Abstract
211Po ions in the ground and isomeric states were produced via 238U projectile fragmentation at 1000 MeV/u. The 211Po ions were spatially separated in flight from the primary beam and other reaction products by the fragment separator FRS. The ions were energy-bunched, slowed-down and thermalized in a gas-filled cryogenic stopping cell (CSC). They were then extracted from the CSC and injected into a high-resolution multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS). The excitation energy of the isomer and, for the first time, the isomeric-to-ground state ratio were determined from the measured mass spectrum. In the subsequent experimental step, the isomers were spatially separated from the ions in the ground state by an ion deflector and finally collected with a silicon detector for decay spectroscopy. This pioneering experimental result opens up unique perspectives for isomer-resolved studies. With this versatile experimental method new isomers with half-lives longer than a few milliseconds can be discovered and their decay properties can be measured with highest sensitivity and selectivity. These experiments can be extended to studies with isomeric beams in nuclear reactions.
Highlights
Isomers are excited metastable states of nuclei with relatively long half-lives, due to hindrance in their decay modes [1]
The excitation energy, i.e., the mass difference, of the isomeric state could be determined for the first time using mass spectrometry; it is (1472 ± 120) keV, which is in good agreement with the literature value of (1462 ± 5) keV [35]
Ions of the nuclide 211Po in the ground and isomeric states were produced by projectile fragmentation at 1000 MeV/u and separated in flight with the FRS. 211gPo and 211mPo ions were resolved in the mass spectrum measured with the MR-TOF-MS
Summary
Isomers are excited metastable states of nuclei with relatively long half-lives, due to hindrance in their decay modes [1]. Experimental methods employing pure isomeric beams can reveal new nuclear properties Such experiments can be isomerresolved decay spectroscopy or investigations via nuclear reactions with isomers. Multiple-reflection time-of-flight mass spectrometers (MR-TOFMS) [20,21] have recently been developed into powerful tools for research with exotic nuclei [22,23,24] They are mass spectrometers, and in combination with a Bradbury–Nielsen Gate (BNG) [25,26], a fast electrostatic ion deflector, they can be employed as high-resolution spatial mass separators [27]. Because of their high mass resolving power, measurement speed, sensitivity and broadband characteristics, they have recently been proposed for efficient experiments with isomers [21]. In this work, such pioneering measurements have been exemplarily performed with 211Po ions for the first time
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