Instantaneous radon measurement method

Early instantaneous radon meters used ionization chambers to measure the alpha rays emitted by radon, such as FD- 1 18 instruments. The measurement method is to drill a hole first, then extract a certain amount of gas from the soil with a sampler, and then measure the number of α particles released by radon or daughters in the sampled gas. Different instruments have different methods to extract gas and measure alpha particles. As shown in Figure 3-7, the pumping circulation system of RE-279 emanator adopts circulation mode. As shown in Figure 3-8, RM- 1003 gas injection device adopts unidirectional air extraction.

There are many measuring instruments in soil radon, such as FD- 1 18G, FD-30 16, RM- 1003, RD-200 and RE-279. There are many instruments on the market now, such as FD-30 17 RaA radon meter.

FD-30 17 RaA radon meter explains the instantaneous radon meter method.

(1) The basic principle of FD-30 17 RaA radon measurement method

FD-30 17 RaA radon meter is the representative of instantaneous radon meter method. It can directly extract the measured concentration of radon in underground soil or water, so as to determine the cause of underground radon anomaly and infer the possibility of underground geological minerals or geological bodies.

The basic principle is that a certain amount of soil radon gas is pumped into the air pump by a pump, and when radon decays into RaA, it is positively charged. A negative high voltage (-2800V) is applied to a special aluminum sheet to collect the decay daughter RaA of radon, and then the counting rate of α particles released by RaA on the aluminum sheet is measured, which is proportional to the radon concentration extracted from the soil, so the α particles produced by the decay daughter RaA of radon can be measured.

Fig. 3-7 schematic diagram of pumping mode of re 279 gas injection device.

Figure 3-8 RM 3-8 RM- 1033 Non-cyclic Measurement Method of One-way Air Extraction by Barometer

(2) The structure of FD-3017RAA radon meter.

The appearance of the instrument is shown in Figure 3-9, which adopts unidirectional pumping mode and consists of two parts: the pumping system and the measuring system. The measuring system applies high voltage to the collecting sheet through the cable to realize the efficient and rapid collection of radon daughter RaA. The measuring system uses a gold-silicon surface barrier detector to measure α particles on the collecting sheet regularly.

Figure 3-9 FD-30 17 RaA Radon Detector

(3) instrument calibration

The measurement of radon in soil radon or water is mostly relative. In order to convert the readings of the instrument into radon concentration values, it is necessary to calibrate the instrument under the condition of consistent measurement conditions and ensure that each reading of the measuring instrument is equivalent to radon concentration values. If they are linear, the conversion coefficient can be determined. The calibration methods of radon meter mainly include circulation method and vacuum method. Radon chamber rose in 1970s, and was built and put into use in China on 1988. The so-called radon chamber is essentially a large volume radon source with stable radon concentration. The volume of 8505-I radon chamber in China is1000 l; Double-layer structure, the upper layer is 200L, and the lower layer is 800L L. Radon concentration starts from 28Bq/L (solid radium non-point source activity providing radon source is 60495bq (3%)). Seven gas nozzles are installed on both sides of the radon chamber, which are specially used for calibration by circulation method and vacuum method. The top cover is equipped with 14 round holes with a diameter of 5.6cm, which are specially used for calibrating silicon semiconductor detectors and accumulated radon detectors.

1. cycle method

Connect the detector (chamber) to be calibrated with the radon chamber through the gas nozzle to form a circulating loop where radon can flow, as shown in Figure 3- 10(a). Open all valves to make the circulation smooth. Blow with a double hammer for 2 minutes or blow with a mechanical pump 1 minute, and close the valve. Read for 6 minutes continuously, take the average value, and calculate the calibration coefficient according to the following formula.

Radioactive exploration method

Where: NRn is the indoor radon concentration value, BQ/(L CPM)；); N is the average of six consecutive readings per minute, cpm；; The bottom of n is the background reading, cpm.

Figure 3- 10 Cycle Calibration System

If the radon chamber is not used, a liquid standard radon source with known radon concentration can be used instead of the radon chamber and connected to the circulation system. As shown in Figure 3- 10(b), close the valves on both sides of the detector with a double hammer 10 minutes. After the airflow is stable, reading can be started after 1 minute. Generally, the number of 10 is taken continuously and the average value is taken.

Radioactive exploration method

Where: q is the activity of radon in the liquid source, bq; V is the total volume of the circulating system (detector+dryer+diffuser+double hammer), L; N is the average value of readings, cpm；; Is the cumulative amount of radon; T is the accumulation time of radon in the diffuser.

If liquid radon source is not used, solid radon source can also be used.

2. Vacuum method

The essence of vacuum method is to connect the detector to the radon chamber (Figure 3- 1 1) and close the valve K2; Vacuum the detector (chamber) with Kl and close k1; Open K2, inhale radon in the radon chamber, close K2 after the air pressure is balanced, and start reading.

Calculate the conversion coefficient according to formula (3-25).

Figure 3- 1 1 vacuum calibration system

1-ionization chamber or scintillation chamber; 2- dryer; 3— Liquid Radium Standard Source

It is also possible to use liquid or solid radon sources instead of radon chambers like the recycling method.

(4) Field working methods

1. Application conditions

The instantaneous measurement method of radon can be effectively applied to the general survey of 0.5 ~ 1.0m thick floating soil area. Generally speaking, radon measurement is the most effective way to find exogenous uranium deposits in sedimentary rocks or sedimentary metamorphic rocks. In the magmatic area, if the metallogenic conditions are closely related to the structural fracture zone, the application effect is also good. In volcanic areas, sometimes ore-bearing and non-ore-bearing structures are dense, and the ore bodies are deep and small, so the application of this method will be affected to some extent.

Smooth terrain and uniform residual soil composition are the most favorable conditions for the application of instantaneous radon measurement method. As for other terrain conditions, the application effect is poor. However, other types of radon methods, such as alpha track measurement, can be effectively used in swamp areas, frozen soil and underwater measurement.

2. Work scale

Using different scales can be effectively applied to all stages from reconnaissance to exploration. In the general survey and detailed investigation, area measurement and square grid are generally adopted, with the distance between points ranging from tens of meters to several meters and the distance between lines ranging from several hundred meters to several tens of meters, as shown in Table 3-7.

Table 3-7 Scale and Point and Line Spacing

3. Field working method of FD-3017RAA radon measurement method

(1) Instrument inspection

Routine inspection should be carried out on the instrument before work every day to check whether the sealing system of the instrument is in good condition, whether the battery voltage value and calibration signal are normal, and whether the scale of the threshold knob is in the original position. Check the stability with the working source before and after work every day. The relative error between each count and the standard count is not more than 65438 00%, and draw the instrument stability check curve.

(2) Working procedures of measuring points

(a) After arriving at the measuring point, check the signs on the measuring point and record the soil quality and landscape conditions;

(b) Drill a hole of about 65,438+000 cm (generally 80cm or 65,438+000 cm) with a steel drill, sledgehammer or special puncher, insert it into the sampler, and stabilize the soil around the upper cone of the sampler in time to prevent the atmosphere from escaping into the hole to dilute the radon concentration;

(c) Put it into an aluminum collector, turn the three-way switch of the instrument to the "inspiratory" position, and lift the inspiratory cylinder evenly, with the inspiratory volume of 1.5L, and complete the inhalation in 45s;

(d) After air extraction, turn the instrument switch to "OFF" and press the "Increase High Pressure" button. High pressure time is generally 2 minutes;

(e) When the high voltage is over, the instrument gives an alarm, takes out the collecting piece from the drawing tube, and puts it into a detector to measure the number of alpha particles released by RaA for 2 min;

(f) After the measurement, the instrument gives an alarm and records the reading;

(g) converting the reading into radon concentration, where NRn=k n, where k is the calibration coefficient of the instrument and n is the count value of 2min on the acquisition table;

(h) Then measure the next point and repeat steps (c) ~ (h).

(3) Exception handling

Three times higher than the background is abnormal. In case of abnormality, check the working state of the instrument in time and do the following work:

(a) Drill a new hole near the original hole and make a second measurement to determine whether the radon source is sufficient;

(b) Qualitative analysis of radon and thorium emanations;

(c) Dense measuring points and survey lines, and delineate the abnormal range;

(d) Observing and recording geological and geomorphological conditions;

(e) Collect specimens, set temporary abnormal signs, and fill in the abnormal registration form.

(5) Quality requirements

In order to check the quality of field observation, several representative sections must be selected for inspection and measurement. Inspection workload accounts for 5% ~ 10% of the total workload.

Inspection and measurement are generally carried out by skilled workers using instruments with good performance. In the process of inspection and observation, attention should be paid to making the sampling depth and air extraction as consistent as possible with the basic measurement.

The inspection and measurement results shall be drawn on the same map as the basic measurement results. If the variation trend of radon concentration on the profile obtained twice repeats quite well, the measurement result is considered satisfactory.

(6) Organize information

Calculation of radon concentration.

Calculate the concentration of radon according to the measured value of the instrument;

Radioactive exploration method

Where: n is the reading of the transmitter; JRn is the calibration coefficient of the emanator.

2. Statistical distribution of radon concentration in the survey area.

Determine the background value and abnormal lower limit of radon concentration in the survey area, draw the radon concentration histogram, and determine its distribution type country.

3. Draw the result map

(a) Survey of radon concentration in the survey area;

(b) Plane contour map of radon concentration in the survey area;

(c) Plane distribution map of radon concentration in the survey area;

(d) Explain the comprehensive result diagram.

(7) Evaluation of abnormal radon emanation.

Concentration values higher than normal visual field 1.5 ~ 3.0 times can be classified as abnormal. It is necessary to make a comprehensive analysis of gas injection anomalies, in order to reasonably explain the anomalies and provide basis for mountain projects. Comprehensive analysis includes:

1. Determine the abnormal properties

The anomalous nature here refers to whether the emanation concentration is caused by uranium or thorium, which can be determined according to the half-life difference between Rn and Tn. After pumping the soil air into the scintillation chamber, we can observe the change of the instrument reading with time in the first 5 ~ 10 minutes. See figure 3- 12.

Fig. 3- 12 I/I0-t relation curve

2. Determine the abnormal range

In order to determine the abnormal range, the grid should be arranged according to a certain proportion. The direction of the survey line should be perpendicular to the extension direction of the anomaly. If its direction is not obvious, you can choose a square measuring network. The size of the network depends on the scale and complexity of anomalies, such as 2m× 1m, 2m×5m, 10m×2m, 10m×5m, etc.

3. Abnormal vertical variation

The purpose is to find out the downward extension of anomalies, which can be achieved by measuring different depths; The measuring points should be arranged at high concentration points. Use an extended sampler at each point to measure at depths of 0.5m, 0.8m, 1.2m, 1.5m, 2.0m, 2.5m and 3.0m respectively. The variation of radon concentration with depth is shown in Figure 3- 13. Curves 1 and 2 in the figure represent shallow and deep coal seams; Curve 3 shows the case of inhomogeneous mechanical dispersion halo; Curve 4 represents non-mineral anomalies. The figure shows the different laws of concentration changing with depth.

Fig. 3- 13 schematic diagram of concentration changing with depth

Because the measurement of different depths can reduce the influence of some accidental factors such as meteorology, the relationship between anomaly and mineralization can be more clearly reflected. This helps to explain abnormal work.

4. Determine the size of the gas source

The size of the air source can be determined by multiple suction. Air holes were punched in the abnormal center, and the air inlet device was inserted to obtain the measured values of different pumping time. With the increase of pumping times, it is a promising anomaly that the concentration of injected gas does not weaken, otherwise it is meaningless.

5. Determine the cause of the abnormality

In order to provide the distribution of radioactive materials on the surface, borehole measurement (or β+γ measurement) can be carried out. The anomaly of emanation caused by local radon accumulation is usually not significantly reflected in borehole gamma measurement.

It is also meaningful to measure the uranium content in the borehole to determine the cause of the anomaly. This method is to analyze the contents of uranium and thorium in soil samples taken from air holes. If the anomalies shown in the isoline map of emanation concentration, γ isoline map and uranium distribution map can overlap (or have certain displacement), they can be determined as favorable areas. This is because there are generally scattered mineralization halos in the overlying strata of concealed ore bodies, accompanied by radon anomalies, gamma anomalies in holes and uranium anomalies.

6. Assessment of abnormal gas emissions

Classify and register the found anomalies; Expose and study meaningful anomalies; This is the main content of anomaly evaluation. Table 3-8 is a comparison table of various abnormal characteristics of gas emission.

Table 3-8 Comparison Table of Various Ejection Abnormal Characteristics