Efficiency Extrapolation Research Articles

Activity standardization of 134Cs and 137Cs

The paper presents the results from a primary standardization of 137Cs using two independent methods – efficiency tracing using PC-NaI coincidence and the TDCR method. The nuclides 60Co and 134Cs were used as the tracers. Primary standardization of the 134Cs is also discussed. The efficiency extrapolation was carried out by measuring samples of varying mass and using the wet extrapolation method. The results obtained are in good agreement; the differences did not exceed 0.5%. The advantages, pitfalls and also possibilities for improvement of the procedures are discussed.

Gamma geometry dependency of efficiency functions in the 4πβ-γ coincidence measurements of complex decaying nuclides

In order to clarify the γ-efficiency dependency of 4πβ-γ efficiency functions, a series of 4πβ-γ efficiency extrapolation measurements of a 134Cs source were carried out for a wide variety of γ-geometries using a 4πβ(PS)-4πγ detector configuration. As the source is situated in the plastic scintillator (PS) β-detector, the γ-efficiency of the system can be readily changed by extracting the β-detector from the well-hole in a series of stages. For data acquisition and analyses, a list-mode two-parameter data acquisition system was employed. The forms of the extrapolation curves were monitored with decreasing γ-geometry, eventually exhibiting a similar behavior to those obtained in a usual 4πβ-γ coincidence counting system. The experimental results and considerations suggested that the γ-geometry dependency of the efficiency functions were due to summing effects in the γ-channel, and some qualitative remarks on the form of the extrapolation functions are given.

Standardisation of 64Cu using a software coincidence counting system

The activity of the radionuclide 64Cu was determined by the efficiency extrapolation method applied to 4π(PC)−γ coincidence counting. The standardisation was performed by software coincidence counting—a digital method for primary activity measurement that simplifies the setting of optimal coincidence parameters. The γ-ray-energy window, characterised by identical gamma detection efficiency related to the sum of EC and to the sum of beta decay branches, was found. This setting ensured a linear and zero slope extrapolation curve.

Standardization of 125I and 109Cd by the photon–photon coincidence method in PTKMR-BATAN

A photon–photon coincidence system was constructed for the standardization of 125I and 109Cd in PTKMR-BATAN, Indonesia. Two NaI(Tl) detectors of 76mm diameter × 6mm thickness with 0.5mm aluminum window were used, which were positioned approximately symmetrically to the source holder. The electronic chain was almost the same as for a 4πβ-γ system. The CANBERRA Multiport II multi channel analyzer was used for energy calibration and a Philips type PM3092 oscilloscope for visualization of the pulses. A polyethylene plastic was used as the source substrate for the 125I and 109Cd samples. The activity of a 125I solution was measured by the photon–photon coincidence and the efficiency extrapolation method (Schrader and Walz, 1987), whereas the activity of a 109Cd solution was determined by a tracer method using 125I (Schrader, 2006). The result of the 125I activity showed good agreement with the result of measurements using a calibrated ionization chamber, and the result of 109Cd also showed good agreement with the measurements result using a LEGe detector.

Improvement of software coincidence counting system for standardisation of EC-β+ radionuclides

Since 2000 the software coincidence counting (SCC) system has been used for activity standardisation of about 15 radionuclides. Their activities were determined by the efficiency extrapolation method applied to 4π (PC)-γ coincidence. Some standardisations, mainly standardisations of EC-β+ radionuclides, required optimal setting of coincidence parameters, for which new procedures based on “the coincidence to total PC count ratios”, “a multiple gamma window combined from subwindows fractions” and “a source test for PC detection efficiency ” were introduced. The paper summarises the development of SCC system related to findings from detailed analyses of recorded data. These findings are not practically achievable by a conventional approach.

Digital Coincidence System for beta-gamma Coincidence Counting

The 4 π β − γ coincidence efficiency extrapolation method is the most popular for the absolute determination of radioactivity and has been successfully applied to a number of radionuclides. It requires highly experienced technicians and niggling operation to perform the radioactivity measurement via the classical analog coincidence counting (ACC) system. The digital coincidence counting (DCC) can provide the more easy and flexible data acquirement process than ACC. This brief paper present the development and validation of a new DCC system that was realized in 2004. To compare with other DCC systems, the advantages of our DCC system included high throughput rate of pulse and adapt to pulse width from 50ns to hundreds ms.

Standardisation of 124SB and 152EU using software coincidence counting system

Activities of the radionuclides (124)Sb and (152)Eu were determined by the efficiency extrapolation method applied to 4pi(PC)-gamma coincidence counting. The (124)Sb sources were prepared from a solution with the chemical form of 50 microg g(-1) SbCl(3) in 2 M HCl. To inhibit the volatility of antimony chlorides, the sources were slowly dried in a H(2)S atmosphere with relative humidity of 76% for about 48 h. This procedure increased the beta detection efficiency up to 0.98, which simplified the standardisation. In the (152)Eu standardisation, the optimal gamma-ray energy window setting to achieve a linear dependency and the correct slope of the extrapolation curve were derived by means of software coincidence counting system using offline evaluation of data with different coincidence parameter settings. The results obtained by the software coincidence counting system were compared with those obtained by the conventional coincidence method.

Determination of 198Au X-rays emission probabilities

This work describes the measurements of the K X-ray and gamma-ray emission probabilities per decay of (198)Au performed at the Nuclear Metrology Laboratory (LMN) at the IPEN, São Paulo. The radioactive sample was obtained by means of (197)Au(n, gamma)(198)Au reaction irradiating an Au foil in a thermal neutron flux near the core of the IPEN 3.5 MW research reactor. The activity of samples was determined in a 4pibeta-gamma coincidence system, setting the gamma window at the 411.80 keV total energy absorption peak. The same samples were measured in two different spectrometers: a HPGe planar spectrometer with Be window, suitable for measurements in the low energy range and a coaxial REGe spectrometer. Both spectrometers were previously calibrated in a well defined geometry by means of standard sources calibrated in a 4pibeta-gamma coincidence system. MCNP4C Monte Carlo code was used for simulating the REGe spectrometer calibration curve, and a new version of code ESQUEMA was adopted for simulating the detection processes in the coincidence system, in order to predict the efficiency extrapolation curve.

Standardization of 241Am by digital coincidence counting, liquid scintillation counting and defined solid angle counting

The nuclide (241)Am decays by alpha emission to (237)Np. Most of the decays (84.6%) populate the excited level of (237)Np with energy of 59.54 keV. Digital coincidence counting was applied to standardize a solution of (241)Am by alpha-gamma coincidence counting with efficiency extrapolation. Electronic discrimination was implemented with a pressurized proportional counter and the results were compared with two other independent techniques: Liquid scintillation counting using the logical sum of double coincidences in a TDCR array and defined solid angle counting taking into account activity inhomogeneity in the active deposit. The results show consistency between the three methods within a limit of a 0.3%. An ampoule of this solution will be sent to the International Reference System (SIR) during 2009. Uncertainties were analysed and compared in detail for the three applied methods.

Standardization of 18F using the 4 π( β+ γ) integral counting technique

Α 4 π( β+ γ) integral counting technique using a 4 πβ−4π γ detector configuration was adopted for the standardization of 18F. In this technique, the β-detector is composed of two thin plastic scintillators sandwiching the source, coupled with a slender photomultiplier tube. The β-detector part with the source was inserted into a large well-type NaI(Tl) scintillation detector for γ-ray detection, making a 4 πβ−4π γ coincidence counting system. In this work, positron particles were detected with high efficiency in the β-channel and annihilation quanta were also detected with high efficiency in the 4 πγ channel. The very small inefficiency of the 4π( β+ γ) integral counter for the β-plus branch has been confirmed by EGS5 Monte Carlo simulation. The result using this technique agreed within the uncertainties with the result obtained by the conventional 4 πβ− γ coincidence counting with the efficiency extrapolation technique using the same detector configuration and a conventional 4 πβ− γ coincidence counter.

Standardisation of 56Co and 57Co using software coincidence counting system

Activities of the radionuclides 56Co and 57Co were determined by the efficiency extrapolation method applied to 4 π(PC)- γ coincidence counting. Solutions of 56Co usually contain a significant amount of 57Co and 58Co, so the measured activity of 56Co requires correction. When the conventional coincidence method is used for 56Co standardisation, the corrections are derived from the dependence of Proportional Counter (PC) detection efficiencies for 57Co and 56Co measured using sources with mixture of 56Co and 57Co, which is complicated. These difficulties were reduced by means of a software coincidence method, with a HPGe detector in gamma channel, where the detection efficiencies were evaluated directly from the records of coincidence measurements of standardised sources. In the case of 57Co standardisation, the software coincidence counting system was applied for the evaluation of optimal setting of coincidence parameters. The results obtained by software coincidence counting system were compared with those obtained by the conventional coincidence method.

The efficiency variation method for 4 πβ− γ coincidence counting by ink-jet printing

In order to vary the counting efficiencies in the 4 πβ− γ coincidence extrapolation technique, a radioactive source was coated directly with varying amounts of an electrical conducting pigment using an ink-jet printer. This method can be used to efficiently prepare the multiple sources needed to generate efficiency extrapolation curves, and was successfully applied to the standardization of a 54Mn source.

Assurance of the traceability chain for 131I measurement

The paper presents some recent results of the Radionuclide Metrology Laboratory (RML) from IFIN-HH, concerning the assurance of the international traceability for the measurement of 131I solution aimed to continue the national chain for nuclear medicine applications in Romania. The following sequences of the traceability are presented: the absolute standardization of solution by the 4πβ-γ coincidence, efficiency extrapolation method and participation in the BIPM.RI(II)-K1.I-131 key comparison; and recalibration of the CENTRONIC IG12/20A ionization chamber secondary standard. The participation in the IAEA/SSDL (SSRL) 131I comparison is also presented.

Primary standardization of activity using the coincidence method based on analogue instrumentation

Widely implemented at national metrology institutes (NMIs), the coincidence method is a technique to assay a wide variety of radionuclides which decay through two or more types of radiation. Through a survey of the literature, this paper seeks to describe the main aspects of one of the most powerful direct methods available in radionuclide metrology. The basics of coincidence counting and the efficiency extrapolation method are covered. The problem of non-linearities in the extrapolation curve is also considered. The main characteristics of variants to the conventional coincidence instrumentation are presented.

Calculation of electron deposition in proportional counters

Efficiency extrapolation is a common technique in the standardization of radionuclides by the technique of 4 πβ– γ coincidences. It can be achieved in two ways: adding thin films onto the radioactive source or by electronic discrimination in a pressurized 4 πβ– γ system. The last case is the most extensively used actually in metrology laboratories. There is a need to know the electron energy loss inside the proportional counter as a function of the electron energies and working pressures. Calculations of electron energy loss have been performed for electron energies in a range from a few keV to 2 MeV in a mixture of Ar(90%) CH 4(10%) and for pressures ranging from 100 to 2000 kPa in a 5 cm radius proportional counter. Wall effects, such as electron backscattering, are included in the calculations. Results are provided in the form of graphs and some recommendations are given about the optimal values of the energies that can be used to perform the extrapolation in the standardization of some radionuclides.

Standardisation of 54Mn and 65Zn using a software coincidence counting system

The activities of 54Mn and 65Zn have been determined by 4 π(PC)- γ coincidence counting, with efficiency variation performed by the conventional method of altering the self-absorption in the sources as well as by the computer discrimination method. The standardisation of 65Zn presents some complications requiring optimisation of the γ-ray energy window settings to achieve a linear efficiency–extrapolation curve. Determination of these optimal settings by the conventional coincidence method is a tedious task. These difficulties have been reduced by the utilisation of a software coincidence counting system that records time and amplitude information of individual pulses from coincidence measurements, where the coincidence parameters are set after the data collection process has completed, facilitating multiple data evaluations on a single data set. The optimal γ-ray energy window settings for the 65Zn standardisation were derived from the results of the 54Mn standardisation, as well as from studies of the 65Zn data itself. The setting of the PC channel thresholds for K and both (K+L) electrons is also discussed. The results are compared with those attained using conventional coincidence counting.

Determination of disintegration rates and γ-ray emission probabilities of 65Zn and 241Am

Radioactive solutions of 65Zn and 241Am were standardized within the frame of the CCRI (II)/BIPM key comparisons carried out recently in 2003. The activity concentrations (in Bq g −1) were determined using the 4 π (proportional counter)- γ coincidence counting method with efficiency extrapolation. Liquid scintillation methods were also used for 241Am activity determination. The 1115.5 keV of 65Zn and the 26.35 and 59.54 keV of 241Am γ-ray emission probabilities were determined as 0.4971, 0.0206 and 0.3587, respectively, using a planar germanium detector. The associated relative uncertainties were 0.33%, 1.56% and 0.47%, respectively.

Standardization of 192Ir solution at BARC

A radioactive solution of 192Ir was standardized at Bhabha Atomic Research Centre (BARC) under the international intercomparison programme of the Bureau International des Poids et Measures (BIPM) by 4 π β - γ coincidence and 4 π γ counting techniques, which are primary and secondary methods of standardization, respectively. The activity concentration of the 192Ir solution obtained by the efficiency extrapolation technique and as reported to BIPM was 203.15 kBq g −1 with a combined uncertainty of ±0.3%. The activity concentration determined by 4 π γ counting was 202.88 kBq g −1 with a combined uncertainty of ±1.3%. These measurement results are comparable within the uncertainty limits. Detailed uncertainty budget for both the primary and secondary methods are included.

Standardization of 65Zn by 4π PC- γ coincidence counting method with efficiency extrapolation

A 65Zn solution was standardized by the 4 πPC– γ efficiency-extrapolation coincidence counting method. Theoretical aspects of coincidence equations, efficiency equations and linearity conditions are reviewed. Experimental measurements were performed for two low level discrimination thresholds for the PC channel (counting K+ L or K X-rays or Auger electrons from EC decay) and for three different settings of the γ window. Requirements on γ channel set-up for linear extrapolation were established by using a Pb absorber or by proper γ window setting. The measured activity values were discussed and found in good agreement with those obtained with a calibrated ionization chamber.

Direct activity determination of 54Mn and 65Zn by a non-extrapolation liquid scintillation method

The measurement of 54Mn and 65Zn by liquid scintillation coincidence counting results in low detection efficiencies. The activity obtained from the extrapolation of efficiency data can therefore become problematic if curvature is present. The simple decay scheme exhibited by these radionuclides, with the emission of an energetic gamma ray, allows the absolute activity to be determined from 4πe–γ data by direct calculation without the need for efficiency extrapolation. The method, which relies on determining the probability of the γ-ray interacting with the scintillator solution, is described and validated by measurements made on 60Co.