An automated ionization chamber for secondary radioactivity standards

Activity standard and calibrations for 227Th with ingrowing progeny

Thorium-227 was separated from its progeny and standardized for activity by the triple-to-double coincidence ratio (TDCR) method of liquid scintillation counting. Confirmatory liquid scintillation-based measurements were made using efficiency tracing with 3H and live-timed anticoincidence counting (LTAC). The separation time and the efficiency of the separation were confirmed by gamma-ray spectrometry. Calibrations for reentrant pressurized ionization chambers, including commercial radionuclide calibrators, and a well-type NaI(Tl) detector are discussed.

Absolute emission intensities of the gamma rays from the decay of 224Ra and 212Pb progenies and the half-life of the 212 Pb decay

Absolute gamma-ray emission intensities for 36 characteristic gamma rays from the decay of 224Ra, 212Pb, and their progeny were determined by measuring sources calibrated for activity by means of primary methods based on well-defined high-purity germanium (HPGe) detectors at both NIST and NPL. Results from the two laboratories agree with recent data evaluations, except for gamma rays with low emission intensities. The decay schemes have been re-balanced based on the new results. In addition, the half-life for 212Pb was measured using several HPGe detectors, ionization chambers, and a well-type NaI(Tl) detector.

Liquid scintillation efficiencies, gamma-ray emission intensities, and half-life for Gd-153

Gadolinium-153 was standardized for activity by live-timed anticoincidence counting and an ampoule was submitted to the international reference system (SIR). Absolute emission intensities for the main γ rays were determined with calibrated high-purity germanium (HPGe) and lithium-drifted silicon (Si(Li)) detectors. A revised decay scheme is indicated, with no probability of direct electron capture to the 153Eu ground state. Triple-to-double coincidence ratio (TDCR) efficiency curves indicate that the revised decay scheme is consistent with experiment. Half-life measurements agree with a previous NIST determination and show no sensitivity to chemical environment.

Primary standardization of 212Pb activity by liquid scintillation counting

An activity standard for 212Pb in equilibrium with its progeny was realized, based on triple-to-double coincidence ratio (TDCR) liquid scintillation (LS) counting. A Monte Carlo-based approach to estimating uncertainties due to nuclear decay data (branching ratios, beta endpoint energies, γ-ray energies, and conversion coefficients for 212Pb and 208Tl) led to combined standard uncertainties ≤ 0.20 %. Confirmatory primary measurements were made by LS efficiency tracing with tritium and 4παβ(LS)-γ(NaI(Tl)) anticoincidence counting. The standard is discussed in relation to current approaches to 212Pb activity calibration. In particular, potential biases encountered when using inappropriate radionuclide calibrator settings are discussed.

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.

Primary standardization of 224Ra activity by liquid scintillation counting

A standard for activity of ²²⁴Ra in secular equilibrium with its progeny has been developed, based on triple-to-double coincidence ratio (TDCR) liquid scintillation (LS) counting. The standard was confirmed by efficiency tracing and 4παβ(LS)-γ(NaI(Tl)) anticoincidence counting, as well as by 4πγ ionization chamber and NaI(Tl) measurements. Secondary standard ionization chambers were calibrated with an expanded uncertainty of 0.62% (k = 2). Calibration settings were also determined for a 5 mL flame-sealed ampoule on several commercial reentrant ionization chambers (dose calibrators).

Standardization of I-124 by three liquid scintillation-based methods

A solution of ¹²⁴I was standardized for activity by 4πβ(LS)-γ(NaI) live-timed anticoincidence (LTAC) counting, with confirmatory measurements by triple-to-double coincidence ratio (TDCR) and CIEMAT-NIST efficiency tracing (CNET) liquid scintillation counting. The LTAC-based standard was shown to be in agreement (within k = 1 uncertainties) with previous measurements at NIST and elsewhere. Calibration settings for radionuclide calibrators were determined and a discrepancy with literature values, partially due to a calibration methodology dependent upon an erroneous setting for ¹⁸F, was identified and explained.

Standardization of 64Cu activity

The complex decay scheme that makes 64Cu promising as both an imaging and therapeutic agent in medicine also makes the absolute measurement of its activity challenging. The National Institute of Standards and Technology (NIST) has completed a primary activity standardization of a 64CuCl2 solution using the 4πβ(LS)-γ(NaI) live-timed anticoincidence (LTAC) counting method with a combined standard uncertainty of 0.51 %. Two liquid scintillation (LS) counting methods were employed for confirmatory measurements. Secondary measurements were made by high-purity germanium detectors, pressurized ionization chambers (IC), and a well-type NaI(Tl) counter. Agreement between the LTAC-based standard and standards from other laboratories was established via IC calibration factors. Poor agreement between methods and with theoretical IC responses may indicate a need for improved β+/- branching probabilities and a better treatment of β+/- spectra.

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.

Determination of photon emission probability for the main gamma ray and half-life measurements of 64Cu

The National Institute of Standards and Technology (NIST) performed new standardization measurements for 64Cu. As part of this work the photon emission probability for the main gamma-ray line and the half-life were determined using several high-purity germanium (HPGe) detectors. Half-life determinations were also carried out with a NaI(Tl) well counter and two pressurized ionization chambers.

Two determinations of the (223)Ra half-life

Ra-223 is an alpha-emitter that is being used as a bone-seeking radiotherapeutic agent. The relatively large uncertainty on its evaluated half-life (0.26%, Bé et al., 2011) is an impediment to precision activity assays, which often involve measurements by various methods over time spans of days or weeks. We have performed two series of measurements using an ionization chamber (IC) and a NaI(Tl) well counter (γ-wc) to determine new, precise values for the (223)Ra half-life. We have endeavored to realistically assess the uncertainties on the derived half-lives, looking beyond the fit uncertainties to identify uncertainty components acting on multiple timescales. We recovered respective values of 11.447(6)d and 11.445(13)d from the IC and γ-wc measurements. Our values are in accord with the evaluated value of 11.43(3)d, but with smaller combined uncertainties.

Revision of the NIST Standard for (223)Ra: New Measurements and Review of 2008 Data

After discovering a discrepancy in the transfer standard currently being disseminated by the National Institute of Standards and Technology (NIST), we have performed a new primary standardization of the alpha-emitter (223)Ra using Live-timed Anticoincidence Counting (LTAC) and the Triple-to-Double Coincidence Ratio Method (TDCR). Additional confirmatory measurements were made with the CIEMAT-NIST efficiency tracing method (CNET) of liquid scintillation counting, integral γ-ray counting using a NaI(Tl) well counter, and several High Purity Germanium (HPGe) detectors in an attempt to understand the origin of the discrepancy and to provide a correction. The results indicate that a -9.5 % difference exists between activity values obtained using the former transfer standard relative to the new primary standardization. During one of the experiments, a 2 % difference in activity was observed between dilutions of the (223)Ra master solution prepared using the composition used in the original standardization and those prepared using 1 mol·L(-1) HCl. This effect appeared to be dependent on the number of dilutions or the total dilution factor to the master solution, but the magnitude was not reproducible. A new calibration factor ("K-value") has been determined for the NIST Secondary Standard Ionization Chamber (IC "A"), thereby correcting the discrepancy between the primary and secondary standards.

A Review of NIST Primary Activity Standards for (18)F: 1982 to 2013

The new NIST activity standardization for (18)F, described in 2014 in Applied Radiation and Isotopes (v. 85, p. 77), differs from results obtained between 1998 and 2008 by 4 %. The new results are considered to be very reliable; they are based on a battery of robust primary measurement techniques and bring the NIST standard into accord with other national metrology institutes. This paper reviews all ten (18)F activity standardizations performed at NIST from 1982 to 2013, with a focus on experimental variables that might account for discrepancies. We have identified many possible sources of measurement bias and eliminated most of them, but we have not adequately accounted for the 1998-2008 results.