Adam Weltz

Growth of hexagonal boron nitride on (111) Si for deep UV photonics and thermal neutron detection

Hexagonal boron nitride (hBN) growth was carried out on (111) Si substrates at a temperature of 1350 °C using a cold wall chemical vapor deposition system. The hBN phase of the deposited films was identified by the characteristic Raman peak at 1370 cm−1 with a full width at half maximum of 25 cm−1, corresponding to the in-plane stretch of B and N atoms. Chemical bonding states and composition of the hBN films were analyzed by X-ray photoelectron spectroscopy; the extracted B/N ratio was 1.03:1, which is 1:1 within the experimental error. The fabricated metal-hBN-metal devices demonstrate a strong deep UV (DUV) response. Further, the hBN growth on the vertical (111) surfaces of parallel trenches fabricated in (110) Si was explored to achieve a thermal neutron detector. These results demonstrate that hBN-based detectors represent a promising approach towards the development of DUV photodetectors and efficient solid-state thermal neutron detectors.

Solid-state neutron detectors based on thickness scalable hexagonal boron nitride

This paper reports on the device processing and characterization of hexagonal boron nitride (hBN) based solid-state thermal neutron detectors, where hBN thickness varied from 2.5 to 15 μm. These natural hBN epilayers (with 19.9% 10B) were grown by a low pressure chemical vapor deposition process. Complete dry processing was adopted for the fabrication of these metal-semiconductor-metal (MSM) configuration detectors. These detectors showed intrinsic thermal neutron detection efficiency values of 0.86%, 2.4%, 3.15%, and 4.71% for natural hBN thickness values of 2.5, 7.5, 10, and 15 μm, respectively. Measured efficiencies are very close (≥92%) to the theoretical maximum efficiencies for corresponding hBN thickness values for these detectors. This clearly shows the hBN thickness scalability of these detectors. A 15 μm thick hBN based MSM detector is expected to yield an efficiency of 21.4% if enriched hBN (with ∼100% 10B) is used instead of natural hBN. These results demonstrate that the fabrication of hBN th...

Anisotropic charge carrier transport in free-standing hexagonal boron nitride thin films

The in-plane and out-of-plane mobility–lifetime products of electrons and holes in free-standing hexagonal boron nitride (hBN) films are extracted from current–voltage characteristics of metal–hBN–metal structures measured under external excitations. The in-plane mobility–lifetime products for electrons and holes are ~2.8 × 10−5 and ~4.85 × 10−6 cm2/V, measured from lateral carrier collection, whereas the out-of-plane mobility–lifetime products for electrons and holes are ~5.8 × 10−8 and ~6.1 × 10−9 cm2/V, measured from vertical carrier collection, respectively. The mobility–lifetime product is a few orders of magnitude higher along the plane than along the out of plane in hBN films.

Nondestructive Assay Measurements Using the RPI Lead Slowing-Down Spectrometer

Abstract The use of a lead slowing-down spectrometer (LSDS) is considered as a possible option for nondestructive assay of fissile material of used nuclear fuel. The primary objective is to quantify fissile isotopes, particularly 239Pu and 235U, via a direct measurement distinguishing them through their characteristic fission spectra in the LSDS. In this paper, we present several assay measurements performed at the Rensselaer Polytechnic Institute (RPI) to support ongoing feasibility studies of the method and to provide benchmark experiments for Monte Carlo calculations of the assay system. A fresh uranium oxide fuel rod from the RPI Walthousen Reactor Critical Facility, a 239Pu-Be source, and several highly enriched 235U disks were assayed in the LSDS. The characteristic fission spectra were measured with 238U and 232Th threshold fission chambers, which are primarily sensitive to fission neutrons with energies above the threshold. Despite the constant neutron and gamma background from the Pu-Be source and the intense interrogation neutron flux, the LSDS system was able to measure the characteristic 235U and 239Pu responses. All measurements were compared to Monte Carlo simulations complementing previous modeling-based studies. It is shown that the available simulation tools and models are well suited to simulate the assay. An absolute calibration technique of the LSDS, which is required to perform quantitative measurements of the assayed fissile materials, is presented.

Boron-10 nanoparticles filled silicon trenches for thermal neutron detection application

This paper reports on the use of 10B nano/microparticles in order to fill microstructures of deep trenches fabricated in n-type Si (110) bulk wafers for the development of solid-state thermal neutron detectors. The high aspect-ratio trenches were fabricated in the wafer by wet etching, with a trench width of 3.5 to 6 μm and a maximum depth of 120 μm. Boron was diffused at a temperature of ∼1000 °C in order to convert the entirety of the delicate Si microstructures into a p+-n junction diode. The deep trenches of the diode were completely filled with 10B nanoparticles using a simple room-temperature process involving the pumping and venting of a vacuum chamber containing the etched wafer with 10B nanoparticles atop. The simple filling process was reproduced consistently, and the best 2.5 × 2.5 mm2 device demonstrated an intrinsic thermal neutron (En < 0.5 eV) detection efficiency of 32.2 ± 1.5% under a self-biased condition. This result is promising as it demonstrates a complete, low-cost fabrication proce...

Low-cost fabrication of high efficiency solid-state neutron detectors

The development of high-efficiency solid state thermal neutron detectors at low cost is critical for a wide range of civilian and defense applications. The use of present neutron detector system for personal radiation detection is limited by the cost, size, weight and power requirements. Chip scale solid state neutron detectors based on silicon technology would provide significant benefits in terms of cost, volume, and allow for wafer level integration with charge preamplifiers and readout electronics. In this paper, anisotropic wet etching of (110) silicon wafers was used to replace deep reactive ion etching (DRIE) to produce microstructured neutron detectors with lower cost and compatibility with mass production. Deep trenches were etched by 30 wt% KOH at 85°C with a highest etch ratio of (110) to (111). A trench-microstructure thermal neutron detector described by the aforementioned processes was fabricated and characterized. The detector—which has a continuous p+-n junction diode—was filled with enriched boron (99% of 10B) as a neutron converter material. The device showed a leakage current of ~ 6.7 × 10-6 A/cm2 at -1V and thermal neutron detection efficiency of ~16.3%. The detector uses custom built charge pre-amplifier, a shaping amplifier, and an analogto- digital converter (ADC) for data acquisition.

Low-Noise Preamplifier Design Considerations for Large Area High Capacitance Solid-State Neutron Detectors

In this paper, the design of a low noise charge sensitive preamplifier, suitable for high capacitance solid-state neutron detector arrays, is described. Noise considerations related to arraying multiple detectors in either series or parallel are also presented. Using the designed preamplifier, an energy resolution of 12 keV FWHM in Si is achieved with 1 nF equivalent detector capacitance and 10 μS shaping time. Additionally, a novel preamplifier topology, which utilizes a capacitance canceling input stage, is presented. Detailed analysis of the capacitance cancelling stage is discussed with special consideration given to its use with a charge sensitive preamplifier. Up to a 15% decrease in noise is demonstrated using the capacitance cancelling technique.

Lead Slowing Down Spectrometry Analysis of Data from Measurements on Nuclear Fuel

Abstract Improved nondestructive assay of isotopic masses in used nuclear fuel would be valuable for nuclear safeguards operations associated with the transport, storage, and reprocessing of used nuclear fuel. Our collaboration is examining the feasibility of using lead slowing-down spectrometry techniques to assay the isotopic fissile masses in used nuclear fuel assemblies. We present the application of our analysis algorithms to measurements conducted with a lead spectrometer. The measurements involved a single fresh fuel pin and discrete 239Pu and 235U samples. We are able to describe the isotopic fissile masses with root-mean-square errors over seven different configurations to 6.3% for 239Pu and 2.7% for 235U. Significant effort is yet needed to demonstrate the applicability of these algorithms for used-fuel assemblies, but the results reported here are encouraging in demonstrating that we are making progress toward that goal.

Development of a large area micro-structured solid-state neutron detector at low cost

The use of (He-3, BF-3) gas-based thermal neutron detector systems is limited by the cost, size, weight, and power requirements for many of the emerging applications. Therefore, the development of low-cost large-area solid-state neutron detectors for a wide range of civilian and defense applications is desirable. Solar cell type solid-state neutron detector using highly matured silicon technology would provide significant benefits in terms of cost, size, weight, and volume, and allow for wafer level integration with charge preamplifier and readout electronics. We present here the recent efforts on the fabrication of large area 10B filled honey-comb structured solid state thermal neutron detectors and detector modules with detection area up to 16 cm2 that requires only a single pre-amp and shaping amplifier for data acquisition. The average leakage current density of 1 cm2 area device is about 3×10-8 Acm-2 and neutron detection efficiency of ~29% for 0.0253 eV neutrons. For two stacked 1 cm2 detector, the efficiency is about 46% with gamma efficiency of 0.001% above the discrimination level of 0.5 MeV.

Lead Slowing Down Spectrometer Research Plans

The MPACT-funded Lead Slowing Down Spectrometry (LSDS) project has been evaluating the feasibility of using LSDS techniques to assay fissile isotopes in used nuclear fuel assemblies. The approach has the potential to provide considerable improvement in the assay of fissile isotopic masses in fuel assemblies compared to other non-destructive techniques in a direct and independent manner. The LSDS collaborations suggests that the next step to in empirically testing the feasibility is to conduct measurements on fresh fuel assemblies to understand investigate self-attenuation and fresh mixed-oxide (MOX) fuel rodlets so we may betterto understand extraction of masses for 235U and 239Pu. While progressing toward these goals, the collaboration also strongly suggests the continued development of enabling technology such as detector development and algorithm development, thatwhich could provide significant performance benefits.