RU152877U1 - COMBINED NEUTRON FLOW SPECTROMETER MONITOR - Google Patents

Abstract

Combined neutron flux monitor spectrometer containing a fast neutron detector, comprising a scintillator and a photomultiplier tube (PMT), and an amplitude-time analyzer, characterized in that the scintillator is made of plastic, additionally adjacent to the fast neutron detector on the opposite side of the neutron flow, a thermal neutron detector with a low sensitivity to gamma radiation is attached, the amplitude-time analyzer contains a signal counting unit, a signal charge measuring unit , coincidence unit, spectrum registration unit, thermal neutron detector is connected to the signal counting unit, PMT of the fast neutron detector is connected to the signal charge measuring unit, the coincidence unit is connected to the signal counting unit and the signal charge measuring unit, the coincidence unit is connected to the spectrum registration unit.

Description

The utility model relates to the field of measurement of nuclear radiation and can be used for simultaneous spectrometry and monitoring of neutron flux in the background of gamma radiation.

Known fast neutron spectrometer, consisting of an organic scintillator, a photomultiplier tube (PMT) [1] and a spectrometric information processing unit with signal separation by pulse shape. In the scintillator, when neutrons are elasticly scattered by hydrogen nuclei, recoil protons are formed, which, when braked in the scintillator, produce light radiation detected by a PMT. The problem of eliminating the influence of the gamma background is solved by discriminating the gamma background by the shape of the pulse.

The disadvantage of such spectrometers is the impossibility of detecting neutrons with energies less than 0.5 MeV, which is associated with a sharp decrease in the light yield with a decrease in the recoil proton energy, which depends on the neutron energy.

A known neutron flux monitor in which neutrons are slowed down in hydrogen-containing moderators to thermal energies and recorded by a thermal neutron detector [2]. Such a monitor measures neutron fluxes in a wide range of energies - from fast to thermal.

The disadvantage of this analogue is that spectrometry of neutron fluxes with good energy resolution in such devices is impossible.

Closest to the device proposed in this application is a neutron spectrometer [3] (prototype) containing an organoboron scintillator (an organic scintillator with ¹⁰ B boron isotope additives), a PMT, and an amplitude-time analyzer. In a scintillator, fast neutrons experience multiple elastic scattering by hydrogen nuclei with the emission of light radiation until their energy reaches thermal values. Thermal neutrons react ¹⁰ B (n, α) ⁷ Li with the formation of charged particles, resulting in light emission. Simultaneous registration by an amplitude-time analyzer of signals from elastic scattering of fast neutrons in a scintillator and capture of a thermal neutron by a boron nucleus in a given time window is a sign of absorption of a neutron that has transferred all its energy to the scintillator. Although such scintillators can detect neutrons of low and medium energies when they are slowed down in the scintillator to thermal energies and the nucleus traps the ¹⁰ B boron isotope, signals during absorption of background gamma rays in the scintillator are almost impossible to discriminate from signals during neutron absorption.

Therefore, the disadvantage of the prototype is the impossibility of simultaneous spectrometry and monitoring of the neutron flux in conditions of high gamma background.

The technical result of the claimed proposal is the possibility of simultaneous spectrometry and monitoring of the neutron flux in conditions of high gamma background.

The technical result is achieved by the fact that a combined neutron flux spectrometer-monitor containing a fast neutron detector incorporating a scintillator and a photomultiplier (PMT), and an amplitude-time analyzer, scintillator made of plastic, are additionally adjacent to the fast neutron detector from the opposite neutron side a thermal neutron detector with a low sensitivity to gamma radiation is connected to the flow, the amplitude-time analyzer contains a signal counting unit, a measurement unit signal charge unit, coincidence unit, spectrum registration unit, thermal neutron detector attached to the signal counting unit, PMT of fast neutron detector connected to the signal charge measuring unit, coincidence unit connected to the signal counting unit and signal charge measuring unit, coincidence unit connected to the registration unit spectrum.

In the proposed device, the energy determination (spectrometry) of neutrons is carried out by the energy transferred to the recoil protons in a plastic scintillator, and the simultaneous registration of thermal neutrons in a given time window is a sign that the neutron transferred almost all of its energy to the recoil protons. A plastic scintillator is also a neutron moderator in which neutrons are slowed down to thermal energies and the resulting thermal neutrons are detected by a thermal neutron detector, so the proposed device can serve as a neutron monitor. The thermal neutron detector has a low sensitivity to gamma rays, and since fast neutrons are detected only if thermal neutrons are simultaneously detected in a narrow time window, the proposed device can operate under gamma-ray background conditions.

The essence of the utility model is illustrated in FIG. 1, 2 and 3.

In FIG. 1 shows a block diagram of a sketch of the proposed device.

Figure 2 shows a block diagram of an experimental model of the proposed device with an example of a thermal neutron detector and an amplitude-time analyzer.

In FIG. Figure 3 shows the measured neutron spectrum of a ²⁵² Cf source. The energy in MeV is plotted on the abscissa, and the number of readings in relative units on the ordinate. Lead lines indicate the hardware spectrum, the calculated neutron spectrum, and the reconstructed neutron spectrum.

The positions in the illustrations indicate:

1 - fast neutron detector,

2 - plastic scintillator,

3 - PMT,

4 - thermal neutron detector,

5 - scintillation screen,

6 - amplitude-time analyzer,

7 - unit for measuring the signal charge,

8 - block counting signals

9 - block matches,

10 - block registration spectrum

11 - integrated discriminator,

12 - one-shot,

13 is a programmable logic integrated circuit,

14 - analog-to-digital Converter,

15 - integrated discriminator,

16 - one-shot,

17 - computer

18 is a digital-to-analog Converter,

19 - microcontroller.

The device comprises a fast neutron detector 1, consisting of a plastic scintillator 2 and a PMT 3, a thermal neutron detector 4 located close to the plastic scintillator 2 from the side opposite to the neutron flux, an amplitude-time analyzer 6, which has a unit 7 for measuring the signal charge from the fast neutron detector 1 , block 8 counting signals from the detector 4 thermal neutrons, block 9 matches, block 10 registration spectrum.

The device operates as follows.

When a fast neutron enters a plastic scintillator 2 as a result of collisions with hydrogen nuclei (protons), part of the neutron energy is transferred to protons, which due to ionization losses create optical radiation in a plastic scintillator 2. A PMT 3 converts optical radiation into an electrical signal from a fast neutron detector 1, the charge of which depends on the energy lost by the neutron in the plastic scintillator 2. This signal goes to the unit 7 for measuring the charge of the signal from the detector 1 of fast neutrons. Part of the fast neutrons of the neutron flux completely loses energy in the plastic scintillator 2 and slows down to thermal energy. When registering the generated thermal neutrons by the detector 4, a signal is generated that goes to the signal counting unit 8 from the thermal neutron detector 4. The time of deceleration of a fast neutron in a plastic scintillator 2 to thermal energy τ depends on the size of the plastic scintillator 2 and is several microseconds to tens of microseconds. Coincidence unit 9 transmits the measured value of the signal charge from the fast neutron detector 1 from the signal charge measurement unit 7 to the spectrum recording unit 10 when signals from the fast detector 1 and the thermal neutron detector 4 simultaneously appear over a time of the order of τ. Simultaneous registration of a signal from a fast neutron detector 1 and a thermal neutron detector 4 means that the value of the charge of the signal received in the spectrum recording unit 10 corresponds to the initial fast neutron energy, since the fast neutron energy is completely absorbed in a plastic scintillator 2. As a thermal neutron detector 4 a detector that is sensitive to gamma radiation, for example, based on a semiconductor detector with a thin sensitive layer or a thin scintillator with an additive lithium and boron or [2]. Since the value of the signal charge from the fast neutron detector 1 is transmitted to the spectrum recording unit 10 only in coincidence with the signal from the thermal neutron detector 4 in a narrow time range, the probability of detecting a false signal from the fast neutron detector 1 when the background gamma-quanta hits is small.

The neutron flux falls from the side of the plastic scintillator 2. Neutrons passing through the plastic scintillator 2 slow down to thermal speeds and are recorded by thermal neutron detector 4, the signals from which are sent to signal counting unit 8. In this case, neutrons of all energies from fast to thermal are recorded, thus, the neutron flux is monitored.

A sketch of the experimental model for implementing the proposed device is presented in FIG. 2. The fast neutron detector 1 consists of a scintillator 2, which is made of polystyrene with a size of мм74 × 30 mm and has optical contact with a PMT 3 of the type PMT-56. The thermal neutron detector 4 consists of a scintillation screen 5 made of material ⁶ LiF / ZnS (Ag) with a diameter of 60 mm and a thickness of 1 mm and a PMT 3, for example, a PMT-56 type, coupled to screen 5. The signal from the fast neutron detector 1 passes an integral discriminator 11 and one-shot 12, generating pulses of standard duration and amplitude. Upon receipt of these pulses in a programmable logic integrated circuit (FPGA) 13, a time window of 50 μs in duration is formed to register the signal from the thermal neutron detector 4. As the unit 7 for measuring the signal charge from the fast neutron detector 1, a conveyor analog-to-digital converter (ADC) 14 connected to the FPGA 13 is used. The signal charge from the fast neutron detector 1 is measured by adding the ADC 14 samples to the FPGA 13 and is stored in the FPGA memory 13 in the form of a spectrum, thus, the ADC 14 and the FPGA 13 function as a spectrum recording unit 10. The signal from the thermal neutron detector 4 is fed to an integrated discriminator 15 and a single-vibrator 16, generating pulses of standard duration and amplitude. Coincidence block 9 is implemented on the basis of FPGA 13. If a pulse from a single-shot 16 enters a time window opened by a signal from a fast neutron detector 1, the signal charge from a fast neutron detector 1 is transmitted to a computer 17. If a signal is received during a time window (50 μs) no thermal neutrons were received from detector 4; the signal charge from the fast neutron detector 1 is reset to zero.

FPGA 13 carries out the counting of signals from a single-vibrator 12, performing the functions of a block 8 of counting signals from a thermal neutron detector 4 for monitoring a neutron flux.

Setting the discrimination threshold of integrated discriminators 11 and 15 is carried out by a digital-to-analog converter 18. Control and data transfer to a computer 17 is carried out via a USB highway using a microcontroller 19.

The measured hardware spectrum F (E) from the fast neutron detector 1 depends on the neutron spectrum (φ (E) and can be determined by the expression [4]:

Here ε (E) is the detection efficiency of a neutron slowed down to thermal energies by the detector, G (E ′, E) is the response function of the detector (spectral distribution of detector signals from monoenergetic neutrons with energy E ′), E _pore and E _max are threshold and maximum neutron energy. The neutron flux spectrum 8 of the spontaneous fission source ²⁵² Cf was measured, which is well described by a function of the form [5]:

where B is the coefficient of proportionality, E ₀ = 1.43 MeV.

The neutron spectrum was reconstructed from the measured hardware spectrum using the directed divergence method [6]. In FIG. Figure 3 shows the hardware spectrum from the fast neutron detector 1, the reconstructed neutron spectrum, and the neutron spectrum calculated by formula (2). One can see a good agreement between the calculated and measured neutron spectrum.

Measurements with a neutron source of ²⁵² Cf showed that the registration efficiency of the fission neutron counting channel of the experimental model of the proposed device is about 9%, which allows it to be used as a neutron flux monitor. To determine the sensitivity of the experimental model to gamma radiation, it was irradiated with gamma radiation from a ⁶⁰ Co source when measuring the spectrum and monitoring the neutron flux. The measurements showed that since the signals from the fast neutron detector 1 are recorded in coincidence with the signals from the thermal neutron detector 4 in a narrow time window, the gamma-ray flux practically does not affect the results of measurements of the neutron spectrum of the ²⁵² Cf source up to a dose rate of 10 μGy / h. Due to the low sensitivity of the thermal neutron detector 4 to gamma rays, the neutron count rate of the fission of a ²⁵² Cf source by the counting channel does not change to a dose rate of 0.5 mGy / h.

INFORMATION SOURCES

1. Kukhtevich V.I., Trykov L.A., Trykov O.A. Single-chip scintillation spectrometer (with organic phosphorus). M .: Atomizdat, 1970, 119 p.

2. Reilly D., Ansselin N., Smith X. ml., Krainer S. Passive non-destructive analysis of nuclear materials, M .: Binom, 2000, 720 p.

3. Stolyarova E.L. Neutron spectrometers and their application in applied problems. M .: Atomizdat, 1968.

4. Yu.I. Kolevatov, V.P. Semenov, L.A. Trykov. Spectrometry of neutrons and gamma radiation in radiation physics. M., Energoatomizdat 1990, 296 p.

5. Tables of physical quantities. Directory. Ed. I.K. Kikoin, - M.: Mir, 1976, - 1008 p.

6. Tarasco M.Z. Kramer-Ageev E.A. Tikhonov E.B. Application of the directed divergence method for reconstructing the spectrum of fast neutrons. In the book: "Questions of dosimetry and radiation protection." Vol. 11. M. Atomizdat 1970, p. 125-133.

Claims (1)

Combined neutron flux monitor spectrometer containing a fast neutron detector, comprising a scintillator and a photomultiplier (PMT), and an amplitude-time analyzer, characterized in that the scintillator is made of plastic, additionally adjacent to the fast neutron detector on the opposite side of the neutron flow, a thermal neutron detector with a low sensitivity to gamma radiation is attached, the amplitude-time analyzer contains a signal counting unit, a signal charge measuring unit , coincidence unit, spectrum registration unit, thermal neutron detector is connected to the signal counting unit, PMT of the fast neutron detector is connected to the signal charge measuring unit, the coincidence unit is connected to the signal counting unit and the signal charge measuring unit, the coincidence unit is connected to the spectrum registration unit.

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