Beta Decay of Highly Charged Ions

F. Bosch^a), K. Becker^b), T. Faestermann^b), B. Franzke^a), J. Friese^b), H. Geissel^a), F. Heine^b), P. Kienle^b), O. Klepper^a), H.J. Kluge^a), C. Kozhuharov^a), G. Menzel^a), G. Münzenberg^a), F. Nolden^a), H. Reich^a), C. Scheidenberger^a), B. Schlitt^a), M. Steck^a), Th. Stöhlker^a), K. Takahashi^b), E. Wefers^b), T. Winkler^a), K. Zeitelhack^b)
^a)GSI-Darmstadt, Germany; ^b)TU-München, Germany

Why do we investigate Beta decay of HCI?

nucleosynthesis proceeds in hot stellar plasmas mainly via Beta^- decay and "slow" neutron capture (s-process) or via "rapid" neutron capture (r-process in exploding supernovae). At temperatures of about 30 keV (s) or above 100 keV (r-process) the atoms get highly ionized.
the atomic charge state q can significantly alter the Beta lifetime (the Fermifunction depends on q; orbital electron capture (EC) depends on number and orbit of bound electrons).
for HCI a "new" Beta decay mode, "bound beta decay", (Beta _b) occurs with the electron bound in an inner shell (Fig.1, r.h.s.). There the decay energy is enlarged by about the electron binding energy; and the lifetime might alter drastically.

Where can we measure Beta lifetimes of HCI?

in ion storage rings/ion traps beta-unstable ions are preserved at a well-defined charge state for hours mainly due to electron cooling.
at GSI beta-unstable HCI are provided by either the fragmentseparator FRS or (if long-lived) by the synchrotron SIS and injected into the experimental storage ring ESR (Fig.2).

How to determine Beta lifetimes of highly charged ions in the ESR?

for electron-cooled ions (all of them get the same sharp velocity) there is a one-to-one correspondence between revolution frequency f, measured via Schottky noise spectroscopy (Fig.3), and mass-to-charge ratio m/q. Both, Beta⁺ and Beta^- decays change the atomic charge state q by one unit and, therefore trajectory as well as revolution frequency f. The Beta-daughters may be recorded either by particle detectors (Fig. 2) or by Schottky spectroscopy.
Example: Beta⁺lifetime of bare ¹⁹Ne ions, measured by a detector moved into the ring aperture and recording the Beta⁺ daughters ¹⁹F (Fig.4). From a precise lifetime one gets the decay energy, the Fermifunction and the EC/Beta⁺ ratio.

First observation of Beta_b decay [1]

stored, bare ¹⁶³Dy ions decay to hydrogen-like ¹⁶³Ho by Beta_b decay.
As the charge state q remains unchanged and as the mass difference of mother and daughter is very small (decay energy only 50 keV), the daughter ions cannot be resolved by Schottky spectroscopy (Fig.3).
"indirect" detection needed: stripping off the electron by turning on a gas jet , changing therewith charge state q and detecting the bare ¹⁶³Ho ions by either a particle detector (Fig.5) or by Schottky spectroscopy (Fig.6).
result: the half-life of bare dysprosium gets 50 days, whereas neutral dysprosium is absolutely stable.

Detecting Beta_b decay of bare ¹⁸⁷Re: Setting a cosmic clock with HCI [2]

from the abundance ratio of ¹⁸⁷Re and its Beta daughter ¹⁸⁷Os and from the half-life of neutral ¹⁸⁷Re (42 billion years) a lower limit for the age of our Milky Way galaxy – and a fortiori – for the age of the universe of 14 billion years can be derived.
during galactic history, however, ¹⁸⁷Re – as all other nuclides – gets "astrated" into newly originating stars and might be, depending on location in the stellar plasma, highly ionized there. Then it can decay by Beta_b decay to the first excited state of ¹⁸⁷Os (Fig.7). The half-life of bare ¹⁸⁷Re has been determined in the ESR to 33 years only, that is by more than 9 orders of magnitude shorter than in the neutral charge state (Fig.8)

Recalibrated rhenium clock and Hubble constant set constraints for the standard cosmological model

from the half-life of bare ¹⁸⁷Re the half-lives for all charge states q can be reliably calculated. Within well-established models of the chemical evolution of our galaxis "lifestory" of ¹⁸⁷Re can be modeled and the "effective" half-life determined. From it a lower limit of 12 billion years for the age of the universe has been derived.
the 12 billion years are corroborated by almost all "astronomical" clocks like globular clusters. Together with a new lower limit of 63 km/s and Megaparsec for the Hubble constant from the first observation of delta-Cepheids in the Virgo cluster, narrow constraints for the matter density in the universe follow which are in disagreement to the Einstein-de Sitter standard model, a universe with "critical" density (Fig.9).

[1] M. Jung et al., Phys. Rev. Lett. 69, 2164 (1992).
[2] F. Bosch et al., Phys.Rev. Lett. 77, 5190 (1996).