Liquid metal spallation targets using mercury as target material are used in state-of-the-art high power pulsed neutron sources that have been constructed in the USA and Japan within the last decade. Similar target concepts were also proposed for next generation ISOL, β-beam and neutrino facilities. A large amount of radioactivity will be induced in the liquid metal during operation caused by the interaction of the target material with the intense proton beam. This radioactivity – carried by a wide range of radioisotopes of all the elements of the periodic table from hydrogen up to thallium – must be considered for the assessment of safe operation and maintenance procedures as well as for a final disposal of the used target material and components. This report presents an overview on chemical investigations performed in our laboratory that deal with the behavior of radionuclides in proton irradiated mercury samples. The solubility of elements in mercury was calculated using thermodynamical data obtained by a semi-empirical method. It is found that several important nuclear reaction products will reach their solubility limit in a high power mercury spallation target. It is also experimentally proven that the radionuclides produced by proton irradiation are generally not homogeneously distributed in the liquid metal but tend to accumulate on vessel walls and free surfaces. The consequences resulting from this behavior for the operation of mercury spallation targets are discussed.

Summary The release of iodine from liquid eutectic lead-bismuth alloy (LBE) under a flowing Ar/7%H2 atmosphere has been studied in a temperature range from 428 to 1223 K using γ-ray spectroscopy. During short-term experiments, significant amounts of iodine evaporate from liquid LBE at temperatures higher than 800 K. Long-term experiments reveal that the release of iodine at temperatures of relevance to technical applications, such as liquid metal spallation targets or Accelerator Driven Systems (ADS), is below our detection limits for periods up to 7 days. The release rate is determined by the desorption/evaporation process rather than by diffusion within the liquid alloy.

Summary Two experiments aiming at the chemical investigation of element 112 produced in the heavy ion induced nuclear fusion reaction of 48Ca with 238U were performed at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt, Germany. Both experiments were designed to determine the adsorption enthalpy of element 112 on a gold surface using a thermochromatography setup. The temperature range covered in the thermochromatography experiments allowed the adsorption of Hg at about 35 °C and of Rn at about -180 °C. Reports from the Flerov Laboratory for Nuclear Reactions (FLNR), Dubna, Russia claim production of a 5-min spontaneous fission (SF) activity assigned to 283112 for the 238U(48Ca,3n) 283112 reaction. Hence, Experiment I was designed to detect spontaneously fissioning (SF) isotopes of element 112 with half-lives (t 1/2) longer than about 20 s. 11 high-energy events were detected. 7 events exhibit a deposition pattern resembling a chromatographic peak in the vicinity of Rn deposition. However, the energy of the events observed in Experiment I was lower than expected for a SF-decay of 283112. Therefore, these events could not be unambiguously attributed to the decay of 283112. In contradiction with earlier publications newer reports from FLNR Dubna claim that 283112 decays by α-particle emission (E α = 9.5 MeV) with t 1/2 = 4 s followed by a SF-decay of 279Ds (t 1/2 = 0.2 s). Therefore, Experiment II was designed to be sensitive to both claimed decay properties of 283112. However, during this experiment neither short α-SF correlations nor SF coincidences were detected. The conclusion is that 283112 was not unambiguously detected, neither in Experiment I nor in Experiment II.

R. Eichler, W. Brüchle, R. Buda, S. Bürger, R. Dressler, Ch.E. Düllmann, J. Dvorak, K. Eberhardt, B. Eichler, C.M. Folden III, H.W. Gäggeler, K.E. Gregorich, F. Haenssler, H. Hummrich, E. Jäger, J.V. Kratz, B. Kuczewski, D. Liebe, D. Nayak, H. Nitsche, D. Piguet, Z. Qin, U. Rieth, M. Schädel, B. Schausten, E. Schimpf, A. Semchenkov, S. Soverna, R. Sudowe, N. Trautmann, P. Thörle, A. Türler, B. Wierczinski, N. Wiehl, P.A. Wilk, G. Wirth, A.B. Yakushev, A. von Zweidorf Paul Scherrer Institut, CH-5232 Villigen, Switzerland 1

Summary The release of mercury and thallium from liquid eutectic lead-bismuth alloy (LBE) under a flowing Ar/7%-H2 atmosphere has been studied in the temperature range from 408 to 1292 K using γ-ray spectroscopy. For technical applications such as liquid metal spallation targets or accelerator driven systems, where liquid LBE is planned to be used as target material, the release of radioactive mercury isotopes produced by spallation is expected to be one of the major safety issues. During short-term experiments significant amounts of mercury begin to evaporate from liquid LBE at temperatures starting from about 475 K. 80% of the mercury present in the sample is released from samples of approximately 1.5–3 g within one hour at temperatures higher than 625 K. Thallium release in the temperature range investigated is below experimental error. Long-term experiments reveal that even at temperatures as low as 476 K about 25% of the mercury present in the samples is released per day under a flowing Ar/7%-H2 atmosphere.

Abstract In preparation for the experimental investigation of chemical properties of element 112 model studies were conducted based on the assumed similarity of element 112 to either the noble gas Rn or the transition metal Hg, its supposed lighter homologue in group 12. The adsorption behavior of elemental Hg on the transition metals Ag, Au, Ni, Pd, and Pt were investigated experimentally by off-line gas thermochromatography. The deduced adsorption data of Hg were compared with new values calculated using the Eichler–Miedema model. The observed sequence of increasing Hg-metal-interactions for Ag < Ni < Au < Pd < Pt confirms the predicted trend. The only exception was Pd, on which Hg was calculated to adsorb at a higher temperature than on Pt. Difficulties to obtain reproducible clean surfaces of Ag, Ni, Pd, and Pt led to the choice of Au as the best metal surface suitable to adsorb Hg. For fast on-line gas thermochromatography studies on metallic surfaces a new set-up was developed based on the In-situ Volatilization and On-line detection technique (IVO). This set-up was tested in on-line thermochromatographic investigations with short-lived Hg isotopes and 219Rn, using Au or Pd as stationary surfaces. An overall efficiency of about 60% and a transportation time less than 25 s was determined for this newly designed IVO. A separation factor of more than 106 was estimated for non-volatile species.

Summary We present results of the second experiment on the chemical identification of element 112. Similar to the first test in 2000, we aimed at the production of the spontaneously fissioning 283112 nuclei with T 1/2≈3min. A natU3O8 (with some Nd) target, 2mg of U/cm2 thick, was bombarded with 233-MeV 48Ca ions (the energy in the middle of the target layer). The nuclei recoiling from the target were thermalized in flowing helium. The target chamber was connected through a 25m long capillary to detectors of α-particles and fission fragments. All the equipment and detectors were kept at ambient temperature. According to the test experiments, of all the heavy elements produced in the bombardment, only Hg, Rn and At could be transported to the detectors. The first detecting device was similar to that used earlier – an assembly of 8 pairs of PIPS detectors coated with gold. Here one would observe the decay of element 112 atoms if they like Hg adsorbed on gold. The atoms which were not retained and freely passed through the PIPS detectors entered a new, flow-through ionization chamber, 5000 cm3 in volume, optimized for detecting fission fragments. The PIPS detectors and the ionization chamber were placed inside a large assembly of 3He – filled neutron counters to detect prompt neutrons from the fission events. In 22.5 days, a beam dose of 2.8×1018 ions was accumulated. More than 95 of the simultaneously produced α-active 185Hg (T 1/2=49 s) were found deposited already on the first pair of PIPS detectors; meanwhile, all the PIPSs did not detect any fission event. In the ionization chamber, eight fission events were observed in coincidence with neutron counts while the expected background was insignificant. Hence, the spontaneous fissions of the volatile activity can be conclusively attributed to the decay of element 112 produced in the fusion reaction 48Ca+ 238U, and formerly observed in Dubna physical experiments. Evaluation of the experimental data in terms of the adsorption enthalpies indicates much weaker interaction of element 112 with Au than that of Hg. One can conclude that in the given chemical environment, element 112 behaves like Rn rather than like Hg. The formation cross section of 283112 estimated from the data amounts to several pb. The experiments were carried out at the Flerov Laboratory of Nuclear Reactions at JINR in November–December 2001.

After the discovery of the neutron by Chadwick in 1932, the group led by Fermi in Rome began bombarding different elements with neutrons to study the radioactivity induced through (n,γ) reactions. The decay characteristics of the radioactivity in neutron-irradiated uranium was interpreted to indicate that some of the products were probably transuranium elements. Similar conclusions were reached by other research groups; for example in 1937 Meitner, Hahn and Strassman in Berlin proposed the following reaction/decay series (omitting mass numbers)

Summary The evaporation behaviour of polonium and its lighter homologues selenium and tellurium dissolved in liquid Pb-Bi-eutecticum (LBE) has been studied at various temperatures in the range from 482 K up to 1330 K under Ar/H2 and Ar/H2O-atmospheres using γ-ray spectroscopy. Polonium release in the temperature range of interest for technical applications is slow. Within short term (1 h) experiments measurable amounts of polonium are evaporated only at temperatures above 973 K. Long term experiments reveal that a slow evaporation of polonium occurs at temperatures around 873 K resulting in a fractional polonium loss of the melt around 1% per day. Evaporation rates of selenium and tellurium are smaller than those of polonium. The presence of H2O does not enhance the evaporation within the error limits of our experiments. The thermodynamics and possible reaction pathways involved in polonium release from LBE are discussed.

The adsorption enthalpy of silver and gold atoms on fused silica surfaces was measured using vacuum-thermochromatography. Carrier-free silver, gold and platinum nuclides were produced by irradiation of cadmium, lead and mercury targets at the Swiss Spallation Neutron Source SINQ, followed by a thermochromatographic separation. For the calculation of the adsorption enthalpy from experimental data a thermodynamic model and a kinetic model (Monte Carlo simulation) were used. Our measurements revealed an adsorption enthalpy of -279 +/- 10 kJ/mol for gold, -232 +/- 13 kJ/mol for silver and an upper limit of -430 kJ/mol for platinum. These values, determined by the thermodynamic model, are in good agreement with literature values based on the same model. Additionally, Monte Carlo simulations have been performed to demonstrate the influence of temperature and size of quartz columns on the transport of model-isotopes with different half-lives.

The thermochromatographic behavior of berkelium chlorides in various chlorinating gases was studied. Using Cl-2 or a mixture Of Cl-2/SOCl2, two peaks were observed in a thermochromatographic column. The "high temperature" peak corresponds to BkCl3 and the "low temperature" one corresponds most probably to BkCl4. Using both the model of mobile adsorption and the of Monte Carlo method, the adsorption enthalpies were determined. The possible chemical reactions in a chromatographic column are discussed.

Carrier-free short-lived nuclides are employed in many different fields of modern nuclear chemistry. The two main production strategies are either thermal neutron-induced fission of 235U or 239Pu at nuclear reactors or spallation neutron sources or charged particle-induced nuclear reactions at accelerator facilities. An alternative method is to use a spontaneously fissioning nuclide. A facility applying this technique (“Miss Piggy”) was built at the University of Berne (Switzerland). Californium-252 (252Cf), which has a 3% fission branch and a half-life of 2.645a, is used for the production of short-lived fission products that are stopped in an adjacent recoil chamber. Short-lived nuclides are transported out of the recoil chamber using the well-known gas-jet technique. Over 100 nuclides have been identified so far and used in different applications. Since such a device does not require any large facility and is easy to operate it serves well the needs of typical university laboratories.

In bombardments of 248Cm with 143.7-146.8 MeV 26Mg ions the nuclides 269Hs and presumably 270Hs were produced. After chemical isolation, Hs atoms were identified by observing genetically linked nuclear-decay chains. Three chains originating from 269Hs confirmed the decay properties observed previously in the decay of 277112. Two chains exhibited the characteristics expected for the new nuclide 270Hs, which was predicted to be a deformed ”doubly magic” nucleus. From the measured \(E_\alpha =9.16^{+0.07}_{-0.03}\) MeV an \(\alpha\)-decay half-life of 3.6+0.8-1.4 s was estimated.

Recently, the first successful chemical investigation of element 108, hassium (Hs) has been reported [1]. Based on 7 detected atoms, Hs was shown to form a higly volatile oxide, most probably HsO4. Therefore it behaves similarly to Os, which is known to form highly volatile OsO4. The enthalpies of adsorption-ΔHa O(T) of the compounds on silicon nitride were evaluated as (46±2) kJ/mol for HsO4, compared to (39±1) kJ/mol for OsO4 under identical experimental conditions. Hs should therefore be considered as a member of group 8 of the periodic table.

Recently, an experiment to chemically characterize for the first time a compound of the transactinide element hassium (Hs, Z = 108) has been performed. On the basis of the assumption that Hs will belong to group 8 of the periodic table and will thus be homologous to ruthenium (Ru) and osmium (Os), the presumably very volatile HsO(4) was isolated by gas adsorption chromatography. The experiment allowed the determination of the enthalpy of adsorption (-DeltaHa(0)((T))) of HsO(4) on a silicon nitride surface. In this work, the trend in standard enthalpies of sublimation (DeltaH(S0)((298))) of group 8 element tetroxides established by ruthenium tetroxide and osmium tetroxide was extrapolated to hassium tetroxide. The Delta(S)(0(298))(HsO(4)) was found to be very similar to DeltaH(S)(0(298))(OsO4). On the basis of an empirical correlation between the sublimation enthalpies and the enthalpies of adsorption of oxide (and oxyhydroxide) molecules on quartz surfaces, -DeltaH(a)(0(T))(HsO(4)) was predicted to be 46 +/- 15 U mol(-1). A trend for the values of the adsorption enthalpies of RuO4 approximate to OsO4 approximate to HsO(4) was found. A different theoretical approach makes use of the relation between the interaction distance of a physisorbed, nonpolar molecule on a dielectric surface and the interaction energy. From literature values of the adsorption of noble gases on metal surfaces, it was found that this distance is independent of the adsorbed species, as well as of the adsorbent material. With the use of this universal interaction distance, -DeltaH(a)(0(T))(OsO4) was calculated only from molecular geometries Of OsO4, the polarizability, and the first ionization potential and was found to agree with experimental results found in gas adsorption chromatography experiments, The -DeltaH(a)(0(T)) values of RuO4 and HsO4 were calculated using the same procedure; where no experimental values of molecular properties were available. these were extrapolated or published values obtained from density funcional calculations were used. A value of -DeltaH(a)(0(T))(HsO(4)) = 47 +/- 11 U mol(-1) is predicted, in agreement with the extrapolated value.

Summary Gas adsorption chromatography experiments were carried out with Es and Am in a temperature gradient tube using chlorinating carrier gas. From the observed deposition temperatures the adsorption enthalpies of AmCl 3 and EsCl 3 on the chromatographic surface (quartz glass surface, dynamically modified by the chlorinating carrier gases) were deduced for the first time. In good agreement, both the model of mobile adsorption and the Monte Carlo model resulted in:

The periodic table provides a classification of the chemical properties of the elements. But for the heaviest elements, the transactinides, this role of the periodic table reaches its limits because increasingly strong relativistic effects on the valence electron shells can induce deviations from known trends in chemical properties 1 , 2 , 3 , 4 . In the case of the first two transactinides, elements 104 and 105, relativistic effects do indeed influence their chemical properties 5 , whereas elements 106 and 107 both behave as expected from their position within the periodic table 6 , 7 . Here we report the chemical separation and characterization of only seven detected atoms of element 108 (hassium, Hs), which were generated as isotopes 269 Hs (refs 8 , 9 ) and 270 Hs (ref. 10 ) in the fusion reaction between 26 Mg and 248 Cm. The hassium atoms are immediately oxidized to a highly volatile oxide, presumably HsO 4 , for which we determine an enthalpy of adsorption on our detector surface that is comparable to the adsorption enthalpy determined under identical conditions for the osmium oxide OsO 4 . These results provide evidence that the chemical properties of hassium and its lighter homologue osmium are similar, thus confirming that hassium exhibits properties as expected from its position in group 8 of the periodic table.

Selenium nuclides are available from thermal neutron induced nuclear fission of 235 U at the gas-jet facility at the Swiss spallation neutron source (SINQ) at Paul Scherrer Institute, Switzerland. The formation of stable selenium compounds, their transport yields using the gas-jet system and their relative thermal decomposition temperature were investigated under oxidizing and reducing conditions in the target chamber. Using O 2 , H 2 , CO, and propene as additional gases, the selenium isotopes are suggested to form H 2 SeO 3 , H 2 Se, COSe, and C 3 H 6 Se, respectively, with overall 84 Se yields of 1.5%, 4.7%, 6.3%, and 21.9%, respectively. Adsorption enthalpy, vapour pressure, solubility and acidity data for these species were collected from the literature or estimated from other known thermochemical properties. Carrier free bromine isotopes ( 84 Br, 86 Br) in the form of HOBr were obtained by thermally decomposing H 2 SeO 3 and retaining elemental Se under oxygen rich conditions on quartz at 400 K.

The partitioning of nitrogen oxides between ice and air is of importance to the ozone budget in the upper troposphere. In the present study, adsorption of nitrogen oxides on ice was investigated at atmospheric pressure using a chromatographic technique with radioactively labelled nitrogen oxides at low concentrations. The measured retentions solely depended on molecular adsorption and were not influenced by dimerisation, formation of encapsulated hydrates on the ice surface, dissociation of the acids, nor by migration into a quasi-liquid layer or grain boundaries. Based on the chromatographic retention and the model of thermo-chromatography, the standard adsorption enthalpy of -20 kJ mol-1 for NO, -22kJ mol-1 for NO2, -30kJ mol-1 for peroxyacetyl nitrate, -32kJ mol-1 for HON} and -44 kJ mol-1 for HNO3 was calculated. To perform those calculations within the model of thermo-chromatography, the standard adsorption entropy was calculated based on statistical thermodynamics. In this work, two different choices of standard states were applied, and consequently different values of the standard adsorption entropy, of either between -39 kJ mol-1 and -45kJ mol-1, or -164 kJ mol-1 and -169 kJ mol-1 for each nitrogen oxide were derived. The standard adsorption enthalpy was identical for both standard adsorption entropies and thus shown to be independent of the choice of standard state. A brief outlook on environmental implications of our findings indicates that adsorption on ice might be an important removal process of HNO3. In addition, it might be of some importance for HONO and peroxyacetyl nitrate and irrelevant for NO and NO2.