Joseph J. Suter

Fiber optic ionizing radiation detector

The authors describe a novel design for an ionizing radiation sensor using coiled optical fibers, which can be placed within or near a radioactive source. Coiled fiber samples were placed inside metallic and metal-matrix composite cylinders to evaluate the sensitivity of the materials. The system has been shown to have a sensitivity range of 10 to 100 kRad(Si). Further study has shown that the proposed experimental set-up can be used to determine the mass attenuation coefficient. The mass attenuation coefficient was then used to optimize the number of windings that would make the detector most effective for radiation detection. Another major advantage of this detector is that it will be possible to repair the damage caused by ionizing radiation. This can be done by heating the fibers to a moderate temperature and holding the temperature for a specific time in order to eliminate the color centers.<<ETX>>

Results from Gamma Ray and Proton Beam Radiation Testing of Quartz Resonators

This paper summarizes the results of radiation tests on cultured quartz resonators. The tests were conducted to determine the frequency shifts (¿f/f) of the resonators as a result of high and low levels of proton and gamma irradiation. All resonators tested were 5 MHz, 5th overtone, AT cut units manufactured in different lots by Bliley Electric Company from nine bars of Premium-Q, swept, cultured quartz produced by Sawyer Research Products. Generally, the results were as follows: Resonators manufactured from the same bar of cultured quartz showed consistent frequency changes (¿f/f) when subjected to high level gamma radiation. However, this consistency did not exist when the resonators were exposed to low level gamma radiation. For low level radiation no apparent correlation exists between the aluminum content of a resonator and its radiation induced frequency shift (¿f/f). A correlation exists between the low level radiation induced frequency shift (¿f/f) of a resonator and the manufacturing lot in which the resonator was produced. The low level radiation sensitivity of a resonator cannot be extrapolated from high level radiation data. The radiation sensitivities of resonators subjected to low level radiation may be grouped into one of four distinctive signatures. The signatures and the frequency shifts (¿f/f) of resonators subjected to low level proton and gamma radiation are very similar. Radiative preconditioning of resonators reduces the radiation induced frequency shifts (¿f/f) at both high and low levels of gamma and proton radiation.

Simulation of Low-Earth-Orbit Radiation Environments with a 5 to 120 MeV Proton Cyclotron Beam Using a Proton Beam Modulator

In this paper we will describe the experimental results of a novel method for simulating the proton radiation of a low earth orbit with a cyclotron using a specially designed proton beam modulator. We will present details on the design of the proton beam modulator wheel and the experimental results of the irradiation of actual spaceflight electronic components with the modulated beam. Electronic devices on board spacecraft are generally susceptible to penetrating radiation encountered in the space environment. Since the proton radiation spectrum of a low earth orbit covers a rather wide range from 1 to 500 MeV, a proton beam range modulator was designed that simulates a proton spectrum from 5 to 120 MeV. This proton radiation energy range simulates the most damaging part of the spectrum. The spectrum from the cyclotron was mapped by means of a sodium iodide scintillator; those measurements confirmed our initial calculations of the proton spectral density and showed that the mean energy of the protons is centered at about 40 MeV. In this paper, we will present an analysis of the simulation of the low-earth-orbit proton energy spectrum and details concerning the design of the proton beam modulator. In order to verify the operation of the modulator used for simulating low-earth-orbit radiation, power conditioners using MOSFET power switches were subjected to the proton radiation. They were allowed to accumulate a total of 30 krads (Si) of proton radiation over the specified energy range.

Low and Medium Dose Radiation Sensitivity of Quartz Crystal Resonators with Different Al-Impurity Content

The results from low (4.0 rad) and medium (1000 rad) radiation tests on ATquartz crystal resonators with aluminum impurity content of 0.79 and 8.3 PPM are discussed. The radiation tests were conducted with two cobalt-60 sources that generate photons with a mean energy of 1.25 MeV. The dose rate at the quartz crystal blanks was approximately 4.0 rad (%) /h , which represents approximately a factor of ten times the dose rate experienced in the radiation environment o f a low earth orbit. A mathematical analysis of the frequency susceptibility versus total accumulated dose revealed that a least-squares fit of the d f / f versus accumulated dose R data is of the type d f / f ( R ) = ARB, where A and B are real rational coefficients. Some modest interpretation of this result will be presented, together with data on the Allan variances and aging rates of the oscillators during and after the radiation tests.

Positioning of GPS antennas in time-keeping laboratories of North America

The problems with the use of the Global Positioning System (GPS) for time transfer are discussed. The Bureau International des Poids et Mesures (International Bureau of Weights and Mesures, BIPM) has developed a method of differential positioning using the data of time comparisons themselves. The consistency of the coordinates is within 30 cm for distances up to 1000 km. The method was applied to the European laboratories one year ago (1988). The consistency of time comparison improved from about 10 ns to about 2 ns. The principles of this technique and the results of its application to the North American time laboratories are presented. Work on differential positioning by geodetic double-frequency receivers between the US Naval Observatory and the Maryland VLBI (very long baseline interferometry) point are reported.<<ETX>>

Total-Dose Hardness Assurance for Low Earth Orbit

The Low Earth Orbit radiation environment has two significant characteristics that make laboratory simulation exposures difficult: (1) a low dose rate and (2) many cycles of low dose accumulation followed by dose-free annealing. Hardness assurance considerations for this environment are discussed and related to data from the testing of Advanced Low Power Schottky and High-speed CMOS devices.

First Results of the University of Maryland Electron Beam Transport Experiment

The University of Maryland electron beam transport experiment, in collaboration with the Rutherford Laboratory, is designed to study emittance growth in periodically focused intense beams. For initial studies, the electron gun consists of a 1-cm diam., dispenser-type cathode and an anode covered with a wire mesh. To avoid neutralization, 5 ¿s, 60 Hz pulses are used and the current is 230 mA at 5 kV. By varying the voltage from 10 kV to 500 volts the space charge depression, ¿/¿o, of the particle oscillation frequencies in the focusing channel can be changed from ~ 0.04 to ~ 0.2. Further increase of ¿/¿o should be possible with modified guns and the use of emittance control grids. Four prototype solenoids have been built, and the results of experiments with the first two are presented in this paper. Beam profile measurements show the nonlinear effects due to the space charge and the magnetic field, and also the effects of the anode mesh on the beam distribution.

The susceptibility of electrodeless quartz crystal BVA resonators to proton ionization effects

The radiation susceptibility of an electrodeless (BVA) quartz crystal resonator of the AT crystalline cut is reported. It is shown that its sensitivity to low and high levels of proton radiation is less than that of an AT resonator with adhered electrodes. The effectiveness of preconditioning techniques is discussed, and an interpretation of the test results in terms of the materials aspects of the quartz electrode interface is given. >

Barometric pressure-induced frequency offsets in hydrogen masers

Barometric pressure variations have been found to induce significant frequency shifts in atomic hydrogen masers by changing the compressive forces exerted on the masers electromagnetic cavity. The observed frequency shifts, which can be as large as 1 part in 10/sup -13/, track rising and falling pressures symmetrically. The barometric pressure fluctuations control the masers aging rate for periods as short as a few hours and as long as two weeks.<<ETX>>

The effects of ionizing and particle radiation on precision frequency sources (Proposal to IEEE Standards Project P1193)

The susceptibility of quartz oscillators and atomic frequency standards to natural and enhanced ionizing and particle radiation is an important parameter in predicting the short- and long-term performance of these standards in spacecraft. A discussion of radiation environments (proton, electron, neutron, flash X-ray, gamma, and single-event upset) is presented. Selection criteria for radiation sources, including dosimetry and procedures for the radiation testing of frequency standards, are discussed. Directions for the methods by which the radiation sensitivity of frequency standards can be assessed are provided by examining the results of previous studies.<<ETX>>