JPS5840822B2 - X-ray detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an X-ray detector, particularly a multi-element ion chamber X-ray detector, used in an X-ray cross-sectional inspection apparatus.

The X-ray cross-sectional inspection device uses a scanner consisting of a multi-element X-ray detector arranged in an arc shape, facing a fan-shaped X-ray source, with the subject at the center. The object is rotated around the center to irradiate the object from multiple directions, and electrical signals corresponding to the X-ray intensity distribution are reproduced as a cross-sectional image through computer processing.

The detector used in this device must be able to detect with high accuracy a dose range of about three orders of magnitude, from the X-ray dose that has passed through the air space to the attenuated figure dose that has passed through the bones of the human body.

In order to obtain a good reproduced image, the X-ray dose detection accuracy is required to be about 5 in the most attenuated condition.

It has been found that the dominant noise sources mixed into the 000 signal are leakage current flowing from the high voltage electrode through the insulator and noise due to vibration of the electrode plate generated when the scanner rotates.

Since the output of the ionization chamber X-ray detector is proportional to the effective volume of the ionized space between the electrodes into which the X-rays are incident, the spacing between the electrodes must be uniform.

Furthermore, as a characteristic of a multi-element detector for an X-ray transversal inspection device, if an output due to the amount of X-rays incident on a negative element is also generated from surrounding elements, a good image cannot be obtained.

A detector with such characteristics will be explained.

As shown in FIG. 1, a multi-element electrode 2 is installed inside an airtight container 1, and an output signal can be taken out through an airtight terminal 4.

The X-rays enter the container through the entrance window 3 and ionize the gas filled therein.

The filling gas is a high-pressure gas that has a large absorption coefficient for incident X-rays.

It is generally used as an inert gas alone or as a mixed gas.

As shown in FIG. 2, the structure of the electrode 2 is a metal holding frame 5.
, 6 and an electrode plate 7 electrically insulated from them.
．． 8 is fixed.

The electrode plates 7 and 8 are arranged alternately with signal electrodes and high voltage electrodes, and one element of the detector is composed of one signal electrode and high voltage electrodes on both sides thereof.

The signal electrode is usually used at ground potential, the high voltage electrode is applied with a positive or negative voltage, and the potential between the electrodes is 1000 ■/cIn.
It is used at ~10,000 ■/CIrL.

FIG. 3 is a diagram showing the structure of an embodiment of the present invention for fixing the holding frame and the electrode plate.

In the figure, a groove 10 is machined in the holding frame 5, and two holding plates are fixed in parallel to each other.

An insulator 9 is attached to the electrode plates 7 and 8 at the portions to be fitted into the grooves to maintain electrical insulation from the metal holding frame.

Therefore, leakage current from the high voltage electrode flows into the holding frame without directly flowing into the signal electrode, thereby preventing noise.

This corresponds to a guard ring in a normal ionization frost detector.

The grooves formed in the holding frame are formed at a predetermined pitch determined by the thickness of the element.

This dimensional accuracy is important as it relates to variations in signal output, and can be easily reduced to 1/10 or less by machining, thereby preventing pitch variations when assembling the electrodes.

As the material for the electrode plates 7 and 8, it is effective to use a material that has a large absorption coefficient mainly for fluorescent X-rays in order to block scattered X-rays and fluorescent X-rays and prevent them from entering peripheral elements.

In the example, a 200 μm thick iron plate was coated with 25
A plate plated with silver to a thickness of μm was used.

As a result, the effects of preventing ionized electrons from flowing into adjacent elements and leaking of X-rays were sufficient, and output signals with good separation between elements were obtained.

Microphonic noise due to vibrations generated when the scanner rotates does not occur because the two long sides of the electrode plate are fitted into the grooves and fixed, and the noise due to vibrations can be ignored.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the appearance of an X-ray detector according to the present invention, and FIGS. 2 and 3 are diagrams explaining the structure of an electrode according to the present invention. 5... Metal holding frame, 7, 8... Electrode, 9... Insulator, 10... Groove.

Claims (1)

[Scope of Claims] 1. An ionization chamber type X-ray detector comprising: a plurality of metal plate electrodes arranged substantially parallel to each other at predetermined intervals; a gas medium that is impermeable; a metal holding frame disposed perpendicularly to the electrodes and parallel to each other in order to airtightly hold the gas medium together with the electrodes; and the electrodes are disposed on the metal holding frame. An X-ray detector characterized in that the X-ray detector is supported in a recess through an insulator. 2. In the X-ray detector according to claim 1,
An X-ray detector characterized in that the electrodes are made of an iron plate plated with silver on both sides.