Identify the following features and specifications for the ESP1

Transcription

1 LEARNING OBJECTIVES: List the factors which affect an RCT's selection of a portable contamination monitoring instrument, and identify appropriate instruments for contamination monitoring. (Also see the Contamination Monitoring Standard.) Identify the following features and specifications for the ESP1 a. Types of detectors available for use b. Operator adjustable controls c. Specific limitations/characteristics Identify the following features and specifications for the ESP1 count rate meter and HP210 and HP260 probes used at LANL for beta contamination surveys: b. Detector shielding and window c. Types of radiation detected/measured d. Energy response for measured radiation e. Specific limitations/characteristics Identify the following features and specifications for the Ludlum-139 used at LANL for alpha contamination surveys: b. Detector shielding and window c. Types of radiation detected/measured d. Specific limitations/characteristics Identify the following features and specifications for commonly used personnel contamination monitors at LANL. b. Detector shielding and window c. Types of radiation detected/measured d. Energy response for measured radiation e. Operator-adjustable controls f. Specific limitations;characteristics -1- January 5, 1999

2 Introduction Contamination Monitoring Instrumentation Lesson (2.17) This lesson covers contamination monitoring instruments. The RCT uses information from these monitoring instruments to identify and assess the hazards presented by contamination and establish protective requirements for work performed in contaminated areas. Many factors can affect how well the measurement reflects the actual conditions, such as: Selection of the appropriate instrument based on type, energy and intensity of radiation, etc. Correct operation of the instrument based on the instrument operating characteristics and limitations. Calibration of the instrument to a known radiation field similar in type, energy and intensity to the radiation field to be measured. Other factors that affect the instrument response, such as radioactive gases, mixed radiation fields, humidity and temperature. -2- January 5, 1999

3 Factors Affecting Instrument Selection Contamination vs radiation survey instruments The selection of the proper instrument is critical to ensure the data obtained are accurate and appropriate. A distinction should be made between instruments used to measure external radiation (lesson 2.16) and those used for contamination. The following table highlights some important differences. EXTERNAL RADIATION CONTAMINATION typical units mrem/hr cpm (dpm) ideal mode current (integrating dose) pulse (discriminating types) typical window 7 mg/cm 2 (dead skin) 1 mg/cm 2 (thin as possible) types of radiation gamma, beta, neutron beta, alpha, (gamma) ideal materials tissue equivalent varies A contamination instrument measures counts per minute (cpm), and counts the total number of photons, regardless of their energy, in order to deduce disintegrations per minute (dpm) from cpm. Radiation that does not penetrate a 7 mg/cm 2 window also does not penetrate the dead layer of skin, therefore it should not be reported as external radiation. In contrast, contamination that is ingested, inhaled, or injected through wounds contributes a dose even if the radiation is not penetrating, consequently contamination monitors have windows that are as thin as possible, typically about 1 mg/cm 2. Ideally, an external radiation monitor is tissue equivalent, responding in the same way as human tissue, and so reports a smaller dose from low energy gammas than from high energy gammas. -3- January 5, 1999

4 Selection of contamination monitoring instrument Several factors should be considered when selecting an instrument to monitor contamination. The type of radiation to be measured. The Ludlum-139 air proportional counter is suitable for alphas. The ESP-1 with HP210 or HP260 pancake hand probes are suitable for most betas. There are a very small number of radionuclides that emit a gamma only, e.g. Be-7; these require special care. Neutrons are not monitored during contamination surveys. The energy of the radiation to be measured. The HP210 and HP260 are calibrated with Cl- 36 (0.7 MeV beta), and are less sensitive to low energy beta. Interference from a mixed radiation field and background radiation. The Ludlum-139 proportional counter discriminates against beta and gamma. The Eberline AC-3 ZnS scintillation discriminates against beta, and detects gammas with reduced efficiency. Environmental factors, such as radioactive gases, moisture, or temperature, affect instrument response. The air vented Ludlum-139 could, in principle, respond to radon, though in practice this is small, typically 1 cpm. To ensure the proper selection and operation of instruments, the instrument operator must understand the operating characteristics and limitations of each instrument available for use Eberline Smart Portable, ESP a Types of detectors available for use The "Eberline Smart Portable" ESP is a very versatile instrument because it can be used with different probes to measure alpha, beta, gamma, x-ray, or neutron radiation. However, it can only be calibrated (by HS4) to one type of detector probe at a time, e.g.: 1) HP-210,-260 (GM pancake): beta 2) AC-3 (ZnS scintillation): alpha 3) SPA-3 (NaI scintillator): gamma 4) NRD (polyethylene rem ball) neutron -4- January 5, 1999

5 b. Operator adjustable controls The ESP-1 has two modes of operation, the Rate Meter Mode, and the Scaler Mode, When the ESP is switched on, it should come on in Rate Meter Mode, with units of counts per minute (cpm). If it comes on in some other mode, something is wrong, e.g. the batteries may be weak. Rate Meter Mode (cpm) is useful for taking direct measurements on surfaces, and swipes or smears in the field. In the Scaler Mode the instrument collects data for a selected time period. The units can be either "counts" or "events". The detector sends a pulse, or event, to the electronics package (ESP). These pulses or events are then converted into counts by the ESP, which takes into account the detector efficiency by multiplying the number of events by a calibration constant set by ESH-4. The user records the number of counts per minute (cpm) and converts to disintegrations per minute (dpm) as described in section 3e. The speaker can be turned on or off. Note, as discussed above, that each click is one event, not one count. Alarm set points can be selected for either of the operational modes. c. Specific characteristics and limitations Power for the ESP-1 is supplied by six "C" cell batteries which provide approximately 250 hours of continuous operation. A low battery condition is indicated when the first character on the display starts to blink. The ESP-1 adjusts the updating time to give a standard deviation of ±5% (see lesson ), if possible within a maximum update time of 10 sec. This is only possible for count rates >2500 cpm; below 2500 cpm the standard deviation is greater than 5%. -5- January 5, 1999

6 Model HP-260, Hand Probe The Model HP-260 hand probe is a sensitive beta detector, featuring a "pancake" GM tube with a thin mica window. The open window, which is protected by an etched stainless steel screen, permits useful beta sensitivities down to 40 kev. The detector is alpha sensitive above 3 MeV. b. Detector Window and Shielding The thin detector window is mg/cm 2 mica and is protected by the screen which is 79% open. Mica windows must be used instead of Mylar, because Mylar will react with the halogen quench gas. The HP210 probe is the same as the HP260, but is shielded on the back to reduce interference from an external gamma radiation field. c. Types of radiation Pancake GM probes such as the HP210 and HP260 are designed to measure betas (with energies greater than 40 kev). They will also respond to gammas (>6 kev) with less than 1% efficiency, and alphas greater than 3 MeV if the probe is held very close (1/4 inch) to the source of alphas. d. Energy response The detector is designed, calibrated and used to measure beta radiation. Efficiencies for the detector are dependent on the type and energy of the radiation. At LANL, the instrument is calibrated to read 100% of the two-pi surface emission from Cl-36. This means that for 0.7 MeV betas (e.g. from Cl-36) the four-pi activity (dpm) will be exactly twice the meter reading (cpm). Cl-36 emits a 0.71 MeV beta. The instrument will over respond to higher energy betas and under respond to lower energy betas. Since the four-pi efficiency is calibrated to be 50% for Cl-36, then for Sr-90 (Y-90) it is about 75%, and for C-14 it is about 10%. Gamma efficiency is about 0.5%. Low energy betas from tritium will not be detected at all. -6- January 5, 1999

7 e. Specific characteristics and limitations The operating voltage for the GM detector is 900V± 50V. The detector has a 50 µs maximum dead time which is defined as the time from initial pulse until another pulse can be produced by the detector. When using the HP-260 probe for direct surface contamination surveys you must multiply your results by a correction factor of 12. The number comes from dividing 100 cm 2 (the desired reporting area) by 15.5 cm 2 (the overall area of the probe window) and by the calibrated efficiency (50% for 36 Cl): direct reading in cpm cm2 x 15.5 cm 2 = results in dpm 100cm 2 This formula applies to contamination uniformly distributed over a surface area that is > 100 cm 2. For contamination that is not uniformly distributed or has a surface area of < 100 cm 2, use the measured value corrected to dpm, i.e. dpm = 2*cpm, when calibrated at LANL as described in section d Ludlum-139 Alpha detector The Ludlum-139 detector is an air vented proportional detector for measuring alpha radiation. The detector is "proportional" (lesson ) because the output pulse is proportional to the amount of ionization in the air. Since alphas have a large specific ionization and LET (lesson ) they deposit more energy in the detector and therefore produce larger pulses than betas and gammas. The electronic discriminator then discriminates against (rejects) the beta and gamma pulses, so the device measures only alphas. b. Detector window Alphas will not penetrate a sheet of paper, or the dead layer of skin, therefore the window of an alpha detector must be very thin. The Ludlum-139 detector has a thin window of 0.25 mil aluminized Mylar, i.e. density-thickness = 1 mg/cm 2. This is equivalent to about a quarter inch of air, and allows alphas to penetrate into the detector, if the detector is held close (1/4 inch) to the source of alphas. -7- January 5, 1999

8 c. Types of radiation The proportional detector system selects only alpha particles. Betas and gammas are discriminated against, as described in section 4a. d. characteristics and limitations The detector is vented to air, so humidity can cause spurious pulses or instability Personnel Contamination Monitor PCM-1B The Eberline Personnel Contamination Monitor, Model PCM-1B is a microprocessor-based radiation detection system. The central processing unit (CPU) includes an Intel 8085 microprocessor, memory, and input-output lines. It will perform a two-part personnel whole body survey by performing a right-side then left-side personnel body survey. The Hand and Foot Monitor HFM-7 is essentially similar to the PCM-1B. The difference is that the HFM-7 has 7 detectors (two for each hand, one for each foot, and a frisker) whereas the PCM-1B has 15 detectors. The PCM-1B has fifteen independent P-10 gas-flow proportional detectors. The proportional mode allows discrimination between alphas, betas and gammas, based on the pulse height from the detectors. P-10 gas is 10% methane and 90% argon. b. Detector window Thin windows are an important feature of all contamination monitors, allowing the detectors to respond to alphas and low energy betas. The windows are the same as in the Ludlum-139 and Eberline AC-3: 0.25 mil aluminized Mylar, density-thickness = 1 mg/cm January 5, 1999

9 c. Types of radiation Two options are available and in use at LANL. The standard PCM-1B only checks for betagamma contamination. Alphas are discriminated against, to reduce false alarms from radon daughters, which typically clings to clothing. The PCM-1B OPT-9 checks for alpha, beta and gamma radiation, with separate background subtraction for alpha counts. Alphas are stopped by an inch or two of air, so alpha contamination will only be detected if it is very close to the detector. Typical efficiency for a source 3 inches from the detector is 10% for beta, 1% for gamma. As with the HP210/260, low energy beta from C-14 is detected with a lower efficiency, and betas from tritium will not be detected. The manufacturer states that with a 10 sec count time, contamination of 5000 dpm can typically be detected, with about one false alarm for every 500 people counted. e. Operation Mode The PCM continuously monitors and stores background values for all detectors, and subtracts background. There are checks for high background, low or high count rates, and low gas pressure. The PCM stops accumulating background when its ultrasonic motion sensor detects movement of a person towards the monitor. Thus if the person is contaminated, the background count is not affected. Counting begins when infrared beams sense that the person is in position. Alarms * Display reads- "Step up - Insert Right Arm" * Place arm in arm cavity to initiate personnel monitoring * Display reads "Counting Right Side" * If no alarm levels are detected, the unit beeps * Display reads "Right Side OK, Insert Left Arm" * Place left arm in cavity to initiate monitoring * Display reads "Counting Left Side" * If no alarm levels are detected, the unit beeps * The display reads "Count Complete, You May Pass" * The display is accompanied by a chime and the LED extinguishes * If there is a premature withdrawal of arm an alarm sounds. * The display will read "Count Incomplete ** Recount" * Reinsertion of the arm will restart the count -9- January 5, 1999

10 Contamination detection: * the alarm sounds at the end of the count time * An appropriate display appears "Alarm: Zone 1-Zone 2, etc. * The display reads "Contaminated, Please Step Out". Troubleshooting The PCM-1B message display includes trouble or diagnostic lights to identify various monitor malfunctions. The basic malfunction conditions are listed below: High Background If the background count rate in any zone is above the selected limit, the alarm light, high background light, sonalert, and Channel Designation (e.g. Zone 1) will activate and a "High Background" message will activate. The area should be checked for radioactive sources and the detector checked for dirt, moisture, or radioactive contamination. High/Low Count Fail If the count rate in any zone is high or low, HS4 should be contacted. Alarm lights, trouble lights, sonalert, and channel designation messages will be activated. A low count may be the result of component failure or loss of counting gas. The detector should be checked for a leak in the mylar. A leak in the mylar can be sealed with scotch tape or replaced by HS4. Contaminated Detector A light will be activated along with a message. Operation will continue with the detector light on. The detector should be checked for contamination. Loss of Gas Pressure There are two cylinders; No. 1 will be used until it is empty and then a "Bottle No. 1 Empty" light is activated and cylinder No. 2 is put in use automatically. If both cylinders are empty, the trouble light "Bottle No. 2 empty" will activate, and the visual display will indicate "Failure ** Out of Gas". Each gas bottle should last for about 60 days January 5, 1999

11 f. Specific characteristics and limitations The standard PCM-1B does not detect alphas. PCM-1B-OPT-9 detects alphas, if the source is very close to the detector; it could alarm from radon daughters clinging to clothing. Nuclides such as C-14 (low energy beta) or Be-7 (electron capture) are detected with low efficiency. The very low energy betas from tritium are not detected. The PCM monitors and subtracts background. The count time is varied by the computer according to the background conditions. If the background rate is high (e.g. because the detectors are contaminated) the computer will increase the count time in order to achieve the required accuracy (see lesson ). Summary In this lesson, we have covered contamination monitoring instruments in relation to types used, purpose for, radiation monitored, operational requirements, and specific limitations and characteristics for use. The RCT uses this information to identify and assess the hazards presented by contamination and establish protective requirements for work performed in contaminated areas January 5, 1999