Identification of the unknown shielding parameters with gammaray spectrum using a derivative-free inverse radiation transport model

Identifying the unknown geometric and material information of a multi-shield object by analyzing the radiation signature measurements is always an important problem in national and global security. In order to identify the unknown shielding layer thicknesses of a source/shield system with gamma-ray spectra, we have developed a derivative-free inverse radiation transport model based on a differential evolution algorithm with global and local neighbourhoods(IRT-DEGL). In the present paper, the IRT-DEGL model is further extended for estimating the unknown thicknesses with random initial guesses and material mass densities of multi-shielding layers as well as their combinations. Using the detected gamma-ray spectra,the illustration of inverse studies is implemented and the main influence factors for inverse results are also analyzed.     

Using the Schwinger inverse method for solutions of inverse transport problems in two-dimensional cylindrical geometries

The Schwinger method for solving inverse transport problems is applied to the problems of interface location identification, shield material identification, source isotope weight fraction identification, and material mass density identification (separately) in multilayered two-dimensional cylindrical gamma radiation source/shield systems. The method is iterative and estimates unknown interface locations, source isotope weight fractions, and material densities directly, while the unknown shield material is identified by estimating its total macroscopic gamma-ray cross sections. The energies of discrete gamma-ray lines emitted by the source are assumed to be known, while the unscattered flux of the lines is assumed to be measured at points external to the system. In numerical test cases, the Schwinger method correctly identifies the unknowns when the same deterministic ray-tracing code is used for both the parameter estimation process and simulation of the measured data. With realistic simulation of the measured data using a Monte Carlo code, the method produces more ambiguous results for interface location, shield material identification, and material density identification. The method works well for source weight fraction identification with measured data simulated by Monte Carlo. In addition to the application to more realistic (two-dimensional) problems, this paper extends previous work on the Schwinger inverse method by using surface formulas for unknown interface locations, automatic correction attempts for violated constraints, and ray-tracing instead of discrete-ordinates for transport calculations.     

A comparison of the Covariance Matrix Adaptation Evolution Strategy and the Levenberg–Marquardt method for solving multidimensional inverse transport problems

The Covariance Matrix Adaptation Evolution Strategy (CMA-ES), a powerful optimization algorithm that mimics the process of evolution in nature, is applied to the inverse transport problems of interface location identification, source composition identification, and material mass density identification (both separately and combined) in cylindrical radioactive source/shield systems. The energies of discrete gamma-ray lines emitted by the source are assumed to be known, while the uncollided line fluxes are assumed to be measured at points external to the system. CMA-ES is compared to the Levenberg–Marquardt method, a standard gradient-based optimization algorithm, on numerical test cases using both simulated data that is perfectly consistent with the optimization process and with realistic data simulated by Monte Carlo. Numerical results indicate that the Levenberg–Marquardt method is more adept at problems with few unknowns (i.e. ?3), but as the number of unknowns increases, CMA-ES becomes the superior strategy. Results also indicate that a parallel version of CMA-ES would be more robust than, and have competitive run times with, the Levenberg–Marquardt method for many inverse transport problems.     

近地空间强流脉冲中子源大气传输散射γ特性研究

基于射线与物质的相互作用原理和辐射输运理论，建立了近地空间强流脉冲中子源在大气中输运时与气体散射产生γ射线传输到同步轨道的数学物理模型，应用MCNP程序对散射γ输运至地球同步轨道的特征参数进行了数值模拟，获得了一些规律性的辐射现象，并依据这些辐射特征，初步提出了散射γ探测器相关指标，为相关研究提供了一定参考。     

Fission products and secondary gamma-ray spectra in a plutonium-fueled fast reactor

The fission products' gamma-ray and gamma-ray energy source spectra for a gas-cooled fast breeder reactor (GCFR) are calculated for different times after shutdown by modifying the RIBD computer code. The secondary gamma-ray energy source spectrum in the core of a GCFR, from fission, inelastic scattering, and capture reactions, is calculated using a typical GCFR neutron spectrum. The computer code LAPHANO is used to generate the multigroup (n, xγ) neutron-coupled gamma-ray transfer matrix. The weak dependence of capture and inelastic gamma ray source spectrum on the neutron flux spectrum has been noted. The fission products and secondary gamma-ray source spectra obtained can be used to calculate heat generation and refueling shielding requirements, etc.     

泊松克里金算法在核辐射场重构中的应用研究

为验证泊松克里金法在核辐射场重构中的应用效果,本文对泊松克里金算法在辐射场剂量率插值计算中的计算原理进行了研究,结合代理模型对泊松克里金算法进行了求解。通过SuperMC软件建立虚拟辐射场,对泊松克里金算法的参数影响进行了分析,筛选出二次多项式基函数和GAUSS误差相关模型。采用6150AD-b闪烁体探测器对实际放射源进行测量,并利用实测辐射场剂量率数据和福岛周边空间剂量率数据对泊松克里金算法在核辐射场重构中的可行性进行了验证。结果表明：小范围简单辐射场中,样本数据量为网格点数的1/10时,平均相对误差在10%以内;大范围辐射场中,辐射热点区域略有偏移,平均相对误差为141.69%。以上结果表明,该方法在快速、低成本解决未知放射源辐射场的重构问题中有一定的潜力。     

Feasibility study on determining the conventional true value of gamma-ray air kerma in a minitype reference radiation

A minitype reference radiation(MRR) with dimensions of only 1 m × 1 m × 1 m has been developed for the in situ calibration of photon dosimeters.The present work conducts a feasibility study on determining the conventional true value of gamma-ray air kerma at the point of test in the MRR.Owing to its smaller dimensions,the scattered gamma-rays in the MRR are expected to induce a non-negligible interference with the radiation field compared with conditions in the standard reference radiation stipulated by ISO4037-1 or GB/T12162.1.A gamma-ray spectrometer was employed to obtain the spectra of scattered gamma-rays within the MRR,and the feature components of the spectra were extracted by principal component analysis to characterize the interference of a dosimeter probe in the radiation field.A prediction model of the CAK at the point of test was built by least squares support vector machine based on the feature component data obtained from nine sample dosimeters under five different dose rates.The mean prediction error of the CAK prediction model was within ±4.5%,and the maximum prediction error was about ±10%.     

The propagation in air of gamma radiation from a momentary point source

In this article the Monte Carlo method is used to solve the problem of the propagation in an infinite air medium of gamma quanta with an initial energy of 1 Mev from a momentary isotropic point source. We obtain the radiation intensities as functions of time for gamma radiations arriving at the observation point from various solid angles, as well as the time-energy spectra of the intensity of the scattered gamma radiation at various distances from the source. By integration of the time distributions with respect to time we obtained the values of the buildup factors and the differential energy spectra of the scattered gamma radiation for a continuously acting source, as well as the angular distribution of the intensity and the angular energy spectra of the intensity of the scattered radiation for three distances of the observation point from the source.In conclusion the authors express their deep gratitude to I. M. Gel'fand for his attention to the work and his collaboration in its execution.     

γ扫描测量等效活度的校准方法研究

本文提出了采用标准线源代替标准体源对装置进行效率校准的等效活度校准方法。利用蒙特卡罗模拟方法,研究了不同介质密度和特征γ射线能量对线源校准位置的影响。测量实验结果与模拟计算结果吻合,证实了这种方法是一种介于常规效率校准与无源效率校准两种方法之间的等效校准技术。     

Spatial distribution of reflected gamma rays by Monte Carlo simulation

In nuclear facilities, the reflection of gamma rays of the walls and metals constitutes an unknown origin of radiation. These reflected gamma rays must be estimated and determined. This study concerns reflected gamma rays on metal slabs. We evaluated the spatial distribution of the reflected gamma rays spectra by using the Monte Carlo method. An appropriate estimator for the double differential albedo is used to determine the energy spectra and the angular distribution of reflected gamma rays by slabs of iron and aluminium. We took into the account the principal interactions of gamma rays with matter: photoelectric, coherent scattering (Rayleigh), incoherent scattering (Compton) and pair creation. The Klein-Nishina differential cross section was used to select direction and energy of scattered photons after each Compton scattering. The obtained spectra show peaks at 0.511^∗ MeV for higher source energy. The Results are in good agreement with those obtained by the TRIPOLI code [J.C. Nimal et al., TRIPOLI02: Programme de Monte Carlo Polycin?etique à Trois dimensions, CEA Rapport, Commissariat à l’Energie Atomique. [1]].     

Gamma-ray spectral penetrations: A review

This review discusses gamma-ray penetration through matter, particularly the differential, angular and energy spectra which have been reported. Experimental spectra are obtained by unfolding measured data and the features of unfolding methods and codes are discussed. A large proportion of the published data is for point and parallel beam sources of gamma radiation from the decay of the isotopes ¹³⁷Cs and ⁶⁰Co. Data for higher energy sources show the importance of secondary radiations of Bremsstrahlung and positron annihilation photons in shield penetrations. Reasons why it is difficult to make direct comparisons of reported data are outlined. The need for benchmarking methodology is emphasized.     

Monte-Carlo simulation of gamma-ray backscatter soil density gauges

The Monte-Carlo code OGRE-G is modified to calculate spectral and spatial distributions of the z-component of the backscattered gamma-ray current. Calculation l procedures are developed and demonstrated for translating this current into the response of the most common types of gamma-ray soil density gauges. Results were obtained for the four materials average soil, chalk, aluminum, and magnesium. The source considered was ¹³⁷Cs and six photon energies and 10 spatial distances were considered. Parameters are given for an empirical model determined by a linear regression analysis of all the predicted current results. The explicit dependence of the model on density could be specified by a technique involving translating the differential current to cumulative current and then reversing this process. The results and procedures developed in this study should prove useful for the calibration of gamma-ray soil density gauges by determining the model constants from experimental responses on chalk, aluminum, and magnesium. The results should also prove useful in the optimum design of these gauges.     

Calculation of Energy Deposition Spectra for Radioactivity Induced in Germanium Gamma-Ray Spectrometers by Various Space Environments

Radioactivity induced in detector materials by cosmic rays and radiation belt protons provides an ultimate limit to the sensitivity of gamma-ray spectrometers used for gamma-ray astronomy and remote sensing spectroscopy of planetary bodies. The increasing use of high resolution germanium spectrometers has made it worthwhile to adapt predictive techniques for this component based on those previously used for alkali halide scintillators. Decay rates of the most significant radionuclides have been calculated using semiempirical cross-sections and combined with Monte Carlo calculations of energy depositions to yield background spectra for both interplanetary space and low earth orbit locations.     

Calculation of neutron and gamma-ray energy spectra for fusion reactor shield design: Comparison with experiment-II

Measured and calculated neutron and gamma-ray energy spectra resulting from the transport of 14 MeV neutrons through a 0.30-m-thick lithium hydride slab and through a 0.05-m-thick lead slab followed by 0.30 m of lithium hydride are compared. Also reported are comparisons of the measured and calculated neutron energy spectra behind an 0.80-m-thick assembly comprised of stainless steel type 304 and borated polyethylene. The spatial dependence of the gamma-ray energy deposition rate measured using thermoluminescent detectors is compared with calculated data. The calculated data obtained using two-dimensional radiation transport methods and ENDF/B-IV cross-section data are in good agreement for all of the experimental configurations. Calculated integral neutron energy spectra agree with the measured data within 5 to 20% depending on neutron energy for the LiH and Pb plus LiH assemblies. The gamma-ray spectra agree within 20% for these slabs. The measured and calculated neutron energy spectra behind the SS-304-borated polyethylene assembly agree within 5% except at neutron energies below 5 MeV where background radiation influences the measured spectra. The gamma-ray energy deposition rates as a function of depth agree within a factor of two at all detector locations.     

航空γ能谱仪无源效率刻度方法研究

因人工放射性核素的航空γ能谱仪实物刻度模型匮乏,导致难以依据航空γ能谱准确反演地面人工放射性核素的含量。本文基于窄束γ射线指数衰变规律与微积分的思想建立了任意形状的γ辐射源上空航空γ能谱仪无源效率刻度的数值计算模型。通过低空探测实验、高空变化趋势分析、5-100 m高空探测实验证明该模型适用于任意位置点源航空γ能谱仪全能峰探测效率数值计算。同时计算发现在低空探测时不同γ辐射的面源与体源的航空γ能谱仪全能峰探测效率与MCNP5模拟值的相对偏差在±1.5%以内,且含1 460.83 ke V或2 614.533 ke Vγ射线的无限大体源90-150 m探测高空计算结果与石家庄动态带上的实验值相对偏差为8.33%-15.82%。上述实验充分证实该无源效率刻度计算模型适用于航空γ能谱探测实践,为利用航空γ能谱仪寻找丢失放射源及核事故应急监测提供技术支持。     

A digital spectrometer approach to obtaining multiple time-resolved gamma-ray spectra for pulsed spectroscopy

Neutron-induced gamma-ray emission and its detection using a pulsed neutron generator system is an established analytical technique for quantitative multi-element analysis. Traditional gamma-ray spectrometers used for this type of analysis are normally operated either in coincidence mode - for counting prompt gamma-rays following inelastic neutron scattering (INS) events when the neutron generator is ON, or in anti-coincidence mode - for counting prompt gamma-rays from thermal neutron capture (TNC) processes when the neutron generator is OFF. We have developed a digital gamma-ray spectrometer for concurrently measuring both the INS and TNC gamma-rays using a 14 MeV pulsed neutron generator. The spectrometer separates the gamma-ray counts into two independent spectra together with two separate sets of counting statistics based on the external gate level. Because the TNC gamma-ray yields are time dependent, additional accuracy in analyzing the data can be obtained by acquiring multiple time-resolved gamma-ray spectra at finer time intervals than simply ON or OFF. For that purpose we are developing a multi-gating system that will allow gamma-ray spectra to be acquired concurrently in real time with up to 16 time slots. The conceptual system design is presented, especially focusing on considerations for tracking counting statistics in multiple time slots and on the placement of pulse heights into multiple spectra in real time.     

An inversion decomposition method for better energy resolution of NaI(Tl) scintillation detectors based on a Gaussian response matrix

NaI(T1) scintillation detectors have been widely applied for gamma-ray spectrum measurements owing to advantages such as high detection efficiency and low price.However,the mitigation of the limited energy resolution of these detectors,which detracts from an accurate analysis of the instrument spectra obtained,remains a crucial need.Based on the physical properties and spectrum formation processes of NaI(T1) scintillation detectors,the detector response to gamma photons with different energies is represented by photopeaks that are approximately Gaussian in shape with unique full-width-at-half-maximum(FWHM) values.The FWHM is established as a detector parameter based on resolution calibrations and is used in the construction of a general Gaussian response matrix,which is employed for the inverse decomposition of gamma spectra obtained from the detector.The Gold and Boosted Gold iterative algorithms are employed to accelerate the decomposition of the measured spectrum.Tests of the inverse decomposition method on multiple simulated overlapping peaks and on experimentally obtained U and Th radionuclide series spectra verify the practicability of the method,particularly in the low-energy region of the spectrum,providing for the accurate qualitative and quantitative analysis of radionuclides.     

Measurements of keV-neutron capture cross sections and capture gamma-ray spectra of 105Pd

The neutron capture cross sections and capture gamma-ray spectra of ¹⁰⁵Pd were measured in the region from 15 to 100 keV and at 585 keV. A neutron time-of-flight method was utilised with an anti-Compton NaI(Tl) spectrometer and a 1.5-ns pulsed neutron source by the ⁷Li(p,n)⁷Be reaction. The capture yields were obtained by applying a pulse-height weighting technique to the observed net capture gamma-ray pulse-height spectra. The capture cross sections of ¹⁰⁵Pd were derived with errors less than 5%, using the standard capture cross sections of ¹⁹⁷Au. The evaluated capture cross sections of JENDL-4.0 and ENDF/B-VII.1 were compared with the present results. The evaluations of JENDL-4.0 and ENDF/B-VII.1 were larger than the present results by 3%–15% in the region from 15 to 100 keV and at 585 keV. The capture gamma-ray spectra of ¹⁰⁵Pd were also derived by unfolding the observed net capture gamma-ray pulse-height spectra. The multiplicities of capture gamma rays of ¹⁰⁵Pd were obtained from the capture gamma-ray spectra.     

Use of 4.7 MeV alpha particles in elemental analysis and fusion reactor materials studies

The possibilities for using 4.7 MeV alpha particles produced at the U-120 CIP Cyclotron for charged particle induced X- and gamma-ray emission applications (PIXE and PIGE, respectively) and for fast neutron radiation damage simulation are presented.The combined analysis using PIGE and PIXE methods either separately or simultaneously is an excellent means of determining the relative abundances of lighter elements with gamma-ray spectra and of heavier elements (Z ⩾ 16) with X-ray spectra.Some aspects of surface deformation effects by neutrons were simulated by means of medium-energy helium ions. An investigation of three types of commercial stainless steel (Romanian W 4016, Soviet 12KH18N10T and Japanese W 4541) was started using 3.0 [1], 4.7 and 6.8 MeV helium ions. The main post-irradiation effects observed are discussed.     

Evaluation of Neutron Capture Gamma-ray Spectra for 89Y, 93Nb, 127I, 133Cs, 141Pr, 197Au,natTl, and 209Bi

The neutron capture gamma-ray spectra for 8 nuclides, ⁸⁹Y, ⁹³Nb, ¹²⁷I, ¹³³Cs, ¹⁴¹Pr, ¹⁹⁷Au, ^natTl, and ²⁰⁹Bi, were calculated by using the Hauser-Feshbach statistical model, and their results were compared with the available experimental data. Two dominant ingredients to perform the statistical calculation were the level densities described by the Gilbert-Cameron approach with an improved systematics, and the gamma-ray transmission coefficients described by gamma-ray strength functions. Although various gamma-ray strength functions with a Lorentzian formula have been developed by using the photonuclear data or a microscopic analysis, they have failed to reproduce the occasional anomalous bumps observed near or below a neutron binding. In this work, we could reproduce the bumps well by adding a Lorentzian with an energy-temperature dependent width into a giant electric dipole resonance with an enhanced generalized Lorentzian. In addition, we introduced a correction function so as to compensate for the shortcomings due to missing levels or level-cuts.