A Basic Theoretical Research On Light Nuclear Reaction And The Nuclear Data Calculation

A basic theoretical research on light nuclear reaction and the nuclear data calcu-lation, as the two contents of this thesis have been performed. The basic theoreticalresearch is engaged in the establishment of the pre-formation probability of unstable⁵He cluster with the Iwamoto-Harada model, and the applied research is focused on thedevelopment of the neutron data libraries including file of the double differential crosssections(file-6) for ¹⁹F and ¹⁶O at incident neutron energy up to 20MeV, respectively.To meet the needs the incident neutron energy has been expanded to 30MeV for theneutron data library of ¹⁶O.The data of double differential cross sections for light nucleus are important forthe nuclear engineering applications. However, the file of the double differential crosssections including information of all outgoing particles as well as keeping full energybalance are generally absent in international neutron data libraries because absentthe appropriate theoretical approach. The new light nuclear reaction model developedfrom the unified Hauser-Feshbach and exeiton model could solve this problem well. Thismethod could describe the main reaction mechanism of light nucleus, which are the pre-equilibrium emissions from compound nucleus to discrete levels. Unfortunately, thisreaction mechanism is just absent in the other nuclear reaction codes widely used, sothey are unable to describe the behavior of the light nuclear reaction well. In developingof the new light nuclear reaction model, the possibility of ⁵He cluster emission hasbeen proposed theoretically, therefore the preformation probability of ⁵He cluster inpre-equilibrium emissions needs to be established.The composite particles emissions have been taken into account in the former nu-clear reaction statistical model are stable cluster, such as d, t, ³He andαparticles, butthe possibility of unstable ⁵He cluster emission is ignored for a long time. Althoughthe ⁵He cluster is unstable and separated into a neutron and an alpha particle sponta-neously, the threshold energy of ⁵He emission is smaller than ³He emission in neutroninduced nuclear reaction. Thus, the possibility of ⁵He cluster emission has been affirmedtheoretically. The ⁵He cluster emissions mainly come from the pre-equilibrium reactionprocesses in nuclear reaction. The angular-momentum-dependent exciton model is ableto describe the pre-equilibrium reaction successfully. In the exciton model, it is one of the important factor to describe the ⁵He emission that the pre-formation probability of⁵He cluster is taken into account theoretically. The Iwamoto-Harada model has beenproposed to give the pre-formation probabilities of d, t, ³He andαparticle reasonably,which only include ls shell nucleons. The basic physical idea of this model is that thepick-up mechanism in pre-equilibrium processes of composite particles emissions is em-ployed, namely, the emitted single nucleon picks up other nucleons to form compositeparticle before emitting. The nucleon inside nucleus could be described by the wavefunction of the shell model, and the associate condition of momentum and positionof every degree of freedom can be given by the shell model. Then the pre-formationprobability of each kind of configuration [λ,m], whichλand m stand for the numberof nucleons above Fermi surface and below Fermi surface respectively, for a compositeparticle could be obtained by means of the volume of the phase space occupied by thecomposite particles. In the statistical nuclear reaction models, only the configuration[1,4] of the ⁵He cluster is taken into account. It means that a emitted neutron picksup an alpha cluster in compound nucleus before emitting to form a ⁵He cluster to emitin pre-equilibrium process. The outgoing neutrons from ⁵He separation are contributeto the low energy region of the total neutron double-differential cross sections, and thecalculated results could improve the fitting with the double differential measurementsonce the ⁵He emission is taken into account.The double-differential cross sections of total outgoing neutron for 14.1MeV and18MeV neutron induced ¹⁶O reactions have been calculated and analyzed by usingabove new light nuclear reaction model, which agree fairly well with the experimentalmeasurements for 14.1MeV neutron induced ¹⁶O reaction. The deviation from calcu-lated results and experimental data at low energy region of total outgoing neutronenergy-angular spectrum in 18MeV neutron induced ¹⁶O reaction has been analyzed.Up to now there is only one neutron double-differential measurement in the world at18MeV, so the further accurate measurement is expected. The contributions of neutronpartial spectra from different reaction channels to the total outgoing neutron energy-angular spectrum are analyzed in detail, particularly the outgoing neutrons from ⁵Heseparations. The analysis indicate the neutrons emitted from ⁵He separations play animportant role at the low energy region of the total outgoing neutron energy-angular spectrum in neutron induced ¹⁶O reactions. The direct reaction mechanism is takeninto account during the calculation of neutron induced ¹⁶O reaction, but the calcu-lated results show that the percentage of direct process is small. The analysis for thereaction mechanism indicate the main reaction mechanism in light nuclear reactionis pre-equilibrium emission from compound nucleus to discrete levels of residual nu-clei. Based on the good agreement between the calculated results and experimentalmeasurements, the neutron data libraries including the file of the double differentialcross sections(file-6) have been set up for neutron induced ¹⁶O reactions at the inci-dent neutron energy below 20MeV. To meet the needs, the incident neutron energyis expanded to 30MeV for the neutron induced ¹⁶O reactions, and the neutron datalibrary has been established up to 30MeV. The Kalbach systematic has been employedto give file-6 for ¹⁶O in the eighth release of American ENDF/B-â…¥library. However,the Kalbach systematic is applied to calculate the double differential cross sections ofthe continuous state emissions, so they do not suit to describe many particles emissionsof discrete levels in the light nuclear reactions and could not give the outgoing neutronenergy-angular spectra reasonably. Moreover, it is need to be pointed that the file-6with full energy balance could not be given by using the Kalbach systematic.The new light nuclear reaction model has been applied to the 1p shell nucleisuccessfully. In order to further verify the applicable range, this method is expandedto ¹⁹F for 2sld shell. The double differential cross sections of total outgoing neutronfor 14.2MeV neutron induced ¹⁹F reaction have been calculated and analyzed, whichagree fairly well with the experimental measurements. The (n,2n) reaction channel inneutron induced ¹⁹F reaction has low threshold and the cross sections are larger thanother light nuclei, so the secondary neutrons emitted from (n,2n) reaction dominate thelow energy region in double differential cross sections, whereas the neutrons emittedfrom ⁵He separations contribute to this region indistinctively. Based on the goodagreement between the calculated results and experimental measurements, the neutrondata libraries including the file of the double differential cross sections(file-6) have beenset up for neutron induced ¹⁹F reactions at the incident neutron energy below 20MeV.The TNG code and the Kumabe systematic have been employed to give file-6 for ¹⁹Fin American ENDF/B-â…¦and Japan JENDL-3.3 libraries respectively. In fact, The Kumabe systematic is developed from the Kalbach systematic, therefore, it is not suitto describe light nuclear reaction and could not give the file-6 with full energy balancetoo. The file-6 with full energy balance could be given by using TNG code, whichis based on the Hauser-Feshbach theory and angular-momentum-independent excitonmodel. Only the continuous states of the residual nuclei are taken into account inthe angular-momentum-independent exciton model. Therefore, the TNG code couldnot describe the main reaction mechanism in light nuclear reaction. The analysisindicate the outgoing neutron characteristic spectrum calculated by using discrete levelscompletely could not be given by using the above approaches, which only the smoothspectrum shape is given.The continuous level is taken into account in American ENDF and Japan JENDLlibraries for ¹⁶O and ¹⁹F. But, the energy of discrete levels taken from level schemeis generally high for light nucleus, therefore the discrete levels are taken into accountcompletely in calculations whereas the continuous level is not considered. In casesof using the discrete levels completely or using the continuous level to calculate, thephysical analysis about the level density parameter of the continuous level has beenperformed. In fact, it is an approximate description for nuclear structure to considerthe continuous level, and the more information about particles emissions and the com-petitions among the charged particles and the neutrons as well as theγrays couldbe given by using the discrete levels completely, therefore, the much more accuratephysical picture could be given accordingly.