Precise Efficiency Calibration of an HPGe Detector Using the Decay of 180m Hf

Superallowed 0+ → 0+ nuclear beta decays provide both the best test of the Conserved Vector Current (CVC) hypothesis and, together with the muon lifetime, the most accurate value for the up-down quark-mixing matrix element, Vud , of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. This matrix should be unitary, and experimental verification of that expectation constitutes an important test of the Standard Model. In aiming for a definitive test of CKM unitarity we have mounted a program at the Texas A and M University to establish (or eliminate) the discrepancy with unitarity. One correction accounts for isospin symmetry breaking and its accuracy can be tested by measurements of the ft-values of Tz = - 1 superallowed emitters (e.g. 22 Mg and 30 S) to a precision of about ±0.1%. A requirement for these measurements is a detector whose detection efficiency is known to the same precision. However, calibration of a detector's efficiency to this level of precision is extremely challenging since very few sources provide γ-rays whose intensities (relative or absolute) are known to better than ±0.5%. The isomer 180m Hf (t 1/2 = 5.5 h) provides a very precise γ-ray calibration source in the 90 to 330 keV energy range. The decay of 180m Hf to the 180 Hf ground state includes a cascade of three consecutive E2 γ-ray transitions of energies 93.3, 215.2 and 332.3 keV with no other feeding of the intermediate states. This provides a uniquely well-known calibration standard since the relative γ-ray intensities emitted are dependent only on the calculated E2 conversion coefficients. The 180m Hf isomer was produced by irradiation of a 0.91 mg sample of HfO2 isotopically enriched to 87% in179 Hf, at the TRIGA reactor in the TAMU Nuclear Science Center. In order to minimise the self-absorption of γ-rays in Hf we required a thin source that was prepared following a procedure described by Kellog and Norman. The activated HfO2sample was dissolved in 0.50 ml of hot 48% HF acid to produce HfF4 , which remained in solution. A 0.03--0.04 ml drop of the solution was deposited on a 50μg/cm2 carbon foil which had been premounted on a 75-μm-t hick Mylar foil backing and coated with insulin. The initial activity of this source was about 1 μCi. (authors)