An X-ray device capable of measuring titanium tritide film thickness from 0.1 to 30 micrometers has been built and tested. The monitor was designed for use in a rotating target system which used thick targets and incorporated a sputtering electrode to remove depleted layers from the target surface. The thickness measurement can be done in the presence of an intense background of bremsstrahlung and characteristic titanium X-radiation. A measurement can be accomplished in situ in two hours with reasonable accuracy

Alger, D. L.

Steinberg, R.

English

NASA Technical Reports Server

/VT IS
NASA TECHNICAL
MEMORANDUM
S4c 3. X
NASA TM X- 68170
oor-
X1
(NASA-TM-X-68170) AN X-RAY MONITOR FOR
1- MEASUREMENT OF A TITANIUM TRITIDE TARGET
THICKNESS (NASA) 14 p HC $3.00 CSCL 14B
Z \ . G3/
AN X-RAY MONITOR FOR MEASUREMENT OF A +
TITANIUM TRITIDE TARGET THICKNESS
by Donald L. Alger and Robert Steinberg L
Lewis Research Center
Cleveland, Ohio
N73-15475
Unclas
14 52514
TECHNICAL PAPER presented at
International Meeting of the American Nuclear Society
.Washington, D.C., November 12-16, 1972
I
https://ntrs.nasa.gov/search.jsp?R=19730006748 2020-03-23T05:42:44+00:00Z
AN X-RAY MONITOR FOR MEASUREMENT OF A
TITANIUM TRITIDE TARGET THICKNESS
by Donald L. Alger and Robert Steinberg
Lewis Research Center
National Aeronautics and Space Administration
Cleveland, Ohio
ABSTRACT
An X-ray device capable of measuring titanium tritide film thickness
from 0.1 to 30 ?lm has been built and tested. The monitor was designed for
use in a rotating target system which used thick targets and incorporated a
+c sputtering electrode to remove depleted layers from the target surface. The
thickness measurement can be done in the presence of an intense background
of bremsstrahlung and characteristic titanium X-radiation. A measurement
can be accomplished in situ in two hours with reasonable accuracy.
IAN X-RAY MONITOR FOR MEASUREMENT OF A
TITANIUM TRITIDE TARGET THICKNESS
by Donald L. Alger and Robert Steinberg
Lewis Research Center
National Aeronautics and Space Administration
Cleveland, Ohio
SUMMARY
An X-ray device capable of measuring titanium tritide film thick-
nesses from 0. 1 to 30 /gm has been built and tested. The monitor was de-
signed for use in a rotating target system which used thick targets and in-
corporated a sputtering electrode to remove depleted layers from the target
surface. The thickness measurement can be done in the presence of an in-
tense background of bremsstrahlung and characteristic titanium X-radiation.
A measurement can be accomplished in situ in two hours with reasonable
accuracy.
INTRODUCTION
The measurement of a thin film thickness on a substrate has been ac-
complished for a variety of materials by the excitation of the characteristic
X-rays of the substrate with a radioactive source. 1, 2, 3 However, these
materials were not radioactive. The measurement of the thickness of a
titanium tritide film, used as a target for neutron production, is compli-
cated by the X-ray background created by the radioactive decay in the film
itself.
The need for a monitor to measure the thickness of a tritide film
followed the development, by these authors, of a rotating high-yield neutron
target (HYNT) for a 300 kV and 30 mA deuteron accelerator, The acceler-
ator utilized the T(d,n)He 4 reaction for the production of neutrons, and in-
corporated a sputtering electrode in the target design which provides for the
use of tritide targets up to 30 jim thick. These targets had a thickness about
ten times the range of a 300-keV deuteron. After a target film has been
2depleted of tritium to about a 3 gm depth (the range of the deuteron), the
excess titanium is sputtered off in situ. A new target surface is thus ex-
posed. By this technique the lifetime of a target surface can be extended con-
siderably. The thickness monitor provides an accurate measurement of the
amount of titanium removed.
THE THICKNESS MONITOR
Principle of Operation
The titanium tritide thin film target is formed on a copper substrate.
The characteristic 8 keV (1. 28 fJ) K X-rays of the copper substrate are ex-
cited by 24 keV (3.84 fJ)X-rays emitted by a cadmium-109 source. These
8 keV X-rays are attenuated by passing through the titanium tritide film. The
X-rays then pass through the collimator system and are counted by a propor-
tional counter which is coupled to a pulse-counting system. The copper
filter over the face of the proportional counter passes the 8 keV X-rays while
absorbing the majority of the background X-rays. The ratio of the 8 keV
X-ray beam intensity transmitted through the titanium tritide film to the in-
tensity of the X-ray beam from the uncoated target substrate is a measure of
the film thickness.
Description
The monitor is constructed mostly of 6061-T6 aluminum alloy. This
material is best suited for use in the high neutron environment of the acceler-
ator target. The aluminum rapidly loses its activity following neutron acti-
vation, thus minimizing the gamma ray background of the proportional counter
within the monitor.
Figure 1 shows a drawing of the thickness monitor. Figure 2 shows a
photograph of the monitor attached to the HYNT that illustrates its size. The
monitor uses two collimator discs, one fixed and one rotatable. Two small
stepper motors rotate and aline, separately, the rotatable collimator and
shutter. A cadmium-109 source is attached to the underside of the rotatable
collimator and faces the target. Figure 3 shows a photograph of the 2. 5 cm
3diameter portion of the monitor that inserts into the HYNT port. The
cadmium-109 source can be seen surrounding the aperture. The end window
of a 0.63 cm diameter Xenon-filled proportional counter faces the aperture
of the upper collimator. A 0. 005 cm thick copper filter covers the window of
the proportional counter. The proportional counter, electrical leads, and
triax feedthrough connector are electrically insulated from the target housing.
Panel lights, connected to position indicator wafer switches attached to the
stepper motor shafts, show the position of the rotatable collimator and shutter.
Background Radiation
The background radiation consists of the 4. 5 keV characteristic titanium
K X-rays and bremsstrahlung spectra, both excited by the tritium beta ra-
diationo The beta radiation has a maximum energy of 18 keV. In addition to
the reduction in background caused by the copper filter, a further reduction
is possible by discriminating against all but the 8 keV X-rays by selective
pulse sorting. Figure 4 shows the pulse height spectra from the proportional
counter after pulse shaping and amplification. Pulses below 1 volt were
eliminated by a threshold descriminator.
A Thickness Measurement
A film thickness measurement can best be illustrated by use of figure 5,
which shows the four rotatable collimator and shutter positions 1 through 4.
Three separate counts are required for each thickness measurement. A
fourth count, obtained by monitor configuration 1 as shown in figure 5(a), is
the isolated counter background count. This count, I,, is independent of the
cadmium-109 or tritium sources and, therefore, this measurement is not
necessary for every thickness determination. The background count is a
function of the gamma activity of the surrounding accelerator environment.
Thickness measurements are usually made several hours after an accelerator
run when the residual background activity is at a relatively constant value.
One I1 measurement can then be made prior to a series of thickness meas-
urements,
4Figure 5(b) shows monitor configuration 2 which is used for measure-
ment of the tritium caused background intensity 12. Note that the source is
isolated so that only tritium caused.X-rays are counted.
Figure 5(c) shows monitor configuration 3 which is used for measure-
ment of count rate I3. The rotatable shutter, when positioned beneath the
source and counter, provides a copper reference surface for generation of
8 keV X-rays. The 8 keV X-ray source intensity, IOS, due to the
cadmium- 109 source is then calculated from the equation:
IOS = I3 - I1 (1)
Source intensity IOS is related to the intensity of X-rays from the un-
coated copper target cylinder, IOT, by a constant factor of 1. 46. This factor
is a function of the different source-counter geometry involved for the in-
tensity measurements of Ios and IOT. The factor was measured as a ratio
of IoS/IoT for the uncoated copper HYNT cylinder and copper reference
shutter of the monitor.
Finally, figure 5(d) shows monitor configuration 4 for obtaining a count
rate 14. Substraction of the tritium caused background 12 from 14 results
in the number of 8 keV X-rays transmitted through the film, Ix, where:
I X = 14 - 12 (2)
The fraction of 8 keV X-rays transmitted through a titanium tritide film
is:
IoX T IOIOSI T
(3)
IX " 1.46
5where
IX intensity of 8 keV X-rays transmitted through a titanium layer of
thickness X
IOT 8 keV X-ray intensity from uncoated copper target cylinder
IOS 8 keV X-ray intensity from rotatable copper shutter
The fraction of 8 keV X-rays transmitted through the tritide film can be
related to the thickness of the film by the exponential equation:
ex= eX (4)
IOT
where
k the absorption coefficient for the film, cm1
X the thickness of the film
Use of the reference shutter count permits determination of both IX
and IOT for each thickness measurement. Therefore, changes in counting
system amplifier gain, source decay, and other long-term variables are
eliminated.
EVALUATION AND TESTING
Calibration
The thickness monitor was calibrated for the measurement of titanium
thicknesses by use of a 3. 45 gm titanium-coated HYNT target cylinder. The
cylinder was coated'by a radiofrequency sputtering technique. ,The,:thidkness
of 'the titanium film was measured by. an interferometer.
By use of equation (4) of the previous section, a one-point measurement
of the intensity ratio, IX/IoT, for the 3. 45 gm titanium thickness will de-
fine a calibration line that can be used for the measurement of other titanium
thicknesses. The transmission measurement for X = 3.45 gm corresponds
to- an I/IoT ratio of 0. 785. The calibration plot of IX/IoT as a function
6of titanium thickness, X, is shown in figure 6. The absorption coefficient
calculated from equation (4) is 702 cm - 1 . Other titanium film thicknesses
are determined by measuring the ratio TX/IOT then obtaining the corre-
sponding thickness from figure 6. This figure was also used for the titanium
deuteride and titanium tritide film thickness measurements that are done in
the next section. Because our 3. 45 Im standard calibration film is titanium,
our titanium deuteride and titanium tritide thickness measurements are
slightly in error. The error, however, is small and can be neglected because
the mass absorption coefficient of hydrogen (deuterium or tritium) for 8 keV
X-radiation is much smaller, by a factor or more than 1000, than the co-
efficient for titanium.4
Target Film Thickness Measurement
The thickness monitor was used to measure titanium deuteride film
thicknesses during an experiment with the HYNT. The measurements were
made with the monitor attached to the HYNT and the deuteron accelerator
beam tube as shown in figure 2. An entire 3.45 gum thick titanium deuteride
film was removed in situ, in approximately 0. 1 to 0. 8 Mim increments, by-
a sputtering electrode. Following the removal of each increment, the thick-
ness of deuteride remaining was measured with the thickness monitor by
the method explained in "The Thickness Monitor" section and by use of
figure 6. Details of the experiment are reported in reference 5.
The thickness of a commercially-made titanium tritide target was also
measured. The target was in the form of a 4.6 cm diameter disc with a
tritium activity of approximately 0. 7 Ci/cm2 . A special target holder,
shown in figure 7, was made to accommodate the flat disc target for the
measurement. A thickness of 3. 5 Jim, by figure 6, was indicated. For this
measurement, the tritium caused background intensity, I2, was approxi-
mately 60 percent of the IOS intensity.
7DISCUSSION OF COUNTING ERRORS
A 1 mCi (37 M dis/s) cadmium- 109 source was used for the above thick-
ness measurements. The IOS intensity was 3000 counts per minute. This
intensity was too low to make an accurate thickness measurement in a prac-
tical time period. For example, to resolve a 0. 1 Mm change in a 3. 5 Mim
thick titanium tritide film to an accuracy of ±30 percent requires on the
order of 20 hours of counting time. We, therefore, recommend the use of a
100 mCi cadmium-109 source. The area of our cadmium- 109 source is about
1 cm2 . A 100 mCi source can be electroplated onto this area by using
carrier-free cadmium- 109.
We performed an error analysis based on our measured absorption co-
efficient of 702 cm-l and an IOT count of 107 and determined how ac-
curately an incremental AX film thickness could be resolved for various
titanium tritide total film thicknesses to 30 gm. The statistical standard de-
viations in the X-ray intensity counts of equations (1) through (3) were used.
The calculated percent error is plotted as a function of total film thickness
in figure 8. For example, a 0.2 Mim change in a 15 jIm thick film can be
measured to ±10 percent, whereas a thicker 0. 5 gm increment is required
for the same ±10 percent accuracy at a 30 gm film thickness. The error
associated with the isolated counter background count, I1, was assumed to be
negligible in comparison with the other counts and was neglected in this anal-
ysis.
The 100 mCi source will cause an IOS count rate of 3x105 counts per
minute. Assuming use of this source to accumulate an IOT of 107 , a thick-
ness measurement to the accuracy shown in figure 8 can be made in about
2 hours.
CONCLUSIONS
The X-ray thickness monitor described herein is capable of measuring
the thickness of a titanium tritide film on a copper substrate to a thickness
of 30 jim. The monitor is designed for use in a rotating target system that
uses very thich titanium tritide films. Its purpose is to measure the thick-
ness of depleted titanium layers removed by a sputtering electrode in the
target system.
8The thickness measurement can be done in the presence of the intense
background of bremsstrahlung and characteristic titanium X-radiation that
results from the tritium beta radiation. A measurement can be accomplished
in 2 hours with reasonable accuracy. For example, a 0.2 Mm change in a
-15 Alm thick film or a 0. 5 Am change in a 30 Aim thick film can be measured
to an accuracy -of ±10 percent in that time.
REFERENCES
1. J. F. CAMERON, J. R. RHODES, and P. F. BERRY, "Tritium
Bremsstrahlung and Its Applications, " AERE-R-3086, Atomic Energy
Research Establishment (Oct. 1959).
2. G. B. COOK, C.. E. MELLISH, and J. A. PAYNE, Anal. Chem., 32,
590 (1960).
3. S. I. LOBON, Instr. Exp. Tech., No. 1, 195 (1969).
4. G. L. CLARK, ed., The Encyclopedia of X-Rays and Gamma Rays,
Reinhold Publishing Corp., New York (1963), p. 388.
5. D. L. ALGER, and R. STEINBERG, "A High Yield Neutron Target for Cancer
Therapy, " Paper for presentation at the 1972 Nuclear Science Symposium
on Nuclear Instrumentation and Development, IEEE, at Miami Beach,
Florida (Dec. 6-8, 1972).
E-7246 
ROTATABLE COLLIMATOR—-
STEPPER 
MOTORS 
Zf— POSITION INDICATOR 
LIGHT SWITCHES 
OS 
XENON F I L L E D ^ ^ 
PROPORTIONAL COUNTER 
FIXED COLLIMATOR 
<<,—"CADMIUM-109 SOURCE 
BENEATH COLLIMATOR) 
ROTATABLE SHUTTER 
CD-11301 
TITANIUM TRITIDE FILM 
ON COPPER SUBSTRATE-
Figure 1. - The X-ray thickness monitor. 
Figure 2. - The HYNT with the thickness monitor attached. 
Figure 3. - X-ray thickness monitor. 
- 8 keV X-RAY PHOTOPEAK 
PULSE HEIGHT VOLTAGE CS-65112 
Figure 4. - Pulse height spectra of X-radiation passing 
through the 0.005 cm thick filter from a titanium 
deuteride target with copper substrate. Excitation by 
24 keV cadmium-109 X-rays. 
1. ISOLATED COUNTER BACKGROUND
X
(a) ISOLATED COUNTER BACK-
GROUND, 11
2.TRITIUM CAUSED BACKGROUND
SC COUNTER
SOURCE-- -. ROTATABLOR
ROTATABLE . COLLIMATOR
SHUTTER - _ : TITANIUM
/ TRITIDE
'-COPPER SUBSTRATE CYLINDER
(b) TRITIUM CAUSED BACKGROUND, 12
XX
(c) REFERENCE COUNT, 13.
~SOURCE
CS-65118
(d) TOTAL COUNT, 14.
Figure 5. - The four rotatable collimator and shutter configurations, 1 to 4, of the
thickness monitor.
1- = I ) x 146
WHERE
IX = INTENSITY OF 8 keV X-RAYS AS ATTENUATED BYA TITANIUM LAYER OF THICKNESS X
IOT =8 keV X-RAY INTENSITY FROM UNCOATED
COPPER TARGET SURFACE
Ios =8 keV X-RAY INTENSITY FROM ROTATABLE
COPPER SHUTTER
z .8
1 2 3 I
x . 1 2 3
TITANIUM FILM THICKNESS, X, pM CS-65115
Figure 6. - X-ray intensity ratio, IX11oT, as a function of titanium
film thickness.
C-72-1895 
Figure 7. - The thickness monitor with adapter for measuring the thickness of a titaniun 
tritide disc target. 
0 5 10 15 20 25 30 
TITANIUM TRITIDE FILM THICKNESS, X, uM 
Figure 8. - Percent error in measurement of an incremental 
film thickness, AX, as a function of total film thickness for 
I 0 j = 107 counts. 
N A S A - L e w i s - C o m ' l 


An X-ray monitor for measurement of a titanium tritide target thickness

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server