X-ray detector unit with solid converter

Details X-ray detector unit with solid converter X-ray detector unit with solid converter

1999-11-19

2001-12-25

Epps, Georgia (Department: 2873)

Radiant energy

Invisible radiant energy responsive electric signalling

Including ionization means

Reexamination Certificate

active

06333506

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a detector unit for two-dimensional detection of incoming radiation from an X-ray source. The invention also concerns a solid material structure for use in a detector unit, primarily intended to be used in X-ray radiography, especially for medical imaging.
BACKGROUND OF THE INVENTION
Such a detector unit, which is previously known from U.S. Pat. No. 5,308,987 (Wuest et al) comprises:
a solid material structure including a conversion medium in which incoming radiation causes the emission of electrons into at least one drift region,
an anode and cathode for generating an electric field in said at least one drift region, and
a detector sensitive in two dimensions adjacent to said solid material structure so as to detect electrons originating from said at least one drift region in response to said incoming radiation.
In the previously knowndetector unit the solid material structure is formed by a solid layer made of a conversion medium. The incoming radiation interacts with the conversion medium and generates electrons which are transported by means of the electric field into an adjacent gas chamber where a detector is located. Normally, secondary electrons are generated in the gas chamber, possibly under avalanche amplification. The detector is coupled to readout electronics.
Accordingly, a two-dimensional detection of the radiation can be accomplished. However, the efficiency of the prior detector unit will not be very high because the absorption probability of the incoming radiation is low, and only a small fraction of the electrons, which are generated in the vicinity of the surface of the conversion medium, will be emitted into the gas chamber.
Also, the low efficiency and a necessarily long absorption length for the radiation in the gas results in the positional resolution being relatively low.
SUMMARY OF THE INVENTION
The present invention improves the efficiency and the positional resolution of a detector unit of the kind stated above.
More particularly, the present invention, is directed to a detector unit, wherein a plurality of passages, arranged in a two-dimensional array, are located close to each other and extend within the solid material structure, said passages having surface portions comprising said conversion medium and being inclined at an angle relative to the direction of the incoming radiation, whereby said incoming radiation will impinge onto said surface portions at an acute angle and cause the emission of electrons directly from said surface portions into said drift regions.
By such a structure, the electrons will be emitted from the same surface portions as those struck by the incoming radiation. The acute angle of impinging X-ray photons will lead to a high efficiency for electrons escaping from the surface. Moreover, some photons will enter into the solid material structure between neighbouring passages, close to one of these passages, and cause the generation of electrons escaping from a neighbouring surface portion into a drift region.
The emitted electrons will thus enter into a gas in the drift region. Here, secondary electrons may be generated by electron avalanche multiplication in the gas, and some of the electrons will also hit other surface portions of the passage and generate further electrons, some of which will escape back into the drift region. Accordingly, a high number of electrons will drift towards the detector, which can be located immediately adjacent to the solid material structure, possibly integrated therewith, or at a distance therefrom.
Thus, the detector is operative in two dimensions and will register the drifted electrons in response to the incoming X-ray photons, with high efficiency and improved positional resolution.
If there is a gap between the solid material structure and the detector, the electrons will be accelerated further and may collide with gas molecules and cause ionization with associated secondary electrons, possibly under avalanche amplification.
In any case, the positional resolution can be maintained at a very high level being determined by the two-dimensional array of passages in the solid material structure.
In one embodiment, the passages are formed by channels in a unitary plate of solid material. The solid material may form the conversion material or, alternatively, be coated with such conversion material on the surface portions of the passages.
In a second embodiment, the solid material structure is constituted by a two-dimensional array of columns with said passages formed therebetween, each column extending from a support plate so as to be held in a fixed position relative to the other columns. The support plate may be located on a front side or on a rear side of the solid material structure, the front side being the side facing the X-ray source.
Preferably, the passages extend from the front side of the solid material structure to a rear side thereof, the detector means being located adjacent to the front or the rear side of the solid material structure. The drift electrodes are then arranged on the front and rear sides, respectively, of the solid material structure so as form drift regions in the passages.
The detector may be constituted by a micropattern gas chamber detector employing avalanche amplification and an arrangement of read-out elements for detection of electron avalanches in said two-dimensional pattern, e.g. of the kind disclosed in the co-pending Swedish applications Nos. 9704015-8, 9901324-5, 9901325-2 and 9901326-0 filed on Apr. 14, 1999.
It is also possible to use a detector of a solid material, e.g. a CCD or TFT detector, e.g. integrated with a support plate for the solid material structure provided with passages.
The invention will now be described more fully with reference to the appended drawings illustrating two preferred embodiments.


REFERENCES:
patent: 5308987 (1994-05-01), Wuest et al.
patent: 5693947 (1997-12-01), Morton
patent: 6118125 (2000-09-01), Carlson et al.

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