2.18 Air Sampling Equipment RCT Study Guide Identify the factors that affect the operator's selection of an air sampling system.

Transcription

1 Learning Objectives: Identify the factors that affect the operator's selection of an air sampling system Define the terms "fixed-head air sampling" and "continuous air monitoring" (CAM) Identify the physical and operating characteristics and the limitations of the most common alpha air monitors (CAMs) used at LANL Identify the physical and operating characteristics, and the limitations of the two most common beta/gamma air monitors used at LANL Identify the physical and operating characteristics, and the limitations of the three most common tritium air monitors used at LANL Identify the physical and operating characteristics, and the limitations of the most common motor air pumps used at LANL List the steps for a preoperational checkout of a portable air sampler. Procedures and standards ESH Surveying for tritium contamination ESH Air Monitoring Standard ESH Air monitoring procedure ESH Responding to CAM alarms ESH Operating the Canberra ASM 1000 alpha CAM ESH Operating the Eberline AMS-4 beta CAM ESH Operating the Eberline AMS-3 beta CAM ESH Operating the Eberline Alpha-6 alpha CAM ESH Operating the Johnston Triton 110/111 tritium sniffer -1- July 3, 1995

2 Introduction As Radiological Control Technicians, one of your major concerns is preventing radiation exposure due to internal deposition of radioactive material in the body. The primary pathway for such deposits is through inhalation of microscopic particles of radionuclides suspended in the air we breath. If we breath air contaminated with radioactive material, some of the material will be trapped in the lung, and from there will be distributed to other organs within the body. To limit such exposures it is very important to identify how and where radioactive materials become airborne, and how to effectively sample the air to measure their concentrations. Alpha emitters in a solid form with high specific activities inherently become airborne even when no operations are being performed on them. This results from the high rate of energy release in the form of the relatively massive alpha particles causing microscopic pieces of the material to break away and become airborne. The amount becoming airborne depends upon the total amount of material present, the specific activity, and also, to some degree, upon the chemical composition of the material. This characteristic is responsible for the rapid spread of airborne contamination when materials such as plutonium or americium are not adequately contained. Beta and gamma emitters do not normally become airborne on their own accord because the energy released per unit mass is insufficient to break away pieces of the material. In this lesson you will gain a general overview of the operations of the most commonly used air sampling monitors used at LANL. -2- July 3, 1995

3 Selection of air samplers and monitors The following factors affect the RCT's selection of an air sampling or monitoring system. a) Type of radiation emitted by airborne contaminant in question Alpha: use an alpha CAM such as the Alpha-5, Alpha Sentry, etc. Beta: use Eberline Air Monitoring System, AMS-3 or 4. Tritium (and other low energy beta): use a tritium monitor. b) Physical state of the airborne contaminant solids: use filters HTO: use silica gel I-131: use activated charcoal c) Type and duration of job being performed Fixed-head sampling, and continuous air monitoring Air sampling at the Los Alamos National Laboratory can generally be divided into two broad categories: fixed-head air sampling and continuous air monitoring (CAMs). Fixed-head air sampling draws air through a filter that is changed and analyzed periodically to determine average concentrations of airborne radionuclides in a room, duct, or exhaust stack. In this case, there is no radiation detector to continuously monitor the filter, so the results are only obtained after the filter has been sent away to be analyzed. This is also referred to as "retrospective sampling", in contrast to "real time monitoring". Continuous Air Monitors (CAMs) include a radiation detector to continuously monitor the filter and analyze airborne contaminant concentrations. CAMs signal an immediate alarm if the concentration exceeds pre-set values, so they can immediately warn personnel in a work area of the presence of airborne radionuclides, thereby initiating evacuation to prevent the intake of significant quantities of hazardous material Alpha Continuous Air Monitors CAMs (Continuous Air Monitors) continuously monitor ambient air for the presence of airborne radioactive particulate contamination and can signal an evacuation alarm to personnel when a significant airborne release occurs. Several variations of alpha CAMs manufactured by Eberline are in use at LANL: the ALPHA-2, 3, 4, 5, the TA-55 CAM, and the ALPHA-6. The ALPHA-2, -3, -4, - 5 and the TA-55 CAM operate similarly with minor variations in the electronics -3- July 3, 1995

4 and accessories, whereas the ALPHA-6 is microprocessor based and uses a keypad for operator control. The TA-55 CAM is used only at the Plutonium Facility, Technical Area 55, while the others are in use throughout the Laboratory. The latest CAM is the Alpha Sentry Continuous Air Monitor (ASM 1000 Controller) designed by Canberra, which reduces many of the problems associated with Alpha CAMs such as false alarms due to radon and its daughters. The most important problem with all alpha CAMs is the detection of small amounts of contamination from alpha emitters such as plutonium or americium, in the presence of a large background from natural radon and its daughters. These are distinguished by pulse height analysis. Pu-239 emits a 5.15 MeV alpha, Am-241 emits a 5.47 MeV alpha, while the radon daughters emit 5.99 MeV, 7.68 MeV and 8.78 MeV alphas. In principle these may be distinguished by a high-resolution detection system. The standard LANL settings for detecting the 5.15 MeV alphas from Pu-239 are: threshold=4.65mev, window=1.00 MeV. To detect uranium also, the settings are: threshold=4.15mev, window=1.50mev. Silicon semiconductor detectors are used to obtain good energy resolution. But the resolution is limited by the energy loss of the alpha particles emerging from the filter. Alphas emitted by radionuclides embedded deeper in the filter have to pass through more of the filter material than those near the surface, and so lose more energy. This energy loss produces a long tail on the low energy side of the peak. The low energy tail from the 5.99 MeV peak produces background in the region where the plutonium and americium would be detected. All of the alpha CAMs have some method of subtracting this radon background. -4- July 3, 1995

5 Alpha Air Monitor: Alpha-2 (3, 4, 5), and the TA-55 CAM The model ALPHA-2, -3, -4, -5, and TA-55 CAMs provide a silicon solid state (semiconductor) detector, and single-channel pulse-height analyzer for the detection of alpha particles of a specific energy and the rejection of other energies and types of particles. The rejection of radon daughters greatly increases the sensitivity to other alpha emitters such as plutonium. The advantage of the semiconductor detector is that it has good resolution to alpha particles while showing little sensitivity to gamma and beta. The count rate is read on the front panel recorder which has three switch selected ranges of 50, 500, and 5000 counts per minute full scale. The response time is controlled by a switch providing fast response for calibration, or slow response for minimum meter fluctuation. The background subtraction circuit employs a separate adjustable window so that the radium-a (5.99 MeV) peak may be used to determine the amount of subtraction needed. A 1-inch diameter filter head is standard on the ALPHA-3 and TA-55 CAM, however, a 47-mm diameter head is available when specified. The larger head allows more air flow, but has a lower counting efficiency. Specifications Detector: The Eberline alpha CAMs use a silicon (Si) semiconductor detector to provide good alpha resolution. (These are diffused-junction detectors with 490 mm 2 area. Diffused-junction detectors are similar to, but more rugged than, surface-barrier detectors. Both provide a thin active region near the surface, suitable for alphas. When alpha particles bombard the detector, they enter the depletion layer of the back-diode junction. While these particles are in the depletion region, they give up energy, causing ion pairs to be created. One ion pair is created for each 3.23 ev of energy given up by the alpha particle. As soon as the ion pairs are created, they are swept out of the depletion region and the total resulting charge is applied to the preamplifier input.) Filter: One inch in diameter. A millipore SM 5µm or equivalent is recommended. The resolution is highly dependent on the type of filter paper used to collect the air sample, because if the alpha emitters become embedded deep in the filter, the energy loss of the alphas emerging from the filter produces a low energy tail to the left of the peak. Counting Efficiency: Approximately 25% of 4 for 239 Pu. Background: The natural background level will vary with the locale. Around the Santa Fe area it rarely exceeds 400 counts per minute with a wide pulse-height-analysis window, at an air flow rate of 30 liters per minute (LPM). This is approximately 1 cfm This results in approximately -5- July 3, 1995

6 8 cpm in the window when the instrument is calibrated for 239 Pu. When the optional 47 mm sample holder is used, similar performance is obtained with an air flow rate of 60 LPM (2 cfm). Alarm: The alarm point is adjustable from 0 to 100% of full scale. The setting is read directly on the recorder by interrupting the cpm reading. A red alarm light and a squealer are provided. Response Time: Fast response is approximately 30 seconds while the slow response is approximately 180 seconds. Alpha Air Monitor- Eberline ALPHA-6 General Description The ALPHA-6 is a continuous alpha air monitor that uses a 256 channel analyzer, and a set of parameters and equations which accurately measure activity by subtracting out counts due to other radionuclides. In addition, the ALPHA-6 archives historical data, checks for alarms, and responds to user commands. The ALPHA-6 is comprised of a detector assembly, a 256 channel analyzer, two 8-bit microprocessors, a dot matrix liquid crystal display, a keypad, air flow meter, two serial ports, a real time clock, lithium battery, rotating beacon, bell, beeper, and external alarm and failure contacts. Specifications Detector: Silicon semiconductor (diffused-junction type), area = 490 mm 2, diameter = 25 mm Counting Efficiency: Approximately 25% of 4 (gross count) from a plated one inch diameter 239 Pu source in the filter holder. The efficiency is about half that amount when an unmasked full diameter source is used in the large filter holder. Computers: the ALPHA-6 contains two 8-bit 80C31 microprocessors. P1 functions as the main processor and is primarily responsible for acquiring the spectrum from P2, using the data to calculate values, archiving data at regular intervals, interfacing with an operator and checking alarms. The P2 is referred to as the detector processor. It receives incoming counts from the detector and sorts them according to pulse height to form a 256 channel energy spectrum. The detector processor (P2) can store up to 16,000,000 counts per channel in its internal memory. The maximum display counts per channel should be limited to 4,000,000 counts. Losses Approx, Dead Time (16 µsec) -6- July 3, 1995

7 416,000 cpm 10% 936,000 cpm 20% 1,605,000 cpm 30% Air Flow: The recommended flow rate is one cfm through the one inch diameter center of the filter paper. A mass air flow measurement system continuously measures flow in the range 0.5 to 2.0 cfm with an accuracy within ± 10%, precision within ± 2%. Background. The default ALPHA-6 configuration is set up to measure the level of airborne 239 Pu activity. The program uses an algorithm to accurately measure 239 Pu in the presence of radon and thoron daughters. Time constant. The CAM checks for both gradual (chronic) and sudden (acute) increases in activity. Typical time constants are less than a minute for acute increases, and about 30 minutes for gradual increases. Alarm. There are six alarm setpoints which are defaulted to check for: (1) flow failure, (2) excessive fluctuations in the background, (3) a sudden increase in plutonium activity, (4) a gradual plutonium increase, (5) air flow out of limits, and (6) airborne Pu concentration above limit (e.g pci/l). Instrument Operation The ALPHA-6 presents the operator with two types of graphic displays. The first type is a spectrum display which presents the entire spectrum, or portions of the spectrum called regions of interest (ROIs). The second type simulates a strip chart and presents a graph of an alarm variable and its reference value versus time. The operation of the ALPHA-6 is entirely dependent on a set of operating parameters which if changed would immediately affect the monitor's ability to measure alpha activity in air and to alarm. Consequently, the ALPHA-6 protects itself by requiring the operator to enter a code before allowing access to certain menus and functions. Canberra Alpha Sentry Continuous Air Monitor General Description The Alpha Sentry CAM addresses the problem of false alarms due to radon daughters with a two-fold approach: the physical removal through a patented radon reduction screen, and mathematical subtraction of the remainder. -7- July 3, 1995

8 Reduction of the radon daughters not only lowers the false alarm rate, it also increases the sensitivity. The ALPHA SENTRY can have up to eight sampling heads operated from a single ASM1000 controller, so an entire room can be monitored for a potential release from a point safely outside the room. A silicon semiconductor detector is mounted close to the filter to detect alphas. (The detector is called a Passivated Implanted Planar Silicon or PIPS detector; it is similar in principle to the silicon diodes used in the other CAMs.) Good energy resolution is essential, so the filter must be mounted correctly, otherwise energy loss in the filter and the air will broaden the peaks. -8- July 3, 1995

9 Each sampling head continuously collects data into its multichannel analyzer. These data are continuously monitored for the presence of an acute release. Periodically the ASM1000 reads out the spectrum from each sampling head, corrects for background, and determines if a chronic release has occurred. Canberra recommends that this "cycle count time" be at least minutes for reasonable counting statistics and sensitivity. Specifations Detector. Silicon semiconductor (PIPS) Background. Background from radon daughters is measured by fitting mathematical functions to the peaks in the multi-channel-analyzer spectrum, and subtraction by the micro-processor. Air flow. 1 or 2 cfm. Time constant. Typically 30 seconds for the fast alarm, and 30 minutes for the slow alarm. Alarm. Typical alarm setting is 40 counts above background, inside the window (3 to 5.4 MeV) and within the specified count time (30 seconds or 30 minutes). The Radon Reduction Screen The manufacturer states that the patented radon reduction screen in the sampling head removes over 95% of newly formed radon daughters, but not "aged" radon that is attached to dust particles. Relatively large plutonium particles, and radon that is attached to dust, are not easily deflected from the air stream and so they pass through the screen. In contrast, isolated radon atoms adhere to the screen by Van der Waals forces, the electronic attraction between molecules. The manufacturer further states that this does not work in a "dusty" environment. In a typical work environment, radon daughters attach to dust particles within a minute, so this radon reduction screen may be of limited use Beta/Gamma Air Monitoring Systems The two most commonly used beta/gamma air monitors at LANL are the AMS-3 and the AMS-4. The differences between the two are as follows. The AMS-3 includes lead shielding, weighs 160 lbs, and uses two GM detectors The AMS-4 uses two proportional detectors, and includes micro-processors. The beta/gamma air monitors continuously monitor the quality of particulate beta/gamma airborne activity in selected areas. As with the alpha CAMs, the major problem is background, both from radon and from cosmic rays. -9- July 3, 1995

10 Both the AMS-3 and -4 have two detectors, one to measure the activity on the filter, the other to measure the ambient background. In addition, the AMS-3 includes 160 lb of lead shielding to reduce the background. The AMS-4 uses proportional counters to monitor the alpha and beta background separately. General Characteristics There are two detectors, one monitors the filter, the other measures background. An air flow meter is included There are alarm lights for high activity, and low air flow. Limitations of the AMS-3 and AMS-4 Low air flow - CAMs must be placed near or downwind of the suspected source Poor response to low energy beta -10- July 3, 1995

11 Eberline Model AMS-3 Beta Air Monitor General Description The AMS-3 (Air Monitor System 3) is designed for the detection and measurement of beta emitting particulate matter. It consists of a lead-shielded filter paper and detector, and has a four decade count rate meter and recorder. An alarm indication is given by a red rotating beacon and bell. Relay contacts are provided for remote alarm indication. A second detector provides for subtraction of the gamma background. Specifications Detectors: pancake Geiger-Mueller (GM) 1 3/4 inches in diameter with mg/cm 2 mica windows. The GM chambers are identical to those used in the HP210 and HP260 hand probes. Shield: Equivalent to 2 inches of lead. Filter paper: 47 mm diameter Hollingsworth and Vose Type LB5211 is supplied, however, other types are equally suitable. Range: k counts per minute (cpm) Counting Efficiency: Approximate efficiencies for the 47 mm-diameter standard plated sources are 99 Tc = 12 % of 4 and 90 Sr- 90 Y = 25% of 4. Ambient Gamma Response: Approximately 200 cpm per mr/h of 60 Co. The subtraction circuit compensates for ambient gamma under normal conditions. Natural Background Response: The natural background is caused by radon and/or thoron daughters which vary considerably with the location and weather conditions. In Santa Fe, New Mexico the equilibrium background does not normally exceed 500 cpm with 60 liters per minute (LPM) airflow (approx 2 cfm). With airflow off and a clean filter in place, the background is approximately 30 cpm July 3, 1995

12 Theory of Operations The high voltage power supply develops +900 V, which is applied to the anodes of the GM tubes. When radiation reacts in the GM tubes, negative voltage pulses are generated. These pulses are coupled to the trigger circuits where they are converted to standard pulses. The pulses are then integrated to produce a current proportional to the input count rate. At this stage, the pulses from the subtraction GM tubes may be subtracted, so that the output current is proportional to the net beta activity on the filter paper. The integrated current is coupled to a logarithmic converter whose output voltage is proportional to the logarithm of the input current. This voltage drives the meter, recorder, and alarm-sensing circuits. Operation Check Turn the power switch on. The power indicator lamp should light. Either the Counting or Failure lamp should light. Check the meter indication to see that background radiation is a low number. If an appreciable reading exists, it indicates that the detector is contaminated, is in a high gamma radiation field, or an electronic failure has occurred. Place a calibration source into the detector filter holder. The meter should move upscale and stabilize. The actual reading should be the source value multiplied by the detector efficiency. Remove the source from the detector. After the proper connections are made and an operational check is performed, the unit is ready to operate. Place a clean filter paper in the detector and turn on the pump. The alarm must be set to indicate if airborne contamination exists, yet not alarm on background radiation. There are two major sources of background radiation to which the detector will respond. These are: naturally occurring radon daughter products in the air; external gamma radiation which penetrates the lead shield. Preventive Maintenance The instrument should not be exposed to rain, snow, or extreme temperatures. Always keep the instrument as dry as possible. Inspect the instrument for physical damage. Eberline Model AMS-4, Beta Particulate Monitor The Eberline AMS-4 (Air Monitor System 4) is a microprocessor based radiation detection instrument designed for the detection and measurement of airborne beta emitting particulate matter. It consists of a main processing unit and a sampling head. An integrated pump is available as an option July 3, 1995

13 The main processing unit acts as the central element which monitors all inputs and controls all outputs. Three serial ports support communications with the sampling head, printer, and host computer. In addition, the main processing unit provides a keyboard, display, and status lights. The multiple micro-computer system provides a sophisticated and flexible means of acquiring and manipulating data, and presenting operational conditions, current readings, and alarms on the alphanumeric vacuum fluorescent display and status indicator lights. The status indicators located on the front panel inform the user of its Ready and Malfunction status. The LED bar graph display indicates the current Slow Concentration value as a percentage of the Slow Concentration Alarm setpoint. The red strobe on top of the unit indicates that one or more of the five enabled alarms have been exceeded. The alarms supported by the AMS-4 are Slow Concentration, Fast Concentration, Net Beta Count Rate, DAC Hour, and Stack Release Rate. Specifications Detectors: the AMS-4 contains two sealed gas-proportional detectors, Argon-CO 2 (radial head), and Neon-CO 2 (in-line head). Gas pressure runs at 600 Torr and the window material is mica. The window density-thickness is 2 to 3 mg/cm 2. Efficiency (4): 99 Tc = 9%, 90 Sr - 90 Y = 18%. Background. The AMS-4 uses two proportional detectors, and counts alphas and betas separately. One detector monitors the filter for airborne contamination, the other measures background. The microprocessor then corrects for background. Time constant. Fast response, to an acute release of airborne contamination, is typically less than one minute. Slow response, to a chronic release, is typically 30 minutes. Alarms. Typical settings are 60,000 cpm for the fast alarm, and 600 cpm for the slow alarm. The standard AMS-4 is supplied with a radial inlet head which can be remoted. A fixed installation in-line head, required for stack and duct monitoring, is available as an option Tritium Air Monitors Tritium emits a very low energy beta having a maximum energy of 18.6 kev and an average energy of 5.7 kev. These low-energy betas will not penetrate even the thinnest window, so the methods used to detect tritium require that no window is present between the sample and the detector. For example, removable contamination is detected by a liquid scintillation system, as described in lesson 2.19 section 3. Airborne tritium is detected by allowing the air to enter an ion chamber, and detecting the ions produced by tritium decaying inside the chamber July 3, 1995

14 These methods do not distinguish between tritium gas, HT, and tritiated water, HTO. HTO is absorbed into the body more easily than HT, so the DAC for HT is about 25,000 times that for HTO. If there is doubt then the RCT must make the most conservative assumption and assume that all the tritium is in HTO. There are three types of portable tritium monitor (PTM) in use at LANL: the Femto-Tech, the Overhoff and the Triton. All use an air ionization chamber to measure the low-energy betas from HT and HTO, but these ion chambers also detect gamma background. The Triton is the only one that is gamma compensated; it uses four ion chambers, two to measure tritium and two to measure the external gamma background. All tritium monitors could also be used to measure other radioactive gases, provided the appropriate calibration factor is applied to the meter reading July 3, 1995

15 Femto-Tech Portable Tritium Monitor PTM-1812 General Description The Femto-Tech, Inc. model PTM-1812 Portable Tritium Monitor is a precision airborne beta radiation detection instrument that serves as two instruments in one. The user can select between a perforated ion-chamber shell for passive monitoring or a solid ion-chamber shell for active monitoring. Both ion chamber shells are supplied with the instrument and are easily exchangeable in the field. In the passive configuration the PTM-1812 serves as a portable, continuous, real-time area monitor. Tritium gas diffuses into the ion chamber from the surrounding air. In the active configuration, the PTM uses an internal pump to pull samples from tritium containers, glove boxes, etc. Specifications Radiation Detected: HT and HTO beta (5.7 kev average). Background from alpha, beta and gamma is also detected. Background: the PTM-1812 is not gamma compensated and is therefore sensitive to external gamma radiation. Internal alphas, betas and gammas from radioactive gases are also detected. Detector: Air Ionization chamber. Dynamic Range: 0 to 20,000 µci/m 3 in one continuous range. Sensitivity: 1µCi/m 3 Accuracy: ± 5% of reading Power: 12 VDC rechargeable batteries will operate continuously for 7 days in the passive mode with the pump off. The pump battery will operate continuously for up to 2 days. The instrument can be operated while connected to the charger July 3, 1995

16 Operation The user should first decide whether active- or passive-monitoring will be utilized, and install the appropriate shell. It is normal for the meter to show a very large value, or the most significant digit (left most) on until the electrometer circuit comes into balance and the stray static charges accumulated while the unit was off are neutralized. Depressing and holding the zero switch for about 2 seconds will speed up the process. Zeroing The Instrument With the instrument on and the meter stabilized, the instrument should be zeroed. The zero switch should be pressed and held and the zero adjust knob rotated slowly until the meter displays This adjustment should be made gradually to allow the instrument to react. Alarm Read/Adjustment When the PTM-1812 is zeroed and the instrument operation verified, the alarm adjustment should be made. The ALARM READ rocker switch should be held and the ALARM SET knob turned until the desired level in micro curies is reached. Tritium Monitoring Due to the sensitivity of the instrument, movement while attempting a measurement should be minimized because of gravitational effects (geotropism). The PTM-1812 utilizes a direct current ion chamber and a differential amplifier with a 7 second time constant (to reach 66%, 35 seconds to reach 100%). The ion chamber readings can be affected by smoke, aerosols, and ambient ions, as well as background from fission product gases, radon, or external gamma fields. In the case of ions, elimination of the ions or filtering are the best solutions to reduce the background. External gammas may be reduced by shielding. If this is not possible, the gamma background must be subtracted from the reading. In addition, memory effects may occur due to moisture and condensation, tritiated ammonia or tritiated organics July 3, 1995

17 Measurement When exposed to tritium the PTM-1812 reading will rapidly rise to the level of the tritium present. In the active mode (pump on) equilibrium will be reached in less than a minute. In the passive mode, the PTM-1812 will sense the tritium immediately (in less than 7 seconds) but equilibrium will depend upon the uniformity of the tritium concentration and the air convection currents in the vicinity of the instrument. Tritium naturally diffuses very rapidly in air, due to its low molecular weight. A stable reading may not occur because of continuing diffusion. In these circumstances, the user may need to observe both peak and average readings. A reading may have to be made that approximates the average of the values displayed. For example, if the meter displays values between 120 and 130, with neither end of the range being dominant, the average could be estimated at 125 µci/m 3. Calibration The calibration factor of the PTM-1812 or ionization chamber is expressed in units of amperes per curie per cubic meter. This is derived using first principles and has been verified in the laboratory with tritium gas standards. With the ion current known and the high megaohm feedback register known, the value of microvolts per microcurie per cubic meter has been determined. Overhoff Portable Tritium Monitor PTM-394C The Overhoff Portable Tritium Monitor PTM-394C is a light weight Tritium monitor designed to detect airborne radioactive matter. The instrument is calibrated directly in terms of activity of Tritium in the following full scale ranges: µci/m 3 0-1,000 µci/m ,000 µci/m 3 The instrument may be used to detect other radiogases, or to monitor gamma radiation provided that appropriate calibration factors are applied to the meter reading. The PTM-394C is not gamma compensated and will therefore respond to gamma radiation. A field of 1 mr/hr will produce an equivalent display reading of approximately 90 µci/m 3. Specifications Radiation measured: HT and HTO (low energy beta). Background: the PTM-394C is not gamma compensated and is therefore sensitive to external gamma radiation. Internal alphas, betas and gammas from radioactive gases are also detected July 3, 1995

18 Detector: air ionization chamber Accuracy: ± 15% of full scale over the temperature range of -28 C to +55 C Alarm: Adjustable over 120% of scale, latching, and with ON-OFF switch. Operation: Inability of the equipment to operate properly on the 100 µci/m 3 range indicates that moisture and/or dirt has entered the ion chamber. It may be possible to remove this moisture by letting the instrument run on its pump for several hours. Becton Dickinson Tritium Monitor Triton 111 The Triton Model 111 detects, monitors, and measures tritium in air. Many other radioactive gases may also be monitored and measured. A calibration scale factor must be used for each new gas, since the instrument is only calibrated for tritium. The option of open and closed-loop operation enables monitoring and analysis of controlled environments, such as glove boxes and environmental chambers. The monitor alarm contains an adjustable threshold for visual and audible alarm signals as well as a separate reset or acknowledge switch for the audible alarm indicator. The Triton 111 is gamma compensated. The unit contains four ion chambers. The sample air is drawn through two ion chambers to measure the tritium. Two additional ion chambers measure the external gamma radiation. Other types of tritium monitor are often referred to as gamma compensated, when in fact the instrument has only been re-zeroed to subtract out the gamma contribution. The gamma background is then assumed not to change over time. The Triton 111 is the only model discussed which is truly gamma compensated, and should not be confused with the Triton 110 which looks similar but is not gamma compensated. Specifications Radiation measured: HT and HTO (low energy beta) Background: the Triton 111 is gamma compensated for external gamma radiation. Internal alphas, betas and gammas from radioactive gases are detected. Detectors: gamma compensated ion chambers -18- July 3, 1995

19 Accuracy: ±10% of full scale on the linear ranges; ±20% of reading on log ranges. Response Time: On the most sensitive range, the monitor response time is less than 30 seconds. This is the overall exponential response time for cleanout, including electrical response, chamber pump-out with the internal pump, and tritium desorption from the chamber. Equivalent response time on other ranges is less than 20 seconds. Operating Principles A positive displacement pump draws the radioactive gas into an ion chamber. Ionization due to radioactivity within the chamber is detected and amplified by the electrometer. Background from external gamma fields is measured by separate ion chambers and is subtracted. To assure maximum reliable sensitivity, particulate matter and ions (atomic and molecular) must be removed from the air. A submicron filter removes these particles July 3, 1995

20 Calibration The Tritium Calibrator is adjusted to provide a known quantity of tritium activity, then attached to the monitor in the open-loop mode and flushed through the monitor. 6. Motor Air Pumps ( ) There are two common motor air pumps used at LANL: the Gast and the Sutorbilt. These units are usually used to sample air for extended periods of time at low flow rates (common flow rate at LANL is 2 cfm). The Gast motor air pumps are generally used in the portable "giraffe" configuration, with a long necked sampling tube, which can be adjusted to sample air at the same height that it is breathed. The Sutorbilt is a fixed pump used in a weather-tight enclosure in an outside location, connected to a system of fixed CAMs or stack monitors. Operational Characteristics The flow rate is maintained relatively constant by a regulator. The most common flow rate used at LANL is 2 cfm (56.6 liters/minute). The sample heads used are designed to accept 2 inch diameter media: filters for particulates, activated charcoal for iodine and vapors, or silica gel for tritiated water. Common components of the motor air pumps are the constant flow air regulator, and the flow meter. Air flow through air sampling systems must be provided by a house vacuum or external pump and can be regulated by a valved rotameter on the face of the instrument. The rotameter (air flow) must be calibrated at Los Alamos since the rotameter readings relate specifically to air flow at sea level. All rotameters must be calibrated in place with a mass flow meter, upon installation and on a periodic basis. Gast Motor Air Pump Specifications: The most common motor air pumps used at LANL are the Gast Model 0522 and the Model 0523, (which is the newest model in use) July 3, 1995

21 Sutorbilt Motor Air Pumps Specifications: The most common motor air pump used on stacks is the Model 2MVF and the Model CAAMDLA (which is the newest model). These models are commonly used outside in a weather-tight enclosure. The maximum operating limitations for most are 4165 RPM, at 10 PSI with a maximum vacuum of 12" Hg at 280 F discharge temperature Preoperational Checkout of Portable Air Samplers When performing a preoperational checkout, the RCT should: 1. Verify the air sampler has a current Calibration Sticker. 2. Check for physical damage All gaskets in place General physical condition of housing and controls. Verify the instrument is operating properly by checking for sound (no unusual noises), sight (no smoke, no excessive sparking from motor brushes), smell (no burning), and feel (no unusual vibrations, not overly hot to touch). 3. Check the battery, or power (whichever is appropriate) 4. Air Check the air flow, usually 2 cfm Ensure filters and cartridges are loaded in proper orientation to air flow prior to sampling July 3, 1995

22 Glossary Note: scientists use the word "adsorption" when the process takes place at the surface of a solid. The word "absorption" should be used for a different process, e.g. with liquids, but the precise definition below is not universal. The following definitions are taken from the EPA publication, EPA 450/ , L. Theodore,1984. Absorption: A process by which a material, usually liquid, is used to remove one or more soluble (absorbate) components, usually gas, and usually without chemical reaction. Typical absorbents are: water, dilute basic or acidic solution, and lean (low molecular weight) hydrocarbon oils. Adsorption: A process by which a solid material is used to remove one or more components from a liquid or gaseous stream, usually without chemical reaction. The removal takes place through adherence to the surface. Typical adsorbents are activated carbon, molecular sieves, silica gel, and activated alumina. Aerosol: A two-phase medium consisting of a gas (usually air) and either particulates or small droplets of liquid. Dusts: Powdered materials of earth or other matter. Usually formed by crushing, grinding, combustion, or detonation. Mist: Liquid particles that are formed by condensation of vapor. Particle: Fine liquid or solid matter such as dust, smoke, mist, fumes, or smog, found in the air or gaseous emissions. Spray: Liquid droplets created by mechanical disintegration; found in absorbers and some particulate control devices. Stack: A vertical duct or conduit that discharges exhaust gases into the atmosphere July 3, 1995