Toward new frontiers : Encounter and synergy of state-of-the-art astronomical detectors and exotic quantum beams

Program D01-5	Search for nuclear two-photon decay in highly charged ions
Principal Investigator	YAMAGUCHI, Takayuki (Saitama University)

The present study aims to study nuclear two-photon decay by taking advantage of the unique feature of stored highly charged ions and to establish a new technique to investigate nuclear structure beyond the conventional gamma-ray spectroscopy. In a pilot experiment, we will address the low-lying 0⁺ state of fully-ionized ⁷²Ge³²⁺.

An excited state of a nucleus cannot undergo gamma decay when the spin-parity is 0⁺ in both the ground and excited states due to the angular momentum conservation law. Instead, an internal conversion electron (ICE) decay occurs in the neutral atom. Alternatively, an internal pair creation (IPC) occurs if the excitation energy is larger than the sum of the rest masses of electron and positron. However, when such dominant, first-order electromagnetic processes are forbidden, a rare decay which simultaneously emits two photons can appear as the second-order effect of quantum mechanics. So far, the two-photon decay studies have been reported only for three nuclides. The measurements in neutral atoms, however, suffered from a large background of electrons and annihilation gamma-rays, often suggesting controversial results. By contrast, a relativistic-energy heavy-ion storage ring provides a unique experimental condition where fully-stripped ions can be stored for an extended period of time and the background due to bound electrons is just absent. Thus, a clear identification of the nuclear two-photon decay can be realized.


Fig. 1. Experimental Storage Ring (ESR) at GSI
The experiment will be conducted at the accelerator facility of GSI. The secondary ion beam of fully-stripped ⁷²Ge³²⁺ is produced via the projectile fragmentation of ⁷⁸Kr primary beam accelerated to 460 MeV/u. Contaminating particles caused by the nuclear reaction are removed by the magnetic rigidity selection. Produced ⁷²Ge³²⁺ beam is expected to contain the low-lying state ⁷²Ge^* (spin 0⁺, 691 keV). It is injected and stored in the experimental storage ring ESR, see Fig. 1. The ion optical mode of the ESR is tuned to be precisely isochronous such that the revolution frequency of the stored ions is proportional to their mass-to-charge ratio and is independent from their momentum spread. Thanks to excellent mass resolving power of ∼10⁵, the ground and excited states are in-flight identified by a non-destructive resonant Schottky detector. The shift of the revolution frequency of the excited state to that of the ground state during storage shows a clear evidence of the two-photon decay of ⁷²Ge^*.

The storage ring, therefore, will be a new tool for exploring the structure of low-lying 0⁺ states, complementarily to the gamma-ray and electron spectroscopy techniques. Accurate knowledge of low-lying 0⁺ levels will contribute to the elucidation of shape coexistence appearing in exotic nuclei. The rare two-photon decay will also contribute to precision testing of quantum theory.

The present research will be performed as part of the ILIMA collaboration at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt (Germany) in the context of the FAIR (Facility for Antiproton and Ion Research) Phase-0 program.

Members

Reference Materials

• F. Bosch, Yu. A. Litvinov, T. Stöhlker, “Nuclear physics with unstable ions at storage rings,” Prog. Part. Nucl. Phys. 73, 84 (2013).