Phonon dispersion and phonon densities of states for ZnS and ZnTe

Neutron scattering data are reported for II–VI zincblende crystals, which are believed to be of sufficient precision to refine earlier ZnS ambiguities and to provide a basis for model fitting comparable to existing III‐V results. Valence shell models, including 9–12 parameters (VSM) and a variable shell charge extension (VCM), were fit to the data and used to generate phonon density of states and Debye temperatures. Very good fits to the neutron data were obtained, but no model was found that also predicts an accurate set of electric and mechanical constants. It is concluded that an unambiguous ionic charge Z cannot be assigned from the neutron results in either case.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71346/2/JCPSA6-60-9-3613-1.pd

Direct measurement of product of the electron mobility and mean free drift time of CdZnTe semiconductors using position sensitive single polarity charge sensing detectors

This article describes novel techniques to directly measure the electron mobility and mean free drift time product μeτeμeτe in semiconductor detectors. These methods are based on newly developed single polarity charge sensing and depth sensing techniques. Compared with conventional methods based on the Hecht relation, the new methods do not involve curve fitting, are less sensitive to the variation of pulse rise times, and allow the use of higher energy γ rays typical of many applications. © 1998 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70289/2/JAPIAU-84-10-5566-1.pd

Measured radiation fields in commercial nuclear power plants

This paper presents a compilation of experimental data on radiation fields found in commercial nuclear power plants. Exposure rates for systems and components are described by average values, although these averages may be significantly low (by orders of magnitude) near localized "hot spots". Conditions for, and exposure rates from, important fields are cited for both normal and abnormal operating conditions. Away from hot primary components, exposure rates for both [gamma] and neutron fields typically range from 0 to 1000 mrem h-1. Photon fields are most frequently the result of decay from neutron activated materials or fission product deposits. Neutron fields exhibit well-moderated spectra, with few neutrons above 750 keV. There seems to be no correlation between [gamma] and neutron fields in a typical nuclear power plant outside the reactor core.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/28128/1/0000578.pd

Efficiency and angular resolution calculations for a prototype multiple Compton scatter camera

Monte Carlo simulations were performed to optimize the layer thickness of a Compton camera which utilizes multiple Compton scatterings. The optimum layer thickness was found to be less than 0.5 mm for 150 keV, 2 mm for 511 keV gamma rays and approximately 5 mm for 1000 keV gamma rays. The efficiencies of two multiple Compton scatter cameras which consist of a 20 layer stack of either 1 mm or 4 mm thick Si strip detectors surrounded by a 2 cm thick CsI(Tl) side counter are presented for 150-1000 keV gamma rays. The expected angular resolution of the proposed design (1 mm thick, 20 layer Si) is presented for different assumed noise levels over the energy range 150-1000 keV.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31504/1/0000426.pd

Instantaneous flux measurements using the background signal of the rhodium self-powered neutron detector

The background current produced by a rhodium self-powered neutron detector (RSPND) with an integral background lead has been investigated to determine its relationship to the neutron flux seen by the detector. The background current and steady state flux were measured at the core midplane for all fuel assemblies, and good linearity was seen. Under transient conditions, the background current, when delayed fission product decay contributions are included, follows the flux reasonably well although with a slight lag. This opens the possibility of using the RSPND during transient conditions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30122/1/0000498.pd

Evaluation of wavelength shifters for spectral separation of barium fluoride emissions

Application of the barium fluoride (BaF2) scintillator for characterization of intense radiation fields has been pursued since the fast component was discovered in the early 1980's. Unfortunately, ~ 80% of the scintillator emissions have a slow ~ 600 ns decay time constant. The long decay time hampers the use of BaF2 at high count rates because the slow emissions appear as an afterglow which has an intensity that varies with the interaction rate in the crystal. The temporal separation of BaF2 emissions by spectral separation using wavelength shifting techniques has been explored in this work. Of the wavelength shifting techniques that were examined, the solvent excitation technique showed the greatest potential, but was insufficient by itself to suppress the slow emissions to the desired degree. All the wavelength shifting techniques yield an enhanced ratio of fast to slow photons, but a decreased fast photoelectron yield.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/31384/1/0000297.pd

Evaluation of p-terphenyl and 2,2" dimethyl-p-terphenyl as wavelength shifters for barium fluoride

p-Terphenyl (PTP) and 2,2" dimethyl-p-terphenyl (DMT) have been investigated for use as wavelength shifters (WLSs) for barium fluoride (BaF2) scintillation emission. These two organic fluors are attractive due to their high quantum efficiency, fast fluorescence decay time, and emissions above the wavelength cut-off of borosilicate glass. Measurements of these WLSs dissolved in cyclohexane are presented. Coincidence time spectra and fluorescence decay spectra measured with BaF2 as the pump source, are shown. The fluorescence decay time and the ratio of fast to slow intensities were determined from the fluorescence decay spectrum. The increase in signal resulting from using a WLS between the borosilicate end-window photomultiplier tube and the BaF2 crystal is measured. The motivation behind this work is the development of a gamma-ray spectrometer suitable for high count rate applications. We are also investigating the possibility of separating the fast and slow emissions of BaF2 using optical techniques. PTP and DMT both can spectrally separate BaFe2 emissions to a marginal extent.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30880/1/0000545.pd

Hand‐Held Gamma‐Ray Imaging Sensors Using Room‐Temperature 3‐Dimensional Position‐Sensitive Semiconductor Spectrometers

This paper demonstrates the capability of compact gamma‐ray imaging devices using 3‐dimensional position sensitive CdZnTe semiconductor gamma‐ray spectrometers, developed at the University of Michigan. A prototype imager was constructed and tested using two 1 cm cube 3‐dimensional position sensitive CdZnTe detectors. Energy resolutions of 1.5% FWHM for single pixel events at 662 keV gamma‐ray energy were obtained on both detectors, and an angular resolution of about 5° FWHM was demonstrated. The capabilities of proposed devices, which can cover a wider energy range up to 2.6 MeV, are discussed. © 2002 American Institute of PhysicsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87581/2/209_1.pd

Mobile robot positioning: Sensors and techniques

Exact knowledge of the position of a vehicle is a fundamental problem in mobile robot applications. In search of a solution, researchers and engineers have developed a variety of systems, sensors, and techniques for mobile robot positioning. This article provides a review of relevant mobile robot positioning technologies. The article defines seven categories for positioning systems: (1) Odometry, (2) Inertial Navigation, (3) Magnetic Compasses, (4) Active Beacons, (5) Global Positioning Systems, (6) Landmark Navigation, and (7) Model Matching. The characteristics of each category are discussed and examples of existing technologies are given for each category. The field of mobile robot navigation is active and vibrant, with more great systems and ideas being developed continuously. For this reason the examples presented in this article serve only to represent their respective categories, but they do not represent a judgment by the authors. Many ingenious approaches can be found in the literature, although, for reasons of brevily, not all could be cited in this article. © 1997 John Wiley & Sons, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34938/1/2_ftp.pd

Charge calibration of CsI(Tl)/photodiode spectroscopy systems

A charge calibration method for spectroscopy systems that use inorganic scintillators, photodiodes, and charge-sensitive preamplifiers is presented. The shaped square wave (SSW) method accounts for ballistic deficit when long decay time constants are present. The SSW method is demonstrated for CsI(T1) and compared to other calibration methods.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30115/1/0000491.pd