Application of time-dependent processes in the prompt-gamma activation analysis

Prompt-gamma activation analysis (PGAA, or PGNAA) is a nuclear analytical technique for non-destructive determination of elemental and isotopic compositions. In the present work the time-dependent processes of PGAA were investigated and utilized to attain better precision and more reliable analysis. The characteristic times of these vary between picoseconds and several hours; hence different methods have to be involved.

My first task was to test the capabilities of three digital signal processors when used for PGAA, and to compare their performance to our existing analog electronics. The energy resolution at different gains, as well as the time stability and noise sensitivity were examined. It was concluded that the digital spectrometers have worse resolution than the analog system, and seem to be incompatible with detectors having non-ideal preamplifiers. However, I managed to utilize many existing advantages of the digital spectrometry. The principle of the list-mode data acquisition, as implemented in a digital spectrometer of XIA (X-Ray Instrumentation Associates), was used with success in investigation of radioactive decays. It was shown that this kind of acquisition preserves very detailed time and energy information which can be utilized with great flexibility at the evaluation stage. A computer program was prepared to process the raw data, by sorting the events into energy-spectra, time spectra or two-dimensional histograms. A dead-time correction model was worked out, and was validated with measurements. Methods for background correction of decay spectra and a fitting procedure for 3D regions were invented. I proved that the method gives correct half-life data for various medium-lived radionuclides. I compared the precision of the off-line processing techniques and demonstrated that they give more accurate results than the conventional multichannel scaler (MCS) technique on the same dataset. I demonstrated that by simultaneous use of energy and time information, peaks overlapping in the energy spectrum can be decomposed. The elemental compositions of halogenide samples were determined with combined use of PGAA and post-irradiation counting. The latter method was able to reduce the limit of detection for Th.

A new beam-chopper was also installed at the PGAA facility. This is an effective tool to investigate short-lived nuclides, to reduce the limits of detection and improve the selectivity for a group of elements. I set up new controller and data acquisition electronics, to acquire the beam-on and beam-off spectra concurrently. I derived simplified formulas for use in chopped-beam PGAA and calculated the in-beam saturation factor for the case of branching activation. Partial gamma-ray production cross-sectionsand k ₀-factors of 16 short-lived nuclides were determined with this technique. The experimental results were compared to the k₀-NAA literature. A thorough estimation of uncertainties was also done, following the guidelines of ISO. I proved that the technique can offer better selectivity and lower detection limits for many elements than conventional PGAA.

Finally, a new fitting procedure for the Doppler-broadened peak of Boron was worked out and validated. I used a more sophisticated peak-shape function and a modified background-shape in the model function. I also paid attention to the energy-dependent efficiency. The sample method was extensively checked on model samples, representing typical peak shapes. The effect of the matrix on the peak shape was also studied. Peaks of Boron and interfering elements were successfully decomposed in geological and dosimetrical samples.

1. L. Szentmiklósi, T. Belgya , Zs. Révay, G. L. Molnár: Digital signal processing in prompt-gamma activation analysis, J. Radioanal. Nucl. Chem. 264 (1) (2005) 229-234.

2. Zs. Révay, T. Belgya, L. Szentmiklósi, G. L. Molnár: Prompt gamma activation analysis using a chopped neutron beam, J. Radioanal. Nucl. Chem. 264 (2) (2005) 277-281

3. G. L. Molnár, Zs. Révay, L. Szentmiklósi: New perspectives for very short-lived neutron activation analysis, J. Radioanal. Nucl. Chem. 262 (1) (2004) 157-163

4. L. Szentmiklósi, T. Belgya, G. L. Molnár, Zs. Révay: Time resolved gamma-ray spectrometry, J. Radioanal. Nucl. Chem. 271 (2) (2007) 439-445

5. L. Szentmiklósi, K. Gméling, Zs. Révay: Fitting the boron peak and resolving interferences in the 450-490 keV region of PGAA spectra, J. Radioanal. Nucl. Chem. 271 (2) (2007) 447-453

6. L. Szentmiklósi, Zs. Révay, G. L. Molnár: Three-dimensional data processing for the time resolved gamma-ray spectrometry, J. Radioanal. Nucl. Chem. 265 (2) (2005) 213-219.

7. L. Szentmiklósi, Zs. Révay, R. Chobola, P. Mell, S. Szakács, I. Kása: Characterization of CaSO₄-based dosimeter materials with PGAA and thermoluminescent methods, J. Radioanal. Nucl. Chem, 267 (2) (2006) 415-420

8. L. Szentmiklósi, Zs. Révay, T. Belgya: An improved beam chopper setup at the Budapest PGNAA-NIPS facility (accepted for publication, IRRMA-6 proceedings of Nucl. Instr. Meth. B.)

9. L. Szentmiklósi, Zs. Révay, T. Belgya: Investigation of nuclides activated in-beam with chopped-beam PGAA, Nucl. Instr. Meth. A 564 (2006) 655-661