Isotope identification software for Gamma spectra taken with CdZnTe detectors

Abstract

The identification of isotopes in a sample emitting gamma rays is a complex and difficult task particularly if the number of gamma lines is high, detector resolution and statistics are limited and little is known about the history of the sample. The better the resolution the easier this task can be accomplished. High-purity germanium detectors (HPGe) are in this respect the best choice, however, they are difficult to use under field conditions and in spent fuel ponds, since they require liquid nitrogen for cooling. NaI detectors do not require cooling, but have limitations in their resolution. For field applications, e.g., the verification of nuclear material, the use of CdZnTe detectors has advantages over NaI detectors since their resolution is much better. CdZnTe detectors, often exhibit asymmetric peak shapes, particularly at high energies making automated peak fitting methods and sophisticated isotope identification programs difficult to use. In this paper the design and use of a semi-automated isotope identification program is described. It is based on an interactive graphical method to find a simple solution to identifying various isotopes which may be present in a sample. The spectrum to be analyzed and an isotopic library containing a list of suspected isotopes is loaded. After selection of detector type , measurement geometry and starting values for the resolution, a peak search is performed. This can also determine the FWHM of the gamma peaks as function of energy. For each gamma peak found, the software suggests possible candidate isotopes to which the gamma peak may belong. The next, most decisive step to determine to which of the sometimes many choices of isotopes, a certain gamma line belongs to, is visual and interactive. Once one candidate's gamma line is marked, the program overlays the expected gamma peak pattern of this isotope with the measured one. Based on the presence or absence of certain lines in the spectrum (checked visually, aided by a numeric indicator), and the observed pattern of coincidence, the user can easily make a decision on whether a certain isotope is present in the sample or not. This approach proved to be simple, effective not requiring specialized knowledge in gamma spectrometry. Although the software works also for low-resolution NaI and high-resolution HPGe detectors, it is in particular well suited for use with spectra of CdZnTe detectors for which peak fit methods are more difficult to use. The deficiency of not using a strict peak fit is compensated by fast and straightforward human interaction. Application examples are given to identify isotopes in various samples, typical for qualitative safeguards verification tasks – attribute tests.