The Triangle Universities Nuclear Laboratory is located on the campus of Duke University and employs a model FN tandem Van de Graaff accelerator to provide beams of light ions up to ~20 MeV for Nuclear Structure Research. The laboratory is divided into five controlled areas, each of which is monitored by neutron and gamma area monitors. The monitors are used to monitor the radiation in each area from the control room and also to activate a safety interlock circuit when the radiation detected exceeds a preset level. By regulation, these area monitor readings must be recorded daily when the accelerator is in operation - this is presently done manually by the operators. In an effort to provide a better picture of the overall radiation levels, a computer-based system is under construction to record the readings from these monitors as frequently as once per minute.
Description
The tandem laboratory utilizes two types of area monitors: Nuclear Chicago type 9177 and Ludlum model 306. The Nuclear Chicago monitors are being phased out and replaced with the more modern Ludlum instruments. The Ludlum model 306 units have a serial output port on which the onboard processor transmits a status string every two seconds. This fourteen-character status string includes the six-digit display of the present radiation level (0 to 99,999 mrem/hr), status bits: Alert, Alarm, Monitor, and Over Range, and error codes. The Alert bit is set to '1' if the radiation level is above the "Alert" setting on the 306, likewise the Alarm bit is set to '1' if the radiation level is above the Alarm setting on that particular instrument. The Monitor bit indicates that the Monitor is working and is detecting background counts. The Over Range bit indicates that the display is not recording a true radiation level as the detector is in an over-range condition.
The laboratory presently has five Ludlum 306/307 systems installed and will eventually have twelve, when all of the Nuclear Chicago units are replaced. To log the output from each of the twelve monitors is an easy task for a small computer. The standard approach would be to purchase a multiple port serial interface to plug into the PC and to run a serial cable from each port to each monitor. In our case however, we decided to locate the PC away from the laboratory and therefore the wiring issue became more complex. We located the computer in an adjoining building for two reasons. Firstly, we did not want to require the programming to be done in a controlled area where access could be a problem when the programmer is an undergraduate student and not a badged staff member. Secondly, we wanted the computer located where it would not be used by others - interrupting its primary task. The solution to the wiring problem was to purchase a commercial "Smart Switch" (model 232XS5 made by B&B Electronics, Inc.). These devices are readily available, and inexpensive. This one has five ports, which can be addressed via a single serial line to the PC. These smart switches can be connected in series - up to a maximum of four, which provides up to seventeen addressable serial I/O ports for our Area Radiation Monitors.
The switches operate by decoding a port select command made up of three characters sent from the PC to the Smart switch. The first character is an attention command, the second is the Switch selection character and the third is the port select character. Each Smart Switch is configured to respond to an individual Switch Selection character (A, B, C, D). The Baud rate, number of data bits, stop bits, parity, etc are all set up in the Smart Switches by DIP switches - to match the Ludlum 306 requirements. The 306 has a maximum baud rate of 9600 - which is more than adequate for the small amount of data being transmitted.
The computer that we are using for this project is a Pentium II based, 400 MHz processor running Windows 98 and LabView 5.1.1 for Windows. The programming for this system was done using LabView to communicate with the Serial switches and to produce an interactive display of the connected monitors. The main window displays five model 306s using pictures downloaded from Ludlum's online catalog. The appearance is therefore very familiar to our personnel. The program when started initializes the Smart Switches with a pre-coded command string. The program then selects the first serial port on the first switch and listens for six seconds. As the 306 instruments transmit about every two seconds, the computer should see two strings during this 'window'. If it does not, it illuminates a red "Link Failure" lamp on the picture of the 306 Unit selected. After six seconds, the LabView updates the display window, and sends a new port select string to the serial switch. In this fashion, the VI continuously loops through all connected monitors, updating the display. At present, we have set up a ten-minute interval for logging the data to disk. The data is written to an Excel spread sheet for convenience when reprocessing. The program also writes the contents of the window to a .htm file on our web server where authorized users can have remote access to the present readings from anywhere. To conserve disk space, the program will - after a preset interval of perhaps 3 months - compress the data older than three months down into a daily total and/or average. Data files will be offloaded to Zip disks after a period of time for archiving and backup.
Continuing Developments
The program is being expanded to include fifteen monitors. Monitors not yet connected will simply show up in the log as "Link Failures" for the time being. A window is being developed which will permit authorized users to access the logs remotely and to display charts of the radiation levels from selected monitors over any specified interval. A third window displays the laboratory floor plan with the location of the area monitors and the present readings as an overlay. Red lights will indicate areas in which the readings exceed the Alarm condition.
A secondary logging system is also being incorporated into this program. We wished to record various accelerator-operating parameters simultaneously in order to provide a better record of operating conditions with the logs. For this task, we chose to use the National Instruments Field Point hardware. This DIN rail based hardware is connected via a serial line to the second RS232 port on our computer. The Field Point hardware we installed provides 8 - 16-bit Analog to Digital inputs and 32 digital inputs. The analog inputs are presently being used to record the FN tandem terminal potential, the Polarized Ion Source frame voltage, and the mini-tandem terminal potential. Other inputs may be used to record vacuum readings or an analyzing magnet field or other analog signals of interest. The digital lines can record the status of interlocks, beam stops, gate valves, etc. At present only one digital line is being used - to record the status of a switch used to mark the radiation logs whenever a source is being used to check the area monitors. This status bit is logged and will be used on playback to indicate that monitor checks were being done during this period and that the radiation readings may be due to a check source and not to accelerator induced radiation.
Conclusions
Using an inexpensive PC with minimal external hardware and National Instruments LabView data acquisition software, we have succeeded in creating a means to continuously log multiple area radiation monitors and associated data from a remote location. Simple web interfacing made possible by LabView permits remote access to the real time data as well as the logs by authorized personnel from any online computer. These records will provide our staff and the regulators with a much clearer understanding of the radiation levels throughout the laboratory.