The ionization chamber had a stainless steel shell 5 inches in diameter, with walls only 1/100-inch thick. The chamber was filled with argon gas into which was projected a quartz fibre next to a quartz rod.
A charged particle entering the chamber would leave a wake of ions in the argon gas. Negative ions accumulated on the rod, reducing the potential between the rod and the spherical shell, eventually causing the quartz fibre to touch the rod. This action discharged the rod, producing an electrical pulse which was amplified and transmitted to the Earth. The rod was then recharged and the fibre returned to its original position.
In order to penetrate the walls of the chamber, protons required an energy of 10 million electron volts (Mev), electrons needed 0.5 Mev, and alpha particles 40 Mev.
The particle flux detector incorporated three Geiger-Mueller tubes, two of which formed a companion experiment to the ionization chamber; each generated a current pulse whenever a charged particle was detected. One tube was shielded by an 8/1,000-inch-thick stainless steel sleeve, the other by a 24/1,000-inch-thick electron-stopping beryllium shield. Thus, the proportion of particles could be determined.
The third Geiger-Mueller tube was an end-window Anton-type sensor with a mica window that admitted protons with energies greater than 0.5 Mev and electrons, 40,000 electron volts. A magnesium shield around the rest of the tube enabled the instrument to determine the direction of particles penetrating only the window.
The three Geiger-Mueller tubes protruded from the box on the superstructure of the spacecraft. The end-window tube was inclined 20 degrees from the others and 70 degrees from the spacecraft-Sun line since it had to be shielded from direct solar exposure.
THE MAGNETOMETER
Mariner carried a magnetometer to measure the magnetic field in interplanetary space and in the vicinity of Venus. Lower sensitivity limit of the instrument was about 5 gamma. A gamma is a unit of magnetic measurement and is equal to 10⁻⁵ or 1/100,000 oersted, or 1/30,000 of the Earth’s magnetic field at the equator. The nails in one of your shoes would probably produce a field of about 1 gamma at a distance of approximately 4 feet.
Housed in a 6- × 3-inch metal cylinder, the instrument consisted of three magnetic core sensors, each aligned on a different axis to read the three magnetic field components and having primary and secondary windings. The presence of a magnetic field altered the current in the secondary winding in proportion to the strength of the field encountered.
The magnetometer was attached near the top of the superstructure, just below the omni-antenna, in order to remove it as far as possible from any spacecraft components having magnetic fields of their own.