Irradiating the Mail Is It Safe?
Not for electronics
By: Steve Fowler
The Postal Service has ordered 8 Irradiation
systems from Titan Scan (Surebeam) (news
release) These systems may provide up to 10 Mega ElectronVolt
(MeV) electron beams or up to 7.5 MeV X-ray beams. The electron
systems may have a power level of 14 kW. The X-ray conversion systems
will be considerably less power in the delivered beam. These units
will be installed over the next year. The Postal Service is asking
for many more units in the near future. For now, the mail is being
irradiated at a Titan Facility in Lima, Ohio and an IBA facility
in Bridgeport, New Jersey.(news
release) Both of these facilities are operating as
very high MeV electron beam irradiation units.
Is irradiation of the mail safe? That
depends on what is meant by safe. It is like any other industrial
process dealing with energy. After all, radiation is just energy.
It has the same inherent dangers as any process- no more - no less.
From the human standpoint, it is very safe. However, from the packaged
product standpoint, there are some drawbacks with which we will
have to live.
Some things to keep in mind
about this process:
Electronics Probably will
Pathogens will be killed
Biological Samples will
The terrorist mail threat
will be eliminated
Seeds will be Damaged
The Mail will not become
Plants will be Damaged
Photographic Film will
Glassware & Jewelry
May be Changed
Magnetic Media May be
Costs of Postage will
In other words the use of radiation
to sanitize the mail is justified from a safety standpoint but does
come with some adjustments to our concepts of the postal system.
If the total mail volume is irradiated or if the mail is randomly
irradiated, the products which may be damaged will no longer be
able to be shipped by standard mail.
It is our opinion that the sanitizing
of the mail needs to be from a systems approach using many methods
of pathogen elimination. Some suggestions have been ETO gas such
as now used in medical sterilization. Also Chlorine dioxide may
be used. Some have suggested Ozone. The problem is that we do not
have a good understanding of the lethal dose for Anthrax spores
for these gases. More work needs to be done in this area. Our attempts
to get such information was unsuccessful. We were told by the Center
for Disease Control in Atlanta that this was the responsibility
of the EPA. Calls to the EPA have not yielded any useful information
as of this time.
We are sure that radiation doses of
15 to 25 kiloGrays (1.5 - 2.5 MegaRads) will kill most of the spores.
A study by the Centre for Applied Microbiology and Research in Salisbury,
Wilkshire, UK suggests doses as high as 41 kGy may be required for
sterilization. The good news about this is that almost any well
designed system will eliminate the threat to a very workable level.
It will make the effort by terrorists not justified and therefore
be very successful.
The mail should be collected differently,
sorted differently and have sanitizing methods applied for the level
of distribution. There were no deaths from Anthrax in the offices
of the News personnel or Senators who received the letters. The
only deaths outside of the 1st death at AMI in Florida (Bob Stevens)
were among postal workers and possibly associates. This means if
we now know of the threat and handle the mail with some caution,
the receiving party of a letter containing a pathogen may be able
to be contained and treated. However, those who handle the letter
are at risk due to the volume of mail and the lack of knowledge
about each piece. What this should mean is that the mail must be
sanitized and contained as early in the collection process as possible.
While we need massive radiation facilities for the accumulated volume
of mail in central areas, we must not neglect the need for sanitizing
mail near the collection point. This is true whether the mail is
collected on a street, in an urban area or from a rural mailbox.
The mail needs to have some measure of control of the release of
spores or other pathogens while it is being collected and handled
early in the system.
The use of plastic bags would help
contain and segregate the mail.
The use of smaller radiation systems may be more economic to place
nearer the collection point.
The use of other methods such as gases in the shipping process may
be economically justified.
Electron Beam Radiation Systems
Electron beams use the same electrons
which we use everyday for electricity. The main difference is
that the electrons are traveling at relativistic speeds (near the
speed of light) and are free to travel in the air after exiting
the electron beam accelerator titanium window. These electrons give
up their energy by ionizing the materials within their range. The
distance (range) an electron will travel is shown in the following
depth dose curves. These curves are for water which is 1 gm/cc density.
Anything else that is irradiated by these electrons would have a
proportional range. If the material is less dense such as paper,
the electrons would travel further. If the material is more dense
the electrons would not travel as far. In fact, their range in any
material can be ratioed to the density of water. Paper has a lower
density than water. Some sheets of paper may have densities of 0.7
gm/cc. However, a book's density could fall to 0.5 gm/cc with the
interpage air gaps. Envelopes may have very low densities allowing
the electron beam to penetrate several inches of letters depending
on the energy level of the electron beam.
One of the problems with electron beams and its effects on materials
is that when the beam is stopped in an insulating material such
as a polymer, the electrons become "Static Electricity."
Electrons which are not moving are "static." This means
that very large static fields may develop in materials which may
cause undesired changes to the material. It is common for plastics
subjected to electron beam which have a range shorter than the thickness
of the plastic to be damaged by discharges of the static charges.
This damage may be seen as holes or fractures. These fractures are
called Lichtenberg trees (or frozen lightning) and can be quite
beautiful. Of course this is a problem for electronic circuits.
X-ray Radiation Systems
X-rays are produced when high energy
electrons are stopped abruptly in a dense material such as Tungsten,
lead or Gold. X-rays penetrate much greater distances than electrons.
This means they give up less of their energy in the product. Therefore
the doses needed to kill pathogens will take longer than in an electron
beam system. X-rays by their greater penetration allow more dense
products to be irradiated such as catalogues and large boxes or
bags of mail. The production of X-rays is very inefficient. At 1
MeV electrons, the production may yield only a percent or so of
useful X-rays. At 10 MeV the yield may be as high as 10%. This means
for a 100 kW electron beam system converting its output to X-rays,
the power of the X-rays will be between 1 and 10 kW. If the system
has 10 kW output the X-rays would be between 100 Watts and 1000
Gamma-ray Radiation Systems
Gamma rays are very simiar to X-rays
in their penetrating ability. They are both electromagnetic energy
as opposed to the electron which is an energetic partical. Gamma
rays come from radioactive isotopes such as Cobalt-60 and Cesium-137.
Cobalt-60 produces gamma rays which have an energy of about 1.25
MeV where Cesium-137 has rays with about 660 keV energy. The Titan
and IBA systems can produce X-rays much more energetic than these
two isotopes. Cobalt-60 and Cesium-137 are radioactive and can not
be turned off. They must always be shielded. X-ray machines as well
as electron accelerators are not a radiation source when their power
Cobalt-60 has a half-life of about
5 years. Cesium has a half-life of about 30 years. this means that
after every half-life period the original activity of the source
has been reduced by one half. This is a drawback for the economics
of isotopes as sanitizing units. Some large gamma radiation systems
have as many as 3.5 Mega Curies of Cobalt-60. In order to keep the
source at strength the facility must replace about 1/3rd of the
material each year at a cost of well over $1Million.
Also, the Postal Service must consider
that the large proliferation of gamma sources might make attactive
targets for terriorists to spread a "dirty" bomb.
The calculation of the dose need for
sanitizing the mail may be made from the following relationships:
1 Gray (1 Gy) = 1 Joule/kg -----------
25 kGy = 25E3 J/kg
kWatts X 3.6E6 = Joules/hr
a 100 kW system would present 3.6E8 J/hr.
at 25 kGy this would sanitize 14,400 kg/hr
a 1 kW system would present 3.6E6 J/hr.
at 25 kGy this would sanitize 144 kg/hr
Of course these are maximums.
There are many entrepreneurs who have
begun to advertise solutions to the mail issue. These range from
desk top irradiators to herbal potions. We recommend that if it
sounds too good to be true, it probably is. Beware.
Radiation Safety is an important issue
for the installation of these proposed systems. Most states require
that a Radiation Safety Officer be on staff for each facility. These
persons must have knowledge and experience with radiation safety.
They typically must attend radiation safety training courses which
are specific to the use of electron beams and X-rays for industrial
uses. These systems are capable of being operated with little or
no safety issues. In fact some users of electron beam systems have
now operated almost 50 years with no safety problems.
Fowler is the publisher of the Rad Journal and the ESD Journal as
well as a contributing editor for the magazines. He is also the
President of Fowler Associates, Inc.