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Au+Au三分裂反應的輸運理論研究(40)質(zhì)子引起的散裂反應及介質(zhì)內的核子－核子有效相互作用和兩體散射截面(32)
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本報告是歐立博士在博士后工作期間主要研究工作的總結，共分三個(gè)部分。第一部分是核子入射核反應中同位旋效應的研究；第二部分是對中高能質(zhì)子引起的散裂反應的研究；第三部分是利用極限溫度對有限溫度下不對稱(chēng)核物質(zhì)核態(tài)方程進(jìn)行約束的研究。
散裂反應的研究在實(shí)際應用和理論研究?jì)煞矫娑加泻苤匾囊饬x。與重離子碰撞相比，散裂反應在入射能量不太高的情況下反應過(guò)程的溫度比較。所以利用散裂反應核數據抽取到的核物質(zhì)的核態(tài)方程比從重離子碰撞中得到的核態(tài)方程更接近零溫時(shí)的核態(tài)方程。本工作中，我們利用改進(jìn)的量子分子動(dòng)力學(xué)模型ImQMD 結合兩種統計衰變模型，GEM2 及GEMINI 研究了能量在1 GeV 以下的質(zhì)子入射56Fe、136Xe、197Au、208Pb、238U等靶核引起的散裂反應。給出的出射中子、質(zhì)子雙微分截面與實(shí)驗符合得非常好。同時(shí)還給出反應產(chǎn)物的質(zhì)量、電荷、同位素分布。通過(guò)調整統計衰變模型里的參數，ImQMD+GEM2 能夠很好地再現重靶上的散裂反應產(chǎn)物的截面，而ImQMD+GEMINI 則很好地再現了輕靶上的散裂反應產(chǎn)物的截面。由于ImQMD 模型沒(méi)有可調的參數，如果實(shí)驗能提供足夠的精確數據用于GEM2 與GEMINI 中可調參數的系統化，則可以大大地提高模型的預言能力。這對散裂反應的實(shí)際應用非常有意義。
在量子分子動(dòng)力學(xué)模型框架下，研究了核子入射112−132Sn 及其相應穩定線(xiàn)上的同量異位素誘發(fā)核反應中的同位旋效應。研究表明質(zhì)子入射112−132Sn 反應截面的體系依賴(lài)明顯偏離擬合質(zhì)子入射 穩定核得到的Carlson 經(jīng)驗表達式，并且明顯依賴(lài)于對稱(chēng)能的硬度。大碰撞參數下質(zhì)子入射112−132Sn 的彈性角分布對對稱(chēng)能密度依賴(lài)形式非常敏感，這完全是對稱(chēng)勢的效應而沒(méi)有混雜核子-核子散射截面的同位旋依賴(lài)的效應。在入射能量為100 MeV，碰撞參數7.5 fm 條件下，質(zhì)子入射與中子入射彈散的彈散角分布峰值對應的角度差對于軟的對稱(chēng)能是正值，對于非常硬的對稱(chēng)能是負值，而對于線(xiàn)性的對稱(chēng)能則接近零。這樣，大碰撞參數下，能量為100MeV 的質(zhì)子和中子在豐中子靶核上的彈散角分布峰值對應的角度間的差別可以提供一個(gè)非常好的觀(guān)測量，來(lái)約束對稱(chēng)能的密度依賴(lài)形式。質(zhì)子入射Sn 同位素反應截面對體系的依賴(lài)對對稱(chēng)能密度依賴(lài)敏感而對靶核中子皮厚度變化不敏感，而中子入射Sn 同位素反應截面對體系的依賴(lài)對對稱(chēng)能密度依賴(lài)不敏感而對靶核中子皮厚度變化敏感。因此，測量低能中子入射豐中子核的反應截面將能夠為我們提供中子皮厚度的信息。有限核體系的極限溫度，即核體系在破裂前所能承受的最高溫度，與核物質(zhì)的臨界溫度有關(guān)。因此極限溫度的實(shí)驗信息可以用來(lái)檢驗微觀(guān)理論給出的有限溫度下的核態(tài)方程，這是其它方法很難實(shí)現的。我們從核極限溫度出發(fā)，研究了有限溫度下的不對稱(chēng)核物質(zhì)的核態(tài)方程。采用Skyrme相互作用，結合不同對稱(chēng)能密度依賴(lài)的形式，同時(shí)考慮不同表面張力的影響，計算了有限核體系極限溫度，以及Z=38 的同位素的極限溫度，A=93的同量異位素的極限溫度。通過(guò)與實(shí)驗比較，對低密區域的對稱(chēng)能的密度依賴(lài)形式進(jìn)行了約束。結果表明軟的對稱(chēng)能更為合理。此外，我們用極限溫度檢驗了幾套Skyrme 相互作用在有限溫度、高不對稱(chēng)度條件下的適用情況。
關(guān)鍵詞：散裂反應，雙微分截面，核態(tài)方程，對稱(chēng)能，同位旋效應，極限溫度
This report contains the main postdoctoral work of the author. It consists three parts. The first part is the investigation on dynamical isospin effects in nucleon-induced reactions. The second part is the study on proton-induced spallation reactions by the improved quantum molecular dynamics model plus statistical decay models. The last part is about the investigation on the state of asymmetry nuclear matter at finite temperature constrained from limiting temperature.
It is important to study the spallation reactions both on applications and basic research. Comparing with the temperature in the heavy ion collisions, the temperature in the intermediate energy spallation reaction is lower when the incident energy is not very high. So we can extract the information of the equation of state for nuclear matter which is more close to the zero-temperature case than the one from HIC. Intermediate energy proton-induced spallation reactions with the targets 56Fe、136Xe、197Au、208Pb、238U , etc. are studied by the improved quantum molecular dynamics model (ImQMD) incorporated two statistical decay model GEM2 and GEMINI. The double differential cross sections of emitted protons and neutrons are found to be in good agreement with experimental data. By readjusting two respective parameters in GEM2 and GEMINI, the cross sections for products in proton-induced reactions on heavy targets can be reproduced quite well by both models, especially by the ImQMD+GEM2 model. However, the GEM2 model is not very competent to describe the reactions on light targets compared with the GEMINI model which can describe these reactions quite well. Because there is no adjustable parameter in the ImQMD model, for the future, with more precise experimental data to be available, we expect that the systematic adjustable parameters in GEM2 and GEMINI can be obtained with the present approach to improve the prediction power of this hybrid model. That is very significative for the applications of the spallation reactions.
The isospin effect in nucleon-induced reactions on Sn isotopes has been applied to explore the sensitive observable for the density dependence of the symmetry energy. With the ImQMD model, the various forms of density dependence of the symmetry energies are applied in the calculations. Our study shows that the system size dependence of the reaction cross sections for proton-irradiated 112−132Sn depends sensitively on the density dependence of the symmetry energy, so does the angular distributions of elastic scattering for proton-irradiated 132Sn at large impact parameters, which show that this behavior originates uniquely from the effect of the symmetry potential. Furthermore, The isospin effect in neutron-induced reactions is also studied and it is found that the effect is just opposite to that in proton-induced reactions. The difference between the angular distributions of elastic scattering for proton and neutron-induced reactions at the same incident energy positive for soft symmetry energy, negative for extra-stiff symmetry energy and close to zero for linear density dependent symmetry, which will be possibly very useful for constraining the density dependence of the symmetry energy. Based on the same model, we study the effects of the thickness of neutron skin and symmetry potential on nucleon-induced reactions on Sn isotopes in order to find new sensitive observables to the thickness of neutron skin. The investigation shows that the target size dependence of the reaction cross sections for neutron-induced reactions on Sn isotopes is sensitive to the thickness of neutron skin of target nucleus but not sensitive to the density dependence of the symmetry energy, which may be helpful to extract the thickness of neutron skin of neutron-rich nuclei.
The limiting temperatures of finite nuclei, i. e., the maximum temperatures nuclei can sustain before they become unbound, are correlated with the critical temperature of nuclear matter. Experimental information on limiting temperatures will thus permit tests of microscopic calculations of the nuclear equation of state at finite temperature which cannot be easily obtained by other means. The EOS of asymmetry nuclear matter at finite temperature has been investigated from the view of limiting temperature. With Skyrme effective interaction plusvarious density dependence of symmetry energy, and considering the effect of surface tension on the limiting temperature, we calculated the size dependence of limiting temperature, the isotope distribution of limiting temperature for element of Z=38, and the isobar distribution of limiting temperature for element of A=93. By comparing these results to the experimental data, constraints on the density dependence of symmetry energy at subnormal density are obtained.
The applicability of several Skyrme interactions at finite temperature and large asymmetry degree are test by the view of limiting temperature in this work.
Keywords: spallation reaction, double differential cross section, equation of state, symmetry energy, isospin effect, limiting temperature
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