A fast and sensitive resonant Schottky pick-up for heavy ion storage rings
Introduction
Schottky spectra of low intensity heavy ion beams have played a major role in experiments at the experimental storage ring ESR at GSI. The method of Schottky Mass Spectrometry [1] has been developed to measure unknown masses of exotic nuclei [2]. Such experiments allow also the identification of nuclear isomers [3] or nuclear decay studies [4]. All these kinds of measurements often rely on the identification of single particles of a certain nuclear species circulating (among others) in the storage ring. Till now, a broad-band detector in connection with a resonant circuit has been used for such measurements. In case of extremely well-cooled beams featuring ordering effects [5], [6], [7] the relative spread of the revolution frequency of such beams can be as low as . Taking as an example fully stripped Samarium ions with a charge state of 62, the signal to noise ratio is approximately 1:2 which is just enough to allow for precision measurements after sufficient averaging.
The idea of using rf resonators for Schottky measurements is not new. For example, a measurement at CERN of the antiproton lifetime using a resonator running at 129 MHz was published already in 1978 [8]. Compared to these measurements, nowadays the advancement of digital rf processing has decreased enormously the measuring time, giving access to investigations of short transient processes.
We have designed and built a cavity-like device with a resonant frequency of roughly 245 MHz in order to extract the Schottky signal with a high signal to noise ratio. We discuss its mechanical layout, its rf properties and its measured performance with various kinds of beams.
The working frequency of the resonator of 245 MHz is higher by a factor of four with respect to the standard Schottky pick-up mentioned above, which has its maximum sensitivity near 60 MHz. Therefore the frequency differences of neighbouring peaks are enhanced by the same factor. Due to the Nyquist theorem this decreases the time it takes to distinguish neighbouring peaks by a factor of four, which is very beneficial for observing fast changes (as nuclear decays) or allows for more averaging if the same temporal resolution is required.
A similar device will be installed at the cooler storage ring CSRe at the Institute of Modern Physics in Lanzhou [9].
Section snippets
Basic design considerations
The design of the pick-up is based on a ceramic gap connected to the metallic vacuum chamber on both sides of the gap. Originally this type of gap has been used for both accelerating cavities of the ESR. On one hand, the gap provides the necessary rf coupling between the beam pipe and the air-filled inner volume of the resonant cavity. On the other hand, it provides the vacuum inside the beam chamber. The advantage of such a design is the possibility to remove the resonant cavity from the gap
Basic definitions
The performance of resonating cavities is characterised by their resonant frequency , their quality factor Q and their shunt impedance Rs. A fourth important parameter is the ratio . As there are many slightly different definitions of the shunt impedance in the scientific literature, it is necessary to mention which ones are used in this article. We assume that resonator is used as an accelerating structure, producing in the centre a sinusoidally varying longitudinal electric field
Equivalent circuit
We model the resonator near the resonance by means of a parallel RLC circuit. This equivalent circuit is shown in Fig. 7. The beam is described as an ideal current source I(t), i.e. a particle crossing the resonator induces a pulse . The output coupler is represented by means of a transformer with the load impedance ZL in its secondary circuit. The impedance of the unloaded RLC circuit can be written with the parameters R, , and Q0 only [11]:In order to define
Measurement of shunt impedance with calibrated neon beams
As an alternative to field measurements as presented in Section 3, the -value was determined from Schottky spectra using an 20Ne10+ beam with currents measured by a dc beam current transformer. The spectra were determined with currents of 18, 47, and . The amplification between the output port of the resonator and the spectrum analyzer had to be determined separately. At the specific energy of 400 MeV/u, we deduce from Fig. 6 a transit time factor . The Schottky power turned out to
Conclusions
We have built a resonant Schottky pick-up featuring an excellent signal to noise ratio for the measurement of single heavy ions. Different measurement techniques have yielded consistent values of the electromagnetic properties of the resonator. Its rather high resonant frequency is beneficial for precise measurements of the revolution frequency in remarkably short time, allowing precise mass measurements of short-lived nuclei as well as the observation of fast processes which was out of reach
Acknowledgement
One of the authors, M.S.S., acknowledges the support by the Helmholtz International Centre for FAIR within the framework of the LOEWE program launched by the State of Hesse.
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