RU104733U1 - DETECTOR FOR COORDINATE REGISTRATION OF X-RAY RADIATION AND ANNIHILATION GAMMA QUANTUM - Google Patents

Abstract

 A detector for coordinate registration of x-ray radiation and annihilation gamma rays, comprising a multiwire proportional chamber and two layers of metal foil converters located on both sides of the chamber, characterized in that the converters are composed of foil strips with a length equal to the sensitive height of the chamber, while the foil strips stretched in a vertical direction with a small step along the width of the camera at an acute angle relative to the direction of movement of the gamma ray, and the camera is equipped with a spectroscope for registration of optical fibers electroluminescent signals from drifting electrons.

Description

The proposal relates to techniques for measuring the coordinates of the interaction of x-rays and annihilation gamma rays in a substance and can be used in medicine, biology, and in the radiography of industrial products.

Currently known devices, collimators used in conjunction with gas-filled multi-wire proportional cameras [A.P. Jeavons, C. Gate. The Proportional Chamber Gamma-Gamma. IEEE Trans. Nucl. Sci., 1975, v. NS-22, N 1, p. 640; Yu.V. Zanevsky. Elemental particle detectors. M. Atomizdat, 1978].

In these devices, the drift space in front of the camera is filled with solid converters with channels. The collimator channels can be round or square with a diameter of 0.8 mm to 3 mm. Photoelectrons formed by the interaction of γ-quanta with the walls of the converter-collimator drift through the channels under the influence of an electric field into the sensitive region of the chamber.

A significant disadvantage of the device is that the spatial resolution of the detector is determined by the channel spacing. With an increase in γ-ray energy of more than 50 keV due to absorption in photoelectron converters, the detection efficiency drops to 10%. In general, such detectors have a rather complicated design, low spatial resolution and do not provide high detection efficiency, which leads to the use of large doses of radiation per unit area of the investigated object.

The closest technical solution in essence and the achieved result is the design of a detector for coordinate registration of x-rays and annihilation gamma rays, containing a multi-wire proportional camera and two layers of metal foil converters located on both sides of the camera. [J.E. Bateman and JFConnolly. AMWPC gamma camera for imaging ^99m Tc radionuclide distributions. RL 77-031 / B, 1977]. The material used in the device depends on the radiation energy, and the detection efficiency is determined by the probability of the release of photoelectrons from the cathode foil. In an assembly consisting of 25 proportional chambers with tin foil cathodes 30 μm thick, the detection efficiency of γ quanta with an energy E _γ = 140 keV is only 16.5%.

Significant disadvantages of the prototype are manifested in the low efficiency of X-ray detection in the energy range E _γ = (50 ÷ 200) keV due to the incorrect position of the converters relative to the direction of γ-quanta, which contradicts the photoelectric effect, due to which the γ-quantum is detected in the camera.

A beam of gamma rays passing through matter is attenuated exponentially

N = N ₀ · e ^-µx ,

where N ₀ is the number of γ-quanta in the initial beam; N is the number of gamma rays passing through a layer of matter with thickness x; µ is the linear attenuation coefficient (in cm ² g ^-1 ).

The linear attenuation coefficient µ is related to the total cross section σ of scattering or absorption per atom of a substance by the expression

μ = n

where n is the number of atoms of the absorber in 1 cm ³ and σ is measured in cm ² .

If the energy of the quantum beam is ≤511 keV, then it is attenuated as a result of photoelectric absorption, and in order to significantly attenuate the flux of γ-rays, it is necessary to use large thicknesses of the converters. Absorption is significantly enhanced if the converter material has a large atomic number. However, photoelectrons formed in this case, due to low energy and high ionization losses, are not able under these conditions to leave the substance and get into the sensitive gas of the detector - this determines the unreliability of the readings.

The technical task and the positive result of the claimed detector is to eliminate these drawbacks and increase the efficiency of registration of x-rays and annihilation gamma-quanta with gas-filled coordinate detectors, increase the accuracy of measuring the coordinates of the interaction points of γ-quanta in the converter.

This task and technical result are achieved due to the fact that the detector for coordinate registration of x-rays and annihilation gamma rays, containing a multiwire proportional chamber and two layers of metal foil converters located on both sides of the chamber, while the converters are made up of foil strips of length equal to the sensitive height of the chamber, while the foil strips are stretched in the vertical direction with a small step along the width of the chamber at an acute angle relative to the direction of zheniya gamma quantum, wherein the camera is equipped with optical fibers for wavelength-registration electroluminescent signals from drifting electrons.

The described detector is disclosed on the attached graphic materials, where:

figure 1 shows a schematic structural diagram of the proposed detector;

figure 2 is a simplified diagram of the detector of figure 1;

figure 3 - distribution of photoelectrons in the detector;

figure 4 - the assembly process of technical and electronic circuits of the detector;

figure 5 is a General view of the detector assembly.

The detector for coordinate registration of x-rays and annihilation gamma-quanta contains a multiwire proportional chamber 1, two layers of converters 2 and 3, made of metal foil and located on both sides of the camera. These converters are composed of strips 4, 5 foil, their length is chosen equal to the sensitive height of the chamber 1; the strips are stretched in the vertical direction and with a small step along the width of the chamber at an acute angle relative to the direction of movement of the gamma ray. The chamber is equipped with cathode 6 and anode 7 elements in the form of tensioned cathode and anode wires, while the wires of the first cathode 8 are stretched in the direction perpendicular to the direction of the anode wires 9, and the second cathode 10 is parallel to the anode wires. In the detector, to determine the place of interaction of the gamma-quantum and increase the efficiency of gamma-gamma coincidences, spectroscopic optical fibers 11 are used, which record the electroluminescent signal from drifting electrons resulting from the operation of the x-ray emitter 12; charges are recorded by charge-sensitive amplifiers 13, 14, then, by processing signals using a computer (not shown in the drawing), the horizontal "X" and vertical "Y" coordinates of the gamma-quantum interaction point in the 2-3 converter are taken into account taking into account the angular distribution (Fig. .3) photoelectrons and taking into account different values of velocities, gamma-ray path length in the detector technical circuit.

The developed detector is superior to its analogues in radiation resistance, in counting speed and spatial resolution, which allows for light radiation loads to obtain clearer images of the object under study.

The operation of the detector, as it is partially described above, is carried out as follows. On the cathode 6 and anode 7 wires supply potential; X-ray radiation - 12 act on the layers of the converters 2 and 3 and their sensitive strips 4, 5, made of metal with a high value of "Z" ("Z" is the atomic number of the absorbing substance), in particular, from tantalum, while the effective length the gamma-ray path in the converter layers will be 230-350 microns. Ensuring the drift of ionization electrons to the anode wires 7 is ensured by several strips 4, 5 of each layer of the Converter; The location of the interaction of the gamma quantum with the strips is used by spectroscopic optical fibers 11 and amplifiers 13, 14, from which the signals are fed to a computer for processing. The installation of converters 2, 3 at an angle of about 5 ° (Fig. 1, 2) to the direction of travel of gamma rays allows with absolute accuracy to read and recognize the angular distribution of photoelectrons at several speeds (Fig. 3). A complete picture of the process is formed by the working space of this detector (Figs. 4, 5), which allows working with minimal radiation loads and at the same time obtaining more accurate and clear images of the studied object with superior counting speed (more than an order of magnitude compared to the best samples of such detectors) and 2-4 times more accurate in spatial distribution.

Claims (1)

A detector for coordinate registration of x-ray radiation and annihilation gamma rays, comprising a multiwire proportional chamber and two layers of metal foil converters located on both sides of the chamber, characterized in that the converters are composed of foil strips with a length equal to the sensitive height of the chamber, while the foil strips stretched in a vertical direction with a small step along the width of the camera at an acute angle relative to the direction of movement of the gamma ray, the camera is equipped with a spectroscope for registration of optical fibers electroluminescent signals from drifting electrons.