Radiant energy – Invisible radiant energy responsive electric signalling – Including a radiant energy responsive gas discharge device
Reexamination Certificate
2000-07-21
2002-09-17
Ham, Seungsook (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Including a radiant energy responsive gas discharge device
C250S375000, C250S385100
Reexamination Certificate
active
06452190
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an apparatus for radiation analysis by means of analyzing ionizing radiation, including a radiation detector for detecting the analyzing radiation, which detector includes:
a gas-filled absorption chamber for absorbing the radiation to be detected, which absorption chamber is provided with an entrance window which is formed in a wall of the absorption chamber and is transparent to the radiation to be detected, and
at least one counting wire which is arranged in the gas atmosphere, the surface of the entrance window being oriented transversely of the longitudinal direction of the counting wire.
The invention also relates to a radiation detector for use in such an apparatus.
A radiation detector for use in such an apparatus is described in U.S. Pat. No. 3,952,197. The radiation detector described therein includes a gas-filled, elongate chamber, the wall of which constitutes a first electrode. A rod-shaped or wire-shaped second electrode is arranged in a slit-shaped cut-out in the wall in such a manner that it extends parallel to the longitudinal direction of said chamber. A voltage difference exists between the two electrodes, so that a very inhomogeneous electrical field is present in the elongate chamber. The elongate chamber is closed at both its ends by end plates which extend transversely of the longitudinal direction and in which there is provided an entrance window which is permeable to the radiation to be detected.
DESCRIPTION OF PRIOR ART
A radiation detector for use in such an apparatus is described in U.S. Pat. No. 3,952,197. The radiation detector described therein includes a gas-filled, elongate chamber, the wall of which constitutes a first electrode. A rod-shaped or wire-shaped second electrode is arranged in a slit-shaped cut-out in the wall in such a manner that it extends parallel to the longitudinal direction of said chamber. A voltage difference exists between the two electrodes, so that a very inhomogeneous electrical field is present in the elongate chamber. The elongate chamber is closed at both its ends by end plates which extend transversely of the longitudinal direction and in which there is provided an entrance window which is permeable to the radiation to be detected.
In this known radiation detector the ion current to be detected is not amplified in the radiation detector itself. Consequently, for a given radiation intensity the current to be measured is very low or a very high intensity is required.
In radiation detectors of the kind generally known from prior art the problem imposed by an inadequate detection current is solved by producing an avalanche effect, i.e. the particles released upon ionization are accelerated by the electrical field, prior to collision with another gas particle, in such a manner that such a collision produces a new ionization; this process is repeated many times with the particles released by the new ionizations. The avalanche of released particles ultimately reaches the counting wire in which the large number of particles produces a current impulse which is much larger than that produced by a single particle.
Radiation detectors utilizing the avalanche effect, however, have the drawback that the shape of the current impulse is dependent on the location where the ionization, i.e. the beginning of the avalanche, occurs. This phenomenon is due to the fact that the incident X-ray quanta in such radiation detector require a long path through the gas so as to make the probability of ionization high enough for adequate X-ray detection. This means that ionizations occur both close to the counting wire as well as at a comparatively long distance therefrom. An ionization in the gas atmosphere of the detector causes a cloud of electrons whose size is dependent on the energy, i.e. the wavelength, of the X-rays to be detected. Such an ionization-induced cloud travels to the counting wire under the influence of the electrical field in the vicinity of this wire. While traveling to the counting wire the electrons of said cloud are driven apart from one another by mutual electrical repulsion, so that not only gas amplification of the current impulse occurs but also widening of this impulse. Consequently, ionization close to the counting wire produces a sharp impulse whereas, due to said repulsion, an ionization remote from the counting wire causes broadening of the impulse. Because the charge content of the impulse remains the same, the impulse is then also flattened proportionally. Consequently, it may occur that two wide impulses in rapid succession are not distinguished from one another but interpreted as a single impulse of higher energy by the processing electronics, thus leading to incorrect interpretation of the measurements. This problem can be circumvented by inhibiting the detection of a second impulse within a given period of time after a first detected impulse; this given period of time must then be chosen to be equal to the longest possible impulse duration. However, this makes the detector much slower and the duration of measurements will be prolonged proportionally.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a radiation detector of the kind set forth which offers impulses of comparatively high current intensity without the counting speed of the radiation detector being degraded.
To this end, the apparatus for radiation analysis according to the invention is characterized in that the radiation detector is also provided with at least two avalanche chambers which adjoin the absorption chamber, are in atmospheric contact therewith and are intended to produce an avalanche of released charged particles, each of said avalanche chambers being provided with:
a counting wire which is present in the gas atmosphere,
a grid which is present in the gas atmosphere and is arranged around the counting wire,
the counting wires of said avalanche chambers extending substantially parallel to one another.
Because the surface of the entrance window is oriented transversely of the longitudinal direction of the counting wire, the radiation to be detected is incident substantially parallel to the counting wire in the absorption chamber. The absorption chamber can be constructed so as to have an arbitrary length within broad limits (i.e. the dimension in the direction parallel to the counting wire, so parallel to the incident radiation). Consequently, the absorption of the incident radiation can be proportionally high. The distance between the ionization and the counting wire may then be approximately constant and small, irrespective of the distance between the entrance window and the location of the ionization. The grids of the avalanche chambers are electrically adjusted relative to the absorption chamber in such a manner that an electron cloud formed in the absorption chamber will travel in the direction of the counting wires without the electrons in this cloud causing an avalanche of ionizations in the absorption chamber. Because of the small distance between each ionization and the grid, the electron cloud will hardly be dispersed during this short travel, so that no pulse broadening will be induced in this space. When the electron cloud enters the space between the grid and the counting wires (i.e. the avalanche chamber), it causes an avalanche of ionizations. This is due to the fact that the counting wires in the avalanche chambers are electrically adjusted relative to the grids in such a manner that an adequately strong electrical field is present in the avalanche chamber. Thus, for all electrons entering the avalanche chamber the avalanche commences at substantially the same distance from the counting wire. Because of the design of the avalanche chamber, this distance can be chosen so as to be sufficiently small to prevent broadening of the current impulse to be detected, so that the impulse duration is always short and hence the counting speed may be high.
Because several avalanche chambers are provided, the count rate of the detector may be higher than in the case
Bethke Klaus
Bolk Hendrik Johannes Jan
Ham Seungsook
Koninklijke Philips Electronics , N.V.
Moran Timothy
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