Investigating Thermal Shock*
Fran Morrissey
DOE ERULF
Kutztown University
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37830
April 29, 1999
ABSTRACT
With the use of various sensing equipment, a neutron spallation target was studied via experiments performed at the Los Alamos Neutron Science Center. The goal is to determine the characteristics of the target container and cavity which contains the liquid mercury. The sensing equipment developed by the Photonics and Measurement Systems Group within Engineering Technology Division includes fiber optic strain sensors, phosphor temperature sensors, and high sensitivity fiber optic diaphragm pressure sensors. The pressure and temperature of the mercury, along with the strain on the cavity, were measured during interaction with a 10^13 proton pulse having energies of 1 GeV.
*Research sponsored by the U.S. Department of Energy at ORNL under contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corporation.
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The Fabry-Perot Interferometric Strain Sensor, Phosphor Temperature Sensor, and the High Sensitivity Diaphragm Pressure Sensor were used to record the characteristics of the target during the spallation experiments.
BACKGROUND
The most common means for producing copious amounts of neutrons involves nuclear fission in a reactor. Another means for neutron production is neutron spallation. Neutron spallation refers to a nuclear reaction where subatomic particles interact with the nucleus of an atom. This interaction produces many particles which are ejected from the atom's nucleus. The experiment done in conjunction with the Los Alamos Neutron Research Facility used a high energy proton beam directed at a mercury target.
The spallation process is believed to under go two stages. The first is the interaction of the proton with the target nucleus. The incoming particle produces a high energy particle cascade within the target nucleus. This results in some high energy particles and some low energy cascade particles escaping the nucleus. The end result leaves the target nucleus in a highly excited state. The second phase is where the nucleus relaxes. This is done by emitting low energy neutrons by a process called evaporation. In some nuclei, fission can also occur. This results in the fission process competing with the evaporation of the highly excited nucleus.
Spallation differs from fission by the number of useful low energy neutrons produced per event. The number of neutrons produced per fission event is about 2.5 (and at least one is needed to sustain the fission reaction), which is far less than the tens of neutrons produced in a single spallation event. Once these neutrons are produced in the target, they must be released before they can be moderated to useful energies. The control of the leakage of high energy neutrons is done through target geometry.
At Los Alamos, a radiofrequency (RF) linac (linear accelerator) was used to produce the energetic protons for spallation. The intensity of the proton beam can be increased by a factor of 2000 by compressing the length of the pulse from millisecond to microsecond range, which is achieved with a proton storage ring.
A target will contain a source of mercury which will be bombarded by a beam of 10^15 proton fired in pulses of 60 Hz at the proposed Spallation Source to be constructed at Oak Ridge by 2005. At Los Alamos, sensors were located on the target to measure: temperature, pressure, and deformation of the walls of the container. The sensors where fiber optic based, using phosphors and diaphragms.
The experimentation done at Los Alamos will lead to important insights on future target geometry. By better understanding the beams effects on the target cavity and the mercury, the linac community will be better equipped to design smarter and more robust targets. As beam energies increase and target geometries become more complex, there will be a greater need for target diagnostics to ensure stability of the target cavity. There are economic and environmental issues concerning a ruptured target cavity containing mercury; both the delay for spallation experiments and the clean up of the mercury hazard. As future spallation facilities are erected, along with long term targets that utilize flowing metals, there will be an increasing need for diagnostics to monitor target conditions.
THEORY
Fabry-Perot Interferometric Strain Sensor
The measurements of strain is based on an interferometric approach which utilizes what is known as a Fabry-Perot Cavity. A Fabry-Perot cavity is formed when two partially reflective planar mirrors are situated near each other so that light can bounce back and forth between them. This is implemented by placing two fibers closed together since the fiber endfaces are partially reflecting and thus serve as mirrors. The point of sensing is depicted in figure I. Light is injected into the fiber on the left. Approximately 4% of that light is reflected at the output fiber endface as seen due to Fresnel Reflection. Then it travels to the second fiber where a similar reflection occurs. This reflected light returns and enters the first fiber. When both reflected signals emerge from the delivery fiber and impinge on a detector surface, an interference between them is produced. When the distance between the two fibers, the Fabry-Perot spacing, is an integral number of half-wavelengths of the incident light, they interfere constructively and the signal detected is a maximum. As the spacing changes, the signal detected will oscillate appropriately with maxima and minima every half-wavelength.
As seen in the figure, the ends of the fiber are secured in a short length of capillary tubing. The fibers also are tacked onto the surface whose strain is to be measured. When the surface is stressed, the distance between the two fibers changes, and therefore, the interference signal is changed also. The optoelectronic package containing the light source, detection, and associated electronics is known as a FOSS I unit (for Fiber Optic Support System). It converts the interferometric optical output signal and translates it into a voltage signal that can be displayed on a digital oscilloscope. The approach to data acquisition and analysis for the present project is to save the digitized signal and later send it to a personal computer subsequent study.
Phosphor Temperature Sensor
Phosphors are made up of inorganic oxides, oxysulfides, orthophosphates, and rare earth metals. They all contain a small concentration of a dopant. These compounds when excited by a certain wavelength of light, will fluoresce and the decay time of the fluorescence is proportional to the temperature of the phosphor. As the temperature increases the decay time decreases at a particular emission wavelength. The phosphors used included Manganese doped Magnesium Fluorogermanate and Europium doped Lanthanum Oxysulfide, which had peak emission intensities at 514nm, 538nm, and 619nm.
The phosphor sensors measures the change in temperature of the mercury. The system uses an ultraviolet nitrogen laser that emits a pulse that excites the phosphors which fluoresces. This light is routed by fiber through a narrow bandpass filter to a photomultiplier tub, which amplifies the light and converts it to a voltage signal that is inverted when sent to a digital oscilloscope. This system is depicted below in figure II. After excitation, the fluorescence decays at an exponential rate to where it is no longer luminous. The decay time is both a function of temperature and pressure. However, it is very sensitive to temperature. Very high pressure are required in order to affect the decay time.
When there is a change in the state of the mercury due to the proton beam pulse, the wave form of the phosphor decay will rise or fall in comparison to the exponential decay curve. This is due to the phosphors becoming more luminous or less luminous with longer or shorter decay time as a result of the interaction of the mercury and dependent on the actual phosphor compound used.
High Sensitivity Diaphragm Pressure Sensor
The pressure sensor uses light emitted from the FOSS I unit which strikes a diaphragm. There are two types of diaphragms being used. The thick diaphragm was used to monitor the mercury at high pressure while the thin diaphragm was used to monitor it at low pressure. Not only do the properties of the diaphragms determine the range of pressure that can be measure be the initial distance to the end of the optical fiber probe affects the range. This optimum distance was determined using the calibration curves generated in the laboratory. A mechanical limit is set by the distance between the housing that holds the end of the fiber and the diaphragm. This limit is set by the point of deformation on the diaphragm when it is resting against the fiber probe. Pressures greater than this cannot be measured.
For the pressure sensor, a Fabry-Perot cavity is formed by the diaphragm surface and the output end of the fiber. Teh diaphragm flexes toward the fiber with an increase in cavity pressure. The FOSS I unit output, as displayed on an oscilloscope, will oscillate between maxima and minima as the distance changes by a half-wavelength. For the high pressure sensor, the inital distance is about 250 microns. For the calibration runs it was determined that a pressure change of 500 psi will cause the diaphragm to move about 25 microns. Similarly, the diaphragm-to-fiber distance for the low pressure sensor was 125 microns. A change in pressure of about 20 psi causes a 25 micron motion of the diaphragm. The principles of the diaphragm sensor is detailed in Figure III presented below.
BIBLIOGRAPHY
1. R.O. Claus, M.F. Gunther, A.B. Wang, K.A. Murphy and D. Sun, "Extrinsic Fabry-Perot Sensor for Structural Evaluation," Applications of Fiber Optic Sensors in Engineering Mechanics, 1993, pp. 60-70.
2. S.W. Allison, G.J. Capps, D.B. Smith, M.R. Cates, W.D. Turley, J. Gleason, "Thermographic Phosphor strain Measurements," Research Project 8004-3, U.S. Department of Energy, February, 1994, pp. 1-8.
3. S.W. Allison, G.T. Gillies, "Remote thermometry with thermographic phosphors: Instrumentation and applications," Review of Scientific Instruments, American Institute of Physics, Vol. 68, No. 7, July 1997, pp. 2615-2650.
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