In order to examine the potential of using SSME exhaust plume radiation in the soft X-ray spectrum as an early warning system of imminent engine failure, a low cost, low risk experiment was devised. An approach was established, equipment was leased, the system was installed and checked out, and data were successfully acquired demonstrating the proof-of-concept. One spectrum measurement of the SSME plume was acquired during a 300 second burn on the A-1 Test Stand. This spectrum showed a prominent, line emission feature at about 34.5 KeV, a result which was not expected, nor can it be explained at this time. If X-ray spectra are to be useful as a means of monitoring nominal engine operation, it will be necessary to explore this region of the EM spectrum in greater detail. The presence of structure in the spectrum indicates that this technology may prove to be useful as an engine health monitoring system

Olive, Dan F.

English

NASA Technical Reports Server

NASA CR-185308
X-RAY SPECTROSCOPY
OF THE SSME PLUME
(NASA-CR-ld_308) X-RAy 5PECT_GSC_Py OF THE
S_;ME PLUME Renorte May - Oct. 1989
(_vordrup Technology) 20 p CSCL 2IN
N90-10168
Unclas
G3/20 0224570
Dan Olive
NOVEMBER 1988
IW A
https://ntrs.nasa.gov/search.jsp?R=19900000852 2020-03-20T00:23:02+00:00Z
X-RAY SPECTROSCOPY OF THE SSME PLUME
Dan Olive _ J_-
November 1988
Prepared Under _CB-RADV by
Sverdrup Technology, Inc.
SSC Group
for
JOHN C. STENNIS SPACE CENTER
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
STENNIS SPACE CENTER, MISSISSIPPI 39529
SUMMARY
The first x-ray spectrum of the SSME plume was obtained
during a 300 second test firing on the A-I Test Stand on
October 3, 1988. The spectrometer used covered a range of 2Kev
through 41 Key. Data was integrated throughout the full 300
seconds of burn time. The spectrum is dominated by a prominent
line feature which peaks at 34.5 Kev, equivalent to a wavelength
of 0.36 angstroms. The physical process responsible for
generating this essentially monochromatic radiation has not been
determined. As of this writing plans to repeat the experiment
have been frustrated by failure of the x-ray detector.
uev -
FET -
Ge -
Kev -
SiLl -
TCC -
SYMBOLS
Angstrom (i0 -I0 meters) unit of measure for wavelengths
Electron Volt - unit of measurement for energy
Field Effect Transistor
Germanium
Kiloelectron Volt (10 3 ev)
Silicon Lithium
Test Control Complex
November 1988
CDDF REPORT
X-Ray Spectroscopy of the SSME Plume
TWR # CB-RADV
;NTRODUCTION
Much data has been collected on the electromagnetic
emissions of the Space Shuttle Main Engine (SSME) in the ultra-
violet (UV), visible (VIS), and near infrared (IR) portions of
the spectrum. This report summarizes initial efforts intended to
extend the database above the UV into the x-ray portion of the
spectrum.
BACKGROUND
Agreement was reached with the benefitor that TWR #CB-RADV
would concentrate on collecting data in the X-ray portion of the
spectrum. It is known through earlier experiments that the plume
does emit in the soft x-ray range of i0 Kev through 60 Kev. The
efforts under this TWR were designed to yield spectra over this
range to evaluate the feasibility of deriving real-time SSME
"health-monitoring" data in the soft x-ray portion of the EM
spectrum.
ACCOMPLISHMENTS TO DATE
Initial efforts were concentrated on acquiring and
evaluating technical data on currently available x-ray detection
instrumentation. A telephone market survey was conducted wherein
manufacturers of x-ray detection equipment were polled to
determine if they could supply equipment which would provide the
desired spectra. A total of 16 different vendors were contacted,
five of whom mailed technical data packages.
Concurrent with the market survey, the physics of x-ray
spectroscopy was reviewed with particular emphasis on detection
methods. Detectors employing the various detection methods were
evaluated for the specific task of acquiring spectra of the SSME
plume emission. Of particular concern was the capability of the
detectors to function properly in the severe acoustic,
vibrational and temperature environment associated with the SSME
firing.
All detection methods rely on the capacity of x-rays to
ionize matter. The amount of ionization is proportional to the
original energy of the x-ray photon, or inversely proportional to
the wavelength.
Available detector types include: i) Proportional Chambers
2) Scintillation Counters
3) Solid State Detectors
In proportional chambers the chamber is filled with "inert"
gases, typically argon (Figure 1 shows a typical chamber). A
very thin wire in the center of the chamber is maintained at a
high voltage. Ionization produced by x-ray photons is multiplied
by "avalanche" in the vicinity of the wire. The resulting pulse,
proportional to the X-ray photon energy, is collected. Pulses
are sorted by pulse height analyzers and the resulting histogram
of number of pulses versus pulse height yields the desired
spectrum.
In scintillation counters special materials are used which
fluoresce due to recombination of electrons freed by ionization
with the atoms in the material. Figure 2 shows a typical scin-
tillator, in this case doped sodium iodide, which is optically
coupled to a PM tube. The fluorescence is within the portion of
the EM spectrum which is detectable by photomultiplier (PM)
tubes. Pulse height analysis of the resultant pulses from the PM
tubes again yields the desired spectrum.
Solid state detectors are operated so that the x-ray induced
ionization is directly collected from specially doped semiconduc-
tor materials. These detectors must be operated at very low
cryogenic temperatures so that thermally generated noise is mini-
mized. They are cooled with liquid nitrogen (LN2) and are
operated within a hard vacuum to insulate them from ambient heat.
Figure 3 depicts a typical solid state detector assembly with in-
tegral preamp FET.
All types of x-ray detectors are, to some extent,
microphonic. Thus, a certain amount of the signal from them will
be attributable to acoustic and/or vibrational input.
e
All the x-ray detectors are also sensitive to charged par-
ticles as well as EM radiation outside the x-ray region. For
this reason, all have "windows" of material (typically beryllium)
which will absorb or stop these particles and are opaque to EM
radiation at longer wavelengths. The thickness and material type
of this window sets the lower limit of x-ray photon energies
which can be detected. Figures 4 A & B show x-ray transmission
through various windows. Here, the trade-off is window strength,
required to protect the detector, and the requirement to minimize
window thickness so as to obtain as much of the lower end of the
spectrum as possible.
FLYING
/-.-14-40 X _ D.P.
1.25__///
4-HOLES
/I
t,,T2"6
.31
.03
1.00
1
1.812
l
1
BERYLLIUN
WINDOg
].00
DIA THRU
4-HOLES
FIGURE I - PROPORTIONAL CHAMBER (REF. 1)
JI
/----- kl,_'T. _ _q,,eub'm,.,
F
FIGURE 2 - SCINTZLLATION COUNTER (REF. 2)
4
s_(Li)
detector -
X-rays
Quantum Of '_
Beryllium
window
J ]1
||
f Vacuum
i_/ l'l
/
, L
L
Electrical Field effect Copper
insulator transistor cold finger
Liquid
nitrogen
FIGURE 3 - SOLID STATE DETECTOR (REF. 3)
5
X.RAY • G_,_t._ RAy TP_MISSION _ _ _OIq WIN()_qtS
¢©
2O
3 4 $ 4
EI_RGY IN K_
FZCURE 4A (REF. 2)
lO0 I I I....I IIIII
l_ _t _I ltll
FIGURE 4B (REF. 4)
6
10
ORIC-LNAL PAC,F IS
OF P(X_ Q_J.ALr'rY
The upper end of the attainable spectrum is a function of
the detector material and its thickness. Higher energy photons
are more "penetrating" and require more thickness for any
material to absorb the energy. In this respect, the Germanium
solid-state detector performs better than the silicon (SiLi) or
scintillators. Figures 5 A & B show the reduction in absorption
efficiency as a function of photon energy.
SYSTEM CONFIGURATION
The equipment utilized in obtaining the x-ray spectrum was
leased for a 60 day period from Applied Detector Corporation of
Fresno, California. Figure 6 is a block diagram of this system.
The detector is the Applied Detector Corporation Model ADC
400 x-ray spectrometer with a model PS-3 power supply.
Characteristics of this spectrometer are summarized below.
• Detector type: Germanium, Solid State, 5mm dia.
• Window: 0.005" Beryllium
• Range: _ 4 Kev thru 200 Key
• Resolution: 180 ev @ 5.9 Kev; 550-600 ev @ 122 Kev
• Low Noise Preamp, built into detector assembly
LN 2 Dewar hold time: 8-10 hours
Size: 3" dia. X 18" long
• Compact, sturdy construction
The scaling amplifier is a California American Scientific
Instrument X-ray Pulse Processor. This amplifier permitted
adjustment of gain in order to set the upper energy limit for ten
volt output pulses to match the input range of the pulse height
analyzer.
Sorting of the pulses by amplitude was accomplished by the
Model 1024 D Pulse Height Analyzer (1024 channels) from
Nucleus Inc. of Oak Ridge, Tenn. The resultant spectra were
displayed on a CRT with a controlled cursor to enable digital
readout of accumulated counts by channel number.
The detector was housed in a section of eight inch diameter
steel pipe fabricated on-site. It was cushioned in acoustic foam
rubber and protected to some extent by an aluminum endplate with
a small hole to admit x-rays to the detector window.
7
10_
80
6O
4O
2O
IO
ENERGY OF GAMMA
ABSORPTION EFFICIENCY NaI{TI)
FIGURE 5A (REF. 2)
10,000
!
!
¥
loo )
I0'
40
0
10
SI(L_) Cry_taI__
5 me ThlckJ
3 0= T_lc_, -/
2 =m Thick /
] I
I I I I I I
\
\
\
\
1 I I ! I1_
IEI_,_lv (keVl
FIGURE
8
5B (REF. 4)
_L PAGE !$
OF Poor {_IJALIT_
DETECTOR
POWER
SUPPLY
I CRT i
]
SCAL IN G
AMPLIFIER
PULSE HEIGHT
ANALYZER
AI TEST STAND
TEST CONTROL
COMPLEX (TCC}
SYSTEM C ONF IGUR AT ION
FIGURE 6
This assembly was mounted on the southwest slam ring
mounting bracket on the A-I Test Stand. This location was chosen
because it was the closest practical position to the plume where
the assembly would not interfere with normal operations.
The power supply and scaling amplifier were located within
the hardstand at level 4. Amplifier output pulses were routed
over existing coaxial cable to the TCC where the pulse height
analyzer could be operated and monitored during engine tests.
pROCEDURE
Operation of the system was verified by placing a Cobalt 60
source at the housing window and checking the resultant spectra.
This also yielded pretest calibration spectra which compensated
for the unknown cable attenuation between the A-I Stand and the
TCC.
The single SSME spectrum was obtained on October 3, 1988.
This 300 second burn yielded the spectrum shown in Figure 7.
On the day of the test, additional pretest data was taken to
verify proper operation and obtain pretest calibration
verification. Background data, integrated over several five to
ten minute intervals prior to the burn, showed essentially no
counts.
Just prior to ignition the analyzer was started and data
accumulated over the full 300 seconds of the burn. The resultant
spectrum, with the prominent line emission feature, was
unexpected. Prior to the test it was expected that a low level
continuum which would decrease in intensity at higher energies
would be seen. This is seen in the data with the line feature
superimposed.
CALIBRATION ANALYSIS
As noted, the system was calibrated by using a Cobalt 60
source with lines at 5.90 Kev and 6.49 Key. For the system
settings at which the test spectrum was obtained, these lines
peaked at bins 125 and 140 respectively. Extrapolation from
these readings produce a zero at bin 25 with a bln width of 391/3
ev per bin. Thus, adjusting for this offset yields bin numbers
of 150 and 165 for the calibration peaks.
(391/3) ev/bin X 150 = 5.90 Kev
(391/3) ev/bin X 165 = 6.49 Key
i0
Eo_
.__.J 0
C5_
u_
I
X
I I I I I I I I
8 _ 8 8 _ _ 8
[]
0
0
0
i
v
i,l
Z t._
sJ,unoo
11
The prominent line feature was centered at bin 877, adjusted
bin 902, yielding a peak at 35.48 Kev. Utilizing the equation
E = hc/_ this energy is equivalent to a wavelength of 0.36 .
Data for the plot was laboriously taken for each I0 th bin
utilizing the cursor. Scope camera photographs of the CRT
readout were taken and have been included as Appendix A.
The large "spike" near zero is presumed to be noise,
probably acoustically or vibrationally induced. A count of
38,000 was registered in bin 43. No counts were registered in
bin 25 or lower.
Attempts to obtain post-test calibration data were delayed
until the stand was "safed" and reopened for access. When the
calibration source was finally installed the LN 2 dewar was
apparently empty. A second attempt the following day was
unsuccessful also. The equipment was removed to the electronics
lab and checked. All indications pointed to a loss of vacuum in
the detector housing, preventing proper cooling of the detector.
The detector was returned to the vendor for repair or
replacement.
CONCLUSION
The single, prominent feature noted in the only x-ray
spectrum of the SSME plume obtained to date is extremely
interesting. The author feels strongly that:
i) Additional spectra should be taken
2) Further efforts be made in analysis of subsequent data in
order to understand the origin of the feature.
The lease on the equipment terminates in mid November. It
is recommended that the PC-based spectrometer (with analysis
software) be procured to permit in-depth analysis of this
phenomenon. A verbal quote of $16,000 was provided by Applied
Detector Corp. for this system.
12
FIGURE REFERENCES
The figures referenced are taken from publicly available
manufacturers' brochures and are intended to illustrate typical
characteristics of available x-ray detection equipment.
1
2
3
4
Cianflone Scientific Instruments Corporation
228 RIDC Park West Drive
Pittsburgh, PA 15275
Bicron Corporation
12345 Kinsman Road
Newbury, OH 44065
Kevex Corporation
355 Shorway Rd.
San Carlos, CA 94020
HNU Systems, Inc.
Oak Ridge Technical Center
271 Midway Road
Oak Ridge, TN 37830
13
, ORIGINAl:: P_131t
BLACK AND WHITE PHOTOGRAPH
Photo #4
- Expanded horizontal near 0
- Full scale 1,000 counts
- Low energy peak overranged
Photo # 5
- Expanded horizontal scale of feature
- Full scale 1,000 counts
- Peak of 738 counts at bin 877
APPENDIX A
Page I of 2
ORIGINALPAGE"
BLACK AND WHITE PHOTOGRAPH
Photo #6
- Full scale 10,000 counts
- Very high (38,000) low energy count
- Cursor at 512 (center)
Photo #7
- Full scale 1,000 counts
- Peak of feature at bin 877
- 738 counts at peak
- "Noise" about 50 counts PK-PK
- Continuum about 190-200 counts in neighborhood of peak
- Width of feature (FWHM) about 35 bins or about 1.4 Kev
APPENDIX A
Page 2 of 2
1. Report No.
NASA CR- 185308
Report Documentation Page
2, Government Accession No.
4. Title and Subtitle
Engine Diagnostics/Soft X-Rays
(X-Ray Spectroscopy of the SSME Plume)
7. Author(s)
D. F. Olive
9. Performing Organization Name and Address
Sverdrup Technology, Inc.
Stennis Space Center, MS 39529
12. Sponsoring Agency Name and Addre_
National Aeronautics & Space Administration
Stennis Space Center, MS 39529
3. Recipient's Catalog No.
5. Report Date
November 1988
6. Performing Organization Code
8. Performing Organization Report No,
10. Work Unit No,
11. Contract or Grant No,
13. Type of Repo_ and Period Covered
Contractor Report
May-October 1988
14. Sponsoring Agency Code
15. Supplementaw Notes
i
NASA Technical Monitor: D_. Chenevert
16. Abstract
In order to examine the potential of using SSME exhaust plume radiation in the
soft x-ray spectrum as an early warning system of imminent engine failure, a
low-cost, low-risk experiment was devised. An approach was established, equipment
was leased, the system was installed and checked out, and data were successfully
acquired demonstrating the proof-of-concept. One spectrum measurement of the
SSME plume was acquired during a 300 second burn on the A-I Test Stand. This
spectrum showed a prominent, line emissin feature at about 34.5 Key, a result
which was not expected, nor can it be explained at this time. If x-ray spectra
are to be useful as a means of monitoring nominal engine operation, it will be
necessary to explore this region of the EM spectrum in greater detail. The
presence of structure in the spectrum indicates that this technology may prove
to be useful as an engine health monitoring system.
17. Key Words (Sugge_ed by Author(s))
X-ray Spectroscopy
Engine Health Monitoring
Engine Diagnostics
Plume Diagnostics
18. Distribution Staiement
Unclassified--Unlimited
Subject Category 20
19. SecuriW Classif. (of this report) _. Securiw Cla_ff. (of this page)
Unclassified Unclassified
NASA FORM 1626OCT
Fo_ sale by the National Technical Information Service,
21. No. of pages
17
2.2. Price
A02
Springfield, Virginia 22161-2171


X-ray spectroscopy of the SSME plume

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