We have developed a surface ionization ion-source as part of the JAEA-ISOL (Isotope Separator On-Line) setup, which is coupled to a He/CdI2 gas-jet transport system to determine the first ionization potential of the heaviest actinide lawrencium (Lr, Z = 103). The new ion-source is an improved version of the previous source that provided good ionization efficiencies for lanthanides. An additional filament was newly installed to give better control over its operation. We report, here, on the development of the new gas-jet coupled surface ion-source and on the first successful ionization and mass separation of 27-s 256Lr produced in the 249Cf + 11B reaction.

The first ionization potential (IP) is a fundamental physical and chemical property of an element. Information on the IP of the heaviest elements can provide a test and better understanding of relativistic effects which are significantly noticeable for heavy elements. A precise and accurate determination of the IP of the heaviest elements allows us to challenge modern theoretical calculations on the electronic structure of heavy atoms.

The IP values of heavy actinides up to einsteinium (Es, atomic number Z = 99), produced in a nuclear reactor, were successfully measured by resonance ionization mass spectroscopy (RIMS).1–4 Recently the atomic level structure of fermium (Fm, Z = 100) was also investigated by RIMS using a sample of 2.7 × 1010 atoms of 255Fm with a half-life (T1/2) of 20.1 h.5,6 The elements heavier than Fm, however, must be produced at accelerators using reactions of heavy ions with heavy target materials. Moreover, both half-lives and cross sections of the isotopes of the still heavier elements are rapidly decreasing. Thus, they are usually available in quantities of a few atoms only at a time. Consequently, beginning with about the end of the actinides, properties of the heaviest elements must be studied on an atom-at-a-time scale. The IP values of the heavier actinides with Z ⩾ 100, therefore, have not been measured using well-established methods like RIMS.

The ground-state electronic configuration of the heaviest actinide, lawrencium (Lr), is predicted to be [Rn]5f147s27p1/2, which is different from that of the lanthanide homolog Lu [Xe]4f146s25d. The reason for this change in ground-state configuration is because the 7p orbital of Lr is stabilized below the 6d orbital by strong relativistic effects.7 The weakly-bound outermost electron results in a significantly lower IP of Lr as compared with its neighboring heavy actinides.8 With an experimentally determined IP value of Lr, we can contribute to a better understanding of shell effects and how relativistic effects play a role in the electronic structure of heavy atoms.

The surface ionization is a process in which an atom is ionized via the interaction with a solid (metal) surface at high temperature. According to the Saha-Langmuir equation, the ionization strongly depends on the surface temperature of the solid material, a work function which is element dependent and can be very specific dependent on any modification of the surface substrate, and the ionization potential of the element of interest.9 Since the process takes place between an atom and a surface, it is applicable to atom-at-a-time scale experiments. Some attempts to measure ionization potentials of actinides up to curium (Cm, Z = 96) have been performed based on the surface ionization process.10–12 The technique was, however, applied to isotopes with macro-amounts of about 1 μg.

In order to determine the IP value based on the surface ionization method at the atom-at-a-time scale, we have improved the existing surface-ionization type ion-source13 installed in the JAEA-ISOL setup.14 The ion-source is coupled to a target recoil chamber via a capillary for gas-jet transportation of short-lived isotopes produced in heavy-ion induced nuclear reactions.

This paper reports on the improved version of the gas-jet coupled surface-ionization type ion-source and the first successful ionization of 256Lr (T1/2 = 27 s) produced in the 249, 250, 251Cf + 11B reaction.15 

A schematic diagram of the experimental setup is shown in Fig. 1. It consists of a target recoil chamber, a gas-jet transport system, a surface-ionization type ion-source in ISOL, mass-separator, and α/γ detection systems. Among the various available detection systems, we applied two rotating catcher wheel apparatuses of the MANON (Measurement system of Alpha particle and spontaneous fissoN events ON-line),16 MANON(A) being installed behind the mass-separator and MANON(B) being used for a direct-catch experiment, and used a tape collection system.

FIG. 1.

Schematic diagram of the experimental setup.

FIG. 1.

Schematic diagram of the experimental setup.

Close modal

Figure 2(a) shows a schematic drawing of the ion-source chamber including the double-skimmer alignment and the extraction electrode to accelerate ions and inject them into the mass analyzer. The orifices of the first and second skimmers are 2 mm and 6 mm in diameter, respectively. The outlet of the stainless steel capillary was located 4 mm upstream from the first skimmer and the distance between the first and second skimmers was 9 mm. A roots pump, 1195 m3/h, pumped the first skimmer compartment, while a 680 l/s turbo-molecular-pump (TMP) pumped the second skimmer compartment, both pumping the target recoil chamber through the capillary (1.5 mmϕ × 8.8 m). Nuclear reaction products attached on CdI2 aerosols were transported from the target recoil chamber with a He gas flow through the capillary. The aerosols passed through the skimmers and flew into the ionizer of the ion-source. At a He gas-flow rate of 1.4 l/min, the pressure in the second skimmer compartment was 2 × 10−2 Pa while that in the target recoil chamber was 95 kPa.

FIG. 2.

(a) Schematic drawing of the surface ionization type ion-source with the skimmer system and the extraction electrode of the ISOL. (b) Photograph of the new arrangement of the ionizer. An additional filament is installed as indicated Filament (2).

FIG. 2.

(a) Schematic drawing of the surface ionization type ion-source with the skimmer system and the extraction electrode of the ISOL. (b) Photograph of the new arrangement of the ionizer. An additional filament is installed as indicated Filament (2).

Close modal

The previously developed ion-source that already provided good ionization efficiency for lanthanides13 was further improved; the cylindrical ionizer made of a tantalum (Ta) crucible of 44 mm length and 4 mm inner diameter was newly surrounded by two tungsten (W) filaments. In addition, the heat shields were rearranged to effectively heat the ionizer. The newly added filament is indicated as Filament(2) in Figs. 2(a) and 2(b). The ionizer was heated by both thermal radiation from the filaments and the electron bombardment. For thermal heating, we typically used an electrical power of about 200 W for each filament. Mainly by slightly changing the input power for Filament(1), we modified the temperature of the ionizer through differences in the power of the electron bombardment from Filament(1). The voltage for the electron bombardment was always kept constant at 450 V. The electric power for heating each filament and for the electron bombardment was controlled independently. In the present study, the input power of the electron bombardment from the Filament(2) was kept constant at 99 W (450 V × 0.22 A).

When the electric power of the electron bombardment from Filament(1) reached 450 W (450 V × 1.0 A), the temperature near the outlet of the ionizer was approximately 2600 K measured with an optical pyrometer. To compare the ionization efficiency on a Ta surface with that at a different ionizer surface, a rhenium (Re) surface was also employed. In the case of Re, the inner wall of another Ta ionizer was covered with a thin Re foil (50 μm). CdI2 was used as an aerosol material for transportation because (a) it is heavy enough to give a good skimmer efficiency, (b) the elements are tolerable in the hot ionizer, and (c) it has a sufficiently low boiling point of 713 °C17 to rapidly evaporate all aerosol material. The CdI2 aerosols carrying nuclear reaction products passed through the skimmers, and were immediately vaporized at the surface of the ionizer. Then, the reaction products were desorbed and ionized via the surface ionization process at the surface, and were extracted by the extraction electrode and accelerated.

The isotope 256Lr (T1/2 = 27 ± 3 s)18,19 was produced in the reaction of 249, 250, 251Cf(11B, xn)256Lr.15 A 249, 250, 251Cf target (249Cf: 63%, 250Cf: 12%, and 251Cf: 25%) with 185 ± 25 μg/cm2 was prepared by electrodeposition of Cf nitrate from 2-propanol onto a 1.85 mg/cm2 thick Be backing foil. The Cf target was irradiated with a 67.9-MeV 11B4 + beam from the JAEA tandem accelerator. The beam energy was calculated to be 63 MeV in the middle of the target. The lutetium isotopes 168mLu and 168gLu, with a half-life of 6.7 min and 5.5 min, respectively, were synthesized in the 162Dy(11B, 5n) reaction using an enriched 162Dy target with 230 μg/cm2 in thickness electrodeposited on a 1.80 mg/cm2 Be foil. The Dy target was positioned downstream of the Cf target (see Fig. 1). As a reference material for the measurement of the ionization efficiency, we used 80Rb (T1/2 = 30 s) produced in the reaction of natGe(11B, xn). A natGe target of 1 mg/cm2 was prepared by sputtering of natGe metal onto a 1.80 mg/cm2 thick Be foil. Reaction products recoiling out of the targets were stopped in the target recoil chamber in He gas loaded with CdI2 aerosols, and were transported within 0.6 s into the ion-source of the ISOL by a gas-jet stream through the Teflon capillary. All reaction products, which were ionized in the ion-source, were accelerated with 30 kV and were mass-separated with a mass resolution of MM ∼ 900.13 In the analyses of this experiment, we are assuming similar efficiencies for Lr and the lighter elements in the mass separator part of our ISOL system.

Mass-separated 256Lr was measured with 8-pairs of Si PIN photodiodes of MANON(A) at the end of the ISOL (see Fig. 1). An additional Si PIN photodiode was set at a sample collection port to monitor mass-separated products. All 256Lr ions were implanted into polyethylene terephthalate (PET) foils of a thickness of 120 μg/cm2 and 20 mm in diameter at the perimeter of a 40-position stainless steel wheel of 42 cm diameter. The wheel was periodically rotated at 10-s intervals to move a collected sample to a detection position. Each detector pair had an efficiency of 80 ± 2% for the detection of an α-particle. The α-particle energy resolution (FWHM) was 30 keV for the top detectors and was 100 keV for the bottom ones. All α-energies and detector numbers were registered in an event-by-event mode together with time information.

A tape transport system was installed between the end of the ISOL beam line and the MANON(A) set-up to perform γ-ray measurements of mass-separated ions of 168m, gLu and 80Rb. These ions were implanted into an aluminized Mylar tape and were transported to the detection port with stepping times of 60 s. A high-purity germanium (HP-Ge) detector with a relative efficiency of 36% (ORTEC 36% GAMMA-X) was placed at the collection position to monitor the implanted ions and another HP-Ge detector (ORTEC 28% GAMMA-X) was set at a detection port.

To determine the amount of each of the reaction products transferred to the ion-source, they were transported to a separate collection site where their radioactivity was measured prior to the ISOL experiments. Here, the measurement of 256Lr was performed using the MANON(B) (see Fig. 1). The procedures, such as cyclic times and data taking, were the same as those with MANON(A). The reaction products were transported within 2.1 s with the flow of CdI2 aerosols in the He gas at a flow rate of 1.8 l/min from the target recoil chamber through a Teflon capillary. The Teflon capillary had a length of 25 m and a 2.0 mm i.d. The reaction products on the CdI2 aerosol particles were collected for 10 s onto PET foils of 120 μg/cm2 thickness. Then, the collected sample was rotated with a 10-s interval under the active area of the first one of 12 successive PIN photodiodes (Hamamatsu S3204) for α-particle energy measurement. Each detector had 30 ± 2% detection efficiency, and the α-particle energy resolution was about 20 keV (FWHM). For the measurement of 168m, gLu and 80Rb, aerosols from the target chamber were collected on a glass fiber filter for 1 min. Then, the γ-rays of each nuclide on the glass fiber filter were measured by using a HP-Ge detector (ORTEC 28% GAMMA-X).

A sum of α-spectra measured with all 8 Si PIN photodiodes of MANON(A) facing the implantation side of samples for mass number A = 256 ionized with the Re surface at 2600 K is shown in Fig. 3. Alpha particles originating from 256Lr and its decay products, 256No and 252Fm, are clearly observed (the decay scheme of 256Lr is indicated in the inset of Fig. 3). This clearly demonstrates the first successful observation of mass-separated Lr ions with an ISOL technique. In the α-energy range of 8.25–8.65 MeV,20 a total of 80 single α-events were registered both in the top and bottom detectors of MANON(A) for a total beam dose of 2.5 × 1017 while 18 events were registered during a total beam dose of 9.5 × 1016 in the direct collection with MANON(B), which was only equipped with top detectors.

FIG. 3.

Measured α-particle spectrum of mass-separated ions with mass number A = 256 with the Re surface.

FIG. 3.

Measured α-particle spectrum of mass-separated ions with mass number A = 256 with the Re surface.

Close modal

The ionization efficiency (εion) is defined as the ratio of the number of ions formed (Yion) to that of atoms injected into the ion-source (Yin). We can only measure, however, the number of ions at the end of ISOL, YISOL with MANON(A) or the tape system. Yin is evaluated from measuring reaction products directly at MANON(B) or a glass fiber filter (Ydirect). In addition, we define εtransport as the transport efficiency from the ion-source (the extraction part) through the mass separator all the way to the collection site at the end of ISOL. If Ydirect can be assumed to be linearly related with Yin with a proportional factor, a, we obtain the following equation:

\begin{equation}\epsilon _{\rm ion} = \frac{Y_{\rm ion}}{Y_{\rm in}}=\frac{Y_{\rm ISOL}/\epsilon _{\rm transport}}{a Y_{\rm direct}} = \frac{1}{a \epsilon _{\rm transport}}\frac{Y_{\rm ISOL}}{Y_{\rm direct}},\end{equation}
ε ion =Y ion Y in =Y ISOL /ε transport aY direct =1aε transport Y ISOL Y direct ,
(1)

where a reflects the transport efficiency from the target recoil chamber to the ion-source, the efficiency from the target recoil chamber to the direct collection in MANON(B) or the glass fiber filter, and the transmission in the skimmer section at the entrance of the ion source.

Because of the low ionization potential of Rb, namely, εion(Rb) = 1, it is justified to assume that the ionization efficiency of Rb is 100% on the surface of both Ta and Re at those high temperatures used in our experiment. Based on this assumption we can derive Eq. (2),

\begin{equation}\epsilon _{\rm ion}{\rm (Rb)} = \frac{Y_{\rm ion}{\rm (Rb)}}{Y_{\rm in}{\rm (Rb)}}=\frac{Y_{\rm ISOL}{\rm (Rb)}/\epsilon _{\rm transport}}{a Y_{\rm direct}{\rm (Rb)}}=1.\end{equation}
ε ion ( Rb )=Y ion ( Rb )Y in ( Rb )=Y ISOL ( Rb )/ε transport aY direct ( Rb )=1.
(2)

From Eqs. (1) and (2), the following equation results:

\begin{equation}a\epsilon _{\rm transport} = \frac{Y_{\rm ISOL}{\rm (Rb)}}{Y_{\rm direct}{\rm (Rb)}}.\end{equation}
aε transport =Y ISOL ( Rb )Y direct ( Rb ).
(3)

We can obtain the following relation (Eq. (4)) which will be used for the determination of the Lr ionization efficiency,

\begin{equation}\epsilon _{\rm ion} = \frac{Y_{\rm ion}}{Y_{\rm in}}=\frac{1}{a \epsilon _{\rm transport}}\frac{Y_{\rm ISOL}}{Y_{\rm direct}} = \frac{Y_{\rm direct}{\rm (Rb)}}{Y_{\rm ISOL}{\rm (Rb)}} \times \frac{Y_{\rm ISOL}}{Y_{\rm direct}}.\end{equation}
ε ion =Y ion Y in =1aε transport Y ISOL Y direct =Y direct ( Rb )Y ISOL ( Rb )×Y ISOL Y direct .
(4)

The ionization efficiency of an atom can be deduced from the yield ratio of the mass-separated ions to the directly collected atoms together with the yield ratio of Rb. We measured a ratio of YISOL(80Rb)/Ydirect(80Rb) in a temperature range of the ion source from 2300 K to 2600 K at the beginning of the experiment. As expected, the obtained ratios were nearly constant and about 1 for both Re and Ta surfaces, and they were independent of the temperature of the ion-source.

Taking account of detection efficiencies of each measurement system and the decay-loss of the isotopes, the YISOL/Ydirect ratio of 256Lr was obtained to be

$0.46^{ + 0.15}_{\rm - 0.13}$
0.460.13+0.15 at 2600 K on Re. The errors given here and in the following are one-sigma c.i. taking into account statistical errors only. The rather large error on the Lr results from the limited number of observed α-decays in the Lr-spectra.21 The YISOL/Ydirect value of Lu, measured at the same ionization condition, was 0.219 ± 0.004. Using YISOL(Rb)/Ydirect(Rb) = 1.10 ± 0.04 for this measurement, the ionization efficiency of Lr was evaluated to be
$42^{+\;20}_{ -\;19}$
4219+20
% while that of Lu was 19.9 ± 7.0%. The errors include the statistical error, that from the half-life uncertainty, that of the detection efficiencies, and that of the gas-jet transport efficiencies.

In addition to the experiments performed with a Re surface in the ionizer, we measured ionization efficiencies of Lu and Lr on a Ta surface at 2400 K and 2600 K. At 2400 K, εion of Lr and Lu were

$19^{+\;9}_{-\; 8}$
198+9% and 4.0 ± 1.4%, respectively, and at 2600 K, 24 ± 10% and 6.6 ± 2.3%, respectively. Again, the εion of Lr was higher than that of Lu at both temperatures. The obtained ratio YISOL/Ydirect of 168m, gLu, 256Lr, and 80Rb, and the deduced ionization efficiencies of Lu and Lr are shown in Table I. The volatility of Lr is predicted to be similar to that of Lu.22 Assuming that the adsorption-and-desorption behavior of Lr in the ionizer is the same as that of Lu, the ionization efficiency inversely correlates with the ionization potential on the same metal surface. Thus, the higher ionization efficiency of Lr compared with that of Lu suggests that the ionization potential of Lr is lower than that of Lu. This result is consistent with the theoretical prediction from a coupled cluster (CC) calculation8 that takes into account relativistic effects.

Table I.

Ratios of the number of ions at the end of ISOL (YISOL) to that of isotopes caught directly at the collection site (Ydirect) for 80Rb, 168m, gLu, and 256Lr, and ionization efficiencies of 168m, gLu and 256Lr deduced from these ratios (see text).

  YISOL/Ydirect Ionization efficiency [%]
Surface80Rb168m, gLu256Lr168m, gLu256Lr
Re (T = 2600 K) 1.10 ± 0.04 (21.9 ± 0.4)×10−2 
$0.46^{+\;0.15}_{-\;0.13}$
0.460.13+0.15
 
19.9 ± 7.0 
$42^{+\;20}_{-\;19}$
4219+20
 
Ta (T = 2600 K) 0.91 ± 0.02 (6.0 ± 0.4)×10−2 0.22 ± 0.06 6.6 ± 2.3 24 ± 10 
Ta (T = 2400 K) 0.91 ± 0.02 (3.6 ± 0.2)×10−2 
$0.17^{+\;0.06}_{-\;0.05}$
0.170.05+0.06
 
4.0 ± 1.4 
$19^{+\;9}_{-\;8}$
198+9
 
  YISOL/Ydirect Ionization efficiency [%]
Surface80Rb168m, gLu256Lr168m, gLu256Lr
Re (T = 2600 K) 1.10 ± 0.04 (21.9 ± 0.4)×10−2 
$0.46^{+\;0.15}_{-\;0.13}$
0.460.13+0.15
 
19.9 ± 7.0 
$42^{+\;20}_{-\;19}$
4219+20
 
Ta (T = 2600 K) 0.91 ± 0.02 (6.0 ± 0.4)×10−2 0.22 ± 0.06 6.6 ± 2.3 24 ± 10 
Ta (T = 2400 K) 0.91 ± 0.02 (3.6 ± 0.2)×10−2 
$0.17^{+\;0.06}_{-\;0.05}$
0.170.05+0.06
 
4.0 ± 1.4 
$19^{+\;9}_{-\;8}$
198+9
 

We have developed the surface ion-source coupled to the He/CdI2 gas-jet transport system to measure the ionization potential of Lr at atom-at-a-time conditions. We successfully ionized and mass-separated 256Lr ions for the first time by applying the present ion-source and the ISOL technique. The ionization efficiencies of Lr were estimated to be approximately 42% and 24% at 2600 K on Re and Ta surfaces, respectively. These values were higher than those of Lu in all of ionization condition. The results indicate that the ionization potential of Lr would be lower than that of Lu, 5.4 eV. Therefore, it is concluded that the surface ion-source is a promising apparatus to measure the first ionization potential of Lr. Using the present system, determination of the ionization potential of Lr is being performed.

The authors would like to thank the JAEA tandem accelerator crew for supplying intense and stable beams for the experiments. They are also indebted to Prof. H. Kudo and Prof. T. Mitsugashira for providing us with a mixed Cf material which was chemically purified at International Research Center for Nuclear Materials Science, Institure of Materials Research (IMR), Tohoku University.

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