I
—'\1
I0):
(DO'KO

RELATION BETWEEN TEE BETH DWUHON
OF EQNEZATION AND THE LETHAL
EFFECTS ON BACTERTA

Thesis Ear the Dawn 0? pin D.
MKCRHGEN BUTTE UNWERSITY

1. Leon Newcomer
1957

JHESIS

L.

I “)

47V

This is to certify that the

thesis entitled

Relation between depth distribution
of ionization and the lethal effects
on bacteria.

presented by
J. Leon Newcomer

has been accepted towards fulfillment
of the requirements for

Ph.D. Agricultural Engineering

degree in

owed. W

Major professor

Date 9% ' g .3”! /é\5‘7

 

0-169

 

RELATION BETWEEN THE DEPTH DISTRIBUTION OF IONIZATION
AND THE LETHAL EFFECTS ON BACTERIA

by

J. Leon Newcomer

AN ABSTRACT

Submitted to the School of Advanced Graduate Studies of Michigan
State University of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY
Department of Agricultural Engineering
Year 1957

/- I .’ _ i ,
Approved by CM” 144/646 ah}. £57 157

 

J. Leon Newcomer
ABSTRACT 1

Numerous investigators have shown that cathode rays
exhibit a lethal effect on bacteria. The lethal action is at-
tributed to ionization occurring within the substance irradiated.
Since the rays are able to penetrate matter, it has been proposed
that cathode rays be used to sterilize foods.

The distribution-in-depth of the ionization within a
substance irradiated by cathode rays, determined by absorption
methods, is known to be non-linear.

The investigation was conducted to ascertain the dis-
tribution-in-depth of the lethal effect of one million electron-
volt cathode rays on bacteria and to compare the characteristics
of the distribution with the ionization-in-depth curve obtained
by absorption methods. The test organism used was Scrratia
marcescens.

The depth distribution of the lethal effect was found
to be non-linear. The general features of the lethal curve
were comparable to the absorption curve. It was shown that
the lethal effect of one million electron—volt rays was dis-
proportionate to the dose administered. The absorption curve

alone is not a reliable measure of the lethal effect within

a substance.

 

RELATION BETWEEN THE DEPTH DISTRIBUTION OF IONIZATION
AND THE LETHAL EFFECTS ON BACTERIA

by
/
J. Leon Newcomer

A THESIS

Submitted to the School of Advanced Graduate Studies of Michigan
State University of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Agricultural Engineering

195 7

 

/--/J--54

'n‘ alib- I
.- 9m

ACKNOWLEDGMENTS

Although bibliographies and footnotes indicate many
of the sources of information, such devices can not do justice
to all persons who have been both instrumental in guiding our
thinking and helpful throughout the project. It is a pleasure
to note a few of these persons here by name.

The author wishes to express his sincere thanks to
Dr. Carl W. Hall for his inspirational supervision and con-
tinued ready help in this investigation. He also wishes to
thank the other members of the guidance committee, Doctors
D. J. Montgomery, B. H. Dickinson, and J. R. Brunner, for their
suggestions and c00peration throughout this endeavor.

He is also indebted to Professor A. W. Farrell and
his entire staff for making materials, equipment and facili-
ties available for this study.

The writer expresses his appreciation to Dr. 0. W.
Kaufmann of the Department of Microbiology and Public Health,
for his generous assistance both in his technical guidance and
for his bacteriological analysis phase.of the project.

Additionally, to Mr. Richard Nicholas of the Department
of Agricultural Engineering, is extended a word of thanks for
his patient and courteous council, his numerous constructive

suggestions and his Operation of the electron beam accelerator.

 

J. Leon Newcomer
candidate for the degree of

Doctor of Philosophy

Final examination, February 25, 1957, 10:00 A.M., Conference
Room, Agricultural Engineering Building

Dissertation: Relation between the Depth Distribution of
Ionization and the Lethal Effects on Bacteria

Outline of Studies

Major Subject: Agricultural Engineering
Minor Subject: Physics

Biographical Items
Born, September 20, 1910, Hamilton County, Indiana

Undergraduate Studies, Purdue University 1929-1933
(B. s. Degree 19335

Graduate Studies, Michigan State University, l95u—1955
(M.S., Agricultural Engineering, 1955);
Michigan State University, 1955-1957

Experience: Engineer with Indiana Condensed Milk Company,
' 1933-hl; Consulting Engineer in private
practice, 19h1-5h

Member of American Society of Agricultural Engineers,
Sigma Pi Sigma, Physics Honor Society, Society
of the Sigma Xi

Registered Professional Engineer in Indiana,
Ohio, Illinois, Tennessee, Wisconsin.

 

I.
II.
III.
IV.
V.
VI.
VII.
VIII.
IX.
X.
XI.

INTRODUCTION .

REVIEW OF LITERATURE

OBJECTIVES . .
APPARATUS . .
PROCEDURE . .

DATA AND DISCUSSION

CONCLUSIONS .
SUMMARY . .

TABLE

OF CONTENTS

OF RESULTS . .

SUGGESTIONS FOR FURTHER RESEARCH .

GLOSSARY OF TERMS .

LITERATURE CITED

Page

18
19
26
29
no
hl

us
he

 

LIST OF TABLES

Page

I. Percentage of bacteria surviving at various depths
in the absorber when irradiated at a dosage of
5000 rep OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO0.0.000. 36

 

LIST OF FIGURES

FIGURE ' PAGE

1. Relative ionization densities of l, 2, and 3 mev.
electrons in matter.....0000...OOOOOOOOOOOOOOOOOOO 7

2. Relative ionization density of l mev. non-
monoenergetic cathode rays in matter.............. 18

3. Cutaway view of a l mev. cathode ray apparatus..... 23
u. Irradiation—absorption chamber..................... 2h
5. Membrane filter diSCOOOOOOOCOOOOOOOOO00.00.000.000. 25

6. Irradiation-absorption chamber loaded with bacteria-
laden filter. discs...................OOOOOOOOOOOOO 25

7. Relative number of bacteria destroyed at various
depths within the absorber........................ 37

8. Colonies growing on membrane filter discs.......... 38

9. Survival curve for §. marcescens irradiated with
1mev. cathOde raySOOOCOOOO0.....00000000000000000 39

 

 

INTRODUCTION

It has been observed that o< and (5) particles, ya, X—
and cathode rays and ultraviolet light exhibit a lethal effect
on biological systems. These radiations penetrate matter to
varying depths and cause ionization within the substance. The
lethal effect is attributed to the ionization.

It was observed early that although Xprays exhibited
a lethal effect on biological systems, the efficiency was only
approximately 5 percent. Ninety-five percent of the energy of
the electrons was dissipated in the form of heat from the tar-
get. By removing the target and permitting the electron beam,
(cathode rays) to impinge upon the substance, not only were
there lethal effects but also considerable improvement in beam
utilization.

Insofar as cathode rays have been proposed as a means
for killing bacteria within foods and food products, it be-
comes significant to evaluate their effectiveness as a lethal
agent.

Two aspects of the problem must first be briefly re-
viewed, namely, 1) the manner in which bacteria respond to a
lethal agent, and 2) the depth distribution of ionization
within a substance irradiated by energetic electrons.

When bacteria are subjected to any lethal agent, such

as heat, disinfectants, X-rays, cathode rays etc., they do

 

 

not all die at once, but a constant fraction of those present
dies in each increment of time. The fraction of the number
initially present which survives at any given time is called
the survival ratio. The fraction killed is one minus the
survival ratio. The survival ratio is an exponential function
of the time of exposure and the intensity of the lethal agent.

It takes the form

-KIt

N‘Noe 000.0(1)
where No 3 the number of bacteria initially present
N I the number surviving at time (t)

t C the time of exposure to the lethal agent
K t a constant

= the intensity of the lethal agent

The effectiveness of the lethal agent is accordingly
expressed in terms of the dose required to produce a given
survival ratio. A dose of I x t t l/K reduces the exponent
of e to -l , and the corresponding survival ratio is
l/e 8 0.368 or 36.8 percent. The dose required to give a
survival ratio of 36.8 percent is called the mean lethal dose,
lethal exposure, or inactivation dose. Usually, the whole
number, 37 percent, is used.

Within wide limits the Bunsen-Roscoe reciprocity law (13)
applies to the killing of bacteria. That is, a given dose
results in a given survival ratio, regardless of whether the

dose consists of a low intensity for a long time or a high

 

intensity for a correspondingly shorter time. For bacteria,
this law has been found to apply over a thousandfold range

in intensity. Probably the failure of the law at very low
intensities is because the exposure time becomes so long that
bacteria reproduce during the time of exposure.

There is no dispute that ionizing radiations can pro-
duce a lethal effect on bacteria but there is considerable
disagreement among biologists regarding the exact mechanism
of the cause of death. A number of theories have been pro-
posed and presently a preponderance of experimental evidence
supports the "single hit" theory. It is characterized by the
following: 1) the survival curve is exponential, 2) destruc-
tion is independent of dose rate, 3) destruction is independent
of temperature, and u) the concentration of organisms does not
effect the percentage survival.

Rahn, especially, treated the theoretical aspects as
well as the experimental evidence in a comprehensive manner
and concluded that so far as single-celled organisms were
concerned, the single hit theory is the only plausible explan-
ation for the fact that the survival curve is exponential.

Although this work is not especially concerned with
the exact mechanism of death, some attention must be given to
the shape of the survival curve insofar as it indicates that
the death of a bacterium is evidently due to the energy dissi-

pated by the radiation in the bacterium.itself and is not an

 

an indirect action due to the dissipation of energy in the

surrounding media.
For this purpose it will be more convenient to express

equation (1) in the following form:

N/NO I e"D/Do ‘ .....(2)
.where N/NO c the survival ratio
No I the original number of bacteria
N t the number of survivors of dose D
D0 = dose at which there is an average of one

effective hit per organism.
Cathode rays consist of parallel beams of high velocity
electrons, having attained their high velocity from being
accelerated through a high electric potential. Thus, they
possess kinetic energy and are able to traverse matter. The
depth of penetration (range) of an energetic electron is a
function of its initial kinetic energy and the density of the
substance.

In their passage through matter, cathode rays
(primary electrons) ionize atoms along their paths. The elec-
trons which are ejected in this process, called secondary elec-
trons, may themselves possess sufficient energy to ionize still
other atoms. If the substance is sufficiently thick, i.e.,
equal to or greater than the maximum range of the electrons
the primary electron plus all its secondary electrons will

eventually lose energy and be absorbed within the substance.

 

The mass of the electron is very small compared to the mass

of the atoms with which it collides, consequently, its path I
is a tortoruous one. Not all electrons travel the maximum
range but dissipate their energies at different depths through-
out the substance. Therefore, the depth distribution of ion-
ization within a substance being bombarded with cathode-rays
would not be expected to be uniform.

The usual procedure for determining the actual depth
distribution of ionization is to place varying thicknesses of
an absorber between a source of cathode rays and an ionization
chamber and to relate the measured activity to the thickness.
-This relation is called an absorption curve. Absorber thick-
nesses are usually reported in terms of areal density (grams
per square centimeter, etc.) instead of actual thickness.

This is done partly because it is easy to find areal density

by weighing thin foils but more importantly because thicknesses
so expressed are roughly independent of the nature of the ab-
sorber.

Figure 1 shows the distribution of ionization—in—depth
in aluminum produced by mono-energetic cathode rays of differ-
ent initial energies. It will be observed that the relative
ionization increases from approximately 60 percent at the sur-
face of the absorber to a maximum of 100 percent and then
falls off rapidly to zero at the maximum range for the partic-

ular energy level. Although the maximum range for an

 

 

electron is determined by its initial kinetic energy, the

Iionization distribution depends upon the irregular paths of

the primary and secondary electrons.

A resume of the foregoing discussion focuses attention
upon the following observations: 1) cathode rays act as a
lethal agent on bacteria, 2) the lethal action is associated
with ionization, 3) the depth of penetration of cathode rays
is a function of their initial kinetic energy and the density
of the absorber, and.u) the distribution of ionization within
an absorber is non-uniform.

If foods are to be sterilized by irradiation with cathode
rays, then the significance of such non-uniform distribution
of the lethal agent becomes evident. This investigation was
undertaken to determine the depth distribution of the lethal

effect of cathode rays within an absorber.

 

 

 

.._ ._.-..__.

 

 

(U

IONIZAYmu

RELATIVE

 

 

 

 

0 02 04 0.6 0.8 I0 I2 I.4
RANG E (gms/cmz)

 

Fig. 1. Relative ionization densities
of l, 2, and 3 mev. electrons

in matter (23).

 

REVIEW OF LITERATURE

The bactericidal effects of ultraviolet light were
observed as early as 1903 (Rentschler gt 31., 20), but
they were not studied with any degree of quantitative accu-
racy until recent years. Gates (8, 9) showed that the wave-

length for germicidal action was 2660 A. He showed, further

- that bacteria exhibited widely different resistances to ultra-

violet radiation, depending on the different stages in their
life cycles. A sublethal dose was shown to retard the rate
at which colonies developed after irradiation.

Soon after the discovery of radium and X—rays, inves-
tigators in the field of bacteriology became interested in
the effect of these radiations on microorganisms (Pacinotti
and Porcelli, 1898); Prescott, 1904). Schmidt (21) studied_
the effects of radioactive phosphorus (P32) on E. ggii. He
found that the figradiations of P32 had a lethal effect on
E, 221; and that, in general, this effect was related to the
initial concentration of the P32.

X-rays have been of interest to the engineer, the
chemist, the bacteriologist, and the food technologist almost
since their discovery by Roentgen in 1895. Dunn £3 E}. (6)

determined the effects of high voltage X-rays on a number of

 

bacteria, yeasts, and molds. They found that these micro-
organisms could be destroyed by X-rays and that the dosage
necessary for destruction varied with different types of
microorganisms.

The.mechanism of X-ray destruction of bacteria is ex-
plained according to the "direct hit" theory, also called the
"target" or "Treffler" theory. This theory, which is thor-
oughly discussed by Lea (15), states that bacteria are des-
troyed by direct hits of the photons on the'cells. 0n the
assumption that the destruction of bacteria is brought about
by direct hits, the survival curve would be exponential and
therefore a straight line when plotted on semi-logarithm paper.
According to Lea (15) and Rahn (19) the destruction of bac-
teria by photons is based on the mathematical probability of
a direct hit taking place.

Proctor, Van deCEraaff, and Fram (18) found that
ground meat could be sterilized by irradiation with high
voltage X-rays. Since X-rays produced at high voltage are”
able to sterilize food materials without raising the tempera-
ture to an appreciable extent, this might appear to be an ef-
ficient method of "cold" sterilization. However, approximately
95 percent of the electron energy goes into heat in the X—ray
equipment when it strikes the target interposed between the
electron source and the object irradiated and only approximately

5; percent into the production of X-rays. Hence, interest has

 

 

 

'10

.more recently been directed to high speed electrons, usually
called cathode rays.

In recent years, several types of apparatus for pro-
ducing high voltage cathode rays have been developed. To name
a few: Van de Graaff Electrostatic Generator (Trump gt 31.,
19MB), Capacitron developed by Brasch and Huber (3), and the
resonant transformer type (Charlton e} 31., l9h0) which shall
be more fully described later. _ - ’

Coolidge and Moore (5) report that as early as 1926
cultures of S, aureus, g. ggli,‘§. subtilis and §._prodigiosus
were killed by short exposures to cathode rays. Wyckoff and
Rivers (25) reported that the absorption of a single 155 kilo—
volt electron was sufficient to cause the death of'g. 321$ and
‘g. aertryke and that all, or nearly all the electrons were
lethal to the bacteria. Dunn gt gl. (6) showed that energetic
electrons are capable of destroying bacteria.

According to Dunn, Campbell, Fram and Hutchins (6)
high energy cathode rays offer an effective means of pro-
ducing biological, bacteriological, and even chemical changes
on a practical scale.‘Phey report that these changes are brought
about by excitation and ionization of constituent atoms of the
absorber.

Several investigators have studied the effect of tem-
perature upon irradiation. Rentschler §£_§1. (20) report
temperature has little if any effect on radiation between

5°C. (u1.c°F.) and 37°C. (98.6°F.). Bachofer (1) experimented

 

 

 

11

in the same temperature range and concluded that temperature
has no effect on the dose inactivation constant of Q. ggli by
Xfirays.

Koller (13) reported that when bacteria are subjected
to any lethal agent, such as heat, disinfectants, X—rays,
ultraviolet light, and (5 rays they do not all die at once
but a constant fraction of those present dies in each increment
of time. The fraction of the number initially present which
survives at any given time is called the survival ratio (N/No).
.The survival ratio is an exponential function of time of exp
posure and the intensity of the lethal agent. It takes the

form:

N/N ‘e-Klt 0.000(3)

0

where N t the number of bacteria initially present

c number surviving after time (t)

2'.

C,-

= the time of exposure to the lethal agent

K - dose inactivation constant

I 3 the intensity of the lethal agent
Th6 dose is the product of the intensity of the radiation and
the exposure time. Koller (13) further reports that within
wide limits the Bunsen-Roscoe reciprocity law applies to the
killing of bacteria. The Bunsen-Roscoe reciprocity law states
that a given dose results in a given survival ratio, regard-
less of whether the dose consists of a low intensity for a

long time or a high intensity for a correspondingly shorter

 

 

 

 

 

 

 

 

 

 

 

 

12

time. For bacteria, this law has been found to apply over a
thousand fold range in intensity.

Lea (15) concluded that the survival of bacteria is
not dose—rate dependent. This fact has been confirmed with
g,‘ggli over a dose range of 8,000-fold (2). These results
are similar to those reported by Hollaender, 23 21. (12) for
x-rays.

The inactivation of bacteria by ionization is not
clearly understood. Although first regarded as actual killing
of the organism, it is now generally regarded to be a process
which prevents the multiplication of the cells so that normal
dying occurs before multiplication has taken place. This
theory is thoroughly discussed by Lea (15) who considers the
lethal action (in single—celled organism, e.g., bacteria) to
be a "lethal mutation." Since it had been determined that
the survival of bacteria is not dose-rate dependent, Lea (15)
and others have emphasized that not only is the survival curve
exponential but that consequently the "single hit" theory is
valid. -

It has been pointed out that the most convenient
method for expressing the sensitivity of organisms to irradi-
ation is as "mean lethal dose," 1.6., the dose necessary to

reduce the number of survivors to 37 percent of the original.

The relation is expressed by
N/No - e-D/D° coo-0(a)

 

13

where N - number of survivors of dose D
No 8 original number of bacteria

Do I dose at which there is an average of one effective
hit per organism

Although Lea proposes the "single hit" theory he
recognizes the possibility of other mechanisms as well. How-
ever Rahn (l9) proceeds to bring considerable experimental
evidence to bear in favor of the single hit theory in the case
of one celled organism and relegates the other theories con-
cerning the mechanism of death to multi-cellular organisms.
He concluded that a logarithmic order of death can be obtained
only if the death of the bacteria is brought about by the re-
action of a single molecule. The logarithmic order of death

is entirely impossible if more than one molecule must be inac-

 

tivated to produce the death of the cell. The logarithmic
order of death of bacteria can be established only by counting
the survivors, and the only practical method in use is the
plate count method which gives the number of colonies develop-
ing from a known volume of bacterial suspension. The number
of colonies represent the number of original bacteria only
when the bacteria are single. Clumping should be avoided.

It has been demonstrated by Sherman and Albus (22)
that young bacterial cells succumb more readily to harmful
influences than old cells. The term "old cells" is applied
to cells of cultures which have nearly or completely reached

the maximum population.

 

According to Hahn (19), another characteristic of
the direct hit theory is the concentration of organisms does
not effect the percent survival. Lee (15) states that the
fraction of the organisms in an aqueous suspension which are
killed by a given dose of radiation is independent of the con-
centration of the organisms in the suspension, indicating that
the death of a bacterium is due to the energy dissipated by the
radiation in the bacterium itself, and is not an indirect ac-
tion due to the dissipation of energy in the water.

Occasionally survival curves deviating systematically
from.the exponential shape have been reported. Gates (8)
explains that the deviation takes the form of the fraction of
organisms killed by a given increment of dose being less at
small doses than at large doses, so that a sigmoid survival
curve is obtained. It is probable that exponential survival
is the typical result, and that the occasional sigmoid curve
is due to some disturbing factor. Hahn (l9) mentions that‘
one such disturbing factor is clumping of the organisms. If
the proportion of organisms which survive a given dose is the
exponential function e'x , where x is proportional to the
dose, then the probability that an individual organism be
killed by this dose is (l-e‘x). But if the organism exists
in clumps of n individuals, the probability of all n
organisms of a clump being killed will be (l—e'x)n. There-

fore the proportion of the clumps which produce colonies after

 

 

15

n’). This func-

a dose proportional to x will be [l-(l-e'x)
tion represents a sigmoid curve, not an exponential one.

Trump and Van de Graaff (2h) report that the photo-
chemical and the biological reactions produced by the absorp-
tion of Xprays and cathode-rays are similar in their physical
nature and require closely equivalent energies. In cathode-
ray irradiation, electrons are directly projected into the
material and produce ionization similar in its general charac-
teristics to that produced by X-rays wherein the energetic
electron originates within the absorber. Each high energy
electron in the cathode-ray stream proceeds into the material
losing energy by collision with atoms in its path. These pri-
mary electrons thus distribute the energy of cathode rays
through the volume of the absorber. Many of the secondary
electrons produced in these encounters may themselves possess
sufficient energy to act as biological agents by the ionization
of other atoms. They further report that in the case of ini-
tially parallel cathode rays of homogeneous energy, the maxi-
mum ionization density produced in an absorber occurs at about
one-third the maximum range. This location of the region of
maximum ionization toward the forward part of the maximum
range for any given voltage is due to the naturally high
scattering tendencies of electrons.

Absorption of ionizing radiations has been measured
by numerous investigators using various absorbing substances.

It is reported by Halliday (11) that electrons involved in

 

l6

radioactive decay (0.05 to 10 mev.) are penetrating enough
so that solid absorbers are convenient. Aluminum is the usual
choice, although mica, cellophane or colloid films are useful
at very low energies. The technique is to place absorber foils
between a source and a thin-windowed detector, usually a
Geiger counter or an ionization chamber. By relating the
activity to the thickness of the absorber, for various ab-
sorber thicknesses, the relative distribution-in-depth can
be determined. For electrons (cathode rays) there is a maxi-
mum absorber thickness, beyond which electrons will not pene-
trate. This thickness is called the "range." According to
Glendenin (10) absorber thicknesses are often given in terms
of an areal density (grams per square centimeter etc.) partly
because it is easy to find this quantity by weighing thin
foils and partly because thicknesses so expressed are roughly
independent of the nature of the absorber.

The penetration (maximum range) of cathode rays into
matter can be determined by the following equation (Evans, l9h7).

Rmax - 0.5hE - 0.15 .....(S)

where Rmax I the maximum range expressed in grams per square
centimeter
E - the voltage expressed in megavolts
Cathode rays may be either moncenergetic or non-mono-

energetic depending upon the characteristic of the accelerating

 

 

 

 

 

 

 

17

potential. In either case the distribution of energy within
an absorber has been found to be non-uniform.

The non-uniform distribution of ionization with depth
and the maximum range for an absorber (aluminum) is shown in
Figure l by Trump g3 gl. (23). Each of these curves represent
monoenergetic, cathode rays at the three different accelerating
potentials (l, 2 and 3 mev.). The range for each energy is
found to agree very well with the empirical equation reported
by Evans (7).

The resonant transformer type of electron accelerator,
used in this investigation, produces non-monoenergttic cathode
rays. The accelerating potential is supplied by a transformer
having a 1 mev. peak voltage, 180 cycle per second, sinusoidal
wave form. Figure 2 shows the ionization distribution-in-depth
for this apparatus plotted from data furnished with the machine
by the manufacturer. Their data were obtained by absorption

procedure using aluminum foil absorbers.

 

 

100
90
80

70
6‘1
5d

ad

30

Relative ionization density in percent

20

10

 

 

O I I I
O 0.1 0.2 0.3 Oeh UtE
Penetration depth in Gm/cm2

Fig. 2. Relative ionization density of 1 mev.
non-monoenergetic cathode rays in matter.

 

 

OBJECTIVES

To determine the depth distribution of the lethal effect

of one million electron-volt cathode rays on bacteria.

To compare the depth distribution of the lethal effect

with the relative ionization-in-depth curve obtained by

other methods.

 

 

 

20

APPARATUS

Cathode Ray Unit (Electron Beam.Accelerator)

The source of cathode rays was a one million electron-
volt, resonant-transformer type of electron accelerator, manu-
factured by General Electric Company. The apparatus has three
basic electrical components: the transformer and tube unit,
control stand, and motor-generator set.

Figure 3 shows a cutaway view of the transformer and
cathode-ray tube unit assembled within the steel housing.

Flat pancake coils form the transformer primary and the tuned
secondary. The secondary coils are connected by pressure con—
tacts through flat phosphor-bronze terminals; spacing allows
for cooling, and decreases towards the top to maintain the
required potential gradient. Each coil develops about 10,000
volts. Filament emission and current in the central cathode
ray tube is controlled by a variable reactor in a coil that
inductively couples the filament to the primary. The entire
unit is mounted in a steel tank; shielded with overlapping
silicon iron strips to prevent eddy current energy losses.

The secondary (high voltage) circuit is tuned to resonance at
180 cycles per second. Power for the transformer is obtained
from a synchronous motor-generator set. In the twelve-section,
permanently evacuated, cathode ray tube, electrons are pro-

duced by a hot filament, accelerated through the tube by

 

 

 

 

(‘0
H

intermediate electrodes connected to the terminals of the
secondary, and focused by a coil into a beam that passes
through a window to the outside air and irradiation area.

It was mentioned in the Review of Literature that
cathode rays may be either monoenergetic or non-monoenergetic'
depending upon the nature of the accelerating potential. blee-
trons accelerated by a constant potential are monoenergetic.
An absorption curve for these is shown in Figure 1,

The accelerating voltage in the cathode—ray apparatus
used for this work varied sinusoidally, consequently the elec-
tron beam is non-monoenergetic. Figure 2 shows the relative
ionization densities for one million electron-volt electrons.
This curve was obtained by absorption procedure using aluminum

foil absorbers.

Irradiation-Absorption Chamber

A chamber, shown in Figure A, consists of a plastic
(plexiglass) holding fixture (A) in which are assembled alter-
nate layers of bacteria-laden filter discs (B) and thin sheets
of polyethelene separators (C). This laminated stack of 20
filter discs and 20 separators form a cathode-ray absorber
wherein ionization occurs. The total depth of the stack ex-
ceeds the maximum range for the most energetic electron. by
depositing equal bacterial populations on each disc and keeping
each disc isolated from adjacent ones by the polyethelene
separators, the lethal effect of ionization at various depths

could be determined.

 

m
m

The depth of the chamber in which the stack of filter

  

discs and separators fit was h.0 millimeters. The membrane
filter discs shown in Figure Slused to retain the bacteria,
were h7.0 millimeters in diameter and 0.129 millimeters thick.
When an aqueous suspension of bacteria was filtered through
these discs, the organisms were retained. A grid is printed
on the discs to facilitate colony counting. The separators,
cut from sheet polyethelene, measure h7.0 millimeters in diam-
eter and 0.0559 millimeters thick.

A stack of 20 (iisczs and 20 separators weighs 7.703
grams, when the ditscs were saturated with aqueous suspension.
The bulk density of the water saturated discs and separators,
was 1.20 grams per cubic centimeter. The thickness of one
separator and one saturated disc, when using areal density as
a measure of thickness, was 0.02220 grams per square centimeter.
The thickness of the entire stack of 20 separators and 20
filter discs then becomes 0.hhh0 grams per square centimeter.

A chamber loaded with filter discs and separators is shown in

Figure 6.

 

 

  

  
  
 
 
  
 
 
  
 
 
  

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envelope

drive motor

   

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I

Fig. 3. Cutaway view of a l mev.
cathode ray apparatus.*

 

*Permission to reprint this drawing was granted Yip
by General Electric Company by letter from their 'g . _ ‘
~, f‘“ -' i

 

Milwaukee office, 1-21-57. . .9 .

 

 

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Fig. A. Irradiation-absorption chamber

 

 

 

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Fig. 5. Membrane filter disc. . ' ;

       

Fig. 6. Irradiation-absorption chamber
loaded with bacteria-laden
filter discs.

26

PROCEDURE

Determining the distribution-inpdepth of the lethal

 

effect involved four main steps: 1) preparing the culture,
2) preparing the irradiation-absorption chamber, 3) irradiating

the organisms, and h) incubating and counting the colonies.
Preparing the Culture

The test organism selected for this work was Scrratia
marceggggg, which produces a red pigment. It was chosen be-
cause it is nonpathogenic and possesses a distinctive color.
The numerous steps in the procedure made contamination of the
filter discs a likely possibility. To obviate this difficulty
the color characteristic was relied upon to distinguish the
test organism from contaminants.

Organisms from refrigerated cultures were transplanted
into 5 milliliters of nutrient broth in test tubes. These
were shaken on a Burrell, wrist-action shaker at 28 to 39°C
(82.h° to 86.0°F.) for 8 hours. One milliliter was trans-
ferred to another fresh test tube containing 5 milliliters of
nutrient broth. This was shaken for 2h hours at 28-30°C. A
one to 20 million dilution in phosphate buffer was used as
the stock inoculum. This technique produced a bacterial popu-
lation of approximately 80 to 120 organisms per milliliter of

solution. Three milliliters of the stock solution were filtered

27

'through the filter discs. Tests on the filtrate indicated

100 percent retention of the organisms on the filter disc.

'

Preparing the Chamber

Alternate layers of bacteria-laden filter discs and
sterilized polyethylene separators were placed in the chamber.
.All filter discs were placed in the chamber with their inocu-
lated surface towards the chamber window. In this position,
the cathode rays were incident to the inoculated surfaces. The
pressure was just sufficient to hold the laminae firmly together,
yet not tightly enough to force solution from.the discs. The
back of the chamber was then fixed into place. Thus.the chamber
when loaded with 20 filter discs and 20 separators constituted
a bacteria—laden, cathode ray absorber. The depth distribution
of the lethal effect of cathode rays could be ascertained by
comparing the survivors in each layer to the number of bacteria
in non-irradiated control discs. A loademi chamber is shown in

Figure 6.
Irradiating the Organisms

The chamber was positioned in the cathode ray beam
with the chamber window facing the beam source. In this posi-
tion the cathode rays entered the laminae through a polyethylene
sheet 0.0559 millimeters thick before impinging on the first
disc. The chamber rested on a wooden block 1.0 inch thick to

minimize any back scattering of electrons that might occur.

 

 

 

 

 

Several preliminary tests were made to ascertain the optimum
dose. The ideal dose was that at which some organisms were
killed throughout the stack except on the 19th and 20th filter
disc which were farthest from the beam source and beyond the
maximum range for one million volt electrons. The optimum dose
was found to be 5000 rep.

This dose was administered by locating the chamber
M7 centimeters from the cathode ray machine window. The
central axis of the chamber was parallel and in alignment
with the longitudinal axis through the cathode ray tube. In
this position the beam was incident at a right angle to the
face of the chamber and filter discs. The beam-out current
was maintained at 10 milliamperes for 10 seconds. The tube

potential was one million volts peak.
Incubating and Counting Colonies

After the chambers were irradiated, they were disas-
sembled and the filter discs were placed in miniature Petri
dishes on pads moistened with 2 milliliters of nutrient broth.
The plates were incubated at 28—32°C for h8 to 72 hours.
Colonies were counted at hB hours and checked at 72 hours.

The only noticeable difference at 72 hours was a more pronounced
pigmentation and slightly larger colonies. Although a pigmented
organism was selected to obviate contamination difficulties no

such problem was encountered.

 

 

29

DATA AND DISCUSSION OF RESULTS

Ten irradiation-absorption chambers, each prepared as
described in the Procedure were irradiated by one mev., none
monoenergetic cathode rays at a dosage of 5000 rep., at the'
incident surface. The colonies, which developed upon each
filter disc were counted and related to the colony count of
the controls. The percentage of the bacteria surviving on
each disc within each chamber is reported in Table I. The
ten experiments are noted as A through J. Since this work
concerns the lethal effect at various depths within an
irradiation-absorber, the depth from the face of the chamber
to each inoculated filter disc is also given in Table I.

The observations are shown graphically in Figure 7.
The experimental data are reported in percent of bacteria
surviving. These were converted to percent destroyed and both
the relative survivors and the relative bacteria destroyed
were plotted against‘the depth.

In Figure 8 are shown 15 filter discs bearing bacterial
colonies. The top left one is a top filter disc, the next one
in the top row is the next disc below it in the chamber and
so on, counting from left to right. Some of the discs in the
bottom strata of the chamber are not shown since their appear-

ance is similar to the last two or three in the series.

 

30

Two objectives were established, namely, 1) to deter-

  

mine the depth distribution of the lethal effect of one million
electron-volt cathode rays on bacteria, and 2) to compare the
depth distribution of the lethal effect with the relative
ionization-in-depth curve obtained by absorption methods.

These shall be discussed in that order.

In Figure 7 it is observed that the relation between
the percentage of the bacteria destroyed and the depth is
non-linear. At the surface, 95.5 percent were killed, in-
creasing to a maximum of 96.5 percent at approximately one-
fifth of the depth of the absorber, and thereafter the percent-
age killed gradually decreased to zero at the maximum depth.

The calculated maximum range for l mev. cathode rays,
using equation (5), was found to be 0.39 grams per square cen-
timeter, whereas the curve shown in Figure 7 indicates the
range to be approximately 0.h3 grams per square centimeter.
There are three main reasons for this apparent discrepancy.
Equation (5) is an empirical one and may not accurately give
the maximum range for electrons having 1 mev. kinetic energy.
Although the accelerating potential applied to the cathode-ray
tube was reported as one million volts, this value can vary,
plus or minus, 50,000 volts (5 percent) owing to uncertainties
in reading the volt meter. Additionally there were chances
for errors in counting colonies when the number of colonies
was large. The latter reason is suspected to be of greater

importance. Observe the scatter of the counts in Figure 7,

 

 

 

 

31

especially from point B, to the point where the curve inter-
sects the depth axis. There are at least two reasons for
widely varying observation in this area, namely coincidence

of multiple bacteria producing only one colony and starving

of the bacteria. The former is obvious. The latter, although
perhaps more subtle, is in evidence from Figure 8. Note that
as the colony count increased beyond a certain population their
size was diminished. The food supply was fixed by the quantity
of nutrient capable of reaching each cell. When the population
was large some organisms starved and did not develop into col-
onies.

The problem of accurately accounting for all bacteria
was anticipated early in the experimental procedure and every
precaution was exercised to minimize it. Such precautions
were limited, however, since when the discs were inoculated
with too few bacteria so few survived the maximum dose that
inaccurate percentages resulted, yet when tne initial popula-
tion was large the difficulty referred to above was experienced.
Since the main interest concerned the upper portion of the curve
it was decided to tolerate the troublesome situation in the
lower portion.

The curve in Figure 7 was drawn through the average
of the observations at each depth. From the above discussion
it becomes obvious that a curve falling somewhere below the
drawn curve, in its lower portion, would more nearly represent

iflne bacteria killed by cathode rays alone. Also such a proposed

 

 

32

displaced curve would probably intersect the depth axis at

a point more nearly in agreement with the calculated maximum
range. However, it was believed that the evident discrepancy
would not effect the qualitative aspects of the work.

Before continuing with the discussion of the first
objective it will be well to consider the second one. .In
Figure 2 is shown the ionization-in-depth relation for one
million electron-volt, non-monoenergetic cathode rays emitted
from the experimental apparatus. Observe that the percentage
ionization increases from approximately 85 percent at the
surface of the absorber to a maximum of 100 percent at near
one-ninth of the maximum range, then decreases gradually to
zero at the maximum range. The lethal distribution curve

shown in Figure 7 possesses these same general characteristics,

'in that the lethal effect was less at the incident surface

than at some distance within the absorber and thereafter de-
creased gradually to zero at or near the maximum range.

From.a comparison of the general shape of the two
curves it might be inferred that an absorption curve for a
given cathode ray source is a faithful measure of tne lethal
distribution-in-depth to be expected within an irradiated
bacteria-laden surface. There are, however, distinct differ-
ences between the two curves which cannot be overlooked.

The ionization curve exhibits a peaked upper portion
While the lethal curve is much broader in that area. Though

130th curves diminish gradually from their maxima, the lethal

 

 

 

 

 

33

curve, as a consequence of its broader top, decreases more
sharply. If the compensating curve proposed earlier were
adopted the falling-off would be even more abrupt. It must
be pointed out that the dashed line portion of the curve in
Figure 3 was extrapolated beyond the available data. Here
again this portion of this curve suffers from the same uncer-
tainties as the experimental curve.

From the likeneSses of the two curves it was inferred
that the ionization curve was a measure of the lethal distri-
bution, yet their differences, just pointed out, cast some
doubt on such a conclusion. Superimposing one curve upon the
other is meaningless and does not eliminate the dilemma. The
lethal effect is related logarithmically to the ionization
density (dose) by equation (2). The plot shown in Figure 9
was prepared, using equation (2) to relate the logarithm of
the dose, taken from Figure 2 to the corresponding percent
bacteria surviving, taken from Figure 7. The dose administered
to the incident surface of the absorption chamber was 5000 rep.
This is represented in Figure 2 as 85 percent relative ioniza-
tion density. The dosages at other relative ionization density
values were obtained. The following example is given to illus-
trate the procedure used to develop Figure 9. The dose repre—
sented by 30 percent relative ionization density in Figure 2
is 1765 rep. Line A-B was drawn on Figure 2. This dimension
‘was transferred to Figure 7. The length of A-B fitted the

curve in Figure 7 at A'-B' indicating that 37 percent of the

 

 

 

31L

    
    
 
  
   
   
  
  
  
  
   
   
  
  
  
  
  
   
   

bacteria survived a dose of 1765 rep. The point representing
37 percent bacteria surviving for a dose of 1765 rep was
plotted in Figure 9. This procedure was repeated at 5 percent
relative ionization density intervals.

It is observed that this semi-logarithmic plot of dose
versus percent bacteria surviving is not a straight line as
predicted by the "single hit" theory. A number of reasons
for this deviation must be explored.

Assume first that the "single hit" theory is a valid
one. The digression from a straight line would occur if there
were clumping of bacteria on the filter discs. In such a case
when an ionization occurred within one organism its attached
companion or companions might not experience a lethal influence.
Even though a bacteria were destroyed a colony would develop
and no accounting of the destruction would have been made. As
a consequence the colony count would have been unduly great.

A direct hit‘supposedly destroys a bacterium. Bacteria
are able to survive in a media which has suitable properties
to sustain life. Consider what might happen when an ionization
does not occur within an organism but in its near vicinity.
Ionization can alter the properties of the environment. If
this alteration changes the environment to one not conducive
to the support of the bacterium it might die as an indirect
result and not because of a direct hit. It could be argued
that this too could be considered a "direct hit" in so far

as one ionization destroyed one bacterium. But when two or

 

 

 

35

more bacteria, not clumped, but closely adjacent, were sub-
jected to a poisonous environment and two or more were des-
troyed, the lethal effect would then be disproportionate to
a "single hit" event. That is, the lethal effect would be
unaccountably higher than predicted by the "direct hit"
theory.

Consider next the "multiple hit" theory, wherein it
is considered necessary that two or more ionization events
must occur within a bacterium to produce death. It is obvious
without belaboring the point that this would contribute to a
colony count disproportionately low with respect to the single
hit concept.

It was not an objective herein to lend support to or
to take issue with any of the theories concerning the mechan-
ism of death of bacteria subjected to the lethal action of
cathode rays. The foregoing conjectures serve only to empha-
size that there are evidently a number of subtle factors which
can contribute to a disproportionate relation between the lethal
dosage administered and the lethal results observed.

Regardless, the digression from a linear relation in
Figure 9 implies that the ionization-in-depth curve obtained
by absorption methods is probably not a reliable measure of
the lethal distribution-in-depth inasmuch as this has been

demonstrated for this test organism.

 

_
_
_
_
_
_
_
_
,.

 

36

0m.a0a 0.a0a 0.00a 0.00 m.s0 0.H0H 0.m0H 0.00 0.00 --- --- mmm:.0 0N
am.:0 0.m0a 0.H0H m.s0 0.Hn 0.50 --- --- 0.0a 0.00 0.00 00H:.0 0H
mm.m0 0.00 --- H.00 --- --- --u --u 0.00s m.00 0.00s 05mm.0 0H
M0.sm --u --- --n --u --u --u --- 0.ma 0.00H m.m0 mom.0 NH
m0.0s --- --- --- --- --- --- 0.m0 0.0a m.00 ~.0~ m:m.0 ea
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00.0m e.m: a.m: :.m: 0.em m.e: --- --- 0.em :.m: «.wm e:mm.0 ma
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sa.0a m.Hm 0.00 0.mm m.0a H.0m --- 0.:H 0.0m m.mm s.aa m0am.0 0H
00.0 e.0 m.ma a.m :.e a.0 0.0a o.0 a.ma m.aa :.0 000H.0 0
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we.a m.0 m.0 0 0 a.m H.m 0.0 0.m 0.: 0 0sa0.0 :
em.m m.m m.a N.H m.a 0.: 0.m 0.0 m.m m.: 0.0 0:m0.0 m
00.: 0.: e.m m.m :.m m.: 0.: :.m 0.m 0.a m.m emm0.0 m
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- .So .Um\mEmao
.25 h. H m o m m G o m « omen .Ho 908.32
comm on Space omen aepaam

a

 

0mm 000m a0 m0<m00 a a4 ameaHaemmH 2mm:
mahm0mn< has 2H headed mpoame> ea ozH>H>m0n «Hmaeoqm a0 meaazaommd

H 3mg.

'Elrr. Ly “rt-bu.) I II[

 

 

 

 

 

 

 

 

 

"7

Bacteria surviving in percent

37
o - 100‘ —
lo— 90
20— so
30—- 7o

0‘
O

11.0"

50-

U1
0

4:-
O

Bacteria destroyed in percent

to
O

90-— 10

 

 

 

100d 0 L 1 1 i .L: j
0 0.1 002 003 00,43 2 005

Penetration depth in Gm/cm2

 

Fig. 7. Relative number of bacteria destroyed
at various depths within the absorber.

 

‘64
v00
4‘. .

c
.4 2
a. .0
.~ .

d

.0

.5 " ,.
-5 n ‘

 

Colonies growing on membrane
filter discs. The number of
colonies on each succeeding
disc, reading from left to
right and top to bottom, il-
lustrates the relative sur-
vivors in adjacent increments
of depth within the absorber.

 

 

 

Dose in rep.

 

$— 39

2000

1000
900
800

700
600

500

 

too

300-

200_

uan I 1 1 l I
a 20 no 60 80 100

Relative number of survivors in percent

 

Fig. 9. Survival curve for g. marcescens
, irradiated with 1 mev. cathode
rays.

 

 

   
   
  
  
  
  
  
  
  
  
  
  
   
   

2.

3.

LL.

ho

CONCLUSIONS

One million electron-volt cathode rays exhibit a lethal
effect on Scrratia marcescens.

When a bacteria-laden substance is irradiated with one
million electron volt, non-monoenergetic, cathode rays

the depth-distribution of the lethal effect is non-linear.
The percentage of bacteria killed reaches a maximum at or
near one-ninth the maximum range of the electron beam and
then gradually decreases to zero at or near the maximum
range for one million electron-volt cathode rays.

When the lethal distribution is compared to the ionization
density distribution the lethal effect is disproportionate
to the ionization density (dose).

Using S. marcescens for a test organism, it is indicated
that a cathode-ray absorption curve alone is not a reli-
able measure of the relative lethal distribution within

a bacteria-laden absorber.

 

 

  

41

SUMMARY

  

Numerous investigators have shown that cathode rays
exhibit a lethal effect on bacteria. Cathode rays consist of
parallel beams of high velocity electrons, whose high velocity
was attained by having been accelerated through a high electric
potential. Their maximum depth of penetration, called range,
is a function of their initial kinetic energy and the density
of the substance in which they are absorbed. The lethal property
is attributed to ionization that occurs within the absorber.
Because cathode rays possess these properties they have been
proposed as a means for sterilizing foods.

When a high velocity electron traverses a substance,
1t ionizes atoms along its path and ejects other electrons,
called secondary electrons, in the process. The secondary
electrons may also possess sufficient energy to ionize still
other atoms in their paths. The mass of an electron is very
small compared to the mass of the atoms with which it collides,
consequently it takes a zig-zag course within the absorber.
As a consequence of a meandering path and secondary and ter-
tiary ionization along the way the depth distribution of ioni-
zation in a substance being bombarded with cathode rays is non-
linear.

The actual distribution of the ionization density has

been measured by absorption techniques. The lethal property

 

  

1+2

  

of cathode rays is attributed to ionization and since the dis-
tribution of ionization within a substance is known to be non-
uniform it becomes significant to determine the distribution-

in-depth of the lethal effect and to compare the distribution

with the ionization-in-depth distribution.

Thin filter discs were inoculated with known popula-
tions of bacteria. Twenty discs were assembled into a chamber
with extremely thin plastic separators between each disc. These
laminae thus constituted a bacteria-laden cathode-ray absorber.
Ten chambers were assembled and irradiated with one million
electron-volt cathode rays at an incident dosage of 5000 rep.
The lethal effect was determined in each successive stratum by
counting the bacterial colonies developing on each disc. By
relating the number of colonies on each disc to the number of
colonies on inoculated, unirradiated control discs, the per—
centage survivors (or percentage destroyed) were determined
for twenty increments of depth. The data obtained by this pro-
cedure, when shown graphically, described the distribution-
in-depth of the lethal effect of cathode rays.

The curve shows that the lethal effect is non-linear.
The relative number of bacteria destroyed increased from a
certain value at the incident surface to a maximum value near
one-ninth the maximum range for one million electron-volt
cathode rays and then gradually diminished to zero at or near

the maximum range.

LLL

f— e

s

The distribution-in-depth of the lethal effect curve
was compared with the distribution-in-depth of ionization curve.
* The likenesses between the two erroneously indicated that an
absorption curve might be a reliable measure of the lethal dis—
tribution within a bacteria-laden substance.

The two curves could not be directly compared. It was
L necessary to graphically relate the dosage values taken from

the ionization density curve to the corresponding percentage

 

of bacteria surviving taken from the lethal curve, by using
a logarithmic relationship. When this was done the relation-
ship between the log dose and the relative survivors was shown
to be non-linear. The digression from a linear relation became
the discriminating criterion which denoted that the ionization

- density was disproportionate to the lethal effect. The cathode-
ray absorption curve alone is not a reliable measure of the

relative lethal distribution within a substance.

 

 

  

  

SUGGESTIONS FOR FURTHER RESEARCH

During the course of this work certain unanticipated
technical problems developed. These were principally con-
cerned with the bacteriological aspect of the undertaking.

It was mentioned in the section on Procedure that an organism
was selected which developed a characteristic pigmentation so
as to preclude, what was at first believed to be, a potential
contamination problem. However, during the investigation it
appeared that another characteristic would have saved consider-
able trouble. This added characteristic should have been that
the bacterium be highly heat resistant. There is evidence

that bacteria which are highly heat resistant are also highly
resistant to cathode rays. If this is true, then the bacterial
suspensions would probably have been more stable, that is, the
initial bacterial population used to inoculate the filter

discs would probably have been kept constant more easily.

It is therefore recommended, that if this work is re-
peated, the researcher select a strain of bacterium which is
highly heat resistant.

This reasoning leads to still a second recommendation;
that, instead of using vegetative bacteria as a test organism,
a spore-former organism be used. This might provide an even

greater degree of stability.

.n

 

  

15'

GLOSSARY OF TERMS

  

Ionization

 

An atom consists of a positively charged nucleus and
a surrounding constellation of negative electrons, the whole
being electrically neutral. Ionization is the loss by an atom
of one or more of these electrons. The principal means of
energy dissipation by an ionizing radiation in its passage
through matter is the ejection of electrons from atoms through
which it passes. An atom so ionized is left positively charged,
and is called an ion.

The electron which is ejected from an atom in the
process of ionization eventually becomes attached to another
atom and makes it a negative ion. As far as the physical
measurement of ionization is concerned, the positive and
negative ions are equally significant, thus one usually speaks
of an ion-pair. But since the energy involved in the attach-
ment of an electron to an atom to form a negative ion is usually
even less than the energy of excitation, according to Lea (15)
it is probably safe to neglect negative-ion formation as a
factor of biological importance. Thus ionization refers to
the production of a positive ion by the ejection of an electron,
therefore it is the positive ions which are of biological im—

portance.

 

he

Excitation

Radiations dissipate energy in matter by another process
in addition to ionization, i.e., by excitation. This means
the raising of an electron in an atom or molecule to a state
of higher energy, but by an amount insufficient to free it
from the atom or molecule. According to Lea (15) it appears
probable that when dealing with an ionizing radiation, excita—
tion may usually be neglected as a cause of biological effect

by comparison with ionization.

Primary ion

The electron originating at the source. In the case
of cathode-rays it is the electron which is accelerated in the

cathode ray accelerator and hurled into matter.

Secondary ion

The ion formed as a result of ionization by the primary
ion. In the case of cathode rays, it is the ion formed by the
ionization of the electrons emitted by the accelerator as it
passes through the matter. Secondary ions (electrons) may
themselves have sufficient energy to ionize still other atoms.
These resulting ions are called tertiary ions. In this work
all ions except the primary electron shall be considered as

secondary ions. Thus, secondary ions include tertiary ions as

well.

 

 

 

Qggg (dosage)

The radiation delivered to a specified area or volume,
or to the whole body. The unit for dose specifications is
roentgen for X- or \’-rays; and the unit used for (aparticles

is rep (roentgen equivalent physical) (lh)«A

  
 
   
  
   
  
  
  
  
  
   
 
  

Roentgen (r)
That quantity of X- or gamma radiation for which the

associated corpuscular emission per 0.001293 gram of air
produces, in air, ions carrying one electrostatic unit of

electricity of either sign (1h).

Roentgen equivalent physical (rep)

 

A unit to apply to doses of ionizing radiations not
covered by the roentgen. There is some confusion as to its
definition, but it is best taken as the absorption of 93 ergs
of energy per gram of body tissue. The choice of 93 ergs per
gram is not arbitrary but is made by assuming that all energy
absorption is proportional to the electron density (electrons
per gram) of the absorber. Electron density is essentially
proportional to Z/A and hence for air I 1/2. For simplicity,
we assume tissue to be equivalent to water for which Z/A 8 10/18.
Then since it has been shown that one roentgen is equivalent
to the absorption of 83 ergs per gram of air we have energy

absorption per gram of water

10 18 _
_lé2-_ x 83 92.2 ergs.

A more exact calculation for tissue yields 93 ergs per gram (1h).

 

  

1+8

  

Electron-volt
Is the amount of work done when an electron is accel-
erated by a potential difference of one volt. One electron-

volt is equal to 1.6018 x 10'12 erg.

Cathode rays

Are streams of electrons accelerated to high velocity,
under the influence of an applied potential difference. At
this point, the distinction between cathode rays and (5rays
should be clarified. Both are electrons. Their difference
lies in their source. Beta rays are electrons emitted from radio-
active substances while cathode rays are electrons induced to
high velocity by man-made devices. Beta rays and cathode rays
having the same energies exhibit the same properties in their

passage through matter.

29am
Death cannot be defined by positive criteria; it can

be characterized only by the absence of some property which

is essential to life. The death of a cell is not always

determined by the same method. Thus, a cell can be alive ac-

cording to one definition, and dead according to another. The

criterion almost universally used by bacteriologists to define

death is the loss of reproduction. For purposes herein, a cell

is dead when it is sterile, i.e., permanently unable to repro-

duce, and death shall be in evidence if a colony will not

develop when a bacterium is transplanted to a suitable media (19).

 

  
 
  
   
    
  
   
   
   
    
    
   
    
   
  
   
   
 
   
   
    
     

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10.

 

11.

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12.

13.

1h.

 

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