The Isochronous Mass Spectrometry With Two Time-of-flight Detectors At HIRFL-CSR

Like few other parameters,mass of a nucleus,with its inherent connection with the nuclear binding energy,is one of the most important fundamental quantity.Precision nuclear mass measurements lead us to a deeper understanding of the basic properties of matter.The discovery of isotopic nature of the chemical elements in late 19^th century,and the exploration to nuclear internal structure,promote the development of nuclear models and the research in the origin of elements in the cosmos.In recent years,the advent and application of radioactive ion beam facilities and novel mass spectrometers,such as penning traps and storage rings,have strikingly enlarged the number of measured exotic nuclei which lie far from the valley of?-stability.With the successful commissioning of the Cooler Storage Ring at the Heavy Ion Research Facility in Lanzhou?HIRFL-CSR?,a series of precision mass measurements of short-lived nuclides have been carried out by employing Isochronous Mass Spectrometry?IMS?.The obtained mass values have been successfully applied to nuclear structure and astrophysics studies,taking the IMS at HIRFL-CSR to the forefront of the mass measurement in the neutron-deficient nuclides region.In present IMS experiments with single TOF detector,the mass resolving power is relatively low for the nuclides far from the transition point,and large systematic deviation has been found between different series of nuclides in mass calibration.The systematic deviation is probably due to the asymmetric momentum distribution of different ion species.Besides,the difference of energy loss in the TOF detector for different ion species also contributes to the disparity of momentum distribution.In order to solve this problem and achieve higher mass precision,a new isochronous mass spectrometry with double Time-of-Flight?TOF?detectors has been proposed.Thusm two TOF detectors have been installed in the straight section of CSRe.The mass measurements of ³⁶Ar projectile fragments,which aimed at testing the IMS with double TOF detectors,has been performed in 2015.With the flight time of the stored ions measured by the two TOF detectors separately,the velocity of each ion is calculated,in addition to the revolution time.By employing the velocity information,the revolution time of the ion in any orbit is corrected to the revolution time corresponding to the same reference orbit.After the time correction of magnetic field,the mass calibration results show that the correction of momentum spread can not only reduce the systematic deviation,but also lead a reduction in systematic error.The mass precision for almost all ions are improved,especially for the ions far from the,whose mass precision are improved by nearly 3 times.The results of this test experiment indicate that the IMS with double TOF detectors can reduce the systematic deviation caused by the momentum spread effectively,and improve mass resolution of IMS.Thus,by employing this new IMS method,a larger number of nuclei can be measured or used as references in a single setting of the future RIBLL2-CSRe system.The improved method is more competitive in the mass measurements of the exotic nuclei close to the drip lines.