Simple atom, extreme nucleus - laser trapping and probing of (sup 6)He and (sup 8)He.
Helium-8 ({sup 8}He) is the most neutron-rich helium isotope that can be synthesized on Earth: it consists of two protons and six neutrons, and remains stable for an average of 0.2 seconds. Unlike the abundant helium isotope, {sup 4}He, which has the two neutrons packed closely with two protons, the additional neutrons in {sup 8}He form a 'halo' around a compact, {sup 4}He-like core. A similar neutron-halo structure also exists for {sup 6}He (half-life = 0.8 s). Because of their intriguing properties, {sup 6}He and {sup 8}He have the potential to reveal new aspects of the fundamental forces among the constituent nucleons. We have recently succeeded in laser trapping and cooling these exotic helium isotopes, and have performed precision laser spectroscopy on individual trapped atoms. Based on atomic frequency differences measured along the isotope chain {sup 4}He-{sup 6}He-{sup 8}He, the nuclear charge radius of {sup 8}He has been determined for the first time to be 1.96 fm and that of {sup 6}He to be 2.07 fm. The result shows that the charge radius of {sup 8}He is smaller than that of {sup 6}He, but notably larger than that of {sup 4}He (1.68 fm). This may be understood by noting that the four extra neutrons are more evenly distributed around the {sup 4}He core than the extra neutron pair in {sup 6}He as illustrated in Figure 1. The result is in good agreement with recent predictions of state-of-the-art nuclear structure theories, and provides a critical test of the present understanding of these loosely bound halo nuclei. The {sup 6}He measurement was first performed at the ATLAS accelerator facility of Argonne National Laboratory in 2004. An improved measurement on {sup 6}He and the first measurement on {sup 8}He were performed at GANIL in 2007.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC02-06CH11357
- OSTI ID:
- 992391
- Report Number(s):
- ANL/PHY/JA-62741; TRN: US1007731
- Journal Information:
- Nucl. Phys. News, Vol. 19, Issue 1 ; Jan. 2009
- Country of Publication:
- United States
- Language:
- ENGLISH
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