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Self-supporting thin tin targets fabricated by ultra-high vacuum evaporation for heavy-ion induced reactions
Journal Pre-proof Self-supporting thin Tin targets fabricated by ultra-high vacuum evaporation for heavy-ion induced reactions Arshiya Sood, G.R. Umapathy, Arzoo Sharma, R. Abhilash S, S. Ojha, D. Kabiraj, Akashrup Banerjee, Pushpendra P. Singh PII:
S0042-207X(19)32361-9
DOI:
https://doi.org/10.1016/j.vacuum.2019.109107
Reference:
VAC 109107
To appear in:
Vacuum
Received Date: 17 August 2019 Revised Date:
22 November 2019
Accepted Date: 24 November 2019
Please cite this article as: Sood A, Umapathy GR, Sharma A, Abhilash S R, Ojha S, Kabiraj D, Banerjee A, Singh PP, Self-supporting thin Tin targets fabricated by ultra-high vacuum evaporation for heavy-ion induced reactions, Vacuum, https://doi.org/10.1016/j.vacuum.2019.109107. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
Self-supporting thin Tin targets fabricated by ultra-high vacuum evaporation for heavy-ion induced reactions Arshiya Sooda,*, G. R. Umapathyb, Arzoo Sharmaa,1, Abhilash S. R.b, S. Ojhab, D. Kabirajb, Akashrup Banerjeec,1, Pushpendra P. Singha a
Department of Physics, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India
b c
Inter-University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110067, India
Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India Abstract 116,118
Self-supporting thin films of Sn isotopes have been fabricated using ultra-high vacuum (UHV) facility at the Inter-University Accelerator Centre (IUAC), New Delhi. Owing to the requirements for fusion and quasi-elastic backscattering studies, several self-supporting thin target films with areal density
∼ 250-600 μg/cm 2 were fabricated. Attempts were also made to prepare thin films by cold rolling 2 technique, and an areal density of ∼ 1.8 mg/cm could be achieved after several rolling cycles. Various advanced characterization techniques viz., Rutherford Backscattering Spectrometry (RBS), Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared (FTIR) Spectroscopy and Field Emission Scanning Electron Microscopy (FE-SEM) were employed to (i) ascertain levels of trace- and heavy- impurities, if present, in the fabricated target films, (ii) determine exact thickness of the films, and (iii) study morphological changes, if any, in the films after irradiation with high energy ion beam fluence of ∼ 10 ions/cm . The characterization results manifested that negligible amount of impurities was present in the films, and they were quite stable under energetic ion-beam irradiations. The fabricated films were successfully used as targets in online nuclear physics experiments to measure 15
2
quasi-elastic backscattering cross-sections in
7
Li induced reactions.
Keywords: Self-supporting enriched 116,118 Sn thin films, Ultra-high vacuum deposition, Rutherford Backscattering Spectrometry (RBS), Energy Dispersive X-ray Spectroscopy (EDS), Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared (FTIR) Spectroscopy
1. Introduction Over the last many decades, heavy-ion (HI) induced reactions have captivated nuclear physicists across the globe, and prodigious theoretical and experimental efforts have been made to understand the underlying dynamics. The HI induced fusion reactions play a significant role in nuclear astrophysics, generation of energy in the stellar environment through nucleosynthesis, production of exotic nuclei away from stability in the territory of proton-rich side of mass valley, quest for synthesis of superheavy elements, radiochemistry for national security and development of next-generation nuclear reactors for the production of nuclear energy and transmutation of commercial nuclear waste[1, 2, 3, 4, 5]. On the other hand, HI peripheral reactions are considered as a promising spectroscopic tool to achieve high spin states of the stable as well as complex nuclear configurations, and also provide a sensitive probe for nuclear surface studies[7, 6]. An interesting strand of HI induced reactions is the one involving weakly bound projectiles ( 6,7 Li and 9 Be) at energies around the Coulomb barrier as they offer widespread opportunities to explore different aspects of nuclear structure and reactions[9, 8]. Weakly bound nuclei are characterized by their cluster/halo structure and low breakup thresholds, which makes incomplete/breakup fusion a dominant reaction process. The fusion cross-sections are sensitive to the internal structure of interacting nuclei and coupling to other reaction channels like inelastic excitations, breakup, and *
Corresponding author
Email address: [email protected] (Arshiya Sood) Present address: GSI Helmholtzzentrum für Schwerionenforschung, Planckstrasse 1, 64291 Darmstadt, Germany
1
direct nucleon transfer. The couplings of non-fusion channels substantially modify the effective interaction potential, and leads to the splitting of single, uncoupled fusion barrier into multiple barriers which result in the distribution of barriers. The barrier distribution can be experimentally obtained by two complementary processes - fusion and large angle quasi-elastic (QEL) scattering and are expected to be similar[11, 12, 10]. However, for weakly bound projectiles, the distribution obtained from QEL has been found to be broader than that derived from fusion, and a relative shift in peak has also been observed between the two distributions indicating strong influence of breakup or breakup-like processes on fusion at energies near or below the barrier[14, 15, 13]. To further investigate this discrepancy, experiments have been proposed to derive barrier distributions from fusion and quasi-elastic back-angle 6,7 Li+ 116,118 For precision of data measurements in such experiments, appropriate target thickness, high-quality self- supporting films in sufficient number, homogeneity and well characterized isotopic composition of the films are among the indispensable requirements. Self-supporting and isotopically enriched target films are preferred as they provide reaction products of interest with minimal interference from other impurities. The target film thickness can affect energy resolution of spectra as spread in energy loss for incoming and outgoing particles within the thickness of target foils contributes to the resolution. The target films should be thick enough to obtain appreciable reaction cross-section, but thin enough to achieve better energy resolution so that all the peaks of interest are well separated. Moreover, for barrier distribution measurements, in particular, energy loss in the target film must be less than the energy steps used to carry out the experiment. Therefore, all these factors necessitate the need for self-supporting homogeneous thin target films for the proposed experiments. In the present work, we aimed to fabricate self-support-ing 116 Sn and 118 Sn thin films of areal density in the range of 250-400 μg/cm 2 and 500-700 μg/cm 2 for quasielastic backscattering and fusion cross-section measurements, respectively using different facilities available in the Target Laboratory of Inter-University Accelerator Center (IUAC), New Delhi. Several reports are available in the literature illustrating distinct techniques for fabricating thick and thin tin films either as selfsupporting samples or on a backing [17, 18, 19, 16]. However, thin tin films are reported to be successfully fabricated mostly on Al and C backings[20], and seldom as self-supporting films[18], to the best of our knowledge. Besides aiding basic nuclear physics research to explore various aspects of nuclear reactions and structure, tin thin films have been extensively used to assess applied research domains, including optical, electronic, optoelectronic, and medical. Thin tin dioxide films behave as n-type semiconductors with wide bandgap and have attracted resurgent interest due to their particular properties like high optical transmission and high reflectivity in visible and infra-red range, respectively, and an excellent chemical resistance which can be used to constitute transparent and chemically stable thermal barriers. These films are widely used in solar cells, catalytic support materials, solid-state chemical sensors, and high-capacity lithium-storage[22, 21]. Moreover, tin dioxide films are used as sensors for volatile organic compounds and when combined with nanocarbon materials such as carbon nanotubes and graphene create electronic gas sensors capable of functioning at room temperature[24, 25, 23]. These detectors are efficient in trace detection and find their applications in environmental monitoring, industries, transportation, energy, agriculture, and medical diagnosis[26]. Tin-tellurium and tin-tellurium-phosphide have emerged as a new class of crystalline topological insulators which have proven to be promising materials for widespread applications in spin-electronics and quantum computing[28, 27]. Recently, in the domain of medical sciences, a sensor has been developed for detecting 1-nonanal gas present in the breath of lung cancer patients by combining tin dioxide nanosheets with tin dioxide nanoparticles and noble metal catalysts[29]. In this communication, Section 2 discusses practical techniques used to fabricate self-supporting 116 Sn and 118 Sn thin films. Characterization methods employed to precisely measure thickness and purity level as well as to study the morphology of the films are given in Section 3. The application of fabricated thin films and summary of fabrication and characterization methods are included in Section 4 and 5, respectively.
2. Target Preparation Tin is a post-transition element belonging to group 14, period V, p-block, and is a member of the carbon family of the periodic table. It is a soft, malleable and easily pliable silvery-white metal generally non-toxic except for few organotin compounds that are highly toxic. Tin is known to exist in two allotropic forms - (i) metallic β-tin (white tin) which is stable at room temperature and malleable above it, and (ii) powdery amorphous non-metallic α-tin (gray tin) which is stable below 13.2 D C and brittle[30]. The white tin is transformed into the gray tin when the former is cooled to temperatures below 13.2 D C. This process is termed as tin pest which, however, takes place rather slowly. The white tin has a tetragonal crystal structure with a density of 7.286 g/cm 3 , whereas gray tin has a facecentered cubic structure having a density of 5.765 g/cm 3 . Melting and boiling points of tin are 232.06 D C and 2603 D C, respectively. Tin is usually not affected by water and oxygen at room temperatures, but it tends to react with both at high temperatures to form tin oxide. Tin is amphoteric and reacts with strong bases and acids with evolution of hydrogen. Tin has 10 stable naturally occurring isotopes including 116 Sn (14.54%) and 118 Sn (24.22%). The raw material to fabricate thin films was procured from ISOFLEX USA in the form of 99.95% and 99.6% isotopically enriched metal ingot of 116 Sn and sheet of 118 Sn, respectively. After careful consideration of the aforementioned form and properties of tin, both mechanical reshaping and vacuum thermal deposition were attempted for the fabrication of thin tin films [31]. Over the last few decades, cold rolling technique has been extensively used to fabricate selfsupporting thin/ thick films particularly for expensive isotopically enriched materials. This technique is quite economical in terms of minute amount of starting material required and minimal wastage of material as the oddments left during film fabrication can be put to use in other techniques like vacuum deposition. Owing to these advantages, initially, extensive trials were carried out to fabricate selfsupporting thin films using cold rolling technique, however, limited success was achieved. After repetitive efforts, the focus was then shifted to fabricate thin films using ultra-high vacuum thermal deposition (evaporation-condensation) technique and multiple self-supporting thin films of 116 Sn and 118 Sn were successfully fabricated. The details of extensive trials conducted and practical intricacies involved during the thin film fabrication stages using these techniques are delineated in subsequent sub-sections.
2.1. Cold Rolling Isotopically enriched tin isotopes were rolled using heavy -force exerting Heiss Maschinenbau GmbH ™[32] rolling machine installed in target lab at IUAC. It is a motor controlled rolling machine with a provision to control rotary motion of two 85 mm diameter cylindrical rollers manually by a foot pedal. This machine belongs to two high reversing rolling machine category in which rollers can first rotate in one direction and then in the other so that the material to be rolled can be passed back and forth many times through rollers. In this process, the sheet becomes thinner with each rolling cycle, and finally, a film of uniform surface density and desired thickness can be achieved. Moreover, the lower roller is fixed in the vertical plane whereas the upper roller can be moved up and down using a top-mounted circular lever, thus enabling adjustment of force imparted on the material being rolled. Before rolling enriched 116 Sn and 118 Sn isotopes, the cold rolling technique was tried out with naturally abundant isotope of tin. The rolling of natural tin metal sheet was quite feasible, and areal density of ≈ 2.5 mg/cm 2 could be achieved in a few rolling cycles. Thus, following the methodology practiced to roll natural tin, enriched tin metal sheet (initial areal density ≈ 240 mg/cm 2 ) to be rolled was placed inside a rolling pack made up of a mirror-polished stainless steel sheet folded over symmetrically to form an envelope with three open edges. The rolling pack with the metal sheet in-between was rolled back and forth under a gradually increasing force exerted when the gap between the rollers is reduced using the circular lever. During rolling, care was taken not to over roll
the sheet. Meticulous cleanliness was maintained to avoid appearance of pin-holes in the film due to dust particles, and new packs were used whenever necessary. However, it was observed that when a thickness of ≈ 13 mg/cm 2 was reached cracks started developing around edges of the film. At this point, the rolling force was reduced before opening the pack to avoid any damage to the film like blistering, sticking or breaking. The film was then taken out of the pack carefully and cropped properly to a rough square to restrict any further ingrowth of cracks. The rolling cycles were then repeated with a new rolling pack and a modified pack-rolling technique described by Banerjee et al.[33]. Using this technique, in addition to controlled and gradual increase of force on the rolling pack, the film was rotated 90 D and flipped over after each rolling cycle to flatten all the edges uniformly. The cracked edges were snipped off as and when required. After a thickness of ≈ 3 mg/cm 2 was achieved, the film started sticking on upper face of the rolling pack. Parchment paper was used to peel off the film without damaging it, and also for cropping the cracked edges of the film to avoid its possible adhesion to scissors. After many rolling cycles and adopting all the measures mentioned above, a minimum thickness of ≈ 1.8 mg/cm 2 and area 1.9 cm × 2.6 cm was achieved using this technique. Since for our proposed measurements, thin films in large number were required, therefore, cold rolling was not considered as a viable technique to obtain very thin films of tin. In order to fabricate a sufficient number of thin films required for the experiments, alternate methods of thin film fabrication were explored.
2.2. Ultra-high vacuum thermal deposition Vacuum thermal deposition is an evaporation- condensation technique in which a volatile material to be evaporated is heated to a high temperature in an evacuated chamber followed by condensation of vaporized material on an appropriately chosen substrate. The growth of evaporated films depends on many factors, viz. nature of the substrate and its temperature, contamination from various sources, deposition rate, method of heating material, source-to-substrate distance, and choice of parting agent (particularly for self-supporting thin films)[34]. Figure 1: (top) Ultra-High Vacuum (UHV) evaporator at IUAC New Delhi, and (bottom) top view of arrangement inside UHV chamber for self-supporting thin target foil deposition. To fabricate self-supporting tin thin films, ultra-high vacuum (UHV) evaporator facility of target lab at IUAC, as shown in Figure 1, was used. The evaporation unit consists of a scroll pump, a turbo molecular pump, and a cryopump which aid in maintaining contamination less high vacuum environment for thin-film deposition[35]. It houses a multi-pocket electron gun with 6kW power supply and resistive heating setup. A typical vacuum of ∼ 10−8 mbar can be achieved and maintained throughout the deposition procedure in UHV evaporator. To monitor deposition rate and thickness of the growing film in-situ, a piezo-electric quartz crystal monitor [36], based on the principle of change in vibration frequency of crystal with deposited mass, is provided. Tin isotopes have low abundance resulting in a very high cost of enriched isotopic material, therefore, to ensure the economical usage of this material, the deposition method and parameters were optimized in as many as 10-15 trials with natural tin. The inside view of the UHV unit showing an arrangement for tin thin film deposition is presented in Figure 1. A set of ultrasonically cleaned mirror-polished glass slides were used as a substrate for deposition of tin thin films. After cleaning, the slides were immediately mounted on the substrate holder inside the vacuum chamber to minimize the possibility of pin-holes in thin films due to dust particles. The water-soluble parting agents play a crucial role in the fabrication of self-supporting thin films as they govern the easy separation of film layers from the substrate. The choice of parting agent depends on many factors like their temperature stability, crystal structure and lattice constant, and interference with the experiment being undertaken as they may leave elemental traces. In the present work, (i) alkali halide : NaCl (sodium-chloride), BaCl 2 (barium chloride) and KCl (potassium chloride) , and (ii) Detergent : Teepol were evaluated for use as a parting agent. As per requirement, 3mm thick pellets of alkali halides were prepared by powder pressing with a hydraulic pellet press and were used immediately to avoid appreciable absorption of moisture. Thin films of alkali halide parting agents were prepared using electron beam gun heating of
the pellet placed in copper hearth of multi-pocket e-gun setup in UHV evaporator. A monoatomic layer of Teepol was obtained by putting a drop on center of the glass slides which were then uniformly covered with Teepol by spreading the drop over the entire surface and polished until no visible accumulation was left. After multiple iterations, parameters like deposition rate, source-tosubstrate distance and thickness of deposited films were optimized for different parting agents. Resistive heating technique was used to evaporate natural tin material owing to its melting point. The tin metal sheet was snipped into tiny metal flakes and kept in tube-type tantalum (Ta) boat, which is a refractory metal with a melting point of 3017 D C, for evaporation (see Figure 2(a) and (b)). To precisely optimize different deposition parameters required to achieve the desired thickness of thin films, including the amount of material and distance of substrate from Ta boat, several trials were carried out using natural tin material. Depending on the primary requirements discussed earlier in this section, empirically KCl was found to be the best parting agent for fabrication of self-supporting tin targets. Figure 2: (a) Tantalum (Ta) tube-type boat used for material evaporation, (b) enriched tin metal flakes, (c) tin thin film deposited on glass substrate, (d) scored and floated thin films in distilled water, (e) tin thin film mounted on stainless steel frame, and (f) enriched tin thin films fabricated in single batch of evaporation. After ascertaining different parameters, the deposition with enriched isotopic material was performed using 45-50 mg of source material in every deposition cycle. First, a KCl layer of ≈ 200 nm was deposited at a deposition rate of 0.1-0.5 nm/s on the glass substrates placed at a distance of 10 cm from the pellet kept in the copper hearth of electron beam gun setup. The emission current was applied slowly to assist in outgassing, and when the vacuum had recovered, the evaporation of material was undertaken until the desired thickness was achieved. The maximum emission current for deposition of 200 nm KCl was 1 mA. The thickness of KCl was measured in-situ using the quartz crystal monitor. The chamber was allowed to cool down for sufficient time before venting it with inert Argon gas. After venting, Ta boat with enriched isotopic tin metal flakes was clamped between highcurrent electrodes of resistive heating setup. The substrate holder carrying the glass slides coated with KCl layer was fixed manually on the support rods to place the slides directly above the Ta boat at a distance of 9.5 cm to ensure optimal use of the material and uniform thickness of thin films. The current was ramped up gradually, and the vacuum was continuously monitored. In case of abrupt changes in vacuum, the ramping up of current was interrupted several times during the deposition cycle to allow the vacuum to recover sufficiently. This aided in fabricating thin films of uniform thickness and also helped in reducing the contamination level. The current was increased to a maximum of 225 A at voltage 2 V, and it took ≈ 1 hour to complete the evaporation of enriched material. Vacuum inside the chamber was maintained in the range of 10 −6 - 10 −7 mbar during the complete cycle of deposition of KCl and tin. For instance, an isotopically enriched tin thin film deposited on a glass substrate is shown in Figure 2 (c). The chamber was allowed to cool down for ≈ 8 hours after the deposition was completed and subsequently vented. After each evaporation cycle, every part of the working chamber and support rods were cleaned scrupulously with acetone and propanol to avoid contamination in the successive evaporations. Table 1: Comparison of Sn targets fabricated in the present work with different works reported in the literature Target Target description
Fabrication method
Thickness (mg/cm 2 )
Refs.
116
Sn
Self-supporting
UHV evaporation (Resistive heating)
0.25-0.6
Present work
118
Sn
Self-supporting
UHV evaporation (Resistive heating)
0.25-0.6
Present work
112
Sn
Self-supporting
Cold Rolling
1
[17]
112
Sn
Self-supporting
Cold Rolling
8.4
[19]
116
Sn
Carbon (C) backed
HV evaporation (Resistive heating)
0.15
[20]
124
Sn
Aluminium (Al) backed
HV evaporation (Resistive heating)
0.2-0.35
[37]
112
Sn
Lead (Pb) backed
Cold Rolling
2.44, 10.26
[38]
2.2.1. Separation and Mounting of thin target foils After deposition, the glass slides were cautiously taken out of the chamber to separate the deposited film from the substrate. A cut was scribed on all edges of the film to ensure a better contact with water surface and was scored precisely into appropriate sizes using a diamond tip pen for mounting on stainless steel (SS) target frames with a hole in the center which is carefully deburred on both sides. Since the parting agent used was water-soluble, therefore, distilled water was used to float off the films. The substrate was inclined at an angle of ≈ 45 D to the water surface and immersed slowly into the water as the scored films began to release and started to float on the surface (see Figure 2(d)). To facilitate mounting of a free section of floated film, a suitable SS target frame was lowered beneath it. The film was then gently nudged over to the frame and positioned properly on it. Subsequently, the target frame was cautiously raised keeping it vertical to the water’s surface, with the film adhering to the frame [34]. The mounted films were then kept inside a laminar airflow fume hood to drain excess water on the surface, and when dried, all the frames were stored in a vacuum desiccator. Figure 2 (e) and (f) show the self-supporting enriched tin thin film mounted successfully on the SS frame after floating on the water surface and several thin films fabricated in a single batch of evaporation, respectively. With this technique and optimized deposition parameters, ≈ 53 self-supporting thin films were fabricated in different batches consuming ≈ 250 mg of enriched material in total. The thickness of prepared thin films was found to range between 250-600 μg/cm 2 which may be attributed to variation in material deposition occurring due to different positions of the glass slides with respect to Ta boat in resistive heating setup. It is essential to mention here that even after altering different deposition parameters including the amount of material to be deposited, thin films of thickness greater than 600 μg/cm 2 could not be achieved, and an appreciable amount of material could be found left in the boat. A comprehensive comparison of the present work with reports available in the literature on fabrication of isotopically enriched tin targets is given in Table 1. As apparent from this table, present work is the only detailed report available on fabrication of self-supporting tin thin targets for nuclear physics research, to the best of our knowledge. Further, the fabricated tin targets are compared in Table 2 with other elemental targets, in general, and elements of Group 14, period V, in particular, available in the literature for similar nuclear reaction studies as in the present work. From this comparison, it can be inferred that in the present work self-supporting thin targets have been fabricated successfully considering the requirements to perform high accuracy measurements as discussed in Section 1 of this paper. Table 2: Comparison of Sn targets (present work) with different elemental targets reported in the literature for nuclear reaction studies. Target
Target description
Fabrication method
Thickness (μg/cm 2 )
Refs.
116,118
Self-supporting
Resistive heating
250-600
Present work
Sn
92
Mo
Self-supporting
Cold rolling
5000
[39]
92
Zr
Carbon (C) backed
Resistive heating
80-270
[40]
Carbon (C) backed
Resistive heating
139-260
[41]
130
Te
Gold (Au) backed
Resistive heating
1050
[42]
Self-supporting
Resistive heating
500-800
[43]
Carbon (C) sandwiched
Resistive heating and e-gun
200-250
[44]
Ir, 187 Re, 182 W, 181 Ta
Carbon (C) backed
e-gun
80, 60, 70, 175
[35]
175
Carbon (C) backed
Resistive heating
110, 270
[35]
127
I
206,208 138
Pb
Ba
193
Lu, 169 Tm
3. Characterization 3.1. Purity Analysis and Thickness measurement 3.1.1. Rutherford Backscattering Spectrometry (RBS) RBS is an outstanding ion beam analysis technique that has been widely used by nuclear physicists for over the decades to examine purity and thickness of the targets. Figure 3: RBS spectra of enriched 116 Sn and 118 Sn thin films. (Blue) solid lines through the spectra are SIMNRA simulation fitting to experimental data. The most attractive features of RBS are - the quantitative and accurate measurement of film thickness and impurities within the thin film and, in most cases, with no radiation damage to the film. In order to ascertain the exact thickness and composition of fabricated self-supporting tin thin films used in the online experiments, ion beam analysis was carried out using RBS with a 2 MeV He probe in backscattering configuration at1.7 MV Tandem accelerator (Model 5SDH, National Electrostatics Corporation) at Pelletron Accelerator RBS-AMS (Accelerator Mass Spectroscopy) System (PARAS), IUAC [45]. Data reduction was performed using SIMNRA software [46] by fitting simulations over the experimental data to obtain information regarding areal densities of the films and impurities present, if any. The RBS spectrum for enriched 116 Sn and 118 Sn thin films along with the simulated spectra are presented in Figure 3. It is evident from the figure that no heavy-impurities and a negligible amount of trace-impurities like carbon and oxygen were present in 116 Sn and 118 Sn thin films. It is pertinent to mention here that RBS measurements of the enriched tin thin films have been carried out after a span of several weeks of their preparation and irradiation, which signifies that these films are not prone to oxidation. The areal density of the thin films obtained from the SIMNRA simulation fitting is ≈ 430 μ g/cm 2 and ≈ 380 μ g/cm 2 for 116 Sn and 118 Sn, respectively.
3.1.2. Alpha-transmission method Multiple self-supporting tin thin films were fabricated to perform another set of offline experiments to measure fusion excitation function employing recoil-catcher activation technique. The thickness of these thin films, including the ones used in the online experiment to measure quasi-elastic back scattering, was determined using alpha-transmission method [47].The alpha particles of 5486 KeV from 241 Am source were used to estimate the thickness of thin films by measuring the energy loss suffered by them while traversing through the films. The energy of alpha-particles was measured using a Silicon Surface Barrier Detector (SSBD) before and after transmission through thin films. For instance, the energy spectra of alpha-particles obtained from sample-less (blank) frame and after transmission through irradiated 116 Sn and 118 Sn thin films are shown in Figure 4 (a) and (b), respectively. As evident, all the spectra were best fitted with a Gaussian distribution. The centroids obtained from the fitting for blank, 116 Sn and 118 Sn thin films are 5486 KeV, 5367 KeV and 5385 KeV, respectively. The stopping power for 5486 KeV alpha particles in 116 Sn and 118 Sn material, calculated using SRIM[48], is 0.318 and 0.312 KeV/(μg/cm 2 ), respectively. Consequently, the energy shift observed from the respective centroids divided by the stopping power of the materials
was used to determine the thickness of self-supporting thin films. The thickness of these films was found to be ≈ 374 μg/cm 2 and ≈ 325 μg/cm 2 for 116 Sn and 118 Sn, respectively, and is in reasonable agreement (13 % ) with the RBS measurements. Moreover, the thickness of thin films from each prepared batch was measured using this technique and was found to vary within 250-600 μ g/cm 2
, which met our requirements.
3.1.3. Energy Dispersive X-ray Spectroscopy (EDS) To substantiate the elemental composition results obtained from RBS, EDS measurements were performed for the self- supporting enriched tin thin films. EDS is a sensitive probe to obtain a qualitative assessment of elemental composition in thin films and quantitative information of the elements present. Since every element has characteristic X-rays associated with it, therefore, EDS employs a finely focused electron beam to excite atoms on surface of the film, making it possible to identify and quantify the elemental composition from the emitted X-rays. In the present work, EDS measurements of tin thin films were performed using JEOL’s 7610F Field Emission Scanning Electron Microscope (FE-SEM) equipped with EDAX’s EDS at IUAC[49]. The EDS elemental analysis of 116 Sn and 118 Sn thin films before and after irradiation is shown in Figure 5. As can be observed from this figure, elements C and O are insignificantly present in the thin films with weight % of 2.62 and 0.76, respectively, relative to 96.63 of 116 Sn, and 3.1 and 0.77, respectively, relative to 96.13 of 118 Sn. Figure 4: Energy calibrated alpha-particle transmission spectra obtained with and without targets of (a) 116 Sn and (b) 118 Sn isotopes. Red lines through the spectra are Gaussian fit to the data. The EDS measurements corroborate the results obtained from RBS, indicating the purity of fabricated self-supporting tin thin films. Also, no appreciable difference has been observed in the EDX spectra taken before and after the experiment which manifests the stability of thin films under energetic ion-beam irradiations. Figure 5: EDS elemental analysis of enriched 116 Sn and 118 Sn thin films before and after irradiation. Characteristics X-rays of different elements identified in the analysis are marked.
3.1.4. Fourier Transform Infrared (FTIR) Spectroscopy Fourier Transform Infrared (FTIR) Spectroscopy is a novel analytical technique that allows closeexamination of sample’s chemical composition. In this technique absorption of various infrared light wavelengths by different functional groups is measured. These absorption bands manifest the presence of specific molecular components in the sample. In order to have better insights into purity and composition, we examined the presence of chemical bonding in fabricated selfsupporting Sn thin films by FTIR Spectrometer in the mid-infrared frequency range of 400 to 4000 cm −1 . The samples used for FTIR studies were in the form of self-supporting thin films mounted on stainless steel frames as shown in Figure 2(e). The characteristic FTIR spectra of pristine and 7 Li irradiated 116,118 Sn thin films are shown in Figure 6 (a) and (b), respectively. The absorption peaks centered at 603 cm −1 (pristine) and 616 cm −1 (irradiated) for 116 Sn and 615 cm −1 (pristine) and 623 cm −1 (irradiated) for 118 Sn thin films can be attributed to the Sn-O stretching vibrations [50]. The very weak peaks observed between 2280-2400 cm −1 have their probable origin from reaction with carbon dioxide in air [51]. Since peak intensity in infrared spectra depends on concentration of molecules present in a sample, the feebly intense peaks observed for Sn-O lattice vibration in Figure 6 indicate negligible presence of contaminants like C or O, and corroborate the findings of RBS and EDS characterizations. It is important to mention here that the FTIR analysis of self-supporting tin thin films is done after months of their fabrication which not only establishes the purity of films but also reinstates that these films are not prone to oxidation.
Figure 6: FTIR spectra of (a) 116 Sn and (b) 118 Sn thin films before and after irradiation. In the insets feebly intense Sn-O stretching vibration peaks observed around 624 cm −1 are zoomed out for better visualization.
3.2. Morphology In order to study the morphological changes in tin thin films after irradiation with 7 Li beam, the surface morphology of pristine and irradiated films were studied using FE-SEM at IUAC. The SEM images for pristine and irradiated 116 Sn and 118 Sn films obtained at an accelerating voltage of 15kV are shown in Figure 5 (a) and (b), and (c) and (d), respectively. The 116 Sn film was irradiated with high energy ion beam fluence of 1.25× 10 15 ions/cm 2 , and 118 Sn film at a fluence of 8× 10 14 ions/cm 2 . As evident from Figure 5, granules in the pristine film appear to melt slightly, form bigger granules and agglomerate after ion beam irradiation at high fluences. However, this does not have appreciable effects on the structural integrity and physical stability of the films which reinstates the fact that tin remains undefiled in the irradiated samples. Figure 5: SEM images for pristine (P) and irradiated (I) thin films (top) to study the morphological changes after irradiation with 7 Li ion beam.
116
Sn and (bottom)
118
Sn
4. Application The fabricated 116 Sn and 118 Sn thin films have been successfully used as a target in online experiments to study quasi-elastic (QEL) backscattering in 7 Li+ 116,118 Sn systems from sub- to abovebarrier energies. For weakly bound systems 7 Li, QEL scattering includes breakup ( 7 Li → α + t ), elastic, inealstic and transfer process. The experiments were performed in General Purpose Scattering Chamber (GPSC) with beams of 7 Li in the energy range 15-29 MeV from Pelletron accelerator (Model 15UD, National Electrostatics Corporation) at IUAC. Beam current was maintained between 2-3 nA throughout the experiment. To detect and identify the charged particles produced in the reactions, HYbrid Telescope ARray (HYTAR) detector facility was used[52]. The quasi-elastic backscattering events were measured using four telescopes arranged in symmetrical cone geometry, two in- and out- of the plane each, at an angle of 173 D . Additionally, six telescopes were placed at angles +60 D to +160 D and three telescopes were placed at angles -36 D to -60 D with an angular separation of 20 D and 12 D , respectively. Figure 6: (top) Experimental set-up for quasi-elastic scattering measurement at IUAC New Delhi, and (bottom) Δ E-E res spectrum at θlab = +140 D for 7 Li+ 118 Sn at E lab = 21 MeV. Two SSBD monitor detectors were placed at ± 10 D with respect to the beam direction for normalization and beam monitoring purposes. The experimental set-up is shown in Figure 6(top). For instance, a typical two-dimensional Δ E-E residual spectrum obtained from the telescope at +140 D for 7
Li+ 118 Sn system is presented in Figure 6 (bottom). The events corresponding to multiple elastic + inelastic ( 7 Li) scattering and breakup (α) processes at E lab = 21 MeV are marked as Z=3 and Z=2, respectively.These experiments act as a sensitive probe to determine the purity of samples as any likely uptake of residual gas contamination like, C, O or N during target fabrication would have severely meddled with our actual measurements, and appeared as a separate lobe in Z=3 band in Figure 8(bottom). Moreover, to further understand the effect of impurities, if any, on the present analysis, the projection of Z=3 and Z=2 band on total energy axis is shown in Figure 9. The observed energy peaks of our interest for Z=3 and Z=2 are events are also checked with the kinematics. In this figure, the Z=3 band peaks at 16.57 MeV which agrees reasonably well with the value 16.94 MeV obtained from the kinematics. Also, it has been calculated that the breakup alpha peaks from the contamination of 12 C, 14 N and/or 16 O fall below 6 MeV. While for reaction with 118 Sn the breakup alphas peak at 11 MeV which is in agreement with the experimental
measurements given in Figure 9. These sensitive measurements corroborate that there are no traces of contamination in the fabricated films as 7 Li and α particles are purely produced from the reactions of 7 Li with 118 Sn in accordance with the kinematical calculations[53]. Figure 7: Projection of Z=2 and Z=3 events of Figure6 (bottom) on the total energy axis.
5. Summary Isotopically enriched multiple self-supporting thin films of 116,118 Sn with areal density ≈ 250-600 μg/cm 2 were successfully fabricated using ultra-high vacuum thermal deposition technique. For two sets of experiments to derive barrier distributions from fusion and quasi-elastic backscattering crosssection measurements, thin films of mentioned areal density range were required in large numbers. The cold rolling technique was attempted to fabricate the required thin films. Areal density of ≈ 1.8 mg/cm 2 was achieved by rolling down enriched isotope metal sheet of ≈ 241 mg/cm 2 . However, after repetitive trials, it was concluded that cold rolling was not a viable technique to obtain very thin films of tin. In order to fabricate a sufficient number of thin films required for the experiments, an alternate technique of high vacuum deposition was used. To optimize different deposition parameters, multiple iterations were carried out by using various parting agents, including alkali halides and detergent, and naturally abundant tin isotope. Moreover, quite a few attempts were also made to fabricate thin films of areal density higher than 600 μ g/cm 2 by varying different parameters including the amount of material used for evaporation. It was observed that an appreciable amount of material residue was left behind in the boat after evaporation, and the thin target foils of required areal density could not be fabricated. Purity and stability of fabricated self-supporting tin thin films were established by various characterization techniques viz., RBS, EDS, FTIR and FE-SEM. It was observed that negligible amount of trace- and no heavy- impurities were present in the fabricated thin films even after irradiation and weeks of their preparation. Thickness of the films was measured using RBS and alpha transmission methods. The fabricated films were successfully used as targets in online experiments to measure quasi-elastic backscattering cross-sections in 7 Li+ 116,118 Sn reactions. The experiments were performed to study the effect of the internal structure of colliding nuclei and coupling of different reaction channels on fusion from sub- to above- barrier energies by deriving barrier distributions from the cross-section data. The convincing quality of data obtained in these experiments validates the purity of these films and bolsters the importance of using self-supporting thin films in such experiments.
Acknowledgment The authors are grateful to Director, IUAC for providing all necessary facilities required to carry out the present work, including FE-SEM facility, funded by MoES under Geochronology project. One of the authors A.S. acknowledges Indian Institute of Technology Ropar, India for funding her research work, Dr.Akhil Jhingan for his useful suggestions and discussions, Dr. Subhendu Sarkar for his inputs in characterization section of this article and Dr. Saif A. Khan for EDS measurements of natural tin samples. P.P.S. acknowledges a startup grant from IIT Ropar for purchasing the isotopically enriched material required for the thin film fabrication.
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[31] A. Stolarz, J. Radioanal. Nucl. Chem. 299 (2014) 913. [32] https://www.heiss-sondermaschinenbau.de/. [33] A. Banerjee, et al., Nucl. Inst. Meth. Phys. Res. A 887 (2018) 34. [34] A. H. F. Muggleton, Vacuum 37 (1987) 785. [35] T. Banerjee, et al., Vacuum 144 (2017) 190. [36] K. H. Behrndth, R. V. Love, Vacuum 12 (1962) 1. [37] P. K. Giri, et al., Proc. DAE-BRNS Symp. on Nucl. Phys. 59 (2014) 984. [38] H. Pai, et al., Vacuum 167 (2019) 393. [39] N. Y. Kheswa, et al., Nucl. Inst. Meth. Phys. Res. A 613 (2010) 389. [40] Khushboo, et al., Vacuum 145 (2017) 14. [41] R. N. Sahoo, et al., Nucl. Inst. Meth. Phys. Res. A 935 (2019) 103. [42] S. S. Tiwari, et al., Vacuum 167 (2019) 336. [43] S. Goyal, et al., Nucl. Inst. Meth. Phys. Res. A 777 (2015) 70. [44] K. K. Rajesh, et al., Vacuum 141 (2017) 230. [45] G. R. Umapathy, Proc. DAE-BRNS Symp. on Nucl. Phys. 61 (2016) 1038. [46] M. Mayer, AIP Conf. Proc. 144 (1999) 190. [47] D.C.Santry, R.D.Werner, Nucl. Inst. Meth. 159 (1979) 523. [48] The Stopping and Range of Ions in Matter (SRIM-2008.04), http://www.srim.org/. [49] http://www.jeoulusa.com/PRODUCTS/Scanning-Electron-Microscopes-SEM/FE-SE M/JSM7610F. [50] H. Yang, et al., J. Alloys Compd. 363 (2004) 271. [51] R. Sangeetha, S. Muthukumaran, Ceramics International 42 (2016) 5921. [52] A. Jhingan, et al., Nucl. Inst. Meth. Phys. Res. A 903 (2018) 326. [53] A. Sood, et al., manuscript to be submitted.
Highlights 1. Several 116,118Sn self-supporting thin films of areal density ≈250-600 μg/cm2 have been fabricated using ultra-high vacuum evaporation facility at Inter-university Accelerator Centre, New Delhi. 2. Cold rolling technique has also been attempted to fabricate thin tin films. After several rolling cycles, an enriched metal sheet of ≈240 mg/cm2 could be rolled down to a minimum of ≈1.8 mg/cm2. 3. Prepared thin films have been successfully used as targets in online experiments to study quasi-elastic backscattering in 7Li induced reaction. The convincing quality of experimental data obtained signifies the importance of self-supporting thin films for such experiments. 4. Results of various advanced characterization techniques – RBS, EDS, FT-IR and SEM point towards purity and stability of these thin films in high fluence ion beam nuclear physics experiments.
Declaration of interests 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. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0042-207X(19)32361-9
DOI:
https://doi.org/10.1016/j.vacuum.2019.109107
Reference:
VAC 109107
To appear in:
Vacuum
Received Date: 17 August 2019 Revised Date:
22 November 2019
Accepted Date: 24 November 2019
Please cite this article as: Sood A, Umapathy GR, Sharma A, Abhilash S R, Ojha S, Kabiraj D, Banerjee A, Singh PP, Self-supporting thin Tin targets fabricated by ultra-high vacuum evaporation for heavy-ion induced reactions, Vacuum, https://doi.org/10.1016/j.vacuum.2019.109107. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.
Self-supporting thin Tin targets fabricated by ultra-high vacuum evaporation for heavy-ion induced reactions Arshiya Sooda,*, G. R. Umapathyb, Arzoo Sharmaa,1, Abhilash S. R.b, S. Ojhab, D. Kabirajb, Akashrup Banerjeec,1, Pushpendra P. Singha a
Department of Physics, Indian Institute of Technology Ropar, Rupnagar 140001, Punjab, India
b c
Inter-University Accelerator Center, Aruna Asaf Ali Marg, New Delhi 110067, India
Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India Abstract 116,118
Self-supporting thin films of Sn isotopes have been fabricated using ultra-high vacuum (UHV) facility at the Inter-University Accelerator Centre (IUAC), New Delhi. Owing to the requirements for fusion and quasi-elastic backscattering studies, several self-supporting thin target films with areal density
∼ 250-600 μg/cm 2 were fabricated. Attempts were also made to prepare thin films by cold rolling 2 technique, and an areal density of ∼ 1.8 mg/cm could be achieved after several rolling cycles. Various advanced characterization techniques viz., Rutherford Backscattering Spectrometry (RBS), Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared (FTIR) Spectroscopy and Field Emission Scanning Electron Microscopy (FE-SEM) were employed to (i) ascertain levels of trace- and heavy- impurities, if present, in the fabricated target films, (ii) determine exact thickness of the films, and (iii) study morphological changes, if any, in the films after irradiation with high energy ion beam fluence of ∼ 10 ions/cm . The characterization results manifested that negligible amount of impurities was present in the films, and they were quite stable under energetic ion-beam irradiations. The fabricated films were successfully used as targets in online nuclear physics experiments to measure 15
2
quasi-elastic backscattering cross-sections in
7
Li induced reactions.
Keywords: Self-supporting enriched 116,118 Sn thin films, Ultra-high vacuum deposition, Rutherford Backscattering Spectrometry (RBS), Energy Dispersive X-ray Spectroscopy (EDS), Field Emission Scanning Electron Microscopy (FE-SEM), Fourier Transform Infrared (FTIR) Spectroscopy
1. Introduction Over the last many decades, heavy-ion (HI) induced reactions have captivated nuclear physicists across the globe, and prodigious theoretical and experimental efforts have been made to understand the underlying dynamics. The HI induced fusion reactions play a significant role in nuclear astrophysics, generation of energy in the stellar environment through nucleosynthesis, production of exotic nuclei away from stability in the territory of proton-rich side of mass valley, quest for synthesis of superheavy elements, radiochemistry for national security and development of next-generation nuclear reactors for the production of nuclear energy and transmutation of commercial nuclear waste[1, 2, 3, 4, 5]. On the other hand, HI peripheral reactions are considered as a promising spectroscopic tool to achieve high spin states of the stable as well as complex nuclear configurations, and also provide a sensitive probe for nuclear surface studies[7, 6]. An interesting strand of HI induced reactions is the one involving weakly bound projectiles ( 6,7 Li and 9 Be) at energies around the Coulomb barrier as they offer widespread opportunities to explore different aspects of nuclear structure and reactions[9, 8]. Weakly bound nuclei are characterized by their cluster/halo structure and low breakup thresholds, which makes incomplete/breakup fusion a dominant reaction process. The fusion cross-sections are sensitive to the internal structure of interacting nuclei and coupling to other reaction channels like inelastic excitations, breakup, and *
Corresponding author
Email address: [email protected] (Arshiya Sood) Present address: GSI Helmholtzzentrum für Schwerionenforschung, Planckstrasse 1, 64291 Darmstadt, Germany
1
direct nucleon transfer. The couplings of non-fusion channels substantially modify the effective interaction potential, and leads to the splitting of single, uncoupled fusion barrier into multiple barriers which result in the distribution of barriers. The barrier distribution can be experimentally obtained by two complementary processes - fusion and large angle quasi-elastic (QEL) scattering and are expected to be similar[11, 12, 10]. However, for weakly bound projectiles, the distribution obtained from QEL has been found to be broader than that derived from fusion, and a relative shift in peak has also been observed between the two distributions indicating strong influence of breakup or breakup-like processes on fusion at energies near or below the barrier[14, 15, 13]. To further investigate this discrepancy, experiments have been proposed to derive barrier distributions from fusion and quasi-elastic back-angle 6,7 Li+ 116,118 For precision of data measurements in such experiments, appropriate target thickness, high-quality self- supporting films in sufficient number, homogeneity and well characterized isotopic composition of the films are among the indispensable requirements. Self-supporting and isotopically enriched target films are preferred as they provide reaction products of interest with minimal interference from other impurities. The target film thickness can affect energy resolution of spectra as spread in energy loss for incoming and outgoing particles within the thickness of target foils contributes to the resolution. The target films should be thick enough to obtain appreciable reaction cross-section, but thin enough to achieve better energy resolution so that all the peaks of interest are well separated. Moreover, for barrier distribution measurements, in particular, energy loss in the target film must be less than the energy steps used to carry out the experiment. Therefore, all these factors necessitate the need for self-supporting homogeneous thin target films for the proposed experiments. In the present work, we aimed to fabricate self-support-ing 116 Sn and 118 Sn thin films of areal density in the range of 250-400 μg/cm 2 and 500-700 μg/cm 2 for quasielastic backscattering and fusion cross-section measurements, respectively using different facilities available in the Target Laboratory of Inter-University Accelerator Center (IUAC), New Delhi. Several reports are available in the literature illustrating distinct techniques for fabricating thick and thin tin films either as selfsupporting samples or on a backing [17, 18, 19, 16]. However, thin tin films are reported to be successfully fabricated mostly on Al and C backings[20], and seldom as self-supporting films[18], to the best of our knowledge. Besides aiding basic nuclear physics research to explore various aspects of nuclear reactions and structure, tin thin films have been extensively used to assess applied research domains, including optical, electronic, optoelectronic, and medical. Thin tin dioxide films behave as n-type semiconductors with wide bandgap and have attracted resurgent interest due to their particular properties like high optical transmission and high reflectivity in visible and infra-red range, respectively, and an excellent chemical resistance which can be used to constitute transparent and chemically stable thermal barriers. These films are widely used in solar cells, catalytic support materials, solid-state chemical sensors, and high-capacity lithium-storage[22, 21]. Moreover, tin dioxide films are used as sensors for volatile organic compounds and when combined with nanocarbon materials such as carbon nanotubes and graphene create electronic gas sensors capable of functioning at room temperature[24, 25, 23]. These detectors are efficient in trace detection and find their applications in environmental monitoring, industries, transportation, energy, agriculture, and medical diagnosis[26]. Tin-tellurium and tin-tellurium-phosphide have emerged as a new class of crystalline topological insulators which have proven to be promising materials for widespread applications in spin-electronics and quantum computing[28, 27]. Recently, in the domain of medical sciences, a sensor has been developed for detecting 1-nonanal gas present in the breath of lung cancer patients by combining tin dioxide nanosheets with tin dioxide nanoparticles and noble metal catalysts[29]. In this communication, Section 2 discusses practical techniques used to fabricate self-supporting 116 Sn and 118 Sn thin films. Characterization methods employed to precisely measure thickness and purity level as well as to study the morphology of the films are given in Section 3. The application of fabricated thin films and summary of fabrication and characterization methods are included in Section 4 and 5, respectively.
2. Target Preparation Tin is a post-transition element belonging to group 14, period V, p-block, and is a member of the carbon family of the periodic table. It is a soft, malleable and easily pliable silvery-white metal generally non-toxic except for few organotin compounds that are highly toxic. Tin is known to exist in two allotropic forms - (i) metallic β-tin (white tin) which is stable at room temperature and malleable above it, and (ii) powdery amorphous non-metallic α-tin (gray tin) which is stable below 13.2 D C and brittle[30]. The white tin is transformed into the gray tin when the former is cooled to temperatures below 13.2 D C. This process is termed as tin pest which, however, takes place rather slowly. The white tin has a tetragonal crystal structure with a density of 7.286 g/cm 3 , whereas gray tin has a facecentered cubic structure having a density of 5.765 g/cm 3 . Melting and boiling points of tin are 232.06 D C and 2603 D C, respectively. Tin is usually not affected by water and oxygen at room temperatures, but it tends to react with both at high temperatures to form tin oxide. Tin is amphoteric and reacts with strong bases and acids with evolution of hydrogen. Tin has 10 stable naturally occurring isotopes including 116 Sn (14.54%) and 118 Sn (24.22%). The raw material to fabricate thin films was procured from ISOFLEX USA in the form of 99.95% and 99.6% isotopically enriched metal ingot of 116 Sn and sheet of 118 Sn, respectively. After careful consideration of the aforementioned form and properties of tin, both mechanical reshaping and vacuum thermal deposition were attempted for the fabrication of thin tin films [31]. Over the last few decades, cold rolling technique has been extensively used to fabricate selfsupporting thin/ thick films particularly for expensive isotopically enriched materials. This technique is quite economical in terms of minute amount of starting material required and minimal wastage of material as the oddments left during film fabrication can be put to use in other techniques like vacuum deposition. Owing to these advantages, initially, extensive trials were carried out to fabricate selfsupporting thin films using cold rolling technique, however, limited success was achieved. After repetitive efforts, the focus was then shifted to fabricate thin films using ultra-high vacuum thermal deposition (evaporation-condensation) technique and multiple self-supporting thin films of 116 Sn and 118 Sn were successfully fabricated. The details of extensive trials conducted and practical intricacies involved during the thin film fabrication stages using these techniques are delineated in subsequent sub-sections.
2.1. Cold Rolling Isotopically enriched tin isotopes were rolled using heavy -force exerting Heiss Maschinenbau GmbH ™[32] rolling machine installed in target lab at IUAC. It is a motor controlled rolling machine with a provision to control rotary motion of two 85 mm diameter cylindrical rollers manually by a foot pedal. This machine belongs to two high reversing rolling machine category in which rollers can first rotate in one direction and then in the other so that the material to be rolled can be passed back and forth many times through rollers. In this process, the sheet becomes thinner with each rolling cycle, and finally, a film of uniform surface density and desired thickness can be achieved. Moreover, the lower roller is fixed in the vertical plane whereas the upper roller can be moved up and down using a top-mounted circular lever, thus enabling adjustment of force imparted on the material being rolled. Before rolling enriched 116 Sn and 118 Sn isotopes, the cold rolling technique was tried out with naturally abundant isotope of tin. The rolling of natural tin metal sheet was quite feasible, and areal density of ≈ 2.5 mg/cm 2 could be achieved in a few rolling cycles. Thus, following the methodology practiced to roll natural tin, enriched tin metal sheet (initial areal density ≈ 240 mg/cm 2 ) to be rolled was placed inside a rolling pack made up of a mirror-polished stainless steel sheet folded over symmetrically to form an envelope with three open edges. The rolling pack with the metal sheet in-between was rolled back and forth under a gradually increasing force exerted when the gap between the rollers is reduced using the circular lever. During rolling, care was taken not to over roll
the sheet. Meticulous cleanliness was maintained to avoid appearance of pin-holes in the film due to dust particles, and new packs were used whenever necessary. However, it was observed that when a thickness of ≈ 13 mg/cm 2 was reached cracks started developing around edges of the film. At this point, the rolling force was reduced before opening the pack to avoid any damage to the film like blistering, sticking or breaking. The film was then taken out of the pack carefully and cropped properly to a rough square to restrict any further ingrowth of cracks. The rolling cycles were then repeated with a new rolling pack and a modified pack-rolling technique described by Banerjee et al.[33]. Using this technique, in addition to controlled and gradual increase of force on the rolling pack, the film was rotated 90 D and flipped over after each rolling cycle to flatten all the edges uniformly. The cracked edges were snipped off as and when required. After a thickness of ≈ 3 mg/cm 2 was achieved, the film started sticking on upper face of the rolling pack. Parchment paper was used to peel off the film without damaging it, and also for cropping the cracked edges of the film to avoid its possible adhesion to scissors. After many rolling cycles and adopting all the measures mentioned above, a minimum thickness of ≈ 1.8 mg/cm 2 and area 1.9 cm × 2.6 cm was achieved using this technique. Since for our proposed measurements, thin films in large number were required, therefore, cold rolling was not considered as a viable technique to obtain very thin films of tin. In order to fabricate a sufficient number of thin films required for the experiments, alternate methods of thin film fabrication were explored.
2.2. Ultra-high vacuum thermal deposition Vacuum thermal deposition is an evaporation- condensation technique in which a volatile material to be evaporated is heated to a high temperature in an evacuated chamber followed by condensation of vaporized material on an appropriately chosen substrate. The growth of evaporated films depends on many factors, viz. nature of the substrate and its temperature, contamination from various sources, deposition rate, method of heating material, source-to-substrate distance, and choice of parting agent (particularly for self-supporting thin films)[34]. Figure 1: (top) Ultra-High Vacuum (UHV) evaporator at IUAC New Delhi, and (bottom) top view of arrangement inside UHV chamber for self-supporting thin target foil deposition. To fabricate self-supporting tin thin films, ultra-high vacuum (UHV) evaporator facility of target lab at IUAC, as shown in Figure 1, was used. The evaporation unit consists of a scroll pump, a turbo molecular pump, and a cryopump which aid in maintaining contamination less high vacuum environment for thin-film deposition[35]. It houses a multi-pocket electron gun with 6kW power supply and resistive heating setup. A typical vacuum of ∼ 10−8 mbar can be achieved and maintained throughout the deposition procedure in UHV evaporator. To monitor deposition rate and thickness of the growing film in-situ, a piezo-electric quartz crystal monitor [36], based on the principle of change in vibration frequency of crystal with deposited mass, is provided. Tin isotopes have low abundance resulting in a very high cost of enriched isotopic material, therefore, to ensure the economical usage of this material, the deposition method and parameters were optimized in as many as 10-15 trials with natural tin. The inside view of the UHV unit showing an arrangement for tin thin film deposition is presented in Figure 1. A set of ultrasonically cleaned mirror-polished glass slides were used as a substrate for deposition of tin thin films. After cleaning, the slides were immediately mounted on the substrate holder inside the vacuum chamber to minimize the possibility of pin-holes in thin films due to dust particles. The water-soluble parting agents play a crucial role in the fabrication of self-supporting thin films as they govern the easy separation of film layers from the substrate. The choice of parting agent depends on many factors like their temperature stability, crystal structure and lattice constant, and interference with the experiment being undertaken as they may leave elemental traces. In the present work, (i) alkali halide : NaCl (sodium-chloride), BaCl 2 (barium chloride) and KCl (potassium chloride) , and (ii) Detergent : Teepol were evaluated for use as a parting agent. As per requirement, 3mm thick pellets of alkali halides were prepared by powder pressing with a hydraulic pellet press and were used immediately to avoid appreciable absorption of moisture. Thin films of alkali halide parting agents were prepared using electron beam gun heating of
the pellet placed in copper hearth of multi-pocket e-gun setup in UHV evaporator. A monoatomic layer of Teepol was obtained by putting a drop on center of the glass slides which were then uniformly covered with Teepol by spreading the drop over the entire surface and polished until no visible accumulation was left. After multiple iterations, parameters like deposition rate, source-tosubstrate distance and thickness of deposited films were optimized for different parting agents. Resistive heating technique was used to evaporate natural tin material owing to its melting point. The tin metal sheet was snipped into tiny metal flakes and kept in tube-type tantalum (Ta) boat, which is a refractory metal with a melting point of 3017 D C, for evaporation (see Figure 2(a) and (b)). To precisely optimize different deposition parameters required to achieve the desired thickness of thin films, including the amount of material and distance of substrate from Ta boat, several trials were carried out using natural tin material. Depending on the primary requirements discussed earlier in this section, empirically KCl was found to be the best parting agent for fabrication of self-supporting tin targets. Figure 2: (a) Tantalum (Ta) tube-type boat used for material evaporation, (b) enriched tin metal flakes, (c) tin thin film deposited on glass substrate, (d) scored and floated thin films in distilled water, (e) tin thin film mounted on stainless steel frame, and (f) enriched tin thin films fabricated in single batch of evaporation. After ascertaining different parameters, the deposition with enriched isotopic material was performed using 45-50 mg of source material in every deposition cycle. First, a KCl layer of ≈ 200 nm was deposited at a deposition rate of 0.1-0.5 nm/s on the glass substrates placed at a distance of 10 cm from the pellet kept in the copper hearth of electron beam gun setup. The emission current was applied slowly to assist in outgassing, and when the vacuum had recovered, the evaporation of material was undertaken until the desired thickness was achieved. The maximum emission current for deposition of 200 nm KCl was 1 mA. The thickness of KCl was measured in-situ using the quartz crystal monitor. The chamber was allowed to cool down for sufficient time before venting it with inert Argon gas. After venting, Ta boat with enriched isotopic tin metal flakes was clamped between highcurrent electrodes of resistive heating setup. The substrate holder carrying the glass slides coated with KCl layer was fixed manually on the support rods to place the slides directly above the Ta boat at a distance of 9.5 cm to ensure optimal use of the material and uniform thickness of thin films. The current was ramped up gradually, and the vacuum was continuously monitored. In case of abrupt changes in vacuum, the ramping up of current was interrupted several times during the deposition cycle to allow the vacuum to recover sufficiently. This aided in fabricating thin films of uniform thickness and also helped in reducing the contamination level. The current was increased to a maximum of 225 A at voltage 2 V, and it took ≈ 1 hour to complete the evaporation of enriched material. Vacuum inside the chamber was maintained in the range of 10 −6 - 10 −7 mbar during the complete cycle of deposition of KCl and tin. For instance, an isotopically enriched tin thin film deposited on a glass substrate is shown in Figure 2 (c). The chamber was allowed to cool down for ≈ 8 hours after the deposition was completed and subsequently vented. After each evaporation cycle, every part of the working chamber and support rods were cleaned scrupulously with acetone and propanol to avoid contamination in the successive evaporations. Table 1: Comparison of Sn targets fabricated in the present work with different works reported in the literature Target Target description
Fabrication method
Thickness (mg/cm 2 )
Refs.
116
Sn
Self-supporting
UHV evaporation (Resistive heating)
0.25-0.6
Present work
118
Sn
Self-supporting
UHV evaporation (Resistive heating)
0.25-0.6
Present work
112
Sn
Self-supporting
Cold Rolling
1
[17]
112
Sn
Self-supporting
Cold Rolling
8.4
[19]
116
Sn
Carbon (C) backed
HV evaporation (Resistive heating)
0.15
[20]
124
Sn
Aluminium (Al) backed
HV evaporation (Resistive heating)
0.2-0.35
[37]
112
Sn
Lead (Pb) backed
Cold Rolling
2.44, 10.26
[38]
2.2.1. Separation and Mounting of thin target foils After deposition, the glass slides were cautiously taken out of the chamber to separate the deposited film from the substrate. A cut was scribed on all edges of the film to ensure a better contact with water surface and was scored precisely into appropriate sizes using a diamond tip pen for mounting on stainless steel (SS) target frames with a hole in the center which is carefully deburred on both sides. Since the parting agent used was water-soluble, therefore, distilled water was used to float off the films. The substrate was inclined at an angle of ≈ 45 D to the water surface and immersed slowly into the water as the scored films began to release and started to float on the surface (see Figure 2(d)). To facilitate mounting of a free section of floated film, a suitable SS target frame was lowered beneath it. The film was then gently nudged over to the frame and positioned properly on it. Subsequently, the target frame was cautiously raised keeping it vertical to the water’s surface, with the film adhering to the frame [34]. The mounted films were then kept inside a laminar airflow fume hood to drain excess water on the surface, and when dried, all the frames were stored in a vacuum desiccator. Figure 2 (e) and (f) show the self-supporting enriched tin thin film mounted successfully on the SS frame after floating on the water surface and several thin films fabricated in a single batch of evaporation, respectively. With this technique and optimized deposition parameters, ≈ 53 self-supporting thin films were fabricated in different batches consuming ≈ 250 mg of enriched material in total. The thickness of prepared thin films was found to range between 250-600 μg/cm 2 which may be attributed to variation in material deposition occurring due to different positions of the glass slides with respect to Ta boat in resistive heating setup. It is essential to mention here that even after altering different deposition parameters including the amount of material to be deposited, thin films of thickness greater than 600 μg/cm 2 could not be achieved, and an appreciable amount of material could be found left in the boat. A comprehensive comparison of the present work with reports available in the literature on fabrication of isotopically enriched tin targets is given in Table 1. As apparent from this table, present work is the only detailed report available on fabrication of self-supporting tin thin targets for nuclear physics research, to the best of our knowledge. Further, the fabricated tin targets are compared in Table 2 with other elemental targets, in general, and elements of Group 14, period V, in particular, available in the literature for similar nuclear reaction studies as in the present work. From this comparison, it can be inferred that in the present work self-supporting thin targets have been fabricated successfully considering the requirements to perform high accuracy measurements as discussed in Section 1 of this paper. Table 2: Comparison of Sn targets (present work) with different elemental targets reported in the literature for nuclear reaction studies. Target
Target description
Fabrication method
Thickness (μg/cm 2 )
Refs.
116,118
Self-supporting
Resistive heating
250-600
Present work
Sn
92
Mo
Self-supporting
Cold rolling
5000
[39]
92
Zr
Carbon (C) backed
Resistive heating
80-270
[40]
Carbon (C) backed
Resistive heating
139-260
[41]
130
Te
Gold (Au) backed
Resistive heating
1050
[42]
Self-supporting
Resistive heating
500-800
[43]
Carbon (C) sandwiched
Resistive heating and e-gun
200-250
[44]
Ir, 187 Re, 182 W, 181 Ta
Carbon (C) backed
e-gun
80, 60, 70, 175
[35]
175
Carbon (C) backed
Resistive heating
110, 270
[35]
127
I
206,208 138
Pb
Ba
193
Lu, 169 Tm
3. Characterization 3.1. Purity Analysis and Thickness measurement 3.1.1. Rutherford Backscattering Spectrometry (RBS) RBS is an outstanding ion beam analysis technique that has been widely used by nuclear physicists for over the decades to examine purity and thickness of the targets. Figure 3: RBS spectra of enriched 116 Sn and 118 Sn thin films. (Blue) solid lines through the spectra are SIMNRA simulation fitting to experimental data. The most attractive features of RBS are - the quantitative and accurate measurement of film thickness and impurities within the thin film and, in most cases, with no radiation damage to the film. In order to ascertain the exact thickness and composition of fabricated self-supporting tin thin films used in the online experiments, ion beam analysis was carried out using RBS with a 2 MeV He probe in backscattering configuration at1.7 MV Tandem accelerator (Model 5SDH, National Electrostatics Corporation) at Pelletron Accelerator RBS-AMS (Accelerator Mass Spectroscopy) System (PARAS), IUAC [45]. Data reduction was performed using SIMNRA software [46] by fitting simulations over the experimental data to obtain information regarding areal densities of the films and impurities present, if any. The RBS spectrum for enriched 116 Sn and 118 Sn thin films along with the simulated spectra are presented in Figure 3. It is evident from the figure that no heavy-impurities and a negligible amount of trace-impurities like carbon and oxygen were present in 116 Sn and 118 Sn thin films. It is pertinent to mention here that RBS measurements of the enriched tin thin films have been carried out after a span of several weeks of their preparation and irradiation, which signifies that these films are not prone to oxidation. The areal density of the thin films obtained from the SIMNRA simulation fitting is ≈ 430 μ g/cm 2 and ≈ 380 μ g/cm 2 for 116 Sn and 118 Sn, respectively.
3.1.2. Alpha-transmission method Multiple self-supporting tin thin films were fabricated to perform another set of offline experiments to measure fusion excitation function employing recoil-catcher activation technique. The thickness of these thin films, including the ones used in the online experiment to measure quasi-elastic back scattering, was determined using alpha-transmission method [47].The alpha particles of 5486 KeV from 241 Am source were used to estimate the thickness of thin films by measuring the energy loss suffered by them while traversing through the films. The energy of alpha-particles was measured using a Silicon Surface Barrier Detector (SSBD) before and after transmission through thin films. For instance, the energy spectra of alpha-particles obtained from sample-less (blank) frame and after transmission through irradiated 116 Sn and 118 Sn thin films are shown in Figure 4 (a) and (b), respectively. As evident, all the spectra were best fitted with a Gaussian distribution. The centroids obtained from the fitting for blank, 116 Sn and 118 Sn thin films are 5486 KeV, 5367 KeV and 5385 KeV, respectively. The stopping power for 5486 KeV alpha particles in 116 Sn and 118 Sn material, calculated using SRIM[48], is 0.318 and 0.312 KeV/(μg/cm 2 ), respectively. Consequently, the energy shift observed from the respective centroids divided by the stopping power of the materials
was used to determine the thickness of self-supporting thin films. The thickness of these films was found to be ≈ 374 μg/cm 2 and ≈ 325 μg/cm 2 for 116 Sn and 118 Sn, respectively, and is in reasonable agreement (13 % ) with the RBS measurements. Moreover, the thickness of thin films from each prepared batch was measured using this technique and was found to vary within 250-600 μ g/cm 2
, which met our requirements.
3.1.3. Energy Dispersive X-ray Spectroscopy (EDS) To substantiate the elemental composition results obtained from RBS, EDS measurements were performed for the self- supporting enriched tin thin films. EDS is a sensitive probe to obtain a qualitative assessment of elemental composition in thin films and quantitative information of the elements present. Since every element has characteristic X-rays associated with it, therefore, EDS employs a finely focused electron beam to excite atoms on surface of the film, making it possible to identify and quantify the elemental composition from the emitted X-rays. In the present work, EDS measurements of tin thin films were performed using JEOL’s 7610F Field Emission Scanning Electron Microscope (FE-SEM) equipped with EDAX’s EDS at IUAC[49]. The EDS elemental analysis of 116 Sn and 118 Sn thin films before and after irradiation is shown in Figure 5. As can be observed from this figure, elements C and O are insignificantly present in the thin films with weight % of 2.62 and 0.76, respectively, relative to 96.63 of 116 Sn, and 3.1 and 0.77, respectively, relative to 96.13 of 118 Sn. Figure 4: Energy calibrated alpha-particle transmission spectra obtained with and without targets of (a) 116 Sn and (b) 118 Sn isotopes. Red lines through the spectra are Gaussian fit to the data. The EDS measurements corroborate the results obtained from RBS, indicating the purity of fabricated self-supporting tin thin films. Also, no appreciable difference has been observed in the EDX spectra taken before and after the experiment which manifests the stability of thin films under energetic ion-beam irradiations. Figure 5: EDS elemental analysis of enriched 116 Sn and 118 Sn thin films before and after irradiation. Characteristics X-rays of different elements identified in the analysis are marked.
3.1.4. Fourier Transform Infrared (FTIR) Spectroscopy Fourier Transform Infrared (FTIR) Spectroscopy is a novel analytical technique that allows closeexamination of sample’s chemical composition. In this technique absorption of various infrared light wavelengths by different functional groups is measured. These absorption bands manifest the presence of specific molecular components in the sample. In order to have better insights into purity and composition, we examined the presence of chemical bonding in fabricated selfsupporting Sn thin films by FTIR Spectrometer in the mid-infrared frequency range of 400 to 4000 cm −1 . The samples used for FTIR studies were in the form of self-supporting thin films mounted on stainless steel frames as shown in Figure 2(e). The characteristic FTIR spectra of pristine and 7 Li irradiated 116,118 Sn thin films are shown in Figure 6 (a) and (b), respectively. The absorption peaks centered at 603 cm −1 (pristine) and 616 cm −1 (irradiated) for 116 Sn and 615 cm −1 (pristine) and 623 cm −1 (irradiated) for 118 Sn thin films can be attributed to the Sn-O stretching vibrations [50]. The very weak peaks observed between 2280-2400 cm −1 have their probable origin from reaction with carbon dioxide in air [51]. Since peak intensity in infrared spectra depends on concentration of molecules present in a sample, the feebly intense peaks observed for Sn-O lattice vibration in Figure 6 indicate negligible presence of contaminants like C or O, and corroborate the findings of RBS and EDS characterizations. It is important to mention here that the FTIR analysis of self-supporting tin thin films is done after months of their fabrication which not only establishes the purity of films but also reinstates that these films are not prone to oxidation.
Figure 6: FTIR spectra of (a) 116 Sn and (b) 118 Sn thin films before and after irradiation. In the insets feebly intense Sn-O stretching vibration peaks observed around 624 cm −1 are zoomed out for better visualization.
3.2. Morphology In order to study the morphological changes in tin thin films after irradiation with 7 Li beam, the surface morphology of pristine and irradiated films were studied using FE-SEM at IUAC. The SEM images for pristine and irradiated 116 Sn and 118 Sn films obtained at an accelerating voltage of 15kV are shown in Figure 5 (a) and (b), and (c) and (d), respectively. The 116 Sn film was irradiated with high energy ion beam fluence of 1.25× 10 15 ions/cm 2 , and 118 Sn film at a fluence of 8× 10 14 ions/cm 2 . As evident from Figure 5, granules in the pristine film appear to melt slightly, form bigger granules and agglomerate after ion beam irradiation at high fluences. However, this does not have appreciable effects on the structural integrity and physical stability of the films which reinstates the fact that tin remains undefiled in the irradiated samples. Figure 5: SEM images for pristine (P) and irradiated (I) thin films (top) to study the morphological changes after irradiation with 7 Li ion beam.
116
Sn and (bottom)
118
Sn
4. Application The fabricated 116 Sn and 118 Sn thin films have been successfully used as a target in online experiments to study quasi-elastic (QEL) backscattering in 7 Li+ 116,118 Sn systems from sub- to abovebarrier energies. For weakly bound systems 7 Li, QEL scattering includes breakup ( 7 Li → α + t ), elastic, inealstic and transfer process. The experiments were performed in General Purpose Scattering Chamber (GPSC) with beams of 7 Li in the energy range 15-29 MeV from Pelletron accelerator (Model 15UD, National Electrostatics Corporation) at IUAC. Beam current was maintained between 2-3 nA throughout the experiment. To detect and identify the charged particles produced in the reactions, HYbrid Telescope ARray (HYTAR) detector facility was used[52]. The quasi-elastic backscattering events were measured using four telescopes arranged in symmetrical cone geometry, two in- and out- of the plane each, at an angle of 173 D . Additionally, six telescopes were placed at angles +60 D to +160 D and three telescopes were placed at angles -36 D to -60 D with an angular separation of 20 D and 12 D , respectively. Figure 6: (top) Experimental set-up for quasi-elastic scattering measurement at IUAC New Delhi, and (bottom) Δ E-E res spectrum at θlab = +140 D for 7 Li+ 118 Sn at E lab = 21 MeV. Two SSBD monitor detectors were placed at ± 10 D with respect to the beam direction for normalization and beam monitoring purposes. The experimental set-up is shown in Figure 6(top). For instance, a typical two-dimensional Δ E-E residual spectrum obtained from the telescope at +140 D for 7
Li+ 118 Sn system is presented in Figure 6 (bottom). The events corresponding to multiple elastic + inelastic ( 7 Li) scattering and breakup (α) processes at E lab = 21 MeV are marked as Z=3 and Z=2, respectively.These experiments act as a sensitive probe to determine the purity of samples as any likely uptake of residual gas contamination like, C, O or N during target fabrication would have severely meddled with our actual measurements, and appeared as a separate lobe in Z=3 band in Figure 8(bottom). Moreover, to further understand the effect of impurities, if any, on the present analysis, the projection of Z=3 and Z=2 band on total energy axis is shown in Figure 9. The observed energy peaks of our interest for Z=3 and Z=2 are events are also checked with the kinematics. In this figure, the Z=3 band peaks at 16.57 MeV which agrees reasonably well with the value 16.94 MeV obtained from the kinematics. Also, it has been calculated that the breakup alpha peaks from the contamination of 12 C, 14 N and/or 16 O fall below 6 MeV. While for reaction with 118 Sn the breakup alphas peak at 11 MeV which is in agreement with the experimental
measurements given in Figure 9. These sensitive measurements corroborate that there are no traces of contamination in the fabricated films as 7 Li and α particles are purely produced from the reactions of 7 Li with 118 Sn in accordance with the kinematical calculations[53]. Figure 7: Projection of Z=2 and Z=3 events of Figure6 (bottom) on the total energy axis.
5. Summary Isotopically enriched multiple self-supporting thin films of 116,118 Sn with areal density ≈ 250-600 μg/cm 2 were successfully fabricated using ultra-high vacuum thermal deposition technique. For two sets of experiments to derive barrier distributions from fusion and quasi-elastic backscattering crosssection measurements, thin films of mentioned areal density range were required in large numbers. The cold rolling technique was attempted to fabricate the required thin films. Areal density of ≈ 1.8 mg/cm 2 was achieved by rolling down enriched isotope metal sheet of ≈ 241 mg/cm 2 . However, after repetitive trials, it was concluded that cold rolling was not a viable technique to obtain very thin films of tin. In order to fabricate a sufficient number of thin films required for the experiments, an alternate technique of high vacuum deposition was used. To optimize different deposition parameters, multiple iterations were carried out by using various parting agents, including alkali halides and detergent, and naturally abundant tin isotope. Moreover, quite a few attempts were also made to fabricate thin films of areal density higher than 600 μ g/cm 2 by varying different parameters including the amount of material used for evaporation. It was observed that an appreciable amount of material residue was left behind in the boat after evaporation, and the thin target foils of required areal density could not be fabricated. Purity and stability of fabricated self-supporting tin thin films were established by various characterization techniques viz., RBS, EDS, FTIR and FE-SEM. It was observed that negligible amount of trace- and no heavy- impurities were present in the fabricated thin films even after irradiation and weeks of their preparation. Thickness of the films was measured using RBS and alpha transmission methods. The fabricated films were successfully used as targets in online experiments to measure quasi-elastic backscattering cross-sections in 7 Li+ 116,118 Sn reactions. The experiments were performed to study the effect of the internal structure of colliding nuclei and coupling of different reaction channels on fusion from sub- to above- barrier energies by deriving barrier distributions from the cross-section data. The convincing quality of data obtained in these experiments validates the purity of these films and bolsters the importance of using self-supporting thin films in such experiments.
Acknowledgment The authors are grateful to Director, IUAC for providing all necessary facilities required to carry out the present work, including FE-SEM facility, funded by MoES under Geochronology project. One of the authors A.S. acknowledges Indian Institute of Technology Ropar, India for funding her research work, Dr.Akhil Jhingan for his useful suggestions and discussions, Dr. Subhendu Sarkar for his inputs in characterization section of this article and Dr. Saif A. Khan for EDS measurements of natural tin samples. P.P.S. acknowledges a startup grant from IIT Ropar for purchasing the isotopically enriched material required for the thin film fabrication.
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Highlights 1. Several 116,118Sn self-supporting thin films of areal density ≈250-600 μg/cm2 have been fabricated using ultra-high vacuum evaporation facility at Inter-university Accelerator Centre, New Delhi. 2. Cold rolling technique has also been attempted to fabricate thin tin films. After several rolling cycles, an enriched metal sheet of ≈240 mg/cm2 could be rolled down to a minimum of ≈1.8 mg/cm2. 3. Prepared thin films have been successfully used as targets in online experiments to study quasi-elastic backscattering in 7Li induced reaction. The convincing quality of experimental data obtained signifies the importance of self-supporting thin films for such experiments. 4. Results of various advanced characterization techniques – RBS, EDS, FT-IR and SEM point towards purity and stability of these thin films in high fluence ion beam nuclear physics experiments.
Declaration of interests 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. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: