Lab #13: MEASUREMENT OF RADIATION DOSES

Transcription

1 Lab #13: MEASUREMENT OF RADIATION DOSES THEORETICAL BACKGROUND In order to estimate the radiation exposure it is essential to determine the quantity of radiation. If we identify the dose of radiation to which human body was exposed, we are able to predict the biological effects of irradiation. One of the oldest methods of quantitative determination of radiation field is the method of electrostatic charge measurement generated in the air by ionizing radiation. The measurement, which allows to estimate the value of exposure dose, marked as X, is carried out under highly specific conditions. According to the definition: the value of exposure dose is the relation of dq to dm, where dq is the absolute value of the sum of ion charge of one sign generated in the air, under the conditions of electric equilibrium. X d Q = (1) d m The SI unit of exposure dose is C/kg. More often an extra-systemic unit roentgen is used, where 1R= 2.58 x 10-4 C/kg. At present, we use in dosimetry the notion of absorbed dose instead of exposure dose (applied only to the air). Absorbed dose is defined as the relation of average energy de emitted via the ionizing radiation to the matter contained in a part of volume with mass = dm. de D = (2) dm The SI unit of absorbed dose is J/kg. When the body with the mass of 1 kg has absorbed ionizing radiation energy with the value of 1J we call it the dose of 1 Gy (Grey). The extra systemic unit is rad = 100 Gy. The following dependence between the exposure dose and the absorbed dose is valid for ß- radiation: 1R=87 Gy. Information about the current radiation exposure in the place of measurement is delivered by the value named the power of dose. The increment of exposure dose in the defined period of time is called the power of exposure dose: dx MX = (3) dt The SI unit of exposure dose is A/kg. The extra systemic unit is mr/h. The power of absorbed dose is the relation of increment of absorbed dose (dd) to the time in which this increment took place (dt):

2 dd MD = (4) dt The SI unit of power of absorbance is Gy/s [m 2 /s 3 ]. The biological effect of radiation depends not only on the absorbed dose, but also on the type of radiation. That is why to estimate this effect you should multiply the absorbed dose by the RBE (relative biological effectiveness) factor. For the outside irradiation with the X-, α-, and β-radiation the RBE factor = 1. For the irradiation with the same type of rays resulting from the inside radioactive contamination the RBE factor = 2. For the outside radiation of neutrons and protons with energy up to 10 MeV, the RBE factor = 10 and for the heavy recoil nuclei the RBE factor = 20. For other types of radiation and internal contaminations, the values of RBE factor are given in special tables. The equivalent of absorbed dose is the product of the RBE factor and absorbed dose. The SI unit of the equivalent of absorbed dose is the same as the unit of absorbed dose - J/kg. Unlike the unit of absorbed dose (Grey), the unit of the equivalent of absorbed dose is called sivert (Sv). The extra systemic unit of the equivalent of absorbance is rem = 100 Sv. DIGITAL RADIOMETER The EKO-D digital radiometer is a multifunctional measuring apparatus used for dosimetric measurements and as a warning device. It is applied for the measurement of dose and power of dose of ionizing radiation, and for counting the impulses from the Geiger- Muller counter. Using the EKO-D radiometer you can read the equivalent of absorbed dose (the equivalent of absorbed dose is equal to the absorbed dose under our experimental conditions). You can also determine the power of the equivalent of absorbed dose (under our experimental conditions the power of equivalent of absorbed dose is equal to the power of absorbed dose). Units of the measured values are expressed respectively as msv and µsv/h. Measurement results can be also expressed in extra systemic, traditional units mrem and mr/h. In the front part of the radiometer casing there is a window for the measurement of β- radiation. The Geiger-Muller counter is energetically compensated, which is why its detection performance is equalized in the wide range of gamma quantum energy. The application of microprocessor allows to correct the nonlinearity of the Geiger-Muller counter performance, reducing the error of measurement in the upper range of the power of doses.

3 BUTTON FUNCTIONS: ON - turn on the device OFF - turn off the device R - turn on the power of dose display E - turn on the dose display AR - set/ display the dose power alarm level AE - set/ display the absorbed dose alarm level + - start counting impulses - - stop counting impulses Simultaneously press the buttons: R & E - to turn on/off the speaker of impulse counter AR & AE - to turn on/off the impulse counter + & - - to reset the counter display and dose memory RADIOMETER OPERATION: From the moment of turning on the radiometer, it records both the value of absorbed dose and that of power of dose. Press the E button to read the value of absorbed dose. When the value of absorbed dose is displayed in mrem (which is signalized by a red LED indicator beside the unit) and you want to change it to msv, press the E button once again. Each consecutive pressing of the E button changes the dose units. When you press the R button you can read the value of the power of dose. Selecting the unit of the power of dose is parallel to that of absorbed dose. NOTE: Turning the device off does not reset the dose memory. Press the + and buttons simultaneously to reset the dose memory. To watch the results continuously select the type of measurement and then press the + button. Press the button to turn off continuous display. After you have completed the work, press the OFF button to turn off the device.

4 I. DOSE MEASUREMENT EXPERIMENTAL PROCEDURE 1. Uncover the 137 Cs source of radiation. Press the ON button to turn on the radiometer. Press the E button (the red LED indicator beside the mrem or msv unit will go on) to select the type of dose measurement. Press the E button again and, just after that, the + button. In order to reset the dose memory press + and - buttons simultaneously. Turn the radiometer off. 2. Place the radiometer at the appointed distance from the source of radiation. The GM counter should be directed towards the source of radiation. 3. Simultaneously turn on the radiometer and the stopper. Leave the radiometer at the appointed distance for 5 minutes. 4. After 5 minutes press the OFF button to stop the measurement. Place the radiometer out of the reach of radiation, behind the lead cover. Press the ON button, next the E button, and just after that the + button. Select the mrem dose unit with the E button. Put the result in Table I. Simultaneously press the + and buttons, check if the value is projected. If not, press the + and buttons again. Turn the radiometer off. Repeat the procedure from point no. 2 to Place the radiometer out of the reach of radiation, behind the cover of lead bricks and measure the dose value repeating the procedure from points no. 3 and 4. Cover the source of radiation after you have completed the measurements. Table 1 No. Distance R (m) Dose D (mrem) D R 2 (mrem m 2 ) , 0.8 Mean value 7. Measurement behind the cover 6. Using the mean value of D R 2 calculate the dose values at the distances from the source of radiation that are mentioned below: D(0.05m)= ; D(1m)= ; D(10m)= ; D(50m)= Draw the plot of the results.

5 II. POWER OF DOSE MEASUREMENT 1. Uncover the 137 Cs source of radiation. Press the ON button to turn on the radiometer. Press the R button (the red LED indicator beside the mr/h or µsv/h unit will go on) to select the type of dose measurement. Press the R button again and, just after that, the + button for the continuous display of the measurement result. 2. Place the radiometer at the appointed distance from the source of radiation. The GM counter should be directed towards the source of radiation. 3. Using the R button select µsv/h as the power of dose unit. Watch the display of the measurement values, and after they become stable (after ca. 1 minute), read out the displayed value and put it in Table Repeat the procedure from the points no. 2 and 3 for all distances from Table Place the radiometer out of the reach of radiation, behind the cover of lead bricks and measure the power of dose value repeating the procedure from the points no. 2 and 3. Table 1 No. Distance R (m) Power of dose M (µsv /h) , 0.8 Mean value 7. Measurement behind the cover Dose for 2 hours D 2 (µsv) M R 2 (µsvm 2 /h) 6. Using the mean value of MR 2 calculate the power of dose values at the distances from the source of radiation mentioned below: M(0.05m)= ; M(1m)= ; M(10m)= ; M(50m)= Draw the plot of the results. 7. Using the power of dose values calculated above (point no. 6), calculate the values of doses for 2 hours at the distances from the source of radiation mentioned below: D 2 (0.05m)= ; D 2 (1m)= ; D 2 (10m)= ; D 2 (50m)= Draw the plot of the results (logarithms of the values).