MINIRAY SM 2000 X. Dose Rate Measuring Instrument for Radiation Protection Purposes.

Transcription

1 1121 Budapest, Konkoly Thege M. u HUNGARY Phone: Fax: sute@izotop.hu MINIRAY SM 2000 X Dose Rate Measuring Instrument for Radiation Protection Purposes 1

2 1. Application The Miniray SM 2000 X is a dose rate measuring instrument for radiation protection purposes. Principally it is suitable for measuring gamma and roentgen radiations. The detector applied in the instrument is a Geiger-Mueller counting tube supplied with thin glass window allowing sensing of beta radiation as well. The suitable measuring range is adjusted automatically. The Miniray SM 2000 X is a typical pocket instrument needing only the continuous pushing of the attendance button which makes the operation of the instrument easy. The LCD display of three and a half digit provides good reading and allows displaying the automatic measuring range by modifying place of the decimal point. Application fields: industrial and research laboratories, educational institutions, civil and police organisation, fire brigades dealing with radioactive materials industrial radiation measurements by plant and factor radiation protection supervisors radiation protection measurements in medical Roentgen and isotope laboratories background measurements made by individuals 2. Technical data Detector type VA-Z Geiger-Mueller tube dose limit 0.5 R/h (beta and gamma radiation) proposed anode voltage 450 V zero effect approx. 35 imp/min life span approx imp operation temperature range -50 to +70 C wall material surface mass 35 to 55 mg/cm 2 active length 27 mm gas filling neon-halogene Measuring unit µsv/h (pronounciation: micro Sievert per hour). This measuring unit relates to the dose equivalent. Measuring ranges three automatically adjusted measuring ranges: range I 0,00 to 15,00 µsv/h range II 13,0 to 150,0 µsv/h range III 130 to 2000 µsv/h Display 3z digits LCD display with low battery voltage indication a single figure 1 appearing at the highest decimal value indicates a bad function. Time constant approx. 5 seconds. Measuring accuracy within ± 20% in all measuring ranges (for 137 Cs) Energy dependence within ± 20% between 80 kev and 3 MeV Direction dependence within ± 20% for a space angle of 120 degree Temperature range: 0 C to +50 C Temperature change effect: below 10% Humidity: 0 to 85% relative Pressure: 600 to 1300 mbar Operating position: Neutral Warm up time: No Sound signal: Ceramic buzzer 2

3 In measuring range I (0.00 to µsv/h) each detected particle causes a sound signal. In the two higher measuring ranges the sound signal is continuous and serves for indicating the operation of the instrument. Stopping the sound signal indicates incapability of the instrument for measuring (for instance if too high level unprocessable for the detector should be measured). Power supply: One piece of 9 V radio battery (6LR61, 6AM6, 6LF22 type) Operating time: approx. 40 hours for standard battery / approx. 100 hours for alkali-manganese battery Size: 150*80*30mm Weight: approx. 300 g (without battery) 3. Operation manual 3.1 Measuring procedure The instrument can be switched on by pushing the button on the front panel. The button must be kept pushing while measuring. In laboratories using open radioactive isotopes as well as in places where contamination is possible it is advisable to put the instrument into a plastic bag which can be replaced easily if contaminated. In this way decontamination time can be saved. The detector (GM tube) is located parallel to the front panel of the instrument above the LCD display. For this reason the front panel must be approached to the place to be measured as close as possible. Self adjusting of the instrument happens within 8 to 10 seconds after pushing the button while it must be kept pushing continuously. After the mentioned time elapsed the measuring result can be read on the display. For instance the dose rate on the surface of a container must be measured. The front side of the instrument should touch to the surface of the container at the point -or line - where dose rate is intended to measure. The display will show figures like that: Results show slight fluctuation because of the statistic character of radiation. More accurate results can be achieved by forming an average: % % % % % 5174:5=1035 0% 3

4 As it can be seen a mean value of 1035 µsv/h was obtained. This calculation is proposed especially for those cases when fluctuation is extremely high. Considering the present measured values one can conclude that any of them could have been given as final result because deviations from the mean value are less than 5%. If measurements with legal consequences are to be made care is needed. The following condition must be taken into consideration: As the detector of the instrument senses beta radiation too, in spaces where dose rate is to be measured in mixed radiation space (presence of gamma and beta rays) following procedure is proposed. The detector should be shielded by an approximately 5 to 10 mm thick plexi or plastic sheet by which the possible beta radiation is filtered Background measurement If no radiation source is in the vicinity the instrument will measure the background radiation when pushing the button. As the background radiation is rather low very different figures will appear on the display. The reason for that is the time elapsing between arriving particles and the time constant of the instrument is in the same range. In order to get the appropriate measuring result the above mentioned average formation method should be applied. Example: Push the button and keep it pushing e. g. for 100 seconds. Read and record the figures appearing on the display in every 10 seconds. No. Time [sec] Read out Total 1.89 Accordingly the dose rate caused by the background radiation is 0.19 µsv/h on the given place and in the measuring time. By comparing the mean value with the individual results very high differences can be seen giving reason for accepting only the mean value as reliable measuring result. 3.2 Battery replacement When using alkali-manganese batteries the instrument can operate more than 100 hours. Appearance of the LO BAT signal when the instrument is switched on indicates exhausting of the battery. In this case the displayed value can not be considered as measuring result. Measurement can only be followed after replacing the battery. 4

5 Steps of replacing the battery: a. Lay the instrument onto its front panel. b. Unscrew the two bolts in the bottom of the case c. Remove the cover of battery holder d. Replace battery according the appropriate polarity indicated in the holder e. Screw the cover of the battery holder back f. Check the battery is ready for operation by pushing the button for several seconds. When a particle of the background radiation is detected a sound signal can be heard and simultneously a figure other than zero appears on the display. 3.3 Maintenance In addition to the battery replacement the instrument needs the following maintenance works: GM-tube replacement It is nessecary by 5 years independently from the wearing out or earlier depending on the rate of wearing out. PRECAUTION: The instrument may be disassembled only after removing the battery because there is a high voltage inside it in operation Cleaning: Wiping with soapy or alcoholic wet rag is possible. Do not wash! 4.1 Radioactivity and radioactive isotopes 4. Principal ideas of radioactivity There are variations of elements with different atomic numbers but they are identical in most physical and chemical properties. These variations are called isotopes. The name is originated from the Greek denomination of 'identical place'. This refers to the same place of these atomic cores in the periodic table. Isotopes are distinguished from each other by indicating the number of particles constructing the atomic core that is the mass number near the symbol of the element. Potassium has for instance isotopes with 37, 38, 39, 40, 41, 42, 43 and 44 mass numbers. Denotations are 37 K, 38 K K, respectively. Isotopes of a given element differ from each other in the number of neutral particles (neutrones) found in the atomic core. This difference has small influence on the chemical properties of the material but causes big changing in the atomic core structure. With certin particle numbers the atomic cores become instable and get transferred to another state after a certain time. During the transformation process radiation is emitted which is not sensible by human organs of sense but well detectable by suitable instruments. This phenomenon is the radioactivity and isotopes endured such transformation are called radioactive isotopes. The transformation process is commonly called radioactive decay. Among potassium isotopes listed above the variatons 37 K, 38 K, 40 K, 42 K, 43 K and 44 K emit radiation so they are radioactive isotopes while the others are stable ones (not emitting radiation). Transformations taking place in the atomic core of radioactive isotopes are manifold. Among radiations accompanying transformations alpha, beta, gamma and neutron radiations can be distinguished. From among these the beta and gamma radiation can be detected by Miniray SM 2000 X. Beta radiation consists of fast electrons. Gamma radiation consists of photons similarly to common light. 5

6 4.2 Statistical fluctuation Transformation of the radioactive isotopes takes place not simultaneously so the life-span of atomic cores to be transformed is not identical. The transformation takes place according to statistical rules. The process can be caracterized by the probability of transformation related to a time unit instead of the life-span of atomic cores. The statistical character of the process has great consequences on measurements and on interpretation of measured data. When making radioactivity measurements repeatedly different results will be obtained. By repeating the measurement several times it can be observed that results are concentrated in the vicinity of a certain value. This is the expectable value of the measured quantity. Measured values are approaches to that. The phenomenon that measured values fluctuate around the expectable value is called statistical fluctuation. This is not an insturment error but a consequence of the nature of radioactivity. Statistical fluctuation can not be eliminated but its significance is the smaller the more transformations are registered. If the statistical fluctuation is too high individually measured results give only a rough approach to the expectable value. In this case several measurements should be carried out and by averaging their results a better approach to the expectable value of the measured quantity can be achieved. 4.3 Half-life Transformation of radioactive isotopes takes place in a way that in every time unit an identical rate of cores is transformed within a given material volume. If the transformation in the atomic core of a radioactive isotope has been performed that core will not take place in the same transformation any more. Consequently the transformations decrease the number of atomic cores to be transformed. It can occur that the new state is not stable. In this case a new type of transformation will take place but it has no influence on the previous one. Let us see for example a radioactive isotope of which twothird is decayed in one week. So thenumber of atomic cores to be transformed decreases to one-third of the initial number after one week. In another week one-third of this that is oneninth of the initial number will remain. The time period under which one half of the radioactive cores decays is called half-life of the isotope. The half-life is an important feature for radioactive isotopes. There are isotopes with a half-life of second and others with more than tenthousand years. In the following table the half-life of some radioactive isotopes used in the industrial measuring technique is given. Element Symbol Half-life Radiation Cobalt 60 Co 5,26 years gamma Strontium 90 Sr 28,1 years beta Caesium 137 Cs 30 years gamma Americium 241 Am 458 years gamma 4.4 Activity Intensity of the radiation emitted by a radioactive material is directly proportional to the number of transformations taking place in the material. The measure for the rate of core transformations is called activity. This is an important feature for radioactive materials. The activity is equal to the number of transformations in a short time period divided by the duration of this time period. The time period must be short compared to the half-life. According to the statements given in the previous section the activity of a radioactive source decreases with time. 6

7 Unit of the activity is called becquerel1 denoted by Bq. A radioactive source has an activity of one becquerel if per unit time one transformation takes place in the source in average. The becquerel unit is too low for the practice and for this reason its multiples kbq (kilobecquerel), MBq (megabecquerel) and GBq (gigabecquerel) are often used. These units denote one thousend, one million and one milliard becquerels, respectively. 4.5 Absorbtion of radiation Shielding By colliding with atoms of the surrounding material the particles of radiations gradually lose their energy and will be absorbed. Interactions taken place during collisions are manifold depending on the type and energy of the radiation and on the type of atoms participating in the process. In certain cases the interaction between radiation and material is extraordinarily intensive. In these cases radiation is absorbed quickly. This phenomenon is called shielding. For shielding of beta radiation materials with low atomic number e. g. plastics, water or glass should be applied. For shielding the gamma radiation just in the contrary, materials with high atomic number and density e. g. lead or concrete are the most suitable. Other materials also absorb beta and gamma radiation but greater quantities are needed for reaching the same effect Half-layer Absorbtion of the radiation is a statistical process. Between certain limits it is true both for beta and gamma radiation that a given material thickness absorbs always the same part of the yet not absorbed radiation. The thickness of a material shield absorbing just one half of the radiation is called half-layer. The thickness of a half layer depends on the radiation type and energy and on the absorbing material. 4.6 Measuring effects of radiation Dose Dose is the absorbed quantity of radiation energy in a given substance. This quantity is characteristic to the interaction taking place between material and radiation. The dose describes the overall radiation exposure in a certain time period. From among dose units only one will be emphasized now: The absorbed dose is a quotient of the energy absorbed in a volume unit of absorbing material and the mass of that material. The unit of absorbed dose is the gray denoted as Gy Dose rate Rate of the actual radiation exposure is given by the dose rate. This is a quotient of the dose and the exposing time. The dose can be obtained by multiplying the dose rate with the time spent in the radiation space. The unit for dose rate is Gy/h Dose equivalent The absorbed dose measures the effect of the radiation by the absorbed energy. But the interaction between radiation and materials are manifold. Physical effects of various radiations are described uniformly by the dose equivalent. This is the product of the absorbed dose and a certain quality factor depending on the character of radiation. 7

8 For living organs even the same absorbed dose equivalent can have different biological effects depending on the character of absorbing tissue and that of the radiation. For characterizing the biological effect the effective dose equivalent is used. This applies a further multiplication factor for considering radiation resistance of various tissues. Both quantities have the same unit of measure called sievert denoted by Sv. Determination of the multiplication factors relating to various radiations and organs is an extraordinarily complicated and diverging task. Miniray SM 2000 X measures the dose equivalent rate. The measured value is displayed in µsv/h (micro sievert per hour) units. The dose equivalent can be obtained by multiplying this value with the exposure time. Far from radiation sources Miniray SM 2000 X displays values fluctuating around 0,1 µsv/h. This corresponds to the natural background radiation occuring everywhere. By multiplying this with the number of hours in a year the annual dose equivalent originating from natural background can be obtained which is approximately 1000 µsv: 0,1 µsv/h * 365 * 24h = 876 µsv. 4.7 Biological effect of radiation During absorbtion of radiation the molecular structure of the absorbing material can be damaged. This is a negligable effect for inorganic materials except extreme cases. But organic materials are more sensitive from this respect and are generally damaged by the radiation of radioactive isotopes. The rate of deterioration depends on the received dose. However, it must be taken into consideration that during the evolution process living creatures accomodated to natural background radiation being constantly present. According to our knowledge effects of radiations for living organisms can be classified into two groups: casual and noncasual effects. Non-casual effects appear only when certain dose limits are exceeded. They appear short after the exposure and seriousness of symptoms is proportional to the received dose. For preventing such effects care must be taken on not exceeding the given dose limits. Casual effects ensue with certain probabilities. The received dose has no influence on the seriousness of symptoms but the frequency of occurence among exposed persons. Casual effects occure only after long periods. Knowledge on such effects is approximate and relates for high doses only. Casual effects at levels of some multiples of natural background dose are very rare and they can not be distinguished from symptoms of other origin. 4.8 Dose limits for population In Europe the annual effective dose equivalent originating from background radiation is estimated to approximately 1 msv. This value has no deleterious effect. The human organism accomodated to such radiation exposure. Radioactivity is utilised from industrial measuring technique through energy production to medical sciences. In order to avoid health damages during working with radioactive materials the International Commitee for Radiation Protection publishes recommendations for protecting requlations and dose limits for such works. The two main aspects in determining dose limits are to avoid unnecessary risk and unnecessary restrictions. Different limits relate to people working with radioisotopes professionally and other (lower) limits to the population. Every limit is determined to be lower than that of relating to noncasual effects and not increasing significantly the frequency of casual effects. The International Commitee for Radiation Protection recommends an annual individual limit of 5 msv (that is 5000 µsv) for population and 1 msv for prolonged exposure of a large group. 8

9 4.9 Protection methods Although living organisms accomodated to harmful effects of radiation by having developed certain protection mechanisms the avoiding unnecessary exposure is a principal rule. Radiation exposure can be decreased basically in three ways: by decreasing exposure time, by increasing distance from the radiation source, and by applying absorbing layers. These possibilties are generally applied in combination Reducing exposure time For reducing radiation exposure avoiding places exposed to radiation is the most obvious method. If work should be done in a radiation space the residence time must be decreased to the possible shortest duration by good organization of work and by exercising working processes far from the radiation space. Let us assume that by installing a radiation source in an industrial plant the dose rate has been increased to the double background level in the source vicinity. If somebody would stay his full working time close to this radiation source his radiation exposure would be by approximately thirty percent of natural background higher which is still within the above mentioned dose limit. But if somebody stays only a quarter of hour in this space his radiation exposure will increase by one percent only which value can be practically neglected Keeping distance By moving away from the radiation source the radiation intensity decreases rapidly. This is a very simple and effective protection method against unnesessary radiation exposure. As a basic rule one can assume that doubling of distance will decrease the exposure to a quarter. In this way one can stay in safe without any protection keeping a certain distance. Howerer a radiation source even with a relatively low activity can cause serions radiation exposure in its close surrounding and for this reason keeping radiation sources in hand or putting them into the pocket is prohibited. For moving radiation sources long pinceers, tongs and manipulators are used in the practice depending on the type and activity of the source Shielding Radiation exposure can be effectively decreased by using absorbing layers suitable for the nature of radiation. Dimensions of shielding strongly depend on the type of radiation to be shielded and the shielding material used. Effect of the applied shielding can be easily tested by measuring the dose rate behind the shielding by Miniray SM 2000 X. By knowing the neccesary duration of exposure the suitability of shielding can be checked. Let us assume that a mechanic must stay for maximal 20 minutes in a dose equivalent rate of 3 µsv/h. This results in a maximal dose equivalent of 20/60 h * 3 µsv/h = 1 µsv which is one thousandth of the limit given by the most rigorous regulation referring to population. That means the actual shielding is satisfactory and the mechanic can carry out his work without fear. 9