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Vacuum Chromatography of Tl on SiO at the Single-Atom Level Patrick Steinegger, Masato Asai, Rugard Dressler, Robert Eichler, Yusuke Kaneya, Akina Mitsukai, Yuichiro Nagame, Dave Piguet, Tetsuya K. Sato, Matthias Schädel, Shinsaku Takeda, Atsushi Toyoshima, Kazuaki Tsukada, Andreas Tuerler, and Alessio Vascon J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b12033 • Publication Date (Web): 06 Mar 2016 Downloaded from http://pubs.acs.org on March 14, 2016

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The Journal of Physical Chemistry

Vacuum Chromatography of Tl on SiO2 at the Single-Atom Level Patrick Steinegger,†,‡ Masato Asai,¶ Rugard Dressler,‡ Robert Eichler,∗,†,‡ Yusuke Kaneya,¶ Akina Mitsukai,¶ Yuichiro Nagame,¶ Dave Piguet,‡ Tetsuya K. Sato,¶ Matthias Schädel,¶ Shinsaku Takeda,¶ Atsushi Toyoshima,¶ Kazuaki Tsukada,¶ Andreas Türler,†,‡ and Alessio Vascon¶ Department of Chemistry and Biochemistry, University of Bern, Bern, Switzerland, Laboratory for Radiochemistry and Environmental Chemistry, Paul Scherrer Institute, Villigen PSI, Switzerland, and Advanced Science Research Center, Japan Atomic Energy Agency, Tokai-mura, Japan E-mail: [email protected]

∗

To whom correspondence should be addressed University of Bern ‡ Paul Scherrer Institute ¶ Japan Atomic Energy Agency †

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ACS Paragon Plus Environment


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Abstract An isothermal vacuum chromatography setup for superheavy element chemistry studies was developed and tested on-line at the one-atom-at-a-time level. As a model system, the adsorption behavior of thallium on quartz was chosen with respect to a future chemical characterization of its superheavy homolog, element 113 (E113, Z = 113), using the described setup. Short-lived duced in the reaction

152 Gd(35 Cl,

184 Tl

(t1/2 = 10.1(5) s) was pro-

3n)184 Tl and delivered as mass-separated ion beam

to the chemistry experiment: A sub-surface implantation and a subsequent fast thermal release from a metal matrix was followed by an isothermal vacuum chromatography section as the chemical separation stage. Single atomic species passing this chromatographic separation were finally identified by time- and energy-resolved event-by-event α-spectroscopy using a diamond-based solid state detector. The derived adsorption SiO2 enthalpy of −∆Hads (Tl) = 158 ± 3 kJ · mol−1 significantly exceeds available data,

but correlates well with the adsorption of other elements studied on the same surface. The described technique enables chemical experiments with short-lived transactinide elements (t1/2 < 1 s), surpassing the rapidity of today’s state-of-the-art gas phase experiments by at least one order of magnitude.

Introduction The chemical characterization of superheavy elements (SHEs) has proven to be a challenging but mandatory task to gain deeper insights into the structure of the periodic table of elements. Due to their large nuclear charge, direct and indirect relativistic effects increasingly influence the electronic structure of these elements and, thus, their chemical behavior. 1,2 Experimental studies with SHEs have been exclusively performed using artificially produced isotopes formed in nuclear heavy-ion fusion reactions. So far, only neutron-deficient, rather short-lived SHE nuclides are available. The short lifetimes, together with extremely low production cross-sections, has led to so-called one-atom-at-a-time experiments, requiring both 2


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a fast processing time as well as the highest achievable transport and detection efficiencies. In recent decades, various on-line and off-line liquid-phase and gas-phase experiments have proven to be successful tools to tackle this task (see 3–5 for a more detailed review). State-ofthe-art gas phase experiments 6–8 can address relatively short-lived isotopes of rather volatile SHEs in the elemental state or in the form of volatile compounds. As no isotopes of elements heavier than flerovium (Fl, Z = 114) are known to have half-lives longer than one second, faster gas phase experimental setups need to be designed. This can be achieved with a transition from the commonly applied continuum flow conditions (i.e., laminar flow) to a rarefied gas flow regime in a so-called vacuum chromatography experiment. Negligible particle-particle interactions and high particle velocities overcompensate the otherwise undirected molecular flow. Additional advantages of this transition include lower background due to an absence of aerosol-bound transport of non-volatile nuclear reaction by-products (i.e., transfer products), cleaner stationary surfaces and improved spectroscopic resolution of radioactive decays. The first vacuum thermochromatographic studies were carried out by L. Westgaard and others, 9–11 aiming for a quantitative rapid chemical separation stage within the on-line preparation of mono-isotopic beams of short-lived nuclear reaction products at the isotope-separator-on-line (ISOL) facility OSIRIS and in preparation of the ISOLDE facility at CERN. Later, B. Eichler 12 seized upon this method and developed it further to discern the thermochemical side of the process. By means of a model describing migration in an evacuated tube, the adsorption enthalpies of different elements on quartz were determined, and the possible behavior of SHEs in the region Z = 112 − 117 was predicted. 12 The description of the transport process was further simplified, and the limits of the method were explored and defined by H. W. Gäggeler et al. 13 More recently, a further vacuum thermochromatography experiment was carried out by R. Eichler and M. Schädel 14 aiming for the determination of the adsorption behavior of radon on various metal surfaces. Nevertheless at that time, in case of the high-temperature experiments, the extensive IR/vis/UV-radiation background or directly occurring heat loads prevented any on-line 3



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α-spectroscopic detection of short-lived radionuclides using Si-based semiconductor detectors. This impracticability arose as a consequence of the relatively low band gap of silicon, amounting to 1.11 eV at room temperature. However, almost in parallel, an alternative detector development started to gain pace, potentially being able to overcome this evident setback. Although the applicability of natural diamond as a radiation sensor had been realized long before, 15 it was not until the mid-seventies that a spectroscopic application came into reach. 16 This application eventually almost surpassed the performance levels of Si-based detectors with later production advances related to high-purity single-crystal synthetic diamonds grown by chemical vapor deposition (CVD). 17–19 The larger band gap of 5.47 eV at room temperature provides low intrinsic charge carrier densities and thus allows, in principle, for α-spectroscopic measurements up towards the material’s high Debye temperature and the sensor to withstand high IR/vis/UV-loads. 20 These characteristics, together with the recent progress in production processes (e.g., related to the reduction of bulk impurities, available maximum surface area, and costs), makes the single-crystal electronic-grade CVD diamond a superior material for detectors to be used in future SHE chemistry experiments. 21 Here, we report a first-ever on-line isothermal vacuum chromatography (IVAC) experiment demonstrating a proof of principle for fast chemistry experiments with SHEs at the one-atom-at-a-time level. The adsorption behavior of thallium on quartz was chosen as a model system. The first vacuum thermochromatography experiment studying this physicoSiO2 (Tl) = 112 ± 5 kJ · mol−1 chemical system was performed by B. Eichler, 12 yielding −∆Hads SiO2 as the adsorption enthalpy for elemental thallium on quartz (hereinafter, −∆Hads refers

to the standard adsorption enthalpy at 298 K and zero surface coverage). Following this, A. Serov et al. 22 set out to verify the adsorption interaction of thallium on gold and quartz, using off-line gas-phase thermochromatography. In the case of quartz as a stationary surface, similar deposition temperatures were found independently of the redox potential of the used carrier gas, thus indicating the formation of a single species under all chosen chemical conditions. The observed deposition was attributed to TlOH, resulting in an adsorption 4


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SiO2 (TlOH) = 134 ± 5 kJ · mol−1 and simultaneously illustrating the exenthalpy of −∆Hads

treme sensitivity of thallium to trace amounts of oxygen and water, despite the otherwise inert or reducing atmosphere. Although the deposition temperatures of the latter study agreed very well with those measured for elemental thallium on quartz by B. Eichler, 23 the evaluated adsorption enthalpies differed significantly from each other. Theoretical reference values for the discussed system of thallium on SiO2 are still non-existent, mainly due to the complexity of a fused silica surface. Hence, additional experiments are needed to determine the interaction energy of elemental thallium on quartz, as well as with respect to a future SHE experiment with E113 (eka-thallium), its heavier homolog in group 13 of the periodic table. An isothermal vacuum chromatography experiment is a plausible way to achieve this goal, as the possibilities to form TlOH are reduced to a surface-bound reaction with residual oxygen and water impurities adsorbed on the surface or with the terminal silanol-groups of the quartz surface itself. Both reactions can be largely suppressed, as will be discussed in this article. The current experiment combines earlier findings related to the fast thermal release of various elements from solid catcher materials 24,25 with gas-phase chromatography studies of thallium species on quartz 22 as well as with the recently established diamond detector technology. 26 This proof-of-principle experiment paves the way towards cleaner and faster SHE experiments that allow access to more short-lived and only moderately volatile SHEs. Due to the reasonable production rates of ≈ 7 atoms·day−1 in the nuclear fusion reaction 243

Am(48 Ca, xn)291−x 115, 27–32 E113, as the α-decay daughter of E115, seems to be the most

promising candidate to be studied in the first SHE vacuum chromatography experiment.

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Experimental Section 184

Tl - Production & Delivery

184

Tl (t1/2 = 10.1(5) s, %α = 2.1(7), %ε+%β + = 97.9(7) 33 ) was produced in the reaction

152

Gd(35 Cl, 3n)184 Tl using a gadolinium target enriched in 152 Gd to 34.8(2)% (Trace Science

International ). The target was deposited on a 2 µm thick titanium backing foil by molecular electroplating to produce a final thickness of ≈ 1 mg/cm2 with respect to gadolinium. The gadolinium target was irradiated by a 186 MeV

35

Cl10+ beam from the JAEA tandem ac-

celerator (JAEA: Japan Atomic Energy Agency, Tokai-mura, Japan) with an average beam intensity of 150 pnA (particle-nA). The beam energy at the center of the target, after passing the 2.5 µm HAVAR vacuum window and a 5 mm thick He cooling gas gap (p = 108 kPa), was calculated to be 161 MeV (E ∗ = 49 MeV) using SRIM-2013. 34 The expected cross section for the reaction at the stated energy amounts to ≈ 7 mb. 35,36 The recoiling reaction products, pre-stopped by the target backing foil, were thermalized in the adjacent recoil chamber, being constantly flushed by a He/CdI2 aerosol gas jet (see Fig. 1). Attached to the aerosol particle surfaces, 184 Tl was transported within 0.6 s through a Teflon capillary to the JAEA-ISOL system. 37,38 After a surface ionization process inside the ionizer (rhenium tube) of the ISOL setup, the singly positively charged thallium ions effusing from the ionizer were extracted with 30 kV and mass-separated with a resolution of M/∆M ∼ 900 (see 39 for a more detailed description). Subsequently, this mass-separated ion beam was guided into the entrance orifice of the IVAC setup.

IVAC setup The IVAC setup consisted of three parts encountered sequentially by the arriving thallium isotopes (Fig. 2): (a) The hot catcher (hereinafter abbreviated as HC), (b) the isothermal chromatography, and (c) the final event-by-event α-spectroscopic detection stage. The pressure in the setup during the entire experiment was maintained at psys ≈ 1 · 10−5 mbar. 6


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Figure 1: Schematic representation of the production and transport processes using the JAEA-ISOL system 37,38 with the final delivery of the 184 Tl+ ion beam to the isothermal vacuum chromatography setup (IVAC). Considering the inner diameter of the chromatography column din = 6 mm to be the most critical experimental dimension, the molecular flow conditions (Knudsen number Kn > 1), where particle-particle interactions become negligible, were well-established for this pressure range. The mean free path for thallium atoms at room temperature amounted to λ ≈ 6 m and thus to Kn = λ/din ≈ 1000. Hot catcher A 25 µm thin hafnium foil (30 × 48 mm2 , Goodfellow Cambridge Ltd.) was heated to a temperature > 1500℃ by Joule heating to serve as a catcher and release matrix for the arriving thallium isotopes (number 1 in Fig. 2a). The low kinetic energy of 30 keV leads to a shallow sub-surface implantation depth of only ≈ 7.2 nm (longitudinal range) in hafnium, 34 presumably facilitating a fast thermal release by evading slow bulk diffusion. Following the suggestions described in 24 and striving for lowest vapor pressures under the experimental conditions, hafnium was chosen as the most promising release matrix, ensuring both a fast thermal release as well as a sufficiently long operation cycle of approximately 24 h (pvap ≈ 8.8· 10−9 mbar, i.e., Φmax {Hf} = 6.84 nMol·m−2 ·s−1 for hafnium, see Fig. 3). Zirconium, another promising candidate, 24 has an approximately one order of magnitude higher vapor pressure than hafnium and thus cannot ensure the aimed-for long operation times (Φmax {Zr} = 7



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Figure 2: Cross sectional view of the IVAC setup in the direction of the incoming beam with 1 hot catcher, 2 quartz insert, 3 Ta heat shield, 4 Ni heat shield, 5 ceramic end caps, 6 & 7 electrical connections for Joule heating, 8 ceramic oven holders and shielding, 9 water-cooled oven heat shield, 10 quartz chromatography column (llong = 247 mm and lshort = 27 mm), 11 isothermal oven, 12 start/end oven, 13 CVD diamond detector, 14 detector cooling block, 15 detector readout connector. The entrance orifice is shown true to scale on the right side (flanked red dot), whereas (a) is the hot catcher, (b) marks the chromatography section and (c) is the detection stage.

8


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51.9 nMol · m−2 · s−1 ). 0.01 Hf (mp. 2233°C)

Vapor pressure, mbar

1E-3

Zr (mp. 1854°C)

1E-4 Exp. pressure level 1E-5

1E-6

1E-7

Zr Hf

1E-8

HC temp.

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1E-9

1E-10 1000

1200

1400

1600

1800

2000

2200

2400

Temperature, °C

Figure 3: The vapor pressures of hafnium and zirconium are given as functions of the temperature. The dots mark the melting points of the corresponding element, and the arrows indicate the vapor pressures at the applied HC operating temperature of THC = 1500℃. To ensure the lowest possible adsorption losses of the released thallium on the exposed inner metal surfaces, a quartz insert was used as the innermost layer of the HC containment. This volume was further confined by two cylindrical heat shields (Ta on the inside and Ni on the outside) and ceramic end caps on the top and bottom (Fig. 2a). The 30 keV 184

Tl+ -beam, delivered from the JAEA-ISOL system, was fed through an open orifice (d =

3 mm) in the heat shield. The isothermal chromatography column was attached to a second hole in the heat shield, placed perpendicular to the beam axis. As the transport of the released atoms is governed exclusively by random walk, the number of surface interactions is directly proportional to the geometric area. This means that the probability for an atom to leave the HC containment through the 6 mm exit hole into the chromatography column is four times greater (80%) than for it to exit back through the 3 mm entrance orifice (20%). However, as the conductance of the chromatography column is rather low (i.e., ≈ 2.7%), the bulk of the species under observation re-enters the HC containment multiple times and eventually leaves it through the small entrance orifice, leading to its ultimate loss. The maximum transmission efficiency through the entire setup was estimated using the

9



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COMSOL Multiphysics® mathematical particle tracing package 40 and is discussed below. The beam stability characteristics were addressed prior to the actual experiment using a twodimensional position sensitive detector (PSD) with an active area of 9 × 9 mm2 (S5991-01, Hamamatsu Photonics K. K.; the protective silicone resin was removed from the active area). The overall beam panning was found to be of the order of the entrance orifice radius (Fig. 4), which consequently may led to an altered

184

Tl-input rate over time. Nevertheless, over a

measurement time of two hours, the beam reproducibly fluctuated around a mean value without drifting away in one direction. This led to the to the applied

35

184

Cl10+ -ion beam current. The average

Tl input rate being proportional

184

Tl input rate through the HC

entrance orifice was measured with the IVAC in-beam monitor. For this purpose, another single-crystal CVD diamond detector was lowered just in front of the IVAC entrance orifice, with the active area facing the incoming beam. The average input rate was then measured by passing the

184

Tl+ -beam through a 3 mm collimator just in front of this detector. In either

case, the ion beam was optimized through current pick-up measurements on the in-beam monitor front electrode or the HC foil. Note here that the measurements with the IVAC in-beam detector are solely applicable for rough balancing statements. To deduce an absolute number of injected atoms per measurement would require the exact knowledge of various parameters such as the in-beam detector efficiency, beam panning and focusing stability, beam intensity fluctuations, changes in target thickness and beam energy scatter. Chromatography A one-sided kinked quartz chromatography column (din = 6 mm, total axis length l = 247 + 27 mm, length to radius ratio l/r ≈ 91) was placed inside a three-section thermocoax® oven (Fig. 2b). The temperature at the start (HC side) and at the end (detector side) of the column in the oven (hereafter referred to as S/E-oven) was controlled simultaneously, using an electrically coupled oven assembly. Meanwhile, the middle section was independently op10


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Time, s

x-position, mm

0 1.2

1000

2000

3000

4000

5000

6000

(a)

0.8 0.4 0.0

1.5 mm

-0.4

0.0

(b)

-0.4 -0.8

rate, counts / 30 s

1.3 mm

-1.2

40

-1.6

(c)

y-position, mm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 20 10 0 0

1000

2000

3000

4000

5000

6000

Time, s

Figure 4: Measured JAEA-ISOL beam stability in (a) the horizontal and (b) vertical directions using a two-dimensional PSD (central moving average over 20 consecutive events); minimum-maximum deviations for both directions are indicated for the given time frame of 6000 s. (c) presents the count rate variation over the measurement time with a bin size of 30 s. erated and used for changing the temperature of the isothermal section (hereafter referred to as ISO-oven). Extensive heat sinks with Tend Tiso towards the end of the chromatography column compete with the isothermal section with respect to the chromatographic process. Therefore, the S/E-oven was always kept at higher temperatures compared to its isothermal counterpart. Nevertheless, a temperature drop towards the detector side could not be completely prevented (right side in Fig. 5). This isothermal chromatography setup provides a temperature range suitable for the investigation of the adsorption of thallium on quartz or fused silica surfaces. Using a kinked section towards the end of the chromatography column guarantees at least one surface interaction for each transmitted species, thereby fulfilling the minimum requirement for a chromatographic process. The direct flight probability for a straight column with the same l/r ≈ 91 ratio amounts to < 0.02%. Detection A diamond detector served as an escape detector to record the decay of the

184

Tl atoms

emerging from the far end of the chromatography column (Fig. 2c). These radionuclides 11



354°C

1000

800

600

Temperature, °C

403°C

431°C

494°C

457°C

518°C

571°C

1200

640°C

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400

200

25

Det.

20

15

10

5

Chromatography column, cm

0

HC

Figure 5: Measured temperature profiles along the chromatography column with the detector on the left hand side and the HC on the right side. The curve indices refer to the plateau temperature measured at 14 cm distance from the HC exit hole (i.e., the entrance to chromatography column). are detected via the emitted α-particles during the decay of

184

Tl to

180

Au. As introduced

earlier, diamond detectors can endure high loads of IR/vis/UV-radiation without losing spectroscopic performance. Thus, they can be brought very close to the hot chromatography column exit, enabling on-line α-spectroscopic measurements at this position for the first time. The segmented diamond detector consists of four single-crystal electronic-grade CVD diamond plates (4.5 × 4.5 × 0.5 mm3 ). 41 The total gold-covered active area was 9 × 9 mm2 , facing the end of the chromatography column in close proximity (i.e., 3 mm). The four fully metalized front electrodes were put on ground potential and interconnected using aluminum wire bonding, whereas the detector high voltage (HV = 0.6 V · µm−1 ) was applied from the back, the signal read-out side. Such a design ensures measurements at low noise levels. 42 Each of these detectors was read out separately, using a dedicated type of the CIVIDEC Cx spectroscopic shaping amplifier 43 with a Gaussian pulse shaping time of 1 µs (rise time = 190 ns, gain = 10.9 mV/fC). The signals were analyzed using an event-by-event NIKI GLASS ADC/MCA readout system. 44 See 21,26 for a more detailed description of the four-fold diamond detector.

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Simulation and data evaluation The chromatographic process was simulated using a Monte-Carlo (MC) simulation approach based on the microscopic kinetic model described by I. Zvara, 45–47 which integrates experimental conditions such as the applied temperature profiles (Fig. 5) and the radioactive half-life of

184

Tl (i.e., t1/2 = 10.1 s), as well as the column dimensions and the mentioned

kinked section at the end of the chromatography column. This MC model describes the transmission of an atom through the chromatography column as a function of its particu˜ ads with the surface material. ∆Hads hereafter refers to larly chosen adsorption enthalpy ∆H the deduced most likely adsorption enthalpy approaching the true interaction energy of the adsorbed species with the stationary phase. In other words, a MC simulation with the obtained value for ∆Hads describes the experimental external chromatogram best. Note here, that symbols denoted with a tilde indicate particularly chosen parameters in a specific simulation run. Without a tilde however, the symbols refer to the best estimate of the respective physical or chemical property. The model of Zvara was slightly adjusted with respect to the lattice phonon vibration frequency νb , used in the Frenkel-type Eq. (1) for the calculation of the average surface adsorption time tads in [s]. Instead of the commonly applied theoretical prediction for νb according to F. A. Lindemann, 48,49 measured values for the most prominent vibrational mode (phonon) of the quartz surface (main band) were implemented in the form of a linearly temperature dependent function νb (T ) in [Hz] obtained from 50 and confirmed by. 51

tads

◦ ∆Hads 1 · exp − = νb R·T

13

νb (T ) = 1.3424 · 10

2.8776 · 108 Hz ·T Hz − K

(1)

(2)

where T is the temperature in [K] and R in [J · mol−1 · K−1 ] is the ideal gas constant. The multiple back-migration into the HC containment was considered in the form of a derived 13



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residence time function of the HC volume, amounting to

Ncoll = 0.8 · Ntot · {1 − exp(−83.89 · tx )}

(3)

where Ncoll is the fraction of collected particles at time tx [s] after the injection or reinjection into the HC containment. A total number of Ntot = 10000 particles with mass 184 u was simulated using the COMSOL Multiphysics® mathematical particle tracing module. 40 All particles were released out from the chromatography column into the HC containment. The simulations were carried out in the molecular flow regime with diffusive scattering on the walls and Maxwell-Boltzmann distributed gas velocities (Twall = 1100 K). Each of the simulated atoms was finally collected either at the entrance orifice or at the exit hole, leading back into the chromatography column (Fig. 6). The cumulative distribution function from Eq. (3) was then derived based on the individual residence times inside the HC volume of those particles arriving back at the point of release. This equation was finally implemented in the simulation using the inverse transform method. 52 Additionally, the 80% re-entering chance (see also Fig. 6) into the chromatography column (see previous section) has been taken into account. Due to the temperature drop inside the column just in front of the detector (see right side in Fig. 5), a minor portion of the thallium is deposited in this particular section. Nevertheless, due to the close proximity to the active area of the detector, there is a certain probability for detecting the α-particles emitted from this region. Therefore, this process was likewise considered in the simulation of the experiment. As a result of ˜ ads ), can be such a MC simulation, an external chromatogram with absolute yields, η(Ti |∆H obtained. These yields correspond to the fraction of individual atoms of the radionuclide under investigation (from a total number of 106 particles per MC simulation run) surviving the passage through the chromatography setup. This number of transmitted species depends ˜ ads . on the applied isothermal temperature Ti and the selected adsorption enthalpy ∆H Two problems arise when trying to link the simulation to the experimental outcome. First, the absolute number of atoms injected into the HC volume during an experiment can 14


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100 Chromatography column

x

Collected yield at time t , %

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Entrance orifice 80

60

40

20

0 0

20

40

60

80

100

Time t , ms x

Figure 6: Collected yield of the injected particles at the exit hole leading back into the chromatography column (black) and at the entrance orifice (red) respectively at time tx after re-injection into the HC volume. Maxwell-Boltzmann distributed particle velocities were taken into account after each wall interaction (Twall = 1100 K). be neither controlled nor determined. In order to compare different experiments, only the ratio of observed events to the number of applied beam particles can be used as a meaningful quantity. This will be referred to in the following as the yield of an experiment. Hence, the yields of an experimentally obtained external chromatogram y(Ti ) = yi have a different unit (e.g., events per collected beam integral) than the ones resulting from the simulation (i.e., number of transmitted particles). The second problem concerns the true saturation value, i.e., the maximum achievable transmitted amount of the investigated species, which is hitherto unknown. Thus, linking the simulations to the experimental outcome is not straight-forward and requires special attention. In previously applied isothermal chromatography evaluation techniques all measurements were simply normalized to the experimental data point at the highest isothermal temperature, assumed to be sufficiently close to the corresponding saturation. The following method shall overcome this uncertain assumption. The experimentally deduced final result, the adsorption enthalpy ∆Hads , is derived using the above introduced MC simulation model in combination with the minimum least squares method (MLS) 53 as the goodness of fit. Generally, the functional dependence of the ab˜ ads solute yield at a given temperature Ti on a particularly chosen adsorption enthalpy ∆H 15



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˜ ads ). Unfortunately, there is no known analytical expression for this is denoted as f (Ti |∆H function and the full implementation of the MC simulation using available non-linear leastsquares-fit computer codes represents an enormous effort. Thus, the MLS method is used for the determination of ∆Hads , whereas the MC simulations provide the separately calcu˜ ads ). In particular, these MC simulation results lated necessary values describing f (Ti |∆H ˜ ads ), which have to be converted by a scale factor κ provide absolute yields, η(Ti |∆H ˜ into the experimentally used unit of yi :

˜ ads ) = κ ˜ ads ) f (Ti |∆H ˜ · η(Ti |∆H

(4)

As seen from Eq. (5), this leads to a two-dimensional χ2 -surface using the experimen˜ ads and κ tal yields yi together with selected combinations of ∆H ˜ as independent variables (Fig. 7a).

˜ ads ) = χ2 (˜ κ|∆H

X

˜ ads )}2 {yi − κ ˜ · η(Ti |∆H

(5)

i

˜ ads ) can be obtained for each ∆H ˜ ads by locally minimizing An optimum scale factor κ(∆H ˜ ads as a fixed paχ2 of Eq. (5) with respect to κ ˜ . Using the MLS method and treating ∆H rameter in the fit, one obtains a closed expression for κ from setting the first order derivative of Eq. (5) to zero. P ˜ i yi · η(Ti |∆Hads ) ˜ κ(∆Hads ) = P 2 ˜ i η(Ti |∆Hads )

(6)

˜ ads ) represents an unambiguously defined value, one for each individually seThus, κ(∆H ˜ ads (Fig. 7). It can be used in a second step to derive the value of ∆Hads as the best lected ∆H ˜ ads ) with η(Ti |∆H ˜ ads ) estimate of the adsorption enthalpy. Combining the results for κ(∆H from the MC simulation sweeps over a range of temperatures Ti and adsorption enthalpies ˜ ads yields f (Ti |∆H ˜ ads ). Thus, by applying Eq. (6), the former two-dimensional problem ∆H from Eq. (5) simplifies to two individually solvable one-dimensional problems (Fig. 7b). 16


Page 17 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


˜ ads ) = χ2 (∆H

X

˜ ads )}2 {yi − f (Ti |∆H

(7)

i

Figure 7: Example of a hypothetical evaluation with the pure two-dimensional case (a) and the solution with two individually solvable one-dimensional problems (b). Purple on the right hand side corresponds to the minimum of the χ2 -surface, which increases towards ˜ ads (i.e., a vertical cut through the χ2 -surface in a), the red. For a specifically chosen ∆H 2 ˜ ads ). corresponding χ -values minimize for the case of κ(∆H The minimum of the χ2 -values indicates the best estimate of the adsorption enthalpy ˜ ads ) can be approximated as a matter of form using a first order ∆Hads . Thus, f (Ti |∆H Taylor expansion, developed in the vicinity of the adsorption enthalpy ∆Hads . ˜ ∂f (T |∆ H ) i ads ˜ ads ) = f (Ti |∆Hads ) + f (Ti |∆H ˜ ads ∂∆H ∆H

˜ ads − ∆Hads ) · (∆H

(8)

ads

Upon substituting Eq. (8) into (7), a parabolic function (9) is obtained. Besides the two pure quadratic terms, the mixed one vanishes as a consequence of minimizing χ2 , ultimately ˜ ads −∆Hads )2 can be factored out from the sum due to its leading to ∆Hads . 54–56 The term (∆H independence from the measurements taken. Introducing χ2min and σf2it for the substitution of the respective sums with useful quantities for the uncertainty estimation, Eq. (9) simplifies 17



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Page 18 of 34

to (10). As obvious from these equations, both quantities do not functionally depend on ˜ ads . The sum over the first derivatives of f (Ti |∆Hads ) depends only on the value of ∆H ˜ ads ) and does not depend on particular values of ∆H ˜ ads . ∆Hads at the minimum of χ2 (∆H

2 X X ∂f (Ti |∆H ˜ ads ) ˜ ads ) = χ2 (∆H {yi − f (Ti |∆Hads )}2 + ˜ ads ∂∆H i

i

˜ ads ) = χ2 · χ2 (∆H min

˜ ads − ∆Hads )2 · (∆H

(9)

∆Hads

˜ ads − ∆Hads )2 (∆H 1+ (n − 2) · σf2it

! (10)

˜ ads ) from where n denotes the number of experimental data points. Plotting χ2 (∆H Eq. (7) using the corresponding values of the MC simulations yields a parabolic function profile. The value of ∆Hads and the parameters χ2min and σf2it are then derived by a fit of the parabolic function from Eq. (10) to these values. The contribution of the resulting uncertainties of the fit parameters themselves to the overall uncertainty of ∆Hads depends ˜ ads -values in the proximity of the minimum (Fig. 7b). Therefore, on the number of used ∆H choosing this number sufficiently large leads to a reasonably small uncertainty in the order of a few [J · mol−1 ] and thus is irrelevant for the overall uncertainty in the [kJ · mol−1 ] range. Finally, taking into account the experimental uncertainties εi , the 1σ-combined confidence interval amounts to s σ∆Hads =

σf2it

P 2 n−2 ε · 1+ · 2 i 2 n χmin

(11)

Eq. (11) is a conservative estimate of the standard uncertainty, shown by transferring ˜ ads parameter space (see e.g., 57 the calculated χ2 -values to likelihood values in the κ ˜ vs. ∆H or 55 ). The isohypsometric line where the probability drops to 1/e, i.e., the 1σ confidence region, results in a somewhat smaller uncertainty than the one obtained wit Eq. (11).

18


Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Results and discussion IVAC setup A measurement with the escape detector at the highest chromatography oven temperatures ensured a maximized throughput of 184 Tl (i.e., minimum retention time). The resulting yield was then compared to the subsequently recorded 100% measurements of the IVAC in-beam monitor, revealing a total yield of 2.2% of

184

Tl, as measured by the IVAC escape detector.

The theoretical transport efficiency through the system onto the detector was estimated using the COMSOL Multiphysics® mathematical particle tracing module. 40 The simulations were conducted neglecting any surface retention, considering Maxwell-distributed particle velocities for all infinitely long-lived thallium atoms with mass 184 u at room temperature and obeying all geometric dimensions. Approximately 6% of all atoms injected through the entrance orifice of the HC reach the detector, 2% end up beside of it, and 92% irreversibly leave the experiment back through the HC entrance orifice (Fig. 8). The simulated dwell time needed for 95% of the 184 Tl atoms to either escape through the HC entrance hole or reach the detector side takes approximately 340 ms. Due to the low temperature and thereof resulting smaller gas velocities, this number reflects only a conservative estimate of the transmission time, neglecting any adsorption retention. Comparing the experimentally determined transmission efficiency through the system to the simulated one reveals a relative transport yield of 37%. The discrepancy can be partly explained by the suboptimal transition between the exit of the chromatography column and the detector front face as well as the non-ideal geometry of all encountered inner surfaces. As another important difference, the simulation does not consider any retention losses occurring as a consequence of the radioactive decay of

184

Tl. Thus, its outcome can be regarded as

an overall conservative estimate. Losses from a potentially prolonged thermal release out of the HC matrix or localized retention losses along the way to the detector were ruled out by qualitative post-experimental γ-spectroscopy analysis of various parts of the setup (e.g., HC, 19



Y

= 92%

Y

= 8%

Y

= 6%

max

100

10

max

95% in final position

Relative yield, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

max

Page 20 of 34

HC entrance hole chrom. col. exit ads. on detector

0.1 0.0

0.2

0.4

0.6

0.8

Time, s

Figure 8: Temperature independent transmission efficiency through the setup as simulated with the COMSOL Multiphysics® mathematical particle tracing module. 40 A major portion (92%) of all inserted thallium atoms escape through the HC entrance orifice (black line), whereas 8% (red line) reach the end of the chromatography column, 6% (blue line) adsorbing on the active area of the detector (9 × 9 mm in 3 mm distance from the column exit). The dashed line at 340 ms indicates the time at which 95% of all particles are in their final positions. electrical connectors, quartz insert) with respect to the long-lived ε/β + -decay product

184

Ir

(t1/2 = 3.0 h). The direct comparison between a γ-measurement of the HC foil operated at lower release temperatures (THC ≈ 1200℃) and another one after having applied the actual experimental conditions reveals the sensitivity of such a measurement (Fig. 9) for immobilized activity. However, this screening process might fail to detect activity being spread-out over larger areas, potentially acting as loss sites. Hence, such additional losses cannot be excluded and contribute to the overall transmission efficiency. The current setup was designed according to the given experimental situation behind the JAEA tandem accelerator and the JAEA-ISOL system. However, the evident drawback of having an open 3 mm orifice as a major loss site can be eliminated in a later SHE experiment by moving either directly behind the target or to the focal plane of a physical pre-separator (e.g., TASCA 58 ). In these two cases, the high kinetic energies of the recoiling evaporation residues allow them to pass through a thin entrance window, closing up the HC containment.

20


Page 21 of 34

Energy, keV

Counts / 0.5 keV

230 120

Counts / 0.5 keV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

100

240

250

260

(a)

270

184

Ir (E

280

290

300

= 263.97 keV)

80 60 40 20

50

(b)

40 30 20 10 0 230

240

250

260

270

280

290

300

Energy, keV

Figure 9: Post-experimental γ-spectroscopy measurements of two hafnium HC foils kept at different temperatures: A at THC ≈ 1200℃ (tlive = 1800 s) and B under the actual experimental conditions (THC > 1500℃, tlive = 1300 s). The dashed line indicates the full-energy peak of 184 Ir. The first option, with the projectile beam passing through the HC foil, leads to radiation (damage) enhanced diffusion, 24,25 which can severely speed up the thermal release process. For the second case, however, the current catcher setup with its 30 × 48 mm size can provide relatively high efficiencies even with the rather broad product distribution in the focal plane of the pre-separator. The pre-stopping of the recoiling evaporation residues in such a preseparator, and the selectable HC containment entrance foil thickness will help to keep the implantation depth as shallow as possible, thereby possibly promoting a subsequent fast thermal release.

Isothermal vacuum chromatography of thallium on quartz Quartz presents itself as a rather complicated surface. Differently attached terminal silanol groups in varying mutual positions define the surface of fused silica under atmospheric conditions. The dehydration of all physically adsorbed water molecules was found to occur at moderate temperatures exceeding 200℃ under vacuum conditions, whereas for promoting the dehydroxylation, which is mainly the condensation of silanol groups to form less reactive siloxane bridges, temperatures close to and above 1000℃ are required. 45,59 Eq. (12) and (13) 21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

show sample reaction schemes for forming TlOH under the given conditions, either together with residual impurities in the gas phase and on the surface (12) or through a surface-bound reaction equal or similar to (13). Both the negligible particle-particle interactions in the molecular flow regime as well as the high temperatures of ≈ 1000℃ applied all along the quartz chromatography column prior to the experiment make neither (12) nor (13) a probable formation path for TlOH in the current vacuum chromatography experiment.

4 Tl + O2 + 2 H2 O −→ 4 TlOH

(12)

Tl + ·OH −→ TlOH

(13)

The measurements were taken at the isothermal temperatures shown in Fig. 5. For each data point of the external chromatogram (Fig. 12), an integral

35

Cl10+ beam dose of about

6.2 · 1015 ions (i.e., collected charge of Qcoll ≈ 9930 µC) was collected within an approximately 2 h duration for each. All measurements were normalized to the respective collected beam integrals to account for variations in the measurement times and beam intensities (as seen in Fig. 4). The measurement sequence was obtained by adjusting the isothermal temperature in both directions, i.e., to higher and lower temperatures (see measurement sequence indicated in Fig. 12). Only recorded α-events with an energy higher than 5.75 MeV were considered (dashed line in Fig. 10), taking into account the three α-lines 60 of Eα = 5.988 (40%), 6.066 (6%), 6.162 MeV (54%).

184

184

Tl:

Hg (t1/2 = 30.87(26) s, %α = 1.11(6), 33

Eα = 5.535 MeV 60 ) as the mainly populated β + -daughter of

184

Tl has a low retention on

SiO2 quartz of only −∆Hads (Hg) = 42±2 kJ·mol−1 and therefore is supposed to pass through the

chromatography column at fairly low temperatures. 61 Furthermore, mercury exhibits a weak Au adsorption interaction with gold surfaces (i.e., −∆Hads (Hg) = 98 ± 3 kJ · mol−1 , see 62 ), as

encountered on the diamond detector. The measured diamond detector surface temperature close to the highly heated chromatography column exit amounts to Tdet ≈ 200℃, which pre-

22


Page 23 of 34

vents any direct deposition of mercury on the detector under vacuum conditions and might even facilitate the desorption from the detector surface of the non-implanted ε-/β + -decay daughter of

184

Tl (see Fig. 10). 1E+04 in-beam meas. 180

Au

Sum spectrum Integration line

1E+03

counts / 20 keV

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


184

Hg

184

Tl

1E+02

1E+01

1E+00

4.5

5.0

5.5

E

6.0

6.5

, MeV

Figure 10: 184 Tl measurement using the in-beam monitor shown in red (Qcoll = 18199 µC) together with a sum spectrum (black) of all events recorded using the 4-fold diamond detector at the chromatography column exit (Qcoll = 189413 µC). Despite the severe depletion of 184 Hg in the case of the sum spectrum compared to the direct catch measurement in front of the IVAC system, only events with Eα = 5.75 MeV (dashed line) were considered for the data evaluation. The measurements were analyzed according to the procedure described above. The ˜ ads )-surface for the current experiment was derived through a two-dimensional paraχ2 (˜ κ|∆H ˜ ads and the scale factor κ metric sweep over ∆H ˜ (in preparation of Fig. 11b). ∆Hads and the corresponding 1σ-uncertainties were extracted based on Eq. (10) and (11), respectively (Fig. 11a). According to, 57 the χ2 -surface can be transferred into a likelihood (i.e., a probability), enabling the determination of the 1σ uncertainty region (white surrounded and hatched area in Fig. 11b). The comparison of the two 1σ confidence ranges reveals a consistent error estimation. Hereafter, the uncertainty based on Eq. (11) is used. SiO2 Using this evaluation procedure, an adsorption enthalpy of −∆Hads (Tl) = 158 ± 3 kJ ·

mol−1 was determined. The corresponding simulation and the associated confidence bands are shown together with the experimental data in Fig. 12. This adsorption enthalpy is in SiO2 disagreement with earlier findings by 12 (−∆Hads (Tl) = 112 ± 5 kJ · mol−1 ) and differs from

23



153

155

Hads

~ SiO2 -1 (Tl), kJ mol 157

159

161

163 0.10

(a) fit 0.09

0.08

2

-value, events mC

2

0.07

2

-values

Parabola-fit

-2

10

0.06 1.0

9 0.8

Likelihood ratio

~

(b)

Scale factor,

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

8

7

6 153

0.6

0.4

0.2

0.0 155

157

-

159

161

163

Hads

~ SiO2 -1 (Tl), kJ mol

Figure 11: Uncertainty evaluation of the adsorption enthalpy. Panel (a): the resulting ˜ ads )-values from Eq. (7) running along the minimum parabolic curve described by the χ2 (∆H valley as shown in panel (b). The parabolic function from Eq. (10) is fitted in the close range of ±3 kJ · mol−1 around the χ2min (dashed line and cross features); the resulting σf it is shown as gray area. Panel (b): the likelihood ratio contour plot is shown for the two-dimensional ˜ ads and κ parameter space defined by ∆H ˜ , together with the minimum valley-line (dashed line) as well as the 1σ confidence region (hatched area).

24


Page 25 of 34

SiO2 (TlOH) = the recently reported adsorption enthalpy for the monohydroxide of −∆Hads

134 ± 5 kJ · mol−1 . 22 Both experiments were re-evaluated using the lattice phonon vibration SiO2 frequency given by M. Guerette et al., 50 moving the values closer together: −∆Hads (Tl) =

122 ± 6 kJ · mol−1 were obtained for the vacuum thermochromatography experiment, 12 and SiO2 (TlOH) = 137±5 kJ·mol−1 for the gas-phase thermochromatography experiment 22 −∆Hads

(Fig. 13). - H

ads

68% confidence bands

MCS with -

H

ads

mC

-1

Exp. data (68% c.i.)

2

Rate, events

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

5 6 7

3

1

8 - H

ads

= 158

-1

4 kJ mol

Temperature, °C

Figure 12: External chromatogram (black dots, error bars: 68% c.i.) together with the MC SiO2 simulation using the adsorption enthalpy −∆Hads (Tl) = 158 ± 3 kJ · mol−1 (red line) and the corresponding confidence bands (red area); the bold black numbers indicate the sequence of the conducted measurements. Based on this apparent shift from the adsorption enthalpy of TlOH on quartz together with the negligible formation possibilities of TlOH, we conclude that the result obtained in the presented experiment indeed represents the interaction energy of elemental thallium on quartz. The discrepancy from the value presented by 12 is not fully understood. However, the temperature gradient applied in the experiments conducted by 12 may lead to incomplete silica surface condensation and remaining O2 and H2 O adsorbents, especially towards the colder end of the column. Thus, by omitting a complete heating of the entire column to above 1000℃, an increased thallium hydroxide formation might be expected. The newly derived value is in better agreement with the established semi-empirical correlation of the adsorption 25



1200

100 -1

Exp., 15 ml min

carrier gas H

1000

2

Temperature gradient

Hads

SiO2

60

800

-1

(TlOH) = 137 kJ mol

600

400 40

200

Temperature, °C

MC simulation

80

Relatvie yield, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

0 20

Tdep

204

(

-200

0

TlOH) = 296 C

-400

0 0

5

10

15

20

25

30

35

40

45

50

55

Column length, cm

Figure 13: Re-evaluated experiment from 22 with H2 as carrier gas (15 ml · min−1 ). The gray bars indicate the experimental results, whereas the solid step line represents the MC simulation output, and the dashed line the applied temperature gradient (right hand scale) for this thermochromatography experiment. enthalpy of various elements on a quartz surface as a function of the respective sublimation enthalpy of these elements, as given in Eq. (14), thereby bolstering our conclusion regarding the chemical speciation (see Fig. 14, adapted from 4 ).

SiO2 ◦ −∆Hads = (0.84 ± 0.07) · ∆Hsubl − (2.8 ± 8.9) kJ · mol−1

(14)

Conclusions The feasibility of an on-line isothermal vacuum chromatography experiment with short-lived 184

Tl at the one-atom-at-a-time level has been successfully demonstrated. A four-fold dia-

mond detector was utilized as an escape detector, allowing for the first time a successful online α-spectroscopy measurement in a vacuum chromatography experiment with short-lived SiO2 radionuclides. The adsorption enthalpy of −∆Hads (Tl) = 158 ± 3 kJ · mol−1 for elemental

thallium on quartz was determined. The result agrees very well with the semi-empirical corSiO2 ◦ relation between ∆Hsubl and −∆Hads of various elements, supporting the identified chemical

state (Fig. 14). The overall efficiency reached approximately 37% with respect to the theoretically calculated yield. The estimated average dwell time of 340 ms can be shortened in 26


Page 27 of 34

Pt [lower limit] Excluded Tl values

In

Ag

Tl [this work]

SiO2

, kJ mol

-1

95% c.i. of fit

Hads

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


At

Au

Zn Cd

Te

Hg

Sb

Pb Po I

Tl [Eichler, 1977]

Hsubl o

-1

, kJ mol

Figure 14: Correlation between the sublimation enthalpies of various elements and their respective adsorption enthalpies on a quartz surface. The measurement of platinum, stating SiO2 only a lower limit of −∆Hads (Pt), was excluded from the linear fit as well as the newly derived value for thallium together with the old one, found by. 12 All results were adjusted with respect to the newly applied phonon lattice vibration frequency. 50 The correlation equation is given in the text. future experiments by further geometric optimizations. Thus, the current setup is ready for the chemical identification of transactinide elements.

Acknowledgement This project is supported by the Swiss National Science Foundation (grant: 200020_144511) and CIVIDEC Instrumentation GmbH.

References (1) Pyykkö, P.; Desclaux, J.-P. Relativity and the Periodic System of Elements. Accounts of Chemical Research 1979, 12, 276–281. (2) Pyykkö, P. The Physics Behind Chemistry and the Periodic Table. Chemical Reviews 2012, 112, 371–384. (3) Schädel, M. Chemistry of Superheavy Elements. Angewandte Chemie - International Edition 2006, 45, 368–401. 27



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(4) Schädel, M., Shaughnessy, D., Eds. The Chemistry of Superheavy Elements, 2nd ed.; Springer-Verlag Berlin Heidelberg, 2014. (5) Türler, A.; Pershina, V. Advances in the Production and Chemistry of the Heaviest Elements. Chemical Reviews 2013, 113, 1237–1312. (6) Eichler, R.; Aksenov, N. V.; Belozerov, A. V.; Bozhikov, G. A.; Chepigin, V. I.; Dmitriev, S. N.; Dressler, R.; Gäggeler, H. W.; Gorshkov, V. A.; Hänssler, F. et al. Chemical Characterization of Element 112. Nature 2007, 447, 72–75. (7) Eichler, R.; Aksenov, N. V.; Belozerov, A. V.; Bozhikov, G. A.; Chepigin, V. I.; Dmitriev, S. N.; Dressler, R.; Gäggeler, H. W.; Gorshkov, A. V.; Itkis, M. G. et al. Thermochemical and Physical Properties of Element 112. Angewandte Chemie - International Edition 2008, 47, 3262–3266. (8) Yakushev, A.; Gates, J. M.; Türler, A.; Schädel, M.; Düllmann, C. E.; Ackermann, D.; Andersson, L. L.; Block, M.; Brüchle, W.; Dvorak, J. et al. Superheavy Element Flerovium (Element 114) is a Volatile Metal. Inorganic Chemistry 2014, 53, 1624– 1629. (9) Westgaard, L.; Rudstam, G.; Jonsson, O. C. Thermochromatographic Separation of Chemical Compounds. Journal of Inorganic and Nuclear Chemistry 1969, 31, 3747– 3758. (10) Grapengiesser, B.; Rudstam, G. Use of Thermochromatography for Rapid Chemical Separations. 1. Application to Steady-State Separation of Fission-Product Isobars Obtained with an Isotope Separator Online a Reactor. Radiochimica Acta 1973, 20, 85–90. (11) Rudstam, G.; Grapengiesser, B. Use of Thermochromatography for Rapid Chemical Separations. 2. Determination of Deposition Temperatures. Radiochimica Acta 1973, 20, 97–107.

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(12) Eichler, B. Das Verhalten Flüchtiger Radionuklide im Temperaturgradientrohr unter Vakuum; 1977. (13) Gäggeler, H. W.; Eichler, B.; Greulich, N.; Herrmann, G.; Trautmann, N. VacuumThermochromatography of Carrier-Free Species. Radiochimica Acta 1986, 40, 137–143. (14) Eichler, R.; Schädel, M. Adsorption of Radon on Metal Surfaces: A Model Study for Chemical Investigations of Elements 112 and 114. Journal of Physical Chemistry B 2002, 106, 5413–5420. (15) Gudden, B.; Pohl, R. Das Quantenäquivalent bei der lichtelektrischen Leitung. Zeitschrift für Physik 1923, 17, 331–346. (16) Kozlov, S. F.; Stuck, R.; Hage-Ali, M.; Siffert, P. Preparation and Characterization of Natural Diamond Nuclear Radiation Detectors. IEEE Transactions on Nuclear Science 1975, 22, 160–170. (17) Isberg, J.; Hammersberg, J.; Johansson, E.; Wikström, T.; Twitchen, D. J.; Whitehead, A. J.; Coe, S. E.; Scarsbrook, G. A. High Carrier Mobility in Single-Crystal Plasma-Deposited Diamond. Science 2002, 297, 1670–1672. (18) Kaneko, J. H.; Tanaka, T.; Imai, T.; Tanimura, Y.; Katagiri, M.; Nishitani, T.; Takeuchi, H.; Sawamura, T.; Iida, T. Radiation Detector Made of a Diamond Single Crystal Grown by a Chemical Vapor Deposition Method. Nuclear Instruments and Methods in Physics Research Section A 2003, 505, 187–190. (19) Pomorski, M.; Berdermann, E.; Caragheorgheopol, A.; Ciobanu, M.; Kis, M.; Martemiyanov, A.; Nebel, C.; Moritz, P. Development of Single-Crystal CVD-Diamond Detectors for Spectroscopy and Timing. Physica Status Solidi (a) 2006, 203, 3152–3160. (20) Tsubota, M.; Kaneko, J. H.; Miyazaki, D.; Shimaoka, T.; Ueno, K.; Tadokoro, T.; Chayahara, A.; Watanabe, H.; Kato, Y.; Shikata, S. et al. High-Temperature Charac29



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