RU2484554C1 - Method of detecting ionising radiation - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method of detecting ionising radiation is based on the ionisation effect, which involves use of at least one ionisation cell to the electrodes of which an electric potential difference is applied; ionising radiation is detected through continuous measurement of electric current flowing through the ionisation cell, wherein the volume V of the ionisation cell is selected according to defined relationships which link the average free path before ionisation of current carriers knocked out by a quantum of penetrating radiation; the average number of current carriers knocked out by a quantum of penetrating radiation; the equilibrium concentration of current carriers in the semiconductor at given measurement temperature; a coefficient equal to the ratio of the amplitude of the detected, depending on the equipment used and mathematical signal processing techniques, to noise level, wherein for subsequent processing, independent amplification of a signal from each ionisation cell is performed using field effect.

EFFECT: high detector sensitivity.

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Description

Technical field

The invention relates to the field of detection of ionizing radiation using semiconductor devices and can be used in research equipment and radiation protection.

State of the art

Semiconductor detectors of ionizing radiation are solid-state analogues of gas-filled ionization chambers. An ionizing particle entering the detector produces electron-hole pairs, which are collected by an electric field applied to the detector electrodes. The magnitude of the corresponding electric pulse is proportional to the energy lost by the particle or γ-ray in the detector. It is important that the detector collects all the charges formed in it.

Semiconductor detectors have a number of significant advantages over gas-filled ionization chambers:

1. The energy required to produce one pair of carriers in a semiconductor detector is much lower (2.96 eV in Ge and 3.66 eV in Si) than in gases filling the chambers (~ 30 eV). Therefore, the number of pairs formed in the detector is correspondingly larger and less susceptible to statistical fluctuations.

2. The density of the material of a semiconductor detector is much higher than the density of the gases filling the ionization chambers. Therefore, even small detectors can detect high-energy particles and γ-quanta.

However, the known semiconductor detectors themselves are not without a number of disadvantages. The main problem is a semiconductor material that would combine such properties as a large band gap and high carrier mobility. This is necessary in order to, on the one hand, reduce the number of intrinsic (thermal) electrons present in the semiconductor and without exposure to radiation, so that the appearance of additional electrons caused by radiation is noticeable. On the other hand, electrons knocked out by radiation quanta must be sufficiently mobile to create a noticeable current pulse. Unfortunately, currently unknown materials that fully satisfy the specified requirements. Therefore, when conducting, for example, physical experiments, germanium detectors that have all the necessary qualities, except for the presence of a large forbidden zone, cool at least to the temperature of liquid nitrogen.

To increase the electrical resistivity of the detectors, various methods have been developed to reduce the number of carriers caused by the presence of impurities in Si and Ge. These methods are based on creating a pn junction with a small number of carriers in the detector.

Two main types of detectors based on p-n junctions are known:

1) diffuse and surface-barrier detectors;

2) drift detectors.

The diffuse detector is a pn junction switched in the opposite direction. In this case, a region depleted in carriers is formed. The ionization of this region by a particle of penetrating radiation leads to the formation of a current pulse. Surface-barrier detectors are similar to diffuse, but the depletion region is formed at the metal-semiconductor interface.

The drift detector is characterized in that the depletion region is formed by doping p-type semiconductors (Ge or Si) with donors (lithium ions). Note that after the advent of the technology for creating ultra-pure semiconductor materials (Electronic Grade Silicon, High Purity Germanium Crystals) with an impurity density of ~ 10 ¹⁰ cm ^-3 (approximately one impurity atom per billion atoms of a semiconductor), it became possible to use them in radiation detectors.

Another significant problem is the amplification of a weak electric signal generated by a single radiation particle in case the detector is designed to operate at relatively low radiation dose rates and is a particle counter.

Gas-discharge counters are similar in design to an ionization chamber, but secondary ionization due to collisions of primary ions with atoms and molecules of the gas and walls plays the main role in them. We can talk about two types of gas-discharge counters: proportional (in them the gas discharge is non-self-sustaining, i.e., it goes out when the external ionizer ceases to function) and Geiger-Muller counters (in them the self-sustaining charge, i.e. is maintained after the termination of the action of the external ionizer).

In proportional counters, the operating voltage is selected so that they work in the region of the current-voltage characteristic corresponding to a non-self-sustained discharge, in which the output pulse is proportional to the primary ionization, i.e. energy flew into the particle counter. Therefore, they not only register a particle, but also measure its energy. In proportional counters, pulses caused by individual particles are amplified 10 ³ -10 ⁴ times.

The Geiger-Muller counter is not significantly different in design and operation principle from the proportional counter, but it works in the area of the current-voltage characteristic corresponding to an independent discharge, when the output pulse is independent of primary ionization. Geiger-Muller counters register a particle without measuring its energy. The gain of these counters reaches 10 ⁸ . To register separate pulses, the resulting discharge should be extinguished. For this, for example, a resistance is switched on sequentially so that a discharge occurring in the counter causes a voltage drop on the resistance sufficient to interrupt the discharge.

A similar avalanche signal amplification is used in semiconductor devices.

To date, several basic types of photodetectors with built-in avalanche signal amplification have been developed:

- avalanche photodiode;

- silicon photomultiplier tube

The avalanche process itself is a typical phenomenon for all semiconductor devices. It is avalanche breakdown that is a common cause of failure of transistors and diodes and other semiconductor devices. Avalanche photodiodes retain all the useful properties of conventional silicon detectors. However, the operation of the detector in avalanche mode imposes special requirements on the stability of the operating point, since the avalanche multiplication coefficient has a strong dependence on voltage and temperature. These requirements limit the use of avalanche detectors.

The appearance of avalanche photodiodes with negative feedback, which quenches the avalanche process, allowed us to create an avalanche photodiode operating in the so-called “Geiger” mode (APDg).

Like a Geiger-Muller gas discharge counter, which is capable of detecting only the fact of the passage of an ionizing particle, APDg is also capable of detecting only the fact of the production of photoelectrons under the influence of external light, but not their quantity.

A similar but somewhat improved design has a PIN photodiode, and a metal-dielectric semiconductor photodiode.

In order to solve the problem of detecting radiation intensity, another type of photodetector has been developed - silicon micropixel avalanche photodiode MAPD (Micropixels Avalanche PhotoDiode). A similar device is also called SiPM (Silicon PhotoMultiplier), MPPC (multi-pixel photon counter), SiPHE.

This type of detector is a photodetector based on an ordered set (matrix) of pixels (approximately 10 ³ mm ^-2 ) made on a common substrate. Each pixel is an APDg operating in the "Geiger" mode, with a multiplication factor of the order of 10 ⁶ , but the entire MAPD is an analog detector, since the MAPD output signal is the sum of the signals from all the pixels that triggered when they absorbed photons.

As a rule, semiconductor devices with avalanche signal amplification for detecting γ-radiation are used together with a scintillator module that converts high-energy particles (with an energy of ~ 1 MeV) into beams of light quanta (with an energy of ~ 1 eV). This is due to the fact that the sensitive elements of semiconductor devices are quite thin and the probability of absorption of a γ-quantum in it is small. Meanwhile, light quanta in p-n junctions are absorbed with high probability, and scintillators, in principle, can have large sizes, which, on the one hand, is an advantage and, on the other, a disadvantage, when the dimensions of the detector are limited in size.

The patent of the Russian Federation No. 2061282 dated November 30, 1993, published on May 27, 1996 for a “semiconductor ionizing radiation detector”, which implements a method for detecting ionizing radiation based on the ionization effect, which consists in the use of semiconductor electrodes, is known from the prior art. a voltage of 50 V is applied to the detector in order to form two reverse biased pn junctions and, accordingly, two regions of space charges. The incident radiation, interacting with the semiconductor material, due to Compton scattering leads to the appearance of secondary electrons, which, in turn, create electron-hole pairs, which leads to an electric current pulse that can be detected by connecting to the electrodes of the measuring device.

The method of detecting penetrating radiation, implemented in a semiconductor ionizing radiation detector, is the closest in technical essence to the proposed one and is therefore selected as a prototype.

The disadvantage of the prototype is the low sensitivity to ionizing radiation.

Disclosure of invention

The problem to be solved is to create a method for detecting ionizing radiation with enhanced functionality.

Achievable technical result is to increase the sensitivity of the detector of ionizing radiation with the possibility of radiation spectrometry by recording single particles of ionizing radiation with the ability to determine their energy.

To achieve a technical result in a method of detecting ionizing radiation based on the ionization effect, which consists in using at least one ionization cell with an electric potential difference applied to the electrodes, ionizing radiation is recorded by continuously measuring the electric current flowing through the ionization cell, it is new that the volume V of the ionization cell is selected from the relations:

V ^1/3 > l _e ,

N _e > k · (n ₀ · V) ^1/2

where l _e is the average path length to ionization of current carriers knocked out by an ionizing particle;

N _e is the average number of current carriers knocked out by an ionizing particle;

n ₀ is the equilibrium concentration of current carriers in a semiconductor at a given measurement temperature;

k is a coefficient equal to the ratio of the amplitude recognized, depending on the equipment used and the methods of mathematical processing of the signal, to the noise level,

and for subsequent processing, independent amplification of the signal from each ionization cell is performed using the field effect.

The essence of the invention is to isolate the volume of the ionization cell in the radiation detector, which contains the number of thermal current carriers such that fluctuations in the number of carriers will be commensurate with the additional number of carriers that are formed in one act of scattering of the ionizing particle. In this case, it can be expected that it will be relatively easy, without involving cryogenic technology and direct avalanche amplification of the signal in a strong electric field (at voltages of hundreds of volt-kilovolts), to record using the field transistor amplification single events of scattering of quanta of penetrating radiation. Further, by creating according to planar technology an array of elementary sensors working in parallel, creating a cellular structure of the sensor as a whole, it is possible to achieve the required total sensitivity of the device. The linearity of the detector characteristics, provided by the use of field-effect transistor amplifiers and the proportionality of the number of knocked out electrons of the energy of the ionizing particle, will determine the energy spectrum of the radiation.

Consider the expression for the electrical resistance R of the sensing element with a cross section S, length l, containing current carriers with a concentration n, mobility μ and having an electric charge e.

$$R=\frac{l}{e\cdot \mu \cdot n\cdot S}\left(\text{one}\right)$$

As noted, to increase the sensitivity of the detector, it is necessary to increase the electrical resistance of the ionization cell. The mobility of the carriers cannot be reduced due to the need to provide a noticeable ionization current. The minimum carrier concentration at a given temperature is determined by the properties of the semiconductor (band gap) and cannot be reduced. However, reducing the scale of the ionization cell of the device (proportionally reducing all its sizes), according to (1) we increase the value of R, because the length l decreases linearly, and the area S quadratically. By creating an array of similar elementary ionization cells, each of which is connected to a separate signal amplifier (based on, for example, a field-effect transistor), essentially a new material is constructed with high electrical resistance and high sensitivity to penetrating radiation.

The prior art does not know how to create ionizing radiation detectors with a choice of selected volumes of ionization cells based on the criterion for matching the number of current carriers knocked out by the ionizing particle, the value of the rms thermal fluctuation of the number of current carriers in the cell, with field amplification of the ionization signal of each cell, therefore, the invention meets the criterion " novelty".

The absence of distinguishing features in the known analogues means that the claimed invention obviously does not follow from analogues and prototype, in view of which it meets the criterion of "inventive step".

The scope of industrial application of the claimed invention can be very broad - it is both research equipment and radiation protection equipment, including household ones. The latter circumstance is due to the fact that the devices according to the claimed invention can be fully implemented using microelectronic technologies, which ensure low cost of products during mass production. Accordingly, the claimed invention meets the criterion of "industrial applicability".

The inventive method is implemented by the device shown in the figures.

List of drawings

Figure 1 presents a sketch of the ionization cell. 1 - selected volume of a semiconductor with its own conductivity; 2 - insulator; 3 - electrodes.

Figure 2 presents a number of ionization cells loaded on separate amplifiers, and the whole device is made on one semiconductor wafer. 4 - amplifier (field-effect transistor); 5 - substrate; 6 - device for collecting, processing and transmitting data: 7 - communication line with external devices.

Information confirming the possibility of carrying out the invention.

The method is implemented as follows.

The device for detecting quantization of ionizing radiation contains an ionization cell, which includes the allocated volume of a semiconductor with its own conductivity 1, surrounded by an insulator 2, with deposited electrodes 3. The volume of the semiconductor is V. As a semiconductor 1 is used undoped silicon (Electronic Grade Silicon) with its own carrier concentration n ₀ = 1.5 · 10 ¹⁰ cm ^-3 , which corresponds to its own specific volume resistance ρ = 2.3 · 10 ⁵ Ohm · cm. As an insulator, silicon oxide SiO ₂ . The electrodes 3 form ohmic contacts with silicon.

, соответственно среднеквадратичная амплитуда флуктуаций количества электронов δN₀ в объеме V, будет равна;On average, N ₀ = n ₀ · V electrons will be in this allocated volume. The average relative value of fluctuations in the number of electrons is proportional $$\raisebox{1ex}{$\mathrm{one}$}\!\left/ \!\raisebox{-1ex}{$\sqrt{N_0}$}\right.$$

, respectively, the rms amplitude of fluctuations in the number of electrons δN ₀ in volume V will be equal to;

$$\delta N_0=\sqrt{N_0}\left(\text{2}\right)$$

To ensure the normal operation of the entire detecting system, the number of conduction electrons N _e created by the ionizing particle should be noticeable against the background of thermal fluctuations in the number of electrons in the volume V - δN ₀ . The possibilities of extracting a useful signal against a background of noise are determined both by the electronic recording equipment used and the methods used for mathematical processing of the signal. The task here is facilitated by the fact that, in principle, it is known what type of signal should be sought. In such conditions, it is possible to isolate the signal deep below the noise level. In general, using (2) this condition can be written in the following form:

$$N_e>k\cdot {\left(n_0\cdot V\right)}^{\raisebox{1ex}{$\mathrm{one}$}\!\left/ \!\raisebox{-1ex}{$2$}\right.},\text{Where}\left(\text{3}\right)$$

N _e is the average number of current carriers knocked out by an ionizing particle, n ₀ is the equilibrium concentration of current carriers in a semiconductor at a given measurement temperature, k is a coefficient equal to the ratio of the amplitude recognized, depending on the equipment used and the methods of mathematical processing of the signal to the noise level.

The rms amplitude of thermal fluctuations of the voltage δV at the resistance R in the frequency band Δν can be estimated by the Nyquist formula:

$$\delta V=\sqrt{\mathrm{four}kT\cdot R\cdot \Delta \nu },\left(\text{four}\right)$$

where k is the Boltzmann constant, T is the temperature.

The configuration of the semiconductor volume can be optimized in each case by numerical calculations.

An essential condition for the optimal operation of the device is to minimize the number of avalanche electrons initiated by an ionizing particle (for example, a γ-quantum) that leave the selected volume of semiconductor 1. If the direction to the radiation source is not known in advance, then it is preferable that the size of volume 1 along all three coordinate axes be would be equal. Accordingly, the condition must be met:

$$V^{\raisebox{1ex}{$\mathrm{one}$}\!\left/ \!\raisebox{-1ex}{$3$}\right.}>l_e,\left(\text{5}\right)$$

where l _e is the average path length before ionization of current carriers knocked out by a quantum of penetrating radiation.

Let the semiconductor volume V be a cube with a height of h = 1 mm and a base area of S = 1 mm ² . Then its electrical resistance is R = 2.3 · 10 ⁶ Ohms, and the capacitance is C = 0.12 pF.

A voltage U = 1 V is applied to the electrodes 3 of semiconductor 1. In this case, the electric field strength in the volume of the cube is E = U / h = 10 V / cm. The drift velocity of the main current carriers, the electrons in the field E, will be v _e = µ _e · E = 1.3 · 10 ⁴ cm / s. Accordingly, the time the electron crosses the interelectrode gap h will be t _h = h / v _e = 8 · 10 ^-6 s. Moreover, all processes associated with carrier transfer should fit into the frequency band Δν = 1 · 10 ⁶ Hz. From here one can estimate the rms amplitude of thermal noise δV = 3.5 · 10 ^-4 V. The corresponding rms thermal fluctuations of the current will be δI = 1.5 · 10 ^-10 A.

Let an incident γ-quantum with an energy of 0.5 MeV knock out 1 Compton electron in the volume, which created N _eγ conduction electrons. In this case, N _eγ = 4.5 · 10 ⁴ (the energy of formation of an electron – hole pair in silicon is E _eh = 3.8 eV). These excess electrons will leave the semiconductor volume in time t _h , creating a current pulse with amplitude I _γ = e · N _eγ / t _h = 0.9 · 10 ^-9 A. Note that for V = 1 mm ³ we have N ₀ = 1.5 · 10 ⁷ and δN ₀ = 1.2 · 10 ⁴ .

The electrodes 3 of semiconductor 1 are connected to the input of an amplifier 4 based on a field-effect transistor with an input capacitance C _in = 1.5 pF (for example, type BF998). The charge of a cloud of electrons knocked out by a γ-quantum will be Q _γ = e · N _eγ = 0.7 · 10 ^-14 K. The voltage of the pulse signal at the transistor input corresponding to the charge Q _γ released at the input capacitance C _in will be U _γ = Q _γ / C _in = 5 mV. By measuring the amplitude of the signal of the sensing element, it is possible to determine the number of electrons knocked out by the γ quantum and, accordingly, estimate the energy of the γ quantum. Accordingly, the amplified signal is fed to a data acquisition, processing and transmission device 6, which transmits data via a communication line 7 to external devices for further processing (measuring signal amplitudes), data storage, spectral characteristics construction and visualization.

In the mass production of quantum registration devices by the present method, it may be economically feasible to produce allocated volumes 1, insulators 2, electrodes 3, amplifiers 4, a device for collecting, processing and transmitting data 6 on one substrate 5 using a single microelectronic technology.

. В этих равенствах учтено, что удельная плотность кремния ρ_m=2.3 г/см³. Зная заряд электрона, можно определить объемную скорость ионизации в кремнии $${\dot{n}}_e=1.1\cdot {10}^5\text{ 1/с}{\text{м}}^{\text{3}}\cdot с$$

. Пусть, в среднем, энергия γ-квантов равна 1 МэВ. Тогда одному γ-кванту будет соответствовать N_eγ электронов. В данном случае N_eγ=1.2·10⁵. Скорости радиационной генерации электронов $${\dot{n}}_e$$

соответствует скорость поглощения γ-квантов $${\dot{n}}_{\gamma }$$

Let us evaluate the characteristics of the radiation acting on the sensitive element. Let a silicon-based recording device be in a gamma-ray flux with an exposure dose rate $$\dot{D}=\mathrm{one}00\text{μR / hour}=\text{7}\text{.2}\cdot {\text{10}}^{\text{-fifteen}}\text{Cl / g}\cdot \text{from}=\text{one}\text{.7}\cdot {\text{10}}^{\text{-fourteen}}{\text{C / <figure-callout id="3" label="cm" filenames="00000015.png,00000016.png" state="{{state}}">cm</figure-callout>}}^{\text{3}}\cdot \mathrm{from}$$

. In these equalities it is taken into account that the specific density of silicon is ρ _m = 2.3 g / cm ³ . Knowing the charge of an electron, it is possible to determine the volume rate of ionization in silicon $${\dot{n}}_e=1.1\cdot {10}^5\text{1 / s}{\text{<figure-callout id="3" label="m" filenames="00000015.png,00000016.png" state="{{state}}">m</figure-callout>}}^{\text{3}}\cdot \mathrm{from}$$

. Let, on average, the energy of gamma rays be 1 MeV. Then one γ-quantum will correspond to N _eγ electrons. In this case, N _eγ = 1.2 · 10 ⁵ . Electron radiation generation rates $${\dot{n}}_e$$

corresponds to the absorption rate of gamma rays $${\dot{n}}_{\gamma }$$

$${\dot{n}}_{\gamma }=\frac{{\dot{n}}_e}{N_{e\gamma }},\left(\text{6}\right)$$

The density of the incident flux of γ-quanta j _γ is related to the rate of their absorption through the mean free path l or the reciprocal of this parameter, µ.

$$j_{\gamma }=\frac{{\dot{n}}_{\gamma }}{\mu }=\frac{{\dot{n}}_e}{\mu \cdot N_{e\gamma }},\left(\text{7}\right)$$

With the considered values of the parameters, we obtain that a dose of 100 μR / h corresponds to a radiation flux j _γ = 12 quanta / cm ² · s. Further, we can estimate the sampling frequency ν _γ gamma quanta with an energy of ~ 1 MeV by a silicon sensitive element with a volume of V = 1 mm ³ .

$${\nu }_{\gamma }=\mu \cdot j_{\gamma }\cdot V,\left(\text{8}\right)$$

Substituting the parameter values, we obtain ν _γ ≈2 · 10 ^-3 s ^-1 .

The sampling rate can be increased by creating a series of ionization cells on the surface of the silicon wafer. Their number N can be chosen from the following considerations. Let P ₁ be the probability of detecting a single particle by one ionizing cell, depending on the spectral characteristics of ionizing radiation, and P _{Σ the} given total probability of detecting a particle of ionizing radiation. Then the number of ionization cells N is selected from the relation:

$$N\ge \frac{P_{\sum }}{P_{\mathrm{one}}}\left(9\right)$$

So, if for 1 cm ^{2 we} produce 100 parallel-operating ionization cells, then the average counting frequency will be already 0.2 s ^-1 . In one radiation detector, it is possible to arrange several layers of plates with ionization cells. This solution allows you to create a compact device with a high probability of particle registration.

The additional cost in this case will be practically determined only by the cost of the additional silicon area (the cost of an Electronic Grade Silicon silicon wafer with a diameter of 300 mm is approximately $ 100, respectively, the cost of 1 cm ² is approximately $ 0.15). The device for detecting ionizing particles by the present method can be created on the basis of other wide-gap semiconductors - gallium arsenide (GaAs) and gallium phosphide (GaP), etc.

Thus, in the claimed invention, it is possible to register and measure the energy of individual particles of ionizing radiation (γ-quanta, α and β-particles), which solves the stated technical problem of increasing the sensitivity of the detector of ionizing radiation with the possibility of spectrometry of the radiation flux.

Claims (1)

A method for detecting ionizing radiation, based on the effect of ionization, which consists in using at least one ionization cell with an electric potential difference applied to the electrodes, recording ionizing radiation by continuously measuring the electric current flowing through the ionization cell, which differs the fact that the volume V of the ionization cell is selected from the relations:
V ^1/3 > l _e ,
_Ne > k (n ₀ V) ^{l / 2} ,
where l _e is the average path length before ionization of current carriers knocked out by a quantum of penetrating radiation;
N _e is the average number of current carriers knocked out by a quantum of penetrating radiation;
n ₀ is the equilibrium concentration of current carriers in a semiconductor at a given measurement temperature;
k is a coefficient equal to the ratio of the amplitude recognized, depending on the equipment used and the methods of mathematical processing of the signal to the noise level,
and for subsequent processing, independent amplification of the signal from each ionization cell is performed using the field effect.

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