IEC Device as a Tuneable X-Ray Source

Abstract

A low cost small-scale tunable X-ray source, comprising a spherical-electron injected inertial electrostatic confinement (IEC) device. Within a spherical containment vessel recirculatory focusing electrons are accelerated by a spherical grid within the vessel, and cause electron--electron collisions in a dense, central plasma core region of the sphere. The IEC synchrotron source (IEC-SS) in a mechanism for generating tunable X-ray radiation is essentially equivalent to that for conventional synchrotron sources. The IEC-SS operates at a much lower electron energy (<100 kev compared with>200 Mev in a synchrotron), but still gives the same X-ray energy due to the small-scale bending radius associated with the electron--electron interactions. The X-rays can be filtered for particular purposes using diffraction gratings, prisms or the like.

Background

The development of a compact, tunable, hard x-ray source would have profound and wide ranging applications in a number of areas. These areas include x-ray diagnostics, medical imaging, microscopy, nuclear resonance absorption, solid-state physics and material science.

Currently, varieties of x-ray generators exist. The most modern devices are generally based on one of three methodologies: laser and discharge plasmas, electron impact sources, and synchrotron. The spectrum of these sources can be divided into two categories: characteristic x-rays and continuum x-rays. The characteristic x-ray sources are dependent on the particular atomic structure of the gas or target material in use. Among all the types of x-ray sources, only synchrotron produces continuum radiation.

Synchrotron radiation is the electromagnetic radiation emitted by electrons moving at relativistic velocities along a curved trajectory with a large radius of curvature, for example, several meters to tens of meters. The energy of the photons ranges from a few electron volts to 10.sup.5 Ev. This corresponds to the binding energy of electrons in atoms, molecules, solids, and biological systems. Thus, synchrotron radiation photons have the right energy to probe the properties of such electrons and of the corresponding chemical bonds to understand their physical and chemical properties. The uses of electron accelerators as sources of synchrotron radiation have grown enormously during the last two decades. Unique features such as tunability and wide x-ray spectrum tend to render the synchrotron irreplaceable for many applications. Presently, third generation synchrotron sources are being pursued that are based on high-energy electron storage rings and bending magnets.

A typical electron accelerator can be tuned to emit synchrotron radiation in a very broad range of photon energies, from microwaves to hard-x-rays. Thus, it provides electromagnetic radiation in spectral regions for which no other usable sources exist, e.g., most of the ultraviolet/soft-x-ray range. Furthermore, it is by far the best source of hard-x-rays, even though other sources exist for this range. The system has met most application needs, but fails with respect to physical size and cost for individual laboratory use. They are inevitably large and expensive devices requiring complex supporting facilities, costing tens to hundreds of millions of dollars. The nature of synchrotron x-ray sources means that they are expensive, remote multi-user facilities, and are therefore not suited for use with a laboratory scale. The alternative x-ray sources, such as electron impact systems, laser and discharge plasmas, cannot match synchrotron in terms of its tunability and continuum x-rays.

Summary

In the IEC-based x-ray source design, the electron storage ring of the synchrotron is replaced by recirculatory focusing electrons in a sphere that are accelerated by a grid, and the bending magnets are replaced by the electron--electron collisions in the sphere center. This arrangement results in an IEC synchrotron source (IEC-SS), wherein the mechanism for generating tunable x-ray radiation is essentially the same as in the bending magnet synchrotron sources. The IEC-SS operates at a much lower electron energy (<100 keV compared with>200 MeV in a synchrotron) while still giving a same radiated x-ray energy compensated by a bending radius of much smaller scale from electron--electron interactions. In short, electrons are accelerated 10's to 100 kev by the anode grid. Due to spherical (or other) convergence, the energetic electrons scatter in the center of the sphere. The scattering interactions create intense bremsstrahlung x-rays. The emitted x-ray energy is controlled by the grid bias.

Some Possible Applications

An object of the invention disclosed is to provide a small compact tunable x-ray source for laboratory use. For applications where a relatively small sample is practical, the availability of a laboratory-scale source would be very advantageous. Another object is to provide a compact tunable x-ray source for security inspection applications such as a more sensitive baggage x-ray inspection systems. As a supplement, this x-ray source can be combined with an IEC neutron source to provide a versatile inspection station. This concept was recently discussed in some detail in a paper titled “IEC-based Neutron Generator for Security Inspection System”, presented at the Annual Am. Nuclear Society Meeting, San Diego CA, June 2003.

Introduction

Large nuclear reactors are widely employed for electricity power generation, but small nuclear radiation sources can also be used for a variety of industrial/government applications.  In this paper we will discuss the use of a small neutron source based on Inertial Electrostatic Confinement (IEC) of accelerated deuterium ions.

With more stringent requirements on airport security, highly sensitive and precise detecting systems for explosives are in urgent need.  While current airport inspection systems are strongly based on x-ray technique, neutron activation including Thermal Neutron Analysis (TNA) and Fast Neutron Analysis (FNA) is more powerful in detecting certain types of explosives in luggage and in cargoes.  Basic elements such as oxygen, carbon, nitrogen and chlorine present in the explosives can be measured through the (n, n'g) reaction initiated by fast neutrons, in which gamma rays characteristic of associated element are produced.  An improved version of FNA, Pulsed FNA (PFNA) uses pulsed neutron source and the time-of-flight (TOF) technique to reduce the "noise" effect resulting from the interaction between neutrons and backgrounds and to obtain localization information.  Therefore, it is considered as one of the best explosive detecting techniques. The difference between different techniques will be given in later section.   Fission reactors, accelerators and radioisotopes such as ²⁵²Cf are major neutron sources for neutron activation analysis.   Among these neutron source generators, the IEC is an ideal candidate to meet the neutron activation analysis requirements. Compared with other accelerators and radioisotopes, the IEC is simpler, can be switched on or off, and can reliably produce neutrons with minimum maintenance.

Theoretical and experimental studies of a spherical IEC have been conducted at the University of Illinois.   In a spherical IEC device, 2.54-MeV neutrons of ~10⁸ n/s via DD reactions over recent years or 14-MeV neutrons of ~2´10¹⁰n/s via DT reactions can be obtained using an ion gun injection technique.

In this paper we will examine the possibility of using an alternative cylindrical IEC configuration.  Such a device was studied earlier at the University of Illinois and it provides a very convenient geometry for security inspection.  However, to calculate the neutron yield precisely with this configuration, an understanding of the potential wall trapping and acceleration of ions is needed.   The theory engaged is an extension of original analytic study by R.L. Hirsh on the potential well structure in a spherical IEC device, i.e. roughly a "line" source of neutrons from a cylindrical IEC is a "point" source from the spherical geometry.  Thus our present study focuses on the cylindrical IEC for its convenient application in an FNA detecting system.  The conceptual design and physics of ion trapping and re-circulation in a cylindrical IEC intended for neutron-based inspection system will be presented.  We will also discuss the concept of a security inspection system based on the combination techniques of TNA/PFNA, FNA and x-ray. Calculation of the neutron yield through numerically solving a Possion-Valsov equation for the specified IEC device will be given. Possible technical issues are to be discussed.

Such an inspection station is illustrated schematically, below:

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