Fabrication and characterization of thin Nd targets for the study of dynamics of heavy-ion induced reactions
Introduction
Several studies related to the experimental investigations on the different aspects of the heavy-ion induced nuclear reaction dynamics have been made possible over the last three decades due to the development of heavy-ion accelerator facilities at different parts of the world. In the heavy ion nuclear reaction studies at the low and medium beam energies, the energy spread of the incident beam is mainly determined by the successive energy loss of the beam as it passes through the target material. The more the target thickness, the more would be the energy degradation of the beam and this would further lead to the production of different reaction products at the various stages of beam degradation, thereby causing unwanted complicacies in the observed data. Thus, for a meaningful extraction of the experimental observables related to the underlying reaction dynamics, it is the utmost importance of carrying out the investigations using high quality thin targets. However, the preparation of thin isotopic targets with uniform thickness is indeed a challenging task and requires a lot of effort. Various kinds of deposition techniques[1], [2], [3], [4], [5], [6] are routinely being used to prepare the thin targets suitable for nuclear physics experiments. The thin targets of different materials are usually prepared either in the form of self-supporting types[7], [8] or with thin backings[9], [10]. Also, depending on the chemical properties (such as the oxidation, hygroscopic nature etc.), thin sandwiched targets of a few specific materials are prepared with thin layers of capping and backing materials. It is to be pointed out here that some of the rare earth elements are chemically very active. Hence, the preparation and storage of such types of targets are very much challenging[11]. Special care is needed for their fabrication and preservation. In fact, the fabrication of different types of sandwiched lanthanide targets have been reported recently in a number of occasions[12], [13] keeping in mind the specific chemical properties that the concerned target materials possess.
In the present contribution, fabrication and characterization of thin sandwiched Nd targets are reported. We had to prepare the Nd targets with an experimental motivation for the measurements of excitation functions and the follow up extraction of barrier distributions for the 28Si + Nd systems (see Ref.[14] for details) from the quasi-elastic scattering data. To the best of our knowledge, no such measurements were performed in the past using these systems. Also the experiments were planned to carry out with a spherical target (142Nd) and a deformed target (150Nd) to look into the possible effects of target deformation on the underlying barrier distributions of the systems. It is worthwhile mentioning that purity and thickness of the targets are of utmost importance for this type of measurements. It is essential to carry out the measurements using high purity thin (having thickness in the order of g/cm2) targets in order to minimize the unwanted contaminating events in the acquired data. Also, it is to be noted that the quasi-elastic excitation function measurements are to be carried out in small steps of incident beam energies. In this context, the staggering in energy of the incident beam, which determines subsequently the energy resolution of the beam, should be somewhat less than that of the required energy steps. Hence, for a narrow binning of the energy steps, it is very much essential to prepare the Nd targets as thin as possible so that the measurements can effectively be carried out within the desirable limits of binning of energies of the incident beams. It is to be pointed out here that Nd reacts slowly with oxygen and forms NdO powder at room temperature. Furthermore, due to the hygroscopic nature of Nd[15], it also quickly reacts with water and forms Nd(OH). Hence, in order to avoid the possible direct contact of Nd material with air and water during the floating process of preparation, a coating of very thin layer of carbon were deposited on both the sides of the Nd layer. Due to the low-Z value of the carbon, the energy degradation of the incident beam and the scattered beam like particles would be minimum. Hence the use of carbon material seems to be the best option for backing and capping purposes while preparing the thin Nd targets. It may be pointed out here that the use of carbon material has some other additional advantages as well such as the carbon is amenable to fabrication, mechanically strong, stable under beam bombardment due to having high sublimation point and low vapour pressure, and possess high chemical stability comparable to that of noble metals. In the present work, attempts have been made to prepare the targets (Nd) using the highly enriched materials in metallic form and the subsequent detail characterization of the targets have been put forward. The level of enrichment for the 142Nd and 150Nd target materials were 98.26% and 97.56%, respectively (see Table1). As these target materials are expensive, special care has been undertaken in optimizing the preparation procedure so that the extremely pure targets (free from oxidation as much as possible) of required thickness can be prepared using the minimum amount of the target materials.
Section snippets
Fabrication of targets
Thin sandwiched and highly enriched Nd-targets with carbon backing and capping were prepared using the diffusion pump based coating unit (HV Evaporator) available at the target laboratory of IUAC, New Delhi. The details about the experimental set up and the fabrication procedure of the targets have been presented in the following subsections.
Characterization of targets
Different characterization techniques have been adopted to characterize the purity and thickness of the prepared targets. The impurity and thickness check of the targets are of utmost importance prior to their use for the experiments using the heavy-ion accelerated beams. The use of targets having impurity and improper thickness would lead to poor quality experimental data. The analysis for extracting the features of the targets using different characterization techniques are described in the
Conclusions
Twenty four numbers of thin sandwiched Nd targets each of thicknesses /cm, with layer of carbon backing thickness, /cm and carbon capping thickness, /cm, have successfully been fabricated using the limited amount of enriched isotopic target materials. The electron gun bombardment method and thermal evaporation technique was utilized for fabrication of the targets under a high vacuum ambiance using a diffusion pump based HV evaporation
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful to the operating staff of the accelerator machine at IUAC, New Delhi for their efforts in delivering the excellent quality of beams during the experiment. The help and co-operation received from the various persons of the different laboratories of IUAC, New Delhi during the present course of work is also gratefully acknowledged. The authors are also thankful to IUAC, New Delhi for extending FESEM facility, procured under Geochronology project [MoES/P.O.(Seismic)8(09)-
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