about half over this time period. The areas of the 1s peaks, expressed as the Na ls,’F 1s ratio per atom, decreased with NaBF4 from 2.94 to 2.74 and with NaF from 2.04 to 1.73 over this period. The direction of change is that expected, with Na 1s having the lower kinetic energy and the lower escape probability, but the effect is too small to explain the large difference in the ratio between the compounds. Mounting each sample without Scotch tape on a roughened stainless steel sample cylinder and heating at 200 “ C did not change the ratios for the compounds by more than 57,. Heat treatment neither drives off a surface contaminant nor changes the elemental composition of the surface layers. The ratios seem to be characteristic of the materials themselves. There must be an inherent difference in probability for energy loss by an electron, either by collective excitation or in “shakeup” or “shakeoff” processes.
This variability in response among compounds makes it doubtful that the technique is capable of better than semiquantitative application on any routine basis. Nevertheless, its applicability to analyze the surface of insulating materials, or surfaces easily destroyed by electron beams, such as those beams used in the electron probe apparatus, makes it of considerable potential value for elemental analysis. ACKNOWLEDGMENT
The author is pleased to acknowledge helpful discussions with T. Novakov and J. W. Otvos of Emeryville Research Centre, and with M. S. de Groot, P. Biloen, and C. la Lau of Koninklijke/Shell-Laboratorium, Amsterdam. RECEIVED for review July 26, 1971. Accepted December 21, 1971.
~
Detection of Chlorine on Aluminum by Means of Nuclear Reactions A. R. Knudson and K. L. Dunning Nacal Research Laboratory, Washington, D.C.20390
IDENTIFICATION AND MEASUREMENT of thin films on various surfaces is a problem which is important in many areas of science. In the course of corrosion studies at this laboratory, it became important to be able to measure small quantities of chlorine on aluminum surfaces. A number of methods of analysis, all involving nuclear reactions induced by the bombardment of specimens with positive ions from a 5-MV Van de Graaff, were investigated ( I ) . In these methods no preparation of the sample is necessary. The only limitation on the samples is that they must be of a size convenient for mounting in an accelerator target chamber. By the use of a beam of small spatial extent, it is possible to measure chlorine content as a function of position on the aluminum surface. These methods do not alter the sample as long as heating by the incident ion beam does not cause evaporation of the sample constituents into the accelerator vacuum system. A further limitation under some conditions is the fact that the incident ions cause some damage to any crystalline lattice structure that may be present. Also, if the substrate is thicker than the range of the incident ions, they will be implanted into the substrate. Studies were made both of radiation emitted while the sample was being bombarded and radiation emitted by induced radioactivity. Both photons and particles were detected. The most promising of the methods tested will be described here. EXPERIMENTAL
The ion beam from the Van de Graaff was deflected through a 55’ angle by an analyzing magnet to ensure that no undesired components were present and then focused by either an electrostatic or magnetic quadrupole lens. In most cases, the beam was not focused to the smallest spot possible in order to reduce localized heating of the specimen. Small beam currents were also used to reduce heating effects and possible deterioration of the target. (1) 0 . U. Anders, ANAL.CHEM., 38, 1442 (1966).
3.89
37CI* d - p
37.2 min 38
3 8100
CI
p - , 3 6 % , I I1 Mev 2.1677
I
P-,
0.06
1.
I
I /
-p-,
1 L -J4 . 9 2
II %
,2
77 Mev
5 3 % , 4.90Mev
38
Ar
Figure 1. Decay scheme of 38C1 Signals from the various detectors were routed through appropriate amplifiers to an analog-to-digital converter which was interfaced to a data acquisition and processing system developed around an SEL 840A computer. All pulse height spectra were recorded on magnetic tape for possible additional analysis in the future. Samples for use in the present studies were prepared by the evaporation in a vacuum of NaCl or BaCh onto various substrate materials. The angular distribution of 3.0-MeV 4He ions elastically scattered by chlorine was determined to obey the Rutherford law and the cross section predicted by the Rutherford law was then used to determine the amount of chlorine on the various samples. Method I. An attractive method for detecting traces of chlorine on or near the surface of aluminum is based on the observation of y rays emitted in the decay of 38Clformed in the 37Cl (d,p)WI reaction. Chlorine consists of 75.53 35C1and 24.47x 37Cl. Figure 1 indicates the decay scheme ( 2 ) for 38Cl. For every 100 decays, 38Clproduces thirty-six (2) P. M. Endt and C . Van der Leun, Nircl. Phys., A105, 1 (1967). ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, M A Y 1972
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5 z
1
.B .... *... ..... ....
y-RAY ENERGY ( 5 ” x 5” N a I )
Figure 4. This is a two parameter spectrum obtained by requiring coincidence between a 3-in. X 3-in. and a 5-in. X 5-in. NaI crystal 0‘ I .5
1.7 I .9 2.1 GAMMA-RAY ENERGY ( M e V )
2.3
Figure 2. Gamma-ray spectrum obtained with a Ge(Li) detector from a sample containing 2 pg/cm2 of chlorine, after irradiation with 5.0 M e V deuterons The notation s.e. indicates a single and d.e. a - escape - peak double escape peak 10,000
750C J
z
W
f U I
5a
5000-
v,
z
8
2500-
0
200
4 00 600 CHANNEL NUMBER
800
I1
Figure 3. Electron spectra for aluminum with no chlorine and for tantalum with 40 pg/cm2 of chlorine on the surface Essentially all of the electrons from the aluminum fall below channel 400
1.642-MeV y rays, forty-seven 2.168-MeV y rays, and fiftythree electrons with an end-point energy of 4.92 MeV as well as other electrons with smaller end-point energies. The 37.2-minute half-life is convenient in that i t allows the start of counting to be delayed until the intense **A1activity with its 2.27-minute half-life has decayed to a tolerable level. The elements carbon, nitrogen, and oxygen, which are generally present, provide no interference with the observation of the 38Cl decay modes. However, studies performed using 125-p thick commercial aluminum foil indicated considerable interference with the chlorine y-ray spectrum as observed with a NaI(T1) detector. Later measurements with a Ge(Li) detector indicated the presence of manganese and gallium in these commercial foils. The cross section for production of 38Clwas determined by using a target consisting of BaC12evaporated onto a thin (about 15 pg/cm2)carbon foil. Elastic scattering of 3.0-MeV 4He ions by chlorine was used to determine the areal density 1054
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
Each channel that contains between 26 and 26 counts is presented at the minimum intensity levels. Each channel containing between 26 and 2’ counts is presented at the next intensity level and so on. The “islands” marked A are due to the decay of 38CIand those marked B are due to the decay of 24Na
of chlorine on the target. The target was then bombarded with 5.0-MeV deuterons and the resulting y-ray activity measured by using a 5-in. X 5-in. NaI(T1) well-type crystal. Photopeak efficiencies for this crystal were estimated by using previous data (3, 4 ) for large well-type crystals in conjunction with a measurement on the present crystal with a 6OCo source of known intensity. It was determined that the cross section for producing 38Clis 100 i 20 mb. Although impurities in the aluminum prevented the use of this method with a NaI(T1) detector except at high chlorine concentrations, the high resolution obtainable with a Ge(Li) detector circumvented this problem. Figure 2 shows a spectrum obtained by bombarding a 25-p A1 foil onto which had been evaporated 2 pg/cm2 of C1 in the form of NaCl. The specimen had been bombarded for 38 minutes with a 37-nA beam of 5.0-MeV deuterons. After a delay of 38 minutes, the sample was counted for 21 minutes with the sample on the window of a 30 cm3 Ge(Li) detector to obtain the spectrum shown in Figure 2. Method 11. The possibility of detecting the high energy electrons emitted by the 38Cl was also explored. A cylinder of NE-102 plastic scintillator 1 inch in diameter by 1.25 inches long mounted on a photomultiplier tube was used as a detector. Figure 3 shows two pulse height spectra obtained for electrons from activated specimens. One is for aluminum with no chlorine on the surface and the other is for tantalum with 40 pg/cm2 of chlorine on the surface. Tantalum was used to provide a situation in which only the chlorine activity would be present, since the Coulomb barrier of tantalum is essentially impenetrable ( 5 ) to a 5.0-MeV deuteron. Also 1s2Ta emits only low energy electrons with an end-point energy of 0.52 MeV. Figure 3 indicates that the aluminum sample does not emit high energy electrons, while the chlorine sample does. In both cases, the samples were irradiated for 31 minutes with a 1-pA beam of 5.0-MeV deuterons and then counted after a delay of about 20 minutes. (3) L. J. Colby, Jr., and J. W. Cobble, ANAL.CHEM.. 31,798 (1959). (4) J. H. Neiler and P. R. Bell, “Alpha-. Beta-, and Gamma-Ray Spectroscouy,” K. Siegbahn, Ed., Vol. 1. North Holland, Amsterdam, 1968, p 245. (5) J. B. Natowitz and R. L. Wolke, Phys. Rec;., 155, 1352 (1967).
Table I. Summary of Experimental Conditions
Length
Integrated Count- Amount irradia- beam Delay ing of tion, current, time, time, chlorine, min pC min min pg/cm2 of
Method I I1
I11 IV
38 31 60 60
84 1800
144 160
38 20 35 O
Detection limit, pg/cm2
21
2
0.3
10 27 b
40 8 0.2
a 0.2 0,015
a A detection limit was not determined for this method for reasons given in the text. b
Figure 5. Spectrum of 3.0-MeV 4He ions elastically scattered from an aluminum foil with 0.2 pg/cm2 of chlorine on the surface The two barely resolved peaks on the right are due to 35Cland 37Cl
Counting was done simultaneously with irradiation.
23 keV for 5.48-MeV a particles from an 241Amsource. This detector subtended an angle of 3” in the reaction plane producing a kinematic energy spread of 11 keV for 4He ions scattered from chlorine. RESULTS AND DISCUSSION
Since the energy of the scattered 4He ion is a function of the mass of the scattering nucleus, the energy spectrum of the scattered particles can be used to investigate the nature of the specimen from which they have been scattered. Figure 5 shows the energy spectrum of “e ions, whose initial energy was 3.0 MeV, after being scattered through an angle of 150” by a specimen consisting of 0.2 pg/cm2 of chlorine on a 25-p aluminum foil. This spectrum was obtained in 1 hour using a beam of about 45 nA for a total integrated charge of 160 pC. The peaks due to 35Cland V l are just barely resolved. The resolution of the surface barrier solid state device used to detect the 4He ions was about
The experimental parameters and results for the methods investigated are summarized in Table I. Calculations based on the y-ray spectrum of Figure 2 yielded a detectability limit of 0.3 pg/cm2 of chlorine. This was obtained by requiring that the area of the peak integrated from one half-maximum point to the other should be at least three times the standard deviation of the background integrated over the same interval. A spectrum obtained with a specimen having 0.2 pg/cm2 of chlorine showed definite evidence of chlorine y rays after counting for 60 minutes, but obviously was very close to the minimum detectable density. Unfortunately, the electron spectrum observed in the decay of W l is rather featureless and it is difficult t o ensure that no unforeseen contaminants are contributing to the spectrum. The rate at which the intensity in the upper portion of the spectrum decreases with time could be measured to determine that it has the proper half-life, but this is difficult t o do with any accuracy when only trace amounts of chlorine are present. This was judged to be the least practical method discussed here of determining chlorine on aluminum. The technique of requiring a coincidence between two y rays allows one to detect amounts of chlorine down to about 0.2 pg/cm2. This method requires a more complex electronic configuration than any of the other procedures discussed here and accordingly requires more time for set-up and testing. The use of 4He elastic scattering is the simplest, quickest, and most sensitive of the methods described here. The present measurements indicate a detectability limit of 0.015 pg/cm2of chlorine for a 160-pC bombardment of the sample. However, the effectiveness of this method would be greatly reduced if the chlorine were not present as a surface layer, but instead had diffused several microns into the aluminum, so that the 4He would have lost energy before scattering from the chlorine. The other techniques would not be so severely affected since there the energy of the detected radiation is not affected by the energy of the bombarding particle. Only the cross section would be affected and this is not a sensitive function of energy for the 37Cl(d,p) W l reaction.
( 6 ) J. B. Marion and F. C . Young, “Nuclear Resction Analysis Graphs and Tables,” North Holland, Amsterdam, 1968, p 142.
RECEIVED for review September 17, 1971. Accepted December 7, 1971.
Method 111. It was felt that y-y coincidence measurements might be effective in detecting chlorine since any products of the activation which decayed by processes other than the emission of cascade y rays would be excluded by this method. Also, since the spectrum is spread out in a two-dimensional space, there should be less chance of various cascade y-ray emitters interfering with each other. In these measurements, the irradiated foil was placed between a 5-in. diameter by 5-in. long and a 3-in. diameter by 3-in. long NaI(T1) crystal. The time resolution of the system was about 50 nsec. In the computer-generated display, y-ray energy in the 5-in. X 5-in. detector is represented by distance along the horizontal axis and y-ray energy in the 3-in. X 3-in. detector by distance along the vertical axis. The number of counts is indicated by the intensity of the dot. The sample contained 8.0 pg/cmZof C1 and was bombarded for 60 minutes with a 40 nA beam of 5.0-MeV deuterons. After a delay of 35 minutes, the spectrum shown in Figure 4 was accumulated in about 27 minutes. Method IV. With the purpose of finding a prompt technique for the detection of chlorine on aluminum, 4He elastic scattering was investigated. The energy of 4He ions of initial energy E,, scattered through an angle $ by nuclei of mass M (in amu), is given by (6)
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