Alpha spectrometric characterization of thin ²³³U sources for ^229(m)Th production

2.1 Stopping power in materials

The stopping powers of the alpha particles and the recoil daughter ions differ by about one order of magnitude. Furthermore, they depend on the chemical nature of the mother nuclide material. For the example nuclide ²³³U (half-life: 1.59 × 10⁵ a, E_α: 4824 keV (82.7%), 4783 keV (14.9%), 4729 keV (1.85%)), the kinetic energy E_k of the ²²⁹Th ions is about 84 keV while the alpha particle energy is about 4.8 MeV [derived from E_k (²²⁹Th) = Q_α (²³³U)–E_α(²³³U), where Q_α is the Q-value of the alpha decay and E_α the kinetic energy of the alpha particle]. The stopping powers and ranges in different materials can easily be calculated with SRIM-2013 [24]. Due to the fact that the exact uranium compound is only known for DoD-produced sources, but not for the others, a number of different uranium compounds were used for the SRIM simulations. The stopping powers and ranges of both the ²²⁹Th and the alpha particles in these compounds are given in Table 1 and Table 2. SRIM defines a monolayer of any material as 10¹⁵ atoms cm⁻². Therefore, the stopping powers describe the energetic loss per atomic layer. The data show how much more the ²²⁹Th recoil ions are influenced by the mother nuclide material with ranges below 500 Å compared to the alpha particles with ranges above 10 µm in the same materials. Thus, alpha spectra of ²³³U recoil ion sources cannot give detailed information on the kinetic energy distribution of the recoil ions. Even very thin sources with 10 atomic layers of the mother nuclide material decelerate the emitted recoil ions by several keV. For a first semi-quantitative screening, a high-resolution alpha spectrum of an uranium recoil ion source allowing for a detailed evaluation of the peak shape may give a hint about the thickness of the source, which can help determining the recoil efficiency and energy distribution of the thorium ions.

2.2 Theoretical areal density

As previously mentioned, a single layer of any material is defined in SRIM-2013 as 10¹⁵ atoms cm⁻², which is too crude for an approximation of all elements in one layer in order to be directly applicable in the present case. As an example, an areal density of about 2.23 × 10¹⁴ atoms cm⁻² is calculated for a monolayer of uranium atoms in uranium dioxide. The areal uranium densities of this and other materials are given in Table 3. As the true chemical species and crystal structure is difficult to investigate for sources prepared by MP and DoD, a value of 10¹⁴ atoms cm⁻² will be used for further discussions and for the calculation of the concentration of solutions as used in the experimental section. The estimation of an areal uranium density gets more complicated for methods like chelation by sulfonic acid groups and SA, because there the areal uranium densities depend on the density and availability of binding partners on the surface of the substrate.

For the chelation method, the areal density of sulfonic acid groups is important. An areal uranium density of about 4 × 10¹⁵ atoms cm⁻² is achievable on PIPS detectors functionalized in that way [13]. For SA, the density of oxygen binding partners on the surface of the substrate influences the amount of adsorbed uranium atoms. Investigations in the field of nuclear waste disposal have shown the highest adsorption rate of uranium for titanium dioxide [17]. These investigations were always performed with titanium dioxide colloids, whereas source fabrication is performed on a titanium foil, which provides a lower density of oxygen binding partners than colloids do. To increase the source activity, the density of these binding partners can be increased, e.g., by passivation. There are several methods to achieve titanium passivation, e.g., by anodic oxidation or by heating [27]. Thermal treatment allows investigating the adsorption yield (and thus the achievable areal uranium density) by specifically producing one of two obtainable titanium dioxide modifications. This is carried out by the irreversible transformation of anatase→rutile at temperatures >600 °C at atmospheric pressure [28], [27]. The exact transformation temperature depends on impurities in the titanium. D. Velten et al. found transformations for amorphous→anatase and anatase→rutile in sol-gel powder samples after heat treatment [27]. Anatase, the modification found naturally on a passivation layer of titanium, is the most promising species for high adsorption yields. The areal oxygen density of both, anatase and rutile was calculated for some ideal crystal faces to derive an average number. Calculations based on crystal structure data [26], [29], [30] are given in Table 4. The mean areal density of oxygen atoms on the surface of anatase and rutile is similar and is about 1.3 × 10¹⁵ atoms cm⁻². Due to the large ionic radius and coordination sphere of uranyl ions in carbonate solutions [14], [17], the uranyl ion will be coordinated to two oxygen atoms on the surface. This leads to an U:O ratio of 1:3–1:2, corresponding to a maximum areal uranium density of 6.5 × 10¹⁴ atoms cm⁻² with SA. The real areal density of adsorbed uranium is probably lower due to defects in the crystal surface. 10¹⁵ atoms cm⁻² of ²³³U have an activity of 138 Bq cm⁻², corresponding to an emission rate of ²²⁹Th ions into a 2π solid angle of about 69 ions s⁻¹cm⁻².

4.1 General instrumentation and methods

A light microscope (Bresser LCD Micro, 40x–1600× magnification) was used for visual inspection of the sources. Scanning electron microscopy (SEM) was performed with a Philips XL 30 for the investigation of the topography of the sources in high resolution. Autoradiographic images (RI) weren taken with a Fujifilm FLA-7000 for information about the spacial distribution of radioactivity. Atomic force microscopy (AFM) was performed with a Semilab DS95 SPM System for the investigation of the topology (root mean square roughness) of the sources. For alpha spectrometry, different PIPS detectors and source-detector-distances were used for quantitative yield determination and for qualitative measurements. These are specified for each method in the following sections. A 16 K channel analog-digital-converter (Canberra Nuclear Data ND581 ADC, used in 1 K and 2 K mode) with amplifier (Canberra 2015A AMP/TSCA) was used for data acquisition. A certified ²⁴¹Am source (Amersham Buchler, 290 Bq homogeneously distributed on an anodized aluminum disc with 16 mm in diameter and 150 µm thickness) was used for efficiency calibration of the used PIPS detectors.

4.2 Molecular plating

4.3 Drop-on-demand inkjet printing

4.3.3 Source fabrication and characterization

Five sources were fabricated with a printing sequence containing 1413 drops within a circular area of 30 mm in diameter (see Figure 2(a)). For this, the 74.2 μg mL⁻¹ solution and a drop volume of 5 nL at a stroke velocity of 100 μm ms⁻¹ were used. Two additional sources were fabricated with a printing sequence containing 5637 drops within a circular area of 30 mm in diameter (see Figure 2(b)). Drop volume and stroke velocity were kept the same at 5 nL and 100 μm ms⁻¹, but the solutions with concentrations of 18.6 μg mL⁻¹ and 138 μg mL⁻¹ were used for these latter two sources. The sources were dried at air in the fume hood after fabrication. Alpha spectra, SEM pictures and RI were processed as described in Section 4.2.3.

Figure 2:

Printing sequences used for source fabrication. Panel a: Sequence containing 1413 coordinates. Panel b: Sequence containing 5637 coordinates. The distances between two coordinates are 0.7 mm (a) and 0.35 mm (b), respectively.

4.4 Chelation by sulfonic acid groups

4.5 Self-adsorption

4.6 Investigation of the Th daughter recoil efficiency

5.1 Molecular plating

The substrates, plating durations, deposit areas and quantitatively determined areal density of the MP sources are given in Table 5. The deposition area had a shiny surface. No material layer was observable with a light microscope. The qualitative alpha spectrum of source MP2 is shown in Figure 3 together with an RI and a SEM picture. The three peaks of the ²³³U multiplet are well resolved. The RI shows some inhomogeneities of the areal activity distribution. The surface of both, the coated wafer and the deposition area, look very smooth in the SEM picture except for some residues of the solvent.

Figure 3:

Qualitative alpha spectrum including a multiplet peak fit with confidence band (a), RI (b) and SEM picture (c) of a molecular plated ²³³U recoil ion source (MP2). The source was fabricated on a titanium-coated silicon wafer from a mixture of 10% isopropanol and 90% isobutanol. Irradiation time for the RI was about 2 h.

5.2 Drop-on-demand inkjet printing

The number of drops, drop volume, printed area and yield of the DoD sources are given in Table 6. The droplets dried within seconds during the printing process. No residues were observable with a light microscope. The qualitative alpha spectrum of the printed source DoD1 containing 1413 drops as well as its RI and SEM picture are given in Figure 4. The three peaks are resolved. The RI shows a macroscopically homogeneous areal distribution of the activity. The size of the deposits varies within a range of about 200 µm due to surface effects of the titanium foil. The deposits of the single drops are faintly visible in the SEM picture thanks to a high height contrast by the rough surface of the titanium foil.

Figure 4:

Qualitative alpha spectrum (a) of a DoD-printed ²³³U recoil ion source (DoD1) on titanium foil as well as RI (b) and SEM picture (c). Irradiation time for the RI was about 2 h.

5.3 Chelation by sulfonic acid groups

The contact times, areas of the wafer pieces and areal densities of the chelated ²³³U deduced from the quantitative alpha spectra are given in Table 7. No visible change occurred on the surface of the wafer pieces due to the fabrication process. The qualitative alpha spectrum of the source Ch6 including the multiplet peak fit is given in Figure 5. Due to the very low areal density on the wafers, the count rate is very low. The three peaks are well-resolved and very narrow. The RI of the same source in Figure 5 shows a homogeneous, yet not very dense areal distribution of the activity.

Figure 5:

Qualitative alpha spectrum (a) of a ²³³U recoil ion source (Ch6) fabricated by chelation on a functionalized silicon wafer piece as well as RI (b) and SEM picture (c). Irradiation time for the RI was about 1 h.

5.4 Self-adsorption

Figure 6 shows the ²³³U density of the SA sources as functions of the pH value of the solution (panel a) and the pretreatment temperature (panel b). The maximum yield was reached from a solution with a pH value of about 5, resulting in about 40 ng cm⁻² (10¹⁴ atoms cm⁻²) on thermally oxidized titanium foils at 500 °C. This was increased to about 80 ng cm⁻² (2 × 10¹⁴ atoms cm⁻²) by changing the pretreatment temperature of the titanium foils to 450 °C. Sand-blasted titanium foils showed 270 ng cm⁻² (7 × 10¹⁴ atoms cm⁻²) of ²³³U when prepared under the same conditions. To inspect a potentially big influence of different surface roughnesses, the corresponding values are listed in Table 8.

Figure 6:

(a) Areal density of ²³³U atoms on Ti foil thermally oxidized at 500 °C for 1 h as a function of pH value. (b) Areal density of ²³³U atoms obtained at pH 5 on Ti foils as a function of the oxidation temperature applied before the adsorption exposure.

The parameters for fabrication and yields of the SA sources are given in Table 9. No visible change occurred on the surface of the titanium foils after fabrication. The qualitative alpha spectrum of a source on a non-sand-blasted titanium foil (SA1) including the multiplet peak fit is given in Figure 7(a). The two peaks at lower energies are visible as shoulders of the peak at 4824 keV and are not well-resolved. The RI in Figure 7(b) shows a homogeneous areal distribution of activity. In the SEM picture in Figure 7(c), the height contrast of the titanium foil dominates, as this has a quite rough surface even without sand-blasting. No signal originating from the ²³³U can be seen. The qualitative alpha spectrum of a source on a sand-blasted titanium foil (SA3) as well as its RI and SEM pictures are given in Figure 8. The left shoulder of the main alpha peak at 4824 keV is larger and the two peaks at lower energies can barely be identified. RI shows a quite homogeneous areal distribution of the activity but also shows artifacts due to the very rough surface structure of the foil, which is clearly visible in the SEM picture.

Figure 7:

Qualitative alpha spectrum (a) as well as RI (b) and SEM picture (c) of a ²³³U recoil ion source (SA1) fabricated by self-adsorption on a smooth titanium foil thermally oxidized at 450 °C for 1 h. Irradiation time for the RI was about 2 h.

Figure 8:

Qualitative alpha spectrum (a) as well as RI (b) and SEM picture (c) of a ²³³U recoil ion source (SA3) fabricated by self-adsorption on a sand-blasted titanium foil thermally oxidized at 450 °C for 1 h. Irradiation time for the RI was about 2 h.

5.5 Comparison of fitting results of all four methods

A complete list including all results of the fit parameters is given in Table 11 in the appendix. The average values of the most significant parameters, η₁ and τ₁, are plotted against the average areal ²³³U density for each fabrication method in Figure 9. The fit parameter σ₁ is also significant for the qualitative evaluation, but is identical within error bars for all fabrication methods. Therefore, σ₁ is not included in the plots. The other parameters, σ₂ and τ₂, are less significant and have in general much higher values (see Table 11 in the appendix).

Figure 9:

Plotted average fit parameters η₁ (left) and τ₁ (right) for each fabrication method against the average areal ²³³U density of the sources.

5.6 Recoil efficiency of MP and SA prepared sources

The active areas of both fabricated sources were visible with the naked eye. The contact area of the SA source turned from copper-colored to violet and the deposition of the MP source had a bright yellow color. In contrast, no visible change occurred on the catcher foils. The geometric detection efficiency of the catcher foils was determined by Monte-Carlo simulations with AASI for the quantitative measurements. The determined ²³²U source activities, the parameters and yields for the ²²⁸Th recoil collection and the resulting recoil efficiencies are given in Table 10. The error of the recoil efficiency was calculated by a Gaussian error propagation, using the counting errors of the quantitative measurements as well as the errors of the geometric efficiencies for recoil collection and alpha detection. In Figure 10, the qualitative spectra as well as the RI of the SA source and the corresponding catcher foil are depicted.

Figure 10:

Qualitative alpha spectrum (a) and RI (b) of the ²³²U SA source and the corresponding ²²⁸Th recoil catcher foil. The spectra were measured for 1 h in case of the SA source and 5 h for the corresponding catcher foil, to obtain sufficient counts. The additional line at 5685 keV belongs to ²²⁴Ra, the daughter nuclide of ²²⁸Th. Irradiation time for the RI was about 15 min for the SA source and 2 h for the catcher foil.

7.1 AASI simulations

The simulation results of Figure 11 show that it is not simple to differentiate between a ²³³U source containing a monolayer and one with 10 atomic layers based on the recorded alpha spectra. The difference can be seen by an energy shift of the peak maximum in the fit evaluation, because the error of the fitted peak position μ is about 0.5 keV. Both simulated spectra have the same shape, except for a slight shift of one bin (5 keV) to lower energies in case of the alpha source with 10 atomic layers. The systematic uncertainty of the alpha spectra collected with our used alpha-spectrometry setup is estimated to be about 10 keV; we are therefore not able to extract any information on the number of atomic layers from the peak positions in our experimental spectra. The Monte-Carlo simulations suggest this shift to occur predominantly because of alpha particles originating from inside the source material and less of these occurring from the top-most layers, as inferred from the fact that the number of decays did not increase proportionally to the number of atomic layers. At 20–30 atomic layers, the three single peaks are increasingly less well-resolved. At 50 atomic layers, only a single broad peak is visible. This indicates that with an increasing number of atomic layers in the source material, the alpha peak becomes broader, corresponding to an increase of σ₁ from (4.8 ± 0.1) keV–(14.4 ± 1.0) keV.

The simulation results depicted in Figure 12 show a significant increase of the tailing with increasing RMS roughness. Also in this case it is not possible to differentiate between a source with a smooth surface and one with an RMS roughness of 8.5 nm with τ₁ at about 10 keV. The influence only becomes significant at higher RMS roughnesses. At 17 nm RMS roughness, the three single peaks start to merge due to a higher tailing τ₁ of about (16.1 ± 0.6) keV. At 51 nm RMS roughness, the three peaks are not distinguishable any more due to the very high tailing τ₁ of about (24.9 ± 1.6) keV. Additionally, the peak height starts to decrease but the peak area remains constant. The parameter σ₁ is linked to the increasing tailing, indicated by an increase from (4.8 ± 0.1) keV up to (10.2 ± 0.3) keV.

The simulation results of Figure 13 pertain to the case of different shapes (convex/concave rotational paraboloid) of the source layer. The simulations show a minor, but clear difference of the alpha peak shape for the three cases. The fit parameters η₁, σ₁ and τ₁ are not significantly affected by the convex or concave layer shape. A slight broadening is indicated by an increase of σ₁ from (4.8 ± 0.1) keV to (7.0 ± 0.1) keV (convex) and (5.9 ± 0.2) keV (concave). Additionally, the tailing parameter τ₁ is increased in both cases from (10.1 ± 0.3) keV to about (12.5 ± 0.5) keV. More alpha particles reach the detector from a convex source while a concave source shows the opposite effect.

7.2 Molecular plating

Thin ²³³U sources with areal densities between 7 × 10¹³ and 2 × 10¹⁵ atoms cm⁻² were produced by MP. The investigations by alpha spectrometry, RI and SEM showed no significant differences of the sources in this range of areal densities. The area in contact with the electrolyte was completely covered with active material, but differences in homogeneity occurred (see Figure 3 (b)). These were not correlated with the concentration, substrate or electrolyte. The SEM picture shows a clearly visible contrast of the deposited material but a surface structure similar to the one of the pristine substrate. This is in agreement with measurements of A. Vascon et al. [10], who found the surfaces of thin sources on titanium-coated silicon wafers to be smoothest. The substrate roughness has a significant impact on the alpha spectra, as visible in Figure 12 as an increase of the tailing τ₁ with increasing RMS roughness. Therefore, a smooth surface should be preferred for the source fabrication by MP. Furthermore, the alpha lines of the MP sources are well-resolved (see Figure 3), with average σ₁ of (7.7 ± 1.6) keV. The average fit parameter η₁ is very close to 1 (0.95 ± 0.1) and the average tailing parameter τ₁ is (6.5 ± 1.0) keV, which is one of the lowest values when compared with those of the samples produced by the other fabrication methods (see Figure 9). These results are in good agreement with AASI simulations of ²³³U sources with less than 20 atomic layers (see Figure 11). In summary, the MP sources show very good properties; with probably less than 20 atomic layers they should reach a recoil efficiency of ²²⁹Th close to 100%. Nevertheless, the investigation of the recoil efficiency from a ²³²U source only showed an efficiency of (10.5 ± 1.1) %. This discrepancy to theoretical suggestions based on the ²³³U experiments may have different reasons. First, the deposition of the ²³²U was visible with the naked eye in contrast to all of the ²³³U sources. Therefore, the deposited layer must be much thicker with lesser uranium atoms. This was checked by AASI simulations and comparison with a qualitative 4 k alpha spectrum of the MP source. These AASI fits (with an areal uncertainty of 0.7% and a χ² of 1.749) gave only a thickness of about 35 nm with an RMS roughness of about 90 nm and a density of 6.25 g cm⁻³ for the ²³²U MP source. The unexpected high density can only be caused by metallic, inactive impurities, since the total amount of all determined ²³²U and daughter atoms give a far lower weight (0.04 µg of a total simulated amount of 10.9 µg, including also lighter atoms like hydrogen and oxygen). These impurities might be palladium from the anode or other metals like iron, tin or zinc, which were not investigated beforehand. Another reason for the discrepancy could be sputter effects. The high activity of the MP source corresponds to a high recoil ion rate. High energetic recoil ions, e.g., ²⁰⁸Pb from the alpha decay of the daughter nuclide ²¹²Po (E_kin = 166.9 keV), may also ablate collected ²²⁸Th from the catcher foil. This has to be investigated by further simulations before a final statement on the recoil efficiency of the MP sources can be made. The energy loss of the recoil ions is difficult to estimate, since the exact chemical composition of the MP sources is not known and, e.g., organic impurities from the electrolyte, which do not substantially affect alpha particles, or impurities by daughter nuclides could have a significant impact on the recoil ion energy.

7.3 Drop-on-demand inkjet printing

The DoD printed sources were very thin with areal densities in the range of 1−7 × 10¹⁴ atoms cm⁻². The deposits were not visually observable. SEM analysis was quite challenging, because the deposits were barely visible due to the height contrast of the titanium foils. The shape of the deposits on different regions of the titanium foils varied, probably due to differences in the surface properties. This also had an impact on the fit parameters η₁ and τ₁. Some spectra of sources with large deposits had smaller values of the fit parameters (see DoD1, DoD2, DoD4 and DoD6 in Table 11), whereas others with smaller deposits showed broader alpha peaks (see DoD3, DoD5 and DoD7 in Table 11). The broadening is probably an effect of the more convex lateral shape of the deposits, as indicated by AASI simulations (Figure 13). In dependence on the surface properties (hydrophilic or hydrophobic) of the backing, the lateral cut of deposits of evaporated drops have a concave or convex shape [12]. The recoil efficiency for ²²⁹Th might be close to 100%, because the sources do not contain contamination of light elements in contrast to sources prepared by MP. Also the macroscopic areal distribution of the activity is homogeneous as shown in the RI in Figure 4. The energy loss of the ²²⁹Th recoil ions can be estimated more easily than for MP sources, because the chemical species, uranyl nitrate, is known. As the number of atomic layers is estimated to be below 20 from simulated spectra (Figure 11) compared with the experimental alpha spectrum in Figure 4, the maximum energy loss of the ²²⁹Th recoil ions might be about (9 ± 0.2) keV based on the values of Table 1.

7.4 Chelation by sulfonic acid groups

The sources fabricated by chelation have the best quality by far as judged from the value of η₁ of about 1.0. The silicon surface provides a perfectly smooth surface for the sulfonic acid groups and therefore for the chelated ²³³U. This can be seen both in the SEM picture and in the RI by a very homogeneous areal distribution of the activity (see Figure 5). The RI and also the areal ²³³U density indicate, though, that the densest occupation of the silicon surface by functional groups of about 10¹⁵ –OH–groups cm⁻² [37] has not yet been reached. The reason for the low areal density below 10¹⁵ atoms cm⁻² could be due to two things. On one hand, preventing the formation of functional multilayers was problematic during the functionalization of the silicon surfaces. Therefore, conservative fabrication conditions were selected [37]. These may have led to fewer sites than needed to reach the theoretically possible limit of 10¹⁵ atoms cm⁻². The optimal conditions for these silicon surfaces would have to be approached iteratively. On the other hand, the second step of the functionalization, the oxidation of the thiol groups to sulfonic acid groups, is also complex for extremely thin layers. The process is quite delicate, as a specific concentration of hydrogen peroxide is required to oxidize the thiols to sulfonic acid groups. If the concentration is too low or the contact time too short, the thiol groups on the surfaces will be only partially oxidized and there might an intermediate stage containing disulfide bridges as a possible reaction product. If the hydrogen peroxide concentration is too high, the intermediate disulfide bridges are oxidized to sulfonic acid groups, but the functional layers are also destroyed. Still, the alpha spectra, as judged by the corresponding fit parameters, are the best when compared with those from sources fabricated by any other method. The fit routine had to be performed with a fixed η₁ = 1, because the multiplet peaks are too narrow and do not have a long tailing (parameterized by τ₂) at all. A problem with the alpha spectra was the very low statistics due to the low areal uranium content, which is reflected in the larger error bars of σ₁ and τ₁. The ²³³U multiplet in the alpha spectra is even better resolved than suggested by the AASI simulation (see Figure 11, black line). All this indicates that the sources fabricated by chelation might have an almost 100% recoil efficiency of ²²⁹Th recoil ions. We estimate the energy loss to just a few eV, due to breaking of chemical bonds.

7.5 Self-adsorption

The parameter studies for source fabrication by SA show optimal conditions at a pH value of 5 and on titanium foils thermally oxidized at 500 °C for 1 h. In this way, a maximum areal density of about 80 ng cm⁻² (2 × 10¹⁴ atoms cm⁻²) was reached. The findings of uranium adsorption on anatase fit well with results reported in [17]. The evolution of the adsorption rate as a function of treatment temperature of the titanium foils (Figure 6) cannot be explained by an increase of the RMS roughness, as the AFM data in Table 8 show. Moreover, the evolution shows the adjustment of the titanium modification on the foil surface in the variation of the uranium adsorption. Two peaks of adsorbed uranium are visible and correspond to anatase (at 450 °C) and to rutile (at 700 °C). At room temperature up to 400 °C, amorphous titanium is present. Between 450 and 700 °C, a (semi-amorphous) mixture of both anatase and rutile is present. This fits well with X-ray diffraction measurements of sol-gel coated titanium films [27]. It was possible to increase the amount of adsorbed uranium from 80 to 270 ng cm⁻² on titanium foils that were sand-blasted and then pretreated at 450 °C. However, the roughness produced by sand-blasting was not well reproducible, as inferred from the large standard deviation of the amount of adsorbed uranium. The properties of sources prepared on untreated vs. sand-blasted Ti foils were very different. The areal distribution, shown on untreated foils in Figure 7(b), is very homogeneous. Only few artifacts are visible, probably caused by the quite rough surface of the titanium foils (see SEM picture in Figure 7(c)), as it was also observed on the titanium foils of the DoD sources (see Figure 4). The three single peaks of the ²³³U multiplet are visible in the alpha spectra as shoulders of the next highest peak, due to the large average tailing τ₁ of (10.96 ± 2.26) keV, due to the rough surface of the titanium foil. The average τ₁ value is similar to that of the DoD sources, but τ₂ of (50.35 ± 5.39) keV has a higher weighting factor due to a lower value of η₁ of (0.75 ± 0.6) keV in the case of the untreated foils (see Figure 9). The comparison with an AASI simulation in Figure 14(b) indicates that the reason for the differences in the peak shape is likely connected to the rough surface of the titanium foils. The RI and SEM pictures of the SA sources on sand-blasted foils show a much rougher surface and, therefore, more artifacts in the RI. Also the tailing in the alpha spectra is increased, leading to even lower values of η₁ of (0.48 ± 1.4) keV and higher values of τ₁ of (14.76 ± 10.68) keV. This supports the assumption that the tailing of the SA sources on untreated foils can be explained solely by the roughness of the foils. If the roughness of the substrates could be reduced, the SA sources would have the same quality as the sources fabricated by chelation with sulfonic acid groups concerning the recoil efficiency and the energy loss of ²²⁹Th recoil ions. However, the SA method provides currently a higher areal density. The investigation of the recoil efficiency from a ²³²U source produced on a limited area by SA gave an efficiency of (94.2 ± 3.3) %, which fits very well with the theoretical suggestions. The small discrepancy to an optimum value of 100% might be caused by sputter effects due to high energetic recoil ions like ²⁰⁸Pb coming from the source. The spectra depicted in Figure 10(a) support the theoretical suggestion that the layers produced by SA are single atomic layers, since no ²³²U is visible in the spectrum of the catcher foil. In thicker deposits with several atomic layers, clusters of the source material are expected to be sputtered due to alpha decay collision cascades in deeper layers [38]. Furthermore, the areal distribution of ²³²U in the RI is as homogeneous as the distribution of ²³³U in the RI of Figure 7(b). Therefore, self-adsorption can also be carried out well on confined surfaces.