Decay scheme of erbium-168 : a coincidence study of the electron capture decay of thulium-168
Herath, Lawrence Walter
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The electron capture decay of ¹⁶⁸Tm to ¹⁶⁸Er is examined by using a 20 cm³ Ge(Li) detector with a full width at half maximum resolution of 2.7 to 3.6 keV. Energy levels are determined by the gamma-ray transitions between the energy levels. Cascade gamma-ray transitions are examined using coincidence techniques with a 3" x 3" Nal detector. A new energy level in ¹⁶⁸Tm decay at 1403 keV is established with the placement of the 1324 keV gamma ray. The 853 keV gamma ray and its placement recently reported is confirmed by coincidence data. The 1489 keV gamma ray is also seen but is not placed in the decay scheme. As a result of coincidence data, the 1167 keV gamma ray is placed in the decay scheme and the 1351 keV line is placed in two locations in the decay scheme. Weak gamma rays at 74 and 75 keV are detected by choosing the appropriate coincidence gates demonstrating a possible method of increasing the detector system sensitivity. The possibility that the two unresolved levels at 1541 keV with spins and parity of 3⁻ and 4⁻ are both populated in ¹⁶⁸Tm decay is discussed on the basis of inconsistent gamma-ray intensities. The possibility of two unresolved energy levels at 1431 keV is discussed on the basis of average resonance neutron capture data and an energy discrepancy. The unresolved problem of possible equal energy transitions between the evenly spaced 1193/ 1094/ 994/ and 896 keV energy levels is also discussed.