Search

The main features of code SUMCOR developed for cascade summing correction for volumetric sources are described. MCNP6 is used to track histories starting from individual points inside the volumetric source, for each set of cascade transitions from the radionuclide. Total and FEP efficiencies are calculated for all gamma-rays and X-rays involved in the cascade. Cascade summing correction is based on the matrix formalism developed by Semkow et al. (1990). Results are presented applying the experimental data sent to the participants of two intercomparisons organized by the ICRM-GSWG and coordinated by Dr. Marie-Cristine Lépy from the Laboratoire National Henri Becquerel (LNE-LNHB), CEA, in 2008 and 2010, respectively and compared to the other participants in the intercomparisons.

The procedure followed by the Nuclear Metrology Laboratory (LMN) at the IPEN for the standardization of activity of 64Cu is described. The measurement was carried out in a 4π (PC) β-γ coincidence system. The activity per unit mass of the solution was determined by the extrapolation technique. The events were registered using a Time to Amplitude Converter (TAC) associated with a Multi-channel Analyzer. The gamma-ray emission probability per decay of the 1345.7keV transition of 64Cu determined with a calibrated REGe gamma-ray spectrometer was (0.472 ± 0.010) %, in agreement with the literature.

In this work we present a combined experimental and theoretical study of $K$-shell ionization by electrons with energies close to the threshold. The ionization cross sections of the $K$ shells of Te, Ta, and Bi atoms have been measured up to 100 keV with uncertainties ranging from 4% to 8%. In turn, calculations have been done using the subconfiguration average distorted-wave (SCADW) method, which includes the full two-body retarded electromagnetic interaction between the projectile and target electrons. The predictions of the SCADW method are in good agreement with the experimental data. In contrast, theoretical cross sections based on first-order perturbation theory where the transverse interaction is computed with plane waves instead of distorted waves underestimate the SCADW values as well as the experimental data. The difference between the two investigated ab initio formalisms grows with atomic number, being 3% for Te, 15% for Ta, and 25% for Bi. An additional comparison of both theoretical approaches with recent measurements for Au $K$ supports the conclusion that the SCADW method reproduces well the experimental $K$-shell ionization cross section of atoms with intermediate to large $Z$ near the threshold.

The procedure for determining the (67)Ga disintegration rate by a primary method is described. The proposed triple 4πβ-γ coincidence system consists of a thin window gas-flow 4π proportional counter (PC) coupled to a NaI(Tl) scintillator and a HPGe crystal. Independent pulse height and occurrence time information is provided for the three detector outputs by means of a Software Coincidence System. Separate spectrometry measurements with a n-type reversed electrode coaxial Ge detector (REGe) were performed for obtaining gamma-ray emission probabilities per decay. Accurate values of disintegration rate, gamma-ray emission probabilities and the metastable half-life were achieved.

The measurement of the gamma-ray emission probabilities per decay of 1077 keV of 68Ga is presented. The standardization system consists of a gas-flow proportional counter in 4π geometry coupled to an HPGe detector for the gamma-ray detection. The gamma-ray emission probabilities per decay were measured in an HPGe gamma-ray spectrometer. The weaker gamma-ray intensities of 68Ga were measured in a relative way, making use of an uncalibrated ampoule of 68Ge–68Ga in radioactive equilibrium, and considering the absolute result from the 1077 keV gamma-ray.

In the present work, the determinations of k0 and Q0 for 74Se, 113In, 186W and 191Ir targets were performed. The irradiations were conducted near the core of the IEA-R1 4.5 MW swimming-pool nuclear research reactor of the Instituto de Pesquisas Energeticas e Nucleares (IPEN-CNEN/SP – Nuclear and Energy Research Institute), in Sao Paulo, Brazil. The irradiation position was chosen where the neutron spectrum shape parameter α is very close to zero. For this reason, the correction to be applied for the determination of Q0 is very close to one, thus improving the accuracy of the results. For each experiment, two irradiations were carried out in sequence: the first one with bare samples and the second with a cadmium cover around the samples. All partial uncertainties were considered, applying the covariance matrix methodology. The final results were compared with the literature.

A Monte Carlo simulation code known as ESQUEMA has been developed by the Nuclear Metrology Laboratory (Laboratorio de Metrologia Nuclear—LMN) in the Nuclear and Energy Research Institute (Instituto de Pesquisas Energeticas e Nucleares—IPEN) to be used as a benchmark for radionuclide standardization. The early version of this code simulated only β−γ and ec−γ emitters with reasonably high electron and X-ray energies. To extend the code to include other radionuclides and enable the code to be applied to software coincidence counting systems, several improvements have been made and are presented in this work.

The procedure followed by the Nuclear Metrology Laboratory (LMN), at the Nuclear and Energy Research Institute (IPEN), for the primary standardization of (177)Lu is described. This radionuclide is widely used in radiopharmacy due to its convenient half-life and emitted beta ray energies. The (177)Lu solution was supplied during an international comparison sponsored by BIPM in 2009 and the primary standardization has been accomplished by the 4pibeta-gamma coincidence method using a proportional counter in 4pi geometry coupled with two NaI(Tl) scintillation counters. The beta efficiency was varied by placing Collodion and aluminum absorbers over and under the radioactive source. The (177)Lu calibrated sources were also measured in a previously calibrated HPGe spectrometer, in order to obtain the emission probability per decay for the selected gamma-ray transitions. The experimental extrapolation curves were also compared with Monte Carlo simulations by means of code ESQUEMA developed at the LMN.

The present work describes the procedure developed for the standardization of 18F by means of a coincidence system using a plastic scintillator detector in 4pi geometry, named 47pi(PS)beta - gamma, which was developed at the Nuclear Metrology Laboratory in IPEN, Sao Paulo, Brazil. The main advantage of this detector system is the ability to perform primary standardizations without the need for a coating of a metal layer on the radioactive source film for rendering it conductive, as usually necessary for proportional counter measurements. The measurements were also performed with a conventional 47pi(PC)beta - gamma coincidence system, which makes use of a 4pi proportional counter for charged particles or X-ray detection coupled to a pair of Nal(Tl) scintillation counters. The gamma-ray window was set to measure the 511 keV gamma-rays produced by positron annihilation. The detector efficiency was changed by moving the electronic discriminator threshold. The observed activity values were extrapolated to 100% efficiency and the results showed good agreement between the two detector systems.

Improvements recently developed at the Nuclear Metrology Laboratory of IPEN-CNEN/SP in São Paulo were performed in order to increase the detector efficiency of a 4pibeta-gamma coincidence primary system using plastic scintillators in 4pi geometry. Measurements were undertaken and compared to the original system and Monte Carlo simulations of the extrapolation curves were calculated for this new system and compared to experimental results. For this purpose, the code Penelope was applied for calculating response functions for each detector and the code Esquema, developed at LMN, was used for simulating the decay scheme processes.

This work describes the procedure followed by the Laboratório de Metrologia Nuclear (LMN) for the standardization of (55)Fe by the tracing method. This technique was applied using two radionuclides, which decay by the electron capture process followed by a prompt gamma-ray, namely (51)Cr and (54)Mn, as tracers. The calibration was performed in a 4pibeta-gamma coincidence system. The efficiency was obtained by selecting a gamma-ray window set at the 320keV total absorption peak for (51)Cr and at 834keV for (54)Mn.

A modified version of a Monte Carlo code called Esquema, developed at the Nuclear Metrology Laboratory in IPEN, Sao Paulo, Brazil, has been applied for simulating a 4πβ(PS)–γ coincidence system designed for primary radionuclide standardisation. This system consists of a plastic scintillator in 4π geometry, for alpha or electron detection, coupled to a NaI(Tl) counter for gamma-ray detection. The response curves for monoenergetic electrons and photons have been calculated previously by Penelope code and applied as input data to code Esquema. The latter code simulates all the disintegration processes, from the precursor nucleus to the ground state of the daughter radionuclide. As a result, the curve between the observed disintegration rate as a function of the beta efficiency parameter can be simulated. A least-squares fit between the experimental activity values and the Monte Carlo calculation provided the actual radioactive source activity, without need of conventional extrapolation procedures. Application of this methodology to 60Co and 133Ba radioactive sources is presented and showed results in good agreement with a conventional proportional counter 4πβ(PC)–γ coincidence system.

The procedure for the standardization of (59)Fe using a 4π(PC)β-γ software coincidence system is described. The standardization was performed with an experimental setup consisting of a thin window gas-flow proportional counter (PC) in 4π geometry coupled to a NaI(Tl) scintillator and to a HPGe detector. The data acquisition was carried out by means of a Software Coincidence System (SCS). The beta efficiency was changed by using Collodion films and aluminum foils as external absorbers.

A calibration system composed of a 4 π β - γ gas-flow proportional counter coupled to a pair of NaI(Tl) crystals operating in coincidence was used for absolute measurement of Am 242 g sources activities. The samples were produced by irradiating dried aliquots of 241Am solution with epithermal neutrons in a research reactor. Efficiency tracing methodology using 60Co as tracer was applied. Thick external absorbers were used to change the Am 242 g beta detection efficiency and to avoid counting alpha particles from the dominating activity of 241Am. The resulting complex behavior of the beta detection efficiency curve was simulated by the Monte Carlo method, and the Am 242 g activity was obtained by least-squares method fitting. The activity was determined with an uncertainty of about 1%.

The methodology recently developed by the Nuclear Metrology Laboratory (LMN) from IPEN, in Sao Paulo, Brazil, for simulating all detection processes in a 4/spl pi/(/spl beta/,X)-/spl gamma/ coincidence system by means of the Monte Carlo technique is described. This procedure makes possible to predict the behavior of the observed activity as a function of the 4/spl pi//spl beta/ detector efficiency. The present paper describes its application to the standardization of a typical beta-gamma radionuclide, namely /sup 134/Cs. In this approach, information contained in the decay scheme is used for determining the contribution of all radiations emitted by the selected radionuclide to the measured spectra of each detector. This simulation yields the shape of the coincidence spectrum, allowing the choice of suitable gamma-ray energies for which the activity can be obtained with maximum accuracy. The theoretical work applies the MCNP Monte Carlo code to a gas-flow proportional counter with 4/spl pi/ detection geometry, coupled to a pair of NaI(Tl) crystals. The calculated extrapolation curve showed good agreement when compared to experimental values obtained at the LMN of IPEN.

A gamma-gamma coincidence experiment was performed to check the 152Eu 13-year decay scheme and the placement of the observed gamma-ray transitions. The multi-detector array for residual activity measurement of the Linear Accelerator Laboratory was used. The source activity was 1 MBq, and about 10(9) coincidence events were observed. About 30 gamma's were placed in the decay scheme and four 152Sm levels were added to the previously known set of levels fed in 152Eu 13-year decay.

The procedure to determine the statistical correlations between γ -ray intensities is described here, as well as that required to take these correlations into account. The advantages of using branching-ratios and feeding fractions instead of γ -ray intensities in the γ -ray standards are discussed.

A coincidence system using a plastic scintillator detector in 4π geometry has been developed and applied for the standardization of radionuclides. The scintillator shape and dimensions have been optimized for maximum charge particle detection efficiency, while keeping background low and a nearly constant gamma-ray efficiency for different points from the radioactive source. The gamma-ray events were measured with a NaI(Tl) scintillation counter. The electronic system for processing pulses consisted of logic gates and delay modules feeding a time-to-amplitude converter with output to a multichannel analyzer. The alpha detection efficiency measured with 241Am was around 95% and the beta detection efficiency for 60Co was around 67%. Activity measurements of 241Am and 60Co were performed and the results showed good agreement when compared with a conventional coincidence system employing a 4π proportional counter.

The variance matrix between the emission probabilities of the strong gamma-rays following the β decay of 152Eu were determined in a specially designed experiment. The emission probabilities are deduced from the decay scheme, whose branching-ratios and beta feeding fractions were fitted to the observed peak areas.

We measured the cross sections for Au Lα, Lβ, Lγ, Ll and Lη x-ray production by the impact of electrons with energies from the L3 threshold to 100 keV using a thin Au film whose mass thickness was determined by Rutherford Backscattering Spectrometry. The x-ray spectra were acquired with a Si drift detector, which allowed to separate the components of the Lγ multiplet lines. The measured Lα, Lβ, , L, , , and Lη x-ray production cross sections were then employed to derive Au L1, L2 and L3 subshell ionization cross sections with relative uncertainties of 8%, 7% and 7%, respectively; these figures include the uncertainties in the atomic relaxation parameters. The correction for the increase in electron path length inside the Au film was estimated by means of Monte Carlo simulations. The experimental ionization cross sections are about 10% above the state-of-the-art distorted-wave calculations.

We have used the low-energy beam line of the Sao Paulo Microtron accelerator to study the maximum energy transfer point (tip) of electron–atom bremsstrahlung spectra for C, Al, Te, Ta and Au. Absolute cross sections differential in energy and angle of the emitted photon were measured for various electron kinetic energies between 20 and 100 keV, and photon emission angles of 35◦, 90◦ and 131◦. The bremsstrahlung spectra were collected with three HPGe detectors and their response functions were evaluated analytically. Rutherford backscattering spectrometry allowed us to obtain the thicknesses of the targets with good accuracy. We propose a simple model for the tip region of the bremsstrahlung spectrum emitted at a given angle, whose adjustable parameters are the mean energy of the incident beam and its spread as well as an amplitude. The model was fitted simultaneously to the pulse-height distributions recorded at the three angles, determining the doubly differential cross sections from the corresponding amplitudes. The measured values have uncertainties between 3% and 13%. The agreement of the experimental results with the theoretical partial-wave calculations of Pratt and co-workers depends on the analyzed element and angle but is generally satisfactory. In the case of Al and Au, the uncertainty attributed to the theory is probably overestimated.

The covariance methodology applied to uncertainties in 126I disintegration rate measurements is described. Two different coincidence systems were used due to the complex decay scheme of this radionuclide. The parameters involved in the determination of the disintegration rate in each experimental system present correlated components. In this case, the conventional statistical methods to determine the uncertainties (law of propagation) result in wrong values for the final uncertainty. Therefore, use of the methodology of the covariance matrix is necessary. The data from both systems were combined taking into account all possible correlations between the partial uncertainties.

The procedure followed by the Laboratorio de Metrologia Nuclear (LMN) at the IPEN, in Sao Paulo, for the standardization of the 45Ca is described. The activity measurement was carried out in a 4pb-g coincidence system, by the tracing method. The radionuclide chosen as the b-g emitting tracer nuclide was 60Co because of its end-point beta-ray energy which is close to 45Ca. Six sources were prepared using a 1:1 ratio (b-pure and b-g) dropped directly on the Collodion film, and other two solutions of 45Ca + 60Co were mixed previously using a 1:1 and 1:2 ratio before making the radioactive sources. The activity of the solution was determined by the extrapolation technique. The events were registered using a Time to Amplitude Converter (TAC) associated with a Multi-channel Analyzer.

The methodology for simulating all detection processes in a 4…fl i ∞ coincidence system by means of the Monte Carlo technique is described. The goal is to predict the behavior of the observed activity as a function of the 4…fl detector efficiency. In this approach, the information contained in the decay scheme is used for determining the contribution of all radiations emitted by the selected radionuclide, to the measured spectra by each detector. This simulation yields the shape of the coincidence spectrum, allowing the choice of suitable gamma-ray windows for which the activity can be obtained with maximum accuracy. The simulation can predict a detailed description of the extrapolation curve, mainly in the region where the 4…fl detector efficiency approaches 100%, which is experimentally unreachable due to self absorption of low energy electrons in the radioactive source substrate. The theoretical work is being developed with MCNP Monte Carlo code, applied to a gas-flow proportional counter of 4… geometry, coupled to a pair of NaI(Tl) crystals. The calculated efficiencies are compared to experimental results. The extrapolation curve can be obtained by means of another Monte Carlo algorithm, being developed in the present work, to take into account fundamental characteristics of a complex decay scheme, including different types of radiation and transitions. The present paper shows preliminary calculated values obtained by the simulation and compared to predicted analytical values for a simple decay scheme.

The correction for accidental summing (pile-up) in the activity determination of 125I by the Sum-Peak method is described. The results are compared with the X-(X, γ) coincidence method. Theoretical estimates of this correction have been performed by the Monte-Carlo method and predictions were made for other detector efficiencies and electronic shaping times.

The present work aims the standardization of 68Ga, a positron emitter of short half-life used in PET (Positron Emission Tomography). The 68Ga standardization was performed in a triple 4πβ-γ coincidence system that consists of a thin window gas-flow proportional counter (PC) in 4π geometry coupled to a NaI(TI) scintillator and a HPGe crystal, for gamma-ray detection. The data acquisition was carried out by means of a Software Coincidence System (SCS) developed at the Nuclear Metrology Laboratory (Laboratorio de Metrologia Nuclear - LMN) at the IPEN-CNEN/SP. The final results were obtained from a multiple curve fitting applying a covariance matrix methodology combining experimental results with those determined by the Monte Carlo simulation.

In this paper the application of Monte Carlo simulation for standardizing 111In in a 4π(PC)-NaI(Tl) coincidence system is presented. The Monte Carlo simulation is applied using ESQUEMA code, which was developed by the Nuclear Metrology Laboratory (LMN) in IPEN, Sao Paulo, Brazil. The experimental results obtained by means a linear fitting were compared with those determined by the Monte Carlo prediction and they are in good agreement within the uncertainty.

The procedure followed by the Nuclear Metrology Laboratory (Laboratorio de Metrologia Nuclear — LMN) at the IPEN-CNEN/SP, in Sao Paulo, for the standardization in a 4πβ-γ coincidence system and the determination of gamma ray emission probabilities per decay of 166mHo are described. The standardization by means of a 4πβ-γ system was performed selecting two γ-ray windows: one set at the 184 keV total absorption peak and another one covering 700–830 keV energy range, both following β-decay processes. The 166mHo sources were also measured in a ReGe spectrometer previously calibrated with standard sources of 241Am, 133Ba, 57Co, 60Co, 152Eu and 54Mn standardized in the 4πβ-γ coincidence system. The experimental results were compared with data from literature.

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.

The present work describes the methodology for predicting the behavior of extrapolation curves obtained in radionuclide standardization by 4pibeta-gamma coincidence measurements, applied to (22)Na, developed at the Laboratorio de Metrologia Nuclear of IPEN-CNEN/SP (LMN-Nuclear Metrology Laboratory). The LMN system consists of a proportional counter (PC) in 4pi geometry coupled to a single or a pair of NaI(Tl) scintillation crystals. Two standardization techniques were used: the Sum-Peak and the Nuclear-Peak methods. The theoretical response functions of each detector have been calculated using the MCNPX Monte Carlo code. The code ESQUEMA, developed at LMN, has been used for calculating the extrapolation curve in the 4pibeta-gamma coincidence experiment. Modifications were performed in order to include response tables for positrons and coincidences with annihilation photons. From the calibration results it was possible to extract both the activity value and the positron emission probability per decay. The latter was compared with results from the literature.

This work describes the method developed by the Nuclear Metrology Laboratory in IPEN, São Paulo, Brazil, for the standardization of a (57)Co radioactive solution. Cobalt-57 is a radionuclide used for calibrating gamma-ray and X-ray spectrometers, as well as a gamma reference source for dose calibrators used in nuclear medicine services. Two 4pibeta-gamma coincidence systems were used to perform the standardization, the first used a 4pi(PC) counter coupled to a pair of 76 mm x 76 mm NaI(Tl) scintillators for detecting gamma-rays, the other one used a HPGe spectrometer for gamma detection. The measurements were performed by selecting a gamma-ray window comprising the (122 keV+136 keV) total absorption energy peaks in the NaI(Tl) and selecting the total absorption peak of 122 keV in the germanium detector. The electronic system used the TAC method developed at LMN for registering the observed events. The methodology recently developed by the LMN for simulating all detection processes in a 4pibeta-gamma coincidence system, by means of the Monte Carlo technique, was applied and the behavior of extrapolation curve compared to experimental data. The final activity obtained by the Monte Carlo calculation agrees with the experimental results within the experimental uncertainty.

In this paper results of X-ray and gamma-ray emission probabilities per decay of (51)Cr and (241)Am are presented. The measurements were carried out by means of HPGe planar and REGe spectrometers. The activity of (51)Cr and (241)Am samples was determined in a 4pibeta-gamma coincidence counting system. The HPGe spectrometers were calibrated in a well defined geometry by means of (54)Mn, (55)Fe, (57)Co, (133)Ba, (152)Eu, (166m)Ho and (241)Am sources, previously standardized in a 4pibeta-gamma coincidence system. The MCNP Monte Carlo code was used for simulation of the REGe spectrometer calibration curve, for the selected geometry, and compared with the experimental curve. The experimental results were compared with data from literature.

Specific activity results of 51 Cr, 67 Ga, 99m Tc and 201 Tl radioactive solutions, measured in different dose calibrators located at nuclear medicine services and in a calibrated HPGe gamma spectrometer, were compared. The HPGe spectrometer was calibrated in a well defined geometry by means of 60 Co, 133 Ba, 152 Eu, 166m Ho and 241 Am sources, previously standardized in a 4πβ−γ coincidence system. Despite the observed differences, the results may be accepted within the 10% uncertainty range, established by Brazilian regulatory standards.

In this paper the procedure followed by the Nuclear Metrology Laboratory (Laboratorio de Metrologia Nuclear - LMN) at the IPEN, in Sao Paulo, for the standardization of the 186Re and determination of the gamma emission probabilities per decay of the main gamma-ray energies of 186Re, 137.15 keV from β− decay and 122.6 keV from EC are presented. The activity measurement was carried out in a 4π (PC) β-γ coincidence system. The contribution from electron capture events in the proportional counter has been estimated by means of additional 4π(PC)β-γ HPGe coincidence measurements. The activity of the solution was determined by the extrapolation technique. The events were registered using a Time to Amplitude Converter (TAC) associated with a Multichannel Analyzer. The gamma emission probabilities per decay were determined by means of an HPGe spectrometer calibrated in the 81 keV to 1408 keV energy range, by measuring sealed ampoules of 60Co, 133Ba, 137Cs and 152Eu, standardized at the LMN.

Measurement of the activity of (182)Ta sources produced by irradiation at the IPEN research reactor was performed in a 4pibeta-gamma coincidence system by using the extrapolation technique. The measurements were undertaken selecting two windows in the gamma-channel, in order to check the consistency of the results. A Monte Carlo calculation was performed in order to predict the behavior of the observed activity as a function of the 4pibeta detector efficiency and the results were compared with experimental values.

A technique for simulating all detection processes in a 4 π ( β ,e,X)– γ coincidence system by means of the Monte Carlo technique is described. This procedure yields a more realistic behaviour of the extrapolation curve as compared to the usual polynomial fit. The present paper describes its application to the standardisation of a typical pure beta emitter, namely 35 S, by the efficiency tracing technique, and an EC-gamma radionuclide, namely 133 Ba. The calculated extrapolations were compared to experimental values obtained at the IPEN.

The activity of 72 Ga sources produced by irradiation at the IEA-R1 reactor have been performed in a 4 π β – γ coincidence system by using the extrapolation technique. The measurements were undertaken selecting two windows in the γ -channel, in order to check the consistency of the results. A Monte Carlo calculation was performed in order to predict the behavior of the observed activity as a function of the 4 πβ detector efficiency and the results were compared to experimental values.

The methodology recently developed by the Nuclear Metrology Laboratory (LMN) from IPEN, in Sao Paulo, Brazil, for simulating all detection processes in a coincidence system by means of the Monte Carlo technique is described. This procedure makes possible to predict the behavior of the observed activity as a function of the detector efficiency. The present paper describes its application to the standardization of a typical beta-gamma radionuclide, namely Cs. In this approach, infor- mation contained in the decay scheme is used for determining the contribution of all radiations emitted by the selected radionuclide to the measured spectra of each detector. This simulation yields the shape of the coincidence spectrum, allowing the choice of suit- able gamma-ray energies for which the activity can be obtained with maximum accuracy. The theoretical work applies the MCNP Monte Carlo code to a gas-flow proportional counter with de- tection geometry, coupled to a pair of NaI(Tl) crystals. The calcu- lated extrapolation curve showed good agreement when compared to experimental values obtained at the LMN of IPEN.

The 232Th(n,γ)233Th thermal neutron‐capture reaction cross section was measured using targets of ∼ 1.5 mg of high‐purity metallic thorium irradiated in the IPEN IEA‐R1m 5 MW pool research reactor. The 197Au(n,γ)198Au reaction was used to monitor the thermal and epithermal neutron fluxes in the irradiation position, which was found using the Westcott formalism. The residual gamma‐ray activity was followed with an HPGe detector. The detector efficiency curve was fitted by the least‐squares method applying covariance analysis to all uncertainties involved. The experimental result is σ0 =7.20±0.20 b, in agreement with previous published values.

The {sup 57}Co(n,{gamma}){sup 58}Co thermal and resonance integral cross section were measured as 51(5) b and 20.0(19) b, respectively, by irradiating aliquots of {sup 57}Co solution sealed inside quartz bottles near the core of the IEA-R1 IPEN research reactor and counting the gamma-ray residual activity. The irradiations were monitored using Au-Al alloy foils, with and without Cd cover. The gamma-ray measurements were performed with a shielded HPGe detector. Westcott formalism was applied for the average neutron flux determination. The cross section energy dependence was evaluated using the multilevel Breit-Wigner expression considering the first two resonances and the statistical model for energies above the second resonance. Maxwellian averaged neutron capture cross section with neutron temperatures between 5 and 100 keV were also evaluated.

A nonlinear function in combination with the method of mixing activity-calibrated and uncalibrated gamma-ray sources is applied for fitting the experimental peak efficiency of HPGe spectrometers in the 59-2754 keV energy range. In addition, a step function was developed for fitting the gamma-ray background under the peak. Calibrated 241Am, 133Ba, 137Cs, 152Eu, 60Co and 88Y sources, as well as one 24Na source (treated as uncalibrated) were used for obtaining the experimental peak efficiency curve. The results were compared to the conventional linear polynomial fitting. From the fitted parameters, the 24Na activity was determined and compared with the result obtained from absolute measurements in a 4pibeta-gamma coincidence system.

Cascade summing corrections for application in HPGe gamma ray spectrometry have been calculated numerically by the Monte Carlo method. An algorithm has been developed which follows the path in the decay scheme from the starting state at the precursor radionuclide decay level, down to the ground state of the daughter radionuclide. With this procedure, it was possible to calculate the cascade summing correction for all gamma ray transitions present in the decay scheme. Since the cascade correction requires the values of peak and total detection efficiencies, another code has been developed in order to estimate these parameters for point and cylindrical sources. The radionuclides 60Co, 133Ba and 131I were used for testing the procedure. The results were in good agreement with values in the literature.

In the present work, the gamma-ray emission probabilities per decay of 42K transitions have been measured. The activity measurement was carried out in a 4pi (PC)beta-gamma coincidence system. The gamma-ray measurements were performed in a HPGe system. All the uncertainties involved were treated rigorously, by means of covariance analysis. The value of the 1524 keV gamma-ray intensity per decay of 42K obtained in the present work was (0.1813+/-0.0014). This result is compared to the data from other authors.

A system for the standardization of alpha-gamma or electron-X radionuclide emitters is described. The system consists of one or two surface barrier detectors for alpha or electron detection which are coupled to thin-window NaI(Tl) crystals suitable for low-energy X- or gamma-ray detection. The performance of the system has been verified by the standardization of 241 Am, 137 Cs and 109 Cd solutions. The activity has been obtained using the extrapolation method applied to the 4π α-γ and 2π e-X coincidence techniques. The surface barrier detection efficiency was varied by placing absorbers over the radioactive source or by changing the source-to-detector distance. The results were compared to those obtained using conventional absolute systems based on gas-flow and pressurized 4π proportional counters, or using radioactive solutions standardized in international comparisons sponsored by the Bureau International des Poids et Mesures, France. The expected and measured activities agreed within the experimental uncertainties, which were: 0.2% for 241 Am, 0.7% for 137 Cs and 0.6% for 109 Cd.