RU2523611C1 - Method of measuring fast neutron fluence with semiconductor monocrystalline detector - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method includes calibrating a detector, measuring electrophysical parameters of the detector before and after irradiation and irradiating the detector with fast neutrons. The detector is made in form of a plate with plane-parallel base surfaces. Electrical resistance between bases of the plate is measured before and after irradiation, for which ohmic contacts are deposited on the entire surface of each base of the plate before measurements, and fast neutron fluence F is determined from the change in electrical conductivity between contacts before and after irradiation of the plate $$F=\frac{Kd}{S}(\frac{1}{R_0}-\frac{1}{R}),$$

where K is a coefficient of proportionality, which is constant for the measured neutron spectrum and does not depend on the initial electrical resistance; coefficient K is determined when calibrating the detector; d is the thickness of the plate; S is the surface area of each base of the plate; R₀, R denote the initial and final electrical resistance between ohmic contacts before and after irradiation, respectively.

EFFECT: simple and cheaper method of detecting fast neutron fluence.

1 tbl

Description

The invention relates to the field of radiation technology, as well as to the operation of nuclear installations and accelerators.

The principle of operation of semiconductor detectors is based on a change in the electrophysical parameters of semiconductors under the influence of radiation. When fast neutrons are irradiated in single crystals of semiconductors, along with other types of defects, complex defects of both donor and acceptor nature are formed, which are the result of the interaction of vacancies and interstitial atoms with each other and with the atoms of the initial chemical impurities. Moreover, the introduction of complex compensating centers leads to compensation of the main dopant. This property of semiconductors is used to measure fast neutron fluence.

A known method for detecting fast neutrons, based on the change in the voltage drop on the direct branch of the current-voltage characteristics of the diode under the action of fast neutrons [Kramer-Ageev EA, Mironov Yu.A., Sinitsyn AD, Troshin VS Emergency neutron dosimeters based on silicon industrial semiconductor diodes. “Questions of dosimetry and radiation protection”, Moscow, No. 19, 1980, p. 61-66].

There is also a known method for detecting fast neutron fluence by a semiconductor detector, which includes calibrating the detector, measuring the electrical resistance of the detector before irradiation, irradiating the unknown fast neutron fluence, measuring the electrical resistance of the detector after irradiation [SU a.s. No. 934402, published 07.06.82, BI No. 21]. In this case, n-type silicon is used as a detector, for example, a portion of doped silicon between the first and second bases in a single-junction transistor, otherwise called a double-base diode, KT117. The formula connecting the fast neutron fluence with the change in interbase resistance has the form

R = R ₀ · exp (K · F), ( 1)

where K is the coefficient of proportionality, which is determined during the calibration of each specific detector, R ₀ , R are the interbase resistance of KT117, respectively, before and after irradiation, F is the fluence of fast neutrons.

The main disadvantage of these methods is associated with a significant scatter of the initial parameters even for the same type of serial production devices. Therefore, each such device requires individual calibration, after which the restoration of the initial parameters during high-temperature annealing is often impossible due to the destruction of the internal structure of the devices.

Closest to the claimed is a method of measuring the fluence of fast neutrons with a semiconductor detector, including calibrating the detector, measuring the electrophysical parameters of the detector before and after irradiation, irradiating the detector with fast neutrons [RU No. 2339975, published November 27, 2008. Bull. No. 33]. The formula connecting the fast neutron fluence F with the change in the electrical resistivity has the form

, (2) $$F=K(\frac{\mathrm{one}}{{\rho }_0}-\frac{\mathrm{one}}{\rho })$$

, (2)

where K is the coefficient of proportionality, which is constant for the measured neutron spectrum and does not depend on the initial electrical resistivity; coefficient K is determined during detector calibration; ρ ₀ , ρ are the initial and final electrical resistivities of the semiconductor before and after irradiation, respectively.

This method has several advantages: the use of a simple semiconductor without p-n junctions as a detector; wide range of measured fast neutron fluence; one initial calibration of the detector for a given neutron spectrum with any initial specific resistance.

The main disadvantage of the prototype is the need for a sufficiently complex and expensive installation for measuring the electrical resistivity of semiconductor single crystals. Most often, for these purposes, installations with a four-probe measurement method are used. To service the installation and conduct correct measurements of electrical resistivity, an appropriate room, the availability of standards for the calibration of this installation, and the appropriate qualifications of the maintenance staff are required.

The technical result of the invention is to simplify the method for detecting fast neutron fluence. At the same time, all the advantages of the prototype are preserved, but the availability of the method for its use in reactors and accelerators is significantly expanded.

This is achieved by the fact that the method for measuring fast neutron fluence with a semiconductor single crystal detector, as in the prototype, includes calibrating the detector, measuring the electrophysical parameters of the detector before and after irradiation, and irradiating the detector with fast neutrons.

According to the invention, a plate-shaped detector with plane-parallel base surfaces is used, before and after irradiation, the electrical resistance between the base of the plate is measured, for which ohmic contacts are applied to the entire surface of each base of the plate before measurements, and the fast neutron fluence F is determined by the change in electrical conductivity (inverse electrical resistance) between the contacts before and after irradiation of the plate

, (3) $$F=\frac{Kd}{S}(\frac{\mathrm{one}}{R_0}-\frac{\mathrm{one}}{R})$$

, (3)

where K is the coefficient of proportionality, which is constant for the measured neutron spectrum and does not depend on the initial electrical resistance; coefficient K is determined during detector calibration;

d is the plate thickness,

S is the area of each base of the plate,

R ₀ , R are the initial and final electrical resistances between the ohmic contacts before and after irradiation, respectively.

The essence of the invention lies in the fact that in the proposed method, the detector is made in the form of a plate of a single-crystal semiconductor with ohmic contacts over the entire surface of each base of the plate, and the surfaces of the bases are plane-parallel. This allows you to establish a unique relationship of electrical resistance between the bases of the plate and the electrical resistivity in the volume of a semiconductor single crystal. Thus, to obtain exactly the same information about the fast neutron fluence, as in the prototype, but using simple recording equipment. At the same time, all the advantages of the prototype are preserved. In fact, under the influence of fast neutrons in semiconductor single crystals radiation defects are formed, including electrically active ones, i.e. ionized at ordinary temperature. The concentration of these defects is proportional to the fast neutron fluence and depends on the neutron energy. These defects compensate for the main admixture of the single crystal, thereby increasing its electrical resistivity. If the plane of the base of the plate is plane-parallel, and the side surface of the plate is perpendicular to the plane of the base, then in accordance with Ohm's law, the electrical resistivity can easily be determined through the electrical resistance between the bases of the plate:

, (4) $$\rho =\frac{SR}{d}$$

, (four)

where ρ is the electrical resistivity,

R is the electrical resistance between the bases of the plate,

S is the base area of the plate,

d is the plate thickness.

Substituting (4) into expression (2) we obtain expression (3). Note that the base of the plate can have any configuration: circle, ring, triangle, polygon, etc. To implement this method of determining the fluence of fast neutrons, it is necessary to correctly determine the electrical resistance R between the ends of the washer. At the metal-semiconductor interface, a potential barrier arises and an associated barrier layer. Therefore, these contacts will be rectifying. In some cases, this potential barrier is negligible and the current-voltage characteristic of such a contact is a straight line. The connection between the current through such a contact and the voltage across it is expressed, thus, by a linear law - Ohm's law - regardless of the polarity of the voltage applied to this contact. Such a contact is ohmic (not rectifying). To include single-crystal plates in an electric circuit, ohmic contacts are applied to the entire plane of each base of the plate. The easiest way to do this is with an aluminum-gallium pencil or indagallium paste [for example, page 233 of the manual: Nashelsky A.Ya. The production of semiconductor materials. - M.: Metallurgy, 1989 - 272 p.]. Any simple semiconductor can be used as a semiconductor single crystal: silicon Si, carbon C, germanium Ge, gray tin Sn-Sn, arsenic As ,, boron B, phosphorus P, selenium Se (red), sulfur ά-S, antimony β- Sb, tellurium Te, iodine J.

The possibility of implementing the method is confirmed by experiments conducted on a research reactor of the IRT-T type with a capacity of 6 MW in the city of Tomsk. Silicon was used as a single crystal. The experiments were carried out on the extracted neutron beam of the horizontal experimental channel GEK-10 (at the exit from the channel). The fast neutron fluence was controlled using threshold sulfur activation detectors showing the integrated neutron flux density with an energy above 3 MeV. The detectors were calibrated previously, i.e. according to the readings of sulfur detectors F _S and changes in conductivity (reciprocal of the electrical resistance) of single-crystal washers, the proportionality coefficient K was determined in expression (3):

. (5) $$K=\frac{S{F}_S}{d}{(\frac{\mathrm{one}}{R_0}-\frac{\mathrm{one}}{R})}^{-\mathrm{one}}$$

. (5)

For the HES-10 channel, K = 9.94 · 10 ¹⁴ Ohm / cm. Let us make a few comments about detector calibration. The threshold for defect formation (the energy that needs to be reported to an atom in order to knock it out of the crystal lattice site) in silicon single crystals is about 25 eV. The maximum energy E transmitted by a neutron with energy E _{n to an} atom in a head-on collision is

, (6) $$E=\frac{\mathrm{four}\cdot M\cdot M_n}{{(M+M_n)}^2}{E}_n$$

, (6)

where M, M _n are the masses of the atom and neutron, respectively.

The energy E = 25 eV corresponds to the neutron energy E _n = 194 eV. In this case, the atom can recombine with the vacancy formed. Therefore, he needs to communicate great kinetic energy. It can be approximately assumed that defect formation in silicon occurs at E> 400 eV. Sulfur detectors provide information about the fluence of neutrons with energies above 3 meV. Therefore, despite the fact that defects in silicon are formed at neutron energies above 400 eV, the calibration of silicon detectors in accordance with expression (5) allows us to judge only the fluence of neutrons with energies above 3 meV. Therefore, for each channel of the reactor with its own neutron spectrum (known or not known), individual calibration of the detectors is necessary. Obviously, with the known spectrum in the reactor channel, it is possible to determine the neutron fluence of each energy group of the spectrum from the readings of both sulfur activation detectors and (after calibration), from the readings of silicon detectors. At the same time, silicon detectors without calibration can be used to determine in relative units the fluence of all neutrons with energies above 400 eV, i.e. precisely those neutrons that are involved in defect formation.

When determining the flux density of neutrons with energies above 3 meV, 9 washers of n-type single-crystal silicon were irradiated. The silicon washers were made in the form of a regular cylinder with a diameter of 1.33 cm and a height of 0.5 cm. At the same time, 3 silicon washers were irradiated together with a sulfur detector. Before measuring the electrical resistance, thin layers of indagallium paste were applied to the ends of the washers and titanium plates were applied. The measurement results are shown in the table. In the table R ₀ , R are the electrical resistances between the bases of the washers before and after irradiation, respectively; F _S , F _Si — fast neutron fluences with energies above 3 meV according to the readings of sulfur and silicon detectors, respectively; δ = 100 · (F _Si —F _S ) / F _S %. F _{Si was} calculated in accordance with expression (3) at K = 9.94 × 10 ¹⁴ Ω / cm. According to the data presented, it is possible to estimate the detection error - ≈10%.

A useful result is that to obtain information about the fast neutron fluence it is enough to have a simple recording equipment - an ohmmeter. This makes the method available for use in any relevant enterprise. Calibration of the detector can, as in the prototype, be carried out even in one single exposure of a single-crystal washer with any initial electrical resistance. In this case, the calibration remains the same for a single crystal with any other initial resistance. In addition, each single crystal can be used repeatedly, either by annealing radiation defects to convert the resistance to its original value, or by irradiating a previously irradiated washer, taking as the initial resistance the one that the irradiated single crystal had before the next irradiation.

Table. The results of exposure.

R ₀ R F _s F _si δ Ohm Ohm cm ^-2 cm ^-2 % 420 440 3.70 · 10 ¹⁰ 4.0410 ¹⁰ 9.3 412 429 3.70 · 10 ¹⁰ 3.5910 ¹⁰ 2.9 417 434 3.70 · 10 ¹⁰ 3.51 10 ¹⁰ 5.1 277 352 2.8210 ¹¹ 2.8710 ¹¹ 1.9 282 356 2.8210 ¹¹ 2.7510 ¹¹ 2,3 279 360 2.8210 ¹¹ 3.0110 ¹¹ 6.9 152 448 1.5610 ¹² 1.6210 ¹² 4.1 149 431 1.5610 ¹² 1.6410 ¹² 5.2 155 411 1.53 · 10 ¹² 1.50 · 10 ¹² 1.9

Claims (1)

A method for measuring fast neutron fluence with a semiconductor single crystal detector, which includes calibrating the detector, measuring the electrophysical parameters of the detector before and after irradiation, irradiating the detector with fast neutrons, characterized in that the detector is made in the form of a plate with plane-parallel surfaces of the bases, before and after irradiation, the electrical resistance between the bases is measured plates, for which, before measurements, ohmic contacts are applied to the entire surface of each base of the plate , and the fast neutron fluence F is determined by the change in the electrical conductivity between the contacts before and after irradiation of the plate

,
where K is the coefficient of proportionality, which is constant for the measured neutron spectrum and does not depend on the initial electrical resistance, the coefficient K is determined during calibration of the detector;
d is the plate thickness;
S is the area of each base of the plate;
R ₀ , R are the initial and final electrical resistances between the ohmic contacts before and after irradiation, respectively.