4.3 Implanted Targets
4.3.4 Implanted Target Yields
Na - Ta Targets
Early tests showed thatENa = 50 keV would be a sufficient implantation energy to meet the
required target thickness. The yield curve of an implanted Na - Ta target is shown in Figure 4.5. Two rounds of implantation occurred on this target, both atENa= 50 keV. The estimated
total dose collected on the target was 0.5×1018atoms/cm2 after both implantations. Between the implantations, a yield curve was measured, testing the stoichiometry and thickness (Test #1). After the second implantation another yield curve was measured (Test #2). As a way to test the target for stability, the target was bombarded by a beam of Elabp = 150 keV protons with a current of ∼50 µA for an accumulated charge of 1 C. A subsequent yield curve was measured (Test #3).
The yield curves of the Na - Ta targets were very similar to those of Reference [Seu87], displaying a high concentration of23Na at the surface of the target as seen in Figure4.5. The
maximum stoichiometry of this target was estimated after the first implantation, yielding NNa:NTa = 1 : 6.5 (excluding the surface peak). An additional implantation was performed
in hopes of increasing the23Na concentration. Between the first and second tests, the surface
concentration increased with the additional implantation, but the concentration decreased beyond the surface, suggesting that23Na was migrating within the target. After the stability
test, the surface concentration increased from additional beam charge collected on the target, which suggested that even under modest beam currents,23Na was migrating within the target. Other targets of 23Na implanted into Ta showed similar results. Ultimately, Na - Ta lacked the23Na concentration and the uniformity necessary. Therefore, they were not used for the experiment.
Na - Ni Targets
Figure 4.6shows yield curves of 23Na implanted into a Ni backing. It was implanted a total of three times, the first and second implantations with ENa = 50 keV beam and the third
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Test #1 Test #2 Test #3Figure 4.5: Measured yield curves of the Erlab = 309 keV resonance in 23Na(p, γ)24Mg for a target fabricated by implanting Na ions into a Ta backing. Test #1 was performed after the initial implantation. Test #2 was performed after a second implantation of roughly same dose and energy as the first implantation. Test #3 was performed after prolonged beam bombardment to study the target degradation using a proton beam ofEplab= 150 keV energy and 50µA intensity. Notice the peak in Na concentration at the target surface. It increased in magnitude after proton bombardment, indicating that Na ions diffused to the surface.
#1), was not uniform throughout the thickness of target, which suggested that saturation had not been reached. We expected this since previous work showed no saturation of 23Na
implanted in Ni [Seu87]. The stoichiometry was calculated based on the maximum yield resulting in a concentration of NNa:NNi = 1 : 1. Another implantation was performed with
the hopes of increasing the concentration in the high energy region of the yield curve. The second yield curve (Test #2) showed the maximum yield stayed constant, and an increased concentration of target material in the high energy region, but the thickness of the target was also increased. Unlike the Ta case, target material seemed to migrate further into the target rather than towards the surface. A final implantation was performed at half the initial energy, ENa= 25 keV to again see increase concentration of the high energy region of the yield curve,
thus increasing the uniformity. Unfortunately, the yield curve shows (Test #3) significant damage to the target in terms of sputtering. The stoichometry at maximum decreased to NNa : NNi = 2 : 3. Table 4.8 displays the characteristics of targets implanted into a Ni
backing. Results of the Na - Ni implantations did not correspond with the results of those in Reference [Seu87]. Since a uniform yield could not be achieved, these targets were not suitable for use in the experiment.
Table 4.8: Experimental Na - Ni implantation parameters and stoichiometries. Implantation ENa Charge Dose Stoichiometry
(keV) (C) (×1018 atoms/cm2) NNa:NNi 1 50 0.67 0.83 1 : 1 2 50 0.26 0.32 1 : 1 3 25 0.12 0.15 2 : 3 total 1.05 1.30 Na - Cu Targets
Implantation of23Na into Cu showed similar results of those from23Na implanted into Ta, very low23Na yield, but high concentration at the surface of the target. Figure4.7shows the mea- sured yield curves for a target, which was implanted only once at an energy ofENa= 50 keV
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Test #1 Test #2 Test #3Figure 4.6: Measured yield curves of the Erlab = 309 keV resonance in23Na(p, γ)24Mg for a target fabricated by implanting Na ions into a Ni backing. Test #1 was performed after an initial implantation atENa= 50 keV. Test #2 was performed after a subsequent implantation
at ENa = 50 keV. Test #3 was performed after a final implantation at ENa = 25 keV, which
utively, i.e., Test #1 was performed after implantation, Test #2 was performed immediately after Test #1, and Test #3 was performed immediately after Test #2. The stoichiometry was calculated based on the maximum yield after Test #1, resulting inNNa :NCu= 1 : 16. If an
implantation efficiency of 100% is assumed, the Cu target was implanted 24 times the amount required to obtain a stoichiometry of NNa:NCu = 1 : 16. This stoichiometry was drastically
different from the theoretically expected stoichiometry of NNa : NTa = 3 : 2. Clearly, im-
planted material was sputtered away during implantation and the implantation efficiency was very low. After Test #1 there was no evidence for a higher concentration of target material at the surface of the target. However, after only modest charge collected on the target (< 0.2 C), a surface peak became apparent after Test #2. Test #3 showed an increase in the surface concentration after more beam charge was collected on target. The migration of23Na within Cu was apparent as was the case for Ta. The low yield and non-uniform Na profile in Cu were not be suitable for use in this experiment.
Na - NiTa targets
Since the results of Reference [Seu87], could not be reproduced for Na implanted into thick Ni foils, an attempt was made to implant into an evaporated Ni layer on a Ta sheet, as was done in the previous work. Reference [Seu87] states that they implanted 23Na into “thick
layers evaporated on Ta sheets.” No other information was provided about the evaporated thickness of Ni. To estimate the thickness of the evaporated Ni layer needed, sputtering was taken into account. Sputtering information for Ni was found in Reference [Mat84]. Although sputtering of 23Na in Ni was not listed, sputtering of Ne in Ni was included. To first order,
since Na and Ne have similar masses, Ne was assumed to estimate the sputtering of Na in Ni. For23Na implanted at an energy ofENa = 50 keV for 0.26 C, the sputtering ratio yielded 1
Ni atom to 1 Na ion, corresponding to 35 nm of Ni material, thus sufficiently thick layers of Ni (i.e., 100 – 150 nm) were needed.
Nickel was electron-beam evaporated onto Ta at the Shared Materials Instrumentation Facility (SMiF) at Duke University. Two NiTa target backings were produced with Ni evap- orated thicknesses of 125 nm. A maximum thickness of 125 nm was advised by the SMiF
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Test #1 Test #2 Test #3Figure 4.7: Measured yield curves of the Erlab = 309 keV resonance in 23Na(p, γ)24Mg for a target fabricated by implanting Na ions into a Cu backing. The backing was implanted into once atENa = 50 keV. The yield curve tests shown were performed consecutively on the
target with no further implantation into the target. The target was not stable and became nonuniform under modest amounts of proton beam current and charge collected.
technician to prevent the evaporated layer from peeling or cracking.
Figure 4.8 shows the yield curves of a Na - NiTa target. The 23Na was implanted into
NiTa a total of three times. The first two implantations were performed at ENa = 50 keV
and the last atENa= 70 keV. The yield curves after the first implantation (Test #1) showed
a target structure very similar to that for the Ni foil backing. The second implantation was performed to see if the concentration in the high energy region of the yield curve would increase. The yield curve (Test #2) concentration did increase slightly, but not by a sizable amount considering that the dose was doubled. After the third implantation, the yield curve (Test #3) showed substantial reduction of the maximum yield, attributable to sputtering of the target material.
During the course of this work in October of 2010, a paper was published by Brown et al. [Bro09] at the University of Washington on the study of 23Na implanted targets. Their
goal was much like ours, to analyze the structure and stability of 23Na implanted into Ta,
Ni, and Cu. They obtained very similar results to our Ta results. For their Ni results, they did not see a long, high-energy tail on their yield curves. However they implanted at a lower energy ofENa= 30 keV. For Cu, they did not see a high concentration of23Na at the surface
of their targets after accumulating beam charge, but their implantation energy was lower at ENa= 10 – 30 keV. A major difference between these works was the amount of beam current
used for the implantation. At times, the beam current was as high as 50µA for this study, compared to their currents of 30 – 80 nA.