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Features

• Patented radon reduction screen
• Patented mass flow meter for accurate air flow measurement
• High sensitivity lowers false alarm rate
• Optional interface to laboratory-wide computer for data transfer
• Precise radon stripping algorithm
• Cleanable, rugged high resolution detector
• Patented disposable filter cartridge assembly saves time and labor
• Distributed architecture reduces sampling location cost
• LCD screen displays spectral data and historical trends
• Continuous self diagnostics ensure system health
• Full compliance with ANSI N42.17B specifications

Description

The Alpha Sentry System

The Alpha Sentry Continuous Air Monitor (CAM) System resolves many of the issues facing Alpha CAM users today. A patented diffusion screen removes 95% of the unattached radon daughters from the air sample (as much as 90% of the radon daughters are unattached in typical laboratory environments).

In addition, the spectroscopic algorithm (which uses a stripping method instead of ROIs) is extremely effective at subtracting out the radon daughter interference from the transuranic region of interest.

This physical and mathematical radon reduction increases the sensitivity of the Alpha Sentry, which in turn translates into a lower false alarm rate.

The distributed architecture approach taken by the Alpha Sentry CAM system enables the user to monitor potential leak situations from a point safely outside the affected area. Multi-dropping up to eight sampling heads off a single operator interface (ASM1000), enables the location of the sampling heads in the room while the ASM1000 resides in the hallway.

Sharing the operator interface resource lowers the cost per sampling location. It also makes networking to a lab-wide computer easier - there's only one connection per eight sampling heads.

In the Alpha Sentry network, the detector and multichannel analyzer electronics in each sampling head count the activity on the filter and determine if an acute release has occurred. In addition, the ASM1000 reads the spectrum from each head at regular time intervals, applies a precise spectroscopic algorithm, and determines if a chronic release has occurred. In either case, both the sampling head and the ASM1000 signal an alarm.

Figure 2:
Sampling Head Diagram

The Sampling Head

Based on research conducted by Los Alamos National Laboratory and Texas A&M University's Aerosol Laboratory, CANBERRA's sampling head represents the most advanced technology available in Alpha CAMs today.

The sampling head contains a patented diffusion screen that removes up to 95% of the newly formed radon daughters particles from the air. This is ideal for laboratory environments with HEPA filtered air which have a low concentration of condensation nuclei and hence a low attachment rate. Such facilities typically have high air exchange rates as well, further increasing the fraction of unattached radon decay products.

Representative air sampling is ensured by a radial 360^o inlet and air channels optimized for uniform particulate deposition on the filter paper.

In-line monitoring is available with an optional specially designed manifold. The use of the manifold guarantees results consistent with the open radial inlet application. The in-line adapter (Figure 3) connects to any duct or pipe via 1 in. NPT thread.

The sample is uniformly deposited on a filter encased in a cartridge. The patented filter cartridges are designed to save labor and reduce operating costs. When changing filters, the entire cartridge is easily switched, rather than handling flimsy filter paper while wearing gloves. The filter paper within the cartridge is easily removed for further analysis if needed.

The activity on the filter is counted by a Passivated Implanted Planar Silicon (PIPS®) detector. This rugged, cleanable detector is available in two sizes: 450 mm² and 1700 mm². The larger detector offers greater sensitivity due to increased efficiency and greater possible air flow.

The counts from the detector electronics are stored in a 256 channel MCA, where they are continuously monitored for an acute release. Should such an event be detected, the sampling head annunciates an alarm and sends its alarm status to the ASM1000.

The alarm device can be either the CANBERRA-supplied optional audio/visual package (driven internally), or the standard SPDT relay contacts can be used to drive custom alarm devices. Two LEDs are present on the head that indicate the counting status - normal or alarm.

In addition to collecting the spectrum, the sampling head monitors the flow rate (with a patented mass flow meter), power supply, and detector voltage. It reports these values back to the ASM1000, where it is determined if an alarm condition exists.

The sampling head offer full compliance with ANSI N42.17B, including the rigorous EMI/RFI susceptibility tests.

The Alpha Sentry Manager

The operator interface for either a single sampling head or a network of up to eight sampling heads is the ASM1000 Alpha Sentry Manager. Its function is two-fold: to provide graphical and numerical feedback from the individual heads, and to determine if a chronic release has occurred.

Using spectroscopic techniques, the ASM1000 strips the background component due to the 6.0 MeV, 7.68 MeV and 8.78 MeV radon daughter peaks from the plutonium region. If the counts in the Pu region are found to statistically differ from a zero result, the Pu concentration is calculated, and the DAC-hour value determined. This stripping technique is superior to the common ROI techniques, which use predefined regions and empirical constants that do not change to reflect changing conditions.

The analysis can also be set up to detect uranium instead of plutonium via simple steps in the user interface of the unit.

The background compensation algorithm is applied to each spectrum as it is read in from the sampling head. If the DAC-hour value exceeds the alarm limit, the ASM1000 will sound an alarm itself, as well as at the sampling head location where the condition occurred.

In addition to the DAC-hour alarm, the ASM1000 constantly monitors the flow rate, detector voltage, filter drawer status (open/closed) and other operational parameters of the sampling heads and of the network. If a failure is detected (for example, loss of air flow), a trouble alarm is annuciated at both the ASM1000 and individual sampling head locations.

The alarm annunciation is entirely configurable during system setup. The relay contacts, visual and audible alarms available at both the sampling head and the ASM1000, can be configured to annunciate during a particular condition. For example, during a trouble alarm, the user may choose to illuminate the visual alarm at the head only, and not annunciate any other alarms. The ASM1000 contains an LCD display that provides at-a-glance information for all of the sampling heads connected to it. A pushbutton keypad completes the operator interface.

Setup software (Model S578) is available to let a standard personal computer be used for initial setup, such as entering alphanumeric IDs for each sampling head.

Software (S579) is also available to download new ASM1000 firmware versions, since the firmware is resident in reprogrammable non-volatile memory.

The operation of the system through the ASM1000 is entirely menu-driven. Individual head control, maintenance, configuration and historical trend viewing are all controlled through easy-to-follow menus.

Both filter changes and performance checks are automated sequences, with on-screen help to guide the operator step-by-step. The last filter change date for each sampling head is tracked automatically by the ASM1000. User-entered pass/fail criteria tell the operator whether the unit is still in calibration after completing a performance check.

The ASM1000 offers an entirely configurable security system. There are four possible security levels to control user access. Each menu can be assigned a security level during system setup. When logging in, only those menus assigned at the user's security level will be accessible. Menus can be assigned multiple levels.

During normal operation, the ASM1000 displays a graphical status of all the sampling heads attached to it, showing bar graphs of the flow rate and the DAC-hour value.

In the event that the detectable limit is higher than the alarm limit (due to extreme background conditions), the ASM1000 replaces the user-entered alarm limit with the detectable limit and will annunciate an alarm as soon as the plutonium or uranium is statistically different from the background.

At any time, the user can look at a detailed display of an individual spectrum with the touch of a single button. This screen shows the DAC-hour value, flow rate, concentration, alarm status, CAM ID, and last calibration date, in addition to the spectrum. The ability to view the last alarm spectrum and historical trends is also provided.

Historical data is stored when the ASM1000 obtains the spectrum from each sampling head. Counts, concentration and DAC-hour values are available as graphical trends. The depth of the history file depends on the number of sampling heads on the network and the poll time of the sampling heads. As much as 24 days of historical data can be stored with a single sampling head polled every 30 minutes.

The ASM1000 can be interfaced to a laboratory computer via the optional RS-485 or RS-232 connection. This provides uploading of pertinent data (such as historical trends) for permanent storage and central monitoring as well as the ability to have the computer supply the setup configuration.

Multi-dropped CAM Network

The multi-dropped network approach, where up to eight sampling heads are connected to a single operator interface (the ASM1000) offers several advantages over a single head CAM:

Situations can be monitored from a safe location

Potential leaks can be monitored from an ASM1000 located outside the affected area, instead of at the sampling head location. There is no need to suit up to enter a potentially contaminated area only to find it was a false alarm.

Low cost per sampling location

The cost per sampling location is greatly reduced with up to eight sampling heads sharing the processing power and operator interface of the ASM1000. This reduced cost means additional placements are possible for added protection.

Ease of Networking

With multiple sampling heads already networked to the ASM1000, there are fewer connections to make when networking all the CAMs to a laboratory-wide computer. You need only connect the single ASM1000, and all the pertinent information from its sampling heads can be uploaded for further analysis, trending and long-term storage.

Multiple Alarming Points

Although the sampling heads are connected to a single ASM1000, the workers are not left unprotected if the ASM1000 fails. The intelligent sampling heads, each containing a microprocessor-based MCA, have the ability to detect an acute release on their own, and to drive their own alarms. The ASM1000 annunciates whenever any sampling head on the network is in an alarm state.

Diagnostics

The ASM1000 contains diagnostics that continuously check the health of the network and itself. Should a problem be detected, an alarm will sound to notify users of any network difficulties.

Figure 5
Example Alpha Sentry Network

Network Configuration

There are two different communication scenarios in the Alpha Sentry system: a single head connected to an ASM1000, and multiple heads multi-dropped off an ASM1000.

A single head can be powered directly from the ASM1000, with no need for external connections. The head can be located up to 45 m (150 ft) away under direct power, or up to 1200 m (4000 ft) away via the RS-485 connection. The latter situation requires local power to each sampling head.

Each head in a multiple head configuration requires a Network Tee Box (NTB) which provides in-line RS-485 and sampling head connections. The NTB contains individual coding of each sampling head. This type of connection allows easy installation or removal of sampling heads on the network. A Network Terminator (NT) is also required.

External Communications

A standard RS-232 port is provided on the ASM1000 for output to a printer or setup by a local personal computer such as a laptop. This connection provides keyboard support to enter parameters, alarm configuration, passwords, and alphanumeric CAM identifications into the Alpha Sentry system.

Operating firmware can easily be updated by downloading from a personal computer into the flash resident in the ASM1000.

Host Communications

Host Communications Packages are available options for the ASM1000. These options include the necessary hardware, protocol and documentation to communicate to the ASM1000 via a host computer.