RU2400773C1 - Method of determining concentration of beta-radioactive gases - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention can be used in technical monitoring or scientific research related with the study of the kinetics of interaction of beta-radioactive gases. A gas system having an absolute pressure sensor with an upper limit of 133 Pa also includes an additional sensor (β-electron detector) whose output value is potential (Uout) (dwg.1) which is proportional to the number of β-electrons formed during decay of atoms of beta-radioactive gases. The detector is calibrated using gaseous mixture of beta-radioactive gases with known concentration at absolute gas pressure less than 133 Pa. Pressure and potential values are recorded in pairs for the required number of times; based on the obtained values for calibrating (graduating) the detector, the sensitivity coefficient (K_T) (dwg.2) is calculated as

, where P is the corrected value of pressure of the beta-radioactive gas taking into account concentration of the calibration gas;

is the average of the product of recorded pressure and potential values;

is the product of average values of pressure and potential;

is the average square and square of average values of potential respectively. Concentration of the beta-radioactive gas is calculated as

, where P_Σ is absolute pressure of the analysed gaseous mixture in the gas system; P_β is partial pressure of the beta-radioactive gas calculated using formula (2), P_β=K_β·U_β.

EFFECT: direct determination of concentration of beta-radioactive atoms and molecular compounds based on said atoms in gaseous mixtures with precision and in a wide range of concentration independent of energy of beta-particles.

2 cl, 3 dwg

Description

The invention relates to the field of radiochemistry and can be used in technological control or research work related to the study of the kinetics of interaction of beta-radioactive gases, in particular tritium, with various metals and alloys, isotope-exchange processes in heterogeneous media, in mass spectrometry and other fields of science and technology, where measurements of the concentration of beta-radioactive atoms (T, ¹⁴ C, ¹⁸ F, etc.) and their compounds in the gas phase are required.

There are various methods for detecting beta particles: ionization, photographic, chemical, calorimetric, semiconductor, etc. [1-4]. Structurally, for registration of beta particles thin-walled cylindrical and end beta counters are used, as well as flow counters. Thin-walled cylindrical counters register beta particles with an energy of at least 500-700 keV, since the aluminum and steel cathodes of these counters have a conditional ultimate thickness of 30-50 mg / cm ² . Beta particles with energies of 100-200 keV are recorded by end counters with mica windows having a thickness of 1-5 mg / cm ² [2].

An analogue of the proposed method is a method for detecting charged particles using ionization detector chambers based on measuring the ionization current in gases under the influence of beta particle energy and representing various forms of capacitors. The number of ion pairs formed largely depends on both internal and external factors, in particular: the radioactivity of the detected gas, beta-particle energy, composition, pressure and temperature of the gas in the ionization chamber [1-3]. There are two main modes of operation of ionization chambers: pulsed and current.

Closest to the proposed method is a method of the current mode of operation of the ionization chamber. The principle of operation of ionization chambers in the current mode is as follows: a beta particle, flying through the sensor’s working space, forms a certain number of gas ion pairs, filling the sensor’s volume, depending on energy, which creates a current in the recording circuit of this device. Thus, to quantitatively determine the radioactivity of a gas, the ionization chamber is pre-calibrated by correlating the magnitude of the recorded signal with the known radioactivity of the irradiating standard. Structurally, ionization chambers (flat, cylindrical, spherical, multielectrode, etc.) differ from each other in the kind of gas (air, argon, boron trifluoride, etc.) that fills the chamber, and in the pressure of this gas in the chamber. Beta radiation is measured using ionization chambers mainly for determining the concentration of radioactive elements (T, ¹⁴ C, ¹⁸ F, ³⁶ Cl, ³⁸ Cl, ³⁹ Ar, ⁸⁵ Kr, ¹³³ Xe), for measuring preparations of artificial radioactive isotopes, as well as for measuring dose or dose rate in the field of beta emitters [1].

The disadvantages of this method are:

- determination of the concentration of beta-radioactive gas is based on the registration of the secondary effect - the current generated during beta-particle ionization of the working gas and depending on the energy of the beta particles, which, as a result, affects the accuracy of the results;

- for each energy range of detected beta particles, the development of an individual design of the measuring device is required [2], in particular, the energy range of beta particles can be from 0.0056 MeV (7) to 4.11 MeV ( ³⁶ Cl) [5];

- the dependence of the recorded current value on the gas parameters in the ionization chamber: pressure, temperature and gas composition [1];

- the presence of a threshold value of the saturation current, that is, with an increase in the radiation dose (concentration of beta particles), the magnitude of the recorded current remains constant.

The sum of these factors reflects the complexity and individuality of the approach in the manufacture of a recording device for each radioactive isotope. In addition, each device operates in a strict range of concentrations and energies of beta particles. All this, as a consequence, affects the accuracy of determination of radioactive isotopes in the gas phase, and an increase in accuracy leads to a significant increase in the complexity of the work performed.

Thus, the object of the invention is to provide a method that allows direct measurements of the concentration of beta-radioactive gases in gas mixtures with high accuracy and reliability of the results. In addition, the detected signal should not depend on the energy of beta particles, the composition of the gas and should have a linear dependence on the number of decayed nuclei of a beta-radioactive gas.

The technical result obtained by using the invention is that:

- it is possible to directly determine the concentration of beta-radioactive atoms and molecular compounds based on them (HT, DT, T ₂ , CH _x T _4-x , ^l4 CH ₄ , ³⁹ Ar, ¹³³ Kr, ¹³³ Xe and others) in gas mixtures with precision accuracy and in a wide range of concentrations, regardless of the energy of beta particles, using one device for any radioactive gas.

The problem is solved in that when implementing the method, including calibrating the recording device, registering the output signal and calculating the concentration of beta-radioactive gas, according to the invention, the absolute gas pressure in the system is recorded, and the potential (U _β ) generated at the resistance is recorded as the output signal load during the passage of beta-particles current resulting from the radioactive decay of beta-radioactive gas nuclei in the recording device, and the concentration calculation beta-radioactive gas is performed according to the formula

where P _Σ is the absolute pressure of the investigated gas mixture;

P _β - partial pressure of beta-radioactive gas in the test gas mixture, calculated by the formula

where K _β is the sensitivity coefficient of the sensor [Pa / V], determined during calibration on a gas mixture with a known concentration of beta-radioactive gas;

U _β is the recorded output value of the potential of the recording device [B].

Another difference of the method is that the calibration of the beta-particle detection sensor is carried out using a gas mixture of beta-radioactive gas of known concentration (C _β ), the absolute pressure values (P _∑ ) and the output value of the recording device potential (U _β ) are recorded in pairs times (n), on the basis of the obtained values, the calibration coefficient of sensitivity of the beta particle sensor (K _β ) is determined by the formula

where P _β = P _∑ · C _β ;

- среднее значение суммы произведений величин P_β и U_β;

- the average value of the sum of the products of the quantities P _β and U _β ;

- произведение средних значений величин P_β и U_β;

- the product of the average values of P _β and U _β ;

- среднее значение суммы квадратов величины U_β,

- the average value of the sum of the squares of the quantity U _β ,

- квадрат среднего значения величины U_β.

- the square of the average value of U _β .

The basis of the proposed method is the principle of recording the current of charged beta particles generated during the radioactive decay of beta-radioactive gas in the gas mixture at a pressure not exceeding 133 Pa (1 mmHg). At a given pressure, the beta particle free span will be commensurate with the size of the recording device [1] and the ionization effect of the source gas will be minimal. In addition, the detected output signal (U _out ) of the sensor (Fig. 1) is proportional to (linearly) the number of beta particles formed in accordance with the law of radioactive decay (4), the ideal gas equation of state (5), and the current (6) generated by beta -particles.

The rate of radioactive decay (A) is described by an equation of the form

where λ is the decay constant, s ^-1 ;

N _β is the number of beta-radioactive atoms.

The number of atoms (molecules) is related to the partial pressure by the equation of state of an ideal gas

where N _M is the number of gas molecules (atoms);

V is the volume of the recording system; k is the Boltzmann constant; T is the absolute temperature.

со скоростью A (4) с образованием β-электрона, имеющего заряд е=-1.6-10^-19 Кл. Вследствие этого в регистрирующем устройстве (фиг.1) будет протекать ток, который пропорционален числу распавшихся ядер трития. Таким образом, уравнение для тока β-электронов (I_β) можно представить следующим образом:For example, the nucleus of the hydrogen-tritium isotope decays according to the scheme

with a speed of A (4) with the formation of a β-electron having a charge of e = -1.6-10 ^-19 C. As a result, a current will flow in the recording device (FIG. 1), which is proportional to the number of decayed tritium nuclei. Thus, the equation for the current of β-electrons (I _β ) can be represented as follows:

In accordance with the scheme shown in figure 1, the recorded potential at the output of the amplifier with a gain (K _y ) will be

Introducing the replacement of all constants in equation (7) by Z = λ · e · R · K _y , we obtain

Equation (8) shows that the detected signal (U _out ) linearly depends on the number of tritium atoms. In [6], it was shown that at pressures above 3 mm Hg gas mixture containing tritium (30%), begins a nonlinear region associated with the influence of the process of radiation of ion formation in the gas mixture. Thus, the choice of the upper limit of the working pressure of 133 Pa is considered quite reasonable.

Figure 1 presents a schematic diagram of a device for registering beta particles. Figure 2 shows the calibration characteristic of the device for registering beta particles, performed at a tritium concentration of 97.0%. Figure 3 presents the thermal desorption spectrum of the allocation of tritium and radiogenic helium-3 from titanium tritide during linear heating of the sample.

The device in figure 1 consists of the following parts: 1 - gas inlet flange; 2 - a case made of stainless steel type 12X18H10T and having an external heater to a temperature of at least 200 ° C; 3 - working volume of at least 10 cm ³ ; 4 - anode; 5 is a grid made of solid copper wire in such a way that the free passage of beta particles in the sensor volume is blocked; 6 - load resistance (1-10 MΩ) is consistent with the internal resistance of the amplifier; 7 - thermally stabilized amplifier, with a gain (K _y ) of at least 1000; 8 - high-precision power source.

Figure 2 shows a calibration graph for determining the sensitivity coefficient of the tritium sensor, performed at a tritium concentration of 97%,

K _β = 16.975 · 133.33 · 0.97 = 2.195 · 10 ³ Pa / V.

Figure 3, by way of example, shows the thermal desorption spectrum of the release of tritium and radiogenic helium-3 from titanium tritide by linear heating, 1 is the potential of the beta-electron detection sensor, 2 is the absolute pressure of the gas in the system. As can be seen from figure 3, the start of gas evolution ( ³ He) occurs at a temperature of 450 K. In this case, tritium is not released, as evidenced by the readings of the beta-electron sensor. The release of tritium occurs in the temperature range of 650-950 K.

The method is as follows: according to the invention, the sensor is calibrated using a gas mixture with a known concentration of beta-radioactive gas by simultaneously recording the absolute pressure of the gas mixture and registering the sensor output signal. The calibration of the sensor on the example of tritium is as follows. The balancing of the recording path of the amplifier is performed in order to satisfy the condition: U _out = 0 at P = 0, where U _out = I _β · R · K _y is the potential at the output of the electrometric amplifier, with the gain K _y , R is the load resistance, I _β is the current of beta electrons; P - absolute pressure sensor readings with an upper measurement limit of 133 Pa. Tritium is injected with a known concentration (C _T2 ) into the recording system to a pressure of P≈120 Pa. The values of U _out (j), P (j) are recorded. Further, reducing the pressure by 10 Pa, the values of U _out (j), P (j) are recorded to a residual pressure of 10 Pa. Perform this procedure in the reverse order, that is, start with a minimum pressure, gradually increasing, reach a maximum and repeat the entire cycle at least three times. Based on the obtained array of values U _out (j = 1 ... n), P (j = 1 ... n), where n is a statistically significant number of measurements, the arithmetic mean values of U, P and their standard deviations are calculated. A correction is made for the values of P, taking into account the actual concentration of tritium C _T2 , that is, P _T2 = C _T2 · P.

Based on the obtained values (U _out (j = 1 ... n), P _T2 (j = 1 ... n)), the correlation coefficient (ρPV), calibration coefficient (K _T2 ) and its standard deviation (σK) are determined by the least squares method by the formulas (9-12)

- дисперсия результатов измерения;

; P_j - среднеарифметическое значение величины давления; аналогично вычисляются и остальные величиныWhere

- variance of the measurement results;

; P _j is the arithmetic mean value of pressure; the remaining quantities are calculated similarly

,

,

,

,

.

,

,

,

,

.

The assessment of the range of recorded concentrations is as follows. The specific activity of gaseous tritium at a maximum working pressure of 133 Pa (1 mmHg) is 1.266 · 10 ⁸ Bq · cm ^-3 . Given the free volume of the sensor V ₀ = 10 cm ^{3 the} maximum activity in the working volume of the sensor will be A _max = 12.66 · 10 ⁸ Bq. Given the obtained value of A _max and the parameters of the electrical circuit (R = 1.0 MΩ, K _y = 10 ³ ), the boundary values of the tritium concentration are found. To do this, determine the maximum possible current (I _max ) in the sensor circuit and the voltage at the output of the amplifier (U _out )

I _max = A _max · e = 12.66 · 10 ⁸ · 1.6 · 10 ^-19 = 2 · 10 ^-10 A;

U _max = I _max · R = 2 · 10 ^-10 · 10 ⁶ = 2 · 10 ^-4 B,

U _out = K _y · U _max = 10 ³ · 2 · 10 ^-4 = 0.2 V.

Thus, under the condition of 100% tritium concentration and a pressure of 133 Pa, the recorded signal from the sensor will be 0.2 V. The analog signal registration system consists of a 12-bit ADC (4095 gradations) of the PCL-818HG type with a minimum measurement range of ± 0.005 V. Then taking the ratio of the minimum signal (U _min ) to the least significant bit of the ADC equal to 2, and discarding the least significant bit, get

Therefore, the minimum detectable concentration of tritium will be 2.5-10 ^-3 %. That in absolute units will correspond to the limit of sensitivity of the sensor for the partial pressure of tritium and will be 3.3 · 10 ^-3 Pa (2.5 · 10 ^-5 mm Hg). Based on the obtained values, the dynamic range for measuring tritium concentration, respectively, is 10 ⁵ , which is comparable in sensitivity to the mass spectrometric method of analysis, in particular, for the MX-7304 mass analyzer.

Evaluation of the methodological relative error in determining the concentration of tritium in the gas mixture is performed as follows.

Initial data:

- the relative error of the pressure sensor (σ _p ) from Varian does not exceed 0.15% [7];

- the relative error of the tritium content in the calibration mixture (σ _P ) is not more than 0.2%;

- the relative error of the electrometric amplifier (σ _T ), according to [8, 9], does not exceed 5-10 ^-4 %.

The relative error in measuring the tritium concentration (σ _DT ) depends on the following main components of its errors: σ _K is the error of the tritium content in the reference gas (refers to the systematic error), σ _P is the error in determining the pressure (when calibrating the sensor and during measurements), σ _U is the total error of the recording channel. In addition, the error of the obtained calibration coefficient K _β (σ _GK ) will serve as the dominant and systematic measurement error.

Substituting the values, we obtain

;

;

.

.

The relative methodological error in measuring tritium concentration is not more than 0.3%.

Using the invention will allow determination of the concentration of atoms of beta-radioactive gases located in any molecular structure in the gas mixture, regardless of the composition of this gas and the energy of the resulting beta particles, while the accuracy of the measurements depends only on the accuracy of pressure and potential measurements.

The application of this method both in combination with mass spectrometric analysis and separately is a promising tool in the study of hydrogen isotope exchange processes and its interaction with metals and alloys.

Information sources

1. Rossi B., Staub G. Ionization chambers and counters. Ed. Zhdanova G.B. M .: Foreign lit., 1951.

2. Sidorenko V.V., Kuznetsov Yu.A., Ovodenko A.A. Detectors of ionizing radiation. Directory. L., Shipbuilding, 1984.

3. Lukyanov VB Measurement and identification of beta-radioactive drugs // Gosatomizdat, 1963.

4. Methods for measuring tritium. USA: Per. from English Ed. Yu.V. Sivintseva. - M.: Atomizdat, 1978. - USA, 1976.

5. Isotopes and properties of elements. Ref. ed. / Kulikov I.S., Metallurgy. 1990.

6. David R. Voorhees, Richard L. Rossmassler, Gretchen Zimmer, Tritium analysis at TFTR, Prinston Plasma Physics Laboratory, Prinston University, Contract # DE-AC02-76-CH03037, 1995.

7. Calibration Test Report. Varian CMR 401, Accuracy max. 0.15% of reading.

8. Low Noise Preamplifier Model 5113. AMETEK Advanced Measurement Technology, Inc.

9. Manoilov V.V., Meleshkin A.S. et al. Hardware for automation systems of isotopic mass spectrometers. Instruments and experimental equipment, No. 3, 1997.

Claims (2)

1. The method of determining the concentration of beta-radioactive gas in the gas mixture, which consists in calibrating the recording device, registering the output signal and calculating the concentration of beta-radioactive gas,
characterized in that the absolute pressure of the gas in the system is additionally recorded, the potential (U _β ) generated by the load resistance during the passage of beta particles generated by the radioactive decay of beta-radioactive gas in the recording device is recorded as an output signal, and the concentration calculation beta radioactive gas is determined by the formula:

,
where P _∑ is the absolute pressure of the investigated gas mixture;
P _β is the partial pressure of beta-radioactive gas in the test gas mixture, calculated by the formula:
P _β = K _β · U _β ,
where K _β is the sensitivity coefficient of the sensor, Pa / V, determined during calibration on a gas mixture with a known concentration of beta-radioactive gas;
U _β is the recorded output value of the potential of the beta particle sensor, V. 2. The method according to claim 1, characterized in that the calibration of the beta particle detection sensor is carried out using a gas mixture of beta-radioactive gas of known concentration (C _β ), the absolute pressure values (P _∑ ) and the output value of the beta sensor potential are recorded in pairs particles (U _β ) the required number of times (n), on the basis of the obtained values determine the calibration coefficient of sensitivity of the beta particle sensor (K _β ) by the formula:

,
where P _β = P _∑ · C _β ;

- the average value of the sum of the products of the quantities P _β and U _β ,

- the product of the average values of P _β and U _β ,

- the average value of the sum of the squares of the quantity U _β ,

- the square of the average value of U _β .

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