Nanostructured target materials development

Y2O3targets were also tested at ISOLDE, for the first time [50]. Nanometric yttria was com-mercially acquired and heat treated to 1380^◦C in order to assess the stability of the nanos-tructure [56]. These particles were agglomerated in a plate like snanos-tructure, as shown on figure 1.17a, which after heat the agglomerates turns into a micrometric/submicron target as shown of figure 1.17b. During the heat treatment the surface area changes from 17.8 to 0.6 m²g⁻¹, changing the nanostructure into a micrometric one [56]. Nonetheless, this target provided higher intensities on Co and Cu beams than other targets and also many isotopes of Cr and Fe which haven’t been seen before at ISOLDE [50]. This material revealed decreasing yields over

(a) SiC microstructure obtained by ice templating [87].

(b) BeO microstructure used as target material at ISOLDE [52].

Figure 1.16 – Different microstructured materials developed at ISOLDE: SiC ice templating structure and micrometric BeO. Reproduced from [87] and [52].

time, down to 20 % of the initial value over 120 h of operation, likely due to the microstructure degradation [50].

(a) Yttria as supplied. (b) Yttria heat treated at 1380^◦C.

Figure 1.17 – Yttria nanomaterial tested at ISOLDE before and after heat treatment. Repro-duced from [56].

Calcium oxide has been used at ISOLDE as a powder target, since 1985 to produce beams of C (as CO), N (as N₂), Ar, Ne and He [54]. However, even though the beam intensities provided from these targets were good, they were known to either reduce over time or even be low from the beginning. Since this was a problem suspected to come from the microstructure (sinter-ing), either during its production and/or operation, material investigations were done [55].

The new mesoporous nanometric CaO material produced, shown on figure 1.18a was devel-oped from the decomposition of CaCO3in vacuum (CaCO3→ CaO + CO2) and has with about 30 nm particle size and 12 nm pore size [55] [54, 55]. Sintering studies were done following the specific surface area evolution (see figure 1.18b for an example), in order to assess the right operation temperature for the ISOLDE target in order to keep a stable nanometric CaO structure [55]. It was reported that surface diffusion controlled the sintering kinetics at low temperatures while at higher temperatures volume diffusion was the dominating mechanism for CaO sintering [55]. A conservative operation temperature was defined to maximum of 800^◦C in order to keep the nanostructure stable over time and to account for irradiation ef-fects, contrarily to before where there was no temperature limit [54]. This was the first target nanomaterial operated at ISOLDE. Even though a lower temperature was used, high yields

of Ar, Ne, He and CO beams were extracted from this target and is now a standard unit at ISOLDE [54].

(a) Mesoporous nanometric CaO material de-veloped for ISOLDE.

(b) Sintering behaviour, in terms of specific surface area of the nanometric CaO.

Figure 1.18 – Mesoporous nanometric CaO material developed for ISOLDE and its sintering behaviour. Reproduced from [54].

An interesting fact was noticed in the isotope release time structure of nanometric CaO, which was also seen in all the following nanomaterials at ISOLDE. As exemplified, on figure 1.19c and d, the isotope release is apparently longer from nanometric form (figure 1.19b) than micrometric form (figure 1.19a) of the same solid compound [57]. The term apparently longer is used since the yields obtained are usually much higher than those of micrometric materials with "fast release": in the example of figures 1.19c and d, while a yield of 2.0× 10⁶μC⁻¹was obtained for the micrometric target, one of 3.7× 10⁷μC⁻¹was obtained for the nanometric one. This is likely due to the fact that the release is not any longer only limited by diffusion, due to the small particle sizes of the target material. In fact, the release is likely limited by both diffusion and effusion due to the complex nanometric pore network which will multiply by orders of magnitude the number of atom collisions before they are extracted. This effect has also been reported for UCxmaterials [20].

Nanometric CaO has also been used to unexpectedly produce radioactive argon while cold, meaning without any temperature supplied by the target oven [57]. The beams provided by this cold target were only a factor 2 to 3 lower than the ones with the target at 800^◦C, which doesn’t follow classical diffusion laws predicting that the fall should be orders of magnitude in beam intensity [57]. In this report the release was attributed to the spallation recoil mo-mentum from the nuclear reaction, during production, which could eject the isotopes from their production positions. The recoil energy was estimated to be 9.2± 1.8keV which would make a projected ion range of 10.9 nm, more than enough to escape the nanometric CaO particles [57].

Uranium carbide is usually developed by mixing micrometric forms of UO₂ and graphite, pressing them into pellets and heat treat them up to 2000^◦C to promote the carbothermal re-duction of the UO2into UCx(UO2+ 6C→ UC2− x+ (2+x)C + 2CO, where 0 ≤ x ≤ 1). By milling down the UO₂to a median size of 160 nm and replacing the graphite by multiwall carbon

(a) Old CaO micrometric material used at ISOLDE.

(b) Newly developed nanometric CaO for ISOLDE.

(c) Release profile of³⁵Ar from micrometric CaO targets.

(d) Release profile of³⁵Ar from nanometric CaO targets.

Figure 1.19 – Calcium oxide microstructure used before at ISOLDE and the newly developed nanometric CaO in [55] and respective release profiles. Reproduced from [57].

nanotubes (MWCNT) and following the same thermal treatment procedure, a nanocompos-ite was obtained [51, 20]. This yielded an extremely stable structure at high temperatures, which was able to deliver short lived isotope intensities (³⁰Na - t1/2= 48ms) over more than 12 days without any decrease. Such isotope in standard UC_xtargets would reduce by almost two orders of magnitude [20, 51] in the same period of time. The beams delivered by this target were all of higher intensity than the standard uranium carbide and stable over time [51].

A similar composite, but developed through a different process was developed by SPES and tested at the HRIBF-ORNL [88]. The isotope release results were inferior to those of standard (micrometric grains) target (tested in the same conditions), contrarily to what was obtained at ISOLDE. Comparison is difficult between the two nanometric UCxtargets since the mate-rial processing, TISS and also primary beam energy were all different. The low yield results were attributed to possibly low efficiency ion source due to target outgassing or possibly high effusion times [88].

A LaC₂-MWCNT nanocomposite was also developed at ISOLDE, following the studies of Bi-asetto et al. [89] at INFN where an LaC2-carbon nanotubes (CNT) nanocomposite was de-veloped by mixing La2O3with different volume ratios of graphite and CNT. Using a similar process to the UC_xnanocomposite and La(OH)₃and MWCNT as starting materials, a LaC₂

-MWCNT was produced [45], and tested online [46] to produce beams of Cs and Ba. The same apparently long release profiles were seen with high release efficiencies, however this proto-type was not able to deliver as high intensities as the molten La target [46]. This is thought to be due to the very low densities (when comparing to La molten targets) and mainly the limitation in operation temperature at which the LaC2starts to sublime.

The last nanometric target to be tested at ISOLDE was developed to extract boron beams, a highly refractory and reactive element which was never before extracted in ISOL-type facilities.

Due to the very low vapor pressure and high reactivity of this element, its extraction as^ABF2

was seen as a very efficient mechanism [43]. After release studies and in order to have a low as possible diffusion times, pressed pellets of MWCNT were used as target material. The huge surface area of MWCNT also helps to promote the reaction of the CF4with the B and extract it [43, 44].