CN114496340B - A radiographic image screen based on the Cherenkov effect - Google Patents

Abstract

The invention provides a radiation image screen based on the Cherenkov effect, which provides a scheme for the ultra-fast radiation imaging technology of energy clamping threshold, according to the scheme, through optimizing the conversion target and the light guide array, the problem that detection efficiency and spatial resolution based on the Cherenkov effect imaging are compatible is solved. The invention provides a radiation image screen based on a Cherenkov effect, which comprises a conversion target and a light guide array; the conversion target is arranged on the light inlet side of the light guide array, and the distance between the conversion target and the light guide array is within 2 mm; the light guide array has high transparency and light guide image transmission function, and is mainly composed of a plurality of light guides which are arranged, wherein the light guides are made of radiation-resistant optical materials with high optical transparency, and a black anti-crosstalk layer is coated between the light guides; the core diameter of the light guide is 0.5-1 mm.

Description

A radiographic image screen based on the Cherenkov effect

Technical Field

The present invention belongs to the technical field of high-energy ray imaging, and specifically relates to a ray image screen based on the Cherenkov effect, which is mainly used for gamma or X-ray and electron beam imaging, and particularly relates to an ultrafast ray image conversion screen with energy card threshold characteristics.

Background Art

With the development of high-energy-density physical devices, their intensity is getting higher and higher, and their duration is getting shorter and shorter. For example, the ultrafast gamma-ray device and inertial confinement fusion device under construction often have physical process durations of nanoseconds or even subnanometers. Rays of different energies reflect different mechanisms of action. It is an indispensable and important means to diagnose the evolution of morphological parameters in the radiation zone of the device and distinguish the mechanisms of action of different physical processes through ultrafast ray imaging technology. At the same time, due to the complexity of ray action, the source area rays often form a low-energy scattering background after scattering with the environmental medium, which interferes with the acquisition of effective images. The application of effective energy card threshold detection can obtain higher quality target images.

At present, scintillators are mostly used as radiation conversion screens for ray detection and imaging. Most scintillators respond to rays of various energies, but the time response is not fast enough, mostly above the nanosecond level, and there is a certain slow decay time component. The Cherenkov converter has both ultra-fast time response capability (hundreds of ps level) and energy threshold characteristics, and can be used for imaging rays in a specific energy range. However, due to the low luminous efficiency of the Cherenkov effect, it is usually necessary to increase the thickness of the Cherenkov radiator to improve the detection efficiency and light output. However, increasing the thickness of the Cherenkov radiator will reduce the spatial resolution of the imaging.

Summary of the invention

The present invention provides a radiation image screen based on the Cherenkov effect, and provides a solution for ultrafast radiation imaging technology with an energy card threshold. The solution solves the problem of balancing detection efficiency and spatial resolution of imaging based on the Cherenkov effect by optimizing conversion targets and applying light guide arrays.

In order to achieve the above object, the technical solution of the present invention is as follows:

The ray image screen based on the Cherenkov effect provided by the present invention is mainly suitable for MeV energy ray imaging, which includes a conversion target and a light guide array; the conversion target is arranged on the light incident side of the light guide array, and the distance from the light guide array is within 2 mm; the light guide array has high transparency and light guide image transmission function, and is mainly composed of a plurality of light guides arranged, the light guides are made of radiation-resistant optical materials with high optical transparency, and the light guides are coated black to add an anti-crosstalk layer; the core diameter of the light guide is 0.5-1 mm; the thickness x of the conversion target is determined by the following formula:

In the formula, is the energy loss rate of electrons in the conversion target material; e is the charge of the electron, v is the velocity of the electron; _m0 is the rest mass of the electron; N and Z are the number of atoms and atomic number per unit volume of the conversion target material, respectively; E is the electron energy; β = v/c; I is the average excitation and ionization potential of atoms in the conversion target material.

Furthermore, the thickness of the light guide array is 3 cm to 5 cm.

Furthermore, the plurality of light guides are cured by using thermal infrared curing adhesive to form an array.

Furthermore, the light guide array is arranged inside an external package made of a light and high-hardness material.

Furthermore, the external package is blackened and frosted.

Furthermore, the external packaging is made of aluminum material.

Furthermore, the conversion target is made of beryllium and has a thickness of 2 mm; the conversion target is made of organic glass and has a thickness of 3 mm to 5 mm.

Compared with the prior art, the present invention has the following beneficial effects:

1. The radiation image screen provided by the present invention has an energy threshold characteristic, which can image radiation images in a specific energy range or an energy range of interest, and reduce the interference of background scattered low-energy rays on imaging.

2. Compared with the existing scintillator image screen, the radiation image screen provided by the present invention has a shorter luminous duration and a faster response time capability, reaching the order of hundreds of picoseconds, and has no slow component decay time.

3. The radiation image screen of the present invention adopts a radiation-resistant light-conducting array and is suitable for use in a high-intensity radiation environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a radiation image screen based on the Cherenkov effect of the present invention;

FIG2 is a schematic diagram of the arrangement of the light guide array of the present invention;

FIG3 is a schematic diagram of testing a quartz optical fiber array image screen in an embodiment of the present invention;

FIG4 is a schematic diagram of the luminescence performance of the quartz fiber array image screen 4 measured on a cobalt-60 gamma-ray source in an embodiment of the present invention;

FIG5 is a schematic diagram of the imaging result of the tungsten block resolution card measured on the quartz fiber array image screen 4 on the cobalt 60 gamma ray source in an embodiment of the present invention;

FIG6 is a schematic diagram of the time response waveform of the quartz fiber array image screen 4 measured by a pulsed electron beam of less than 10 ps in an embodiment of the present invention.

Figure numerals: 1 - conversion target, 2 - light guide array, 21 - light guide, 3 - external packaging, 4 - quartz fiber array image screen, 5 - cobalt source, 6 - collimator, 7 - shielding body, 8 - tungsten block resolution card, 9 - reflector, 10 - CCD camera.

DETAILED DESCRIPTION

The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments. It should be understood by those skilled in the art that these embodiments are only used to explain the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

The present invention provides a ray image screen based on the Cherenkov effect, which is implemented based on the principle of optical waveguides and is a thick array-type Cherenkov image conversion screen. The ray image screen is mainly composed of a conversion target 1, a light guide array 2 and an external package 3. For the incident charged particle beam, the conversion target 1 can be used as a primary energy threshold. For the ray beam, the conversion target 1 can form a secondary charged particle beam image. The charged particle image that meets a certain threshold energy is transported in the light guide array 2 to generate a Cherenkov optical image, which is then collected and processed by the CCD camera at the back end. The ray image screen provided by the present invention has an ultra-fast time response and an energy threshold characteristic, which can be used for both measurement of ray images in a specific energy range and ultra-fast time-resolved ray image diagnosis.

Based on the Cherenkov effect generated by the transport of charged particles in the material, the ray image is converted into a visible light image that can be collected and transmitted. According to the Cherenkov effect, when the speed of the charged particle group in the medium exceeds the speed of light in the medium, Cherenkov light radiation will be generated, that is, only charged particles greater than a certain energy threshold can generate Cherenkov light in the material, and its energy threshold is related to the refractive index of the material. Because high-energy rays (such as gamma) can generate secondary charged particles when interacting with the medium, high-energy rays can also realize imaging based on Cherenkov light radiation. According to the theory of optical waveguides, only light that meets the critical angle of transmission can be transmitted in the optical waveguide, forming the so-called bound light. Under the constraint of the waveguide, the transmission of optical images with certain spatial resolution can be realized. Therefore, combining the principles of Cherenkov radiation and optical waveguides can solve the problem of insufficient imaging detection efficiency and realize energy threshold imaging of rays; at the same time, because the duration of Cherenkov luminescence is extremely short, it has ultra-fast response capability.

According to the above principle, the radiation image screen based on the Cherenkov effect provided by the present invention mainly comprises two parts. The first part is the charged particle conversion target 1 at the front end, and the second part is the image array detector as a Cherenkov radiation converter.

The design of the conversion target 1 is based on the Compton scattering formula (1) to (3) of gamma rays and matter to calculate the energy and angular distribution of electrons:

Where, E _e is the energy of the recoil electron; _Er is the energy of the incident gamma ray; m ₀ is the electron rest mass; c is the speed of light in vacuum; is the recoil angle of scattered electrons; θ is the scattering angle of scattered gamma; is the differential cross section of the gamma ray interaction with the conversion target; Z is the atomic number of the target material; a＝E _r /m ₀ c ² ; r ₀ is the classical electron radius;

The energy loss formula (4) of the interaction between fast electrons and matter can be used to calculate the relationship between the electron emission efficiency and the thickness, and determine the thickness x of the conversion target 1 when the maximum electron yield is emitted;

In the formula, is the energy loss rate of electrons in the conversion target material; e is the charge of the electron, v is the speed of the electron; N and Z are the number of atoms and atomic number per unit volume of the conversion target material, respectively; E is the electron energy; β = v/c; I is the average excitation and ionization potential of atoms in the conversion target material.

Beryllium is a commonly used conversion target. For gamma with MeV energy, the above theory combined with Monte Carlo method simulation software (MCNP) is used to simulate that when the beryllium target thickness is about 2 mm, the secondary electron conversion efficiency is optimal. If organic glass is used, 3 mm to 5 mm is preferred.

For the design of image array detector, a light guide array 2 with high transparency and high radiation resistance is selected. A single light guide 21 is used as a pixel, and the whole constitutes an array image screen. At present, the glass material of optical light guide 21 with high transparency has a refractive index of about 1.4, corresponding to the minimum energy threshold of electrons that produce Cherenkov radiation light of about 0.2MeV. The image screen of the present invention is mainly suitable for MeV energy ray imaging. For gamma rays with MeV energy, according to the above theoretical formulas (1) to (4), combined with the Monte Carlo method and with the help of large-scale ray particle transport software MCNP simulation, the spatial distribution of secondary electron transport in optical glass and the imaging spatial resolution characteristics can be obtained. According to the simulation results, the diameter of the single pixel light guide 21 in the present invention is 0.5mm~1mm, and the thickness of the light guide array 2 is 3cm~5cm, which can achieve the result of comprehensive optimization of spatial resolution and detection efficiency.

For charged particle beams, the conversion target 1 can also be used as a threshold medium. The loss law of different materials (commonly used are metal materials, such as copper and aluminum) for electrons of different energies can be calculated by formula (4) to determine the selection and size of the material.

As shown in FIG. 1 and FIG. 2 , the radiation image screen based on the Cherenkov effect of the present invention mainly includes a conversion target 1, a light guide array 2 and an external package 3. For a charged particle beam, the conversion target 1 can be used as a primary card threshold, or it can be omitted, and there is no restriction on its necessary use. The conversion target 1 needs to be close to the light guide array 2 to reduce the influence of the spatial dispersion of the emitted charged particle beam on the spatial resolution of the overall image screen. The light guide array 2 includes a plurality of light guides 21, and the light guides 21 are made of a material with high optical transparency ... c is the speed of light in a vacuum, and n is the refractive index of the material), and an optical lightguide material with the required refractive index is selected. The material of the lightguide array 2 selects radiation-resistant optical materials to reduce the impact of the optical transparency of the lightguide 21 material being affected by the irradiation. The parameters of the lightguide array 2 are based on the specific selection principles mentioned above. The outer cladding of the single-pixel lightguide 21 in the lightguide array 2 needs to be blackened to reduce the crosstalk of optical signals between the lightguides 21. At the same time, the lightguides 21 and 21 are cured with thermal infrared curing glue to form an array. The external packaging 3 of the entire array screen adopts a lightweight and high-hardness material, and aluminum material can be specifically selected. In order to reduce the background stray light of the image, the material of the external packaging 3 needs to be blackened and frosted, and blackened and frosted.

When the radiation image screen of the present invention detects and images based on the Cherenkov effect, the incident source is a charged particle beam or a charged particle beam is generated by a radiation beam passing through a conversion target 1, and the charged particles are transported in a light guide array 2 to generate Cherenkov light to form an optical image; the energy threshold of the radiation beam that generates Cherenkov light is determined by the formula of the Cherenkov effect and the physical process of radiation generating secondary charged particles, and meets the general principle of the interaction between radiation and matter.

The imaging experiment of FIG3 is used to evaluate the feasibility of the ray image screen based on the Cherenkov effect of the present invention. The experimental system includes a quartz fiber array image screen 4, a cobalt source 5, a collimator 6, a shield 7, a tungsten block resolution card 8, a reflector 9 and a CCD camera 10. In the experiment, a cobalt 60 gamma ray source is used. The ray source emits through the collimation hole. A quartz fiber array image screen 4 is placed at a certain distance from the gamma source. A 3mm organic material plate is placed close to the incident surface of the fiber array screen as a conversion target 1. The entire input surface of the array image screen is uniformly irradiated by the ray. The optical image output by the array screen is refracted by the reflector 9, and the image is collected and recorded by the lens-coupled CCD camera 10. In order to reduce the direct irradiation and spatial scattering of the CCD camera 10 by the ray, the array screen and the CCD camera 10 are shielded by lead bricks except for the ray incident channel.

FIG4 shows the experimental results of the present invention, indicating that the image screen has uniform light output. A tungsten block resolution card 8 is placed at the incident surface of the image screen, and the tungsten block resolution card 8 is close to the incident end surface of the image screen. Gamma rays pass through the tungsten block resolution card 8 and are incident on the image screen. Due to the different intensity of the rays after transmission, a grayscale image similar to the tungsten block resolution card 8 is formed on the image screen. FIG5 is an image formed by the tungsten block resolution card 8 through the image screen under gamma ray irradiation, indicating that the present invention is feasible for using ray imaging.

FIG6 is a time response waveform of the quartz fiber array image screen 4 measured with a pulsed electron beam of less than 10 ps. The waveform includes various responses of the array screen, photodetector, transmission cable, etc., but its half-width is still less than 1 ns, indicating that for a pure quartz array screen, the luminescence duration will be shorter.

Claims (5)

1. A radiation image screen based on the Cherenkov effect, characterized in that: it is mainly suitable for MeV energy radiation imaging, and comprises a conversion target (1) and a light guide array (2); The conversion target (1) is arranged on the light incident side of the light guide array (2), and the distance between the conversion target and the light guide array (2) is within 2 mm; The thickness of the light guide array (2) is 3 cm to 5 cm; the light guide array (2) has high transparency and light guide image transmission function, and is mainly composed of a plurality of light guides (21) arranged in an array, wherein the plurality of light guides (21) are cured by thermal infrared curing adhesive to form an array, and the light guides (21) are made of radiation-resistant optical materials with high optical transparency, and the light guides (21) are coated black to add an anti-crosstalk layer; the core diameter of the light guide (21) is 0.5 to 1 mm; The thickness x of the conversion target (1) is determined by the following formula: In the formula, is the energy loss rate of electrons in the conversion target material; e is the charge of the electron, v is the speed of the electron; _m0 is the rest mass of the electron; N and Z are the number of atoms and atomic number per unit volume of the conversion target material, respectively; E is the electron energy; β = v/c; I is the average excitation and ionization potential of atoms in the conversion target material. 2. The radiation image screen based on the Cherenkov effect according to claim 1, characterized in that the light guide array (2) is arranged inside an external package (3) made of a light and high-hardness material. 3. The radiation image screen based on the Cherenkov effect according to claim 2, characterized in that the external package (3) is blackened and frosted. 4. The radiation image screen based on the Cherenkov effect according to claim 3, characterized in that the external package (3) is made of aluminum material. 5. The radiation image screen based on the Cherenkov effect according to claim 4, characterized in that: the conversion target (1) is made of beryllium and has a thickness of 2 mm; the conversion target (1) is made of organic glass and has a thickness of 3 mm to 5 mm.