Accurate Background Correction Research Articles

Neutron Coincidence Measurements and Monte Carlo Modelling of Waste Drums Containing Reference Nuclear Material

Within the EC-funded CHANCE project several non-destructive techniques are being considered for the assay of waste bearing drums. Such techniques include calorimetry, gamma-ray spectrometry and neutron coincidence counting. The aim is to quantify uncertainties on the inventory of radionuclides, and how these are potentially reduced by combining the signatures from different techniques in the data analysis. In this framework, neutron coincidence measurements were carried out with two slab counters based on 3He detectors coupled to shift register electronics. Such a system consists of two identical slabs with 6 detectors each, and is transportable, rather compact and flexible in terms of sizes and geometries that can be measured. With this system three 200 L drums containing certified reference nuclear material and different filling materials were measured. The certified nuclear material was in the form of 21 pellets of mixed oxide of U and Pu with a total mass of about 10.5 g; in addition, a single pellet of about 10.05 g was also available. The pellets could be placed in predefined positions within the drum in a reproducible way. The geometry and composition of the three drums was well characterized and consisted of Ethafoam, a mixture of Ethafoam, stainless steel and PVC, and mortar with an inner core of extruded polystyrene. The measurement setup was arranged such that the drum was placed between the two slab counters. The positions of the slab counters relative to the drum were accurately measured before each measurement, and a dedicated system was used to minimize the uncertainty on the detector positioning. The measurement data were first analysed by applying the point model of Hage and the mass of nuclear material in the drum was determined from the rate of totals and reals and the radionuclide composition. Due to the fact that not all the point model conditions were met, we found that the point model overestimates the mass up to about 50%. In addition, a Monte Carlo model of the measurement geometry was developed using the MCNP code. The model was used to determine a calibration factor between the reals rate and the mass of the sample. Measurements with a calibrated 252Cf source were used to verify the model. With a Monte Carlo based approach the mass of the mixed oxide pellets is within a few percent from the nominal values, except for strongly asymmetrical configurations where the deviation is up to about 20%. The results reveal the importance of an accurate background correction and of accounting for surrounding materials of the building such as walls, floor and ceiling in the Monte Carlo model.

Accurate background correction in neutron reflectometry studies of soft condensed matter films in contact with fluid reservoirs.

Neutron reflectometry (NR) is a powerful method for looking at the structures of multilayered thin films, including biomolecules on surfaces, particularly proteins at lipid interfaces. The spatial resolution of the film structure obtained through an NR experiment is limited by the maximum wavevector transfer at which the reflectivity can be measured. This maximum is in turn determined primarily by the scattering background, e.g. from incoherent scattering from a liquid reservoir or inelastic scattering from cell materials. Thus, reduction of scattering background is an important part of improving the spatial resolution attainable in NR measurements. Here, the background field generated by scattering from a thin liquid reservoir on a monochromatic reflectometer is measured and calculated. It is shown that background subtraction utilizing the entire background field improves data modeling and reduces experimental uncertainties associated with localized background subtraction.

New transverse Zeeman effect method for mercury detection based on common mercury lamp

The accurate background correction can determine the minimum limit of trace mercury measurement in atmosphere by the cold vapor atomic absorption method. This paper studies a new method of mercury detection using the common mercury lamp as sources which correct the background according to the transverse Zeeman effect. The resonance spectral line (253.65 nm) of the meccury lamp generates σ-, σ+, and π linear polarized light in the vertical direction of the magnetic field. This study obtains mercury absorbance of σ-, σ+, and π light in different magnetic field intensity by using ultra-high resolution spectrometer, then gets the minimum field intensity of the method. We discuss the existing possible interference caused by benzene with narrow-band absorption and acetone with broadband absorption under 1.78 T magnetic field intensity. Taking σ- and σ+ as background light, and π as absorption light, we quantify the saturated mercury vapor cell with different lengths. With the accurate background correction, the R value of absorption fitting curve can achieve 0.99. Results indicate that the method can accomplish the job of accurate background correction and can be applied to trace mercury measurement in atmosphere.

Determination of Cd in urine by cloud point extraction–tungsten coil atomic absorption spectrometry

Cadmium concentrations in human urine are typically at or below the 1 μg L −1 level, so only a handful of techniques may be appropriate for this application. These include sophisticated methods such as graphite furnace atomic absorption spectrometry and inductively coupled plasma mass spectrometry. While tungsten coil atomic absorption spectrometry is a simpler and less expensive technique, its practical detection limits often prohibit the detection of Cd in normal urine samples. In addition, the nature of the urine matrix often necessitates accurate background correction techniques, which would add expense and complexity to the tungsten coil instrument. This manuscript describes a cloud point extraction method that reduces matrix interference while preconcentrating Cd by a factor of 15. Ammonium pyrrolidinedithiocarbamate and Triton X-114 are used as complexing agent and surfactant, respectively, in the extraction procedure. Triton X-114 forms an extractant coacervate surfactant-rich phase that is denser than water, so the aqueous supernatant is easily removed leaving the metal-containing surfactant layer intact. A 25 μL aliquot of this preconcentrated sample is placed directly onto the tungsten coil for analysis. The cloud point extraction procedure allows for simple background correction based either on the measurement of absorption at a nearby wavelength, or measurement of absorption at a time in the atomization step immediately prior to the onset of the Cd signal. Seven human urine samples are analyzed by this technique and the results are compared to those found by the inductively coupled plasma mass spectrometry analysis of the same samples performed at a different institution. The limit of detection for Cd in urine is 5 ng L −1 for cloud point extraction tungsten coil atomic absorption spectrometry. The accuracy of the method is determined with a standard reference material ( toxic metals in freeze-dried urine) and the determined values agree with the reported levels at the 95% confidence level.

Quantitative planar imaging method for measurement of renal activity by using a conjugate-emission image and transmission data.

We are proposing a method to accurately measure renal activity in renography using Tc-99m labeled tracers. This method uses a conjugate-view image and transmission data for attenuation correction, the triple energy window (TEW) method for scatter correction, and background correction techniques that consider the source volume for accurate background activity correction. To examine this method in planar imaging, we performed two renal phantom studies with various uniform background activity concentrations. One study used two ideal box-shaped kidney phantoms with a thickness of 2 or 4 cm in a water tank and the other study employed two real kidney-shaped phantoms in a fillable abdominal cavity. For these studies the kidney phantom-to-background activity concentration ratio (S) was changed from 5 to infinity. The transmission data were obtained with an external Tc-99m line array source. The anterior- and posterior-view emission images were acquired with a dual-headed gamma camera simultaneously and the TEW method was used to correct scatter for the emission and transmission images. The results showed that this method with both the accurate background correction and scatter correction could give depth-independent count rates and could estimate the true count rate with errors of less than 5% for all S values. However, if either accurate background correction or scatter correction was performed alone, the absolute error increased to about 50% for the smaller S values. Our proposed method allows one to accurately and simply measure the renal radioactivity by planar imaging using the conjugate-emission image and transmission data.

Accurate Measurement of Trace Elements Using an Innovative Fixed Goniometer for a Simultaneous Spectrometer

Advancements in trace clement analysis require improvements in both the signal-to-noise ratio and accurate background correction. With a sequential spectrometer, one can obtain detection limits of around 0.1 ppm for medium to heavy Z elements. Conditions can be individually optimized for each element, for example, selection of filters, collimators, crystals and background subtraction. The disadvantage is that the analysis time may become “long” if many elements are to be analyzed. This long exposure time can lead to the deterioration of some samples.

A square-wave wavelength modulation system for automatic background correction in carbon furnace atomic emission spectrometry

A square-wave wavelength modulation system, based on a rotating quartz chopper with four quadrants of different thicknesses, has been developed and evaluated as a method for automatic background correction in carbon furnace atomic emission spectrometry. Accurate background correction is achieved for the residual black body radiation (Rayleigh scatter) from the tube wall and Mie scatter from particles generated by a sample matrix and formed by condensation of atoms in the optical path. Intensity modulation caused by overlap at the edges of the quartz plates and by the divergence of the optical beam at the position of the modulation chopper has been investigated and is likely to be small.

A microanalytical method for the analysis of gold in biological media by flameless atomic absorption spectroscopy.

A simple and sensitive method is presented for the determination of gold in various biological matrices by flameless atomic absorption spectrometry (FAAS). The effects of the biological media on the atomic absorption of gold and the application of some of the important fundamental principles in relation to the measurement of the analyte are also discussed. The results demonstrate that, in FAAS analysis, to obtain maximum peak height sensitivity the samples must be confined to the centre of the graphite atomiser, and for accurate background correction the light paths of the analyte and the continuum lamps through the atomised sample must be synchronised. for measurement of gold in the presence of sodium chloride, the concentration of the salt in the gold standards and the samples must be matched to neutralise the severe depression of the gold signal by sodium.

Application of a Wavelength Modulation Inductively Coupled Plasma-Echelle Spectrometer to Cd and Pb Determinations in Marine Samples

The installation and performance of a wavelength modulation system in a custom-built inductively coupled plasma-echelle spectrometer is described. The system, based on a computer-controlled rotatable quartz plate mounted just behind the entrance slit, permits extremely reproducible short (∼0.1 nm) wavelength scans in the vicinity of any chosen wavelength, as well as automatic two-point background subtraction during analyses. Its effectiveness, both in facilitating the characterization of spectroscopic interferences and in enabling accurate background correction, is demonstrated by determinations of trace quantities of cadmium and lead in biological tissues and sediments. Its potential use for wavelength modulation plasma emission spectrometry is discussed.

Some Design Considerations in Background Corrected Atomic Absorption Instrumentation

Two recent papers have described modifications to single-beam Varian Techtron AA5 atomic absorption spectrometers which are purported to allow accurate background correction to be performed with that instrument. Unfortunately, the simple modifications described will not, generally, give an accurate atomic absorption signal under conditions often encountered in nonflame atomic absorption experiments. The reason for this is that emission light from either the atomizer or its contents cannot be accurately corrected for with the equipment described.