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Fowler Associates Labs

 

 

Static Fire Stories Articles & Technical Papers Current News

Can Radioactive Sources Produce Ozone?

by Steve Fowler - Fowler Associates


In the February - March 1999 of Threshold (the ESD Association's Newsletter) my good friend Neils Jonassen answered a question by Ben Baumgartner on "had Neils ever smelled ozone from a radioactive source?"  Neils nailed the answer for small sources. This is an expansion on Neils' answer.  The main answer to the question is a qualified yes.  Yes, radioactive sources can produce ozone.  However it takes a large source to build up enough ozone to smell.


First let's review some radiation physics and units:

Neils prefers to use the SI units of Becquerel.  These are more correct today.

Antoine Becquerel

However, I am from the old USA school and have a hard time giving up Marie's unit: the Curie.

Marie Curie

A Becquerel (Bq) is one disintegration per second.  A Curie (Ci) is 3.7E10 disintegrations per second or the activity of one gram of radium. All electrical engineers should remember (you do, don't you?) that one (1) Ampere is a Coulomb per second and that a Coulomb is about 6E18 electrons. Also don't forget that J.J. Thompson discovered the electron

J.J. Thompson

and named it after the Greek word meaning "amber."

My experience is primarily with much larger radiation sources than those used in ionizers and radon generating systems.  With large sources such as those used for sterilization and medical applications the production of ozone is real.  Most of my experience is with electron sources and electron accelerators.  These produce outputs in the kilowatt or higher levels.  They produce output beams and radiation streams of several orders of magnitude above the small sources normally seen in ionizing or other minor process applications.

Neils talked about 10 MBq of activity.  This is the same as saying 10E6 disintegrations per second. Some medical sources are sometimes much greater than 3 Curies.  This means they have a radiation output of greater than 1E11 disintegrations per second.  A sterilizer unit may have 3 or more MCi.  This is greater than 1E17 disintegrations per second. Electron accelerators used for plastics processing have outputs up to 1 Ampere ( at 300kv or 100 mA at 1 Mv).  This equates to 6E18 disintegrations per second. By the way, all these are lethal sources by several orders of magnitude.

1 Million Volt Electron Beam Accelerator

The production of ozone (concentration) is related to several factors such as oxygen concentration, production efficiency of the ionizing event, radiation beam intensity, stopping power of the radiation beam, distance traveled in the medium, time of irradiation, and the volume of the space being considered. For a discussion on the production of ozone by radiation sources please review the NCRP or ICRP reports.

The following is the formula used by accelerator facilities for the design of ozone removal systems:

the fractional concentration of oxygen in air (0.232

is the G-value for O3 production by electron irradiation of oxygen (~6 molecules per 100 electron volts)

is the number of electronic charges per milliampere-second of electron beam current (6.28E15 per MA -sec)

is Avagadro's Number (6.02E23 molecules per 22.4 liters of gas at NTP)

is the collision stopping power of electrons in air at NTP (keV per cm)

is the external electron beam current (mA)

is the distance in air traversed by the external electron beam (cm)

is the irradiation period (s)

is the volume of the room containing the external electron beam (l)

As an example of ozone production, a 1 mA external electron beam of 10 MeV electrons, transversing a 300 cm distance in a room 4m X 4m X 3m, can produce an ozone concentration of about 15 ppm after a 5 minute period.

The threshold of perception for ozone differs among people but is about 0.1 ppm.

Therefore an electron beam of about 10 microAmps at 1 MeV would cause that room to have a threshold level of ozone in about 5 minutes.  10 microAmps is equivalent to 6E13 electrons per second or about the same as a beta source of 6E13 Bq. (1.6kCi). The difference of an electron accelerator and a radioactive beta source is that all the electrons exiting the accelerator are basically of one energy while a beta source gives off a spectrum of electrons.  Kr-85 gives off about a 500keV beta particle (electron).  I have not run the numbers but it would probably take a few Curies to produce perceptible ozone in a relatively small space.

Sources such as described by Neils at 10MBq. (270 microCuries) even of radium would most likely not produce enough ozone to be perceptable even in a small space. This is the same for most sources used in applications such as ionizers, smoke detectors, watch dials, lantern mantles, etc.  It just takes more hits per second than these can deliver. If they could we probably would not need to worry about salmonella if we ate out of the old Festaware dishes (Uranium paint).

As for high voltage ionizers the amount of ions produced

 

 

 

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