LEARNING OBJECTIVES: 2.17.01 List the factors which affects an RCT's selection of a portable contamination monitoring instrument. 2.17.02 Describe the following features and specifications for commonly used count rate meter probes used for beta/gamma and/or alpha surveys: a. Detector type b. Detector shielding and window c. Types of radiation detected/measured d. Energy response for measured radiation e. Specific limitations/characteristics. 2.17.03 Describe the following features and specifications for commonly used count rate instruments. a. Types of detectors available for use b. Operator-adjustable controls c. Specific limitations/characteristics. 2.17.04 Describe the following features and specifications for personnel contamination monitors. INTRODUCTION a. Detector type b. Detector shielding and housing c. Types of radiation detected/measured This lesson covers contamination monitoring instruments in relation to types used, purpose for, radiation monitored, operational requirements, and specific limitations and characteristics. 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. General Discussion Measurements using portable contamination monitoring (count rate) instruments provide the basis for assignment of practical contamination and internal exposure controls. To establish the proper controls, the contamination measurements must be an accurate representation of the actual conditions. Measurements using non-portable contamination -1- FH 05/2003
monitors, such as an Eberline PCM-1B or HFM-6, are used to identify personnel contamination prior to exiting controlled areas or facilities. Measurements using counter scalers to determine the levels of transferable contamination on specific location samples are the basis for contamination postings and material releases from controlled areas. Many factors can affect how well the measurement reflects the actual conditions, such as: $ Selection of the appropriate instrument based on type and energy of radiation, radiation intensity, and other factors $ 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 radiological and non-radiological factors that affect the instrument response, such as radioactive gases, mixed radiation fields, humidity and temperature. FACTORS AFFECTING INSTRUMENT SELECTION 2.17.01 List the factors which affects an RCT's selection of a portable contamination monitoring instrument. The selection of the proper instrument is critical to ensure the data obtained is accurate and appropriate. Instrument selection is based on the characteristics and specifications for that instrument as compared to the required measurements. Several factors should be considered when selecting the instrument. $ The type of radiation to be measured $ The energy of the radiation to be measured $ The intensity of the radiation (dose rate or activity levels) $ Interference from a mixed radiation field $ Background radiation conditions -2- FH 05/2003
$ Environmental factors, such as radioactive gases or temperature, affecting instrument response $ Procedural requirements. 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. COUNT RATE METER HAND PROBES 2.17.02 Describe the following features and specifications for commonly used count rate meter probes used for beta/gamma and/or alpha surveys: a. Detector type b. Detector shielding and window c. Types of radiation detected/measured d. Energy response for measured radiation e. Specific limitations/characteristics. EBERLINE MODEL HP-210/260 (Figure 1 and 2) Models like the Eberline Model HP-210/260 hand probe are sensitive beta detectors using a thin window "pancake" Geiger-Mueller (GM) detector. These detectors are designed for contamination surveys of personnel, table tops, floors, equipment, etc. The difference between the HP-260 and HP-210 is the inclusion of some type of shielding on the HP-210. Figure 1 - Eberline Model HP-260-3- FH 05/2003
Detector responds to alpha, beta, gamma and X-ray radiation of minimum energies. $ alpha > 3 MeV Detector must be close enough to the source of alpha particles to prevent alpha particle attenuation in the air between the source and the detector. $ beta > 40 kev This precludes the detection of low energy beta particles, such as the beta particle from the decay of tritium (E max = 18.6 kev). $ gamma > 6 kev Photon radiation, such as gamma or X-ray, can interact in the detector walls and the fill gas to create a pulse. However, the probability of interaction is small due to the shallow depth of the detector and therefore the efficiency for photon radiation is small. GM tube has mica window of 1.4 to 2.0 mg/cm 2 density Gamma sensitivity is approximately 3,600 counts per minute (cpm) per mr/hr for Cs-137. Figure 2 Eberline HP-210 Probe Available with either high-density tungsten or aluminum housings $ HP-210AL - aluminum housing with a low shielding factor for low background use $ HP-210T - tungsten shield covering the top and sides of the detector allows use in high background area -4- FH 05/2003
Detector Type The detector is sealed Geiger-Mueller (GM) "pancake" detector. A "pancake" detector has a radius or width that is much larger than the depth of the detector. The shielded hand probe contains the GM detector which has the mica window protected by a wire or stainless-steel etched screen. The fill gas in the GM tube is halogen-quenched argon. The operating voltage for the GM detector is 900V + 50V. The detector has a 50 μs resolving time which is defined as the minimum time that must elapse after the measurement of an ionizing particle before a second particle can be measured. Detector Window and Shielding The thin detector window is 1.4-2.0 mg/cm 2 mica and is protected by the screen which is 79% open. Mica windows must be used instead of Mylar, because the Mylar will react with the halogen quench gas. The window has an effective surface area of 2.4 inch 2 (15.5 cm 2 ). Efficiencies for the detector are dependent on the type and energy of the radiation. The detector is designed, calibrated and used to measure beta radiation 22% for Cs-137 16% for Co-60 32% for Sr-90 -Y-90 15% for Tc-99 6% for C-14 Typically, a conservative beta efficiency of 10% is assigned for these types of problems. Therefore, to convert the cpm reading to a dpm value, the meter reading is multiplied by ten (dpm = cpm x 10). Efficiencies for alpha and photon radiation are not typically quoted because the probes are not calibrated for either type of radiation. However, gamma efficiencies are low, about 1-2%, because of the shallow detector depth. Alpha efficiencies are highly dependent on the particle energy and distance from the source, but can be as high as 20%. Specific Limitations and Characteristics Generally, environmental conditions, such as humidity and temperature, do not affect the response of the detector because it is sealed at a pressure slightly less than atmospheric pressure. -5- FH 05/2003
Use of the hand probe at proper frisking speeds and distances is extremely important to ensure accurate results. The probe should be used at a distance of no more than 1/4 inch and at a speed of about 2 inches per second. The mica window is extremely fragile and sufficient care must be taken to prevent any punctures which will ruin the detector. The detector probe is not calibrated for alpha radiation; however, it may be used for indication of alpha emission from contamination, if used properly. -6- FH 05/2003
COUNT RATE INSTRUMENTS 2.17.03 Describe the following features and specifications for commonly used count rate instruments. a. Types of detectors available for use b. Operator-adjustable controls c. Specific limitations/characteristics. PORTABLE ALPHA METER (PAM) The portable alpha count rate meter used at the Hanford Site consists of a count rate meter and an alpha scintillation detector. The count rate meter is a Bicron Surveyor X. The rate meter scale indicates a count rate range of 0 to 1,000 cpm. The scale selector switch may be selected to X1, X10, or X100. The resulting cpm and response times are: Range X1 (0 to 1000 cpm) X10 (0 to 10,000 cpm) X100 (0 to 100,000 cpm) Response Time 12 seconds 5 seconds 2 seconds The PAM response is fairly independent of alpha energy if the alpha particle has enough energy to penetrate the window. Detector The scintillation probe consists of a zinc sulfide activated with Silver (ZnS (Ag)) decal backed with a 1/16 inch sheet of Lucite which is mounted on the face of the detector. The zinc sulfide is covered two layers of 0.41 mg/cm 2 aluminized polycarbonate film for a total window density thickness of 0.82 mg/cm 2. The polycarbonate is topped with two spacers and protected by a fine copper beryllium screen. Two detector sizes are available, the most commonly available has an active area of 54 cm 2. A 100 cm 2 probe is also available which offers improved sensitivity for distributed sources. The light pulses from the scintillator are reflected to the photomultiplier tube which is in the probe handle. Operator-adjustable controls -7- FH 05/2003
The PAM has a six-position switch arranged as follows $ OFF (turns instrument off) $ High Voltage (checks voltage supplied to detector) $ BATT (checks status of batteries) $ X1 (meter reading x 1 equals cpm) $ X10 (meter reading x 10 equals cpm) $ X100 (meter reading x 100 equals cpm) A separate switch is provided to turn the speaker on and off. Specific Limitations/Characteristics The PAM utilizes the following correction factors: 1) To obtain correct count rate (cpm) multiply the meter reading by the selector switch setting. 2) To convert cpm to dpm, multiply the cpm by the inverse of the instrument efficiency. (7 would be the inverse of an efficiency of 14%) The instrument has a radioactive source attached to the side of the case. This source is typically 0.85 nci ( 2000 dpm) Th 230. The PAM is not generally affected by temperature shocks or changes in ambient humidity or pressure. It may be sensitive to electromagnetic fields, but the PMT is shielded to reduce this affect. The PAM responds to alpha energies > 4 MeV. This instrument also responds to light such as might happen if a small hole exists in the probe face. The PAM uses four 9 volt batteries and battery life is about 400 hours. Each PAM probe is individually matched to a count rate meter. The probes can not be interchanged between instruments in the field without invalidating the calibration. -8- FH 05/2003
EBERLINE E-140 COUNT RATE METER Eberline E-140 is a beta-gamma contamination survey instrument. The normal detector is a pancake GM probe (HP-210/HP-260). This probe has a thin mica window approximately 1.4 mg/cm 2 thick and 15.5 cm 2 in area. This window is very fragile and is protected from damage by a fine mesh screen. Tungsten and lead shielded GM detectors are available for use in areas with high gamma backgrounds. Figure 3 Eberline E140N The instrument=s metal case has an external speaker mounted on the side. A phone output is provided for the use of earphones The instrument case has a 0-1000 cpm meter and a five-position switch. $ OFF $ BATT $ X1 $ X10 $ X100 The Response Control knob adjusts the response from SLOW to FAST (2-10 seconds). A RESET button allows the meter needle to return to a lower reading faster following a source check or other higher reading. -9- FH 05/2003
The pancake GM detector responds to alpha, beta, and photon (gamma and x-ray) radiations. Because of the limited range of alphas and the poor interaction of photons, the majority of the instrument=s response will be due to beta in a mixed field. The E-140 uses the following correction factors: $ To obtain the correct count rate, multiply the meter reading by the factor shown on the range selector switch. $ To convert an instrument response (cpm) to an activity (dpm), the general rule of thumb is to multiply the reading by a factor of 10. This is an acceptable (and conservative) practice for all measurements performed by the GM survey instrument. $ If the radionuclide of the source (contamination) is known, then a more accurate measurement can be made by dividing the count rate meter response by the appropriate instrument efficiency. Efficiencies for a few radionuclides are printed on the calibration label of each detector. The responses observed during an instrument=s initial source check should be evaluated to determine if the values are reasonable. For the GM, methods to determine whether the initial response is reasonable include: $ Response is within the acceptable ranges provided with a calibrated source assembly. $ Response is within 20% of the mean instrument response for that source. This instrument is not affected by temperature shocks. It is good practice to allow the instrument to equalize with the ambient temperature before performing a survey if time permits. The E-140 uses two D cell batteries and has a battery life of approximately 200 hours. Extremely high count rates may cause an apparent decrease in the counting efficiency of pancake GM detectors because of long resolving times. Measured count rates can be as much as 40% below actual count rates. Correction for this effect can be performed and generally needs to be considered above 50,000 cpm. -10- FH 05/2003
PERSONNEL CONTAMINATION MONITORS 2.17.04 Describe the following features and specifications for personnel contamination monitors. a. Detector type b. Detector shielding and housing c. Types of radiation detected/measured The PCM-1B, HFM-6/6A, and HFM-7A are installed devices designed to monitor for radioactive contamination on part or all of a person's body. The HFMs are capable of monitoring only the hands and the bottom of the feet, while the PCM monitors the entire body, one side at a time. The instruments described in this section utilize gas-flow proportional detectors. The carrier gas is P-10 (90% Argon, 10% methane). These detectors can detect both alpha and beta particles. However, PCM-1Bs may not contain the necessary circuitry to record, and alarm on, alpha particles. Likewise, HFM-6s may or may not have their alpha recording/alarming capabilities enabled. A bias (~1400 V) is applied to each detector. Alpha and beta particles interacting with the carrier gas create electrical pulses. The electrical pulses are amplified and routed through discriminators to determine if the pulse is from beta or alpha (as equipped) radiation. The signal is finally registered in the corresponding channel as a count. -11- FH 05/2003
PCM-1B Physical Description Figure 4 Eberline PCM-1B/C The PCM-1B is 228 cm (89.6 in.) high by 75 cm (29.5 in.) wide by 112 cm (44 in.) deep. Without any shielding or gas cylinders, the instrument weight is 250 kg (550 lbs). The 15 detectors in the PCM-1B are gas-flow proportional type consisting of a metal housing with a mylar covered window. The hand (palm and back of hand) detectors are 323 cm 2 with a 60% open cover, the remaining probes are 503 cm 2 with a 54% open probe cover, depending on the installed metal housing. Operational Description Due to the short range of alpha particles in air, PCM-1Bs can effectively monitor only the hands and bottom of feet for alpha contamination. -12- FH 05/2003
Unlike the HFM-6, PCM-1Bs maintain an activity (dpm) alarm. During calibration, the Master Reliably Detectable Activity (RDA) and a parameter called the RDA Confidence Factor are entered. These values establish an activity that will post an alarm with a specific confidence. The RDA is set to the appropriate HNF-5173/HNF-5183, Table 2-2, limit and the RDA Confidence Factor is set to 1.65, establishing 95% confidence. Like the HFM-6, all PCMs have three operating modes: Preset All (Mode 1), Maximum Sensitivity (Mode 2), and Minimum Count Time (Mode 3). The recommended mode of operation for PCMs within PHMC is Mode 3, with a fixed Master Reliably Detectable Activity (RDA) equal to the facility's HNF-5173/HNF-5183, Table 2-2, total contamination limit. Operating modes other than Mode 3 are used at the discretion, and direction, of facility technical staff. In Mode 3, the PCM computes its count time based on the RDA setting for each channel, a sigma factor (false alarm rate), confidence level, and detector background. Once the constants are entered, the PCM varies its count time with background. While unoccupied, PCMs automatically measure the background count rate. A high background alarm occurs if the background in any channel is high enough that the computed count time exceeds a preset maximum count time. Mode 3 is recommended as the site-wide standard because it minimizes survey times and is more flexible in environments with changing background rates, including fluctuating radon levels. The PCM RDA setting is not an absolute alarm set point (i.e., the RDA represents a probability of detecting contamination at the RDA value). The confidence level entered during the calibration establishes this detection probability. All PCMs within PHMC operate at the 95% confidence level (RDA confidence factor of 1.65). Therefore, PCMs set to alarm at 5,000 dpm 137 Cs will detect that activity level 95% of the time. This same instrument alarms on less than 5,000 dpm 137 Cs with a reduced probability of detection. As a result, it is not strictly correct to state that a given PCM has a fixed portable instrument equivalent detection level. Instead, PCM alarms are possible, with varying frequency, over a range of portable instrument count rates. This situation is evident when a PCM alarms and subsequent manual frisks fail to find contamination. In this situation, the PCM alarmed at a contamination level below its RDA (a level that portable instruments cannot detect), or the instrument has experienced a false alarm based on the ~0.0 5% probability of this occurrence. Due to the large detector area and the PCM-1Bs channel summing capabilities, these instruments can detect lower levels of distributed contamination when compared to portable instrument capabilities. HFM-7A Physical Description -13- FH 05/2003
The instrument measures 146 cm (58 in) high, 55 cm (22 in) wide, and 84 cm (33 in) deep. Without internal gas bottles, the instruments weigh 114 kg (250 lbs). Two #2 P-10 bottles add 68 kg (150 lbs). The six detectors are gas-flow proportional type consisting of a metal housing with a mylar covered window. The hand (palm and back of hand) detectors are 323 cm 2 with a 60% open cover, the foot are 503 cm 2 with a 54 to 58% open probe cover, depending on the installed metal housing. Operational Description The Eberline HFM-7A is a microprocessor-based radiation detection system providing evaluation of β-γ and α contamination on the hands and soles of the feet. This monitor operates in the minimum count time mode (Mode 3 for HFM-6 and PCM-1B units). This operational mode is discussed in the PCM-1B section above. Like all HFMs discussed in this procedure, the HFM-7A employs six gas-flow proportional detectors to monitor the hands and feet. The HFM-7A is designed to house two #2 (0.59 ft 3 ) P-10 gas bottles. An optional 100 cm 2 hand-held probe and belly probe are available to add monitoring capabilities to the basic hand and foot operation. HFM-6/6A Physical Description The instrument measures approximately 150 cm (59 in) high, 70 cm (27 in) wide, and 104 cm (42 in) deep. Without shielding installed, the instruments weigh 107 kg (235 lbs). The optional lead shielding adds 204 kg (450 lbs) if all of it is installed. The six detectors are gas-flow proportional type consisting of a metal housing with a mylar covered window. The hand (palm and back of hand) detectors are 323 cm 2 with a 60% open cover, the foot probes are 503 cm 2 with a 54% open probe cover, depending on the installed metal housing. -14- FH 05/2003
Operational Description Figure 5 Eberline HFM-6/6A For a chosen count time, HFM-6s calculate an alarm setting (R A ) for a given detector as a function of that detector's background. This value, R A, is the count rate above background at which a source of activity will trigger an alarm in 50% of the count cycles (50% confidence). Activity greater than R A will cause alarms more than 50% of the time, and lesser activity will cause alarms less than 50% of the time. As such, a R Amax value must be selected, and entered into the software, so that the probability of detecting contamination at the appropriate HNF-5173/HNF-5183, Table 2-2, contamination value (e.g., 5000 dpm/100 cm 2 ) is at least 67%. All HFM-6s have three operating modes: Preset All (Mode 1), Maximum Sensitivity (Mode 2), and Minimum Count Time (Mode 3). Due to the manner in which HFM-6s determine their alarm setting, the recommended mode of operation within PHMC is Mode 2. In this operating mode, all HFM-6s post a high background alarm if the systemcalculated R A value exceeds the R Amax value entered (typically during the calibration). Operating modes other than Mode 2 are used at the discretion, and direction, of facility technical staff. -15- FH 05/2003
While not in use, HFM-6s count and store alpha (as configured) and beta background count rates. The instrument uses background data to evaluate personnel for contamination above background. The HFM-6 can have separate alarm set points and indications for each of the following channels: left hand palm, left hand back, right hand palm, right hand back, left foot, and right foot. Instrument Specifications and Limitations Temperature The APMs are intended for use indoors. They have not been tested for operation in conditions other than normal laboratory conditions. The instruments are designed to operate in temperatures between 0C (32F) and 50C (122F). Temperature Shock These instruments are not intended to be moved between areas of different temperatures. Allow APMs to reach equilibrium with the ambient temperature (usually about 1 hour) before using. P-10 gas bottles should also be thermally acclimatized before connecting to an instrument. Humidity and Pressure APMs should not be used in condensing environments. Because the APMs are calibrated at the same altitude at which they are used, no changes in pressure due to elevation are anticipated. The changes in ambient pressure caused by changing weather are minor and will not significantly affect the APM s response. RFI/EMI Interference APMs are not generally affected by external non-ionizing radiation fields such as microwaves, portable radios, or cellular phones. Energies and Types of Radiation The APMs are designed for the detection of low levels of alpha and beta contamination. Alpha particles above ~3 MeV and beta particles above ~200 kev are measured. Specific calibration adjustments can impact energy response. These instruments are not intended to measure "hard to detect" beta emitting radionuclides such as 3 H and 14 C. The detectors are energy dependent (i.e., efficiency increases with incident radiation energy). The -16- FH 05/2003
average distance between the detector surface and the surface being surveyed is three inches. As such, the instruments have limited utility as alpha contamination monitors. A typical Hanford alpha particle (~5 MeV) travels only 1.5 in air. Interfering Ionizing Radiation Response Gamma: These instruments are sensitive to gamma radiation. Typical response for 137 Cs gamma emissions is 105,000 cpm/mr/hr and 135,000 cpm/mr/hr for 60 Co (503 cm 2 detectors). For the 323 cm 2 detectors, typical gamma response is 50,000 cpm/mr/hr ( 137 Cs) and 66,000 cpm/mr/hr ( 60 Co). As a result, instruments may require shielding or relocation to obtain a sufficiently low background count rate. Neutron: These instruments do not respond to neutron radiation. Battery Life Installed 3.6 V Lithium batteries will last for years. Battery strength should be assessed annually during the calibration and replaced based on age or voltage. Performance Testing Refer to contractor-specific procedures for performance testing. Operational Check The APMs constantly monitors functionality and background levels. If errors are encountered, the instruments will post a trouble message or indicator, and may take themselves out of service, depending upon the failure (or problem), instrument, and setup configuration. Operational checks are therefore performed by the monitors and require no effort on the part of the user or maintaining organization. Source Check APMs should be periodically inspected and source checked to ensure proper operation. Frequencies vary depending upon location, operating history, and requirements base. These inspections are commonly performed daily or weekly, as a minimum. -17- FH 05/2003
Applications APMs are used to monitor personnel for contamination on their skin and clothing SUMMARY In this lesson, we have covered contamination monitoring instruments in relation to types used, purpose for, radiation monitored, operational requirements, and specific limitations/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. REFERENCES 1. Radiation Detection and Measurement, Glenn F. Knoll 2. Basic Radiation Protection Technology, Daniel A. Gollnick 3. Operational Health Physics, Harold J. Moe 4. ANSI N323 5. HNF-5173, Project Hanford Radiological Control Manual 6. HNF-5183, Tank Farms Radiological Control Manual 7. GM: GM Portable Survey Instrument, HNF-13536, 6.1.1 (FH); RPP-5779, RCI- 80 (CHG) 8. PAM: Portable Alpha Meter (PAM), HNF-13536, 6.1.2 (FH); RPP-5779, RCI-81 (CHG) 9. APM: Eberline HFM-4/4A, HFM-6, HFM-7A, and PCM-1B, HNF-13536, 6.2.8 (FH), RPP-5779, RCI-88 (CHG) -18- FH 05/2003