Proton induced gamma-ray production cross sections and thick-target yields for boron, nitrogen and silicon

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Abstract

The excitation functions for the reactions 14N(p,p′γ)14N, 28Si(p,p′γ)28Si and 29Si(p,p′γ)29Si were measured at an angle of 55° by bombarding a thin Si3N4 target with protons in the energy range of 3.6–6.9 MeV. The deduced γ-ray production cross section data is compared with available literature data relevant for ion beam analytical work. Thick-target γ-ray yields for boron, nitrogen and silicon were measured at 4.0, 4.5, 5.0, 5.5, 6.0 and 6.5 MeV proton energies utilizing thick BN and Si3N4 targets. The measured yield values are put together with available yield data found in the literature. The experimental yield data has been used to cross-check the γ-ray production cross section values by comparing them with calculated thick-target yields deduced from the present and literature experimental excitation curves. All values were found to be in reasonable agreement taking into account the experimental uncertainties.

Introduction

Particle Induced Gamma-ray Emission (PIGE) is an analytical technique based on the use of MeV charged particle beams. It can be utilized to determine the composition of the surface region of solids for the light elements by measurement of characteristic prompt γ-rays.

Previous thick-target gamma-ray yield measurements on natural nitrogen (see e.g. [1]) have shown that there are practically only two γ-rays which can be utilized for analytical purposes. These are the 2313 keV γ-rays from the 14N(p,pγ1–0)14N reaction and the 4439 keV γ-rays from the 15N(p,αγ1–0)12C reaction. The available γ-ray production cross section and thick-target yield data for these reactions are scarce. In this work we concentrate on the utilization of the former reaction and restrict to proton energies ⩽7 MeV. Phillips et al. [2] have determined normalized cross sections within the energy range of 3.75–6.36 MeV. Dyer et al. [3] provide data for proton energies from 3. 7 MeV up to 23 MeV and H. Benhabiles-Mezhoud et al. provide cross section data for energies 6.55–26.2 MeV [4]. The available thick-target gamma-ray yields for this reaction are even scarcer. Only the individual data points at proton energies of 3.8 MeV [1], 7 MeV [5] and 3.537–4.13 MeV [6] can be found.

Silicon can be analysed by PIGE employing either the 28Si(p,p′γ)28Si reaction or the 29Si(p,p′γ)29Si reaction by detecting the γ-rays of 1779 keV and 1273 keV, respectively. Although a considerable body of data already exists in the nuclear physics literature, in relation to cross sections for nuclear reactions with gamma-rays in the exit channel, this has not been compiled aiming at ion beam based analytical use. The study of Ref. [3] provides γ-ray production cross section data for the reaction 28Si(p,p′γ)28Si for proton energies from 3.7 MeV up to 23 MeV, in the study of Ref. [7] data for energies 2.2–3.8 MeV is provided and in Ref. [8] for energies 2.0–3.2 MeV. For the reaction 29Si(p,p′γ)29Si no ion beam analysis relevant cross section data can be found in the literature for the presently considered energy range. Even more, the only available previous data is those of Jokar et al. for energies 2.0–3.2 MeV [8]. For silicon the following individual thick-target gamma-ray yield data points are available in the literature; at 2.4 MeV [9], 3.1 and 3.8 MeV [1], 7 MeV [5], 3.05–4.09 MeV [10] and 2.65–4.13 MeV [6]. A more systematic approach has recently been taken by Jokar et al. showing yield values from 2.4 MeV up to 3 MeV with 10 keV energy steps [8].

Boron isotopes can be detected by the reactions 10B(p,αγ), Eγ = 429 keV; 10B(p,p′γ), Eγ = 718 keV and 11B(p,p′γ), Eγ = 2125 keV. The corresponding available thick-target gamma-ray yields are for energies 3.1–4.2 MeV [1]; 2.4 MeV [9]; 7 MeV [5]; 2.49–4.13 MeV [6] and 1.77–4.08 MeV [10].

Here we provide γ-ray production cross section data for the reactions 14N(p,p′γ)14N, 28Si(p,p′γ)28Si and 29Si(p,p′γ)29Si as well as thick-target gamma-ray yields for boron, nitrogen and silicon in the energy range of 3.6–6.9 MeV. It should be noted that the yield values provided in this work correspond to samples with natural isotopic abundance, i.e. they are not corrected to correspond to pure isotopic composition. The present study is part of an extensive effort, coordinated by the International Atomic Energy Agency (IAEA), to generate a reference database for PIGE spectroscopy.

Section snippets

Accelerator

The proton beam was generated by the 5-MV belt-driven tandem accelerator of the University of Helsinki. All the run parameters of the accelerator are controlled by industrial PLC’s via an operator interface coded in NI Labview™. The energy of the proton beam at target is defined by a 90° analyzing magnet with a bending radius of 1500 mm. The flux density of the magnetic field is measured using a Group3™ teslameter with the Hall probe located between the magnet poles near the midpoint of the

Cross sections

The excitation functions for the reactions 14N(p,p′γ)14N, 28Si(p,p′γ)28Si and 29Si(p,p′γ)29Si were measured at an angle of 55° by bombarding the thin Si3N4 target with protons in the energy range of 3.586–6.920 MeV with 100 keV energy steps except for the energy regions close to reaction 14N(p,p′γ)14N resonances where the energy steps were more narrow. The collected charge for each measurement point was ∼12 μC with beam currents varying from 5 nA to 10 nA. A tantalum collimator (with a circular

Cross sections for the reaction 14N(p,p′γ)14N

The resonance decay through the 14N 2313-keV, Jπ = 0+, T = 1 first-excited state (mean lifetime τm = 98.7 ± 4.5 fs) was followed. The γ-ray decay to the ground state has an isotropic angular distribution. The γ-ray energy is sufficiently high to ensure rather low background contribution in the measured spectra. It should be also noted that the respective γ-ray peak is clearly Doppler broadened providing unambiguous interpretation.

The obtained cross sections are compared with the available previous cross

Concluding remarks

In conclusion, the new γ-ray production cross section data for the reaction 14N(p,p′γ)14N has been cross-checked by measurements of nitrogen thick-target gamma-ray yields and comparing them with calculated thick-target yields deduced from the present and literature experimental excitation curves. All values are in reasonable agreement taking into account the experimental uncertainties. From the thin target measurement data also the cross sections for the reactions 28Si(p,p′γ)28Si and 29Si(p,p′γ)

Acknowledgements

International Atomic Energy Agency (IAEA) Nuclear Data Section is acknowledged for the support under the research agreement no. 16692 as part of the Coordinated Research Project “Reference Database for Particle Induced Gamma Ray Emission”. This work was supported also by the Finnish Centre of Excellence in Atomic Layer Deposition.

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