Proton elastic scattering for light element cross section enhancement with Ep > 2.5 MeV

Nuclear Instruments North-Holland

and Methods

in Physics Research

861 (1991) 175-177

175

Proton elastic scattering for light element cross section enhancement with Ep > 2.5 MeV Yang Guohua, Zhu Dezhang, Xu Hongjie and Pan Haochang Shanghai Institute of Nuclear Research, Academia Sink, Received

P.O. Box 800-204, Shanghai 201800, China

1 April 1991

The proton elastic scattering (PES) technique in the energy range of 2.5-3.6 MeV for light elements C, N and Cl detection has been investigated by measuring the enhanced scattering cross sections at 17OO. It is found that at the incident proton energy near 3.0 MeV the scattering cross sections for all three light elements vary rather smoothly. The results show that PES technique with proton energy near 3.0 MeV may be used for simultaneously profiling carbon, nitrogen and oxygen over a reasonable depth in a heavy matrix.

Over the past years proton elastic scattering (PES) technique has been acknowledged to be a very useful tool for light element detection in material analysis. The general characteristics of the technique have been well established (l-31. Above a few hundred keV, the proton scattering cross section is influenced by nuclear force interaction and it may be significantly enhanced compared with the Rutherford value. Up to now, most studies on proton backscattering were limited to the energy range below 2.5 MeV. At higher energies (i.e. ei, > 2.5 MeV), the PES technique has not been extenstvely exploited and few experimental proton scattering cross sections for light elements have been measured, especially near 170° [4-61. In this paper, we aimed to find a reasonable proton energy interval in which the enhanced proton elastic scattering cross sections varied smoothly for simultaneous detection of common light elements C, N and 0 and to determine the cross sections for these light elements in terms of the ratio of enhanced value to Rutherford value. The ion backscattering experiments were performed in a typical setup using well collimated ion beams from the 4 MV pelletron at the Shanghai Institute of Nuclear Research. The proton energy was calibrated with the “AI(p, ~)‘*Si resonances. The backscattered ions were detected at a scattering angle of 170” using a silicon surface barrier detector with an energy resolution of 16 keV (at E, = 2 MeV). The target-to-detector distance was 80 mm and the aperture in front of the detector was 3 mm in diameter. The target used to measure the cross section e~~~rnen~ of carbon and oxygen was a 15 pg./cm’ layer of La,O, deposited on a 20 pg/cm* film of carbon. The characteristics of the target were carefully checked before and after PES measurements by 2 0168-583X/91/$03.50

0 1991 - Elsevier Science Publishers

MeV He+ RRS at the same beam spot. No significant changes in the target during the measurements were detected. The results revealed that the oxide stoicbiometry La : 0 is 0.658 f 0.02. The method used to derive the scattering cross sections from the experimental data was the same as that used in ref. [7]. In the energy range from 2.5 to 3.6 MeV, most reported values were not obtained near B = 170 O. In order to cross-check the accuracy of our measurements, we also measured the proton cross sections of C and 0 in the energy range of 2.2-2.5 MeV, which have been accurately measured [7,8]. The experimental proton cross sections from carbon and oxygen as a function of incident proton energy are shown in fig. 1. Good agreement with the results of refs. [7,8] is found in the energy range of 2.2-2.5 MeV. The uncertainty of our results consists mainly of statistical and background subtraction (3%) and the content of the target used (3%). Our results are estimated to have a total accuracy of 4% in the nonresonant energy regions. In the energy interval of 2.7 to 3.1 MeV, both measured p-C and p-0 cross sections appear rather smooth. This energy interval may be used for accuracy stoichiometric determination in “thicker-than-usual” films. Table 1 presents the measured laboratory differential scattering cross section enhancement for carbon and oxygen in this energy interval. In the case of nitrogen, we chose a 180 nm thick TiN layer on a graphite substrate as the target. The uniform target was prepared by 2 keV Xe+ ion implantation during tit~um deposition in a chamber with nitrogen gas to a pressure of 5 x 10e6 Tot-r [9]. In order to separate completely the peaks of C,N and 0 in the TiN layer, 2.2 MeV proton backscattering was used to quan-

B.V. (North-Holland)

176

Yang Guohua et al. / PES for light element cross section enhancement

.;.r;;;‘-:.,:t:,:,jr,

E

v:

l

P

‘““I qk ’

2.2

E,, @eV)

Carbon

Nitrogen

Oxygen

2700 2800 2900 3ooo 3100

14.23 15.12 16.24 17.31 17.96

10.41 10.86 11.43 11.92 12.63

6.43 7.23 7.64 8.18 8.73

Luomajarvy et al(8)

t

'I

Table 1 Proton elastic scattering cross section enhancement factor for C,NandOat 8=170°

&

2.4 2.6 PROTON

1

I

2.8 3.0 ENERGY

I

3.2

3.4

3.t

(MeV)

Fig. 1. Tbe experimental laboratory differential scattering cross

sections of carbon (a) and oxygen (b) as a function of incident proton energy.

tify the compositions of the target. The PEB spectrum for the target used is shown in fig. 2. The average stoichiometry of the layer was determined in atomic fraction to be: N, 0.412; 0, 0.051; C, 0.041; Ti, 0.473;

‘:!‘ANNEL

and Xe, 0.023. In data processing, Xe was used as the reference element. Unlike C and 0, nitrogen shows strong broad resonant structure in the energy range studied. However, the scattering cross section of nitrogen in the energy range from 2.7 to 3.1 is also weakly dependent on the proton energy. The proton scattering cross section enhancement for nitrogen is also listed in table 1. The benefits concerned with the choice of 3.0 MeV protons are: (1) The local variation of cross section is very small for all three elements and it can be used for depth profiling of a sample with a sufficient depth range. for all three ele(2) The cross section enhancements ments are higher compared with those below 2.5 MeV. (3) The inelastic scattering and other nuclear reaction interferences from C, N and 0 in the energy range of the present investigation are energetically impossible or insignificant in comparison to the proton elastic scattering [3]. In conclusion, energy

proton

of 3.0 MeV

elastic scattering

may be suitable

with incident

and sensitive

for

NUMRER

Fig. 2. Tbe PES spectrum obtained using 2.2 MeV protons on the target consisting of TiN layer deposited on a graphite substrate.

177

Yang Guohua et 01. / PES for light element cross section enhoncemenr

simultaneously profiling C, N and 0 over a sufficient depth range in a heavy matrix.

References [l] W.K. Chu, J.W. Mayer and M.-A. Nicolet, Backscattering Spectrometry (Academic Press, New York, 1978). [2] K.M. Yu, J.M. Jaklevic and E.E. Haller, Nucl. Instr. and Meth. B30 (1988) 551. [3] E. Rauhala, Nucl. Instr. and Meth. B40/41

(1989) 790.

[4] J.W. Mayer and E. Rimini, Ion Beam Handbook for Material Analysis (Academic Press, New York, 1977).

[5] G. Goldhaber and R.M. Williamson, Phys. Rev. 82 (1951) 495. [6] F.K. Mcgowan, W.T. Milner, H.J. Kim and W. Hyatt, Nucl. Data Tables A6 (1969) 353. [7] E. Rauhala, Nucl. Instr. and Meth. B12 (1985) 447. [8] M. Luomajarvy, E. Rauhala and M. Hautala, Nucl. Instr. and Meth. B9 (1985) 255. [9] Wang Xi, Chen Youshan, Yang Genqing, Zhou Zuyao, Zheng Zhihong, Huang Wei, Liu Xianghuai and Zou Shichang, presented at the Conference of Ion Beam Modification of Materials,

Knoxville,

Tennessee

(1990).