Adam M. Conway

6:1 aspect ratio silicon pillar based thermal neutron detector filled with B10

Current helium-3 tube based thermal neutron detectors have shortcomings in achieving simultaneously high efficiency and low voltage while maintaining adequate fieldability performance. By using a three-dimensional silicon p-i-n diode pillar array filled with boron-10 these constraints can be overcome. The fabricated pillar structured detector reported here is composed of 2μm diameter silicon pillars with a 4μm pitch and height of 12μm. A thermal neutron detection efficiency of 7.3+∕−0.6% and a neutron-to-gamma discrimination of 105 at 2V reverse bias were measured for this detector. When scaled to larger aspect ratio, a high efficiency device is possible.

High aspect ratio composite structures with 48.5% thermal neutron detection efficiency

The pillar structured thermal neutron detector is based on the combination of high aspect ratio silicon p-i-n pillars surrounded by the neutron converter material 10B. By etching high aspect ratio pillar structures into silicon, the result is a device that efficiently absorbs the thermal neutron flux by accommodating a large volume fraction of 10B within the silicon pillar array. Here, we report a thermal neutron detection efficiency of 48.5% using a 50 μm pillar array with an aspect ratio of 25:1.

Fabrication of Pillar-structured thermal neutron detectors

Pillar detector is an innovative solid state device structure that leverages advanced semiconductor fabrication technology to produce a device for thermal neutron detection. State-of-the-art thermal neutron detectors have shortcomings in achieving simultaneously high efficiency, low operating voltage while maintaining adequate fieldability performance. By using a 3-dimensional silicon PIN diode pillar array filled with isotopic boron 10, (10B) a high efficiency device is theoretically possible. The fabricated pillar structures reported in this work are composed of 2 mum diameter silicon pillars with a 4 mum pitch and pillar heights of 6 and 12 mum. The pillar detector with a 12 mum height achieved a thermal neutron detection efficiency of 7.3% at 2 V.

Numerical Simulations of Pillar Structured Solid State Thermal Neutron Detector: Efficiency and Gamma Discrimination

This paper reports numerical simulations of a three-dimensionally integrated, Boron-10 (10B) and Silicon p+, intrinsic, n+ (PIN) diode micropillar array for thermal neutron detection. The inter-digitated device structure has a high probability of interaction between the Si PIN pillars and the charged particles (alpha and 7Li) created from the neutron-10B reaction. In this paper, the effect of both the 3-D geometry (including pillar width, separation and height) and energy loss mechanisms are investigated via simulations to predict the neutron detection efficiency and gamma discrimination of this structure. The simulation results are demonstrated to compare well with the experimental data available at this time, for 7- and 12-mum tall micropillar arrays. This indicates that upon scaling the pillar height, a high efficiency thermal neutron detector is possible.

Fabrication Methodology of Enhanced Stability Room Temperature TlBr Gamma Detectors

Thallium bromide (TlBr) is a material of interest for use in room temperature gamma ray detector applications due to is wide bandgap 2.7 eV and high average atomic number (Tl 81, Br 35). Researchers have achieved energy resolutions of 1.3% at 662 keV, demonstrating the potential of this material system. However, these detectors are known to polarize using conventional configurations, limiting their use. While high quality material is a critical starting point for excellent detector performance, we show that the room temperature stability of planar TlBr gamma spectrometers can be significantly enhanced by treatment with both hydrofluoric and hydrochloric acid. By incorporating F or Cl into the surface of TlBr, current instabilities are eliminated and the longer term current of the detectors remains unchanged. In addition the choice of electrode metal is shown to have a dramatic effect on the long term stability of TlBr detector performance 241Am spectra are also shown to be more stable for extended periods; detectors have been held at 4000 V/cm for 50 days with less than 10% degradation in peak centroid position.

X-ray photoemission analysis of chemically treated GaTe semiconductor surfaces for radiation detector applications

The surface of the layered III-VI chalcogenide semiconductor GaTe was subjected to various chemical treatments commonly used in device fabrication to determine the effect of the resulting microscopic surface composition on transport properties. Various mixtures of H3PO4:H2O2:H2O were accessed and the treated surfaces were allowed to oxidize in air at ambient temperature. High-resolution core-level photoemission measurements were used to evaluate the subsequent chemistry of the chemically treated surfaces. Metal electrodes were created on laminar (cleaved) and nonlaminar (cut and polished) GaTe surfaces followed by chemical surface treatment and the current versus voltage characteristics were measured. The measurements were correlated to understand the effect of surface chemistry on the electronic structure at these surfaces with the goal of minimizing the surface leakage currents for radiation detector devices.

Si pillar structured thermal neutron detectors: fabrication challenges and performance expectations

Solid-state thermal neutron detectors are desired to replace 3He tube tube-based technology for the detection of special nuclear materials. 3He tubes have some issues with stability, sensitivity to microphonics and very recently, a shortage of 3He. There are numerous solid-state approaches being investigated that utilize various architectures and material combinations. Our approach is based on the combination of high-aspect-ratio silicon PIN pillars, which are 2 μm wide with a 2 μm separation, arranged in a square matrix, and surrounded by 10B, the neutron converter material. To date, our highest efficiency is ~ 20 % for a pillar height of 26 μm. An efficiency of greater than 50 % is predicted for our device, while maintaining high gamma rejection and low power operation once adequate device scaling is carried out. Estimated required pillar height to meet this goal is ~ 50 μm. The fabrication challenges related to 10B deposition and etching as well as planarization of the three-dimensional structure is discussed.

Nine Element Si-based Pillar Structured Thermal Neutron Detector

Solid state thermal neutron detectors are desirable for replacing the current 3He based technology, which has some limitations arising from stability, sensitivity to microphonics and the recent shortage of 3He. Our approach to designing such solid state detectors is based on the combined use of high aspect ratio silicon PIN pillars surrounded by 10B, the neutron converter material. To date, our highest measured detection efficiency is 20%. An efficiency of greater than 50% is expected while maintaining high gamma rejection, low power operation and fast timing for multiplicity counting for our engineered device architecture. The design of our device structure, progress towards a nine channel system and detector scaling challenges are presented.

Long-term room temperature stability of TlBr gamma detectors

TlBr is a material of interest for use in room temperature gamma ray detector applications due to is wide bandgap 2.7 eV and high average atomic number (Tl 81, Br 35). Researchers have achieved energy resolutions of 1.3 % at 662 keV, demonstrating the potential of this material system. However, these detectors are known to polarize using conventional configurations, limiting their use. Continued improvement of room temperature, high-resolution gamma ray detectors based on TlBr requires further understanding of the degradation mechanisms. While high quality material is a critical starting point for excellent detector performance, we show that the room temperature stability of planar TlBr gamma spectrometers can be significantly enhanced by treatment with both hydrofluoric and hydrochloric acid. By incorporating F or Cl into the surface of TlBr, current instabilities are eliminated and the longer term current of the detectors remains unchanged. 241Am spectra are also shown to be more stable for extended periods; detectors have been held at 2000 V/cm for 52 days with less than 10% degradation in peak centroid position. In addition, evidence for the long term degradation mechanism being related to the contact metal is presented.

Surface processing of TlBr for improved gamma spectroscopy

Planar detectors have been fabricated on 0.5 mm thick TlBr crystals grown by Radiation Monitoring Devices (RMD). The crystals have been characterized by microhardness measurements. A surface damage layer resulting from mechanical polishing has been measured to be approximately 3.7 μm thick. We have removed this layer with H2O2 chemical etching and compared device performance with and without the presence of the surface damage layer and found significant differences in the initial and long term current-voltage behavior and radiation response. Detectors treated with H2O2 to remove this layer have been shown to display superior performance as compared to unetched detectors followed a period of “field annealing”.