Calibration and Thick and Thin Al (p,γ) and (p,p'γ) Cross Sections

In relation to cross section measurements, the procedure to normalize cross sections by RBS procacitons scattered by a heavy component of the target was adopted; hence proton spectra were collected at 160o and 140o, simultaneously with the acquisition of gamma-ray spectra. Thin films (<50 μg/cm2) were evaporated on thin (∼50 μg/cm2) Ag films. Alpha RBS was done to get the atomic ratio between the desired element and silver. As an example the results for the 23Na(p,p'γ) 23Na; 2 - 4 MeV, γ - 440 keV measurement are shown. Present results (blue) are compared with previous results of the group (red) and of other authors (green). The 25% discrepancy between red and green values persists along the entire energy range between blue and green values. It was suggested that this discrepancy is due to the instability of NaBr, the compound used by the other authors, as Br evaporates readily from the film even at low beam intensities (50 nA). It was emphasized that the large amount of spectra and lines to be analysed led to the need to develop automatic spectra analysis (using root - our choice) which caused some delay in data analysis. Calibration and Thick Al (p,γ) Exercise: Being the Tandem accelerator at the ITN/CFNUL Lab a General Ionex Cockroft-Walton tandetron (3 MV), with a very good high voltage stability (Regulation: +/- 0.1%, High frequency Ripple: 200 V peak to peak (p-p) at 2.5 MV, Low frequency Ripple: 1.0 kV p-p at 2.5 MV, Stability: <1.5 kV at 3.0 MV), it was brought to discussion the approach of calibrating directly the high voltage versus calibrating a deflecting magnet by magnetic resonance. This is especially pertinent due to the fact that two deflecting magnets lead the beam to the reaction chamber and an interplay between them is possible, so that at a given energy more than one value of magnetic field (for each magnet) is possible. The approach of calibrating directly the high voltage was taken. Thin target nuclear reaction resonances, presented in the table below, were employed to the purpose. The first resonance was used to get the thickness of the target, in order to obtain the experimental resonance energy. A good straight line is obtained. Using the obtained calibration equation, the resonance of Al (p,γ) at 992 keV, for a thick Al target, is deviated by 2 keV. This deviation gets smaller (∼1 keV) if this resonance is included in the calibration procedure. In relation to energy calibration we may add: from proton spectra pertaining to the 23Na(p,p'γ) 23Na; 2 - 4 MeV, γ - 440 keV measurement, Ag peak areas were extracted. Normalized to the collected charge, these areas follow a 1/E2 behaviour (not shown). For the thick Al (p,γ) exercise the following conditions were used: Target - Thick polished Al foil; Currents - 100-150 nA; Collected Charge -100 μC. Using the same methodology referred before, gamma-ray and scattered proton spectra were collected simultaneously. The conditions were: Target - Thin Al on thin Ag; Currents - 100 nA; Collected Charge - 40 μC per point. Alpha RBS was not done yet to normalize the results. In arbitrary units, the excitation function is presented. Finally, the ERYA code and it's capability to analyze mass concentrations of an arbitrary number of elements in a homogeneous sample was demonstrated