RU175876U1 - SCYNTIC DETECTOR WITH A PICOSECOND TIME RESOLUTION BASED ON CsF - Google Patents

Abstract

The utility model relates to the field of registration of ionizing particles, in particular, fast electrons and positrons, gamma rays with energies from 100 keV to 10 MeV with high temporal resolution, namely, to scintillation detectors, and is intended for registration of particles in high-energy physics with high flows and in medical equipment, such as positron emission tomographs with time-of-flight registration. The technical result is to improve the accuracy of measuring the moment of hit (registration) of an ionizing particle to 100 picoseconds. A scintillation detector for detecting ionizing particles and gamma rays with energies from 100 keV to 10 MeV, the scintillation component of which provides a temporal accuracy of at least 10 picoseconds, contains a scintillating crystal plate in a transparent sealed shell and a silicon photomultiplier. A scintillating crystalline plate up to 1 mm thick is made of pure CsF, which is a hygroscopic material; therefore, the plate is placed in an airtight shell. The silicon photomultiplier is in optical contact with the wafer. 1 ill.

Description

Technical field

The utility model relates to the field of scintillation technology, in particular, to high-speed scintillation detectors designed to detect ionizing particles (fast electrons and positrons, gamma rays with energies from 100 keV to 10 MeV) with a high temporal resolution (the scintillation component provides temporal accuracy not worse than 10 ps), in experiments with high fluxes in high-energy physics and in medical equipment, in particular, positron emission tomographs with time-of-flight registration.

State of the art

The requirements for materials for ionizing particle detectors are numerous. The most important properties of such materials are the output of the scintillation signal, the decay time of the luminescence, the energy resolution, the spectral composition of the emitted glow, etc. For scintillation detection of ionizing particles during the registration of events in accelerators with high luminosity, as well as in medical positron emission tomographs with time-of-flight registration, parameters related to recording the moment of arrival of the ionizing particle, in particular, the time of arrival of the first photon and the time of luminescence ignition, come to the fore . In such tasks, it is required to improve the temporal resolution to 10 picoseconds. Due to this, it is possible to significantly improve the spatial resolution of the positron emission tomograph (up to 1.5 mm) and increase its signal-to-noise ratio, and, in the case of detectors in high-energy physics, to distinguish between individual events that occur during the collision of two bunches in high-luminosity colliders ( P. Lecoq, M. Korzhik, A. Vasil'ev, IEEE Trans. Nucl. Sci., 61 (2014) 229-234).

It is known that in many transparent media, when an ionizing particle passes through a substance, a part of the energy is converted to Cherenkov radiation, which is the fastest way to convert a part of the energy of an ionizing particle into photons (Lindenbaum S, Yuan L., Cherenkov counters // Principles and methods of recording elementary particles, transl. from English., M, 1963). However, such converters have a number of disadvantages, in particular, the luminescence lies in the short-wavelength (ultraviolet and vacuum ultraviolet) spectral regions, which does not correspond to the spectral sensitivity of modern fast optical radiation detectors, for example, silicon photomultipliers (SiPM) (RU 1702831 C published on June 27, 1997, CL H01L 31/06). In addition, unlike luminescence, Cherenkov radiation is non-isotropic with respect to the passing particle, and the radiation process has a threshold character in energy of the ionizing particle, the glow is observed only for particles flying at a speed greater than the phase velocity of light in the crystal.

It is known that most fast scintillators with luminescent centers in the form of activators have a characteristic burnup time of the order of hundreds of picoseconds, with characteristic decay times of more than 20 nanoseconds, and for this reason they can be used to record the time of flight of a particle through the first photon only with a large scintillation yield (S Gundacker et al., J. Instrum., 8 (2013) 07014). Currently, such methods give a temporal resolution of no better than 150 picoseconds, because luminescence centers begin to be excited only after thermal relaxation of electronic excitations, which lasts in many scintillation crystals for at least several picoseconds, and the transfer of energy from electronic excitations to an activator with a characteristic time of several hundred picoseconds. In addition, the use of scintillation materials with a large integral (over time) scintillation yield worsens the conditions for recording by the first photon under conditions of intense fluxes of ionizing particles due to the large number of delayed photons emitted with a delay of the order of the emission time (several tens of nanoseconds).

Substances with ultrafast fluorescence constant are known in which this luminescence is associated with transitions between different energy zones (cross-luminescence), in particular, BaF ₂ with a fluorescence time constant of 0.8 nanoseconds (US patent No. 4510394, published 04/09/1985, class G01J 1 / 58). However, this substance has two drawbacks, namely, there is a slow luminescence component with a duration of hundreds of nanoseconds, and the spectral composition of ultrafast luminescence is limited to the ultraviolet and vacuum ultraviolet regions of the spectrum, which does not correspond to the spectral sensitivity region of radiation-resistant SiPM based receivers.

There is also a known method for producing BaF ₂ -based material by adding Тm (patent RU 2467354 C1 published on November 20, 2012, CL G01T 1/202) or Sc (patent RU 2436123 C1 published on 12/10/2011, cl. G01T 1/202), which suppresses slow components of the glow, and due to this, only the subnanosecond component remains in the material of the scintillation detector. However, this method does not change the spectral composition of the glow, which remains outside the sensitivity region of SiPM.

Closest to the claimed solution is a scintillation detector, in which crystals are used to detect ionizing particles, in which so-called intraband luminescence is observed due to the emission of hot secondary electronic excitations (patent RU 165998 U1 published November 10, 2016, class G01T 1/20). The decay time of such a glow does not exceed the lifetime of a hot electronic (or hole) state, i.e., several picoseconds. However, the efficiency of intraband luminescence is 10 ^-4 -10 ^-3 photons per one secondary electronic excitation, which may be insufficient to obtain a high temporal resolution in the time of registration of the first photon.

Thus, scintillation detectors based on crystals with activator luminescence have a significantly lower temporal resolution, scintillators based on intraband luminescence do not have a sufficiently high light output, and scintillators based on BaF ₂ crystals have a luminescence in the spectral region that does not coincide with the sensitivity region of silicon photomultipliers.

Utility Model Disclosure

The objective of this utility model is to develop a scintillation detector for detecting the moment of arrival of an ionizing particle, in particular fast electrons and positrons, gamma rays with energies from 100 keV to 10 MeV, the scintillation component of which provides a temporal accuracy of no worse than 10 ps.

The technical result is to improve the accuracy of measuring the moment of entry (registration) of an ionizing particle to 10 picoseconds through the use of a material having luminescence with a picosecond burn-up time and a spectrum that matches the sensitivity region of silicon photomultipliers.

The problem is solved by a scintillation detector for detecting ionizing particles and gamma rays with energies from 100 keV to 10 MeV, the scintillation component of which provides a temporal accuracy of at least 10 picoseconds, which contains a scintillating crystal plate (scintillator) up to 1 mm thick made of pure CsF, placed in a sealed transparent shell, and a silicon photomultiplier in optical contact with the scintillator. CsF crystals are hygroscopic material, so the plate is placed in a sealed enclosure. The shell can be made of quartz up to 0.5 mm thick. The limitation on the thickness of the CsF plate is imposed by the requirement of 10 ps time resolution. In this case, ~ 10% of gamma rays with energies of 511 keV will be absorbed in 1 mm CsF. In high-energy physics at high radiation fluxes, this approach does not pose a problem. In positron emission tomography, where an increase in flux will require an increase in the patient's radiation dose, another geometry is proposed in which gamma radiation is directed to the end face of the CsF plate. A SiPM matrix is used for reading, which makes it possible to determine the localization (distance from the gamma-ray entry point) of the scintillation flash.

Thus, the problem is solved through the use of CsF crystals for recording ionizing particles, in which cross-luminescence is observed with a subpicosecond rise time and a luminescence spectrum that well corresponds to the spectral sensitivity region of silicon photomultipliers.

In the interaction of an ionizing particle (a primary ionizing particle or a photoelectron created by absorption of a gamma quantum) with a substance, a cascade of electronic excitations lasting up to several tens of femtoseconds develops, which ends when the energy of electrons and holes in the cascade becomes less than the threshold of strongly inelastic scattering with the formation of electronic excitations. After this, the stage of thermalization of excitations begins, which lasts from fractions to units of picoseconds. Thermalization of excitations is due to the emission of optical phonons. In the case of cross-luminescence, the thermalization time of electrons and valence holes does not affect the temporal characteristics of the glow, and the ignition time is determined by the thermalization time of a hole in the upper core zone, which is very short due to the low mobility of such holes and amounts to τ _r ≤1 ps (T. Sekikawa, et al., J. Opt. Soc. Am. 19 (2002) 1941).

The CsF scintillator emits only fast cross-luminescence without the admixture of any slow glow. The cross-luminescence decay time in CsF is τ _d = 2.8 ns (M. Moszinski et al., Nucl. Instrum. Meth. 179 (1981) 271), and the center of the luminescence band is at 390 nm (VN Makhov, M. Kirm, G. Zimmerer, Proceedings of the Eighth Int. Conf. On Inorganic Scintillators and their Use in Scientific and Industrial Applications SCINT 2005, Alushta, Crimea, Ukraine, September 19-23, 2005, pp. 74-76.). The photoelectron yield of the CsF scintillator is N _phe ~ 500 / MeV (M. Moszinski et al., Nucl. Instrum. Meth. 205 (1983) 239), which provides the ultimate time resolution of the scintillator Δt = τ _d / N _phe ~ 5 ps. The difference in the times of arrival of photons at the detector from the front and back faces of the CsF crystal is no more than 2 picoseconds with a crystal thickness of 1 mm. Thus, the resolution of registration of single ionizing particles and gamma rays is 10 picoseconds (excluding the time resolution of SiPM). Utility Model Implementation

In FIG. 1 schematically shows a scintillation detector of ionizing particles, designed to detect ionizing particles, in particular fast electrons, positrons and gamma rays with energies from 100 keV to 10 MeV.

The detector contains a scintillating crystal plate (1) in a sealed transparent shell (2) and a silicon photomultiplier (3).

The plate (1) is made of pure (without impurities) CsF. Plate thickness (1) up to 1 mm. The plate (1) is enclosed in a transparent sealed enclosure (2). The silicon photomultiplier (3) is in optical contact with the plate (1) through a transparent shell (2).

Modern silicon photomultipliers (SiPM) have a signal rise time of less than 100 ps, which provides a temporary resolution of the entire detector also less than 100 ps.

Typically, the plate (1) and the silicon photomultiplier (3) are attached to each other by gluing. However, it is also possible to make a scintillation detector inside an airtight housing (2) from an optically transparent substance with a wall thickness of up to 0.5 mm, for example, quartz containing a plate (1) and a silicon photomultiplier (3), while the plate (1) and a silicon photomultiplier (3 ) are glued using optically transparent glue.

The detector operates as follows.

An ionizing particle or gamma quantum with an energy of 100 keV to 10 MeV when it enters the scintillating crystal plate (1) from CsF causes the generation of optical photons within ~ 2.8 nanoseconds, however, the first photon is generated on average for ~ 5 picoseconds.

Thus, this scintillation detector allows you to record the moment of arrival of an ionizing particle, in particular fast electrons and positrons, gamma rays with energies from 100 keV to 10 MeV, with a time accuracy of 100 ps.

An example is a design in which the plate area of an AdvanSiD ASD-SiPM3S-M-50 silicon photomultiplier is 3 × 3 mm and the temporal resolution is 120 ps (V. Puill, et al., Nuclear Instruments & Methods In Physics Research A (2012)), and the CsF crystal has a size of 1 × 3 × 3 mm. In this case, the temporal resolution of such a system will be determined only by the characteristics of the silicon photomultiplier, since the front of rise of the optical response is much smaller than the latter.

Claims (1)

A scintillation detector for detecting ionizing particles and gamma rays with energies from 100 keV to 10 MeV, the scintillation component of which provides a temporal accuracy of at least 10 picoseconds, containing a scintillating crystalline plate with a thickness of up to 1 mm made of pure CsF, the transverse dimensions of which coincide with the size of the silicon window a photomultiplier placed in an airtight transparent shell; and a silicon photomultiplier in optical contact with the plate.

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