Specifications and Performance of the Compton Suppression Spectrometer at the Pennsylvania State University

Open Access
Author:
CETINER, NESRIN OZGAN
Graduate Program:
Nuclear Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Kenan Unlu, Thesis Advisor
Keywords:
  • CSS
  • compton suppression
  • Compton
  • neutron activation analysis
  • NAA
Abstract:
A Compton Suppression Spectrometer (CSS) consists of a primary high-purity germanium (HPGe) detector and surrounding secondary detectors with appropriate data acquisition and processing electronics. The CSS reduces the noise content from Compton scattered photons to the gamma energy spectrum. The CSS operates in anticoincidence mode, in which events that take place within a predetermined time window result in elimination of the event from the spectrum. A custom-designed Compton Suppression Spectrometer was purchased from Canberra for Penn State Radiation Science and Engineering Center (RSEC). A detailed listing of the physical specifications of the Penn State Compton Suppression Spectrometer is presented. Various performance measurements were performed; among them are peak height-to-Compton ratio (P/C) for suppressed and unsuppressed spectra, suppression factor (SF) and reduction factor (RF). Peak height-to-Compton ratio (P/C) is defined as the ratio of the peak count in a photopeak to the average count per channel in a relatively flat region in the corresponding Compton continuum that avoids the Compton edge. This ratio is calculated for both the suppressed and unsuppressed spectra. Suppression factor is defined as the ratio of the suppressed peak height-to-Compton ratio to that of unsuppressed. Preliminary measurements showed that a peak height-to-Compton ratio of (P/C)sup=1001 is obtained with the Cs-137 source in the Compton suppressed mode, and (P/C)uns=100.4. This corresponds to a suppression factor of SF≅9.97. Performance measurements cover a series of peak-to-Compton ratio calculations of various sources, the corresponding suppression factor and reduction factor calculations. Natural environmental background spectra were collected with a long acquisition period in both the unsuppressed mode and the Compton suppressed mode. A standard reference material NIST SRM-1571 Orchard Leaves was irradiated and counted. Peak height-to-Compton ratio and the suppression factor were calculated. Another standard reference material NIST SRM-1570 Spinach sample was irradiated, and INAA calculations were performed. Among the identified elements are Na-24, Br-80, Mg-27, V-52 and K-42. An example application of the Compton Suppression Spectrometer is the dendrochemistry study. The dendrochemistry research involves comprehensive conventional neutron activation analysis of dated tree rings collected from various regions of the world. It was shown that the Compton Suppression Spectrometer can determine certain peaks that correspond to elements such as gold, calcium, zinc and iron that cannot be easily identified and analyzed with the conventional NAA. Concentrations of these elements from each year were calculated, from which their chronologic variations were obtained and plotted. For accurate determination of elemental concentrations, cascade corrections must be taken into account in concentration calculations. Therefore, peak-to-total calibration schemes must be incorporated into the analyses for future research.