Performance Studies And Improvements Of The Time-of-flight Detector For Isochronous Mass Measurements At CSRe

Isochronous Mass Spectrometry has been developed to measure masses of exoticnuclei with lifetimes as short as a few tens of microseconds at HIRFL-CSR acceleratorat IMP in Lanzhou. In order to further improve the ability to measure masses of theexotic nuclei of HIRFL-CSR accelerator, the performance characteristics of thedetector were investigated in detail and significantly improved.In mass measurement experiment, considerable saturation effects can occur whentoo many ions are injected into CSRe at the first few turns. The saturation effect is adead time effect, which will result in a decrease of the gain, the amplitude of thesignals and the detection efficiency and thus deteriorate the timing performance of thedetector. In order to eliminate such effects, a fast, pulsed high-voltage power supply(PHVPS) has been employed which makes the detector switched-on after the firstseveral revolutions, when most of the ions lying outside the storage acceptance are lostfrom the CSRe. This procedure could save necessary channels for ions that are movingisochronously.The PHVPS is constructed by Hefei Bolei Electricity CO.,LTD in Hefei, Chinaand its performance has been tested by offline experiment in our laboratory. Theresulting high performance allowed it to be tested in the experiment on massmeasurements of112Sn projectile fragments at the end of2012. The online resultsdemonstrated that the detection efficiency and the averaged signal amplitudes of the detector were significantly improved by using the PHVPS instead of the conventionalhigh voltage power supply. Particularly the charge-resolution was greatly improvedwhich provides essential additional information for the particle identification that canbe used to address nuclides with very close mass-over-charge rations, e.g., the N=Znuclei. The PHVPS can be used as well in future experiments at HIAF, where theprimary beam intensities will be increased by several orders of magnitude.Since the storage ring is a large-acceptance machine, it allows for storing ions ina broad range of mass-to-charge ratios. However most of the ions are lying outside theisochronicity window, which affects significantly the achievable mass resolving power.A measurement of the velocity or magnetic rigidity of each particle in addition to itsrevolution time would allow correction for this “non-isochronicity” effect.In the CSRe, we are aiming at the in-ring measurement of the velocity of eachstored ion. For this purpose two ToF detectors will be installed in one of the straightsections of the CSRe. It is obvious that the timing characteristics of thedouble-ToF-system are crucial for the success of the method. For better performanceof the time-of-flight detector, the performance characteristics have been investigatedand improved in offline experiments. A falling time of the negative signals as short as230ps has been achieved due to the optimization of the bandwidth of the signal cablesand the transit time of the avalanche electrons. The non-isochronocity in the SEtransport from the carbon foil to the MCP was reduced by increasing the acceleratingelectric field. The best value of18.5ps was obtained with E=180V/mm, which is animprovement by almost a factor of three as compared to50ps measured for theprevious detector. In the experiment on mass measurements of78Kr projectilefragments at the end of2013a78Kr36+stable beam was used to investigate theperformance of the further improved detector. The online results have shown that thetime resolution has been improved significantly compared to the previously used detector and the measured time resolutions for offline and online tests agree wellwithin1statistical uncertainties. The better timing performance of the TOF detectorwill be of a great benefit for future storage ring mass measurement experiments.