Μελέτη του συστήματος p+11B σε ενέργειες πρωτονίων και γωνίες ανίχνευσης κατάλληλες για EBS και NRA

Abstract

Μέτρηση ενεργών διατομών των αντιδράσεων 11Β(p,po)11B και 11Β(p,α0)8Βe για σκοπούς IBA.

The present work aims at studying the p + 11B system and at acquiring data for the differential cross sections of the 11B(p,po)11B and 11B(p,αo)8Be reactions over a wide range of energies and detector angles suitable for material analysis. Boron is a very important technological element and is widely used in various areas. For example, it is used in the semiconductor industry as dopant for silicon and germanium substrates and, moreover, it is an essential ingredient in the hard coatings on the walls of thermonuclear plants. Thus, the quantitative determination of boron depth profiling in heavy and light substrates is of great scientific and technological importance. Charged particles and techniques based on the interactions of such particles with matter have been traditionally used for material analysis. Nuclear Reaction Analysis (NRA) and Elastic Backscattering Spectroscopy (EBS) are two of the most widely used methods in Ion Beam Analysis (IBA) due to the high cross sections involved, the simultaneous study of various elements in the same sample and their high isotopic selectivity. However, the use of these techniques is impeded by the fact that differential cross section datasets of the reactions involved are generally sparse in literature and at the same time, a theoretical evaluation is still pending. In the past, several reactions have been implemented, like e.g. the 11Β(d,p) and 11Β(d,α) reactions using deuterons, for several beam energies and detector angles. However, it is the author’s belief that implementing a proton beam for material analysis could be – in principle – beneficial, since greater depths can be probed while at the same time the radiation safety issues are less demanding. Unfortunately, until now, there are only a few available data in literature concerning the 11B(p,po)11B and 11B(p,αo)8Be reactions and, furthermore, there are big discrepancies between different datasets, rendering their implementation in material analysis inefficient. For this reason, these two reactions have been studied in the present work. The experiments were conducted at N.C.S.R. ―Demokritos‖, Athens, Greece, using the 5.5 MV TN11 Tandem accelerator. The protons were accelerated to Ep,lab=2200-4200 keV and were led to a large cylindrical scattering chamber which was kept in vacuum. Four Si surface barrier detectors (1000 κm thickness), set at a distance of 9-10 cm from the target, were implemented to collect the data. The target, manufactured in Garching, Munich, consisted of an isotopically enriched (~99.9%) 11B layer, deposited on a thick tantalum backing. The thickness of the boron layer was experimentally verified with a deuteron beam, as well as, with low energy p-RBS measurements on both sides of the target. The charge on the target and the solid angle were experimentally determined together, as a product (Q*Ω), by the RBS (Rutherford Backscattering Spectrometry) spectra, in order to avoid excessive uncertainties. Finally, two different algorithms were used for peak fitting/integration and background subtraction. The differential cross section values for each angle and energy were obtained using the formula for absolute measurements. The combined statistical uncertainty varied between ~4-5% for the 11B(p,pν)11B reaction and between ~5-7% for the 11B(p,αo)8Be one. The above mentioned values do not include the error in the determination of the target thickness (~3.5%). 4 From these experiments, differential cross section values for both reactions in the already mentioned energy range, and for detector angles between 135o-160o in steps of 5o have been determined (differential cross section values for detector angles 165o and 170o have also been determined but due to uncertainties these results were not published) and are presented in the next pages along with more details about the experiments and along with comparisons with already existing data. No ―plateau‖ has been observed in the differential cross section values over the whole energy range studied, and this sets limits to the analyzing power of both reactions. However, there is a slow variation with angle and a rather smooth variation with energy (absence of strong, narrow resonances) which may facilitate analytical measurements. Both reactions could be used simultaneously for greater accuracy. An attempt to explain the results in the framework of the resonance mechanism is also presented. The results from this work have been accepted for publication in NIM B (for detector angles 135o-160o), under the title ―Differential cross sections for the 11B(p,αo)8Be and 11B(p,pν)11B reactions, suitable for ion beam analysis‖, and are available to the scientific community through IBANDL (Ion Beam Analysis Nuclear Data Library, http://www-nds.iaea.org/ibandl/).