Geometry Dash Radioactive

[Rosette Allaband]

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The presented paper discusses the production of radioactive ion beams of francium, radium, and actinium from thick uranium carbide (UC\(_x\)) targets at ISOLDE, CERN. This study focuses on the release curves and extractable yields of francium, radium and actinium isotopes. The ion source temperature was varied in order to study the relative contributions of surface and laser ionization to the production of the actinium ion beams. The experimental results are presented in the form of release parameters. Representative extractable yields per \(\mu\)C are presented for \(^222-231\)Ac, several Ra and Fr isotopes in the mass ranges 214\(\le\)A\(\le\)233 and 205\(\le\)A\(\le\)231 respectively. The release efficiency for several isotopes of each of the studied elements was calculated by comparing their yields to the estimated in-target production rates modeled by CERN-FLUKA. The maximal extraction efficiency of actinium was calculated to be 2.1(6)% for a combination of surface ionization using a Ta ion source and resonant laser ionization using the two-step 438.58 nm, and 424.69 nm scheme.

The field of nuclear medicine is rapidly evolving. Nowadays, the most common radioisotope used for medical diagnostic imaging is \(^99m\)Tc, representing approximately 80% of all nuclear medicine procedures every year1. Radionuclides may also be used for medical treatment. For instance, the \(\beta\)-particle-emitting \(^177\)Lu-based drug Lutathera was approved in 2015 by the Food and Drug Administration and in 2017 by the European Medicines Agency and is today widely used for cancer therapy2. Worldwide investigations are ongoing to produce novel radionuclides whose chemical and nuclear properties are optimised for producing radiopharmaceuticals for the treatment of a range of cancer types3,4. In the last decade, much interest has been focused on the supply of \(^225\)Ac for targeted alpha therapy (TAT)4. This isotope has been proven to be more efficient than \(^177\)Lu as it decays by 4 consecutive \(\alpha\) particles with energies ranging between 5 - 8.4 MeV. Since they have a higher linear energy transfer (LET) than \(\beta\) radiation, more energy is deposited in cancer cells4. This is beneficial not only to patients but also to hospital staff due to a shorter radiation range and a smaller dose needed for the same treatment5. The demand for this radionuclide is expected to rise every year (unmatched with supply), thus finding a sustainable production way is crucial6. Many research groups are trying to find the optimal solution in terms of price, purity, availability and production efficiency7. Radionuclide purity is crucial for TAT in order to ensure a high radiolabeling efficiency, to prevent long-lived isotopes from damaging other organs or healthy tissue, and to minimize long-lived waste management in hospitals.

The present work discusses one of the possible production paths for \(^225\)Ac and other radionuclides by inducing nuclear reactions in thick uranium targets with highly energetic protons in combination with the Isotope Separation On-Line (ISOL) method8. Furthermore, this study is very important for other fields that are experimentally studied at ISOLDE. Altogether, Fr, Ra, and Ac isotopes were investigated between A = 205-233 in order to characterize their release from the target, extractable yields and their release efficiencies. The release of an isotope from a target can be described as a delay time distribution of the ions after the proton interaction with target. Several different processes influence the isotope release from target and the final extractable yields: in-target production; diffusion to the target surface; desorption from the surface; effusion through the target pores to the target container and transfer line and ionization - surface, electron impact and resonant laser ionization.

Various factors can affect yields extracted at ISOLDE, including laser ionization, target temperature, thermal gradient between ion source, transfer line and target, and ion source temperature itself. Francium (Z = 87) belongs to the alkali elements, which ionization potential is typically low (4.07 eV)9. Francium is expected to be released rapidly from target. For radium (Z = 88), which belongs to the alkali earth elements, the ionization potential is higher (5.28 eV)10 but still low enough for surface ionization. However, we expect slower release from target compared to Fr. The surface ionization efficiency for elements with a low ionization potential is high at nominal ion source temperature (\(\approx\)2000 \(^\circ\)C)11. Within the framework developed by Kirchner, a surface ionization efficiency for Fr of 98.1% and for Ra of 31% was calculated (assuming an ion survival of 1 and a number of wall collisions 42)12.

The last and most relevant studied element is actinium (Z = 89). Actinium has an ionization potential of 5.38 eV13, slightly higher compared to Ra, which does not result in efficient ionization on a hot surface, the efficiency being estimated to 3.3% for Ac at 2000 \(^\circ\)C (ion survival of 1, 42 wall collisions)12. For elements with a high ionization potential, the use of different ion sources may be required for efficient ionization, such as resonant laser ionization. Furthermore, due to its similar physico-chemical properties to the UC\(_x\) target material, it is not released from the target easily. Diffusion and effusion from target grains take much longer compared to an alkali element like Fr. Based on these properties, it is reasonable to expect that the release from the target will take longer for Ac than Ra or Fr. Release properties and extractable yields were therefore systematically measured in this experiment to gain a global understanding of the production of those beams, particularly as this was the first laser-ionization of Ac beams at the ISOLDE radioactive ion beam facility.

The first part used the ISOLDE High Resolution Separator (HRS) made of 2 dipole separator magnets with resolving power of approx. 7000 to separate the beam16. Surface ionized beams of Fr and Ra were then delivered to the Alpha Setup (ASET) chamber where alpha spectroscopy of the implanted beam was performed. This setup consists of two silicon detectors: an annular (partially depleted silicon surface barrier - Ortec C Series) detector with active area of 450 mm\(^2\) and 6 mm diameter hole through which the ion beam passes before implantation onto the foil, and a full (partially depleted Passivated implanted planar silicon - Canberra PD Series) detector with active area of 300 mm\(^2\) placed behind the foil holder. The geometric efficiency of annular detector is 25(5)% (4 mm from ladder) and that of full detector 36(5)% (3 mm from ladder). Nine 20 \(\mu\)g/cm\(^2\)-thick carbon foils were placed on a ladder to collect the radioactive ion beam as well as to remove the accumulated radioactivity between measurements. The data were collected by a digital acquisition system based on a CAEN V1724 module and read out by the MIDAS software from Daresbury (UK), enabling event-by-event data collection for offline analysis. This combination of HRS and ASET is suitable for selection of an ion beam at desired mass-to-charge ratio and identification of its isobaric composition by the detection of their decay products. This setup was used to study radioactive isotopes of surface-ionized francium, radium and partially actinium through their \(\alpha\) and \(\beta\) decays, though actinium was also observed as a decay product.

The second part focused on actinium isotopes. The beam was separated by the ISOLDE General Purpose Separator (GPS) featuring a single dipole magnet with mass resolving power of approx. 240016. The actinium beam was produced with the Resonance Ionization Laser Ion Source (RILIS)17. This combination provided element selectivity of resonant laser ionization combined with m/q selection of ions to evaluate production of laser-ionized products vs surface-ionized ones. Resonance laser ionization enhances ionization of the element of interest by resonance excitation of its specific atomic levels, which are unique for each element. The isotopes were then detected with either a Faraday cup for ion beam current measurements, or using the ASET in the same configuration as for the first part. For actinium, only laser ionization resulted in a measurable ion yield. In the two-step process of laser ionization, the valence electron was promoted from the atomic ground state 6d7s\(^2\) \(^2\)D\(_3/2\) to the 6d7s7p \(^4\)P\(^\circ\) \(_5/2\) level with a laser at 438.58 nm, and then a subsequent excitation to an autoionizing state with a laser at 424.69 nm18. By turning on and off the laser beams it was possible to easily switch between different ionization mechanisms and thus to investigate their impact on the radioactive ion beam production. The characteristics of produced ion beams were studied by measuring the ion current using a Faraday cup (FC) in dependence of the mass and ion source temperature.

The radioactive ion beam transmitted through the mass separators contains species with the same mass-to-charge ratio. Complementary use of resonance laser ionization allowed us to study an enhancement of a particular element, in our case Ac. The impact of ion source temperature and laser ionization of actinium was studied on mass A = 227 during the second part of the campaign using ASET in collaboration with RILIS. All the measurements were performed with the target held at 2000 \(^\circ\)C. The ion source temperature can be changed indirectly by changing the current applied for the resistive heating, and the temperature is derived from a calibration performed prior to the experiment for each target-ion source unit. In this experiment the ion source temperature was varied from 1790 to 2300 \(^\circ\)C and the effect of blocking and not blocking the first-step laser was investigated by ion current detected in FC. Bunches of \(3\times 10^13\) protons were delivered in 2.4 \(\mu\)s duration every 4.8 s19. The results are presented in Fig. 1a. Each of these points represent a single measurement on a specific ion source heating temperature. An offset FC reading is subtracted from both data sets, as measured when the ion beam is blocked, which explains why one point scatters in the negative.

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