Gas stopping cell efficiency @ SHIPTRAP
In the SHIPTRAP experiments, at GSI, precise mass measurements of the heaviest nuclei are performed. The SHIPTRAP research group currently holds the world record with direct mass spectrometry on 256Lr. To be able to measure the first superheavy nucleus, it is necessary to improve the efficiency of the entire set-up. The gas stopping cell is essential as it concerns the efficiency. In this report the prospects of changing the buffer gas from helium to argon is evaluated.
The main goal of the Penning trap mass spectrometer SHIPTRAP is to perform precise mass measurements on exotic heavy nuclei. The set-up is located behind the velocity filter SHIP, Separator for Heavy Ion reaction Products. The heavy elements are created in a fusion evaporation reaction. Here, the fusion of typically a lighter projectile with a heavier target is followed by an evaporation of neutrons. SHIP separates out the “slow” reaction products from the “fast” primary beam by their velocity with a symmetric composition of dipole magnets and dipole electrical fields .
The transmitted heavy ions from SHIP typically have kinetic energies of several 10 MeV. To be able to trap the ions in the subsequent penning trap system, it is necessary to slow them down to a few eV, otherwise they would just fly through the trapping potential. Therefore, the ions are decelerated in a thin foil of titanium before they interact with an inert gas, classically helium, in the gas stopping cell. The stopped ions are then extracted from the cell using a combination of electric fields and a gas jet, before they are guided to the mass measurement set-up. The mass measurements at SHIPTRAP are performed with a penning trap based system. A superconducting magnet confines the trapped ions in the radial direction and the ions are axially confined in a quadrupole electric field. In the trap the ions perform a circular motion with a frequency characteristic to their mass. Through different techniques the characteristic frequency can be measured very precise and the mass of the ion can be determined with a high precision .
SHIPTRAP holds the world record of direct mass measurement of the heaviest nucleus, 256Lr (Z = 103). The combination of significantly lower production cross sections and steadily shorter half-lives for even heavier nuclei, presents a challenge for the SHIPTRAP set-up. Towards the first mass measurement of a superheavy element, i.e. Z ≥ 104, an improvement on the set-up efficiency, and especially that of the gas stopping cell, is crucial [2, 1].
The efficiency of the gas cell is governed by the fraction of ions which are stopped within the geometry of the cell, denoted as stopping efficiency, and the fraction of ions that are stopped which then are extracted from the gas cell, denoted as extraction efficiency. A new, more efficient cell, denoted as CryoCell, has been under development since 2008 and its first online commissioning was conducted in 2015 . The combined stopping and extraction efficiency is anticipated to approach 70% .
With the current Ti-foil thickness a significant fraction of the ions already come to a stop in the entrance window and are lost . Therefore, a thinner foil is desired, which in turn requires either a larger helium pressure or a buffer gas with larger stopping power to obtain a better stopping efficiency. The pressure with helium is limited, but argon represents a buffer gas with a larger stopping power.
In the following, the prospects of argon as the stopping cell buffer gas at SHIPTRAP is evaluated, based on the experimental settings of the 2015 online commissioning experiment. After a description of the CryoCell, simulations of the stopping efficiency with argon using the program SRIM are discussed. Here, simulation settings; the choice of ion and the energy and spatial distributions of the incoming ion are examined, followed by a discussion on the simulation results. Finally, the benefits and limitations for the usage of argon are concluded and an outlook for future prospects is given.
 M. G. Dworschak (2014) (Doctoral dissertation).
 C. Droese et. al. Nucl. Instr. and Meth. B 338, (2014) 126-138.
 O. Kaleja et. al. (2015) (GSI Scientific Report).
 O. Kaleja (2016) (Master thesis).