# Production of <sup>203</sup>Pb from enriched <sup>205</sup>Tl using deuteron beams

Thomas Sounalet, Arnaud Guertin, Ferid Haddad, Keerthana Kamalakannan, Etienne Nigron

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Thomas Sounalet, Arnaud Guertin, Ferid Haddad, Keerthana Kamalakannan, Etienne Nigron. Production of <sup>203</sup>Pb from enriched <sup>205</sup>Tl using deuteron beams. Applied Radiation and Isotopes, 2024, 205, pp.111190. 10.1016/j.apradiso.2024.111190. hal-04416109

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# Production of <sup>203</sup>Pb from enriched <sup>205</sup>Tl using deuteron beams

Thomas Sounalet<sup>a,*</sup>, Arnaud Guertin<sup>a</sup>, Ferid Haddad<sup>a,b</sup>, Keerthana Kamalakannan<sup>a,b</sup>, Etienne Nigon<sup>b</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

### Keywords:

* <sup>203</sup>Pb production

* Enriched thallium target

* Cross section measurement

* Deuteron beams

* Thick target yield

## ABSTRACT

Lead-203 is a SPECT emitter that can be used in theranostic applications as an imaging counterpart of lead-212 which is intended to be used for alpha therapy as lead-212/bismuth-212 in-vivo generator. In our study, we explore the production of lead-203 using enriched thallium-205 target irradiated by a deuteron beam. Excitation functions of deuteron induced reactions leading to the formation of <sup>204m,203m2+m1+g,202m,201m+g</sup>Pb, <sup>202</sup>Tl and <sup>203m+g</sup>Hg isotopes were determined experimentally in the energy range from 21 MeV to 34 MeV. Cross sections were measured using the stacked foils technique and a set of two monitor foils, <sup>nat</sup>Ni and <sup>nat</sup>Ti for beam intensity evaluation. The experimental excitation functions of the investigated reactions were compared with the published data and also with the TENDL-2021 nuclear database. From our experimental data, we calculated lead-203 thick target yield in the energy range between 30 MeV and 32.5 MeV to be 56.7 MBq/µAh ±6.1 MBq/µAh. This value is compatible with large batch production showing that deuteron beams can be used for a routine production process. However, special attention must be paid to <sup>203</sup>Hg and other lead contaminants.

## 1. Introduction

<sup>203</sup>Pb and <sup>212</sup>Pb are promising candidates for radiopharmaceuticals and can be used for theranostic applications. <sup>203</sup>Pb emits a strong 279.19 keV gamma line and has a half-life of 51.9 h ("National Nuclear Data Center," n.d.). <sup>203</sup>Pb is then adapted for SPECT imaging while <sup>212</sup>Pb (T<sub>1/2</sub> = 10.65 h) is used for alphatherapy thanks to its emission of two &beta;<sup>-</sup> particles and one &alpha; particle during its decay chain. The production of <sup>212</sup>Pb is well known, the radionuclide is obtained after several decays from the <sup>228</sup>Th generator (Hassfjell, 2001; Hassfjell and Hoff, 1994; Zucchini and Friedman, 1982). <sup>203</sup>Pb is produced by light charged particle (proton and deuteron) bombardments of natural thallium or enriched <sup>203</sup>Tl or <sup>205</sup>Tl. Several authors have studied two different paths of <sup>203</sup>Pb production with protons (Sakai et al., 1965; Lagunas-Solar et al., 1980; Qaim et al., 1979; Bonardi et al., 1983; Al-Saleh et al., 2007; Tárkányi et al., 2013; Hermanne et al., 1992) or deuterons (Blue et al., 1978; Adam Rebeles et al., 2012). Natural thallium is composed of two stable isotopes, <sup>203</sup>Tl and <sup>205</sup>Tl, with isotopic abundances of 29.52% and 70.48% respectively. The presence of <sup>203</sup>Tl in the target lies in the impurity production of <sup>200</sup>Pb (T<sub>1/2</sub> = 21.5 h) and <sup>201</sup>Pb (T<sub>1/2</sub> = 9.3h) after irradiation even at low energy and chemically separating them from <sup>203</sup>Pb is not possible. To eliminate these impurities, it is necessary to

apply a long cooling time at the cost of a decrease in the final <sup>203</sup>Pb activity. In order to obtain a high purity <sup>203</sup>Pb radiopharmaceutical with a shorter cooling time, the production of <sup>203</sup>Pb by deuteron beams using enriched <sup>205</sup>Tl has been investigated because it is possible, to a large extent, to avoid the production of these contaminants. The stack foils technique is used to study the excitation functions of deuteron induced reactions of enriched <sup>205</sup>Tl leading to the formation of <sup>204m,203m2+m1+g, 202m,201m + g</sup>Pb, <sup>202</sup>Tl and <sup>203m + g</sup>Hg isotopes in the energy range between 21 MeV and 34 MeV. The present results are compared with Adam Rebeles et al. who used natural thallium (Adam Rebeles et al., 2012) and Blue et al. who used enriched <sup>205</sup>Tl (Blue et al., 1978) as well as with the values of the TENDL-2021 nuclear database which is based on the TALYS nuclear reaction code [8].

## 2. Materials and methods

### 2.1. Target preparation of <sup>205</sup>Tl

Thin deposits (15 µm) of enriched <sup>205</sup>Tl, purchased from Isoflex (USA) in oxide form with an isotopic composition <sup>205</sup>Tl (99.75%) and <sup>203</sup>Tl (0.25%), were obtained by electroplating on thin high-quality gold foil (15 µm thick from Goodfellow, 99.9%). The preparation of the

<footnote><sup>a</sup> Subatech, UMR 6457, IMT Atlantique, CNRS/IN2P3, Nantes Université, 4 Rue Alfred Kastler BP20722, 44307, Nantes, France</footnote>

<footnote><sup>b</sup> GIP ARRONAX, 1 Rue ARRONAX, CS10112, 44817, Saint-Herblain, France</footnote>

<footnote>* Corresponding author. 4 rue Alfred Kastler, 44307, Nantes, France.</footnote>

<footnote>E-mail address: thomas.sounalet@subatech.in2p3.fr (T. Sounalet).</footnote>
plating solution and the parameters of electrodeposition to obtain a thin, compact and homogeneous deposit have been already published (Sou- nalet et al., 2023). After having obtained a $^{205}$Tl deposit on gold backing, the deposit was protected from oxidation by putting it in a desiccator with low moisture. To carry out our experiments, 10 smooth and dense deposits of $^{205}$Tl were manufactured on gold backing. Fig. 1 shows one example of a typical $^{205}$Tl deposit on gold backing.

## 2.2. Experimental set-up

Five irradiation runs were performed at the ARRONAX facility (Haddad et al., 2008) in the energy range between 21 MeV and 34 MeV with deuteron beams, using the stacked foils method, which consists of the irradiation of a set of thin foils, grouped as patterns. Each stack was composed of two identical patterns of enriched $^{205}$Tl targets electro- deposited on thin gold foil, two different monitors, Ni and Ti (20 µm and 15 µm thick respectively), placed between a $^{205}$Tl target to measure the effective beam flux by considering a reference reaction recommended by IAEA ("Cross-Section database for medical radioisotope production: IAEA-CRP," n.d.) and Al degrader foils (100 µm, 500 µm and 800 µm thick) to change the incident beam energy for the irradiation of the next pattern with lower energy. All foils mentioned were provided by Goodfellow (99.99% purity). Each stack was positioned in air, 6 cm from the beam output window which is made of a 50 µm thick Kapton foil.

Each irradiation lasted for 60 min. The mean beam current for the stacks from 1 to 4 was 50 nA and 100 nA for the last stack.

## 2.3. Data analysis

All the activity values of the radionuclides produced in the $^{205}$Tl deposits and monitors were calculated from decay and spectrometric characteristics ("National Nuclear Data Center," n.d.) and are summarized in Table 1 and Table 2 respectively. Table 1 gives complementary information concerning the main contributing nuclear reactions of the two stable isotopes of Tl ($^{203}$Tl and $^{205}$Tl) and their associated threshold energies. The gamma ray energy of $^{203}$Pb at 279.19 keV, which corresponds to the highest intensity at 80.9%, is omitted from Table 1 because of the interference with $^{203}$Hg.

<figure><img src="image_4.png" /><figcaption>Fig. 1. Photograph of a typical $^{205}$Tl target electrodeposited on gold foil.</figcaption></figure>

<table><caption>**Table 1**

Decay data and main contributing production reactions of $^{203}$Tl and $^{205}$Tl.</caption><thead><tr><th>Nuclide</th><th>Half-life</th><th>γ energy (keV)</th><th>γ intensity (%)</th><th>Nuclear reaction</th><th>Threshold energy (MeV)</th></tr></thead><tbody><tr><td rowspan="2"><sup>204</sup>Pb</td><td>66.93</td><td>374.76</td><td>94.2</td><td><sup>203</sup>Tl(d,n)</td><td>0</td></tr><tr><td>m</td><td>899.15</td><td>99.17</td><td><sup>205</sup>Tl(d,3n)</td><td>9.88</td></tr><tr><td rowspan="2"><sup>203</sup>Pb</td><td>51.92</td><td>401.32</td><td>3.35</td><td><sup>203</sup>Tl (d,2n)</td><td>4.02</td></tr><tr><td>h</td><td></td><td></td><td><sup>205</sup>Tl (d,4n)</td><td>18.36</td></tr><tr><td rowspan="2"><sup>202</sup>Pb</td><td>3.54 h</td><td>422.12</td><td>84</td><td><sup>203</sup>Tl (d,3n)</td><td>10.99</td></tr><tr><td></td><td>960.70</td><td>89.9</td><td><sup>205</sup>Tl (d,5n)</td><td>25.35</td></tr><tr><td rowspan="2"><sup>201</sup>Pb</td><td>9.33 h</td><td>331.15</td><td>77</td><td><sup>203</sup>Tl l(d,4n)</td><td>19.81</td></tr><tr><td></td><td></td><td></td><td><sup>205</sup>Tl (d,6n)</td><td>34.17</td></tr><tr><td rowspan="2"><sup>202</sup>Tl</td><td>12.31</td><td>439.51</td><td>91.5</td><td><sup>203</sup>Tl (d,t)</td><td>1.61</td></tr><tr><td>d</td><td></td><td></td><td><sup>203</sup>Tl (d,n + d)</td><td>7.92</td></tr><tr><td rowspan="7"><sup>201</sup>Tl</td><td rowspan="7">3.04 d</td><td rowspan="7">167.43</td><td rowspan="7">10</td><td><sup>203</sup>Tl (d,2n + p)</td><td>10.18</td></tr><tr><td><sup>205</sup>Tl (d,3n + d)</td><td>22.24</td></tr><tr><td><sup>205</sup>Tl (d,4n + p)</td><td>24.52</td></tr><tr><td><sup>203</sup>Tl (d,n + t)</td><td>8.54</td></tr><tr><td><sup>203</sup>Tl (d,2n + d)</td><td>14.85</td></tr><tr><td><sup>203</sup>Tl (d,3n + p)</td><td>17.12</td></tr><tr><td><sup>205</sup>Tl (d,3n + t)</td><td>22.86</td></tr><tr><td rowspan="5"><sup>203</sup>Hg</td><td rowspan="5">46.59 d</td><td rowspan="5">279.19</td><td rowspan="5">81.56</td><td><sup>205</sup>Tl (d,4n + d)</td><td>29.17</td></tr><tr><td><sup>205</sup>Tl (d,5n + p)</td><td>31.46</td></tr><tr><td><sup>205</sup>Tl (d,α)</td><td>0.00</td></tr><tr><td><sup>203</sup>Tl (d,2p)</td><td>1.95</td></tr><tr><td><sup>205</sup>Tl (d,p + t)</td><td>7.72</td></tr><tr><td rowspan="5"><sup>204</sup>Ni</td><td rowspan="5"><sup>204</sup>Ti</td><td rowspan="5"><sup>58</sup>(m)Co</td><td rowspan="5">70.86 d</td><td rowspan="5">983.52</td><td><sup>205</sup>Tl (d, n + n+<sup>3</sup>He)</td><td>8.49</td></tr><tr><td><sup>205</sup>Tl (d,2d)</td><td>11.79</td></tr><tr><td><sup>205</sup>Tl (d,n + p + d)</td><td>14.03</td></tr><tr><td><sup>205</sup>Tl (d,2p+2n)</td><td>16.27</td></tr></tbody></table>

<table><caption>**Table 2**

Decay data used for $^{nat}$Ti and $^{nat}$Ni monitors.</caption><thead><tr><th>Monitor</th><th>Nuclide</th><th>Half-life</th><th>γ energy (keV)</th><th>γ intensity (%)</th></tr></thead><tbody><tr><td><sup>204</sup>Ni</td><td><sup>58</sup>(m)Co</td><td>70.86 d</td><td>810.76</td><td>99.45</td></tr><tr><td><sup>204</sup>Ti</td><td><sup>48</sup>V</td><td>15.97 d</td><td>983.52</td><td>99.98</td></tr></tbody></table>

The Fitzpeak software (Fitzgerald and Servives's, n.d.) has been used to determine the activity at end of bombardment, EOB. Energy and efficiency calibrations of the HPGe detector were carried out with a multi-gamma liquid source in two geometries at 0 cm (contact with detector) and 19 cm and with a $^{152}$Eu sealed solid source at 19 cm from the detector head.

To ensure a reasonable dead time during counting, the irradiated enriched $^{205}$Tl deposit and gold foil must be separated. The deposit was dissolved using 10 mL of concentrated nitric acid (10 mol/L) and an aliquot of 5 mL was used for counting on the HPGe detector. The solutions were counted 12 h after the irradiation. The dilution factor of 2 was taken into account to determine the produced activities in Table 1. The distance between the sample and the detector was 19 cm except for dedicated $^{203}$Hg counting for which the activity was measured 40 days after EOB at contact.

Solid titanium monitor foils were measured by the same detector 19 days after EOB (Nigron et al., 2021) to allow the full decay of $^{48}$Sc (T$_{1/2}$ = 43.67 h, E = 983.526 keV, I$_{γ}$ = 100.1%) and avoid interferences. Similarly, nickel monitor foils were measured 2 months after EOB
(Hermanne et al., 2013) to avoid interferences of <sup>56</sup>Ni (T<sub>1/2</sub> = 6.075 d, E = 811.85 keV, I<sub>y</sub> = 86%). The distance between the sample and the detector was always 19 cm. All targets and foils were counted for 24 h and the dead time for liquid samples and solid samples was below 5% for all cases.

The energy loss through each thin foil and the enriched <sup>205</sup>Tl deposit was determined using SRIM software (Ziegler, 2004) and taking into account the different material crossed by the projectiles. The energy associated with experimental points was taken in the middle of each foil. Table 3 gives the deuteron energy in each <sup>205</sup>Tl deposit for each stack. A 0.25 MeV incident beam energy error was considered as provided from simulations by the cyclotron provider. Each target of enriched <sup>205</sup>Tl deposit was weighed and scanned before irradiation to precisely determine its area. Thus, from these values, the thickness was accurately deduced.

## 2.4. Cross-section calculations

The production cross sections of produced radionuclides in a thin foil can be calculated using the activation formula with the appropriate projectile flux:

$$ \sigma(E) = \frac{\text{Act}(E) \cdot A}{\varphi \cdot \chi \cdot N_A \cdot M \cdot (1 - e^{-\lambda t})} \quad (1) $$

In Equation (1), the production cross section $\sigma(E)$ at the (1)energy E depends on the activity of a radionuclide corrected to the EOB, Act(E) (Bq), and its decay constant $\lambda$ (s<sup>-1</sup>). It also depends on the parameters of the target as the target areal density, M (g.cm<sup>-2</sup>), its isotopic abundance $\chi$ and its molar mass, A (g.mol<sup>-1</sup>), and the Avogadro constant (N<sub>A</sub>). And it finally depends on the irradiation parameters as the irradiation duration t (s) and the projectile flux $\varphi$ (particle s<sup>-1</sup>).

The incident projectile flux $\varphi$ is determined thanks to the use of monitor foils. Each thin target foil received the same flux as the monitor foil placed behind it. For each monitor foil, a monitor reaction is well known allowing to precisely determine the irradiation flux $\varphi$. We used the natNi monitor for that purpose and the natTi monitor foil allowed us to crosscheck irradiation parameters. The values of incident projectile flux converted in mean beam current from both monitor foils, natNi and natTi, are given in Table 4.

A propagation error calculation is used to estimate the cross section uncertainties and its formula is expressed in Equation (2):

$$ \frac{\Delta\sigma(E)}{\sigma(E)} = \sqrt{\left(\frac{\Delta\text{Act}(E)}{\text{Act}(E)}\right)^2 + \left(\frac{\Delta\varphi(E)}{\varphi(E)}\right)^2 + \left(\frac{\Delta M}{M}\right)^2} \quad (2) $$

## 2.5. Corrected data from literature

Enriched <sup>205</sup>Tl are used in our experiments. Our data can be directly compared that of Blue et al. (1978) who used also enriched material. Data from Adam Rebeles et al. used natural thallium (Adam Rebeles et al., 2012). The isotopic abundances of natural thallium are 70.48% for <sup>205</sup>Tl and 29.52% for <sup>203</sup>Tl. To compare with our values, we have rescaled their values by a factor of 1.4 or 3.4 to take into account the isotopic composition of natTl. A factor of 1.4 has been used for nuclear reactions taking place mainly on <sup>205</sup>Tl (factor 1.4 = 1/0.7048). This

<table><caption>**Table 3**
Deuteron energy in each <sup>205</sup>Tl deposit for each stack.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Deuteron energy (MeV)</th></tr><tr><th>Foil 1</th><th>Foil 2</th></tr></thead><tbody><tr><td>Stack 1</td><td>26.0 ± 0.3</td><td>21.5 ± 0.5</td></tr><tr><td>Stack 2</td><td>29.8 ± 0.3</td><td>24.3 ± 0.4</td></tr><tr><td>Stack 3</td><td>33.1 ± 0.3</td><td>29.5 ± 0.3</td></tr><tr><td>Stack 4</td><td>27.9 ± 0.4</td><td>22.2 ± 0.5</td></tr><tr><td>Stack 5</td><td>31.6 ± 0.3</td><td>27.8 ± 0.4</td></tr></tbody></table>

<table><caption>**Table 4**
Values of mean beam current for both monitors, natNi, and natTi in each stack.</caption><thead><tr><th></th><th>Mean beam current (nA) from<br>natNi monitor foil</th><th>Mean beam intensity (nA) from<br>natTi monitor foil</th></tr></thead><tbody><tr><td>Stack 1</td><td>58.9 ± 5.8</td><td>57.1 ± 4.2</td></tr><tr><td>Stack 2</td><td>57.8 ± 5.8</td><td>61.2 ± 4.5</td></tr><tr><td>Stack 3</td><td>54.3 ± 6.0</td><td>55.7 ± 4.3</td></tr><tr><td>Stack 4</td><td>54.6 ± 4.8</td><td>53.4 ± 3.9</td></tr><tr><td>Stack 5</td><td>102.5 ± 8.0</td><td>104.7 ± 7.7</td></tr></tbody></table>

concerns mainly <sup>204</sup>m, <sup>203</sup>m, <sup>202</sup>Tl. A factor of 3.4 has been used for nuclear reactions taking place on <sup>203</sup>Tl (factor 3.4 = 1/0.2952). This concerns mainly <sup>201m+g</sup>Pb.

# 3. Results

The measured cross sections with the estimated total errors for each investigated radionuclide at a certain energy are given in Table 5.

## 3.1. Production of <sup>204m</sup>Pb

Two states exist for <sup>204</sup>Pb: the ground state (T<sub>1/2</sub> ≥ 1.4 10<sup>17</sup> y) and a short-lived metastable (T<sub>1/2</sub> = 66.93 min) which decays by IT to the ground state. Only the short-lived metastable state is measured by using two intense gamma rays at 374.76 keV (94.2%) and 899.15 keV (99.17%). Due to the short half-life of <sup>204</sup>mPb, its presence is low after 12 h of cooling, resulting in significant measurement uncertainties, up to 45.1% at 33.1 MeV. We were able to extract only 6 out of 10 targets irradiated during our experiment.

No publications using enriched <sup>203</sup>Tl or <sup>205</sup>Tl have studied the nuclear reactions of <sup>203</sup>Tl(d,n) <sup>204</sup>mPb and <sup>205</sup>Tl(d,3n) <sup>204</sup>mPb. Our results are compared with those of Adam Rebeles et al. corrected as mention earlier and also with the values extracted from TENDL-2021, specifically considering the <sup>205</sup>Tl(d,3n) <sup>204</sup>mPb reaction. From Fig. 2, the value of cross sections rises when decreasing the energy reaching 463 mb at 22 MeV. The prediction of TENDL-2021 data is in accordance with our results and is slightly lower than that of Adam Rebeles et al. corrected as

<table><caption>**Table 5**
Measured cross sections of deuteron induced reactions on enriched <sup>205</sup>Tl with an enrichment of 99.75%. <sup>201m + g</sup>Pb is produced on <sup>203</sup>Tl.</caption><thead><tr><th>E<sub>d</sub><br>(MeV)</th><th>204mPb<br>(mb)</th><th>203m2+m1+gPb<br>(mb)</th><th>202mPb<br>(mb)</th><th>201m +<br>gPb<br>(mb)</th><th>202Tl<br>(mb)</th><th>203m +<br>gHg</th></tr></thead><tbody><tr><td>21.5 ± 0.5</td><td>-</td><td>46 ± 5</td><td>1 ± 0.4</td><td>-</td><td>0.3 ± 0.0</td><td>4 ± 0.5</td></tr><tr><td>22.2 ± 0.5</td><td>463 ± 41</td><td>67 ± 7</td><td>0.9 ± 0.1</td><td>-</td><td>-</td><td>4 ± 0.5</td></tr><tr><td>24.3 ± 0.4</td><td>408 ± 137</td><td>353 ± 39</td><td>0.9 ± 0.3</td><td>174 ± 58</td><td>0.4 ± 0.0</td><td>3 ± 0.4</td></tr><tr><td>26.0 ± 0.3</td><td>320 ± 111</td><td>694 ± 77</td><td>0.8 ± 0.3</td><td>469 ± 83</td><td>0.4 ± 0.1</td><td>3 ± 0.3</td></tr><tr><td>27.8 ± 0.4</td><td>-</td><td>913 ± 86</td><td>-</td><td>721 ± 116</td><td>-</td><td>-</td></tr><tr><td>27.9 ± 0.4</td><td>260 ± 23</td><td>1059 ± 108</td><td>0.8 ± 0.3</td><td>939 ± 119</td><td>0.8 ± 0.2</td><td>3 ± 0.4</td></tr><tr><td>29.5 ± 0.3</td><td>-</td><td>1117 ± 137</td><td>0.8 ± 0.3</td><td>1215 ± 179</td><td>1 ± 0.1</td><td>3 ± 0.3</td></tr><tr><td>29.8 ± 0.3</td><td>157 ± 34</td><td>1044 ± 116</td><td>1 ± 0.2</td><td>1116 ± 139</td><td>1 ± 0.2</td><td>3 ± 0.3</td></tr><tr><td>31.6 ± 0.3</td><td>-</td><td>1333 ± 124</td><td>11 ± 1</td><td>1485 ± 176</td><td>-</td><td>-</td></tr><tr><td>33.1 ± 0.3</td><td>163 ± 73</td><td>1180 ± 144</td><td>74 ± 9</td><td>1509 ± 232</td><td>10 ± 0.9</td><td>2 ± 0.3</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Excitation function for the $^{205}\text{Tl}(d,3n)^{204}\text{Pb}$ nuclear reaction.</figcaption></figure>

previously mentioned.

## 3.2. Production of $^{203}\text{m}^{2+\text{m}+\text{g}}\text{Pb}$

$^{203}\text{Pb}$ possesses two short-lived metastable states, with respective half-lives of 480 ms and 6.21 s, decaying by IT to the ground state. The ground state has a half-life of 51.92 h. The irradiated samples are counted using a HPGe detector 12 h after EOB. In these conditions, these metastable states cannot be measured and only the cumulative production cross section for the $^{203}\text{m}^{2+\text{m}+\text{g}}\text{Pb}$ can be determined.

The most significant gamma ray intensities were 80.9% and 3.35% for 279.19 keV and 401.32 keV gamma rays respectively. However, an interference at 279.19 keV exists coming from $^{203}\text{Hg}$ produced by the nuclear reactions of $^{203}\text{Tl}(d,2p)^{203}\text{Hg}$ and $^{205}\text{Tl}(d,x)^{203}\text{Hg}$. We then decided to only use the 401.32 keV to measure the activity of $^{203}\text{m}^{2+\text{m}+\text{g}}\text{Pb}$.

Fig. 3 reports the $^{205}\text{Tl}(d,4n)^{203}\text{m}^{2+\text{m}+\text{g}}\text{Pb}$ cross section measured in this work in the energy range from 21 MeV to 34 MeV. Two set of data are available in the EXFOR library measured by Blue et al. (1978) and Adam Rebeles et al. (Adam Rebeles et al., 2012). The uncertainties from Blue et al. are not reported in the database. These data seem slightly shifted to higher energy when compared to our data and those of Adam Rebeles et al. corrected as previously mentioned. These two set of data

are in accordance except in the energy region of 32 MeV where our data point is 16% higher. Data of $^{205}\text{Tl}(d,4n)^{203}\text{m}^{2+\text{m}+\text{g}}\text{Pb}$ reaction from values present in the TENDL-2021 database [3] are also plotted in Fig. 3. These values are in agreement with our data up to 30 MeV and are lower at higher incident energy.

## 3.3. Production of $^{202}\text{m}^{2+\text{m}}\text{Pb}$

$^{202}\text{Pb}$ is produced in the sample either as a metastable state ($T_{1/2} = 3.54$ h), $^{202}\text{mPb}$, or as a ground state ($T_{1/2} = 52.5 \text{ 10}^3 \text{ y}$). $^{202}\text{mPb}$ decays at 90.5% into to the long-lived ground state and at 9.5% into $^{202}\text{Tl}$. Due to the long half-life of $^{202}\text{Pb}$, this radionuclide could not be measured in the present experiment. Only the short-lived $^{202}\text{mPb}$ was measured with our HPGe detector 12 h after EOB.

From the emitted gamma rays 422.12 keV, 657.49 keV, 786.99 keV, and 960.70 keV with respective intensities of 84%, 31.7%, 49%, and 89.9%, only the 422.12 keV and 960.70 keV gamma lines were used for the evaluation of the production of $^{202}\text{mPb}$ due to their high intensity. In Fig. 4, our results are compared with the two existing set of data, Blue et al. (1978) and Adam Rebeles et al. corrected [13] and the data extracted from TENDL-2021 database, specifically $^{205}\text{Tl}(d,5n)^{202}\text{mPb}$ reaction. The points of these two authors are congruent and are in agreement to our values (Fig. 4). A notable piece of information from

<figure><img src="image_6.png" /><figcaption>Fig. 3. Cross section of the $^{205}\text{Tl}(d,4n)^{203}\text{m}^{2+\text{m}+\text{g}}\text{Pb}$ nuclear reaction.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. Cross section of the $^{205}$Tl(d,5n)$^{202}$mPb nuclear reaction.</figcaption></figure>

Adam Rebeles et al. corrected is that above 30 MeV, the nuclear reaction of $^{205}$Tl(d,5n)$^{202}$mPb is predominant. TENDL-2021 data for the $^{205}$Tl(d, 5n)$^{202}$mPb reaction do not reproduce the data even if the trend in the energy range of interest is similar to that of the data. Our data show a small contribution below the energy threshold of the $^{205}$Tl(d,5n)$^{202}$mPb (25.35 MeV) coming from the 0.25% content on $^{203}$Tl in the target.

### 3.4. Production of $^{201m+gPb}$

The energy range of our study between 21 MeV and 34 MeV is below the threshold energy associated to the nuclear reaction of $^{205}$Tl (d,6n)$^{201}$Pb (34.175 MeV). So, only the contribution of $^{203}$Tl (0.25%) is measured in our experiment. $^{201}$Pb has a very short half-life ($T_{1/2}$ = 60.8 s) metastable state, and a middle half-life, $T_{1/2}$ = 9.33 h, ground state. The metastable state decays solely by IT to the ground state. As our measurement are performed 12 h after EOB, we only access to the cumulative production of the cross section, $^{201m+gPb}$ after total decay of the metastable state.

Only one gamma ray energy at 331.15 keV with high intensity at 77% is used to measure the activity of $^{201m+gPb}$. Our values have been rescaled for a 100% $^{203}$Tl target.

Blue et al. (1978) and Adam Rebeles et al. corrected (rescaled to a 100% enriched $^{203}$Tl material) (Adam Rebeles et al., 2012) measured the

excitation function of the $^{203}$Tl(d,4n)$^{201m+gPb}$ and their values are in agreement with our values (Fig. 5) until 30 MeV. Above this energy, our values are 1.2 time higher than the corrected values by Adam Rebeles et al. corrected. The extracted values of TENDL-2021 for the $^{203}$Tl(d, 4n)$^{201m+gPb}$ reaction are smaller for the whole energy range (Fig. 5). Maximum of the excitation function for the production of $^{201m+gPb}$ occurs at deuteron energy of 33.1 MeV, where it reaches cross section of 1508 mb.

### 3.5. Production of $^{202}$Tl

$^{202}$Tl, which has a half-life of 12.31 d, is produced indirectly by the decay of its parent $^{202}$mPb and directly through the nuclear reactions of $^{205}$Tl(d,x)$^{202}$Tl and $^{203}$Tl(d,x)$^{202}$Tl. Among different energy of gamma rays, only one has a high intensity of 91.5% at the energy of 439.51 keV and this energy is used to measure the activity of $^{202}$Tl. Our measurement were performed 40 days after EOB to have the complete decay of $^{202}$mPb.

Our data are presented in Fig. 6 together with the values extracted from TENDL-2021 and the values from Adam Rebeles et al., which have been corrected for the contribution of the $^{203}$Tl(d,x)$^{202}$Tl nuclear reaction at a proportion of 0.25%. Our measured values primarily stem from the $^{203}$Tl(d,x)$^{202}$Tl nuclear reaction, which were obtained within the

<figure><img src="image_8.png" /><figcaption>Fig. 5. Cross section of the $^{203}$Tl(d,4n)$^{201m+gPb}$ nuclear reaction.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 6. Cross section of the $^{205}$Tl(d,x)$^{202}$Tl.</figcaption></figure>

deuteron energy range from 21 MeV to 32 MeV. However, beyond 32 MeV, our measured value deviates from the corrected data of Adam Rebeles et al. for this nuclear reaction and the $^{205}$Tl(d,x)$^{202}$Tl nuclear reaction becomes predominant. The values extracted from TENDL-2021 for the $^{205}$Tl(d,x)$^{202}$Tl reaction do not align with our measured values in the energy range from 21 MeV to 32 MeV; instead, they provide lower values.

### 3.6. Production of $^{201m+gTl}$

$^{201}$Tl is produced directly either as a short-lived metastable state ($T_{1/2}$ = 2.01 ms) decaying to the ground state by IT or a long-lived ground state ($T_{1/2}$ = 3.04 d) and indirectly through the decay of its parents $^{201}$Pb. For all energy range from 21 MeV to 34 MeV, our HPGe detector did not detect $^{201m+gTl}$.

### 3.7. Production of $^{203m+gHg}$

$^{203}$Hg has a short-lived metastable state ($T_{1/2}$ = 22.1 µs) which decays by IT at 100% to the long-lived ground state with a half-life 46.594 d. Only one gamma of 279.19 keV with a branching ratio of 81.56% is emitted during the decay of $^{203}$Hg. This gamma is common with $^{203}$Pb. To limit interferences with $^{203}$Pb, we did a measurement 40 days after

EOB.

The excitation function is presented on Fig. 7 and shows low values for all energy between 4 mb and 2 mb and the TENDL 2021 data are from 1.5 to 2 times weaker.

### 3.8. $^{203}$Pb production yield

$^{203}$Pb is produced by the $^{205}$Tl(d,4n)$^{203m2+m1+gPb}$ reaction from 18.36 MeV and $^{203}$Tl is not contributing. However, even with a low percentage (0.25%), we could not avoid the production of $^{201m+gPb}$ by the $^{203}$Tl(d,4n)$^{201m+gPb}$. The shape of the excitation functions of both nuclear reactions is close, which means that the production of $^{201m+gPb}$ is inevitable in the deuteron energy range from 21 MeV to 34 MeV. Using the data measured during this set of experiments, we were able to calculate the production yield at EOB of $^{203}$Pb and $^{201}$Pb coming from $^{205}$Tl enriched to 99.75% and natural Tl respectively (see Fig. 8). The production yield values from natural Tl are extracted from the measurement of Adam Rebeles et al. (Adam Rebeles et al., 2012). The production yield in the range of irradiation energy between 30 MeV – 32.5 MeV and 28.5 MeV – 31 MeV respectively, ranges corresponding to a thallium deposit of 40 µm, are presented to Table 6. From 30 MeV to 32.5 MeV, the production yield of $^{203}$Pb coming from $^{205}$Tl is high, 56.7 MBq/µAh contrary to those coming from natural Tl, 36.3 MBq/µAh.

<figure><img src="image_10.png" /><figcaption>Fig. 7. Cross section of the $^{205}$Tl(d,x)$^{203m+g}$Hg nuclear reaction.</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 8. Production yield of $^{203}$Pb and $^{201}$Pb from $^{205}$Tl enriched to 99.75% and natural Tl.</figcaption></figure>

<caption>**Table 6**</caption>

Production yield of $^{203}$Pb and $^{201}$Pb from $^{205}$Tl enriched to 99.75% and natural Tl with deuteron beams. r is the activity ratio of $^{203}$Pb on $^{201}$Pb, expressed in percentage.

<table><thead><tr><th rowspan="2"></th><th colspan="3">From 30 MeV to 32.5 MeV</th></tr><tr><th>203m1+m2+gPb</th><th>201m +<br>gPb</th><th>r</th></tr></thead><tbody><tr><td>From <sup>205</sup>Tl with 99.75% enrichment (MBq/<br>μAh)</td><td>56.7</td><td>0.9</td><td>1.5%</td></tr><tr><td>natTl</td><td>36.3</td><td>86.4</td><td>237.8%</td></tr></tbody></table><table><thead><tr><th rowspan="2"></th><th colspan="3">From 28.5 MeV to 31 MeV</th></tr><tr><th>203m1+m2+gPb</th><th>201m +<br>gPb</th><th>r</th></tr></thead><tbody><tr><td>From <sup>205</sup>Tl with 99.75% enrichment (MBq/<br>μAh)</td><td>52.4</td><td>0.7</td><td>1.4%</td></tr><tr><td>natTl</td><td>33.3</td><td>73.2</td><td>220.2%</td></tr></tbody></table>

These values decrease with the energy, 52.4 MBq/μAh and 33.3 MBq/μAh respectively. Furthermore, the ratio between $^{201}$Pb and $^{203}$Pb yield is very low if $^{205}$Tl enriched material is used, decreasing with the energy from 1.5% to 1.4% whereas using natural Tl, the values expected

are high, from 237.8% to 220.2%. Using enriched $^{205}$Tl, we can greatly limit the decay time as compare to natural Tl target. Using deuteron beams, it is possible to obtain a production yield of 56.7 MBq/μAh in the energy range from 30 MeV to 32.5 MeV, which allow the production of large batches.

Fig. 9 and Table 7 show the comparison of production yield of $^{203}$Pb and $^{201}$Pb with deuteron and proton beams. To produce $^{203}$Pb with proton beams, the energy must be lower than 24 MeV to avoid producing $^{200}$Pb (T$_{1/2}$ = 21.5 h). Indeed, the threshold energy of $^{203}$Tl (p,4n)$^{200}$Pb is 24.5 MeV. Considering that the Tl thickness is 40 µm, the input energy is 24 MeV with proton beams, the output energy will be

<caption>**Table 7**</caption>

Production yield of $^{203}$Pb and $^{201}$Pb from $^{205}$Tl enriched to 99.75% with proton beams. r is the activity ratio of $^{203}$Pb on $^{201}$Pb, expressed in percentage.

<table><thead><tr><th rowspan="2"></th><th colspan="3">From 24 MeV to 22.2 MeV with proton beams</th></tr><tr><th>203m1+m2+gPb</th><th>201m +<br>gPb</th><th>r</th></tr></thead><tbody><tr><td>From <sup>205</sup>Tl with 99.75% enrichment (MBq/μAh)</td><td>46.2</td><td>0.4</td><td>0.9%</td></tr></tbody></table>

<figure><img src="image_12.png" /><figcaption>Fig. 9. Production yield of $^{203}$Pb and $^{201}$Pb from $^{205}$Tl enriched to 99.75% with deuteron and proton beams.</figcaption></figure>
22.2 MeV. The production yields of $^{203}$Pb and $^{201}$Pb are 46.2 MBq/µAh and 0.4 MBq/µAh. Production yield obtained for $^{203}$Pb is slightly weaker (56.7 MBq/µAh and 46.1 MBq/µAh for deuteron and proton beams respectively) but at the same time the $^{201}$Pb production yield is 2.2 times lower with the proton beam. The activity ratio between $^{201}$Pb and $^{203}$Pb with proton beams at EOB is 0.9%, 1.7 times lower than if $^{205}$Tl is irradiated with deuteron beams.

$^{203}$Pb is also produced using enriched $^{203}$Tl (McNeil et al., 2021, 2023) at low energy at 13 MeV with protons. At this energy, the production of $^{201}$Pb is not produced due to its threshold energy of 17.56 MeV (Tárkányi et al., 2013). The production of $^{203}$Pb is then high purity and accessibility with a medical cyclotron. However, its production yield is very low. Instead, using enriched $^{203}$Tl the cross section value at 13 MeV is reported as 98.2 mb (Tárkányi et al., 2013) while for protons or deuterons on enriched $^{205}$Tl, the cross section values are higher, reaching 1379 mb at 26.6 MeV (Tárkányi et al., 2013) and 1333 mb at 31.6 MeV (our work), respectively. Producing $^{203}$Pb with a medical cyclotron is feasible but quantities will be limited restricting its use at the local scale.

GIP ARRONAX has a rabbit system that can accommodate a 15° tilted target and is cooled by water to efficiently dissipate the heat during irradiation. This system will be used to produce $^{203}$Pb in a large batch mode. Using this rabbit system, the forecast of $^{203}$Pb and $^{201}$Pb activities for 8 h hours of irradiation with 50 µA of deuteron beams using a deposit of $^{205}$Tl, enriched at 99.75%, with a thickness of 40 µm (energy range from 30 MeV to 32.5 MeV), is that 20.7 GBq and 0.3 GBq can be produced respectively. $^{203}$Hg is also produced in this case, its expected activity is 2.18 MBq (production yield expected is 0.01 MBq/µAh). A specification of $^{203}$Pb from NIDC ("Product Catalog | NIDC: National Isotope Development Center," n.d.) indicates that the radionuclidic purity must be above 98%. This percentage can be reached with the previous irradiation parameters after a 12 h day time.

The separation of lead and thallium is well known using a lead resin (Li et al., 2017; Nelson et al., 2023; McNeil et al., 2021) to suppress $^{202}$Tl, $^{201}$Tl and stable Tl and to allow the recycling of $^{205}$Tl enriched material. In this case, the separation yield is documented to be more than 99%. A study must be conducted to eliminate traces of $^{203}$Hg and stable Hg to obtain a high purity of $^{203}$Pb solution.

## 4. Conclusion

The radionuclide productions of the $^{204m}$, $^{203m}$+gPb, $^{202}$, $^{201m}$+gTl and $^{203m}$+gHg from irradiation of enriched $^{205}$Tl target with deuteron have been experimentally determined in the energy range from 21 MeV to 34 MeV. Our cross section values are slightly above those of Blue et al. (1978) and Adam Rebeles et al. corrected as previously mentioned (Adam Rebeles et al., 2012). However, we have used enriched $^{205}$Tl in contrast to Adam Rebeles et al. who have used natural thallium. To compare the values from Adam Rebeles et al. with our measured values, these values have been corrected by the factor of 1.4 or 3.4 which correspond to rescaled the natural isotope of $^{205}$Tl from 70.48% to 100% for produced radio-isotopes of $^{204m}$,$^{203m}$+m+g,$^{201m}$+gPb and $^{202}$Tl and of $^{203}$Tl from 29.52% to 100% for $^{201m}$+gPb.

In general, the TENDL-2021 database indicates lower values compared to our experimental results.

Using $^{205}$Tl deposit enriched to 99.75%, the production yield estimation in the 30 MeV – 32.5 MeV energy interval, which corresponds to a 40 µm thick thallium target, indicates that 56.7 MBq/µAh ± 6.1 MBq/µAh and 0.9 MBq/µAh ± 0.1 MBq/µAh of $^{203m}$+m+gPb and $^{201m}$+gPb respectively will be produced at EOB. Due to the quite short half-life of $^{201m}$+gPb, after 12 h of cooling, 98% radionuclidic purity of $^{203}$Pb will be reached. Using the deuteron route, contrary to the production with proton beams, the $^{203}$Hg radio-impurity is produced with a yield of 0.01 MBq/µAh. This means that it is necessary to separate Pb from Tl but also Hg elements to have a high purity $^{203m}$+m+gPb solution after 12h of cooling. The activity of $^{203}$Pb can reach up to 11.9 GBq at the end of

bombardment (EOB) with 3 h of irradiation at 70 µA. After 12 h of cooling, 10.1 GBq of $^{203}$Pb with more than 98% radionuclidic purity can be available for radiolabelling. This level of activity is suitable for preclinical applications, and the solution can be distributed to multiple customers. To our knowledge, people us 1–2 GBq as starting activity to produce a patient dose. Our production level is then compatible with clinical trial. This work confirms that $^{203}$Pb large-scale production is feasible with deuteron beams and the production will be done soon at GIP ARRONAX.

## CRediT authorship contribution statement

**Thomas Soualet:** Writing – original draft. **Arnaud Guertin:** Writing – review & editing, Validation, Methodology, Investigation. **Ferid Haddad:** Writing – review & editing, Validation, Resources, Methodology, Investigation. **Keerthana Kamalakannan:** Validation, Methodology, Investigation. **Etienne Nigon:** Writing – review & editing, Validation, Methodology, Investigation.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Data availability

No data was used for the research described in the article.

## Acknowledgements

The cyclotron Arronax is supported by CNRS, Inserm, INCa, the Nantes University, the Regional Council of Pays de la Loire, local authorities, the French government and the European Union. This work has been, in part, supported by grants from the French National Agency for Research called "France 2030 investment plan", Equipex Arronax-Plus (ANR-11-EQPX-0004), Labex IRON (ANR-11-LABX-0018) and ISITE NExT (ANR-16-IDEX-0007).

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