The influence of fission on the fragmentation of relativistic Pb and U projectiles in peripheral collisions

The projectile fragmentation in peripheral nuclear collisions was studied using ≈ 1 GeV·A of ²⁰⁸Pb and ²³⁸U from the SIS at GSI, impinging on a copper target. Lead and thallium isotopes from the fragmentation of ²⁰⁸Pb as well as uranium and protactinium isotopes from ²³⁸U, were selected by the fragment separator FRS. In contrast to the fragmentation of ²⁰⁸Pb, the cross sections for the production of heavy fragments from ²³⁸U are strongly reduced by fission. The measured isotopic yields may be explained quantitatively by calculations in the framework of the abrasion-ablation model, if fission is allowed to compete with particle evaporation in the deexcitation of the prefragments.     

An Isoratio Method to Study Free Energy and Temperature Effects in Intermediate Mass Fragments Produced in Heavy-Ion Collisions

An isoratio method, i.e., the isotopic (isotonic) ratio among three isotopes (isotones), is proposed to study the free energy and temperature effects in the intermediate mass fragments produced in heavy-ion collisions. The parameterizations for the free energy of nucleus at low temperature, which have been proposed in the framework of the density functional theory using the SKM skymre interaction, are adopted to calculate the temperature-dependent free energy of fragment. By analyzing the measured yields of fragments in the 140A MeV ^58,64Ni + ⁹Be reactions, it is verified that the free energy in the isoratio is almost the same for different reactions. A temperature-dependent pairing-energy is introduced into the parameterizations for free energy, which reveals that the weakened pairing energy at the low temperature accounts for the weakened or disappearing odd-even staggering in isoratio.     

AFM observations of latent fission tracks on surfaces: amorphous SiO2 and quartz

Preliminary results of a systematic AFM experimental investigation of the surface ‘track’ effects produced by the passage of fission fragments from a californium (²⁵²Cf) source into amorphous SiO₂ and quartz are described. Fission fragments from the source were collimated using a 10 μm thick aluminum foil and comprised fragments with the usual binary distribution of energies—light and heavy—79.4 and 103.8 MeV. Irradiations and AFM measurements were carried out in air at normal room temperature and pressure. Remarkably high sputtering yields/fragment were discovered, and in the case of crystalline quartz the ejecta was found to be arranged in an ordered manner. A brief discussion is given of a part likely to be played by electronic energy loss induced Coulomb explosion of target atoms for each point of fragment entry.     

46．7MeV／u~（12）C轰击~（197）Au、~（115）In、~（58）

用气体电离室－半导体位置灵敏探测器望远镜测量了４６．７ＭｅＶ／ｕ（１２）Ｃ离子轰击（１９７）Ａｕ，（１１５）Ｉｎ，（５８）Ｎｉ靶时，在大角区发射的从Ｌｉ到Ｍｇ的复杂碎片能谱；由各碎片的能谱提取了蒸发源的温度和碎片发射的最可几动能Ｅｐ，并与＝Ｖｃｏｕｌ＋２Ｔ计算的平均动能进行了比较，发现实验上提取的最可几动能Ｅｐ总是低于计算的．用Ａ．Ｆｒｉｅｄｍａｎ简单的统计公式对复杂碎片的产额进行粗糙拟合，拟合结果令人满意．     

Nucleon Finite Volume Effect and Nuclear Matter Properties in a Relativistic Mean-Field Theory

Effects of excluded volume of nucleons on nuclear matter are studied, and the nuclear properties that follow from different relativistic mean-field model parametrizations are compared. We show that, for all tested parametrizations, the resulting volume energy al and the symmetry energy J are around the acceptable values of 16 MeV and 30 MeV, and the density symmetry L is around 100 MeV. On the other hand, models that consider only linear terms lead to incompressibility Ko much higher than expected. For most parameter sets there exists a critical point （pc, δc）, where the minimum and the maximum of the equation of state are coincident and the incompressibility equals zero. This critical point depends on the excluded volume parameter r. If this parameter is larger than 0.5 fm, there is no critical point and the pure neutron matter is predicted to be bound. The maximum value for neutron star mass is 1.85M⊙, which is in agreement with the mass of the heaviest observed neutron star 4U0900-40 and corresponds to r = 0.72 fm. We also show that the light neutron star mass （1.2M⊙） is obtained for r ≌ 0.9 fro.     

Residue Coulomb Interaction Among Isobars and Its Influence in Symmetry Energy of Neutron-Rich Fragment

The residue Coulomb interaction (RCI), which affects the result of symmetry-energy coefficient of neutron-rich nucleus in isobaric yield ratio (IYR) method, is difficult to be determined. Four RCI approximations are investigated: (i) The M1-RCI adopting the a_c/T (the ratio of Coulomb energy coefficient to temperature) determined from the IYR of mirror-nucleus fragments; (ii) The M2-RCI by fitting the difference between IYRs; (iii) The M3-RCI adopting the standard Coulomb energy at a temperature T=2 MeV; and (iv) Neglecting the RCI among isobars. The M1-, M2- and M3-RCI are no larger than 0.4. In particular, the M2-RCI is very close to zero. The effects of RCI in a_sym/T of fragment are also studied. The M1- and M4-a_sym/T are found to be the lower and upper limitations of a_sym/T, respectively. The M2-a_sym/T overlaps the M4-a_sym/T, which indicates that the M2-RCI is negligible in the IYR method, and the RCI among the three isobars can be neglected. The relative consistent low values of M3-a_sym/T (7.5±2.5) are found in very neutron-rich isobars.     

Dynamical fluctuations in fragmentation reactions in a Boltzmann–Langevin approach

Based on the isospin-dependent Boltzmann–Langevin model, the dynamical fluctuations in the fragmentation reaction of ¹¹²Sn+¹¹²Sn are investigated. The quadrupole moment and octupole moment with zero magnetic quantum number have large fluctuations in the early time of the collisions. The dynamical fluctuations in momentum space show a strong dependence on the incident energy. The effects of using different fluctuations on the fragment cross sections are also studied in the fragmentation reactions. The results by using Q₂₀ + Q₃₀ fluctuation have a better agreement with the experimental data. Calculations using Q₂₀ + Q₃₀ fluctuation produce more proton-rich and neutron-rich nuclei than those using Q₂₀ fluctuation only. Besides, the difference between the production cross sections of fragments calculated by using Q₂₀ and Q₂₀ + Q₃₀ fluctuations is larger in the vicinity of the projectile. These results present that the dynamical fluctuations may affect the whole dynamical process of fragmentation reactions including the production of fragments, due to the nonlinear nature of the Boltzmann–Langevin equation.     

乏燃料贮运用铝基碳化硼复合材料的屏蔽性能计算

采用蒙特卡罗方法, 利用MCNP程序计算了在中子能量为0.5–20 MeV, ²³⁵U核热中子裂变源条件下, 厚度为3–9 cm、碳化硼含量5%–15%的铝基碳化硼复合材料在空气、水、200–1400 ppm (1 ppm=10^-6) 硼酸溶液介质中的中子透射系数. 结果表明: B₄C/Al复合材料的透射系数随碳化硼含量和材料厚度的增加而减少, 随中子能量的升高而增大, 而硼酸浓度的改变对中子透射系数影响不大. B₄C/Al复合材料在水中比硼酸中更能发挥其屏蔽效果, 在空气中屏蔽特性显现出“反转”现象, 中子能量高于5 MeV时透射系数几乎没有变化. 在裂变源条件下的B₄C/Al复合材料中子透射系数均比稳定源20 MeV 低. 介质的中子屏蔽效果是硼酸溶液>水> 空气, 水介质的中子透射系数与介质厚度呈指数下降关系, 裂变源和稳定源条件下分别近似为e^-0.71x和e^-0.669x, x为厚度(cm). 关键词： 蒙特卡罗 乏燃料设备 中子吸收材料 4C/Al')" href="#">B₄C/Al     

Pion-transfer (n, d) and (d, He) reactions leading to deeply bound pionic atoms

Theoretical studies are given on the (n, d) and (d, ³He) reactions leading to deeply bound pionic atoms in heavy nuclei of configuration [(nl)_π·j_n⁻¹]J. The cross sections for various pionic and neutron-hole configurations in the case of a ²⁰⁸Pb target are calculated at incident energies 300–1000 MeV/u by using the effective number approach and the eikonal approximation for distortion. The effective number with a pion in the 1s or 2p state and a neutron hole in the orbit peaks around the same incident energy (T_n = 600 MeV) as the elementary cross section n+n→d+π⁻, where the momentum transfer matches the angular-momentum transfer of L = 5–7. The DWIA cross section for (n,d) producing a pion in the 1s or 2p orbit at T_n = 600 MeV is found to be around 42 or 75 μb/sr, respectively. At T_n = 350 MeV, where the momentum transfer is small, quasi-substitutional states of configurations and are preferentially populated with (n, d) cross sections of 95 and 190 μb/sr, respectively. The (d, ³He) cross sections are estimated to be an order of magnitude smaller than the (n, d) cross sections. Thus, the (n, d) and (d, ³He) reactions are found to be suited for the production of deeply bound pionic atoms.     

Angular momentum in peripheral fragmentation reactions

The angular momentum induced in peripheral fragmentation reactions at relativistic energies is described on the basis of shell-model considerations. An analytical expression for the spincutoff parameter of prefragments is obtained, and numerical calculations in the framework of the abrasion-ablation model are performed in order to estimate the angular-momentum distribution of prefragments and final fragments. Additionally, an approximative formula is given to calculate the angular-momentum distribution of final fragments.     

Temperature and excitation energy of hot nuclei in the reaction of 40Ar+197Au at 25 MeV/nucleon

The coincidence measurements between heavy fission fragments and light charged particles with Z ⩽ 2 were carried out for the ⁴⁰Ar+¹⁹⁷Au reaction at 25 MeV/nucleon, to study the properties of hot nuclei in heavy ion induced reactions. The linear momentum transfers (LMTs) were deduced from the folding angle and the time-of-flight difference between two fission fragments of heavy residues. The relationship of the nuclear temperature (slope parameter of the energy spectrum) and the excitation energy was determined independently from the measurement of the kinetic energy spectra in the frames of the emitting sources and from the LMT analysis. Both the temperature and the excitation energy increase with decreasing impact parameter, which suggests that a plateau temperature of 5.5 MeV is reached at an excitation energy of 3.1 MeV/nucleon. The result was also compared with various statistical models that explain the plateau by the multifragmentation process, where the excitation energy is assumed to be stored in compression and expansion effects.     

Polarization of neutrons from the O(d, n)F reaction

The polarizations of neutrons from the ¹⁶O(d, n₀)¹⁷F and ¹⁶O(d, n₁ ¹⁷F reactions have been measured at 30° (lab) in steps of approximately 0.15 MeV from E_d = 3.96 to 5.35 MeV. Polarization angular distributions have been obtained at 3.96 and 5.35 MeV. It is determined that the random phase approximation for the scattering amplitudes is not appropriate for the compound nucleus contributions. The analysis indicates probable interference between the compound nucleus and direct interaction reaction mechanisms.     

Study of S11 resonance in nuclei through (γ, η) reactions

In order to study the S₁₁(1535) resonance in the nuclear medium, total cross sections of the (γ, η) reactions on C, Al and Cu have been measured for photon energies between 680 and 1000 MeV. A broad resonance due to the excitation and decay of the S₁₁ resonance in the nucleus has been clearly observed for the first time. The apparent energy and width of the resonance are 900 and 300 MeV, respectively. Model calculations based on the quantum molecular dynamics have been performed. A comparison of the calculations with the data indicates that the S₁₁ resonance in nuclei should have a larger width; the width of about 250 MeV is required while the elementary one is about 150 MeV.     

Correlation phenomena in the fragmentation of 32s-induced reactions

Fragmentation data from the reactions ³²S + ²⁸Si, ⁴⁰Ca, ⁵⁸Ni at 10 MeV/u are used to deduce the scission configurations. Assuming three fragments the possible scission configurations are expanded into two basic configurations (closed and stretched configurations) which can be distinguished by means of the relative fragment momenta. The majority of the analyzed reactions is in agreement with the assumption of random fragmentation. However, in a few reactions consisting of one heavy and two light fragments the fragmentation appears to pass through the closed configuration, whereas the stretched configuration is formed in reactions containing one light and two heavy fragments.     

136Xe+198Pt多核子转移反应机制的理论研究

如何生成丰中子重核是目前原子核物理领域的热点问题之一,多核子转移反应是生成丰中子重核的一种可行途径.本工作采用时间相关的Hartree-Fock(TDHF)方法研究了136Xe+198Pt在Ec.m.=645 MeV的多核子转移反应.对动力学过程中的核子转移、能量耗散、形变核的朝向效应等进行了细致的分析,并结合统计衰变...     

Cross sections for the photoproduction of Al and Al from phosphorus

Bremsstrahlung activation curves were measured for the reactions ³¹P(γ, 2p)²⁹Al and ³¹P(γ, 2pn)²⁸Al in 0.5 MeV steps from 23.0 to 62.0 MeV. The cross sections obtained from these curves show peaks at 32.6 ± 0.7 MeV, 41.2 ± 0.8 MeV and 51.0 ± 1.5 MeV for both reactions. The cross section integrated to 62 MeV for the (γ, 2p) reaction is 5.6 ± 0.6 MeV · mb and for the (γ, 2pn) reaction is 7.6 ± 0.8 MeV · mb.     

The response of thermally and optically stimulated luminescence from Al2O3:C to high-energy heavy charged particles

The thermoluminescence (TL) and optically stimulated luminescence (OSL) response of Al₂O₃ dosimeters to high-energy heavy charged particles (HCP) has been studied using the heavy ion medical accelarator at Chiba, Japan. The samples were Al₂O₃ single-crystal chips, of the type usually known as TLD-500, and Luxel^TM dosimeters (Al₂O₃:C powder in plastic) from Landauer Inc. The samples were exposed to ⁴He (150 MeV/u), ¹²C (400 MeV/u), ²⁸Si (490 MeV/u) and ⁵⁶Fe (500 MeV/u) ions, with linear energy transfer values covering the range from 2.26 to 189 keV/μm in water and doses from 1 to 100 mGy (to water). A ⁹⁰Sr/⁹⁰Y beta source, calibrated against a ⁶⁰Co secondary standard, was used for calibration purposes. For OSL, we used both continuous-wave OSL measurements (CW-OSL, using green light stimulation at 525 nm) and pulsed OSL measurements (POSL, using 532 nm stimulation from a Nd:YAG Q-switched laser). The efficiencies (η_HCP,γ) of the different HCPs at producing OSL or TL were observed to depend not only upon the linear energy transfer (LET) of the HCP, but also upon the sample type (single crystal chip or Luxel^TM) and the luminescence method used to define the signal—i.e. TL, CW-OSL initial intensity, CW-OSL total area, or POSL. Observed changes in shape of the decay curve lead to potential methods for extracting LET information of unknown radiation fields. A discussion of the results is given, including the potential use of OSL from Al₂O₃ in the areas of space radiation dosimetry and radiation oncology.     

Stimulated acceleration and confinement of deuterons in focuseddischarges. II

For pt.I see ibid., vol.16, no.3, p.368-73, June 1988. Methods of increasing, by a factor greater than five, the neutron yield/short Y _n from D-D fusion reactions in a plasma focus (PF) enhance both the D⁺-ion acceleration to energy values E _d>1-8 MeV and the ion confinement in the pinch region. Nuclear activation of C and N in the (doped) filling gas of the discharge chamber and of solid targets of C and BN bombarded by the ion beam in the direction of the electrode axis (0°) confirms earlier determination of the energy spectrum of the trapped ions (dφ_t/dE∝φ_0tE ^-m) and of the ejected beam (dφ_b/dE∝φ_0bE _d^-m, m=2.5±0.5 for 0.1 MeV≲E≲3 MeV). A Thomson (parabola) spectrometer with nanosecond time resolution determines the time of emission t( E) of the beam at 0°. Ion acceleration and trapping occur within the small (filamentary) elements of the magnetic fine structure of the pinch, which can be dispersed on a relatively large confinement volume after the pinch disintegration. It has been found that φ_t/φ_b≳10-10³ for E_d≳1 MeV, depending on Y_n     

Complex Fragments Emitted in Reactions of 46．7MeV/u 12C on 197Au,115In and 58Ni

Energy spectra of complex fragments emitted in the reactions of 46.7MeV/u ¹²C with different targets ¹⁹⁷Au,¹¹⁵In and ⁵⁸Ni have been measured by telescopes,consisting of a gis ionization counter followed by a position Sensitive semiconductor detector.Tha most probable kinematical energies.E_p of complex fragments and temperature parameters of long－live intermediate systems have been deduced by complex fragment ernergy spectra.The experimental values of E_p are lower than the calculated ones by E=V_coul+2T.The experimental values of complex fragment yields can be reproduced by simple A.Friedman statistical evaporation model satisfactorily.     

Study on υμ, υγ, Decays and CP Non-Invariance at Early Epoch of the Universe

We study the υ_μ and υ_γ decays in the early epoch of the universe. If m_υγ > 2m_e, there would be a CP asymmetry between υ_γ→e⁺+e^-+υ_e and υ_γ→e⁺+ e^-+ υ_e. The resultant CP non-invariance is a function of temperature and density and can reach l0^-7 for a reasonable temperature range, but it is noticed that if m_υγ > 2υ_μ～200 MeV, the CP non-invariance can be much larger.