Half-life measurements at the PTB

Ra-224 activity, half-life, and 241 keV gamma ray absolute emission intensity: A NIST-NPL bilateral comparison

The national metrology institutes for the United Kingdom (UK) and the United States of America (USA) have compared activity standards for 224Ra, an α-particle emitter of interest as the basis for therapeutic radiopharmaceuticals. Solutions of 224RaCl2 were assayed by absolute methods, including digital coincidence counting and triple-to-double coincidence ratio liquid scintillation counting. Ionization chamber and high-purity germanium (HPGe) γ-ray spectrometry calibrations were compared; further, a solution was shipped between laboratories for a direct comparison by HPGe spectrometry. New determinations of the absolute emission intensity for the 241 keV γ ray (Iγ = 4.011(16) per 100 disintegrations of 224Ra) and of the 224Ra half-life (T1/2 = 3.6313(14) d) are presented and discussed in the context of previous measurements and evaluations.

Two determinations of the Ge-68 half-life

In nuclear medicine, 68Ge is used to generate 68Ga for imaging by positron emission tomography (PET) and sealed sources containing 68Ge/68Ga in equilibrium have been adopted as long-lived calibration surrogates for the more common PET nuclide, 18F. We prepared several 68Ge sources for measurement on a NaI(Tl) well counter and a pressurized ionization chamber, following their decay for 110 weeks (≈ 2.8 half-lives). We determined values for the 68Ge half-life of T1/2 = 271.14(15) d and T1/2 = 271.07(12) d from the NaI(Tl) well counter and ionization chamber measurements, respectively. These are in accord with the current Decay Data Evaluation Project (DDEP) recommended value of T1/2 = 270.95(26) d and we discuss the expected impact of our measurements on this value.

57Co half-life determination

The National Laboratory for Metrology of Ionizing Radiation (LNMRI)/Brazil participated in the last (57)Co activity measurement comparison organized by IAEA, in 2008. This was made in order to realize primary standardization of this radionuclide and to reduce the measuring uncertainties in the laboratory. Additionally, one sample with 2.6g was taken from the IAEA master solution of (57)Co and was prepared to be used in the measurement of the radionuclide's half-life in an ionization chamber system. The (57)Co half-life was measured to be 271.82 (17) days.

Half-life measurement of 124Sb

The half-life of (124)Sb was determined experimentally by following the decay of a source from a radiopure solution with a Centronic IG12 ionisation chamber. Thousands of measurements were performed over a period of 358 days, i.e. about six half-life periods. However, the data analysis was restricted to the first 221 days, in order to limit the dominant uncertainty component associated with the hypothetical possibility of a systematic error on background subtraction. The resulting value for the (124)Sb half-life, 60.212 (11) days, is found to be in very good agreement with published values, but carries a lower uncertainty. Major uncertainty contributions pertain to possible systematic errors in background correction, long-term changes in source-detector geometry and medium- and long-term instability of the instrument. Additional measurements were performed with a high-purity germanium detector to confirm the above value.

Standardization and nuclear decay data of 65Zn

Standardization by means of 4pibeta-gamma coincidence counting was applied to calibrate a 65Zn solution within the scope of EUROMET project no. 721. The activity result was combined with gamma-ray spectrometry measurements to investigate the photon emission probabilities. The half-life of 65Zn was measured with improved accuracy by means of 4pi ionization chamber measuring systems that were also used for secondary activity determination. In addition, we tested a new secondary standardization procedure by means of liquid scintillation counting.

Standardization and decay data determinations of 125I, 54Mn and 203Hg

The absolute 4pi-betagamma coincidence counting has been used to measure the activity concentrations of 54Mn and 203Hg, and the Sum-Peak method was used for 125I. 54Mn and 125I radionuclides have been part of international key-comparisons coordinated by the Bureau International des Poids et Measures (BIPM) in 2003/2004, while 203Hg is part of the traceability programme of the National Laboratory for Ionizing Radiation Metrology (LNMRI)/Brazil. Three different detectors were used for the Sum-Peak method: 3''x3'' NaI(Tl) and 5''x5'' well NaI(Tl) scintillation detectors and a planar germanium detector. Direct measurements were made of the photon emission probabilities of the 35.5-, 834.8- and 279.2-keV gamma-rays of 125I, 54Mn and 203Hg to give values of (0.0667+/-0.0014), (0.9997+/-0.0055) and (0.8161+/-0.0005), respectively. The half-lives of 203Hg and 54Mn were also determined by means of a 4pigamma ionization chamber (203Hg) and by the reference source method using a HPGe detector (54Mn) to give values of (46.639+/-0.023) days and (312.1+/-0.9) days, respectively.

Half-life determination of 88Y and 89Sr

Half-life measurements have been carried out at LNHB for 88Y using a 4pi gamma-ionisation chamber and for 89Sr using a proportional counter. The determined half-life values and associated standard uncertainties are 106.63 +/- 0.05 d for 88Y and 50.65 +/- 0.05 d for 89Sr, being consistent with relevant values reported in literature. Based on the present results and relevant literature values revised recommended half-life values and associated standard uncertainties are proposed, viz. 106.626 +/- 0.021 d for 88Y and 50.57 +/- 0.03 d for 89Sr.

Activity standardization by liquid scintillation counting and half-life measurements of

Radioactive 90Y was standardized by means of 4piBeta liquid scintillation efficiency tracing with a two-phototube counting system. The efficiencies of 90Y and the tracer 3H are related by the CIEMAT/NIST method. The dependence of the calculated efficiency on various shape factors and the importance of accurately measuring the radionuclide impurity of 90Sr are discussed. The half-life of 90Y was measured with improved accuracy by means of a liquid scintillation counting and a 4pi ionization chamber measuring system, and an average value T1/2 = 64.053(20) h was found.

Half-life measurements with ionization chambers—A study of systematic effects and results

Accurate half-lives have been determined at the Physikalisch-Technische Bundesanstalt (PTB) using ionization-chamber measuring systems to assure the quality of radioactivity standards over the period of their use as detector calibrants or other purposes such as nuclear medicine and environmental monitoring. Some long-lived radionuclides have been measured for up to 28 years: 85Kr, 90Sr, 108Agm, 133Ba, 152Eu, 154Eu, and more recently 65Zn, 106Ru, 109Cd and 137Cs; short-lived radionuclides have also been studied such as: 18F, 67Ga, 81Rb, 99Mo, 99Tcm, 103Pd, 111In, 123I, 131I, 153Sm, 169Er, 177Lu, 186Re, 188Re, 201Tl, 222Rn and 224Ra. Relative uncertainties of half-life values of the order of 5 x 10(-4) were obtained. These values are compared to the results of other measurements and evaluations, and the limiting systematic effects are discussed.

Experimental verification of the half-life of 65Zn

The half-life of 65Zn was determined experimentally and compared with literature values. Two ampoules of a 65Zn solution were measured at regular intervals in two ionisation chambers over a period corresponding to nearly two half-lives. The result of 243.8+/-0.3 days is in agreement with the value of 244.0+/-0.2 days obtained by De Roost et al. (Z. Phys. 250 (1975) 395) at the same institute (previously called CBNM) in 1972. Moreover, this half-life is supported by most of the other measurements reported in the literature.

Absolute determination of activity per mass and half-life measurements of 152Eu

A solution of 152Eu was absolutely standardized by the 4pi beta (PPC)-gamma (HPGe) coincidence counting method as part of an intercomparison program coordinated by Bureau International des Poids et Mesures. The measured activity has been used to determine the calibration factor of the LNMRI reference ionization chamber for 152Eu. The half-life of 152Eu was also determined by means of a 4pi gamma ionization chamber to be (4963.6+/-15.3) days, and compared with others published values.

Standardization and half-life measurement of antimony-124

The half-life of 124Sb has been measured using two solutions prepared from 124Sb obtained from the nuclear reactions: 124Sn(p,n) 124Sb (cyclotron) and 123Sb(n,gamma) 124Sb (reactor). These solutions were standardized by the 4pibeta-gamma-coincidence extrapolation method with a total uncertainty of 0.12%. A 124Sb half-life of (60.11+/-0.07) days was measured.

Search for an optimum approach to the evaluation of data of varying consistency: half-live evaluations for 3H, 35S, 55Fe, 99Mo and 111In

A common procedure is proposed for the evaluation of both consistent and discrepant data in which adjustments are made to the uncertainties in the measured data that are dependent on the chi2 values of the data set. A computer program EVINEW has been developed and applied to the evaluation of 3H, 3S, 55Fe, 99Mo and 111In half-lives to give the following values: 12.32(2) yr, 87.32(16) d, 2.732(14) yr, 2.7480(9) d and 2.8047(4) d, respectively.