Maximizing the brightness of an electron beam by means of a genetic algorithm

We present the architecture and some applications of a genetic algorithm developed for solving problems in beam dynamics of high brightness beams, where highly non-linear behaviour of the beam characteristics are optimized through a multi-dimensional variation technique based on genetic evolution criteria.     

Neutron interrogation of high-enriched uranium by a 4 MeV linac

For revealing unauthorized transport (illicit trafficking) of nuclear materials, a non-destructive method reported earlier, utilizing a 4 MeV linear accelerator for photoneutron interrogation, was further developed. The linac served as a pulsed neutron source for assay of highly enriched uranium. Produced in beryllium or heavy water by bremsstrahlung, neutrons subsequently induced fission in the samples. Delayed neutrons were detected by a newly designed neutron collar built up of 14 ³He counters embedded in a polyethylene moderator. A PC controlled multiscaler served as a time analyzer, triggering the detector startup by the beam pulse. Significant progress was achieved in enhancing the detector response, hence the sensitivity for revealing illicit material. A lower sensitivity limit of the order of 10 mg ²³⁵U was determined in a 20 s measurement time with a reasonable amount of beryllium (170 g) or of heavy water (100 g) and a mean electron current of 10 μA. Sensitivity can be further enhanced by increasing the measurement time.     

Design study of a multibeam type IH-RFQ linac to accelerate high intensity and low energy heavy ion beams

A multibeam Radio Frequency Quadrupole (RFQ) linac with an Interdigital H-mode (IH) structure has been developed for high intensity heavy-ion beam acceleration. The defocusing force of space charge effect is proportional to the beam current and to the inverse square of the beam velocity. Therefore, multibeam acceleration is an attractive scheme to suppress this in the low energy region. The electromagnetic field distribution and the beam dynamics were studied for a prototype. We discuss the configuration of the multibeam IH-RFQ linac and the results of these simulations.     

Photoneutron interrogation of low-enriched uranium induced by bremsstrahlung from a 4 MeV linac

Revealing smuggled nuclear material by passive γ-detection is hindered, because the weak radiation can easily be shielded. Neutrons, as penetrate shielding, represent a detection potential, by inducing fission in the nuclear material. A 4 MeV linear accelerator was used as a pulsed neutron source for active interrogation of U-bearing material. Produced in heavy water by bremsstrahlung, neutrons subsequently induced fissions in UO₂ samples. Delayed fission neutrons were detected in a neutron collar built up by ³He counters in a polyamide container. The counters were gated to be detached from high voltage during the electron pulse. Irradiation-measurement cycles were carried out with a 25 Hz pulse repetition rate as optimum setting. The time analyser start-up was externally triggered and synchronised by the electron beam pulse. The response of the system was studied as a function of the intensity of the electron current, the amount of heavy water, U enrichment, and total U content. Sensitivity limit was achieved as 0.5 g ²³⁵U and/or 30 g ²³⁸U in a 20 s measurement time (500 cycles) with the amount of heavy water of 100 g and a mean electron current of 2 μA. Because of the long decay time of the prompt (interrogating and fission) neutron pulse, about a half of the time interval (40 ms) between pulses is only available for counting delayed neutrons.     

Influence of some variable parameters on bremsstrahlung dosimetric characteristic for electron linac

The paper presents some important problems related to the practical aspect of employing the bremsstrahlung radiation generated by linear accelerators used in the irradiation technological processes, namely:

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The optimization of the electron-bremsstrahlung conversion output by optimizing the target thickness and

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the study on the influence of some variable parameters of the accelerator (i.e. the beam current, the magnetron frequency and the injection voltage) on the dosimeter characteristics of the bremsstrahlung radiation.

The measurements have been performed with the 3.5 and 10 MeV linear electron accelerators in NILPRP-Electron Accelerator Laboratory.     

Dependence of PXR beam performance on the operation of the pulsed electron linac

Advanced applications of parametric X-ray radiation (PXR) such as energy-dispersive X-ray absorption fine structure (DXAFS) analysis and phase-contrast imaging have been developed at the Laboratory for Electron Beam Research and Application (LEBRA) of Nihon University. To improve the electron beam quality and geometrical stability of the target crystal, the cooling-water system for the linac and the PXR target was replaced with a new one capable of more precise control. As a result, the reliability of the experimental data in PXR applications, especially in X-ray imaging, has improved. The effect of the electron beam focusing on the target crystal was also investigated. The results of X-ray imaging with a long propagation distance and measurement of the time-structure of the PXR intensity suggest that the correlation between the electron beam profile and the X-ray coherence is rather complicated. It is possible that incident electrons cause some deformation of the target crystal, becoming the dominant factor restricting the quality of intense PXR.     

Quantification of sodium in ancient Roman glasses with ion beam analysis

Concentrations of light elements (Na, Al, Si) in glass have been profiled by comparing the PIGE yields as a function of beam energy to those of a glass standard in combination with PIXE measurements which are used to determine the specific energy loss of protons in the unknown glasses. The technique has been tested on a known sample and applied to a few Roman natron glasses giving results compatible with the morphology of the archaeological samples, as deduced from microscopy. The results have suggested some considerations on the systematic use of the PIGE technique in the determination of bulk sodium concentration in ancient materials.     

IBA analyses and lead concentration measurements of AMS-C dated bones from two medieval sites in Italy

AMS (accelerator mass spectrometry) radiocarbon dated human bone samples recovered in the deserted medieval village of Quattro Macine, Lecce, in Southern Italy, were analyzed by PIXE (particle induced X-ray emission) and PIGE (particle induced gamma ray emission). The optimization of the PIXE-PIGE experimental set-up in term of beam energy and detection geometry resulted in the possibility to obtain information about major, minor and trace elements of the samples. Unexpectedly, all the analyzed samples showed a Pb concentration higher than 50 ppm. Two possible explanations of the measured high Pb concentration are discussed in the paper: the pre-mortem intake associated with the use of Pb-rich glazed pottery and the post-mortem intake from soil as the result of diagenetic processes.     

An analytic formalism for the emission of coherent transition radiation from an oblique finite thin metallic target screen

Long-wavelength coherent transition radiation is commonly used in electron beam diagnostics for the determination of bunch lengths. Typically the spectrum of coherent transition radiation for a short bunch accelerator is settled in the low or sub-THz regime. Hereby, we present a theoretical model based on physical optical techniques in order to calculate emission characteristics for transition radiation, for both the radiating near-field and the far-field. This approach yields analytic solutions for the emitted electromagnetic fields without the need to solve integral equations. The simulated intensity distribution is compared with measurements showing good agreement.     

Thermodynamic properties and equation of state of liquid lead and lead-bismuth eutectic

Since the 1950s, liquid lead (Pb) and lead-bismuth eutectic (Pb-Bi) have been studied in the USA, Canada and in the former-USSR as potential coolants for nuclear installations due to their very attractive thermophysical and neutronic properties. However, experimental data on the thermal properties of these coolants in the temperature range of interest are still incomplete and often contradictory. This makes it very difficult to perform design calculations and to analyse the normal and abnormal behaviour of nuclear installations where these coolants are expected to be used. Recently, a compilation of heavy liquid metal (HLM) properties along with recommendations for its use was prepared by the OECD/NEA Working Party on Fuel Cycle (WPFC) Expert Group on Lead-Bismuth Eutectic Technology. A brief review of this compilation and some new data are presented in this article. A set of correlations for the temperature dependence of the main thermodynamic properties of Pb and Pb-Bi(e) at normal pressure, and a set of simplified thermal and caloric equations of state for the liquid phase are proposed.     

Measurement of electrical conductivity of Pb-Bi alloys in the melting-solidification region

The electrical conductivity, σ(T), of Pb-Bi alloys of eutectic and near eutectic compositions was investigated in the melting-solidification temperature region. The revealed discrepancies between the heating and cooling σ(T) curves as well as a hysteresis observed in course of heating-cooling cycles suggest a metastable microheterogeneous structure of the Pb-Bi melts.     

Study of chaotic behaviors in intrinsic photoconductors

In intrinsic photoconductors, the photoconduction process can be described with the help of a set of nonlinear differential equations. This paper investigates numerically chaotic behaviors in radiated intrinsic photoconductors under different parameter conditions, and some useful results are presented.     

辐照食品直线加速器电子枪设计

本文介绍了中国科技大学国家同步辐射实验室研发的一台食品辐照保鲜用直线加速器中电子枪的设计.通过枪体内电磁场模拟计算、优化电子枪形状、并且在考虑空间电荷效应情况下对电子束发射过程进行模拟,得到了满足设计要求的束斑尺寸.另外对电子枪的加工和调试过程也进行了介绍,通过电子枪高压老炼及出束实验表明,其各项参数性能均达到设计要求.另外对直线加速器螺线管聚焦线圈漏场对束流的影响也作了讨论,采用相应措施可增加磁屏蔽,减小漏场影响,保证束流品质.     

Charge transfer in collisions of protons with water molecule at high projectile energies and small scattering angles

The electron capture process induced by fast protons impinging upon water has been theoretically studied for very small scattering angles. Singly differential and total cross sections have been calculated for scattering angles ranging from 0 to 0.5 mrad and compared to the existing experimental data. For the sake of comparison, we have also presented the theoretical charge transfer cross sections for proton-helium collisions.     

Structure and electrophysical properties of liquid Pb83Mg17 and Pb83Li17 eutectics

Short range order structure of Pb₈₃Mg₁₇ and Pb₈₃Li₁₇ liquid alloys has been studied by means of X-ray diffraction method. The structure factors and pair correlation functions are analyzed. Experimental structure data were used to calculate the partial structure characteristics by means of Reverse Monte Carlo method. It is shown also that Li₄Pb significantly affects the structure of Pb₈₃Li₁₇ eutectic melt. For Pb₈₃Mg₁₇ eutectic melt the electrical resistivity and thermo-e.m.f. were measured in the temperature range of 550-1300 K. Their analysis confirms the diffraction data concluding the heterocoordinated atomic distribution Pb and Mg atoms.     

Quantification of unavailability caused by random failures and maintenance human errors in nuclear power plants

Risk analysis of any equipment or system estimates the unavailability of redundant components due to hardware failure, periodic test and repair work, and human errors in maintenance tasks. A model has been developed in this study to estimate the unavailability of a periodically repairable component of a system that considers hardware failure and maintenance errors due to human failure, and which is applicable to estimate system unavailability in nuclear power plants as well as other industries. A sensitivity analysis has been performed based on the newly developed model considering multiple levels of human errors in maintenance tasks to observe the effects of maintenance errors on overall unavailability of a component. The effect of human error on the optimal frequency of periodic maintenance of a device has been analyzed based on the newly developed unavailability quantification model.     

Ion beam induced defects in solids studied by optical techniques

Optical methods can provide important insights into the mechanisms and consequences of ion beam interactions with solids. This is illustrated by four distinctly different systems.X- and Y-cut LiNbO₃ crystals implanted with 8 MeV Au³⁺ ions with a fluence of 1 × 10¹⁷ ions/cm² result in gold nanoparticle formation during high temperature annealing. Optical extinction curves simulated by the Mie theory provide the average nanoparticle sizes. TEM studies are in reasonable agreement and confirm a near-spherical nanoparticle shape but with surface facets. Large temperature differences in the nanoparticle creation in the X- and Y-cut crystals are explained by recrystallisation of the initially amorphised regions so as to recreate the prior crystal structure and to result in anisotropic diffusion of the implanted gold.Defect formation in alkali halides using ion beam irradiation has provided new information. Radiation-hard CsI crystals bombarded with 1 MeV protons at 300 K successfully produce F-type centres and V-centres having the structure as identified by optical absorption and Raman studies. The results are discussed in relation to the formation of interstitial iodine aggregates of various types in alkali iodides. Depth profiling of and aggregates created in RbI bombarded with 13.6 MeV/A argon ions at 300 K is discussed.The recrystallisation of an amorphous silicon layer created in crystalline silicon bombarded with 100 keV carbon ions with a fluence of 5 × 10¹⁷ ions/cm² during subsequent high temperature annealing is studied by Raman and Brillouin light scattering.Irradiation of tin-doped indium oxide (ITO) films with 1 MeV protons with fluences from 1 × 10¹⁵ to 250 × 10¹⁵ ions/cm⁻² induces visible darkening over a broad spectral region that shows three stages of development. This is attributed to the formation of defect clusters by a model of defect growth and also high fluence optical absorption studies. X-ray diffraction studies show evidence of a strained lattice after the proton bombardment and recovery after long period storage. The effects are attributed to the annealing of the defects produced.     

Study by positron annihilation of radiation damage in an aluminium collimator of a 30 MeV electron linac

A positron annihilation lineshape study was made of samples cut from an aluminium commercial alloy collimator of a 30 MeV electron LINAC. The same material was studied after 60% deformation and both measurements were compared with the results of deformed pure aluminium samples. It was concluded that after irradiation, in addition to dislocations other trapping centres also have to be present. The annealing behaviour of the different samples is discussed.     

Proton/antiproton collisions with positronium in the presence of an electric field

Effects of aligned electric field on the hydrogen formation mechanism have been studied. Classical trajectory Monte Carlo (CTMC) method is used to simulate the proton/antiproton collisions with positronium (Ps) in the energy range of 1-200 keV. Total cross section for an electron/positron capture by colliding proton/antiproton has been calculated using the simple Coulomb potential. The hydrogen and antihydrogen formation in these capture processes, as expected, is reported to be charge conjugate. The capture cross sections are in reasonable agreement with the recently reported experimental and theoretical results. The effects of the external electric field are seen to be quite prominent.     

Measurement of the total neutron cross-section and resonance parameters of molybdenum using pulsed neutrons generated by an electron linac

The neutron total cross-sections of molybdenum were measured in the neutron energy region from 0.01 eV to 200 eV by using the time-of-flight method at the Pohang Neutron Facility, which consists of an electron linear accelerator, a water-cooled tantalum target with a water moderator, and a 12-m long time-of-flight path. A ⁶Li-ZnS(Ag) scintillator with a diameter of 12.5 cm and a thickness of 1.6 cm was used as a neutron detector, and a high purity natural molybdenum metallic disc with a diameter of 6.2 cm and a thickness of 3 mm thickness was used for the neutron transmission measurement. Notch filters composed of Co, In, Cd were used to estimate the background level and to calculate the length of neutron flight path. In order to reduce the gamma-ray background from Bremsstrahlung and from neutron capture, we employed a neutron-gamma separation system based on their different pulse shapes. The present measurement was compared with the existing experimental and the evaluated data. The resonance parameters of Mo isotopes were extracted from the transmission by using the SAMMY code and were compared with other previous reported results.