Thin, Free-Standing Films For High Resolution Neutron Imaging
Open Access
- Author:
- Trivelpiece, Cory Luke
- Graduate Program:
- Nuclear Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- February 19, 2010
- Committee Members:
- Jack Brenizer Jr., Dissertation Advisor/Co-Advisor
Jack Brenizer Jr., Committee Chair/Co-Chair
R Gregory Downing, Committee Member
Arthur Thompson Motta, Committee Member
Carlo G Pantano, Committee Member
Kenan Unlu, Committee Member - Keywords:
- stopping and range of ions in matter
geometric uncertainty
charged particle
neutron imaging
BPSG
boro-phosphosilicate glass
free-standing films
SRIM
neutron depth profiling
NDP - Abstract:
- Thin, free-standing boro-phosphosilicate glass (BPSG) films were fabricated at PSU Nanofab to serve as prototype neutron converters for a proposed high resolution neutron imaging system (HRNIS). The <sup>10</sup>B isotope contained within the BPSG network will capture thermal neutrons and undergo an (n,α) reaction producing a charged particle and recoiling nucleus that will be detected in coincidence to determine the original point of neutron interaction. A 1.5 μm thick BPSG layer was deposited via plasma enhanced chemical vapor deposition at Cornell Nanofabrication Facility. The BPSG composition was: 3.5 w% P, 4.5 w% B, 92 w% SiO<sub>2</sub>. The BPSG layer was stacked between two Si<sub>3</sub>N<sub>4</sub> layers, which functioned as etch stops. The wafers were patterned by photolithography and cleaved into smaller samples containing 6 – 11 window-like features per sample. The Si substrate was removed from patterned areas using a high-temperature potassium hydroxide/de-ionized water wet etch. The result was a Si substrate base structure containing exposed, free-standing BPSG windows. The Si3N4 etch stop layer was removed from the exposed windows by magnetically-enhanced reactive ion etching. Properties of the films were characterized using mechanical profilometry, optical profilometry, and infrared absorption spectroscopy (FTIR). A diameter of uncertainty (D<sub>u</sub>) was derived from a geometric uncertainty model describing the error that would be introduced into imaging (position-sensitive, coincidence) measurements by charged-particle transport phenomena and experimental setup. The transport of α and Li ions, produced by the <sup>10</sup>B(n,α)<sup>7</sup>Li reaction, through the BPSG thin films was modeled using the Monte Carlo code SRIM, and the results of these simulations were used as input to determine D<sub>u</sub> for the proposed HRNIS. The results of these calculations showed that D<sub>u</sub> is dependent on the angle of charged particle emission, encoder separation, and film thickness. Based on the anticipated timing resolution of the HRNIS instrumentation, emission events that yield large D<sub>u</sub> can be discriminated by logical arguments during spectral deconvolution.