Speakers
Dr
Danyal Turkoglu
(NIST)
Dr
Heather Chen-Mayer
(NIST)
Description
Isotopes emit characteristic prompt gamma (γ) rays immediately after neutron capture. In prompt gamma-ray activation analysis (PGAA), a high-resolution spectrometer measures γ-ray energies and intensities that are used for quantitative analysis of elemental composition of a bulk sample. In addition to the prompt γ rays, decay γ rays can be used for elemental assay.
To enhance the measurement of the relatively-weak decay γ rays, a beam chopper that cyclically blocks the cold neutron beam – thus eliminating the prompt γ rays when the beam is off – has been implemented. Unlike traditional instrumental neutron activation analysis (INAA) that transports the sample post-irradiation for counting, the stationary sample and detector configuration in chopped-beam PGAA enables measurement of very-short lived activation products with half-lives (T1/2) less than one minute. Many, possibly thousands, of chopper cycles allow for accumulation of signal. Irradiation and decay lengths, ti and td, respectively, in the beam chopper cycle are tunable parameters for maximizing the signal-to-background ratio for a half-life. The effect of ti and td can be determined with analytical modeling of irradiation and decay cycles.
Short-lived activation products from commonly-determined elements include: 20F, 24Na, 28Al, 38Cl, 56Mn, 46Sc, 77mSe, 80Br, 82Br, 127I, 179m1Hf, 187W, 108Ag, 110Ag, as well as a huge number of isotopes produced from fissile elements. Determinations of 77mSe and unfolding of interfering γ rays of Dy 515.467 keV (T1/2= 75.4 s) and Yb 514.868 keV (T1/2= 0.068 s) by their half-life have been demonstrated. The decay spectra, with lower noise baselines and fewer γ-ray lines, are valuable for samples producing complex γ-ray spectra, potentially lowering detection limits and increasing selectivity for some elements. Current developments for routine application include an optimized detector geometry, and energy and time stamps of every detected γ ray saved in list mode to memory.
These time-stamped, list-mode (TLIST) data present the opportunity for investigating the temporal behavior of signals and optimizing for signal-to-background ratio in particular time windows depending on the sample matrix. TLIST data are sparse when binned to a time histogram with 1 ms bins, making it necessary to combine data from many cycles together to view decay properties. However, frequency domain analysis by Fourier transform of the list mode data has shown signature peaks above the noise background at the chosen chopper operation frequency. We report preliminary results of frequency domain analyses for the 24mNa (T1/2= 0.020 s) and the Dy+Yb TLIST data. Simulation of ideal TLIST data in the time and frequency domain has been performed to deconvolve the excitation function (neutron beam irradiation) and the response function (decay emission). This technique might be useful in identifying weak signatures of short-lived radionuclides in TLIST data for chopped-beam PGAA, while further development could lead to quantitative analysis.
Primary author
Dr
Danyal Turkoglu
(NIST)
Co-authors
Dr
Heather Chen-Mayer
(NIST)
Dr
Rolf Zeisler
(NIST)
