Thin, free-standing films for high resolution neutron imaging

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 10B 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% SiO2. The BPSG layer was stacked between two Si3N4 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/deionized 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 (Du) 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 10B(n,α)7Li 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 Du for the proposed HRNIS. The results of these calculations showed that Du 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 Du can be discriminated by logical arguments during spectral deconvolution.