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

Raphael Haas  Michelle Hufnagel  Roman Abrosimov  Christoph E. Düllmann
Dominik Krupp  Christoph Mokry  Dennis Renisch  Jörg Runke  Ulrich W. Scherer
Department of Chemistry - TRIGA Site, Johannes Gutenberg University Mainz,
Mainz, Germany  Helmholtz Institute Mainz, Mainz, Germany  GSI
Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany  PRISMA
Cluster of Excellence, Johannes Gutenberg University Mainz, Mainz, Germany
Institut für Physikalische Chemie und Radiochemie, Hochschule Mannheim -
University of Applied Sciences, 68163 Mannheim, Germany

###### Abstract

Four different techniques were applied for the production of 233U alpha recoil
ion sources, providing 229Th ions. They were compared with respect to a
minimum energy spread of the 229Th recoil ions, using the emitted alpha
particles as an indicator. The techniques of Molecular Plating, Drop-on-Demand
inkjet printing, chelation from dilute nitric acid solution on chemically
functionalized silicon surfaces, and self-adsorption on passivated titanium
surfaces were used. All fabricated sources were characterized by using alpha
spectrometry, radiographic imaging, and scanning electron microscopy. A direct
validation for the estimated recoil ion rate was obtained by collecting 228Th
recoil ions from 232U recoil ion sources prepared by self-adsorption and
Molecular Plating. The chelation and the self-adsorption based approaches
appear most promising for the preparation of recoil ion sources delivering
monochromatic recoil ions.

##  1 Introduction

Radioactive sources of alpha-decaying or spontaneously fissioning
radionuclides are used in many experiments in nuclear chemistry and nuclear
physics as a source for their daughter nuclei. In alpha-decaying recoil ion
sources, the daughter nuclei are emitted from thin active layers owing to the
short residual range of the daughters in matter. Such recoil ion sources were
recently used in physics experiments, e.g., for investigations of the nuclear
clock isomer 229mTh [1, 2, 3] and for optimizations of the extraction of
short-lived isotopes in buffer gas cells [4, 5]. The 233U is used as example
nuclide in the source fabrication, as it is of high current interest due to
the production of 229mTh and will be used as a recoil ion source in the
experiment of the TACTICa111Trapping And Cooling of Thorium Ions with Calcium
collaboration [6, 7, 8]. In the TACTICa experiment, the 229(m)Th recoil ions
will be decelerated by an electric field to less than $1\text{\,}\mathrm{keV}$
kinetic energy to allow their trapping in a linear Paul trap. In contrast to
the approach employed, e.g. in [1, 2, 3], where recoil ions were thermalized
in buffer gas, stringent vacuum requirements and the desire to retain the
initial charge state distribution [8] render purely electrostatic deceleration
favorable for TACTICa. Ideal recoil ion sources for application in TACTICa
consist of a monolayer of the mother nuclide 233U to gain a high recoil
efficiency with a minimal energy dispersion of the recoil ions. Due to the
approximately fixed areal density of a monolayer, the activity of such an ion
source is defined by the active area.

Different methods exist to fabricate targets or radioactive sources in many
different geometries and thicknesses. A well established method is the
Molecular Plating (MP) technique [9]. This method was well characterized and
enhanced by Vascon et al. to fabricate smooth crack-free large-area thin films
of alpha particle emitting 147Sm [10]. These layers were investigated with
X-ray photoelectron spectroscopy (XPS) and alpha spectrometry and showed that
specific MP parameters, like the choice of plating solvent and the roughness
of the substrate, lead to effects which influence the recoil efficiency and
energy dispersion of the alpha-recoil ions. The thickness and morphology of
the produced salt layer as well as cracked solvent fragments deposited on top
of the radioactive source proved to influence the relative alpha detection
efficiency by as much as $15\text{\,}\mathrm{\char 37\relax}$ [11]. Such alpha
detection losses imply a critical influence on the recoil efficiency of the
alpha daughter ions and their energy distribution. The MP technique needs
further developments to fabricate more efficient recoil ion sources.

A novel fabrication technique is the Drop-on-Demand ink-jet printing method
(DoD) [12], which is able to deposit dissolved material onto a variety of
solid surfaces and is not restricted to electrically conducting substrates. It
has a high printing accuracy as well as a reliable reproducibility of the
deposited material [12]. This method has not yet been used to fabricate recoil
ion sources consisting of a few atomic layers. Printing as well as solution
parameters are well adjustable and, therefore, it should be possible to
produce recoil ion sources suitable for application in, e.g. the TACTICa
experiment.

In contrast to MP and DoD, where the material adheres mainly to the substrate
by physisorption, there are two other possible ways to achieve effectively a
monolayer of radionuclides, which is chemisorbed to the substrate. Krupp et
al. developed a method to functionalize silicon surfaces of, e.g. passivated
implanted planar silicon (PIPS) detectors for alpha spectrometry [13]. The
silicon surfaces are functionalized with sulfonic acid groups, which chelate
radionuclides from an acid solution. The chelation creates a chemisorption of
the radionuclides, so that the surface can be rinsed afterwards with distilled
water to remove non-bound radionuclides without loosing bound material.
Theoretically, a saturated functionalized surface contains a single layer of
radionuclides and seems promising for the production of ideal recoil ion
sources.

Another way to produce chemisorbed monolayer films is to make use of the
chemical equilibrium of radionuclides in solution on metal oxide surfaces,
well known as self-adsorption (SA) process. SA processes are well investigated
for many radionuclides on a wide variety of metal oxides and soil material in
the field of nuclear waste disposal [14, 15, 16, 17]. Self-adsorption of
thorium was used for the production of thin 229Th films on CaF2 surfaces [18]
and is also promising for the production of ideal recoil ion sources for a
high recoil efficiency with small kinetic energy dispersion of the emitted
recoil ions.

In this article, the theoretical aspects of recoil ion sources concerning the
stopping powers of alpha particles and recoil daughters in the source material
as well as the achievable areal density of uranium depending on the
fabrication method is discussed. Furthermore, Advanced Alpha spectrometric
Simulations (AASI) [19, 20, 21] were performed for the investigation of the
influence of different source parameters on the alpha spectra. The
experimental and simulated spectra were parameterized using an adapted fit
routine [22, 23] for a quantitative evaluation, allowing a comparison of
recoil sources fabricated by different methods. At last, the recoil
efficiencies were investigated of two sources produced by MP and SA by using
the shorter-lived 232U (half-life: $68.9\text{\,}\mathrm{a}$).

##  2 Theoretical aspects

###  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: $1.59\text{\times}{10}^{5}\text{\,}\mathrm{a}$, Eα:
$4824\text{\,}\mathrm{keV}$ ($82.7\text{\,}\mathrm{\char 37\relax}$),
$4783\text{\,}\mathrm{keV}$ ($14.9\text{\,}\mathrm{\char 37\relax}$),
$4729\text{\,}\mathrm{keV}$ ($1.85\text{\,}\mathrm{\char 37\relax}$)), the
kinetic energy Ek of the 229Th ions is about $84\text{\,}\mathrm{keV}$ while
the alpha particle energy is about $4.8\text{\,}\mathrm{MeV}$ [derived from
Ek(229Th) = Q$\alpha$(233U) - E$\alpha$(233U), where Q$\alpha$ is the Q-value
of the alpha decay and E$\alpha$ 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
${10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$. 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 $500\text{\,}\mathrm{\SIUnitSymbolAngstrom}$
compared to the alpha particles with ranges above
$10\text{\,}\mathrm{\SIUnitSymbolMicro m}$ 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
ten atomic layers of the mother nuclide material decelerate the emitted recoil
ions by several $\text{\,}\mathrm{keV}$. 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.

Table 1: Stopping powers and ranges of 229Th recoil ions in different 233U compounds. The initial kinetic energy of the recoil ions is set to $84\text{\,}\mathrm{keV}$. The data were obtained with SRIM-2013 [24]. Compound |  density / $\text{\,}\mathrm{g}\text{\,}{\mathrm{cm}}^{-3}$ [25, 26] |  Stopping Power of 229Th / $\text{\,}\mathrm{eV}\mathrm{/}\mathrm{(}\mathrm{${10}^{15}\text{\,}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}$}\mathrm{/}\mathrm{c}\mathrm{m}\mathrm{{}^{2}}\mathrm{)}$ |  Range / $\text{\,}\mathrm{nm}$
---|---|---|---
UO2(OH)2$\times$H2O |  5.0 |  348 |  28
UO2CO3 |  5.7 |  510 |  25
UO2(NO3)2 |  2.8 |  457 |  45
UO2 |  11.0 |  731 |  15
U3O8 |  8.4 |  664 |  19
Table 2: Stopping powers and ranges of alpha particles in different 233U compounds. The maximum kinetic energy of the alpha particles is set to $4.824\text{\,}\mathrm{MeV}$. The data were obtained with SRIM-2013 [24]. Compound |  density / $\text{\,}\mathrm{g}\text{\,}{\mathrm{cm}}^{-3}$ [25, 26] |  Stopping Power of alpha particle / $\text{\,}\mathrm{eV}\mathrm{/}\mathrm{(}\mathrm{${10}^{15}\text{\,}\mathrm{a}\mathrm{t}\mathrm{o}\mathrm{m}\mathrm{s}$}\mathrm{/}\mathrm{c}\mathrm{m}\mathrm{{}^{2}}\mathrm{)}$ |  Range / $\text{\,}\mathrm{\SIUnitSymbolMicro m}$
---|---|---|---
UO2(OH)2$\times$H2O |  5.0 |  20.6 |  17.3
UO2CO3 |  5.7 |  29.5 |  15.6
UO2(NO3)2 |  2.8 |  27.5 |  28.9
UO2 |  11.0 |  43.1 |  10.7
U3O8 |  8.4 |  38.9 |  13.1

###  2.2 Theoretical areal density

As previously mentioned, a single layer of any material is defined in
SRIM-2013 as ${10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$,
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
$2.23\text{\times}{10}^{14}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$
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
${10}^{14}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$ 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.

Table 3: Calculated areal uranium densities in various compounds. Basis of the calculations are crystal structure data from [25, 26]. Compound |  areal uranium density / ${10}^{14}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$
---|---
UO2(OH)2$\times$H2O |  1.41
UO2CO3 |  0.32
UO2(NO3)2 |  0.51
UO2 |  2.23
U3O8 |  0.93

For the chelation method, the areal density of sulfonic acid groups is
important. An areal uranium density of about
$4\text{\times}{10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$ 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$\rightarrow$rutile at temperatures
¿$600\text{\,}\mathrm{\SIUnitSymbolCelsius}$ at atmospheric pressure [28, 27].
The exact transformation temperature depends on impurities in the titanium. D.
Velten et al. found transformations for amorphous$\rightarrow$anatase and
anatase$\rightarrow$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
$1.3\text{\times}{10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$.
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 to 1:2, corresponding
to a maximum areal uranium density of
$6.5\text{\times}{10}^{14}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$
with SA. The real areal density of adsorbed uranium is probably lower due to
defects in the crystal surface.
${10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$ of 233U have an
activity of $138\text{\,}\mathrm{Bq}\text{\,}{\mathrm{cm}}^{-2}$,
corresponding to an emission rate of 229Th ions into a $2\pi$ solid angle of
about
$69\text{\,}\mathrm{ions}\text{\,}{\mathrm{s}}^{-1}\text{\,}{\mathrm{cm}}^{-2}$.

Table 4: Calculated areal densities (ad) of oxygen available for self-adsorption on ideal crystal faces of anatase and rutile based on crystal structure data from [26, 29, 30]. The Miller indices of the individual crystal faces are given. The areal densities of oxygen are given for each crystal face and as average values for each modification. |  lengths / $\text{\,}\mathrm{\SIUnitSymbolAngstrom}$ |  crystal face |  ad / ${10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$ |  mean ad / ${10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$
---|---|---|---|---
|  a: 3.785 |  (001) |  1.40 |
Anatase |  c: 9.514 |  (100) |  1.11 |  1.18
|  |  (101) |  1.02 |
|  a: 4.594 |  (001) |  1.51 |
Rutile |  c: 2.962 |  (110) |  0.98 |  1.36
|  |  (100) |  1.60 |

##  3 Peak fit model and AASIFIT

Alpha spectrometry is an important radioanalytical technique for the
quantitative and qualitative investigation of alpha-emitting radionuclide
samples in environmental and nuclear research [31]. High-resolution spectra of
high-quality sources help characterizing samples with regard to alpha-decaying
impurities, isotopic compositions and even qualitative characteristics like
thickness and homogeneity of the sample [23]. Often, nearby peaks in alpha
spectra overlap at least partially. For the deconvolution and interpretation
of such spectra, several peak fit models were developed and improved in the
last decades. One of the most successful models to represent a mono-energetic
alpha peak consists of a Gaussian joined with an exponential distribution [22,
32]

$\begin{gathered}f\left(u-\mu;\sigma,\tau\right)=\frac{A}{2\tau}\mathrm{exp}\left(\frac{u-\mu}{\tau}+\frac{{\sigma}^{2}}{2{\tau}^{2}}\right)\\\
\times\operatorname{erfc}\left[\frac{1}{\sqrt{2}}\left(\frac{u-\mu}{\sigma}+\frac{\sigma}{\tau}\right)\right]\end{gathered}$

where $A$ is the peak area, $u$ is the distance to the peak position $\mu$,
$\sigma$ is the standard deviation of the Gaussian and $\tau$ is the tailing
parameter. This model was improved by subtraction of the long tail
distribution and a mix of two exponential functions with different lengths,
$\tau_{1}<\tau_{2}$, and a normalized weighting factor $\eta$ [33]. In the
present case of monolayer alpha recoil sources, the peaks are expected to be
very narrow and the long tail distribution is expected to be non-existent, in
which case it can be neglected. Therefore, equation 3 was modified with the
normalized weighting factor $\eta$ and by a sum of three functions for the
present nuclide of interest, 233U, showing a multiplet of the three
significant peaks

$\begin{gathered}F\left(u\right)=\sum_{i=1}^{2}\sum_{j=1}^{3}{\eta_{i}}{f_{j}}\left(u-{\mu_{i}};{\sigma_{i}},{\tau_{i}}\right)\\\
1={\eta_{1}}+{\eta_{2}}\end{gathered}$

with individual peak positions $\mu_{1}$, $\mu_{2}$ and $\mu_{3}$ and two
shared standard deviations of the Gaussian $\sigma$ and tailing parameters
$\tau$, each pertaining to all three peaks. This mixed function contains eight
free parameters

$\begin{gathered}F\left(u\right)=\\\
\left[{\eta_{1}}{f_{1}}\left(u-{\mu_{1}};{\sigma_{1}},{\tau_{1}}\right)+{(1-\eta_{1})}{f_{1}}\left(u-{\mu_{1}};{\sigma_{2}},{\tau_{2}}\right)\right]\\\
+\left[{\eta_{1}}{f_{2}}\left(u-{\mu_{2}};{\sigma_{1}},{\tau_{1}}\right)+{(1-\eta_{1})}{f_{2}}\left(u-{\mu_{2}};{\sigma_{2}},{\tau_{2}}\right)\right]\\\
+\left[{\eta_{1}}{f_{3}}\left(u-{\mu_{3}};{\sigma_{1}},{\tau_{1}}\right)+{(1-\eta_{1})}{f_{3}}\left(u-{\mu_{3}};{\sigma_{2}},{\tau_{2}}\right)\right]\end{gathered}$

which was used for the determination of the individual, characteristic tailing
parameters $\eta_{1}$ and $\tau_{1}$ of spectra from 233U sources produced by
the four different methods. The 233U spectra were fitted with QtiPlot [34]. An
example of the applied fit function is given in Fig. 1.

Figure 1: Exemplary QtiPlot fit of a 2k channel alpha spectrum of purified
233U. A mixed fit including all three peaks of the multiplett (Eq. 3) as well
as independent fits (Eq. 3) of the single peaks are shown.

Furthermore, for the investigation of effects of different source
characteristics on the alpha spectrum, e.g., the thickness and RMS roughness,
the simulation and fitting software AASIFIT was provided by STUK, Finland [19,
20, 21]. Numerous parameters of the detector setup, the source material and
dimensions can be set in the software to give iterative Monte Carlo-simulated
alpha spectra. Experimental data can be loaded into the software and can then
be fitted on the basis of simulated spectra of specific isotopes [21]. Effects
of source layer thickness, root mean square (RMS) roughness, homogeneity and
source material on simulated alpha spectra will be discussed in Section 7.

##  4 Experimental

###  4.1 General instrumentation and methods

A light microscope (Bresser LCD Micro, 40x - 1600x 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 16K channel analog-digital-converter (Canberra Nuclear Data ND581
ADC, used in 1K and 2K mode) with amplifier (Canberra 2015A AMP/TSCA) was used
for data acquisition. A certified 241Am source (Amersham Buchler,
$290\text{\,}\mathrm{Bq}$ homogeneously distributed on an anodized aluminum
disc with $16\text{\,}\mathrm{mm}$ in diameter and
$150\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness) was used for efficiency
calibration of the used PIPS detectors.

###  4.2 Molecular Plating

####  4.2.1 Reagents and materials

A stock solution of column separated (in July 2017) 233U as uranium nitrate in
$0.1\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HNO3 (Merck,
$65\text{\,}\mathrm{\char 37\relax}$) with a concentration of
$2.02\text{\,}\mathrm{mg}\text{\,}{\mathrm{mL}}^{-1}$ was used for the
preparation of all solutions. For MP, three solutions with concentrations of
$38.9\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$,
$21.5\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$,
$5.25\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$ were
prepared by dilution of aliquots from the stock solution in
$0.1\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HNO3. Either
N,N’-dimethylformamide (DMF) (Merck, $99.8\text{\,}\mathrm{\char 37\relax}$)
or a mixture of $10\text{\,}\mathrm{\char 37\relax}$ isopropanol (Merck,
$99.8\text{\,}\mathrm{\char 37\relax}$) and $90\text{\,}\mathrm{\char
37\relax}$ isobutanol (Merck, $99\text{\,}\mathrm{\char 37\relax}$) was used
as electrolyte in the plating cells. A palladium foil or a 1-mm thick
palladium wire (¿$99.98\text{\,}\mathrm{\char 37\relax}$) was used as anode.
Silicon wafers ($34\text{\,}\mathrm{mm}$ in diameter,
$300\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness) with a titanium coating
($100\text{\,}\mathrm{nm}$ thickness) produced by magnetron sputtering as well
as titanium foils ($99.6\text{\,}\mathrm{\char 37\relax}$,
$25\text{\,}\mathrm{mm}$ in diameter, $25\text{\,}\mathrm{\SIUnitSymbolMicro
m}$ thickness) were used as substrates. For cleaning of the titanium foils,
distilled water, $6\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HCl
(Merck, $37\text{\,}\mathrm{\char 37\relax}$), isopropanol and acetone (Merck,
$99.5\text{\,}\mathrm{\char 37\relax}$) were used.

####  4.2.2 Instrumentation and methods

For depositions on the titanium-coated silicon wafers, a horizontal cell
design of A. Vascon et al. [10, 35] was used in combination with a HV power
supply (Heinzinger LNC 3000-10 pos.,
$0\text{\,}\mathrm{V}3000\text{\,}\mathrm{V}$,
$0\text{\,}\mathrm{mA}10\text{\,}\mathrm{mA}$). For depositions on the
titanium foils, a vertical cell design [36] was used in combination with a
different HV power supply (FUG MCP 350-1250,
$0\text{\,}\mathrm{V}1250\text{\,}\mathrm{V}$,
$0\text{\,}\mathrm{mA}250\text{\,}\mathrm{mA}$). Alpha spectra were recorded
with a $2000\text{\,}{\mathrm{mm}}^{2}$ PIPS detector (Canberra
PD2000-40-300AM) in $5\text{\,}\mathrm{mm}$ distance for yield determination
and a $25\text{\,}{\mathrm{mm}}^{2}$ PIPS detector (Ortec ULTRATM,
$11\text{\,}\mathrm{keV}$ FWHM intrinsic resolution) in
$100\text{\,}\mathrm{mm}$ distance was used for recording high resolution
spectra.

####  4.2.3 Source fabrication and characterization

The titanium foils were cleaned with
$6\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HCl, distilled water,
isopropanol and acetone. The coated silicon wafers were cleaned with
isopropanol to avoid damages to the titanium coating. An aliquot of
$100\text{\,}\mathrm{\SIUnitSymbolMicro L}$ of the
$5.25\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$ 233U
solution was mixed with $35\text{\,}\mathrm{mL}$ DMF using a vortex mixer for
deposition on one silicon wafer. Another aliquot of
$100\text{\,}\mathrm{\SIUnitSymbolMicro L}$ of the
$34.4\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$
solution was mixed with $3.5\text{\,}\mathrm{mL}$ isopropanol and
$31.5\text{\,}\mathrm{mL}$ isobutanol using a vortex mixer for deposition on
another silicon wafer. For deposition on the titanium foils, aliquots of
$20\text{\,}\mathrm{\SIUnitSymbolMicro L}$ of the
$21.5\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$
solution were mixed with $1\text{\,}\mathrm{mL}$ isopropanol and
$9\text{\,}\mathrm{mL}$ isobutanol. MP was performed at constant current of
about $0.75\text{\,}\mathrm{mA}\text{\,}{\mathrm{cm}}^{-2}$ for
$1\text{\,}\mathrm{h}2\text{\,}\mathrm{h}$. After deposition, the sources were
carefully dried under an IR lamp in a fume hood. Alpha spectra for yield
determination were taken with peak areas of at least
${10}^{4}\text{\,}\mathrm{cts}$ to reduce statistical counting errors below
$1\text{\,}\mathrm{\char 37\relax}$. Qualitative alpha spectra were taken for
several days to obtain adequate statistic. SEM pictures and RI were taken for
visual inspection of the sources. Irradiation time for RI was about
$2\text{\,}\mathrm{h}$.

###  4.3 Drop-on-Demand inkjet printing

####  4.3.1 Reagents and materials

An aliquot of the stock solution was diluted to give three solutions with
concentrations of $138\text{\,}\mathrm{\SIUnitSymbolMicro
g}\text{\,}{\mathrm{mL}}^{-1}$, $74.2\text{\,}\mathrm{\SIUnitSymbolMicro
g}\text{\,}{\mathrm{mL}}^{-1}$ and $18.6\text{\,}\mathrm{\SIUnitSymbolMicro
g}\text{\,}{\mathrm{mL}}^{-1}$ of 233U in
$0.1\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HNO3 (Merck). Circular
titanium foils ($99.6\text{\,}\mathrm{\char 37\relax}$,
$34\text{\,}\mathrm{mm}$ in diameter, $25\text{\,}\mathrm{\SIUnitSymbolMicro
m}$ thickness) were used as substrates. For cleaning of the titanium foils,
distilled water, $6\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HCl
(Merck, $37\text{\,}\mathrm{\char 37\relax}$), isopropanol, and acetone
(Merck, $99.5\text{\,}\mathrm{\char 37\relax}$) were used.

####  4.3.2 Instrumentation and methods

A DoD printer setup as described in [12] was used for printing. Tips with an
inner diameter of $200\text{\,}\mathrm{\SIUnitSymbolMicro m}$ were used. Alpha
spectra were recorded with a $2000\text{\,}{\mathrm{mm}}^{2}$ PIPS detector
(Canberra PD2000-40-300AM) in $5\text{\,}\mathrm{mm}$ distance for yield
determination and a $25\text{\,}{\mathrm{mm}}^{2}$ PIPS detector (Ortec
ULTRATM, $11\text{\,}\mathrm{keV}$ FWHM intrinsic resolution) in
$100\text{\,}\mathrm{mm}$ distance was used for recording high resolution
spectra.

####  4.3.3 Source fabrication and characterization

Five sources were fabricated with a printing sequence containing 1413 drops
within a circular area of $30\text{\,}\mathrm{mm}$ in diameter (see Fig. 2
(a)). For this, the $74.2\text{\,}\mathrm{\SIUnitSymbolMicro
g}\text{\,}{\mathrm{mL}}^{-1}$ solution and a drop volume of
$5\text{\,}\mathrm{nL}$ at a stroke velocity of
$100\text{\,}\mathrm{\SIUnitSymbolMicro m}\text{\,}{\mathrm{ms}}^{-1}$ were
used. Two additional sources were fabricated with a printing sequence
containing 5637 drops within a circular area of $30\text{\,}\mathrm{mm}$ in
diameter (see Fig. 2 (b)). Drop volume and stroke velocity were kept the same
at $5\text{\,}\mathrm{nL}$ and $100\text{\,}\mathrm{\SIUnitSymbolMicro
m}\text{\,}{\mathrm{ms}}^{-1}$, but the solutions with concentrations of
$18.6\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$ and
$138\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$ 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 $0.7\text{\,}\mathrm{mm}$ (a) and
$0.35\text{\,}\mathrm{mm}$ (b), respectively.

###  4.4 Chelation by sulfonic acid groups

####  4.4.1 Reagents and materials

An aliquot of the stock solution was diluted to give a solution with a
concentration of $38.9\text{\,}\mathrm{\SIUnitSymbolMicro
g}\text{\,}{\mathrm{mL}}^{-1}$ of 233U in
$0.1\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HNO3 (Merck). Pieces of
silicon wafers ($300\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness) in
various forms with areas of about
$6.4\text{\,}{\mathrm{mm}}^{2}10\text{\,}{\mathrm{mm}}^{2}$, functionalized
with sulfonic acid groups according to [13], were used as substrates.

####  4.4.2 Instrumentation and methods

Alpha spectra were recorded with a $2000\text{\,}{\mathrm{mm}}^{2}$ PIPS
detector (Canberra PD2000-40-300AM) in $5\text{\,}\mathrm{mm}$ distance for
yield determination and a $25\text{\,}{\mathrm{mm}}^{2}$ PIPS detector (Ortec
ULTRATM, $11\text{\,}\mathrm{keV}$ FWHM intrinsic resolution) in
$100\text{\,}\mathrm{mm}$ distance was used for recording high resolution
spectra.

####  4.4.3 Source fabrication and characterization

The functionalized wafers were placed in a crystallizing basin and covered
with $10\text{\,}\mathrm{mL}$ of the 233U solution. After
$5\text{\,}\mathrm{min}$, $15\text{\,}\mathrm{min}$,
$30\text{\,}\mathrm{min}$, $60\text{\,}\mathrm{min}$ and
$120\text{\,}\mathrm{min}$ the wafers were carefully removed from the
solution, rinsed with distilled water and dried under an IR lamp. Alpha
spectra, SEM pictures and RI were processed as described in section 4.2.3.

###  4.5 Self-adsorption

####  4.5.1 Reagents and materials

An aliquot of the stock solution was diluted with distilled water to give a
concentration of $5.89\text{\,}\mathrm{\SIUnitSymbolMicro
g}\text{\,}{\mathrm{mL}}^{-1}$ of 233U. A titanium foil
($99.6\text{\,}\mathrm{\char 37\relax}$,
$25\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness) was used for preparation
of the substrates. $0.1\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HNO3
(Merck) was used for the adjustment of different pH values in the aqueous
solutions. For cleaning of the titanium foils, distilled water,
$6\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HCl (Merck,
$37\text{\,}\mathrm{\char 37\relax}$), isopropanol and acetone (Merck,
$99.5\text{\,}\mathrm{\char 37\relax}$) were used.

####  4.5.2 Instrumentation and methods

A muffle furnace (Nabertherm, LE 1/11/R7, max.
$1100\text{\,}\mathrm{\SIUnitSymbolCelsius}$) was used for thermal oxidation
of the titanium foils at air. Alpha spectra were recorded with a
$450\text{\,}{\mathrm{mm}}^{2}$ PIPS detector (Ortec, CR-SNA-450-100) in
$5\text{\,}\mathrm{mm}$ distance for yield determination and a
$25\text{\,}{\mathrm{mm}}^{2}$ PIPS detector (Ortec ULTRATM, CU-011-025-300,
$11\text{\,}\mathrm{keV}$ FWHM intrinsic resolution) in
$100\text{\,}\mathrm{mm}$ distance was used for recording high resolution
spectra.

####  4.5.3 Source fabrication and characterization

In a first experimental series, solutions with different pH-values of 2 to 7
were generated as mixtures of
$0.1\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HNO3 with distilled
water. Pieces with dimensions of $10\text{\,}\mathrm{mm}$ x
$10\text{\,}\mathrm{mm}$ were pretreated in the muffle furnace at
$500\text{\,}\mathrm{\SIUnitSymbolCelsius}$ for one hour. The titanium pieces
were put upright into small polyethylene flasks, containing a mixture
consisting of $250\text{\,}\mathrm{\SIUnitSymbolMicro L}$ of the 233U solution
and $1\text{\,}\mathrm{mL}$ of the pH-adjusted solution, for one day.
Afterwards, the titanium pieces were rinsed with distilled water and dried
under an IR lamp in the fume hood. Yield determination was performed by alpha
analysis of both sides of the titanium pieces as described in section 4.2.3.
In a second experimental series, Ti pieces with dimensions of
$10\text{\,}\mathrm{mm}$ x $10\text{\,}\mathrm{mm}$ were thermally oxidized in
the muffle furnace at different temperatures between
$100\text{\,}\mathrm{\SIUnitSymbolCelsius}900\text{\,}\mathrm{\SIUnitSymbolCelsius}$
for one hour. The pieces were processed as described before in a solution with
a pH value of 5. Yield determination was performed by alpha analysis of both
sides of the titanium pieces as described in section 4.2.3. Three titanium
foils, pretreated at temperatures of
$20\text{\,}\mathrm{\SIUnitSymbolCelsius}$,
$450\text{\,}\mathrm{\SIUnitSymbolCelsius}$, and
$700\text{\,}\mathrm{\SIUnitSymbolCelsius}$, were investigated by AFM.
At last, two circular titanium foils with a diameter of
$30\text{\,}\mathrm{mm}$ were processed at
$450\text{\,}\mathrm{\SIUnitSymbolCelsius}$ and kept in 233U solution at a pH
value of 5. Another two titanium foils with the same dimensions were sand
blasted in advance and processed exactly like the two other foils. Alpha
spectra were recorded of one side of those foils. SEM pictures and RI were
performed as described in section 4.2.3.

###  4.6 Investigation of the Th daughter recoil efficiency

####  4.6.1 Reagents and materials

A stock solution with $50\text{\,}\mathrm{kBq}$ of 232U in
$6.5\text{\,}\mathrm{mL}$ $0.1\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$
HNO3 (Merck) was evaporated under an IR lamp and dissolved again in
$8\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HCl (Merck). This process
was repeated three times. A column ($50\text{\,}\mathrm{mm}$ $\times$
$3\text{\,}\mathrm{mm}$ with Dowex AG 1x8 anion exchanger) was heated up to
$55\text{\,}\mathrm{\SIUnitSymbolCelsius}$ and washed several times with
$8\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HCl. The 232U solution was
given onto the column and the daughter nuclides were eluted by 4 $\times$
$2\text{\,}\mathrm{mL}$ $8\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$
HCl. The 232U was stripped with 5 $\times$ $2\text{\,}\mathrm{mL}$
$0.5\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HCl. The 232U-containing
eluate was evaporated to dryness again and the residue was dissolved in
$1\text{\,}\mathrm{mL}$ HNO3 solution at pH 5. Circular titanium foils
(Goodfellow, $99.6\text{\,}\mathrm{\char 37\relax}$,
$25\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness, $25\text{\,}\mathrm{mm}$
in diameter) were used as substrates. For cleaning of the titanium foils,
distilled water, $6\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HCl
(Merck, $37\text{\,}\mathrm{\char 37\relax}$), isopropanol and acetone (Merck,
$99.5\text{\,}\mathrm{\char 37\relax}$) were used. A mixture of
$10\text{\,}\mathrm{\char 37\relax}$ isopropanol (Merck,
$99.8\text{\,}\mathrm{\char 37\relax}$) and $90\text{\,}\mathrm{\char
37\relax}$ isobutanol (Merck, $99\text{\,}\mathrm{\char 37\relax}$) was used
as electrolyte for MP in the plating cells. A 1-mm thick palladium wire
(¿$99.98\text{\,}\mathrm{\char 37\relax}$) was used as anode for MP.

####  4.6.2 Instrumentation and methods

For depositions (SA and MP) on a limited area with $8\text{\,}\mathrm{mm}$ in
diameter, a vertical cell design [36] was used. For MP, the cell was used in
combination with a HV power supply (FUG MCP 350-1250,
$0\text{\,}\mathrm{V}1250\text{\,}\mathrm{V}$,
$0\text{\,}\mathrm{mA}250\text{\,}\mathrm{mA}$). Alpha spectra were recorded
with a $450\text{\,}{\mathrm{mm}}^{2}$ PIPS detector (Ortec CR-SNA-450-100) in
$8\text{\,}\mathrm{mm}$ distance for yield determination and
$450\text{\,}{\mathrm{mm}}^{2}$ PIPS detectors (Canberra A450-18AM) in
$50\text{\,}\mathrm{mm}$ distance inside an Alpha Analyst system (Canberra,
Dual Alpha Spectrometer Upgrade Module 7200) were used for recording high
resolution spectra with 4k channels.

####  4.6.3 Source fabrication and characterization

##### Self-adsorption

For the preparation of an SA source, a titanium foil was thermally oxidized in
a muffle furnace for $1\text{\,}\mathrm{h}$. Afterwards, it had a copper-
colored surface. The oxidized titanium foil was fixed in the plating cell
without a spacer disk. The 232U solution was filled into the plating cell and
Parafilm© was used to prevent evaporation. After $24\text{\,}\mathrm{h}$, the
solution was carefully removed with a pipette and the titanium foil was rinsed
with distilled water and dried under an IR lamp.

##### Molecular Plating

For the preparation of the MP source, the residual solution was evaporated to
dryness again and the residue was dissolved in
$40\text{\,}\mathrm{\SIUnitSymbolMicro L}$
$0.1\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HNO3. The solution was
mixed with $1\text{\,}\mathrm{mL}$ isopropanol and $9\text{\,}\mathrm{mL}$
isobutanol using a vortex mixer. A titanium foil was etched with
$6\text{\,}\mathrm{mol}\text{\,}{\mathrm{L}}^{-1}$ HCl and cleaned with
distilled water, isopropanol and acetone. Both the titanium foil and the
electrolyte solution were given into a plating cell and MP was performed at
constant current of about
$0.75\text{\,}\mathrm{mA}\text{\,}{\mathrm{cm}}^{-2}$ for
$2\text{\,}\mathrm{h}$. After deposition, the source was carefully dried under
an IR lamp in a fume hood. Alpha spectra for yield determination were taken of
both sources with peak areas of at least ${10}^{4}\text{\,}\mathrm{cts}$ to
reduce statistical counting errors below $1\text{\,}\mathrm{\char 37\relax}$.

####  4.6.4 Recoil collection and investigation of the recoil efficiency

Both sources were kept in $5\text{\,}\mathrm{mm}$ distance to a titanium foil
($25\text{\,}\mathrm{mm}$ in diameter) under vacuum at
$8\text{\times}{10}^{-3}\text{\,}\mathrm{mbar}$. Monte-Carlo simulations
performed with AASI gave $30.9\pm 0.4\text{\,}\mathrm{\char 37\relax}$ of
geometric efficiency for this setup. The collection times were
$26\text{\,}\mathrm{d}$ for the SA source and $5.16\text{\,}\mathrm{d}$ for
the MP source. After the collection, alpha spectra for yield determination
were measured of both catcher foils with peak areas of at least
${10}^{4}\text{\,}\mathrm{cts}$ to reduce statistical counting errors below
$1\text{\,}\mathrm{\char 37\relax}$. Furthermore, qualitative alpha spectra
were taken of both sources and the corresponding catcher foils.

##  5 Experimental results

###  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 Fig. 3 together with a 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.

Table 5: Parameters and yield of the MP-produced 233U recoil ion sources. N,N’-Dimethylformamide (DMF) and mixtures of $90\text{\,}\mathrm{\char 37\relax}$ isopropanol and $10\text{\,}\mathrm{\char 37\relax}$ isobutanol were used as electrolytes. The areal density refers to 233U atoms. sample |  conc. / $\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$ |  substrate |  electrolyte |  duration / h |  area / $\text{\,}{\mathrm{cm}}^{2}$ |  areal density / ${10}^{14}\text{\,}{\mathrm{cm}}^{-2}$
---|---|---|---|---|---|---
MP1 |  5.25 |  wafer |  DMF |  1.0 |  7.1 |  0.69(1)
MP2 |  34.4 |  wafer |  IP/IB |  1.5 |  7.1 |  3.82(4)
MP3 |  21.5 |  Ti foil |  IP/IB |  1.0 |  0.5 |  2.48(3)
MP4 |  21.5 |  Ti foil |  IP/IB |  1.5 |  0.5 |  19.1(2)
MP5 |  21.5 |  Ti foil |  IP/IB |  2.0 |  0.5 |  19.8(2)
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 $10\text{\,}\mathrm{\char 37\relax}$
isopropanol and $90\text{\,}\mathrm{\char 37\relax}$ isobutanol. Irradiation
time for the RI was about $2\text{\,}\mathrm{h}$.

###  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 Fig. 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
$200\text{\,}\mathrm{\SIUnitSymbolMicro m}$ 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.

Table 6: Parameters for DoD printing and yield of the 233U recoil ion sources. The areal density refers to the 233U atoms on the area given in the table. sample |  conc. / $\text{\,}\mathrm{\SIUnitSymbolMicro g}\text{\,}{\mathrm{mL}}^{-1}$ |  drops |  drop volume / $\text{\,}\mathrm{nL}$ |  area / $\text{\,}{\mathrm{cm}}^{2}$ |  areal density / ${10}^{14}\text{\,}{\mathrm{cm}}^{-2}$
---|---|---|---|---|---
DoD1 |  74.2 |  1413 |  5.0 |  7.1 |  4.45(4)
DoD2 |  74.2 |  1413 |  5.0 |  7.1 |  4.60(5)
DoD3 |  74.2 |  1413 |  5.0 |  7.1 |  1.83(2)
DoD4 |  74.2 |  1413 |  5.0 |  7.1 |  1.81(2)
DoD5 |  74.2 |  1413 |  5.0 |  7.1 |  1.82(2)
DoD6 |  18.6 |  5637 |  5.0 |  7.1 |  1.23(1)
DoD7 |  138 |  5637 |  5.0 |  7.1 |  6.84(7)
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 $2\text{\,}\mathrm{h}$.

###  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 Fig. 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 Fig. 5 shows a
homogeneous, yet not very dense areal distribution of the activity.

Table 7: Parameters for chelation by sulfonic acid groups on silicon wafer pieces and areal densities. Contact time with the radioactive solution and areas of the silicon wafers are given. The areal density refers to 233U atoms. sample |  contact time / min |  area / $\text{\,}{\mathrm{cm}}^{2}$ |  areal density / ${10}^{14}\text{\,}{\mathrm{cm}}^{-2}$
---|---|---|---
Ch1 |  5 |  7.40 |  0.210(2)
Ch2 |  15 |  9.97 |  0.122(1)
Ch3 |  30 |  7.98 |  0.149(1)
Ch4 |  30 |  8.74 |  0.109(1)
Ch5 |  60 |  6.48 |  0.235(2)
Ch6 |  120 |  9.89 |  0.238(2)
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
$1\text{\,}\mathrm{h}$.

###  5.4 Self-adsorption

Fig. 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 $40\text{\,}\mathrm{ng}\text{\,}{\mathrm{cm}}^{-2}$
(${10}^{14}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$) on thermally
oxidized titanium foils at $500\text{\,}\mathrm{\SIUnitSymbolCelsius}$. This
was increased to about $80\text{\,}\mathrm{ng}\text{\,}{\mathrm{cm}}^{-2}$
($2\text{\times}{10}^{14}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$)
by changing the pretreatment temperature of the titanium foils to
$450\text{\,}\mathrm{\SIUnitSymbolCelsius}$. Sand-blasted titanium foils
showed $270\text{\,}\mathrm{ng}\text{\,}{\mathrm{cm}}^{-2}$
($7\text{\times}{10}^{14}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$)
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 $500\text{\,}\mathrm{\SIUnitSymbolCelsius}$ for $1\text{\,}\mathrm{h}$ 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. Table 8: Root mean square (RMS) roughness of titanium foils pretreated at different temperatures for $1\text{\,}\mathrm{h}$. Atomic force microscopy was used for the investigation of the RMS roughness. pretreatment temperature / $\text{\,}\mathrm{\SIUnitSymbolCelsius}$ |  RMS roughness / $\text{\,}\mathrm{nm}$
---|---
20 |  39(2)
450 |  24(10)
700 |  40(10)

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 Fig. 7 (a).
The two peaks at lower energies are visible as shoulders of the peak at
$4824\text{\,}\mathrm{keV}$ and are not well-resolved. The RI in Fig. 7 (b)
shows a homogeneous areal distribution of activity. In the SEM picture in Fig.
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 Fig. 8.
The left shoulder of the main alpha peak at $4824\text{\,}\mathrm{keV}$ 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.

Table 9: Parameters for self-adsorption on preheated titanium foils and yields. The areal density refers to 233U atoms. sample |  sand-blasted |  pretreatment temperature / $\text{\,}\mathrm{\SIUnitSymbolCelsius}$ |  area / $\text{\,}{\mathrm{cm}}^{2}$ |  areal density / ${10}^{14}\text{\,}{\mathrm{cm}}^{-2}$
---|---|---|---|---
SA1 |  no |  450 |  7.1 |  2.33(2)
SA2 |  no |  450 |  7.1 |  1.90(2)
SA3 |  yes |  450 |  7.1 |  6.73(7)
SA4 |  yes |  450 |  7.1 |  1.58(2)
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
$450\text{\,}\mathrm{\SIUnitSymbolCelsius}$ for $1\text{\,}\mathrm{h}$.
Irradiation time for the RI was about $2\text{\,}\mathrm{h}$. 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
$450\text{\,}\mathrm{\SIUnitSymbolCelsius}$ for $1\text{\,}\mathrm{h}$.
Irradiation time for the RI was about $2\text{\,}\mathrm{h}$.

###  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,
$\eta_{1}$ and $\tau_{1}$, are plotted against the average areal 233U density
for each fabrication method in Fig. 9. The fit parameter $\sigma_{1}$ is also
significant for the qualitative evaluation, but is identical within error bars
for all fabrication methods. Therefore, $\sigma_{1}$ is not included in the
plots. The other parameters, $\sigma_{2}$ and $\tau_{2}$, are less significant
and have in general much higher values (see Table 11 in the appendix).

Figure 9: Plotted average fit parameters $\eta_{1}$ (left) and $\tau_{1}$
(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 Fig. 10, the qualitative spectra
as well as the RI of the SA source and the corresponding catcher foil are
depicted.

Table 10: 232U activities of the SA and MP source and parameters and yields for the 228Th recoil collection. source |  232U activity / $\text{\,}\mathrm{Bq}$ |  collection time / $\text{\,}\mathrm{d}$ |  geometric efficiency (AASI) / $\text{\,}\mathrm{\char 37\relax}$ |  collected 228Th / $\text{\,}\mathrm{Bq}$ |  recoil efficiency / $\text{\,}\mathrm{\char 37\relax}$
---|---|---|---|---|---
SA |  1300(9) |  26.0 |  16.6(5) |  10.7(1) |  94.2(33)
MP |  18800(60) |  5.16 |  16.6(5) |  3.13(2) |  10.5(11)
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
$1\text{\,}\mathrm{h}$ in case of the SA source and $5\text{\,}\mathrm{h}$ for
the corresponding catcher foil, to obtain sufficient counts. The additional
line at $5685\text{\,}\mathrm{keV}$ belongs to 224Ra, the daughter nuclide of
228Th. Irradiation time for the RI was about $15\text{\,}\mathrm{min}$ for the
SA source and $2\text{\,}\mathrm{h}$ for the catcher foil.

##  6 Simulation results

As two fabrication methods (MP and DoD) cannot produce specifically
monolayers, simulations are necessary to show the influence of several atomic
layers on the alpha spectra of 233U sources. The effects of different numbers
of source layers (Fig. 11), different RMS roughnesses (Fig. 12) and different
macroscopic layer shapes (Fig. 13) on simulated alpha spectra are shown.
Convex and concave source shapes are described with a paraboloid of
revolution, as described in [19]. The most important fit parameters,
$\sigma_{1}$ and $\tau_{1}$, are given in a legend for some simulated spectra
to show quantitatively the effect of different simulation parameters on the
alpha spectrum. The residual fit parameters are given in Table 11. The
influence of different chemical species (see Table 3) on simulated alpha
spectra was also investigated, for sources having 10 atomic layers. The
simulated spectra were identical and are therefore not plotted individually.
The information on source material, backing and detector, which were used as
parameters in AASI, is given in the caption of the figures. The varied
parameters are given in the legends of the figures. A constant number of
${10}^{6}\text{\,}$ simulated decays and a constant bin size of
$5\text{\,}\mathrm{keV}$ was used for all simulations. The source-detector-
distance was kept constant at $100\text{\,}\mathrm{mm}$ and the PIPS detector
properties of the Ortec ULTRATM, as employed to obtain the qualitative
spectra, were used to enable a direct comparison between experimental and
simulated results. For a qualitative comparison of experimental spectra with
these simulations, two exemplary alpha spectra of a molecular plated source
(MP2) and a self-adsorbed source (SA1) were fitted with simulations in AASI.
These are shown in Fig. 14.

Figure 11: AASI simulations of 233U sources with different numbers of atomic
layers. The source material was set to UO2(OH)2$\times$H2O with a diameter of
$30\text{\,}\mathrm{mm}$ on a Ti backing with
$25\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness. The layer thickness was
varied from $1.7\text{\,}\mathrm{nm}$ (representing a monolayer) up to
$85\text{\,}\mathrm{nm}$. The source-detector-distance was set to
$100\text{\,}\mathrm{mm}$ and a number of ${10}^{6}\text{\,}$ decays were
simulated for each parameter variation. Figure 12: AASI simulations of 233U
sources with different root mean square roughnesses. The source material was
set to UO2(OH)2$\times$H2O with a diameter of $30\text{\,}\mathrm{mm}$ and a
thickness of $17\text{\,}\mathrm{nm}$ (representing 10 atomic layers) on a Ti
backing with $25\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness. The source-
detector-distance was set to $100\text{\,}\mathrm{mm}$ and a number of
${10}^{6}\text{\,}$ decays were simulated for each parameter variation. The
RMS roughness was varied in the range of
$0\text{\,}\mathrm{nm}51\text{\,}\mathrm{nm}$. Figure 13: AASI simulations of
233U sources with different layer shapes. Normal represents a cylindrical-
shape, as the central thickness and the thickness at the side of the source
layer are equal. Convex and concave source shapes are described with a
paraboloid of revolution, as described in [19]. The source material was set to
UO2(OH)2$\times$H2O with a diameter of $30\text{\,}\mathrm{mm}$ and a
thickness of $17\text{\,}\mathrm{nm}$ (representing 10 atomic layers) on a Ti
backing with $25\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness. The source-
detector-distance was set to $100\text{\,}\mathrm{mm}$ and a number of
${10}^{6}\text{\,}$ decays were simulated for each parameter variation. Figure
14: Qualitative comparison of experimental alpha spectra with AASI
simulations. For the simulations, the source material was set to
UO2(OH)2$\times$H2O with a diameter of $30\text{\,}\mathrm{mm}$ and a
thickness of $17\text{\,}\mathrm{nm}$ (representing 10 atomic layers) on a Ti
backing with $25\text{\,}\mathrm{\SIUnitSymbolMicro m}$ thickness. The source-
detector-distance was set to $100\text{\,}\mathrm{mm}$ and a number of
${10}^{6}\text{\,}$ decays were simulated. In (a), for comparison with the
molecular plated source (MP2), the RMS roughness was set to
$0\text{\,}\mathrm{nm}$, while for comparison with the self-adsorbed source
(SA1) in (b) it was set to $34\text{\,}\mathrm{nm}$.

##  7 Discussion

###  7.1 AASI simulations

The simulation results of Fig. 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 $\mu$ is about $0.5\text{\,}\mathrm{keV}$. Both simulated
spectra have the same shape, except for a slight shift of one bin
($5\text{\,}\mathrm{keV}$) 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
$10\text{\,}\mathrm{keV}$; 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 to 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 $\sigma_{1}$ from $4.8\pm 0.1\text{\,}\mathrm{keV}$ to $14.4\pm
1.0\text{\,}\mathrm{keV}$.
The simulation results depicted in Fig. 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 a RMS
roughness of $8.5\text{\,}\mathrm{nm}$ with $\tau_{1}$ at about
$10\text{\,}\mathrm{keV}$. The influence only becomes significant at higher
RMS roughnesses. At $17\text{\,}\mathrm{nm}$ RMS roughness, the three single
peaks start to merge due to a higher tailing $\tau_{1}$ of about $16.1\pm
0.6\text{\,}\mathrm{keV}$. At $51\text{\,}\mathrm{nm}$ RMS roughness, the
three peaks are not distinguishable any more due to the very high tailing
$\tau_{1}$ of about $24.9\pm 1.6\text{\,}\mathrm{keV}$. Additionally, the peak
height starts to decrease but the peak area remains constant. The parameter
$\sigma_{1}$ is linked to the increasing tailing, indicated by an increase
from $4.8\pm 0.1\text{\,}\mathrm{keV}$ up to $10.2\pm
0.3\text{\,}\mathrm{keV}$.
The simulation results of Fig. 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 $\eta_{1}$, $\sigma_{1}$ and $\tau_{1}$ are not
significantly affected by the convex or concave layer shape. A slight
broadening is indicated by an increase of $\sigma_{1}$ from $4.8\pm
0.1\text{\,}\mathrm{keV}$ to $7.0\pm 0.1\text{\,}\mathrm{keV}$ (convex) and
$5.9\pm 0.2\text{\,}\mathrm{keV}$ (concave). Additionally, the tailing
parameter $\tau_{1}$ is increased in both cases from $10.1\pm
0.3\text{\,}\mathrm{keV}$ to about $12.5\pm 0.5\text{\,}\mathrm{keV}$. 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
$7\text{\times}{10}^{13}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$ to
$2\text{\times}{10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$
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 Fig. 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 Fig. 12 as an increase of the tailing $\tau_{1}$ 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 Fig. 3), with average $\sigma_{1}$ of $7.7\pm
1.6\text{\,}\mathrm{keV}$. The average fit parameter $\eta_{1}$ is very close
to 1 ($0.95\pm 0.01\text{\,}$) and the average tailing parameter $\tau_{1}$ is
$6.5\pm 1.0\text{\,}\mathrm{keV}$, which is one of the lowest values when
compared with those of the samples produced by the other fabrication methods
(see Fig. 9). These results are in good agreement with AASI simulations of
233U sources with less than 20 atomic layers (see Fig. 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
$100\text{\,}\mathrm{\char 37\relax}$. Nevertheless, the investigation of the
recoil efficiency from a 232U source only showed an efficiency of $10.5\pm
1.1\text{\,}\mathrm{\char 37\relax}$. 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 4k alpha spectrum of the MP source. These AASI fits (with an
areal uncertainty of $0.7\text{\,}\mathrm{\char 37\relax}$ and a $\chi^{2}$ of
1.749) gave only a thickness of about $35\text{\,}\mathrm{nm}$ with a RMS
roughness of about $90\text{\,}\mathrm{nm}$ and a density of
$6.25\text{\,}\mathrm{g}\text{\,}{\mathrm{cm}}^{-3}$ 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 ($0.04\text{\,}\mathrm{\SIUnitSymbolMicro g}$ of a
total simulated amount of $10.9\text{\,}\mathrm{\SIUnitSymbolMicro g}$,
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\text{\,}\mathrm{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
$17\text{\times}{10}^{14}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$.
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 $\eta_{1}$ and $\tau_{1}$. 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 (Fig. 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 $100\text{\,}\mathrm{\char
37\relax}$, 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 Fig. 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 (Fig. 11)
compared with the experimental alpha spectrum in Fig. 4, the maximum energy
loss of the 229Th recoil ions might be about $9\pm 2.\text{\,}\mathrm{keV}$
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 $\eta_{1}$ 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 Fig. 5). The RI and
also the areal 233U density indicate, though, that the densest occupation of
the silicon surface by functional groups of about
${10}^{15}\text{\,}\mathrm{-OH-groups}\text{\,}{\mathrm{cm}}^{-2}$ [37] has
not yet been reached. The reason for the low areal density below
${10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$ 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 ${10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$. 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
$\eta_{1}=1$, because the multiplet peaks are too narrow and do not have a
long tailing (parameterized by $\tau_{2}$) 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 $\sigma_{1}$ and $\tau_{1}$.
The 233U multiplet in the alpha spectra is even better resolved than suggested
by the AASI simulation (see Fig. 11, black line). All this indicates that the
sources fabricated by chelation might have an almost
$100\text{\,}\mathrm{\char 37\relax}$ recoil efficiency of 229Th recoil ions.
We estimate the energy loss to just a few $\text{\,}\mathrm{eV}$, 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
$500\text{\,}\mathrm{\SIUnitSymbolCelsius}$ for $1\text{\,}\mathrm{h}$. In
this way, a maximum areal density of about
$80\text{\,}\mathrm{ng}\text{\,}{\mathrm{cm}}^{-2}$
($2\text{\times}{10}^{14}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$)
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 (Fig. 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
$450\text{\,}\mathrm{\SIUnitSymbolCelsius}$) and to rutile (at
$700\text{\,}\mathrm{\SIUnitSymbolCelsius}$). At room temperature up to
$400\text{\,}\mathrm{\SIUnitSymbolCelsius}$, amorphous titanium is present.
Between $450\text{\,}\mathrm{\SIUnitSymbolCelsius}$ and
$700\text{\,}\mathrm{\SIUnitSymbolCelsius}$, 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
$80\text{\,}\mathrm{ng}\text{\,}{\mathrm{cm}}^{-2}$ to
$270\text{\,}\mathrm{ng}\text{\,}{\mathrm{cm}}^{-2}$ on titanium foils that
were sand-blasted and then pretreated at
$450\text{\,}\mathrm{\SIUnitSymbolCelsius}$. 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 Fig. 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 Fig. 7 (c)), as it was
also observed on the titanium foils of the DoD sources (see Fig. 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
$\tau_{1}$ of $10.96\pm 2.26\text{\,}\mathrm{keV}$, due to the rough surface
of the titanium foil. The average $\tau_{1}$ value is similar to that of the
DoD sources, but $\tau_{2}$ of $50.35\pm 5.39\text{\,}\mathrm{keV}$ has a
higher weighting factor due to a lower value of $\eta_{1}$ of $0.75\pm
0.06\text{\,}\mathrm{keV}$ in the case of the untreated foils (see Fig. 9).
The comparison with an AASI simulation in Fig. 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 $\eta_{1}$ of $0.48\pm 0.14\text{\,}\mathrm{keV}$ and higher
values of $\tau_{1}$ of $14.76\pm 10.68\text{\,}\mathrm{keV}$. 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 $94.2\pm 3.3\text{\,}\mathrm{\char 37\relax}$, which fits very
well with the theoretical suggestions. The small discrepancy to an optimum
value of $100\text{\,}\mathrm{\char 37\relax}$ might be caused by sputter
effects due to high energetic recoil ions like 208Pb coming from the source.
The spectra depicted in Fig. 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 Fig. 7 (b). Therefore, self-adsorption can also be carried out
well on confined surfaces.

##  8 Conclusion

Four different fabrication methods were used to produce 233U recoil ion
sources delivering 229(m)Th in a most narrow kinetic energy distribution. They
were analyzed both quantitatively and qualitatively with respect to the
resolution of the alpha spectra and the areal density of the 233U layer.
Simulations performed with AASI helped to understand the effects of different
source characteristics like layer thickness, roughness, homogeneity and
chemical species on the 233U alpha spectrum. Both the information from the
simulations and the experimental alpha spectra were used to make predictions
on recoil efficiency and energy loss of 229Th recoil ions. Additionally,
experiments were performed with two 232U sources produced by MP and SA to
investigate their 228Th recoil efficiency. Sources produced by chelation with
sulfonic acid groups are likely the best recoil ion sources with respect to
energy sharpness of the 229Th recoil ions. However, their areal density and
hence the recoil ion rate is quite low. MP sources are very close in their
quality to sources produced by chelation. However, they consist of several
atomic layers and are therefore probably not ideal concerning the energy loss
of recoil ions. This was proven with a 232U source, for which a recoil
efficiency of roughly $10\text{\,}\mathrm{\char 37\relax}$ was determined.
Effects by metallic, inactive contaminants in the source material, which have
caused a greater density in the deposit layer, have to be investigated by,
e.g., neutron activation analysis in further experiments. They have to be
removed before source production to achieve a higher recoil efficiency with MP
sources. Also the methods of DoD and SA yielded promising sources. In contrast
to MP and sources produced by chelation, they were fabricated on quite rough
substrates. This induces a more pronounced tailing in the alpha spectra and
decreases their quality. A method to control the shape of deposits produced by
DoD is, e.g., to print on hydrophilic or even superhydrophilic surfaces. Such
surfaces can be fabricated by anodic oxidation of titanium foils or by sol-gel
coating of TiO2 particles [27]. The deposits would then result in very broad
and regular shapes and therefore the quality of DoD sources could probably be
improved. The suggested method will also improve the quality of the SA
sources. After the implementation of these improvements, the sources produced
by chelation and by self-adsorption may well be the best recoil ion sources
due to their guaranteed monolayer thicknesses. The investigation of a 232U
source produced by SA showed actually a 228Th recoil efficiency close to
$100\text{\,}\mathrm{\char 37\relax}$, which fits perfectly to the proposed
theoretical efficiency by alpha spectrometry analysis. MP and DoD would
produce high qualitative recoil ion sources, but with a source thickness that
can be less well controlled, and may, therefore, result in a larger energy
spread and lower recoil efficiency of the 229Th ions. All the presented data
fit well to the experiments and theoretical suggestions of Pohjalainen et al.
[39], where sources with much larger areal uranium densities of
$74\text{\times}{10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}530\text{\times}{10}^{15}\text{\,}\mathrm{atoms}\text{\,}{\mathrm{cm}}^{-2}$
were investigated and recoil efficiencies of about $16\text{\,}\mathrm{\char
37\relax}$ were measured. For the TACTICa experiment, the fabricated sources
by SA appear well suited. The actual energy distribution of the 229Th ions can
be measured in the TACTICa ion source setup [8] to gain further insights into
the characteristics of the produced sources.

##  9 Acknowledgement

The authors acknowledge the local support of the mechanical workshop and the
electronics workshop at the TRIGA site of the Department of Chemistry at JGU
Mainz. We also acknowledge the help of P. Thörle-Pospiech with the 232U
separation. This work is supported by the Helmholtz Excellence Network
ExNet020, Precision Physics, Fundamental Interactions and Structure of Matter
(PRISMA+) from the Helmholtz Initiative and Networking Fund.

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## Appendix

Table 11: Fit parameters of the qualitative alpha spectra of all fabricated
sources, sorted by the fabrication method, and of some simulated spectra.
Constants are marked with a $*$ inside the error brackets because their error
could not be determined. When $\eta_{1}$ was set to 1.0, the peak had no
significant tailing and therefore had to be fitted only with $\tau_{1}$ and
$\sigma_{1}$. As the second term of the fit function is multiplied with
$\eta_{2}$=0, $\sigma_{2}$ and $\tau_{2}$ can be neglected. “DoF” is short for
“degrees of freedom”. Experimental spectra
---
sample |  channels |  $\eta_{1}$ |  $\sigma_{1}$ / keV |  $\sigma_{2}$ / keV |  $\tau_{1}$ / keV |  $\tau_{2}$ / keV |  $\chi^{2}/\text{DoF}$ |  R2
MP1 |  2048 |  1.00(*) |  7.51(137) |  – |  15.4(51) |  – |  1.55 |  0.7131
MP2 |  2048 |  0.94(1) |  5.28(9) |  12.5(21) |  5.21(21) |  49.7(50) |  0.74 |  0.9965
MP3 |  2048 |  1.00(*) |  10.6(21) |  – |  2.4(107) |  – |  1.41 |  0.8370
MP4 |  2048 |  0.96(1) |  9.63(31) |  104(52) |  6.32(91) |  54.4(235) |  0.75 |  0.9884
MP5 |  2048 |  0.96(1) |  8.13(22) |  84(32) |  8.04(56) |  65.1(173) |  0.68 |  0.9915
DoD1 |  2048 |  0.96(1) |  7.13(23) |  93.2(590) |  11.6(7) |  95.4(381) |  0.72 |  0.9897
DoD2 |  1024 |  0.95(1) |  6.57(19) |  30.0(*) |  9.94(45) |  125(14) |  5.75E-6 |  0.9911
DoD3 |  1024 |  0.72(6) |  5.92(38) |  12.3(15) |  15.8(16) |  63.0(57) |  3.52E-6 |  0.9881
DoD4 |  2048 |  0.93(2) |  9.27(55) |  93.2(*) |  8.29(152) |  72.3(438) |  1.09 |  0.9660
DoD5 |  2048 |  0.93(2) |  6.31(48) |  93.2(*) |  19.9(19) |  80.3(475) |  0.996 |  0.9662
DoD6 |  1024 |  0.96(1) |  7.04(24) |  75.0(*) |  7.98(52) |  259(84) |  2.73E-6 |  0.9872
DoD7 |  2048 |  0.67(7) |  8.34(40) |  14.2(12) |  11.0(20) |  52.9(52) |  0.97 |  0.9903
Ch1 |  2048 |  1.00(*) |  8.55(83) |  – |  7.95(206) |  – |  3.58 |  0.8840
Ch2 |  1024 |  1.00(*) |  10.9(14) |  – |  5.12(334) |  – |  2.72 |  0.7269
Ch3 |  1024 |  1.00(*) |  8.73(140) |  – |  9.32(344) |  – |  3.40E-6 |  0.8499
Ch4 |  1024 |  1.00(*) |  10.1(22) |  – |  3.75(682) |  – |  5.87E-6 |  0.7034
Ch5 |  1024 |  1.00(*) |  6.86(106) |  – |  11.1(22) |  – |  5.15E-6 |  0.8578
Ch6 |  2048 |  1.00(*) |  9.24(96) |  – |  9.38(247) |  – |  1.70 |  0.8974
SA1 |  2048 |  0.75(6) |  10.8(4) |  5.00(258) |  11.1(22) |  50.7(56) |  0.92 |  0.9912
SA2 |  2048 |  0.75(6) |  9.79(48) |  16.8(17) |  10.8(24) |  50.0(52) |  1.38 |  0.9886
SA3 |  2048 |  0.62(3) |  12.6(10) |  30.1(43) |  25.4(56) |  215(57) |  0.90 |  0.9800
SA4 |  2048 |  0.33(9) |  8.17(601) |  8.92(569) |  4.08(300) |  75.7(45) |  3.80 |  0.9798
Simulated spectra
L1 |  2048 |  0.91(1) |  4.8(1) |  6.6(21) |  10.1(3) |  65.7(44) |  1.18 |  0.9973
L30 |  2048 |  0.96(3) |  9.1(3) |  11.0(*) |  18.7(19) |  116(74) |  6.93 |  0.9846
L50 |  2048 |  0.94(*) |  14.4(10) |  17.5(*) |  18.5(121) |  96.5(491) |  33.2 |  0.9297
R0 |  2048 |  0.91(1) |  4.8(1) |  6.6(21) |  10.1(3) |  65.7(44) |  1.18 |  0.9973
R17 |  2048 |  0.94(*) |  6.7(1) |  6.8(*) |  16.1(6) |  95.0(67) |  2.05 |  0.9954
R51 |  2048 |  0.95(2) |  10.2(3) |  13.5(*) |  24.9(16) |  124(62) |  4.47 |  0.9896
norm |  2048 |  0.91(1) |  4.8(1) |  6.6(21) |  10.1(3) |  65.7(44) |  1.18 |  0.9973
conv |  2048 |  0.93(*) |  7.0(1) |  7.0(*) |  12.5(4) |  86.5(43) |  1.98 |  0.9970
conc |  2048 |  0.91(*) |  5.9(2) |  5.9(*) |  12.5(5) |  67.6(38) |  1.65 |  0.9933