Validation of gamma-ray true coincidence summing effects modeled by the Monte Carlo code MCNP-CP

When radionuclides decay by cascading photons, the accuracy of the measured nuclide activity may be affected by true coincidence summing effects. The effects can be quantified by Monte Carlo simulations that can handle correlated γ-and X-ray emissions from a radionuclide. Analysis techniques are also available commercially to correct for the effects due to cascading γ-rays. The MCNP-CP code was used to compute the effects in high purity germanium detectors for several commonly used nuclides and geometries and the results were compared to measurements and an analysis technique. Excellent agreement in true coincidence summing corrections predicted by MCNP-CP and the analysis technique was obtained. In addition, the X-ray true coincidence summing effects were evaluated.     

Stability of metallofullerenes following neutron capture reaction on the metal ion

To achieve the highest possible sensitivity of analysis for environmental samples it is common practice to use both a high efficiency detector and a close measurement geometry with a large sample size (e.g. Marinelli beaker). Under such conditions, the typical efficiency calibration procedure results in a biased activity value for many nuclides due to the true coincidence summing effect. While there are a few methods to correct for this effect with special calibration standards, such calibrations can be both time consuming and expensive. Due to these calibration difficulties, the true coincidence summing effect is often simply ignored. Recently, it has been demonstrated that the coincidence summing correction can be performed mathematically even for voluminous sources. This new method consists of an integration of the coincidence correction factor over the sample volume while taking into account its chemical composition and the container. In this paper, we will discuss the latest approaches for establishing the peak efficiency and peak-to-total efficiency curves, which are required for this method. These approaches have been tested for HPGe detectors of two different relative efficiencies.     

True coincidence summing corrections in point and extended sources

The true coincidence summing (TCS) effect on the full energy peak (FEP) efficiency calibration of an HPGe detector has been studied as a function of sample-to-detector distance using multi-gamma sources. Analytical method has been used to calculate coincidence correction factors for ¹⁵²Eu, ¹³³Ba, ¹³⁴Cs and ⁶⁰Co for point and extended source geometry at close sample-to-detector distance. Peak and total efficiencies required for this method have been obtained by using MCNP code by using the optimized detector geometry. The correction factors have also been obtained experimentally. The analytical and the experimental correction factors have been found to match within 1–5%. The method has been applied to obtain the activity of the radionuclides (¹⁰⁶Ru, ¹²⁵Sb, ¹³⁴Cs and ¹⁴⁴Ce) present in a fission product sample.     

DÖME: revitalizing a low-background counting chamber and developing a radon-tight sample holder for gamma-ray spectroscopy measurements

There was an emerging need at the Nuclear Analysis and Radiography Department of the Centre for Energy Research, Hungarian Academy of Sciences, Budapest to be able to perform low level radioactivity measurements of various samples from in-beam activation and from environmental studies. Important aspects of reusing the low-background chamber called DÖME, which had been unused for some years, were the development of an easily reusable radon-tight sample container and the setup of a measurement system capable of counting extended samples in close-in geometry. As a result of our efforts a special sample container made of HDPE (High-density Polyethylene) was developed, and it is proved that the probability of a radon loss larger than the 2 % of its radioactive decay constant is <95 %. Due to the lack of reference samples, containing the same radionuclides as the unknown sample, the absolute method of measuring the activity concentration of nuclides in the sample had to be applied, which implied the reliable determination of the full-energy peak efficiency. A method called efficiency transfer combined with the correction for true coincidence summing effects is proven to be providing appropriate results and applied.     

Characterisation of cascade summing effects in gamma spectroscopy using Monte Carlo simulations

A GEANT4 based Monte Carlo simulation has been successfully utilised to calculate peak efficiency characterisations and cascade summing (true coincidence summing) corrections in two source geometries commonly used for environmental monitoring. The cascade summing corrections are compared with values generated using an existing (validated) system, and found to be in excellent agreement for all radionuclides simulated. The calculated correction factors and peak efficiencies were also tested by analysing well defined sources used in the operation of the International Monitoring System, which undertakes radionuclide monitoring for verification of the Comprehensive Nuclear-Test-Ban Treaty. All abundances of the radionuclides measured matched the values that were previously determined using proprietary software. Using GEANT4 in this way, cascade summing corrections can now be extended to complex detector models and source matrices, such as Compton Suppression systems.     

Validation testing of the Genie 2000 Cascade Summing Correction

Summary To validate the accuracy and precision of the Cascade Summing Correction method, over 800 archived measurements of calibrated sources (filter paper, 20 cm³ liquid scintillation vial, 400 ml beaker and Marinelli beaker) containing cascading (⁸⁸Y and ⁶⁰Co) and non-cascading isotopes from 133 different ISOCS/LabSOCS characterized high purity germanium detectors have been analyzed. Comparing the corrected results for the cascading isotope activities to the known activities shows the method is effective and accurate. Evaluation of the accuracy as a function of the amount of correction reveals a small systematic error for which a variable precision adjustment is recommended. Requirements to filter true coincidence X-rays by are verified.     

True coincidence summing correction determination for 214Bi principal gamma lines in NORM samples

The gamma lines 609.3 and 1,120.3 keV are two of the most intensive γ emissions of ²¹⁴Bi, but they have serious true coincidence summing (TCS) effects due to the complex decay schemes with multi-cascading transitions. TCS effects cause inaccurate count rate and hence erroneous results. A simple and easy experimental method for determination of TCS correction of ²¹⁴Bi gamma lines was developed in this work using naturally occurring radioactive material samples. Height efficiency and self attenuation corrections were determined as well. The developed method has been formulated theoretically and validated experimentally. The corrections problems were solved simply with neither additional standard source nor simulation skills.     

Simple method for true coincidence summing correction

For the correction of losses due to true coincidences summing and edge effect, a simple method which is based on the ratio of a reference single -ray energy to that of cascade energies at near and far geometry is developed. The correction factors for several radioactive sources with simple and complex decay schemes are experimentally determined for three types of germanium detectors. It is shown that coincidence summing can be a complex effect and depends on the individual detector, the counting geometry and on the decay scheme of the radionuclide concerned.     

Measurement of radioactive samples in Marinelli beakers by gamma-ray spectrometry

Radioactivity measurement of environmental samples is frequently assayed by gamma-ray spectrometry using Marinelli beakers. In this work, self-absorption and coincidence summing effects arising in activity measurements for Marinelli beaker geometry have been studied with a Ge detector. Three types of Marinelli beakers which have capacities of 450 mL, 1 L, and 2 L were developed. Self-attenuation effects for density variation of radioactive samples in each type of the Marinelli beakers were measured as a function of gamma-ray energy, and also the results were compared with calculated values by mathematical model. Meanwhile, the coincidence summing effects of¹²⁵Sb and¹⁵⁴Eu nuclides were obtained from the determination of the full-energy peak and total efficiencies for a Ge detector.     

Total versus effective total efficiency in the computation of coincidence summing corrections in gamma-ray spectrometry of volume sources

For the evaluation of coincidence summing effects for volume sources an effective total efficiency (ETE) is used instead of the common total efficiency (TE). In this paper ETE is computed by the Monte Carlo method. The differences between ETE and TE are analyzed and their origin is discussed. Measured values for the coincidence summing correction factors for a standard solution containing ¹⁵²Eu in a one liter Marinelli beaker are compared with computed values obtained from appropriate values of ETE. It is shown that the procedure for the evaluation of the coincidence effects is reliable. As a consequence it can be concluded that ¹⁵²Eu volume sources can be successfully used for efficiency calibration even in the case of high-efficiency detectors and close source-to-detector distances.     

Evaluation of Three Software Programs for Calculating True-Coincidence Summing Correction Factors

True-coincidence summing correction is an essential element in k ₀-based NAA¹ and becomes important when samples are counted with a high efficiency detector. This may be the case where large detectors are used or where samples are counted in or in the vicinity of the detector in order to achieve low detection limits in conjunction with low-flux reactors. In some laboratories coincidence correction is accomplished by calculating the coincidence correction factors. Since experimental validation of the calculations will reveal only the most significant errors and is a laborious task due to the high number of radionuclides involved, three laboratories decided to compare their calculated coincidence factors. Each laboratory uses a different software package. A comparative performance analysis was made of COINCALC developed at the INW of the University of Gent (implemented in SOLCOI by DSM Research), the software of the IRI, University of Delft, the Netherlands, and the software of the Ecole Polytechnique, Montreal, Canada. The overall approach, data and algorithms were chosen independently by each institute as the software was being developed and, so, the comparison has yielded a number of interesting conclusions. A follow-up investigation of the discrepancies found will probably allow the performance of each program to be improved.     

Implementation of the coincidence method for determining125I activity with an estimate of error

The established coincidence method of determining¹²⁵I activities has been adapted for use with a multisample gamma counter. The method has been extended to include a formula which has been derived to indicate the precision of the measured activities. The validity of this formula has been tested and found to be satisfactory. The techniques used to select appropriate counting conditions are described.     

A semi-empirical method for calculation of true coincidence corrections for the case of a close-in detection in γ-ray spectrometry

In this paper, a semi-empirical method is proposed to determine true coincidence-summing (TCS) correction factors for high resolution γ-ray spectrometry. It needs the knowledge of both full energy peak (FEP) efficiency and total-to-peak (TTP) efficiency curves. The TTP efficiency curve is established from the measurements with a set of coincidence-free point sources. Whereas for a volume source, the coincidence-free FEP efficiency curve is obtained iteratively by using the peaks from almost the coincidence-free nuclides and those from the coincident nuclides in the mixed standard sources. Then the fitting parameters obtained for both TTP and FEP efficiency curves are combined in a freely-available TCS calculation program called TrueCoinc, which yields the TCS correction factors required for any nuclide. As an application, the TCS correction factors were determined for the particular peaks of ²³⁸U, ²²⁶Ra and ²³²Th in the reference materials, measured in the case of a close-in detection geometry using a well-type Ge detector. The present TCS correction method can be applied without difficulty to all Ge detectors for any coincident nuclide.     

Calculation of peak-to-total ratios for high purity germanium detectors using Monte-Carlo modeling

Summary In order to estimate by calculation the magnitude of the true coincidence summing losses that may be affecting the observed gamma-ray spectrum of a given nuclide, measured using a spectrometer, knowledge of the total detection efficiencies at the gamma-ray energies within the cascades is essential. The total efficiency can be determined from the full energy peak efficiency, provided the peak-to-total ratio is known. For a given high purity germanium (HPGe) detector, one can establish an intrinsic peak-to-total (P/T) efficiency curve using a set of measurements performed with “single” (ideally monoenergetic) gamma-emitting nuclides (e.g., ²⁴¹Am, ¹⁰⁹Cd, ⁵⁷Co, ¹¹³Sn, ¹³⁷Cs, ⁶⁵Zn). Some of these nuclides are short lived and so have to be replaced periodically. Moreover, the presence of low energy gamma-rays and X-rays in most of the decay schemes complicate the empirical determination of the P/T ratios. This problem is especially severe if measurements are made using HPGe detectors that have a very thin dead layer. The problems posed by low energy gamma-rays and X-rays can be avoided by using absorbers, but then one has to be careful not to perturb the intrinsic value of the P/T ratio being sought. This paper addresses these problems. Measurement related limitations are avoided if one can use a computational technique instead. In the work presented here, the feasibility of using a Monte-Carlo based technique to determine the P/T ratios at a wide range of energies (60 keV to 2000 keV) is explored. The Monte-Carlo code MCNP (version 4B) is used to simulate gamma-ray spectra from various nuclides. Measured P/T ratios are compared to calculated ratios for several HPGe detectors to demonstrate the generality of the approach. Reasons for observed disagreement between the two are discussed.     

Correction equations of coincidence summing using75Se radionuclide in the efficiency of HpGe detector

Correction equations of the coincidence-summing effect for efficiencies of HpGe detector based on the decay scheme were developed by considering the summing up to triple coincidence. The correction equations which do not dependent on the kind of the Ge detector are very useful for efficiency calibrations of a Ge detector in the energy region from 60 to 400 keV by using⁷⁵Se radionuclide even with very short source-to-detector distances.     

Coincidence summing corrections factors calculated for volume 152Eu sources in γ-ray spectromtery using Monte Carlo techniques

Journal of Radioanalytical and Nuclear Chemistry - True coincidence summing (TCS) correction factors were determined for 152Eu volume sources used for γ-ray efficiency calibration as the ratio...     

Count rate characteristics and count loss correction of Positologica II: a whole body positron emission tomograph

This paper describes evaluation and correction of count rate characteristics of POSITOLOGICA II, a multi-slice whole body positron emission tomography system. The present study was performed using three phantoms; a 5 cm inner diameter, water-filled lucite cylinder, a 20 cm inner diameter, water-filled lucite cylinder and a chest phantom. After injection of high activity (about 1.85 GBq (50 mCi] of 13N ammonia into each phantom, rates of true coincidence, random coincidence and single photon detections were measured during decay of the isotope through more than two orders of magnitude of activity. At very high levels of activity, count rate characteristics of the system were saturated and limited to 660 kcps of total coincidence rate, which was the sum of rates in on-time and off-time windows, by the FIFO (first-in first-out) output frequency. Below those levels of activity the relationship between count loss and true coincidence rate was not unique but depended on the phantom configurations, suggesting that count loss correction using the above relationship was inadequate for quantitative study. However, the relationship between count loss and single rate was almost independent of the phantom configurations. Thus in conclusion count loss could be corrected using single rate for POSITOLOGICA II. A practical method of count loss correction was also proposed.     

Parametric normalization for full-energy peak count rates obtained at different geometries

The Effective Interaction Depth (EID) law has been systematically studied and applied to parametric normalization for peak count rates obtained at different source-detector distances (S-D). The errors caused by EID normalization are less than 4% over the full range of S-D (from to several mm) for true coincidence-free -rays. Parametric corrections for the true coincidence (summing) effect are also established, based on simplified decay schemes and P/T ratio determinations. The total response of Ge detector for single-energy -rays (T) is clearly defined with scattering contributions from surroundings included. Errors from summing effect corrections are also less than 4%. The combined EID normalization and summing effect corrections give an error no greater than 5.7% for the worst situations (several mm S-D and cascade-crossover decay scheme), acceptable for most practical K₀ NAA.     

Experimental validation of corrections factors for γ–γ and γ–X coincidence summing of 133Ba, 152Eu,and 125Sb in volume sources

True coincidence summing correction factors for ¹³³Ba, ¹⁵²Eu and ¹²⁵Sb were determined experimentally for a small volume source and compared with correction factors obtained with three softwares (EFFTRAN-X, GESPECOR and VGSL). The radionuclides investigated have a relatively challenging decay scheme and their spectra are known to suffer from losses due to summation (γ–γ, γ–X and X–X) when measured at close distances on a HPGe detector sensitive to low energy photons. This study shows that the softwares were in good agreement with each other and the experimental data and the calculated activity was consistent with the activity in the volume source.

     

Coincidence counting for PGNAA applications: Is it the optimum method?

Summary One of the main advantages of γ-γ coincidence counting is the reduction of the background spectrum, pulse pile-up, and summing effects (for simple schemes). For prompt gamma-ray neutron activation analysis (PGNAA), the sources of background include the gamma-rays from the natural background, from surrounding materials, from the neutron source, and from detector neutron activation. While this counting approach effectively increases the signal-to-noise (S/N) ratio, it also decreases the signal counting rate. This adds some practical limitations to using this approach. In this work, two examples are presented for the efficient use of the coincidence counting approach.