Radiant energy – Invisible radiant energy responsive electric signalling – Including a radiant energy responsive gas discharge device
Reexamination Certificate
2000-05-24
2002-05-28
Anderson, Bruce (Department: 2881)
Radiant energy
Invisible radiant energy responsive electric signalling
Including a radiant energy responsive gas discharge device
C250S387000, C250S310000, C250S397000, C250S374000
Reexamination Certificate
active
06396063
ABSTRACT:
TECHNICAL FIELD
This invention pertains to the technical field of physical sciences and technology. A novel detector is disclosed which can be used with instruments generating ionising radiations in a gaseous environment. This detector can be used also for particle measurements such as in the field of nuclear methods and instruments.
BACKGROUND ART
Conventionally, instruments employing electron and ion beams utilise a vacuum envelope into which the electron or ion beam is generated and applied. Such beams usually impinge on a specimen with which they interact and generate various products, or signals, such as electrons, ions, molecular fragments and x-rays or other photons. These instruments are used for imaging, testing, analysis, or modification of the surface of a specimen. To study a specimen, we need to detect the products of the beam-specimen interaction by suitable detection means usually placed inside the vacuum envelope of the instrument. However, a more recent generation of related instruments allows the examination of specimens inside a gaseous envelope with substantial pressure.
One electron beam instrument that allows the examination of a specimen in a gaseous environment is the environmental scanning electron microscope (ESEM) as disclosed by references 1 through to 10 below. This instrument employs a scanning electron beam that is generated in the vacuum envelope of an electron optics system. The vacuum envelope communicates with a specimen chamber via two small apertures which limit the flow of gas in the vacuum column to a negligible amount. Practically, most of the gas leaking through the first aperture from the specimen chamber is pumped out and only a very small amount of gas escapes into the vacuum of the electron optics column which is also pumped out and maintained in vacuum. The electron beam reaches the specimen through the two apertures with sufficient current still in a focussed condition. The focussed electron beam scans a very small surface area of the specimen in a raster form and releases various signals. One type of signal is electrons of low energy, called secondary electrons (SE). The SE are released in the gaseous environment and are detected with suitable means as disclosed by Danilatos in the references cited. Another type of signal generated is electrons of high energy, called backscattered electrons (BSE). The BSE, in turn, release free electrons of low energy from the gas molecules. The latter free electrons are also detected by suitable means as disclosed by Danilatos and summarised below.
The detection of free electrons of low energy in the gas of an ESEM is achieved by subjecting the electrons to a static electric field of sufficient intensity. As a result of the action of the external field, a controlled electrical discharge develops that multiplies the initial free electrons. The avalanche multiplication of the electrons is also accompanied by an avalanche of photon generation and multiplication in the gas. The discharge is controlled by the applied bias and develops as long as there is an initial supply of free electrons, whilst the discharge is extinguished when the supply source is eliminated. In other words, the amplified output signal is practically proportional to the input intensity of the source. The output signal is collected by suitable means, such as biased electrodes and photon detectors. In prior art, the same principle has been applied for detection and counting of ionising particles in particle physics, namely, by the proportional counter.
Although the principle on which proportional counters operate has been known art in the field of particle physics and nuclear instruments for a long time, the application of the same principle for the development of a detector in an ESEM has led to a series of new inventions and patents given by references 1 through to 6.
The present invention discloses a novel detector for free electrons of low energy in a gaseous environment of instruments employing electron and ion beams, or in the gaseous environment of a particle counter, in general. The novelty of this invention is primarily based on the introduction of an alternating electromagnetic field which can generate an electron and photon discharge in a gaseous environment. The use of an alternating field instead of a static field provides for novel and alternative means of detection for several instruments and has certain advantages over previous art.
The principle and application of an alternating electromagnetic field to create an electrodeless self-sustained discharge for completely different purposes has been previously presented by other workers as in references 11 through to 15. The same principle is also used for the generation of ion sources in various instruments. In all prior art, this principle has been used mainly in a self-sustained discharge mode for generating ions; however, it has never previously been disclosed or used as a proportional amplifier and detector of ionising radiations in a controlled discharge regime which takes place below the breakdown voltage of a self-sustained discharge.
DETAILED DISCLOSURE OF THE INVENTION
This invention relates to a novel detection system for instruments employing focussed charged particle beams such as electron or ion microscopes and for technologies generally employing focussed electron or ion beams. Electron and ion microscopes are a sub-class of the latter instruments and ESEM is one form of an electron microscope. The ionising radiations emanating from the specimens under examination in those instruments as well as the ionising particles used in other fields of science can be detected by a novel method and apparatus disclosed by the present invention herein.
According to the present invention, a volume of gas is acted upon by an alternating (oscillating) electromagnetic field in the radiofrequency range, preferably but not limited, anywhere between 1 MHz and 1 GHz. Both the frequency and amplitude of oscillation are variable within a particular working range for each technological case. The working range depends on the nature of gas, pressure of gas and geometry of the detection volume. When one or more electrons are released in the gas by any source, the electrons are acted upon by the alternating field and are forced to oscillate and collide with the gas molecules or atoms. Some electron collisions ionise the gas molecules or atoms and create electron/ion pairs, while some other electron collisions excite the gas molecules or atoms and release photons. The electrons may also collide repeatedly with the surrounding walls generating new electrons. By such mechanisms the initial electron signal is multiplied in an avalanche form. The predominant mechanism of electron multiplication is determined by the chosen frequency, pressure, geometry and nature of gas, and the effects can be controlled accordingly. Now, it is an essential feature and requirement of the present invention that the amplitude of oscillation of the applied field is kept below the sparking value, i.e. below the point where the avalanche becomes self-sustained and uncontrollable. When this requirement is met, the discharge is controlled and is said to operate in the proportional region, where the amplified current is proportional to the initial number of free electrons released in the gas. The sparking value of the field depends on the chosen frequency, geometrical configuration, gas pressure and nature of gas. The preferred values of all these parameters depend on the instrument and application with which this invention is used and do not restrict the scope and spirit of the invention.
For the purposes of the present description of the invention the term “electromagnetic” refers to either (a) the field generated between two electrodes biased with an alternating voltage and creating a predominantly alternating electric component of field with negligible magnetic component, or (b) the field created by an alternating electric current through a coil with both an alternating electric and magnetic component present
Anderson Bruce
Bednarek Michael
Shaw Pittman LLP
Wells Nikita
LandOfFree
Radiofrequency gaseous detection device (RF-GDD) does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Radiofrequency gaseous detection device (RF-GDD), we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Radiofrequency gaseous detection device (RF-GDD) will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2831197