14C Counting Research Articles

Assessment of Radioecologically Relevant Radionuclides 14C, 99Tc, and 129I in Vitrified High-Level Nuclear Wastes from Spent Fuel Reprocessing

A method is proposed for quantifying the specific activities of radioecologically significant radionuclides, including 14C, 99Tc, and 129I, in vitrified high-level wastes. The conditions for the separation and purification of these radionuclides from potential interfering components are optimized and the yield coefficients and purification factors are determined. The quantification of 14C entails a three-fold distillation in CO2 followed by the measurement of the 14C counting rate in the purified solution by liquid scintillation spectrometry. 129I was determined by dissolving a sample in HNO3, five-fold extraction, and the subsequent measurement of the 129I counting rate in the purified solution using liquid scintillation spectrometry. The determination of 99Тс included dissolving a sample in the presence of ClO–, double extraction-chromatographic separation of 99Тс with an adsorbent impregnated with methyltrioctylammonium nitrate, followed by the determination of its activity by inductively coupled plasma–mass spectrometry. We have applied this approach to a simulator of vitrified high-level waste, labeled with radionuclides, and achieved satisfactory results. This approach will be instrumental in the analysis of the accumulated vitrified high-level wastes at the “Mayak” Production Association (Mayak PA). The results will be important for modeling the engineering safety barriers of a deep disposal site for radioactive wastes.

Improved C− efficiency and ion beam currents by modifying SNICS cathode material

We investigated the possibility of increasing the 14C AMS efficiency and maximizing C− beam currents by modifying sample cathode geometry and material. Previously, we reported results which showed an increase in C− beam current and improved ionization efficiency when reducing the cathode sample well diameter from 1 mm to 0.75 mm and using zinc inserts, pressed into the aluminum cathode holder [4]. In this study we expand that effort by using cathodes with inserts made of Fe, Mo, Ni, Ti, Cu, and Zn. We pressed cathodes with 100 μg of Oxalic Acid I (OX-I) and ran them to exhaustion. 12C beam currents, 14C counts, and graphite weight were used to determine ion source efficiency.

BaCO3 targets produced from dissolved carbonate in groundwater for direct AMS measurement

This paper describes a technique of making carbonate by rapid CO2 capture from groundwater for direct measurement of the 14C content by accelerator mass spectrometry (AMS). The DIC in groundwater was extracted from acidified water in the form of carbon dioxide (CO2), and transferred into saturated barium hydroxide solution to form barium carbonate (BaCO3). After the BaCO3 precipitate was freeze-dried, it was mixed with a metal powder and pressed into an AMS target for measurement. The behaviors of Si, Al, Fe and Ta powder as the binder with carbonate were evaluated. The C3+ currents from carbonate targets mixed with Fe or Ta were found generally larger than those with Si or Al by a factor of two. The baked Ta was shown to produce fewer contaminating 14C counts than all the others, with or without baking. The CaCO3/Ta and BaCO3/Ta mass mixing ratios in the range from 1:2 to 1:4 and in 1:1.5, respectively, produced optimized currents. The precision and accuracy of the measurement by direct CaCO3 or BaCO3 sputtering using Ta as the binder, were evaluated in comparison to a selected reference material. The agreement was reasonably good between the direct carbonate measurement and the high precision measurement through combustion and graphitization. These findings support the further development of a rapid assessment method directly from field work to measurement.

Method validation and verification in liquid scintillation counting using the long-term uncertainty method (LTUM) on two decades of proficiency test data

Results from proficiency tests gathered over the past two decades by the laboratory for low level radioactivity measurements for liquid scintillation counting of 3H (184 results) and 14C (74 results) are used to verify the validated measurement methods used by the laboratory, in particular the estimated uncertainty budget of the method and its reproducibility and stability. A linear regression approach is used for the analysis of the results, described in the literature as the long term uncertainty in measurement method. The present study clearly indicates the advantages of using proficiency test results in identifying possible constant or proportional bias effects as well as the possibility to compare the laboratory performance with the performance of peer laboratories.

Amelioration of photo-assimilates translocation pattern by the application of phytohormones in Indian mustard under abiotic stress environment

Detailed studies were carried out to find out the effect of various phytohormones on 14C translocation pattern in mustard under abiotic stress conditions. IAA100 μM enhanced 14C accumulation in siliquae 19.66%and 26.84% in waterlogged and high temperature stressed plants respectively in respect to control (3.67%) while, IAA500 and Kin50 reduced the accumulation in siliquae 1.41% and 1.46% in water logging (WL) environments and 2.30% in high temperature condition in compared to control 5.78% and 14.27%respectively in water logging and high temperature (HT) stress. All the concentrations of phytohormones significantly enhanced the translocation of photo assimilates in siliquae 6.5%, 22.31%, 13.85% and 6.29% respectively in plant sprayed with IAA 100, IAA 500, Kin 5 and Kin 50 uM in respect to control 2.67% under WL+HT stress, roots except Kin5 that decreased the accumulation in roots. After a revival period of ten days, the effect of all the hormonal concentrations was equivalent to water spray with regard to the 14C counts in siliquae of waterlogged plants. Further, in siliquae of high temperature stressed plants, IAA 100 was at par with water, while IAA500, Kin5 and Kin50 reduced the incorporation of 14C assimilates.

High-Precision Atmospheric 14CO2 Measurement at the Rafter Radiocarbon Laboratory

This article describes a new capability for high-precision 14C measurement of CO2 from air at the Rafter Radiocarbon Laboratory, GNS Science, New Zealand. We evaluate the short-term within-wheel repeatability and long-term between-wheel repeatability from measurements of multiple aliquots of control materials sourced from whole air. Samples are typically measured to 650,000 14C counts, providing a nominal accelerator mass spectrometry (AMS) statistical uncertainty of 1.3‰. No additional uncertainty is required to explain the within-wheel variability. An additional uncertainty factor is needed to explain the long-term repeatability spanning multiple measurement wheels, bringing the overall repeatability to 1.8‰, comparable to other laboratories measuring air materials to high precision. This additional uncertainty factor appears to be due to variability in the measured 14C content of OxI primary standard targets, likely from the combustion process. We observe an offset of 1.4‰ in our samples relative to those measured by the University of Colorado INSTAAR, comparable to interlaboratory offsets observed in recent intercomparison exercises.

High-Precision Atmospheric 14 CO 2 Measurement at the Rafter Radiocarbon Laboratory

This article describes a new capability for high-precision 14 C measurement of CO2 from air at the Rafter Radiocarbon Laboratory, GNS Science, New Zealand. We evaluate the short-term within-wheel repeatability and long-term between-wheel repeatability from measurements of multiple aliquots of control materials sourced from whole air. Samples are typically measured to 650,000 14C counts, providing a nominal accelerator mass spectrometry (AMS) statistical uncer- tainty of 1.3‰. No additional uncertainty is required to explain the within-wheel variability. An additional uncertainty factor is needed to explain the long-term repeatability spanning multiple measurement wheels, bringing the overall repeatability to 1.8‰, comparable to other laboratories measuring air materials to high precision. This additional uncertainty factor appears to be due to variability in the measured 14 C content of OxI primary standard targets, likely from the combustion process. We observe an offset of 1.4‰ in our samples relative to those measured by the University of Colorado INSTAAR, comparable to interlaboratory offsets observed in recent intercomparison exercises. This article describes the high-precision atmospheric 14 CO 2 measurement capability at the Rafter Radiocarbon Laboratory at GNS Science, New Zealand. We discuss the two different methods we commonly use for CO 2 collection and subsequent extraction. We describe our recently upgraded graphitization system and detail our measurement protocols for samples requiring high precision. We use replicate measurements of CO 2 from a number of different control materials, all derived from whole air, to examine mean values and offsets between different standardization methods within our laboratory, as well as interlaboratory offsets based on an ongoing intercomparison with the University of Colorado INSTAAR. We then examine short-term within-wheel and long-term repeatability of our measurements, and the sources of uncertainty that contribute to these. METHODS CO 2 from air is collected either by in situ absorption of CO 2 into sodium hydroxide (NaOH) solution or by collection of whole air into flasks or pressurized cylinders. In addition, control materials, pri - mary standards, and process blanks are routinely analyzed. In this section, we describe the collection methods and protocols for extracting and purifying the CO 2 in the laboratory.

Automated combustion accelerator mass spectrometry for the analysis of biomedical samples in the low attomole range.

The increasing role of accelerator mass spectrometry (AMS) in biomedical research necessitates modernization of the traditional sample handling process. AMS was originally developed and used for carbon dating, therefore focusing on a very high precision but with a comparably low sample throughput. Here, we describe the combination of automated sample combustion with an elemental analyzer (EA) online coupled to an AMS via a dedicated interface. This setup allows direct radiocarbon measurements for over 70 samples daily by AMS. No sample processing is required apart from the pipetting of the sample into a tin foil cup, which is placed in the carousel of the EA. In our system, up to 200 AMS analyses are performed automatically without the need for manual interventions. We present results on the direct total (14)C count measurements in <2 μL human plasma samples. The method shows linearity over a range of 0.65-821 mBq/mL, with a lower limit of quantification of 0.65 mBq/mL (corresponding to 0.67 amol for acetaminophen). At these extremely low levels of activity, it becomes important to quantify plasma specific carbon percentages. This carbon percentage is automatically generated upon combustion of a sample on the EA. Apparent advantages of the present approach include complete omission of sample preparation (reduced hands-on time) and fully automated sample analysis. These improvements clearly stimulate the standard incorporation of microtracer research in the drug development process. In combination with the particularly low sample volumes required and extreme sensitivity, AMS strongly improves its position as a bioanalysis method.

Degradation of p12 Subunit by CRL4Cdt2 E3 Ligase Inhibits Fork Progression after DNA Damage

After acute DNA damage, the cell arrests S-phase progression by inhibiting origin initiation and fork progression to repair damaged DNA. The intra-S-phase checkpoint kinase Chk1 phosphorylates Cdc25A to target the latter for degradation by CRL1(β-TrCP) and so inhibit origin firing. The mechanism for inhibiting fork progression, however, has not been identified. Here, we show that degradation of p12, the fourth subunit of DNA polymerase δ, is critical for inhibiting fork progression. CRL4(Cdt2) is an E3 ligase that ubiquitinates and degrades p12 after UV treatment. Cells expressing a stable form of p12 exhibit UV-resistant DNA synthesis. DNA fiber assay and alkaline-sucrose gradient assay demonstrate that the impairment of fork progression after DNA damage requires p12 degradation. These results suggest that ubiquitination of p12 through CRL4(Cdt2) and subsequent degradation form one mechanism by which a cell responds to DNA damage to inhibit fork progression.

Shoot labelling with 14CO2: a technique for assessing total root carbon exudation under phosphorus stress

Root exudation is one of the mechanisms adopted by plants to acquire fixed phosphorus (P) from soil. We report a rapid method for evaluating P-efficient genotypes under low-P availability by measuring root exudation capacity after labelling shoots with 14CO2. Forty-four greengram (Vigna radiata) genotypes were raised with low (2 μM) and sufficient (100 μM) P levels for 15 days in hydroponics. Shoots were exposed to 14CO2 and root exudate sampling was done at 24, 48 and 96 h after exposure. On the basis of relative 14C exudation and biomass, genotypes were categorized into four groups viz. efficient and responsive, inefficient and responsive, efficient and non-responsive and inefficient and non-responsive. Carboxylic acids were measured in exudates and root apices of selected genotypes. Amount of carboxylates in root exudate was significantly correlated with total 14C exudation under low-P. Highest proportion of oxalic acid was detected in genotype PDM-139 belonging to efficient and responsive category under low-P. This suggests that total 14C count can be used as a rapid method as compared to quantification of carboxylates using chromatographic techniques to measure the root exudation capacity of genotypes. Thus, this simple technique can be employed to screen genotypes at an early stage of plant growth.

High-Current 14C Measurements on an HVE 1MV AMS System

High radiocarbon sample throughput is an increasingly important requirement of accelerator mass spectrometry (AMS) systems, which in turn requires high source output. To accomplish this, HVE AMS systems use a model SO-110B ion source, which can run at outputs of more than 200 μA 12C. It is known that high source output may compromise precision. To quantify this effect, we have tested an HVE 1MV AMS system for the influence of medium (100 μA) and high (200 μA) source outputs on the precision. These early measurements indicate for a 200 μA 12C source output, equivalent to a ≃1 kHz 14C count rate for Ox II samples, a 13C/12C precision of 1% and a 14C/12C precision of 3% can be obtained.

Optimizing a microwave gas ion source for continuous-flow accelerator mass spectrometry

A 2.45 GHz microwave ion source coupled with a magnesium charge exchange canal (C × C) has been successfully adapted to a large acceptance radiocarbon accelerator mass spectrometry system at the National Ocean Sciences Accelerator Mass Spectrometry (AMS) Facility, Woods Hole Oceanographic Institution. CO(2) samples from various preparation sources are injected into the source through a glass capillary at 370 μl∕min. Routine system parameters are about 120-140 μA of negative (12)C current after the C × C, leading to about 400 (14)C counts per second for a modern sample and implying a system efficiency of 0.2%. While these parameters already allow us to perform high-quality AMS analyses on large samples, we are working on ways to improve the output of the ion source regarding emittance and efficiency. Modeling calculations suggest modifications in the extraction triode geometry, shape, and size of the plasma chamber could improve emittance and, hence, ion transport efficiency. Results of experimental tests of these modifications are presented.

Balanced-Energy Counting Window for Stable Liquid Scintillation Radiocarbon Dating

This paper describes an optimal radiocarbon counting window for liquid scintillation (LS) 14C dating that secures for unquenched as well as for heavily quenched dating samples maximal stability of 14C counting efficiency and theoretically minimal quench correction. In high-precision dating, a balanced counting window with fixed channel limits is frequently used, where about 3% of the highest part of the 14C spectrum is sacrificed for high 14C counting stability. The stability is, however, diminished for quenched samples. Therefore, this window is here replaced by a balanced fixed-energy 14C counting window where the channel limits depend on the quench level. The LS system used must have a linear amplifier and a multichannel analyzer. All samples are measured at a fixed high voltage. For energy calibration and determination of the quench level, the channel number of the middle of the 59.5-keV peak from an external 241Am gamma source is determined before and after measuring each sample. This counting mode is valuable in high-precision dating. It could be widely applied if adapted to systems with a logarithmic amplifier, generally used in LS dating.

Application of the Triple-Photomultiplier Liquid Spectrometer Hidex 300 Sl in Radiocarbon Dating

The Hidex 300 SL is a liquid scintillation analyzer with an automatic sample changer and a triple-photomultiplier tube detection assembly that registers triple- as well as double-coincidence spectra. In the triple mode, the background in the 14C window is 13.7 cpm (14C standard 30.8 cpm; =154.3 cpm/5.01), so the factor of merit equals 8.7. The triple-to-double coincidence ratio (TDCR) allows for determining the 14C counting efficiency, the quench level, and quench correction. However, in the case of very low-activity samples, which is the case even for modern 14C samples, the TDCR is not the best method for the correction of benzene impurities. We propose using the position (channel) of the right slope of the sample (14C) logarithmic pulse-height spectrum. In the case of near-background samples, the cosmic muon peak can be used instead. The Monte Carlo modeling of spectra gave the 14C level below which the muon peak is a better quench correction parameter than the position of the 14C spectrum. The spectrometer, with the proposed quench correction method, was tested with wood samples dated dendrochronologically. For 21 samples, there is no systematic bias observed, and the standard deviation of the age differences scaled by the Poisson errors is 1.24 ± 0.15, which means that the counting statistics account for 80% of the total variability (including sample preparation).

Role of fatty acid elongases in determination of de novo synthesized monounsaturated fatty acid species

Enhanced production of monounsaturated fatty acids (FA) derived from carbohydrate-enriched diets has been implicated in the development of obesity and insulin resistance. The FA elongases Elovl-5 and Elovl-6 are regulated by nutrient and hormone status, and have been shown using intact yeast and mammalian microsome fractions to be involved in the synthesis of monounsaturated FAs (MUFA). Herein, targeted knockdown and overexpression of Elovl-5 or Elovl-6 was used to determine their roles in de novo synthesis of specific MUFA species in mammalian cells. Treatment of rat insulinoma (INS)-1 cells with elevated glucose increased de novo FA synthesis and the ratio of MUFAs to saturated FAs. Elovl-5 knockdown decreased elongation of 16:1,n-7. Elovl-5 overexpression increased synthesis of 18:1,n-7; however, this increase was dependent on stearoyl-CoA desaturase-driven 16:1,n-7 availability. Knockdown of Elovl-6 decreased elongation of 16:0 and 16:1,n-7, resulting in accumulation of 16:1,n-7. Elovl-6 overexpression preferentially drove synthesis of 16:0 elongation products 18:0 and 18:1,n-9 but not 18:1,n-7. These findings demonstrate that coordinated induction of FA elongase and desaturase activity is required for balanced synthesis of specific n-7 versus n-9 MUFA species. Given the relative abundance of 16:0 to 16:1,n-7 and the specificity of Elovl-6 for 16:0, Elovl-6 is a major elongase for 18:1,n-9 production.

2D-NMR reveals different populations of exposed lysine residues in the apoB-100 protein of electronegative and electropositive fractions of LDL particles

Several potentially atherogenic LDL subfractions present low affinity for the LDL receptor, which result in impaired plasma clearance. Electronegative LDL [LDL(-)] is one of these minor subfractions and the molecular basis for its reduced receptor affinity is not well understood. In the present study, high-resolution 2D-NMR spectroscopy has been employed to characterize the surface-exposed lysine residues of the apolipoprotein (apo)B-100 protein in both LDL(-) and LDL(+) subfractions. LDL(+) showed two populations of lysine residues, similar to those previously described in total LDL. "Normal" Lys have a pk(a) of 10.4 whereas "active" Lys have a pk(a) of 8.8 and have been suggested to be involved in receptor binding. In contrast to LDL(+), the LDL(-) subfraction presented a third type of Lys, named as "intermediate" Lys, with a different microenvironment and higher basicity (pk(a) 10.7). These intermediate Lys cannot be reliably identified by 1D-NMR. Because the abundance of normal Lys is similar in LDL(+) and LDL(-), the intermediate Lys in the apoB-100 molecule of LDL(-) should come from a group of active Lys in LDL(+) particles that have a less basic microenvironment in the LDL(-) particle. These differences between LDL(+) and LDL(-) are indicative of a distinct conformation of apoB-100 that could be related to loss of affinity of LDL(-) for the LDL receptor.

Continuous-flow laboratory simulation of stream water quality changes downstream of an untreated wastewater discharge

In regions of the world with poor provision of wastewater treatment, raw sewage is often discharged directly into surface waters. This paper describes an experimental evaluation of the fate of two organic chemicals under these conditions using an artificial channel cascade fed with a mix of settled sewage and river water at its upstream end and operated under continuous steady-state conditions. The experiments underpin an environmental risk assessment methodology based on the idea of an “impact zone” (IZ) – the zone downstream of wastewater emission in which water quality is severely impaired by high concentrations of unionised ammonia, nitrite and biochemical oxygen demand (BOD). Radiolabelled dodecane-6-benzene sulphonate (DOBS) and aniline hydrochloride were used as the model chemical and reference compound respectively. Rapid changes in 14C counts were observed with flow-time for both these materials. These changes were most likely to be due to complete mineralisation. A dissipation half-life of approximately 7.1 h was observed for the 14C label with DOBS. The end of the IZ was defined as the point at which the concentration of both unionised ammonia and nitrite fell below their respective predicted no-effect concentrations for salmonids. At these points in the cascade, approximately 83 and 90% of the initial concentration of 14C had been removed from the water column, respectively. A simple model of mineral nitrogen transformations based on Michaelis–Menten kinetics was fitted to observed concentrations of NH 4, NO 2 and NO 3. The cascade is intended to provide a confirmatory methodology for assessing the ecological risks of chemicals under direct discharge conditions.

ARF Induces Autophagy by Virtue of Interaction with Bcl-xl

The ARF tumor suppressor controls a well-described p53/Mdm2-dependent oncogenic stress checkpoint. In addition, ARF has recently been shown to localize to mitochondria, and to induce autophagy; however, this has never before been demonstrated for endogenous ARF, and the molecular basis for this activity of ARF has not been elucidated. Using an unbiased mass spectrometry-based approach, we show that mitochondrial ARF interacts with the Bcl2 family member Bcl-xl, which normally protects cells from autophagy by inhibiting the Beclin-1/Vps34 complex, which is essential for autophagy. We find that increased expression of ARF decreases Beclin-1/Bcl-xl complexes in cells, thereby providing a basis for ARF-induced autophagy. Our data also indicate that silencing p53 leads to high levels of ARF and increased autophagy, thereby providing a possible basis for the finding by others that p53 inhibits autophagy. The combined data support the premise that ARF induces autophagy in a p53-independent manner in part by virtue of its interaction with Bcl-xl.

A performance comparison of nine selected liquid scintillation cocktails

Sample load capacity, sample compatibility, influence of sample load on 3H/ 14C counting efficiency, background count rate, figure of merit, quench resistance, and sample stability for nine selected, commercially available, liquid scintillation cocktails are presented in this study. The cocktails tested are Ecoscint A, Insta-Gel Plus, OptiPhase Hisafe 3, OptiPhase Trisafe, Ready Gel, SafeScint 1:1, Ultima Gold, Ultima Gold LLT, and Ultima Gold XR. For the data acquisition a Packard TriCarb Model 1900CA and a Quantulus 1220 liquid scintillation counter is used. All samples are prepared in either 20 mL low potassium, borosilicate glass vials or 20 mL high density, polyethylene vials. The aim of the full comparison study is to determine a single cocktail that best suits all measurement requirements of the liquid scintillation laboratory at SCK·CEN for the determination of low levels of radioactivity in biological and environmental samples. The results published in this report are intended to help other laboratories in selecting a cocktail fit for their purposes or to set up a similar study.

A shot at a Bayesian model for data analysis in AMS measurements

A new Bayesian model for analysis of AMS-measurement data is presented. A first-order continuous autoregressive process is used to model the drift in the machine throughput or the modern reference level. Uncertainty for individual measurements is given by the 14C counting statistics. With the use of the autoregressive process the standard level for an unknown sample is estimated probabilistically and the parameters for this process are determined from the measurement data as a whole. Thus, the model is able to give reliable uncertainties even when the number of measurements is small. Simulations have been made to compare the novel method with the conventional mean-based method of data analysis. Problems related to the non-normality of the end result in the mean-based method are discussed. The present Bayesian model seems to give more accurate results and more rigorous error bounds.