Characterization and Validation of a-Si Magnetron-Sputtered Thin Films as Solid He Targets with High Stability for Nuclear Reactions.

Vanda Godinho,Asunción Fernández,Joaquin Gómez-Camacho,Francisco Javier Ferrer,Jaime Caballero-Hernández,Begoña Fernández

doi:10.1021/acsomega.6b00270

Abstract

In this work, we present our magnetron sputtering based methodology to produce amorphous silicon coatings with closed porosity, as a strategy to fabricate solid helium targets, in the form of supported or self-supported thin films, for nuclear reactions. We show how by changing the He working pressure it is possible to obtain highly porous homogeneous structures incorporating different He amounts. These porous coatings (a-Si:He) are very reproducible from run to run, and the high He amount incorporated makes them excellent candidates for solid He targets. The possibility of producing self-supported films is illustrated here, and its potential use in inverse kinematics experiments with radioactive beams is shown through the dispersion in forward geometry of a stable 6Li beam. Also the elastic scattering cross-sections for proton from helium were determined using an a-Si:He coating. The results agree well with the ones reported in the literature. These two examples validate our coatings as good candidates to be used as solid He targets in nuclear reactions. The stability of He inside the coatings, fundamental for its use as solid He targets, was investigated, both over time and after irradiation. The coatings proved to be very stable, and the amount of He inside the pores remains unaltered at least 2 years after deposition and after high irradiation fluence (5 × 1017 particles/cm2; with a dose rate of 5 × 1012 particles/(cm2 s)).

Highlights

Nanostructured thin films and surfaces obtained by low-energy helium plasma treatments or by He incorporation via magnetron sputtering have been paid attention as strategies to modify and optimize materials properties.[1−4] Irradiation of metals with light ions, and in particular He, has been a subject of study for many years due to its technological interest in nuclear reactor materials.[5−7] Helium has been reported to be insoluble in different metals and tends to accumulate forming small bubbles causing damage in nuclear reactors.[7]
As an alternative route to top-down methods for producing porous materials, we have recently proposed a bottom-up methodology to grow porous silicon-based coatings, with closed porosity by magnetron sputtering.[11−13] The approach is a very versatile methodology based on magnetron sputtering technology, and extensible to a wide variety of materials and substrates, which in the presence of He plasmas allows the deposition of porous coatings with a decreased refractive index.[12,14]
As mentioned in the Introduction section, one of the major requirements of thin films to be used as solid He targets is a high He concentration

Summary

■ INTRODUCTION

Nanostructured thin films and surfaces obtained by low-energy helium plasma treatments or by He incorporation via magnetron sputtering have been paid attention as strategies to modify and optimize materials properties.[1−4] Irradiation of metals with light ions, and in particular He, has been a subject of study for many years due to its technological interest in nuclear reactor materials.[5−7] Helium has been reported to be insoluble in different metals and tends to accumulate forming small bubbles causing damage in nuclear reactors.[7]. To explore the possibility of producing a selfsupported a-Si:He film to be used as a solid He target, some considerations must be taken into account regarding the required high He amount, but a reasonable thickness must be selected to have a compromise between low thickness and still to ensure easy handling and mechanical stability during measurements In this case, as described in the Experimental, a coating was deposited on a NaCl substrate under similar deposition conditions to those of sample a-Si:He_MP, presenting the highest deposition rate, with a thickness of 2300 nm. To demonstrate the potential use of the porous silicon coatings as solid helium targets in nuclear reactions, EBS spectrometry was employed to determine the elastic scattering cross-sections of protons from helium For this purpose, an Au overlayer of 1.1 × 1016 atoms/cm[2] was deposited by evaporation over a porous Si film (prepared at MP conditions) to prepare an Au/a-Si:He/Si target for the cross-section measurements. The stability of He content throughout the experiment was checked. p-EBS measurements, using 2.0 MeV, at both the beginning and the end of the experiments were performed

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES