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A Proof-of-Principle Investigation for a Neutron-Gamma Discrimination Technique in a Semiconductor Neutron Detector

Kandlakunta, Praneeth

Abstract Details

2012, Master of Science, Ohio State University, Nuclear Engineering.

Gadolinium (Gd) is an efficient thermal neutron conversion material due to the superior thermal neutron absorption cross-section predominantly composed of 157Gd, which is also 15.65% abundant in natural Gd. A thermal neutron capture by 157Gd results in an excited 158Gd nucleus. The de-excitation of 158Gd* involves the emission of prompt gamma rays with competing internal conversion (IC) electrons. Following the expulsion of conversion electrons from the atomic shells of 158Gd, the excitation energy of the atom is released through the emission of Auger electrons and characteristic x-rays. The low energy conversion electrons and Auger electrons are considered the principal component of neutron-induced signal in a Gd-based thin film semiconductor. Besides possessing a high thermal neutron absorption cross-section, Gd also has a good interaction probability for high/medium energy gamma rays, owing to its high Z (64) number. A Gd atom activated by an external high energy gamma ray leads to the emission of characteristic x-rays that come primarily from the K-shell. These x-rays have a fairly low energy (43.0 keV, 42.3 keV for Kα1, Kα2 respectively) compared to those of the prompt gamma rays that are emitted following a neutron capture.

Thin film semiconductors, although transparent to high energy gamma rays, are comparatively sensitive to low energy gamma rays and x-rays. Hence, it is supposed that a thin film semiconductor neutron detector using Gd as a neutron convertor receives greater interference from low energy x-rays that are emitted following gamma ray activation in Gd, than that from high energy background gamma rays. Thus, due to the presence of an inherent gamma ray background, separation of the neutron-induced signal from a gamma/x-ray induced signal is central to a semiconductor neutron detector employing Gd as the conversion material. A method of separation of these two signals by means of a current subtraction technique has been proposed. This gamma ray rejection scheme presents two identical semiconductor detectors separated by a thin Gd foil and a polyethylene thin layer. In the presence of a mixed neutron and gamma ray environment, a subtraction of signals resulting from these two detectors generates a ‘neutron only’ induced signal.

Nevertheless, an experimental validation will reinforce the abovementioned supposition and provide substantiation of the same. The objective of this research is thus to validate the principle proposed for gamma ray rejection in a thin film semiconductor neutron detector based on Gd. As the first stage, an experimental setup was designed and constructed to perform the required radiation measurements. In the second phase, preliminary measurements were performed to calibrate the instrumentation system and to gain expertise on using the signal processing electronics. In the final phase, a mixed beta-gamma measurement using two silicon detectors was performed in order to simulate a neutron-gamma discrimination scenario in a Gd based semiconductor detector. The output energy spectra encompassed a mixed beta-gamma spectrum from an unshielded silicon detector and a gamma ray only spectrum from a shielded silicon detector. Subtraction of the two spectra generated a beta-only spectrum representing a detector’s response to the IC and Auger electrons from Gd.

Lei Cao (Advisor)
Don Miller (Committee Member)
93 p.

Recommended Citations

Citations

  • Kandlakunta, P. (2012). A Proof-of-Principle Investigation for a Neutron-Gamma Discrimination Technique in a Semiconductor Neutron Detector [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1332447196

    APA Style (7th edition)

  • Kandlakunta, Praneeth. A Proof-of-Principle Investigation for a Neutron-Gamma Discrimination Technique in a Semiconductor Neutron Detector. 2012. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1332447196.

    MLA Style (8th edition)

  • Kandlakunta, Praneeth. "A Proof-of-Principle Investigation for a Neutron-Gamma Discrimination Technique in a Semiconductor Neutron Detector." Master's thesis, Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1332447196

    Chicago Manual of Style (17th edition)