Radial efficiency gradients in Ge(Li) gamma detectors

Counting of radioactive sources in contact with the vacuum container of a Ge(Li) detector may lead to errors because of large efficiency gradients. In order to explore the radial dimension of this problem, several point sources were stepped across in contact with the cap of several detectors, and curves of absolute efficiency against radius were measured. The insensitive core of an open-end coaxial detector reduced the central point-source efficiency at 122 keV at the cap to 20% less than a comparable closed-end detector. In compensation, however, there was a reduction in the radial efficiency gradient. The radial efficiency gradient was approximately proportional to the radius, with the central flattening for the open-end detector superimposed on the trend.     

A new improved expression for gamma-ray detection efficiency of Ge(Li) detectors

In the present paper, a new improved expression for -ray detection efficiency of Ge(Li) detectors, ⁰ , is given. It is represented as a continuous function of x (viz. E ^–1) with a maximum and decreases very rapidly to a small positive value as -ray energy, E, drops to 40 keV or lower, but slowly as E rises to 1.7 MeV or higher. Since it can well represent the whole physical process of the -ray detection, this expression may be one of the simplest and most precise representations, for ⁰ at the present time.     

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.     

Precise Ge(Li) spectrometry at high and variable counting rate

New circuits are presented to determine precisely the counting losses suffered in the entire gamma-ray spectrometer and to allow automatic correction for them even in the case of time-dependent counting rates as encountered in the measurement of short-lived radioisotopes. Experimental proof is given that the proposed circuitry allows accurate quantitative measurements in gamma-ray spectrometry. With counting rates up to 20,000 cps losses amount to less than 1.5%.     

A computer program for calculating Ge(Li) detector counting efficiencies for Marinelli beakers

A novel computing method has been developed to calculate the absolute photopeak efficiency of a Ge(Li) detector for Marinelli beakers of different heights and diameters and with variable density. For each point in the cylindrical sample the detection efficiency is calculated taking into account the distance from the detector and gamma-ray attenuation and the efficiency is integrated numerically over the volume of the sample. The detector is approximated as a point detector with an experimentally determined effective interaction depth. It is necessary to measure the absolute efficiency for a point source located on the detector axis and on a line beside the detector parallel to the axis. The computer program calculates the absolute counting efficiency for Marinelli beakers of any geometry and for any density. The measured and calculated values for three different densities give a good (–2.2%) overall agreement.     

The relative sensitivity and accuracy of NaI(Tl) and Ge(Li) detectors for minor components

A mixture of²³³Pa,¹⁴⁰La,⁸²Br,⁶⁴Cu.⁷²Ga,⁶⁰Co, and²⁴Na of known disintegration rates was counted on a NaI(Tl) and a large Ge(Li) detector. Both systems detected²³³Pa when present at 0.2%, and accurate results were observed for²³³Pa,¹⁴⁰La, and⁶⁰Co when present at greater than 0.5% of the total disintegration rate. The accuracy of the results from either system was not significantly different since the mean ratio of the observed to true results was 0.99±0.05 and 1.06±0.06, respectively. However, equal sensitivity and accuracy were achieved only when the counting interval for the germanium system was 1000 min, or 60 to 600 times that for sodium iodide.     

A least squares spectrum fitting method for the measurement of Ge(Li) gamma-ray peak areas

A method is described for the measurement of very small peak areas and for the resolution of multiplet peaks based upon least squares fitting procedures. The method does not use an analytical function to describe the peaks, but instead uses an actual peak shape derived from a γ-spectrum. Analysis of the IAEA. Intercomparison Spectra, G-1, is used to illustrate the technique.     

Relative efficiency calibration of Ge(Li) detector in low energy region

The relative gamma-ray intensities in the energy region between 122 and 411 keV in the decay of¹⁵²Eu were measured by using a Ge(Li) detector. Its efficiency calibration was carried out with the radioactive sources of²⁴¹Am,⁵⁷Co,²⁰³Hg,¹³⁷Cs,¹³³Ba,⁷⁵Se,¹⁶⁹Yb and¹⁹²Ir.     

Photoefficiency of gamma-radiation recording and limits of detection for individual elements in activation analysis using Ge(Li) semiconductor detectors

The photoefficiency and source utilization coefficients for 3.5, 28 and 45 cm³ Ge(Li) detectors were determined. Values of the source utilization coefficient were obtained as a function of the measurement geometry. Limits of detection were determined for 30 elements with long-lived isotopes and 38 elements with short-lived isotopes.     

Parametric normalization for full-energy peak count rates obtained at different geometries

The Effective Interaction Depth (EID) law has been systematically studied and applied to parametric normalization for peak count rates obtained at different source-detector distances (S-D). The errors caused by EID normalization are less than 4% over the full range of S-D (from to several mm) for true coincidence-free -rays. Parametric corrections for the true coincidence (summing) effect are also established, based on simplified decay schemes and P/T ratio determinations. The total response of Ge detector for single-energy -rays (T) is clearly defined with scattering contributions from surroundings included. Errors from summing effect corrections are also less than 4%. The combined EID normalization and summing effect corrections give an error no greater than 5.7% for the worst situations (several mm S-D and cascade-crossover decay scheme), acceptable for most practical K₀ NAA.     

Chemical reactions and adsorption geometries of pyrrole on Ge(100)

The adsorption structures of pyrrole (C(4)H(5)N) on a Ge(100) surface at various coverages have been investigated with both scanning tunneling microscopy (STM) and ab initio density-functional theory (DFT) calculations. Three distinct features are observed in the STM images at low coverages. The comparison of the STM images with the simulation reveals that the most dominant flowerlike feature with a dark side is that the adsorbed pyrrole molecules with H dissociated form bridges between two down Ge atoms of neighboring Ge dimer rows through N-Ge bonding and beta-carbon-Ge interaction. The flowerlike feature without a dark side is also observed as a minority, which is identified as nearly the same structure as the most dominant one where a dissociated H is out of the feature. The third feature showing bright protrusions may be due to a C- and N-end-on (CN) configuration, where the pyrrole molecule is located on one dimer row. At higher coverages, the number of localized configurations increases.     

Cross-correlation analysis of Ge(Li) spectra

A sensitive technique is proposed for activation analysis using cross-correlation and improved spectral orthogonality achieved through use of a rectangular zero area digital filter.     

A method for systematically analyzing Ge(Li) photopeaks

A relatively straight forward method of fitting Ge(Li) photopeaks with a Gaussian histogram is presented. The method, which is expecially suited to photopeaks which are distributed over a relatively small number of channels, determines photopeak area, pulse height, resolution and the residual background. Results are given to indicate the Gaussian distribution of the experimental data from −1σ to +6σ.     

Spline and polynomial models of the efficiency function for Ge gamma-ray detectors in a wide energy range

Summary In one of our recent papers, the applicability of linear parameter functions for fitting the full-energy peak efficiency of n-type Ge gamma-ray detectors has been examined over a wide energy range of 50-8500 keV. In that paper we compared six different analytical functions and showed that higher-order polynomial functions on a log-log scale gave the best performance. However, there is a drawback to using the log-log scale when an additive function of efficiency at different energies or of the inverse efficiency has to be used in a fitting procedure. In the present study, the applicability of higher-order polynomial and spline functions to linear and inverse efficiency, but logarithmic energy scales, is examined.     

Analysis for the rare-earth elements by neutron activation and Ge(Li) spectrometry

A procedure for analysis of 12 of the rare-earths, tested on more than a hundred samples of rocks and minerals, is described. Carrier is added for each element to be determined. Samples are fused with Na₂O₂. The rare-earths are separated as a group, then precipitated and mounted as oxalates for Ge(Li) spectrometry. Following radioassay, chemical yields for the individual rare-earths are obtained by irradiation and counting of the carrier. About two months are needed to obtain complete data for a set of four samples, but many samples are analyzed concurrently. The precision and accuracy exceed ±5 per cent for 10 of the elements in 0.1 to 0.5 g samples of common rocks.     

Sum peak in gamma-energy spectra of NaI(T1) detectors

The factors affecting the sum peak of NaI(T1) detectors has been studied experimentally, namely the intrinsic photopeak efficiency, the geometry factor, the energy and the intensity of the incident photon beam. Several radioactive sources were used for the measurement of the intrinsic photopeak efficiency of the two NaI(T1) detectors used in this work (51 mm × 51 mm and 76 mm × 76 mm).⁸⁸Y,⁶⁰Co and²²Na, were used for sum peak determination. Different source to detector distances, to produce different values of geometry factor, and different source activities were used. The effect of geometry on the intrinsic photopeak efficiency is very pronounced at high geometry factor. The linear relation between the relative intensity of the sum peak and both the geometry factor and the intensity of the incident photons, facilitate the quantitative prediction of the sum effect on the energy response function of NaI(T1) detectors.     

Evaluation of Ge(Li) Compton suppression spectrometers for non-destructive radiochemical analysis

Ge(Li) Compton suppression spectrometers are evaluated relative to the specific needs of the analytical radiochemist. Their application to the analysis of radionuclides found in neutron activation, fission product, and environmental radionuclide analyses is discussed. This paper is based on work performed under United States Atomic Energy Commission Contract AT(45-1)-1830.     

Ge(Li) gamma-ray spectrometry as a pilot for NaI(Tl) gamma-ray spectrometry

Lithium-drifted germanium semiconductor detectors give much better resolution than do thallium-activated sodium iodide detectors, but much lower sensitivity. They can often advantageously be used in conjunction with NaI(Tl) detectors, to show whether corrections must be applied for activities other than the one to be measured and to provide the necessary information for calculation of the corrections.     

Verwendung eines Ge(Li)-Detektors in der radiochemischen Analyse

Zusammenfassung Zur Bestimmung der U- und Th-Gehalte in 0,5–1,5mg wiegenden Mikroproben von ThO₂-UO₂-Kernbrennstoffen wurde die Neutronenaktivierungsanalyse mit zerstörungsfreier-Spektrometrie eingesetzt. Bei Gehalten über 0,1% beträgt die relative Standardabweichung 6%. Der Th-Gehalt der Mikroproben wird durch-spektrometrische Messung des²³³Pa ermittelt; bei Natur-Uran enthaltenden Proben wird der U-Gehalt über²³⁹Np bestimmt, bei hochangereichertes UO₂ enthaltenden Proben über¹³¹J. Mit der beschriebenen Methode können sowohl unbestrahlte als auch bestrahlte ThO₂-UO₂-Kernbrennstoffe untersucht werden.

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Use of a Ge(Li) detector in radiochemical analysisII. determination of U and Th contents in microsamples of ThO²-UO² nuclear fuels

Neutron activation analysis with non-destructive-spectrometry has been used to determine U and Th in microsamples (0.5–1.5 mg) from ThO₂-UO₂ nuclear fuels. With U contents of more than 0.1% the relative standard deviation amounts to 6%. Th in the microsamples is estimated by measuring²³³Pa; in natural UO₂ containing samples²³⁹Np ist measured for U determination, in highly enriched UO₂ samples¹³¹J. Fresh und spent ThO₂-UO₂ nuclear fuels can be investigated by the method described.

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Teil I siehe [3].

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Herrn G. Hofmann danken wir für die Entnahme der Mikro-Bohrproben aus den bestrahlten und unbestrahlten Brennstoffschliffen.