First storage and cooling of secondary heavy-ion beams at relativistic energies

Abstract

Secondary beams of radioactive heavy ions have been stored and cooled in a storage ring for the first time. Relativistic beams of bare l9 Ne and ,8 F ions were produced by fragmentation of 310 MeV/nucleon 20 Ne at the heavy-ion synchrotron at GSI. They were separated in flight with the highresolution forward spectrometer and injected into the storage-cooler ring. The feasibilities of in-flight lifetime and direct mass measurements for radioactive beams are demonstrated.PACS numbers: 29.20.Dh, 25.75.+r, 29.27.Eg Recently considerable progress has been made in the production and isotropic separation of secondary beams of exotic nuclei which are created by fragmentation of heavy ions in peripheral collisions [1-3]. The first experiments using relativistic secondary beams have demonstrated new possibilities for studying nuclear structure [4]. Another frontier has been opened with the advent of storage-cooler rings for heavy ions [5]. Both new experimental features are incorporated in the high-energy accelerator complex at GSI. The system consists of the synchrotron SIS [6] providing ions of all elements up to uranium with energies of up to 2 GeV/nucleon, and the experimental storage ring ESR [7] equipped with an electron-cooler facility [8]. Heavy-ion beams up to bismuth have been stored and cooled in the ESR. The cooled beams were characterized by a relative momentum spread of Ap/p < 10~5 and by a transverse emittance of £<0.1/r mmmrad [9]. The high-resolution forward spectrometer FRS [10] interconnects the synchrotron and the storage ring (see Fig. 1). It was demonstrated that the FRS efficiently separates isotopically pure fragment beams up to the heaviest projectiles extracted from the SIS.In the two pilot experiments presented here, the FRS and the ESR were combined for the first time. Secondary beams produced and separated with the FRS were injected and accumulated in the ESR. The experiments aimed at exploring two fields of research: (i) Production and accumulation of bare short-lived /3-unstable ions, followed by in-flight determination of their lifetime. This type of experiment was explored with ,9 Ne ions (half-life: T , i/2 == 17 s) as an example, (ii) Production, accumulation, and cooling of both stable and radioactive bare nuclei, followed by a precise direct mass measurement of the circulating beam. Fully ionized ,8 F ions (7^/2 = 1.8 h) were selected to demonstrate that the ESR can serve as a high-resolution spectrometer for direct mass measurements of secondary beams.In our experiments a 310 MeV/nucleon ^Ne 10 " 1 " beam was extracted from the SIS with a pulse length of about 1 s and focused onto a 4 g/cm 2 beryllium target placed at the entrance of the FRS (Fig. 1). In one SIS cycle about 2x 10 9 neon particles were extracted. As a result of reaction kinematics and the slowing-down processes in the production target, the selected fragments had a mean en-TARGET FRS PARTICLE IDENTIFICATION DETECTORS (TOF.AE) / START ESR: ELECTRON COOLER PARTICLE DETECTOR FIG. 1. Layout of the new high-energy facilit...

Studies at the border between nuclear and atomic physics: Weak decays of highly charged ions

“…The selection of the atomic charge state is done by optimising the primary projectile energy, target material and its thickness [36]. Secondary beams are separated in flight in the FRS within about 300 ns and are injected into the ESR [37]. By employing the magnetic rigidity analysis the cocktail beams can efficiently be transmitted to the ESR, which has a maximum magnetic rigidity Bρ = 10 Tm [38].…”

“…The first measurement of a pure three-body β + c decay channel was conducted already at the commissioning of the FRS-ESR in 1992. A beam of fully-ionised 19 Ne was stored in the ESR and the decay constant of 19 Ne 10+ was measured [37]. Later, in a dedicated study, the β + c decay rates of fully-ionised 52,53 Fe 26+ nuclei were measured and compared to theoretical expectations [62].…”

Magnetic hyperfine structure of the ground-state doublet in highly charged ionsTh89+,87+229and the Bohr-Weisskopf effect

“…We have taken into account the Bohr-Weisskopf effect and the mixing of the states, and calculated the probabilities of the transitions between the sublevels. Our calculations can be instrumental in experimental studies of the magnetic hyperfine (MHF) interaction in highly ionized atoms investigated in a storage ring of accelerator complex [46][47][48][49].…”

Direct mass measurements of exotic nuclei at the FRS-ESR facility at GSI

“…The investigation of exotic nuclei in a heavy-ion storage and cooler ring opens up a new field of precision experiments. Already the first experiments showed the discovery potential for nuclear structure and astrophysics [2][3][4]. The high charge states strongly affect those decay channels of radioactive ions, which involve the atomic cloud, such as electron conversion and electron capture branches, but also led to the discovery of new decay modes like the beta-decay to bound states of the released electron [5].…”