Jonen

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Negative and positive ions

 

Almost all natural + ions come from radioactivity.  About 40% of natural air ions come from radioactive minerals in the ground. Each time a radioactive atom decays near the air, it produces 50,000 to 500,000 air ion pairs.  Another 40% comes from radon in the air (which produces about 250,000 ion pairs for each radon atom), and 20% comes from cosmic rays (high-energy protons from distant supernovas).  Negative ions come from radioactivity and evaporating water.

duration & distribution patterns

Indoors, ions “live” on average about 30 seconds before touching a surface and shorting to ground.  Outdoor, ions usually “live” several minutes.  Lightning, thunderstorms, and forest fires contribute both + and - ions. Since these ions are not produced during fair weather, it is usually only radioactivity and evaporating water that produce ions outdoors. Normal fair-weather concentrations are 200-800 negative and 250-1,500 positive ions per cubic centimeter.  Indoor levels are usually lower.  Hours before a storm, + ion concentration will increase dramatically, often exceeding 5,000 ions/cm³. During a storm, - ions increase to several thousand while + ions decrease, to below 500. 

Because a large concentration of + ions can attract - ions, high concentrations of + and - ions are often found together. Typically, a high concentration (1,000 or more) of both may be found in one area outdoors while low concentration (300 or less) is found a city block away. A cloud of pure + ions (with no -), with a concentration of 1,000 ions/cm³  would be very unstable and would fall apart if its diameter were more than about 30m (100'). For this reason, high concentrations of exclusively + (or exclusively -) ions tend to be compact, and don't extend more than about 30 m. Indoors, one may find high - in a room area and high + in another, with variations depending on time of day.

collisions

The lifetime of “fast” ions (the most common type) is determined by how long they last before colliding with a solid (or dust) that usually neutralizes their charge. Electric fields indoors are stronger than outside. Plastic surfaces charge to a typical potential of negative 1,000 volts, producing electric fields of 500-5,000 volts per meter near the plastic surface. The electric field repels negative ions (air molecules with an extra O- or OH-). The mobility of fast - ions is about 1/12,000 m/s per v/m, so at 2,000 v/m, - ions are repelled at a speed of 2,000 X 1/12,000 = 0.24m/s (meters per second).  Positive ions (air molecules with an extra H+ or positive ammonia molecule) are attracted to the plastic by the same field. Their mobility is slightly lower (about 1/10,000 m/s per v/m) and they have a slightly slower speed of 0.2 m/s.

indoor ion events

When the + ions touch plastic, they give up their + charge. This partially neutralizes the - charge on plastic. Under typical conditions, complete neutralization of the - charge on the plastic would occur in a few weeks.  However, dust “blowing by” will rub against the plastic and acquire a + charge.  This dust carries the + charge away (ultimately to earth ground).  As a result, the plastic always retains a negative charge. In typical electric fields near synthetic fabric, negative ions are repelled at a speed of around 3 to 30 cm/sec (about 1-10 inches/sec). Positive ions are similarly attracted to the fabric.

A good way to standardize (and lengthen) the lifetime of indoor ions is to put them in a large cardboard box. Lifetime then is about 50 sec, regardless of humidity, so if, for example, 4 pCi/L of radon is in the box, it will produce a continuous 1,600 + ions/cm³ in the box.      

You can produce negative ions directly by combing your hair with a plastic comb. If you then blow air past the comb, the air will have between 1,000 and 10,000 - ions/cm³ immediately next to the comb. The number is lower in high humidity. 

ions & breath

Breath contains 20,000 to 50,000 - ions/cm³ (from the evaporating water). One must be grounded to exhale a concentration this high.  When insulated from ground, one becomes more and more positively charged with each exhalation (by about 5 volts) since the breath is removing negative charge: eventually, the person will become sufficiently positive (after exhaling about 20 times), that the negative ions will immediately return. 

ions & cooling systems

This same effect occurs in building cooling systems that use an evaporating water tower.  When not properly grounded, the water pump and vents will become very positive. If the inside vents are isolated from the evaporating water via a heat exchanger, the vents may become very positive and produce a large number of + ions. This can be corrected simply by grounding the vent.

ions & combustion

Both + and - ions come from combustion (flame, wood burning, cigarette smoke, and car exhaust) and from very hot surfaces (hot enough to glow).

ions & radon 

Indoors, near ground level (basement), most + ions come from radon, and a reading of 1,000 + ions/cm³ means about 4 pCi/L of radon: the maximum allowable amount in the U.S. (This number of ions is directly proportional to radon concentration multiplied by average ion lifetime: strong electric fields indoors will reduce the ion lifetime.)  Because it is unlikely that a level so high (1,000) can come from anything else (other than flame, smoke, or a hot electric heating element), it is likely that 1,000 ions/cm³ in a basement means about 4 pCi/L of radon are present (or 2,000 ions/cm³ = 8 pCi/L, etc.).  Note that if radon is the source of the ions, then the concentration of ions will be approximately equal throughout the basement. If it is instead 1,000 near a hot water heater but only 100 ions/cm³ elsewhere, it is not radon. A higher concentration of + ions near cracks in the concrete foundation or near corners indicate the radon is coming in there. If the average + ion count is low (for example, less than 100), then there is essentially no radon present. It is not possible to “hide” the ions that radon produces. “No ions” means “no radon”.

other sources of ions: heat, voltage

Ions are produced by high-energy events, such as an open flame or a very hot object (hot enough to glow).  Hot objects usually emit equal numbers of + and - ions.  In addition, high DC voltage (over 1000 Volts), especially when connected to pointed metal edges or needles, will produce ions of the same polarity as the voltage source. This is the basis of home ionizers.

evaporating water

Evaporating water will produce - ions in the air and as a consequence leave + charges behind in the water that hasn't yet evaporated. If the excess + charges left behind are not conducted back to ground, the water will become + enough that - ion production will cease. For example, a fountain that has a motor that plugs into the wall will continuously produce - ions (until the water runs out) but a battery operated fountain will stop producing - ions after a few minutes if the fountain is well insulated from ground. The same is true of a battery-powered air ionizer.  In general, for every 3x10¹³ water molecules that evaporate, 1 water molecule carries an excess - charge.

alpha particle sources & neutrons

An ion counter near any alpha particle source (Uranium, Thorium, etc.) will produce very high ion readings, especially +. 

Some ion counters can therefore directly be used in place of a Geiger Counter. Hold the Ion Counter, in the positive ion detection mode as close to the test source of possible radioactivity.  If 0.1 microCurie of 5 to 8 MeV alpha (Uranium, Thorium, Radium) is entering the probe, the display will read 250,000 ions/cm³. These alpha particles can only travel through about 5 cm (2”) of air, so hold the top of the counter very near the suspected source. The display is proportional to the radioactivity present. Neutrons can also be detected by putting a thin layer of plastic (a hydrogen source) over the rectangular slot. This will convert hi-energy neutrons to protons, which can be detected because protons create ion pairs.  Sensitivity is a few orders of magnitude less than sensitivity to alpha particles (as described above).

William Lee