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UBC Theses and Dissertations

Design and calibration of a precise ion energy control system for a Van De Graaff electrostatic accelerator and its use in the study of resonant reactions in some light elements Aaronson, David Andrew


A precise energy control system has been constructed for the U.B.C. electrostatic accelerator. Over the past six months it has provided analyzed beams of protons as large as 4½ microamperes on a target with an energy homogeneity of 0.1%. In the system adopted, the accelerated positive ions are analyzed by a 90° deflection magnet provided with entrance and exit slits to define the beam path. The magnetic field is stabilized to a few parts in 100,000, and controlled by a nuclear magnetic resonance method. A fraction of the emergent beam falls on two insulated slits, "sniffers", connected to a differential amplifier, the output of which varies as the beam impinges more on one than the other. Thus an error signal is obtained according to the shift in energy and hence position of the beam, which is used to modulate a reverse beam of electrons sent up the differential pumping tube of the generator. This beam loads the generator so as to maintain its voltage, and hence the energy of the ions, constant. The main central part of the beam passes through the slits onto the target mounted beyond. One-dial control over a range of 20 KeV is achieved by simply tuning the oscillator controlling the frequency of the nuclear magnetic resonance fluxmeter head. The energy of the ions can be varied in steps as fine as 0.2 KeV in 1,000 KeV. The generator's voltage scale (the generating volt-meter) and energy scale (the magnetic field of the analyzing magnet) have been calibrated relative to the currently accepted standard value of Herb, Snowdon, and Sala of 0.8735 MeV for the strong F¹⁹(p, α ɤ )O¹⁶ resonance and checked with the 0.3404 MeV resonance occurring in the same reaction. Additional calibration points were obtained using mass 2 and 3 beams. The complete gamma ray excitation curve for the reactions from F¹⁹ bombarded with protons has been taken up to 2 MeV and new resonances found at 1.62 and 1.84 MeV. The 1.355, 1.381 MeV doublet was resolved with a peak to trough value of 9/1 which is excellent confirmation of the homogeneity of the proton beam. The resonances in the N¹⁵¹⁹(p, α ɤ )C¹² reaction have also been investigated and background yields from various target backing materials measured up to 2 MeV.

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