IVO, a device for In situ Volatilization and On-line detection of products from heavy ion reactions
Introduction
Investigations of the nuclear decay properties of the heaviest elements (transactinides, Z⩾104) require fast on-line separators with high separation factors and overall yields. Due to the generally very low production rates of such elements in heavy ion induced fusion reactions, presently reaching levels as low as one atom per month [1], any suitable device has to act as a filter for the much more abundant, unwanted by-products from transfer reactions and to detect final products with an efficiency close to 100%.
Two different classes of devices have successfully been applied, physical separators and chemical set-ups.
Physical separators for instance take advantage of the different velocities of wanted fusion products and interferring transfer products. This may be achieved by a velocity filter (Wien filter) [2] or a gas-filled magnetic separator [3], in which velocity dependent charge states of the ions lead to different trajectories.
Chemical separators employ the difference in chemical properties of the element to be separated from any unwanted element. Hence, chemical separators are Z specific. So far, only one on-line technique proved to be able to continuously isolate the desired transactinide element and forward it to an on-line detection system. This technique is called on-line isothermal gas chromatography. Two devices based on this principle have been developed, the On-Line Gas chemistry Apparatus (OLGA) [4] and the Heavy Element Volatility Instrument (HEVI) [5], a modified version of OLGA.
Reaction products recoiling from a target are thermalized in helium (He) gas and adsorbed to the surface of carbon aerosol particles (≈106 cm−3) suspended in the He. Within a few seconds, the products are continuously transported through a thin capillary from the recoil chamber to the OLGA chemistry device, where the particles are collected on a quartz wool filter kept at an elevated temperature of up to 1000°C. Reactive gases and oxygen (O2) are added to convert the carbon aerosol particles to CO2 and to form volatile molecules with the reaction products. The chromatographic separation takes place in the adjoining isothermal section of the column (usually quartz), kept at a cooler temperature. Depending on the isothermal temperature setting, only species reaching a threshold volatility pass through the column. At the exit of the column, the products are reattached to new aerosol particles and then transported to a counting device where the nuclear decay of the separated nuclides is registered. Rotating wheel systems such as the Rotating Wheel Multidetector Apparatus (ROMA) [6] or the MG-wheel [7], but also tape devices [8] have proven to be well suited. Typical separation times with the current version of OLGA, OLGA III [9] are about 5 s.
One obvious shortcoming of this on-line gas chemistry system is the large number of steps involved, each having a yield less than 100%. These include the thermalization of highly energetic reaction products in a gas, the attachment of the products in ionic or atomic form to the surface of sub-micrometer particles, the transport of these particles through a capillary, a chemical reaction of the attached species with reactive gases to form volatile compounds, a gas adsorption chromatography of the compounds with the surface of the chromatography column, a reattachment of the molecules leaving the column to new aerosol particles followed by a transport to the detection system, and, finally, the deposition of the particles on thin foils via impaction. The overall yield of all these processes reaches values of only 10–20%.
From this point of view it is highly desirable to develop a technique with a much simpler reaction scheme between synthesis and detection of final products. In this work such a device was developed.
Section snippets
IVO, a device for in-situ volatilization and on-line detection
One obvious shortcoming of the OLGA technique is the aerosol-borne transport of reaction products from the recoil chamber at the irradiation position to the chromatography set-up. The adsorption of the thermalized products to the surface of aerosol particles is poorly understood. The yield of this process and the efficiency for the aerosol particles to exit the recoil chamber depend on several parameters such as (i) plasma effects caused by the beam, (ii) particle number concentration and size
First experiments with volatile osmium tetroxide (OsO4), to model a chemistry with Hs (element 108)
The transactinide element hassium (Hs, Z=108) is expected to be a member of group 8 of the periodic table of the elements and thus a homologue of osmium (Os) and ruthenium (Ru). The longest-lived known isotope which decays by emission of an α-particle is 269Hs with a half-life of about 10 s and an α-particle energy of 9.2 MeV [11]. It can be produced by the reaction 248Cm(26Mg,5n) with an estimated production cross-section of about 7 pb [12]. If the chemical properties of Hs are comparable to
First experiments with mercury (Hg) to model a chemistry with elements 112 and 114
Recent experiments gave evidence that elements Z=112 [21] and 114 [22] have neutron-rich isotopes with lifetimes long enough for their chemical characterization. These elements are presumably very noble metals and expected to be highly volatile. Gas adsorption chromatographic investigations in the elemental state seem to be feasible [23], [24]. These superheavy elements are predicted to have rather high adsorption enthalpies on metal surfaces such as palladium (Pd) or platinum (Pt) [23].
Conclusions
A new chemical separator set-up was built to separate volatile elements or molecules containing short-lived nuclides produced in heavy ion reactions which was tested with short-lived nuclides of Os and Hg in the form of OsO4 and elemental Hg, which served as model species for HsO4 and element 112, respectively. The chemical yield of the separator in the Os experiment was about 50% and the lower limit of the decontamination factor from Po was 2×104. According to the test results, the apparatus
Acknowledgements
The preparation of the targets by Mrs. E. Rössler and the delivery of a stable beam by the operators of the PSI cyclotron are highly appreciated. This work was supported by the Swiss National Science Foundation.
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