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United States Patent |
5,028,794
|
Miller
|
July 2, 1991
|
Method for radiation detection and measurement
Abstract
M-center luminescence is used to measure radiation. An LiF crystal is
excited with a 442 nm He-Cd laser because absorption measurements in LiF
indicate the peak of the M-center absorption occurs at 443 nm. Laser
stimulation produces an excited state of the M-center, which undergoes a
very strong Stokes' shift. The peak of the M-center emission spectrum
occurs at 665 nm with a half-width of 0.36 ev. Since the excitation
wavelength differs significantly from the emission wavelength, measurement
of the deep red emission can be done simultaneously with the excitation.
The population of M-centers grows with increasing radiation damage, and
therefore M-center lunimescence provides a basis for radiation dosimetry.
Inventors:
|
Miller; Steven D. (Richland, WA)
|
Assignee:
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Battelle Memorial Institute (Richland, WA)
|
Appl. No.:
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431307 |
Filed:
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November 3, 1989 |
Intern'l Class: |
G01T 001/105 |
Field of Search: |
250/484.1 C,484.1 B,484.1 A,484.1 R,337
|
References Cited
U.S. Patent Documents
4507562 | Mar., 1985 | Gasiot et al. | 250/484.
|
4636642 | Jan., 1987 | Simons et al. | 250/337.
|
4638163 | Jan., 1987 | Braunlich et al. | 250/337.
|
4792691 | Dec., 1988 | Morlotti et al. | 250/484.
|
4801806 | Jan., 1989 | Nakamura et al. | 250/484.
|
4825084 | Apr., 1989 | Braunlich et al. | 250/484.
|
Other References
R. K. Gartia, "Optical Bleaching-a Technique to Clean Thermoluminescence
Peaks", Phys. Stat. Sol. (a), vol. 42, No. 2 (Aug. 16, 1977) pp.
K155-K158.
McLaughlin et al., "Electron and Gamma-Ray Dosimetry Using
Radiation-Induced Color Centers in LiF", Radiat. Phys. Chem. 14, 467-480
(1979).
Seitz, "Color Centers in Alkali Halide Crystals", Rev. Mod. Physics 18, 348
(1946).
Knox, "Inversion Symmetry of the M-Center", Phys. Rev. Letters 2(3), 87
(1959).
Klick, "Luminescence of Color Centers in Alkali Halides", Phys, Rev. 79,
894 (1950).
Bosi et al., "Lifetime Studies on Excited (F.sub.2.sup.+)* and M Centers in
NaF Doped with Magnesium", Phys. Stat. Sol. (b) 140, pp. 355-360 (1987).
Boxi et al., "New Results on the Decay Properties of Perturbed and
Unperturbed M-Centers in MaCl:CdCl.sub.2 ", Phys. Stat. Sol. (b) 123, pp.
519-524 (1984).
|
Primary Examiner: Hannaher; Constantine
Attorney, Agent or Firm: Dellett, Smith-Hill and Bedell
Parent Case Text
REFERENCE TO GOVERNMENT CONTRACT
This invention was made with government support under contract number
DE-AC06-76RLO 1830, awarded by the U. S. Department of Energy. The
government has certain rights in the invention.
Claims
I claim:
1. A method of measuring radiation dose, comprising:
exposing a body of crystalline alkali halide material to ionizing
radiation,
exciting the material with optical radiation at a first wavelength, and
measuring optical energy emitted from the material by luminescence at a
second wavelength that is longer than the first wavelength.
2. A method according to claim 1, wherein the alkali halide material is
lithium fluoride.
3. A method according to claim 1, wherein the material is excited by laser
light in the visible region of the spectrum.
4. A method according to claim 1, wherein the material is excited by laser
light in the blue region of the optical spectrum and the optical energy
emitted by luminescence is measured in the red region of the spectrum.
5. A method of measuring radiation dose, comprising:
exposing a body of crystalline material to ionizing radiation,
exciting the material with optical radiation at a first wavelength, and
measuring optical energy emitted from the material by luminescence at a
second wavelength that is longer than the first wavelength and is in the
visible region of the spectrum.
6. A method according to claim 5, wherein the crystalline material is an
alkali halide material.
7. A method according to claim 6, where the alkali halide material is
lithium fluoride.
8. A method according to claim 5, wherein the material is excited by laser
light in the visible region of the spectrum.
9. A method according to claim 5, wherein the material is excited by laser
light in the blue region of the optical spectrum and the optical energy
emitted by luminescence is measured in the red region of the spectrum.
10. A method of measuring radiation dose, comprising:
exposing a body of crystalline alkali halide material to ionizing
radiation,
exciting the material with laser light at a wavelength in the blue region
of the optical spectrum, and
measuring optical energy emitted from the material by luminescence in the
red region of the spectrum.
11. A method according to claim 10, wherein the alkali halide material is
lithium fluoride.
Description
BACKGROUND OF THE INVENTION
When solid-state crystals are subjected to ionizing radiation, several
absorption bands make their appearance at increasingly higher levels of
radiation damage.sup.(1). In the case of the alkali halides, the F-band is
the first of the radiation damage centers to produce a detectable
absorption band. Although the F-center provides the greatest concentration
of damage centers for a given radiation exposure, its physical properties
preclude its use in quantifying ionizing radiation exposure using
luminescence techniques. The luminescence of the F-center is thermally
quenched below room temperature.
With increasing radiation exposure, a second damage center builds up in the
absorption spectrum. The second damage center is known as the M-center and
is generally thought to consist of two adjacent F-centers.sup.(2,3).
Unlike the F-center, however, excitation into the longer wavelength
M-center absorption band at room temperature produces
luminescence.sup.(4). M-center luminescence involves a Stokes' shift,
allowing the luminescence to be observed at a significantly different
wavelength from the exciting wavelength.
Highly purified lithium fluoride (LiF) crystals have long been used as
optical windows. Optical grade LiF is known for its excellent transmission
mission from the deep ultraviolet through the infra-red. Radiation
dosimetry applications have so far been restricted to high-level Mega-Rad
gamma dosimetry using radiation-induced absorption peaks measured with a
spectrophotometer. However, absorption measurements are a very insensitive
way to measure these radiation damage centers.
SUMMARY OF THE INVENTION
A more sensitive method of measuring radiation damage than absorption
measurement can be accomplished by measuring the M-center luminescence.
Excitation of an LiF crystal with a 442 nm He-Cd laser has been found to
significantly amplify the luminescence yield of the M-center of LiF. An
He-Cd laser was the preferred excitation laser because absorption
measurements in LiF indicate the peak of the M-center absorption occurs at
443 nm. Laser stimulation produces an excited state of the M-center, which
undergoes a very strong Stokes' shift. The peak of the M-center emission
spectrum occurs at 665 nm with a half-width of 0.36 ev. Since the
excitation wavelength differs significantly from the emission wavelength,
measurement of the deep red emission can be done simultaneously with the
excitation. Therefore, optical filtration provides a means of measuring
the M-center luminescence simultaneously with laser excitation into the
M-center absorption band. The population of M-centers grows with
increasing radiation damage, and therefore M-center luminescence provides
a basis for radiation dosimetry. Readout of the LiF can be accomplished in
a fraction of a second.
While all members of the alkali halide family are candidates for M-center
luminescence dosimetry, lithium fluoride (LiF) was chosen for a number of
reasons. First, the crystal is commercially available in large quantities
through its use as optical window material. Secondly, LiF is nearly tissue
equivalent in its energy response to photons, making it a suitable
material for mixed field dosimetry applications. Finally, LiF can be
excited by a low cost, commercially available blue-light laser and its
M-center luminescence occurs significantly within the visible spectrum.
This feature simplifies light detection since phototubes that are
sensitive to the visible light spectrum are readily available. Data
describing the dosimetric properties of the M-center and other general
physical data concerning the M-center are presented in the detailed
description, which also discusses the possibilities for M-center dosimetry
in LiF.
M-center luminescence exists in almost every solid-state crystal which can
be artificially grown at the present time and is not restricted to the
alkali halide group. Each crystal has a characteristic M-center absorption
band which can be measured following a large radiation exposure (around 1
Mega R). Illumination into the M-center absorption band with laser light
stimulates M-center luminescence which occurs at a significantly different
wavelength from the exciting laser light beam. Easy separation and
detection is possible simultaneously with the exciting light by using an
optical filter which absorbs the exciting light and transmits the M-center
luminescence.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram of the M-center luminescence readout system. The He-Cd
laser produces 70 mW at 442 nm with a beam diameter of 1 mm. The reader
consists of a light tight metal box with a sample drawer for insertion and
removal of the LiF crystals. The chamber surfaces are made of a black
plastic to reject and shield stray light. An RCA 8852 phototube with a
broadband optical interference filter is used to reject the laser light
and transmit the M-center luminescence. A personal computer houses a
multichannel scaling board for photon counting capable of 50 MHz.
FIG. 2 shows an LiF M-center luminescence optical emission spectrum
measured using a Tracor Northern optical multichannel analyzer. The
M-center luminescence emission was excited by a 442 nm He-Cd laser. The
LiF crystal was given an exposure of 2.6 C/kg using a .sup.60 Co source.
The peak of the emission spectrum is 665 nm with a half-width of 0.36 ev.
FIG. 3 shows an optical absorption spectrum for a LiF crystal exposed to
260 C/kg. The optical path length of the LiF crystal was 6 mm. The optical
absorption spectrum was taken using a Hewlett-Packard diode array
spectrometer. The peak of the absorption spectrum was measured to be 443
nm.
FIG. 4 shows the optical bleaching rate of the M-center luminescence as a
function of 442 nm He-Cd laser energy incident upon the crystal. The
crystal used in this figure was exposed to 26 mC/kg. The time constant for
the optical bleaching was found to be 32 s. The total reduction in
M-center luminescent intensity was 20% for a delivered energy of 30J.
FIG. 5 is a schematic diagram of a personnel dosimeter based on an LiF
crystal.
DETAILED DESCRIPTION
Optical grade LiF single crystals (6 mm.times.6 mm .times.6 mm) were
selected for the M-center measurements. Since an appreciable population of
M-centers are present in commercially available crystals, the centers are
erased by heat treatment. A heat treatment in air at 873K for 1 h was
found satisfactory in eliminating the M-center background. After heat
treatment, some of the crystals were kept as controls, while the remainder
were exposed to 0.26, 2.6, 26, and 260 mC/kg from a .sup.60 Co source.
After irradiation, the crystals were stored in paper envelopes to prevent
unwanted light exposure
As shown in FIG. 1, the readout apparatus consisted of three distinct
functional units: a 70 mW He-Cd 442 nm laser 2, a light-tight box 4 that
contained the sample chamber 6 and phototube 8, and a photon counting
system 10. The He-Cd laser beam was focused to beam diameter of 1 mm at
the crystal face and transmitted through the crystal at the centers of two
opposite cube faces. To provide the laser excitation light to the
crystals, the laser beam path and the sample chamber were made of black
plastic which helped shield stray room light and laser scatter. A
broadband interference filter 12 centered at 650 nm was chosen to prevent
the 442 nm He-Cd laser light from reaching the phototube 8 and to transmit
the M-center luminescence. The filter provided a 650 nm peak transmission
of 50% and a bandwidth of 70 nm. Transmission of the scattered 442 nm
He-Cd laser light was reduced to 0.01% by the filter 12. A 1 mm aperture
was placed between the optical filter and the crystal to help reject some
of the background light that accompanies the M-center luminescence. The
phototube 8 chosen for the experiment was an RCA 8852 with a red-extended
multi-alkali photocathode and was mounted at 90.degree. from the axis of
the laser beam. This phototube was chosen for its suitability for photon
counting and its excellent red response. The photon counting system
consisted of a preamplifier, amplifier, discriminator, and a multichannel
scaling board housed in a personal computer. The counting system is
capable of 50 MHz rates.
Readout consisted of passing the 70 mW laser beam through the sample
chamber and into the LiF crystal while simultaneously counting photons
with the multichannel scaling board within the computer. A region of
interest was chosen which consisted of 400 channels with each channel
having a time width of 50 ms. An integration was performed over the
400-channel region of interest which lasted for 20 s.
FIG. 2 is a LiF M-center luminescence optical emission spectrum measured
using a Tracor Northern optical multichannel analyzer. The LiF crystal was
given an exposure of 2.6 C/kg using a .sup.60 Co gamma source. The peak of
the emission spectrum is 665 nm with a half width of 0.36 ev. This result
was used to determine the optimal interference filter to use in the
M-center luminescence reader. An optical absorption spectrum for a heavily
exposed LiF crystal was measured using a Hewlett-Packard diode array
spectrometer. This absorption spectrum is shown in FIG. 3. The exposure
level for the absorption spectrum was 260 C/kg. To resolve the M-center
absorption curve adequately, it was found that 260 C/kg was the minimum
exposure that could be used The absorption curve peaks at 443 nm, which
matches the 442 nm line of the He-Cd laser.
The following table shows the M-center response as a function of .sup.60 Co
gamma exposure of 0.26, 2.6, 26 and 260 mC/kg.
______________________________________
Exposure Level
Background Subtracted
(mC/kg.sup.-1)
(Counts/mC/kg.sup.-1)
______________________________________
Laser Beam at Full Power (50 mW)
0.26 7,040,000
2.6 7,040,000
26 6,190,000
Laser Beam Through a
10% Neutral Density Filter
2.6 5,380.000
26 5,270,000
260 5,620,000
______________________________________
Two laser power levels were used due to counting saturation of the 260
mC/kg exposure using the full 70 mW of laser power. Within experimental
uncertainty, the response follows a linear relationship. The crystals
exposed to 0.26 mC/kg gave an integral response that was twice as high as
a crystal that received no exposure.
FIG. 4 is a plot of the bleaching rate of the M-center luminescence as a
function of time. The crystal was exposed to 26 mC/kg and the laser power
was 70 mW delivered for 600 s. The time constant for the bleaching process
was found to be 32 s and appears to remain constant over time. For the
entire time in which the M-center luminescence was monitored, the total
reduction due to bleaching was only 20%.
The time constant associated with the decay of the excited M-center was
measured in a straightforward manner. The same readout apparatus that was
used for the previous measurements was used in the time-constant
measurements. A nitrogen-pumped dye laser was used to excite an irradiated
LiF crystal within the readout apparatus, the signal was fed into a
Hewlett-Packard digitizing fast storage oscilloscope. The oscilloscope was
allowed to average the M-center decay signal until a smooth set of data
was obtained. The time constant measured was 70 ns. This appears to be
consistent with measurements of the other authors .sup.(5,6).
The M-center luminescence in LiF occurs with a time constant on the order
of 70 ns. Based on this measurements, each M-center in LiF should be
capable of producing around 10.sup.7 photons per second at saturation. A
simple calculation based on the oscillator strength of the M-center in LiF
predicts the M-center luminescence saturates with a 442 nm laser beam at
40 kw. Since the laser used in the present experiment is 70 mW, large
gains in the M-center luminescence signal can be achieved by increasing
the excitation power, for example, through use of a high intensity argon
laser.
Phototubes that record the M-center luminescence must be extremely
sensitive to the red-infra-red emission spectrum if a sensitive dosimeter
capable of personnel monitoring is to be realized. Unfortunately, red
extended phototubes capable of single photon counting have
characteristically large dark counts associated with thermionic emission.
A commercially available dye laser provides an elegant solution to the
problem of high dark counts and also satisfies the need for large peak
powers to achieve saturation of the M-center luminescence. Since dye
lasers typically have extremely short pulse lengths, the red extended
phototube dark count within the duration of the dye laser pulse becomes
negligible. The flashlamp-pumped dye laser can be made to produce very
large peak powers with time constants of 10 to 100 .mu.s, orders of
magnitude longer than the time constant for the decay of excited
M-centers. Therefore, through the use of the flashlamp-pumped dye laser,
the M-center luminescence is maximized by producing saturation, while
allowing a sampling of the luminescence in a time period that is much
longer than the decay constant of the M-center. This condition will
provide better statistical results. Fortunately, the M-center luminescence
yield at saturation is theoretically optimized with the same laser that
produces a negligible dark count.
The most restricting aspect of the M-center luminescence technique in LiF
is the large fluorescence background. The unwanted background, stimulated
by the direct and scattered laser light, emanates from a number of
sources. The first such source is the fluorescence that results within the
chamber surfaces from laser light scatter. A careful chamber design both
in geometry and in material selection can greatly reduce this source of
unwanted light. A second source is the fluorescence that emanates from the
crystal surfaces. The surface fluorescence is clearly visible by the human
eye under the proper conditions and represents the largest source of
unwanted light. The surface fluorescence depends on the nature of the
fluorescing species at the surface. The third source of unwanted light is
from the bulk of the crystal. Impurities within the crystal that are
excited by the laser beam will produce light that interferes with the
desired signal. This source of light has not been observed at the present
time, due to the large fluorescence signals that mask the bulk crystal
effects. The fourth source of background light is from the leakage of the
optical filter used both to condition the laser and to block stray laser
light from reaching the phototube. Since the phototube is highly sensitive
to the exciting laser wavelength, proper filtration is essential. High
quality filters and the use of multiple filters may prevent this source of
background from causing any serious problems. The final source of
background light is perhaps the most troubling.
In the dosimeter application, it is desirable that the dosimeter be
reusable, and this implies the need to anneal the crystal so as to remove
the M-centers. As noted previously, thermal treatment can be used to
eliminate the M-centers. However, at some point of the heat treatment, the
M-center will reach an equilibrium level, at which point further reduction
in M-center concentration becomes difficult. It remains to be seen whether
this concentration level will be suitably low to perform adequate
personnel radiation monitoring.
Based on the foregoing, it will be appreciated that M-center luminescence
in LiF using a .sup.60 Co gamma source has been demonstrated to be a
useful dosimeter in the exposure range of 0.026 to 260 mC/kg. The major
difficulty in using LiF for M-center luminescence dosimetry is the large
background of fluorescence which accompanies the M-center luminescence
signal. Low cost diode lasers emitting in the infra-red are currently
available which could be used to excite those crystals which have their
M-center absorption band in the infra-red.
Many alkali halides have their M-center luminescence absorption band in the
infra-red. An inexpensive infra-red detector can be used to detect the
infra-red luminescence allowing a low cost laser and detection system to
be technically feasible. In this way, the M-center luminescence can be
used as a means of personnel or environmental dosimetry. By coupling a
small infra-red diode laser with an infra-red detector and suitable
optical filters, the combination becomes a radiation monitor which gives a
real-time analysis. FIG. 5 illustrates such a dosimeter, comprising a
crystal 20 to which are mounted a diode laser 22, a detector 24 to detect
the crystal's M-center luminescence, and an optical filter 26 to block the
laser light and transmit the crystal's M-center luminescence.
Optically stimulated luminescence is described in co-pending U.S. Pat.
applications Ser. Nos. 07/213,245, now U.S. Pat. No. 4,954,707, and
07/420,293 the contents of which are hereby incorporated by reference
herein.
It will be appreciated that the invention is not restricted to the
particular embodiment that has been described, and that variations may be
made therein without departing from the scope of the invention as defined
in the appended claims and equivalents thereof.
REFERENCES
1. McLaughlin et al, "Electron and gamma-ray dosimetry using
radiation-induced color centers in LiF", Radiat. Phys. Chem. 14, 467-480
(1979).
2. Seitz, "Color centers in alkali halide crystals", Rev. Mod. Physics 18,
348 (1946).
3. Knox, "Inversion symmetry of the M-center", Phys. Rev. Letters 2(3), 87
(1959).
4. Klick, "Luminescence of color centers in alkali halides", Phys. Rev. 79,
894 (1950).
5. Bosi et al, "Lifetime studies on excited (F.sub.2 +)* and M centers in
NaF doped with magnesium", Phys. Stat. Sol. (b) 140, 355-360 (1987).
6. Bosi et al, "New results on the decay properties of perturbed and
unperturbed M-centers in NaCl:CdCl.sub.2 ", Phys Stat. Sol. (b) 123,
519-524 (1984).
The disclosure of each reference is hereby incorporated by reference herein
.
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