Natural radioactive radiation is the main component of the earth’s radioactive and an important part of the human living environment. With people’s gradual understanding of radionuclides in nature and the wide application of radionuclides in various fields, the radiation of radionuclides pollution has gradually attracted people’s attention   . Soil is one of the main radiations that radionuclides transfer to the environment . Measuring the concentration of radionuclides in the soil in a certain area helps to understand the local radioactivity level. Analysis of the activities of various radionuclides in the soil can also understand the distribution of nuclides and provide suggestions for public radiation protection . As the first choice for leisure and entertainment in people’s daily life, the park is the closest place to nature in the city. Therefore, it is necessary to measure the level of radionuclide in the soil in the park.
2. Investigation Method
2.1. Instrument and Technical Route
The environmental gamma energy spectrum measurement method uses a gamma spectrometer composed of NaI(Tl) scintillator detectors as a tool to directly determine the specific activity of radionuclides in environmental soil on site. Its working principle is that the energy deposited by gamma rays produces fluorescence in the scintillator, and the fluorescence emits electrons on the photocathode of the photomultiplier tube, which are then amplified to form a voltage pulse. The more energy deposited by gamma photons, the greater the pulse amplitude of the probe output. The microcomputer multi-channel system analyzes the height distribution of the pulse, and then the energy distribution of the γ-ray deposited in the scintillator can be obtained. Analyze the obtained gamma spectrum. Then the nuclide information can be extracted .
A portable microcomputer multi-channel gamma spectrometer was used to measure the energy spectrum of the park soil, and the high-purity germanium gamma spectrometer was used to measure the laboratory gamma energy spectrum of the park soil samples. According to relevant standards and methods, 7 typical parks in a certain city were selected as the survey target areas. The number of grid measurement points was determined according to the size of the park and the functional planning area of the park, and 350 on-site gamma spectrometry measurements were determined point.
2.2. Measurement Process
According to the technical specification of ground gamma spectrometry measurement, first turn on the instrument and preheat the instrument. After preheating, check whether the instrument works normally. After the instrument is stable, the instrument is placed vertically at the point to be measured, which is close to the soil surface. The measurement time is 180s, and the data is recorded.
2.3. Quality Assurance
The portable multi-channel gamma spectrometer used in the experiment is measured through the standard saturation model, and the calibration coefficients of K, U and Th are obtained in the calibration certificate issued by the relevant qualification unit. The calibration method is referred to EJ/T 363-2012 . Before the experiment, the short-term stability of the instrument was checked, and the gamma spectrum was continuously measured at a fixed position. The stability measurement data are shown in Table 1.
Table 1. Stability measurement data of portable gamma spectrometer.
From the analysis of the measured data, the relative standard deviation of the instrument is less than 10%, and the instrument is stable and meets the specification requirements.
The multi-channel gamma spectrometer is used to preheat the instrument. After preheating, check whether the instrument works normally. After the instrument is stable, the instrument is placed vertically at the point to be measured, which is close to the soil surface. The measurement time is 180s. The data are recorded and the soil in the park is directly measured, and the contents of three radionuclides 40K, 238U, and 232Th were obtained, and the contents of the three radionuclides were converted into specific activities through conversion coefficients. The average value of these three radionuclides is 544 Bq/kg for 40K, 30.2 Bq/kg for 238U, and 47.2 Bq/kg for 232Th. The specific information is shown in Table 2.
The specific activities of 40K, 238U, 232Th in soil of each park are shown in Figures 1-3.
It can be seen from the figure that the specific activity of radionuclides in the soil of Park E is higher than that of other parks. However, in the analysis of a single nuclide, the specific activity of 40K in the soil of Park E is the highest, which is 640.2 Bq/kg, the specific activity of 238U in the soil of Park F is the highest, which is 33.31 Bq/kg, and the specific activity of the soil in Park E is 232Th. The highest is 54.72 Bq/kg. The specific activity of 40K in the soil of Park A was the lowest at 470.88 Bq/kg, the specific activity of 238U in the soil of Park D was the lowest at 26.77 Bq/kg, and the specific activity of 232Th in the soil of Park C was the lowest at 42.14 Bq/kg.
The survey results of this study show that there are certain differences in the content of radionuclides in the soil of different parks, but the differences are not significant. The radionuclide content in the park is compared with the national average, as shown in Table 3  .
As can be seen from the table, the three types of soil radionuclide content in the city’s open parks are all lower than the national average.
1) After measuring the radionuclide content of 7 parks in the city, the results show that the natural radioactivity level of the city park environment is not high, and the specific activity content of 40K in the park soil ranges from 470.88 Bq/kg
Table 2. Results of radionuclide survey in the park.
Figure 1. 40K Specific activity.
Figure 2. 238U Specific activity.
Figure 3. 232Th Specific activity.
Table 3. Comparison of radionuclide content with national average.
to 640.2 Bq/kg; the specific activity content of 238U ranges from 26.77 Bq/kg to 33.31 Bq/kg; the specific activity content of 232Th ranges from 42.14 Bq/kg to 54.72 Bq/kg.
2) The radionuclide content in soil samples in the park is much lower than that of ordinary building materials.
3) The survey area is a natural radiation safe area suitable for human life. People can walk in the park, relax and exercise in their daily life, without causing harm to the health of residents.