A new detector setup for ISOLTRAP and test of the Isobaric multiplet mass equation

Installed at the on-line isotope separator ISOLDE at CERN, the tandem Penning-trap spectrometer ISOLTRAP is designed to perform high-accuracy mass measurements on short-lived radionuclides. Based on the determination of the ion cyclotron frequency ν$_c$= qB/(2πm) in a strong magnetic field, the typical relative mass uncertainty is $\sigma$(m)/m = 10$^{-8}$ with a resolving power of up to R = 10$^7$. Accurate and precise mass values for specific unstable nuclides are important since they serve e.g. as a stringent test of nuclear models. The main part of this work is the design, installation and characterization of a new Channeltron-detector setup in order to increase the detection efficiency of ISOLTRAP by about a factor of 3. Since a detection efficiency close to 100% is reached, true single-ion experiments can be performed for the first time and exotic nuclides further away from the valley of stability and with very short half-lives are now accessible. Concerning mass measurements on short-lived nuclides, the masses of potassium isotopes from $^{35}$K( $\,\tau\scriptstyle_{1/2}$= 178 ms ) up to $^{46}$K( $\,\tau\scriptstyle_{1/2}$= 105 s ) have been measured with a relative precision of better than 1.6$\times$10$^{-8}$. The accurate value of the 35K mass contributes to a stringent test of the isobaric multiplet mass equation for the A=35, T=3/2 isospin quartet.