Studies on anticoincidence gamma-ray spectrometry in neutron activation analysis.
Date
1997
Authors
Zhang, Weihua.
Journal Title
Journal ISSN
Volume Title
Publisher
Dalhousie University
Abstract
Description
Instrumental neutron activation analysis (INAA) is a well-established analytical technique for the simultaneous determination of multielement concentrations in a variety of sample matrices. One of the problems generally encountered in INAA is the high background activity arising from the scattering of photons, a phenomenon known as the Compton effect. In order to reduce the Compton background events and lower the detection limits, anticoincidence spectrometry can be employed. The spectrometer used here consists of a 25% relative efficiency HPGe detector surrounded by a 10$\sp{\prime\prime}$ x 10$\sp{\prime\prime}$ Nal(Tl) annulus and a 3$\sp{\prime\prime}$ x 3$\sp{\prime\prime}$ Nal(Tl) plug as well as timing electronics. This system has been characterized. A peak-to-background-plateau ratio of 590 has been obtained; an improvement factor of 7 has been achieved compared to a single HPGe detector. Several factors that can influence efficiency of the methodology have been evaluated. The distance of the sample from the HPGe detector surface and the relative position of the Nal(Tl) annulus with respect to the HPGe detector have been investigated to obtain the best efficiency.
A systematic investigation has been carried out on the merits and limitations of anticoincidence counting for the nuclides which are most commonly used in NAA. Short-lived nuclides (half-life $$ 12.5 h) of interest in the present study are: $\sp{76}$As, $\sp{198}$Au, $\sp‚$Br, $\sp{141}$Ce, $\sp{51}$Cr, $\sp{60}$Co, $\sp{59}$Fe, $\sp{197}$Hg, $\sp{203}$HG, $\sp{42}$K, $\sp{140}$La, $\sp{24}$Na, $\sp{147}$Nd, $\sp†$Rb, $\sp{122}$Sb, $\sp{46}$SC, $\sp{182}$Ta, $\sp{160}$Tb, $\sp{187}$W, $\sp{175}$Yb, $\rm\sp{69m}$Zn, and $\sp{65}$Zn.
The benefits of anticoincidence counting are frequently described by the peak efficiency reduction factor (PERF) of the nuclide of interest. Experiments were done in this work to calculate the PERF values of major gamma-rays of the above nuclides. The results showed that about 70% of the nuclides studied have no peak efficiency reduction in anticoincidence spectrometry. An attempt has been made to define an "analytical figure of merit (AFOM)" term for assessing the practical advantages of anticoincidence counting. This term includes parameters, such as resolution, peak area, background around the peak, dead time, counting time, and counting statistics which vary with changes in sample matrix activity. The effectiveness of the AFOM terms has been evaluated by analyzing a number of biological reference materials of diverse types of matrix and varying amounts of major and interfering elements. The variation of AFOM with dead time has also been investigated.
Thesis (Ph.D.)--Dalhousie University (Canada), 1997.
A systematic investigation has been carried out on the merits and limitations of anticoincidence counting for the nuclides which are most commonly used in NAA. Short-lived nuclides (half-life $$ 12.5 h) of interest in the present study are: $\sp{76}$As, $\sp{198}$Au, $\sp‚$Br, $\sp{141}$Ce, $\sp{51}$Cr, $\sp{60}$Co, $\sp{59}$Fe, $\sp{197}$Hg, $\sp{203}$HG, $\sp{42}$K, $\sp{140}$La, $\sp{24}$Na, $\sp{147}$Nd, $\sp†$Rb, $\sp{122}$Sb, $\sp{46}$SC, $\sp{182}$Ta, $\sp{160}$Tb, $\sp{187}$W, $\sp{175}$Yb, $\rm\sp{69m}$Zn, and $\sp{65}$Zn.
The benefits of anticoincidence counting are frequently described by the peak efficiency reduction factor (PERF) of the nuclide of interest. Experiments were done in this work to calculate the PERF values of major gamma-rays of the above nuclides. The results showed that about 70% of the nuclides studied have no peak efficiency reduction in anticoincidence spectrometry. An attempt has been made to define an "analytical figure of merit (AFOM)" term for assessing the practical advantages of anticoincidence counting. This term includes parameters, such as resolution, peak area, background around the peak, dead time, counting time, and counting statistics which vary with changes in sample matrix activity. The effectiveness of the AFOM terms has been evaluated by analyzing a number of biological reference materials of diverse types of matrix and varying amounts of major and interfering elements. The variation of AFOM with dead time has also been investigated.
Thesis (Ph.D.)--Dalhousie University (Canada), 1997.
Keywords
Chemistry, Nuclear.