Passage of fission fragments through thin film plastic scintillators

The slowing down of fission fragments from the spontaneous fission of ²⁵²Cf has been studied in the self-supporting thin films of plastic scintillator NE 102A, using a surface barrier detector. The measured residual energies and hence the energy losses of the mean light and heavy fragments after passing through the thin films of thickness ranging from ～-100 to 1200 μg/cm² of NE 102A are reported. These measurements were carried down to 20.0 MeV and ～- 14.0 MeV for light and heavy fragments respectively. The shape spectrum parameters from the slowed spectra are determined. The measured energy straggling parameters obtained from these spectra show a maximum around 300 μg/cm² of NE 102A and the energy bunching effect due to fission fragments is observed in thicker plastic scintillator films (>300 μg/cm²). The stopping power obtained from these measurements is compared with the theoretical predictions.     

直径30 cm的等高圆柱形载钆液闪探测性能

载钆液闪探测器是高能物理及核物理实验中重要的粒子探测工具。通过研制得到了一台大体积的直径为30 cm等高圆柱形载钆液闪探测器,载钆液闪溶液的载钆量为0.5%wt;利用252Cf中子源进行了中子与γ分辨性能实验测试,结果表明,直径30 cm等高圆柱形载钆液闪的中子与γ分辨性能较差;利用飞行时间技术通过符合测量的方法,分别测量了中子与伽马分辨谱中的中子与γ信号的时间分布,两者峰位之间的时间差为2 ns;利用252Cf裂变电离室的裂变碎片信号作为开门信号,通过符合测量的方法,获得了直径30 cm等高圆柱形载钆液闪的中子俘获时间分布实验数据,中子俘获平均时间为11μs。对于较大体积条件下,载钆液闪的中子与γ分辨性能较差的物理现象,通过实验给出了合理解释和分析。     

基于能量-速度关联技术的裂变产物质量分布测量方法研究

采用能量E-速度v关联技术测量裂变产物碎片的动能和飞行速度能精确测定裂变产物核的质量。本工作主要研究能量-速度关联技术的可行性并解决相关关键技术。实验测量系统由飞行时间测量单元、能量探测器和真空靶室系统组成。实验中用1对微通道板探测器测量粒子飞行时间来确定粒子速度,金硅面垒探测器测量粒子能量。对于~(241) Am放射源5.48 MeVα粒子,飞行时间测量系统时间分辨(FWHM)为186ps,金硅面垒探测器能量分辨(FWHM)为44keV。实验完成了~(252) Cf自发裂变源产物质量分布试测量。初步实验结果显示,裂变产物质量分布在110amu(amu为原子质量单位)的位置时,其质量分辨为1.6amu。     

^252Cf微型快裂变室

本文叙述的快裂变室主要用于~(252)Cf裂变中子飞行时间谱测量。该裂变室具有重量轻(1.65g)、上升时间快(6ns)、对裂变碎片探测效率>99％等特点。针对裂变中子能谱测量的特殊要求,详细论述了裂变室设计应遵循的基本原则及其对能谱测量可能带来的影响。     

采用基于标准谱的热中子俘获瞬发活化伽玛分析方法分析NaCl水溶液

使用ＮａＩ（Ｔｌ）等能量分辨率较低但探测效率高的探测器在复杂环境进行在线热中子瞬发活化伽玛分析时宜采用基于标准谱分析方法。该方法包括标准谱完全集的获得，加权最小二乘解谱，灵敏度及归一化参数刻度等关键环节。我们采用ＮａＩ（Ｔｌ）探测器，使用＾２５２Ｃｆ中子源，对ＮａＣｌ水溶液进行了分析，获得了较好的精度。     

适用于研究低能裂变碎片电荷分布的探测器装置

为研究²⁵²Cf自发裂变碎片电荷分布，建立了由屏栅电离室和ΔE-E粒子望远镜构成的探测器系统。在该系统中，将薄的屏栅电离室作为碎片的ΔE探测器，E探测器是金硅面垒半导体探测器。通过分析实验测量的4参数关联数据，得到了²⁵²Cf自发裂变碎片质量数、动能及碎片在气体ΔE探测器中的能量沉积分布等物理量。用多高斯（multi-Gaussian）分布函数对ΔE探测器的能量响应函数进行最小二乘法拟合，得到了在固定质量数A^*_L和动能条件下轻碎片的电荷分布。结果表明：该探测器系统的电荷分辨能力Z/ΔZ约为40∶1；建立起来的测量技术可用于测定²³⁵U(n,f)和²³⁹Pu(n,f)反应碎片的电荷分布。     

Technique for measuring angular correlations and g-factors of excited states with large multi-detector arrays: An application to neutron rich nuclei produced by the spontaneous fission of Cf

Triple coincidences between prompt γ-rays emitted in the spontaneous fission of ²⁵²Cf were measured with Gammasphere. These data are used to measure the angular correlation of cascades of γ-rays from excited states of neutron rich fission fragments stopped in an unmagnetized iron foil. The hyperfine fields in the iron lattice cause attenuations of the angular correlations between γ-rays emitted from the excited states which have sufficiently long lifetimes. This attenuation is measured and used to calculate the g-factors of excited states in many neutron rich nuclei.     

用于在线测量反应堆内热中子相对通量密度分布的闪烁体光纤探测器研究

闪烁体光纤探测器采用双探头甄别中子信号,利用252 Cf裂变源对探测器系统进行了测试,并与3 He计数管的计数进行了对比。在启明星1#上进行了热中子相对通量密度分布的测量,结合Geant4得到的不同能量段的中子转化率及MCNPX模拟得到的反应堆中子能谱,对探测器进行了相对效率刻度,测试结果与固体核径迹探测器测得的裂变率分布进行了对比。测量结果表明,闪烁体光纤探测器对于252 Cf中子源的响应基本符合点源的衰减趋势,与3 He计数管的测量结果符合较好。在启明星1#热区测得的热中子相对通量密度分布与固体核径迹探测器测量到的结果一致,快区测得的热中子相对通量密度分布与3 He计数管的测量结果及MCNPX的模拟结果符合较好。测量结果为闪烁体光纤探测器的研究提供了较好的实验依据。     

252Cf自发裂变n/γ TOF和TCC谱的MC模拟

时间关联符合法是主动法测量裂变材料特征信息的有效方法之一。首先使用MCNP程序模拟了单个BC-501A液体闪烁体探测器探测252Cf自发裂变n/γ飞行时间谱,模拟谱与实测谱符合很好;在此基础上,模拟了两个BC-501A探测器探测252Cf自发裂变n/γ时间关联符合谱。结果表明,n/γ时间关联符合谱包含了n-n、γ-γ、n-γ和γ-n符合成分。     

Proton-Induced Nuclear Reactions in Silicon

Measurements of the energy deposited in silicon surface-barrier detectors as a result of proton-induced nuclear reactions were carried out at the Harvard Cyclotron for protons with incident energies ranging from 50 to 158 MeV and detectors with thicknesses of 2.5, 4.2, 24.1, 100, and 200 ?m. The number of events in which a given threshold amount of energy is deposited in a 4.2 ?m detector varied with incident proton energy in a manner similar to previous measurements of the proton-induced soft-error cross section. The number of events in which at least a threshold amount of energy was deposited in the detector fell off in a near exponential manner with increasing threshold energy. The data were found to be in reasonable agreement with a computer simulation model developed in our laboratory. The model is used to illustrate how the mass spectra of the residual nuclear fragments shifts towards lower masses with increasing recoil energy. Lighter recoils have longer ranges and a greater chance of leaving a microscopically thin sensitive volume element before coming to their end of range.     

Direct computation of lss parameters for the prediction of fission-fragment ranges and energy-loss in any substance

Analysis of extensive experimental range and energy loss data of ²⁵²Cf fission fragments in gases and solids reveals large discrepancies between the ranges calculated according to LSS theory and the experimental data. The same applies also to experimental energy loss data. A modified LSS expression which is based on these results enables the prediction of range and energy loss data for ²⁵²Cf fragments in any substance. Both the original LSS expressions and the modified expressions used to involve an interpolation procedure and iterative processes.Based on extensive experimental range and energy loss data a direct method for calculating the LSS parameters is introduced, eliminating the need for the interpolation procedure and the iterative process.     

EJ339A型液体闪烁探测系统光输出响应函数的理论模拟与实验研究

对液体闪烁探测器EJ339A中子与伽马辐射测量问题,采用理论模拟与实验分析方法,结合自主设计搭建的基于Lab VIEW的数字化信号处理系统,分别完成22Na、133Ba、137Cs和60Co等4种不同能量的γ源等效电子能量测量与刻度。在此基础上,利用中子飞行时间测量原理,在不同时间窗下将锎(252Cf)源近似分化成若干个单能中子源,获得1.9 7.8 Me V范围内中子在探测器中的光输出响应函数。结果表明,理论模拟与实验测量值在低能段比较吻合,而高能段存在约7.3%的误差。     

Determination of the subcriticality level using the Cf source-detector method

Measurement and monitoring of reactivity in a subcritical state, e.g. during the loading of a power reactor, has a clear safety relevance. The methods currently available for the measurement of k_eff in stationary subcritical conditions should be improved as they refer to the critical state. This is also very important in the framework of ADS (accelerator driven systems) where the measurement of a subcritical level without knowledge of the critical state is looked for. An alternative way to achieve this is by mean of the ²⁵²Cf source-detector method. The method makes use of three detectors inserted in the reactor: two “ordinary” neutron detectors and one ²⁵²Cf source-detector which contains a small amount of ²⁵²Cf that introduces neutrons in the system through spontaneous fission. By observing fissions through the detection system and correlating the signals of the three detectors, the reactivity ρ (and hence the multiplication factor k) can be determined.     

用薄膜~(252)Cf源测定ST451快中子探测器的相对效率

本文叙述了用飞行时间测量~(262)Cf瞬发裂变中子能谱来刻度ST 451探测器的快中子相对效率响应的方法。在飞行时间谱仪中,一个流气式的微型电离室作为裂变碎块的探测器。文中给出了对实验数据的处理和修正的过程。得到了阈值分别为0.420、0.625、0.825、1.168、1.565和1.882MeV中子能量从几百keV到10MeV范围内的相对探测效率。实验结果与用Monte Carlo方法计算的效率作了比较。     

Transient Response of Surface Barrier Detectors as a Function of Fission Fragment Energy

The transient response of silicon surface barrier detectors to fission fragments of Cf252 was observed over a range of incident fragment energies from 13.6 to 90 MeV. From these observations the effect of the plasma formed by the incident particle on the charge collection time was determined. The time to disperse the plasma, tp, was calculated as a function of incident particle energy, Eo, and applied field, E. It was found that tp ? Eo1/m where 2 ? m ? 3 and tp ? E-1. A simple model gave reasonable agreement with the experiments and indicated that the plasma is dispersed by field-enhanced diffusion.     

Monte Carlo simulation of prompt neutron emission during acceleration in fission

The possibility of prompt neutron emission during acceleration of fission fragments (FFs) was examined by means of Monte Carlo method and statistical neutron emission model. Multimodal random neck-rapture model was used to describe the initial distribution of mass, charge, and total kinetic energy of the primary fragments. Statistical model was used to simulate the de-excitation process of the fragments from the moment of scission until full acceleration. By random number sampling, the fission process was simulated in order to obtain the basic physical quantities, and their correlations were analyzed to verify the adequacy of the model. It was found that, on the average, ～10% and ～16% of prompt neutrons for ²³⁵U(n _th,f) and ²⁵²Cf(sf), respectively, were emitted before reaching 90% of the final fragment kinetic energy.     

Limitations in Application of Spark Counter in Counting of Fission Fragments at Various Angles of Incidence and That of Their Energy Degradation in Air

Reports had been made of a spark counter and this has now been used in the studies of detection and counting of fission fragments, incident on an SSNTD at various angles of incidence, from a ²⁵²Cf source. The effect of their straggling in air has also been observed with the use of this counter in the determination of the number of fission fragments. The implications of these studies may be useful in the measurements of total fission events occurring from a nuclear reaction of a heavy element which is of some interest to the reactor and applied nuclear physicists.     

时间关联符合法探测铀部件质量实验研究

采用²⁵²Cf源中子诱发金属铀部件裂变的主动探测方法对铀部件质量识别进行探索,实验测量了3.8～15.2kg8件高浓缩铀部件的源中子与探测器的互相关函数谱C₁₂(τ),互相关函数谱积分值R₁₂随铀部件质量有明显的变化规律。当铀部件质量为3.8～7.2kg时,质量灵敏系数为8.5%;铀部件质量为7.2～15.2kg时,质量灵敏系数为11.0%。     

Fission cross-sections of U and Pu averaged over Cf neutron spectrum

A series of measurements have been carried out to derive values for the spectrum-averaged fission cross-section of ²³⁵U and ²³⁹Pu for ²⁵²Cf fission neutrons. Two nearly identical target foils were mounted on either side of a Cf source (10⁷ neutron/sec) in a compensated beam geometry. Fission fragments passing through limited solid angle apertures were recorded from each foil by solid-state track-etch techniques. The Cf neutron source strength was calibrated in manganese bath relative to the standard source NBS-II. Values of 1.215 ± 0.022 barn for ²³⁵U and 1.790 ± 0.041 barn for ²³⁹Pu were obtained for the fission cross-sections, corresponding to a ratio value of 1.473 ± 0.041.     

Statistical Estimation of Distributions of Physical Quantities in Nuclear Fission

Making use of a model based on the statistical theory, calculations were performed to obtain the mass yields, the most probable charges, the kinetic energies and the prompt neutron yields of fission fragments, and the mass yields of fission products from thermal-neutron-induced fission of ²³³U, ²³⁵U, ²³⁹Pu, ²⁴¹Pu and from the spontaneous fission of ²⁵²Cf. The calculations are further extended to fast-neutron-induced fission. The scission-point distance is treated as a parameter varying with the mass number of the heavy fragments of fission. This proved successful in approaching the calculated curves closer to the observed values.

It is possible to predict unknown physical quantities in nuclear fission with use made of the method developed in the present work.