Fiber optic dosimeter system using electron trapping materials

A fiber optic dosimeter in which an electron trapping material is coated onto a tip of an optical fiber. The tip is placed in a region where radiation is to be measured, and the opposite end of the optical fiber, from which radiation readings are measured, is placed in a location remote from the radiation source. When radiation impinges upon the electron trapping material, electrons in the material are raised to a higher state where they are trapped and stay indefinitely. When infrared light strikes the material, the stored electrons are released from their traps and, upon falling to a lower energy level, emit visible light which can be detected and measured. Thus, to measure the amount of ambient radiation, the electron trapping material is stimulated with infrared light from an infrared source at the opposite end of the optical fiber. This infrared stimulation releases trapped electrons and causes the emission of visible light, at least a portion of which is collected and directed back down the optical fiber to the visible light detector, where it is converted into an electrical signal and measured.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a system for radiation dosimetry and, more 
specifically, to a fiber optic dosimeter using electron trapping 
materials. 
2. Description of the Related Art 
Two types of devices are commonly used for radiation dosimetry: continuous 
radiation monitors or integrating radiation monitors. The most widely used 
continuous monitors are ionization-type dosimeters, which work by 
repeatedly measuring the current produced when radiation ionizes a known 
volume of a gas, usually air. Integrating dosimeters, on the other hand, 
measure the total radiation dose received over a fixed period of time. The 
most common integrating dosimeters are thermoluminescent dosimeters 
(TLD's). TLD's consist of a phosphor which, after exposure to ionizing 
radiation, produces a luminescence when heated, the magnitude of the 
luminescence being proportional to the radiation exposure. 
The problem with most existing commercial dosimeters is that they are 
usually large in size and thus not suitable for use in small or 
inaccessible areas. Moreover, ionization-type dosimeters require an 
electrical cable between the probe and the read-out instrument. The 
electrical signal passing through this cable is susceptible to 
electromagnetic interference; also, because the current signals to be 
measured are usually quite small, the cable must be very short to minimize 
resistance. Finally, both integrating and continuous radiation monitors 
are generally limited to a narrow dynamic range of radiation intensities 
and thus require several different probe configurations to measure over a 
wide range of radiation intensities. 
SUMMARY OF THE INVENTION 
The present invention overcomes the above-noted problems and deficiencies 
of the prior art by providing a electron-trapping material at the tip of 
an optical fiber. The tip is placed in a region where radiation is to be 
measured, and the opposite end of the optical fiber, from which radiation 
readings are measured, is placed in a location remote from the radiation 
source. 
The electron trapping material utilized in the present invention is a novel 
photoluminescent material which can be "charged" by the radiation to be 
measured; upon such energetic exposure, electrons in the material are 
raised to a higher state where they are "trapped" and stay indefinitely. 
When low energy photons (such as infrared) impinge upon the material, the 
stored electrons are released from their traps, and, upon falling to a 
lower energy level, emit blue-green light which can be detected and 
measured. 
Thus, to measure the amount of ambient high energy radiation, it is not 
necessary to heat the sample (as in prior art TLD's), but only to "read" 
the amount of radiation "stored" by stimulating the exposed electron 
trapping material with infrared light. This infrared stimulation releases 
trapped electrons and supplies an analog of the integrated dose for the 
previous time interval in the form of the returned intensity of blue-green 
light. 
Since the state of the electron trapping sensor can be tested by 
stimulating with light, which then supplies information in the form of yet 
another wavelength of light, it is possible to use an optical fiber to 
measure radiation at remote locations and thereby avoid problems caused by 
electrical interference. Also, since only a small amount of electron 
trapping material need be placed at the tip of the optical fiber, the 
invention provides a "microprobe" which can reach into small and 
ordinarily inaccessible locations. 
The method of the present invention thus involves the following steps: 
(1) exposing electron trapping material on a tip of an optical fiber to the 
radiation to be measured: 
(2) sending infrared light down the optical fiber to release stored 
electrons from the traps; 
(3) measuring the returned visible light (e.g., blue-green light) to 
correlate with the exciting radiation dose; and 
(4) continuing the sequence using low-intensity infrared stimulation, or 
"erasing" between measurements using higher intensity infrared 
stimulation. 
The electron-trapping material used on the tip of the optical fiber 
preferably comprises a base formed of alkaline earth sulfides, doped with 
impurities from the lanthanide (rare earth) series. More specifically, the 
electron trapping material comprises a base material of strontium sulfide, 
a first dopant selected from the group of samarium, samarium oxide, 
samarium fluoride, samarium chloride and samariuim sulfide; a second 
dopant selected from the group of cerium oxide, cerium fluoride, cerium 
chloride, and cerium sulfide; and a fusible salt, preferably lithium 
fluoride. Enhanced performance is realized by the addition of a cesium 
halide. Barium sulfate also may be added to provide an improvement in 
emission efficiency. 
The electron trapping material is mixed with a clear binder, overcoated 
with a carbon filled binder and coated onto the tip of a optical fiber to 
form a microprobe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, the fiber optic dosimeter of the present 
invention, indicated generally by reference numeral 2, includes a small 
amount of election trapping material 4 on the tip of one end of an optical 
fiber 6 to form a microprobe 8. The opposite end of optical fiber 6 is 
coupled by a two leg fiber coupler or beamsplitter 9 to an infrared source 
10 and a visible light detector 12. The probe is situated so that the 
election trapping material on its tip is exposed to radiation emitted from 
a radiation source. 
The electron trapping material is charged by the ambient radiation and is 
read out by infrared pulses emitted from IR source 10 and sent through the 
optical fiber 6. When the infrared light from IR source 10 strikes the 
charged election trapping material 4, the trapped electrons are released. 
As the released electrons in the electron trapping material fall to a 
lower energy state, photons are emitted. At least a portion of these 
photons are collected and directed back down optical fiber 6, and are 
detected at the opposite end of fiber 6 by detector 12. 
Detector 12 preferably comprises off-the-shelf silicon photodetector 
circuitry. Alternatively, if the received visible luminescence from the 
electron trapping material is weak, a photomultiplier with maximum 
sensitivity in the visible emission wavelength spectrum can be used. 
The output of detector 12 is an electrical signal which is proportional to 
the amount of visible light received, which is in turn proportional to the 
amount of radiation impinged upon electron trapping material 4. The manner 
in which this signal is processed depends upon the type of measurement 
desired. For example, if the radiation levels are sufficiently high to 
produce a strong signal from the electron trapping material, measurement 
of immediate scintillation (without infrared "reads") is possible simply 
by connecting the detector to an oscilloscope 14 or a counter 16 (to 
measure the number of received pulses/sec.). 
Alternatively, the electron trapping material can be charged by the 
radiation to be measured and "read out" periodically by pulsing it with 
infrared light from IR source 10. The instantaneous visible output 
received by detector 12 following each infrared pulse can be quantified by 
measuring the pulse height using oscilloscope 14 or digitizer 20. However, 
this method of measurement is not preferred because the amplitude of the 
visible output received by detector 12 depends directly upon the intensity 
of infrared light which reaches electron trapping material 4. Thus, 
variations can occur from one measurement to the next. 
The more preferred method of measurement is to use an integrator 18 coupled 
to infrared detector 12 to obtain an integral of the visible output vs. 
time. This method of measurement becomes time independent because the 
electron trapping material 4 is stimulated continuously with infrared 
until substantially all traps are depleted. 
Another dosimeter method which can be used with the present invention, 
particularly in instances where the radiation to be measured is quite low, 
involves placing the probe in the radiation area and allowing it to 
accumulate dose over a long period of time, e.g. one day. The accumulated 
dose can be then read out by either of the above-described pulse-height or 
integration measurement techniques. One such application might involve 
attaching the microprobe 8 to an individual working in a nuclear facility 
and, at the end of the day, coupling the probe back to the IR source 10 
and detector 12 to read out the radiation dose received. 
The IR source 10 utilized to "read out" the energy stored in electron 
trapping material 4 may comprise a conventional IR LED or laser diode with 
appropriate circuitry for pulsing periodically or upon command. 
Alternatively, a low power Nd:YAG cw laser, which emits light at a 
wavelength of 1.064 microns, can be employed as the IR source. 
The infrared light from IR source 10 is preferably focused by an objective 
lens (not shown) and directed into optical fiber 6. The structure of probe 
8 is analogous to the tip of a match; i.e. the electron trapping material 
4 is mixed with a transparent or translucent binder and coated onto the 
tip of optical fiber 6 as shown in FIG. 2. The tip is then overcoated with 
a carbon filled plastic 7 (e.g. Teflon) which serves to both protect the 
material and shield the material from visible light. 
The energy relations present in electron trapping material 4 will now be 
described. 
Referring to the energy diagram of FIG. 3, the electron trapping material 
has a valence band G full of electrons at a ground state. The material is 
subjected to high-energy particles or photons, generated by the radiation 
dose to be measured. The photons function to energize certain electrons in 
the valence band G. An electron shown on the left is originally in a 
valence band G and is subjected to radiation. Here, electrons absorb 
photons, and they rise to the communication band E and to higher bands C 
created by a dopant of a cerium compound. 
Within a short time, electrons will attempt to return to their ground 
state; those that avoid traps will emit light in the form of fluorescence, 
while others will be captured in trapping level T, all depending upon the 
composition of the material and available trapping sites. Electrons in the 
trapping level T, will remain isolated from other electrons and trapped, 
holding the radiation dose. 
Reading of the radiation dose is accomplished by infrared stimulation that 
provides sufficient additional external energy in order to raise the 
electrons back up to the communication band E from which they recombine, 
emitting blue-green light. As shown on the right side of FIG. 3, trapped 
electrons may be stimulated by infrared electromagnetic energy to move 
them back to the communication band E where they may interact with each 
other and fall back to band G and output photons of visible light, 
reconstructing the radiation dose in the process. 
The materials used in dosimeter of the present invention work upon the 
principle illustrated by FIG. 3, whereby the radiation dose may be 
"stored" by the phenomenon of electron trapping and the dose may be read 
out by application of infrared radiation to push the electrons up above 
the trap and allow them to return to the valence band. The number of 
trapping sites, the depth of the traps, the probability of transitions 
occurring in the communication band and the degree of X-ray absorption are 
all dependent upon the composition of the material used. 
The composition and preparation of the electron trapping materials used in 
the present invention will now be described. 
As previously mentioned, the radiation-sensitive electron trapping material 
of the present invention comprises a composition of a base material, a 
first dopant, a second dopant and a fusible salt. 
The base material is selected from the group of alkaline earth metal 
sulfides. An alkaline earth metal sulfide is a binary compound crystal 
containing a column IIA metal, and sulfur. Strontium sulfide is preferred 
because efficient light trapping and emission can be accomplished in this 
material and it has a high enough Z (atomic number) to absorb a 
significant portion of incident high energy radiation, such as X-rays. The 
first dopant, selected from the group of samarium, samarium oxide, 
samarium fluoride, samarium chloride, and samarium sulfide, provides the 
trapping sites. The second dopant, selected from the group of cerium 
oxide, cerium fluoride, cerium chloride, and cerium sulfide, establishes 
the communication band E and the upper bands C. It may be mentioned that 
while a europium compound provides somewhat better light emission 
efficiency the radiation sensitivity is only 1/1000th to 1/100th of that 
achieved with cerium making the use of europium impractical for low 
radiation doses. 
The addition of a cesium halide to the mixture after the first heat 
treatment tends to improve the light output intensity by 25-50%. 
In order to produce bulk materials, fusing of the components takes place by 
adding about 10% of fusible salts to the mixture. Fusible salts are 
typically compounds of elements of Column 1A and 7A of the table of 
elements or elements of Column IIA and 7A elements. Examples include 
fluorides, chlorides, bromides, or iodides of Li, Na, K, Cs, Mg, Ca, Sr, 
or Ba. 
A number of different electron trapping material combinations may be used 
in the present invention. The following exemplary mixtures, which output 
blue-green light, have been found to be particularly sensitive to 
radiation. 
EXAMPLE 1 
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Strontium sulfide 100 parts 
Barium sulfate 5 parts 
Lithium fluoride 10 parts 
Samarium 100 parts per million 
Cerium oxide 1200 parts per million 
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As used above and throughout this application, "parts" and "parts per 
million" shall refer to parts by weight unless otherwise noted. 
The mixture is placed into a graphite crucible within a furnace flushed 
with a dry nitrogen atmosphere (derived from a liquid source) or other dry 
inert atmosphere such as argon, and heated to between 950.degree. C. and 
1300.degree. C. (preferable 1150.degree. C.) for 30 minutes to one hour 
such that a fused mass is formed. For longer heating times, the fused mass 
could be formed at temperatures as low as 950.degree. C. Temperatures as 
high as 2000.degree. C. could be used to form such a fused mass in shorter 
times. 
After cooling, the fused mass is ground using standard techniques into a 
powder having a particle size of 5 to 100 microns, suitable for matching 
the fiber diameter. A particle size of 2 microns or less is preferable if 
thin film techniques are to be used. 
After grinding, the powdered material is heated to about 300.degree. C. to 
700.degree. C. (preferably 600.degree. C.) in the graphite crucible within 
the nitrogen or other inert atmosphere furnace. This second heating is 
below the fusing temperature of the material (about 700.degree. C.) and is 
maintained for 10 to 60 minutes (preferably 30 minutes) The second heating 
step removes internal stresses and repairs damage done to the crystalline 
surfaces during the grinding step. 
After the second heating, the material is cooled and the powdered material 
is then mixed with a suitable binder or vehicle such as acrylic 
polyethylene, or other organic polymer. After the material has been mixed 
with a transparent binder, it is applied as a thin coating to the tip of 
optical fiber. The coating of the photoluminescent material upon the tip 
of optical fiber 6 will preferably be between 1 micron and 500 microns in 
thickness, but in no event much greater than the diameter of the optical 
fiber. 
In the above mixture, the strontium sulfide serves as a base material, 
whereas the lithium fluoride operates:to provide the fusability 
characteristics useful for the specific embodiment. Alternatively, other 
alkaline earth metal sulfides might be used as the base material. 
The barium sulfate in the above mixture is used to improve the brightness 
of output light from the material. Preferably 5 parts are used as noted 
above, but between 0 and 10 parts may be used of the barium sulfate as 
well as between 2 and 10 parts of lithium fluoride relative to the 100 
parts of strontium sulfide. 
As mentioned previouslY, the dopant cerium oxide in the above mixture is 
used for establishing the communication band E and the upper bands C. 
Samarium is used to establish the electron trapping level T. Preferably 
180 parts per million of samarium are used, but the samarium could 
alternately be between 50 parts per million and 200 parts per million. The 
cerium oxide may be between 300 and 1500 parts per million, with 1200 
parts per million being preferred. Cerium fluoride, cerium chloride or 
cerium sulfide could be used in lieu of cerium oxide. 
The mixture resulting from the above process provides a depth for electron 
traps of 1.2 electron volts below the communication band and has an output 
spectrum as shown in FIG. 4, which illustrates that the center frequency 
of the output has a wavelength of approximately 500 manometers 
corresponding to a blue-green light. The infrared interrogation response 
of the mixture is shown in FIG. 5. 
EXAMPLE 2 
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Strontium sulfide 80 parts 
Lithium fluoride 10 parts 
Barium sulfate 10 parts 
Samarium 150 parts per million 
Cerium sulfide 1200 parts per million 
Cesium iodide 1 part 
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The use of a cesium halide, as in Example 2, is optional; however, it does 
provide a significant improvement in emission efficiency. The cesium 
halide is added after the grinding step and prior to reheating. Preferably 
150 parts per million of samarium are used in Example 2, but the samarium 
could alternatively be between 50 parts per million and 500 parts per 
million, depending upon the specific dosimeter application. For example, 
where long term storage is not required, the samarium concentration could 
be increased significantly. The cerium compound concentration may be 
between 200 and 2000 parts per million, with 1000 and 1500 parts per 
million being preferred and 1200 parts per million being the optional 
value. The cesium compound concentration in parts per hundred may be 
between 0.1 and 5, with 0.5 to 2 being preferred, and around 1 being 
optimum. 
The mixture resulting from the above process provides a depth for electron 
traps of about 1.2 electron volts below the communication band and has an 
output spectrum very similar to that shown in FIG. 4. And an infrared 
interrogation response similar to that shown in FIG. 5. 
The above-described electron trapping materials can be integrated with an 
optical fiber because it has been established that optical functions are 
available in grains as small as 10-20 microns (monolithic crystals are not 
required). The particles of the electron trapping material are fused or 
attached to the optical fiber. In the preferred embodiment, small amounts 
of small-grain phosphor powders are applied directly to the end of a fiber 
optic filament. The powder particles are coupled to the fiber optic 
filament with an organic binder (such as polyeurethane). Direct inorganic 
fusion to the fibers is also possible. 
Fiber microprobe dosimeters have both commercial and governmental 
applications to determine radiation levels in power generating reactor 
environments, medical isotope detection microvolume remote sensing or 
small entry applications, and many other applications. The fiber optic 
dosimeter of the present invention is particularly suitable for these 
applications because of its unique capability to: 
a) do radiation dosimetry without any electronic devices in the radiation 
field, using only fiber optic coupling. 
b) be small enough to penetrate very small openings, leading to 
applications such as medical microcatheter measurements. 
c) allow massive multiplexing of numerous optical fiber dosimeters while 
retaining integrated dose data over long intervals between addressing. 
d) utilize rather low cost sensor probes interrogated by common infrared 
sources and evaluated by usual photodetector circuitry. 
e) be accumulative in response up to a level approaching saturation, at 
which point they can be optically erased by either extended duration or 
higher intensity infrared stimulation. 
Although the present invention has been described in connection with 
preferred embodiments thereof, many variations and modifications will now 
become apparent to those skilled in the art. It is preferred, therefore, 
that the present invention be limited not by the specific disclosure 
herein, but only by the appended claims.