Studies of Thorium and Ytterbium Ion Trap Loading from Laser Ablation for Gravity Monitoring with Nuclear Clocks

Studies of thorium and ytterbium ion trap loading from laser ablation for gravity monitoring with nuclear clocks

Marcin Piotrowski,1, 2, 3, ∗ Jordan Scarabel,2 Mirko Lobino,2, 4 Erik Streed,2, 5 and Stephen Gensemer1, 2 1CSIRO Manufacturing, Pullenvale, Queensland 4069, Australia 2Centre for Quantum Dynamics, Griffith University, Brisbane, QLD 4111 Australia 3Currently at French-German Research Institute (ISL), 68301 Saint-Louis, France 4Queensland Micro- and Nanotechnology Centre, Griffith University, Brisbane, QLD 4111, Australia 5Institute for Glycomics, Griffith University, Gold Coast, QLD 4222 Australia Compact and robust ion traps for thorium are enabling technology for the next generation of atomic clocks based on a low-energy isomeric transition in the thorium-229 nucleus. We aim at a laser ablation loading of single triply ionized thorium in a radio-frequency electromagnetic linear Paul trap. Detection of ions is based on a modified mass spectrometer and a channeltron with single- ion sensitivity. In this study, we successfully created and detected 232Th+ and 232Th2+ ions from plasma plumes, studied their yield evolution, and compared the loading to a quadrupole ion trap with Yb. We explore the feasibility of laser ablation loading for future low-cost 229Th3+ trapping. The thorium ablation yield shows a strong depletion, suggesting that we have ablated oxide layers from the surface and the ions were a result of the plasma plume evolution and collisions. Our results are in good agreement with similar experiments for other elements and their oxides.

I. INTRODUCTION clocks have not reached the accuracy and stability of laboratory-based clocks [12], but advances in the Gravity can be monitored with atomic clocks field are being made rapidly [13]. through a technique known as chronometric lev- The Thorium-229 nucleus possesses a metastable eling [1]. It is a well-known consequence of gen- excited state, 229mTh, which exhibits the lowest eral relativity that clocks sitting in different grav- known excitation energy of all nuclei. The exci- itational potentials have different ticking rates. tation of this transition from the ground state re- Clock networks can provide a new way for ground- quires energy around 8 eV (the corresponding wave- based gravimetry at the Earth’s surface, allowing length is 150 nm) [14–18]. The isomer’s existence us to monitor minute drifts in the gravity anoma- was inferred based on indirect experimental evi- lies. In recent years, rapid progress in atomic dence more than forty years ago [19]. Its exact en- clock networks makes this kind of gravity sensor ergy value was debated over the years before the re- feasible[2]. Such networks have been demonstrated cent final isomer’s direct detection via the internal with two state-of-the-art optical atomic clocks com- conversion decay channel [16]. New ways to prepared via fibre link [3–6], including demonstrations cisely measure the frequency of the nuclear clock of a transportable clock [7]. The best of lab-based transition are also emerging [20–27], with the elec- optical clocks are capable of detecting down to tron bridge process having application to 229Th3+ 1 cm differences between clock altitudes, which is [28, 29]. A clock operating on this transition is already beyond the geodetic limit [8]. The optical an approach pursued to overcome the challenges fiber links are no longer a limiting factor for such of present optical clocks [30]. Its narrow linewidth networks, as stability of frequency transfer down is uncoupled from most of the environmental ef- to 10−20 has been demonstrated [9]. Even optical fects that plague existing atomic clocks. A sin- links hundreds or thousands of kilometers long can gle 229Th ion housed in an electromagnetic trap is perform well enough for requirements of chrono- expected to allow frequency measurements of un- metric leveling with millimeter precision [9, 10]. precedented accuracy reaching 10−19[31]. That is The portability of optical clocks and maintaining almost an order of magnitude better than the best their performance out-of-the lab is one of the main arXiv:2005.13918v3 [physics.atom-ph] 29 Jul 2020 optical electronic-shell clock in use now, namely obstacles we face in the development of a true grav- the clock based on 27Al+ with a systematic un- ity observing clock network. While clocks with sys- certainty of 9.4 × 10−19 and frequency stability of tematic uncertainty below 10−18 have been devel- √ 1.2 × 10−15/ τ [11]. On top of that, the single- oped in a laboratory setting [11], portable optical ion clock has great potential to be highly miniaturized [32]. The 229Th3+ system is also attractive from several technical perspectives [33], design and ∗ [email protected] simulation work has shown that a trapped Th3+ 2 ion could be developed in a compact and relatively it to examine methods of target preparation and low-cost package, making field deployment in a dis- trap loading. We also investigated Yb for tests of persed network achievable. In addition, the 229Th experimental procedures, because more experimen- nucleus is expected to be an excellent system for tal and theoretical data is available about this el- the search for time variation of fundamental con- ement in terms of ion trapping and laser ablation. stants [34–36]. To this date, a few groups world- We used ytterbium along with thorium for com- wide have demonstrated thorium trapping in ion parison of laser ablation yield evolution and trap traps [37–44]. loading efficiency for different charge states of both The nuclear clock is not the only stimulating ap- elements. plication of thorium ion traps. Single ions can be used as very sensitive probes of the electric field, as was recently demonstrated for ytterbium ions [45]. II. METHODS AND EXPERIMENTAL A potential application of such single ion-based SETUP force sensors is the manipulation and investigation of dipole moments in bio-molecules [46]. The UV The experimental set-up consists of an ion trap light used in the ytterbium ion force sensor can with an ion detector and a laser system for abla- be harmful to bio-molecules, and thus might not tion. The ion trap, the ablation targets, and the be very useful. In contrast, Th3+ ion has atomic ion detection system are under ultra-high vacuum, transitions used for cooling and imaging in the red pumped by turbomolecular and ion pumps. and infrared parts of the electromagnetic spectrum Fig. 1 shows the rendering of the experimental [37], making it very attractive as a potential force setup. A spherical cube vacuum chamber (Kim- sensor for large bio-molecules. ball Physics) holds the ion trap and laser ablation Laser ablation of solids is a technique widely used targets. The parts are mounted by grove grabbers in material processing and deposition where precise to the internal grooves of the vacuum ports. The and effective material removal from the surface is optical access to the ion trap is provided through crucial [47]. This technique can also deliver ions to the top and bottom 4.5 inch diameter ports and one an RF trap by vaporization and ionization of mate- smaller side port. The viewports are equipped with rial from the metal surface [48]. Short laser pulses anti-reflection coated glass windows. The vacuum in the nanosecond range are the most effective in pumps, gauges, and detection system are connected terms of the level of ionization of the ablated mate- by three 2.75 inch ports. The remaining fourth rial [49]. Nanosecond laser ablation offers temporal port allows additional optical access for laser cool- control over trap loading and is a particularly at- ing and probing of ions in the trap. tractive method for experiments with micro traps [50–52]. The trap loading of ions into RF traps was demonstrated for Ca [53], Sr [54], and Th atoms A. Ion trap [55, 56]. Systematic studies of the ablation process and trap loading are challenging because of changes Our ion trap is a standard radio-frequency (RF) in the metal surface caused by the removal of the linear Paul trap [57] modified from the one formerly material from the spot. After repeated pulses, a used for experiments with Yb ions [58, 59]. The crater in the metal surface forms and the ablation rod diameters, spacing, and other crucial dimen- yield diminishes. To describe this process quantita- sions are listed in table I. The quadrupole radius tively we performed a series of measurements where refers to the distance from a geometric center of the number of ions from the same spot was moni- a square formed by the quadrupoles (trap axis) to tored over long times. Our findings of the evolution a quadrupole rod. The quadrupole length is the of the yield from the laser ablation are complemen- length of the rods. The wire radius is half the wire tary to previous findings, and we present a more gauge used for the quadrupole and end caps. The thorough examination of pulse yield depletion. end cap distance from the center is half the distance Here, we present efforts to develop effective between the two end caps. The end cap radius is methods for ion trap experiments with thorium-229 the distance from the perimeter of the circular end isotope and future studies of nuclear clock transi- cap nodes to their center. The RF field is provided tion. One of the main difficulties with the 229Th from a digital signal generator (RIGOL DG4162), isotope is that it is radioactive with a relatively amplified by 30 dB, and transmitted to the trap short half-life of fewer than 8000 years compared via a helical resonator for impedance matching. to the much more stable 232Th isotope (a half-life The trap and helical resonator are resonant at a of 1.4 × 1010 years). The latter is practically much frequency around 6.2 MHz, allowing us to confine easier to handle and obtain, that is why we applied different ionic states of thorium and ytterbium by 3

Figure 1. A rendering of the experimental setup. The ion trap is placed inside the central vacuum chamber. The port on the left of the chamber leads to the RGA based ion detection system. The axis of the ion trap is aligned with the axis of the RGA’s mass ﬁlter rods. The port on the right-hand side connects to the vacuum control and detection system. Top and bottom 4.5-inch ports and front smaller port are closed with windows for optical access. The rear smaller port serves for electrical connection to the ion trap.

Quadrupole rods radii r0 4.508 mm C. Thorium and ytterbium targets Quadrupole rods length l0 51.5 mm Rods and end cap wire radius rw 787.5 µm End cap distance from centre z0 20 mm Thorium-229 isotope metal samples are neces- End cap radius rz 7 mm sary for future nuclear clocks, that is why we car- ried out a series of preliminary experiments to Table I. Dimensions of the Paul trap. establish a simple and reliable method of target preparation. The thorium electrodeposition on a stainless steel plate was adopted from the proce- dure used for uranium in [60]. We used a 0.02M tuning the amplitude of the drive RF signal and thorium nitrate water solution to electrodeposit the DC bias applied between adjacent rods. thorium on a stainless plate (Kimpball Physics eV parts). A deposition set-up was composed of a plat- inum electrode, the ammonia hydroxide was used B. Laser system for ablation of metal targets for pH control. According to weighing the plate before and after the process we deposited 0.05 g The laser ablation as a method for trap load- of material on the plate. A laser ablation signal ing is most commonly applied to elements like from a target prepared in this way was observed thorium with very high evaporation temperature. visually and used for tests of our detection system Our laser plasma ablation setup is similar to the at that time. A thorium signature was observed one described in [55]. The ablating laser source on a mass spectrometer after the ablation of the is a nanosecond pulsed nitrogen laser (Stanford target, so this simple method of target prepara- 229 Research Systems NL100). It produces a 3.5 ns tion may serve for Th ion loading in the future. pulse with an energy of 170 µJ. The wavelength Even though targets of thorium-232 prepared this is 337.1 nm. The repetition rate of the laser can way were not used eventually for the results pre- be tuned between 1 - 20 Hz. The output beam sented here, the findings are of general importance was focused to a spot of 350 µm 1/e2 diameter. for future compact thorium-229 ion traps. The laser beam enters via the top window of the For all the results presented here, we changed to chamber and is focused on the targets that are to pure thorium-232 metal. A piece of thorium wire be ablated. Taking into account the absorption co- approximately 1 mm long was spot-welded to the efficient of thorium, our pulsed laser generates a surface of an aluminum strut inserted throughout maximum fluence of about 0.35 J/cm2. the ceramic holder. The thorium 0.22 mm wire is 4

Ablation laser beam Yb/Th targets

+ + + + + + + + + + + +

Channeltron Ion trap RGA mass ﬁlter detector

Figure 2. A schematic illustrating the relative positions of the laser plasma creation, the ion trap, and RGA based ions detector with the channeltron. The ions created in the ablation process travel along the path marked towards the detector. naturally monoisotopic thorium with 99.5% purity Figure 3. Example of the ion current detected in the (The Goodfellow supplier). Similarly, a piece of channeltron as a function of time (green lines). The ytterbium wire with natural abundance is placed repetition rate of the ablation laser is 1 Hz, the ion trap by the side of the Th target. driving frequency 6.24 MHz, ion signal data points were averaged over 500ms. The blue line is a ﬁt with periodic signal sin2(2πf + φ) with frequency 2πf = 198 mHz corresponding to the beat frequency between ablation D. Ion detection and plasma diagnostics and trap frequency.

We developed a simple non-optical detection system that was sensitive enough to detect a single ions initially shortens the quadrupole rods to the ion signal. We utilized a commercial residual gas ground and the following ions can be guided by the analyzer (RGA) to serve as an ion filter and detec- quadrupole field. A similar effect was observed in tor (Extorr XT300M). The ions were detected by a experiments investigating the RF trap loading with channeltron in combination with an electron multi- Ca ions [61]. The first of these potential explana- plier. The mass filter from the RGA has been used tions is more likely as we noticed the oscillation pe- to guide the ions ejected from our ion trap towards riod depends on the trap frequency and repetition the channeltron. The RGA allows filtering ions up rate of the laser, but it might be also a combination to 300 amu. To make the RGA serve as a detector of both. we removed the grid and filament at the RGA’s entrance. Normally, these elements ionize the gas to be analyzed, but in our case, we wanted to detect III. RESULTS already ionized atoms. When the grid and the filament were removed, the ions from our trap were able to enter freely and to be guided by the mass In our experimental setup, we observed laser ab- filter of the RGA. We also added a switching mech- lation from metallic targets of Yb and Th. The anism for the RF field to RGA rods. In this way, we two ablation plumes could be visually easily distin- can switch off the filtering in RGA completely and guished - blue fluorescence from the plasma plume filter charge states only with our Paul trap. The of Yb and white from Th. The ytterbium was much scheme in Fig. 2 illustrates the relative positions of brighter to the eye and lasted longer, while the the ion trap and detection system elements. thorium ablation was much fainter and short-lived. We observed that the number of ions guided by It is consistent with the expected higher thresh- the quadrupole mass filter which reached the de- old for laser ablation of thorium. To develop a tector varied periodically. The first explanation of thorough understanding of these observations we this behavior might be the phase difference of trap present here a detailed quantitative analysis. drive frequency and the repetition rate of the ablation laser, as the phases are not synchronized. In our case, the repetition rate of the pulsed ablation A. Trapping and detection of Yb+, Th+ and laser can be tuned between 1 – 20Hz. An example Th2+ states of the ion current detected in the channeltron as a function of time (green lines) is shown in figure The laser ablation plume contains both neutral 3. Another explanation might be that the flux of and ionized atoms, the ratio of the constituents de- 5

800 50 Ytterbium Direct Signal Thorium Direct Signal 14 800 Ablated Ytterbium (a) + (b) + Natural Isotope Abundance Yb Th 40 12 600 2+ 600 Th 10 30 8 400 400 20 6

Ion Current (fA) Current Ion 4

200 (fA) Current Ion 10 (pA) Current Ion 200 Natural Abundance (%) Abundance Natural 2

0 0 0 0 168 169 170 171 172 173 174 175 176 177 178 150 160 170 180 190 100 150 200 250 - Mass to Charge Ratio (amu/e ) Mass to Charge Ratio (amu/e -) Mass to Charge Ratio (amu/e -)

Figure 4. Comparison of the Yb mass spectrum de- Figure 5. A signal confirming successful creation and tected in the RGA with the natural isotope abundance RF-trap loading of (a) Yb+ and (b) Th+ and Th2+ ions of Yb. using our laser ablation method and detection scheme. The plots show ion current in the electron multiplier during the m/z scan of the RGA detector (horizontal pends on the energy density of the ablation laser. axes). The first check of our ablation setup was to test it on a ytterbium target without the ion trap. The 2 produced plasma plume was first analyzed by the the vaporization threshold, less than 0.7 J/cm . unmodified RGA. The products of ablation were Regardless of the nominally sub-threshold fluence ionized by the RGA filament and were accelerated level, we observed the ablation of thorium targets by the entrance grid voltages. This way we repro- and the creation of singly and doubly ionized atoms 2 duced the expected natural isotope abundance of similarly to [64] where authors reached 1.1 J/cm ytterbium, but the detection, in this case, does not with the same kind of laser for ablation. Suc- 232 3+ resolve neutral and ionized products of the abla- cessful trap loading Th has been reported in tion. The graph in figure 4 shows the result of this [40] but with a more powerful laser source and flu- 2 measurement with Yb atoms ionized at the RGA ence reaching 320 J/cm , calculated from their 50 entrance. mJ pulse energy and 100 µm laser spot size. A We also confirmed the signal from all isotopes of metallic thorium-232 target was used. Success- 232 3+ ytterbium without the high voltage and filament at ful loading of Th via laser ablation has also the entrance of the RGA, while only measuring the been reported by Campbell et al [37] with a flu- +460 2 ions produced by the laser ablation. We detected ence of 60−40 J/cm , calculated from their 200 µJ a clear signal from Yb1+, Th1+ and Th2+ when pulse energy and 15(10) µm spot size. Large un- the ion trap before the RGA transmitted particular certainty in the fluence propagates from laser spot charge states and the RGA scanned around mass size uncertainty. Both groups used a thorium-232 to charge ratio during the laser ablation. These metal target. Campbell et al [38, 65] later used results are presented in figure 5. We have only a thorium nitrate sample and successfully loaded 229 3+ +48 2 confirmed a signal for singly and doubly ionized Th with a fluence of 16−8 J/cm calculated thorium. Each point represented as a vertical line from their 100 µJ pulse energy and 20(10) µm spot is an integration over 50 ms. size. The first assumption about the threshold for the laser-produced plasma is the energy fluence required for vaporization from the metal surface. B. Stability plots for thorium charge states The threshold for thorium (approximated around 2 1 J/cm ) is based on a purely thermal model of We scanned the RF voltage amplitude (VRF ) and laser ablation [62]. Other models predict that the DC bias voltage applied to the ion trap electrodes volume of the material ablated from the metal sur- (UDC ) while the RGA mass filter was only trans- face scales linearly with pulse energy [63]. We mitting to explore the parameter space of our trap. might expect that the fluence required for the ion- The radial motion of the ions on the quadruple trap ization of metal atoms would be even higher; some is governed by the Mathieu equation [57] and we energy from the pulse must be absorbed by the can extract two parameters called a and q that are atoms to release electrons. To estimate the thresh- proportional to UDC and VRF and other constant old for ionization, the energy density in the pulse trap parameters. The expected scaling between must be compared with the ionization energy. We charge states of thorium was confirmed. The trap estimate our peak fluence to be only a fraction of parameters were scanned in 20 V steps for both RF 6

60 7 (a) (b) Experimental Data 6 Experimental Data 50 Decay Fit Decay Fit 5 40 4 30 3

Ion Current (fA) Current Ion Ion Current (pA) Current Ion

+ 2

+ Th Yb 10 1 0 0 0 2000 4000 6000 8000 0 200 400 600 800 1000 1200 Number of Pulses Number of Pulses

Figure 6. The contour plots of the ion current as a Figure 7. Time dependence of the ytterbium (a) and function of the parameters a and q of the trap for Th1+ thorium (b) ions yield. The laser is moved to an un- and Th2+ charge states. The horizontal axes represent damaged location on the ablation target, and a se- the q parameter proportional to the amplitude of the quence of pulses is repeated on the same spot. driving voltage of the ion trap at 6.2 MHz. The vertical axes represent a parameter linked to the DC bias voltage applied to the trap electrodes. The color bar The observation of a significant decrease in the indicates the ion current at the detector (arbitrary log ablation yield is contrary to previous studies which scale), a measure of the number of ions that traveled have suggested that the decay process is not signif- through the ion trap. The predicted boundaries of stability regions from Mathieu equations are indicated by icant and for solid metal targets ablation yield re- white lines. mains constant [49]. Some results of ablation yield evolution for Sr, Yb and their oxides were also reported in [55], the authors recorded the yield evo- and DC fields, while the detector parameters were lution for Yb up to 500 pulses and did not observe fixed. For each data point, we measured the corre- any significant decay. In contrast to these reports, sponding ion current. The results are presented in we have monitored laser ablation for much longer, figure 6. The amplitude (VRF ) is inferred from the up to a few thousand pulses. The significant drop amplitude of the signal generator, accounting for in the yield amount was observed after 1500 pulses the amplifier and independently measured losses in (reduction by 1/e), which is consistent with previ- the helical resonator. ous studies; the authors could have missed a yield The results show that, while there may be a sys- drop when observing only 500 pulses. However, tematic shift in the overall scale due to incomplete the exponential decay for the thorium target was quantitative characterization of the trap, the sta- evident much more quickly (1/e decay after 150 bility regions do scale as expected when comparing pulses). This might seem surprising unless we con- the behavior of different m/z ratios. Of particu- sider that the ablation only comes from the ox- lar importance is the scaling between Th2+ and ide layer on the metal surface. That explains why Th1+ stability regions, which appear to follow the it disappears so quickly, and the decay is in good expected factor of two in the RF and DC voltages. agreement with similar observation for Yb and Sr Despite a rigorous search, we were unable to detect oxides [55]. The thermal diffusivity (which con- any signal from Th3+. trols the heating characteristics) of metals is usu- ally an order of magnitude higher than of oxides [66], so the expected fluence threshold for oxides is C. Laser ablation yield evolution also lower. That may explain the fact that what we ablate is not the thorium metal, but rather the The yield of the ablation changes over the num- layer of oxides on the surface. The thorium ions ber of pulses in the same spot on the target surface. could be created during the plasma plume evolu- The ablation decays are shown in Fig. 7 for the yt- tion afterward. terbium (a) and thorium (b) targets. The ablation laser runs at 1 Hz rate with an integration time of 500 ms in our detector. The blue points rep- IV. CONCLUSIONS resent the data after binning and averaging (bin size of 25 points for Yb and 5 points for Th), while We have suggested utilizing nuclear clocks based the red line is the fitted curve of the exponential on the isomeric transition of the 229Th nucleus for decay. The experimental runs with higher repeti- chronometric geodesy and discussed experimental tion rates and shorter exposures were noisier but efforts toward its realization. An ion trap for trap- showed similar behavior. ping and storing thorium ions was designed and 7 built along with a vacuum enclosure and detection tivity. Once we produce and trap Th3+, we can system. At the initial stage of research, we investi- perform laser cooling working towards the nuclear gated methods of delivering thorium ions into the clock transition spectroscopy. trap. We conducted a series of laser ablation experiments to determine a reliable and efficient way to deliver ions from metal targets. We showed that FUNDING even a simple and fairly weak ablation laser can lead to thorium ionization lasting long enough to be This work was funded by the Commonwealth Sci- effectively injected into an RF trap. Our results are entific Industrial Research Organisation (CSIRO). in good agreement with previously reported trap Mirko Lobino was supported by the Australian loading with thorium, but the extended studies of Research Council Future Fellowship FT180100055. yield from laser ablation suggest that only the oxide Erik Streed was supported by Australian Research layers contribute to the Th ion production by UV Council Future Fellowship FT130100472 and Link- nitrogen laser pulses. The findings are important age Project LP180100096. for optimal loading of miniaturized thorium traps, which are expected to be the crucial component of future nuclear clocks. ACKNOWLEDGMENTS We studied electroplating for thorium target preparation. 232Th was used in this study as a predecessor for 229Th, for reasons of lower radioac- Laboratory space and support services were pro- tivity. Our findings can be directly transferred to vided by Griffith University. We thank Dane Laban the 229Th isotope. Our laser setup cannot produce for his advice in the development of the thorium the fluence required for the ablation of triply ion- deposition apparatus. ized thorium, thus more powerful laser must be em- ployed for this task or the technique presented com- bined with other means of ionization like photo- DISCLOSURES ionization or electron bombardment. We also plan to add time gating of the trap for isotope selec- The authors declare no conflicts of interest.

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