Abstract Short-time instrumental neutron activationanalysis, with high throughput, sensitivity and accuracywithout matrix interferences, can be achieved in spite ofthe initial high count rate from both short and long-livednuclide activation, if the experimental conditions are opti-mized by the combination of techniques. Thus the initialusually high counting rate can be faced by a loss-freecounting system to avoid blocking or distortion of the sys-tem. The rapid radioactive decay and the consequent lowcounting statistics of short lived nuclides can be compen-sated by source-detector distance variation during thecounting period and by cyclic and cumulative activation.Matrix spectral interferences can be reduced by ion ex-change before activation for selective element presepara-tion and by neutron spectrum optimization such as epi-thermal neutron activation for selective element peak en-hancement.

Introduction

In instrumental neutron activation multielement gamma-ray spectroscopy there is often a problem of high countrates, due to the presence of many elements leading topulse-pile up and dead-time distortions. Thus the decaytime has to be prolonged, but then in both short and long-lived nuclide analysis, the shorter lived nuclides will notbe measurable any more and the throughput will be re-duced. On the other hand, because of the rapid radioactivedecay of short-lived nuclides their measurement time hasto be short and thus the counting statistics low. In addi-

tion, even if the irradiation-decay-counting time sequenceis optimized, there might be strong interferences of cer-tain high concentration nuclides. Several techniques havebeen developed and combined to overcome these prob-lems and to enhance the capability of neutron activationanalysis.

Methodology

An electronic loss-free counting system has been developed formeasurements at high decaying count rates [1], correcting for thelost counts even at dead-times higher than 90%. A movable sourceholder system during the counting period reduces gradually thesource-detector distance to increase the geometry factor and thecounting efficiency in order to compensate for rapid radioactivedecay of short lived nuclides [2–4]. Thus the count rate can be keptconstant, the counting time prolonged and in combination withcyclic and cumulative activation the counting statistics can be im-proved. Ion exchange before irradiation for selective element pre-separation reduces the matrix interferences from other elements.Finally modified neutron spectra such as epithermal neutron spec-tra by cadmium screening at the radiation position may reduce theinterference of certain elements of high concentration with lowneutron cross-sections at the epithermal region. A combination ofthe above techniques may enhance the capability of instrumentalneutron activation analysis considerably.

Results and discussion

By using loss-free counting it was possible to measuresamples of about 50 mg of Au reference ores fromCCRMP (Canadian Certified Reference Material Project)[5], as well as an As biological reference material fromBCR (Bureau of Reference Communities) and an As con-taining mussel sample from Elevsina Bay near Athens,Greece. The measurements were performed one day aftera 30 min irradiation. To obtain high counting rates forgood counting statistics this is necessary as the isotopesAs-76 and Au-198 have a t1

⁄2 of 1.09 and 2.7 d, respec-tively. The results are shown in Table 1.

Although in all cases the samples had dead-times ofmore than 60% the measurements caused no problems.The FWHM values of the peaks were in the range of 1.3

K. M. Ochsenkühn · N. N. Papadopoulos ·P. A. Kontopoulos · G. E. Hatzakis · A. C. Salveris ·N. F. Tsagas · M. Ochsenkühn-Petropulu

Short-time neutron activation gamma-ray spectroscopy

Fresenius J Anal Chem (1996) 355 :382–383 © Springer-Verlag 1996

Received: 25 September 1995 / Accepted: 2 November 1995

POSTER

K. M. Ochsenkühn (Y) · N. N. Papadopoulos ·P. A. Kontopoulos · G. E. Hatzakis · A. C. SalverisNational Center for Scientific Research “Demokritos”, Aghia Paraskevi Attikis, 15310 Athens, Greece

N. F. TsagasDemokritus University of Thrace, 67100 Xanthi, Greece

M. Ochsenkühn-PetropuluNational Technical University of Athens, 15773 Athens, Greece

383

to 1.9 keV. The loss-free counting technique is therefore avery good tool to measure samples with higher radioactiv-ity.

References

1. Westphal GP (1987) J Radioanal Nucl Chem 114 :2572. Papadopoulos NN, Tsagas NF (1994) Analyst 119 :6133. Papadopoulos NN, Tsagas NF (1994) Biol Trace Elem Res

43 :754. Papadopoulos NN, Spyrou NM, Tsagas NF, Hatzakis GE (1995)

J Radioanal Nucl Chem 192 :555. Ochsenkühn-Petropulu M, Schramel P (1995) Mikrochim Acta

119 :265

Table 1 Results of Au containing reference materials , an As con-taining biological reference material and a mussel from ElevsinaBay near Athens, Greece

Matrix Au (µg/g)

MA-2 (gold ore) 1.91 ± 0.13CCRMP 1.86 (certified)GTS-1 (gold tailings) 0.38 ± 0.042CCRMP 0.35 (certified)

As (µg/g dry weight)

Mussel 5.1(Elevsina, Greece)

BCR 278 (mussel) 5.67 (measured)5.9 (certified)