摘要
最近，在研究中子诱导的轻核反应总中子出射的双微分截面理论中，预平衡反应中~5He发射的可能性被揭示了出来。确立~5He发射的可能性以后，为了在理论上描述~5He的预平衡发射，便需要给出~5He预平衡发射的双微分截面理论公式。基于“pick-up”机制，利用计算d，t，~3He，α等复杂粒子预平衡出射双微分截面的理论方法，把它推广到用来描述不稳定核~5He的发射，建立了描述预平衡反应中~5He发射的双微分截面理论公式。完善了统计理论中包含~5He在内的预平衡发射各种复杂粒子双微分截面的理论模型。
计算分析表明，对于低能核反应，~5He发射中的[1，4]组态为主要项，即一个出射中子拾取复合核中费米海下的四个核子形成~5He发射．出射粒子分布的朝前性是预平衡反应的特征之一，入射中子能量越高，复杂粒子发射的朝前性越强；在相同的入射能情况下，复杂粒子发射的朝前性弱于单粒子发射的朝前性。这是由于复杂粒子发射中包含了拾取费米海下的核子，而费米海下核子的运动是各向同性的，这使得复杂粒子发射的朝前性减弱。出射的复杂粒子发射朝前性的强弱主要取决于复杂粒子集团中每个核子的平均动量的大小，平均动量越大，朝前性会越强．以n+~(14)N反应为例，讨论了预平衡反应中~5He发射的特点。结果表明~5He出射的朝前性强于α粒子而弱于中子。~5He发射后自发崩裂为一个中子和一个α粒子，计算了n+~(14)N反应中~5He发射后崩裂的中子和α粒子的能谱范围，从~5He崩裂出来的中子主要贡献在总出射中子谱的低能区域。
在统一的Hauser-Feshbach和激子模型理论基础上，发展了一个新的轻核反应理论模型。使用在这个新理论模型基础上编写的LUNF程序，对n+~(14)N反应出射粒子的双微分截面进行了计算和分析。新的理论模型可以严格保证预平衡反应过程中的角动量和宇称守恒。并建立了严格的运动学公式以保证核反应过程中的能量平衡。计算结果显示理论计算能够很好的符合实验测量值，说明新的轻核反应理论模型可以描述好1p壳轻核反应行为。
利用LUNF程序计算了n+~(14)N反应，结果表明n+~(14)N反应的反应机制非常复杂，在入射能En=14.2MeV时，便有超过一百个对应于不同分立能级的分反应道开启，由于与n+~(14)N反应有关的核素的能级纲图给出的分立能级足够高，因此在计算过程中全部使用分立能级，没有使用连续能级。在理论计算中，严格考虑了复合核到剩余核分立能级预平衡发射时的角动量守恒和宇称守恒。对反应中~5He的发射也进行了分析，结果表明~5He发射在中子入射能比较高时有比较重要的作用，从~5He发射崩裂出来的中子贡献于总出射中子谱的低能部分。
对于n+~(14)N反应，预平衡反应机制占主导地位，这也和其它的1p壳轻核相一致，例如~(6，7)Li，~(12)C，~(16)O和~(10)B等。因此仅考虑平衡态发射理论，例如Hauser-Feshbach模型，不能够描述好1p壳轻核反应。将利用新的轻核模型理论计算的结果和ENDF／B-VI库利用Kalbach系统学方法给出的双微分截面谱进行了比较，结果表明利用Kalbach系统学方法不能得到出射粒子的能量特征谱。轻核反应中剩余核的反冲效应非常明显，严格的运动学公式不仅能保证核反应过程中的能量平衡，并且能够给出二次出射粒子的能谱的合理形状。这为建立中子数据库中双微分截面文档提供了运动学的理论基础。
由于缺少理论方法，以往国内外的核数据库中~(14)N的中子核数据文档中没能建立双微分截面文档．现在利用LUNF程序，在符合中子双微分截面实验测量数据的基础上，制作出了以ENDF／B-VI库格式形式的在中子入射能量20MeV以下的双微分截面数据文档．由于核工程应用需要，又将能量范围提高到30MeV。在吸收ENDF／B-VI库数据文档的基础上，给出能量区间10~(-5)eV到30MeV包含双微分截面数据文档的全套~(14)N中子数据．制作出的n+~(14)N全套核数据各项检查均达到要求，可以提供核工程应用。
In recent years, the total outgoing neutron double-differential cross sections of neutron induced light nuclear reactions have been calculated by using a new nuclear reaction model. In these researches, the possibility of 5He emission was revealed. In order to describe the ~5He emission, the theoretical formula of the double-differential cross section of ~5He emission needs to be established. Based on the pick-up mechanism in pre-equilibrium emission processes, used for calculating the formula of d, t, ~3He andαemissions, the theoretical formula of double-differential cross section of ~5He is obtained, which complete the description of the double-differential cross section of composite particle emissions in the pre-equilibrium statistical model.
In the case of low incident energies, the configuration [1, 4] is the dominant part in the reaction processes, which means one nucleon above the Fermi sea and the others in the Fermi sea. The calculated results indicate that for a composite particle, the higher of the incident energy is, the stronger of the forward tendency is. When there is the same incident energy, the forward tendency of a composite particle is weaker than the single particle's. This is because of the motion of the nucleons in the Fermi sea is isotropic; the picked-up particles should counteract the forward tendency of the outgoing single particle. The calculated results show that the forward tendency is determined by the average momentum per nucleon in the emitted composite particles. The larger of the average momentum is, the stronger of the forward tendency is. Besides the reason mentioned above, the forward tendency also depends on the form of integrated factor.
As an example, the reactions of n＋~(14)N have been calculated; the results show that the forward tendency of ~5He is stronger than that of theαparticle but weaker than the single neutron's. ~5He is unstable, it will be separated into one neutron and oneα-particle spontaneously. The double-differential cross sections of the outgoing neutron and theα-particle in the center of mass system have been calculated, the results show that the energy ranges of the outgoing neutron and the a-particle spectra contribute to the low energy spectrum in the total outgoing energy spectrum.
By using a new reaction model for the 1-p shell light nuclei, the double-differential cross sections of n＋~(14)N reactions have been analyzed. This new model keeps angular momentum and parity conservation properly in the pre-equilibrium emission. Meanwhile, the energy balance for variety particles emission is strictly considered by the accurate kinematics. The calculated results agree fairly with the double-differential measurements. It is affirmed that the new model is able to describe the behavior of 1-p shell light nuclear reactions well.
Based on the new model, the LUNF code has been established. Using the LUNF code, the double-differential cross sections of n＋~(14)N reactions have been calculated. The reaction mechanism of n＋~(14)N is very complex, there are more than one hundred partial reaction channels opened even at incident neutron energy of 14.2MeV. In the model calculation, all the residual states are discrete levels without continuous state. The particle emissions from the compound nucleus to the residual nuclei's discrete levels in pre-equilibrium mechanism are taken into account with the angular momentum and parity conservation. ~5He emission in the n＋~(14)N reaction has been considered, when the incident energy is high, the neutron from the ~5He separating contributes to the total outgoing neutron energy spectra in the low energy region.
In the case of n＋~(14)N reaction, the pre-equilibrium mechanism dominates the whole reaction processes, which is the same as other 1-p shell light nuclei, like ~(6,7)Li, ~(10)B, ~(10)C, and ~(16)O. Thus, only the equilibrium theory could not work well, like Hauser-Feshbach model, of the 1-p shell light nuclear reaction. Meanwhile, from the comparison of the outgoing neutron angular-energy spactra obtained by using this new model and Kalbach Systematics, the results of the comparison indicate that the Kalbach Systematics could not give the correct energy spectrum of the outgoing particles. For the 1-p shell light nucleus, the mass of the compound nucleus is so light that the recoil effect of the particle emission is very strong. Thus, the accurate kinemics plays an important role, which not only keeps the energy balance, but also to produce the correct shapes of the partial outgoing spectra in the secondary particle emission processes for reproducing the double-differential measurements. Therefore, the accurate kinemics is the one of the key point to set up file-6 for double-differential cross sections in the neutron library with full energy balance.
Lack of suitable theoretical method, there is no files-6 for double-differential cross sections of ~(14)N in the worldwide neutron data libraries. Now, a light nuclear reaction model is proposed, and the LUNF code is developed accordingly. In terms of fitting the experiment measured data with LUNF code, the files-6 for double-differential cross sections of n＋~(14)N reactions at incident energies up to 30MeV have been established in the ENDF format. And utilizing the data in the ENDF/B-Ⅵneutron library, the total file of neutron induced ~(14)N reactions including file-6 for double-differential cross sections in the energy range 10~(-5)eV to 30MeV can be established, which could provide the application in the nuclear engineering.
引文
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