Topic 8 - Characteristic, Selections and Calibration of Survey Meters.pdf

CHARACTERISTIC, SELECTION AND CALIBRATION OF SURVEY METERS Pusat Latihan Nuklear Malaysia / Nuclear Malaysia Training Centre Contents • • • • INTRODUCTION CHARACTERISTICS OF SURVEY METER a. Radiological Characteristics b. Characteristics related to conditions of use CHOICE/SELECTION a. For sea...

CHARACTERISTIC, SELECTION AND CALIBRATION OF SURVEY METERS Pusat Latihan Nuklear Malaysia / Nuclear Malaysia Training Centre Contents • • • • INTRODUCTION CHARACTERISTICS OF SURVEY METER a. Radiological Characteristics b. Characteristics related to conditions of use CHOICE/SELECTION a. For sealed  -ray and X-ray sources b. For unsealed sources CALIBRATION a. Requirement of calibration b. Calibration certificate c. Requirement of recalibration Introduction Introduction • • No criteria or general standard for selection of survey meter. Variation in conditions of use leading to variety of measuring instruments used in the country (more than 100 different products) Specifications Example: Specifications RADIATION DETECTED: ?? DETECTOR: Built-In GM SENSITIVITY: ~ 3,300 cpm/mR/hr (137Cs gamma) ENERGY RESPONSE: Energy dependent METER DIAL: 0 - 0.15 mR/hr; 0 - 500 cpm; BAT OK MULTIPLIERS: x1, x10, x100 LINEARITY: Reading within ±10% of true value RESPONSE: Typically 5 seconds from 10% to 90% of final reading AUDIO: Built in speaker (quiet position on main switch to silence audio) POWER: 9 volt, typically 250 hours with alkaline batteries (battery condition can be checked on meter ) TEMPERATURE RANGE: -20° C to 50° C SIZE: 4.6 cm H X 8.4 cm W X 13.5 cm L WEIGHT: 0.4 kg including batteries CHARACTERSITICS OF SURVEY METER RADIOLOGICAL CHARACTERISTICS A. RADIOLOGICAL CHARACTERISTIC 1. Measuring Quantity for External Radiation 2. Repeatability 3. Exactness and Linearity 4. Measurement Range 5. Instrument Response Time 6. Energy-Response Dependency 7. Isotropy & Geotropism A. RADIOLOGICAL CHARACTERISTIC 1. Measuring Quantity for External Radiation - Ensure radiation detected: X, gamma or beta - Dose equivalent to an organ or effective dose equivalent. - Specialized equipment for measurements at different depths, e.g: 300 mg/cm2 for the lens of the eye, 1000 mg/cm2 for deep dose. A. RADIOLOGICAL CHARACTERISTIC 2. Repeatability n n S= 2 ( x − x )  i i =1 n n <5 S= 2 ( x − x )  i i =1 n −1 n>5 Good equipment: small standard deviation means reproducible. For radiation protection, error : 5-10% is acceptable. A. RADIOLOGICAL CHARACTERISTIC 3. Exactness and Linearity Exactness • • • Scale Linearity Quality characterizes the capability of giving readings equal to the true value of the measured quantity. Verified at half scale deflection using a reference source, 137cs Good equipment:  5 %; adjust if possible. • Variation of exactness throughout the measuring range. • Exactness measured at 30, 50 and 70 % of full scale (within 20%) A. RADIOLOGICAL CHARACTERISTIC 4. MEASUREMENT RANGE Range covered by a particular survey meter and depends on specific requirements. Typical performance: – Environmental Survey meter – Stationary/fixed Installed Monitor – Area Gamma Survey Meter 0.01 uSv/hr to 1 mSv/hr 1 mSv/hr to 1 Sv/hr 0.1 Sv/h r to 1 Sv/hr For normal radiation leakage, an environmental survey meter is sufficient. A. RADIOLOGICAL CHARACTERISTIC 5. INSTRUMENT RESPONSE TIME This phenomenon is associated with stabilization of reading. Longer response time, more stable. » For low dose rate, use longer response time. » For medical X-ray, use fast response time or integrated mode. » For high dose rate, use variable time constants (Fast, medium and slow). A. RADIOLOGICAL CHARACTERISTIC 6. ENERGY-RESPONSE DEPENDENCY Response or calibration factor varies with energy of radiation. Best if they are independent of the energy. For radiation protection purpose: Variation of more than 30% is unacceptable [unless it is fully calibrated and the meter reading has always to be corrected according to radiation energy employed]. C A B D A. RADIOLOGICAL CHARACTERISTIC 7. ISOTROPY & GEOTROPISM Response at all angles within +30 %, for 137Cs and within +50 % for 241Am. Response at different orientations in gravitational field relative to the reference position should be within 10%. 8. EFFECT OF MIXED RADIATION FIELDS Instrument responses to only one type of radiation at a time. CHARACTERSITIC OF SURVEY METER CHARACTERISTICS RELATED TO CONDITIONS OF USE B. CHARACTERISTICS RELATED TO CONDITIONS OF USE Suitable design to hold up well and give satisfactory service under all expected operating conditions. – Small, light weight, easily carried and of convenient design for making necessary measurements. – Use normal battery that are low cost and light weight and have a long life. – Smooth surface and a finish that does not contaminate readily. The surface should be of material that can be decontaminated easily. – Rugged design and has resistance to abnormal atmospheric conditions, dust, wind, light, immersion, electric & magnetic fields, rough treatment, acid fumes, salt spray etc. CHOICE / SELECTION CHOICE/SELECTION: For sealed  -ray and X-ray sources RATEMETERS used to determine adequacy of shielding and to map variations in the radiation level arising from changes in working habits or changes in radiation energy and radiation output. INTEGRATORS used to determine exposure rates averaged over a period of time, in the case of variable workload, in the case of machine orientation changes, involving several machines or sources. CHOICE/SELECTION: For sealed  -ray and X-ray sources X and - Rays < 100 keV X and - Rays > 100 keV » Most GM and scintillation counters are suitable as rate meters. » Ionization chambers with adequate wall thickness. RATEMETERS INTEGRATORS » Ionization chambers are preferred. » GM and scintillation counters are not advisable, unless fully calibrated. » Medical X-ray - adequately saturated ionization chambers as integrator. CHOICE/SELECTION: FOR UNSEALED SOURCES • Gases - Ionisation chamber and electrometer. – Absorb vapours and count the activity. – Filter particles and count the collected activity. • Liquids - Immerse GM or scintillation detector in liquid. • Environmental Samples – Thin-window GM, windowless proportional and scintillation counters. For highest sensitivity, large NaI:Tl, ZnS or plastic scintillation. • Surface Contamination - Thin-window GM, proportional or scintillation counters. Thin plastic sheet may be wrapped around the probe to prevent contamination. CALIBRATION OF SURVEY METER REQUIREMENT OF CALIBRATION REQUIREMENT OF CALIBRATION ✓ Ensure working properly and functions reliably in the dose rate range specified and in the whole energy ranges employed ✓ Estimate errors in the instrument reading or if possible, to improve overall accuracy of the measurement; ✓ Establish a national network of traceability in radiation measurement ✓ Establish an inventory of meters available in the country for references ✓ Comply with legal requirements as in the Atomic Energy Licensing Act, 1984 and its subsidiary legislations. CALIBRATION OF SURVEY METER CALIBRATION CERTIFICATE Calibration process in Nuklear Malaysia User Application form Mechanical & Electrical Test Repair No Yes > 20% Linearity Test Return manual, certificate, report and sticker < 20% Calibration with X-ray, -ray, -ray and -ray (as requested) Certificate, report & Sticker Calibration of Survey Meter Calibration with gamma sources: • Using Caesium-137 (662 keV) and Cobalt-60 (1250 keV). • One point on each measuring range of the instrument. Calibration Method with X-ray Sources: Measurements by simultaneous irradiation of reference instrument and dosimeter CF = Bc , std Bc ,user = B  K TP  CFs B  K TP Ion chamber M32002#013 (1000 sm3) EXAMPLE OF CALIBRATION CERTIFICATE Table 1 : Correction factor for each dose rate range. Range 0 – 5 R/h Correction factor, J 0.96 0 – 500 mR/h 1.00 0 – 50 mR/h 1.02 0 – 5 mR/h 1.22 Table 2 : Calibration factor. No. Beam quality (keV) Tube potential (kV) HVL (mmCu) Calibration factor (CF) 1. 46 60 0.23 2.19 2. 83 100 1.11 1.64 3. 121 150 2.46 1.56 4. 175 200 4.22 1.57 5. 230 250 5.50 1.61 6. 137Cs(662) 1.72 7. 60Co(1250) 1.56 CALIBRATION OF SURVEY METER REQUIREMENT OF RECALIBRATION REQUIREMENT OF RECALIBRATION ❑ Regular periodic recalibration before expiry of calibration certificate ❑ If equipment damaged or after repair ❑ To extended application of the survey meter for measurement radiation qualities beyond the scope specified by the calibration certificate ❑ If the survey meter gives suspicious reading as indicated in a check source measurements END