Study on the Accelerator Mass Spectrometry Based on a Cyclotron

Abstract

The accelerator mass spectrometry (AMS) based on a cyclotron has been studied. A cyclotron AMS can be more compact and economical compared to the widespread AMS utilizing tandem electrostatic accelerator. In fact, the cyclotron for the AMS was tested earlier than the tandem accelerator. However, the previous efforts were not successful because the cyclotron AMS showed poor performance in the transmission efficiency and the system stability. These difficulties can be overcome with optimal designs of the injection beam line and the cyclotron and also by using highly stable components readily available nowadays. The AMS based on a tandem accelerator uses negative ions to accelerate the ions with a positive terminal voltage. This makes it possible to remove major interfering ions such as 14N- in the case of 14C measurement, but it makes the preparation of target samples time-taking. The cyclotron AMS could ultimately analyze positive ions with the developments of a compact particle detectors for low-number counting.

In the design of the cyclotron AMS, there are two main topics; one is to minimize the size of the total system and the other is to maximize the transmission efficiency. To minimize the AMS system, the injection beam line was designed to have minimal beam elements. Also, we plan to adopt a compact magnet with a high stability and flat-topping RF system in the cyclotron to increase transmission efficiency. The cyclotron used for mass spectrometry is operated with high harmonic and turn number, because the mass resolving power is proportional to both the RF harmonic and the total turn number. It requires a high precision of synchronicity, thus an AVF cyclotron will be used to augment the axial restoring force while the isochronous field is kept. The magnet of the cyclotron was designed using RADIA software. A prototype of the RF cavity was tested, which was designed to be nearly capacitive because of the low accelerating voltage. The flat-topping wave can be achieved by adding the third harmonic component to the original sinusoidal wave.

A prototype of the injection beam line is constructed, which consists of an ion source, an Einzel lens, a saw-tooth RF buncher, a 90° dipole magnet, and a quadrupole triplet. The injection system is designed to have a minimal number of beam elements to keep the system compact using the beam optics codes such as TRANSPORT and TRACE-3D. The positive carbon beam was extracted using CO2 gas, and the positive nitrogen beam was extracted using N2 gas to study the characteristics of 14C+ beam. Some characteristics of the beam were measured such as for example, the beam size, the transverse emittance and the longitudinal beam structure in the injection beam line. The beam phase spaces were calculated and compared with the results of measurement performed at the beam diagnostic chamber.