3.2. Results of Measurements of Radiation and Physical Parameters
As mentioned earlier, in the first stage, the activity of 226Ra was measured daily for 21 days. Based on the results of these measurements, the following were determined: the activity of 226Ra, without taking into account the accumulation of its decay products; the activity of 226Ra, taking into account the accumulation of the daughter products of its decay; the percentage of radon accumulation (in the “free” state); and the period for which the daughter decay products of 226Ra enter the state of radioactive equilibrium. After the experiment, the counting sample was depressurized, and after 1–2 days, the specific activity of 226Ra was measured again.
To establish the equilibrium of all members of the
226Ra series in the samples of the studied rocks, a time interval from 5 to 10 days is sufficient (
Figure 4). When carrying out further studies with rocks of other types, we consider it possible to reduce the measurement period to 15 days.
The emanation coefficient was calculated using two methods: gamma-spectrometric and radiometric (emanation). The coefficient of variation between the results of both methods did not exceed 5–10%. So herein when the emanation coefficient is mentioned it refers to the one calculated only by the gamma spectrometric method.
In general, the research results are presented in
Table 2, in which the following designations are used: A
Ra226—specific activity of
226Ra under sealing conditions; K
emanation—coefficient of emanation; AV
dens—average density; TR
dens—true density; Porosity—porosity. Additionally, these designations are used later in the text of the article. In expanded form the results of the study of radiation and physical parameters are shown in
Table A1,
Appendix A.
We have identified a wide range in the
222Rn emanation coefficient in the studied rock samples from 3 to 40%. The kimberlites of the vent facies, represented by autolithic breccias, are characterized by low values of the emanation coefficient, in the range from 1 to 8%. In only one case, the value of the emanation coefficient exceeded 10%. Kimberlites were also characterized by a low porosity from 0.46 to 4.91%, which agrees with previously obtained results for pipes of the Arkhangelsk diamondiferous province [
32]. At the same time, kimberlite pipes in Yakutia have been characterized by a wider range of porosity variation from 3.6 to 12.9% and in rare cases exceeding 20% [
33,
34]. Vent kimberlites are characterized by the lowest activity of
226Ra among the studied types of rocks, varying from 12.42–15.89 Bq·kg
−1 (including sealing), with the exception of one sample, in which the specific activity of
226Ra was much higher 31.46 Bq·kg
−1 (sample ID 37CA-13). It is known that among the magmatic formations, kimberlites are characterized minimal content of radioactive elements [
35]. In addition, the kimberlites of the pipes of the Zolotitsky field have been characterized by an extremely low average concentration of uranium, 0.67 mg·kg
−1 [
19], which, in terms of the activity of
226Ra under conditions of radioactive equilibrium in the uranium-238 chain, does not exceed 10 Bq·kg
−1.
Tuffaceous-sedimentary rocks were distinguished by a wider range of radiation and physical parameters. The activity of
226Ra in these rocks varies from 11.45 to 48.40 Bq·kg
−1, the emanation coefficient from 9.82 to 34.13% and porosity from 13.20 to 36.74%. However, a different picture was observed in the distribution of the studied parameters in the host Vendian and overlying Quaternary and Carboniferous rocks. Most of these rocks were characterized by significant porosity (up to 41%) which are in good agreement with the data given in [
32] concerning the study of the host rocks of the Arkhangelsk diamondiferous province (from 8 to 40%). The host rocks are distinguished by a high level of specific activity of
226Ra—up to 63.32 Bq·kg
−1 and the overlying rocks have a high level of the emanation coefficient (up to 28%) (
Figure 5).
The rate of radon production was calculated for the different types of rocks (
Table 3). This parameter is one of the most important for assessing radon in its free state.
The minimum level of radon production, from 9.84 to 18.96 Bq∙m
−3∙h
−1 (
Table 3) is characteristic of kimberlites of the vent facies. Considering the high values of the specific activity of radium, the emanation coefficient, the level of radon production and porosity, the enclosing rocks of the near-pipe space stand out among the other types. In the Vendian host rocks a number of mineralogical, structural and geochemical features were found. Postmagmatic endogenous bleaching associated with the influence of kimberlites was found in the host red rocks of the Vendian [
21,
36,
37,
38,
39,
40,
41]. The zones of bleaching occurred at the contact with magmatic bodies and in tectonic faults were formed as a result of the action of reducing fluids [
42]. These rocks with vein bleaching clarification are characterized by an increased content of uranium, thorium and potassium, and they are also characterized by geochemical anomalies, specific mineralogical associations and stable isotope anomalies [
21,
22,
40]. These rocks with vein endogenous bleaching are characterized by high content of K, Fe, Rb, Zn, Sr, Ni and depleted (light) isotopic composition of calcite carbon (δ
13C−4.9 and −5.5‰) Among specific mineralogical associations the saponite and minerals of smectite group (including montmorillonite and beidellite) are observed [
37,
40,
41,
42,
43]. The contacts of kimberlites and host rocks have local tectonic elements: zones of mylonites, steep fractures and low-amplitude thrusts [
21,
40,
43,
44,
45,
46,
47]. The abundance of fracture zones in the near-pipe space of kimberlite bodies is associated with the process of diatreme formation, which influenced the tectonic structure of the adjacent sediment, resulting in the formation of a system of fractures of various types [
48]. The occurrence of zones of fracturing and faults is associated with the formation of diatremes as a result of the significant mechanical effect of penetrating gases and melts on the host rocks [
49,
50]. As a result, a system of radial and concentric zones of fracturing and faults with crushing and the vertical movement of blocks of enclosing rocks has arisen in the near-pipe space [
51]. The impact of kimberlites on the Vendian rocks, which led to the formation of fault zones in the near-pipe space, increased the fracturing and various mineralogical and geochemical changes, influencing the radiation parameters of the host rock. Enrichment with uranium (radium) and the increased fracturing of the near-pipe space created conditions for the production and advection of radon through the host rock mass. As a result, in the soil horizons above the kimberlite bodies of the Lomonosov diamond deposit, an increased volumetric activity of radon can be observed, several times higher than the background values [
20,
52]. In the course of this study, we found that the main source of radon observed in the soil air above kimberlite pipes is the enclosing Vendian rocks of the near-pipe space.
To understand the relationship between the studied radiation and physical parameters, a statistical analysis of the data was performed.
3.3. Statistical Analysis
To study the features of radon emanation in rocks, a correlation analysis of the main radiation and physical parameters of the samples was performed (
Table 4). The following parameter was also added to the correlation matrix: A
Rn222, the volumetric activity of radon in the container, Bq·m
−3. The values of this parameter were obtained as a result of experimental work on the accumulation of radon in a sealed container with test samples.
A significant correlation is observed in rocks for Kemanation-A
Rn222 (r = 0.709), K
emanation-AV
dens (r = −0.666), K
emanation-Porosity (r = 0.753), A
Rn222-AV
dens (r = −0.531), A
Rn222-Porosity (r = 0.691), AV
dens-Porosity (r = −0.648) and TR
dens-Porosity (r = 0.646).
226Ra activity in rock samples has no significant correlations with any of the parameters suggesting that it is not the main parameter influencing the formation of a radon field. The lack of relationships between the content of
226Ra and the volumetric activity of radon is probably due to the form of
226Ra in the minerals that make up the rocks [
30,
53]. Radon formed in a solid can enter the surrounding space due to both radioactive recoil and diffusion. In the case of radioactive decay, radon atoms acquire a certain recoil energy, which they subsequently lose when moving. Some of the atoms remain in the solid phase making up so-called “bound radon”. However, the recoil energy of about 86 keV is enough to release atoms outside the crystal lattice, while forming free radon [
24].
Taking the above into account, to characterize the territory of Arkhangelsk diamondiferous province according to the distribution of radon, it is advisable to use a complex of two parameters—the activity of 226Ra in soils and rocks and their emanation coefficient.
As can be seen from the correlation data, the main factors in the formation of the radon field are the emanation coefficient (r = 0.709) and the porosity of the rock (r = 0.691). At the same time, an increase in porosity leads to an increase in the emanation coefficient (r = 0.753). There is a negative relationship between the average density and the volumetric activity of radon in a free state (r = −0.531). This is due to the fact that the method for calculating the average density takes into account the presence of pores in the rock. At the same time, the true density is characterized by a weak effect on the emanation coefficient and the volumetric activity of radon.
Additional information for the interpretation of the obtained statistical data on rock samples is provided by the results of factor analysis (
Table 5).
On the diagram of factor loads (
Figure 6), three groups of factors are distinguished, which determine the nature of the emanation of radon from rocks. The total variance for the three factors is 91.91%.
The first factor, with a dispersion of 52.81%, includes the radon emanation coefficient, the volumetric activity of radon in the counting sample and the porosity of the sample. This is due to the fact that these parameters are the main parameters in the process of forming a radon field. This conclusion was made in the conditions of the experiment and does not take into account other physical factors that can affect the behavior (gas permeability, humidity, temperature, pressure). The same parameters are involved in calculating the level of radon production.
The second factor, with a dispersion of 21.15%, combines two parameters—true density and porosity. The relationship between these parameters is due to the fact that the porosity is a calculated value and is determined based on the density of the sample.
The third factor is represented by one parameter—the activity of radionuclide 226Ra.
The weak determination of the second and third factors is probably associated with more complex interaction mechanisms during the formation of the radon field; to fully understand them, additional data are required to determine the geochemical and mineralogical compositions [
14].
Based on the results of measuring the volumetric activity of radon in a sealed container and calculating the level of radon production, we built a regression model (
Figure 7), which is a linear function of the dependence of two parameters (dependent variable and regressor) and is characterized by regression coefficients (i.e., slope, coefficient of determination).
In our experiment, the regression model has the form y = 19.138x − 32.696 and is characterized by a positive slope equal to 19.138 and a coefficient of determination R2 = 0.8786. The positive slope indicates that with an increase in the level of radon production by 1 unit, the volumetric activity increases by 19.138 Bq·m−3. This value is theoretical and can vary depending on a number of parameters. For this model, it is calculated based on the results of the measurements of parameters. The value of the constant a = −32.696 in this case is not taken into account, because under ideal conditions (for example, the absence of extraneous sources of radon) at a level of radon production P = 0 Bq·m−3 h−1, the volumetric activity will also be equal to 0 Bq·m−3.
The coefficient of determination shows that the change in the volumetric activity of radon in the container (dependent variable) by 87.9% is described by the independent variable (regressor)—the level of radon production—which indicates a sufficient justification for choosing this model. This model more clearly predicts the distribution of radon based on the results of calculating the radon production rate.