25-MV Tandem Accelerator Research Articles

24Mg(p, α)21Na reaction study for spectroscopy of 21Na

The 24Mg(p, α)21Na reaction was measured at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory in order to better constrain the spins and parities of the energy levels in 21Na for the astrophysically important 17F(α, p)20Ne reaction rate calculation. 31-MeV proton beams from the 25-MV tandem accelerator and enriched 24Mg solid targets were used. Recoiling 4He particles from the 24Mg(p, α)21Na reaction were detected by a highly segmented silicon detector array which measured the yields of 4He particles over a range of angles simultaneously. A new level at 6661 ± 5 keV was observed in the present work. The extracted angular distributions for the first four levels of 21Na and the results from distorted wave Born approximation (DWBA) calculations were compared to verify and extract the angular momentum transfer.

Spin assignments to excited states inNa22through aMg24(p,He3)Na22reaction measurement

The level structure of $^{22}\mathrm{Na}$ has been studied at the Holifield Radioactive Ion Beam Facility in Oak Ridge National Laboratory using the $^{24}\mathrm{Mg}$($p$,$^{3}\mathrm{He}$)$^{22}\mathrm{Na}$ reaction. Proton beams of 41 and 41.5 MeV were generated by the 25-MV tandem accelerator and bombarded isotopically enriched $^{24}\mathrm{Mg}$ targets. Angular distributions of recoiling $^{3}\mathrm{He}$ particles were extracted by using a segmented annular silicon strip detector array. Spins and parities for 10 levels were constrained through a distorted-wave Born approximation analysis of angular distributions, including three levels above the proton threshold at 6.739 MeV.

Pushing the limits of accelerator mass spectrometry

A renewed interest in Accelerator Mass Spectrometry (AMS) from nuclear physics laboratories is emerging in connection with Radioactive Ion Beams (RIBs). At the Holifield Radioactive Ion Beam Facility (HRIBF) at Oak Ridge National Laboratory (ORNL) we are exploring the AMS capabilities of the 25-MV tandem accelerator. Behind this effort is the realization that two fields of research – AMS and RIBs – complement each other in techniques. Development of effective and efficient beam purification techniques is of common interest to both AMS and the RIB program. Two main characteristics of the 25-MV tandem provide unique opportunities for performing the highest sensitivity measurements of AMS; namely (i) the highest operating voltage in the world, and (ii) a folded geometry which involves a 180° magnet in the terminal. For the RIB program, we have used AMS techniques to improve the sensitivity of detection of some radioactive species in the measurement of unknown masses of n-rich nuclei. For AMS, we have concentrated in exploring two important isotopes, 14C and 36Cl, for applications that require the highest sensitivity. We have successfully measured 36Cl/Cl ratios as low as a few times 10−16 in seawater samples demonstrating that our setup has the highest sensitivity for this isotope and proving that 36Cl can be measured at the levels required for a tracer in oceanography.

Measuring the mass of fission fragments using the HRIBF 25-MV tandem accelerator

A method whereby mass differences can be measured using the 25-MV tandem accelerator at the Holifield radioactive ion beam facility (HRIBF) is presented. The technique is unconventional in that mass differences will be measured as position differences at the image of the energy-analyzing magnet following the accelerator and will be made at energies where independent isobaric determination will be possible by a ΔE measurement. The technique is appropriate where the unknown mass is the most neutron-rich isobaric constituent of the beam, such as for neutron-rich fission fragments.

Measuring the mass of fission fragments using the HRIBF 25-MV tandem accelerator

A method whereby mass differences can be measured using the 25-MV tandem accelerator at the Holifield radioactive ion beam facility (HRIBF) is presented. The technique is unconventional in that mass differences will be measured as position differences at the image of the energy-analyzing magnet following the accelerator and will be made at energies where independent isobaric determination will be possible by a ΔE measurement. The technique is appropriate where the unknown mass is the most neutron-rich isobaric constituent of the beam, such as for neutron-rich fission fragments.

Development of the HRIBF 25-MV tandem accelerator as a RIB accelerator

The Holifield facility 25URC tandem accelerator will begin accelerating radioactive ion beams (RIBs) for nuclear structure and astrophysics research in 1996. This paper addresses the development of the accelerator to allow optimum operation with the particular challengesassociated with RIBs. New diagnostics for ultra-low-intensity beams are being installed and the terminal potential stabilization system is being studied to optimize control with these low beam intensities. A new resistor-based voltage-grading system has resulted in more stable operation, as well as allowing operation at the very low terminal potentials which are required for some astrophysics experiments. Also addressed is beam transmission optimization, particularly at low terminal potentials, and operation of the accelerator at high terminal potentials.

Target selection for the HRIBF project

Experiments are in progress at the Oak Ridge National Laboratory (ORNL) which are designed to select the most appropriate target materials for generating particular radioactive ion beams for the Holifield Radioactive Ion Beam Facility (HRIBF). The 25-MV tandem accelerator is used to implant stable complements of interesting radioactive elements into refractory targets mounted in a high-temperature FEBIAD ion source which is on-line at the UNISOR facility. These experiments permit selection of the target material most appropriate for the rapid release of the element of interest, as well as realistic estimates of the efficiency of the FEBIAD source. From diffusion release data information on the release times and diffusion coefficients can be derived. Diffusion coefficients for Cl implanted into and diffused from CeS and Zr 5Si 3 and As, Br, and Se implanted into and diffused from Zr 5Ge 3 have been derived from the resulting intensity versus time profiles.

Selection and design of ion sources for use at the Holifield Radioactive Ion Beam Facility (abstract)a)

The Holifield Radioactive Ion Beam Facility now under construction at the Oak Ridge National Laboratory will use the 25-MV tandem accelerator for the acceleration of radioactive ion beams to energies appropriate for research in nuclear physics; negative ion beams are, therefore, required for injection into the tandem accelerator. Because charge exchange is an efficient means for converting initially positive ion beams to negative ion beams, both positive and negative ion sources are viable options for use at the facility. The choice of the type of ion source will depend on the overall efficiency for generating the radioactive species of interest. Although direct-extraction negative ion sources are clearly desirable, the ion formation efficiencies are often too low for practical consideration; for this situation, positive ion sources, in combination with charge exchange, are the logical choice. The high-temperature version of the CERN-ISOLDE positive ion source has been selected and a modified version of the source designed and fabricated for initial use at the facility because of its low emittance, relatively high ionization efficiencies and species versatility, and because it has been engineered for remote installation, removal and servicing as required for safe handling in a high-radiation-level ISOL facility. The source will be primarily used to generate ion beams from elements with intermediate to low ionization potentials. Prototype plasma-sputter negative ion sources and negative surface-ionization sources are under design consideration for generating radioactive ion beams from high-electron-affinity elements. The design features of these sources and expected efficiencies and beam qualities (emittances) will be described in this report.

Some Aspects of Radiation Safety at the Holifield Heavy Ion Facility

A new 25-MV tandem accelerator will be combined with the existing Oak Ridge Isochronous Cyclotron (ORIC). This facility will be capable of providing useful beams of heavy ions with masses up to A = 160 for nuclear physics research. A discussion of past radiation exposure problems at ORIC as well as a projection of exposures that will occur when the new accelerator comes on line are given. The radiation safety system for the new facility is described and a brief safety analysis of the new insulation gas system for the tandem will be given.