Dead-time Correction Research Articles

High definition large area mapping of geological samples using a Maia detector array in the Nuclear Microprobe

A Maia detector array has been adapted and installed on the CSIRO-MARC Nuclear Microprobe. Maia uses an annular array of 384 silicon detectors to provide a solid-angle of 1.3 sr to improve the collection of X-rays induced by Particle Induced X-ray Emission (PIXE) using a micron focussed MeV proton beam. It features event-by-event data acquisition with dead-time correction and pileup rejection and a total count-rate capacity above 10 M/s, versatile stage and beam scanning modes (coupled to the DAQ-36 system described previously) to cater for a variety of sample configuration and experimental needs, precise measurement of transit time and integration of beam fluence per pixel, and spectral deconvolution of event data in real-time using the Dynamic Analysis method to provide element images during data collection. Here we demonstrate the system on thin sections made from geological samples extracted from active seafloor hydrothermal vents in the PACMANUS hydrothermal field in Papua New Guinea.

Clinical Applications of Technetium-99m Quantitative Single-Photon Emission Computed Tomography/Computed Tomography.

Single-photon emission computed tomography/computed tomography (SPECT/CT) is an already established nuclear imaging modality. Co-registration of functional information (SPECT) with anatomical images (CT) paved the way to the wider application of SPECT. Recent advancements in quantitative SPECT/CT have made it possible to incorporate quantitative parameters, such as standardized uptake value (SUV) or %injected dose (%ID), in gamma camera imaging. This is indeed a paradigm shift in gamma camera imaging from qualitative to quantitative evaluation. In fact, such quantitative approaches of nuclear imaging have only been accomplished for positron emission tomography (PET) technology. Attenuation correction, scatter correction, and resolution recovery are the three main features that enabled quantitative SPECT/CT. Further technical improvements are being achieved for partial-volume correction, motion correction, and dead-time correction. The reported clinical applications for quantitative SPECT/CT are mainly related to Tc-99m-labeled radiopharmaceuticals: Tc-99m diphosphonate for bone/joint diseases, Tc-99m pertechnetate for thyroid function, and Tc-99m diethylenetriaminepentaacetic acid for measurement of glomerular filtration rate. Dosimetry before trans-arterial radio-embolization is also a promising application for Tc-99m macro-aggregated albumin. In this review, clinical applications of Tc-99m quantitative SPECT/CT will be discussed.

87. Lesion dosimetry in metastatic thyroid cancer treated with 131I: Method and preliminary results

Purpose 131I treatment of metastatic thyroid cancer should take into account doses to metastasis for an optimized approach as highlighted by EU 2013/59 Directive. A SPECT-TC dose calculation method is proposed and verified to standardize the operating protocol; patients’ preliminary results are shown. Methods SPECT-TC was calibrated with a phantom (spheres 11.5, 5.6, 1.1 ml, cylinder 130 ml) filled with liquid 131I (13 MBq/ml). Acquisition (64views, circular-orbit, 256 × 256, Iterative SCAC) on successive days allowed partial volume effects, dead-time and detector radius (25 ÷ 33 cm) corrections to calculate the dose, whose accuracy was verified. For 8 patients after the therapeutic administration and for 2 patients also in pre-therapy phase 4 SPECT-TC (4 ÷ 6, 24, 48, 96 ÷ 144 h) were performed to calculate the dose to 30 lesions (24 bone,7 lymph node,6 pulmonary) with the MIRD spheres model. Results Calibration factors (cylinder 1.4 kcts/MBq) confirmed partial volume effects (130 ml:100%, 11 ml:75%, 5.5 ml:58%, 1.1 ml:6%) and radius dependence ( ∼ 10%). Dead-time strongly influences counts in early acquisition (up to 17 %). Mean accuracy on activity −15% [−9% ÷ −24%] and dose −13 % [−9% ÷ −18%] for volumes larger than 2.5 cc validates the dosimetric method, according to literature (MIRD24). Dose to lesions per activity unit varies among patients (mean ± 1dev.st 26 ± 38, range 0.1 ÷ 189 Gy/GBq) and within the same patient (up to +583 %) and uptake seems to decrease in following treatments: individualized dosimetry is needed. CT-based method is recommended for VOIs segmentation (mean – 4.9% compared to the isocontour threshold one). In-therapy dose values show high differences (up to −126%) compared to pre-therapy results, whose efficacy must be further investigated with more patients also in terms of optimal activity to be administered to simulate the treatment. Conclusions Lesion dosimetry of metastatic thyroid cancer treated with radioiodine is accurate with SPECT-TC and provides to the physician a fundamental tool to optimize the treatment. Download : Download high-res image (119KB) Download : Download full-size image Download : Download high-res image (118KB) Download : Download full-size image

New Dead-Time Compensation Method of Power Inverter using Carrier Based Sinusoidal Pulse-Width Modulation

<p>A new dead-time compensation method of power inverter circuits is suggested and presented in this paper. The proposed method utilizes carrier based sinusoidal pulse width modulation technique to produce driving signals of the inverter power switches with dead-time correction capability. The proposed method able to eliminate dead-time effects such as reducing the waveform distortion of the inverter output current, and increasing the fundamental component amplitude of output current. An analysis of the proposed method is presented. Some computer simulations were carried out to investigate the principle operation, and to test performance of the new method. The developed method was validated through experimental test of H-bridge voltage source inverter circuits. The data obtained from the computer simulation and prototype experiments have confirmed that that the proposed method worked well compensating the dead-time in the voltage source power inverter circuits.</p>

Design and performance of the micro-dose calibrator

A new class of dose calibrator called a micro-dose calibrator, designed to measure radioactivity rates in the range between 1 kBq up to 370 kBq, has been designed and an 8 NaI(Tl)/photo multiplier tube (PMT) module prototype built to evaluate its performance. The design of the micro-dose calibrator is based on the concept of a segmented well counter. By segmenting a single crystal well counter into multiple crystals or segments, with each segment operated and read out independently, one can extend the operational range of a standard single crystal well counter while maintaining its accuracy and spectrographic capabilities. The micro-dose calibrator is operated in N-fold coincidence mode to ensure that a single radioactive decay resulting in multiple segments detecting gamma ray events are counted as a single decay. The N fold coincidence detection is achieved using a pipeline coincidence processing algorithm implemented in software as part of a multi-threaded acquisition system. A tri-functional model pulse pileup dead time correction is applied to extend the operational range of the micro-dose calibrator. The micro-dose calibrator achieved an operation range between 1 kBq up to 3.4 MBq for 18F with a 1% linearity precision using a 60 s integration time window. Over a 9 d period, a daily count rate measure of a 137Cs point source had a coefficient of variation of 0.17% with only one high voltage calibration performed on day 0. The central 20 mm up to 35 mm axial length in the sample well showed less than 1% change in count rate depending on the radionuclide. Using a 0.5 ml insulin syringe, a volume of dose between 0.13 ml up to 0.50 ml showed less than 1% deviation from volumetric linearity. A 20 cc scintillation vial showed similar volumetric linearity performance for radionuclide solution volumes between 4 cc up to 20 cc.

I178] EANM/EARL harmonization strategies in PET quantification: From daily practice to multicentre oncological studies

Quantitative positron emission tomography (PET-) imaging is essential for staging and tracking subtle processes like pathophysiologic change and therapy response in oncological diseases. The accuracy of PET-quantification decisively depends on the correct system calibration involving all corrections that are implicitly necessary using the full ring PET Detector setting in 3D mode. Basically, these are scatter-, random-, attenuation-, dead-time corrections and detector normalization on a regular basis as a requirement for correct activity quantification, that is, for the true measurement of the activity concentration in a given lesion, focus, organ or phantom. Moreover, activity quantification with PET is influenced by noise and by the method of data analysis/image-reconstruction that determine the contrast recovery and resolution. These requirements and parameters need to be addressed, selected and assessed with particular scrutiny if PET systems are to be used for semi- or fully quantitative outcomes for monitoring disease progression and/or therapy response in patient care and research settings. This is of even more importance if data is pooled across imaging sites and/or advanced methods like invasive or non-invasive pharmacokinetic modeling become increasingly considered for a particular trial. Depending on the intended method of quantification, several metrics are derived from that concentration and/or its course over time in blood and tissue. This talk will focus on the EANM/EARL strategy to harmonize and standardize PET-quantification across centers and, thus, give an insight in practical aspects, outcomes and benefits of an independent regular assessment of the calibration and image quality of hybrid PET systems. Sites and systems that get EARL accredited considerably save time and efforts if it comes to setting up the site and the system for inclusion and PET imaging within a particular trial were quantitative hybrid PET is a major part in reaching the endpoints of the multicenter study. Despite being first and foremost intended for accreditation for research settings, the conduct, design and outcome of the EARL accreditation raises awareness of the centers excellence in the patient care scenario also.

Radiation detector deadtime and pile up: A review of the status of science

Abstract Since the early forties, researchers from around the world have been studying the phenomenon of deadtime in radiation detectors. Many have attempted to develop models to represent this phenomenon. Two highly idealized models; paralyzable and non-paralyzable are commonly used by most individuals involved in radiation measurements. Most put little thought about the operating conditions and applicability of these ideal models for their experimental conditions. So far, there is no general agreement on the applicability of any given model for a specific detector under specific operating conditions, let alone a universal model for all detectors and all operating conditions. Further the related problem of pile-up is often confused with the deadtime phenomenon. Much work, is needed to devise a generalized and practical solution to these related problems. Many methods have been developed to measure and compensate for the detector deadtime count loss, and many researchers have addressed deadtime and pulse pile-up. The goal of this article is to summarize the state of science of deadtime; measurement and compensation techniques as proposed by some of the most significant work on these topics and to review the deadtime correction models applicable to present day radiation detection systems.

Image-based SPECT calibration based on the evaluation of the Fraction of Activity in the Field of View

BackgroundSPECT quantification is important for dosimetry in targeted radionuclide therapy (TRT) and the calibration of SPECT images is a crucial stage for image quantification. The current standardized calibration protocol (MIRD 23) uses phantom acquisitions to derive a global calibration factor in specific conditions. It thus requires specific acquisitions for every clinical protocols. We proposed an alternative and complementary image-based calibration method that allows to determine a calibration factor adapted to each patient, radionuclide, and acquisition protocol and that may also be used as an additional independent calibration.ResultsThe proposed method relies on a SPECT/CT acquisition of a given region of interest and an initial whole-body (WB) planar image. First, the conjugate view of WB planar images is computed after scatter and attenuation correction. 3D SPECT images are reconstructed with scatter, attenuation, and collimator-detector response (CDR) corrections and corrected from apparent dead-time. The field of view (FOV) of the SPECT image is then projected on the corrected WB planar image. The fraction of activity located in the area corresponding to the SPECT FOV is then calculated based on the counts on the corrected WB planar image. The Fraction of Activity in Field Of View (FAF) is then proposed to compute the calibration factor as the total number of counts in the SPECT image divided by this activity. Quantification accuracy was compared with the standard calibration method both with phantom experiments and on patient data.Both standard and image-based calibrations give good accuracy on large region of interest on phantom experiments (less than 7% of relative difference compared to ground truth). Apparent dead-time correction allows to reduce the uncertainty associated with standard calibration from 2.5 to 1%. The differences found between both methods were lower than the uncertainty range of the standard calibration (<3%). In patient data, although no ground truth was available, both methods give similar calibration factor (average difference 3.64%).ConclusionsA calibration factor may be computed directly from the acquired SPECT image providing that a WB planar image is also available and if both acquisitions are performed before biological elimination. This method does not require to perform phantom acquisition for every different acquisition conditions and may serve to double check the calibration with an independent factor.

Study of Some Parameters Affecting Efficiency of HpGe Detectors for Accurate Radionuclides Analysis

AbstractIn fact, optimized and adjusted parameters of analysis techniques are critical issues and lead strongly to accuracy and high confidence of results. This study is mainly focus on investigation (validation and verification; VV voltage variation, shaping time, type of detectors (n-type or p-type), geometry (size) of detector, source to detector distance and density of the sample matrix. Three different HpGe detectors with their electronics were used to carry out this work. The standard sources; 137Cs, 60Co, 57Co, 133Ba were used. The source-to-detector distance of 25cm was optimized to avoid the coincidence summing and dead-time correction .The efficiency curves of the spectrometers have been measured and presented at the above-mentioned parameters. A simulated reference efficiency curve at different parameters using standard point sources was also produced by Angel-3 software. The noises of electronics due to the electro-mechanical cooling system were also investigated. The optimum parameters were selected for precise analysis of some samples (Black sand and El-Dabaa site) at 3300V and 4000V for p-type and n-type detector respectively. It was observed that n-type detector has better efficiency than p-type especially at low energy below 120 KeV. It covers energies from 10 keV to 3 MeV. The analyses found also that the density has a strong effect on efficiency curve below energy 250 keV. The results obtained at different parameters are discussed, interpreted and presented as figures

Energy-dependent dead-time correction in digital pulse processors applied to silicon drift detector's X-ray spectra.

Dead-time effects in X-ray spectra taken with a digital pulse processor and a silicon drift detector were investigated when the number of events at the low-energy end of the spectrum was more than half of the total, at counting rates up to 56 kHz. It was found that dead-time losses in the spectra are energy dependent and an analytical correction for this effect, which takes into account pulse pile-up, is proposed. This and the usual models have been applied to experimental measurements, evaluating the dead-time fraction either from the calculations or using the value given by the detector acquisition system. The energy-dependent dead-time model proposed fits accurately the experimental energy spectra in the range of counting rates explored in this work. A selection chart of the simplest mathematical model able to correct the pulse-height distribution according to counting rate and energy spectrum characteristics isincluded.

Deadtime effects in quantification of 177Lu activity for radionuclide therapy

BackgroundThe aim of this study was to investigate the deadtime (DT) effects that are present in 177Lu images acquired after radionuclide therapy injection, assess differences in DT based on the full spectrum and the photopeak-only measurements, and design a method to correct for the deadtime losses.A Siemens SymbiaT SPECT/CT camera with a medium energy collimator was used. A 295-mL bottle was placed off-center inside a large cylinder filled with water, and 177Lu activity was sequentially added up to a maximum of 9.12 GBq. The true count rates vs. observed count rates were plotted and fitted to the DT paralyzable model. This analysis was performed using counts recorded in the full spectrum and in other energy windows. The DT correction factors were calculated using the percentage difference between the true and the observed count rates.ResultsThe DT values of 5.99 ± 0.02 μs, 4.60 ± 0.052 μs, and 0.19 ± 0.18 μs were obtained for the primary photons (PP) recorded in the 113- and 208-keV photopeaks and for the full spectrum, respectively. For the investigated range of count rates, the DT correction factors of up to 23% were observed for PP corresponding to the 113-keV photopeak, while for the 208-keV photopeak values of up to 20% were obtained. These values were almost three times higher than the deadtime correction factors derived from the full spectrum.ConclusionsThe paralyzable model showed to be appropriate for the investigated range of counts, which were five to six times higher than those observed in the patient post-therapy imaging. Our results suggest that the deadtime corrections should be based on count losses in the scatter-corrected photopeak window and not on the deadtime determined from the full spectrum. Finally, a general procedure that can be followed to correct patient images for deadtime is presented.

Pseudo-fixed dead time circuit for designing and implementation of JEOL-type X-ray counting systems

Quantitative electron probe microanalysis (EPMA) of trace elements using wavelength dispersive spectrometers (WDS) requires high probe current and/or accelerating voltage and long X-ray sampling times to measure weak X-ray signals from target materials. The X-ray count rate of a standard material is extremely high when measured under the same conditions. Consequently, the accuracy of the dead time correction becomes a significant issue for quantification. Accurate dead time correction requires an exact knowledge of the X-ray counting system, including the behavior of the proportional counter and electronic circuits. Each spectrometer has a specific dead time that depends on factors such as electronic circuit design, counter bias, and X-ray energies. The dead time also changes with aging degradation. A pseudo-fixed dead time correction circuit with non-extendable approximation is described that prevents systematic errors in quantitative EPMA caused by inappropriate dead time corrections. The circuit, which is added to the output of the single channel analyzer, includes one extendable and two non-extendable dead time generators with dead times of 0.68μs, 1.08μs, and 1.92μs, respectively. The behavior of the whole system can be treated as non-extendable for most applications, with a dead time of approximately 2.01μs. The pseudo-fixed dead time circuit suppresses systematic error of the dead time correction to 0.6% or less when the original dead time is between 0.8–2.0μs and the true X-ray count rate is <200kcps. This makes it possible to perform accurate dead time corrections without periodic measurement of the dead time throughout the life of the proportional counter by implementing a high-precision crystal oscillator in the system.

A Comparison of 2D and 3D Kidney Absorbed Dose Measures in Patients Receiving 177Lu-DOTATATE.

Objective(s):To investigate and compare quantitative accuracy of kidney absorbed dose measures made from both 2D and 3D imaging in patients receiving 177Lu-DOTATATE (Lutate) for treatment of neuroendocrine tumours (NETs). Methods: Patients receiving Lutate therapy underwent both whole body planar imaging and SPECT/CT imaging over the kidneys at time points 0.5, 4, 24, and 96-120 hours after injection. Planar data were corrected for attenuation using transmission data, and were converted to units of absolute activity via two methods, using either a calibration standard in the field of view or relative to pre-voiding image total counts. Hand drawn regions of interest were used to generate time activity curves and kidney absorbed dose estimates in OLINDA-EXM. Fully quantitative SPECT data were generated using CT-derived corrections for both scatter and attenuation, before correction for dead time and application of a camera specific sensitivity factor to convert data to units of absolute activity. Volumes of interest were defined for kidney using the co-registered x-ray CT, before time activity curves and absorbed dose measures were generated in OLINDA-EXM, both with and without corrections made to the model for patient specific kidney volumes. Quantitative SPECT data were also used to derive dose maps through dose kernel convolution (DKC), which was treated as the gold standard.Results:A total of 50 studies were analysed, corresponding to various cycles of treatment from 21 patients. Planar absorbed dose estimates were consistently higher than SPECT derived estimates by, on average, a factor of 3.Conclusion:Quantitative SPECT is considered the gold standard approach for organ specific dosimetry however often relies on in house software. As such planar methods for estimating absorbed dose are much more widely available, and in particular, are often the only source of reference in previously published data. For the case of Lutate dosimetry, planar measures may lead to a three-fold increase in measures of kidney absorbed dose.

38. Image-based SPECT calibration based on the Fraction of Activity in Field of view (FAF)

Introduction SPECT quantification is particularly important for dosimetry in Targeted Radionuclide Therapy (TRT) and the calibration of SPECT images is a crucial stage for image quantification. The current standardized calibration protocol (MIRD 23 [1] ) makes use of phantom acquisitions in specific conditions and require an additional phantom calibration for every clinical acquisition protocol. We proposed an alternative and complementary image-based calibration method that allows obtaining a single calibration factor adapted to each patient, radionuclide and acquisition protocol, without specific phantom acquisition. Methods This method consists in SPECT images quantification from WB planar images. First, the conjugate view of WB planar images is computed after scatter and attenuation corrections. 3D SPECT images are acquired in the same patient position, reconstructed with scatter, attenuation and Collimator Detector Response (CDR) corrections, and finally corrected from apparent dead-time. The Field of View (FOV) of the SPECT images is then projected on the corrected WB planar image. The fraction of activity located in the area corresponding to the SPECT FOV is calculated based on the counts on the corrected WB planar image. The Fraction of Activity in Field of View (FAF) is then proposed to compute the calibration factor as the total number of counts in the SPECT image divided by this activity. Quantification accuracy was compared with the standard calibration method both with phantom experiments and on patient data. Results Both standard and our image-based calibrations give good accuracy (ranges of respectively [−0.97; 2.86]% and [0; 3.16]%) on large region of interest on phantom experiments. Apparent dead-time correction allows reducing the uncertainty associated with standard calibration with a coefficient of variation of the calibration factor of 0.17% instead of 2.39%. The differences on accuracy found between both methods on phantom study were lower (1.01% in average) than the uncertainty range with the standard calibration (2.90% in average). On patient study, although the ground truth is unknown, the standard calibration factor is also close (3.96% in average) to the image based patient specific one. Conclusions Calibration was performed directly on the image acquisition of interest. This method allows overriding the errors caused by imperfect attenuation and scattering corrections due to different conditions. This protocol is as accurate as standardized calibration protocol but only requires an additional WB planar acquisition. The main limitation relies on the need of performing the acquisition before patient biological elimination.

Dead time corrections on the Feynman-Y curve using the backward extrapolation method

ABSTRACT Dead time losses in neutron detection, caused by both the detector and the electronics dead time, is a highly nonlinear effect, known to create high biasing in physical experiments as the power grows over a certain threshold. Analytic modeling of the dead time losses is a highly complicated task due to the different nature of the dead time in the different components of the monitoring system (paralyzing vs. non-paralyzing), and the stochastic nature of the fission chains. The most basic analytic models for a paralyzing dead time correction assume a non-correlated source, resulting in an exponential model for the dead time correction. While this model is often used and very useful for correcting the average count rate in low count rates, it is totally impractical in noise experiments and the so-called Feynman-α experiments. In the present study, a new technique is introduced for dead time corrections, based on backward extrapolation of the losses, created by imposing increasing artificial dead time on the data, back to zero. The method is implemented on neutron noise measurements carried out in the MINERVE reactor, demonstrating high accuracy in restoring the corrected values of the Feynman-Y variance-to-mean-ratio.

A general dead-time correction method based on live-time stamping. Application to the measurement of short-lived radionuclides

Dead-time correction formulae are established in the general case of superimposed non-homogeneous Poisson processes. Based on the same principles as conventional live-timed counting, this method exploits the additional information made available using digital signal processing systems, and especially the possibility to store the time stamps of live-time intervals. No approximation needs to be made to obtain those formulae. Estimates of the variances of corrected rates are also presented. This method is applied to the activity measurement of short-lived radionuclides.

Experimental evaluation of the extended Dytlewski-style dead time correction formalism for neutron multiplicity counting

Over the past few decades, neutron multiplicity counting has played an integral role in Special Nuclear Material (SNM) characterization pertaining to nuclear safeguards. Current neutron multiplicity analysis techniques use singles, doubles, and triples count rates because a methodology to extract and dead time correct higher order count rates (i.e. quads and pents) was not fully developed. This limitation is overcome by the recent extension of a popular dead time correction method developed by Dytlewski. This extended dead time correction algorithm, named Dytlewski–Croft–Favalli(DCF), is detailed in reference Croft and Favalli (2017), which gives an extensive explanation of the theory and implications of this new development. Dead time corrected results can then be used to assay SNM by inverting a set of extended point model equations which as well have only recently been formulated. The current paper discusses and presents the experimental evaluation of practical feasibility of the DCF dead time correction algorithm to demonstrate its performance and applicability in nuclear safeguards applications. In order to test the validity and effectiveness of the dead time correction for quads and pents, 252Cf and SNM sources were measured in high efficiency neutron multiplicity counters at the Los Alamos National Laboratory (LANL) and the count rates were extracted up to the fifth order and corrected for dead time. In order to assess the DCF dead time correction, the corrected data is compared to traditional dead time correction treatment within INCC. The DCF dead time correction is found to provide adequate dead time treatment for broad range of count rates available in practical applications.

GUMAP: A GUPIXWIN-compatible code for extracting regional spectra from nuclear microbeam list mode files

Abstract The newly developed GUMAP software creates element maps from OMDAQ list mode files, displays these maps individually or collectively, and facilitates on-screen definitions of specified regions from which a PIXE spectrum can be built. These include a free-hand region defined by moving the cursor. The regional charge is entered automatically into the spectrum file in a new GUPIXWIN-compatible format, enabling a GUPIXWIN analysis of the spectrum. The code defaults to the OMDAQ dead time treatment but also facilitates two other methods for dead time correction in sample regions with count rates different from the average.

Accuracy of Rhenium-188 SPECT/CT activity quantification for applications in radionuclide therapy using clinical reconstruction methods

The main applications of 188Re in radionuclide therapies include trans-arterial liver radioembolization and palliation of painful bone-metastases. In order to optimize 188Re therapies, the accurate determination of radiation dose delivered to tumors and organs at risk is required. Single photon emission computed tomography (SPECT) can be used to perform such dosimetry calculations. However, the accuracy of dosimetry estimates strongly depends on the accuracy of activity quantification in 188Re images.In this study, we performed a series of phantom experiments aiming to investigate the accuracy of activity quantification for 188Re SPECT using high-energy and medium-energy collimators. Objects of different shapes and sizes were scanned in Air, non-radioactive water (Cold-water) and water with activity (Hot-water). The ordered subset expectation maximization algorithm with clinically available corrections (CT-based attenuation, triple-energy window (TEW) scatter and resolution recovery was used). For high activities, the dead-time corrections were applied. The accuracy of activity quantification was evaluated using the ratio of the reconstructed activity in each object to this object’s true activity. Each object’s activity was determined with three segmentation methods: a 1% fixed threshold (for cold background), a 40% fixed threshold and a CT-based segmentation. Additionally, the activity recovered in the entire phantom, as well as the average activity concentration of the phantom background were compared to their true values. Finally, Monte-Carlo simulations of a commercial -camera were performed to investigate the accuracy of the TEW method.Good quantification accuracy (errors <10%) was achieved for the entire phantom, the hot-background activity concentration and for objects in cold background segmented with a 1% threshold. However, the accuracy of activity quantification for objects segmented with 40% threshold or CT-based methods decreased (errors >15%), mostly due to partial-volume effects. The Monte-Carlo simulations confirmed that TEW-scatter correction applied to 188Re, although practical, yields only approximate estimates of the true scatter.

Extension of the Dytlewski-style dead time correction formalism for neutron multiplicity counting to any order

Neutron multiplicity counting using shift-register calculus is an established technique in the science of international nuclear safeguards for the identification, verification, and assay of special nuclear materials. Typically passive counting is used for Pu and mixed Pu-U items and active methods are used for U materials. Three measured counting rates, singles, doubles and triples are measured and, in combination with a simple analytical point-model, are used to calculate characteristics of the measurement item in terms of known detector and nuclear parameters. However, the measurement problem usually involves more than three quantities of interest, but even in cases where the next higher order count rate, quads, is statistically viable, it is not quantitatively applied because corrections for dead time losses are currently not available in the predominant analysis paradigm. In this work we overcome this limitation by extending the commonly used dead time correction method, developed by Dytlewski, to quads. We also give results for pents, which may be of interest for certain special investigations. Extension to still higher orders, may be accomplished by inspection based on the sequence presented. We discuss the foundations of the Dytlewski method, give limiting cases, and highlight the opportunities and implications that these new results expose. In particular there exist a number of ways in which the new results may be combined with other approaches to extract the correlated rates, and this leads to various practical implementations.