Toward a New Primary Standardization of Radionuclide Massic Activity Using Microcalorimetry and Quantitative Milligram-Scale Samples.

We present a new paradigm for the primary standardization of radionuclide activity per mass of solution (Bq/g). Two key enabling capabilities are 4π decay-energy spectrometry using chip-scale sub-Kelvin microcalorimeters and direct realization of mass by gravimetric inkjet dispensing using an electrostatic force balance. In contrast to traditional traceability, which typically relies on chemical separation of single-radionuclide samples, 4π integral counting, and additional spectrometry methods to verify purity, the system described here has both 4π counting efficiency and spectroscopic resolution sufficient to identify multiple radionuclides in the same sample at once. This enables primary standardization of activity concentrations of mixed-radionuclide samples. A major benefit of this capability, beyond metrology, is in assay of environmental and forensics samples, for which the quantification of multiplenuclide samples can be achieved where presently inhibited by interferences. This can be achieved without the need for chemical separations or efficiency tracers, thereby vastly reducing time, radioactive waste, and resulting measurement uncertainty.

Natural Uranium Radioactivity Solution Standard: SRM 4321d.

A new natural uranium solution standard has been produced and will be disseminated by the National Institute of Standards and Technology (NIST) as Standard Reference Material 4321d. The standard is certified for the massic activities of 234U, 235U, and 238U in solution, and it is based on isotopic mass data for the metallic Certified Reference Material (CRM) 112-A (originally issued as SRM 960) that was obtained from THE U.S. Department of Energy, New Brunswick Laboratory. The metallic CRM was chemically cleaned, dissolved, and gravimetrically diluted to prepare a master solution, which was quantitatively dispensed into 5 mL aliquots that were contained within flame-sealed glass ampoules for each SRM unit. Homogeneity among SRM units, verifying solution homogeneity, was substantiated by photonic-emission integral counting with a NaI(Tl) well counter. Confirmatory measurements were performed by liquid scintillation counting for the total massic activity, and by isotope dilution α spectrometry for the 234U and 238U massic activities.

Identification of phase boundaries in surfactant solutions via Compton spectrum quenching.

The critical micelle concentration and the phase boundary between isolated surfactant molecules and aggregates are probed via fluorescence spectroscopy and a Compton spectrum quenching technique for aqueous and toluenic solutions of Triton X-100 (TX-100). The internal fluorophore of TX-100 provides a convenient probe for the fluorescence measurements, and the appearance of redder bands in the fluorescence spectra and their relationship with aggregation (clustering of TX-100) phenomena is addressed. The Compton spectrum quenching approach makes use of quench indicating parameters (QIPs) commonly measured in liquid scintillation counting experiments. Phase boundaries identified by the QIP-based approach are in excellent accord with the fluorescence-based approach.

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.

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.

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.

Comparison of calibration coefficients for a vinten ionization chamber simulated using four Monte Carlo methods.

The Vinten 671 ionization chamber (VIC) was modelled using Monte Carlo (MC) programs EGSnrc, Penelope, and TOPAS. Several national measurement institutes have VICs with well-characterized response relationships and have measured calibration coefficients for many radionuclides. Twelve radionuclides with various decay emissions were assessed as well as 14 monoenergetic photon sources and 10 monoenergetic electron sources. Calibration coefficients were calculated based on the energy deposited in the simulated VIC nitrogen gas volume and compared to experimental values from the literature.

Gravimetric deposition of microliter drops with radiometric confirmation.

A manual microliter gravimetric dispensing technique is demonstrated using a micropipettor modified for use with removeable microcapillaries. Liquid scintillation sources were prepared from a well-characterized 241Am reference solution, providing a radiometric check of dispensed masses. Further experiments confirmed controlled dispensing of drops onto gold foils with losses ≤0.34(4) % of the total drop activity. A detailed measurement equation for the weighing technique, including the corrections for evaporation, is presented with a full accounting of associated uncertainties.

The international reference system for beta-particle emitting radionuclides: Validation through the pilot study CCRI(II)-P1.Co-60.

The Bureau International des Poids et Mesures (BIPM) is developing a new transfer instrument to extend its centralized services for assessing the international equivalence of radioactive standards to new radionuclides. A liquid scintillation counter using the triple/double coincidence ratio method is being studied and tested in the CCRI(II)-P1.Co-60 pilot study. The pilot study, involving 13 participating laboratories with primary calibration capabilities, validated the approach against the original international reference system based on ionization chambers, which has been in operation since 1976. The results are in agreement and an accuracy suitable for purpose, below 5×10-4, is achieved. The pilot study also reveals an issue when impurities emitting low-energy electrons are present in the standard solution, which have a different impact on liquid scintillation counting compared to other primary measurement methods.

Realization and dissemination of activity standards for medically important alpha-emitting radionuclides.

Interest in targeted cancer therapy with alpha-emitting radionuclides is growing. To evaluate emerging radiotherapeutic agents requires precise activity measurements for consistent dose-response relationships and patient-specific dosimetry. National metrology institutes around the world have reported on the development and comparison of activity standards for medically important alpha emitters. This review describes the relevant methods and models underpinning these standards, the generation of new nuclear decay data, and the impacts on preclinical and clinical activity assays using radionuclide calibrators and γ-ray spectrometry.

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 4παß(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.

Radionuclide calibrator responses for 224Ra in solution and adsorbed on calcium carbonate microparticles.

A suspension of 224Ra adsorbed onto CaCO3 microparticles shows promise for α-therapy of intracavitary micro-metastatic diseases. To facilitate accurate activity administrations, geometry-specific calibration factors for commercially available reentrant ionization chambers (ICs) have been developed for 224RaCl2 solutions and 224Ra adsorbed onto CaCO3 microparticles in suspension in ampoules, vials, and syringes. Ampoules and vials give IC responses consistent with each other to <1%. Microparticles attenuation leads to a ≈1% to ≈2.5% reduction in response in the geometries studied.

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 4παß(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).

Identification of active sites of biomolecules II: Saccharide and transition metal ion in aqueous solution.

We discuss the coordination mechanism of Fe(III) and methyl-alpha-mannopyranoside in aqueous solution using a recently presented integrated approach comprising ab initio electronic structure calculations, molecular dynamics simulations, and mass spectrometric measurements. First principles Car-Parrinello molecular dynamics (CPMD) simulations find that a single Fe(III) ion interacts with specific hydroxyl groups of the saccharide in aqueous solution. Specifically, we find that one Fe(III) ion complexed to methyl-alpha-mannopyranoside also associates with two water molecules. These simulations are in accord with electrospray ionization mass spectrometry measurements involving guided ion beam hydration measurements, which reveal an optimal coordination number of four about the Fe(III) ion. CPMD simulations identified specific intramolecular and intermolecular hydrogen bonding interactions that have an impact on the conformation of the saccharide and on the coordination with Fe(III); in contrast, classical molecular dynamics simulations were insensitive to these effects. This study illustrates the strength of ab initio molecular dynamics simulations, chemical reactivity calculations, and natural partial charge analysis coupled with mass spectrometric measurements in identifying the active sites of biomolecules toward ligands and for studying the complexation and coordination chemistry associated with substrate and ligand interactions relevant to the design of biochemical syntheses, drugs, and biomarkers in medicine.

Identification of active sites of biomolecules. 1. Methyl-alpha-mannopyranoside and Fe(III).

Car-Parrinello molecular dynamics (CPMD) simulations, DFT chemical reactivity index calculations, and mass spectrometric measurements are combined in an integrated effort to elucidate the details of the coordination of a transition-metal ion to a carbohydrate. The impact of the interaction with the FeIII ion on the glycosidic linkage conformation of methyl-alpha-d-mannopyranoside is studied by classical molecular dynamics (MD) and CPMD simulations. This study shows that FeIII interacts with specific hydroxyl oxygen atoms of the carbohydrate, affecting the ground state carbohydrate conformation. These conformational details are discussed in terms of a set of supporting experiments involving electrospray ionization mass spectrometry, and CPMD simulations clearly indicate that the specific conformational preference is due to intramolecular hydrogen bonding. Classical MD simulations proved insensitive to these important chemical properties. Thus, we demonstrate the importance of chemical reactivity calculations and CPMD simulations in predicting the active sites of biological molecules toward metal cations.

Glycosidic linkage conformation of methyl-alpha-mannopyranoside.

We study the preferred conformation of the glycosidic linkage of methyl-alpha-mannopyranoside in the gas phase and in aqueous solution. Results obtained utilizing Car-Parrinello molecular dynamics (CPMD) simulations are compared to those obtained from classical molecular dynamics (MD) simulations. We describe classical simulations performed with various water potential functions to study the impact of the chosen water potential on the predicted conformational preference of the glycosidic linkage of the carbohydrate in aqueous solution. In agreement with our recent studies, we find that results obtained with CPMD simulations differ from those obtained from classical simulations. In particular, this study shows that the trans (t) orientation of the glycosidic linkage of methyl-alpha-mannopyranoside is preferred over its gauche anticlockwise (g-) orientation in aqueous solution. CPMD simulations indicate that this preference is due to intermolecular hydrogen bonding with surrounding water molecules, whereas no such information could be demonstrated by classical MD simulations. This study emphasizes the importance of ab initio MD simulations for studying the structural properties of carbohydrates in aqueous solution.

A Hadamard transform electron ionization time-of-flight mass spectrometer.

We describe the first Hadamard transform time-of-flight mass spectrometer (HT-TOFMS) that incorporates an electron (impact) ionization source. This implementation was realized in an existent TOF instrument using commercially available components and simple modifications to the ion source. In the present apparatus, a Hadamard mask is expressed by modulating the ion generation process within the ion source; thus, the present approach differs from previous designs that use external electrostatic devices to modulate a continuous ion stream. The present implementation may be operated in conventional TOF mode at 12.5 kHz and in HT-TOF mode at 20-40 MHz. In Hadamard mode the design can operate using any circulant simplex code, allowing the operator much flexibility for optimizing resolution and mass range and for eliminating nonstochastic fluctuations, e.g., encoding errors and signal hum. We demonstrate typical performance of the HT-TOFMS in standard and reflectron geometries using sequences of three constructions and of varied length, generating HT-TOF mass spectra of molecules that match conventional reference spectra. The auxiliary material includes an electrical schematic for the floating high-speed encoding amplifier, which is also of use in other high-speed electrostatic optics applications, and a list of 537 validated vectors comprising the first row of each circulant simplex sequence (S(n)=3-8219) derived using maximal shift register (n=2(m)-1), quadratic residue (n=4m-3), and twin prime constructions [n=p(p+2)].

Monte Carlo modelling of live-timed anticoincidence (LTAC) counting for Cu-64.

The radionuclide copper-64 is a promising candidate for nuclear medicine, but its complex decay creates challenges in the primary standardization of its activity. Monte Carlo simulations of live-timed anticoincidence (LTAC) counting of 64Cu were used to calculate corrections to extrapolation intercepts, resulting in improved activity determinations. A small correction (-0.33%) to the linear extrapolation of LTAC data acquired with a γ-gate over the 1346keV gamma peak was determined. We discuss the physical origin of the correction. We also use experimental data to demonstrate a Monte Carlo scaling that allows for inclusion of data acquired with a γ-gate set over the annihilation photon peak(s).

An update on 'dose calibrator' settings for nuclides used in nuclear medicine.

BACKGROUND: Most clinical measurements of radioactivity, whether for therapeutic or imaging nuclides, rely on commercial re-entrant ionization chambers ('dose calibrators'). The National Institute of Standards and Technology (NIST) maintains a battery of representative calibrators and works to link calibration settings ('dial settings') to primary radioactivity standards. Here, we provide a summary of NIST-determined dial settings for 22 radionuclides. METHODS: We collected previously published dial settings and determined some new ones using either the calibration curve method or the dialing-in approach. RESULTS: The dial settings with their uncertainties are collected in a comprehensive table. CONCLUSION: In general, current manufacturer-provided calibration settings give activities that agree with National Institute of Standards and Technology standards to within a few percent.