Loading of mass spectrometry ion trap with Th ions by laser ablation for nuclear frequency standard application

Abstract

We describe an original multisectional quadrupole ion trap aimed to realize nuclear frequency standard based on the unique isomer transition in thorium nucleus. It is shown that the system effectively operates on Th⁺, Th²⁺ and Th³⁺ ions produced by laser ablation of metallic thorium-232 target. Laser intensity used for ablation is about 6 GW/cm². Via applying a bias potential to every control voltage including the RF one, we are able not only to manipulate ions within the energy range as wide as 1–500 eV but to specially adjust trap potentials in order to work mainly with ions that belong to energy distribution maximum and therefore to effectively enhance the number of trapped ions. Measurement of energy distributions of ²³²Th⁺, ²³²Th²⁺, ²³²Th³⁺ ions obtained by laser ablation allows us to define optimal potential values for trapping process. Observed number of ions inside trap in dependence on trapping time is found to obey an unusually slow – logarithmic decay law that needs more careful study.

Keywords

Frequency standards optical clocks thorium nuclear transition ion trap optical spectroscopy nuclear spectroscopy mass spectrometry

Introduction

Recent progress in the field of frequency standards led to the development of such fundamental areas of science as accurate tests of general relativity,¹ determination of fundamental constants,² development of new quantum technologies.³ Further progress can be reached using nuclear transitions instead of the atomic ones.^4,5 It can increase the precision of measurements by several orders of magnitude.⁶ Unfortunately, typical energies of nuclear transitions are in the range of keV to MeV, making it difficult to use for frequency standards applications. The unique exception is the isomeric low-lying nuclear state of the ^229Th.⁷ To date, experimental value of indirect registration of the isomeric transition energy accepted by physical community is 7.8 ± 0.5 eV.⁸ The low-lying nuclear transition of Thorium-229 is located in the region of vacuum ultraviolet, which is accessible for laser spectroscopy, and therefore is of special interest due to many potential applications.⁹ However, it is difficult to determine the transition energy precisely because energy width of this transition is very narrow.

One of the promising approaches to solve this problem is to study ²²⁹Th ions of different charge in quadrupole ion trap.^10,11 Ions of ²²⁹Th⁺ and ²²⁹Th²⁺ are used in experiments to excite the nuclear ^229gTh – ^229mTh transition and to determine accurately its frequency.¹² Such opportunity is connected to the complicated overlapping electron level systems of the ground and isomeric ²²⁹Th²⁺ and ²²⁹Th⁺ states. It might significantly increase the probability of excitation of the isomeric ²²⁹Th nuclear state via electronic bridge process.¹³ At the same time, ²²⁹Th³⁺ ion has only one valence electron and thereby relatively simple system of electron levels, and thus this ion is preferable for laser cooling. Moreover, the direct detection of the nuclear transition is related to the process of spontaneous relaxation of the nucleus isomeric state by γ-quant emission. For ²²⁹Th²⁺ and ²²⁹Th⁺ ions, such decay channel is significantly suppressed in comparison to ²²⁹Th³⁺. That’s why the scheme of nuclear clocks is based on ²²⁹Th³⁺ ions⁹. Thus, the possibility to work with ²²⁹Th ions of different charge is very important to investigate the low-lying isomeric nuclear state.

Production, trapping and laser cooling of ²²⁹Th ions of different charge are challenges and require a significant experimental work. High radioactivity, chemical activity and limited accessibility make the laser ablation a common method to produce ²²⁹Th³⁺, ⁺²²⁹Th²⁺ and ²²⁹Th⁺ ions. Unfortunately, ions obtained by laser ablation technique are characterized by wide energy distribution with maximum located at energy varying with charge from several tens of eV up to hundreds eV. This fact constricts the possibilities of usual ion traps generally used for precision measurements. However, the knowledge of energy distribution allows us to adjust trapping parameters to work effectively with required ions.

In this paper, we report about optimization of trapping process of Th⁺, Th²⁺, Th³⁺ ions obtained from laser plasma. We suggest the original method of the production and loading of Th ions of different charge into multisectional quadrupole linear Paul trap. The results of the study of energy distribution of Th ions obtained from laser plasma are presented. The obtained energy spectra were described with the shifted Maxwell–Boltzmann–Coulomb (MBC) distribution that allowed us to obtain the parameters of laser plasma at the late stage of expansion. Also, the results of the preliminary studies of trapping and keeping of high energy thorium ions in the original multisectional quadrupole linear trap are presented.

Experimental

Thorium ions were obtained with laser ablation method. Laser ablation allows one to ionize a small amount of sample in controllable way¹⁴ that is important for work with radioactive materials. Q-switched Nd:YAG laser was used to perform ablation with power density equal to 6 GW/cm² (laser pulse duration is 25 ns, pulse energy is 50 mJ, laser spot radius on the sample is 100 µm). Thorium-232 target was placed in front of the entering section of quadrupole in the vacuum chamber.¹⁵

In order to prevent short circuit in the ion trap electronics while working with very dense laser plasma, several changes were made comparing with our previous works.¹⁶ To reduce plasma density at the entrance of the trap, a 0.5-mm diameter diaphragm was installed. Next to the diaphragm, an electrical potential 50 V was applied in order to destroy plasma. It was found that the laser plasma causes the supply voltages breakdown not only through the diaphragm, but also from the side of entrance quadrupole section. So a shield was installed around the quadrupole. The resulting configuration allowed us to operate plasma, obtained by laser ablation with various power densities from a wide range.

The original multisectional quadrupole linear ion trap¹⁷ is used for trapping ions. Quadrupole mass spectrometry is a well-established analytical method allowing very compact analyzers.^18–20 The trap (Figure 1) consists of five separated quadrupole segments. The potential of each quadrupole can be varied separately. Such feature allows us to form a complex potential profile for trapping, keeping and spatial manipulation on ions. The trap performs a mass-selective trapping of desired ions. Power supplies allow mass filtering in a range of 2−250 Da with resolution 1 Da.

The main advantage of the trap is the ability to apply a bias potential to all control voltages including the RF voltage (RF voltage frequency is 1.22 MHz). Bias potential changes the potential on the quadrupole axis and provides sampling ions with desired kinetic energy within a wide energy range of 1–500 eV. In this way, we can adjust trapping parameters to work with ions belonged to energy distribution maximum. Trapping of ions belonged to the maximum of the distribution function instead of its low-energy tail allows us to use lower laser pulse intensity and thus to improve an efficiency of thorium sample usage.

Energy distributions are measured with a Hughes–Rozhansky energy analyzer installed just next the exit quadrupole section. Energy analyzer window keeps constant, while varying bias potential is applied to all control voltages including the RF voltage. Energy analyzer is adjusted to pass ions in sampling range 0–10 eV. Bias potential is being scanned in the range 0–300 eV.

To implement trapping process, a DC potential synchronized (synchronization time 1 μs) with the laser pulse is supplied to the end cap electrodes. The electrodes have 2 mm diameter apertures coaxial with the ion trap axis. The value of “trapping potentials” applied to the electrodes is about 100 V.

It should be noted that usually ²²⁹Th sample is a thorium deposit formed on different substrates, which is not the same to the bulk ²³²Th metal sample. As to trap loading, the main problem with thorium deposits is to perform efficient mass filtering of substrate particles and chemical compounds produced by laser ablation. Nevertheless, standard techniques of producing and trapping of pure ²³²Th from metal targets and ²²⁹Th ions from deposited ones are very similar. As soon as the statistics accumulation of mass-energy spectra needs a large amount of material ablated within the framework of the current research, we use a stable thorium-232 isotope bulk target. It allows us to lower as well the radioactivity down to the level acceptable for laboratory conditions.

Results and discussion

Typical mass spectrum of thorium ions generated via laser ablation of bulk metallic ²³²Th target is shown in Figure 2. There are well pronounced peaks corresponding to Th⁺, Th²⁺ and Th³⁺ ions. Small peak at 248 Da might be corresponded to the ThO⁺ compound. Mass spectrum shows that triply and double charged thorium ions are effectively produced by laser ablation of metallic thorium-232 target. The intensity ratios Th³⁺/Th⁺ and Th²⁺/Th⁺ are about 17% and 68%, respectively. It means that partial intensities are sufficient enough to adjust the quadrupole parameters for working with selected ions of the desired charge and to carry out measurements of their energy distribution.

Figure 3 shows energy distributions of Th⁺, Th²⁺ and Th³⁺ ion beam. All energy distributions are measured at the same setup settings, only mass-filtering potentials have been adjusted to the desired mass-to-charge ratio. All distributions are normalized to equal area. Energy spectra of Th⁺, Th²⁺ and Th³⁺ have a bell-shaped form with maximum at about 120 eV, 200 eV and 375 eV, respectively. The full width at half maximum for each distribution is equal to 140, 220 and 390 eV, respectively. Thus, the average energy and dispersion increase with the ion charge. Note that the observed energy distributions are typical for ions obtained by laser ablation.^21,22 In order to identify plasma physical parameters at the late expansion stage, we can use the so called shifted MBC distribution function²³

\begin{matrix} F (v) = A (\frac{m}{2 π kT})^{\frac{3}{2}} v^{3} exp (- \frac{m}{2 kT} (v - (v_{k} + v_{c}))^{2}) \end{matrix}

where A is a normalization constant, kT is the ion distribution temperature, m and v are the ion mass and velocity, v_k and v_c are the center-of-mass and Coulomb velocities. Solid lines in Figure 3 correspond to the experimental spectra fitted by the MBC distribution function with parameters presented in Table 1. Fitting procedure cannot distinguish v_k and v_c as independent parameters of the fitting function, but v_k can be estimated as

v_{k} = \sqrt{\frac{γ kT}{m}}

, where γ = 5/3 for metal monatomic samples.²³ It should be noted that the experimental data obtained for Th²⁺ and Th³⁺ are fitted quite well, while Th⁺ spectra observe some reproducible fine structure, that cannot be described by the MBC distribution. Fine splitting of Th⁺ peak (spectra 3, inset) can be associated with recombination processes in the early-stage plasma when multiply charged ions being accelerated in Coulomb field experience multiple collisions and become single-charged. Thus, an ion might be initially accelerated as a multiple charged particle, but later on, it accelerates as a single charged one. The different Coulomb velocities of the initially ionized single-charged ions and those effected by recombination processes contribute to the same resulting distribution and split its pure bell-shaped form. Detailed analysis of the process can be found in Wang et al.²⁴ The observed fine splitting of the Th⁺ ion peak in energy distribution is less than 10% of the overall intensity and therefore the effect cannot sufficiently influence the process of ion’s trapping.

Table 1.

Summary of parameters obtained by fitting the experimental results to the MBC distribution function.

	Th⁺	Th²⁺	Th³⁺
A	0.49	0.11	0.08
kT, eV	18.5	21.1	40.8
v_k, m/s	3.8 × 10³	8.0 × 10³	9.7 × 10³
v_c, m/s	3.6 × 10³	3.8 × 10³	5.3 × 10³

Note: A is a constant of normalization to the experimental data, kT is the ion distribution temperature, v is the ion velocity, v_k is the center-of-mass velocity, and v_c is the Coulomb velocity.

MBC: Maxwell–Boltzmann–Coulomb.

Energy distributions are used to adjust quadrupole bias potential for trapping ions with energy close to the energy distribution maximum that significantly enlarges the number of trapped ions. Table 2 presents optimal quadrupole voltage values for trapping Th⁺, Th²⁺ and Th³ ions.

Table 2.

Optimal quadrupole voltage values for ion trapping.

	Zϕ, Volts
Th⁺	120
Th²⁺	200
Th³⁺	375

Z: Ion charge multiplicity; ϕ: bias voltage,

In order to test the possibility of trapping and keeping of high energy thorium ions obtained in laser plasma plume, we carried out some preliminary experiments for the whole ions ensemble. We use the voltages from Table 2 for single-charged Th ions in order to optimize trapping parameters. Therefore, during the set of quadrupoles operate without mass-filtering, the main contribution to the overall amount of ions is mainly determined by single-charged thorium ions as it follows from distribution functions shown in Figure 3. In order to let ions leave the trap, the exit end cap potential is decreased down to the level of the quadrupole axis potential. The dependence of ion number on the moment of their registration time (time of flight) with electron multiplier for different trapping times is measured (see Figure 4). The ion’s time of flight is measured as a time interval between the moment of switching off the end cap electrode potential and the detection event. Time of flight distribution function changes with trapping time. Figure 4 shows that high energy ions (see the front histogram on Figure 4) are detected for 100 μs trapping time. Their time of flight corresponds to the ion energy of 20 eV. When the trapping time enlarges up to 1 s, the time of flight distribution function is dramatically broadening. Moreover, its maximum shifts towards higher value, i.e. the kinetic energy of trapped ions decreases.

The fact that for large enough trapping time (∼1 s), we mainly detect slow ions which can be explained by collision processes inside the trap because of the residual pressure during experiments is ∼10⁻⁷Torr. The collisions result in relaxation of ion velocity distribution function to thermal equilibrium during the characteristic time τ that can be estimated as following²⁵

τ = \frac{3 (M + m_{g})}{4 m_{g} n σ u_{kT}}; u_{kT} = \sqrt{\frac{8 kT}{π m_{r}}}

where M is the ion mass, m_g is the gas molecule mass, m_r is the reduced mass, n is the gas density and σ is the collision cross section. Using the values M = 3.85 × 10⁻²⁵kg, m_g = 2.99 × 10⁻²⁶kg, m_r = 2.77 × 10⁻²⁶kg, n = 1.61 × 10¹⁶m⁻³, σ = 1.94 × 10⁻¹⁹ m², one can obtain that τ is of the order of some seconds, which is close to the relaxation characteristic time observed experimentally (see Figure 4).

The number of ions leaving the trap in dependence on the trapping time is also obtained (see Figure 5). Measurements show an unusually slow logarithmic decrease of trapped ion number with trapping time. It means that − $\frac{dn}{dt} ~ \frac{1}{t}$ . Explanation of such dependence is a subject of further studies.

Figure 5 shows signal from ions from 100 s trapping time. It should be noted that the ion number measured is 100 times less than the real number is due to losses in energy analyzer. This fact complicates the study of the trapping process at trapping times more than 100 s as soon as the signal became undetectable from the noise. Nevertheless, the obtained 100 s trapping time is enough to perform laser cooling in the next stage of our project. The proposed scheme of laser cooling setup is reported in paper.¹⁵

The 100 s limitation of the trapping time can be connected to the following reasons: Coulomb field inhomogeneity caused by the end cap electrodes; the existence of high energy ions, obtained by laser ablation method; the interaction of ions with molecules of residual gases in the trap chamber.

Conclusion

The method of trapping process optimization to enhance ion number trapped by adjusting the average trap potential for working with ions belonged to energy distribution maximum is proposed. Energy distribution functions of ²³²Th⁺, ²³²Th²⁺, ²³²Th³⁺ obtained by laser ablation of bulk metallic ²³²Th target, with maximum at 120 eV, 200 eV and 375 eV for Th⁺, Th²⁺ and Th³⁺ thorium ions, correspondingly were measured. Using this energy distribution functions, the optimal potentials values for trapping process were defined.

The trapping parameters such as the dependence of ion number inside the trap on trapping time were investigated and discussed. Unusual slow logarithmic decrease of trapped ion number with trapping time was shown. Explanation of such dependence is a subject of further studies.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by a grant of Russian Science Foundation (project No 16-12-00001).

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