A novel scintillating optical fiber is presented using a composite micro-structured quartz optical fiber. Scintillating materials are introduced into the multiple inclusions of the fiber. This creates a composite optical fiber having quartz as a cladding with an organic scintillating material core. X-ray detection using these fibers is compared to a collimated cadmium telluride (CdTe) detector over an energy range from 10 to 40 keV. Results show a good correlation between the fiber count rate trend and that of the CdTe detector

DeHaven, Stanton L.

NASA Technical Reports Server

MICROSTRUCTURED OPTICAL FIBER FOR X-RAY DETECTION
S. L. DeHaven
NASA Langley Research Center, Hampton, VA 23681
ABSTRACT. A novel scintillating optical fiber is presented using a composite micro-structured quartz
optical fiber. Scintillating materials are introduced into the multiple inclusions of the fiber. This creates a
composite optical fiber having quartz as a cladding with an organic scintillating material core. X-ray
detection using these fibers is compared to a collimated cadmium telluride (CdTe) detector over an energy
range from 10 to 40 keV. Results show a good correlation between the fiber count rate trend and that of the
CdTe detector.
Keywords: X-ray Detector, Fiber Optic, Scintillator
PACS: 07.85.Fv, 29.40.Mc, 42.81.Wg, 42.81.Pa
INTRODUCTION
Detecting x-rays began with the discovery of Bremstralung radiation and has
steadily improved with advances in technology. Many types of x-ray detectors exist
including films, ionization chambers, Geiger-Mueller counters, scintillation materials, and
semiconductors. Two categories of x-ray detectors addressed in this paper are scintillation
and solid state.
Many types of scintillator and solid state materials exist. Scintillation detectors use
a liquid or crystal which interacts with x-rays to produce light measured with a photon
detector. Solid state detectors use x-ray energy to induce a change in current when x-rays
interact with the material.
Scintillator materials can be inorganic or organic. Anthracene is the standard
organic crystal and all liquid scintillators are organic. However, the
 y are somewhat
fragile, have low efficiency, and require specific environments. Solid state detectors, such
as CdTe, have several benefits but require cooling because of dark noise considerations
and need an evacuated space behind a beryllium window [ 1 ].
The conventional optical fiber for x-ray and particle detection is plastic PMMA
scintillating fiber. The plastic fiber
 uses an organic hydrocarbon scintillator mixed with a
polymer core. Because of the relatively high visible wavelength absorption losses in the
plastic, the fiber core must be 1000 microns in diameter [2].
The novel approach proposed in this paper is combining a scintillator with
microstructured optical fiber to form an x-ray detector. The microstructured optical fiber
presented uses solid crystalline anthracene or a liquid hydrocarbon (1,2,4-trimethyl
benzene) scintillator inside a quartz matrix. The light guiding mechanism is through total
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internal reflection which is the same for plastic fiber. The visible wavelength absorption
for the liquid hydrocarbon and crystalline anthracene is less than plastic scintillating fiber .
A solid state CdTe x-ray detector is used to characterize the x-ray tube source and
compare with the fiber output. The geometry of the microstructured optical fiber is
considerably smaller than plastic fiber. In the fiber presented, each inclusion is 2.5
microns with 168 inclusions inside a 125 micron outside diameter as shown in Figure 1.
This allows standard FC fiber optic connectors to standard multimode optical fiber and
detectors.
FIGURE 1. Microstructured quartz optical fiber cross-section. Total fiber diameter is 125 microns with 168
air filled inclusions. These inclusions become fiber cores when filled with scintillator material.
THEORY
The x-ray detecting fiber microstructure
 is composed of 168 micro-core multi-
mode waveguides , each transmitting 420 nm wavelength (indigo) visible light. The light is
from the energy conversion of short wavelength higher energy x-ray photons to a larger
quantity of lower energy longer wavelength photons. This fluorescence effect generates
spontaneous emission 420nm wavelength photons.
The fluorescence from an organic scintillator comes from a two-level 7C orbital x-
ray absorption and re-emission of visible light as shown in Figure 2 [1]. This same
fluorescence occurs from using ultraviolet (UV) photons instead of x-rays. Figure 3 shows
the liquid scintillator UV photon absorption in the 370-390 nm wavelength range, with re-
emission at 420 nm.
The scintillator material is placed in the fiber inclusions and guides light using total
internal reflection. The scintillator acts as the core with the quartz as cladding. As such,
photons travel through individual 2.5 micron diameter cores and each core behaves as a
separate waveguide.
The liquid filled fiber operates using a weak guidance waveguide theory. The
anthracene crystal fiber can use the same theory but it is less accurate due to anthracene’s
slightly higher index of refraction. The index of refraction difference between the
scintillator and quartz provides the waveguide effect [3].
X-rays and UV light act to provide a pump effect seen in lasers ; except in this case
there is no population inversion because of the two level absorption and re-emission. As
such, this is not a fiber laser but the excitation process to generate light is similar. Photons
generated due to spontaneous emission fluorescence from x-ray absorption are represented
as non-coherent plane waves.
The generation of photons is de-coupled from the waveguide properties. Even
though photons are generated in fiber materials, transmission behaves as though from an
external source because the emission occurs on a sub-atomic level. This same approach
applies to fiber optic lasers, erbium doped fibers, Raman scattering, and Rayleigh
scattering effects in optical fiber [5, 6].
FIGURE 2. Shown is a typical organic scintillator two-level 7r orbital photon energy absorption-reemission
diagram.
FIGURE 3. Shown are the photospectrometer analysis results. UV light is absorbed and 420nm photons are
emitted from the liquid scintillator.
Absorption of the x-rays is based on Beer’s law along the axial fiber length using
Vinyl toluene property coefficients [4]. The small core and large length to diameter ratio
fiber create a geometry where absorption is along the fiber length and provides a
directional absorption property.
EXPERIMENT
Sample preparation for the liquid scintillator fiber was done by putting liquid
scintillator into the microstructured optical fiber. This process used capillary action and a
pressure gradient by pressurizing the liquid at one end of the fiber. Anthracene crystal
fiber was prepared using a modified Bridgman-Stockbarger technique [7]. An
approximately 12cm long fiber was chosen because that length has over 95% x-ray photon
absorption in the scintillat or materials.
Figure 4 shows light generation from a UV lamp source during examination and
testing of prepared fibers before exposure to x-rays. As described in theory, light is
emitted through the scintillator material with the fiber acting as a bundle of 2.5 micron
core sub-fibers. This inspection allows determining expected output from the fiber before
exposure to x-rays.
FIGURE 4. Emission of 420 nm photons using UV excitation on opposite end of fiber, discontinuities in the
scintillator material result in no emission from the fiber end.
An experiment was constructed to measure the relative photon counts from the
microstructured fibers with scintillating materials. The x-ray tube source was
characterized using a CdTe detector and multichannel analyzer. Measurements were taken
between x-ray tube voltages of 10 and 40 kV. A 40 kV, 4 W silver anode x-ray tube was
used as the source.
Figure 5 shows the experimental setup for x-ray tube characterization and
scintillating fiber output measurement. For characterization, the CdTe detector assembly
with a collimator collects x-ray photons. During fiber measurement, a photomultiplier
tube collects light pulses from the optical fiber.
FIGURE 5. Placement of CdTe detector and collimator for characterization of x-ray tube and taking fiber
data
The CdTe detector operates with a multi-channel analyzer using a collimator
having a 5 mm thick tungsten disk with a 100 micron diameter aperture followed by a 200
micron diameter aperture disk. The CdTe detector is 100% efficient at detecting x-ray
photons between the energies of 10-50 keV. The CdTe detector operates in conjunction
with a computer to store and process data.
This x-ray tube characterization was done prior to taking the fiber x-ray
scintillation data to compare with the CdTe detector counts. The tube characterization
was done at five minute intervals over a series of weeks to ensure tube stability and
measurement repeatability. The CdTe detector energy was calibrated done using an Am-
241 radioactive source. The collimator and tube were carefully aligned to maximize
photon counts through the collimator placed 0.5 cm from the tube.
The prepared fiber was placed in the same location as the detector collimator while
varying x-ray tube voltage without changing source current during measurements.
Scintillating fiber was coupled to the photomultiplier tube FC fiber optic adapter with a
bare fiber adapter. The photomultiplier tube module used a bialkali cathode with peak
quantum efficiency at 420nm. A FC fiber optic coupler was supplied with the module.
Scintillation inside the fiber was directed to the photomultiplier tube.
Signals from a single photomultiplier
 tube module went to a discriminator circuit,
which discards voltage pulses below an adjustable level. The remaining pulses were sent
to a digital pulse counter where the total is accumulated over a five minute time period.
This total pulse count was recorded for discrete x-ray tube voltages between 10 and 40kV.
A blackout cloth was used over a leaded Plexiglas three-sided enclosure with a
lead-foil covered aluminum back plate. Power and cables to the photomultiplier tube and
x-ray tube were routed between the Plexiglas enclosure and backing plate. The
photomultiplier tube module and the fiber couplers were wrapped in lead to reduce noise.
The blackout cloth reduced stray photons and their subsequent noise but allowed
easy access to the experimental setup. Additionally, the room was partially darkened
during operation to remove fluorescent light fixture magnetic and photonic noise. Power
for the electronics had a common ground.
RESULTS AND DISCUSSION
Figure 6 shows the x-ray tube photon energy spectrum collected using the CdTe
semiconductor detector with 100 micron collimator to characterize the tube. Tube
characterization and fiber data were taken over five minute intervals. A 30 micron thick
sheet of aluminum foil was placed in front of the tube as a low energy filter. The
collimator and fibers were positioned 0.5 cm from the tube end in the same relative
position.
FIGURE 6. 40 kV silver anode x-ray tube characterization with photon number and energy distribution
curves at various tube voltages. The plot is of photon counts versus energy (keV) collected using a 100
micron collimator with the CdTe detector over a 300 second time interval.
Figure 7 shows the ratio of liquid an  solid filled fiber to CdTe counts vs. tube
voltage as a percent efficiency. This percent efficiency trend for both liquid and solid
filled fibers increases with the tube voltage. The total number of photons counted from the
fiber follows the same trend as the CdTe detector. Anthracene fiber output is higher than
the liquid for tube voltages above 30 kV with a 37% increase at 40 kV. The liquid output
is rated at 60% of anthracene.
Some considerations noted in the data are that the total area of the microstructured
fiber exposed to x-rays is 10.5% of the CdTe collimator. Considering th  area used, the
anthracene fiber efficiency increases to 8.5% and the liquid to 6.7% at 40 kV. Anthracene
has a bulk efficiency of ~5% and organic scintillator efficiencies operate between 1-5%
[8]. This indicates there is increased efficiency associated with having scintillator
material in a microstruc tured geometry.
This paper shows a novel microstructured optical fiber containing scintillator
material can be used to detect x-rays. The fiber quartz protects the scintillator material
from the environment and small diameter fiber crystals tolerate bending inside the fiber.
While the assembled fiber efficiency is low, the signal to noise ratio is over 25:1.
FIGURE 7. Percent efficiency for fiber scintillation photon counts compared to total CdTe photons at
discrete tube voltages.
ACKNOWLEDGEMENTS
The author would like to thank Patty Davis and Warren Kelliher at the NASA
Langley Research Center for assistance in this work.
REFERENCES
1. G. F. Knoll, Radiation Detection and Measurement, publisher, John Wiley & Sons
(2000), Ch 8 & 14.
2. H. Leutz, “Scintillating Fibers”, Nucl. Instr. And Meth., 364, pp 442-448 (1995).
3. A.W Snyder and J.D. Love, Optical Waveguide Theory, publisher, Chapman and Hall
(1983), Ch 1,2,13 & 14.
4. NIST Website: Tables ofX-Ray Mass Attenuation Coefficients and Mass-Energy
Absorption Coefficients,
http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html.
5. L. Shen and J.A. Kong, Applied Electromagnetism, publisher, PWS Engineering
(1987), pp. 225-230.
6. B. Podolsky and K.S. Kunz, Fundamentals ofElectrodynamics, publisher, Marcel
Dekker (1969), pp. 302-313.
7. R.A. Laudise, Single Crystal Growth, publisher, Prentice-Hall Inc. (1970), Ch 5.
8. J.B. Birks , Theory and Practice of Scintillation Counting, publisher, Paragon Press
(1964).


Microstructured Optical Fiber for X-ray Detection

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