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Manuscript Title: EMILIA, the LS counting efficiency for electron-capture and capture-gamma emitters
Authors: A. Grau Carles
Program title: EMILIA
Catalogue identifier: ADWK_v1_0 Distribution format: tar.gz
Journal reference: Comput. Phys. Commun. 174(2006)35
Programming language: FORTRAN 77.
Computer: any IBM PC compatible with 80386 or higher Intel processors.
Operating system: MS-DOS and higher systems.
RAM: 253 kword
Word size: 32
Keywords: Radioactivity, liquid-scintillation counting, electron-capture decay.
PACS: 07.57.Kp, 29.30.Dn.
Classification: 17.4.
Nature of problem: The determination of radioactivity in liquid samples of electron-capture nuclides is demanded in radiation physics, radiation protection, dosimetry, radiobiology and nuclear medicine. The CIEMAT/NIST method has proved to be suitable for radionuclide standardizations when the counting efficiency of the liquid-scintillation spectrometer is sufficiently high. Although the method has widely been applied to beta-ray nuclides, its applicability to electron-capture nuclides nowadays has not the required degree of accuracy. The inaccuracies of the method are mainly induced by the huge number of low-energy electrons and X-ray photons emitted by the atomic rearrangement cascade after the electron-capture process, which are efficiently detected by liquid-scintillation counting, but are of difficult modeling due to the inherent complexity of the atomic rearrangement process and the non-linear response of the spectrometer in the low-energy range.
Solution method: A detailed simulation of the non-linear response of the spectrometer to photoionization must include the radiation emitted by the atomic rearrangement cascade. However, a model considering all possible scintillator de-excitations at atomic level increases exponentially the number of atomic rearrangement detection pathways subsequent to capture. Since the contribution of the non-linear effects to the counting efficiency are only corrective, we can approximate the reduced energy involved in the photoionization process to a sum of only three terms; the photoelectron energy, the mean energy of the KXY Auger electrons and the global energy contribution of the remaining radiation (electrons and X-rays) emitted by the atomic rearrangement cascade. The value of each term depends on the nature of the atom in which photoabsorption is produced and on the atom shell from where the photoelectron is ejected. The non-linear correction required to simulate low-energy X-ray photoabsorption is basically important when the scintillator cocktail contains elements of high atomic numbers. For heavy atoms, the K- and L-shell binding energies can be slightly less than the energy of the colliding photon. For such cases, the non-linear effects can play an important role.
Restrictions: The simulation of all possible detection pathways of atomic rearrangement that follow to photoionization complicates the problem unnecessary. To correct the non-linear effects we consider only significative photoelectric interactions with the K- and L-shells. Also we assume that K-shell photoionizations only generate three types of entities; the photoelectron itself, KXY Auger electrons and a radiation group that includes the remaining emitted particles. For L-shell photoelectrons the radiation emission subsequent to photoionization is considered as a whole.
Running time: A few seconds.