用直接中子法测量（n, 2n）反应截面的实验方法研究

在中国原子能科学研究院HI-13 串列加速器上的非常规多探测器快中子飞行时间谱仪上,采用直接中子法测量了14.3 MeV 中子与169Tm作用的(n,2n) 反应截面。用蒙特卡罗方法模拟了次级中子的产生,以对实验数据进行中子注量率衰减、多次散射和有限几何修正,同时结合SUNF 方法得到的评价能谱,给出了(n,2n) 反应截面的实验测量结果。测量截面以中子弹性散射微分截面作为标准截面来归一,并用反冲质子望远镜测量n-p 反应的反冲质子,以监视中子注量率。用直接中子法测量得到的结果与评价数据进行了比较,讨论了采用这种方法测量(n,2n) 反应截面的可行性。(n,2n) reaction cross section from 169Tm at 14.3 MeV was measured with the direct emittedneutron coincidence detecting method, using abnormal fast neutron TOF spectrometer on the HI-13 Tandem Accelerator at CIAE. Monte-Carlo method was used to simulate the generation of secondary neutron and correct the experimental data considering neutron flux attenuation, multiple scattering and finite geometry correction. Combining with evaluated spectra given by SUNF program, the experimental measurement results of the (n,2n) reaction cross sections were given. Cross sections of measurements were normalized by using neutron elastic scattering differential cross section as a standard section, and a recoil proton telescope was used to measure recoiling proton from the n-p reaction to monitor neutron flux rate. After comparing the experimental results with evaluated data, the feasibility of the direct emitted-neutron coincidence detecting method is discussed.     

加速器驱动次临界系统中堆外中子注量率监测方法（英文）

在加速器驱动的次临界（ADS）系统中,次临界反应堆的功率控制是通过控制束流强度来实现。监测堆外中子注量率,不仅提供了反应堆功率指示,也为反应堆保护系统在启动和运行阶段提供了重要的监测信息,因此,堆外中子注量率的监测在ADS系统的控制与保护中起着非常重要的作用。采用3套裂变室和3套非补偿电离室来监测ADS堆外中子注量率。由于裂变室有脉冲、电流和均方电压3种操作模式,1套裂变室可以监测源量程、中间量程和功率量程等宽范围的反应堆功率。所以,使用的监测方法有3个优点,即:增加了监测通道的冗余度,提高了保护系统的可靠性,以及能提供更多的轴向功率分布信息。由于这些中子探测器对中子能谱很敏感,提出了一种有效的校准方法,即先用一个标准的中子源校准这些中子探测器,然后再将中子注量率除以一个修正因素。基于Geant4仿真结果显示,所提取的裂变室和非补偿电离室的修正因素分别为5和42。In an accelerator driven sub-critical (ADS) system, power control in sub-critical reactor is achieved through the control of the beam current. Excore neutron flux monitoring in an ADS system, not only provides indication of reactor power, but also provides important inputs to reactor protection system during startup and power operation, and thus plays a very important role in the control and protection of ADS system. This paper presents the excore neutron flux monitoring method which uses three fission chambers (FCs) and three uncompensated ion chambers (UICs). With three operation modes, pulse mode, current mode, and mean square voltage mode, an FC can monitor reactor power over a wide range from the source range to the intermediate and power ranges. The proposed monitoring method increases the redundancy of independent monitoring channels, improves the reliability of the protection system, and provides more information on axial power distribution. Since these neutron detectors are sensitive to the neutron energy spectrum, we propose an effective calibration method to provide the exact value of neutron flux, i.e., these neutron detectors are calibrated with a standardized neutron source, and then, a correction factor is added in the calibration by comparing the neutron energy spectrum of the neutron source with that in ADS system. Based on Geant4 simulation, the correction factors of 5 and 42 are extracted for FCs and UICs, respectively.     

基于D-T紧凑型中子源的快中子照相准直屏蔽体设计及中子束特性模拟研究

中子照相是一种重要的无损检测技术,它能用于火工产品、毒品和核燃料元件等的检测。基于紧凑型D-T中子发生器,完成了一个用于快中子照相的准直屏蔽体系统（BSA）的物理设计。根据D-T中子源的能谱和角分布建立了中子源模型,采用MCNP4C蒙特卡罗程序,模拟了准直屏蔽体系统中中子和γ射线的输运,准直中子束相对于单位源中子的中子注量可以达到9.30×10-6 cm-2,准直中子束中主要是能量大于10 MeV的快中子;在设置的样品平面直径14 cm的照射视野范围,准直束中子注量的不均匀度为4.30%,准直束中中子注量与γ注量的比值为17.20,中子通量和中子注量比值J/Φ为0.992,说明准直中子束有好的平行性;准直屏蔽体外的泄露中子注量率与准直束中子注量率相比降低了2个量级。所设计的准直屏蔽体能满足快中子照相的要求。Neutron radiography is an important nondestructive testing technique. It can be used to detect the explosive devices, drug and the nuclear fuel element, etc. A beam-shaping-assembly (BSA) based on a compact D-T neutron generator is designed for fast neutron radiography in this paper. D-T neutron source model is constructed based on the neutron energy spectrum and angular distribution data. The transportation of neutron and γ-ray in the BSA is simulated using MCNP4C code. The neutron fluence of the collimated neutron beam with respect to the neutron source of the unit source is 9.30×10-6 cm-2. The collimated neutron beams is mainly fast neutrons with energies greater than 10 MeV. In the irradiation field range with a diameter of 14 cm, the neutron fluence uniformity of the collimated beam is 4.3%, the ratio of the neutron fluence to the gamma fluence in the collimated beam is 17.20, and the neutron flux and the neutron fluence ratio (J/Φ) is 0.992 which indicates that the collimated neutron beam has good parallelism. The leakage neutron fluence in outside of BSA is two orders of magnitude lower than that of the collimated neutron beam. The designed BSA can meet the need of fast neutron radiography.     

启明星Ⅱ次临界非稳态热中子注量率测量

中子注量率及分布是反应堆的重要参数,本工作通过核数守恒在非稳态情况下的推导和求解,从理论上论证了次临界反应堆非稳态情况下中子注量率测量的可行性。将活化法和固体径迹法有机结合,利用固体径迹探测器标定活化片的测量数据,测量了启明星Ⅱ零功率装置的He-3管实验孔道内及反应堆外壁的中子注量率的分布,并与模拟计算结果进行了比较,利用MCNPX程序得到的模拟计算结果与实验结果的趋势一致,证明了该测量方法可以测量低通量的中子注量率,可实现反应堆不同时刻、不同位置的中子注量率测量,为CiADS技术的研发提供了实验数据与技术支撑。Neutron flux measurements were carried out at VENUS-Ⅱ lead-based zero power reactor by neutron activation method combined with solid-state nuclear track detectors (SSNTD). This experimental method was proposed based on the principle of nuclear number conservation when a foil was irradiated in an unsteady-state neutron field. By this method, thermal neutron flux distributions inside the He-3 duct were measured when VENUS-Ⅱ was operated under unsteady-state. The neutron flux distributions were also calculated with MCNPX code and were consistent with the experimental data. In addition, the neutron fluxes in the outer layer of VENUSⅡ were measured under steady-state. These results would benefit the further study of experimental methods for neutron flux measurement and provide important support for the design of CiADS.     

T(d,n)4He中子源出射中子能量的单色性与最佳出射角

核反应的Ｑ方程出发,讨论了Ｔ（ｄ,ｎ）４Ｈｅ反应出射中子能量的单色性问题,给出了在Ｔ（ｄ,ｎ）４Ｈｅ中子源上获得准单能中子的最佳出射角的计算方法.Based on the Q equation of nuclear reaction, the monochromaticity of neutron energy at different emitted angles in T(d,n) 4He reaction is discussed and the method of calculation the optimum emitted angle is given.     

14MeV中子引起的核反应截面测量中监督反应的研究

讨论了14MeV中子引起的核反应截面测量中监督反应对测量结果的影响,同时列出了常用的一些监督反应及参数,利用截面的评价值给出了一些监督反应的截面随中子能量变化的关系曲线,并对^27Al（n,p）^27Mg反应做了定性分析,说明了监督反应的选取对反应截面测量的重要性。 It was discussed in this article that the effects of different monitors in the cross section measurements of nuclear reactions induced by 14 MeV neutrons, at the same time some monitors and correlative parameters were listed. The excitation functions of monitors are taken from the evaluatied cross sections, and a qualitative analysis has been performed for ^27Al（n,p） ^27Mg reaction. It indicates that the choice of monitor is very important for cross section measurements     

Cross-section measurement for the ~(93)Nb(n,2n)~(92g)Nb reaction at the neutron energy of 14.6 MeV by the AMS method

The cross-section for the~(93)Nb(n,2n)~(92g)Nb reaction has been measured at the neutron energy of 14.6 Me V using neutron activation and accelerator mass spectrometry(AMS)determination of the long-lived product nuclide~(92g)Nb.The neutron energy was generated from the D+T neutron source at the China Institute of Atomic Energy(CIAE).The neutron flux was monitored by measuring the activity of~(92m)Nb produced in the competing reaction channel of~(93)Nb(n,2n)~(92m)Nb.At the neutron energy of 14.6 MeV,the~(93)Nb(n,2n)~(92g)Nb reaction cross-section of(736±220)mb was obtained for the first time.     

Influence of thermal and resonance neutron on fast neutron flux measurement by ~(239)Pu fission chamber

The 239Pu fission chambers are widely used to measure fission spectrum neutron flux due to a flat response to fast neutrons. However,in the meantime the resonance and thermal neutrons can cause a significant influence on the measurement if they are moderated,which could be eliminated by using10 B and Cd covers. At a column enriched uranium fast neutron critical assembly,the fission reaction rates of 239 Pu are measured as 1.791×10-16,2.350×10-16 and 1.385×10-15 per second for 15 mm thick10 B cover,0.5 mm thick Cd cover,and no cover respectively,while the fission reaction rate of 239 Pu is rapidly increased to 2.569×10-14 for a 20 mm thick polythene covering fission chamber. The average 239 Pu fission cross-section of thermal and resonance neutrons is calculated to be 500 b and24.95 b with the assumption of 1/v and 1/E spectra respectively,then thermal,resonance and fast neutron flux are achieved to be 2.30×106,2.24×106 and 1.04×108cm-2·s-1.     

替代反应法测量锕系核(n,f)和(n,xn)截面

对于某些中子反应截面,在直接测量上存在困难。为了获得那些对核能发展极其重要的核数据,人们提出替代反应法,即利用带电粒子代替中子进行反应,从而实现中子反应截面间接测量的方法。现有替代反应一般采用非弹激发、转移等周边反应,涉及的角动量远比中子反应的大,需要理论修正,这造成了一定的困难。有鉴于此,提出利用轻带电离子的俘获反应作为替代反应,其复合核自旋与中子反应的相当,有效避免了自旋修正的困难。根据这个思路,采用236U的（α,f）和（α,2n）反应作为替代反应,成功提取了239Pu的（n,f）和（n,2n）反应截面,与ENDFB7评价库数据在误差范围内符合较好,表明了替代反应法的优越性,可以在核数据测量中推广应用。There are always difficulties in the direct measurement of reactions induced by neutron, in order to obtain the neutron induced nuclear reaction data which are extremely important for nuclear power development, the surrogate reaction method, which uses charged particles instead of neutron, was proposed. The existing surrogate reaction method generally chooses peripheral reaction as the surrogate reaction, such as inelastic scattering excitation or transfer reaction. As a result, the angular momentum involved is always far larger than the neutron induced reaction, which causes difficulties in the theoretical calculation. Because of this, we proposed to use light-ion capture reaction as the surrogate reaction, the compound nuclei spin is similar with that of the neutron induced reaction, so that the difficulties in spin correction can be effectively avoided. Based on this idea, the 239Pu(n, f) and (n, 2n) reaction cross sections were successfully extracted using 236U(α,f) and (α,2n) reactions as the surrogate reaction. The results coincide well with the data of ENDFB7 within the error range, which shows that the surrogate reaction method has its own superiority, and it can be applied in the measurement of nuclear data.     

Determination of neutron capture cross sections of ~(232)That 14.1 MeV and 14.8 MeV using the neutron activation method

The ~(232)Th(n,γ)~(233)Th neutron capture reaction cross sections were measured at average neutron energies of 14.1 MeV and 14.8 MeV using the activation method.The neutron flux was determined using the monitor reaction~(27)Al(n,α)~(24)Na.The induced gamma-ray activities were measured using a low background gamma ray spectrometer equipped with a high resolution HPGe detector.The experimentally determined cross sections were compared with the data in the literature,and the evaluated data of ENDF/B-Ⅶ.1,JENDL-4.0u+,and CENDL-3.1.The excitation functions of the ~(232)Th(n,γ)~(233)Th reaction were also calculated theoretically using the TALYS1.6 computer code.     

Cross-section measurement for Ni(n,x)58(m+g)Co,Ni(n,x)60mCo,Ni(n,x)61Co and Ni(n,x)62mCo reactions induced by neutrons around 14MeV

The cross sections of Ni(n,x)58(m+g)CO,Ni(n,x)80mCo,Ni(n,x)61Co and Ni(n,x)62mCo reactions induced by neutrons around [14]MeV were measured in this work and calculated by a previously developed formula in this work.The neutron flux was determined using the monitor reaction 27Al(n,α)24Na and the neutron energies were measured with the method of cross-section ratios for 89Zr(n,2n)89Zr to 93Nb(n,2n)92mNbreactions.     

Validation of CASCADE code using the monitor reactions

Production cross-sections of monitor reaction ²⁷Al(x, y)²⁴Na for proton, neutron and deuteron projectiles have been estimated from the CASCADE code from tens of MeV/n to tens of GeV/n energy and compared them with the available experimental data. It has also been shown that using the CASCADE code production cross-section of ²⁷Al(d, y₂)²⁴Na reaction can be obtained from the proton and neutron projectiles. Implicitly, this provides an alternative way of knowing production cross-section of ²⁷Al(n, y ₁)²⁴Na reaction which may be used to monitor the neutron flux in a wide range of energy. However, in the paper need of experimental determination of cross-sections for neutron projectile has been stressed. Similarly, cross-section of other monitor reactions like ²⁷Al(p, y ^′)²²Na and ²⁷Al(n, y ^′ ₁)²²Na are also calibrated as function of energy up to several GeV.     

High-spin yrast and yrare structures in 112In

Up to now, no experimental cross-section data exist for the ⁶⁷Zn (n,a \alpha) ⁶⁴Ni reaction in the MeV neutron energy region. In the present work, the cross-section of the ⁶⁷Zn (n,a \alpha) ⁶⁴Ni reaction was measured at E _n = 6.0 MeV. Experiments were performed at the Van de Graaff accelerator of Peking University, China. Fast neutrons were produced through the D (d, n) ³He reaction using a deuterium gas target. Absolute neutron flux was determined by a small ²³⁸U fission chamber and a BF₃ long counter was used as a neutron flux monitor. A twin gridded ionization chamber was employed as the a \alpha -particle detector and two back-to-back ⁶⁷Zn samples were used for a \alpha events measurement. Background was measured and subtracted from foreground. The measured cross-section of the ⁶⁷Zn (n,a \alpha) ⁶⁴Ni reaction was 7.3 (1±15%) mb at 6.0MeV. The present result was compared with existing evaluations and TALYS code calculations.     

Measurements of the Cross-sections of Produced Short-Lived Nuclei Induced by Neutrons around 14 MeV on Isotopes of Tungsten

New experimental cross-section data for the180 W(n,2 n)179 mW,186 W(n,2 n)185 mW and186 W(n,p)186 Ta reactions at the neutron energies of 13.5 and 14.4 MeV are obtained by the activation technique. The neutron beams are produced by means of the3 H(d,n)4 He reaction. The gamma activities of the product nuclei are measured by a high-resolution gamma-ray spectrometer with a coaxial high-purity germanium detector. The neutron fluence is determined using the monitor reaction93 Nb(n,2 n)92 mNb. The results in the current work are discussed and compared with the measurement results found in the literature. It is shown that these higher accuracy experimental cross-section data around the neutron energy of 14 MeV agree with some previous experimental values from the literature within experimental uncertainties.     

RADIATIVE CAPTURE IN THE INELASTIC CHANNEL OF THE COMPOUND NUCLEUS

In this paper, the radiative capture in the inelastic channel of the compound nucleus is discussed.
Starting from the S-matrix theory, the calculation formular for this situation is derived. Then connected with the optical model and Hauser-Feshbach's statistical theory with the width fluctuation correction, a formular convenient for calculations is given.
Making use of the formular obtained above, the numerical calculations are made for the average cross-section of ⁵¹V and other nuclei, and the comparison with the statistical (n, γ) and (n, n′) cross sections is made.
The results show that if the incident energies of the neutron are large enough to open the inelastic channel, the cross sections of this process will have the same order of magnitude as the statistical cross sections. Hence it is an important mechanism in the (n, γ) reactions.     

Monte Carlo方法计算快中子在样品中多次散射和通量衰减修正因子

本文叙述计算快中子在样品中自吸收通量衰减因子以及(n,γ)和(n,n')反应多次散射修正因子的Monte Carlo方法数学处理过程。     

准单能中子单粒子效应研究现状

对国际上用于单粒子效应(SEE)研究的准单能中子源进行了相关调研,对产生准单能中子源的7Li(p, n)7Be核反应、装置布局以及表征中子场性质的中子注量率、中子能谱、中子束流轮廓及其均匀性、热中子本底等参数的理论计算及实验测量进行了系统的介绍。进行准单能中子SEE实验要求中子源有较高的中子注量率水平、较大的束流轮廓、较好的束流均匀性以及较低的热中子本底,并且能测量出精确的中子能谱。对准单能中子SEE实验过程以及三种中子SEE截面的尾部修正方法进行了介绍。