''Magic'' Energies for Detecting Light Elements with Resonant Alpha Particle Backscattering Page: 4 of 7
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"Magic" Energies for Detecting Light Elements
with Resonant Alpha Particle Backscattering
C.J. Wetteland*, C.J. Maggiore and J.R. Tesmer
Center for Materials Science
X-M. He and D-H. Lee
StructurelProperty Relations
Materials Science and Technology Division, Los Alamos National Laboratory
Resonant backscattering is widely used to improve the detection limit of the light elements such as B, C, N and
0. One disadvantage, however, is that several incident energies are normally needed if the sample contains a
number of the light elements. There are "magic" energies at which several light elements can be detected
simultaneously with suitable sensitivities. When these energies are used along with the elastic recoil detection
of hydrogen, multiple elements can be detected without changing the beam energy, and the analysis time is
greatly reduced. These reactions along with examples will be discussed.INTRODUCTION
High-energy alpha-particle resonant backscattering has
become a standard tool in ion beam analysis (1). There ate
several resonances that may be selected (2). Unfortunately
there are also detrimental effects that accompany the
application of resonances to ion beam analysis. Desirable
considerations for choosing the ideal resonance energy for
analysis are:
1) multiple elements can be detected simultaneously,
2) the resonances are broad and slowly varying so that
depth information can be obtained,
3) the resonance cross sections are large compared to
the Rutherford cross sections as well as the
substrate cross section,
4) the energy does not produce prompt radiation from
the sample or unduly activate the sample being
analyzed,
5) there are no unwanted nuclear reactions or
resonances from the sample or substrate that
complicate the analysis.
The experimenter is lucky if even two of these
conditions are met. It is often necessary to change beam
energy several times to completely analyze a sample.
However, there are several energies where more of the
conditions are fulfilled. We refer to these energy regions as
"magic." The very first energy region that gained
popularity for oxygen detection, 8.6-8.8 MeV, (3X4X5) is
also useful for carbon and nitrogen. Another example is
* Now at the Dept. of Ceramics and Materials Engineering.
Rutgers Universitynear 6.6 MeV where boron, carbon and oxygen have
enhanced cross sections. Both cases have the disadvantages
of prompt radiation and target activation, and a heavy
substrate is necessary because of interfering resonances in
light substrates such as silicon
In this paper we explore the (a,a) backscattering cross
sections for 1B, C, N and O as well as the elastic recoil
cross section (a,p) for H in the energy region from 5 to 6
MeV. The energy region near 5.6 MeV allows the
simultaneous backscattering measurement of four elements:
B, C, N and 0, as well as the elastic recoil detection of H
(with a change of scattering geometry.) This energy also
allows the use of silicon as a substrate-a very common
choice.
EXPERIMENTAL METHOD
Sample Preparation
In practice, a sample to determine backscattering cross
sections is made with a light substrate on which a layer of
a heavy element is deposited. The element of interest is
then deposited on this layer. The heavy element, necessary
for the cross section determination, is also used to shift the
energy of the substrate signal to lower energies so that the
signal from the element of interest can be easily observed.
The program RUMP (6X7) was used to aid in the design of
the samples before they were deposited.
Boron & Carbon
The boron and carbon samples were deposited on a 5
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Wetteland, C.J.; Maggiore, C.J.; Tesmer, J.R.; He, X-M. & Lee, D-H. ''Magic'' Energies for Detecting Light Elements with Resonant Alpha Particle Backscattering, article, November 4, 1998; New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc725067/m1/4/: accessed April 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.