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The Model RD-56 is a beta scintillation, movingfilter particulate detector assembly for use as an offline process or effluent continuous air monitor (CAM). All components of the detector are contained within a sealed enclosure to protect them from the environment. The enclosure contains the filter paper, filter transport mechanism, checksource, and scintillator-photomultiplier tube assembly. The scintillator-photomultiplier tube assembly, inlet, outlet, and filtration point are surrounded by 2.5 in. of 4¹ lead shielding. Mounted internally to the shield is a beta-emitting, solenoid actuated checksource to verify detector operation. Filter paper advances in either a continuous mode or a step mode. An adjustable vacuum relief valve is provided to prevent damage in the event of a clogged inlet. Signal processing for the RD-56 is achieved by using a Model RM-80 digital microprocessor. Various scintillator sizes are available, providing a choice of sensitivities and ranges.
Specifications
Filter: 2.75-in. (7.0-cm) —wide HV LB5211 Filter Speed: 0.5, 1.0, 6 in./hr (1.3, 2.5, 15.2 Filter Supply: 50 ft (15.4 m)/roll Shielding: 2.5-in. (5.1-cm) full 4π cast-lead Weight: 200 lb (91 kg) approximate Detector: side-window photomultiplier tube Checksource: <0.05 _Ci Cl36 (beta) SAMPLE PRESSURE: 2 lb/in.2 (0.14 kg/cm2) Sample Temperature: 35° to 120°F (2° to |
Sample Flow:4 ft3/min (1900 cm3/sec) Inlet/Outlet: 0.5-in (1.3-cm) —diam stainless Sampler Material: Type 304 stainless Pressure Drop Across Inlet/Outlet: 1.5 in. (3.8 cm) Electrical Connectors: high voltage — Power: Processing Electronics: Model RM-80 |
Theory Of Operation
In operation, particulate-laden air enters the shield through the sample inlet, passes downward through the filter paper, and then exits the shield through the sample outlet. The collection side of the filter paper is viewed by a side-window beta scintillation detector. The filter collects more than 99% of the particulate matter larger than 0.3 micron from the sample. A C136 checksource is chosen for its low activity (to minimize personal exposure), for its long half-life (to avoid having to replace it or correct for decay in future readings), and because it is almost a pure beta emitter (thereby better simulating the actual sample and yielding improved performance through a negligible gamma background contribution).
Speed Control
The RD-56 utilizes a stepping motor to advance the filter paper. Circuitry to control the stepping motor is provided in a separate enclosure. Two speeds are provided: normal and fast advance. These two speeds are chosen by the customer from seven available speeds and are set by GA Technologies Inc. during manufacture. These speeds can easily be changed later at the customerÕs facility, should speed requirements change. The stepping motor normally advances at a constant rate. Alternatively, the RD-56 can operate in the step advance mode, wherein the filter tape is fast-advancing to a section of clean filter paper and then held for a preset amount of time before again automatically advancing.
Flow Control
Another available feature is automatic flow control, which maintains the sample flow rate (e.g., 4 ft3/min) even with a varying flow of particulate material. Through use of the RM-80 digital microprocessor, the user can also have completely automatic isokinetic flow control to maintain the sample line velocity the same as that of the process stream. For this feature, the user simply provides a process flow signal to the RM-80.
Calibration
Complete calibration reports have been produced for all Sorrento Electronics detectors, including the RD- 56. Energy response curves for the standard RD-56 at three flow rates and a filter tape speed of 0.5 in./hr are shown below. These curves allow the determination of detector sensitivity for most isotopes of interest.
*Average, rather than end-point, beta energies better represent actual detector operation since beta particles from most isotopes are
non monoenergetic. Using end-point beta energies would appear to result in improved, although perhaps misleading, performance.
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| *Includes normal contributions from cosmic, natural, internal and checksource background. |
The minimum detectable activity (MDA) level absorbed by the RD-56 filter (in equilibrium) for a given isotope can be computed using the following based on Gaussian statistics and ANSI NQ3.10-1947:
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The minimul detectable concentration (MDC) level is related to the MDA level by the following equation:
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| where MDC = | minimum detectable concentration in _Ci/cm3 | _ = | filter collection efficiency (e.g., 99% = 0.99) |
| f = | sample flow rate in ft3/min |
| v = | speed of filter paper in in./hr |
| L = | length of sample collector aperture in in. (1.7 in. for RD-56) |
| 60 = | conversion factor from hr to min |
| 2.83 x 104= | conversion factor from ft3 to cm33 |
Example: MDA and MDC calculations
What are the MDA and the MDC for Cs137 in a 2.5-mR/hr Co60 gamma field using a standard RD-56 in equilibrium? Let f = 4 ft3/min, _ = 0.99, and v = 0.5 in./hr. Solve using an RM-80 digital signal processor.
| Solution: |
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Servicing
The Model RD-56 is designed for easy servicing in the field without any special tools. Direct access to all serviceable components is provided by opening the hinged lid of the detector enclosure. A used filter paper roll can be replaced in less than 5 min. Should the scintillator-photomultiplier tube assembly require servicing, it can be easily reached via a removable lead shield. The filter transport mechanism is designed to require minimal mainte nance (check and oil the drive chain every 90 days). The illustration at the right shows a cutaway view of the RD-56, detailing the filter tape path.
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