Massic Activity Research Articles

The standardization of 188W/ 188Re by 4 πβ liquid scintillation spectrometry with the CIEMAT/NIST 3H-standard efficiency tracing method

The massic activity of a solution containing 188W in equilibrium with its daughter, 188Re, has been standardized by the National Institute of Standards and Technology using 4 πβ liquid scintillation counting with efficiency tracing using the CIEMAT/NIST method. Confirmatory measurements were carried out with γ-ray spectrometry using high purity germanium detectors and a 4 πγ NaI(Tl) system. A calibration factor of 1.68 MBq pA −1 ±0.03 MBq pA −1 (expanded, k=2, uncertainty) for the 188W in the solution was determined, along with correction factors for activity determinations using Capintec dose calibrators. A half-life value of 69.78 d±0.05 d (standard uncertainty) was determined by measurement in the NIST 4 πγ ionization chamber and is consistent with currently recommended values.

Traceability of performance evaluation materials

One of the most critical elements of a performance evaluation (PE) program for radioactivity measurements is the traceability of the PE materials to the national standards. The requirements and criteria for the production of traceable environmental and radiobioassay PE materials have been defined by ANSI N42.22 and ANSI N13.30 standards. It is important to note that use of traceable source materials does not necessarily ensure the traceability of subsequently derived PE materials unless verification measurements exist in conjunction with the preparation processes. This paper describes the protocol currently used by NIST for the preparation and verification of air filter, acidified water, spiked soil, synthetic urine, and synthetic fecal PE materials for low-level radioactivity measurements. The process involves gravimetric dilutions and mixing of primary radionuclide NIST Standard Reference Materials (SRMs), addition of the derived master solution to sample matrices, and subsequent verification measurements. Several gamma-emitters were used to trace the gravimetric dilutions and spike addition through an unbroken chain of gamma comparison measurements. The massic activities of alpha- and beta-emitters in the diluted solutions and PE samples were also measured by radiochemical methods and compared with their gravimetric values. A correlation analysis demonstrated that the gamma emitters quantitatively followed 90Sr, 238U, 238Pu, and 241Am throughout the dilution and spiking and can be used as effective process monitors. The statistical results from t-tests, box plots, and normal probability tests suggested that traceability of radionuclides in the PE materials to their primary standards can be verified to within 1%, with an overall precision better than 2% (1s).

An alternative statistical approach for interlaboratory comparison data evaluation

Interlaboratory comparisons are an essential component of method validation, laboratory proficiency and equivalency evaluation, and standard reference material certification. The reliability of the results from interlaboratory comparison largely depends upon analyzing the data with appropriate statistical methods. Intercomparison data are often found to be non-normally distributed as a result of sample heterogeneity, uneven laboratory variance, and methodology bias. The statistical methods that require data normality are inappropriate for analyzing such skewed data. Instead of using data transformation or non-parametric methods, an alternative method which sets aside the assumption of data normality is utilized in the certification of new National Institute of Standards and Technology (NIST) Ocean Sediment Standard Reference Material (SRM-4357). It first determines the best-fit distribution for the data, then calculates appropriate distributional mean and tolerance limits, and finally estimates the uncertainties for the derived statistics by bootstrap analysis. The method was applied to evaluate the data from the SRM intercomparison, and revealed homogeneity for natural radionuclides and inhomogeneity for anthropogenic radionuclides in the SRM. It was found that the data of natural radionuclides in the SRM are normally distributed. In contrast, the data of anthropogenic radionuclides, such as 90Sr, 137Cs, 238Pu, and (239+240)Pu, are best fitted by Weibull distributions. The difference on data distributions for the two types of radionuclides is believed to be related to "hot" particles in the SRM. By determining the best-fit data distribution and applying bootstrap analysis, more reliable and realistic massic activity (Bq·kg-1) for anthropogenic radionuclides were certified in comparison to those obtained using data transformation and nonparametric methods. Although the developed method is computationally intensive, it provides a straightforward, rigorous procedure for computing the statistics that would otherwise be difficult to obtain when the data distribution form is complicated. With the help of advanced personal computers and use of the DataPlot software from NIST, this method becomes very practical and has enhanced interlaboratory comparison data studies and certification of the NIST standard reference materials.

Radioassays and experimental evaluation of dose calibrator settings for 18F

The positron emitter 18F continues to be one of the most important imaging radionuclides in diagnostic nuclear medicine. Assays of radiopharmaceuticals containing this nuclide are often performed in the clinic using commercial reentrant ionization chambers, or “dose calibrators”. Meaningful quantitative clinical studies require accurate knowledge of the injected activity which requires proper calibration of these instruments. Radioassays were performed at the National Institute of Standards and Technology (NIST) on a solution of 18F produced at the National Institutes of Health (NIH) using 4πβ liquid scintillation (fS) counting with 3H-standard efficiency tracing. Cocktails containing water fractions of approximately 0.9 and 9% (both as saline) were used. The massic activity values were measured to be 2.52±0.06 and 2.50±0.03 MBq g −1, respectively, for the 0.9 and 9% water cocktails as of the reference time. The uncertainties on the activity measurements are expanded ( k=2) uncertainties. The largest uncertainty component was found to be the repeatability on a single LS source, with the cocktails containing 0.9% water fraction exhibiting a larger variability by nearly a factor of two. Reproducibility between LS cocktails with the same water fraction was also found to be a large uncertainty component, but with a value less than half that due to measurement repeatability. Radionuclidic impurities consisted of 48V and 46Sc, at levels of 0.11±0.08% (expanded uncertainties) and approximately 2×10 −3% (upper limit) relative to the activity of the 18F, as of the reference time. Dose calibrator dial settings for measuring solutions of 18F were experimentally determined for Capintec CRC-12 and CRC-35R dose calibrators in three measurement geometries: a 5-ml standard NIST ampoule (two ampoules measured), a 12-ml plastic syringe containing 9 ml of solution and a 10-ml Mallinckrodt molded dose vial filled with 5 ml of solution. The experimental dial settings (and the corresponding expanded uncertainties) for these geometries were found to be 477±7, 474±6, 482±6 and 463±7 for the two ampoules, the syringe and the dose vial, respectively, in the CRC-12. The dial settings determined for the CRC-35R were 472±7, 470±7, 464±6 and 456±6 for the two ampoules, the syringe, and the dose vial, respectively. The uncertainties in the dial settings are expanded uncertainties. Comparisons between the empirically determined dial settings and the manufacturer's recommended setting of “439” indicate that use of the manufacturer's setting overestimates the activity by between 3 and 6%, depending upon the geometry used.

Massic Activity Ratios of the NBS/NIST Tritiated-Water Standards Issued Between 1954 and 1999.

As part of the preparation and calibration of three new National Institute of Standards and Technology (NIST) tritiated-water radioactivity Standard Reference Materials (SRMs), the massic-activity (activity of the sample divided by the mass of the sample) ratios of all of the available NBS/NIST tritiated-water SRMs issued between 1954 and 1999 were measured using liquid-scintillation (LS) counting. Four of the tritiated-water standards (SRMs 4361, 4926B, 4927C, and 4927D) were not available for measurement. All of the other tritiated-water standards (SRMs 4361B, 4361C, 4926, 4926C, 4926D, 4926E, 4927, 4927B, 4927E, and 4927F) were available, having been stored in flame-sealed glass ampoules. Where possible, massic-activity ratios measured by liquid-scintillation counting are compared with massic-activity ratios calculated from gravimetric dilution factors. The agreement is well within the stated uncertainties. For two of the tritiated-water standards that were not available for measurement (SRMs 4361 and 4926B), massic-activity ratios calculated from gravimetric dilution factors are available.

Calibration of the National Institute of Standards and Technology tritiated-water standards

Massic activity values obtained for NBS/NIST tritiated water SRMs 4927 and 4927E, using internal gas-proportional counting, are reported. The massic activity ratios of all of the available NBS/NIST tritiated water SRMs and samples of the Level 1 and Level 3 tritiated water standards produced by the National Physical Laboratory (NPL) in 1996 were measured using an extensive series of liquid-scintillation intercomparisons. New massic activity values obtained for the NPL standards, using the gas counting results for SRM 4927E and the results of the liquid-scintillation intercomparison, are also reported. A new determination of the half-life of tritium was made using massic activity data for SRM 4927 over a period of 38 years. The new value is (4504±9) (one standard deviation) days. This value differs significantly from the previous NBS/NIST value but is in good agreement with the value recommended in the Evaluated Nuclear Structure Data File (ENSDF).

A multimethodic and multiparametric system for standardisation of radionuclides

A measuring system with simultaneous application of four independent counting methods for standardization of solutions of different radionuclides is presented. The result obtained using four methods allows for mutual verification and gives the final result with a higher confidence level. The multidetection scintillation head for 4π(LS)-γ coincidence and anticoincidence with double-parametric registration, as well as the electronic diagram, is described. The four methods, comprised in the presented system, are described. Using the multimethodic and multiparametric system, massic activity of the solutions of the following radionuclides with various decay schemes has been measured: 133Ba, 90Sr, 99Mo, 85Sr, 65Zn, 139Ce, 192Ir, 204Tl, 60Co, 241Am, 152Eu, 169Yb. The results of the methods were analyzed using the figure of merit evaluated following the ISO/IEC guide 47-1. The system seems to allow the analysis of the correctness of the extrapolation curves, activity vs. the LSC efficiency, applied in 4π(LS)-γ methods.

Development of the NIST bone ash standard reference material for environmental radioactivity measurement

The bone ash standard reference material (SRM), a blend of 4% contaminated human bone and 96% diluent bovine bone, has been developed for radiochemical method validation and quality control for radio-bone analysis. The massic activities of 90Sr, 226Ra, 230Th, 232Th, 234U, 235U, 238U, 238Pu, (239+240)Pu and (243+244)Cm were certified using a variety of radiochemical procedures and detection methods. Measurements confirmed undetectable radionuclide heterogeneity down to a sample size of 5 g, thereby implying adequate blending of particulate materials with dilution factors of up to 17,900. The results among most of the intercomparison laboratories and their methods were consistent. Disequilibrium was observed for decay chains: 234U(0.67 mBq/g)– 230Th(0.47 mBq/g)– 226Ra(15.1 mBq/g)– 210Pb(23 mBq/g)– 210Po(13 mBq/g) and 232Th(0.99 mBq/g)– 228Ra(6.1 mBq/g)– 228Th(7.1 mBq/g). The disequilibria were the results of mixing occupationally contaminated human bone with natural bovine bone and the fractionation during internal biological processes. The massic activity of 210Pb, 228Th and 241Am were not certified because of insufficient 228Ra and 241Pu data and lack of knowledge in how 222Rn and its daughters will be fractionated in the SRM bottle over time.

Calibration of the massic activity of a solution of 99Tc

A solution of 99Tc was calibrated in terms of massic activity using the NIST 4 πβ(LS)– γ-anticoincidence counting system. 60Co was used as the efficiency-tracing radionuclide. Measurements were also made using conventional 4 πβ liquid-scintillation (LS) counting. 3H was used as the efficiency-monitor radionuclide. The LS detection efficiency ratios, ( ε 99/ ε 3), were calculated theoretically. The calculated detection efficiency ratios depend strongly upon the assumed shape of the beta spectrum of 99Tc.

The standardization of the potential bone palliation radiopharmaceutical 117mSn(+4)DTPA

Solution containing the potential bone pain palliation radionuclide 117mSn, in chloride form and as a diethylenetriaminepentaacetate (DTPA) complex, have been standardized by 4 πβ liquid scintillation (LS) spectrometry and 4 π γ-ray spectrometry. massic activities of the stock solutions were measured in order to determine dose calibrator settings for the solutions using commericial dose calibrators. Excellent agreement in the measurement of solution massic activity between the two techniques was achieved. The massic activity of 117mSnCl 4 stock solution was found to be 38.62±0.23 MBq g −1 and 38.81 ± 0.94 MBq g − with LS spectrometry and 4 π γ-ray spectrometry respectively. The respective values of the massic activity of the 117mSnDTPA stock solution with LS spectrometry and 4 π γ-ray spectrometry were 39.35±0.23 MBq g −1 and 39.70 ± 0.96 MBq g −1. Impurities were analyzed in several solutions and found to have emission rates on the order of 10 −4 to 10 −6 of the rate of the 117mSn emission at the end-of-bombardment. The largest impurities came from 113Sn and 125Sn, the activation products of isotopic impurities present in the 117Sn target. The relative proportions of the various impurities were found to be highly dependent upon the source of 117Sn target material. The implications of choice of half-life used in the decay correction of 117mSn are discussed.

A Compendium on the NIST Radionuclidic Assays of the Massic Activity of 63Ni and 55Fe Solutions Used for an International Intercomparison of Liquid Scintillation Spectrometry Techniques.

The National Institute of Standards and Technology recently participated in an international measurement intercomparison for 63Ni and 55Fe, which was conducted amongst principal national radionuclidic metrology laboratories. The intercomparison was sponsored by EUROMET, and was primarily intended to evaluate the capabilities of liquid scintillation (LS) spectrometry techniques for standardizing nuclides that decay by low-energy β-emission (like 63Ni) and by low-Z (atomic number) electron capture (like 55Fe). The intercomparison findings exhibit a very good agreement for 63Ni amongst the various participating laboratories, including that for NIST, which suggests that the presently invoked LS methodologies are very capable of providing internationally-compatible standardizations for low-energy β-emitters. The results for 55Fe are in considerably poorer agreement, and demonstrated the existence of several unresolved problems. It has thus become apparent that there is a need for the various international laboratories to conduct rigorous, systematic evaluations of their LS capabilities in assaying radionuclides that decay by low-Z electron capture.

Standardization of 63Ni by 4πβ Liquid Scintillation Spectrometry With 3H-Standard Efficiency Tracing.

The low energy (Eβmax = 66.945 keV ± 0.004 keV) β-emitter 63Ni has become increasingly important in the field of radionuclidic metrology. In addition to having a low β-endpoint energy, the relatively long half-life (101.1 a ± 1.4 a) makes it an appealing standard for such applications. This paper describes the recent preparation and calibration of a new solution Standard Reference Material of 63Ni, SRM 4226C, released by the National Institute of Standards and Technology. The massic activity CA for these standards was determined using 4πβ liquid scintillation (LS) spectrometry with 3H-standard efficiency tracing using the CIEMAT/NIST method, and is certified as 50.53 kBq ·g−1 ± 0.46 Bq · g−1 at the reference time of 1200 EST August 15, 1995. The uncertainty given is the expanded (coverage factor k = 2 and thus a 2 standard deviation estimate) uncertainty based on the evaluation of 28 different uncertainty components. These components were evaluated on the basis of an exhaustive number (976) of LS counting measurements investigating over 15 variables. Through the study of these variables it was found that LS cocktail water mass fraction and ion concentration play important roles in cocktail stability and consistency of counting results. The results of all of these experiments are discussed.

Comparison of the French and U.S. National 3H (tritiated H 2O) standards by 4πβ liquid scintillation spectrometry

Radioactivity standards of tritiated water ( 3H 2O)—disseminated by the Laboratoire Primaire des Rayonnements Ionisants (LPRI) and the National Institute of Standards and Technology (NIST), the national radionuclidic metrology and standardization laboratories of France and U.S.A., respectively— have been intercompared by liquid scintillation (LS) spectrometry. The ratio of the certified massic activities for the two standards was compared to that obtained from direct measurements on matched sets of LS cocktails prepared from the standards. Seven experimental trials (involving a total of 21 counting sources for each standard) were performed for the comparison. The trials were performed under a wide range of experimental conditions, including use of two different LS spectrometers and three series of LS cocktail compositions (with systematically varied 3H detection efficiencies). The results exhibited an apparent mean disagreement between standards of < 0.4% on a relative basis. For contrast, the relative combined standard uncertainty on the massic activity ratio for the two standards, as obtained from their respective certified uncertainty assessments, is about 0.7%.

Cocktail volume effects in 4πβ Liquid scintillation spectrometry with 3H-standard efficiency tracing for low-energy β-emitting radionuclides

The effect of total liquid scintillation (LS) cocktail volume (or mass) in the 4πβ LS spectrometry of β-emitting radionuclides has been investigated. The magnitude of such possible volume effects on the apparent activity of low-energy β-emitting radionuclides, as determined by the CIEMAT/NIST 3H-standard efficiency tracing method, as well as any systematic trends in the relative detection efficiencies of low-ebergy β-emitters were studied. The radionuclides chosen for the study, 63Ni and 36Cl, were traced against 3H. Detection efficiency losses, as a function of total LS cocktail volume, were found to be energy-dependent. Because of the low E βmax for the decay of 3H and 63Ni, systematic losses in efficiency with increasing cocktail mass, as well as at extremely low (< 5 g) masses were observed. The determination of quench-indicating parameters, even for cocktails of uniform composition, were also found to be volume-dependent. Despite a relatively large (17%) variation in the 3H counting efficiency over the volume range 1–20 mL in 22 mL vials and a corresponding 7% varition in the 63Ni counting efficiency, the CIEMAT/NIST method was able to trace the observed 63Ni activity to about 0.1%. No volume effect on the observed relative efficiency of 36Cl was found. The best condition for repeatability in the determination of the massic activity of a single sample was found to be when the volume is 8–12 mL in a 22 mL vial. The effect of total LS cocktail volume on the efficiency-traced massic activities was found to have no clear, consistent trend and proved difficult to quantify. Nonetheless, two approaches to estimating the magnitude of the effect are presented and indicate that it is extremely small (< 0.1%) and was within the statistical variation of the measurements.

63Ni half-life: a new experimental determination and critical review

The 63Ni half-life has been determined to be 101.06 ± 1.97 a based on three independent measurements, conducted over the past 27 years, of the massic activity of gravimetrically-related 63Ni sources. The 63Ni assays were performed initially (in 1968) by microcalorimetry using an assumed mean β − energy per decay, and subsequently (in 1984 and 1995) by 4πβ liquid scintillation spectrometry with 3H-standard efficiency tracing. The present result is the first and only determination of the 63Ni half-life which is based on actually following the radioactive decay of 63Ni. All previously reported determinations were derived from specific activity evaluations. These earlier determinations have been scutinizingly reviewed as part of this work. Some of these previous values can be revised in lieu of more recent nuclear data and on consideration of the experimental details. Based on a critical evaluation of the extant data set, a 63Ni half-life value of T=101.1 ± 1.4 a is recommended.