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Target thickness and uniformity measurements using charged particles
NUCLEAR
INSTRUMENTS
A N D M E T H O D S IO2 ( I 9 7 2 )
599-6Io;
©
NORTH-HOLLAND
PUBLISHING
CO.
TARGET THICKNESS AND UNIFORMITY M E A S U R E M E N T S USING CHARGED PARTICLES* H. L. A D A I R
Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A. The change in energy of charged particles as they pass through target foils is a technique that can be readily adapted to the measurement of both target thickness and uniformity. This technique is limited to those target materials whose stopping power is known and whose thickness does not exceed the range of the charged particles. This paper describes how alpha particles,
fission fragments, and beta particles are used in measuring target thickness and uniformity. Stopping power data are presented for 241Am alpha particles incident on carbon, gold, and nickel and for the fission fragments from 25zcf incident on nickel. These data improve the target thickness and uniformity measurements for these materials to _< 4- 7%.
1. Introduction The thickness and uniformity of targets used by the experimental physicist must be determined accurately. For example, the number of target nuclei per unit area of the target is important to the physicist who is making precision cross-section measurements or to the nuclear physicist who is trying to obtain the best resolution possible for a particular reaction. Numerous techniques have been used to determine uniformity and atom density including balance weighing, ellipsometry, the change in frequency of quartz crystals, resistivity measurements, etc. Few of these methods are easily adapted to both thickness and uniformity measurements. The method used most often by the Isotopes Target Laboratory at O R N L is to measure the change in energy of charged particles as they pass through the target foils. This is a relatively simple technique and is readily adaptable to all material whose stopping power is known and whose thickness does not exceed the range of the charged particles. Stopping power data for numerous incident particles and target materials can be found in the literature, but most of the data have an accuracy of + 10% or more ~. 2). This paper describes how alpha particles and fission fragments are used in measuring target thickness and uniformity of samples varying in areal density from a few pg/cm 2 to several mg/cm 2. Since the accuracy of this technique is limited by the accuracy of the stopping power data, stopping power data for several materials have been determined which should increase the accuracy for these materials to _+7% or better. Beta particle absorption techniques have been applied to target uniformity measurements for those
samples whose thicknesses exceed the range of the alpha particles and fission fragments.
* Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation.
2. Description of techniques 2.1. ALPHA PARTICLE METHOD The change in energy of alpha particles as they pass through foils has been used by many investigators to determine target thickness and uniformity3-6). The thin film chamber and electronics used in these measurements described in this paper are shown in fig. 1. A silicon p-n junction diode was used as the alpha detector, the signal output from which was amplified and fed into a 1024-channel pulse height analyzer. Resolution of the system with no foil in place was 15 keV. The thin film chamber and its schematic representation are shown in figs. 2 and 3, respectively. Alpha particles from the source pass through a 1 mm aperture and then impinge on the target foil. The energy of the attepuated alpha beam is determined by observing the alpha energy shift in a calibrated multichannel analyzer system. Using the proper stopping power data (defined as the amount of energy lost by the incident particle per centimeter of target thickness), the target thickness can be determined. A target thickness profile can be obtained by moving the foil between the stationary source and the detector and recording the energy change of the alpha particles when traversing different areas of the foil. 2.2. FISSION FRAGMENTMETHOD Target thickness and uniformity can also be determined by the use of fission fragments. Since these particles are quite massive compared to alpha particles they experience a greater energy loss per unit target thickness and should be more suitable for measuring the thickness of thin foils < 1 mg/cm 2. A 2 5 2 C f
599 SESSION F
600
H.L.
ADAIR
Fig. 1. E q u i p m e n t used for charged particle m e a s u r e m e n t s .
601
TARGET THICKNESS AND U N I F O R M I T Y MEASUREMENTS
Fig. 2. System used for making thickness and uniformity measurements with charged particles.
target, prepared by the self-transfer process, is used as the source of fission fragments. Fig. 4 shows a typical spectrum from such a source. The spectrum parameters indicate a clean source (very little material other than 252Cf) and good detector performance. The fission fragments are detected by using an O R T E C heavy ion silicon surface barrier detector. 2.3. BETA ABSORPTION METHOD
Beta absorption is a technique which is useful for measuring foils in the 40 to 500 mg/cm 2 range. For this thickness range, Faubel indicates that beta absorption appears more suitable than g a m m a absorption for determining the target profile since beta rays of 1-2 MeV end-point energy have precisely the optimum mass absorption coefficients7). Faubel obtains the following expression for the thickness density resolution:
Atmi.ll = K (e~"'llxl),
(1)
where
Atml, = the minimum detectable thickness density variation, = mean target thickness, K = a constant, and /z = the mass absorption coefficient. Thus Attain~i, the thickness density resolution, is at an optimum if i is 2//~. For a target thickness density of 300 mg/cm 2, # would be 6.7 cma/g. The beta energy that will give this optimum mass absorption coefficient for aluminum is given by Evans as 8) E~.14 =
17 /~(AI)'
(2)
which would be 2.3 MeV. The thickness range we have been concerned with most frequently over the past few months has been 50-100mg/cm z. F r o m eq. (1), the optimum mass absorption coefficient for a 75 mg/cm 2 foil is 26 cmE/g. SESSION F
602
H.L. ADAIR,
For an aluminum target, eq. (2) gives a beta energy of 0.689 MeV. Thus, a 2°'*T1 beta source (E o = 0.767) should be ideally suited for this range.
3. Targets To obtain better stopping power data, targets of carbon and gold whose thicknesses were known quite accurately were prepared. Carbon targets (20100 #g/cm 2) were prepared by the standard technique of vaporizing the carbon by maintaining an electric arc between two carbon electrodes. Carbon was deposited, under vacuum, onto a glass slide which was coated with a detergent. The glass slide was mounted in a mask arrangement which contained six well-defined 0.750 in. diam. openings. When the carbon evaporation was completed, the glass slide was removed from the system and the six foils were floated off in water and were mounted on aluminum frames that had been weighed on a Mettler microbalance. After the carbon foils were mounted, they were rinsed with distilled water to remove any excess parting agent, and finally rinsed in alcohol. After the foils dried under a heat lamp, the frames containing the mounted carbon foils were weighed. When the weight of the foils was determined, T
ANSPA
ENT
a light meter was calibrated to read the light intensity passing through a given thickness of carbon. This provides a very fast semiquantitative measurement of carbon foil thickness, but it gives no indication of foil uniformity. The results of these measurements are shown in fig. 5. To provide a very smooth foil surface for the stopping power measurements, glyptal was applied with a hypodermic needle to a small section of the foil (fig. 6). This was applied on the reverse side of the aluminum frame from which the carbon was mounted and was applied around the inner diameter of the target frame. U p o n drying, the glyptal varnish draws the carbon film taut and smooth. Gold foils (100-700#g/cm 2) were prepared by vacuum evaporation. The gold was placed in a carbon crucible and heated by electron bombardment. The glass slide substrate was coated with sodium chloride by vapor condensation, and the gold was then deposited on six well-defined 0.750 in. diam. spots. After the evaporation the glass slide was removed from the vacuum system and the gold foils were floated off in water. The foils were mounted, cleaned, and weighed as described in fig. 6. CHAMBE
X-Y DETECTOR} ADJUSTMENT
SAMPLE~,
'
r
t.5 Cm
SAMPLE H O ~ "'4'~I~-~ m m APERTURE P L A T E ~ - , ~ I i l
' /
"
APERTURE
(~
1
MOLECULAR SIEVE TRAPPED 15 cfm MECHANICAL Fig. 3. Schematic of system used for thickness and uniformity measurements.
i'~ v'~
b- X,
-- VE.T
TARGET
THICKNESS
FISSION
AND
FRAGMENT
UNIFORMITY
SPECTRUM
AL = 9.5 AH=ll.O L-H=250
FROM
603
MEASUREMENTS
0.036
MICROGRAM *=Cf
3 = 281 NV .
(>265)
NK = 2.23 NV
(t
+H
P- 0.36)
TARGET
2.2 1
c-H’=55.0 = 0.36
$= I z=
0.44
(ZO.45)
2.20
(nc2.18)
I
NL = 1.26 NH
(
1.30)
NL I
2000
(1650) -NV ______.
I
__---____---
\ i
_---
1600
- ..--
\
I
\
I
2400
1
-----
I
1200 600
I I ;I
400 I I IO
20
30
40 CHANNEL
Fig. 4. Typical
fission
fragment
spectrum
-0.1
‘;
I ! 1 I 50
60
I ,
I\
I I I
: I 1 m
00
NL
so
NWBER
for a 2ssCf source
prepared
by the self-transfer
process.
SESSION
F
604
n.L. ADAIR
Masks used in the target preparations were knife edge masks to prevent inhomogeneity around the edge of the foils. To insure good target uniformity the substrate was placed at large distances from the source. The purity of the material used in the evaporations was >99.9%. In addition to the evaporated foils, a series of nickel foils from 0.200 to 5.0 mg/cm 2 were prepared by punching out well-defined 0.750 in. diam. disks from rolled nickel foils. These disks were weighed on a Mettler microbalance and mounted on frames.
4. Stopping power measurements To improve the accuracy of the thickness and uniformity measurements, new stopping power data were determined for the main alpha group of 2¢1Am for carbon, gold, and nickel. In addition, stopping power data were obtained for the median-light and medianheavy fission fragments of 252Cf for nickel. The system used for the stopping power measurements is shown in figs. 1 and 2. Fig. 3 shows a schematic of the system except the 2°4T1 source was replaced with an 241Am source for the alpha stopping power measurements and a 252Cf source for the fission fragment measurements. The 241Am source used contained 90
i1
L
I
I 80
!
i
i.
70
I I 6O
] I
A
~L
50
]
t
V
!
40 G i F50
i
] !
20
ii
t0
0
t0
t 20 LIGHT
50 METER
40
50
READING
Fig. 5. Carbon foil thickness measurements determined by using a light meter.
0.8/~g of 24~Am electroplated on a 3 mm spot centered on a 3.8 cm diam. nickel backing. In order to obtain the actual alpha energy incident on the target foils, the system was initially calibrated with "weightless" deposits of Zl°po, 239pu, and 241Am which had energies of 5.305, 5.156, and 5.4858 MeV, respectively9'l°). After the system was calibrated and adjusted to 1 keV/channel, the linearity of the system was checked using a precision pulser. The variation from linearity over the range of interest (5 to 5.5 MeV) was < 1 % . The 0.8 #g 24LAm source was placed in the system and the energy of the main alpha group was determined to be 5.478 MeV, the decrease from 5.4858 being due to energy lost in the source deposit. To determine the spot size of the film being measured in the thin film chamber, a series of apertures of different diameters were placed over the detector and the number of counts in a 1000 s time period was determined. Results are shown in fig. 7. The number of counts begins to decrease considerably for apertures of < 4 mm diam. The carbon foils were placed in the thin film chamber and the system was evacuated to ~ I0/~m. The energy shift of the main alpha group from the 24LAm source was determined for each foil. After each measurement, the unattenuated 24-lAm peak energy was redetermined to check the stability of the system. Because the substrate was located at quite a distance (8-10 in.) from the source during the fabrication of the carbon foils, it was felt that they would be very uniform. However, an attenuated 24aAm energy was obtained from three different positions on the foil and the vertical error bars in fig. 8 represent the energy variation obtained. The horizontal error bars represent thickness variation and include a _+0.5% error in area determination. The curve represents a least-squares fit to the data. The slope of the curve gives a stopping power for carbon of 876 MeV cmZ/g. The maximum error in the measurements was +_7%. Extrapolation of the data from ref. 2 yields a stopping power for this energy alpha on carbon of 750 MeV cmZ/g. Similar measurements were made for gold targets in the thickness range of 100-700 #g/cm 2. The results are shown in fig. 9. Again the curve is a least-squares fit of the data and the slope of the curve gives a stopping power for gold as 337 MeV cmZ/g ___5%; the extrapolated data from ref. 2 yield a value of 215 MeV cm2/g. Fig. 10 gives the energy loss of the main group of alphas from the 0.8 pg Z41Am source for nickel thick-
605
T A R G E T T H I C K N E S S AND U N I F O R M I T Y MEASUREMENTS
nesses of 0.2 to 5.0 m g / c m 2. The stopping power obtained was 410 MeV cm2/g + 2 % . F o r the fission-fragment measurements, the 2 4 1 A m source was replaced in the thin film c h a m b e r (fig. 2) with a 0.03/~g 252Cf source. An unperturbed 252Cf fission fragment spectrum was obtained and is shown in fig. 1 I. The energies o f the median-light and medianheavy fragments were calculated from eq. (3)~1): E = (a+a'M)
where E = ion energy, m = mass, X = pulse height, and a, a', b, b' are constants given by a = 24.0203/(PL--PH),
a' = 0.03574/(PL-- Pn), b = 89.6083--aPL, b' = 0.1370--a'PL,
(3)
X + b + b'M,
CARBON FILM MOUNTING PROCEDURE
FLOATING FOIL IS PICKED UP ON FRAME CENTERED OVER APERTURE, FILM IS ALLOWED TO DRY BEFORE BEING WEIGHED.
TAL IS CAREFULLY SPREAD
/ ~---
",ETWEEN UNCTURE OF FRAME AND FILM WITH HYPODERMIC NEEDLE (~25) TO STRETCH FILM' TAUT
/ / / /
/
APERTURE . . . . .
~
FRAME DETAIL ROTATED 90"
MOUNTED CARBON FILM Fig. 6. Procedure used for mounting thin foils. SESSION
F
H.L.
606
ADAIR
!
!
o
I
l
'
__
L
i
I
I
/ ,-~T-
t
,
~
t
i
,
!
,
=p
O
ol0
a--
--i--t- .....
6--
1
/ L___t
2.0
1.0
3.0
4.0
5.0 6,0 APERTURE DIAMETER (mm)
8.0
7.0
Fig. 7. D e t e r m i n a t i o n o f foil area e x a m i n e d in the thin film c h a m b e r .
~25
..........T - - - 250
, -
.....................
t,d i
~00
zoo 75
.
.
.
.
.
.
.
-
>
s ~r
.......... t
s
50
!
/~,"
w
/"
25
O
.io
0
-t~o
2o.......... 2o
?-
+.~
5o
t20
CARBON TARGET THICKNESS (/.zg/c rn2)
Fig. 8. Americium-241 energy loss measurements for 25-100#g/ cm ~ carbon foils.
o
.
. . . . . . . . . . . . . . . J 400 200 5oo 400 500 GOLD TARGET THICKNESS ( ,u.g fcm 8)
600
700
Fig. 9. Amerieium-241 energy loss measurements for 100-700 # g / c m 2 gold foils.
TARGET
THICKNESS
UNIFORMITY
/
2500
2000
~
AND
1500
where PL = pulse height of median-light fragments and PH = pulse height of median-heavy fragments. The energies obtained from the median-light and median-heavy fragments penetrating a 30 pg/cm 2 gold window on a 100 m m 2 heavy ion detector were 104.1 and 79.9MeV, respectively. A thin carbon foil (23 #g/cm 2) was placed over the 251Cf source to prevent the self-transfer of the 252Cf to the target foils. The energy loss of the median-light and median-heavy fission fragments in the thin carbon foil was 1.4 and 1.3 MeV, respectively. The change in energy of the fission fragments for nickel in the range 0.2 to 2.0 mg/cm 2 was determined and the results are shown in fig. 12. The lines represent smooth curves drawn by eye through the points. There appears to be a departure from linearity above ~ 1.2 mg/cm 2. This agrees very well with the results obtained by Kahn and ForgueS).
,
I000 w
5OO
0
/4,/"" 0
t
2
3
5
4
Ni TARGET THICKNESS (mg/crn2)
Fig. 10. A m e r i c i u m - 2 4 1 e n e r g y loss m e a s u r e m e n t m g / c m 2 n i c k e l foils.
607
MEASUREMENTS
for 0 . 2 - 5 . 0
/ /
/ /
\-4,,,x. / \
/
't/ /
///
/
/
I
kj
\ \ PULSE H[I@HT
Fig. I 1. C a l i f o r n i u m - 2 5 2 fission f r a g m e n t s p e c t r u m o b t a i n e d w i t h n o foil b e t w e e n the s o u r c e a n d the d e t e c t o r .
SESSION
F
608
H . L . ADAIR
In general, fission fragments are used for measuring the thickness of thin foils because the heavier particles experience a greater energy loss as they pass through the foils. This permits more accurate energy measurements. However, whether alpha particles or fission fragments are used for very thin foils ( < 1 mg/cm2), a very stable system and, usually, long counting times are required to accurately determine the energy lost in the foils.
5. Beta absorption measurements
6. Summary The use of charged particles for measuring both target thickness and uniformity is readily adaptable to those materials whose stopping power is known and whose thickness does not exceed the range of the charged particles. The accuracy of this technique is limited by the accuracy of the stopping power data available. This technique will be used more frequently as more accurate stopping power data become available.
References
For the beta absorption method, a well collimated beam of beta particles from a 2°4T1 source is incident on a target which can be moved in both the X and Y directions (fig. 3). A silicon surface barrier detector with a 50 m m z surface area and a depletion depth of 400 pm was used in all the measurements that are described. A counting-rate profile is obtained by scanning the foil point by point. Beta absorption over the thickness range being considered can be described by the exponential absorption law. The normal procedure in obtaining target thickness profiles by this method is to obtain the mean thickness by weighing. The mean counting rate is then assigned to the mean thickness and the target thickness profile is drawn. Figs. 13 and 14 contain thickness density profiles obtained from a 1.5 by 5 cm area of 5~Ni and 6°Ni foils, respectively. The largest error encountered with our system is variation in counting rates, which in some cases is as high as _+2%; however, the previous method has been used to obtain target thickness profiles to better than
1) W. Whaling, Handbuch der Physik, vol. 34 (ed. S. Fliigge; Springer-Verlag, Berlin, 1958) p. 193. 2) C. F. Williamson, J. P. Boujat and J. Picard, Tables of range and stopping power of chemical elements for charged particles of energy 0.5 to 500 MeV, CEA-R-3042 (1966). 3) H. L. Anderson, Nucl. Instr. and Meth. 12 (1961) I l l . 4) j. R. Comfort, J. F. Decker, E. L. Lynk, M. V. Schully and A. R. Quintan, Phys. Rev. 150, no. 1 (1966) 249. 5) S. Kahn and V. Forgue, Phys. Rev. 163, no. 2 (1967) 290. 6) H. L. Adair, A study of the I1B(aHe, p)taC and UBOHe, 3He)liB reactions, ORNL-4339 (Oak Ridge National Laboratory, 1969). 7) Manfred Faubel, Measurement of target thickness density profiles with a beta-absorption technique, Dissertation (Institute of Nuclear Physics, Mainz University~ 1969). s) R. D. Evans, The atomic nucleus (McGraw-Hill, New York, 1955) p. 628. 9) Nuclear Data Tables, A-8 (Academic Press, New York, Oct. 1970). 10) C. M. Lederer, J. M. Hollander and 1. Perlman, Table of isotopes, 6th ed. (Wiley, New York, 1967).
50
1%7).
60
! !
A-LIGHT FRAGMENTS B-HEAVY FRAGMENTS
50
i
L-" 4 0 - E --
fo
~DEVIATION IS 4.59'0 OR 7 4 0 - - ~(/-~g/cm 2) -I
fl
o 3o
z
c¢
I
I
~ zo w
,o/j O O
I 0,4
0.8 t.2 Ni TARGET THICKNESS (rng/cm 2)
t
~.6
Fig. 12. Energy loss measurements for median-light and medianheavy fission fragments from 252Cf for nickel foils in the 0.2-2.0 mg/cm2 thickness range.
I I
l lt l_!
K
40
"1I
0
0
1.0 RELATIVE
2.0 POSITION
3.0 (cm)
Fig. 13. Target thickness profile for SSNi.
TARGET
THICKNESS
AND UNIFORMITY
609
MEASUREMENTS
60
r
/
\ I
l
J
I
J
45
J J
N
E o
E
i
v
w z v (J
t !
~ so
TARGET THICKNESS PROFILE FOR 6O,"i. THE MAXIMUM DEVIATION IS 26 % OR 13.9 m g / c m z
I
15
I.O
2.0 :5.0 RELATIVE POSITION (cm)
I
i
1
I
i
il
l
4.0
Fig. 14. Target thickness profile for 60Ni.
SESSION F
610
H.L. ADAIR
11) H. W. Schmitt, W. E. Kiker and C. W. Williams, Phys. Rev. 137, no. 4B (1965) 837.
ADAIR: NO, I did not reduce them to the Linhart; 1 did compare them with another source with which they were in excellent agreement (see text).
Discussion
NOGGLE: Would you comment on the counting times involved in generating data points by this method, both for alphas and for the fission fragment? Since you are counting over an area of 4 mm, you are talking about thickness variations that have been averaged out over a substantial area of the film already, ls this true ?
QUESTIONER UNKNOWN: Looking at your data plotted in figs. 8-10, it appears that maybe you are getting into a little trouble due to the variation and stopping power with particle energy. The points to the right tend to be above your straight line. ADAIR: There is a slight variation. I think all the variations are within the errors stated, however. GALLANT" With your thickness profile study of films, do you find a very great difference in the surface uniformity of evaporated films? I imagine you did quite a lot of work on this. H o w uniform are your evaporated films? ADAIR: It depends on many variables, of course. Normally, when we get a request for a target the user states how uniform it must be. This determines the source-to-substrate distance you will use; with a particular arrangement you can get practically any variation you want. We use the particular arrangement that will give us the variation within the range the customer specifies. Normally a value of plus or minus 5% can be realized, but better uniformities can be generated. MOORE: My question concerns the fission fragment range energy relations that you measured for nickel. I am curious to know how your results compared with the reduced range and other parameters that Linhart described. Did you reduce these to the Linhart ?
ADAIR: A 4 m m diam. area is a " s u b s t a n t i a l " area to you; you are an electron microscopicist, but our targets are normally 3/4 in. in diam. or larger. For a 4 m m diam. area of evaporated foil, there is very little variation. It all goes back to the initial setup. As to your first question, the counting time for these measurements was of the order of 3 h/foil. SLETTEN: HOW strong is the 25'2Cf source you have to use? ADAIR: 0.03 #g. SLETTEN: The mother source? ADAIR: The mother source was l0/~g. SLETTEN: HOW much californium do you have to have to make such a source in a reasonable a m o u n t of time? ADAIR: We made some measurements on self transfer from a 0.7 #g mother source. We transferred 0.007 #g/h ~o, for a heavier source, of course, you would transfer more. It depends on the area you would want to transfer to, but normally our transfer sources are of the order of 10/~g.
INSTRUMENTS
A N D M E T H O D S IO2 ( I 9 7 2 )
599-6Io;
©
NORTH-HOLLAND
PUBLISHING
CO.
TARGET THICKNESS AND UNIFORMITY M E A S U R E M E N T S USING CHARGED PARTICLES* H. L. A D A I R
Oak Ridge National Laboratory, Oak Ridge, Tennessee, U.S.A. The change in energy of charged particles as they pass through target foils is a technique that can be readily adapted to the measurement of both target thickness and uniformity. This technique is limited to those target materials whose stopping power is known and whose thickness does not exceed the range of the charged particles. This paper describes how alpha particles,
fission fragments, and beta particles are used in measuring target thickness and uniformity. Stopping power data are presented for 241Am alpha particles incident on carbon, gold, and nickel and for the fission fragments from 25zcf incident on nickel. These data improve the target thickness and uniformity measurements for these materials to _< 4- 7%.
1. Introduction The thickness and uniformity of targets used by the experimental physicist must be determined accurately. For example, the number of target nuclei per unit area of the target is important to the physicist who is making precision cross-section measurements or to the nuclear physicist who is trying to obtain the best resolution possible for a particular reaction. Numerous techniques have been used to determine uniformity and atom density including balance weighing, ellipsometry, the change in frequency of quartz crystals, resistivity measurements, etc. Few of these methods are easily adapted to both thickness and uniformity measurements. The method used most often by the Isotopes Target Laboratory at O R N L is to measure the change in energy of charged particles as they pass through the target foils. This is a relatively simple technique and is readily adaptable to all material whose stopping power is known and whose thickness does not exceed the range of the charged particles. Stopping power data for numerous incident particles and target materials can be found in the literature, but most of the data have an accuracy of + 10% or more ~. 2). This paper describes how alpha particles and fission fragments are used in measuring target thickness and uniformity of samples varying in areal density from a few pg/cm 2 to several mg/cm 2. Since the accuracy of this technique is limited by the accuracy of the stopping power data, stopping power data for several materials have been determined which should increase the accuracy for these materials to _+7% or better. Beta particle absorption techniques have been applied to target uniformity measurements for those
samples whose thicknesses exceed the range of the alpha particles and fission fragments.
* Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation.
2. Description of techniques 2.1. ALPHA PARTICLE METHOD The change in energy of alpha particles as they pass through foils has been used by many investigators to determine target thickness and uniformity3-6). The thin film chamber and electronics used in these measurements described in this paper are shown in fig. 1. A silicon p-n junction diode was used as the alpha detector, the signal output from which was amplified and fed into a 1024-channel pulse height analyzer. Resolution of the system with no foil in place was 15 keV. The thin film chamber and its schematic representation are shown in figs. 2 and 3, respectively. Alpha particles from the source pass through a 1 mm aperture and then impinge on the target foil. The energy of the attepuated alpha beam is determined by observing the alpha energy shift in a calibrated multichannel analyzer system. Using the proper stopping power data (defined as the amount of energy lost by the incident particle per centimeter of target thickness), the target thickness can be determined. A target thickness profile can be obtained by moving the foil between the stationary source and the detector and recording the energy change of the alpha particles when traversing different areas of the foil. 2.2. FISSION FRAGMENTMETHOD Target thickness and uniformity can also be determined by the use of fission fragments. Since these particles are quite massive compared to alpha particles they experience a greater energy loss per unit target thickness and should be more suitable for measuring the thickness of thin foils < 1 mg/cm 2. A 2 5 2 C f
599 SESSION F
600
H.L.
ADAIR
Fig. 1. E q u i p m e n t used for charged particle m e a s u r e m e n t s .
601
TARGET THICKNESS AND U N I F O R M I T Y MEASUREMENTS
Fig. 2. System used for making thickness and uniformity measurements with charged particles.
target, prepared by the self-transfer process, is used as the source of fission fragments. Fig. 4 shows a typical spectrum from such a source. The spectrum parameters indicate a clean source (very little material other than 252Cf) and good detector performance. The fission fragments are detected by using an O R T E C heavy ion silicon surface barrier detector. 2.3. BETA ABSORPTION METHOD
Beta absorption is a technique which is useful for measuring foils in the 40 to 500 mg/cm 2 range. For this thickness range, Faubel indicates that beta absorption appears more suitable than g a m m a absorption for determining the target profile since beta rays of 1-2 MeV end-point energy have precisely the optimum mass absorption coefficients7). Faubel obtains the following expression for the thickness density resolution:
Atmi.ll = K (e~"'llxl),
(1)
where
Atml, = the minimum detectable thickness density variation, = mean target thickness, K = a constant, and /z = the mass absorption coefficient. Thus Attain~i, the thickness density resolution, is at an optimum if i is 2//~. For a target thickness density of 300 mg/cm 2, # would be 6.7 cma/g. The beta energy that will give this optimum mass absorption coefficient for aluminum is given by Evans as 8) E~.14 =
17 /~(AI)'
(2)
which would be 2.3 MeV. The thickness range we have been concerned with most frequently over the past few months has been 50-100mg/cm z. F r o m eq. (1), the optimum mass absorption coefficient for a 75 mg/cm 2 foil is 26 cmE/g. SESSION F
602
H.L. ADAIR,
For an aluminum target, eq. (2) gives a beta energy of 0.689 MeV. Thus, a 2°'*T1 beta source (E o = 0.767) should be ideally suited for this range.
3. Targets To obtain better stopping power data, targets of carbon and gold whose thicknesses were known quite accurately were prepared. Carbon targets (20100 #g/cm 2) were prepared by the standard technique of vaporizing the carbon by maintaining an electric arc between two carbon electrodes. Carbon was deposited, under vacuum, onto a glass slide which was coated with a detergent. The glass slide was mounted in a mask arrangement which contained six well-defined 0.750 in. diam. openings. When the carbon evaporation was completed, the glass slide was removed from the system and the six foils were floated off in water and were mounted on aluminum frames that had been weighed on a Mettler microbalance. After the carbon foils were mounted, they were rinsed with distilled water to remove any excess parting agent, and finally rinsed in alcohol. After the foils dried under a heat lamp, the frames containing the mounted carbon foils were weighed. When the weight of the foils was determined, T
ANSPA
ENT
a light meter was calibrated to read the light intensity passing through a given thickness of carbon. This provides a very fast semiquantitative measurement of carbon foil thickness, but it gives no indication of foil uniformity. The results of these measurements are shown in fig. 5. To provide a very smooth foil surface for the stopping power measurements, glyptal was applied with a hypodermic needle to a small section of the foil (fig. 6). This was applied on the reverse side of the aluminum frame from which the carbon was mounted and was applied around the inner diameter of the target frame. U p o n drying, the glyptal varnish draws the carbon film taut and smooth. Gold foils (100-700#g/cm 2) were prepared by vacuum evaporation. The gold was placed in a carbon crucible and heated by electron bombardment. The glass slide substrate was coated with sodium chloride by vapor condensation, and the gold was then deposited on six well-defined 0.750 in. diam. spots. After the evaporation the glass slide was removed from the vacuum system and the gold foils were floated off in water. The foils were mounted, cleaned, and weighed as described in fig. 6. CHAMBE
X-Y DETECTOR} ADJUSTMENT
SAMPLE~,
'
r
t.5 Cm
SAMPLE H O ~ "'4'~I~-~ m m APERTURE P L A T E ~ - , ~ I i l
' /
"
APERTURE
(~
1
MOLECULAR SIEVE TRAPPED 15 cfm MECHANICAL Fig. 3. Schematic of system used for thickness and uniformity measurements.
i'~ v'~
b- X,
-- VE.T
TARGET
THICKNESS
FISSION
AND
FRAGMENT
UNIFORMITY
SPECTRUM
AL = 9.5 AH=ll.O L-H=250
FROM
603
MEASUREMENTS
0.036
MICROGRAM *=Cf
3 = 281 NV .
(>265)
NK = 2.23 NV
(t
+H
P- 0.36)
TARGET
2.2 1
c-H’=55.0 = 0.36
$= I z=
0.44
(ZO.45)
2.20
(nc2.18)
I
NL = 1.26 NH
(
1.30)
NL I
2000
(1650) -NV ______.
I
__---____---
\ i
_---
1600
- ..--
\
I
\
I
2400
1
-----
I
1200 600
I I ;I
400 I I IO
20
30
40 CHANNEL
Fig. 4. Typical
fission
fragment
spectrum
-0.1
‘;
I ! 1 I 50
60
I ,
I\
I I I
: I 1 m
00
NL
so
NWBER
for a 2ssCf source
prepared
by the self-transfer
process.
SESSION
F
604
n.L. ADAIR
Masks used in the target preparations were knife edge masks to prevent inhomogeneity around the edge of the foils. To insure good target uniformity the substrate was placed at large distances from the source. The purity of the material used in the evaporations was >99.9%. In addition to the evaporated foils, a series of nickel foils from 0.200 to 5.0 mg/cm 2 were prepared by punching out well-defined 0.750 in. diam. disks from rolled nickel foils. These disks were weighed on a Mettler microbalance and mounted on frames.
4. Stopping power measurements To improve the accuracy of the thickness and uniformity measurements, new stopping power data were determined for the main alpha group of 2¢1Am for carbon, gold, and nickel. In addition, stopping power data were obtained for the median-light and medianheavy fission fragments of 252Cf for nickel. The system used for the stopping power measurements is shown in figs. 1 and 2. Fig. 3 shows a schematic of the system except the 2°4T1 source was replaced with an 241Am source for the alpha stopping power measurements and a 252Cf source for the fission fragment measurements. The 241Am source used contained 90
i1
L
I
I 80
!
i
i.
70
I I 6O
] I
A
~L
50
]
t
V
!
40 G i F50
i
] !
20
ii
t0
0
t0
t 20 LIGHT
50 METER
40
50
READING
Fig. 5. Carbon foil thickness measurements determined by using a light meter.
0.8/~g of 24~Am electroplated on a 3 mm spot centered on a 3.8 cm diam. nickel backing. In order to obtain the actual alpha energy incident on the target foils, the system was initially calibrated with "weightless" deposits of Zl°po, 239pu, and 241Am which had energies of 5.305, 5.156, and 5.4858 MeV, respectively9'l°). After the system was calibrated and adjusted to 1 keV/channel, the linearity of the system was checked using a precision pulser. The variation from linearity over the range of interest (5 to 5.5 MeV) was < 1 % . The 0.8 #g 24LAm source was placed in the system and the energy of the main alpha group was determined to be 5.478 MeV, the decrease from 5.4858 being due to energy lost in the source deposit. To determine the spot size of the film being measured in the thin film chamber, a series of apertures of different diameters were placed over the detector and the number of counts in a 1000 s time period was determined. Results are shown in fig. 7. The number of counts begins to decrease considerably for apertures of < 4 mm diam. The carbon foils were placed in the thin film chamber and the system was evacuated to ~ I0/~m. The energy shift of the main alpha group from the 24LAm source was determined for each foil. After each measurement, the unattenuated 24-lAm peak energy was redetermined to check the stability of the system. Because the substrate was located at quite a distance (8-10 in.) from the source during the fabrication of the carbon foils, it was felt that they would be very uniform. However, an attenuated 24aAm energy was obtained from three different positions on the foil and the vertical error bars in fig. 8 represent the energy variation obtained. The horizontal error bars represent thickness variation and include a _+0.5% error in area determination. The curve represents a least-squares fit to the data. The slope of the curve gives a stopping power for carbon of 876 MeV cmZ/g. The maximum error in the measurements was +_7%. Extrapolation of the data from ref. 2 yields a stopping power for this energy alpha on carbon of 750 MeV cmZ/g. Similar measurements were made for gold targets in the thickness range of 100-700 #g/cm 2. The results are shown in fig. 9. Again the curve is a least-squares fit of the data and the slope of the curve gives a stopping power for gold as 337 MeV cmZ/g ___5%; the extrapolated data from ref. 2 yield a value of 215 MeV cm2/g. Fig. 10 gives the energy loss of the main group of alphas from the 0.8 pg Z41Am source for nickel thick-
605
T A R G E T T H I C K N E S S AND U N I F O R M I T Y MEASUREMENTS
nesses of 0.2 to 5.0 m g / c m 2. The stopping power obtained was 410 MeV cm2/g + 2 % . F o r the fission-fragment measurements, the 2 4 1 A m source was replaced in the thin film c h a m b e r (fig. 2) with a 0.03/~g 252Cf source. An unperturbed 252Cf fission fragment spectrum was obtained and is shown in fig. 1 I. The energies o f the median-light and medianheavy fragments were calculated from eq. (3)~1): E = (a+a'M)
where E = ion energy, m = mass, X = pulse height, and a, a', b, b' are constants given by a = 24.0203/(PL--PH),
a' = 0.03574/(PL-- Pn), b = 89.6083--aPL, b' = 0.1370--a'PL,
(3)
X + b + b'M,
CARBON FILM MOUNTING PROCEDURE
FLOATING FOIL IS PICKED UP ON FRAME CENTERED OVER APERTURE, FILM IS ALLOWED TO DRY BEFORE BEING WEIGHED.
TAL IS CAREFULLY SPREAD
/ ~---
",ETWEEN UNCTURE OF FRAME AND FILM WITH HYPODERMIC NEEDLE (~25) TO STRETCH FILM' TAUT
/ / / /
/
APERTURE . . . . .
~
FRAME DETAIL ROTATED 90"
MOUNTED CARBON FILM Fig. 6. Procedure used for mounting thin foils. SESSION
F
H.L.
606
ADAIR
!
!
o
I
l
'
__
L
i
I
I
/ ,-~T-
t
,
~
t
i
,
!
,
=p
O
ol0
a--
--i--t- .....
6--
1
/ L___t
2.0
1.0
3.0
4.0
5.0 6,0 APERTURE DIAMETER (mm)
8.0
7.0
Fig. 7. D e t e r m i n a t i o n o f foil area e x a m i n e d in the thin film c h a m b e r .
~25
..........T - - - 250
, -
.....................
t,d i
~00
zoo 75
.
.
.
.
.
.
.
-
>
s ~r
.......... t
s
50
!
/~,"
w
/"
25
O
.io
0
-t~o
2o.......... 2o
?-
+.~
5o
t20
CARBON TARGET THICKNESS (/.zg/c rn2)
Fig. 8. Americium-241 energy loss measurements for 25-100#g/ cm ~ carbon foils.
o
.
. . . . . . . . . . . . . . . J 400 200 5oo 400 500 GOLD TARGET THICKNESS ( ,u.g fcm 8)
600
700
Fig. 9. Amerieium-241 energy loss measurements for 100-700 # g / c m 2 gold foils.
TARGET
THICKNESS
UNIFORMITY
/
2500
2000
~
AND
1500
where PL = pulse height of median-light fragments and PH = pulse height of median-heavy fragments. The energies obtained from the median-light and median-heavy fragments penetrating a 30 pg/cm 2 gold window on a 100 m m 2 heavy ion detector were 104.1 and 79.9MeV, respectively. A thin carbon foil (23 #g/cm 2) was placed over the 251Cf source to prevent the self-transfer of the 252Cf to the target foils. The energy loss of the median-light and median-heavy fission fragments in the thin carbon foil was 1.4 and 1.3 MeV, respectively. The change in energy of the fission fragments for nickel in the range 0.2 to 2.0 mg/cm 2 was determined and the results are shown in fig. 12. The lines represent smooth curves drawn by eye through the points. There appears to be a departure from linearity above ~ 1.2 mg/cm 2. This agrees very well with the results obtained by Kahn and ForgueS).
,
I000 w
5OO
0
/4,/"" 0
t
2
3
5
4
Ni TARGET THICKNESS (mg/crn2)
Fig. 10. A m e r i c i u m - 2 4 1 e n e r g y loss m e a s u r e m e n t m g / c m 2 n i c k e l foils.
607
MEASUREMENTS
for 0 . 2 - 5 . 0
/ /
/ /
\-4,,,x. / \
/
't/ /
///
/
/
I
kj
\ \ PULSE H[I@HT
Fig. I 1. C a l i f o r n i u m - 2 5 2 fission f r a g m e n t s p e c t r u m o b t a i n e d w i t h n o foil b e t w e e n the s o u r c e a n d the d e t e c t o r .
SESSION
F
608
H . L . ADAIR
In general, fission fragments are used for measuring the thickness of thin foils because the heavier particles experience a greater energy loss as they pass through the foils. This permits more accurate energy measurements. However, whether alpha particles or fission fragments are used for very thin foils ( < 1 mg/cm2), a very stable system and, usually, long counting times are required to accurately determine the energy lost in the foils.
5. Beta absorption measurements
6. Summary The use of charged particles for measuring both target thickness and uniformity is readily adaptable to those materials whose stopping power is known and whose thickness does not exceed the range of the charged particles. The accuracy of this technique is limited by the accuracy of the stopping power data available. This technique will be used more frequently as more accurate stopping power data become available.
References
For the beta absorption method, a well collimated beam of beta particles from a 2°4T1 source is incident on a target which can be moved in both the X and Y directions (fig. 3). A silicon surface barrier detector with a 50 m m z surface area and a depletion depth of 400 pm was used in all the measurements that are described. A counting-rate profile is obtained by scanning the foil point by point. Beta absorption over the thickness range being considered can be described by the exponential absorption law. The normal procedure in obtaining target thickness profiles by this method is to obtain the mean thickness by weighing. The mean counting rate is then assigned to the mean thickness and the target thickness profile is drawn. Figs. 13 and 14 contain thickness density profiles obtained from a 1.5 by 5 cm area of 5~Ni and 6°Ni foils, respectively. The largest error encountered with our system is variation in counting rates, which in some cases is as high as _+2%; however, the previous method has been used to obtain target thickness profiles to better than
1) W. Whaling, Handbuch der Physik, vol. 34 (ed. S. Fliigge; Springer-Verlag, Berlin, 1958) p. 193. 2) C. F. Williamson, J. P. Boujat and J. Picard, Tables of range and stopping power of chemical elements for charged particles of energy 0.5 to 500 MeV, CEA-R-3042 (1966). 3) H. L. Anderson, Nucl. Instr. and Meth. 12 (1961) I l l . 4) j. R. Comfort, J. F. Decker, E. L. Lynk, M. V. Schully and A. R. Quintan, Phys. Rev. 150, no. 1 (1966) 249. 5) S. Kahn and V. Forgue, Phys. Rev. 163, no. 2 (1967) 290. 6) H. L. Adair, A study of the I1B(aHe, p)taC and UBOHe, 3He)liB reactions, ORNL-4339 (Oak Ridge National Laboratory, 1969). 7) Manfred Faubel, Measurement of target thickness density profiles with a beta-absorption technique, Dissertation (Institute of Nuclear Physics, Mainz University~ 1969). s) R. D. Evans, The atomic nucleus (McGraw-Hill, New York, 1955) p. 628. 9) Nuclear Data Tables, A-8 (Academic Press, New York, Oct. 1970). 10) C. M. Lederer, J. M. Hollander and 1. Perlman, Table of isotopes, 6th ed. (Wiley, New York, 1967).
50
1%7).
60
! !
A-LIGHT FRAGMENTS B-HEAVY FRAGMENTS
50
i
L-" 4 0 - E --
fo
~DEVIATION IS 4.59'0 OR 7 4 0 - - ~(/-~g/cm 2) -I
fl
o 3o
z
c¢
I
I
~ zo w
,o/j O O
I 0,4
0.8 t.2 Ni TARGET THICKNESS (rng/cm 2)
t
~.6
Fig. 12. Energy loss measurements for median-light and medianheavy fission fragments from 252Cf for nickel foils in the 0.2-2.0 mg/cm2 thickness range.
I I
l lt l_!
K
40
"1I
0
0
1.0 RELATIVE
2.0 POSITION
3.0 (cm)
Fig. 13. Target thickness profile for SSNi.
TARGET
THICKNESS
AND UNIFORMITY
609
MEASUREMENTS
60
r
/
\ I
l
J
I
J
45
J J
N
E o
E
i
v
w z v (J
t !
~ so
TARGET THICKNESS PROFILE FOR 6O,"i. THE MAXIMUM DEVIATION IS 26 % OR 13.9 m g / c m z
I
15
I.O
2.0 :5.0 RELATIVE POSITION (cm)
I
i
1
I
i
il
l
4.0
Fig. 14. Target thickness profile for 60Ni.
SESSION F
610
H.L. ADAIR
11) H. W. Schmitt, W. E. Kiker and C. W. Williams, Phys. Rev. 137, no. 4B (1965) 837.
ADAIR: NO, I did not reduce them to the Linhart; 1 did compare them with another source with which they were in excellent agreement (see text).
Discussion
NOGGLE: Would you comment on the counting times involved in generating data points by this method, both for alphas and for the fission fragment? Since you are counting over an area of 4 mm, you are talking about thickness variations that have been averaged out over a substantial area of the film already, ls this true ?
QUESTIONER UNKNOWN: Looking at your data plotted in figs. 8-10, it appears that maybe you are getting into a little trouble due to the variation and stopping power with particle energy. The points to the right tend to be above your straight line. ADAIR: There is a slight variation. I think all the variations are within the errors stated, however. GALLANT" With your thickness profile study of films, do you find a very great difference in the surface uniformity of evaporated films? I imagine you did quite a lot of work on this. H o w uniform are your evaporated films? ADAIR: It depends on many variables, of course. Normally, when we get a request for a target the user states how uniform it must be. This determines the source-to-substrate distance you will use; with a particular arrangement you can get practically any variation you want. We use the particular arrangement that will give us the variation within the range the customer specifies. Normally a value of plus or minus 5% can be realized, but better uniformities can be generated. MOORE: My question concerns the fission fragment range energy relations that you measured for nickel. I am curious to know how your results compared with the reduced range and other parameters that Linhart described. Did you reduce these to the Linhart ?
ADAIR: A 4 m m diam. area is a " s u b s t a n t i a l " area to you; you are an electron microscopicist, but our targets are normally 3/4 in. in diam. or larger. For a 4 m m diam. area of evaporated foil, there is very little variation. It all goes back to the initial setup. As to your first question, the counting time for these measurements was of the order of 3 h/foil. SLETTEN: HOW strong is the 25'2Cf source you have to use? ADAIR: 0.03 #g. SLETTEN: The mother source? ADAIR: The mother source was l0/~g. SLETTEN: HOW much californium do you have to have to make such a source in a reasonable a m o u n t of time? ADAIR: We made some measurements on self transfer from a 0.7 #g mother source. We transferred 0.007 #g/h ~o, for a heavier source, of course, you would transfer more. It depends on the area you would want to transfer to, but normally our transfer sources are of the order of 10/~g.