Файл: Исследование суточных вариаций поровой активности радона в поверхностных грунтах удк 550. 42 546. 296 551. 51.docx

the series of RA of radon measured by a radiometer ( Rn 0.5 m) and the series of RA of radon (Alpha 0.5 m and Beta 0.5 m) restored by multiplying by the refined correction factors (Table 3.2). The lower diagram shows the reconstructed VA series from the data of soil alpha and beta detectors.






Figure 13 - Calibration results in a 0.5 m well with an alpha

detector

Table 4 - Conversion coefficients to activity units, Bq / imp .

An analysis of the results of the calibration of a 0.5 m well with an alpha detector showed that the temporal changes in the RA of radon, as well as the fluxes of alpha and beta radiation measured at the same depth of 0.5 m, but in different wells, are practically synchronous, have close to sawtooth shape. The amplitude of variations in the flux of alpha radiation changes with time in accordance with the radon field. The amplitude of the flux of beta radiation at 0.5 m depth is almost the same for the period under consideration.

Calibration results in a 0.5 m borehole with a beta detector are shown in Figure 15. The upper diagram of Figure 15 shows a series of radon VA in a 0.5 m borehole, measured by a radiometer (Rn 0.5 m) and restored by multiplying by correction factors (table 5) series of OA of radon for alpha and beta radiation. Here one can clearly see the discrepancy between the amplitudes of RA RA and the flux of beta radiation at a depth of 5 m. Although the diurnal variation is the same. The alpha radiation flux at a depth of 0.5 m changes according to a completely different law than the RA of radon, which is clearly seen in the middle diagram of Figure 15. The lower diagram shows the radon series measured by the radiometer and the reconstructed series according to the beta detector data. In this case, the restoration of the series was carried out not simply by multiplying by the coefficient given in Table 5, but according to the following scheme. The beta-detector

pulse count rates (Nβ) were divided into 2 parts:

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1) constant (Nβs) due

to soil radionuclides not related to the radon component; and 2) the variable (Nβ Rn ) due to beta-emitting radon decay chain radionuclides contained in the borehole air and the topsoil in the lower open basement of the borehole, i.e. Nβ=Nβs +NβRn . Radon VA was calculated using the formula

VA Rn( i )=(N β ( i )-N β s) Kβ 0.5 m . (18)

In this case, the correction factor takes on a different value of Kβ0.5m=4570, and the value of Nβs was 3 pulses / s for a specific scintillation beta detector and specific conditions for installing a beta detector in a 0.5 m well.



Figure 14 - Calibration results in a 0.5 m well with a β-detector






Figure 15 - Calibration results in a 0.5 m well with a β-detector

Table 5 - Conversion coefficients to activity units, Bq / imp .



Calibration results in a 1 m well with a β-detector installed inside are shown in Figure 16.




Figure 16 — Calibration results in a 1 m well with a β-detector

measured by a radiometer ( Rn 1 m) and restored by multiplying by previously determined correction series of radon OA for alpha and beta radiation. The following diagrams in Figure 16 show the dynamics of meteorological quantities: rainfall; Atmosphere pressure; air temperature at a height of 2 m (T) and dew point temperature (Td) .

The last stage of detector calibration only added questions,

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instead of clearing everything up. During the calibration period, 2 abrupt increases in RA of radon were observed. Unfortunately, during this period there were several failures of the detectors. However, the three working detectors responded in the same way as the radon/ thoron radiometer with an abrupt increase in the pulse count rate during the periods of July 13–15 and July 27–28 (Figure 16).

However, when checking the previously determined correction factors (when measured in a well alpha 1 m) with the new ones presented in Table 6, large discrepancies were obtained. The coefficient Kα1m decreased by almost 1.5 times. In addition, this coefficient took different values for the first half of the measurement period (from June 27 to July 19) and the final stage (from July 26 to July 28). The diurnal variation for the "Alpha 1 m" series did not manifest itself, although the RA of radon at a depth of 1 m experienced diurnal variations.

Reconstruction of the radon RA series according to the data of a beta detector at a depth of 1 m was carried out according to the same scheme as described above. The resulting equation is shown in Table 6, and the results of recalculating the beta series into the radon OA series are shown in Figure 18 in the middle diagram. A good agreement with the values measured by the radon radiometer was obtained, however, the fact that the reaction of the beta field to a change in the RA of radon was ahead by approximately 2–2.5 hours was confirmed.

As for the radon VA series reconstructed from data on alpha and beta radiation at a depth of 0.5 m, presented in Figure 3.26, we can conclude that the daily variation is similar, but the average values and amplitudes of the calculated volumetric activity do not match. The fourth calibration experiment also revealed a shift in time between the onset of the maximum at depths of 1 and 0.5 m. However, this time the opposite situation is observed, when the maximum appears 0.5 m earlier than 1 m.
Table 6 - Conversion coefficients to activity units, Bq / imp .

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Figure 17 - Dynamics of radon RA measured and reconstructed from β- and α-radiation at a depth of 1 m





Figure 18 - Dynamics of radon RA measured and reconstructed from β- and α-radiation at a depth of 1 m
Analysis of the results of the performed calibration experiments showed the following.

When calculating correction factors for converting the measured value into units of radon volumetric activity in soil air, one should take into account the fact that forced pumping of air from a well reduces the flow of β- and α-radiations. At the same time, the range of diurnal variations is significantly reduced.

The α-radiation field at a depth of 1 m does not reflect the dynamics of the radon field and, therefore, is not suitable for use in monitoring subsurface radon. However, with anomalous radon releases, the α-radiation field at a depth of 1 m reacts in a noticeable way, which

makes this parameter acceptable for use in the prediction of hazardous phenomena, with some limitations.


Figure 19 - Dynamics of RA at a depth of 1 m and fields of β- and α-radiation at a depth of 0.5 m

4. 1   ...   6   7   8   9   10   11   12   13   ...   32

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Conclusion on the chapter

The field of β-radiation at depths of 0.5 and 1 m quite well reflects the dynamics of the radon subsoil field, the daily variation is well traced. However, the daily course of the β-field in some periods has a shift compared to the daily course of the radon field, i.e. the time of the onset of the maximum in the dynamics of the β-field is ahead/late by several hours.

The dynamics of RA of radon in soil air at the same depth, but at a distance of 1.5–2 m, can differ significantly. The maxima in the daily course of RA of radon at different depths occur at different times, at a depth of 0.5 m - approximately at 16-18 hours, and at a depth of 1 m - at 24 hours. The delay in some periods reaches 8 hours.

Correlation analysis between the radon field and meteorological values revealed only a significant relationship with the amount of rainfall.

A 2-month experiment on the calibration of β- and α-radiation detectors installed in wells did not make it possible to unambiguously determine the correction factors for converting to units of volumetric activity. As a result, it was decided to conduct a second experiment with some adjustment of the experimental design, as well as refinement of the VA detector installation scheme. The requirements for the conditions for calibrating the readings of the VA detector in units of RA of radon are as follows:

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  Wells with VA detectors installed inside should not be opened during calibration, i.e. tubes for pumping air from the well, which are cyclically connected to the radon radiometer, should be installed at least a day before the start of the experiment.
  
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  The VA detectors should not be removed from the well or moved in the well during calibration, as this leads to a distortion of the time series of data.
  
• 
  To calculate the coefficient of decrease in the range of diurnal variations after the start of pumping air from the well, it is necessary to record data from the VA detector at least a week before the start of the experiment, and after its completion.
  

The development of the project infrastructure made it possible to analyze the results of the calibration of soil detectors by 0.5 and 1 мusing a radon radiometer, which showed the following:

at depth, 0,5 мthe temporal changes in the α- and β-fields are practically synchronous, but have different amplitudes ;

in the daily course of radon VA at different depths, the maxima at depth 0,5 мare recorded at 16–18 h, and at depth 1 мat 24 h; the delay in time of the moments of the onset of maxima in radon VA is

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