physics.fsu.edu | Experimental Nuclear Physics

THE MOST UNIQUE ISOMER

Dr. Samuel L. Tabor

A series of experiments has teased out such amazing properties of a long-lived, high-lying isomer in ⁹⁴Ag, that it must be the most unique in the entire Segre chart. First ⁹⁴Ag, with 47 protons and 47 neutrons, lies near the proton drip line and and only 6 particles away from the “holy grail” of doubly magic ¹⁰⁰Sn. The 0⁺ ground state decays by superallowed Fermi beta decay with a short half life of 29 ms. It also has a low lying 7⁺ isomer whose beta-decay half life is over an order of magnitude longer.

But the most interesting and unique isomer in ⁹⁴Ag is lies at about 6.7 MeV with a half life of about 0.6 s. Such a long lifetime for such a high isomer is itself surprising, but not half as surprising as the γ spectrum following its β decay. The spectrum looks like one from a high-spin, in-beam experiment, with γ lines extending up to the known 20⁺ level at 7700 keV in ⁹⁴Pd [1]. This observation and others lead to a tentative assignment of 21⁺ to the 6.7 MeV isomer in ⁹⁴Ag, making it the highest spin known for a spin-gap isomer. The isomeric nature of this state comes from its fully aligned configuration, ie, 21 is the highest spin possible by combining 3 g_9/2 proton holes with 3 g_9/2 neutron holes. Perhaps it is not so surprising for such a high-lying state near the proton drip line that some of the β decay strength extends to proton-unbound levels in ⁹⁴Pd which then decay by proton emission to ⁹³Rh, since β-delayed proton decay has also been observed for the lower 7⁺ isomer [2].

Before proceeding to even more remarkable properties of this isomer, it is worth pointing out that ⁹⁴Ag is exceedingly difficult to make with stable beams and targets. The combined cross section for producing the two isomers in the most favorable reaction (⁴⁰Ca + ⁵⁸Ni) is only 650 nb. So far the isomers have been studied only by ISOL separation at GSI following their production in the heavy-ion reaction which favors high-spin states. After magnetic separation, the ⁹⁴Ag beam impinged on a moving tape in a low-background area, surrounded by an array of segmented Si detectors and a close packed array of Clover and Cluster Ge detectors.

A careful analysis of the proton spectrum in the Si detectors in coincidence with pairs of known γ lines in ⁹³Pd has revealed clear evidence for direct proton decay of the 21⁺ isomer [3] with a 4% branch. Note that direct proton decay is distinct from β-delayed proton decay. The former leads to ⁹³Pd and the latter, to ⁹³Rh. The two nuclei have quite different γ lines. But that is not all! An even more detailed look at pairs of protons in coincidence with pairs of known γ lines in ⁹²Rh in this rich data set led to the discovery of correlated 2-proton decay [4]. Even though the two proton decay branching ratio from the 21⁺ isomer is only 0.5%, it is the first time that 1 and 2 proton decay have been observed to compete (and with β and β-delated proton decay). The multiple decay modes of ⁹⁴Ag are shown in the figure.

Many questions remain after this exciting result. The statistics are low and it is important to repeat the experiment. Unfortunately the ISOL facility at GSI is no longer available. The collaboration is planning to look at the 21⁺ decays at the JYFL facility in Finland. The extrapolated masses in the Audi and Wapstra compilation imply a 2p decay energy lower than what was observed, so precision mass measurements are now planned at both JYFL and GSI to resolve this issue. Also, although 2 isomers have been inferred in ⁹⁴Ag, not a single γ decay line is known. We are proposing to look for the γ lines above the 7⁺ and 21⁺ isomers at ANL using recoil-decay tagging. This is not an easy task since only about one ⁹⁴Ag nucleus is made in a million reactions. It will require the power of GAMMASPHERE and the Fragment Mass Analyzer.

References

[1] C. Plettner et al., Nucl. Phys., A733, 20 (2004).

[2] I. Mukha et al., Phys. Rev. C 70, 044311 (2004).

[3] I. Mukha et al., Phys. Rev. Lett. 70, 044311 (2004).

[4] I. Mukha et al., Nature 439, 298 (2006).

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	FSU Experimental Nuclear Physics Main			THE MOST UNIQUE ISOMER Dr. Samuel L. Tabor A series of experiments has teased out such amazing properties of a long-lived, high-lying isomer in ⁹⁴Ag, that it must be the most unique in the entire Segre chart. First ⁹⁴Ag, with 47 protons and 47 neutrons, lies near the proton drip line and and only 6 particles away from the “holy grail” of doubly magic ¹⁰⁰Sn. The 0⁺ ground state decays by superallowed Fermi beta decay with a short half life of 29 ms. It also has a low lying 7⁺ isomer whose beta-decay half life is over an order of magnitude longer. But the most interesting and unique isomer in ⁹⁴Ag is lies at about 6.7 MeV with a half life of about 0.6 s. Such a long lifetime for such a high isomer is itself surprising, but not half as surprising as the γ spectrum following its β decay. The spectrum looks like one from a high-spin, in-beam experiment, with γ lines extending up to the known 20⁺ level at 7700 keV in ⁹⁴Pd [1]. This observation and others lead to a tentative assignment of 21⁺ to the 6.7 MeV isomer in ⁹⁴Ag, making it the highest spin known for a spin-gap isomer. The isomeric nature of this state comes from its fully aligned configuration, ie, 21 is the highest spin possible by combining 3 g_9/2 proton holes with 3 g_9/2 neutron holes. Perhaps it is not so surprising for such a high-lying state near the proton drip line that some of the β decay strength extends to proton-unbound levels in ⁹⁴Pd which then decay by proton emission to ⁹³Rh, since β-delayed proton decay has also been observed for the lower 7⁺ isomer [2]. Before proceeding to even more remarkable properties of this isomer, it is worth pointing out that ⁹⁴Ag is exceedingly difficult to make with stable beams and targets. The combined cross section for producing the two isomers in the most favorable reaction (⁴⁰Ca + ⁵⁸Ni) is only 650 nb. So far the isomers have been studied only by ISOL separation at GSI following their production in the heavy-ion reaction which favors high-spin states. After magnetic separation, the ⁹⁴Ag beam impinged on a moving tape in a low-background area, surrounded by an array of segmented Si detectors and a close packed array of Clover and Cluster Ge detectors. A careful analysis of the proton spectrum in the Si detectors in coincidence with pairs of known γ lines in ⁹³Pd has revealed clear evidence for direct proton decay of the 21⁺ isomer [3] with a 4% branch. Note that direct proton decay is distinct from β-delayed proton decay. The former leads to ⁹³Pd and the latter, to ⁹³Rh. The two nuclei have quite different γ lines. But that is not all! An even more detailed look at pairs of protons in coincidence with pairs of known γ lines in ⁹²Rh in this rich data set led to the discovery of correlated 2-proton decay [4]. Even though the two proton decay branching ratio from the 21⁺ isomer is only 0.5%, it is the first time that 1 and 2 proton decay have been observed to compete (and with β and β-delated proton decay). The multiple decay modes of ⁹⁴Ag are shown in the figure. Many questions remain after this exciting result. The statistics are low and it is important to repeat the experiment. Unfortunately the ISOL facility at GSI is no longer available. The collaboration is planning to look at the 21⁺ decays at the JYFL facility in Finland. The extrapolated masses in the Audi and Wapstra compilation imply a 2p decay energy lower than what was observed, so precision mass measurements are now planned at both JYFL and GSI to resolve this issue. Also, although 2 isomers have been inferred in ⁹⁴Ag, not a single γ decay line is known. We are proposing to look for the γ lines above the 7⁺ and 21⁺ isomers at ANL using recoil-decay tagging. This is not an easy task since only about one ⁹⁴Ag nucleus is made in a million reactions. It will require the power of GAMMASPHERE and the Fragment Mass Analyzer. References [1] C. Plettner et al., Nucl. Phys., A733, 20 (2004). [2] I. Mukha et al., Phys. Rev. C 70, 044311 (2004). [3] I. Mukha et al., Phys. Rev. Lett. 70, 044311 (2004). [4] I. Mukha et al., Nature 439, 298 (2006). Back to Dr. Tabor's Research Menu