Neutron depth profiling (NDP) is a near-surface analysis technique that is commonly used to obtain profiles of concentration as a function of depth for certain technologically important light elements in nearly any substrate. The technique was first proposed by Ziegler ''et al.'' to determine the concentration profiles of
boron impurities in
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...
substrates, and later improved by Biersack and coworkers to much of its existing capabilities.
Neutron depth profiling
In NDP, a
thermal
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
or cold
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behav ...
beam passes through a material and interacts with
isotopes
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...
that emit monoenergetic
charged particles
In physics, a charged particle is a particle with an electric charge. It may be an ion, such as a molecule or atom with a surplus or deficit of electrons relative to protons. It can also be an electron or a proton, or another elementary pa ...
upon
neutron absorption
Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons ...
; either a
proton or an
alpha, and a recoil
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
* Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
. Since the charged particles are equally likely to be emitted in any direction, reaction
kinematics are straightforward. Because low-energy neutrons are used, there is no significant momentum transfer from the neutron beam to the substrate, and the analysis is practically non-destructive. As charged particles move towards the surface, they are rapidly slowed down, primarily by interacting with the electrons of the substrate. The amount of energy loss is directly related to the thickness penetrated by the particle. The depth of the reaction site can be found by
stopping power
Stopping power is the ability of a weapon – typically a ranged weapon such as a firearm – to cause a target (human or animal) to be incapacitated or immobilized. Stopping power contrasts with lethality in that it pertains only to a weapon ...
correlations.
Profiling
Conventionally, the residual energies of charged particles and recoil nuclei have been measured by a
silicon charged-particle detector; most commonly either a surface barrier detector (SBD) or a passivated implanted planar silicon (PIPS) detector. In this configuration, the semiconductor detector is placed opposite to the surface of the sample being analyzed, and an energy spectrum of charged particles emitted by the neutron-induced reaction is acquired.
See also
*
Neutron activation
Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emitti ...
References
*
*D. Fink, J.P. Biersack and H. Liebl, in ''Ion Implantation: Equipment and Techniques'', (1983), H. Ryssel and H. Glawischnig, eds., Springer-Verlag, Berlin, pp. 318–326.
*{{cite journal , last1=Downing , first1=R.G. , last2=Fleming , first2=R.F. , last3=Langland , first3=J.K. , last4=Vincent , first4=D.H. , title=Neutron depth profiling at the National Bureau of Standards , journal=Nuclear Instruments and Methods in Physics Research , publisher=Elsevier BV , volume=218 , issue=1–3 , year=1983 , issn=0167-5087 , doi=10.1016/0167-5087(83)90953-5 , pages=47–51
External links
NIST Center for Neutron Research: Cold Neutron Depth Profiling
Neutron Physics Laboratory: Neutron Depth Profiling
Neutron Depth Profiling "Chambers" Neutron-related techniques