Electro-Precipitation of Actinides on Boron-Doped Diamond Thin Films for Solid Sources Preparation for High-Resolution Alpha-Particle Spectrometry

Featured Application: We propose a novel approach enabling the fabrication of high resolution alpha sources with interest for a range of spectroscopy applications. Abstract: In this work, we investigate a novel approach to prepare high-performance alpha-particle solid sources fabricated on diamond thin support layers, o ﬀ ering the properties of diamond such as a low-Z material with corrosion and mechanical hardness. As-prepared solid sources onto boron-doped-diamond (BDD) substrate exhibited high performance of the autoradiography and spectroscopic resolution at the level of other more conventional materials such as stainless steel. A straightforward precipitation process in the Na 2 SO 4 or NaNO 3 simple electrolytes under mild experimental conditions with a low current of several mA.cm − 2 were successfully developed onto BDD substrates for deposition of single 241 Am as well as 239 Pu, 241 Am, and 244 Cm mixed radionuclides. The results demonstrate that solid sources deposited onto such BDD substrates can match the performance of those prepared onto stainless steel substrates with excellent uniformity and high-resolution spectroscopy, together combining the robustness, chemical resilience, and X-ray transparence of the diamond. Alpha-particle spectra exhibiting a low full width at half maximum (FWHM) of 12.5 keV at the energy of 5.485 MeV ( 241 Am) could be practically obtained for BDD substrates.


Introduction
Alpha-particle spectroscopy is commonly used for applications in various fields such as nuclear [1][2][3][4], environmental [5][6][7][8], artistic [9,10], and biological [11][12][13][14] samples. Conventional approaches for the preparation of alpha-particle sources generally include (i) radiochemical separation and purification [15,16], (ii) preparation of weightless and thin solid source onto a suitable backing material [17][18][19], and (iii) alpha-particle spectroscopic measurements under vacuum conditions using solid-state semiconductor detectors [20][21][22][23][24]. High-resolution alpha-spectrometry is always needed for qualitative identification and accurate determination of the alpha-emitting radioactive isotopes concerned. For this purpose, the preparation of low mass solid sources with high uniformity that reveals a significant feature is still today extensively investigated. Conventional approaches include vacuum sublimation, drop-casting, co-precipitation, and electro-precipitation [16,17,21,25,26]. While drop-casting generally allows relatively poor resolution with a large low-energy tail contribution, vacuum sublimation requires an elaborated set-up and gives a relatively poor deposition yield, and co-precipitation is very time-consuming since the sample must be pretreated using chromatography for separation and purification of the actinides. Finally, electro-precipitation represents a more straightforward and spreading route allowing a quantitative alpha-emitter deposition with a high-quality solid radioactive source deposited in the form of a very thin and uniform coating.
With regard to electro-precipitation techniques, this method was firstly developed in the early 1950s [27,28] and extensively explored since, following the works published in 1972 by Puphal et al. [29] and Talvitie [30] using ammonium salt with various counter anions as electrolytes. The main principle of this method is based on the use of a negative current for depleting water into OH − hydroxyl ions generated close to the cathode electrode which subsequently reacts with highly and positively charged actinide ions (Ac n+ ) affording actinide hydroxides, very insoluble compounds, precipitated onto cathode electrode surfaces. The electrolyte generally represents a large number of complexing anions (Cl − , ClO 4 − , SO 4 2− , HSO 4 − , NO 3 − ) and acidic medium to prevent dramatic hydrolysis of actinide ions into actinide hydroxides in bulk solution [31,32]. Otherwise, experimental conditions represent significant features to obtain a quantitative deposition of radionuclides. The work published by Puphal et al., [29] demonstrated that using optimized conditions, e.g., ammonium oxalate-chloride mixture, pH value of 2, a current density of 400 mA.cm −2 , and typical 1 h of precipitation onto conductive substrates from 5-8 cm 2 , could enable high precipitation yields of more than 98% for several different actinides ( 230 Th, 237 Np, 239 Pu, 241 Am, and 244 Cm). High-resolution alpha sources can be obtained with typically full width at half maximum (FWHM) values of e.g., 24 keV for 241 Am. Conventional substrates are metals, and often stainless steel. In the precipitation process, another thing should be remarked on, namely the presence of interfering metal ions such as iron, aluminum, natural thorium, and zirconium, etc. Puphal et al. [29] demonstrated that using fluoride-complexing anions enable the overcoming of such interference. However, the use of such fluoride anions dramatically decreases precipitation efficiency proved by Talvitie's work. For the electrochemical precipitation process, some works proposed changing composition and pH value of the electrolytes: (i) Talvitie demonstrated that ammonium perchlorate or sulfate (1 M) and a very acidic pH value exhibited more suitable electrolytes, prompting almost quantitative electro-precipitation up to 99.8% efficiency for various radionuclides 230 Th, 232 U, 239 Pu, and 241 Am using 2 h of deposition duration onto a cathode area of 2 cm 2 [30]; (ii) another approach resulted from simple sodium sulfate or sodium sulfate/hydrogen sulfate mixture, which exhibited robust electrolytes enabling the stabilization of pH value even when using relatively high current densities of several 100s mA.cm −2 and thus obtaining high efficiencies of more than 90% for different radionuclides 230 Th, 237 Np, 239 Pu, 241 Am, and 244 Cm and thus pH values could be tolerant from 1.5-4.5 [33,34]. Still, one limitation is arising, and is necessary for the process of using conductive materials with high corrosion resilience. Several metals offer these properties, several may be noble metals, and thus expensive. Furthermore, they always exhibit a high atomic number, and for some applications in nuclear facilities the metal gets activated and becomes radioactive waste.
Inspired by these electro-precipitation methods, our group has developed an innovative device called "ActiFind" combining the electrochemistry of a thin synthetic layer of conductive boron-doped diamond (BDD), coupled with the radiometric detector made e.g., of silicon that now allows the direct monitoring of alpha-particles in aqueous media using actinide deposition at a low current of several mA.cm −2 [35]. Such innovative device presents a high benefit for fast and on-field monitoring alpha-activity in nuclear and environmental samples due to direct deposition of radionuclide onto radiometric detector through a diamond thin layer. Furthermore, this work demonstrated that multi-actinides including 239 Pu, 241 Am, and 244 Cm could be successfully deposited onto the diamond/silicon sensor at high electro-precipitation yields [36]. Here, the diamond layers used were fabricated using MicroWave Plasma-Enhanced Chemical Vapor Deposition (MWPECVD), where thin diamond layers, so-called nanocrystalline diamond (NCD), can be synthesized using methane at low cost. It is to be noted that diamond is generally insulating, but can be easily doped during growth by the addition of boron impurities, to become BDD. In the current work, we extend the use of diamond as a substrate for electro-precipitating actinide elements for the preparation of spectroscopic alpha-particle sources based on BDD, a substrate that exhibits high corrosion hardness as well as a low atomic number.

Electro-Precipitation Materials
BDD substrates with dimensions of 2 × 2 cm 2 were fabricated with thicknesses of 400 to 500 nm. The BDD layers are grown using conventional CH 4 in H 2 mixture in a microwave high-density plasma, grown after dense seeding on Si substrates. The material exhibits a nanocrystalline nature, with grain sizes of the order of a few tens of nm. Such BDD electrodes have been used for a broad range of electrochemical applications aimed at the detection of analytes in complex media, such as chemicals in sea water [37], neurotransmitters in physiological serums [38], biofluids in urine [39], or a broad range of applications [37,[40][41][42][43][44]. Doping of the layers was tuned at 2 × 10 21 at.cm −3 for optimal electrochemical performance [45]. Bare BDD electrodes are used, i.e., with no particular surface functionalization, as promoted by the ability to clean the surface in complex media [39,46,47]. Electrochemical cells investigated are similar to those used and described in [36]. They are composed of a planchet (here either stainless steel or BDD substrates), and put on a stainless steel support on which an acrylic glass cell equipped with a plastic seal is screwed, as depicted in Figure 1. The estimated planchet exposed area of the cathode is of about 1.2 cm 2 and a platinum pin is used as the anode. Electrolyte solutions are homogenized using a mechanical stirring system composed of a mechanical rotator (rotating rate of 1500 rpm) on which a plastic disposable coffee stirrer is connected.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 3 of 11 actinides including 239 Pu, 241 Am, and 244 Cm could be successfully deposited onto the diamond/silicon sensor at high electro-precipitation yields [36]. Here, the diamond layers used were fabricated using MicroWave Plasma-Enhanced Chemical Vapor Deposition (MWPECVD), where thin diamond layers, so-called nanocrystalline diamond (NCD), can be synthesized using methane at low cost. It is to be noted that diamond is generally insulating, but can be easily doped during growth by the addition of boron impurities, to become BDD. In the current work, we extend the use of diamond as a substrate for electro-precipitating actinide elements for the preparation of spectroscopic alphaparticle sources based on BDD, a substrate that exhibits high corrosion hardness as well as a low atomic number.

Electro-Precipitation Materials
BDD substrates with dimensions of 2 × 2 cm 2 were fabricated with thicknesses of 400 to 500 nm. The BDD layers are grown using conventional CH4 in H2 mixture in a microwave high-density plasma, grown after dense seeding on Si substrates. The material exhibits a nanocrystalline nature, with grain sizes of the order of a few tens of nm. Such BDD electrodes have been used for a broad range of electrochemical applications aimed at the detection of analytes in complex media, such as chemicals in sea water [37], neurotransmitters in physiological serums [38], biofluids in urine [39], or a broad range of applications [37,[40][41][42][43][44]. Doping of the layers was tuned at 2 × 10 21 at.cm −3 for optimal electrochemical performance [45]. Bare BDD electrodes are used, i.e., with no particular surface functionalization, as promoted by the ability to clean the surface in complex media [39,46,47]. Electrochemical cells investigated are similar to those used and described in [36]. They are composed of a planchet (here either stainless steel or BDD substrates), and put on a stainless steel support on which an acrylic glass cell equipped with a plastic seal is screwed, as depicted in Figure 1. The estimated planchet exposed area of the cathode is of about 1.2 cm 2 and a platinum pin is used as the anode. Electrolyte solutions are homogenized using a mechanical stirring system composed of a mechanical rotator (rotating rate of 1500 rpm) on which a plastic disposable coffee stirrer is connected.

Electro-Precipitation Process
Before the electro-precipitation process, BDD and stainless steel substrates were successively sonicated in deionized H 2 O and acetone for 5 min. The electro-precipitation experiments were carried out using a duration from 15-120 min and a very low current density of several mA.cm −2 in an aqueous solution of 0.3 M Na 2 SO 4 or 0.3 M NaNO 3 electrolyte solution (18 mL of total volume) in which 25 µL of stock solution (a single or mixture of actinides 241 Am, 239 Pu, and 244 Cm) was introduced. The pH values of about 4 were chosen for Na 2 SO 4 electrolyte. While this value of about 3 was used NaNO 3 electrolyte due to the increase of the pH during electro-precipitation [36]. There are some minimum values for current density to generate a hydroxide layer for precipitation as actinide hydroxide compounds [48]. Our previous study demonstrated that the current density should be above 3 mA.cm −2 and 1 mA.cm −2 for NaNO 3 pH 3 and Na 2 SO 4 pH 4 electrolytes respectively [36]. The radioactivity corresponding to 25 µl of each stock solution was calculated at 5.96, 5.13 and 4.40 Bq for 241 Am 239 Pu and 244 Cm, respectively. The pH of electrolyte solutions was carefully adjusted to a suitable value by some droplets of diluted solutions of HNO 3 , H 2 SO 4 , and NaOH. Before switching off the current, the pH of electrolyte solutions was adjusted to reach basic values at pH 9-10 using NH 3 solution (2%).

Autoradiography
The FUJIFILM BAS-1800II instrument is used as a bio-imaging analyzer for autoradiography with an imaging plate (IP). It consists of a fast phosphor IP exposed to the radioactive source. The sensitivity of the IP provides a highly efficient, uniform, and sensitive detection system. It is used to check the homogeneity of the deposited source.

Alpha-Particle Spectrometry Measurement
High-resolution alpha-particle spectra were recorded under vacuum (Canberra alpha spectrometer model 7401) equipped with a passivated implanted planar silicon (PIPS) detector (active area of 25 mm 2 ). This device allows obtaining a spectroscopic resolution of 12 KeV at the energy of 5.485 MeV for 241 Am standard. Alpha-particle spectroscopy for a solid source exhibiting radioactivity as low as a few Bq can be practically achieved within 3 days and with a counting statistic uncertainty close to 1%. The spectra analysis is processed using Genie 2000 software (Canberra).

Alpha-Particle Activity Measurements
Radioactivity measurements were performed using a defined solid-angle counting geometry consisting of a vacuum chamber with a silicon semiconductor detector (Canberra PD 2000-40-300 AM) connected to an electronic device. The electronic device is composed of a preamplifier, an amplifier, and an analog-to-digital converter (model ASPEC from ORTEC). The distance between the diagram (which radius is measured with an uncertainty of 0.01%) in front of the detector and the source was measured with an uncertainty of 0.03%. The spectra are analyzed using the MAESTRO software. In this chamber, the uncertainty of the geometrical factor is negligible in comparison with the main uncertainty component, which comes from the counting statistic estimated below 1.2%.

Uniformity of Solid Sources
The 241 Am compound was used as a typical radioactive source for regarding how 241 Am layers were deposited onto BDD substrates and the effect of either Na 2 SO 4 or NaNO 3 electrolyte solution was also regarded. First, in the Na 2 SO 4 electrolyte, the autoradiography image demonstrated that the deposition of the 241 Am was coated onto the entire BDD substrate (in Figure 2A). The alpha-activity derived from autoradiographs exhibited that 241 Am was deposited on the whole substrates in which Appl. Sci. 2019, 9, 1473 5 of 11 radioactivity in the center is slightly more concentrated than at the border of substrates. Such a result could be explained by the fact that the mechanical stirring system representing an oval shape produces a higher mass transport of the center than the border and thus making 241 Am layer thicker. A very similar quality of the 241 Am layer could be achieved on stainless steel substrates, here used as a comparison ( Figure 2B). Such a result demonstrated that a radioactive layer deposited BDD substrate has a good uniformity and same quality as stainless steel. radioactivity in the center is slightly more concentrated than at the border of substrates. Such a result could be explained by the fact that the mechanical stirring system representing an oval shape produces a higher mass transport of the center than the border and thus making 241 Am layer thicker.
A very similar quality of the 241 Am layer could be achieved on stainless steel substrates, here used as a comparison ( Figure 2B). Such a result demonstrated that a radioactive layer deposited BDD substrate has a good uniformity and same quality as stainless steel.
Secondly, electro-precipitation in the NaNO3 electrolyte enables the production of high-quality sources representing a high homogeneity for both BDD and stainless steel substrates ( Figure 3A and B). In this case, it should be noted that the alpha-activity distribution is more homogenized with respect to that prepared in Na2SO4 solution. It was proposed that in Na2SO4, the electrolysis of water generates hydroxyl ions with more hydrogen gas while in NaNO3 electrolysis of nitrate is preferential and thus does not form hydrogen gas. Such bubbling of the hydrogen gas could be the origin of the effect of the radioactive distribution.   Secondly, electro-precipitation in the NaNO 3 electrolyte enables the production of high-quality sources representing a high homogeneity for both BDD and stainless steel substrates ( Figure 3A,B).
In this case, it should be noted that the alpha-activity distribution is more homogenized with respect to that prepared in Na 2 SO 4 solution. It was proposed that in Na 2 SO 4 , the electrolysis of water generates hydroxyl ions with more hydrogen gas while in NaNO 3 electrolysis of nitrate is preferential and thus does not form hydrogen gas. Such bubbling of the hydrogen gas could be the origin of the effect of the radioactive distribution.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 11 radioactivity in the center is slightly more concentrated than at the border of substrates. Such a result could be explained by the fact that the mechanical stirring system representing an oval shape produces a higher mass transport of the center than the border and thus making 241 Am layer thicker.
A very similar quality of the 241 Am layer could be achieved on stainless steel substrates, here used as a comparison ( Figure 2B). Such a result demonstrated that a radioactive layer deposited BDD substrate has a good uniformity and same quality as stainless steel. Secondly, electro-precipitation in the NaNO3 electrolyte enables the production of high-quality sources representing a high homogeneity for both BDD and stainless steel substrates ( Figure 3A and B). In this case, it should be noted that the alpha-activity distribution is more homogenized with respect to that prepared in Na2SO4 solution. It was proposed that in Na2SO4, the electrolysis of water generates hydroxyl ions with more hydrogen gas while in NaNO3 electrolysis of nitrate is preferential and thus does not form hydrogen gas. Such bubbling of the hydrogen gas could be the origin of the effect of the radioactive distribution.   Furthermore, we also investigated how the radioactive source has grown during the electro-precipitation onto the BDD substrate. At this state, electro-precipitation for mixtures of 239 Pu, 241 Am, and 244 Cm were also carried out in Na 2 SO 4 as well as NaNO 3 electrolyte solutions. First, an autoradiograph displays that the precipitation in Na 2 SO 4 solution is preferentially deposited in the center and then extended to the border of BDD substrate following precipitation duration from 15-120 min (see in Figure 4). It was assumed that the radioactivity was first deposited in the center due to higher mass transport and then extended to the near region onto a BDD substrate that has a more conductive layer than actinide oxides layer. This observation explained the radioactive distribution as mentioned above. It should be noted that the radioactive source layers in NaNO 3 electrolyte were grown in the same way as that in Na 2 SO 4 electrolyte ( Figure 5). Furthermore, we also investigated how the radioactive source has grown during the electroprecipitation onto the BDD substrate. At this state, electro-precipitation for mixtures of 239 Pu, 241 Am, and 244 Cm were also carried out in Na2SO4 as well as NaNO3 electrolyte solutions. First, an autoradiograph displays that the precipitation in Na2SO4 solution is preferentially deposited in the center and then extended to the border of BDD substrate following precipitation duration from 15-120 min (see in Figure 4). It was assumed that the radioactivity was first deposited in the center due to higher mass transport and then extended to the near region onto a BDD substrate that has a more conductive layer than actinide oxides layer. This observation explained the radioactive distribution as mentioned above. It should be noted that the radioactive source layers in NaNO3 electrolyte were grown in the same way as that in Na2SO4 electrolyte ( Figure 5).

Alpha-Particle Spectroscopy
To check the quality of prepared solid sources, alpha-particle spectrometry was recorded and the FWHM value was estimated. All FWHM values of 241 Am sources are given in Table 1. It shows that 241 Am spectra displayed a high resolution with FWHM values varying from 13.2-14.5 keV at the energy of 5.485 MeV for 241 Am source onto BDD substrate. These values are very close to those of stainless steel e.g., 13.8 KeV and of 241 Am standard source of 12 keV. This result confirms that (i) BDD could be used as an alternative material for alpha-particle solid-source preparation, and (ii) NaNO3 electrolyte solution could also be used as an alternative electrolyte with respect to Na2SO4 electrolyte to enable high-resolution alpha-particle spectrometry. Typical alpha-particle spectra displays are of very high resolution and thus enable identification of very distinct peaks attributed to four alpha energies of the 241 Am sources at 5388, 5443, 5485.6, and 5544 MeV ( Figure 6A). Furthermore, with such spectroscopic resolution, a clear identification of actinides mixture ( 239 Pu, 241 Am, and 244 Cm) could be practically achieved for solid sources prepared onto BDD substrates using Na2SO4 as well as NaNO3 electrolyte solutions (see in Figure 6B). Furthermore, we also investigated how the radioactive source has grown during the electroprecipitation onto the BDD substrate. At this state, electro-precipitation for mixtures of 239 Pu, 241 Am, and 244 Cm were also carried out in Na2SO4 as well as NaNO3 electrolyte solutions. First, an autoradiograph displays that the precipitation in Na2SO4 solution is preferentially deposited in the center and then extended to the border of BDD substrate following precipitation duration from 15-120 min (see in Figure 4). It was assumed that the radioactivity was first deposited in the center due to higher mass transport and then extended to the near region onto a BDD substrate that has a more conductive layer than actinide oxides layer. This observation explained the radioactive distribution as mentioned above. It should be noted that the radioactive source layers in NaNO3 electrolyte were grown in the same way as that in Na2SO4 electrolyte ( Figure 5).

Alpha-Particle Spectroscopy
To check the quality of prepared solid sources, alpha-particle spectrometry was recorded and the FWHM value was estimated. All FWHM values of 241 Am sources are given in Table 1. It shows that 241 Am spectra displayed a high resolution with FWHM values varying from 13.2-14.5 keV at the energy of 5.485 MeV for 241 Am source onto BDD substrate. These values are very close to those of stainless steel e.g., 13.8 KeV and of 241 Am standard source of 12 keV. This result confirms that (i) BDD could be used as an alternative material for alpha-particle solid-source preparation, and (ii) NaNO3 electrolyte solution could also be used as an alternative electrolyte with respect to Na2SO4 electrolyte to enable high-resolution alpha-particle spectrometry. Typical alpha-particle spectra displays are of very high resolution and thus enable identification of very distinct peaks attributed to four alpha energies of the 241 Am sources at 5388, 5443, 5485.6, and 5544 MeV ( Figure 6A). Furthermore, with such spectroscopic resolution, a clear identification of actinides mixture ( 239 Pu, 241 Am, and 244 Cm) could be practically achieved for solid sources prepared onto BDD substrates using Na2SO4 as well as NaNO3 electrolyte solutions (see in Figure 6B).

Alpha-Particle Spectroscopy
To check the quality of prepared solid sources, alpha-particle spectrometry was recorded and the FWHM value was estimated. All FWHM values of 241 Am sources are given in Table 1. It shows that 241 Am spectra displayed a high resolution with FWHM values varying from 13.2-14.5 keV at the energy of 5.485 MeV for 241 Am source onto BDD substrate. These values are very close to those of stainless steel e.g., 13.8 KeV and of 241 Am standard source of 12 keV. This result confirms that (i) BDD could be used as an alternative material for alpha-particle solid-source preparation, and (ii) NaNO 3 electrolyte solution could also be used as an alternative electrolyte with respect to Na 2 SO 4 electrolyte to enable high-resolution alpha-particle spectrometry. Typical alpha-particle spectra displays are of very high resolution and thus enable identification of very distinct peaks attributed to four alpha energies of the 241 Am sources at 5388, 5443, 5485.6, and 5544 MeV ( Figure 6A). Furthermore, with such spectroscopic resolution, a clear identification of actinides mixture ( 239 Pu, 241 Am, and 244 Cm) could be practically achieved for solid sources prepared onto BDD substrates using Na 2 SO 4 as well as NaNO 3 electrolyte solutions (see in Figure 6B).

Electro-Precipitation Efficiency
The alpha-particle activity of 241 Am deposited was further measured. This value was calculated to 49.7 and 59.7% for 241 Am deposited in Na2SO4 electrolyte solutions onto stainless steel and BDD respectively (see in Table 2, row a). This slight difference in the precipitation yield could be explained by the fact that there is a slight pH difference (3.97 vs. 3.85) for stainless steel and diamond, respectively. With the same current density applied, a less acidic pH induces a thicker hydroxyl layer, making actinide diffusion lesser on the cathode and thus lower precipitation efficiency [48]. Very close electro-precipitation efficiency values (71 and 70%) could be achieved in NaNO3 electrolyte for BDD and stainless steel substrates (see in Table 2, row b). Generally, a slightly higher efficiency is observed when NaNO3 is used with respect to Na2SO4; even when the precipitation processes are onto either diamond or stainless steel substrate ( Table 2) or even onto BDD film combined with a silicon detector in an ActiFind device [35]. This result confirms that BDD can be used as an alternative material for efficient electro-precipitation and that NaNO3 electrolyte enables a higher efficiency than Na2SO4 solution. Furthermore, electro-precipitation of actinides mixtures ( 239 Pu, 241 Am, and 244 Cm) was also investigated onto BDD substrates for varying durations. Precipitation efficiencies were displayed in Figure 7 for both Na2SO4 and NaNO3 electrolytes. The result demonstrated that the precipitation

Electro-Precipitation Efficiency
The alpha-particle activity of 241 Am deposited was further measured. This value was calculated to 49.7 and 59.7% for 241 Am deposited in Na 2 SO 4 electrolyte solutions onto stainless steel and BDD respectively (see in Table 2, row a). This slight difference in the precipitation yield could be explained by the fact that there is a slight pH difference (3.97 vs. 3.85) for stainless steel and diamond, respectively. With the same current density applied, a less acidic pH induces a thicker hydroxyl layer, making actinide diffusion lesser on the cathode and thus lower precipitation efficiency [48]. Very close electro-precipitation efficiency values (71 and 70%) could be achieved in NaNO 3 electrolyte for BDD and stainless steel substrates (see in Table 2, row b). Generally, a slightly higher efficiency is observed when NaNO 3 is used with respect to Na 2 SO 4 ; even when the precipitation processes are onto either diamond or stainless steel substrate ( Table 2) or even onto BDD film combined with a silicon detector in an ActiFind device [35]. This result confirms that BDD can be used as an alternative material for efficient electro-precipitation and that NaNO 3 electrolyte enables a higher efficiency than Na 2 SO 4 solution. Furthermore, electro-precipitation of actinides mixtures ( 239 Pu, 241 Am, and 244 Cm) was also investigated onto BDD substrates for varying durations. Precipitation efficiencies were displayed in Figure 7 for both Na 2 SO 4 and NaNO 3 electrolytes. The result demonstrated that the precipitation efficiency values are approximately similar for 241 Am and 244 Cm in either Na 2 SO 4 or NaNO 3 and are slightly higher than for 239 Pu. The low precipitation yield of 239 Pu could be attributed to the complex chemistry of Pu existing in many oxidation states (III, IV, V, VI) in solution. Again, the result demonstrated a slightly more efficient precipitation in NaNO 3 than in Na 2 SO 4 electrolyte. The precipitation efficiency could be improved up more than 80% for 241 Am and 244 Cm and more than 40% for 239 Pu in NaNO 3 electrolyte solution after 2 h of precipitation.
Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 11 efficiency values are approximately similar for 241 Am and 244 Cm in either Na2SO4 or NaNO3 and are slightly higher than for 239 Pu. The low precipitation yield of 239 Pu could be attributed to the complex chemistry of Pu existing in many oxidation states (III, IV, V, VI) in solution. Again, the result demonstrated a slightly more efficient precipitation in NaNO3 than in Na2SO4 electrolyte. The precipitation efficiency could be improved up more than 80% for 241 Am and 244 Cm and more than 40% for 239 Pu in NaNO3 electrolyte solution after 2 h of precipitation.

Conclusions
This work demonstrated that BDD substrate could be used as an alternative material for the preparation of high-efficiency and high-quality radioactive sources with a high-resolution alphaparticle spectroscopy. Notably, its inherent corrosion hardness could be a valuable behavior for practical applications in the nuclear industry using highly acidic solutions [49,50]. The obtained FWHM value of 13.2 KeV at 5.485 MeV for BDD substrate is close at the state-of-art spectroscopic measurement system e.g., 12 KeV for 241 Am standard, confirms the benefit of the material, to which adds its extreme physiochemical, mechanical, and electrical properties. The used precipitation method also confirmed the high reproducibility using BDD substrate and its high interests using simple electrolytes such as Na2SO4 or NaNO3 and a low current density of few mA.cm −2 with respect to previous works.