In the frame of a long-term research program on the characterization of large radioactive waste packages by photofission, the Nuclear Measurement Laboratory of CEA IRESNE has measured cumulative ...yields of 239Pu, 235U and 238U photofission products by using a Bremsstrahlung photon beam produced by a 17.5 MeV linear electron accelerator. A characterization of the energy of the Bremsstrahlung photon beam has been carried out by photon activation analysis with different samples of gold, nickel, uranium, zinc and zirconium. The contribution of neutron fission in the different samples 2 has also been estimated by MCNP simulations in order to assess as precisely as possible the photofission yields. Finally, 26 cumulative photofission product yields are reported for 239Pu, 28 for 238U and 26 for 235U, with half-lives ranging from 14 min to more than 3 days, some of them being not recorded so far in the literature. Among these reported photofission product yields, 18 have been measured for all 3 actinides, which can thus be used for their discrimination. A differentiation criterion based on delayed gamma-ray ratios has been established to determine the most efficient photofission product couples to estimate the enrichment of a 235U/238U mixture or the fissile fraction (235U+239Pu)/actinide mass in a mixture of uranium and plutonium.
This article reports a detailed description of the integration tests of the first Rapidly Relocatable Tagged Neutron Inspection System (RRTNIS) carried out at the European Commission’s Joint Research ...Centre in Ispra (Italy). This technology allows the detection and identification of suspicious or illicit materials (such as narcotics, explosives, contraband goods, etc.) inside a cargo container using active neutron interrogation, with the so-called associated particle technique. The method is based on the measurement of the gamma photons emitted by the de-excitation of nuclei that undergo, mainly, inelastic scatterings with incident fast neutrons (En∼ 14 MeV). A set of scintillation detectors (NaI:Tl and LaBr3:Ce) is employed to perform gamma spectroscopy. The data acquisition system is based on fast signal digitizers and customized data acquisition software. A general technical description of the detection module and an outline of the data acquisition system (DAQ) are given. Also, we present the results of the integration tests, in particular, some examples of the performance of the system in the laboratory are shown, specifically, when using no target (background measurement) and when using mono-elemental and an explosive simulant target. Obtained results suggest that all technical requirements are achieved, and the next step will be the field trials.
Abstract
The European H2020 project, entitled “effective Container inspection at BORDer control points” (C-BORD), aims to develop a framework of Non-Intrusive Inspection (NII) technologies,
for ...containers and large-volume freight at the EU borders. In this article the first results of the
field trials of the Rapidly Relocatable Tagged Neutron Inspection System (RRTNIS) are reported.
The tests were carried out at the customs administration of the Netherlands' (DCA) facility in the
seaport of Rotterdam, the Netherlands. For the tests mock-up cargo containers were prepared. The containers were analyzed using the whole set of NII subsystems of the C-BORD framework. Each container underwent a session of subsequent scans with all the subsystems, in order to realistically
reproduce the normal flow of inspections at the customs facility. The RRTNIS was tested under these realistic conditions, as well. The system was able to separate and identify the target materials, in mock-up containers, and improve the state-of-the-art in terms of the detected quantities.
Gamma logging for uranium exploration are currently based on total counting with Geiger Müller gas detectors or NaI (TI) scintillators. However, the total count rate interpretation in terms of ...uranium concentration may be impaired in case of roll fronts, when the radioactive equilibrium of the natural
238
U radioactive chain is modified by differential leaching of uranium and its daughter radioisotopes of thorium, radium, radon, etc. Indeed, in case of secular equilibrium, more than 95 % of gamma rays emitted by uranium ores come from
214
Pb and
214
Bi isotopes, which are in the back-end of
238
U chain. Consequently, these last might produce an intense gamma signal even when uranium is not present, or with a much smaller activity, in the ore. Therefore, gamma spectroscopy measurements of core samples are performed in surface with high-resolution hyper-pure germanium HPGe detectors to directly characterize uranium activity from the 1001 keV gamma ray of
234m
Pa, which is in the beginning of
238
U chain. However, due to the low intensity of this gamma ray, i.e. 0.84 %, acquisitions of several hours are needed. In view to characterize uranium concentration within a few minutes, we propose here a method using both the 92 keV gamma ray of
234
Th and the 98.4 keV uranium X-ray. This last is due to uranium self-induced fluorescence caused by gamma radiations of
214
Pb and
214
Bi, which create a significant Compton scattering continuum acting as a fluorescence source and resulting in the emission of uranium fluorescence X-rays. The comparison of the uranium activity obtained with the 92 keV and 98.4 keV lines allows detecting a uranium heterogeneity in the ore. Indeed, in case of uranium nugget, the 92 keV line leads to underestimated uranium concentration due to gamma self-absorption, but on the contrary the 98.4 keV line leads to an overestimation because of increased fluorescence. In order to test this new approach, several tens of uranium ore samples have been measured with a handheld HPGe FALCON 5000 detector.
In the frame of C-BORD project (H2020 program of the EU), a Rapidly relocatable Tagged Neutron Inspection System (RRTNIS) is being developed to non-intrusively detect explosives, chemical threats, ...and other illicit goods in cargo containers. Material identification is performed through gamma spectroscopy, using twenty NaI detectors and four LaBr3 detectors, to determine the different elements composing the inspected item from their specific gamma signatures induced by fast neutrons. This is performed using an unfolding algorithm to decompose the energy spectrum of a suspect item, selected by X-ray radiography and on which the RRTNIS inspection is focused, on a database of pure element gamma signatures. This paper reports on simulated signatures for the NaI and LaBr
3
detectors, constructed using the MCNP6 code. First experimental spectra of a few elements of interest are also presented.
AREVA Mines and the Nuclear Measurement Laboratory of CEA Cadarache are collaborating to improve the sensitivity and precision of uranium concentration evaluation by means of gamma measurements. This ...paper reports gamma-ray spectra, recorded with a high-purity coaxial germanium detector, on standard cement blocks with increasing uranium content, and the corresponding MCNP simulations. The detailed MCNP model of the detector and experimental setup has been validated by calculation vs. experiment comparisons. An optimization of the detector MCNP model is presented in this paper, as well as a comparison of different nuclear data libraries to explain missing or exceeding peaks in the simulation. Energy shifts observed between the fluorescence X-rays produced by MCNP and atomic data are also investigated. The qualified numerical model will be used in further studies to develop new gamma spectroscopy approaches aiming at reducing acquisition times, especially for ore samples with low uranium content.
To inspect cargo containers, X-ray imaging can be complemented by fast neutron interrogation to provide indication concerning the chemical nature of the transported goods through the detection of ...neutron-induced gamma rays. Organic goods are of special interest since they constitute a significant part of the merchandises. In addition, in the context of NRBC-E threats search in cargo containers, a nondestructive inspection system should also be able to detect explosives. Until now, fast neutron interrogation systems identify organic materials in cargo containers using characteristic neutron-induced gamma-ray peaks of carbon, oxygen and nitrogen. However, identifying organics in this way can lead to ambiguities, since no hydrogen gamma ray can be measured with fast neutrons. Nevertheless, it is known that hydrogen strongly modifies the neutron energy spectrum, which in turn affects the amplitudes of gamma-ray peaks. The link between the hydrogen fraction and the amplitude of gamma ray peaks being complex, no attempt has been made to inverse this link until now. Simulations show, however, that a neural network that takes as inputs the heights of carbon, oxygen and nitrogen gamma ray peaks can indeed determine the hydrogen fraction. Simulations of realistic cases show that the use of a neural network indeed allows identifying compounds having similar fractions of carbon, oxygen and nitrogen but different hydrogen fraction, thus opening the way to more accurate materials identification.
•Hydrogen is measurable using gamma rays induced by fast neutrons on other nuclei.•A neural network can determine the hydrogen fraction in organic materials.•MCNP simulations validate the efficiency of the neural network approach.•Organic materials can be better discriminated.
Within the scope of determining the concentration of uranium in ore samples by gamma-ray spectrometry, we tested a series of machine-learning (ML) algorithms with a database including 1288 HPGe gamma ...spectra measured by Orano Mining. Instead of detecting and identifying peaks, a global interpretation of the spectra is carried out. Two different approaches were used. First, we reduced the size and the dimension of the dataset by selecting 728 spectra acquired with a same experimental setup and by resampling their 8192 channels into 168 energy bands according to the important peaks due to the natural uranium, thorium and potassium activity. Classical ML algorithms have been trained on this reduced dataset and the best uranium concentration predictions show Symmetric Mean Absolute Percentage Error lower than 6%. In a second step, the complete dataset with 1288 gamma spectra including six different measurement setups was used to train deep neural network with a re-sampling of the spectra into 2048 channels. Despite the small dataset, a Convolutional Neural Network (CNN) model provides satisfactory results with mean errors lower than 15% on this broader and more complex dataset in terms of uranium concentrations and experimental setups. These outcomes shows that ML methods can predict uranium concentration with similar uncertainties as classical gamma-ray spectroscopy (10% to 20%), but without requiring an expert knowledge to interpret the spectra.
Nuclear materials characterization in 870 L cemented waste drums is challenging due to significant neutron and gamma ray attenuation. Among the existing non-destructive approaches, photon active ...interrogation is of particular interest. A LINAC accelerates electrons to energies greater than 10 MeV, leading to highly penetrating interrogating bremsstrahlung photons that can reach nuclear materials in the depth of the 870 L waste drum and allow the detection of delayed gamma rays emitted by photofission products. Because the interrogating photon and neutron fluxes are extremely high during irradiation, measurements with high purity germanium detectors are usually performed after a long irradiation time, for example by moving the detector or the waste drum. Therefore, such measurements are only sensitive to delayed gamma rays associated to photofission products having sufficient long half-life of at least tens of seconds, which cuts a valuable signal coming from fission products with a shorter half-life. In this paper, we report gamma rays measurements of very short lived photofission products using a 3 in × 3 in cylindrical LaBr3(Ce) fast scintillation detector. A depleted uranium sample was irradiated by a pulsed bremsstrahlung source with maximum energy 17.5 MeV in a macro pulsing mode (1 s irradiation, 2 s cooling). This allowed to acquire a strong delayed gamma signal within a time window of 2 s.
A clear signature of nuclear materials was observed even in the depth of a concrete mock-up, with an excellent signal-to-noise ratio beyond 3.5 MeV where the background of nonnuclear activated materials is negligible. These results are compared to a semi analytical model that qualitatively agrees but underestimates the measurements by a constant factor 1.7, probably caused by a wrong beam intensity normalization. Indeed, in this work the LINAC pulse time structure (macro pulsing) was different from its nominal working point used to characterize the beam (usual pulsing mode). Since it allows direct calculations of the detector response to delayed gamma-ray photofission measurements, this model is a fast calculation alternative to time-consuming Monte Carlo simulations, in view of further studies that will follow this feasibility demonstration.
The measurement of delayed gamma rays following neutron-induced fission is simulated with MCNP 6.1 to investigate the feasibility of fissile material detection in long-lived, medium activity ...radioactive waste in 870 L drums. The signal from homogeneously distributed fissile material in the drum is several hundred counts in the main delayed gamma peaks of interest. In a peripheral position or in the drum center, the signal is however too small to allow for a reliable measurement.
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