Kalevi Valli

Ion guide method for on-line isotope separation

Abstract A new ISOL technique, the ion guide method for isotope separation, has been developed. The method is based on thermalization of primary recoil ions from nuclear reactions in helium and on their transfer by helium flow through a differential pumping system into the accelerating stage of an isotope separator. With this approach, separation times in the submillisecond region are achievable for all elements. The ion guide has no ionizer but instead exploits charge exchange mechanisms related to nuclear reactions and recoil thermalization processes. Operation takes place at room temperature and the simple construction involves no components that can wear out. These properties ensure smooth and stable operating conditions. The overall separation efficiencies measured for heavy nuclides produced by light-ion reactions are up to 10%. The shortest-lived activity identified in an on-line separation is the 182-μs isomeric state of 207Bi. The production rate of the mass separated 440-μs isomeric state of 43Sc is ≈ 2 × 103 ions/μC.

Helium-jet ion guide for an on-line isotope separator

Abstract A new method based on helium-jet techniques permits primary recoil ions produced in radioactive decay or in nuclear reactions to be run directly through a mass separator. Test results with a 227 Ac source give promise of qualities complementary to those obtained with conventional ion sources. This, 69 ± 5 per cent of the recoil particles transported with pure, commercial helium are positive ions, and 66 ± 5 per cent of them are mass separated. Nearly equal efficiencies are expected for most chemical elements and compounds. The mass spectra are remarkably free from groups caused by impurities. The shortest achievable separation time is estimated to be of the order of one millisecond.

The carrier-loaded helium-jet transport method

Abstract The suitability of commercial helium loaded with small amounts of different gases or vapors for the transport of radioactive reaction products over distances of several meters has been investigated experimentally. Negative results were obtained with gaseous carriers. Transport efficiencies of 50 to 100% were obtained with carrier vapors. The vapor appears to form droplets in the system. The recoils are adsorbed physically by the droplets and carried with them through a capillary. There appears to be an active mechanism that forces the heavy constituents to the centre of the capillary cross section. The heavy constituents emerge from the capillary as a very narrow beam. Collection of the transported activity onto a metal foil in a vacuum chamber is possible only when the vapor pressure of the carrier is lower than the pressure of the collection chamber. Vacuum pump oil was found to meet this requirement. The transport and collection efficiency was studied as a function of the relative mass throughput of the oil vapor. A method for determining the optimum mass throughput of the carrier is demonstrated. The internal energetics of the system are discussed. The kinetic energy of a particle to be transported has to be higher than the average thermal energy in order to avoid adsorption onto the capillary walls. This can be achieved by increasing the mass of the particle by means of a carrier or by decreasing the system temperature. In a cooled system transport over long distances is possible even with pure helium. Application of the carrier-loaded transport method in on-line mass separators is discussed.

Transport efficiency of the helium-jet recoil-transport method with pure helium

Abstract New techniques have been developed for measuring the transport efficiency of the helium-jet recoil-transport method in the transport of radioactive recoil atoms. The dependence of the transport efficiency on main equipment parameters was studied experimentally using pure commercial-grade helium. The results are given in terms of helium mass throughput, capillary parameters and location of a catcher foil. High to moderate efficiencies were observed at transport distances shorter than 20 cm. Efficiencies obtained with pure helium at distances longer than 1 m are low, typically less than 1%. Possibilities for further development of the method are briefly discussed.

Separation of fission products by the ion guide fed isotope separator, IGISOL

Abstract The performance of the ion guide method in the on-line mass separation of fission products has been investigated. Activity was produced by bombarding nat. U targets with a beam of 20 MeV protons. All the mass chains from A = 95 through A = 120 have been scanned using conventional methods of nuclear spectroscopy. No chemical selectivity in the separation was observed; isotopes of highly nonvolatile elements from yttrium to palladium were produced with efficiencies equal to those for the more volatile ones. The boundary of known neutron-rich nuclides was reached throughout the mass region from A = 106 to A = 118. During the course of this preliminary study the new isotope 116 Rh was observed and its half-life measured to be (1.0±0.2)s. Several other unknown activities were also detected. A fraction of the order of 15% of Y, Zr and Nb activities were present as oxide ions.

Efficiency of an on-line isotope separator system employing cooled and NaCl-loaded He-jet methods

Abstract A pure helium-jet at liquid nitrogen temperature coupled to a Nielsen type ion source, and a NaCl-loaded helium-jet coupled to a hollow-cathode ion source have been investigated as means to connect a cyclotron target chamber on-line to a mass separator. Technical details and performances of some critical parts of the system are described. Total separation efficiencies measured under various experimental conditions for several nuclides vary between 0.01 and 1.0%.

Transport of recoil atoms in a stream of liquid-air-cooled pure helium

Abstract Pure commercial helium cooled down to the temperature of liquid air (80 K) has been used for rapid transportation of recoil atoms over distances as long as 10 m. Transport efficiencies of several tens of per cent have been obtained for recoil atoms resulting from alpha decay or from decay by fission. The angle of divergence of the beam of recoil atoms at the exit of the transport capillary is measured to be (11±2)° for particles of mass number A = 211. Experimental results are discussed in terms of thermal diffusion. The elimination of carrier vapors will probably simplify the use of the helium-jet technique in on-line mass separators.

Helium-jet transportation of recoil atoms into a liquid phase

Abstract A rapid inexpensive method has been developed for transportation of recoiling short-lived radioactive nuclides from a target chamber over distances of several meters into a liquid phase. This allows rapid radiochemical separation of different elements. The carrier-loaded helium-jet transport technique is employed without a pumping system. The method would appear to be applicable to most chemical elements. The transport efficiency is higher than 50% and the transport time can be made much shorter than 1 s. Radiochemical separation times of a few seconds were achieved.

The Squeezer ion guide

Abstract The properties of an ordinary ion guide can be improved by inserting a grid between the nozzle and the skimmer. When positive ions are transported, an electric potential lower than +10 V is connected to the grid and one of the order of − 10 V to the skimmer. Efficient focusing is achieved between the nozzle and the grid by a combined action of the viscous helium flow field and the weak electric field. The focusing device is called a “squeezer”. It can be extended by adding more grids, a second skimmer, and differential pumping. Contrary to the first grid, the additional elements act more or less like electrostatic lenses in medium vacuum. With the squeezer ion guide, ions created in a helium chamber can be brought into high vacuum in less than 1 ms without accelerating them to kinetic energies higher than the order of 10 eV. The width of the kinetic energy distribution is typically 2.5−3.5 eV (FWHM). Consequently, problems caused by ion scattering are greatly reduced as compared to ordinary ion guides. The squeezer ion guide was developed and tested with 215Po ions from a radioactive 227Ac source at JYFL as part of a quadrupole mass spectrometer. The ion currents were of the order of 100 ions/s. A typical yield through the first skimmer was 75% and through the second skimmer 65% of that through the nozzle. Later on a similar device was installed in the GARIS-IGISOL mass separator at RIKEN. It was tested with Ar and Xe ions froma gas discharge. Separated ion currents up to several nA were used. The mass resolution was improved by more than a factor of two. A squeezer ion guide tested at the JYFL IGISOL gave promising results in on-line experiments with fission products.

A study of a helium-jet ion guide for an on-line isotope separator

Abstract A new method based on helium-jet techniques permits primary recoil ions, produced in radioactive decay or in nuclear reactions to be run directly through a mass separator. Results obtained with a 227 Ac source promise qualities complementary with those obtained with conventional ion sources and indicate an overall efficiency of the order of 10%. Preliminary results with 20 Na recoils from the 20 Ne(p,n)-reaction indicate that most of the ionic species transported out from the target chamber are negatively charged.