Ion beam dose measurement in nuclear microprobe using a compact beam chopper

Complete text of publication follows. The accurate measurement of ion beam dose is a key point of quantitative analytical methods like PIGE, PIXE or RBS. As all of these analytical work running in our Scanning Nuclear Microprobe are carried out both on insulating and conducting material samples, it was desirable to develop an arrangement, which is not sensitive to the conductivity of samples, furthermore, is applicable in the actual grounded analytical chamber. To avoid difficulties with current/charge measurement directly on the sample, the beam intensity is measured within better conditions, before it reaches the specimen. In this case a spinning conductor component periodically interrupts the beam before the target, allowing sampling of the ion dose for a constant fraction, usually 10% or less, of the full measurement time. Sampling is carried out by the collection of signals (backscattered particles. X-rays, etc.) induced by the incident particles on the spinning component, which is proportional to the dose. Calibrating the arrangement, the specimen is replaced by a biased Faraday cup. Though, adopting these ideas, several types of so called beam chopper systems have been announced [1, 2, 3], our solution [4] has some advantageous features compared to them: 1. It is placed on the beam axis, inside the measuring chamber, so its presence does not require any modifications and readjustments on the high precision quadrupole lenses. 2. The original arrangement of detectors, sample holders, etc. could remain unchanged after built-in. 3. While others are using continuously rotating chopper wings, we applied a stepper motor to assure rapid change between the on and off states of the ion beam. In addition, an inhibit signal is derived by an electronic controller unit when the wings stop the beam, practically eliminating the detection of background radiation induced in the wing. 4. The backscattered particles emerging from the wings are detected by an annual Si detector, which is protected with a thin Al foil against visible light coming from optical fluorescence of the target or operating light sources inside the chamber. 5. To avoid extremely high or low counting rates for the annular detector, instead of collimating it, changeable wings covered by different scattering materials such as Al, Ni and Au are used. (author)