RU2616224C1 - Method for monitoring density of radon undisturbed flow from the ground - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: method for monitoring the density of the undisturbed stream of radon from the soil surface includes the steps of performing registration of alpha radiation radon decay products accumulated inside mounted on the surface of the soil collection chamber, in which the body has holes for the partial release of the soil gas. Preliminary on-site storage chamber produces radon flux density measurement and thoron by a radiometer, determine the number of pulses from Thoron and alpha-emitting daughter products of its decay N_Tn. Then set the storage chamber to the soil surface and produce a continuous sequence of measuring the number of pulses with a duration of one measurement τ from 60 to 900 s the collection chamber fastened alpha scintillation detector sensitive surface of which is not less than 0.10 m above the ground surface, define a correction factor K_Rn for the pulse count rate from the transfer of radon and alpha-emitting daughter products of its decay in units of radon flux density, and radon flux density is determined by the expression:

where q_Rn(t) - flux density of radon from the ground surface at time t, Bq m^-2 s^-1; K_Rn - correction factor (Bq m^-2 s^-1)/(Imp. s^-1); N_Rn Tn(t) - the total number of registered for the duration of one measurement τ pulses of radon, thoron and alpha-emitting daughter products of decay in the time t, imp.; N_Tn - the number of pulses from Thoron and alpha-emitting daughter products of its decay for the duration of one measurement τ, imp.; τ - the duration of one measurement, s.

EFFECT: simplification of the method of monitoring, improving the reliability of the results.

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Description

The invention relates to the field of measuring nuclear radiation, namely to measuring the density of an unperturbed radon flux from the soil surface in a monitoring mode, and can be used to study intraday variations in radon flux density, assessing the radon hazard of territories, ionization density of the atmospheric surface layer, studying earthquake precursors, processes and gas exchange mechanisms between soil and atmosphere, lithospheric-atmospheric bonds.

There is a method of measuring the density of radon flux from the soil surface by beta and gamma radiation, which consists in registering beta radiation of radon decay products accumulated inside the storage chamber mounted on the soil surface [RU 2428715 C1, IPC G01T 1/16 (2006.01), publ. 09/10/2011]. According to this method, the total number of pulses from beta and gamma radiation of radon decay products is measured by a beta and gamma radiation counter installed inside the storage chamber, the sensitive surface of which is located so that beta radiation of soil radionuclides does not get on it, and the base the storage chamber is closed by a diffusion filter, preventing the ingress of thoron from the soil. Before measurement, the chamber is ventilated for at least 900 s, then, with the chamber open, background measurements are performed for t _{1 for} at least 300 s, the chamber is closed, and for the time interval t ₂ from 2400 to 48000 s, the number of pulses from beta and gamma radiation. The radon flux density is determined from the expression

where q is the radon flux density from the soil surface, Bq m ^-2 s ^-1 ;

N is the measured total number of pulses over time t ₂ , imp .;

- измеренное фоновое значение количества импульсов за время t₁, имп.;

- the measured background value of the number of pulses over time t ₁ , imp .;

ε - correction factor, imp. s ^-1 Bq ^-1 ;

S is the base area of the storage chamber, m ² ;

λ is the decay constant of radon, c ^-1 .

t ₁ - background measurement time, s;

t ₂ is the accumulation time of radon, s.

The measurement process is complicated due to the fact that you have to open and close the storage chamber, periodically changing the filter at its base, which leads to a change in the measurement conditions. In this way, the density of the perturbed radon flux is measured, that is, an underestimated value is measured in comparison with the true density value of the unperturbed flux. Thus, all this leads to an increase in the total uncertainty of the measurement result.

A known method of measuring the density of the undisturbed flow of radon from the soil surface [Stieff L.R., Kotrappa P., Bigu J. Passive E-PERM® Radon Flux Monitors For Measuring Undisturbed Radon Flux From The Ground // International Radon Symposium. - Fletcher, North Carolina, 1996. - P. II-1.1 - II-1.6.], Selected as a prototype, which consists in the fact that the storage chamber, in the casing of which there are several small openings for a partial exit of soil gas, and located inside a pre-charged electret detector (Teflon disk) is installed on the soil surface, pushing the edges of the base of the chamber to the restrictive ring, for a period of several hours to months. Soil radon enters the storage chamber through a diffusion filter that traps the thoron mounted at the base of the storage chamber. Inside the chamber, radon decays, there is an accumulation of decay products and air ionization. Ionization of air inside the chamber leads to a decrease in the surface charge of the electret. After exposure, after 8 hours, the electret detector is removed and the voltage drop due to radon is measured with a standard meter. The discharge rate of the electret detector is proportional to the radon flux density.

Radon from the storage chamber through openings in its housing partially enters the external atmosphere and a semi-equilibrium concentration of radon is established inside the storage chamber, which varies with time depending on changes in the radon flux density. Such a storage chamber makes it possible to measure the density of the undisturbed radon flux from the soil surface. The storage chamber is pre-calibrated to establish the coefficient converting the meter reading into units of radon flux density.

The disadvantages of the prototype method:

- the long duration of one measurement, which depends on the thickness of the Teflon disk and ranges from 2 days to 12 months, which does not allow the study of intraday variations in the radon flux density;

- inability to use for monitoring, i.e. conducting continuous sequential measurements without operator intervention;

- the need to periodically change the diffusion filter at the base of the storage chamber, which leads to a change in the measurement conditions.

The objective of the invention is to develop a method for monitoring the density of the undisturbed flow of radon from the soil surface.

The problem is solved due to the fact that the method for monitoring the density of the undisturbed radon flux from the soil surface, as in the prototype, is to register the alpha radiation of radon decay products accumulated inside the storage chamber mounted on the soil surface, in the case of which openings for partial soil gas release.

According to the invention, previously, at the installation site of the storage chamber, the flux density of radon and thoron is measured using a radiometer, the number of pulses from the thoron and alpha-emitting daughter products of its decay N _{Tn is} determined. Then, a storage chamber is installed on the soil surface and continuous sequential measurements of the number of pulses are made with a measurement duration of τ from 60 to 900 s, mounted inside the storage chamber by a scintillation alpha detector, the sensitive surface of which is located at least 0.10 m above the soil surface. The correction factor K _{Rn is} determined to convert the count rate of pulses from radon and alpha-emitting daughter products of its decay into units of radon flux density. The radon flux density is determined from the expression:

where q _Rn (t) is the radon flux density from the soil surface at time t, Bq m ^-2 s ^-1 ;

K _Rn - correction factor, (Bq m ^-2 s ^-1 ) / (imp. S ^-1 );

N _{Rn + Tn} (t) ^{_ the} total number of pulses from radon, thoron, and alpha-emitting daughter products of their decay recorded at the time t, imp .;

N _Tn is the number of pulses from the thoron and alpha-emitting daughter products of its decay during the duration of one measurement τ, imp .;

τ is the duration of one measurement, s.

It is known [Yakovleva BC Modeling the influence of the state of the atmosphere and lithosphere on the dynamics of the flux density of radon and thoron from the earth's surface // Izvestiya TPU. 2010, T. 317, No. 2, S. 162-166] that the density of the toron flux from the soil surface (Fig. 1) almost does not change with a change in the advection rate (not more than 2.6% in a wide range of values 0 <υ < 10 ^-5 m / s). Therefore, in expression (2), the value of N _Tn is constant for a particular monitoring location and does not depend on time.

The choice of the duration of one measurement τ from 60 to 900 s depends on the technical characteristics of the selected alpha detector, namely the efficiency of its registration, as well as on the volumetric activity of radon inside the storage chamber, and is determined by the requirement for the uncertainty of the measurement result. The restriction on the distance of the sensitive surface of the detector from the soil surface, equal to 0.10 m, allows you to get rid of the "background", which may be due to registration of alpha particles formed during the decay of radionuclides contained in the soil.

In the proposed method, the number and size of holes are selected based on the condition that the pulse count rate inside the storage chamber should be no less than 10 times higher than the pulse count rate in an open atmosphere at the same distance of the sensitive surface of the scintillation alpha detector from the soil surface . This allows to reduce the statistical error (standard deviation) of the measurement result.

Thus, the proposed method for monitoring the density of the undisturbed radon flux from the soil surface is simple and reliable.

In FIG. Figure 1 shows: the solid curve is the dependence of the density of the toron flux (PPT) from the soil surface on the rate of advection of soil gases; dashed-dotted curve is the dependence of the radon flux density (SPR) from the soil surface on the rate of advection of soil gases.

In FIG. 2 shows a block diagram of a device for monitoring the density of an undisturbed radon flux from a soil surface.

In FIG. 3 presents the results of measuring the density of radon flux from the soil surface.

To implement the proposed method, a device was used to monitor the density of the undisturbed radon flux from the soil surface (Fig. 2), containing a storage chamber 1, six holes made on its upper surface 2. A scintillation alpha detector 3, a sensitive surface, is mounted on the upper part of the storage chamber 1 which is located 0.10 m above the ground surface. The scintillation alpha detector 3 is connected to a signal amplification unit 4 (BUS), to which a counter 5 (C) connected to a computer 6 (computer) is connected.

The storage chamber 1 with a volume of 4.71 l, a height of 0.15 m and a base area of S = 3.14 · 10 ^-2 m ² and with a restrictive ring is made of a material impermeable to radon - polyvinyl chloride. The diameter of the holes 2 is 4 mm.

As a scintillation alpha detector 3, a signal amplification unit 4 (BUS) and a counter 5 (C), the BDPA-01 detection unit (STC RADEK LLC) was used, which is running factory software on a computer 6 (computer).

To monitor the density of the undisturbed radon flux from the soil surface, we chose a site located near the Institute for Monitoring Climatic and Ecological Systems in Tomsk.

Previously, before installing the storage chamber 1 on the soil surface, the flux density of radon and thoron was measured using an RTM 2200 radiometer (SARAD GmbH, Germany). The measured flux densities of radon and toron were: q _Rn (t) = 15.82 mBq m ^-2 s ^-1 and q _Tn (t) = 850 mBq m ^-2 s ^-1 .

Then, at the installation site of the device for monitoring the density of the undisturbed flow of rhodon from the soil surface, the number of pulses N _Tn from the thoron and alpha-emitting daughter products of its decay was determined for the duration of one measurement τ = 600 using an SEA-13P1 spectrometer (CJSC SPC "ASPECT" ), in which a D30 type semiconductor alpha detector is used (LLC SNIIP-Plus). N _Tn was: N _Tn = 7.80 imp.

After that, the accumulation chamber 1 was installed on the ground so that the sensitive surface of the scintillation alpha detector 3 was located 0.10 m above the surface of the soil, while the edges of the accumulation chamber 1 were pressed into the soil to the restrictive ring. Through holes 2, radon partially entered the external atmosphere; therefore, a semi-equilibrium concentration of radon was maintained inside the storage chamber 1 during monitoring.

An alpha particle entering the ZnS (Ag) scintillator layer of the scintillation alpha detector 3 caused light flashes, which, in turn, interacting with the photocathode material of this detector 3, knocked out electrons from it, which were “multiplied” as a result of secondary emission by dynodes, as a result, a voltage pulse appeared at the equivalent load resistance of the anode circuit. Next, the signal was fed to signal amplification unit 4 (BUS), the output of which was fed to the input of counter 5 (C) and transmitted to computer 6 (computer).

Thus, we measured the pulse count rate inside the storage chamber 1 for a duration of one measurement τ 600 s, making sure that the pulse count rate inside the storage chamber 1 satisfies the condition that it should be at least 10 times higher than the pulse count rate in open atmosphere at the same distance of the sensitive surface of the scintillation alpha detector 3 from the soil surface.

Measured for τ = 600 s, the total number of pulses N _{Rn + Tn} from radon, thoron, and alpha-emitting daughter products of their decay was 48.60 pulses. The pulse count rate inside the storage chamber 1 was 48.60 pulses / 600 s = 0.081 pulse / s, which is 40.5 times higher than the pulse count rate in the open atmosphere, which was 0.002 pulse / s.

The correction coefficient K _{Rn was} determined to convert the count rate of pulses from radon and alpha-emitting daughter products of its decay into units of radon flux density from expression 1, which amounted to

Contributed K _Rn to the computer program 6 (computer) to calculate the radon flux density.

The initial countdown time was set at 00:00:00 on 02.09.2011, the duration of one measurement was set to 600 s, and using a computer 6 (computer), expression (1) monitored the density of the undisturbed radon flux from the soil surface. The first measured value of q _Rn was 14.20 mBq m ^-2 s ^-1 . The remaining values of the results of monitoring of the outflow are shown in FIG. 3.

Similarly, measurements were taken at a measurement duration of τ = 60 s and τ = 900 s and the first measured values were q _Rn = 13.90 mBq m ^-2 s ^-1 and q _Rn = 14.24 mBq m ^-2 s ^-1, respectively .

Claims (7)

A method for monitoring the density of an unperturbed radon flow from the soil surface, which consists in recording alpha radiation of radon decay products accumulated inside a storage chamber installed on the soil surface, in the housing of which holes for partial exit of soil gas are made, characterized in that it is preliminarily at the installation site of the storage chamber measure the flux density of radon and thoron using a radiometer, determine the number of pulses from the thoron and alpha-emitting daughter products in its decay N _Tn, then the collection chamber is set at ground surface and produces a continuous sequential measurements of the number of pulses with a duration of one measurement τ of 60 to 900 with a fixed inside the collection chamber alpha scintillation detector sensitive surface of which is not less than 0.10 m above the ground surface is determined correction factor K _Rn for counting pulses from the speed of translation of radon and alpha-emitting daughter products of its decomposition in unit plotnos and the flow of radon and radon flux density is determined by the expression:

where q _Rn (t) is the radon flux density from the soil surface at time t, Bq m ^-2 s ^-1 ; K _Rn - correction factor, (Bq m ^-2 s ^-1 ) / (imp. S ^-1 ); N _{Rn + Tn} (t) is the total number of pulses recorded during the duration of one measurement of τ from radon, thoron, and alpha-emitting daughter products of their decay at time t, imp .; N _Tn is the number of pulses from the thoron and alpha-emitting daughter products of its decay during the duration of one measurement τ, imp .; τ is the duration of one measurement, s.

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