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.     

Application of epithermal neutron activation in multielement analysis of silicate rocks employing both coaxial Ge(Li) and low energy photon detector systems

The instrumental activation analysis of silicate rocks using epithermal neutrons has been studied using both high resolution coaxial Ge(Li) detectors and low energy photon detectors, and applied to the determination of 23 elements in eight new U.S.G.S. standard rocks. The analytical use X-ray peaks associated with electron capture or internal conversion processes has been evaluated. Of 28 elements which can be considered to be determinable by instrumental means, the epithermal activation approach is capable of giving improved sensitivity and precision in 16 cases, over the normal INAA procedure. In eleven cases the use of the low energy photon detector is thought to show advantages over convertional coaxial Ge(Li) spectroscopy.     

Radiochemical separation and determination of europium by Ge(Li) detector in biological tissues

A simple neutron activation method has been developed for the determination of europium in biological tissues and applied in the analysis of marine organism samples with ±9% precision at the nanogram level. The method is based on the separation, by ion-exchange, of the rare earth group from dry or ashed irradiated tissues and subsequent determination of^152mEu, by γ-spectrometry using a lithium drifted germanium detector.^152mEu, separated almost completely from other than rare earth elements, with better than 98% chemical yield, is counted on the 121.8 keV photopeak which then is practically free from any other γ-ray energy interfering in this counting.     

Comparison of a coaxial Ge(Li) and a planar Ge detector in instrumental neutron activation analysis of geologic materials

Instrumental neutron activation analysis was made on three different rock samples and on mineral separates of plagioclase, pyroxene and magnetite. γ-spectra were recorded using both a coaxial and a planar Ge detector. Counting errors were compared for those elements that could be determined with both detectors. In nearly all cases, analyses using the planar detector yielded smaller errors. The use of this detector in instrumental analysis is of particular advantage, where only contents of rare earth elements are to be determined.     

Calculation of the absolute peak efficiency of Ge and Ge(Li) detectors for different counting geometries

A new method is presented to calculate with improved accuracy the absolute peak efficiency of cylindrical Ge and Ge(Li) detectors for point, disk and cylinder sources, positioned at any source-detector distance. Moreover attention was paid to true-coincidence effects. The method is extensively tested and applied for the analysis of reference materials. The accuracy turned out to be 3% or better. Research associate of the “Nationaal Fonds voor Wetenschappelijk Onderzoek”.     

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.     

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.     

Multielement photon activation analysis of coal samples using a Compton suppressed Ge(Li) detector

Multielement photon activation analysis has been carried out using a Compton suppressed Ge(Li) detector. The irradiated samples were N.B.S. standard reference coals and ashes. An increase in sensitivity by as much as a factor of three is attainable for some elements and this is especially valuable when analysing coals with low trace metal concentrations like N.B.S. reference material 1635.     

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.     

A semi-empirical formula for HPGe detector efficiency calibration

The precision obtainable by instrumental neutron activation analysisdepends strongly on the quality of the calibration of the gamma-ray spectrometerused for measuring the irradiated samples. Even when relative standardizationis employed, practical experience has shown the importance of detector calibration.This is especially relevant when radionuclides of varying activities are involved.The problems often encountered are those of intolerable high dead time andthe occurrence of summed peaks in the gamma-ray spectra. A simple but accuratesemi-empirical formula is presented that could effectively predict the efficiencyof a detector at any source-to-detector distance. Experimental data obtainedat 0, 50, 100, 200 mm source-to-detector distances are used to demonstratethe usefulness of this formulation. In addition, nine elements were analyzedin the NIST Orchard Leaves using absolute standardization including four elementsSm, Ta, Au, and La for which no certified values could be obtained.     

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.     

Full energy peak efficiency calibration of HPGe detector for point and extended sources using Monte Carlo code

Monte Carlo simulation is important to get efficiencies for cases where the experimental efficiencies are difficult to get such as for samples with nonstandard geometries and for large samples. In this paper, efficiency of the HPGe detector, routinely used in our lab for a variety of samples, has been computed for point source geometry and its parameters has been optimized to match MCNP and experimental efficiencies within 5% at different sample to detector distances. This optimized geometry was then validated by efficiency transfer to other geometries.     

Low energy tail spectra of characteristic X-rays detected by a Si(Li) X-ray detector

Characteristic X-ray spectra are inherently accompanied by the tail spectra in the low energy side when X-rays are detected with a semiconductor X-ray detector. The tail spectra of Mg, Al, Si, P, S, Cl, K, Ca, Ti, Cr and Mn KX-rays have been observed using two different Si(Li) X-ray detectors. An annular source of 55Fe has been used to excite the Mg (1.25 keV) to Ti (4.5 keV) KX-rays, and 54Mn and 55Fe sources have been used for the detection of Cr (5.4 keV) and Mn (5.9 keV) KX-rays, respectively. Observed intensity ratios of the tail area Nt to the Gaussian X-ray peak area NP have exhibited to change remarkably at the Si-K adsorption edge energy 1.84 ke V. When X-ray spectra detected with different Si(Li) detectors are compared at some specific characteristic X-ray, different values of Nt/Np intensity ratios as well as different line shapes of tail spectra have been observed. Using a simple model, the thickness of Si layer which generates the tail spectrum has been estimated, i.e., the thicknesses are about 0.05 micron for one detector and 0.09 micron for the other detector. The generation of the tail spectrum is known to be partially due to the escape effect of photoelectrons or Auger electrons from the intrinsic region.(ABSTRACT TRUNCATED AT 250 WORDS)     

A program for complete quantitative neutron activation analysis using Ge(Li) detector gamma spectrometry and a small computer

It is shown that the large sophisticated programs designed for big computers can be efficiently used on medium scale machines for complete quantitative neutron activation analysis with Ge(Li) detectors.     

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.     

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

Zusammenfassung Die sehr gute Energieauflösung der Ge(Li)-Detektoren bei der -Spektrometrie ermöglicht in vielen Fällen der radiochemischen Analyse eine merkliche Verkürzung der chemischen Trennarbeit. Bei Photopeakausbeuten zwischen 3% (bei 0,1 MeV) und 0,1% (bei 1 MeV) eignet sich dieser Detektor besonders gut zur Messung von Aktivitäten der Größenordnung 1–100 nCi; für die Bestimmung kleiner Gehalte an Tellur in Selen durch Neutronenaktivierung und Messung des ¹³¹J ist bei Verwendung des Ge(Li)-Detektors eine schnelle einstufige Trennung ausreichend. Tellurgehalte von 5–100 ppm in kleinen, 5–20 mg betragenden Selenproben lassen sich so mit einer relativen Standardabweichung von 5–10% erfassen.

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Use of a Ge(Li) detector in radiochemical analysisI. Activation analysis of ppm amounts of tellurium in selenium

The very high energy resolution of Ge(Li) detectors in -spectrometry enables chemical separation processes to be shortened quite considerably in many cases of radiochemical analysis. At photo peak efficiencies between 3% (at 0.1 MeV) and 0.1% (at 1 MeV) this detector is especially suitable for measuring activities in the order of 1 to 100 nCi. For the determination of small amounts of tellurium in selenium by means of neutron activation analysis and measurement of ¹³¹I, a quick single-step separation suffices if a Ge(Li) detector is used. Between 5 and 100 ppm of tellurium can be thus determined with a relative standard deviation of 5 to 10% in small samples of selenium weighing between 5 and 20 mg.

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Teil des Tagungsvortrages; Teil II wird später veröffentlicht.     

Precision and accuracy of rare earth determinations in rock samples using instrumental neutron activation analysis and A Ge(Li) detector

Some of the rare earth elements have been determined in U.S.G.S. Standard Rocks W1 and G2 using instrumental neutron activation analysis with a Ge(Li) detector. The experimental conditions were chosen to investigate the precision and accuracy to be expected when using the method for routine purposes. The results are compared with data obtained after chemical group separation of the rare earths and with data reported by other workers; the agreement is, in general, good. La, Ce, Nd, Sm, Eu, Tb, Yb, Lu and Se were determined with the instrumental method and, after group separation, data were obtained for Gd, Dy, Ho and Tm in addition. It is concluded that the accuracy of the data obtained with the non-destructive method is, in general, as good as the precision for each element.     

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.     

Effect of cascade coincidences on the efficiency calibration of a gamma-X detector

The sensitivity on n-type gamma-X detectors for low-energy X- and -rays calls for coincidence corrections in the efficiency calibration that do not apply to the calibration of p-type detectors. Corrections were calculated for the effect of cascade coincidences between -rays, X-rays, annihilation radiation, and bremsstrahlung, for 15 radionuclides frequently used for efficiency calibration. Experimental results are presented for a -X detector with 37% relative efficiency at distances from 0.9 to 17.5 cm. After coincidence correction smooth efficiency curves were found for the energy range 12 to 2750 keV, even for the position closest to the detector.