RU2594991C1 - Detector of charged particles with thin scintillator - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to detectors of charged particles based on solid-state organic scintillators. Detector of charged particles with a thin scintillator in the form of a plate contains semiconducting photosensor as a converter of the light flashes initiated by charged particles into electric pulses, where arbitrarily thin and completely polished scintillator plate made in the form of equilateral polygon with at least four angles is optically and mechanically connected to the transparent for scintillations and completely polished substrate of the same shape and light refraction coefficient as that of scintillator, and total thickness of sandwich formed by the scintillator and the substrate equals to the size across sensitive surface of the semiconducting photosensor optically and mechanically connected to the sandwich in one of its angles which is grind down and polished to obtain a contact pad with dimensions of sensitive area of semiconducting photosensor, all sandwich surfaces except the rear and with attached semiconducting photosensor, coated with a mirror reflector, and the rear surface is coated with diffusing reflector.

EFFECT: technical result is higher efficiency of light collecting on the sensitive surface of the photosensor.

1 cl, 4 dwg

Description

The present invention relates to the field of scintillation detectors of ionizing radiation, more specifically to charged particle detectors based on solid-state organic scintillators.

Solid-state organic (crystalline and plastic) scintillators have a number of properties that make them the main working substance of α- and β-detectors:

1) the low effective atomic number determines the minimum backscattering of charged particles and their minimum radiation losses (bremsstrahlung); due to this, the instrumental (measured) energy spectrum is minimally different from the physical one - almost all the energy of the particles is spent on ionizing the plastic material and, accordingly, on converting it into light pulses;

2) the scintillator can be made thin enough to be insensitive to gamma radiation while maintaining high detection efficiency of α- and β-particles - thus it is possible to separately measure α- and β- and γ-radiation in mixed radiation fields;

3) plastic scintillators are easy to machine, and during the manufacturing process they can be given quite arbitrary shapes and sizes.

A photosensor is required to convert scintillation bursts initiated by charged particles into electrical signals. Until recently, a vacuum photomultiplier (Photomultiplier Tube - PMT) was a non-alternative photosensor. The main problem of taking light from a thin plastic scintillator with a large area using PMT is the mismatch between the sizes of the photocathode and scintillator, which leads to the loss of part of the photons of scintillation flashes and, as a result, to a decrease in the amplitudes of the electrical signals at the PMT anode.

Several techniques are known to resolve this contradiction. Most of them are described in detail in the monograph [Akimov Yu.K. Photonic radiation registration methods. Dubna: JINR, 2014, 323 pp.]:

- articulation of a thin scintillation plate with a PMT photocathode using a fiber from a well-polished plexiglass or fiberglass in the form of a fish tail (Fig. 1);

- the same thing, but a light guide in the form of a fan;

- the use of fiber optics (a bundle of optical fibers surrounds the scintillation plate around the perimeter or is laid in a special groove in the form of a snake along the front and / or back surface of the scintillation plate).

There are other, more complex solutions. Their common drawback is the large loss of light and, consequently, the increase in the lower energy limit of the detected β particles, the bulkiness and complexity of the detector design (usually the cost of the fiber exceeds the cost of the scintillator with which it is used). The use of α- and β-detectors in technological installations of the nuclear industry and nuclear power plants (including for monitoring surface contamination of equipment and personnel) requires a high degree of resistance to mechanical and electromagnetic influences. It is clear how difficult it is to meet these requirements with detectors containing vacuum PMT and fragile optical fibers.

A whole series of problems in creating α- and β-detectors for use in the nuclear industry is removed by replacing vacuum PMTs with silicon photosensors [Akimov Yu.K. Photonic radiation registration methods. Dubna: JINR, 2014, 323 pp.]. These include: silicon photodiodes (PhDs), silicon drift detectors (SDDs), avalanche photodiodes (APDs) and silicon photomultipliers (SiPMs). In the last two cases, silicon photosensors have internal amplification. For APD, the gain is 50–200 (depending on the operating conditions), and for SiPM, the gain can be at the gain level of vacuum photomultipliers. SiPMs are most attractive as a photosensor for use with scintillators. SiPMs are silicon sandwiches — layers that form pn junctions that are reverse biased. The dimensions of the light-sensitive surface are from 1 × 1 to 6 × 6 mm. Sandwich thickness ≈5 µm. Each such sandwich contains several thousand micropixels - tiny Geiger-Muller counters with discharge-quenching resistors. The size of one micropixel is from 10 × 10 to 50 × 50 µm. A photon of light, with a probability of 40–70% (the ratio of the sensitive and total areas of the photosensor - FF) that hit the micropixel with a probability of 25 ÷ 75% (quantum efficiency - QE) causes the appearance of an electron-hole pair. Further, with a probability of 70 ÷ 90% (P _G ), moving in an electric field with high intensity, the photoelectron gives rise to an avalanche of electrons with the number of carriers 10 ⁵ ÷ 10 ⁶ (this is consistent with the amplification of vacuum photomultipliers). A value equal to FF × QE × P _G is called the photodetector efficiency of a silicon photomultiplier (PDE). PDE for SiPM has the same meaning as QE for PMT. The process of avalanche formation takes about 1 ns. The arising avalanche current, as in the Geiger-Muller counter, flows through the quenching resistor, the voltage drops, and the avalanche process stops. A signal of standard amplitude is generated at the load. The linear relationship between SiPM flare, scintillation flash and the output current is achieved due to the large number of micropixels connected to the total load, but provided that the number of light photons in the flash is significantly lower than the number of micropixels.

A number of advantages of silicon photomultipliers over vacuum photomultipliers (PMT) make them very promising for the creation of scintillation detectors of ionizing radiation. It is insensitive to the magnetic field; small dimensions and weight; low value of operating voltage (25 ÷ 75 V against 1000 ÷ 2000 V required for PMT); wider than for PMT spectral range of sensitivity to light (from violet to orange).

Known charged particle detectors, where silicon photomultipliers are used as a converter of light flashes in a thin plastic scintillator into electrical pulses. The vast majority of them use optical fibers (simple or spectroscopic), laid in special grooves in the form of a snake on the back of the scintillation plate (Fig. 2) [V. Andreev et al. A high-granularity scintillator calorimeter readout with silicon photomultipliers. Nucl. Instrum. and Meth. in Physics Research, V. A540 (2005) p. 368-380], [M.Y. Kim et al. Beam test performance of SiPM-based detectors for cosmic-ray experiments. Nucl. Instrum. and Meth. in Physics Research, V. A703 (2013) p. 177-182], [P. Buzhan et al. Silicon photomultiplier and its possible applications. Nucl. Instrum. and Meth. in Physics Research V. A504 (2003) p. 48-52]. The disadvantage of such detectors are:

- the complexity, and therefore the high cost of the design;

- Significant light loss at the scintillator-fiber-SiPM boundaries and in the fiber itself due to its large length.

It is possible to remove light flashes on SiPM without using optical fibers, but to ensure a weak dependence of the amplitude of the electric signal on the site of interaction of the charged particle with the scintillator, a large number of SiPMs are installed on the back of a thin scintillation plate. This is unacceptable for economic reasons.

Known scintillation detector [F. Simon, S. Soldner. Uniformity studies of scintillator tiles directly coupled to SiPMs for imaging calorimetry. Nucl. Instrum. and Meth. in Physics Research V. A620 (2010) p. 196-201] charged particles with one SiPM in optical contact with the scintillation plate (is a prototype). The detector is (Fig. 3) a scintillator plate of size 30 × 30 × 5 mm, in the lateral face of which a cavity is made for placement of a silicon photomultiplier with a size of 1 × 1 mm in it. All the walls of the cavity, except for one adjacent to SiPM, are roughened to obtain diffuse light scattering, and the one in contact with the photosensor is polished. The entire scintillation plate is wrapped in aluminum foil to obtain specular reflection of light. The plate was scanned with a step of 0.5 mm by a collimated beam of β particles, and the average amplitude of electric pulses at the SiPM output was measured for each beam position.

The achieved maximum heterogeneity of light collection depending on the place of interaction of β particles with scintillation plastic was 35%. Light collection heterogeneity did not exceed 20% in 98.9% of the beam positions, 10% in 97.1% of the positions, 5% in 87.9% of the positions.

For comparison, measurements were made with SiPM simply glued to the side surface (without being placed in the cavity). The heterogeneity of light collection did not exceed 20% in 90.7% of the positions of the beam, 10% in 80.85 positions, 5% in 57.4% of the positions. Thus, it was shown that the placement of a photosensor in a cavity with diffusely reflecting walls reduces the dependence of the amplitude of the electric signal at the SiPM output on the site of interaction of β particles with a scintillation plate.

A detector with a plate thickness of 3 mm was investigated. SiPM was placed in a similar cavity. A decrease in the thickness of scintillation plastic led to an increase in the heterogeneity of light collection: the heterogeneity of the light collection did not exceed 20% in 98.0% of the beam positions, 10% in 94.0 positions, 5% in 82.5% of positions. It is obvious that a further decrease in the thickness of the scintillator will lead to an even greater increase in the heterogeneity of light collection.

The disadvantages of the prototype detector are as follows:

1. With such a design, it is impossible to use a thin plastic scintillator with a thickness of 1 mm or less, which is important for achieving a detector insensitivity to gamma background.

2. A plastic scintillator in the form of a thin plate, in which a cavity is made for placing SiPM in it, and at the same time its lateral surfaces are roughened, and the front one is polished, it is rather laborious to manufacture and low-tech.

The objective of the invention is to provide a gamma radiation insensitive high-performance detector of short-range charged particles with high uniformity of light collection over a sensitive surface.

This problem is solved by the fact that the working substance of the detector is a sandwich of an arbitrarily thin scintillating plastic plate and optically transparent in the emission strip of the plastic substrate with a total thickness equal to the diameter of the sensitive region of the silicon photomultiplier, which is optically and mechanically articulated with the contact pad of the sandwich having an area equal to the area of the silicon photomultiplier and created in place of one of the corners of the sandwich with the working and back sides in the form of an equilateral m polygon with the number of sides from four to infinity, while all the free surfaces of the sandwich, except the back, are covered with a mirror reflector, and the back side is diffuse.

The implementation of the detector is shown in FIG. 4, where one of the possible designs is given. It contains a detecting medium in the form of a sandwich from an arbitrarily thin scintillating plastic 1 and deposited on its back side over its entire area of optically transparent non-scintillating material 2 in the emission strip, for example, specially selected plexiglass having a refractive index equal to the refractive index of the scintillator. The thickness of the sandwich formed from the scintillation plate and the non-scintillating, optically transparent sandwich material is chosen equal to the diameter of the sensitive surface of the silicon photomultiplier 3. The detecting sandwich has the shape of an equilateral polygon with at least 4 sides (up to a circle). One corner of the sandwich is ground to a pad with dimensions equal to the dimensions of the SiPM sensitive region. The platform is polished and a silicon photomultiplier 3 is optically and mechanically coupled to it. All free surfaces of the sandwich, except the back (from the side of non-scintillating plastic), are covered with a mirror reflector 4, and the mentioned back surface is covered with a diffuse reflector 5.

The creation of such a design is dictated by the desire to collect light flashes arising at any point of the scintillator on a sensitive surface of a silicon photomultiplier which is relatively small relative to the plastic surface.

The transparent non-scintillating substrate under the plastic in the sandwich is designed to use the entire sensitive surface of the silicon photosensor in the collection of light.

It is known [Akimov Yu.K. Photonic radiation registration methods. Dubna: JINR, 2014, 323 pp.], That when the scintillator is very different from the cube, the best conditions for collecting light on one of the faces of the scintillator are created when there is a mirror reflector around the crystal. However, situations may arise when some photons of light, having undergone many reflections, may not reach the photosensor before being absorbed in the scintillator. The diffuse reflector helps to exclude the movement of photons of light along the same path and thereby increases the likelihood of being hit by the photosensor before it is absorbed.

Placing a silicon photomultiplier precisely in one of the corners of the sandwich, and not on one of the faces, also increases the light collection efficiency, since this minimizes the number of faces of the detecting medium perpendicular to the sensitive surface of the photosensor. According to the consequences of Lambert’s law, this is the condition for maximum light collection [V.P. Semynozhenko et al. Recent progress in the development of CsI (Tl) crystal-Si-photodiode spectrometric detection assemblies. Nucl. Instrum. and Meth. in Physics Research V. A 537 (2005) p. 383-388].

An increase in the number of sides of the detecting sandwich also contributes to an improvement in light collection.

The measures taken, ceteris paribus, provide a much more efficient light collection than in the prototype detector.

The technical result of the application of the inventive charged particle detector with a thin scintillator is that it becomes possible to efficiently collect light from arbitrarily thin scintillation plastic plates having a working surface area many times greater than the sensitive surface area of a silicon photomultiplier, and thereby provide a low energy threshold for recording short-range charged particles.

Claims (2)

1. A charged particle detector with a thin scintillator in the form of a plate, containing a semiconductor photosensor as a converter of light bursts initiated by charged particles into electric pulses, characterized in that the arbitrarily thin completely polished scintillator plate is made in the form of an equilateral polygon with at least four optical angles and mechanically connected to a transparent for scintillation fully polished substrate having the shape and refractive index of light the same as the scintillator, and the total thickness of the sandwich formed from the scintillator and the substrate is equal to the diameter of the sensitive surface of the semiconductor photosensor, optically and mechanically attached to the sandwich in one of its corners, which is made ground and polished to obtain a contact area with the dimensions of the sensitive region a semiconductor photosensor, while all surfaces of the sandwich, except for the back and with an attached semiconductor photosensor, are covered with a mirror reflector, and the rear The surface is covered with a diffuse reflector. 2. The detector according to claim 1, characterized in that a silicon photomultiplier is used as a semiconductor photosensor.

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