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~diat. Phys. Chem. 1977, Vol. 9, pp. 1-15. Pergamon Press. Printed in Great Britain.
BASIC CONCEPTS OF RADIATION PROCESSING
by
Joseph Silverman Laboratory for Radiation and Polymer Science
University of Maryland College Park, MD 20742, U.S.A.
Abstract
A brief introductory history of radiation processing is
followed by an analysis, from a practical point of view, of the
following: the properties of ionizing radiation; the funda-
mental chemical effects produced by ionizing radiation; and a
summary of radiation source technology.
Introduction
The potential practical value of ionizing radiation was
recognized soon after the discovery of the x-ray by Roentgen
and of radioactivity of Becquerel. The deposition of a few
millicalories per gram of a tumor was found to have Drofound
therapeutic effects. Cold sterilization was achieved in thick
objects with radiation doses that would raise the temperature
less than 10°C. Endothermic reactions were performed at low
temperatures. Exothermic chain reactions were triggered with
trivial doses. Long before World War II, the basic studies
of S. C. Lind (using radium and its decay products) and the
applied research of W. D. Coolidge (using an electron beam
accelerator) blazed the path leading to almost all the known
practical applications of ionizing radiation. However it was
not until the post-war period that serious industrial inter-
est developed.
The reason for the delay is the lag in the development
of practical radiation sources. Before the War, radionuclides
were available in only small quantities at a cost of $1,000
per milliwatt of gamma power, and accelerators providing ex-
ternal beams of charged particles were research tools for
physicists. The nuclear fission reactor developed in World
War II now provides relatively inexpensive gamma sources
(~ $50,000 per kilowatt of gamma power), and there are now
reliable rugged electron accelerators that can be obtained at
reasonable costs (less than $10,000 per kilowatt of emergent
beam power).
2 J. Silverman
There is another major postwar factor. Since 1945 atomic
energy commissions throughout the world stimulated and fos-
tered research in pure and applied radiation science. These
programs provided the detailed information underlying the post
war advances. Nevertheless, the actual radiation processes
and products and the electron accelerators now in industrial
use were developed almost entirely by private companies led by
determined and imaginative men using risk capital. These ef-
forts have not been in vain.
At present, radiation processes account for products
which by any reasonable reckoning have an annual sales value
over $ 400,000,000. While this constitutes a substantial
accomplishment, it is also clear that ionizing radiation has
not yet had a major impact on modern technology.
We shall now consider the characteristics of this tech-
nology. The purpose is to provide the audience with some of
the background necessary for an understanding of successes
and failures achieved thus far, and of the promise and limi-
tations of proposed applications,
Properties of Ionizing Radiation
Strictly speaking, radiation consists of electromagnetic
quanta or photons, and ionizing radiation consists of high
energy photons. In practice, "ionizing radiation" is a term
used not only for high energy photons, but also for energetic
charged particles (electrons, protons, alphas, ionizing fis-
sion fragments, fast neutrons, etc.) However, as shown below
in the discussion on radiation source technology, a realistic
discussion of radiation processing would limit our consider-
ation almost entirely to only two types of ionizing radiation:
cobalt 60 gammas (% 1.25 MeV), and electrons in the range
0.15 - i0 MeV.
Perhaps the first point to stress is that any product
exposed to these two types cannot become radioactive. Indeed
when electron accelerators are the sources of ionizing radia-
tion, radioactivity is not involved in the process in any way.
The penetrating properties of the two types of radiation
can be best considered in terms of the product of the linear
thickness and the density of an absorber (usually in units of
grams per square centimeter). Expressed in these units, the
Basic concepts of radiation processing 3
penetrating properties of gammas and high energy electrons
are to a first approximation independent of the composition of
the absorber. The "useful" penetration of an absorber by
monoenergetic electrons with any energy of E MeV is 0.33 E -2 g.cm (or 0.33 E/p cm, where 0 is the density in g.cm-3).
Thus 10 MeV electrons can be used to process a 3 cm thickness
of unit density material by irradiation from one side; elec-
trons with energies of a few hundred keV can be used to pro-
cess only coatings, thin films, and foamed plastics; electrons
with intermediate energies are used to irradiate plastic
sheets and tubing, insulated wire, etc. The gammas are far
more penetrating. Their energy is deposited in a manner that
approximates an exponential decrease in dose with increasing
depth in the absorber; the mean free path in unit density
material is about 30 cm. One may readily conclude that gammas
are used most effectively and efficiently for the processing
of thick products such as in the radiation sterilization of
cases of disposable biomedical supplies, the radiation steri-
lization of whole hams, the curing of thick wood-plastic com-
posites, etc.
The chemical and biological effects produced by electrons
are almost independent of the electron energy. The reason is
that the energy deposited per unit length (called LET) along
the path of the electron is almost independent of the elec-
tron energy. Thus the density of the chemically reactive
fragments produced along the pathlength is also independent
of electron energy. As an example the LET of electrons in
water is ~ 0.01 electron volts per angstrom unit over the
electron energy range 0.25 - 10 MeV.
The chemical and biological effects produced by the ab-
sorption of gammas is similar to that of a similar dose of
electrons delivered at the same dose rate. The primary inter-
action of a 60Co gamma with an absorber is Compton scattering,
in which an energetic electron is liberated. Thus the Compton
electrons from the 1.25 MeV photons also have an LET of o- 1
0.01 eV.A in water.
4 J. Silverman
Units, Formulas and Their Implications
The net effect produced by ionizing radiation is princi-
pally a function of the dose, or the amount of energy absorbed
per unit mass of the absorber. The unit of dose is the rad
which corresponds to the deposition of 100 ergs of energy by
a source of ionizing radiation to a gram of absorber. A more
useful unit is the megarad; most practical applications re-
quire doses in the range 0.01-100 Mrad.
Simple unit conversions lead to the following relation-
ship:
production rate -
where D = dose in Mrad;
and f = fractional efficiency.
360f ~ (i) D
Thus if an application requires a dose of 0.01 Mrad, as for
example the inhibition of sprouting in potatoes, a source
emitting one kilowatt of gamma power and absorbed by the pro-
duct with an overall efficiency of 25% can process nine metric
tons per hour. (Compare this with a kilowatt of electric
power which is just good enough to run a laundry iron or a
toaster.) Similarly one kilowatt hour of emitted gamma energy
can process nine metric tons. This demonstrates that a one
kilowatt radiation source can be a powerful industrial tool
and that one kilowatt hour of radiation energy can accomplish
far more than an equivalent amount of heat. An even more dra-
matic example is the fact that lethal doses of radiation (500
rad) involve the absorption of only a millicalorie per gram
of tissue; a similar absorption of heat energy would raise
body temperature perhaps a thousandth of a degree.
Thus it is clear that energy in the form of large quanta
can have more pronounced chemical effects than energy in the
form of small quanta. The reason becomes clear when we recall
that thermal energy is coupled most strongly to the transla-
tion, rotational and vibrational modes; only a small fraction
goes into the electronic modes of the absorber. Thus ioniza-
tion, bond rupture, and all the processes leading to chemical
reaction occur only in the high energy tail of the Maxwellian
distribution. Ionizing radiation is absorbed almost entirely
by the electronic structure of an absorber and is therefore a
Basic concepts of radiation processing 5
very effective and efficient generator of the reactive species
that initiate chemical reactions. In an energy-concious worl~
this advantage could be the basis of major industrial oppor-
tunities, provided of course that the cost of radiation energy
and the capital cost of radiation power are acceptable.
Another energy concept of radiation science is the G
value, which is used to describe the yield of radiation-in-
duced reactions. It is the number of molecules or ions pro-
duced or destroyed per i00 eV absorbed. Here is an example.
The gamma irradiation of methanol produces ethylene glycol
as well as additional products. The G value for glycol syn-
thesis, G(glycol), is 4; i.e. four glycol molecules are pro-
duced for every I00 eV absorbed. G(-MeOH), the G value for
methanol consumption, is about 10.
The following is a useful energy relationship involving
the radiation yield:
k production rate = 3.74 x 10 -4 GMf ~ ,
where M is the molecular weight of the product. Thus, in
order to get a large weight of product per kilowatt hour of
radiation energy absorbed, the product of the radiation yield
and the molecular weight must be high.
Consider once again the radiation-induced dimerization of
methanol to form ethylene glycol, for which M=64. Application
of the formula discloses that 0.i kg of glycol would be pro-
duced for every kilowatt-hour of radiation energy absorbed.
Since the efficient absorption of radiation energy in the re-
action mixture is likely to be about half of that emitted by
the radiation source, it takes the expenditure of 20 kWh of
radiation energy to produce a kilogram of the glycol. At a
price of a dollar per kilowatt-hour, radiation energy is too
costly. Even at a price of $0.10 per kWh it would be too
high.
On the other hand free radical chain reactions, such as
the polymerization of vinyl monomers, often have G values in
excess of 1,000. Such reactions with high kinetic chain
lengths have been studied in detail as possible radiation pro-
cesses. Both Brookhaven National Laboratory and the Takasaki
Radiation Chemistry Research Laboratory (JAERI) had large
6 J. Silverman
projects devoted to the polymerization of ethylene at high
pressures by cobalt-60 gammas. The results confirm that the
G value is high, that the polyethylene produced by gamma irra-
diation is at least as good as the conventional product, that
the processing methods are simple and safe, and that the costs
on a production scale of 50,000 tons per annum would be com-
petitive. Indeed, the BNL group is convinced the cost of the
gamma process would be lower than that of the conventional
process. Nevertheless the radiation process has not been
adopted by industry and the two research projects have been
terminated.
There is an important lesson to be learned from the
ethylene polymerization case. The purpose of the ionizing
radiation in this application is to initiate a free radical
which then continues to propagate without the additional ex-
penditure of energy. Thus the cost of ionizing radiation is
to be compared with that of chemical initiators. Since the
cost of initiation is a small element in the total cost of
production, it can provide only small advantages, at best, in
the net cost per unit of product. The fact that industry
would have to develop a new and strange technology to make a
small gain means there is little industrial incentive to ir-
radiate ethylene.
Does all this mean that ionizing radiation is to have no
role in high G reactions? Not at all. Both the curing of
acrylic paints on automobile dashboards by the Ford Motor Co.
and the grafting of acrylamide derivatives to fabric by the
Deering-Milliken Co. are both successful radiation applica-
tions involving relatively high G reactions. What it does
mean is that a high G is not enough. Either the product of
the radiation reaction must be more valuable or else the use
of radiation must show processing advantages which reduce
costs significantly. Thus there is still the possibility that
ethylene polymerization by radiation will be applied on the
commercial scale. The product is somewhat higher in density
and purer than polyethylene produced by the conventional high
pressure processes. Even more important the radiation reac-
tion is more easily controlled because the initiation rate
can be quenched in seconds by removing the source rods; in
Basic concepts of radiation processing 7
the conventional process there are occasional runaway reac-
tions leading to unscheduled plant shutdowns.
I pause here to point out a significant difference be ~
tween ionizing radiation and ultraviolet light (which is,
generally speaking, nonionizing radiation). These two energy
forms sometimes have the same principal effect on paints, that
is, they cure the paints by initiating a free radical polym-
erization. Thus if one were to consider coating polypropylene
panels with acrylic paints, both UV and electron beams would
be evaluated. While our session on surface applications will
reveal many advantages and disadvantages associated with each
technique, the electron beam process would have one major
advantage. Ultraviolet would cure the coating alone and ad-
hesion would be poor without special treatment of the polyo-
lefin backing; the electron beam would cure the coating and
provide covalent bonding between the coating and backing.
We have already seen that high G reactions do not offer
an easy path to commercial success. Let us now consider re-
actions where M is high. Of course, we are referring to
polymers, whose molecules have molecular weights of the order
of 10,000; even where the G value for a polymer effect is low,
(such as in crosslinking of polyethylene where G ~ i) small
doses can have relatively large effects on the physical and
mechanical properties. This is the basis of the most success-
ful application of radiation processing: the crosslinking of
polymers for use in electrical insulation and packaging.
Dose Rate
Any discussion of the effect of ionizing radiation must
take into account the effect of dose rate. The general prin-
ciple is that all sources of ionizing radiation with the same
LET produce the same chemical effect with the same dose, pro-
vided the energy is delivered at the same dose rate. LET
effects are small with the two principal sources used in in-
dustrial radiation processing, but dose rate effects can have
a major influence on the results.
A simple example of the dose rate effect can be seen in
radiation sterilization. At low dose rates (< 10,000 rad-h -I)
the reproduction rate of the bacteria compete with the rate
8 J. Silverman
of radiation-induced bactericide, so that the sterilization
dose is higher than it is at high dose rates.
Sometimes the dose rate effect is a fundamental aspect of
the reaction kinetics. At low dose rates, free radical polym-
erizations vary directly as the square root of the dose rate.
Thus a hundredfold increase in dose rate produces only a ten-
fold increase in the rate of conversion of monomer to polymer.
As the viscosity of the system increases, the rate dependence
on dose rate becomes linear. At very high dose rates, where
spur overlap phenomena begin to occur, the reaction rate tends
to become independent of dose rate.
The term '~dose rate" is complicated to some extent by the
characteristics of the radiation source. Some accelerators
produce a beam of electrons whose electrical characteristics
are constant with time. Others, such as the linear accelera-
tor, produce a continuous train of short high current pulses
separated by periods of zero current. Thus different devices
can produce different effects even when the time average dose
rate is the same.
Fundamental Chemical Effects
At first glance, it is not clear how ionizing radiation
can excite a molecule much less ionize it. The LET figures
quoted earlier show that an electron traversing a molecular
diameter loses, on the average, a few hundredths of an elec-
tron volt, which is about one percent of the mean ionization
potential. Actually a fast electron does not lose its energy
continuously. It goes down the energy scale dropping its
energy in discrete bundles averaging 100 eV. In a condensed
medium the average distance between successive interactions is
a few thousand angstroms apart and the spherical regionsoin
which the energy is deposited, called spurs, are 10-100 A in
radius. Inside the spur, the energy deposited is sufficient
to produce ions, radicals and excited species. In a low vis-
cosity medium, intraspur reactions occur almost at once and
after about a microsecond, the surviving reactive species and
the reaction products in the spur are sufficiently diffused
throughout the medium so that their subsequent reactions can
be described by homogeneous kinetics.
Basic concepts of radiation processing 9
For these early transient species, the G values are usu-
ally less than ten. For example, the G value for the forma-
tion of propagating radicals in styrene is 0.7; addition of
some methanol can raise it to 6. The G value for ion pair
formation in gaseous n-hexane is 4; G(ip) for the liquid is
0.i. Clearly additives and the state of the substance are im-
portant factors in the yield. For very early processes where
the energy associated with the initial event is very high, the
yields are independent of temperature.
The principal chemical effects are too numerous to cata-
log in an introductory talk but some salient points can be
drawn from a consideration of the following species produced
in polymers by ionizing radiation:
Crosslinks and scissions Free radicals Electron-cation pairs Unsaturation
Crosslinkin~ and scission: Crosslinking and scission are
simultaneous competitive phenomena and it is the ratio of the
two yields that determines the net effect. Regardless of
which predominates, the molecular weight distribution is
broadened by irradiation. Anti-oxidants, plasticizers, dyes,
fillers, oxidized impurities, etc. are common components of
industrial polymers; they affect the net result. Also mor-
phology can be a complicating feature. It appears that while
radiation crosslinking occurs in the amorphous regions of
polyethylene, the long-lived radicals associated with the
crystalline regions can turn into peroxides which initiate
degradation reactions. A practical solution often employed
is to anneal the irradiated polymer soon after the radiation
crosslinking.
Despite the complications, there are sufficient advan-
tages to make radiation-crosslinking a major success. We are
privileged to have as participants at this Meeting the prin-
cipal authors of this achievement: Professors A. Charlesby
and M. Dole whose fundamental work forms the basis of our
knowledge of this field, and Messrs. Paul Cook of Raychem and
William Baird of W. R. Grace who applied and extended this
knowledge with skill and resourcefulness.
i0 J. Silverman
In recent years, radiation-induced scission and degra-
dation have assumed some industrial importance. The degrada-
tion of polyethylene oxide is a commercial process. So is the
degradation of scrap polytetrafluoroethylene to produce a
useful component of spray lubricants. There is promise of
future application of radiation-scission in rayon synthesis
and solid waste treatment. The latter topic is the subject
of Professor Brenner's lecture.
Free radicals: The production of short-lived radicals
on the main chain of a polymer becomes useful only when these
reactive sites become the means of a practical polymer modi-
fication. Graft polymerization, in which the radical on a
chain composed of one set of repeating units can serve as the
initiation site for the addition of a side chain of another
set of repeating units, is a small but significant success in
the textile area, and is potentially one of the most impor-
tant industrial applications of ionizing radiation.
When the grafted side chains are linked to polymer mole-
cules on the surface of an object, surface properties such as
adhesion, wettability, etc. are modified without significant
effect on the bulk properties. If the grafted side chains
are randomly distributed throughout the object, bulk proper-
ties are also affected.
The use of radiation in grafting has several potential
advantages over alternative methods. The yield of the radical
sites on the main chain is independent of temperature and the
radicals are generated uniformly throughout the polymer to
be modified at a rate proportional to the dose rate. This
simplifies control over the reaction rate and the molecular
weight of the product. Thus the radiation method can be ap-
plied to many monomer-polymer pairs that are not easily
grafted by other methods. For example, radiation-induced
grafts can be formed in solid state combinations of monomer
and polymer. Also as a chain polymerization reaction, its
G value is high and the dose requirements are low, about a
megarad; thus the cost of radiation energy involved in graft-
ing processes is usually low.
Basic concepts of radiation processing Ii
Further comment about grafting will be heard shortly from
Dr. A. Chapiro, the imaginative contributor to radiation sci-
ence who invented the method. Also, Professor W. K. Walsh
will tell us in the session on surface applications about the
Deering-Milliken process and other grafting applications in
the textile industry.
Ion pairs: For those interested in using electrical com-
ponents such as coaxial cable and capacitors based on plastics
in the radiation field, the production of charge carriers in
polymers is of great interest. From the point of view of
chemistry they are of trivial interest except when they are
precursors of free radicals. Grafting to radiation-induced
cationic sites on the main chain can be done, in principle,
but such an approach does not appear to be fruitful.
Unsaturation: It often comes as a surprise that the
yield of transvinylene unsaturation in irradiated polyethylene
is as great as the yield of crosslinks, but aside from some
dosimetric applications, the unsaturation remains a somewhat
strange and untapped resource. Incidentally the production of
trans-unsaturation in polyethylene by a single-hit process
appears to be a unique property of ionizing radiation.
Vinyl Monomers
We have had occasion to refer to vinyl monomers and the
practical problems associated with radiation-induced polym-
erization. It is noteworthy that ionizing radiation exhibits
interesting characteristics that may yet be of practical
value. Unlike ultraviolet light, ionizing radiation not only
initiates radical polymerizations but also high yield ionic
polymerizations. Professor Tabata who has a long and produc-
tive career in radiation polymer chemistry will elaborate fur-
ther on the matter.
Biochemical Effects
In addition to the fundamental chemical effects leading
to the synthesis and modification of materials, ionizing radia .
tion induces gross biochemical effects that have been the
basis of intensive studies at major government and industrial
laboratories throughout the world. The most successful appli-
cation has been the radiation sterilization of sutures and
disposable medical supplies; the industrial company that has
been most effective and successful in applying the new
RPC Vol. 9, No. I -3--C
12 J. Silverman
technology is Johnson & Johnson and its subsidiary, Ethicon
Corp.; and the man who had the major role directing the tech-
nology to commercial success, Dr. Charles Artandi, is with us
at this Meeting. He will discuss the problems and progress
in this field in the session on biological applications.
This application was instituted in 1956 when electron machine
costs were $25,000 per kilowatt and the cost of electron
beam energy approached $50 per kWh. Despite the high cost of
the radiation process, the products provided enough advantages
in packaging and customer convenience to permit Ethicon to
dominate the market for sterile sutures with a high price pro-
duct. The success of Johnson & Johnson stimulated similar
ventures throughout the world.
At present, the radiation sterilization of drugs and
medicinal preparations is not practiced due in large measure
to the formidable obstacles associated with clearing such pro-
cedures through the regulatory agencies.
The radiation preservation of food represents one of the
great disappointments and one of the great opportunities of
radiation processing. The radiation can inhibit sprouting,
kill insects, eliminate molds and fungi and destroy bacteria,
with doses ranging from 0.01 Mrad for sprout inhibition to a
few megarad for sterilizing doses. A food preservation system
based on irradiation could require far less energy consump-
tion than that required for steam sterilization and refriger-
ation. As the competition for clean fuels increases and the
cost of energy becomes a more dominant factor it is apparent
that a detailed analysis is needed to determine whether it
will benefit society to emphasize energy conservation and
food preservation by widespread substitution of radiation pro-
cesses for thermal processes. Of course there is a major
problem associated with public acceptance. An even more
important factor is the attitude of the regulatory agencies
toward radiation preservation of food. Their insistence on
treating ionizing radiation as a food additive is unrealistic
at best and imposes unnecessarily harsh restrictions on the
development of radiation preservation. The lectures by Pro-
fessor Diehl and Dr. Dierksheide will be instructive.
Basic concepts of radiation processing 13
Another biochemical application of ~ajor i~portance
is the radiation treatment of sewage. In addition to the de-
crease in the settling time of sewage sludge, ionizing radia-
tion also decreases the population of microflora. This field
has had a long but difficult past. In recent years there has
been an upsurge of serious and reliable work, particularly in
Germany and the United States. It will soon be possible to
determine whether the dose level sufficient to kill enough
microscopic life to satisfy public health authorities can be
performed at an acceptable cost. The lecture by Dr. S~ss on
the pasteurization of sewage sludge will be an interesting
commentary on what could be the first truly large scaIe pro-
cess application of ionizing radiation.
Industrial Effluents
Just two years ago, there was news of a novel and poten-
tially important development in applied radiation science.
Studies sponsored by Ebara Industries and performed at the
Takasaki Laboratory of the Japan Atomic Energy Research Insti-
tute showed that irradiation of stack gases from an oil burner
could achieve major reductions in NO x and SO 2 concentrations
with doses of a few megarads. The radiation chemistry of the
gas reaction is a fascinating puzzle. More important is the
effect itself and the possible practical applications to the
stack gases of metal refineries and power plants. Dr. S.
Machi's lecture on the subject is the first major disclosure
of the details of the JAERI study. It should be a highlight
of the Meeting.
Other processes have been suggested for the irradiation
of industrial effluents, e.g. textile wastes containing dyes
and chemical plant effluents containing traces of phenol.
It appears that the radiation effects destroying the unde-
sirable solutes may produce equally undesirable residues.
None of the proposed processes appears practical.
Source Technology
Perhaps the most important advances over the last 10
years have been in the field of source technology. This in-
cludes not only the sources themselves but also in peripheral
areas such as dosimetry, conveyor systems, materials of con-
struction and the calculation procedures for optimum source
14 J. Silverman
utilization. We now have safe reliable equipment available
at reasonable prices, as well as an array of good quality con-
trol techniques and materials handling equipment.
Developments in electron machine technology are particu-
larly impressive. All the significant accomplishments leading
to the Dynamitron of Radiation Dynamics, Inc. and the ICT of
High Voltage Engineering Corp. have been the result of the
dedicated research effort of the small private companies that
produce the two devices now dominating the radiation process-
ing industry. The machines are high power (i0-i00 kW), high
dose rate (~ 1 Mrad. sec -I) devices in the 0.4 - 3 MeV range.
Western Electric Co. which produces 3x!09 meters per year of
irradiated insulated wire with Dynamitrons finds that the
reliability of the electron accelerators is comparable to
that of the plastic extruders. In the 10-100 kW range, for
low penetration applications the only reasonable choice is
an electron machine.
Recent developments, particularly by Energy Sciences, Inc
in the direction of low energy machines (< 0.2 MeV) have
aroused considerable interest particularly for textile and
film applications. A unique feature of the ESI approach is
the use of a long continuous filament that emits high energy
electrons in the form of a curtain. In competitive devices
the electrons are emitted as a pencil beam that are scanned
in the dimension perpendicular to the motion of the product.
We will have ample opportunity at this Meeting to hear"
people with experience using electron beam facilities describ-
ing their experience. Of special interest should be the
lecture by Mr. William Baird of W. R. Grace concerning the
many electron machines used by the Cryovac Division in the
production of irradiated heat shrinkable packaging.
As the cost of electron beam energy sinks lower there is
once again the possibility that conversion of the kinetic
energy of electrons to photons may provide practical competi-
tion to 60Co. Nevertheless industry still holds to the view
that 60Co with all its limitations and high cost holds an
advantage over electron beam generators especially when the
power requirements are low and the penetration requirements
are high. Gamma sources are low dose rate devices
Basic concepts of radiation processing 15
(< i Mrad.h -l) and ostensibly large inventories produce low
radiation power. For example a megacurie of 60Co produces
less than 15 kW of gamma power. A disadvantage of 60Co
sources is that they are always on and the large mean free
path of the gammas does not permit high utilization efficiency
without a complex conveyor system. With a 60Co source as com-
pared with an electron beam generator the greater simplicity
of the radioactive source is offset by the greater complexity
of the materials handling system. Nevertheless 60Co contin-
ues to dominate the field of radiation sterilization and low
volume products such as wood plastic flooring.
Any serious discussion of source technology soon excludes
separated fission products, spent fuel rods, chemonuclear
reactors, positive ion accelerators, etc. Although each has
some potential merit the costs and the developmental problems
are prohibitive in the light of the potential gains. The
last year has seen an increased emphasis on a new radiation
source: fast neutrons from fusion reactors. Although fast
neutrons have been available from the nuclear fission reactor,
they carry only a small portion of the energy of fission and
they are difficult to get at. Fusion reactors emit some 70%
of their energy in the form of penetrating fast neutrons, and
access to the neutrons from the fusion reactor is potentially
much easier, particularly if it is a pellet fusion reactor.
KMS Fusion, Inc. has taken the lead in the development of
radiation chemistry processes based on neutrons from laser
fusion. At this Meeting, Dr. H. Gomberg of KMS Fusion will
present some of the stimulating and imaginative processing
ideas developed by his company.
This has been a cursory survey of the basic elements
of radiation processing. The papers that follow shall fill
in the details.
