RU2244320C1 - Thermal neutron recording scintillator - Google Patents

Abstract

FIELD: radiation monitoring of environment.

SUBSTANCE: proposed scintillator that can be used for radiation monitoring of lands and water areas and in engineering systems for inspecting primary nuclear fuel and items made of fissionable materials includes calcium fluoride activated by europium and ³ He helium isotope, proportion of ingredients being as follows, atom percent: calcium fluoride, 99.25 - 99.59; europium fluoride, 0.4 - 0.7; ³ He helium isotope, 0.01 - 0.05.

EFFECT: enhanced thermal neutron recording efficiency.

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Description

The invention relates to inorganic scintillation materials for detecting thermal neutrons and suitable for creating radiation detectors based on them for radioecological monitoring of territories and water areas, monitoring of cosmic and man-made neutron background, for creating technical control complexes for primary nuclear fuel and products from fissile materials .

To detect and measure the thermal neutron flux density by the scintillation method, scintillation materials are necessary (substances in which light flashes (scintillations) occur under the influence of thermal neutrons), the main properties of which (substances in general or individual components of their composition) are: increased cross-section (probability) of interaction with thermal neutrons; the optimal radiation wavelength of the scintillation flash, consistent with the spectral sensitivity of the photodetector; high light output of scintillations; a small flashing time within the nano- microsecond range; as well as a fairly wide range of operating temperatures.

Known inorganic scintillator based on crystal ⁶ LiKSО ₄ -Cu (RF patent 2148837), suitable for detecting thermal neutrons. It has scintillations with a wavelength of 435-445 nm and a duration of 90 ns. However, it is not technologically advanced, since the problem of obtaining large homogeneous ⁶ LiKSO ₄ crystals has not been solved, has a low scintillation light yield (30% relative to the light output of LiI-Eu crystals) and, most importantly, an insufficiently high cross section for interaction with thermal neutrons equal to 940 barn by the ⁶ Li reaction (n, α) ³ H. Thus, the scintillator ⁶ LiKSO ₄ -Cu has a low detection efficiency of thermal neutrons. All known scintillators in which thermal neutrons are detected by the reaction of ⁶ Li (n, α) ³ H have insufficiently high thermal neutron detection efficiency not exceeding 910 barn (Akimov Yu.K. Scintillation methods for detecting high-energy particles. M: Publishing House - at Moscow State University, 1963; K. Grupen. Detectors of elementary particles. Novosibirsk: Siberian Chronograph, 1999. 408 p.).

Known combined scintillator for detecting thermal neutrons based on a NaI-Tl crystal with a sheath-radiator made of boron-containing material surrounding the NaI-Tl crystal (RF patent 2189057). The thickness of radiators made of boron carbide or boron nitride is sufficient for complete absorption of thermal neutrons by ¹⁰ V nuclei. The cross section for thermal neutron capture by a natural mixture of boron isotopes is 767 bar and 3837 bar for a pure ¹⁰ V isotope. The neutron registration mechanism is as follows. First, thermal neutrons are captured by ¹⁰ V nuclei, which causes a nuclear reaction of the (n, α γ) -type, which proceeds in two stages:

- first stage:

- second stage (in ~ 10-13 s after the first):

- квант с энергией 0,482 МэВ.

- quantum with an energy of 0.482 MeV.

At the first stage, a part of lithium nuclei is formed in the excited state of ⁷ Li *; they provide the occurrence of the second stage of the reaction with the emission of a γ quantum with an energy of 0.482 MeV. This γ-quantum resulting from the reaction (n, α γ) is detected by the NaI-Tl scintillator. The organic scintillators that make up the combined detector are not capable of detecting thermal neutrons; they can only detect fast neutrons by recoil protons. A disadvantage of the known scintillator (RF patent 2189057) is the high hygroscopicity of the used NaI-Tl crystal, which makes it unreliable in operation. All known potential boron scintillators (Price V. Nuclear radiation registration. M: IIL, 1964. 464 p .; K. Grupen. Detectors of elementary particles. Novosibirsk: Siberian chronograph, 1999. 408 p .; Ogorodnikov I.N., Kruzhalov A .V. // Izv. VUZov, Physics. 1996. T.39, No. 11. P.76-93) have a low thermal neutron detection efficiency, usually at the level of 767 bar.

Known scintillation composition of crystals of Bi ₄ Ge ₃ O ₁₂ , plastic or stilbene (RF patent 2158011) for detecting neutrons and γ-radiation. However, such a scintillation composition is insensitive to thermal neutrons.

Known scintillator for detecting neutrons based on a NaI-Tl crystal with a sheath-radiator made of silver (Price V. Registration of nuclear radiation, M .: IIL, 1964. 464 p.). Silver effectively absorbs neutrons of resonant energies and emits γ quanta by the reaction (n, γ). The latter are recorded by a NaI-Tl crystal. Resonance reactions (n, γ) on a natural mixture of silver isotopes have an interaction cross section of 86.3 barn for slow and intermediate neutrons and 63.3 barn for thermal neutrons. However, the cost of such a detector is high due to high silver prices. The use of the ¹⁰⁹ Ag isotope, which has a fairly intense absorption of thermal neutrons (section - 91 barn), is inappropriate to work in combination with NaI-Tl because of its short half-life, which is only 24.6 days.

Known single-chip scintillator (US patent No. 4448808) for detecting neutrons and gamma rays. However, the scintillator is suitable for detecting fast neutrons and unsuitable for detecting thermal neutrons.

Known scintillation composition of three parallel-connected scintillators (RF patent 2143711), one of which, made on the basis of ⁶ Li-silicate glass, is sensitive to thermal neutrons. However, the detection efficiency of thermal neutrons by such a scintillator, determined by the corresponding reaction cross section ⁶ Li (n, α) ³ H (cross section is 940 barn), is low.

The closest technical solution are scintillators based on CaF ₂ -Eu. They can be made on the basis of ceramics, for example, CaF ₂ -Eu with a europium content of not more than 0.5 mol.% (RF patent 2058957). The scintillator is suitable for detecting ionizing radiation, mainly electrons, β particles and γ quanta with energies up to 100 keV. The use of polycrystalline ceramics CaF ₂ -Eu for neutron registration in the RF patent 2058957 is not described.

Scintillators based on CaF ₂ -Eu are made, as a rule, in the form of single crystals (Stavisky Yu.Ya., Shopar A.V. // PTE. 1962. No. 5. P.177-178; Shulgin B.V. et al. // Atomic energy. 1993. V.75, issue 1. P.28-32; Rogozhin A.A. et al. // Regularities of the distribution of impurity centers in ionic crystals: collection of scientific papers SIMS.M .: VIMS, 1977. S. 40-49; Viktorov L.V., Shulgin B.V. et al. // Inorganic Materials. 1991.V.27, No. 10. P.2005-2029; Scintillation Detector. Harshaw, Catalog. 1982. 112 p.). According to the above sources, CaF ₂ -Eu crystals (density - 3.19 g / cm ³ ; melting point - 1407 ° С; light refractive index - 1.44; Mohs hardness - 4; Z _eff = 16.5) belong to the class promising scintillation materials for registration, dosimetry and spectrometry of x-ray and β-radiation against the background of γ-radiation and neutrons. They have an absolute energy yield of scintillations of 8.4% or 29.103 photons / MeV (which is ~ 50% of the efficiency with respect to NaI-Tl), a wavelength of 435 nm, a duration of β-scintillation of 800 ns and an energy resolution of ¹³⁷ Cs 9-10, 5%, and along the line ²⁴¹ Am 26-30%. The optimal concentration of impurities in CaF ₂ -Eu crystals, which provides the highest absolute yield of scintillations, is 0.5-0.7 wt.% (Viktorov L.V., Shulgin B.V. et al. // Neor. Materials. 1991. T.27, No. 10. S.2005-2029). For the known CaF ₂ -Eu crystals, the scintillation light yield is practically stable in the temperature range from -60 ° С to + 20 ° С; the temperature coefficient of decay of the scintillation light output at T> 20 ° C is 0.4-0.5% / ° C; the afterglow, measured with a delay of 1 ms, is 0.6 · 10 ^-6 %, after 30 ms its value becomes less than 10 ^-9 %, the effective absorption coefficient at the radiation wavelength (435 nm) is 0.3 ± 0, 05 cm ^-1 . However, the CaF ₂ -Eu crystal scintillator, which has very high scintillation light output, is suitable only for detecting β-radiation and γ-radiation. The use of CaF ₂ -Eu crystals as a scintillator for detecting thermal neutrons in the above sources is not described.

The objective of the invention is to obtain scintillators based on CaF ₂ -Eu crystals for detecting neutrons with an increased cross section for thermal neutron capture, i.e. with higher detection efficiency of thermal neutrons. The problem is solved due to the fact that the helium isotope ³ He is additionally introduced by thermal diffusion into a known scintillator, including calcium fluoride activated by europium. The result is a crystal scintillator CaF ₂ - (Eu, ³ He). The effect of the invention is manifested in the fact that with the additional introduction of the ³ He isotope, the efficiency of detecting thermal neutrons by CaF ₂ - (Eu, ³ He) crystals almost doubles compared to CaF ₂ -Eu. It increases due to the fact that the capture of the latter in CaF ₂ - (Eu, ³ He) crystals occurs through two channels:

- by the reaction (n, α) on Eu nuclei with an interaction cross section of 4600 barn for a natural mixture of europium isotopes;

- by the reaction (n, β) on ³ He nuclei with an interaction cross section of 4000 barn (Mashkovich V.P., Kudryavtseva L.V. Protection against ionizing radiation. M: Energoatomizdat, 1995. 494 p.). The essence of the invention lies in the fact that the proposed scintillator has a composition, at.%:

CaF ₂ 99.25-99.59;

EuF ₃ - 0.4-0.7;

³ He - 0.01-0.05.

A decrease in the content of europium activator to a level below 0.4 at.% Or an increase of more than 0.7 at.% Leads to a decrease in the scintillation light output. The light output decreases from 0.5 to 0.2-0.3 and lower relative to NaI-Tl. A decrease in the content of ³ He isotopes below 0.01 at.% (~ 1019 at / cm ³ ) leads to a decrease in the efficiency of registration of thermal neutrons by helium nuclei due to a decrease in their number. An increase in the content of helium isotopes in CaF ₂ - (Eu, ³ He) crystals above 0.05 at.% By the method of thermal diffusion is possible in principle, but it takes a long time and is technically difficult to achieve.

Example 1

The scintillator composition CaF ₂ - 99.25 at.%, EuF ₃ - 0.7 at.% And ³ He - 0.05 at.% Get in two stages. At the first stage, CaF ₂ -Eu crystals are grown in the form of boules with a diameter of up to 45 mm and a length of 80 mm in graphite crucibles by the Stockbarger method in vacuum. To remove traces of oxygen, lead fluoride is added to the mixture in an amount of 1 wt.%. A crystal with a diameter of 40 mm and a height of 6 mm is cut from the central parts of the grown boules. In the second step crystal prepared (⌀ = 40 mm, h = 6 mm) were placed in a special pressure chamber, in which it is administered isotope ³ He via thermal diffusion method on AY Kupryazhkina (Kupryazhkin A.Ya., Kurkin A.D. // FTT. 1990. T.32, No. 8. S.2349-2354). In calcium fluoride crystals, interstitial and vacancy diffusion mechanisms are realized with sufficiently low activation energies - 1.16 eV for impurity and 2.24 eV for intrinsic vacancies - and with even lower dissolution energies - 0.5 and 0.81 eV, respectively. The thermal diffusion modes were selected based on the known temperature dependence of helium solubility and on the known dependence of helium solubility in CaF ₂ crystals on saturation pressure. The thermal diffusion mode was chosen so that the helium ³ He content in the CaF ₂ -Eu crystal was brought to 0.05 at.%. The obtained CaF ₂ - (Eu, ³ He) crystals possessed the following properties: thermal neutron registration efficiency - 95%; effective atomic number - Z _eff = 16.5; relative scintillation efficiency in comparison with NaI-Tl (on the γ-line of 662 keV of the ¹³⁷ Cs isotope) - 52%; the luminescence spectrum has a maximum at 435 nm, the duration of scintillations is 780 ns, and the shape of the scintillation decay curve is described by one exponent; energy resolution through ¹³⁷ Cs - 12%. The temperature effect on the luminescence spectrum of CaF ₂ - (Eu, ³ He) single crystals is insignificant: in the range from -50 to + 50 ° С, the position of the maximum of the emission band shifts by no more than 5 nm, the half-width increases with temperature from 25 to 30 nm, respectively , the duration of neutron scintillations ( ²⁵² Ca was used with a moderator), as well as α-scintillations, was 780 ns. The service life t of the _{service of} scintillation crystals CaF ₂ - (Eu, ³ He) is determined by the relaxation time t _re , during which the helium content in the crystals decreases by an e (exponential) time due to diffusion processes. For CaF ₂ - (Eu, ³ He) crystals at 300 K, the interstitial diffusion coefficient is 10 ^-8 -10 ^-9 cm ² / s. The relaxation time was estimated by the formula

where r is the radius of the scintillation crystal.

For r = 2 cm, we obtain that t _services = t _{rel л} 1.3 · 10 ⁹ -1.3 · 10 ¹⁰ s, i.e. from 40 to 400 years. Thus, even a lower limit estimate gives the service life of scintillation crystals of ~ 40 years, and the average service life of at least 100 years.

Example 2

The scintillator composition CaF ₂ - 99.59 at.%, EuF ₃ - 0.4 at.% And ³ He - 0.01 at.%. The growth of CaF ₂ –Eu single crystals with the subsequent introduction of the ³ He isotope into them by thermal diffusion and operates in the same modes as described in Example 1. Crystal sizes: ⌀ = 40 mm, h = 6 mm. The scintillator has the following properties: thermal neutron registration efficiency ~ 90%; effective atomic number - Z _eff = 16.5; relative scintillation efficiency in comparison with NaI-Tl (on the γ-line of 662 keV of the ¹³⁷ Cs isotope) ~ 45%; maximum emission spectrum - 435 nm; scintillation duration - 800 ns; the energy resolution along the line of 662 keV of the ¹³⁷ Cs isotope is 11%. Operating temperature range from -50 to + 50 ° C. The average service life of 100 years.

Example 3

A scintillator of the composition CaF ₂ - 99.4 at.%, EuF ₃ - 0.58%, ³ He - 0.02 at.% Are also obtained in two stages: growing single crystals of CaF ₂ -Eu, introducing isotope ³ into CaF ₂ -Eu Not by thermal diffusion. Crystal dimensions: ⌀ = 40 mm, h = 8 mm. The production conditions are the same as in example 1. The scintillator has the following properties: thermal neutron registration efficiency - 92%; scintillation light output relative to NaI-Tl - 0.5; effective atomic number Z _eff = 16.5; maximum emission spectrum - 433 nm; scintillation duration - 800 ns; the energy resolution along the line of 662 keV of the ¹³⁷ Cs isotope is 12%. The average service life is 100 years.

An additional advantage of the proposed scintillators described in examples 1-3 are:

- the ability to detect not only thermal neutrons, but also intermediate as well as fast neutrons due to the increased cross section for the interaction of intermediate and fast neutrons with nuclei of the ³ He isotope (tens and units of barns, respectively);

- high efficiency of detection of β-radiation due to insignificant losses on backscattering; the latter is due to low Z _eff = 16.5;

- the ability to selectively detect neutrons on the γ-background (for small crystal thicknesses CaF ₂ - (Eu, ³ He), the probability of detecting gamma radiation is small due to the low Z _eff = 16.5).

Examples 4 and 5

Scintillators of the composition ³ He - 0.2 at.%, EuF ₃ - 0.01 at.% (Example 4) or 2 at.% (Example 5), CaF ₂ - the rest. Scintillators were obtained in the same way as in examples 1-3, in two stages and in the same modes.

Scintillators based on CaF ₂ - (Eu, ³ He) with a content of EuF _{3 of} 0.01 at.% Or 2 at.% Are inferior to scintillators with a content of EuF ₃ in the range of 0.4-0.7 at.% (Examples 1-3 ) by the magnitude of the light yield of scintillations 1.2-1.5 times.

Claims (1)

Scintillator for detecting thermal neutrons, including calcium fluoride activated by europium, characterized in that it additionally contains a helium isotope ³ He in the following ratio of ingredients, at.%: Calcium Fluoride 99.25-99.59 Europium fluoride 0.4-0.7 Helium isotope ³ Not 0.01-0.05