Method and apparatus for measuring ice thickness on substrates using backscattering of gamma rays

The present invention provides a method and apparatus for in situ measuring thicknesses of ice buildup on airfoil. The method and device uses a probe including a high energy radioactive gamma ray source .sup.241 Am producing 60 keV photons which penetrate through the airfoil substrate and a photodetector mounted behind the source for detection of backscattered photons. The probe is mounted on the interior of the airfoil and secondary radiation is backscattered within the ice layer and back through the airfoil substrate to the photodetector. The shape and density of the source holder in addition to the geometrical arrangement of the source and detector with respect to the airfoil substrate are used to block photons backscattered in the airfoil substrate thereby favoring scattering in the ice layer over that in the aluminum.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an in situ and non-destructive method and 
device for measuring the thickness of ice layers on substrates using 
backscattering of gamma rays. 
2. History of the Related Art 
The ability to measure, nondestructively and in situ the thickness of 
growing thin films is very advantageous in many industrial applications. 
For example, of paramount importance to passenger safety is the ability to 
monitor in situ growth of ice coatings on aircraft. Buildup of ice layers 
on aircraft wings or other materials has been and continues to be a cause 
of aircraft disasters. During or after takeoff of the aircraft the added 
weight of the ice, which can be very significant, as well as the 
accompanying change in aerodynamic flow patterns over the airflow surfaces 
can cause crashes. Preventative procedures such as de-icing the aircraft 
typically are carried out when the aircraft is near the hangar after which 
the aircraft taxis to the end of the runway for takeoff. During this 
period ice can again build up on the aircraft depending on the distance 
the aircraft must taxi and the severity of the weather conditions. 
One current method of measuring ice thickness on an airfoil uses microwave 
electromagnetic radiation. The microwave radiation is used to monitor the 
thickness and dielectric constant of the growing layer from which the 
composition is calculated. U.S. Pat. Nos. 4,054,255 and 4,688,185 issued 
to Magenheim and Magenheim et al. respectively disclose using a dielectric 
layer affixed to the wing surface as a surface waveguide into which a low 
power microwave signal is directed. The impedance and reflection 
properties of the waveguide change as ice builds up on the waveguide and 
this change is measured and related to the buildup of ice. 
Drawbacks to microwave monitoring systems are the expense of the power 
supplies and the need for sophisticated software for handling the data. 
Microwave monitoring systems necessitate cutting holes in the wings of the 
aircraft or otherwise modifying the wings to include waveguide elements 
which increase installation costs, disturb the flow pattern over the air 
foil and may reduce structural strength. 
Another known method for monitoring ice build-up involves the use of 
internal reflection to measure ice thickness. U.S. Pat. No. 4,797,660 
issued to Rein Jr. teaches use of internal reflection of EM using a prism 
mounted to the wing surface. A light source and detector are positioned to 
cause light to impinge on the exposed surface of the prism and a detector 
measures internally reflected light from the exposed surface with the 
reflected intensity being a function of the buildup on the exposed prism 
surface. U.S. Pat. No. 5,296,853 issued to Federow et al. is directed to a 
laser ice detector comprising a light source, light detector and 
temperature sensor with the light source and detector embedded in a 
plastic housing mounted flush with the surface of the wing. The system is 
designed to give total internal reflection when ice is absent from the 
plastic surface. The presence of ice on the plastic is accompanied by loss 
of total internal reflection. 
U.S. Pat. No. 4,797,660 issued to Michoud et al. discloses an ice thickness 
measuring technique for aircraft using internal reflection of light. The 
device is designed to discriminate against water and ice with for example 
falling rain acting to modulate the light signal received by the detector 
in a characteristic manner thereby distinguishing it from the signal due 
to ice buildup. As with microwave techniques, a drawback to internal 
reflection is the need for modification of the airfoil surface. 
Patent No. GB 1385279 discloses a device for detecting ice on the surface 
of an aircraft including a radioactive source producing gamma rays or beta 
rays (fast electrons) and a pair of Geiger-Muller detectors located 
laterally of the radiation source with one detector on each side of the 
source. This device has several disadvantages. The radiation detector must 
operate under very cold conditions and since Geiger-Muller counters are 
known to be very inefficient detectors of 60 keV gamma rays, the radiation 
sources must have a very high strength. Further, there is nothing in the 
design of the ice measuring device which makes the system more sensitive 
to scattering in an ice layer as opposed to scattering in the air-foil 
material. 
A rapid and accurate method of measuring the build-up of ice on aircraft in 
flight is required for safety considerations. Ice build-up occurs 
predominantly on the ground and at low altitudes with little build-up 
occurring at normally high cruising altitudes for jet aircraft. However, 
the steady increase in air traffic unaccompanied by construction of more 
airports has resulted in the practice of "stacking up" low priority 
flights before giving clearance to land. This is particularly the case in 
inclement weather and at relatively low altitudes, conditions most 
conducive to icing. Therefore, there is a need for a rapid, accurate, 
economic, in situ and nondestructive method of measuring the thickness of 
growing films on substrates. 
SUMMARY OF THE INVENTION 
The present invention provides a non-destructive, in-situ method of 
measuring thickness of ice layers on metal substrates such as an airfoil. 
In one aspect of the invention there is provided a method for measuring 
ice buildup on an outer surface of a metal substrate comprising a gamma 
ray source in a source holder, the gamma ray source producing a beam of 
primary gamma rays having sufficient energy to penetrate through the metal 
substrate. The method includes providing a photodetection means behind the 
source holder and positioning the source holder adjacent to an inner 
surface of the metal substrate so that the beam of primary gamma rays is 
directed through the metal substrate away from the photodetection means. 
The source holder substantially blocks both primary gamma rays from the 
gamma ray source and secondary photons scattered in the metal substrate 
from impinging on the photodetection means. The method includes measuring 
a total intensity of backscattered secondary photons and determining a 
thickness of the ice buildup from a measured increase in intensity of 
backscattered secondary photons over an intensity measured with said 
substrate alone. 
In this aspect of the invention the step of providing a gamma ray source in 
a source holder may include providing a cylindrically symmetric source 
holder to provide an axially symmetric beam of primary gamma rays emerging 
from the source holder. The preferred gamma ray source is radioactive 
.sup.241 Am and the step of measuring the intensity of secondary photons 
is accomplished using a Nal(TI) X-ray scintillator coupled with a 
photomultiplier detector. 
In another aspect of the invention there is provided an apparatus for 
measuring ice layer buildup on an exterior of an airfoil. The apparatus 
comprises a plurality of spaced probes each mounted at a different 
location on an interior surface of the airfoil. Each probe includes a 
source holder defining a cylindrical axis and a photodetection means 
located behind the source holder for measuring an intensity of 
backscattered photons. The photodetection means is symmetrically aligned 
with the cylindrical axis and a gamma ray source is located in the source 
holder. The gamma ray source in the source holder produces an axially 
symmetric beam of primary gamma rays emerging from the source holder away 
from the photodetection means and the gamma rays have energies 
sufficiently high to penetrate through the airfoil. The apparatus includes 
processing means connected to the photodetection means for calculating a 
thickness of the ice layer from the intensity of backscattered photons. 
The source holder includes a cylindrical post constructed of a metal 
selected from the group consisting of molybdenum, gold, platinum, lead, 
silver, tantalum and tungsten and having dimensions suitable to block 
primary gamma rays from impinging directly on the photodetection means 
from the gamma ray source. The cylindrical post has a preselected diameter 
so that the cylindrical post absorbs secondary photons scattered in a 
preselected volume of the airfoil adjacent to the cylindrical post. In 
this aspect of the invention the photodetection means may include a 
Nal(TI) X-ray scintillator coupled with a photomultiplier detector.

DETAILED DESCRIPTION OF THE INVENTION 
A) Basic Configuration of Source-Detector-Target System 
The basic design and geometric arrangement of an axially or cylindrically 
symmetric detector-source geometry constructed in accordance with an 
aspect of the present invention will be discussed first followed by 
descriptions of preferred embodiments for the monitoring of ice thickness 
on the airfoil of aircraft. The preferred embodiments of this invention 
illustrated in the drawings are not intended to be exhaustive or to limit 
the invention to the precise form disclosed so that the applications cited 
are exemplary in nature and are not intended to limit the scope of the 
invention. 
Referring to FIG. 1, a longitudinal section of a detector-source-target 
arrangement constructed in accordance with the present invention is shown 
at 20. A scintillation detector 22 includes a thin (1.0 mm) Nal(TI) 
scintillator 24 housed in an aluminum cylinder (not shown) of 5.08 cm 
external diameter and 15.24 cm in length which also houses a 
photomultiplier 26. A protective covering 28 such as MYLAR extends across 
the scintillator. A lead shielding 30 is provided around the sides of 
detector 22 to minimize multiple scattering from nearby objects. 
A source holder 32 is provided with a longitudinal cavity 34 extending 
partly therethrough for holding a radioactive source 38. Holder 32 is 
shown as being tubular with a radius R.sub.1 and cavity 34 defines a 
detector axis 36. Holder 32, also referred to as an absorber post, is 
fabricated of a sufficiently thick and dense material so that primary 
radiation from source 38 is blocked or absorbed before hitting detector 22 
below the source. 
Radioactive source 38 is preferably a commercially available sealed source 
of .gamma.-rays typically 3.0 mm in length and diameter. Source 38 sits on 
a threaded stud 42 for changing the position of source 38 in cavity 34. 
Source 38 sits at an adjustable depth Z.sub.1 below the top surface 40 of 
holder 32. The cylindrically symmetric geometry and structure of holder 32 
are such that with a source 38 in the holder, the axially symmetric beam 
of primary radiation moves upwards in a cone whose half angle is 
adjustable by the depth Z.sub.1. The area of a target 46 (airfoil 50 and 
ice layer 48) spaced a distance Z.sub.2 from surface 40 irradiated by the 
source is determined by both the half angle and the spacing Z.sub.1. 
The diameter of source holder 32 may vary from about 5 mm to about 8 mm and 
the holder may be fabricated of gold or other suitable high density 
material depending on the application. For example, platinum, tungsten, 
silver, molybdenum, lead and tantalum may all be used as materials for the 
source holder. The detector assembly may optionally include an iris 54 
defining an aperture 56 and having an inner radius R.sub.2 symmetrically 
disposed with respect to source holder 32. Iris 54 is formed of a material 
which acts to absorb x-rays and .gamma.-rays. Therefore, the backscattered 
photons can reach detector 22 only by passing through the annulus defined 
by radius R.sub.1 of the source holder and R.sub.2 of iris 54. Holder 32 
blocks primary radiation from the source impinging on the detector. Iris 
54 is optional since holder 32 is preferably made of a material having an 
effective density and shape to substantially block photons from the source 
from impinging on the photodetector and so is not required for some 
applications described herein. 
The variables of the detector-source-target system include the dimensions 
R.sub.1, R.sub.2, Z.sub.1, Z.sub.2, the presence or absence of iris 54 and 
the choice of radioactive source. 
B) Measurement of Ice Thickness on Aircraft Using Gamma-Ray Backscattering 
To monitor ice buildup on an airfoil, one or more probes containing a 
.gamma.-ray source are installed on the inside of the leading edges along 
the aircraft wing or tail section with the probe containing a 
photodetector to measure .gamma.-ray backscattering from ice forming on 
the outer surface. The fixed installations may be adapted to produce data 
continuously on the status of ice forming on the wings of the aircraft 
which can be displayed on the flight deck. 
Referring to FIG. 2, an ice thickness monitoring probe shown generally at 
90 is adapted to be secured to an inner surface 96 of an airfoil 94 
(Z.sub.2 in FIG. 1 is essentially equal to zero) for measuring ice 
thickness of an ice layer 92 on the outer surface of airfoil 94. Airfoil 
means aircraft parts with curved or flat surfaces such as the wings or 
other components responsible for keeping the aircraft aloft during flight. 
To use the method of the present invention specifically to measure ice 
thickness on an airfoil or leading edges of the aircraft, the source and 
detector must not be located on the exterior of the airfoil for 
aerodynamic considerations. Aluminum or aluminum based alloys are 
currently the preferred material of construction of airfoils. 
A radioactive source producing primary photons with sufficient energy to 
penetrate the aluminum is required and the geometry and density of the 
source holder is chosen so that photons backscattered in a preselected 
volume of the airfoil substrate adjacent to the source holder 32 are 
substantially blocked or attenuated by the holder and thereby prevented 
from impinging on detector 22, see broken lines in FIG. 2. Probe 90 
preferably comprises an .sup.241 Am radioactive source 38 which produces 
.gamma.-rays of energy 60 keV. The backscattered secondary photons, which 
are reduced in energy to about 48 keV, are also energetic enough to 
penetrate back through the aluminum to impinge on detector 22. Holder 32 
has an effective density and shape to substantially block or attenuate 
primary photons directly from source 98 and backscattered from the 
aluminum from impinging on photodetector 22. Probe 90 abuts against inner 
surface 96 of airfoil 94 with surface 40 of holder 32 preferably abutting 
the inner surface to ensure most of the photons backscattered in the 
aluminum are blocked or attenuated in source holder 32 while photons 
backscattered in ice layer 92 reach detector 22. Brackets 102 or other 
attachment means may be used to secure probe 90 to the interior of the 
airfoil. Tubular or cylindrically shaped source holders made of gold, 
tantalum and molybdenum and the like having a radius in the range from 
about 5 mm to about 8 mm provide suitable results. 
FIG. 3 shows the results of backscattering of 60 keV .gamma.-rays from a 
flat sheet of aluminum placed 3.6 mm from the .gamma.-ray source. With a 
.sup.241 Am source strength of 200 microcuries (Isotope Developments 
Laboratory), the backscattered intensity from the aluminum was 139 
kilocounts per minute (kcpm) for 1.016 mm aluminum; 112 kcpm for 0.762 mm 
thick aluminum and 76 kcpm for 0.47 mm thick aluminum. Ice was simulated 
by placing thin sheets of plastic on the side of the aluminum opposite the 
side on which the source was located. The readings were converted to show 
ice on the x- and y-axis of FIG. 3. The intensity of backscattering is 
expressed as a percentage increase above the thickness reading for 
aluminum alone. The increase is linear in the thickness of plastic for 
each thickness of aluminum. Radioactive source strengths in the range from 
about 1 microcuries to about 1 millicurie are preferred but the higher 
source strengths provide faster measurement times. 
In addition to aluminum, airfoils may also be constructed from titanium or 
carbon based composites. For titanium airfoil of substantially the same 
areal density as the aluminum airfoil it is contemplated that the same 
sensitivity to build-up of ice will be achieved using the probe of FIG. 2. 
The inventor contemplates that for practical airfoil materials of any 
type, the present method can be used in situations in which the substrate 
is a laminar structure comprising more than one material as long as both 
materials can be penetrated by the .gamma.-rays and backscattered 
secondary photons. 
The present method for detecting ice buildup on metal surfaces does not 
depend per se on any properties of ice; it merely detects the additional 
low-Z material adhering to the outer surface of the wing. Therefore, those 
skilled in the art will appreciate that the sensors used on aircraft must 
be strategically located on the airfoil to give the best results. In 
addition, the backscatter intensity is sensitive to changes in geometry so 
that small distortions due to aerodynamic forces may cause a change in 
background signal for an aircraft on the ground and in the air so that 
airborne calibration may be required. A plurality of probes are preferably 
used since the ice in many circumstances may not form a uniform continuous 
layer across the airfoil. With measurement times of seconds (depending on 
the source strength) the present method is an in situ technique so that 
the ice layer can be detected and its thickness determined while it is 
forming as well as after it has formed. The present device works with the 
same efficiency from -40.degree. C. to +40.degree. C. 
Referring to FIG. 4, in another set of studies, the source and detector 
assembly was fixed 4 mm from two sheets of 5052 aluminum alloy, (a) 0.762 
mm and (b) 0.559 mm. The intensity of backscattered .gamma.-rays was 
measured as layers of plastic were firmly secured to the aluminum. Each 
plastic layer had a thickness of 0.089 mm, equivalent to 0.147 mm of ice. 
The backscattering results are displayed in FIG. 5. 
FIG. 5 shows the backscatter intensity of 60 keV photons as a function of 
thickness of the plastic sheets (emulating ice) on a sheet of 6061 
aluminum alloy with a thickness of 2.223 mm. In this configuration the 
source/detector array was bolted directly to the aluminum substrate metal 
in order to eliminate relative motion between the source and substrate. 
In another study a uniform layer of water was frozen on one side of a sheet 
of 5052 aluminum alloy of thickness 0.559 mm. The ice thickness was 
calculated by weighing the aluminum substrate with and without ice and 
measuring the surface area. The ice thickness was then measured using the 
present method. The summary of the results are: 
______________________________________ 
Calculated Ice Thickness 
Measured Thickness 
% Error 
______________________________________ 
51.0 mils 53.4 mils 4.7% 
30.9 mils 33.1 mils 7.1% 
______________________________________ 
FIG. 6 illustrates an aircraft 110 in which a plurality of probes 90 are 
disposed along the interior of the leading edges of the wings 112 and tail 
stabilizers 114. Outputs from each probe 90 are input into a 
microprocessor 116 located on the flight deck 118 for displaying the 
outputs to the flight crew. The method disclosed herein of monitoring ice 
thickness on aircraft is advantageous over prior art methods because it 
does not require drilling holes in the wing thereby avoiding aerodynamic 
problems associated with interference of air flow over the wing and 
weakening of the airfoil. Further, the use of the weak radioactive sources 
provides a very rapid measurement while at the same time it avoids the 
need for expensive RF or microwave generators. 
The present invention is very advantageous over the previous methods of ice 
thickness detection for several reasons. For example, the present method 
and device requires only a very weak radioactive source which is a 
significant advance over GB 1385279 because, in the latter, the very low 
efficiency of the Geiger detector for 48 keV gamma rays means that 
radioactive sources with much stronger source strengths are required and 
hence the detectors require much more extensive shielding to prevent a 
large background from arising from direct penetration of 60 keV primary 
photons to the detector. 
Further, the present method and device shields the detector from secondary 
photons scattered in the airfoil itself while GB 1385279 is not designed 
to screen out counts due to scattering in the aluminum airfoil. In 
addition, because the present system has substantially no interfering 
background (i.e., all the pulses are desired signals from ice build-up and 
not from the airfoil), it is unaffected by changes in gain induced by 
changes in ambient temperature. The present device works with the same 
efficiency from -40.degree. C. to +40.degree. C. Geiger counters, on the 
other hand, contain gas mixtures, some of which will condense at 
extensively low temperatures with a resultant change in the intensity of 
spurious afterpulses due to failure in self-quenching. 
Those skilled in the art will appreciate that the present invention 
advantageously uses hindrance to selectively sample backscattered photons 
to ensure photons backscattered from the layer of interest are probed. 
This use of hindrance can include a) absorption by the specimen; 2) 
blocking or shadowing by the radioactive source holder and the iris to 
block secondary radiation arising from scattering in the body of the 
airfoil itself from reaching the detector; and 3) collimation of the 
primary photon beam by varying the position of the source in the source 
holder. 
The method disclosed herein is very advantageous for detecting the 
presence, and measuring the thickness, of ice buildup on an airfoil using 
relatively simple and inexpensive photodetectors and commercially 
available counting electronics. The thickness of the ice layer can be 
measured very rapidly with the measurement time dependent on the 
radioactive source strength. 
Therefore, while the present invention has been described and illustrated 
with respect to the preferred embodiments for measuring the thickness of 
ice buildup on airfoil surfaces, it will be appreciated that numerous 
variations of these embodiments may be made depending on the application 
without departing from the scope of the invention as described herein.