Gamma-ray spectroscopy in Sn isotopes

Excited states in ¹¹⁰Sn and ¹¹¹Sn nuclei have been investigated using in-beam γ-ray spectroscopic methods. An ¹⁶O beam with energy of 60-80 MeV was used to bombard a ⁹⁸Mo target. On the basis of the relative excitation functions, γ -ray angular distributions, γ-γ and γ-ray multiplicity and total energy data the level schemes of ¹¹⁰Sn and ¹¹¹Sn have been reinvestigated and extended up to E_exc~11.5 MeV, I=24ħ, and E_exc~11.1 MeV and I=(51/2)ħ, respectively. An extension of the intruder, g.s. and negative parity bands in ¹¹⁰Sn in low-spin region as well as 5 new band-like structures are proposed. In the ¹¹¹Sn nucleus an intruder band based on 23/2^- state has been reinvestigated and the extension of the g.s. band and second negative parity band is given. An evidence for neutron-core coupling in ¹¹¹Sn is found.

In this experiments OSIRIS-II was equipped with 10 HPGe Compton suppressed spectrometers combined with 48-element BGO multiplicity filter. The HPGe's were surrounding the filter, about 17 cm from the target and the BGO crystals served as collimators for the Ge detectors. Events were accepted by the data acquisition system if at least two coincident γ rays were detected in the Ge detectors. A total amount of 8x10⁷γγ-coincidence events was recorded in the coincidence experiment. The compound nucleus ¹¹⁴Sn was favouring the evaporation of neutrons, hence the main reaction products were ¹¹⁰Sn (4n), ¹¹¹Sn (3n) and ¹¹²Sn (2n).
The main reaction channels obtained in ¹⁶O + ⁹⁸Mo reaction at 80MeV beam energy are shown below. This γ spectrum is gated by multiplicity from 7 to 22. Various colors correspond to different reaction products. For ¹¹⁰Sn it's green, the red for ¹¹¹Sn, blue for ¹¹²Sn and violet for ¹¹¹In.

These experiments have confirmed levels reported previously for ¹¹⁰Sn, ¹¹¹Sn and ¹¹²Sn [1 2, 5] and several new levels were identified. In the level scheme of ¹¹⁰Sn we proposed four new side bands (including M1-transition band). Moreover, the negative parity band was extended. For ¹¹¹Sn two bands were extended, too.
The partial level schemes of ¹¹⁰Sn, ¹¹¹Sn and ¹¹²Sn presenting the intruder bands are shown below.

These are positive parity bands in ¹¹⁰Sn and ¹¹²Sn, and negative parity band in ¹¹¹Sn.
The ordering of transitions is based on relative γ-ray intensities and coincidence relationships. The transition intensities were determined using ESCL8R program [4]. Analysis of these data enabled a detailed level scheme to be established for the lower-spin states in intruder band (below the 5228 keV state). The level having an excitation energy of 5018 keV, intepreted as I^π=10⁺ state was observed for the first time in this work. The new levels proposed for ¹¹⁰Sn are in agreement with the level systematics of corresponding states in neighbouring even-Sn nuclei (see on figure below).

These data confirmed that the level energies of states in the intruder band increase with decreasing neutron number [3].

References

1. D.R. LaFosse et al., Phys. Rev. C51, R2876 (1995)
   
2. H. Harada et al., Phys. Let. B207, 17 (1988)
   
3. S. Juutinen et al., Nucl. Phys. A617, 74 (1997)
   
4. D.C. Radford, Nucl. Inst. Meth. A361, 290 (1995)
   
5. D.A. Viggars et al., Phys. Rev. C36, 1006 (1987)
   

Further information on experiment, data analysis and results are available here.(as a PDF file)

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