First observation of excited states in ¹⁷⁵Hg

D. O'Donnell, J. Simpson and M. Labiche

The Daresbury Nuclear Physics Group, in collaboration with groups in Liverpool, Stockholm, Sweden and Jyvaskyla, Finland,

has led experimental campaigns in which some of the most exotic atomic nuclei known to exist have been explored.

One such nucleus, the subject of a recent study performed at Jyvaskyla, is mercury-175 (¹⁷⁵Hg). Possessing 21 fewer neutrons

than the lightest stable mercury isotope (¹⁹⁶Hg), this nucleus represents the lightest odd-mass mercury isotope in which any

information about excited states is available. The knowledge of such states is crucial for the theoretical understanding of atomic nuclei,

and the forces which bind the constituent protons and neutrons, at the extremes of nature.

This work also marks the first occasion in which these excited states have been observed.

Once created, the excited states decayed via the emission of g rays which were detected using a high-efficiency g-ray spectrometer array.

The production of ¹⁷⁵Hg nuclei is so low, compared with other reaction products that special techniques are required in

order to highlight the interesting g rays from the background of other decays.

The characteristic a-decay of the nucleus of interest is used to unambiguously assign observed g-ray decays to a

specific reaction product. In this way the following spectra were obtained:

The observation of 'delayed' g rays i.e. half-life of the order of microseconds or longer suggests one of the excited states of ¹⁷⁵Hg is isomeric

(half-life > 1 nanosecond). It was determined, by a direct internal conversion measurement, that the isomer decays via the emission of an

M2 414 keV transition and the decay has a half-life of 0.34 microseconds.

This g ray was also found to be in coincidence with a delayed M1 decay of 80 keV.

Using the spectra shown above it was possible to build a level scheme representing the decay of excited states of ¹⁷⁵Hg.

It has been determined that the nucleus ¹⁷⁵Hg exhibits characteristics of different shapes at different energies, much like many other mercury nuclei.

The phenomenon in which atomic nuclei adopt different shapes at different excitation energies is commonly known as shape coexistence.

At low energy, the ¹⁷⁵Hg nuclei are observed to be near-spherical, with states based on specific single-particle configurations.

However, at higher energies the behaviour is consistent with the nucleus being oblate (shaped like a pumpkin) in shape.

In the heavier odd-mass mercury isotopes, states resulting from a third shape, prolate (rugby ball shaped), have been observed at higher energies [1,2,3].

The presence of prolate states in ¹⁷⁵Hg could not be confirmed in the present work and will require further experimental investigation to higher spins.

[1] Melerangi et al., Physical Review C 68, 041301(R) (2003)

[2] Jenkins et al., Physical Review C 66 011301(R) (2002)

[3] Kondev et al., Physics Letters B 528 (2002) 221

For further details of this work please consult the published paper: First observation of excited states in ¹⁷⁵Hg₉₅.