(a) the principle and characteristics of the instrument
This instrument is a typical portable instrument and also a common instrument for geological exploration. The structural principle block diagram of the instrument is shown in Figure 4-9.
Figure 4-9 FD-30 13 Digital Radiometer Structure Principle Block Diagram
The detector consists of NaI(Tl) crystal (φ30mm×50mm) and GDB-35 photomultiplier tube (FD-3065438+3B crystal φ 30 mm× 25 mm). After gamma rays pass through the detector, they convert light energy into electrical signals for output. After being amplified by amplifier and selected by discriminator, low-energy gamma rays with energy less than 40keV are blocked out of the recorder, and only higher-energy gamma rays with energy greater than 40keV are shaped, recorded and calculated (through subsequent electronic circuits). Finally, the ppm value (apparent content) or count per second (cps) of the U content is read out through the display (liquid crystal display).
The sensitivity of the instrument is 5s- 1 (CPS)/ 10-6EU (equilibrium uranium equivalent), that is, five counts per second can be generated for every one millionth of uranium content in rocks and soil. The frequency divider is actually a frequency divider. After fine-tuning the clock circuit, the 10-6EU signal is output every 5cps. Then the uranium equivalent content at equilibrium is read out by the display.
The monitor is a four-bit LCD. The timer gives the selected measurement time signal. The alarm sends out abnormal alarm and battery shortage alarm according to the set counting rate signal.
The adaptive working temperature of the instrument is-10 ~+50℃; The power consumption is 150mW, and it can be powered by two batteries 1 for 40 hours.
(2) the operation of the instrument
The appearance of FD-30 13 instrument and the names of each part are shown in Figure 4- 10. The appearance of FD-30 13B is similar, but the material of the instrument shell is slightly different. The tail of FD-30 13 is a square plastic shell, and the tail of FD-30 13B is a round aluminum alloy shell.
Fig. 4- 10FD-30 13 external structure diagram of digital radiometer
1-rubber sleeve; 2- buckle; 3- probe; 4- printed circuit board; 5— Aluminum alloy probe shell (gun body); 6- watchband; 7- plastic nut; 8— Plastic shell; 9— Buzzer; 10- LCD screen; 11-ppm/CPS changeover switch; 12 —— (left) ppm calibration potentiometer, (right) abnormal alarm tone threshold adjustment potentiometer; 13-battery box (gun handle); 14- battery box cover; 15- start switch
1. Functions of the instrument
(1) range
According to the requirement of γ -ray intensity and 5% accuracy, the instrument has set five measuring times of 1s, 2s, 4s, 16s and 64s. 1s is the normal operation of monitoring; 2s, 4s and 16s are accurately measured by automatic range conversion, and each reading is adjusted to a ppm value (take 16s as an example: when the gamma irradiation rate is weak, the measuring time will be automatically judged after the instrument is turned on, that is, the number of gamma rays received by the instrument in 16s is divided by 16. 64s is suitable for the measurement of extremely low γ -ray irradiation rate, and is displayed as cpm value.
(2) liquid crystal display
There is a σ character on the LCD screen, which indicates the total quantity measurement. The characters ppm and cps are units of measurement. Ppm stands for 10-6 (uranium content); Cpm stands for the number of pulses of 64s; Cps stands for the number of pulses per second. When the liquid crystal display screen displays the measurement results in cps, cpm or ppm, the surveyors can read the five-digit measurement values by the number of flashes of overflow symbols (trapezoidal black dots with two mouths facing up). Through fine tuning, the micro-roentgen/hour (γ) value can also be expressed by ppm symbol, but at this time, the balance coefficient of uranium and radium must be accurately known. If the equilibrium coefficient of uranium and radium is unknown and the instrument is calibrated, 1γ≈ 1ppm can be used. There are four bar symbols directly below the LCD screen, which show the battery voltage. Four bright bars indicate that the battery voltage is above 2.5V; The light bar indicates that the voltage is between 2.0 and 2.5 volts .. The light bar disappears, indicating that the instrument can no longer work normally and the battery must be stopped and replaced.
(3) Sound alarm
As an instrument sound alarm device, buzzer gives four kinds of sound information: reading alarm, overflow alarm, abnormal γ -ray irradiation rate alarm and battery fault alarm. The alarm threshold of γ -ray abnormality can be adjusted.
2. Measurement operation
(1)FD-30 13 radiometer
Disassembling and assembling the battery: hold the battery box cover at the end of the handle, unscrew the battery box cover in the "open" direction, and put two 1 batteries into the positive electrode; Tighten the battery cover in the "off" direction.
Switching power supply: turn the knob in the groove of the battery box cover. When the knob is turned to "ON", the power supply is turned on; Turning to "Off" means that the power supply is off.
Measurement: Turn the ppm/cps changeover switch (1 1) to the "ppm" position after turning on the power supply. The continuous tone of the instrument is about 1 min. During this period, the instrument automatically changed the battery voltage of 3V into a high voltage of 800V through DC- AC conversion, boosting and AC -DC conversion, and put it on the photomultiplier tube, so the instrument did not read during this period. When the sound stops, the instrument will read, but at this time, the electronic circuit inside the instrument is being fine-tuned (such as charging with a large capacitor), so the reading error of the instrument is large at this time, so don't record it. After 3 ~ 5 minutes, the instrument will automatically stabilize (the new instrument has a short stabilization time and the old instrument has a long stabilization time), so that the reading can be recorded. The data of each measuring point shows 8s. After 8s, the instrument automatically enters the monitoring stage. At this time, the number displayed by the instrument is changing, which is caused by radioactive fluctuation. If the instrument reading is not recorded within 8s, press the start switch (15) (that is, start) to measure and record again. When reaching a new measuring point, press the start switch again (15) to measure and record; If the instrument is not turned off and is under monitoring, a certain number of instrument changes can also be directly recorded as readings; In the process of monitoring, if there is any abnormality, the instrument will automatically give an alarm to remind the operator to pay attention. At this point, the operator should read more numbers, take their average values as recorded values, track the anomalies and describe the geological conditions in detail.
The operation method of the new FD-30 13 is similar to this, except that the ppm/cps changeover switch has become a built-in option, and the function conversion is realized by LCD screen selection. The calculation of normalization is fine-tuned, that is, it can produce 5.23 counting rates per million uranium content. The button starts to turn into a small white button, generally with 1min as the cumulative time, and then it is normalized.
(2) FD-3065438+3B radiometer
Figure 4-11FD-3013b front panel of digital radiometer
The front panel of FD-30 13B instrument is shown in Figure 4- 1 1. This instrument has no FD-30 1 1, 12, 14 devices, and the functions of1,12 devices are realized by "man-machine interaction" on the LCD screen after the instrument is turned on. Device 14 (switch) is located at the position of device 12. The LCD can display 5 digits. When the gamma intensity exceeds 5 digits, the instrument will automatically shift to the high-grade position (that is, the instrument unit will automatically change from μR/h to mR/h). Instrument "∧" means "select up" and "∨" means select down; "→" means "exit" and "←" means "confirm"; RESET means "reset".
FD-30 13B instrument also uses two No.5 batteries. 1. Install the battery in the same way as FD-30 13, just tighten the box cover. Press the red button behind the handle (key position 12 in Figure 4- 10) to turn on the machine. Press the button to indicate "On" and then press the button to indicate "Off". When the instrument is turned on, you will hear the long sound of "Didi ……", and the words "Welcome to the measuring instrument FD-30 13B" will be displayed on the LCD screen. Then it automatically enters the self-test state, and the words "System self-test, please wait ..." are displayed on the display screen, and the instrument itself is stable after 16s. The instrument displays "Please perform healthy operation", and then press →. When the instrument enters the main menu, the LCD screen displays two lines: "Schiavo Measurement" (top) and "Micro-Lun Measurement" (bottom); "Shivo measurement" refers to the measurement of absorbed dose rate, which has the function of measuring the harm of radioactivity to human body; "Microlens measurement" refers to conventional gamma measurement. At this time, the instrument is in the position of "Shivo Measurement" (the font of this function is highlighted). Press the "∨" key twice, the instrument will turn the page, and two menus of "Dose Alarm" and "Unit Conversion" will appear. "Dose alarm" selects the alarm threshold of absorbed dose, which is the same as FD-30 13No. 12. At this time, press ∨ or ∧, and the instrument can select the alarm thresholds of 0.25μSv/h, 2.5μSv/h, 10μSv/h and 20μSv/h, and then press ∧ to confirm. Usually, the instrument will automatically select 0.25μSv/h as the alarm threshold. Under the function of unit conversion, when you press ↓ again, the instrument will display "1μ SV/h =115μ r/h", indicating the conversion relationship between Schiavo Measurement and Mirren Measurement. When you press "∨" again, the instrument displays "1γ = 0.258 NC/kg h", which is the conversion relationship between γ value and international system of units; When you press ∨ again, the instrument displays "1γ= 1μR/h". These data are built-in default data when the instrument leaves the factory, and the operator does not need to change them. Press "→" and "∨" to return to the "micro-measurement" function. Press "↓" to confirm that the instrument will enter the automatic measurement state. After about 2s, the reading of the instrument is stable and jumps around a certain value (this is caused by radioactive fluctuation), and then the value of the jumping center is read as the gamma intensity of this point. Read and record again after reaching the next measuring point. When working in the field, it is generally not turned off, and the instrument is in the detection stage. If you don't work for a long time (you can't measure the route), turn it off.
3. Matters needing attention and instrument maintenance
1) There is a rubber sleeve at the front end of the probe for shock prevention. When measuring on bedrock outcrop, it should be placed on the rock surface or 1 ~ 2 cm away from the surface to avoid violent impact and damage to the instrument. When transporting instruments for a long distance, they should be transported in the instrument box to prevent violent vibration.
2) When the instrument is not used for a long time, the battery should be taken out to prevent the battery from leaking and corroding the battery box, resulting in poor contact of the instrument; When the battery box is corroded, open the battery box and polish the contact points with sandpaper to repair the instrument. When the instrument is not used for a long time, it should be stored in a dry environment.
3) It is forbidden to open the aluminum sleeve of the instrument to prevent it from being damaged by the high voltage of the instrument. Observing the internal structure of the instrument or calibrating the energy threshold of the instrument should be carried out under the guidance of teachers or experienced staff.
4) Try to avoid opening the instrument sleeve in humid air to prevent the NaI(Tl) crystal in the instrument from being decomposed by moisture. When measuring the base number on the water surface, it should be measured at a distance of 1 ~ 2 cm from the water surface, and it is not allowed to put the instrument probe into the water for measurement. Under special circumstances, when measuring the gamma intensity of bedrock in water, it is necessary to check the leakage of the aluminum alloy sleeve of the instrument and make sure that the sleeve is watertight before being immersed in the water for measurement, but the nut must not be submerged on the water surface; If the water is deep, drainage or other measures are needed.
5) Before installing the photomultiplier tube and the NaI(Tl) crystal, gently wipe off the dirt on the photocathode and the crystal surface with mirror paper, and then evenly coat the surface with a layer of silicone oil. Under normal circumstances, it takes one year for the instrument to be repainted with silicone oil for maintenance.