RU2748153C1 - Scintillation detector - Google Patents

Abstract

FIELD: ionizing radiation detecting.

SUBSTANCE: invention relates to the field of technology for detecting ionizing radiation. In the detector, an array of single scintillation cells with wells for collecting light is made in the form of a monolithic unit. The scintillator is divided into cells of any geometric shape by a structure obtained by locally removing the main sheet material to its entire depth along the perimeter of the cell boundaries. The vacant space is filled with a material with a reflective property. The reflective screens of the scintillation unit are made in the form of two separate sheets adjacent to the flat surfaces of the unit on both sides. The scintillation unit, a printed circuit board with an array of silicon photodetectors, reflective screens and the detector module cover have reference holes for precise alignment of the detector components with fasteners during assembly.

EFFECT: simplification of the design, an improvement in the manufacturability of scintillation cells of any geometric shape, and an increase in the accuracy of spatial measurements for large detection areas.

1 cl, 7 dwg

Description

State of the art

The invention relates to the field of technology for detecting ionizing radiation using scintillation segmented detector modules (detectors) and can be applied in its various industries:

- in medical technology, in particular, in positron emission tomography (PET), single-photon emission computed tomography (SPECT); mass spectroscopy and high performance liquid chromatography (HPLC) which are based on the detection of energetic charged particles or photons.

- in introscopy systems for remote inspection;

- in high-energy physics, for example, in scintillation detectors for experiments at accelerators with colliding beams and linear accelerators.

Currently, scintillation detectors based on individual scintillation cells assembled into modules are widely used. The scintillation cells are separated from each other by opaque materials that surround the outer surfaces, which prevents the light produced in each of them from entering adjacent cells. Such detectors make it possible to increase the energy, temporal and spatial resolution, and the sensitivity of the detectors. The modules are assembled from separate scintillation cells, as a rule, square cells with a square side of 20-30 mm and a thickness of 3-5 mm, with a reflective coating made of special paint or wrapped in a reflective material, which are successively element-wise aligned with silicon photodetectors.

The performance of segmented scintillation detector modules is highly dependent on how precisely the well scintillation cells and emitted light collection elements (photodetectors) are assembled in the detector, as well as on the characteristics of the reflective surfaces used to coat the scintillator.

An important task is also to ensure the collection of light in single cells of scintillation detectors to increase the sensitivity of the recording system and improve the parameters of such detectors.

Work on optimizing the design of unit cells of scintillation detectors has been going on for a long time.

So US patent US 8,085,398 [US 8,085,398 B2 Dec 27 2011, Dushkan et al, CONCAVE COMPENSATED CELL FOR THE COLLECTION OF RADIATED LIGHT] describes a scintillation cell with a concave well for collecting emitted light, which is located on one of its flat sides and shows how you can organize the collection of light directly from the well using a photodetector.

Cell for collecting emitted light, which is a body made of a scintillator by casting or extrusion and having a concave well on one of its flat side. The well works as a lens for collecting light scintillated in the cell. The flat surfaces of the scintillation cell are covered with a reflective material. Reflective films, coatings, foils and paints are used as reflective material. The photodetector with its active photosensitive side faces the concave reflective surface of the hole without an intermediate medium (only air). The cell can be in the form of a square or a hexagon. The patent describes well the design of the unit cell. Separate methods and techniques for manufacturing scintillation cells, which can be square or hexagonal, are described. There is an indication that several cells in a detector can be positioned together.

The patent does not show what the design of a detector based on separate scintillation cells can be. Creation of a technological design of segmented scintillation detector modules is a separate engineering problem that had to be solved for the application of segmented scintillation cells in various branches of detection technology.

The amount of registered light emitted by the scintillator depends on the accuracy and stability of the position of the photodetector relative to the well, as well as on the choice of materials for the reflective coating and the design of the reflector, which ultimately determines the temporal, energy and spatial resolution of the detector.

Known created by the international collaboration CALICE scintillation detector unit AHCAL [Felix Sefkow and Frank Simon On behalf of the CALICE Collaboration "A highly granular SiPM-on-tile calorimeter prototype" 18th International Conference on Calorimetry in Particle Physics (CALOR2018) IOP Conf. Series: Journal of Physics: Conf. Series 1162 (2019) 012012]. The AHCAL detector unit consists of a printed circuit board with an array of silicon photodetectors, a set of individual single scintillation cells and signal reading electronics (Fig. 1). The position of the photodetectors on the board is determined by the granulation of the detector. The AHCAL detector unit is completed with separate single scintillation cells of a square shape 30 × 30 mm and a thickness of 3 mm with a well for collecting the emitted light, made by casting. Each unit cell is wrapped in a reflective foil with an opening for light to escape. Specialized robotic packaging equipment has been developed and manufactured for packing scintillation cells into reflective films. Single scintillation cells wrapped in reflective film were sequentially, using robotics, glued to the printed circuit board with the side having a dimple (Fig. 2).

When installing single cells in the detector, it was important to ensure the most accurate position of each scintillation cell relative to its paired photodetector. The use of robotics made it possible to solve the problem.

The AHCAL detector unit used orthogonal segmentation for the array of cells and the scintillation square cells were the same size, which meant that only one set of foundry tooling, one set of tooling for a packaging machine and one set of tooling for a robot performing installation of single cells on a board were manufactured.

The design of the AHCAL detector unit, taken as a prototype, is well suited for mass production in the case when it is required to produce a large number of identical scintillation cells of a square or hexagonal shape. This requires one mold, one size of reflective wrapper, one tooling and one packaging device.

There are cases when detectors with free-form scintillation cells are needed. For example, a polar coordinate system is often used to create an array of cells, where the cell sizes are specified by the values of radii and angles, the so-called "R-ϕ" geometry (Fig. 3).

To create an array of cells with an "R-ϕ" geometry, it is required to manufacture cells of different sizes, which means that there is a need to manufacture a large number of expensive specialized equipment for each standard size of the scintillation cell:

- molds for casting for each standard size of scintillation cells;

- tooling or equipment for the manufacture of individual wrappers for each standard size of the scintillation cell;

- equipment for packing each standard size of scintillation cells into reflective foil for a packaging machine;

- sophisticated equipment for precise positioning of scintillation cells wrapped in reflective foil on a printed circuit board;

The essence of the invention

The technical objective of the invention is to simplify the design, improve manufacturability, manufacture scintillation cells of any geometric shape, increase the accuracy of spatial measurements for large areas of detection.

The technical result is achieved due to the fact that the array of single scintillation cells is made in the form of a monolithic block of single scintillation cells (scintillation block) made of a single scintillator sheet, which is divided into cells of any geometric shape by a structure obtained by local removal of the main material of the sheet for its entire the depth along the perimeter of the boundaries of the cells and filling the vacant space with a material with a reflective property, while the sheet material forming the cells retains its geometric position in the block. The reflective screens of the scintillation unit are made in the form of two separate sheets adjoining the flat surfaces of the unit on both sides. The reflective screen, which is located between the surface of the scintillation unit with holes and the printed circuit board with silicon photodetectors, has holes for light coming out of the holes of the unit cells in the direction of the paired photodetectors. The reflective screen, adjacent to the flat surface of the scintillation unit, which does not have holes, is made solid and is pressed against this surface by a cover. The scintillation unit, printed circuit board with an array of silicon photodetectors, reflective screens and the detector module cover have at least two reference holes for precise alignment of the detector components with fasteners when assembling the scintillation detector module.

Description of figures

FIG. 1. Scintillation detector unit AHCAL.

1 - Printed circuit board with silicon photodetectors;

2 - A separate single scintillation cell wrapped in an individual envelope made of reflective material.

Individual single scintillation cells (2) wrapped in reflective foil are sequentially installed on a printed circuit board with silicon photodetectors (1) and fixed on it with glue.

FIG. 2. Single scintillation cells of the AHCAL detector with holes for focusing the emitted light and placement of photodetectors.

1 - Printed circuit board with silicon photodetectors.

2 Individual single scintillation cell wrapped in an individual envelope made of reflective material.

3 Separate, single scintillation cell with a well, unwrapped in an envelope made of reflective material.

Each scintillation cell has a hole for focusing the emitted light and placing a photodetector paired with it. The position of the well of the cell and the photodetector is clearly visible in the example with the installed scintillation cell without wrapper 2.

FIG. 3. An array of scintillation cells with dimensions specified in the polar coordinate system.

The dimensions of the cell are determined by the radii R1, R2 and the angle ϕ.

Fig. 4. Scintillation detector module.

1- Printed circuit board with silicon photodetectors.

4 - Monolithic unit of single scintillation cells (scintillation unit).

5 - Reflective screen with an array of holes for light coming out of the wells of the unit cells.

6 - Solid reflective screen.

7 - Cover of the detector module.

8 - Scintillation detector module, assembled.

FIG. 5. Monolithic unit of single scintillation cells (scintillation unit).

Reference holes (P1, P2) are made in the block, corresponding to similar holes in the printed circuit board 1, cover 7 and reflective screens 5 and 6.

Fiducial holes are used to accurately align the detector components when assembling the scintillation detector module.

FIG. 6. An example of the positioning and fastening of the assembled scintillation detector module 8.

1 - Printed circuit board with silicon photodetectors.

4 - Monolithic unit of single scintillation cells (scintillation unit).

5 - Reflective screen with an array of holes for light out of the holes for paired photodetectors.

6 - Solid reflective screen.

7 - Cover of the detector module.

9 - Fasteners of the module.

10 - Installation mounting elements.

11 - Mounting plane.

12 - Pins for assembling and positioning the module. The plane for mounting the detector 11 has holes with pins 12, which correspond to the reference holes (P1, P2) on the printed circuit board 1, the scintillation unit 4, reflective screens 5 and 6, the cover of the detector module 7. The scintillation detector module is pre-assembled using technological pins, similar to pins 12 and fasteners 9. The module is fastened to the mounting plane 11 using fasteners 10.

FIG. 7. Samples for research:

2 - A separate single scintillation cell wrapped in an individual envelope made of reflective material.

13 Monolithic unit of single scintillation cells 3 × 3 (scintillation unit 3 × 3)

14 - Array of nine separate single scintillation cells, analogous to the prototype detector unit AHCAL.

A monolithic block of single scintillation cells (photograph) 13, contains nine single scintillation cells, the dimensions of which correspond to the cells of the AHCAL scintillation detector unit (square 30 mm × 30 mm, thickness 3 mm). There are no reflective screens in the photo.

The array of individual scintillation cells (photograph) 14 is assembled from nine separate single scintillation cells wrapped in individual envelopes made of reflective material, similar in design and size to the cells of the AHCAL scintillation detector unit. Reflective material of the cell wrap - ESR of the company 3 M.

Implementation of the invention

For the study, samples of the prototype single scintillation cells of the AHCAL scintillation detector unit, wrapped in individual envelopes made of reflective material, and samples of a monolithic unit of scintillation cells with two reflective screens (one for each side of the scintillation unit) were used, identical in size and material (Fig. 7). ).

In the process of creating samples of a monolithic block of scintillation cells, the local removal of the main material of the scintillator sheet to its entire depth along the perimeter of the cell boundaries was carried out by milling on a CNC machine. The space vacated in the workpiece after milling was filled with a material with a reflective property (polymer modified epoxy glue with a reflective TiO2 additive). For the manufacture, we used a sheet material scintillator BC-408 from Saint-Gobain Crystals.

The reflective screens were made of 3M ESR reflective sheet film material by laser cutting on a special CNC machine.

Comparative results of studying the properties of a monolithic block of scintillation cells 13 consisting of a single array of separate scintillation cells having a single reflective screen on each side with a sample similar to the design of the prototype of the scintillation detector AHCAL 14, consisting of an array of single scintillation cells 3 showed the following:

1. The light output of an unirradiated monolithic unit of scintillation cells practically does not differ from the light output of unirradiated individual single scintillation cells of the prototype. The average of pure photoelectrons (PMEs), obtained by measuring 10 samples of individual single scintillation cells (the material of the cells is VS-408, is similar to the prototype), wrapped in individual envelopes made of reflective ESR material, is 21.9 ± 0.9 PME. The average PMT number measured on 16 cells for a monolithic block of scintillation cells equipped on both sides with reflective screens made of reflective ESR material is 21.4 ± 0.5 PME. The measurement results show that the light yield of scintillation cells made in the form of a monolithic block is 98 ± 4.7% with respect to the light yield of individually wrapped cells. Single reflective screens for the scintillator array, increase the uniformity of light collection compared to scintillators wrapped in individual envelopes made of reflective material. All measurements were carried out under the same conditions on an experimental stand, using silicon photodetectors S12572-015P from Hammatsu as a photodetector.

2. Due to the possible loose fit of the reflective screen to the flat surface of the monolithic block of scintillation cells, light may flow between adjacent cells. The likelihood of such a process was investigated using a ⁹⁰ Sr radioactive source. and cosmic radiation. The probability of detecting light in a cell adjacent to the irradiated cell with a thickness of the bonding and, at the same time, separating layer of 1 mm turned out to be equal to no more than 0.15% for measurements on a radioactive source of ⁹⁰ Sr and less than 0.2% for studies on cosmic radiation.

3. The radiation resistance of a monolithic block of scintillation cells and prototype cells was investigated at the IBR-2 JINR reactor. Scintillator samples were irradiated with a flux of neutrons and gamma quanta to an absorbed dose of 0.65 Mrad (dose rate was 2.1 Crad / hour). The average light yield of the scintillation cells, measured after irradiation, was 14.0 ± 0.2 p.e. for individual scintillation cells and 13.7 ± 0.6 p.e. for cells that form a monolithic block. Thus, the relative light yield (normalized to the light yield of individual scintillation cells before irradiation) when the samples are irradiated to a dose of 0.65 Mrad and a dose rate of 2.1 kRad / h is 63.9 ± 4.3% for individually wrapped scintillation cells and 62.4 ± 4.9% for cells inside a monolithic scintillation unit. cells.

Within the limits of measurement errors, the radiation resistance of individual scintillation cells wrapped in individual envelopes made of ESR reflective material and scintillation cells of a monolithic block of scintillation cells with reflective screens made of the same ESR material practically coincides.

Claims (1)

A scintillation detector, including at least one array of single scintillation cells with light collection wells, isolated by light between themselves and with holes for light to exit towards paired cells of silicon photodetectors, which are arranged in an array on a printed circuit board adjacent to the cells, characterized in that an array of single scintillation cells is made in the form of a monolithic block of single scintillation cells (scintillation block) made of a single scintillator sheet, which is divided into cells of any geometric shape by a structure obtained by local removal of the main sheet material to its entire depth along the perimeter of the cell boundaries and filling the vacated places with a material with a reflective property, while the cell-forming material of the sheet retains its geometric position in the block; reflective screens of the scintillation unit are made in the form of two separate sheets adjacent to the flat surfaces of the unit on both sides; a reflective screen, which is located between the surface of the scintillation unit with holes and a printed circuit board with silicon photodetectors, has holes for light coming out of the holes of the unit cells in the direction of the paired photodetectors; the reflective screen, adjacent to the flat surface of the scintillation unit, which does not have holes, is made solid and is pressed against this surface by a cover; scintillation unit, printed circuit board with silicon photodetector array, reflective screens and detector module cover each have at least two reference holes for precise alignment of detector components with fasteners when assembling the scintillation detector module.

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