Deconvolution of gamma-ray spectra for nuclear level scheme studies

Unfolding or deconvolution of measured γ-ray spectra into full energy spectra is of high importance, particularly in the case of understanding nuclear level densities of nuclides. Furthermore, the method could be applicable for analytical purposes (activation analysis).

The response function of the detector can significantly differ from the true γ-ray spectrum emitted by the measured sample due to the numerous interactions between photons and the detector material. Besides the full energy peak, a background related to pair production and Compton scattering events also contribute to the measured spectra. The single and double escape peaks caused by pair production appear at 511 keV and 1022 keV lower energies than the full energy peak. A continuous background can be also observed due to single and multiple Compton scattering events. The total Compton background is the sum of backgrounds corresponding to the different peaks. The annihilation peak appears at 511 keV, and the backscattering peak occurs at around 200 keV energy.

Various techniques exist in the literature for the deconvolution of measured spectra, such as the stripping method, the inverse matrix method, the neural network method, and the folding iteration method. We developed a modified version of the folding iteration method, and we apply it for spectra obtained using high-purity Ge detector.

The response function of the HPGe detector is determined by applying Monte Carlo simulations of certain monoenergetic gamma spectra. The response functions at all of the different energies are then determined from the spectra obtained by the simulations using a spectrum interpolation method.

We demonstrated our deconvolution technique in the case of unfolding a measured nitrogen (Urea-D) spectrum. The resulting unfolded spectrum is in good agreement with the level densities of nitrogen (Fig. 1). Furthermore, the cross section values of natural gold and cobalt determined using our method are very close to the values in the literature (Table 1). Recently we are working on further improvements regarding the deconvolution technique, and in the future we plan to apply it in nuclear research as well as in neutron activation analysis.

Fig. 1. Relative intensities of the main peaks of nitrogen versus gamma-ray energy. The intensities of peaks determined from the unfolded spectrum are in good agreement with the values in the literature.

σliterature (barn)  σcalculated (barn)   
197Au (n,g)98.65  97.08 
59Co  37.18 36.08

Table 1. Comparison between cross section values provided by the literature and calculated values based on our deconvolution method in the case of gold and cobalt.

Contact Person

For more information about spectrum deconvolution, please contact Tamás Belgya.