Excitation functions for the production of radionuclides by neutron-induced reactions on C, O, Mg, Al, Si, Fe, Co, Ni, Cu, Ag, Te, Pb, and U up to 180 MeV

doi:10.1016/j.nimb.2014.11.006

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms

Volume 343, 15 January 2015, Pages 30-43

Dedicated to the memory of our late colleague Petter Malmborg

https://doi.org/10.1016/j.nimb.2014.11.006 Get rights and content

Abstract

Irradiation experiments with well-characterized, quasi mono-energetic neutrons of energies between 32.7 MeV and 175.4 MeV were performed at UCL/Louvain-la-Neuve and TSL/Uppsala. The abundances of relatively short-lived residual radionuclides from 13 different target elements were determined by γ-spectrometry. More than 100 excitation functions of neutron-induced reactions were unfolded based on the neutron spectra and the radionuclide abundances with the aid of additional information that was provided by “guess” excitation functions calculated by the TALYS 1.0 code. The results are compared with the sparse existing data from other authors. The new excitation functions were validated by calculation of and comparison with experimental thick-target production rates. Consistency with neutron excitation functions up to 1.6 GeV, which were derived earlier by unfolding the thick-target production rates, was so demonstrated.

Introduction

The understanding of the production of radioactive and stable residual nuclides by medium-energy nuclear reactions (0.2–1 GeV/A) is of importance for many applications ranging from astrophysics over space and environmental sciences, medicine, space and aviation technology to accelerator-based technologies such as nuc1ear waste transmutation, energy amplification and spallation neutron sources.

In many of these applications one has to consider residual nuclide production in targets with dimensions which are comparable or larger than the interaction lengths of the incoming primary particles. Consequently, the production of secondary particles and the transport of primary and secondary particles in intra- and inter-nuclear cascades have to be taken into account. Whereas the transport of primary and secondary hadronic particles can well be modeled using basic physical principles, the reliable theoretical prediction of cross sections for the production of residual nuclides is still a problem [1]. Therefore, experimental cross sections and excitation functions provide an important basis for modeling of residual nuclide production at intermediate energies.

During the last decades, many experiments were performed in order to obtain production cross sections for charged-particle-induced reactions. Our earlier work in this field concentrated on cross sections relevant for the proton-induced production of residual nuclides in extraterrestrial matter and for accelerator technologies such as spallation neutron sources and associated applications [2], [3], [4], [5]. These experiments yielded substantial information about nuclide production by proton-induced reactions up to 2.6 GeV on several materials. Published experimental cross section data are included in the nuclear reaction data library EXFOR [6].

The charged-particle-induced cross sections are, however, not sufficient for a reliable modeling of the residual nuclide production induced by irradiation of thick (0.1 μ < d < 10 μ; μ = 1/Σ_a with the macroscopic absorption cross section Σ_a) or extended (10 μ ≪ d) targets with medium-energy charged particles, because in such targets secondary particles, mainly neutrons, dominate the production of residual nuclides. Energy spectra of secondary neutrons in such targets are dominated by neutrons with energies up to about 200 MeV [10].

Until recently, most of the experimental cross sections for neutron-induced reactions were confined to energies below 14.7 MeV (see the respective evaluated neutron data libraries [7], [8], [9]) since mono-energetic neutron beams of higher energies were not available. Since the reliability of theoretical predictions for such cross sections at higher energies was not sufficient for the modeling of cosmogenic nuclides in extraterrestrial matter, our group used a thick-target approach to simulate the production of cosmogenic nuclides in meteorites. Thick-target experiments were performed, in which artificial meteorites of differing sizes and compositions were irradiated by 600 MeV and 1.6 GeV protons [10], [11], [12]. The experimental production rates in these artificial meteoroids allowed to extract the contribution of secondary neutrons and to unfold from these contributions the underlying neutron excitation functions, starting from theoretical neutron excitation functions used as guess functions. These neutron excitation functions [13] proved to be capable – together with our experimental proton excitation functions – to provide a reliable basis for the modeling of cosmogenic nuclide production in meteoroids and on the lunar surface [14], [15], [16].

During the 1990s, neutron beam-lines with quasi mono-energetic neutrons with energies up to 200 MeV became available, e.g., at Uppsala [17] and Louvain la Neuve [18]. So it was tempting to measure cross sections for the production of residual nuclides at these beam lines. However, even with the highest flux densities obtained, the measurement of cross sections for neutron-induced reactions remained experimentally challenging due to the low activities of the produced residual nuclides. Only few data for neutron-induced reactions with energies above 15 MeV were reported by other authors [19], [20], [21], [22], [23], [24], [25]. Moreover, the fact that these neutrons were not purely monoenergetic made it necessary to perform series of experiments and to use unfolding techniques to obtain excitation functions.

We report here on a series of 21 activation experiments with quasi mono-energetic neutrons performed at Uppsala and Louvain la Neuve. Residual nuclides were measured by γ-spectrometry. Excitation functions for a large number of neutron-induced reactions were derived by unfolding techniques from the experimental data starting from theoretical guess excitation functions. The excitation functions derived from quasi-monoenergetic neutron irradiations were validated by calculation of some production depth profiles of residual nuclides in the aforementioned artificial meteorites. Reports about the progress of the work were occasionally given [26], [27], [28], [29], [30], [31], [32], [33] and several Ph.D. theses dealt with these experiments [34], [35], [36].

Section snippets

Methodology

Since purely mono-energetic neutron beams are not available for activation experiments at medium energies, it is not possible to derive the cross sections directly from the experimental production rates obtained in a single experiment. In case of quasi-monoenergetic neutron beams [37] with spectra exhibiting a pronounced high energy peak and a low-energy continuum an indirect method has to be used to determine neutron cross section, namely to unfold an excitation function $σ (E)$ , from a set of

Activation experiments

For the present work, a set of 21 activation experiments was carried out and 13 different targets, C, O, Mg, Al, Si, Fe, Co, Ni, Cu, Ag, Te, Pb and U were irradiated with quasi mono-energetic neutrons produced by the ⁷Li(p,n)⁷Be reaction. The experiments were performed at neutron beam lines at the cyclotrons of the Université Catholique de Louvain la Neuve (UCL) [18] and The Svedberg Laboratory (TSL) at Uppsala [17], [40]. The neutron energies, referring to those of the high-energy peaks, were

γ-Spectrometric measurements

Gamma-spectrometry of the irradiated target foils started between 12 and 14 h after the end of the irradiations and continued for several months. For this purpose, three high-purity germanium (HPGe) detectors and two lithium-drifted germanium (GeLi) detectors were used. The parallel use of the spectrometers helped to analyze the target stacks in a relatively short time, which was advantageous with regard to the different residual nuclides and their half-lives. Furthermore, the use of different

Neutron spectra and neutron transport inside the target stacks

In order to obtain the neutron spectra inside each target foil, not only the spectra of the primary neutrons have to be known but also modifications of the spectra by elastic and inelastic scattering inside the target stack. In addition, secondary particle production and transport have to be taken into account.

In initial experiments, the attenuation of the neutron beam inside the target stack was investigated by placing copper foils between the studied targets and by subsequent measurements of

Theoretical excitation functions

The guess excitation functions needed for the unfolding procedure were calculated by the TALYS 1.0 code [38] using the same 155 energy bins as for the neutron spectra. For each energy bin, the corresponding cross section of the analyzed reaction was calculated. These calculated cross sections relate to the independent production of the respective residual nuclide, whereas the measured residual nuclides represented the cumulative production after the decay of short-lived radioactive progenitors,

Uncertainties

The total uncertainties of the unfolded cross sections originate from several sources, i.e., the uncertainties of the measured activation yields, of the neutron flux monitoring and of the theoretical guess excitation functions, and the uncertainty of the unfolding procedure. They were considered in the covariance matrices $K (\vec{P})$ and $K (J)$ and propagated in the unfolding calculations to obtain the uncertainties of the unfolded cross sections.

Due to the low activities of the produced radionuclides,

Results and discussion

The experiments described here allowed to measure activation yields of more than 100 residual radionuclides from 13 different target elements (C, O, Mg, Al, Si, Fe, Co, Ni, Cu, Ag, Te, Pb, and U) and to unfold the respective excitation functions up to 180 MeV. Not every target-product combination was covered in each experiment. Therefore, the number of experimental production rates available for the unfolding of each excitation functions varied. The numerical data of the resulting unfolded

Conclusions

The results of the present work demonstrate that the experimental determination of excitation functions is in principle feasible employing high-current neutron sources providing quasi-mono-energetic neutrons. An unfolding of the underlying neutron excitation functions was performed from the measured production rates from activation experiments for a wide variety of residual radionuclides for the target elements carbon, oxygen, magnesium, aluminum, silicon, iron, cobalt, nickel, copper, silver,

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¹: Until 31.3.2010 Centre for Radiological Protection and Radioecology.

²: Passed away in spring 2013.

³: Mailing address: Wilh.-Henze-Weg 14, D-31303 Burgdorf, Germany.

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Excitation functions for the production of radionuclides by neutron-induced reactions on C, O, Mg, Al, Si, Fe, Co, Ni, Cu, Ag, Te, Pb, and U up to 180 MeV

Abstract

Introduction

Section snippets

Methodology

Activation experiments

γ-Spectrometric measurements

Neutron spectra and neutron transport inside the target stacks

Theoretical excitation functions

Uncertainties

Results and discussion

Conclusions

Nucl. Instr. Meth. Phys. Res. B

Nucl. Instr. Meth. Phys. Res. A

Nucl. Data Sheets

Nucl. Instr. Meth. Phys. Res.

Nucl. Instr. Meth. Phys. Res.

Nucl. Instr. Meth. Phys. Res. B

Nucl. Instr. Meth. Phys. Res.

Nucl. Instr. Meth. Phys. Res. A

Nucl. Instr. Meth. Phys. Res. A

Nucl. Instr. Meth. Phys. Res. B

Nucl. Instr. Meth. Phys. Res. B

Nucl. Instr. Meth.

Nucl. Instr. Meth. Phys. Res. A

Rad. Meas.

Nucl. Instr. Meth.

Nucl. Instr. Meth. A

J. Korean Physical Soc.

J. Nucl. Sci. Technol.

Meteorit. Planet. Sci.

Meteorit. Planet. Sci.

J. Nucl. Sci. Technol.

Nucl. Sci. Eng.

Nucl. Sci. Eng.