Energy resolving x-ray detector

An energy-resolving x-ray detector for soft x-rays produced by elements having atomic numbers ranging from 9 to 23 includes a charge-coupled integrated circuit radiation detector device having an array of collection regions in a parallel plurality of collection shift registers forming columns of the array; an output amplifier for sequentially amplifying and signalling the charges received by the collection shift register; and a row shift register connected between the collection shift registers and the output amplifier; and a clock circuit having a multi-phase column output connected for sequentially shifting charges between collection regions of the collection shift register and into the row shift register during continuous exposure of the array to incoming radiation, each of the charges received by the output amplifier being sequentially accumulated in each of the collection regions of one collection shift register in response to the radiation, the clock circuit also having a multi-phase row output connected for sequentially shifting the charges from the row shift register to the output amplifier, the output amplifier having a reset connection to the clock circuit for momentarily resetting the input to the output amplifier at a predetermined level prior to receipt of each of the charges into the output amplifier. The output amplifier feeds an analog signal chain providing correlated double sampling. A spectrometer and thickness measurement apparatus suitable for monitoring silicone coatings includes the detector.

BACKGROUND 
The present invention relates to radiation detectors and x-ray energy 
spectroscopy. 
Elemental compositions of many materials are discoverable by the process of 
x-ray fluorescence, wherein atoms excited by high energy x-ray absorption 
re-emit the energy as x-rays having lower energy. The energy distribution 
of the re-emitted x-rays forms an elemental "fingerprint" that is useful 
for identifying the composition. X-ray spectroscopy is particularly 
effective fox determining elemental abundances in that the energies of the 
characteristic x-rays of an element are little changed by the 
environmental matrix of the element, unlike spectroscopy in the 
ultraviolet, visible and infra-red spectrums. 
The energy of these characteristic x-rays generally increases with 
increasing atomic number (Z), as does the energy spacing between the 
characteristic emissions. Higher energy x-rays are generally easier to 
detect than low energy x-rays in that they are more penetrating and 
generate a larger signal in an energy sensitive detector. Thus elements of 
higher Z are easier to isolate and quantify than elements of low Z. For 
example, silicon (Z=14) is used extensively in a wide variety of 
applications, including semiconductor fabrication and lubrication. Silicon 
is also used in thin coatings as a release agent on paper and plastic 
substrates, giving rise to a need for monitoring the thickness and quality 
of the coatings. X-ray spectroscopy directed to silicon has had limited 
success in the prior art because of its low atomic number and because of 
the likely presence of other common low atomic number elements such as 
aluminum and calcium that have similar x-ray fingerprints. 
The performance of an x-ray spectrometer is most often limited by the 
performance of the x-ray detector utilized, x-ray detectors of the recent 
prior art exhibiting tradeoffs between energy resolution, response time, 
and cooling requirements. Available devices having the highest performance 
require complex support equipment; thus spectrometers utilizing these 
devices have large physical size and/or slow response times. High 
performance spectrometers find application in laboratory analysis of 
metals, ceramics, pharmaceuticals, and other products wherein sample 
analysis typically requires extensive preparation and then many minutes to 
hours of data acquisition. Lower performance devices having less 
sensitivity and energy resolution find application in industrial process 
quality control, the industrial units typically operating off-line with 
sample analysis requiring little preparation and only a few minutes of 
data acquisition time. 
Detectors in common use for x-ray energy spectroscopy include scintillation 
detectors, gas-proportional detector tubes, lithium-drifted silicon 
detectors, and large-area PIN photodiodes. Scintillation detectors 
down-shift the x-ray energy to the optical spectrum, requiring further 
detection by an optical detector such as a photo-multiplier tube, whereas 
the remaining detectors provide a direct electrical output. The main 
operational parameters of these typical detectors are given below in Table 
1. 
TABLE 1 
______________________________________ 
Detector Type 
scin./PMT gas Si--Li PIN--PD 
______________________________________ 
Energy Range (kev) 
5-1000 1-100 .1-100 1-100 
Resolution @ 6 kev 
1-2 kev 800 ev &lt;100 ev 
1 kev 
Detection Efficiency 
30% 50% &gt;90% &gt;70% 
Max. Count 1000 200 1000 &gt;1000 
Rate (kHz) 
Cooling none none LN.sub.2 
&lt;25 C 
Requirements (77 K) 
______________________________________ 
To provide a useful output, the detector must generate a signal that is 
significantly above thermal noise limits. An energy-sensitive x-ray 
detector provides a series of pulses that must then be quantified as to 
the energy of the x-ray each pulse represents. The energy resolution of 
this process determines the range and resolution of elements that may be 
effectively identified. The counting statistics of the pulses determine 
the statistics of the final quantitative measurement. 
Conventional detectors are classified as avalanche and non-avalanche. 
Avalanche detectors feature some form of internal gain (such as the 
electron cascade in a gas proportional tube or photo-multiplier tube) to 
raise the signal level above background noise. Since they have no internal 
gain mechanism, non-avalanche detectors require extensive cooling such as 
by liquid nitrogen for reducing background noise. Another class of 
detectors includes charge-coupled array detectors (CCD's), typical 
examples thereof being commercially used in video cameras. A CCD detector 
can provide a non-avalanche effective gain equal to the ratio of the 
equivalent capacitance of the entire imaging area of the device (typically 
10.sup.5 pf) to the capacitance of the output amplifier MOSFET gate 
(typically 0.25 pf). 
Although normally utilized for the detection of infrared or visible light, 
the CCD has two basic characteristics that are advantageous for the 
detection of soft x-rays: A large active area (&gt;5 cm.sup.2 possible); and 
low inherent noise (less than 10 e- possible). The x-ray sensitivity of 
the CCD was first exploited extensively by James Janesick and others as a 
tool to characterize the performance of the CCD because it was observed 
that a good CCD in the x-ray domain was an excellent CCD in the visible 
domain. See, for example, Robinson et al., "Performance Tests of Large 
CCDs," Charge-Coupled Devices and Solid State Optical Sensors II (SPIE 
Vol. 1447, 1991), p. 214. In particular, testing in the x-ray domain led 
to identification of the important CCD characteristic 
"charge-transfer-efficiency" (CTE). 
Conventional CCD's, as used in video cameras, have a number of drawbacks in 
x-ray detector applications, such as a parallel set of "shadow" registers 
that are used in the frame transfer process required for video format 
signals. These shadow registers are opaqued and not available for x-ray 
detection, wasting valuable detector area. Video CCD's operate at a high 
data rate such that low-noise operation is not possible, and thus are not 
optimized for low noise. Also, any window on the device package absorbs 
the low-energy x-rays of interest. 
CCDs more suitable for x-ray detection are also known, being high-quality, 
scientific-grade, windowless devices that do not have shadow registers. 
Since the electrodes used for charge transport across the detector array 
block photon absorption, scientific-grade CCD's are often operated in a 
thinned, backside-illuminated mode. For these devices, the bulk silicon 
from the rear of the CCD chip is removed until the chip is only 10 microns 
thick. The chip is then mounted upside-down to allow maximum photon 
transmission and the resulting assembly is cooled to reduce thermal noise 
contributions. 
Although CCDs have been used in x-ray detection, it is believed that all 
prior art x-ray detection applications of the CCD are for intensity 
sensing, not energy sensing. Also, it is believed that all prior art 
applications of the CCD have utilized the device in an imaging mode. 
Thus there is a need for an x-ray detector that has high energy resolution 
for detecting low-Z elements, high detection efficiency for rapid data 
acquisition, that is statistically accurate, that is inexpensive to 
provide, and that does not have excessively burdensome cooling 
requirements. There is a further need for a spectrometer that exhibits 
these advantages. 
SUMMARY 
The present invention meets this need by providing an energy-resolving 
x-ray detector. In one aspect of the invention, the detector includes a 
charge-coupled integrated circuit radiation detector device having an 
array of collection regions with an associated transport electrode, the 
collection regions forming at least one collection shift register, and an 
output amplifier for sequentially amplifying and signalling the charges 
received by the collection shift register; and a clock circuit connected 
to the transport electrodes for sequentially shifting charges between 
collection regions of the collection shift register and into the output 
amplifier during continuous exposure of the array to incoming radiation, 
each of the charges received by the output amplifier being sequentially 
accumulated in each of the collection regions of the collection shift 
register in response to the radiation. 
The detector device can include a parallel plurality of the collection 
shift registers, each collection shift register forming a column of the 
array, corresponding collection regions of the collection shift registers 
forming rows of the array. The detector device can further include a row 
shift register connected between the collection shift registers and the 
output amplifier, the row shift register having cell regions and 
associated transport electrodes, the transport electrodes of the row shift 
register being connected to the clock circuit, the clock circuit being 
operative for periodically shifting the charges from each of the 
collection shift registers into a corresponding element of the row shift 
register, and for sequentially shifting the charges from the row shift 
register to the output amplifier. Preferably the clock circuit has a 
three-phase row output for driving the transport electrodes of the row 
shift register, and a three-phase column output for driving the transport 
electrodes of the collection shift registers. 
Preferably the output amplifier has a reset connection to the clock circuit 
for momentarily resetting the input to the output amplifier at a 
predetermined level prior to receipt of each of the charges into the 
output amplifier. The detector can further include a correlated double 
sampling signal chain responsive to the output amplifier and the clock 
circuit for generating an analog sample signal for each charge received 
into the output amplifier, the analog sample signal being proportional to 
the energy of the incoming radiation having reached successive collection 
regions of the collection shift register during accumulation of the 
received charge. 
The present invention also provides a spectrometer having the detector in 
combination with a source of x-ray radiation for producing x-ray 
fluorescence of a sample, the detector being responsive to the 
fluorescence of the sample. The source of x-ray radiation can be a 
radioactive material. The radioactive material forms a ring-shaped member 
surrounding a radiation path between the sample and the detector array. 
The array of collection regions can be planar, the ring-shaped member 
being supported parallel to the array, being axially spaced from the array 
not more than approximately 25% of a path length between the sample and 
the array for enhancing a field of view from the array to the sample. The 
radioactive material can include iron-55. The source of x-ray radiation 
can be an x-ray tube. 
Preferably the spectrometer further includes a histogram generator for 
recording a relative frequency of events within predetermined energy 
ranges in response to the output amplifier of the detector device. The 
spectrometer can include an event filter connected between the output 
amplifier and the histogram generator, the event filter including a 
threshold detector for identifying as events received charges having at 
least a predetermined magnitude; an event correlator for determining the 
occurrence of correlated events in adjacent collection regions of the 
detector device; and an integrator for combining as a single event the 
magnitudes of the correlated events. The event filter can further include 
an event discriminator for rejection of ambigously correlated events. 
The adjacent collection regions can be within a moving rectangular 
discrimination window array, a base element of the window array 
corresponding to a collection region of the detector device from which an 
event is detected. The window array can be a 3 by 3 array, a center 
element of the array being the base element. 
The present invention can further provide a coating thickness gauge having 
the spectrometer in combination with an analysis processor, the analysis 
processor receiving intensity ratios within predetermined energy bands 
from the spectrometer, the processor subtracting a base calibration value 
from the intensity of an energy band associated with a material being 
measured for producing a corrected abundance, the processor applying a net 
multiplier to the corrected abundance for generating a net coating 
thickness. 
In another aspect of the invention, a method for detecting energy levels of 
x-ray radiation includes the steps of: 
(a) providing an array of radiation collection regions and associated 
transport electrodes in a charge-coupled integrated circuit; 
(b) continuously exposing the array to x-ray radiation; 
(c) biasing the electrodes for the collection of electron charges in the 
collection regions in response to the x-ray radiation; 
(d) coupling an output amplifier to the array; and 
(e) clocking the transport electrodes for sequentially shifting the 
collecting charges to an input of the output amplifier, the output 
amplifier signalling the energy levels of the x-ray radiation.

DESCRIPTION 
The present invention is directed to an energy resolving x-ray detector 
that is particularly effective in discriminating low level soft x-rays 
such as those produced by fluorescence of elements having relative low 
atomic number (Z) in the range of 9-23. The invention is also directed to 
methods and apparatus utilizing the detector. With reference to FIGS. 1-3 
of the drawings, a detector unit 10 includes a semiconductor integrated 
circuit 12 having a plurality of transfer electrodes 14 for forming an 
array of potential wells 16 in a substrate 18 of the circuit 12. As shown 
in FIG. 2, the substrate 18 includes SiO.sub.2 in a very thin first layer 
18a facing the electrodes 14, N--Si in a second layer 18b, and P--Si in a 
third layer 18c, the substrate 18 having a total thickness T under the 
electrodes 14. As indicated in FIGS. 1 and 2, the electrodes 14 have a 
three-phase configuration. In 3-phase devices the electrodes overlap and 
block incoming radiation, necessitating thinning of the device for 
permitting entry of the radiation from the back side. Accordingly, the 
thickness T made very small, being preferably from approximately 10 .mu.m 
and approximately 20 .mu.m for permitting soft x-ray penetration into the 
potential wells 16 from opposite the electrodes 14. Soft x-rays entering 
collection regions that are formed by the potential wells 16 produce 
multiple electron-hole pairs, the pairs being separated by the presence of 
an electric field using appropriate biasing of the electrodes 14 by 
methods known to those skilled in the art of CCD detectors. In silicon, 
one electron-hole pair is generally produced for every 3.66 ev of x-ray 
energy. Electron charges are thus trapped in the potential wells 16, the 
wells 16 holding respective "pixels" (picture elements) of the detector 
unit 10. 
CCD detection efficiency for soft x-rays is primarily a factor of the depth 
of the depletion region and the thickness of absorbing layers above the 
depletion region. The thicker the depletion region, the higher the 
probability for absorption. Absorption above the depletion layer 
represents a loss mechanism. X-rays absorbed below the depletion region 
generate electron-hole pairs that generally recombine instead of becoming 
trapped in a potential well. Typical detection efficiencies for CCD 
detectors in the soft x-ray region are on the order of 50%. 
The integrated circuit 12 also includes an output amplifier 20 for 
sequentially receiving the pixels as charge signals from a terminal one of 
the potential wells, designated 16.sub.T as described herein. According to 
the present invention, the electrodes 14 are connected to a multi-phase 
transfer clock 22 for sequentially shifting the electron charges between 
the potential wells 16 and into the output amplifier 20, the wells 16 
functioning as a collection shift register wherein the electron charges 
separated from electron-hole pairs in each potential well 16 are 
accumulated in each clocking interval together with the charge, if any, 
having been transferred from the adjacent upstream well 16. Thus the 
electron charges of each pixel are accumulated and transferred in 
bucket-brigade fashion from a first potential well 16.sub.1 to the 
terminal well 16.sub.T and thence to the output amplifier 20 during 
continuous exposure of the array to incoming radiation. Accordingly, the 
detector unit 10 of the present invention provides for accumulation of 
charges in a single pixel over the full area of the potential wells 16, 
for enhanced signal to noise ratio. The detector unit 10 as described 
above is thus characterized as operating in "time-delay-and-integrate" 
mode and is believed to provide significantly enhanced energy 
discrimination of soft x-ray signals. In addition to providing multi-phase 
clocking signals, designated .phi..sub.1, .phi..sub.2, and .phi..sub.3, 
the transfer clock 22 preferably issues a reset clock signal .phi..sub.R 
periodically to the input of the output amplifier 20 for resetting the 
input of the output amplifier 20 to a known voltage prior to a pixel being 
shifted to the output amplifier 20. Although a three phase clocking scheme 
is shown in this exemplary configuration, any clocking scheme which 
ensures a controllable flow of charge to the output amplifier 20 is 
applicable. The reset clock signal .phi..sub.R produces a measurable base 
level output of the amplifier 20, as indicated at A in FIG. 3. When the 
charge contents of the pixel are shifted into the output amplifier 20, 
there is a corresponding signal voltage level output as indicated at B in 
FIG. 3. The difference between the base level A and the signal level B, 
designated energy signal C, is detected by correlated double sampling by a 
signal processor 24 as described below and is directly proportional to the 
accumulated charge in each successive pixel. The other features in the 
output waveform are due to capacitative feedthrough of the various 
clocking signals and are ignored in processing the signal. 
With further reference to FIG. 4, an alternative and preferred 
configuration of the detector unit, designated 10', has the potential 
wells 16 in a two-dimensional pixel array of rows 16R and columns 16C in a 
counterpart of the integrated circuit, designated 12' wherein the electron 
charges are sequentially shifted in parallel from a first row 16R.sub.1 to 
a terminal row 16R.sub.T of the array and serially from the terminal row 
16R.sub.T into the output amplifier 20. The columns 16C also range from a 
first column 16C.sub.1, to a last column 16C.sub.N, the last column 
16C.sub.N corresponding to the potential wells 16.sub.1 to 16.sub.T of 
FIGS. 1 and 2. The detector unit 10' includes a counterpart of the 
multi-phase transfer clock, designated 22', having two sets of outputs, 
designated .phi..sub.P and .phi..sub.S, for appropriately biasing the 
potential wells 16 and transporting the accumulated electron charges to 
the output amplifier 20. More particularly, the clock outputs .phi..sub.P 
effect shifting of the charges sequentially from the first row 16R.sub.1 
to the terminal row 16R.sub.T, and the clock outputs .phi..sub.S effect 
shifting of the charges in the terminal row 16R.sub.T sequentially from 
the first column 16C.sub.1 to the last column 16C.sub.N in each clock 
interval of the clock outputs .phi..sub.P. The detector unit 10' of the 
present invention thus provides for accumulation of charges in a single 
pixel over a relatively large area of the circuit 12' corresponding to one 
of the columns 16C, each pixel integrating received electron charges over 
a respective full column 16C of the integrated circuit 12'. 
The transfer clock 22' also provides a counterpart of the reset clock 
signal .phi..sub.R for resetting the input of the output amplifier 20 to a 
known measurable voltage prior to a pixel being shifted to the output 
amplifier 20 as described above. Thus the output waveform of the amplifier 
20 is characterized as described above in connection with FIG. 3, the 
detector unit 10' also preferably including a counterpart of the signal 
processor 24. 
With a large sensitive surface, shallow depletion region, and low noise 
level, the CCD integrated circuit 12' of the present invention is an 
excellent detector for soft x-rays. Many of the benefits of using a CCD as 
the integrated circuit 12 or 12' for detecting soft x-rays result from the 
very small input capacitance of the output amplifier 20, allowing in low 
noise charge detection. This is in contrast to the avalanche gain 
mechanisms used by conventional radiation detectors for driving the larger 
input capacitances that are associated with external amplifiers. 
The fraction of charge that is actually transferred between two potential 
wells 16 during a shift is called the charge transfer efficiency (CTE). 
Typical CTEs for CCDs are greater than 99.995%. Non-unity CTEs result in 
charge trailing and a subsequent loss of energy resolution. For example, a 
CTE of 0.99995 will result in a 5% signal charge loss in 1024 shifts. Thus 
CTE limits energy resolution and the maximum size of a CCD array that can 
be used. In practice, CTE is very dependent upon the surface quality of 
the CCD and is critically related to the levels of the clocking voltages 
used for transporting charge across the integrated circuit 12 or 12'. 
There is also an ageing effect wherein high-energy radiation creates 
surface defects that alter the optimal clocking voltages and generally 
decreases the CTE. This ageing loss can be quantified by observing the 
charge trails of an x-ray event, and a high CTE can be restored by 
recalibrating the CCD clock voltage levels. 
Noise associated with the detection process itself decreases the x-ray 
energy resolution achievable by the detector unit 10. CCD noise components 
include shot noise, dark current noise, reset uncertainty, and on-chip 
amplifier noise. Additionally, external amplifier noise and A/D sampling 
noise must be considered in a complete noise analysis of the resolution of 
the detector unit 10. 
Shot noise is the quantum-mechanical fluctuation in electron-hole pair 
generation and generally follows the square-root of the number of 
electron-hole pairs generated. However, as the generation of electron-hole 
pairs by absorption of a single x-ray event is correlated, this noise is 
reduced due to conversion-of-energy considerations by a coefficient known 
emperically as the Fano factor F, whereby N.sub.shot 
=F.multidot.(N.sub.pair).sup.1/2. For a silicon detector the Fano factor 
is typically 0.1. 
Dark current results from the thermal generation of electron-hole pairs and 
is primarily a function of device construction and temperature. As the 
average dark current is constant over time, it can be removed by simple 
subtraction, leaving only the shot noise contribution of the dark current 
uncorrected. Dark current is reduced by cooling the CCD and is roughly 
halved for every 7.degree. K. decrease in device temperature. 
Reset uncertainty results from thermally induced variations in the 
precharging of the input capacitance of the output amplifier 20 during 
activation of the reset clock signal .phi..sub.R. This is eliminated by 
externally sampling the base level A after every pixel as described below, 
leaving only the sampling noise of this measurement uncorrected. 
On-chip amplifier noise is thermally induced transistor noise in the output 
amplifier 20. It is dependent upon both temperature and the physical 
transistor characteristics. This noise is minimized by cooling as 
described below. 
With further reference to FIG. 5, the output amplifier 20 preferably feeds 
an analog signal chain 26 of the signal processor 24 for effecting the 
correlated double sampling as described herein. Optimal energy resolution 
using a CCD for soft x-ray detection depends upon minimizing noise sources 
as discussed above. A well manufactured CCD, cooled to -90.degree. C. and 
with the clock voltages and timing optimally adjusted, has a theoretical 
noise floor of only one or two electrons. It is desired to preserve this 
performance by appropriate design and timing of the analog signal chain 
26, shown in simplified form in FIG. 5. 
The analog signal chain 26 includes a resistor R.sub.S that functions as a 
transistor load resistor for the output amplifier 20 of the integrated 
circuit 12 or 12'. The CCD output is A.C. coupled through a high-pass 
filter combination C.sub.T and R.sub.T into a low noise preamplifier, 
designated 28 in FIG. 5, a dominant time constant t.sub.d of the analog 
signal chain 26 being the numerical product R.sub.T C.sub.T. The amplified 
signal from the preamplifier 28 is buffered by a clamp driver 30 that 
feeds a clamp amplifier 32 through a clamp capacitor C.sub.C, the input of 
the clamp amplifier 32 being selectively grounded by a clamp switch 34. In 
successive pixel sampling cycles the clamp capacitor C.sub.C is first 
charged to an upstream offset voltage corresponding to the base level A of 
FIG. 3, the clamp switch 34 being momentarily closed while the reset clock 
signal .phi..sub.R is active for eliminating the reset uncertainty from 
the pre-charging of the [CCD reset] capacitor C.sub.C. This uncertainty, 
being typically on the order of several hundred electrons, is thus avoided 
by the double correlated sampling described herein. The clamp switch 34 is 
then opened prior to termination of the reset clock signal .phi..sub.R, 
the clamp amplifier next being driven by an amplified counterpart of the 
energy signal C for charging a sample capacitor C.sub.S through a sample 
switch 36. The sample switch 36 is preferably closed only while the signal 
voltage level B is present at the output amplifier 20 for charging the 
sample capacitor C.sub.S with an amplified counterpart of the charge of 
the pixel. The resulting voltage on the sample capacitor C.sub.S is 
typically for further processing such as described below. 
Between the sample periods A and B, an additional uncertainty is introduced 
by the "off" state resistance of the CCD on-chip reset capacitor charging 
MOSFET, R.sub.off. The resulting signal uncertainty may be described as 
EQU N.sub.sig =N.sub.pc [2(1-e.sup.-.DELTA.t/R.sbsp.off.sup.C.sbsp.pc)] 
where N.sub.sig =number of noise electrons; 
N.sub.pc =reset uncertainty; 
.DELTA.t=the time interval between samples; 
R.sub.off =the reset MOSFET "off" state resistance; and 
C.sub.pc =on-chip reset capacitance. 
The contribution of R.sub.off noise to the correlated double sampling noise 
is seen to be minimized by minimizing the time between samples A and B. 
However, this implies a larger signal bandwidth and hence a larger white 
noise contribution growing as 1/f.sup.2. The white noise is thus minimized 
by increasing the dominant time constant, t.sub.d (signal 
bandwidth=1/t.sub.d). The signal/noise ratio is then described by 
##EQU1## 
The optimum signal/noise occurs when the derivative of the above relation 
with respect to t.sub.d is zero. This is when .DELTA.t/t.sub.d =1.255. 
With an optimized signal chain a noise floor of less than ten electrons is 
achievable. In practice the pixel rate is preferably first set to the 
slowest rate that still substantially minimizes the number of 
multiple-pixel events. Then the corresponding maximum sample interval 
.DELTA.t is used to set the high-pass filter combination R.sub.T and 
C.sub.T of FIG. 5 such that R.sub.T C.sub.T .apprxeq..DELTA.T/1.255. 
Another "noise" source is distribution of the charge generated by a single 
x-ray absorption event to more than one pixel, such as when an x-ray 
photon is absorbed near a boundary between potential wells 16, resulting 
in the splitting of the accumulated charge. This effect is reduced 
according to the present invention by combining the contents of 
neighboring pixels when accumulating an energy histogram as further 
described below. However, the correction is limited both by CTE charge 
trailing effects and by count rate limitations. 
With further reference to FIG. 6, a spectrometer 40 according to the 
present invention includes an x-ray source 42 (which can be a conventional 
x-ray tube) for fluorescing a sample 44, the detector unit 10 or 10' being 
positioned for receiving fluorescence x-rays from the sample 44 in 
response to primary x-rays from the source 42. As shown in FIG. 6, the 
spectrometer 40 includes a processor 46 and a display 48 for displaying 
the analysis of an energy histogram of the detected fluorescence. 
With further reference to FIGS. 7-12, a preferred implementation of the 
spectrometer 40 has a radioactive counterpart of the x-ray source, 
designated x-ray source 50, combined with a counterpart of the detector 
unit 10' in a sensor unit 52. For high sensitivity the source 50, the CCD 
detector integrated circuit 12' of the detector unit 10', and the sample 
44 are preferably tightly coupled. Accordingly, the x-ray source 50, 
configured as an annulus, is supported concentrically a short distance 
below the array of potential wells 16 of the integrated circuit 12'. In an 
exemplary configuration of the sensor unit 52, the x-ray source 50 is 
formed as a coating of .sup.55 Fe material 54 on a ring-shaped source 
substrate 56, a suitable material for the substrate 56 being copper. The 
x-ray source 50 has an outside diameter S.sub.OD and a clear aperture 58 
of inside diameter S.sub.ID. The source 50 and the integrated circuit 12' 
are located behind an optically opaque, x-ray transmitting window 60 (such 
as beryllium foil) in an evacuated chamber 62. 
In one configuration of the sensor unit 52, the CCD integrated circuit 12' 
and the x-ray source 50 are mounted within a machinable ceramic holder 64. 
As shown in FIG. 7, the integrated circuit 12' is clamped between a 
horizontally disposed plate member 66 and a retainer 68 of the holder 64, 
the geometry of the sensor 52 being arranged for minimizing both a source 
to sample distance D.sub.s and a sample to detector distance D.sub.D as 
indicated in FIG. 8 for achieving a high coupling efficiency with the 
sample 44 between the x-ray source 50 and the CCD integrated circuit 12'. 
The window 60 is sealingly affixed to the underside of the plate member 66 
at an air gap spacing D.sub.G from the sample 44. Suitable materials for 
the plate member 66 and the retainer 68 are mica/glass and alumina, 
respectively. 
As shown in FIG. 9, a preferred configuration of the sensor unit 52 has 
thermoelectric cooling applied to the CCD integrated circuit 12' the 
sensor unit 52 having a counterpart of the plate member 66 configured as a 
cold plate 70 that is insulatingly spaced from a hot plate 72. The 
integrated circuit 12' is clampingly retained on the cold plate 70 by an 
insulating spacer 74 that extends along an outer margin of the integrated 
circuit 12', the spacer 74 being clamped by the hot plate 72. An array of 
thermoelectric coolers 76 also extends between the integrated circuit 12' 
and the hot plate 72 for transferring heat from the integrated circuit 12' 
to the hot plate 72, whereby the integrated circuit 12' is cooled to 
approximately -50.degree. C. for reducing the dark current and the on-chip 
amplifier noise of the integrated circuit 12'. The hot side of the 
thermoelectric coolers 76 contacts the hot plate 76 for conducting 
dissipated heat to a liquid to air heat exchanger (not shown). A suitable 
material for the hot plate 76 is copper for efficient heat transfer. The 
cold side of the thermoelectric coolers directly contacts a package 
outline of the CCD integrated circuit 12'. The cold plate 70 is connected 
to the hot plate 72 by a plurality of insulating fasteners 80, the 
fasteners 80 applying clamping force to the spacer 74 through the 
integrated circuit 12' and maintaining the coolers 76 in good thermally 
conductive contact with the integrated circuit 12' and the hot plate 72. 
The x-ray source 50 is also mounted to the cold plate 70 as shown in FIG. 
9. Other details of the sensor unit 52, including the window 60 and the 
evacuated chamber 62, not shown in FIG. 9, are as described above in 
connection with FIGS. 7 and 8. 
It is desired to provide a high and relatively uniform concentration of 
x-ray radiation onto the sample 44, together with efficient transmission 
of resulting fluorescence radiation back to the CCD integrated circuit 
12'. Sample irradiance is conveniently represented in cylindrical 
coordinates (.rho., .zeta., .phi.) due to polar symmetry of the exemplary 
x-ray source 50 described above. With the x-ray source 50 centered at 
(.zeta.=0, .rho.=D.sub.S) and assuming no absorption in the path, the 
sample irradiance .GAMMA. in the .rho.=0 plane can be described by 
##EQU2## 
where .beta.=the source activity per unit area; 
dA=the area element of the annular ring (.rho.d.phi.d.rho.); and 
A.sub.E =the area of the incident wavefront at the sample point. 
Assuming a spherical emitted wavefront and describing .GAMMA. in Cartesian 
coordinates gives 
##EQU3## 
where (x,y)=the sample point of interest; 
D.sub.S =separation between the source and the sample; 
S.sub.ID =inner diameter of the source annulus; 
S.sub.OD =outer diameter of the source annulus, 
Which then simplifies to 
##EQU4## 
where R.sup.2 =D.sub.S.sup.2 +x.sup.2 +y.sup.2. A plot of this simplified 
irradiance function is shown in FIG. 10. 
This last equation neglects the absorption in transmission through the 
x-ray transmitting window 60 and air between the window and the sample. 
Each absorption may be described by 
EQU I.sub.t =I.sub.0.spsb.l.sup.-k.sbsp..mu..sup.d 
where 
I.sub.t =transmitted amplitude; 
I.sub.0 =incident amplitude; 
k.sub..mu. =linear absorption coefficient; and 
d=path length in material. 
Noting that the path length for the absorption term depends upon the 
vertical angle .phi. between the source point and the sample point, an 
exponential attenuation coefficient may be added to the simplified 
irradiance function, yielding 
##EQU5## 
with L.sup.2 =R.sup.2 +.rho..sup.2 -2.rho.(x cos .phi.+y sin .phi.) and 
k.sub..SIGMA. the total material absorption coefficient given by 
##EQU6## 
where k.sub.n is the absorption coefficient for the nth material of 
thickness d.sub.n. 
The signal at the CCD integrated circuit 12' of the detector unit 10' may 
be described in a fashion similar to the sample irradiance by a signal 
distribution function 
##EQU7## 
where S=the signal detected by the CCD; 
Y.sub.ccd =the quantum efficiency of detection by the CCD; 
.mu..sub.e =absorption coefficient for the element of interest; 
.gamma..sub.e =the quantum efficiency of fluorescence for the element; 
k.sub..SIGMA. =the total linear absorption coefficient, above; 
L=distance from sample point to detector point; and 
.GAMMA.=sample irradiance. 
An example of the resulting signal distribution at the CCD detector is 
plotted in FIG. 11. 
In the configuration of FIGS. 7 and 8, it is desired to keep the difference 
between D.sub.S and D.sub.D small, providing a large field-of-view for the 
detector unit 10' through the annular x-ray source 50, for enhancing the 
detected signal. An optimum D.sub.S is found by maximizing the integrated 
sample intensity within this field-of-view. Accordingly, a preferred 
exemplary configuration of the sensor unit 52 has the following 
dimensions: 
n.DELTA.X=13.8 mm 
S.sub.ID =13.8 mm 
S.sub.OD =19.8 mm 
D.sub.D =4 mm 
D.sub.S =2 mm 
D.sub.G =1 mm 
In detecting the fluorescence signal, x-ray photons are converted to 
electrons in the CCD by electron-hole pair production according to 
##EQU8## 
X-ray energies for the predominant lines of some common light elements are 
listed in Table 2 below. Also shown is the number of electrons generated 
in silicon, where E.sub.pair =3.66 ev. 
TABLE 2 
______________________________________ 
Element Kev .ANG. N.sub.e (Si) 
______________________________________ 
Aluminum Al 1.49 8.3 407 
Silicon Si 1.74 7.1 475 
Sulphur S 2.31 5.4 631 
Argon Ar 2.95 4.2 806 
Calcium Ca 3.69 3.4 1008 
Titanium Ti 4.51 2.7 1232 
______________________________________ 
Collected charge is converted to a voltage at the output amplifier of the 
CCD detector serial output register according to 
##EQU9## 
where V=amplifier gate voltage; 
q=charge on an electron, 1.6.times.10.sup.19 coulomb; 
N.sub.e =number of electrons; and 
C=gate capacitance at output amplifier. 
For a typical CCD capacitance of 0.33 pf this yields a sensitivity of 0.47 
.mu.V/e.sup.-, or 230 .mu.V for a silicon x-ray. 
A detected abundance of particular elements on the sample 44 using the 
spectrometer 40 is related to particular fluorescence coefficients of 
elements. Of particular interest are silicone polymers. 
The quantum efficiency of fluorescence .gamma..sub.e for the silicon 1.7 
Kev line is approximately 5%. An approximate value for the absorption 
coefficient .mu..sub.e is arrived at by considering the following generic 
dimethylsiloxane polymer unit: 
##STR1## 
Given a linear absorption coefficient for pure silicon .mu..sub.Si, the 
absorption coefficient .mu..sub.sx of the above signal distribution 
function (see FIG. 11) for silicone can be approximated 
##EQU10## 
where T.sub.SX =the sample thickness; 
D.sub.SX =density of silicone sub-unit (.congruent.0.92 g/cm.sup.3); 
M=molecular weight of silicone sub-unit (74.1 g/mole); 
D.sub.Si =density of silicon (2.33 g/cm3); and 
M.sub.Si =molecular weight of silicon (28.1 g/mole). 
Silicone coatings are commonly specified in terms of mass per unit instead 
of thickness, the absorption coefficient being correspondingly expressed 
as 
##EQU11## 
where W is the silicone coat weight. This coefficient is suitable for use 
in the signal distribution function to relate coating weight to the 
detected signal. 
When measuring the sample 44 at a location thereon being guided by a roll 
additional or background contributions to the signal at the detector unit 
10' are produced by fluorescence of the roll and by Compton scattering. A 
steel roll is preferable (to aluminum, for instance), as the .sup.55 Mn 
x-ray is not able to fluoresce iron atoms. However, alloying elements such 
as vanadium and manganese in the steel are potential signal contributors. 
Due to the high energy resolution of the CCD detector array (narrow x-ray 
linewidths), the problems associated with interference from other low-Z 
elements present in the base are minimized. The primary background problem 
anticipated is silicon present in the form of clays (aluminum-silicates). 
This contribution to the detected silicon abundance is believed to be 
partially compensatable by tracking the aluminum abundance and subtracting 
a proportional fraction of the silicon abundance. 
Compton scattering from electrons in the sample results in a portion of the 
source signal being reflected back to the sensor. By momentum conservation 
the reflected x-rays are energy shifted as a function of the reflected 
angle according to 
##EQU12## 
where E.sub.back =backscattered x-ray energy; 
D.sub.xray =initial x-ray energy; 
.phi.=reflected angle; and 
.alpha.=E.sub.xray /E.sub.rest, E.sub.rest being 0.511 Mev, the rest mass 
of an electron. 
For low-energy x-rays, the energy shift is small, as shown in FIG. 12 in 
terms of the difference E.sub.xray -E.sub.back for .phi.=.pi., along with 
the theoretical 2.sigma. lower linewidth limit for E.sub.xray assuming a 
Fano factor of 0.1. Although the energy shift is greater than the 
linewidth for x-rays above 1.5 Kev, the absolute value of the shift is 
still small. The .phi.=.pi. shift for the .sup.55 Mn K.sub..alpha. line is 
only 150 ev, allowing the backscatter peak to be resolved from even the 
chromium K.sub..alpha. line at 5.4 Kev. 
The scattering cross section .sigma..sub.c for Compton radiation is 
described as follows by the Klein-Nishina equation ((See O. Klein and Y. 
Nishina, Z Physik 52 (1929) 853): 
##EQU13## 
where d.sigma.=the differential cross-section for the solid angle 
d.OMEGA.; 
Z=the atomic number of the scattering atom; 
r.sub.o =the classical electron radius, 2.82.times.10.sup.-13 cm; and 
.chi.=E.sub.back /E.sub.xray. 
For low energy x-rays .alpha..apprxeq.0, giving .chi..apprxeq.1 and 
yielding 
##EQU14## 
The approximate scattering function is symmetric as shown in FIG. 12. 
It is useful to compare the signal strength due to Compton scattering with 
the signal strength due to photoelectric absorption and fluorescent 
emission. If the Compton signal is too high, it will overload the CCD 
detector with unresolvable x-ray events, distorting the corresponding 
energy histogram. Thorough analysis of the Compton signal strength 
following methods described above is complex; however, the analysis is 
facilitated by splitting the linear absorption coefficient into two parts 
EQU k.sub..mu. =k.sub..tau. +k.sub..sigma. 
where 
k.sub..mu. =total absorption coefficient; 
k.sub..tau. =photoelectric absorption coefficient; and 
k.sub..sigma. =Compton scattering "absorption" coefficient. 
The coefficient k.sub..sigma. is considered an absorption coefficient in 
the sense that it represents an intensity loss to the transmitted beam. 
Thus 
##EQU15## 
where N=Avogadro's constant; D.sub.z =density of element of atomic number 
Z; and 
M.sub.z =molecular weight of element of atomic number, yielding 
##EQU16## 
For a reading off of a steel roller, k.sub..sigma. is evaluated using Z=26 
for iron, with D.sub.z =7.57 g/cm.sup.3, M.sub.z =55.8 g/mole, yielding 
k.sub..sigma. =0.4/cm. For iron and 5.9 Kev x-rays, K.sub..mu. =720/cm. 
Thus k.sub..sigma. accounts for only 0.06% of the signal loss. 
Neglecting the difference in the absorption coefficient for the scattered 
x-rays, an upper bound to the detectable Compton signal from a homogenous 
background material can be set by 
##EQU17## 
where I.sub.0 =the incident intensity; 
I.sub.S =the Compton scattered intensity; 
.OMEGA.=the field-of-view of the detector; and 
L=the thickness of the background material. 
Integrating and letting the material thickness L.fwdarw..infin. yields 
##EQU18## 
as an upper limit for the Compton backscatter signal for low energy 
x-rays. Continuing the steel roller analysis, and conservatively assuming 
a source radioactivity of 50 mCi (1.85 GBq) and a detector field of view 
of 90.degree. would result in a backscatter signal of only 45 counts/sec. 
In addition to silicon, the sensor unit 52 is sensitive to the base 
additives in the form of other low-Z elements possibly present in the 
sample 44. A table of elements, their primary fluorescence line, effective 
fluorescent cross-section per atom at 5.9 Kev, and typical CCD output 
signal sizes are given in Table 3 below. 
TABLE 3 
______________________________________ 
Low-Z Elements 
.sigma. 
Z Element Kev .ANG. (barns) 
.mu.v 
______________________________________ 
11 Sodium Na 1.04 11.9 0.005 133 
12 Magnesium Mg 1.25 9.9 0.010 160 
13 Aluminum Al 1.49 8.3 0.018 191 
14 Silicon Si 1.74 7.1 0.032 223 
15 Phosphorus 
P 2.01 6.1 0.055 257 
16 Sulphur S 2.31 5.4 0.085 296 
17 Chlorine Cl 2.62 4.7 0.137 335 
19 Potassium K 3.31 3.7 0.297 424 
20 Calcium Ca 3.69 3.4 0.428 472 
22 Titanium Ti 4.50 2.7 0.820 576 
______________________________________ 
When measuring silicone coatings, the energy resolution of the sensor unit 
52 is sufficient to eliminate interference from adjacent energy peaks. 
However, if the base contains any compounds with silicon atoms as a 
constituent, the measured silicon abundance must be corrected prior to 
calculating the coat weight. 
The most common interfering material in paper bases is kaolinite, common 
clay, having a composition of Al.sub.2 Si.sub.2 O.sub.5 (OH).sub.4. The 
silicon contribution due to kaolinite is distinguishable using a linear 
regression between the silicon and aluminum abundance peaks on a running 
sample of base stock. A linear correction can the be applied to the 
silicon abundance peak based upon the height of the aluminum peak. This 
correction is not valid without further compensation if there are other 
major sources of either aluminum or silicon. If the composition of such 
other interfering compounds is known and measurable by the sensor unit 52, 
the correction can incorporate the further compensation. 
Measurement uncertainty or noise associated with the sensor unit 52 
includes other effects such as contributions from radiation statistics 
associated with x-ray generations, detection noise associated with the CCD 
integrated circuit 12 or 12', electronic noise associated with the analog 
signal chain 26, and quantization noise associated with generation of an 
energy histogram of the measurements. The uncertainty related to radiation 
statistics directly affects the accuracy of the sensor unit 52. The 
remaining noise sources affect the width of the peaks in the energy 
histogram. To the extent that the peaks are resolved, integration cancels 
their effect on the signal. Thus in use of the sensor unit 52 for 
monitoring silicone coatings, for example, the remaining noise sources are 
believed to have a negligible effect on the computation of the silicone 
coat weight once the histogram peaks have been integrated. 
The radioactive decay process has Poisson statistics where the probability 
that any given atom in the source will decay at any given moment is very 
small. However, the actual output flux is nearly Gaussian in that the 
source comprises a very large number of atoms. Thus the standard deviation 
.sigma. is simply related to the number of decay events N by 
##EQU19## 
With Gaussian statistics, noise sources are added as the square root of 
the sum of the squares following 
##EQU20## 
Relating the fluorescent intensity to the irradiating intensity by I.sub.Si 
=.gamma..sub.Si .mu.SiI.sub.0 and treating only the initial radioactive 
decay process as Gaussian results in 
##EQU21## 
where .sigma..sub.Si =the silicon abundance standard deviation; 
.gamma..sub.Si =the fluorescence efficiency of silicon; 
.mu..sub.Si =the silicon absorption coefficient; and 
I.sub.0 =the nominal incident radiation intensity. 
As the source decays, the noise due to radiation statistics increases due 
to the decreasing number of counts per histogram. The .sup.55 Fe source 
decays with a half-life of 2.6 years according to the following decay 
formula 
EQU A=A.sub.0.spsb.l.sup.-.alpha.t/.tau. 
where A.sub.0 and A are initial and current activities of the source; 
.alpha. is a proportionality constant of 1n 2, approximately 0.693; and t 
and .tau. are the half-life and current age of the source. 
Given a nominal silicone coating weight and a maximum allowable 
uncertainty, the useful life of the source is found by substituting the 
decay formula into that for the silicon abundance standard deviation, 
whereby 
##EQU22## 
Now defining the variance in terms of percent coat weight with P.sub.e 
=2.sigma./.gamma..sub.Si .mu..sub.Si I.sub.0 gives a practical formula for 
useful life 
##EQU23## 
For example, from the appendix an initial signal of 2280 counts/sec is 
calculated for a 1 g/m.sup.2 coating and a 50 mCi source. Assuming a five 
second integration period and using the above formula for silicon 
abundance standard deviation, .sigma..sub.Si =49, or P.sub.e =2.15%. 
Substituting P.sub.e =5% in the formula gives t=1.16 years. 
Due to the strong absorption in air of the exciting .sup.55 Mn x-rays and 
the even stronger absorption of the silicon fluorescence x-rays, air gap 
spacing D.sub.G from the face of the gauge to the sample is critical. 
Although the optimal distance is zero, practical distances are in the 
range of 1-5 mm. Within this range, distance fluctuations produce a strong 
signal modulation. However, the present invention provides compensation 
for this by measuring the quantity of argon present in the air gap, the 
quantity being nominally 1% of the matter between the window 60 and the 
sample 44. Compensation for fluctuations in D.sub.G is effected by 
evaluating detected signals for silicon and for argon, that for silicon 
being described as 
EQU I.sub.Si =I.sub.0 .sigma..sub.Si 
.spsb.l.sup.-(k.sbsp.Mn.sup.+k.sbsp.si.sup.)D.sbsp.G 
where 
I.sub.Si =return signal intensity; 
I.sub.0 =source signal intensity; 
.sigma..sub.Si =silicon fluorescence cross-section; 
k.sub.Mn =linear absorption coefficient in air for .sup.55 Mn K.sub..alpha. 
x-rays; and 
k.sub.Si =linear absorption coefficient in air for Si K.sub..alpha. 
x-rays. 
A normalized plot of I.sub.Si /I.sub.0 .sigma..sub.Si with k.sub.Mn 
=0.025/cm and k.sub.Si =0.90/cm is shown in FIG. 14. 
Argon yields a fluorescent k.sub..alpha. x-ray at 2.9 Kev with a quantum 
efficiency of 11.5%. Thus the detected signal for argon is 
##EQU24## 
where I.sub.Ar =return signal intensity; 
I.sub.0 =source signal intensity; 
.sigma..sub.Ar =argon fluorescence cross-section; and 
k.sub.Ar =linear absorption coefficient in air for argon K.sub..alpha. 
x-rays. 
For argon 1% in air, .sigma..sub.Ar =0.55.times.10.sup.-3 /cm and k.sub.Ar 
=0.22/cm. A normalized plot of I.sub.AR /I.sub.o .sigma..sub.Ar with the 
maximum intensity at L.fwdarw..infin. is shown in FIG. 15. 
Using the above description from of signal distribution (see FIG. 11) and 
assuming a 1 g/m.sup.2 silicone coat weight yields a silicon cross-section 
.sigma..sub.Si =0.11.times.10.sup.-3. This is approximately equivalent to 
the argon cross-section for an air gap D.sub.G of 2 mm. The argon 
abundance in the air gap provides a useful basis for correcting for path 
length variations. Assuming a nominal path line distance of L.sub.0 and 
taking a first-order taylor series expansion of the detected signal for 
argon about this point yields the linear approximation 
EQU A=.sub.l.sup.-L.sbsp.0.spsp.(k.sbsp.MN.spsp.+k.sbsp.Ar) 
[(L-L.sub.0)(k.sub.Mn +k.sub.Ar)-1]+1 
where A is the normalized argon abundance. Solving for L gives 
##EQU25## 
Using the above formula for the detected signal I.sub.Si (see FIG. 14) and 
the above relation, a path-length normalized silicone reading can be 
calculated from 
EQU I.sub.Si =I.sub.M.spsb.l.sup..beta.(1-A)-L.sbsp.0 
where I.sub.M is the measured silicone abundance and .beta. is the argon 
slope about the nominal path line distance L.sub.0, 
##EQU26## 
Correction of the silicone abundance in this manner will increase the 
standard deviation of the measurement. An explicit prediction can be found 
by applying the square root of the sum of the squares to the path-length 
normalized silicone reading I.sub.Si and combining the silicon abundance 
standard deviation .sigma..sub.Si along with the return signal intensity 
for argon I.sub.Ar. For argon abundances on the order of silicon 
abundances, a much simpler result can be obtained by first making the 
intuitive approximation 
##EQU27## 
where .sigma..sub.Ar =expected standard deviation for argon abundance; 
.sigma..sub.Si =expected standard deviation for silicon abundance; 
L.sub.= =path line distance for which abundances are equal; and 
L.sub.0 =the nominal path line distance. 
Using the square-root of the sum of the squares and assuming I.sub.Ar 
=I.sub.Si (L.sub.0 /L.sub.=), the .sigma. for the composite measurement 
reduces to 
##EQU28## 
For example, for L.sub.= =2 mm and L.sub.0 =5 mm, .sigma. is 1.2 times 
.sigma..sub.Si. A review of FIG. 14 shows that any fluctuation in the air 
gap distance D.sub.s would result in a much larger variance, thus 
confirming the validity of using the argon abundance as a correction 
factor. 
With further references to FIGS. 16-21, a practical embodiment of the 
present invention includes the spectrometer 40 having the sensor unit 52 
in a coating thickness measurement apparatus 80. The measuring apparatus 
80 includes three primary modules, a sensor head 82 incorporating the 
sensor unit 52, a processor module 84, and a display module 86, connected 
as shown in FIG. 16, the display module 86 also being connected to a 
display 88. 
The sensor head 82 includes the sensor unit 52 in the thermoelectrically 
cooled configuration of FIG. 9, the analog signal chain 26, and associated 
electronics. As shown in FIG. 17, the sensor head 82 is driven by a set of 
clocking signals and voltage references from the processor module and 
outputs an analog data stream of CCD pixel levels from the analog signal 
chain 26. Thus the clock 22.degree. is implemented partially in the 
processor module 84, the sensor head 82 having clock supplies 90 and clock 
drivers 92 that are connected to the CCD integrated circuit 12'. The 
sensor head 82 further includes analog supplies 94 for the analog signal 
chain 26, and thermoelectric cooler supplies 96 for the coolers 76. 
The processor module 84 generates the clocking signals required by the CCD 
detector integrated circuit 12', monitors the thermoelectric coolers 76, 
and processes the analog data stream from the sensor head 82 for 
determining the abundance of low-Z elements present. As shown in FIG. 18, 
the processor module 84 includes a histogram processor 98 that is 
responsive to the analog signal from the sensor head 82 and to a line and 
pixel generator 100, the analog signal being fed to an analog to digital 
converter 102 and a threshold and first-in-first-out (FIFO) buffer 104. 
The histogram processor 98 delivers a histogram in the form of the 
frequencies of occurrence of events within predetermined energy bands to 
an analysis processor 106, the analysis processor 106 also driving digital 
to analog channels 108 and clocks 110 for supplying, together with the 
timing generator 100, clock timing and levels to the sensor head 82. The 
processor module 84 further includes primary power supplies 112 for the 
sensor head 82 that are monitored by the analysis processor 106 by means 
of an analog to digital channel 114, control being effected through 
parallel channels 116. The output from the processor module 84 is a 
digital data stream from the analysis processor 106 and a programmable 
analog channel from the D/A channels 108. Reference voltages are converted 
to clocking levels by the clock supplies 90 and then applied to the CCD 
integrated circuit 12' of the sensor head 82 by the clock drivers 92. 
As discussed above, the analog signal chain 26 is included within the 
sensor head 82, the analog output thereof feeding the analog to digital 
converter 102 within the processor module 84. Alternatively, the converter 
102 can be in the sensor head 82. Further, a portion of the analog signal 
chain 26 can be an included part of the processor module 84, the output 
from the on-chip CCD output amplifier 20 being amplified by the low-noise 
preamplifier 28, and then sent to the processor module 84 for correlated 
double-sampling by the clamp amplifier 32, the clamp switch 34, and the 
sample switch 36, followed by digital conversion in the A/D converter 102. 
As shown in FIG. 19, the display module 86 contains a low-voltage off-line 
supply 118, a digital panel meter display driver 120 for driving a digital 
display 89, and communication ports 122 providing interface electronics to 
another display system. Reset logic 124 is also included for setting the 
system into a known state upon initial start-up. 
Exemplary communication ports 122 comprise a full-duplex RS-232 DCE serial 
channel operating to 19.2 Kbaud, a half-duplex RS-232 DTE logging channel 
paralleling the DCE channel, and a full-duplex 75.OMEGA. coax high speed 
link. The DPM display driver 120 converts the D/A current output of the 
processor module 84 to a simultaneous digital representation on the 
digital display 89, a frequency output 126, and a RS-232 DTE logging 
channel 128 at up to 9600 baud. 
As described above, the CCD integrated circuit 12' is operated in a 
"time-delay and integrate" mode, not a conventional "frame-transfer" mode. 
In the frame transfer mode, after a given exposure time, the sensor unit 
52 would be shuttered closed and the collected charge read out, then the 
shutter opened and the cycle repeated. In the time-delay and integrate 
(TDI) mode there is no shutter. Instead, read-out occurs continuously and 
the exposure time is equal to the product of the number of the rows 16R 
and the period of the parallel transfer signals .phi..sub.P from the clock 
22'. In conventional imaging, operation in the TDI mode would produce 
smearing of the image. As no imaging is required in the measuring 
apparatus 80, there is no need for a shutter. 
Software in the processor module 84 runs concurrently on the histogram 
processor 98 and the analysis processor 106. A suitable device for use as 
the histogram processor 98 is an Inmos T425 transputer (32 bit integer 
processor), available from Inmos Limited of Bristol, UK. The processor 98, 
programmed as diagrammed in FIG. 20, executes a process 130 for performing 
event filtering and histogram generation as described below. 
A first stage of pixel event filtering is performed by hardware, in the 
FIFO buffer 104. Any pixel events that are below a software-set threshold 
value are discarded. Events above the threshold are received by the 
histogram processor 98 in an event recording step 132 of the histogram 
process 130. The information set for each event includes amplitude, 
relative CCD line number, and pixel number within the line. 
The histogram process 130 also includes an event filter 134, wherein a 
second stage of event filtering is performed for combining split-pixel 
events having the charge generated by the absorption of an x-ray photon 
spread over more than one pixel. Before a pixel event is used in an energy 
histogram, the event filter 134 checks for additional events in adjacent 
pixels. This is done by treating a particular event as a base event in a 
base pixel and searching for other events in pixels within a filter window 
that is referenced to the base pixel. In an exemplary embodiment of the 
event filter 134, the reference window is rectangular, being a three by 
three pixel window surrounding the event. Thus the base pixel is the 
central pixel of the reference window, the reference window moving 
relative to a stored array of event data within the histogram processor 
98. If a second event is found within the reference window, the second 
event is normally summed with the base event and tabulated as one event in 
a hardware FIFO register of the histogram processor, whereby a pair of 
nearest-neighbor events are treated as having been produced by a single 
x-ray photon. In the exemplary implementation of the event filter 134, a 
further test is made for determining the occurrence of events in multiply 
adjacent pixels wherein energy may have been spread across more than two 
pixels. It is believed that the likelihood of events in multiply adjacent 
pixels results from singular photons is doubtful, particularly in that the 
events processed by the event filter 134 have not been excluded by 
threshold detection in the FIFO buffer. Accordingly, events in multiply 
adjacent pixels are discarded. 
It will be understood that as each event is processed by the event filter 
134, every event is considered once and only once as a base event and may 
also be found as an adjacent event. 
Event records from the hardware FIFO of the event filter 134 are 
accumulated in a histogram generate step 136 for forming an energy 
histogram record 138. After an integration interval, this histogram record 
138 is transmitted to the analysis processor 106 for calculating elemental 
abundances. In an exemplary configuration of the processor module 84, 
software for the histogram processor 98, including the histogram process 
130, is dynamically downloaded upon power-up from a code image stored in 
the analysis processor 106. 
The histogram record 138 created from the filtered pixel events consists of 
a vector of the accumulated number of events detected for different energy 
levels during the specified integration time. After the integration time, 
this vector is transmitted over a high speed serial link 140 to the 
analysis processor. 
As shown in FIG. 21, the analysis processor 106 is programmed with an 
analysis manager 142, receiving the energy histogram record 138 and 
communicating with the sensor head 82 and the histogram processor 98. In 
the exemplary configuration of the measuring apparatus 80 described herein 
the analysis processor 106 is an Inmos T800 Transputer (32 bit 
floating-point), also available from Inmos Limited. The software of the 
analysis manager 142 executes as a set of concurrent processes running 
under a transputer allocation manager (TAM) operating kernel 144. In a 
development environment software is downloaded from a development system 
(not shown) after power-on or a system reset, whereas in a stand-alone 
environment the software is automatically copied from an image in read 
only memory. 
The analysis manager 142 includes an analysis process 146 for receiving the 
histogram record 138 and computing a sample element abundance such a 
silicone coating weight per unit of area of the sample 44. A menu process 
148 of the manager 142 accepts user parameters and options and displays 
the computed elemental abundances. The analysis process 146 is based upon 
the intensity ratios of x-ray events at specific energy bands from the 
source 50 and the fluorescence of material of the sample 44. Internal 
calibration of the analysis process 146 is performed using the high 
intensity, high energy peak expected from the .sup.55 Mn K.sub..alpha. 
emission. 
The intensity of the exemplary silicon line is the .sup.55 Mn K.sub..alpha. 
and K.sub..beta. escape peaks in the silicon CCD sensor. The contribution 
due to the escape peaks is subtracted as a fixed ratio of the .sup.55 Mn 
K.sub..alpha. intensity. This ratio is calculated during head calibration 
with no sample present. The contribution due to clays is subtracted as a 
ratio of the measured aluminum intensity. This ratio is calculated during 
base calibration, during which base stock without silicone coating is 
measured. The residual after subtraction of these two contributions is 
assumed to be due to the silicon atoms present in the silicone polymer. 
The analysis manager optionally provides a serial driver 150 for 
interfacing to a standard RS-232 protocol device 152, such as an ASCII 
display terminal; otherwise, communications are assumed to be over a 
transputer serial link to a TAM device-server application executing on a 
development system. The analysis manager further includes a temperature 
monitor and control process 154 for driving the thermoelectric coolers 76 
for the CCD integrated circuit 12' from the parallel channels 116 by 
comparing measured temperatures as communicated through the A/D channels 
114 to predetermined setpoints. A download process 156 downloads a code 
image of the histogram process 130 to the histogram processor 98 and 
initiates execution of the process 130 therein. A sensor configure process 
158 sets CCD timing and voltage parameters and allows a user to display 
and modify these parameters. 
In setting up and calibrating the measuring apparatus 80, internal 
calibration is performed for obtaining "zero" reference levels for silicon 
and aluminum abundances and the maximum argon abundance. The internal 
calibration is performed as a process of acquiring a reference energy 
histogram with no sample present. 
Background analysis is next performed as a calibration procedure wherein 
un-coated base is run in the path of the sample 44 at the nominal air gap 
spacing D.sub.G. During this procedure, low-Z elemental abundances are 
acquired and a multiple-linear regression performed upon the major peaks. 
For the regression to be valid base stock must be moving across the gauge 
aperture during calibration. 
The silicone coat weight computed in the analysis process 146 includes a 
net analysis, which may proceed with or without a prior net calibration 
procedure. In the net calibration procedure, the sample 44 in the form of 
coated stock is measured by the apparatus 80. If a calibration procedure 
is not used, an absorption coefficient based upon the percent silicon 
atoms in the silicone resin is used to calculate a net multiplier. 
The net calibration procedure is best performed with a static (stationary) 
counterpart of the sample 44, the measurement apparatus set to read the 
same coat weight as another gauge used as a reference. The base 
calibration should be executed before running the net calibration 
procedure. 
During net analysis, the measured silicon abundance is corrected first by 
the linear factors computed during base calibration and then using the 
argon abundance for compensating for path line variations. The final 
abundance is then converted to engineering units by a net multiplier 
calculated during net calibration and displayed. 
Although the present invention has been described in considerable detail 
with reference to certain preferred versions thereof, other versions are 
possible. For example, alternative microprocessors such as those 
manufactured by Motorola Corp. or Intel Corp. may be applied. Therefore, 
the spirit and scope of the appended claims should not necessarily be 
limited to the description of the preferred versions contained herein.