Nora L. Maidana

Characterization of an extrapolation chamber for low-energy X-rays: Experimental and Monte Carlo preliminary results

The extrapolation chamber is a parallel-plate ionization chamber that allows variation of its air-cavity volume. In this work, an experimental study and MCNP-4C Monte Carlo code simulations of an ionization chamber designed and constructed at the Calibration Laboratory at IPEN to be used as a secondary dosimetry standard for low-energy X-rays are reported. The results obtained were within the international recommendations, and the simulations showed that the components of the extrapolation chamber may influence its response up to 11.0%.

Ionization cross sections of the L subshells of Au by 50 to 100 keV electron impact

We have measured the Lα, Lβ, , , L, and Lη x-ray production cross sections of Au by 50–100 keV electron impact. From this experimental information we derived the L1, L2 and L3 subshell ionization cross sections with a novel analysis procedure that is based on an overdetermined system of equations and achieve the estimates by the least-squares method. The uncertainties in the atomic relaxation parameters needed to transform the x-ray intensities to ionization cross sections impose a lower limit to the relative standard deviations of the L subshell ionization cross sections, which is found to be 5–10% depending on the selected set of relaxation parameters. Our experimental results are in reasonable accord with most of the measurements carried out by other authors, and they agree with the predictions of the semi-relativistic distorted-wave Born approximation.

Measurement of the thermal neutron capture cross section and resonance integral of Am-241

Measurements of the thermal neutron cross section and resonance integral for the reaction 241Am(n,γ) 241gAm have been performed at the IEA-R1 research reactor operating at 2 MW. A 4πβ-γ coincidence system applying the tracer technique has been used for measuring the activities. Irradiations with and without cadmium cover were performed and the neutron transmission through this cover was calculated by the Monte Carlo method. High resolution γ-ray spectrometry measurements were undertaken in order to check the consistency of the results and to measure activities of the neutron fluence monitor foils. Covariance analysis has been applied to all uncertainties involved. The thermal neutron cross section for 2200 m/s neutrons and the resonance integral resulted (602±9)b and (1665±91)b, respectively. These results are discussed and compared with previous measurements.

Measurement of thermal neutron cross section and resonance integral for the 165Ho(n, γ)166gHo reaction

Abstract The ground state thermal neutron cross section and the resonance integral for the 165Ho(n, γ) 166Ho reaction in thermal and 1/E regions, respectively, of a thermal reactor neutron spectrum have been measured experimentally by activation technique. The reaction product, 166Ho in the ground state, is gaining considerable importance as a therapeutic radionuclide and precisely measured data of the reaction are of significance from the fundamental point of view as well as for application. In this work, the spectrographically pure holmium oxide (Ho2O3) powder samples were irradiated with and without cadmium covers at the IEA-R1 reactor (IPEN, São Paulo), Brazil. The deviation of the neutron spectrum shape from 1/E law was measured by co-irradiating Co, Zn, Zr and Au activation detectors with thermal and epithermal neutrons followed by regression and iterative procedures. The magnitudes of the discrepancies that can occur in measurements made with the ideal 1/E law considerations in the epithermal range were studied. The measured thermal neutron cross section at the Maxwellian averaged thermal energy of 0.0253 eV is 59.0±2.1 b and for the resonance integral 657±36 b. The results are measured with good precision and indicated a consistency trend to resolve the discrepant status of the literature data. The results are compared with the values in main libraries such as ENDF/B-VII, JEF-2.2 and JENDL-3.2, and with other measurements in the literature.

Cross sections of K-shell ionization by electron impact, measured from threshold to 100 keV, for Au and Bi

We have measured cross sections for the ionization of the K shell by electrons with energies from the respective thresholds up to 100 keV, for Au and Bi. The experimental values are obtained by dividing the number of counts in the K? peak by the number of counts in an energy interval near the tip of the bremsstrahlung continuum, and multiplying this ratio by the theoretical estimate of bremsstrahlung emission towards the detector in this energy interval. Although such a procedure has already been described in the literature, here it is implemented avoiding some of the simplifications made in earlier works. Our experimental cross sections, which are the first ones to be reported for atoms with 47

First Experiments with the IFUSP Microtron Injector

SRC=http://ej.iop.org/images/0953-4075/47/15/155201/jpb497279ieqn1.gif/> close to the threshold, are in reasonable agreement with the theoretical predictions of the semirelativistic distorted-wave Born approximation. Hippler?s plane-wave Born approximation with corrections for Coulomb and exchange effects yields cross sections that are closer to the experimental data than those evaluated from the relativistic binary-encounter-Bethe model.

Experimental HPGe coaxial detector response and efficiency compared to Monte Carlo simulations.

The first accelerating structures of the Sao Paulo microtron accelerator presently deliver a continuous‐wave 1.9 MeV electron beam, which can be used in atomic physics studies. An energy‐analyzed beam line with focalization magnets, irradiation chamber, movable target system and Faraday cup was built. Here, we will describe the preliminary results of two experiments. In the first one, upper limits on K‐shell ionization cross sections of Pd and Au atoms by electron impact were obtained. To this end, thin targets of these elements were irradiated with the beam impinging at 45° while the emitted Kα x‐rays were recorded with an HPGe x‐ray detector positioned at 90° with respect to the beam direction. The other experiment was the measurement of the bremsstrahlung energy spectra produced by the aforementioned targets in the same geometrical configuration but using an HPGe γ‐ray detector instead. The latter experiment enabled the determination of the electron‐beam energy as 1.909(5) MeV.

Coincidence Doppler Broadening of Positron Annihilation Radiation in Fe

The peak efficiency for photons hitting the frontal surface of a medium volume n-type HPGe coaxial detector is mapped using acutely collimated beams of energies between 31 and 383 keV from a (133)Ba radioactive source. Simulated values obtained with the Monte Carlo radiation transport code penelope, using a model that respected actual detector dimensions and physical constants while varying dead-layer thicknesses, allowed us to fit the experimental results in the detector bulk but not near its rim. The spectra of a (137)Cs source were measured using the detector shielded from the natural background radiation, with and without a broad angle collimator. The corresponding simulated spectra, using the fitted dead-layer thicknesses, underestimate the continuum component of the spectra and overestimate the peak efficiency, by less than ten percent in the broad angle collimator arrangement. The simulated results are sensitive to the photon attenuation coefficients.

Monte Carlo Simulation of Pileup Effects in the Electron‐Positron Annihilation Peak

We measured the Doppler broadening annihilation radiation spectrum in Fe, using 22NaCl as a positron source, and two Ge detectors in coincidence arrangement. The two-dimensional coincidence energy spectrum was fitted using a model function that included positron annihilation with the conduction band and 3d electrons, 3s and 3p electrons, and in-flight positron annihilation. Detectors response functions included backscattering and a combination of Compton and pulse pileup, ballistic deficit and shaping effects. The core electrons annihilation intensity was measured as 16.4(3) %, with almost all the remainder assigned to the less bound electrons. The obtained results are in agreement with published theoretical values.

Environmental Radioactivity Study in Surface Sediments of Guacanayabo Gulf (Cuba)

The Monte Carlo code PENELOPE is employed to simulate a typical experimental Doppler broadening coincidence spectrum (DBCS) where the energy spectrum of the photons emitted by the positrons interacting in the sample is recorded with two HPGe detectors in coincidence. The simulated spectrum reproduces well some of the structures observed in the measured DBCS, but not the prominent tails on the low‐ and high‐energy sides of the electron‐positron annihilation peak seen in the latter. Ad hoc variations of the cross sections implemented in PENELOPE did not improve the situation. A simple parameterization of the background noise in the DBCS is proposed, and the simulated spectrum is modified to account for pileup effects using this model of the background. The resulting spectrum is in good agreement with the experiment on the high‐energy side of the annihilation peak.