Segmented Gamma Scanner Research Articles

Bayesian optimization of a collimated HPGe detector model for Segmented Gamma Scanning

Segmented Gamma Scanning (SGS) is an important technique for quantifying gamma-emitting isotopes in radioactive waste packages. Collimated High-Purity Germanium (HPGe) detectors are commonly employed due to their superior resolution. However, calibrating HPGe detectors equipped with complex collimators poses challenges, especially regarding the modeling of the spatial response of the detector. This work introduces a calibration methodology for the Full Energy Peak Efficiency (FEPE) of HPGe detectors equipped with variable aperture collimators using Bayesian optimization to handle their complex geometries. The method and the model were then validated for the full range of application of the detector-collimator arrangement using experimental FEPE measurements, demonstrating the model’s potential for accurate SGS simulations.

A density correction method for radioactive waste drum based on SRGS technology

The segmented ringed gamma scanning (SRGS) technique represents an advancement in segmented gamma scanning (SGS) technology used for detecting the density of radioactive waste drums, offering enhanced measurement accuracy. However, significant occur errors in the reconstruction of matrix densities due to the non-uniform distribution of density in radioactive waste and the conical beam emitted from the transmission source collimator. This paper proposes a density correction method based on dichotomy to address this issue. The efficacy of this method was verified through both simulations and experiments on a sample containing five different materials, utilizing 137Cs and 60Co for transmission and emission measurements, respectively. The experimental results demonstrate that the errors in the corrected matrix densities are reduced, falling within a margin of 16.8%. Additionally, the corrected reconstruction error of the activity is approximately 25% of the uncorrected results.

An efficiency calibration method of segmented gamma scanning in reconstructing radioactive waste drum activity

An efficiency calibration method of segmented gamma scanning in reconstructing radioactive waste drum activity

An efficiency function model of segmented gamma scanning for measuring radioactive waste drum

Segmented gamma scanning (SGS) is a fast and effective method for measuring radioactive waste drum. The efficiency calibration is directly related to the accuracy of reconstructed radioactivity. An efficiency function model and SGS efficiency calibration method are proposed for solving existing SGS efficiency calibration problems such as time lag, limited by experimental sources or difficult to effectively combine with SGS system. The SGS system model is established by the Geant4 to calculate the segment efficiency under different linear attenuation coefficients of medium and gamma energies. The efficiency calibration function is established with the function model and parameters. Waste drum samples are constructed with the polyethylene and point sources 137Cs/60Co to complete SGS experimental measurement, efficiency calibration and radioactivity reconstruction. The result shows that the relative deviation of the reconstructed activity of a single point source at different locations in the drum is −50.48% to 43.69% and it of multi-point sources in a segment or a drum is −27.88% to 3.57%. Experimental results verify the effectiveness of this efficiency function model and SGS calibration method.

The MICADO Integrated Gamma Station for Radioactive Waste Packages radiological characterization

The MICADO (Measurement and Instrumentation for Cleaning And Decommissioning Operations) Project of the H2020 Research and Innovation Programme aims to propose a cost-effective solution for non-destructive characterization of nuclear waste, implementing a digitization process that could become a referenced standard facilitating and harmonizing the methodology used for the in-field Waste Management and Dismantling & Decommissioning operations. It employs instruments based on different technologies: an active and passive neutron measurement system, a photo-fission facility, and the ‘Integrated Gamma Station’. Within the MICADO Project, an entire work package has been dedicated to the design and realization of the ‘Integrated Gamma Station’ obtained by combining different gamma detection technologies supporting each other to a comprehensive and effective non-destructive gamma characterization, able to accommodate Radioactive Waste Packages of different sizes. The techniques implemented are i) dosimetry measurements, count rate inspection and raw spectroscopy with the CAEN RadHAND, ii) gamma imaging in open geometry with the CEA Nanopix gamma camera, iii) high resolution gamma spectrometry with the ENEA Tomographic Gamma Scanner (TGS). The latter able to carry out different characterization methodologies, i.e., Open Geometry, Segmented Gamma Scanning, Angular Scanning, and Tomography. This paper describes the layout of the Integrated Gamma Station conceived, its features and detection capabilities, and part of the measurement campaign realized during the MICADO Project.

The design of integrated gamma ray scanning system for the uranium sample

An integrated gamma ray scanning system has been designed and built to extend that localizes and quantifies highly enriched uranium sample. The detection mechanisms of the two modes (Segmented Gamma Scanner and Tomographic Gamma Scanning) are analyzed respectively. Moreover, the parameters influence of speed and the deviation of the measurement accuracy are analyzed for the uranium sample. In view of these problems, the system has improved by the optimization processing, which include the optimization of the collimator opening for detector, the crosstalk and the time step. Finally, the experimental were investigated and discussed in detail. The single measurement time of the system reduced to about 30 min with an error of less than 5%, so it enhance the measurement accuracy and the speed effectively. The system has broad application prospects in the fields of nuclear safety, radioactive recyclables and non-destructive measurement of nuclear waste.

Design and optimization of an integrated gamma ray scanning system for the uranium sample

Abstract An integrated gamma ray scanning system has been designed and built to extend that localizes and quantifies highly enriched uranium sample. The detection mechanisms of the two modes (segmented gamma scanner and tomographic gamma scanning) are analyzed respectively. Moreover, the parameters influence of speed and the deviation of the measurement accuracy are analyzed for the uranium sample. In view of these problems, the system has improved by the optimization processing, which include the optimization of the collimator opening for detector, the crosstalk and the time step. Finally, the experiment were investigated and discussed in detail. The single measurement time of the system can reduce to about 30 min with an error of less than ±5%, so the measurement accuracy and the speed are enhanced effectively. The system has broad application prospects in the fields of nuclear safety, radioactive recyclables and non-destructive measurement of nuclear waste.

A Bayesian method for the evaluation of segmented gamma scanning measurements – Description of the principle

The principle of applying a Bayesian Method using Markov Chain Monte Carlo sampling for the evaluation of segmented gamma scanning data for waste packages containing gamma-emitting radioactive waste including additional a-priori information is described in general and its application is demonstrated on simulated and measured data.

Bayesian inference of 1D activity profiles from segmented gamma scanning of a heterogeneous radioactive waste drum

We present a Bayesian approach to probabilistically infer vertical activity profiles within a radioactive waste drum from segmented gamma scanning (SGS) measurements. Our approach resorts to Markov chain Monte Carlo (MCMC) sampling using the state-of-the-art Hamiltonian Monte Carlo (HMC) technique and accounts for two important sources of uncertainty: the measurement uncertainty and the uncertainty in the source distribution within the drum. In addition, our efficiency model simulates the contributions of all considered segments to each count measurement. Our approach is first demonstrated with a synthetic example, after which it is used to resolve the vertical activity distribution of 5 nuclides in a real waste package.

Improvement of the passive efficiency calibration of the segmented gamma scanner

Abstract The Segmented Gamma Scanner(SGS) technology is specially used for type identification and activity quantitative analysis of radioactive material in sealed containers, which mainly divided into the transmission measurement and the emission measurement. In actual measurement process, it cannot meet the need for rapid analysis because the efficiency calibration time takes up more than 60% of the whole detect time in the data analysis. Whereas most previous research have focused on theory or specific applications, this research groups aim to different aspects of the SGS technique and direct it at an audience with interests in the need for rapid analysis. The Monte Carlo simulation calculation models were established by the passive efficiency calibration method, and then the detection efficiency database was carried out based on the experimental verification. Lots of work was used to analysis the influence of crosstalk between layers and interpolation step. Furthermore, the database was implanted into the control and analysis system to complete the measurement experiments. The results show the measuring time of single waste drums is about 30 min, and the average relative deviation is less than 10%, so the problem that the time taken to calibrate the detection efficiency is too long has been solved effectively. The efficiency in the use of SGS technology has been increased.

Collimator-Less Passive Gamma Scanning for Radioactive Waste Drums

In this article, a new method to quantify the activity of spatially distributed gamma-emitting isotopes (hotspots) in homogenous content waste drums without the use of a collimator is presented. The method utilizes a spatial digital filter derived using maximum likelihood (ML) to determine multiple sources’ positions and then calculate their activities. To solve the multidimensional maximization problem, we use an alternating projection (AP) technique, which transforms the problem into a considerably simpler 1-D maximization problem. A dynamic grid search was developed to further decrease the computational load. The mathematical simulations demonstrate the improved accuracy when compared to that of industrial segmented gamma scanning (SGS) systems and the same accuracy as that of newer methods. Furthermore, the new method offers the benefit of replacing the heavy mechanical collimator with a “virtual collimator” formed by digital filters and an advanced algorithm to create a “digital virtual scan” of drum volume to locate hotspots.

Improving Activity Estimation in Passive Gamma Scanning for Radioactive Waste Drums

A method to improve radioactive waste drum activity estimation in Segmented Gamma Scanning (SGS) systems was developed for homogenous content. We describe a method to quantify the activity of spatially distributed gamma-emitting isotopes (‘hot spots’) in homogenous content waste drums without the use of a collimator. Instead of averaging all the detector's readings we treat it as many different spatial samples as if we have multiple detectors surrounding the waste drum ("virtual detectors"). From these readings, we form a general linear model. Next, we derive the Maximum Likelihood Estimator (MLE) for the multiple sources position and activity. We solve this hyper-dimensional search problem using an Alternating Projections (AP) technique which transforms the problem into a simpler one-dimensional maximization problem. We tested this method using a mathematical simulation with a various number of sources, at random activities and positions for several energy bands. The preliminary results are consistent and show large improvement of the accuracy with comparison to industrial SGS systems and the same accuracy as new methods which exploits the spatial samples. Furthermore, since this method eliminates the need for heavy led collimator, none of the sources is blocked for the whole measurement period, which provides increased count rates and decreases the total measurement time.

QUANTOM® − Optimization of the online neutron flux measurement system

For the final disposal of radioactive waste, the waste packages have to meet the acceptance requirements defined by national licensing and supervisory authorities. Nondestructive methods are very much preferred over destructive methods for the qualification or re-qualification. Existing nondestructive methods as integral or segmented gamma scanning or neutron counting only determine the isotope specific activity but do not allow quantifying other non-radioactive hazardous substances. These should have been documented during creation, conditioning, and packaging of the waste. But especially for legacy waste, this documentation is often poor or even missing. This gap is to be filled by the QUANTOM® measurement device that will determine the mass fraction of elements within a 200-l-drum using the Prompt- and Delayed- Gamma-Neutron-Activation-Analysis. In order to obtain a spatially resolved characterization, it will employ a segmented scanning approach. For the determination of the absolute mass fractions, the neutron flux inside the drum has to be known accurately. As the waste itself will alter the neutron distribution and flux, it is not possible to calculate the latter a priori from the gamma measurement. Hence the neutron flux has to be measured simultaneously with the gamma radiation. In this presentation, we will introduce the system for measuring the thermal neutron flux surrounding the waste drum from which the flux within the waste package has to be reconstructed. We performed a simulation study to score several possible detector placements for an improved reconstruction performance. We will show the outcome of these calculations and present the final design of the detector arrangement.

Segmented gamma-ray assay of large volume radioactive waste drums containing plutonium lumps

Segmented gamma-ray scanning (SGS) is a traditional practice, globally, for the non-destructive assay of special nuclear materials (SNMs) in large volume radioactive waste drums. The conventional SGS is a relative two pass method and requires a standard drum of identical geometry. The present work is focused on identifying the limitations of traditional segmented gamma scanning methodology for the assay of waste drums containing plutonium lumps. It has been observed that, for drums containing Pu lumps, the conventional SGS methodology severely underestimates the assay results (~ 2–6 times depending on the gamma-ray energy) due to attenuation under-correction. An alternate single pass absolute efficiency approach following the principle of infinite energy extrapolation of apparent mass has been proposed for the assay of waste drums containing Pu lumps in various random and biased spatial distributions and has been found to agree within 1–10% with the actual value with a maximum uncertainty of 8%. The method has been further validated at higher collimator widths and it has been demonstrated that an increase in collimator width from 5.1 to 10.3 cm increases the throughput of the present system without much of losing the accuracy.

An Improved Gamma Scanning Assay Method for the 400-L Compacted Radioactive Waste Drum Based on the Segmented Equivalent Ring Source

An improved characterization assay algorithm for the 400-L low- and intermediate-level radioactive waste drum was developed based on the segmented equivalent ring source. The improved method (IM) was validated by experiments on a 200-L waste drum. The assay errors are smaller than that of segmented gamma scanning (SGS). Numerical measurements were performed on a 400-L waste drum with two densities of 1.5 and 2.5 g/cc. Three common nuclides, Ba-133, Cs-137, and Co-60, were selected. The results show that the source positions can be determined using two detectors through this method. The maximum error of the IM is several times smaller than that of SGS if the matrix is uniformly distributed in the waste drum. The mean errors are reduced by one third to half for all nuclides compared with SGS. If IM is applied in assaying the solidified wastes drum that contains one or two cylindrical iron products, the strong attenuation of gamma rays by irons and concretes will affect the determination of source positions. The assay accuracy decreases in this case, but it is overall better than that of SGS. The results indicate that IM is an accurate method for assaying the compacted waste drums with nonuniformly distributed radioactive sources.

Uncertainty estimation in non-destructive characterization of radioactive waste packages: some general remarks

Segmented gamma scanning is a widely used method for identification and quantification of gamma-emitting nuclides in a waste package containing radioactive materials. The quotation of uncertainties nevertheless is challenging, as one has to differentiate between uncertainties based on statistical effects and those on lack-of-knowledge on the real content in the waste package (i.e. bias). Usually only the first is reported, while the effect of the latter might be the most dominant part. We present a simple approach considering the lack-of-knowledge in data evaluation and compare these results with the results of considering statistical uncertainties for a real waste package inspected at the ZTWB Radiochemie Munchen (RCM) of the Technische Universitat Munchen.

Full energy peak efficiency calibration for the assay of large volume radioactive waste drums in a segmented gamma scanner

Large volume radioactive waste drums with low/intermediate level of alpha activity, generated in radiochemical laboratories, are in general screened for special nuclear materials (SNM) in a segmented gamma scanner (SGS) before disposal. The assay methodology traditionally requires a standard drum of identical geometry and thereby making the procedure relying on the availability of a true standard, which is often difficult to organize. Here, we report a non-conventional absolute segmented gamma scanning (ASGS) methodology for the assay of 200 L waste drums, avoiding the use of a standard drum. The present analysis employ the full energy peak (FEP) efficiency, ingeniously determined using a standard 152Eu point source. From combined experiment and Monte Carlo simulation, it has been established that, the FEP efficiencies of the detector for a 200 L cylindrical sample can be well reproduced using a point source. While verifying the applicability of the point source FEP efficiencies for the assay of plutonium in 200 L drums, an energy dependent bias has been seen, which confirms the presence of lump attenuation in addition to the general matrix attenuation. An infinite energy extrapolation of apparent mass approach has been adopted for the assay of large volume waste drums which takes care of the gamma-ray attenuation from all sources that is otherwise difficult to correct for in a sample drum of unknown history.

Semi-Tomographic Gamma Scanning Technique for Non-Destructive Assay of Radioactive Waste Drums

Segmented gamma scanning (SGS) and tomographic gamma scanning (TGS) are two traditional detection techniques for low and intermediate level radioactive waste drum. This paper proposes one detection method named semi-tomographic gamma scanning (STGS) to avoid the poor detection accuracy of SGS and shorten detection time of TGS. This method and its algorithm synthesize the principles of SGS and TGS. In this method, each segment is divided into annual voxels and tomography is used in the radiation reconstruction. The accuracy of STGS is verified by experiments and simulations simultaneously for the 208 liter standard waste drums which contains three types of nuclides. The cases of point source or multi-point sources, uniform or nonuniform materials are employed for comparison. The results show that STGS exhibits a large improvement in the detection performance, and the reconstruction error and statistical bias are reduced by one quarter to one third or less for most cases if compared with SGS.

A de-noising algorithm to improve SNR of segmented gamma scanner for spectrum analysis

An improved threshold shift-invariant wavelet transform de-noising algorithm for high-resolution gamma-ray spectroscopy is proposed to optimize the threshold function of wavelet transforms and reduce signal resulting from pseudo-Gibbs artificial fluctuations. This algorithm was applied to a segmented gamma scanning system with large samples in which high continuum levels caused by Compton scattering are routinely encountered. De-noising data from the gamma ray spectrum measured by segmented gamma scanning system with improved, shift-invariant and traditional wavelet transform algorithms were all evaluated. The improved wavelet transform method generated significantly enhanced performance of the figure of merit, the root mean square error, the peak area, and the sample attenuation correction in the segmented gamma scanning system assays. We also found that the gamma energy spectrum can be viewed as a low frequency signal as well as high frequency noise superposition by the spectrum analysis. Moreover, a smoothed spectrum can be appropriate for straightforward automated quantitative analysis.

A prototype of radioactive waste drum monitor by non-destructive assays using gamma spectrometry

In this work, segmented gamma scanning and the gamma emission tomography were used to locate unknown sources in a radioactive waste drum. The simulated detector response function and full energy peak efficiency are compared to corresponding experimental data and show about 5.3% difference for an energy ranging from 81keV to 1332.5keV for point sources. Computation of the corresponding activity is in good agreement with the true values.