Use of Charged Particles from a 2-Megavolt Van de Graaff Accelerator for Elemental Surface Analysis OSWALD U. ANDERS Radiochemistry Research Laboratory, The Dow Chemical Co., Midland, Mich.
b A method originally introduced in 1957 by S. Rubin has been made accessible to practical analytical problems by introducing a semiconductor particle spectrometer for the collection of the data. Charged particles of 2-m.e.v. energy incident on a solid sample will penetrate the surface only to a shallow depth (2 mg./cm.2). They are thus well suited for surface studies, The particles will undergo elastic scattering and nuclear reactions followed by emission of detectable charged reaction products or gamma rays, From the energies of elastically scattered particles the mass of the scattering atoms in the sample surface can be determined by momentum and energy conversion. It could be demonstrated that this principle is applicable to both qualitative and quantitative analyses using the protons deuterons and He+ particles from the accelerator. As little as 9 kg./cm.z of silver and 15 pg./cm.* of chromium could be determined. Errors were in the range of 4%. The charged particle products induced in lowatomic number elements with deuterons were found to be suited for identifying the target element and their surface concentrations. Comparative studies involved samples of thin PVC-PVA copolymer films, films coated with thin lead deposits, chromate pickled magnesium pieces, plastics, metals, and rocks.
C
from an accelerator have been used for a long time now by nuclear physicists to investigate interactions with the nuclei of the atoms in the surface layers of thin and thick targets. The types of interactions are either simple elastic scattering of the type that Rutherford discussed in his famous paper of 1911 (IO)and developed further, as summarized by Rutherford and Chadwick in 1930 ( I I ) , or inelastic scattering, during which the target nucleus is excited to a higher nuclear state and some of the kinetic energy of the incoming particle is expended for this excitation, or nuclear reactions during HARGED PARTICLES
1442
ANALYTICAL CHEMISTRY
which the incoming particle loses its identity and a new particle or electromagnetic quantum is emitted from the interaction site. In 1957 S. Rubin showed in a classic article the principles and problems involved in the application of charged particle interactions to elemental analysis of surfaces (8,9). Rubin used a 2-M.v. Van de Graaff accelerator with beam selector and scattering chamber and a large movable analyzing magnet for measuring the momenta of the particles stemming from the interactions of the beam with the surface atoms of the targets. Momentum spectra of the particles emitted into a certain direction were obtained by stepwise increasing the magnetic field of the magnet. He collected only those particles, which would be bent through the magnet trajectory by that field for a time, during which a predetermined amount of accelerated beam particles impinged upon the target. I n lengthy experiments spectra with as yet unsurpassed resolution were collected, proving the capabilities of the technique. With the advent of the semiconductor charged-particle detector, it is now possible to apply charged particle surface analysis much more conveniently. During the past few years a few reports have been published on this new technique. Turkevich in 1961 reported on several experiments using alpha particles from plutonium-238 and curium-244 sources for surface analyses by measuring the energy distribution of the scattered particles with a semiconductor detector (IS). He followed this up recently with a more detailed report on a small alpha source analyzer, which might be of interest to the exploration of the lunar surface (6). Robertshaw and coworkers, a t the June 1962 meeting of the American ru’uclear Society ( I , 4) and Peisach (6, 7) at last year’s Conference on Modern Trends in Activation Analysis at Texas h & M, discussed experiments using accelerator sources and semiconductor detectors. The present study employed the particles available from a 2-M.v. Van de
Graaff: protons and deuterons of up to 2 m.e.v. snd 1.57-m.e.v. He+ ions. When using 2-m.e.v. protons one deals primarily with elastic interactions. One can, however, induce resonance scattering in some of the low atomicweight e1ements-e.g. carbon and boron, as well as (p,n) and ( p p ) reactions. Deuterons yield, in addition to elastic scattering, copious nuclear reaction interactions of the types. ( d , p ) , (d,n) and (d,a)with elements of lower atomic number. The energetics involved restricted the present work to elastic scattering interactions with the He+ beam. Only charged particle detectors were employed so that (p,n) and ( d p ) reactions were not observed. THEORETICAL
The semiconductor detectors used for this study measure the energies of the charged secondary particles stemming from the interaction of the beam particles with the surface nuclei. They collect an energy spectrum of the scattered particles rather than a momentum spectrum, which a magnetic analyzer would detect. The equations for the elastic interactions in the laboratory system are thus the classical equations of Rutherford : If Eo is the energy of the incoming particle and M is its mass, while m is the mass of the scattering nucleus and (p the scat,tering angle with respect to the incident beam, then we have E) as the energy of the scattered particle.
If l o is the number of incident particles per second, n the number of scattering nuclei per cc., and t the effective target thickness in cm., then the number of particles scattered through an angle (O and observed by a counter subtending the solid angle 0,in square radians, is given by :
cp sin' -
{cot
p
* Jcosec2
J
cosec2rp
rp
- E}' m2
- M'm2
2
(2)
Z is the charge of the scattering nucleus and z the charge of the incident particle. For the case of light particles scattered by heavy nuclei, expression (2) simplifies to :
(3) where u(,,) represents the crom section for elastic Coulomb scattering through the angle 9. If we substitute the value e = 4.8 X 10-10 e.8.u. as the charge of the electron and the conversion factor 1 m.e.v. = 1.6 X lo4 erg, we get
+ + +
end note that A B C D = 1. The energy for the light product is then :
Ea
=
E T B [COScp
*
d D / B - sin2912 (6) and the energy of the heavy product:
Table 1. Energies of Particles Elastically Scattered through 135'"
Energy Energy
Energy
of
He+ protons, deuterons, ions, m.e.v. m.e.v. m.e.v. 1.220 0.728 0.167 0.304 0.919 1.363 0.996 0.367 1.417 0.425 1.063 1.462 0.478 1.121 1.500 1.219 0.574 1.564 0.655 1.298 1.612 0.756 1.668 1.391 1 ,684 1.416 0.785 1.710 1.462 0.837 0.861 1.722 1.482 0.883 1.500 1.733 1.518 0.904 1 743 1.760 1.549 0.942 0.960 1.768 1.563 1.007 1.601 1.790 1.612 1.021 1.796 1 ,060 1.643 1.813 1.822 1.660 1.083 1.104 1.676 1.831 1.684 1.114 1.835 1.157 1.716 1.853 1.180 1.733 1.861 1.747 1.200 1.869 1.206 1.872 1.752 1.224 1.765 1.879 1.229 1,881 1.769 1,780 1.245 1.887 1.249 1.888 1.783 1.263 1.793 1.894 1.272 1.799 1.897 1.276 1 ,898 1.802 1.295 1.815 1.905 1.298 1.907 1.818 1,308 1.825 1.910 1.320 1.833 1.915 1.323 1.835 1.916 1.837 1.326 1.917 1.920 1 ,843 1.334 1.850 1.344 1.923 1.346 1.924 1.851 1.348 1,853 1.925 1.361 1 ,862 1.930 1.367 1.866 1.932 1.380 1.874 1.936 1.877 1.383 1.937 1.405 1.891 1.945 1.410 1.947 1.895 1,900 1.418 1.949 1.428 1.907 1.953 1.456 1.962 1.925 1.458 1.927 1.963 1.464 1.931 1.965 1.465 1.931 1.965 1.471 1.936 1.967 Incident energies: Protons: 2.00m.e.v. Deuterons: 2.00 m.e.v. He+ ions: 1.57m.e.v. of
I
0
of
with EOexpressed in m.e.v. For most of the work reported below the scattering angle was kept conveniently a t 135', and a computer program was able to evaluate the energies for the elastically scattered particles for this angle and a 2.0 m.e.v. (1.57 m.e.v. for He+) energy of the incident particles. Table I presents the results from this calculation. For interactions resulting in nuclear reactions the equivalent expressions for the energies of the emitted particles contain in addition the internal energy balance or "Q" value characteristic of the reaction, which is defined by:
Q = (Mi + Mz - Ma - MJC'
-e
(4)
Here M I is the mass of the incoming particle, M z the mass of the target nucleus, M S and Md the masses of the reaction products, and C the velocity of light. If the masses are measured in units of the atomic weight of C12 the "&" value is expressed in k.e.v. by multiplying the expression in parentheses by 9.3144 X IO6 k.e.v./ A M U . B is the value of the energy difference between the level in which the residual nucleus is left after the reaction and the ground state of the nuclide. We define the quantities encountered in the interaction formulas by means of a sketch: light
We have as the total energy available:
It is convenient to define according to reference 1%
E4
=
ETA[COSe
*
-
~ C / A sinlep (7) In case B 7 D we have
cpmsx =
sin+
and use the minus sign in Equation 6. In case A
> C we have e,
= sin-l.$
and use the negative sign in Equation 7. The relation of 0 to cp is given by:
Calculation of nuclear reaction data is facilitated by nomograms such as the one by F. J. M. Farley which is reproduced in references 8 and 18. In the case that one or more of the reaction products are left in an excited state less kinetic energy will be available for the product system and the quantity c-i.e., the energy of the excited level-will be greater than 0. It was again possible to write a computer program for Equation 6 to evaluate the energies of the secondary particles emitted at the 135' angle and calculate the energy groups of particles expected from the interaction of 2-m.e.v. deuterons with various elements. Some of these are listed in Table 11. The variation of the scattering yield with angle (Equation 2) would make it appear that the detector should be mounted preferably a t an angle less than 90' with respect t o the incoming beam. The greater dependence of the energy of the elastically scattered particles on the mass of the target nuclei (Equation I ) , however, makes the choice of an angle much greater than goo attractive for mass identification. The choice of 135' was a convenient compromise for our scattering chamber, permitting the proper positioning of collimators between target and detector, as well as allowing minimum cable lengths between detectv and preamplifier. In certain cases, where overlapping peaks are encountered in spectra from multi-element targets, it may be advantageous to measure the particles emitted at several additional angles to VOL 38, NO. 1 1 , OCTOBER 1966
0
1443
gain more information for their resolution and identification. It must be pointed out that for light target atoms deviations occur from Equation 2, which assumed scattering due to the Coulomb field only, since in such collisions the incoming particle approaches the target nucleus closely enough for nuclear forces to become effective. Large variations of the scattering cross sections are thus encountered, especially near the energies of resonances of the target nucleus. Similar rapid changes occur in the reaction cross sections at energies favorable for forming product nuclei in a certain excited state. By choosing the energy of the incoming particles properly the sensitivity for a certain element can thus be enhanced. Conversely this fact requires good energy resolution and energy stability for the accelerator system, if quantitative analyses are to be made. If, as is customary in Table II. Energies of Light Products from (d,p) and (d,a) Reactions"
Energy of protons, m.e.v 4.859 0.870 5.042 9.247 2.256 3.702 6.660 9.158 1.672 3.193 3.094 5.560 5.742 6.024 6.395 10.000 5.603 6.846 7.579 9.643 5.827 7.124 8.021 10.500 6.263 7.808 5.459 7.109 6.875 7.667 7.273 10.384 8.099 8.253 10.896 7.523 6.747 7.377 5.725 6.727 7,095
Energy of particles, m.e.v.
(I
9.551 6.875 4.389 11.309 5.602 0.054 4.211 10.304 6.333 3.190 4.193 8.770 3.251 6.733 2.823 6.983 6.848 2.536 6.374 4.001 8.314 5.516 5.766 8.612 8.264
...
5.604
...
8.731
... ...
... ... ...
9.188
..
Energy of incident deuterons: 2,OO m.e.v. Position of detector: 135' m t h respect to incident beam. States of residual nuclei: ground 8tate.b For exam les of particle grou s from reaction in w&ch the residual nucyei were left in an excited state see Figures 14, 15, and 16. 1444
ANALYTICAL CHEMISTRY
1 PUNCH I Figure 1. Schematic diagram of equipment used for charged particle scattering and reaction experiments
activation analysis, relative measurements are taken and standards used 89 comparators, the exact value of the interaction cross section is not required for quantitative analyses. It is, however, necessary that the standards and unknowns can be irradiated under identical conditions. Only particles scattered or emitted from the immediate surface layer of the target will have the energies calculated from the theoretical expressions given. Well defined energy groups of secondary particles represented by sharp peaks in the collected spectra, limited in width only by the resolution of the detecting equipment, are thus obtained only from elements occurring exclusively in the surface layer of the target-e.g., Figure 5. In the case of a relatively thick target (>20 micrograms/cm.2) the incoming particles will be slowed down as they penetrate through the surface layers and will have less than their incident energy a t the time when they interact with atoms situated in a deeper layer of the target. In turn, the secondary particles emitted in the direction of the detector will have to emerge through the surface and will lose additional energy while penetrating the outer layers of the surface. As a result, secondary particles scattered from deeper-lying nuclei of the same mass will arrive at the detector with less energy than those which were scattered from the surface. Particles scattered from an element occurring in a surface deposit of finite thickness will thus represent a peak in the spectrum which is broader than that due to the resolution of the detecting system-e.g., Figure 1Oc. The peak will have its high-energy edge at the position defined by above Equations 1
or 6; it will be flattened more or less, depending on the thickness of the surface deposit, and extend toward lower energies. For elements occurring homogeneously throughout a thick sampleLe., at least in a layer thicker than the maximum penetration depth of the incoming particles, from which a secondary particle can still reach the detector and be registered, the spectrum consists of a plateau with a sharp step at the high energy side. This step is at the position indicated by the Equations l and 6 mentioned. For a homogeneous material it has the shape of the convolution integral of the detection system resolution function, with the distribution of target atoms 89 a function of energy loss in the target (7). The plateaus of elastically scattered particles from elements distributed homogeneously throughout a sample are generally rather level for elastic scattering interactions. The theoretical explanation for this fact is given by Patterson et al. ( 5 ) . If the incident energy, however, exceeds that of a nuclear resonance level, lower-lying layers, for which' the incident particles have been slowed down exactly the right amount, will scatter more efficiently and there will be a hump in the characteristic plateau curve for this element (cf. Figure 12). Such a resonance hump must not be confused with a peak due to a surface deposit of an element of less atomic weight than the substrate giving rise to the plateau. In the case of nuclear reactions, in particular those induced by 2-m.e.v. deuterons, the reaction cross sections generally drop off so rapidly that these reactions occur practically only with target atoms lying in the vely surface
Figure 2. Photograph of fork-shaped sample holder layer of the sample. The “plateaus,” expected for homogeneously distrihuted elements in thick targets, will thus degenerate into non-Gaussian peaks with sharp high energy edges and more or less steep low energy dropoffs (cf. Figure 1%). iMonoenergetic charged particles lose energy rather uniformly as they penetrate through a homogeneous medium. Significant straggling or spread in energy between particles of an initially monoenergetic beam after penetrating a given thickness of matter occurs only near the end of the range of penetration. When a sample has a surface layer superimpos.4 on a homogeneous material, we observe the particles from the interaction with the deposit as described above. Particles interacting with the substrate, however, have energies degraded by their transition through the surface deposit and the scattered particles in turn have their energy further degraded when re-emerging through the samesurfacedeposit. Dependingon the thickness of the deposit, the plateaus characteristic for the homogeneous thicksubstrateand peaksfromsubstrate films, will he displaced toward lower energies (cf. Figure 1Oc and e.g. the oxygen peak in Figure 13). Themagnitude of this displacement can serve as a measure of the thickness of the surface deposit (film). The displacement of the plateau of the substrate will only occur in spectra that have peaks due to a surface film. The width of these peaks can also serve for an estimate of the deposit thicknes. If unaware, the experimenter may not recognize the displacement and assign the plateau t o an element of too low an atomic number. In case of a surface film of different material, the plateau due to the homogeneous substrate may not only be d i e placed toward lower energies, but the shape of its high energy edge may also be distorted. Such distortion is an indicsr tion of non-uniformity of the surface
film, which permits particles of different energy to interact with the topmost layer of substrate atoms. If the substrate consists of several elements, the high energy edges of the corresponding plateau steps will show the same distortions. If the substrate consists of several elements close to each other in the periodic table, the distortions may he strong enough t o wash out the plateau steps and thus make the spectrum of the substrate almost unintelligible. The same can occur with elements unevenly distrihuted throughout an infinitely thick (>400 micrograms/cm.*) sample. The shapes of the various features of the spectra collected with a semiconductor and presenbday electronics are also dependent on the counting rate. This factor is instrument dependent and variations come ahout due to pile-up of pulses (cf. Figure 17). It is in general desirable, for maximum energy resolution, to use most of the information supplied by the detector and thus employ long time constants for pulse shaping in the amplifier system. Single differentiation and RGlimited pulse shapes are preferred over doubly differentiated and delay-line shaped pulses. At higher counting ratm the amplifier will often still be processing one pulse while a second one is already arriving. If no precaution is taken, the two pulses will add and be recognized by the pulseheight analyzer as a single pulse lager than either of the two components. This effect will tend to lessen the highenergy slopes in the spectra and extend them toward higher energies, thus degrading the resolution and introducing a positive distortion. By choosing shorter time constants and double differentiation, shorter pulses are produced and quiescent conditions of the amplifier restored more rapidly, so that the danger of pileup is correspondingly reduced and its effects are minimized. Less of the available signal is used,however, and at low counting rates the resolution of this system is poorer than that of the slower system. EXPERIMENTAL
A schematic of the scattering chamber
with its differential pumping system is given in Figure 1. The equipment was designed so that samples outgassing during irradiation should not interfere with the vacuum system of the accelerator. Thedifferential pumpingsystenl consists of three orificeplates with 0.475em. I.D. openings, inserted into a 15.25om. I.D. pipe system. The first chamber formed by the oriiice plates is connected to a 1oM) liter/second oildiffu‘on pump with Freon-cooled chevron Baffles. A stainless steel cold trap, filled with liquid NI, serves as a cry+ static pump in the second chamber to condense vapors w a p i n g from the scattering chamber. The scattering chamber itself is equipped with a 5-cm. I.D. oil diffusion pump, rated at 100
Figure 3. Photograph of Scattering chamber top liters/second, with Freon-cooled chevron baffles and its own liquid NI cold finger. An automatic gate valve is inserted between the accelerator vacuum system and the first orifice plate, to separate the entire system from the accelerator, I t is interlocked with a vacuum gauge and closes as soon as the pressure in the first chamber rises above a certain level still safe for operating the accelerator. A second gate valve is inserted between the differential pumping system and the scattering chamber proper, to permit opening the chamber to the atmosphere without interfering with the differential pumping system. A third gate valve is placed between the &em. diffusion pump and the chamber, to permit opening the chamber without cooling down the pump. A rough vacuum is obtained in the target chamber through a pump-out with valve by means of a mechanical roughing pump equipped with its own liquid nitrogen trap. I t was found the hard way, that a rather good vacuum is required in the target chamber during the sample irradiations. If a poor vacuum is allowed to exist there, the beam will deposit silicon and carbon residues from the D.C. silicone pump oil on the target. These deposits originate from recoiling vapor molecules hit by the beam. The chamber itself is a larger, (30-em. diameter) modified version of the one described for semiconductor detector use by Feldl el al. (3). The sample holder consists of a stainless steel fork, onto which up to four samples can be mounted with 4-40 screws and washers (Figure 2). The fork is suspended from a cylinder of outer diameter larger than the maximum dimensions of the fork. The cylinder forms a seal with a compressed “0”-ring positioned in the center of the lid of the chamber. Positioning of the sample in the beam is accomplished by placing various-sized horseshoe shaped spacers between the seal screw and the top Aange of the cylinder (Figure 3). A water-cooled beam collimator is provided as a position 25 cm. from the scattering chamber proper. For quantitative work i t is essential that the total amount of charge striking the sample he measured accurately and reproducibly. The charge collected by VOL 38, NO. 11, OCTOBER 1966
1445
5.477 16.6
w.
1.5
Figure 5.
I .6 I.7 1.8 ENERGY OF SCATTERED PROTONS,
I .9 m.8.v.
2-m.e.v. protons elastically scattered from a thin film
Sample Lead coated film of polyvinyl chloride-polyvinyl acetate copolymer (VYNS). Angle of scattering: 135'. The dashed portions represent the changes in the spectrum when retaken after -me additional radiation exposure
5.30
5.40
5.50
5.60
E N E R G Y OF ALPHA PARTICLES,m.r.v. Figure 4. Spectrum of a-particles from thin AmZ4lsource taken with the surface barrier detector and the associated electronics of Figure 1 Counter was at room temperature.
Source to detector distance -1
an unprotected target represents that of the impinging beam particles minus the charge carried away again by those particles which are scattered back out of the target. The measurement is increased by the loss of negative charge due to secondary electrons knocked out of the sample by the beam particles. A second source of error stems from the buildup of an electrostatic field inside nonconducting samples, which may cause space charge losses and absorption of electrons from the ionization of the residual gas in the scattering chamber. Both sources of error lead to nonreproducibility of the beam measurement. The Faraday cup, seen in Figure 3, was thus mounted around the target holder to protect the target and collect any secondary charges emitted by the target during irradiation with the exception of the small fraction emitted into the direction of the detector and backscattered into the direction of the incoming beam. Holes are provided in the cup for the primary beam to enter and the scattered particles to escape toward the detector. The Faraday cup consists of a copper cylinder closed on the bottom. It has a horizontal slot half-way around the cylinder a t the height of the accelerator beam. A slide, consisting of a short piece of copper pipe with an internal diameter somewhat larger than the outer diameter of the cup, is provided with a 1.27-cm. wide slanted slot as well as a vertical slot. This slide is placed over the Faraday cup and 1446
ANALYTICAL CHEMISTRY
cm.
positioned by means of a set screw in such a way that the overlapping slots define the entrance hole for the beam (vertical slot) and the hole for the scattered beam to emerge toward the detector. Moving the slide up or down will change the position of the hole for the scattered beam to leave at a different angle. This cup is mounted onto the lid of the scattering chamber without electrical contact. The target holder assembly is also electrically insulated from the chamber body. Both target holder and Faraday cup are, however, electrically connected and serve thus together as beam-catcher. A current integrator is connected to the sample holder for measuring the total charge with which the target is being irradiated during a run. The counting equipment used consists of a silicon surface-barrier detector of 500-micron depletion depth and 25 sq. mm. area. I t was mounted with a bulkhead feedthrough on the wall of the scattering chamber behind a variable collimator defining the scattering angle. An ORTEC Model 109 preamplifier, using a field-effect transistor in its input stage, is connected with less than 5 cm. of cabling to the detector. The ORTEC-220 amplifier system of the design of F. S. Goulding, a detector bias supply run a t +lo0 volts and a precision pulse generator complete the detector system proper. The output of the amplifier is monitored by a RIDL Model 12B 400-channel analyzer, which is equipped with z,y-
plotter and fasbprinter readout. The analyzer is also able to provide the output for two scalers, the use of which is discussed below. The resolution of the detector system was tested with an A m 2 ' 1 source and found to yield the spectrum of Figure 4 with 16.6 k.e.v. FWHM resolution of the 5.477 m,e.v. peak. A singledifferentiation RC-pulse shaping with 1-microsecond integrating and differentiating time constants was used. Doubledifferentiation RC pulse shaping with 0.5 microsecond time constants was employed for much of the other work, to provide faster recovery and thus permit faster counting rates. The Amz4] resolution was measured as 19.8 k.e.v. FWHM for these settings. BEAM MEASUREMENT
Depending on the scattering yields of the target materials, wide variations of counting rates are experienced for the same average beam strength (cf. Equation 2). Correspondingly, the pulse height analyzer has different amounts of dead time from sample to sample, for irradiations with the same beam intensity. The irradiations, in terms of total integrated beam per run, must be extended by the fraction corresponding to the dead time of the analyzer throughout the run. An accurate evaluation of the fraction of pulses lost during analyzer dead time is possible, if the total number of pulses arriving from the amplifier is monitored with a fast scaler and compared to the number of pulses actually processed during the same period. Two scalers, one connected to the input discriminator circuit of the ADC of the analyzer and triggered by all arriving pulses meeting the acceptance specifications of the analyzer, the other counting- every pulse
I
I .s
Figure 6. elements
I
I
I
I .7 I .8 I .9 ENERGY OF SCATTERED PROTONS,m.r.v.
1.6
I
I
2.0
I .4
I
1 .I
1.6
I I .7
I
I
1.8
2-m.e.v. protons scattered from thick samples of
Plotinurn, silver, iron, manganese, titanium, silicon, magnesium, and surface oxldized graphite. Angle of scattering: 135'
Figure 8. Spectra of 2-m.e.v. protons scattered from thick samples of limestone (---------) and polymethyl methocrylate
-(
1 processed and stored in the 400-channel memory, permitted this evaluation of the dead time. During a run the two scalers were turned on simultaneously for an arbitrary time. The correction factor: Total number of pulses arriving at analyzer F= Number of pulses processed during this time could then be calculated, and the intended total charge per run multiplied by F . The run was then continued until the current integrator had collected this increased amount of charge. The increase represented the correction for the analyzer dead time. RESULTS AND DISCUSSION
1.5
.*
ENERGY OF SCATTERED PROTONS,m.r.v.
.s
1.6 I.7 1.8 I ENERGY OF SCATTERED PROTONS,m.r.v.
2.0
Figure 7. 2-m.e.v. protons scattered from thick multi-element samples Rock (Epidote-Quartzite-Microllne perthlte), gloss, josper, and quartz
Proton Scattering. A typical spectrum of protons elastically scattered from a thin film is seen in Figure 5 The target consisted of a polyvinyl chloride-acetate copolymer substrate of about 10 micrograms/cm.2 onto which a thin layer of lead had been deposited by vacuum evaporation. Another spectrum was taken after additional irradistion of the same target spot. A reduction in the relative size of the chlorine peak indicated the loss of chlorine by radiation damage of the target. Spectra of elastically scattered protons from thick samples of pure elements are seen as the typical plateau curves with sharp high energy cutoff in Figure 6. The spectra of carbon, magnesium, silicon, titanium, manganese, iron, silver, VOL 38, NO, 1 1 , OCTOBER 1966
1447
S
b
W
-I
Figure 9. 2-m.e.v. protons scattered from coated glass targets
4
$ 8
Silver deposited In different thicknesses on glass substrates (Spectra 2 and 3 are displaced vertically for clarity)
and platinum have their high energy cutoffs at the energies calculated in Table I. Spectra of homogeneous substances are formed by the addition of the partial spectra of the elemental components in proportion to their presence in the sample. Figures 7 and 8 show typical spectra of 2.0-m.e.v. protons scattered on glass, quartz, jasper, limestone, and polymethyl methacrylate. Figure 9 presents spectra of 2-m.e.v. protons scattered on glass substrates of
1.7
I .6
1.5
1.8
2.0
1.9
ENERGY OF SCATTERED PROTONS,m.r.v.
layers to be thinner than the resolution of the detector system. Quantitative results from this series of experiments are presented in Table 111. The various silver-coated glass pieces were exposed to the same number of protons and the spectra collected. The counts falling under the silver peak were summed and used as a measurement of the film thickness. The silver deposit was then dissolved off the glass with HNOl and the
4 Figure 10. 2-m.e.v. protons scattered from coated magnesium samples
0
I 1.5
I
I
I
I
I .6 1.7 I .e 1.9 ENERGY OF SCATTERED PR0TONS.m.r.v.
known area, onto which increasing amounts of silver had been evaporated. The sharp peak due to the thin silver deposit has the same FWHM resolution in curves a, 6, and c, indicating the silver
a 2.0
The samples had been pickled in a chromic acid bath for different lengths of time a. Untreated Mg metal b. M g treated for 1 mln. ta give thin deposit (-20 pg./cm.2) c. M g treated for - 1 0 min. to give thicker deposit (-1 40 pg./cm.*) (Spectra b and c are displaced vertically far clarity)
50
n
0 40 X
m 31
Table 111. Proton Scattering Analyses of Silver Coated Surfaces of Glass Slides rg. C/pcoul./
Ag/cm.2
C/rcoul.
9.1 22 22 39.8 39.8 45 45 49 49
54.39 115.05 129.74 218.45 217.68 250.90 250.50 268.09 269.66
2
0
3. 0
cm.-2
0
5.97 5.22 5.89 5.48 5.46 5.57 5.56 5.47 5.50 Av.: 5.57 & 0.23
In
pg.
(4. I%P
Error given is standard deviation of individual determinations.
30
0
20
\
I-
t
3
3
IO
I
l 20
l
I
I
I
1
1
1
1
60
ANALYTICAL CHEMISTRY
1
1
140
1
160
Figure 1 1 . Counting rote of particles scattered from chromium vs. chromium concentration in surface Sampler; chromate pickled magnesium
1448
1
80 100 I20 M I C R O G R A M S / SO. C E N T I M E T E R
40
4 Figure 12. Spectra of protons of various incident energies scattered from a thick sample of graphite Incident energy,
Spectrum
m.e.v. 1.5 1.7 1.8 1.9 2 .o 2.15
...-.-...-
-__-..--- - -
Figure 13. Charged particles emitted at an angle of 135' from a thick quartz sample (---------) and a leadcoated thin film of WNS )-( when irradiated with 2-m.e.v. deuterons
v
IO'
total silver in the solution analyzed by neutron activation analysis. The average amount of silver per unit glass area was calculated and correlated to the 3 counts of the silver peak in the corre0 sponding proton scattering spectra. *) Figure 10 represents a series of spectra of protons scattered from pieces of mag- $ nesium metal onto which various f amounts of chromic oxide had been 3 deposited in a pickling process. Curve a represents scattering from a piece of 5 magnesium metal without deposit. Curve b has a narrow veak at the vosition characteristic for 'chromium, indicating a thin deposit, and also a narrow peak due to the oxygen in the surface deposit, superimposed on the plateau of the magnesium substrate. Curve c is from a sample onto which a thick chromate film had been deposited. The peaks due to chromium and oxygen are very broad and the high energy edge of the magnesium plateau is considerably displaced. The coating is rather nonuniform as indicated by the decreased slope of the plateau edge for the substrate. Quantitative data from this series of samples are given in Figure 11. The
2
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0.9
1.1
1.2 1.3 1.4 1.5 PARTICLE ENERGY, m.0.v.
1.6.
1.7
1.8
1.9
6
b Figure 14. Charged particles emitted at an angle of 135" from a lead coated thin film of WNS when irradiated with 2-m.e.v. deuterons
Peak 1 2 3 4 5 6
7
a 9
Energy, m.0.v. 0.53 1.02 1.12
1.30 1.65 1.94 2.41 3.19 3.70
Reaction C1z(d,plC1a* C12(d,p)C1a* C'Yd,d'lC'' 016[d,d')016 CI Mla7(d,d')CIa6'a7 Pb %s(d,d'IPb me 0~,~)017* 01w,p)017 C1z[d,p)C1a
Energy level of residual nucleus, rn.e.v. 3.68 3.09 g.r. g.1.
g.r. g.r. 0.87 9.1. g.1.
I
2 3 PARTICLE ENERGY .m.r.v.
VOL 38,
NO.
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1 1 , OCTOBER 1966
1449
2.0
II 0
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Figure 15.
Charged particles emitted at an angle of 135' from thick samples ) when irradiated with 2-m.e.v. deuterons
of transistor grade silicon (--------- ) and quartz -(
Energy level of reriduai Energy. nucleus, Peak" rn.0.v. Reaction rn.e.v. a 1.30 O"(d,d')O" I.& b 1.56 Si28(d,d ')S12S 9.1. C 2.41 01~,~)017* 0.87 d 2.91 1 2 . 9 5 SizS(d,p)Sizg* 4.9314.89 e 3.19 O16(d,p)01' g.r. f 3.70 C1z(d,p)C** g.r. 9 4.15 Si2s(d,p)Sizs* 3.62 h 4.67 SizYd,p)Sllg* 3.07 i 5.27 SiPs(d,p)Sizg* 2.43 i -5.35 ? k 5.65 SIPYd,p)Si'g* 2.03 I 6.36 SizS(d,p)Slzg* 1.28 m 6.45 i n 7.05 ? 0 7.3 ? P 7.58 Sizs(d,p)Slzg g.r. f, i, and rn may be rurface contaminations, n and o may b e due to reronancer in the excitation curve for the Size(d,pISiz9ground rtate transition due to reactionr with Si*8 nuclei lylng below the sample surface.
-----___
counts under the chromium peaks were used for t'he correlation. The deposits from well-defined areas were scraped off the magnesium substrate with a razor blade and dissolved in HC1. The resulting solutions were diluted with HzO and their chromium concentrations were determined by atomic absorption. The average amount of chromium per unit area of substrate was thus obtained and used for the abscissa of the graph. The relatively large errors are due primarily to the unevenness of the deposited film and the losses during the scraping-off procedure. Resonance scattering of protons is demonstrated in Figure 12. A piece of graphite was irradiated with increasingly more energetic protons. The effect of the 1.77-m.e.v. level of carbon is felt in the spectrum for 1.80-m.e.v. protons. It becomes increasingly less important as the incident particle energy is further increased. Quantitative data can be 1450
ANALYTiCAL CHEMISTRY
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gained from the level high-energy p r tion above the resonance disturbance. (The piece of graphite used had a small surface contamination of adsorbed moisture, as is seen from the sharp little peak at the characteristic energy for protons scattered on oxygen nuclei.) Elastic Deuteron Scattering. Elastic scattering with deuterons yields spectra essentially similar to those obtained with protons, as is seen from Figure 13 for the thin VYNS film with lead deposit and the elastic deuteron scatter from quartz. The resolution obtainable is roughly the same as for protons, as seen from the corresponding proton spectra (Figures 5 and 6). Proton and alpha groups stemming from deuteron induced nuclear reactions are, however, superposed on the spectra of elastically scattered deuterons. Due to the higher mass of the deuterons, the energy transfer upon collision is greater, so that the dependence of the energy of the scattered particles on the mass of the scatterer is much greater than for protons. This permits higher mass resolutions throughout the range. Nuclear Reactions Induced by Deuterons. Figure 14 shows the entire spectrum of charge particles from the lead-coated VYNS film irradiated with deuterons. Above the region of elastically scattered deuterons we encounter proton groups of different energies from (d,p) reactions on carbon and oxygen, leaving the reaction products C18 and 0 '7 in either the ground states or excited states of various defined energies. Nuclear reaction products from deuteron irradiated thick samples of magnesium, aluminum, silicon, and quartz are seen in Figures 15 and 16. Although thick samples were employed, we ob-
serve peaks in these spectra. These peaks, as mentioned above, are due to the rapid decrease of the reaction crow sections for less energetic deuterons. They represent degenerate plateaus with trailing edges related to the shape of the excitation functions for the corresponding nuclear reactions. The maximum energy of elastically scattered particles is less than that of the incident particle beam. Reaction products from nuclear reactions having positive “Q” values, however, have energies higher than those of the incident particles, and the corresponding peaks lie above the beam energy in the spectrum. The peaks from exoergic nuclear reactions can thus be used for identifying a low-atomic-weight element present &s small fraction in a sample of high atomic weight elements. In the spectrum of elastically scattered particles such small amounts would not be observable, due to the high scattering yield from the heavier elements and the higher-energy particles scattered from them. The proton groups due to oxygen from the sample of surface oxidized lead seen in Figure 17 are thus readily recognized together with a small carbon deposit from the beam, while the plateau-step due to oxygen in the spectrum region of elastically scattered particles is entirely swamped out. There is one difficulty is using these peaks for quantitative determinations of the surface composition. This is the relatively small yield of the nuclear reactions induced with 2-m.e.v. deuterons and the relatively high yield of elastically scattered particles. This results, if no special precautions are taken, in relatively large amount of pulse pileup. This pile-up gives us an exponentially decreasing “plateau” toward higher energies, the relative height of
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Maximum energy of deuterons elastically scattered from lead Spectrum due to accidental summing of pulses at high counting rates 2.41 -m.e.v. protons from 016(d,p)017*reaction in oxidized surface Reduced spectrum due to occidental pulse summing at lower counting rate 3.1 9-m.e.v. protons from 01e(d,p)017reaction In oxidized surface 3.70-m.e.v. protons from C12(d,p)C18 reaction in a carbon-containing surface contamination of the rampie
which is dependent on the counting rate used for getting the spectrum. The pile-up “plateau” is of similar magnitude as the size of the peaks from the nuclear reaction products, and thus forms a
Figure 16. Charged particles emitted at an angle of 135’ from thick samples of aluminum (--------- ) and magnesium -( ) when irradiated with deuterons
Peak 0
b C
d e f 9
h i i k 1 2 3 4 5 6
7 8
5.93 5.55 5.31 4.82 3.70 3.58 3.46 3.12 8.26 7.19 6.39 5.84 5.47 3.99 3.20 3.0
5
Figure 17. Charged particles emitted at an angle of 135” from a thick sample of surface oxidized lead when irradiated with 2-m.e.v. deuterons
4
Energy, m.e.v. 6.85 6.81
4
Reaction AlZ7(d,p)Aiz8 AlZ7(d,p)AIz8* sloping plateau Alz7(d,p)AIz8* AlZ7(d,p)Al2** Alz7(d,p)Alzs* Alz7(d,p)AIz8* ClZ(d,p)C1a Alz7(d,p)AIz8* AiZ7(d,p)Al2** Alz7(d,p)AIz8* Mgzs(d,p)Mg2‘* MgzS(d,p)Mgz6* MgZ4(d,p)Mgz6 Me24(d,p)Mg2s Me“(d,p)Mg z 6 Mgz4(d,p)MgZs Me *‘(d,p)Mg Is
?
Energy level of residual nucleus, m.e.v. 8.S.
0.031 0.97 1.37 1.62 2.14 9,s.(surface contamination) 3.46 3.59 3.94 1.83 2.97 9.s.
0.58 0.98 2.56 3.40
background for which the quantitative data must be corrected (cf. Figure 17b). Elastically Scattered He+ Particles. He+ particles are copiously available from a RF-ion source of an accelerator. It is possible with special ion sources to doubly ionize helium and operate with an alpha particle beam, which would have twice the energy of the accelerator potential and would be even better suited for elastic scattering experiments than He+ of half the energy. Better mass resolution is obtainable with the more energetic particles. But He++ yields, even of the best ion sources, are rather poor and alpha particle beams are controlled with the same magnetic field strength as deuterons of half their energy, so that small amounts of residual deuterium in the vacuum system of the accelerator will cause serious interferences. The high Coulomb barrier encountered by He+ particles makes them very well suited for scattering experiments, as practically no nuclear reactions are induced with 2-m.e.v. He+ ions, and the spectra observed from scattering experiments are almost exclusively from elastically scattered particles. VOL 38, NO. 1 1 , OCTOBER 1 9 6 6
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Figure 18. sample (-
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Spectra of 1.57-m.e.v. He+ ions scattered through 135” by a thick quartz ) and a lead coated thin film of VYNS (----)
Figure 18 is a spectrum of elastically scattered He+ particles from a thin VYNS film with lead deposit. It is similar in structure to spectra of elastically scattered protons or deuterons except for a rather broad hump near 0.85 m.e.v. This hump is due to a Dz+impurity in the He+ beam. The singly charged D2+ion of mass 4.0282 m.u. are deflected almost as much by the magnetic field of the beam analyzer as the He+ ions of mass 4.0260 m.u. On scattering, the Dz+ molecules break up and the d elastically scattered into the detector behave like deuterons from a poorly defined incident deuteron beam of the energy-Le., 0.8 m.e.v. The hump thus represents the scattering of these deuterons from the lead coated film. This could be shown experimentally with a 1.53 m.e.v. D2+beam scattered from the same film. Flushing the accelerator system with helium for a p proximately 1 hour did not eliminate the affect. Because of the low energy of the scat-
1452
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0.e 0.9 1.0 1.1 ENERGY OF SCATTERED. HI+, m.u.v.
ANALYTICAL CHEMISTRY
tered He+ particles and the high ionization density of He+ particles in the silicon of the detector, the resolution for the He+particles scattered from the thin lead layer is only 23 k.e.v. FWHM. The higher rate of energy loss of He+ ions in matter, as compared to p and d, results also in broader peaks for films of finite thickness in the spectrum of scattered He+, as compared to scattered protons and deuterons. The measurement of the thickness of very thin films and deposits is thus carried out preferentially with He+ ions.
Franzgrate, E. J., J. Geophys. Res. 70, 1311 (1965). (6) Peisach, M., Poole, D. 0. :‘Modern
Trends in Activation Analysls,” Proceedings of 1965 Intl. Conf., . 206, Texas A&M Univereity, College &&ion, Texas, 1965. (7) Peisach, M., Poole, D. O., J. S. African Chem. Inst. 18, 61 (1965). (8) Rubin, S., Passel, T. O., Bailey, L. E., ANAL.CHEM.29, 736 (1957). (9) Rubin, S., “Ion Scattering Methods,” Part I, Section D-1, Chapter 41,: “Treatise on Analytical Chemistry, Kolthoff and Elving, eds., Interscience, New York, 1959. (10) Rutherford, E., Phil. Mag. 21, 669 / , A l l \
(lYll).
(11) Rutherford, LITERATURE CITED
(1) Buechner, W. W., Robertshaw, J. E., Trans. Am. Nucl. SOC.5, 197 (1962). (2) Farley, F. J. M., Nucleonics 12, No. 10, 56 (1954) and erratum Ibid., 13, No. 7, 67 (1955). (3) Feldl, E. J., Meriwether, J. R., Choppin, G. R., Fox, J. D., Nucl. Instr. Methods 22, 333 (1963). (4) Green, F. L., Cooper, M. D., Robertshaw, J. E., Trans. Am. Nucl. SOC.5, 197 (1962). (5) Patterson, J. H., Turkevich, A. L.,
E., Chadwick, J., Ellis, C. D., “Radiations from Radioactive Substances,” p. 244, Cambridge Univ. Press, Cambridge, Eng. 1930. (12) Smith, D. B., Jarmie, N., Seagrave, J. D., Los Alamos Scientific Lab. Document LA-2424and TID-4500,1961. (13) Turkevich, A. L., Science 134, 672 (1961).
RECEIVEDfor review June 10, 1966. Accepted August 1, 1966. Work presented in part at the Fifth National Meeting of the Societ for Applied Spectroscopy, Chicago, Id, June 13-17, 1966.
