Characterization of a Nb-Pb composite target

. A thin Nb target on a Pb backing was fabricated using the rolling technique. In the present work, the characterization of the sample has been carried out using various techniques to study the surface morphology, crystallography, elemental analysis, thickness and electrical conductivity useful for the intended use of the target prepared. The thin Nb target was analyzed using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and Rutherford back-scattering spectroscopy (RBS) techniques. The uniform and smooth surface of the sample was confirmed by SEM imaging. The presence of oxygen in the EDS spectra and peaks corresponding to crystal planes of PbO 2 in the XRD spectrum confirms the oxidation of Pb foil. The XRD spectrum shows clear peaks corresponding to (011), (002), (112) crystal planes of pure Nb. The thickness of the thin Nb foil measured using the RBS techniques was found to be 0.8 mg/cm 2 . The current(I)-voltage(V) curve (relationship between the current flowing through an electronic device and the applied voltage across its terminals) at room temperature for the Pb and Nb/Pb foils was plotted and showed linear relationship between applied voltage and current flowing through the target foil. The measured electrical conductivity indicates an enhancement of 35.9 % for Nb/Pb foil relative to the conductivity measured for the Pb foil.


Introduction
Targets of specific thicknesses and physical properties are crucial for nuclear experiments. Lifetime measurements require different types of targets depending on the lifetime of the states (nanoseconds to sub-picosecond regime) to be measured and the respective technique involved in the measurement. The recoil distance doppler shift (RDDS) method [1] ideally requires thin self-supporting targets (without backing) so that populated nuclei recoils in the vacuum while the doppler shift attenuation method (DSAM) [2] requires thin target with the relatively thick backing of high-Z material so that populating nuclei recoil in the backing medium. The nuclear physics experiment depends heavily on the quality of the target, especially for DSAM lifetime technique; there must not be any vacuum gap between the target and backing foil [3]. In the experiment, the target is bombarded with high energy heavy ion beams and enormous heat is produced on the surface of the target, which needs to be ejected out to the surroundings in time to prevent any possible damage of the foil during the experiment. In view of the complexity of such an experiment, highly pure target foils of appropriate thickness having high melting point as well as high electrical and thermal conductivity is of great importance.
In the reported work, a thin Nb target with a thick Pb backing was fabricated using the mechanical rolling technique [4] for lifetime measurements to extract lifetimes of excited nuclear states in 118 Xe with the DSAM technique. The main objective of this work is to determine the quality of the fabricated target from the viewpoint of its intended use. Purity, elemental analysis, crystallinity, surface morphology and thickness are monitored using different material characterization techniques while electrical conductivity was determined using the I-V curve.

Fabrication of the target
A Nb foil of ∼ 1 mg/cm 2 thickness was prepared using mechanical rolling. The Pb foil of thickness ∼ 13 mg/cm 2 was prepared using the one-way rolling method as Pb is extremely soft in nature and two directional rolling often damages the foil. Finally, both the foils, Nb and Pb, are kept over one another placed inside the stainless steel (SS) pack for further rolling to form the composite target. Detailed experimental procedure of fabrication has been reported previously [3].

Scanning Electron Microscopy (SEM)
The surface morphology of the target foil was imaged using the field emission scanning electron microscopy (FESEM, MIRA3, Tescan, USA) operated at 30 kV, present at University Science Instrumentation Centre (USIC), Department of Physics and Astrophysics, University of Delhi, Delhi. SEM provides the images by raster scanning the surface of the sample using focused beam of electrons. The images produced using electron microscopy are shown in Figure 1(a) and Figure 1(b). The SEM image (Figure 1(a)) suggests the uniform and smooth surface of the foil obtained in the present work while Figure 1(b) shows the magnified part of the foil where two different regions, grey (smooth Nb surface) and bright (granular Pb) were observed. This was also confirmed by the elemental composition study using the EDS technique.

Energy Dispersive X-ray Spectroscopy (EDS) technique for elemental composition
The elemental composition of the target foil was studied using an EDS attachment to the FESEM system. EDS spectra taken on the two different regions marked as area 1 and area 2 in Figure 1  The EDS spectra taken at area 1, Figure 2(a), shows a large peak of Nb(Lα), which along with the Pb(Mα) peak confirms the presence of pure Nb (no major elemental impurity), while the EDS spectra taken at area 2, Figure 2(b), shows a strong peak of Pb(Mα), which along with the peaks corresponds to niobium (Nb Lα), carbon (C Kα) and oxygen (O Kα). The observed O and C peaks may be due to oxidation or carbonization of organic contaminants on the foil surface. The results clearly indicate the uniform Nb surface over the region, because a mostly grey region was visible in the pannedout SEM images. Based on the observed EDS spectra, we can state that the fabricated target is pure (no major elemental impurity was added during the fabrication process).

X-ray Diffraction
In XRD, the incident X-ray beam interacts with the atomic layers of the sample to generate characteristic peaks corresponding to the composition, phase and internal atomic arrangement. Crystallinity and phase purity of the sample were analysed using an EMPYREAN X-ray diffractometer (manufactured by PANanalytical), present at IUAC, Delhi, with 2:1 ratio of Cu-Kα1 (λ = 1.540598 A˚) and Cu-Kα2 (λ = 1.544426 A˚) as X-ray source in the angular range of 2θ = 20 • -80 • at a scan rate of 1 • /min [5].
The resulting XRD spectra of the sample are shown in Figure 3 The estimated lattice spacing (dhkl) value of 1.64 A˚ using Bragg's law [7] corresponding to the (002) plane for the Nb foil, agrees well with the cubic structure of Nb, as mentioned in the JCPDS PDF data #98-007-6264. The evaluated lattice constant value of 3.29 A˚ is consistent with the cubic structure of Nb.

Rutherford Back-scattering Spectroscopy (RBS)
To experimentally determine the thickness of thin foil, the Rutherford back-scattering technique is used. 4 He ions are accelerated using 1.7 MV tandem accelerator at IUAC, Delhi. When the He beam hits the target, the He ions lose energy inside the target and are back-scattered at some point inside the target due to elastic collisions. The back-scattered ion again loses energy while coming out of the target and detected by a silicon surface barrier detector (with 50 mm 2 effective area and 3.6 mSr solid angle) kept at an angle of 165 • with respect to the beam direction. The detector set up is coupled with a 2k multichannel analyzer to process the data. The analysis of RBS spectra is done by Rutherford universal manipulation program (RUMP) [8] and the simulation program for the analysis of nuclear reaction analysis (SIMNRA) [9]. The obtained energy spectrum from the RBS measurement is shown in Figure 4. The thickness of the Nb target layer obtained from the analysis is given in Table 1. The RBS spectra obtained support the results from EDS and XRD analysis, no trace of heavy elemental impurities was observed.

Electrical transport properties
As discussed in the introduction section, targets having high electrical and thermal conductivity are required to prevent any possible damage (target peeling off) during the in-beam nuclear experiment. The electrical conductivity of the sample was evaluated by measuring the current through the foil on applying voltage. Two circular conducting contacts were made on the surface of both the foils using silver paste (Ag contacts) as shown in the inset of Figure 5. Programmable Power Supply (2200-60-2 programmable power supply) was used for applying varying voltage (0 to 0.25V) across these two Ag contacts and corresponding current(I) was measured using Picoammeter (6485 Picoammeter).
The current-voltage (I-V) curves for Pb and Nb/Pb foil are shown in Figure 5(a) and Figure 5(b), respectively. The recorded I-V curve for both Pb and Nb/Pb foil shows linear dependence of current on DC voltage (I ∝ V) [10], exhibiting ohmic conduction for charge flow for lateral device structure configurations. The resistivity was evaluated using Ohm's law as ρ = RA/l, where l is the distance between the Ag electrodes, A is the area of cross section, R is the resistance of the sample estimated from slope of the I-V plot shown in

Conclusion
A thin Nb target (∼ 1.2 µm) on a thick Pb backing (∼ 8.8 µm) was fabricated using the rolling technique. The fabricated target was successfully utilized in the nuclear lifetime measurement experiment. Characterization of the target sample was done to confirm the purity, thickness and surface uniformity. SEM images clearly indicate the surface uniformity of the target. Thickness of the target film was found out to be around 0.8 mg/cm 2 from the RBS technique. EDS, XRD and RBS analyses consistently confirm the purity of the target as no major elemental impurity was observed. Enhanced electrical conductivity by 35.9 % was measured using the I-V curve at room temperature. The results of material and electrical properties of the Nb/Pb composite foil indicate the formation of a good quality target foil that can be utilized in future envisaged nuclear experiments.