Accelerators – Reports 1
Refurbishment of the EN tandem accelerator at iThemba LABS Gauteng
All the major mechanical, electrical, and control systems for the refurbishment of the tandem accelerator at iThemba LABS Gauteng were successfully installed within one month after the estimated completion date. The first beam at the facility was transported from the 860A SNICS ion source via the accelerator to the last Faraday cup (FC4C) in the nuclear physics beam line in the experimental vault on the 31st of May 2007 and the 1st of June 2007. The computer control system and beam diagnostic equipment were fully operational. The stable terminal voltage of 2 MV (measured with the generating voltmeter in the control mode) and power supplies contributed to the production of a very stable beam. The vacuum system performed satisfactorily with pressure readings between 10-6 and 10-7 mbar in all the beam lines. Beam diagnostic and control components were used successfully to steer, focus, and transport the beam. Further testing and debugging still have to be done.
Inauguration of the facility took place on 2nd of July 2007, coinciding with the start of the annual conference of the South African Institute of Physics. This major milestone was attended by many prominent South African scientists.
A major achievement this year was the increase in the intensity of the 66 MeV proton beam from 100 µA to 260 μA for the production of radionuclides. With the flat-topping system of the light-ion injector cyclotron in operation, longer beam pulses and much higher beam intensities can be obtained. Two bunchers operating at different frequencies, in the transfer beam line between the two cyclotrons, instead of the single buncher that was used before, provide a more linear effective voltage for longitudinal focussing of the beam on the injection orbit of the separated-sector cyclotron. Due to the flat-topping system of the separated-sector cyclotron the longer beam pulses can be accelerated with low energy spread, and to obtain small beam widths and good beam separation at extraction. At present a beam intensity of 160 μA is routinely used for the production of 82Sr in the vertical beam line. The maximum beam current available will be used after the beam splitter system is commissioned during June 2008.
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Various non-destructive position monitors were installed in the J, X, and I beam lines to measure and monitor the horizontal and vertical positions of the beam along these lines continuously, without intercepting any beam. The position monitors can also be used for semi-automatic alignment of the beam. This project was the result of a South African Inter-governmental Science and Technology Cooperation Program (GUN 2074723) with Hungary and the KFZ Forschungszentrum Jülich, in Germany. The monitors were designed and manufactured at iThemba LABS and scientists in Hungary and Jülich were responsible for development of the electronic systems and software.
The accelerator group was consulted to investigate the possibility of removing Ag-110, the major unwanted reactor product, from the helium coolant gas in the future PBMR reactor, by application of a magnetic field. This radioactive product emanates from the fission process in the pebbles and activates the inside of the reactor circuit if it is not removed from the coolant gas. For that purpose a series of calculations for different conditions was proposed, from which it was expected to verify whether a magnetic field is feasible for removing the unwanted ions. An extensive report submitted to the PBMR board showed that it is not possible to extract radioactive Ag ions from the coolant with any reasonably practical magnetic field at a gas pressure higher than 1 atmosphere. The main factors contributing to this result are the viscosity of He gas, the short mean time between collisions, and the limited practical magnetic field strength.
De Beers Group Services Company (DBGS)
iThemba LABS was continuously and intimately involved in development work with the DBGS on the Diamond Within Kimberlite (DWIK) project since its beginning in 1995 up to December 2006, when the project was terminated by the board of directors of De Beers. Our main contributions were calculations of the beam transport systems, assistance with the procurement of magnets and special power supplies, development work on the ion source, development of components for beam energy measurements, and help with the RF equipment.
iThemba LABS and NECSA were asked to advise De Beers on future use of the two RFQ accelerators made redundant by project termination. A due diligence report was prepared to evaluate the future use of the accelerators, which could be a great asset for science in South Africa. The report is at present under consideration by DST.
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