A reliable and accurate procedure for preparing low-activity efficiency calibration standards for germanium gamma-ray spectrometers

An empirical procedure is described which is readily capable of producing reliable and accurate efficiency calibration sources of low activity for high-resolution -ray spectrometers. Known activities of a multiisotopic solution are adsorbed onto a mixture of cation and anion-exchange resins and chromatographic-grade cellulose. The dried powder that results can be mixed with any desired solid natural matrix material to produce a homogeneous, extended geometry, efficiency calibration source. Results are presented which enable the method to be critically evaluated.     

Stripping voltammetry using sequential standard addition calibration with the analytes themselves acting as internal standards

A novel two-step standard addition calibration procedure for stripping voltammetry, whereby the analytes under investigation act as internal standards for each other, is described. In this way, the benefits of an internal standard for improving precision are obtained, without the requirement to add internal standard solutions. Only the standard solutions used to perform quantification are required. The proposed methodology has been tested against the traditional standard addition method with no internal standardisation, for the measurement of Zn, Cd, Pb and Cu in synthetic and real ambient air sample digests. The new procedure is shown to improve substantially precision and accuracy compared to traditional standard addition without internal standardisation.     

Nonconventional methods for accurately calibrating germanium detectors

We have developed two nonconventional but complementary methods whereby the detector efficiency of a coaxial germanium detector can be determined within an accuracy of a few percent for the energy region from 0.05 to 4.0 MeV without using conventional radioactive standards.     

Total efficiency of gamma-ray detection by germanium semiconductor detectors

The total efficiency of γ-ray detection by Ge(Li) semiconductors is calculated for the energy range 0.1 to 10 MeV, for cylindrical detectors with radii from 0.25 cm to 2.5 cm and thicknesses from 0.1 cm to 1 cm, at distances between the source and detector ranging from 2 cm to 20 cm.     

Efficiency calibration of Ge(Li) detectors,and the effects of geometry and physical properties of samples on the efficiency of detectors

To perform quantitative analysis of Ge(Li) spectra, not only energy calibration but also efficiency calibration procedures are done. Although the use of standard sample volume, mass, density and geometry is preferable, this optimum is not always possible. So to achieve an optimization is the main objective of the work reported here. Efficiencies have to be determined as a function of energy, density, thickness, mass of sample, intensity, and source-to-detector distance.     

Analytical approximation of the gamma ray spectra from germanium detectors

Several functional forms for shape function are tested using data obtained by a Ge(Li) detector. A function built from a Gaussian, a step function and a symmetric or asymmetric tailing on a linear or polynomial background is needed. A new shape function is proposed and superior fit are obtained.     

The divergence from Gaussian of peak shapes in gamma-ray spectra observed with germanium detectors

A formula for the fitting of gamma-ray peaks produced from solid state detectors is suggested, which takes account of possible distortion from a pure Gaussian shape. Comparative analyses of peaks using the formula, and an assumed Gaussian shape are presented showing the improvement in the fitting. When accurate peak energies are required in order to identify unknown nuclides, it is recommended that the expression provided is used, since the calculated peak positions are influenced by the extent of the distortion present.     

The absolute calibration of mid-infrared transmittance standards

The National Physical Laboratory (NPL) uniquely has 40 years’ experience in pioneering, developing instruments and exploiting the spectroscopic advantages of the Fourier transform technique. This has allowed an objective and knowledgeable appraisal of the Fourier transform technique for ordinate standards work. At present not enough is known of the ways in which the many and complicated categories of systematic error in such instruments can be identified, separated, evaluated and corrected for. Thus at NPL grating spectrophotometers are still used to determine, maintain and disseminate transmittance standards for the mid-infrared spectral region. This allows well-established methods developed in UV/visible region work to be adapted and used to minimise and where necessary correct for the residual systematic errors.

The paper expands on the above considerations, and goes on to describe the techniques used to explore possible systematic errors and to give an absolute regular transmittance calibration to a suitable secondary and transfer standard. This last was designed at NPL and consists of a Schott NG11 optical glass filter mounted in a protective milled frame that allows for reproducible alignment in the spectrophotometer, and this is described, together with the particular properties that were required of it. Seven levels of transmittance are provided, ranging from about 73% down to just above zero. The uncertainties of calibration are of order ±0.1%, varying with transmittance level. The pros and cons of highly reflecting filters are also discussed, and it is concluded that these last should be used initially to explore and minimise the interreflection errors in an instrument by adding stops, etc. and then the NPL-certified NG11 filter should be used to actually calibrate the ordinate scale.     

Calculation of peak-to-total ratios for high purity germanium detectors using Monte-Carlo modeling

Summary In order to estimate by calculation the magnitude of the true coincidence summing losses that may be affecting the observed gamma-ray spectrum of a given nuclide, measured using a spectrometer, knowledge of the total detection efficiencies at the gamma-ray energies within the cascades is essential. The total efficiency can be determined from the full energy peak efficiency, provided the peak-to-total ratio is known. For a given high purity germanium (HPGe) detector, one can establish an intrinsic peak-to-total (P/T) efficiency curve using a set of measurements performed with “single” (ideally monoenergetic) gamma-emitting nuclides (e.g., ²⁴¹Am, ¹⁰⁹Cd, ⁵⁷Co, ¹¹³Sn, ¹³⁷Cs, ⁶⁵Zn). Some of these nuclides are short lived and so have to be replaced periodically. Moreover, the presence of low energy gamma-rays and X-rays in most of the decay schemes complicate the empirical determination of the P/T ratios. This problem is especially severe if measurements are made using HPGe detectors that have a very thin dead layer. The problems posed by low energy gamma-rays and X-rays can be avoided by using absorbers, but then one has to be careful not to perturb the intrinsic value of the P/T ratio being sought. This paper addresses these problems. Measurement related limitations are avoided if one can use a computational technique instead. In the work presented here, the feasibility of using a Monte-Carlo based technique to determine the P/T ratios at a wide range of energies (60 keV to 2000 keV) is explored. The Monte-Carlo code MCNP (version 4B) is used to simulate gamma-ray spectra from various nuclides. Measured P/T ratios are compared to calculated ratios for several HPGe detectors to demonstrate the generality of the approach. Reasons for observed disagreement between the two are discussed.     

Effort on calibration of infrared spark ablation of copper with synthetic copper standards

Two types of copper samples, compact certified copper reference materials and calibration samples prepared from liquid doped, pressed copper powders, were studied in terms of accuracy of obtained calibration functions originating from infrared spark ablation. Additionally, corresponding particle size distributions of the aerosols from infrared spark ablation were recorded. It is shown that the differences in quantification results, originating from the two sets of calibration functions, could not mainly be ascribed to different particle size distributions of the two copper sample types. Possible other causes, as different ablation rates, parts of melting and differences of the chemical constitutions of the two sample types were explored.     

New NIST-traceable standards for calibration and validation of DSC

Several reference materials and associated reference values have been recommended for the purposes of calibration and/or validation of differential scanning calorimetry (DSC). However, issues of available sample purity, reference-property value accuracy, and potential undesired reactions between reference materials and the materials used for sample pans have not been considered sufficiently in some of the recommendations of property values for the materials. These limitations attenuate greatly the usefulness of many of these reference material recommendations. Indeed, the state of uncertainty regarding true reference property values can be shown to be a limiting factor in the uncertainty of measurements made with DSC. NIST has certified the temperatures and enthalpies of fusion of two new Standard Reference Materials, SRMs 2234 and 2235, in order to help alleviate some of the difficulty.     

Efficiency calibration in the program ANA

A new algorithm for calculating efficiency calibration has been implemented. It is based on orthonormal polynomial approximation and is capable of dealing with polynomials of degree up to 12 and to handle up to 20 experimental points. Additional utilities have been provided to control the calibration procedure including restriction of the maximal polynomial degree, applying corrections for coincidence summing, managing experimental data set (rejecting/restoring data from calculation) and graphical representation of the resulting curve and so on.     

The use of internal standards in ICP-MS

Careful study of the matrix effect in ICP-MS showed that, in all cases studied, the magnitude of the signal suppression or enhancement depends in a regular way on the mass number. Hence, accurate correction for non-spectral interferences is only possible using an internal standard with mass number close to that of the analyte element(s). It is also shown that using an internal standard with mass number close to that of the analyte improves the precision. For both cases, the ionization energy of the internal standard seems to be of no or only secondary importance. To obtain optimal precision and accuracy, the internal standard should be selected as close in mass number as possible to that of the analyte element(s). When a number of elements over a considerable mass range are to be determined, several internal standards have to be used.     

Depth-profiling of deuterium in steels: Preparation of calibration standards

The preparation of a set of standards for the calibration of the analysis of deuterium using the²H/³He, p/⁴He reaction is described. Important considerations in the selection of a suitable standard are that it should be stable under irradiation and be simple to prepare with ready control of the amount of analyte. These requirements were met by Ca/OH/₂ which was prepared by the replacement of hydrogen with its isotope in known proportions in the reaction of CaO with water.     

Proposed performance qualification and calibration method for evaporative light scattering detectors

Summary A method for the performance testing of Sedex 55 Evaporative Light Scattering Detectors (ELSD) has been developed as part of our regulatory compliance programme. The method uses a readily available chromatographic column, caffeine as the test probe and can be completed in less than one hour.     

Application of orthogonal space regression to calibration transfer without standards

To transfer a calibration model in cases where the standardization samples are rare or unstable, a method based on orthogonal space regression (OSR) is proposed. It uses virtual standardization spectra to account for response changes between instruments or batches. A comparative study of the proposed OSR, piecewise direct standardization, finite impulse response, orthogonal signal correction, and model updating (MU) was conducted on both pharmaceutical tablet data and chlorogenic acid data. The results of these studies suggest that both the OSR and the MU are superior to the other transfer techniques in terms of root‐mean‐squared error of prediction and ratio of performance to interquartile distance. Moreover, OSR requires no identical standard samples, and it avoids re‐optimizing the transfer models. In conclusion, both the differences among spectra measured on different spectrometers and the differences between different batches can be corrected successfully using the OSR method. Copyright © 2013 John Wiley & Sons, Ltd.     

Efficiency calibration curves in gamma-ray spectrometry for samples with different matrices

A new method for determination of efficiency calibration curves in -spectrometry is described. From the calibration curves for a bulky sample and a point source, the curves for sources with the same geometrical parameters as the bulky sample but different attenuation coefficients may be calculated. The method was tested at an energy of 88 keV on a set of samples with attenuation coefficients between 0.2 and 2 cm² g^–1. The conditions where the new method is useful are discussed.     

Efficiency for close geometries and extended sources of a p-type germanium detector with low-energy sensitivity

Typically, germanium detectors designed to have good sensitivity to low-energy photons and good efficiency at high energies are constructed from n-type crystals with a boron-implanted outer contact. These detectors usually exhibit inferior resolution and peak shape compared to ones made from p-type crystals. To overcome the resolution and peak-shape deficiencies, a new method of construction of a germanium detector element was developed. This has resulted in a gamma-ray detector with high sensitivity to photon energies from 14 keV to 2 MeV, while maintaining good resolution and peak shape over this energy range. Efficiency measurements, done according to the draft IEEE 325-2004 standard, show efficiencies typical of a GMX or n-type detector at low energies. The detectors are of large diameter suitable for counting extended samples such as filter papers. The Gaussian peak shape and good resolution typical of a GEM or p-type are maintained for the high count rates and peak separation needed for activation analysis.