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
