i, Antwerpen, Sept. 6-10 (1982), pp. 744-747. I0. ENDF/B V Third Ed. BNL (1979), 23SU (MAT 1395). Ii. S. Bj#rnholm and J. Lynn, "The double-humped fission barrier," Rev. Mod. Phys., 52, 725 (1980).

Total Neutron Cross Sections of Heavy Nuclei

Total neutron cross sections of the heavy and actinide nuclei 181Ta, 197Au, 232Th, 233U, 235U, 238U, 239Pu, and 240Pu were measured from 30 keV to 4.8 MeV. The experimental procedures emphasized a ...

Pauli correlations in heavy-ion collisions at high energies

We calculate the effects of short-range correlations on the Glauber expansion for nucleus-nucleus collisions using the Fermi gas model for nuclei. When we neglect the Pauli principle for collisions between heavy nuclei, calculation of the optical phase-shift function leads to non-unitary results and we cannot obtain cross sections. When we include Pauli correlations we find important cancellations in the optical phase-shift function, which make possible the calculation of total and differential cross sections for heavy nuclei.

Multi-Level Correction to Breit-Wigner Single-Level Formula

A formula for neutron cross sections of heavy nuclei is derived on the basis of the Wigner-Eisenbud theory. The derived formula is the same as the single-level one introduced by Breit and Wigner except for the inclusion of additional interference parameters (u, v) which represent the contribution of the interference effect between resonances. The present formula is therefore applicable to reactor calculations without much modifications of the existing resonance integral codes. The present formula has been applied to the analyses of the 235U and 233U fission cross sections and the 238U total cross sections in the resolved resonance region. By the use of least squares fits of the experimental data, the interference parameters (u, v) are obtained for resonance levels of these nuclei in their typical resonance regions. It is shown that the present formula well represents the experimental data. In the unresolved resonance region, both the single-level resonance parameters and the present interference parameter...

Coulomb “flux corrections” and total cross sections for pion-nucleus interactions in the Δ-resonance region

A relativistic generalization of the Blair model is shown to give large flux corrections (up to 20%) to pion-nucleus cross sections in the Δ-resonance region. Total cross sections for heavy nuclei are calculated and used in the evaluation of these corrections. The formulas also apply to incident high energy protons.

Radiative capture for intermediate-energy neutrons

A systematic analysis is carried out to test the validity of the direct andcollective mechanisms for the neutron radiative capture by nuclei in a wide range of mass number. It is found that the two processes can reasonably explain the experimental (n, γ) cross sections of heavy nuclei ( A > 40) with magic or near-magic neutron number in the (10–20) MeV neutron energy range, while in the (5–10) MeV energy range, a contribution from the compound nucleus mechanism is also necessary. The results are obtained by using for all the nucleides the same set of optical and bound-state parameters consistent with experimental data. It is also shown that the dependence of the experimental cross sections on the neutron number N of the target nucleus and on the asymmetry factor ( N– Z)/ A seems to reveal some shell effects.

Theory of the effective cross sections of heavy nuclei in the region of partial neutron resonance overlapping

Theory of the effective cross sections of heavy nuclei in the region of partial neutron resonance overlapping

Total Cross Sections of Heavy Nuclei for Fast Neutrons

The total neutron cross sections of twenty-three heavy elements from iron to bismuth have been determined as a function of neutron energy from about 0.05 Mev to 3.2 Mev. Every element whose major isotope (>50 percent abundant) has a closed neutron shell of 50, 82, or 126 neutrons is included among the elements investigated. Below 1.4 Mev, neutrons having an energy spread of 20 kev were employed to take measurements at 20-kev intervals on strontium, yttrium, barium, lanthanum, cerium, and praseodymium. Niobium and molybdenum were also investigated in this region using energy intervals of 70 kev between measurements. These measurements and earlier low energy measurements on fifteen other heavy elements were extended to 3.2 Mev, using an energy spread of 20 kev and intervals between points of about 200 kev. The most unusual feature of the results is a very broad maximum which is clearly observed in the total cross sections of thirteen of the elements studied. This maximum appears to move to higher neutron energies with increasing mass number. The position and shape of the maximum seems to be independent of all other nuclear details, such as the nuclear spin or the binding energy of the added neutron. When averaged over resonances, the total cross sections of the closed-shell nuclei do not appear to be different from the total cross sections of other nuclei of about the same masses. There is some indication that the spacing of resonances in the total cross sections of the closed-shell nuclei is greater than observed in the cross sections of other nuclei.

The Scattering of Fast Neutrons by Heavy Nuclei

The experimental work of Dunning has shown that for fast neutrons the capture cross sections of heavy nuclei are small compared to their cross sections for elastic scattering. While it cannot be concluded from this that polarization effects (i.e., contributions to the elastic scattering from quasi-stationary states of the nucleus-neutron system) are negligible, it seemed worth while to investigate the results of a treatment neglecting polarization. We use the effective potential for a neutron in the field of a heavy nucleus developed by Van Vleck. The values of the constants of the inter-particle interactions are adjusted slightly from those found by Feenberg from binding energy calculations. The problem reduces to that of a neutron in the field of a potential well whose depth is a slowly varying function of the neutron velocity. The scattering problem is solved by the method of partial cross sections. Anomalous resonances due to the attractive potential are found in the higher partial cross sections. Although these resonances have anomalously high peaks it is shown that on account of their sharpness they contribute little to the cross section for a neutron beam having a broad velocity distribution. The cross section is found to vary markedly with both velocity and atomic number. Even for a broad velocity distribution corresponding to Dunning's experimental source oscillations remain in the cross section as a function of atomic number. However, the existing experimental points fit the curve within the experimental error. That the experimental results are in no case much larger than those given by the theory would perhaps indicate that contributions due to polarization effects are small. Possible experimental work to test the theory is mentioned.