Neutral fractions of proton beams backscattered from thick C and Au targets and transmitted through thin C, Si, and Au foils were measured in the energy range 100-310 keV. Proton was more neutralized in the case of the Au target than the C and Si targets. An emergence angle dependence was not found within the experimental uncertainty. Effects of the surface contamination were discussed and the thickness of the surface contamination was estimated to be about 10 A

FUKUZAWA, Fumio

TSUMAKI, Kouji

English

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RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:Neutral Fractions in Proton BeamsPassing through SolidsTSUMAKI, Kouji; FUKUZAWA, FumioTSUMAKI, Kouji ...[et al]. Neutral Fractions in Proton Beams Passing through Solids.Memoirs of the Faculty of Engineering, Kyoto University 1979, 41(3): 268-2751979-10-31http://hdl.handle.net/2433/281108268 Neutral Fractions in Proton Beams Passing through Solids By Kouji TSUMAKI* and Fumio FUKUZAWA* (Received February 14, 1979) Abstract Neutral fractions of proton beams backscattered from thick C and Au targets and transmitted through thin C, Si, and Au foils were measured in the energy range 100-310 keV. Proton was more neutralized in the case of the Au target than the C and Si targets. An emergence angle dependence was not found within the experimental uncertainty. Effects of the surface contamination were discussed and the thickness of the surface contamination was estimated to be about 10 A. 1. Introduction Ion solid interaction is a very interesting field of research. Some information about ion solid interaction can be obtained by measuring the charge states of the ion passing through solids. Proton is adequate as a projectile, because it is the simplest and most fundamental atomic system. Only one electron transfer process is necessary to be taken into account in the high energy region. Furthermore, fractions of ions do not change after emergence from solids in contrast to the case of heavy ions with many electrons. 1 i The charge changing collision of a proton with an isolated atom is fairly well described theoretically.2i However, knowledge about electronic states of a hydrogen atom in solids is far from sufficient. Recently, several theories were presented about the charge exchange mechanism of protons passing through solid materials. They are divided into two categories: One is that the surface has an important role. The other is that the surface plays a role only as a boundary. Theories of Trubnikov and Yavli-nskii3l and of Kitagawa and Ohtsuki come into the first category,'i while theories of Cross5i and of Brandt and Sizmann6i-9i come into the second category. Phillips studied the charge fractions of proton beams in the energy range 3-200 * Department of Nuclear Engineering Neutral Fractions in Proton Beams Passing through Solids 269 keV after passing through thin foils which were freshly coated by evaporation in a scattering chamber.101 He found the target material dependence of the charge frac-tions, and pointed out that the material independence which had been reportedm was due to surface contamination. In the present experiment, the neutral fractions of backscattered particles from thick C and Au targets and those of transmitted particles through C, Si, and Au foils were measured in the energy range 100-310 keV, and the medium dependence of the neutral fraction was studied. The emergence angle dependence of the neutral fraction was also measured. The effects of surface contamination are discussed quantitatively. 2. Experiment The charge state distributions were measured by the backscattering method as well as the transmission method. Experimental arrangements are shown schematically in Figs. l and 2, respectively. The tt+ beam from a Cockcroft-Walton accelerator was used as projectile. WBEAM I-- --I,. LIQUID·N2·COOLED·SHIELD I 0.58m I 0.23m / ' ' TARGET l 2-D-SLI T l \ w A.'- 3* 2-D-SLI yV _.. - t 9_ 55D SLIT o 2-D-SLIT --~ DEFLECTOR~ ~ i J SSD Fig. I Arrangement for the backscattering experiment. H•BEAM > 2-D-SLIT r--o.ssm--fo.23m+o.23m+-o.1amj ' '~'= □ ~ i 0'oi c::::J SSD 2-D-SLI T~ARGETSSD DEFLECTOR 2 D SLIT LIQU D-NrCOOLED·SHIELD Fig. 2 Arrangement for the transmission experiment. 270 Kouji TSUMAKI and Fumio FUKUZAWA In the case of the backscattering experiment, the 250 keV proton beam was collimated to 1mm X 1. 5mm with two 2-dimentional slits. The 90° backscattered parti-cles from thick C and Au targets mounted on a goniometer were selected in their direction with two 2-dimentional slits and were detected with a Si surface barrier detector. In this way both neutral and charged particles were detected. When the electric deflecting field is applied just in front of the detector, only the neutral particles can be detected. From these two measurements the neutral fraction was esti-mated. Another Si surface barrier detector was mounted in a scattering chamber as a beam monitor. Since backscattered particles have various energies according to the scattering angle and pass length in solids, the charge fraction over a wide range of energy below the projectile energy can be obtained at one time.12 > In the transmission experiment, the 100-310 keV proton beam was collimated to 0. 4mm X 0. 4mm with two 2-dimentional slits. The transmitted beams through C, Si, and Au foils were detected after collimation with a 2-dimentional slit, with or without an electric deflecting field, just the same as in the case of the backscattering method. The foil was mounted on a target holder which had an aperture of 2. 5mm in diame-ter and was tilted to measure the emergence angle dependence of the neutral fraction. There is a possibility that the particles are scattered from the holder edge resulting in a change of the measured neutral fraction. The beam course was carefully set so that the collimated beam would pass through the center of the target. The residual gas pressure in the scattering chamber was l-2X 10-s Torr. In the case of the backscattering experiment, the Au target was evaporated on a thick Au substrate just before the measurement, so as to make surface contamination as small as possible. The following procedures were done to avoid a carbon build up and to reduce gas adsorption. The targets were heated up to more than 300'C and surround-ed by a liqiud nitrogen cooled surface. 3. Results and Discussion The neutral fractions of the backscattered and the transmitted particles are shown m Fig. 3 for the Au, Si, and C targets. There is no difference between the results of the two different methods. The neutral fractions of the Si and C targets are almost equal, while those of the Au target become larger than those of the Si and C targets at higher energies. If the description is possible in terms of the capture and the loss cross sections in solids, the neutral fraction can be described as follows, Cross5> used the gas phase cross section to predict the charge states of protons in Neutral Fractions in Proton Beams Passing through Solids <Pllo) 20 z 0 ~ u <( a: u.. 10 .....J <( a: ~ ::> UJ z 0 100 BACKSCATTERED FROM Au(e), c(A) TRANSMITTED THROUGH Au(O),Si(• ),C(A) 200 300 E NERGY(keV) Fig. 3 Neutral fractions of proton beams backscattered from Au Ce) and C (A) targets and transmitted through Au (0) Si (X) and C (6) foils. Solid curves are drawn for eye fit. 20 z 0 1-u <( a: u.. .....J <( a: I-::> w z 1 0 O 100 C TARGET -- GAS MODEL ---- CORRELATION MODEL 200 ENERGY(keV) 300 Fig. 4 Neutral fraction by the C target compared with the gas model (solid line) and the correlation model (dashed line). 271 272 Kouji TsUMAKI and Fumio FUKUZAWA solids. In the case of the C target, the neutral fractions which were obtained from gas phase cross sections3> are shown in Fig. 4. They are in good agreement with the experimental results. Brandt and Sizmann gave the following expressions for the cor-relation gain and loss of electrons in solids,9> • 21• z~ a,=1rao5 V&(V2+ 2a Z1419)3 \/40 2 2 Zf3 4Zf3 (Z2+ l) a,=1ra0 Zf3+V 4Zf3 (Z2+l)+V ' 0. 3MeV<EP ( 2) where Z2 is the target atomic number, a0 is the Bohr radius, and Vis the proton velocity. There is no expression below 300 keV. Application of these cross sections in expression ( l) gives the results shown by the dashed curve in Fig. 4. They are slightly larger than the experimental results. Fig. 5 shows a comparison of the experimental results of Phillips and our methods as well as the calculation based on the cross sections of Brandt and Sizmann. The calculated values are slightly larger than the experimental values. Those by Phillips are also larger than the present results. The reason is considered as follows. Phillips 20 z 0 ~ u <( a: lL. _J <( a: ~ :::, LLJ z 10 Au TARGET PRESENT(e) PHILLIPS(o) .. __ o-...&.... __ __... ___ .__ __ _.__ _ __.___. 100 200 300 ENERGY(keV) Fig. 5 Neutral fraction by the Au target compared with the correlation model (dashed line). Solid lines are estimated neutral fractions by the clean surface by assuming that the present results are obtained with the 5 A, 10 A and 15 A carbon contaminated layers. Neutral Fractions in Proton Beams Passing through Solids 273 measured the neutral fractions just after evaporation and when the target surface was fairly clean. However, the procedure of the surface cleaning was not done in the present experiment. Hence, there is a possibility of surface contamination, which affects the neutral fraction. We can estimate the thickness of the surface contamination as follows. The neu-tral fraction <P.w after passing through contaminated layer of thickness x becomes ( 3) where <P., is the eqilibrated neutral fraction by the contaminated material, <P •• is that by bulk material, a, and a, are the cross sections in the contaminated material, and n is the atomic density of the contaminated material. If the contaminated material is assumed to be carbon, the <P •• by a clean Au target can be calculated. In this way, <P.. by an Au target with a 5 A, 10 A and 15 A carbon contaminated layer was calculated and shown in Fig. 5. When x= IO A, the values of the neutral fractions agree with those of Phillips. If the results of Phillips were obtained with no contami-nation, the surface contamination in the present experiment should be 10 A in thick-ness. A different method of estimation of contamination layer is to measure the emer-gence angle dependence of the neutral fraction. If the emergence angle dependence does not exist under a clean surface condition,* the appearance of an emergence angle dependence should be due to the surface contamination, because the path-length of the beam in the contaminated layer changes with the emergence angle. The dependence of the neutral fraction on the emergence angle is given as follows, (4) Emergence angle dependences of the neutral fraction of transmitted and backscat-tered particles are shown in Figs. 6(a) and (b), respectively. The values of the neutral fraction of the () emergence case are normalized by those of () = 0° ( transmitt-ing) and 0=20° (backscattering). Experimental accuracy of the backscattering tech-nique is poorer than that of the transmitting technique due to the difficulty of an ex-perimental setup. The emergence angle dependences were not appreciably found in any case. The calculations with eq. (4) are shown in Figs. 6(a) and (b) in the cases of the carbon contamination layer of 5 A, 10 A and 20 A on the gold target. Considering the experimental errors, we concluded that the contaminated layer was less than 10 A. * Theories of Brandt and Sizmann and of Cross do not show at all the emergence angle depend ence if the solid surface is clean. The theory of Ki,agawa and Ohtsuki shows only a weak dependence on the emergence angle in the present energy region. 274 Kouji TSUMAKI and Fumio FUKUZAWA I. 5 ,__ __________________ __, 1.0 • 207 keV C 094 keV C "200keV Si "' 9, "208keV Au • 97 keV Au 10 20 30 40 9 (deg) 50 Fig 6(a) Emergence angle dependence of the neutral fraction measured in transmission experiment. Solid lines are the calculations with the contaminations of 5 A, 10 A and 20 A thicknesses. 1.5 ,-----------------. 1.0 • 190keV C oJ00keVC I • 200keV Au • 1 00keV Au 5.4 10~ 20A 0.5.___2.i...0 _ ___.__---1. _ ___,~_.______. 30 40 50 60 0(deg) Fig. 6(b) Emergence angle dependence of the neutral fraction measured in backscattering experiment. Solid lines are the calculations with the conbaminations of 5 A, 10 A and 20 A thicknesses. From the above discussions on the medium dependence and the emergence angle dependence of the Au target, we can conclude that about a 10 A contaminated layer exists on the Au surface. Neutral Fractions in Proton Beams Passing through Solids 275 Acknowledgement The authers would like to thank Prof. M. Sakisaka and Messrs. M. Tomita, Y. Kido, M. Asari, A. Ito and T. Ogawa for valuable discussions and Messrs. Y. Kana-mori, S. Ando, K. Ohnishi and Y. Haruyama for helping with the experiment. The cooperative assistance of our technicians, N. Norisawa, Y. Yoshida and T. Kanazawa, in maintaining the accelerator and the associated instruments is also acknowledged. References I) H. D. Betz; Nucl. Instrum. & Methods. 132, 19 (1976). 2) R. A. Mapleton; Theory of Charge Exchange (Wiley Interscience, New York, 1972). 3) B. A. Trubnikov and Y. N. Yablinskii; Sov. Phys. -JETP 25, 1089 (1967). 4) M. Kitagawa and Y. H. Ohtsuki; Phys. Rev. B 13, 4682 (1976). 5) M. C. Cross; Phys. Rev. B 15, 602 (1977). 6) W. Brandt and R. Sizmann: Phys. Lett. 37A, 115 (1971). 7) W. Brandt and R. Sizmann: in Atomic collisions in solids, (Plenum Press, edited by S. Datz, B. R. Appleton, and C. Moak New York, 1975) vol. I, p. 305. 8) W. Brandt; ibid., p. 261. 9) A. Chateau-Thierry, A. Giadieux, and B. Delaunay; Nucl. Instrum. & Methods. 132, 553 (1976). 10) J. A. Phillips; Phys. Rev. 97, 404 (1955). 11) T. A. Hall; Phys. Rev. 79, 504 (1950). 12) F. Fukuzawa; Phys Lett. 43A, 147 (1973). 13) L. H. Toburen, M. Y. Nakai, and R. A. Langley, Phys. Rev. 171, 114 (1968). 

Neutral Fractions in Proton Beams Passing through Solids

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Neutral Fractions in Proton Beams Passing through Solids

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