A Neutron Monitor for the New Millenium
Cosmic rays are highly energetic charged particles that bombard earth continuously. Because cosmic rays originate outside the solar system, in cataclysmic stellar events, they have to traverse a portion of the heliosphere, the region of space that Earth finds itself in. During this traverse, the intensity and energy of the cosmic rays change. The study of this modulation provides us with information on the outer reaches of the heliosphere. Additionally, some events on the sun produce particles with energy high enough to be detected by the neutron monitor. The intensity distribution on the earth's surface of these rare events also give useful information on the transport of energy from the sun to the earth.
One of the instruments used to detect and count cosmic rays is called a neutron monitor. It consists of a number of counting tubes, encased in polyethylene, lead and wax. The wax serves as a shield against terrestrially produced particles, the lead produces neutrons when it is struck by a cosmic ray, and the polyethylene moderates the neutron. When a neutron is captured by the gas in the counting tube the nuclear reaction results in a small separation of charge. A high voltage (4000 V) between the outside of the tube and the collection wire in the centre causes this charge separation initiate a cascade. This pulse of current is then converted to a voltage pulse that can be detected by standard electronics. This technology is vintage Manhattan Project, at least 50 years old.
The Space Physics Unit of Potchefstroom University for Christian Higher Eductation's School of Physics operates a chain of four neutron monitors. They are located at Tsumeb in Namibia, Potchefstroom, Hermanus, and SANAE IV in Antarctica.
The N2M2 (a neutron monitor for the new millenium) is a neutron monitor with an newly upgraded information system.
Each cosmic ray event in a counting tube produces a 5V pulse. This is the primary information in the system, and the rest of the neutron monitor system constist of recording, controlling and editing these pulses
Signals are collected in a counter box. The actual counting is done by a set of ten 82C54 16-bit counter/timers. Because each 82C54 chip has 3 counters, the box provides the recording of pulses on 30 channels. A BasicStamp computer drives a multiplexer for adressing the chips, and a records the counts via an 8-bit bus. The BasicStamp computer is interfaced to a PC through a RS232 cable. When a signal is received from the PC, the BasicStamp reads the data from the counters, and sends it to the PC.
Because the events observed in a neutron monitor is actually produced in the atmosphere, an accurate knowledge of air pressure is an essential part of the neutron monitor data. Air pressure is measured by Paroscientific's Digiquartz® Portable Pressure Standard, which can easily attain the 0.1 mmHg accuracy required. Communication between the barometer and the PC is again by RS 232 serial communication.
For the data collection PC, the GNU/Linux operating system was chosen. We were very satisfied with its performance as an internet gateway in our satellite communcations system. For ease of installation we chose the Redhat 6.0 distribution.
The data collection program was written in C, compiled with gcc, and makes exclusive use of glibc, the GNU C library. It runs as a user-mode program. At the start of every minute the PC requests the latest count from the counter box, and transfers the counts and the air pressure to a file. At the start of each new day a new file is created.
A Garmin GPS25L unit serves as a stratum-0 timeserver for the network time protocol (NTP), by which the system time is kept in synchronised with UTC.
Every transaction is logged through the system logger.
For most stations data will be transferred by FTP via ethernet. The station at Tsumeb, however, is quite remote, and therefore we investigated the use of GSM to transfer data. It was demonstrated that it is possible to transfer a day's data from the remote computer to a local one through the wireless connection.
It was also demonstrated that it is possible to give a real-time interface to the neutron monitor, using GNUPLOT to generate graphs that is included in HTML documents, which is then presented by the Apache HTTP server.
From the beginning of the program a high level of importance was attached to the inspection of the recorded data. For this a graphical interface is necessary. IDL (Interactive Data Language) is a well-established cross-platform graphical data analysis and visualization computing environment. The control program was written in IDL, and allows the user to view as many days of recorded data as is wished. The use of IDL makes it fairly easy to upgrade and customize this control program, because it is an interpreted language.
To present high-quality data, the raw recorded data must be edited to remove spurious counts, and to correct for long-term drift. At present this editing is done with a custom-written Pascal program, running in the MS-DOS environment. This is the next part of the system that will be upgraded.