Positron source

Details Positron source Positron source

2003-12-03

2004-11-16

Lee, John R. (Department: 2881)

Radiant energy

Ion generation

Electron bombardment type

C250S306000, C250S307000, C250S308000, C250S309000, C250S310000

Reexamination Certificate

active

06818902

ABSTRACT:

TECHNICAL FIELD
This invention relates to a positron source.
It has a very large number of applications, particularly in solid state physics, in material sciences and in surface physics, in which a high counting rate is important for many applications, for example such as a scanning positron microscope, lifetime measurements as a function of the implantation depth or Doppler broadening, and PAES (Positron annihilation induced Auger Electron Spectroscopy).
Other applications of the invention use positronium “atoms” directly (positronium being the bound state of an electron and a positron). However, the production of positronium also requires a large number of positrons.
The invention is also applicable in molecular chemistry and more particularly to the determination of processes involved in superconducting materials with high critical temperature.
It is equally applicable to determination of the aging capacity of paints and coatings.
Furthermore, the invention is also applicable to the detection of defects in a material, as it is known that annihilation of positrons is sensitive to the electron density. For example, small variations of this density are detected when the material thermally expands. Vacancies, in other words single atoms missing from the lattice of a crystalline material, are then very easily detected due to their low electron density. Concentrations of vacant atomic sites of the order of 10
−6
(1 ppm) are observable.
Since a material is analyzed by a contact free positron beam, the material can be heated to a very high temperature. Vacant sites may also be introduced at any temperature by mechanical deformation, sputtering or ion implantation.
The adjustable energy of the positron beam is a means of obtaining in-depth information with a resolution of 10% for structures in thin layers or samples comprising a non-uniform distribution of defects.
Furthermore, electric fields in oxides of microelectronic devices such as MOSs can be used to deviate positrons at the study interface.
Vacancy clusters forming cavities of the order of 0.5 nm can easily be observed by variations of Doppler broadening and the lifetime of positrons.
Observing the formation of positronium demonstrates the presence of broader cavities and can determine their size (up to 20 nm).
For even larger cavities, ortho-positronium (state of the positronium in which electron and positron spins are anti-parallel) survives long enough for it to disintegrate into three photons. In this case, the angular correlation of photons gives an increase on Doppler broadening by a factor of 5.
Note also other applications of the invention:
PRS (Positron Re-emission Spectroscopy),
PAES (Positron annihilation induced Auger Electron Spectroscopy),
REPELS (Re-Emitted Positron Energy Loss Spectroscopy),
LEPD (Low-Energy Positron Diffraction),
PIIDS (Positron Induced Ion Desorption Spectroscopy),
PALS (Positron Annihilation Lifetime Spectroscopy), this technique being extremely important in microelectronics,
VEPLS (Variable Energy Positron Lifetime Spectroscopy), and
PAS (Positron Annihilation Spectroscopy).
This invention relates more particularly to production of a low energy positron beam, less than 10 MeV, with an instantaneous intensity of more than 10
10
positrons per second, and preferably more than 10
12
positrons per second, for example in order to obtain:
a low energy positron beam, with an energy of less than 10 kev by coupling with an appropriate trap, or
positronium atoms, by interaction with an appropriate target.
STATE OF PRIOR ART
Production with a high rate (more than 10
10
per second), of low energy positrons and positronium “atoms” is necessary for industrial applications such as measuring defects in crystals or organic materials, when for example PAS (Positron Annihilation Spectroscopy) or other methods mentioned above are used.
These applications use mainly
22
Na sources as positron beam sources. These compact sources are very suitable for laboratory research. But their maximum activity is about 4×10
9
Bq and their average lifetime is only 2.6 years.
Moreover, there are some accelerators for which part of the activity, frequently minor, relates to production of positron beams. However, these are mainly large and expensive installations since the energy of electrons used is very frequently several tens of MeV, typically 100 MeV. Positrons emitted may have energies of several tens of MeV.
Moreover, positrons useful for industrial applications have a kinetic energy less than at least one thousand times the energy of the production threshold. Conventionally, metallic moderators with very low efficiency (less than 0.001) are used to slow them.
Furthermore, it is known how to trap a positron beam in a device called a Penning-Malmberg trap. An improved trap, called the Greaves-Surko trap, enormously increases the brightness of the beam by dividing the energy dispersion of this beam by a thousand, with an efficiency of the order of 1.
Greaves-Surko traps are commercially available from the First Point Scientific Company. They comprise a solid neon moderator whose efficiency is close to 1%.
These traps are very advantageous for the above-mentioned applications and since their appearance they have become more widely used, but the energy of the positrons must be less than 1 MeV.
Furthermore, four techniques are known for producing positrons. These techniques use radioactive sources (of
22
Na type) or neutron fluxes from nuclear reactors or tandem accelerators (designed to accelerate ions) or electron accelerators.
We will now examine the disadvantages of these techniques.
The positron current output by a radioactive source is limited by the thickness of the material surrounding the source. Furthermore, the intensity of the positron beam emitted by such a source is of the order of 10
8
e
+
/s and therefore of the order of 10
6
e
+
/s after moderation.
The use of neutron fluxes output from a nuclear reactor provides a means of obtaining short lifetime radioactive sources capable of producing low energy positrons. However, this technique cannot be industrialized because it requires a nuclear reactor.
One variant of the previous technique consists of using a tandem accelerator to accelerate ions that are sent to a target. This target becomes radioactive and emits low energy positrons. Although a tandem accelerator is smaller than a conventional particle accelerator, it forms a large and expensive installation that requires protection against activation and a maintenance infrastructure.
Large linear accelerators, more simply called “linacs”, are also used to produce positrons, by accelerating electrons and sending them to a tungsten or tantalum target. However, these large linacs are very large and expensive installations and there are not enough of them to facilitate the development of positron applications of the type mentioned above.
Let us reconsider known interaction chambers containing a target that is capable of generating positrons by interaction with an electron beam.
To produce positrons (denoted e
+
) from an electron beam (denoted e
−
), these electrons have to interact with a target material. The electrons then emit X and gamma photons which sometimes disintegrate in pairs (e
+
e
−
).
Since the number of positrons produced depends on the number of electrons that interacted with the target material, a person skilled in the art will decide to use intense beams like those produced by linac type accelerators.
Since the number of e
+
produced by an electron beam increases with the thickness of the target passed through, a person skilled in the art would tend to increase this thickness.
But two problems then arise.
Firstly, the X rays deposit energy in the form of heat in the target.
Secondly, the e
+
created can be captured in the target and annihilate before exiting from the target. This annihilation may take place according to two reactions, namely direct collision with an electron or the formation of a positronium atom.
A person skilled in

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