Troy Unruh

Characteristics of 3D Micro-Structured Semiconductor High Efficiency Neutron Detectors

Silicon diodes with large aspect ratio perforated micro-structures backfilled with 6LiF show a dramatic increase in neutron detection efficiency beyond that of conventional thin-film coated planar devices. Described in the following are advancements in the technology with increased perforation depths. Perforated silicon diodes with three different etched micro-structure patterns were tested for neutron counting efficiency. The etched micro-structure patterns consisted of circular holes, straight trenches, and continuous sinusoidal waves, with each pattern etched 200 mum deep. Normal incident neutron counting efficiencies were determined to be 9.7%, 12.6%, and 16.2% for circular hole, straight trench, and sinusoidal devices, respectively, at a reverse bias of 3 volts. The perforated neutron detectors demonstrate limited sensitivity to high-energy photon irradiation with a 60Co gamma-ray source. This work is part of on-going research to develop solid-state semiconductor neutron detectors with high detection efficiencies and uniform angular responses.

Perforated Diode Fabrication for Neutron Detection

Excessive leakage current in perforated pin diodes was identified and addressed through simple changes in processing techniques. The first pulse height spectra from a perforated diode operated as a radiation detector is reported. Also, methods to load 6LiF neutron absorbing material into deep perforations are reported.

Angular response of perforated silicon diode high efficiency neutron detectors

Perforated silicon diodes with two different etched patterns were tested for neutron counting efficiency and angular response. The etched patterns consisted of circular holes on a square lattice or continuous sinusoidal waves. Normal incident neutron counting efficiencies were determined to be 21% and 35% for circular hole and sinusoidal devices, respectively. A nonuniform angular response was identified for the circular hole perforated devices. For the circular hole patterns, a reduction in efficiency appeared at azimuthal angles near 0deg and 90deg, each referenced at a 90deg polar angle about the normal axis. The nonuniform angular response is due to neutron streaming paths through the square lattice. The sinusoidal device angular response was uniform and matched well to that of a standard simple planar device.

Micro-structured high-efficiency semiconductor neutron detectors

Perforated semiconductor diode detectors have been under development for several years at Kansas State University for a variety of neutron detection applications. The detectors are fabricated from high purity n-type Si. Sinusoidal trenches are etched into the substrate, into which shallow p-type junctions are diffused. The trenches are then backfilled with 6LiF powder to make the device sensitive to neutrons. Thermal neutron measurements from a 0.0253 eV diffracted neutron beam yielded 17% intrinsic detection efficiency for devices with 50 micron deep trenches and 29% intrinsic detection efficiency for devices with 100 micron deep trenches.

Perforated Semiconductor Neutron Detector Modules for Detection of Spontaneous Fission Neutrons

Compact neutron detectors are being designed and tested for use as low-power real-time personnel dosimeters and for remote neutron sensing. The neutron detectors are pin diodes that are mass produced from high-purity Si wafers. Each detector has thousands of circular perforations etched vertically into the device. The perforations are backfilled with 6LiF to make the pin diodes sensitive to thermal neutrons. The prototype devices delivered over 3.8% thermal neutron detection efficiency while operating on only 15 volts. The highest efficiency devices thus far have delivered over 12% thermal neutron detection efficiency. Devices moderated with high density polyethylene (HDPE) can be used for fast neutron detection. Compact packages with or without remote readout electronics are under construction and characterization.

Method to calibrate fission chambers in Campbelling mode

Fission chambers are neutron detectors which are widely used to instrument experimental reactors such as material testing reactors or zero power reactors. In the presence of a high level mixed gamma and neutron flux, fission chambers can be operated in Campbelling mode (also known as “fluctuation mode” or “mean square voltage mode”) to provide reliable and precise neutron related measurements. Fission chamber calibration in Campbelling mode (in terms of neutron flux) is usually done empirically using a calibrated reference detector. A major drawback of this method is that calibration measurements have to be performed in a neutron environment very similar to the one in which the calibrated detector will be used afterwards. What we propose here is a different approach based on characterizing the fission chamber response in terms of fission rate. This way, the detector calibration coefficient is independent from the neutron spectrum and can be determined prior to the experiment. The fissile deposit response to the neutron spectrum can then be assessed independently by other means (experimental or numerical). In this paper, the response of CEA made miniature fission chambers in Campbelling mode is studied. We use a theoretical model of the signal to calculate the calibration coefficient. Input parameters of the model come from statistical distribution of individual pulses. Supporting measurements have been made in the CEA Cadarache zero power reactor MINERVE and results are compared to an empirical Campbelling mode calibration. The tested fission chamber calibration coefficient is about 2 10−26 A2/Hz/(c/s). Both numerical and empirical methods give consistent results, however a deviation of about 15 % was observed.

Enhanced In-Pile Instrumentation at the Advanced Test Reactor

Many of the sensors deployed at materials and test reactors cannot withstand the high flux/high temperature test conditions often requested by users at U.S. test reactors, such as the Advanced Test Reactor (ATR) at the Idaho National Laboratory. To address this issue, an instrumentation development effort was initiated as part of the ATR National Scientific User Facility in 2007 to support the development and deployment of enhanced in-pile sensors. This paper provides an update on this effort. Specifically, this paper identifies the types of sensors currently available to support in-pile irradiations and those sensors currently available to ATR users. Accomplishments from new sensor technology deployment efforts are highlighted by describing new temperature and thermal conductivity sensors now available to ATR users. Efforts to deploy enhanced in-pile sensors for detecting elongation and real-time flux detectors are also reported, and recently-initiated research to evaluate the viability of advanced technologies to provide enhanced accuracy for measuring key parameters during irradiation testing are noted.

Wireless neutron and gamma ray detector modules for dosimetry and remote monitoring

Compact neutron detectors are being designed and tested for use as wireless low-power real-time personnel dosimeters and for remote neutron sensing. The neutron detectors are pin diodes that are mass produced from high-purity Si wafers. Each detector has thousands of perforations etched vertically into the device. The perforations are backfilled with 6LiF to make the pin diodes sensitive to thermal neutrons. The first prototype devices delivered over 3.8% thermal-neutron detection efficiency while operating at 15 volts. Recent high-efficiency perforated devices have delivered over 20% thermal-neutron detection efficiency. Compact packages with wireless communication capability have been constructed and are under test. The compact packages record counts every second and transmit ten data points every ten seconds. The platforms have been designed to incorporate an additional fast neutron detector and a Frisch- collar CdZnTe gamma-ray spectrometer. The overall goal is to manufacture a compact package capable of remote and wireless neutron dosimetry and gamma ray spectroscopy/dosimetry.

Perforated Semiconductor Neutron Detectors for Battery Operated Portable Modules

Perforated semiconductor diode detectors have been under development for several years at Kansas State University for a variety of neutron detection applications. The fundamental device configuration is a pin diode detector fabricated from high-purity float zone refined Si wafers. Perforations are etched into the diode surface with inductively-coupled plasma (ICP) reactive ion etching (RIE) and backfilled with 6LiF neutron reactive material. The perforation shapes and depths can be optimized to yield a flat response to neutrons over a wide variation of angles. The prototype devices delivered over 3.8% thermal neutron detection efficiency while operating on only 15 volts. The highest efficiency devices thus far have delivered over 12% thermal neutron detection efficiency. The miniature devices are 5.6 mm in diameter and require minimal power to operate, ranging from 3.3 volts to 15 volts, depending upon the amplifying electronics. The battery operated devices have been incorporated into compact modules with a digital readout. Further, the new modules have incorporated wireless readout technology and can be monitored remotely. The neutron detection modules can be used for neutron dosimetry and neutron monitoring. When coupled with high-density polyethylene, the detectors can be used to measure fission neutrons from spontaneous fission sources. Monto Carlo analysis indicates that the devices can be used in cargo containers as a passive search tool for spontaneous fission sources, such as 240Pu. Measurements with a 252Cf source are being conducted for verification.

1-D array of micro-structured neutron detectors

A 1024-channel pixel array has been constructed utilizing the perforated diode neutron detector design currently produced at Kansas State University. In this design a single pixel consists of a pn-junction diode fabricated around a single trench 4 cm long, 30 microns wide and 100 microns deep. The trench is filled with LiF powder to provide conversion of neutrons to energetic charged particles which can be captured in the diode depletion region. A pitch of 100 microns between pixels has been achieved and less than 120 micron spatial resolution has been demonstrated experimentally with a 32-channel prototype in previous work. Also, the first array demonstrated 12% thermal neutron counting efficiency. The 1024-channel array was produced by tiling 16 chips side-by-side, each containing 64 pixels. Signal processing is handled by 16 PATARA chips for amplification and thresholds, developed at University of Tennessee. The entire board assembly and digital communications to PC were handled by the KSU Electronics Design Laboratory utilizing a PCI card developed at ORNL.