DALAT UNIVERSITY
FACULTY OF NUCLEAR ENGINEERING
NGUYEN DANG TRONG PHUC – 1510402
DISINFECTION OF FOODS BY
LOW ENERGY X-RAY IRRADIATION
ENGINEERING THESIS
SUPERVISOR
PROFESSOR TAMIKAZU KUME
TUTOR
PHAM THI NGOC HA
2015 - 2020
REMARKS OF SUPERVISOR
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REMARKS OF LECTURER
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ii
ABSTRACT
The reliable supply of safe food that is free from harmful contaminants is
important for people‘s health and daily life. The contamination microorganism on
food is unavoidable because food are contaminated in production, transportation or
preservation process. Food irradiation is an effective method to eliminate the
harmful microorganisms and extend their shelf-life. Disinfection of foods by
radiation is widely used because of its safety and cost-efficiency.
The low energy X-ray irradiator was installed at the Nuclear Engineering
Department, Da Lat University in 2017. The low energy X-ray irradiator is compact
and cheap compared to the 60Co-gamma-ray irradiator or high energy electron beam
irradiator. However, the effective irradiation condition is required to study because
the penetration and dose rate is low.
The disinfection of foods by low energy X-ray irradiator depends on many
factors such as voltage, current, distance from the irradiation port and thickness of
foods. Experimental data of this research show that the voltage of the X-ray tube
and the filters to cut off the low energy X-rays are two important factors affect the
disinfection of E. coli. The low energy X-ray radiation without filter was effective
for the surface (5 mm thickness) irradiation. For the irradiation of thick materials,
the filter such as 1.0 mm Al or 2.0 mm Al is necessary to obtain a better dose
uniformity ratio (Dmax / Dmin).
Keyword: Food irradiation, disinfection, microbiological contamination,
X-ray, E. coli, survival rate.
iii
ACKNOWLEDGMENT
First of all, this thesis would not have been possible without the advice and
guidance of my supervisor, Professor Tamikazu Kume. Although I am an
unpredictable person and I had broken promises with him many times, he still was
patient and understanding for me. Thank for his generous and kindness even I had
made many mistakes. I also express my sincere thanks to Ms. Pham Thi Ngoc Ha
who supported me with her knowledge to complete my thesis.
To all the Nuclear Engineering Department professors, lecturers and staff
who cheer me up and give me a lot of invaluable advice in difficult situations, I
really appreciate their supports. I would like to thank Dr. Nguyen An Son, the dean
of our department. I could not finish this thesis without his permission and
mentorship. I am especially grateful for Dr. Ha, Ms. Sang who help and encourage
me whenever I need and had bought me food many times since the thesis begun.
Moreover, I could not finish my thesis without the support of the Biology
Department and Chemistry Department. Especially, Miss Lien, Dr. Truong, Mr.
Giang, Mr. Trieu and Faculty of Biology staff who taught me more about
microbiology and allowed me to use their equipment.
I am thankful to my family and my classmate for their encouragement and
support.
Last but not least, I want to thank University Security officers who guard my
motorbike only left at 8 pm weekdays or weekends.
Finally, I do apologize for any inconvenient which I had caused throughout
my thesis and thank to all for supporting me.
iv
COMMITMENT
I assure that the work embodied in this thesis was my own work carried out
by me under the supervision of Prof. Tamikazu Kume. The content and research
results in this thesis are honest and unpublished in any form before.
In addition, the thesis also uses figures, the number of comments, reviews as
well as data from other authors, agencies, and organizations as cited and annotated.
If any fraud is detected, I bear full responsibility for the contents of my
graduation thesis.
Da lat, December 2th, 2019.
Author,
Nguyen Dang Trong Phuc
v
Contents
ABSTRACT .................................................................................................................................... iii
ACKNOWLEDGEMENT ............................................................................................................. iv
COMMITMENT ............................................................................................................................. v
INTRODUCTION ........................................................................................................................... 1
Chapter 1: Overview.......................................................................................................................... 2
1.1 Ionization radiation:................................................................................................................. 2
1.2 X-ray ........................................................................................................................................ 3
1.3 Biological effect of radiation ................................................................................................... 5
1.4 Food contamination ................................................................................................................. 9
1.5 Dose measurement.................................................................................................................. 9
1.5.1 Fricke dosimeter ............................................................................................................... 9
1.5.2 Gafchromic film dosimeter ............................................................................................. 10
Chapter 2: Materials and Methods .............................................................................................. 13
2.1 X-ray irradiation system ........................................................................................................ 13
2.2 Dosimetry .............................................................................................................................. 15
2.2.1 Fricke dosimetry ............................................................................................................. 15
2.2.2 Gafchromic film dosimetry ............................................................................................ 17
2.3 Microorganisms experiment .................................................................................................. 18
2.3.1 Microorganism sample preparation ................................................................................ 18
Chapter 3: Results and discussion ............................................................................................... 20
3.1 Dosimetry .............................................................................................................................. 20
3.1.1. Fricke dosimetry ............................................................................................................ 20
3.1.3 Gafchromic film dosimetry ............................................................................................ 25
3.2 Disinfection of E. coli by radiation ....................................................................................... 29
3.2.1 Filter effect on E. coli radiosensitivity ........................................................................... 29
3.2.2 Energy effect .................................................................................................................. 32
3.2.3 Effect of thickness of the sample ................................................................................... 35
3.3 Discussion.............................................................................................................................. 37
CONCLUSION .............................................................................................................................. 38
REFERENCES .............................................................................................................................. 39
vi
List of Tables
Table 2..1: Specifications of MBR-1618R-BE X-ray irradiator...................................................... 14
Table 3. 1 Dose and dose rate measured with different filter .......................................................... 21
Table 3. 2 Dose and dose rate in Eppendorf tube at 150 and 250 mm H ........................................ 24
Table 3. 3 Parameters of fitting equation with different measured dose rate .................................. 25
Table 3. 4 Dose rate measured by Gafchromic film at 150 mm H at different condition ............... 27
Table 3. 5 Dose rate distribution in PMMA with different filters ................................................... 27
Table 3. 6 Survival ratio inside the PMMA phantom ...................................................................... 35
vii
List of Figures
Figure 1. 1 Typical Bremssrahlung spectrum .................................................................................... 4
Figure 1. 2 Attenuation and energy-absorption coefficient ............................................................... 5
Figure 1. 3 Biological effect of radiation .......................................................................................... 6
Figure 1. 4 Typical dose response curves .......................................................................................... 8
Figure 1. 5 Configuration of Gafchromic HD-V2 dosimetry film .................................................. 11
Figure 2. 1 Irradiation system from external view .......................................................................... 13
Figure 2. 2 Irradiator‘s spectrum at 160kV, 18.7mA with different filter ....................................... 15
Figure 2. 3 Pyrex tube (left) and Eppendorf tube (right) ................................................................. 16
Figure 2. 4 Dosimetry diagram ........................................................................................................ 17
Figure 2. 5 Plastic bag microorganism container ............................................................................ 18
Figure 3. 1 Dose at 250 mm H (height) of irradiation table ............................................................ 20
Figure 3. 2 X-ray irradiator spectrum at 50kV, 100kV, 160kVat same current .............................. 22
Figure 3. 3 Change in doses by energy with 50, 100 and 160 keV ................................................. 22
Figure 3. 4 Dose and dose rate in Eppendorf tube at 150 and 250 mm H ....................................... 23
Figure 3. 5Dose in Eppendorf tube at different voltage .................................................................. 24
Figure 3. 6 Optical density of each color channel ........................................................................... 26
Figure 3. 7 Dose rate distribution in PMMA ................................................................................... 28
Figure 3. 8 F0 Survival curve .......................................................................................................... 29
Figure 3. 9 F1 Survival curve .......................................................................................................... 30
Figure 3. 10 F2 Survival curve ........................................................................................................ 30
Figure 3. 11 F3 Survival curve ........................................................................................................ 31
Figure 3. 12 F4 Survival curve ........................................................................................................ 31
Figure 3. 13 F5 Survival curve ........................................................................................................ 32
Figure 3. 14 Survival curve at 50 keV ............................................................................................. 33
Figure 3. 15 Survival curve at 100 keV ........................................................................................... 33
Figure 3. 16 Survival rate at different voltage at 150mm from the source ...................................... 34
Figure 3. 17 Survival curve at 150mm and 250mm height ............................................................. 35
viii
INTRODUCTION
Food crisis is a concerned issue around the globe because of overpopulation,
climate change, sea level rising which are causing landfills shortage, natural
disaster. Therefore, sustaining food for the increasing population is a critical
problem for human civilization, there are many solutions for this problem such as
genetic mutation, alternative food resource, lab-grown food production, food
irradiation. Food irradiation is the most practical solution since it reduces food
wasted, chemical pesticide, food poisoning, etc.
Food may contain a great deal of microorganism species, some of them are
harmless, or the other may cause many diseases such as Salmonella,
Campylobacter, Escherichia coli, etc. The contamination depends on a lot of factors
such as type of food, manufacture process, transportation, and so on. To disinfect
food, there are many ways such as heated, refrigerated, frozen, treated chemical,
etc. However, these processes change the taste, color, and decrease nutrition, even
produce toxic chemicals. Ultraviolet light can kill the bacteria or fungus without
side effect but its penetration is the only surface of a material. Higher energy
radiation can solve this drawback, food has been irradiated food for around four
decades, the source can be radioactive isotope which will become radioactive
wastes after several decades using and require complex shielding structure or
electron beam irradiator has low penetration but it is very effective to sterilize
bacteria. Besides these irradiation systems, the 500 kilovolts X-ray irradiators also
are used since they have high penetration like gamma-source and effective to
disinfect bacteria like electron beam and its operation cost is much more
economical when compared with the other two.
The food crisis has also drawn a very hot issue in Vietnam, so food
irradiation is important for the future of our country. Nuclear Engineering
Department, Dalat University installed a low energy X-rays MBR-1618R-BE
Hitachi irradiator in 2017. This machine can be used for many applications and
food irradiation is one of them.
The major purpose of this research is estimating the disinfection ability of
low energy X-ray by using irradiation system. The effect of X-ray on
microorganism can be different between distinct conditions. Escherichia coli
bacteria were used to study these effects.
1
Chapter 1: Overview
1.1 Ionization radiation:
Radiation can be described as the emission of energy as electromagnetic
waves or subatomic particles. As a result, it covers a wide range of categories such
as charged particle radiation, non charged particle radiation, or electromagnetic
waves which include radio waves, visible light, infrared light, ultraviolet lights, and
so on. It can be divided into two simple categories non-ionization radiation and
ionization radiation which have more energy than the counterpart and this type of
radiation have sufficient energy to detach an electron from the atom, molecule,
thereby ionizing them. In nuclear physic field and high energy physic, the radiation
is preferred to ionizing radiation which includes high energy electromagnetic wave,
electron beam( beta ray), alpha ray (helium nucleus), etc. Although the effect of
radiation on matter was recognized a few decades ago, the first discovery of
radiation is 1895.
In 1895, the physicist named Wilhelm Röntgen had found out that
something has come out of cathode tube while testing the penetration of cathoderay through glass. Even covering the Crookes tube in heavy black paper, he noticed
a barium platinocyanide screen nearby fluorescing. Through experimentation, he
found that the mysterious lights would pass through most substances but leave
shadows of solid objects. Because he did not know what the rays were, he called
them ‘X’ meaning ‘unknown’ rays.
This discovery was followed in 1896 by Henri Becquerel’s discovery that
uranium salts gave off similar rays naturally. Though originally thinking that the
rays were emitted by phosphorescent uranium salts after prolonged exposure to the
sun, he eventually threw away this hypothesis. Through further experimentation
including non-phosphorescent uranium, he instead came to recognize that it was the
material itself that radiated the rays. Marie Curie, who was his doctoral student at
that time named this phenomenon radioactivity. She continued her Professor‘s
work, she did lots of experiments with radioactive material, including the discovery
of additional radioactive elements: thorium, polonium, and radium.
In 1899, Ernest Rutherford discovered that there were two kinds of radiation
emitted by radium; one is a heavy particle that has a positive charge, which he
2
called an “alpha ray”, and the other is much lighter, with a negative charge, and he
named it a “beta ray”. One year later, by eliminating alpha ray with lead screen, and
sweeping beta ray away with magnetic field, Paul Villard found that radium still
gave off another radiation which have no charge, and mass go into the detector.
Villard did not name his rays, but Rutherford, who confirmed their existence,
appropriately labeled them “gamma rays”, and gamma rays they have been ever
since.
Until 1932, the neutron had been discovered by James Chadwick when he
repeated the Joliot-Curies beryllium emission experiment. By bombarding
hydrogen, helium, nitrogen, and other elements as a target, and comparing the
energies of recoil particles, he proved that the beryllium emissions contained a
neutral component with a mass approximately equal to that of the proton, he called
it the neutron.
After these explorations, many research, and applications of ionizing
radiation have been innovated. Ionizing radiations have been applied in a lot of
fields like energy production, industry, material science, agriculture, etc.
1.2 X-ray
X-rays are known as Röntgen rays which are electromagnetic waves with
the frequency higher than visible light. X-rays are produced by the de-excitation of
orbital electron of the atom, or the Coulomb interaction between high energy
electron and high atomic number target. The de-excitation X-ray is emitted when
the electron from higher-level shell fill the inside orbital vacancy caused by
radiation, or nucleus transformation, this ray is called characteristic X-ray. The
other mechanism is bremsstrahlung X rays emission where the high energy electron
travel close to the nucleus due to the attraction of its positive charge and are
subsequently influenced by its electric field. The course of these electrons would be
deflected, and a portion or all of their kinetic energy would be lost. These kinetic
energy differential is emitted as X-ray photons, specifically bremsstrahlung
radiation or braking radiation. These processes are how the X-ray tube and some
accelerators give off electromagnetic waves.
Low energy irradiator contains the X-ray tube works as a specific energy
converter, receiving the electrical energy and converting it into two other forms of
energy: X-radiation and heat. This process is inefficient with 1% of the resulting
3
energy is emitted as X-rays, and the remaining 99% converted to heat. Due to the
Bremsstrahlung effect, the energy of X-ray emitted is from zero to the kinetic
energy of the incident electron, resulting in a continuous Bremsstrahlung spectrum.
Figure 1. 1 Typical Bremssrahlung spectrum
Ionizing electromagnetic waves interact with matter in four different ways
from photoelectric, Thomson scattering, Compton scattering, pair production. The
probability of each interaction depends on the energy of incident proton, it is called
interaction cross-section in radiation physic. For the low energy X-ray,
photoelectric absorption and Compton scattering is the dominant interaction. As a
result, the X-ray photon transfer more energy to material than the higher than the
high energy photon (gamma-ray or high-frequency X-ray). The energy transfer can
be described as mass energy-absorption coefficient of the photon which is shown in
Figure 1.2.
4
Figure 1. 2 Attenuation and energy-absorption coefficient
(Measured by United States National Institute of Standard and Technology)
In the figure above, the energy-absorption coefficient decreases
exponentially in the low energy photon reason (<50keV), falls gradually at the
higher energy, and nearly constant around 1MeV.
1.3 Biological effect of radiation
All living entities are made up of protoplasm, which consists of inorganic
and organic compounds dissolved or suspended in water. The smallest unit of
protoplasm capable of independent existence is the cell, the basic microscopic unit
of all living organisms. Although we tend to think of biological effects in terms of
the effect of radiation on living cells, in actuality, ionizing radiation, by definition,
interacts only with atoms by the ionization process. Thus, all biological damage
effects begin with the consequence of radiation interactions with the atoms forming
the cells. Even though all subsequent biological effects can be traced back to the
interaction of radiation with atoms, there are two mechanisms by which radiation
ultimately affects cells. These two mechanisms are commonly called direct and
indirect effects.
5
In the direct effect, the radiation interacts with the atoms of the DNA
molecule, or some other cellular component critical to the survival of the cell. The
damage leads to the chain of physical and chemical events that eventually produce
the biological damage. Particularly, an interaction may affect the ability of the cell
to reproduce and, thus, survive. If enough atoms are affected such that the
chromosomes do not replicate properly, or if there is significant alteration in the
information carried by the DNA molecule, then the cell may be destroyed by
“direct” interference with its life-sustaining system. Direct action is the dominant
process in the interaction of high linear energy transfer (LET) particles with
biological material.
In indirect action, the radiation interacts with other molecules and atoms
(mainly water, since about 80% of a cell is composed of water) within the cell to
produce free radicals, which can, through diffusion in the cell, damage the critical
target within the cell. In interactions of radiation with water, short-lived yet
extremely reactive free radicals such as H2O+(water ion) and OH* (hydroxyl
radical) are produced. The free radicals in turn can cause damage to the target
within the cell. The free radicals that break the chemical bonds and produce
chemical changes that lead to biological damage are highly reactive molecules
because they have an unpaired valence electron. However, they could combine to
form toxic substances, such as hydrogen peroxide (H2O2), which can contribute to
the destruction of the cell.
Figure 1. 3 Biological effects of radiation
Figure 1.3 shows that ionizing radiation can (a) directly damage a
biomolecule by ionizing it or breaking its bonds, or (b) create an H2O+ ion, which
reacts with H2O to form a hydroxyl radical, which in turn reacts with the
biomolecule, causing damage indirectly.
6
About two-thirds of the biological damage by low LET radiations (sparsely
ionizing radiations) such as X rays or electrons is due to indirect action.
The consequences of radiation exposure occur immediately, timescale
involved between the breakage of chemical bonds and the biological effect maybe
hours to years, depending on the type of damage. If cell kill is the result, it may
happen in hours to days, when the damaged cell attempts to divide (early effects of
radiation).
A microorganism survival curve describes the relationship between the
surviving fraction of bacteria or fungi cells (i.e. the fraction of irradiated cells that
maintain their reproductive integrity) and the absorbed dose. Microorganism
survival as a function of radiation dose is graphically represented by plotting the
surviving fraction on a logarithmic scale on the ordinate against dose on a linear
scale on the abscissa. The survival curve equation:
S ( D)  e D
Where:
(1.1)
S(D) is the fraction of cells surviving a dose D;
 is a constant describing the initial slope of the cell survival
curve;
The type of radiation influences the shape of the cell survival curve. Densely
ionizing radiations exhibit a cell survival curve that is almost an exponential
function of dose, shown by an almost straight line on the log-linear plot. For
sparsely ionizing radiation, however, the curves show an initial slope followed by a
shoulder region and then become nearly straight at higher doses.
A plot of a biological effect observed (e.g. microorganism mutation, cell
death ) against the dose given is called a dose response curve. Generally, as dose
increases so does the effect.
Three types of dose response relationship are known:

Linear;

Linear quadratic;

Sigmoid.
7
Dose response curves may or may not have a threshold. A threshold dose is
the largest dose for a particular effect studied below which no effect will be
observed.
Various dose response curves are shown in Fig. 1.4, with:

A linear relationship with no threshold;

A linear relationship with a threshold;

A linear-quadratic relationship with no threshold;

A linear relationship (the area below the dashed line indicates
the natural incidence of the effect);

A sigmoid relationship with a threshold.
Figure 1. 4 Typical dose response curves
Curve A represents a linear relationship with no threshold; curve B
represents a linear relationship with threshold DT ; curve C represents a linearquadratic relationship with no threshold (assumed for stochastic effects); curve D
represents a linear relationship with no threshold (the area below the dashed line
represents the natural incidence of the effect, for example, carcinogenesis); and
curve E represents a sigmoid relationship with threshold D1, as is common for
8
deterministic effects in organism tissues. The curves are diagrammatic only and are
separated for clarity (in practice the dashed line would be lower).
The response of different species or strains to radiation varies markedly,
depending primarily on the inherent sensitivity of the microorganism. Microorganism survival does not only depend on the microorganism itself but the
environment and radiation also affect the response. For example, oxygen
concentration in the environment, effect of nearby cell, or dose rate effect.
The higher energy transfer the more damage is caused to the microorganism,
and the response to radiation also depends on the cells. The relative biological
effectiveness (RBE) compares the dose of test radiation to the dose of standard
radiation to produce the same biological effect.
1.4 Food contamination
Food is contaminated by toxic metals, pesticides, and veterinary drug
residues, as well as organic pollutants and mycotoxins. Ensuring food safety and
quality is a central public health concern. The major food contamination is related
to bacteria, fungus, or parasite. Microbiological contamination of food by
pathogens such as Salmonella, Staphylococcus or E. coli has significantly increased
over the past decades. There are many processes to disinfect microbiological
contamination, food radiation is a practical method to sterilize microorganisms.
Sterilization is the complete elimination or removal of all forms of
contaminating microorganisms from the foodstuff, container and food package. A
sterility assurance level (SAL) of 10–6 is currently required for sterilization
procedures, i.e., a probability of not more than one viable microorganism in one
million sterilized items of the final product. The inherent problem with these
requirements is that evaluating the success of such sterilization by means of a final
inspection is all but impossible, since contamination rates on the order of an SAL
of 10–6 cannot be recorded in experiments
1.5 Dose measurement
1.5.1 Fricke dosimeter
To measure absorbed dose in water, Fricke dosimeter is commonly used as a
standard. Fricke solution which has the following composition:
1mM FeSO4 or Fe(NH4)2(SO4)2 (Morh’s salt)
9
0.8N H2SO4
1mM NaCl
As the solution expose to ionizing radiation, ferrous (Fe2+) ion was
converted to ferric (Fe3+) ion. The reaction scheme for this process is assumed as
follows:

H 2O  OH , H , eaq
, H 2O2 , H 2

eaq
 H   H  in acidic medium 
(1.2)
Fe2  OH  Fe3  OH 
Fe2  H 2O2  Fe3  OH   OH
In oxygen presence medium
H  O2  HO2
Fe2  HO2  H   Fe3  H 2O2
(1.3)
In the absence of oxygen
Fe2  H   H   Fe3  H 2
(1.4)
Ferric (Fe3+) ion exhibits a strong absorption peak at 304 nm, whereas
ferrous ions do not show any absorption at this wavelength. Radiation-induced
ferric ion concentration can be determined using spectrophotometry, which
measures the absorbance (in OD units) of the solution.
Fricke dosimeter is affected by organic impurities present in the system
these solutes react with the ferrous ions faster than the radicals. To prevent organic
impurities effect, a small amount of sodium chloride is added in solution.
1.5.2 Gafchromic film dosimeter
Radiochromic film are coated with a special chemical that change properties
under radiation exposure. For example, radiographic X-ray film consists of a base
of thin plastic with a radiation sensitive emulsion (silver bromide (AgBr) grains
suspended in gelatin) coated uniformly on one or both sides of the base. A new type
of film is developed and the films have a wider dose range limit, dose and energy
independence, uniformity. The most commonly used is a Gafchromic film which is
a colorless film with a nearly tissue equivalent composition (9.0% hydrogen, 60.6%
10
carbon, 11.2% nitrogen and 19.2% oxygen) that develops a blue color upon
radiation exposure.
Radiochromic film contains a special dye that is polymerized upon exposure
to radiation. The polymer absorbs light, and the transmission of light through the
film can be measured with a suitable densitometer. Radiochromic film is selfdeveloping, requiring neither developer nor fixer. Since radiochromic film is
grainless, it has a very high resolution and can be used in high dose gradient
regions for dosimetry.
Dosimetry with radiochromic films has a few advantages over radiographic
films, such as ease of use; elimination of the need for darkroom facilities, film
cassettes or film processing; dose rate independence; better energy characteristics
and insensitivity to ambient conditions (although excessive humidity should be
avoided).
Ashland Gafchromic HD-V2 films had been used to measure the absorbed
dose due to its wide dose range from 10 to 1000Gy, low dose and dose rate
dependence. The film is comprised of an active layer, nominally 12μm thick,
containing the active component, marker dye, stabilizers and other components
giving the film its energy-independent response. The thickness of the active layer
may vary slightly from batch-to-batch. The active layer is coated on a clear, 97 μm
polyester substrate.
Figure 1. 5 Configuration of Gafchromic HD-V2 dosimetry film
Ashland company recommend to use 48-bit (16bit per color channel) to
measure optical density. The typical dose response of HD-V2 film on an Epson
color scanner is shown in Figure 2.4. We recommend to fit the calibration data to a
function having the form
d x ( D)  a  b / ( D  c )
11
(1.5)
where dx(D) is the optical density of film in scanner channel x at dose D, and
a, b, c are the equation parameters to be fitted.
Figure 1.6: Response of Gafchromic HD-V2 in all Color Channels
12
Chapter 2: Materials and Methods
2.1 X-ray irradiation system
The irradiator of Nuclear Engineering faculty is an X-ray irradiation system
model MBR-1618R-BE made by Hitachi Corporation. This X-ray irradiator is
mainly designed to emit X-rays for experiments involving cultured cells,
microorganisms, mice, rats, and other specimens. The tube voltage (35 to 160kV),
tube current (1 to 30mA) and irradiation time or irradiation dose are set to emit Xrays.
Figure 2. 1 Irradiation system from external view
(1) Irradiation chamber door
(5) Filter cabinet
(2) Opening/closing handle
(6) LAN connector port
(3) Three-color status indicator lamp
(7) Receptacles
(4) Filter mounting base
(8) Wireless LAN router pocket
Some specifications of MBR-1618R-BE X-ray irradiation system are shown
in Table 2.1.
13
Table 2..1: Specifications of MBR-1618R-BE X-ray irradiator
No.
Item
1 Tube voltage
Specifications
Enables fine-adjustment from 35 to 160kV in 0.1kV
steps.
2 Tube current
Enables fine-adjustment from 1 to 30mA in 0.1mA
steps
3 Maximum power
3kW
4 Turntable
5 Filters
Enable fine-adjustment from 550 to 250 mm height in
1mm steps. ( 150mmH with 100mm pedestal)
F1 Aℓ1mm
F2 Aℓ2mm
F3 Aℓ0.5mm+Cu0.1mm,
F4 Aℓ0.5mm+Cu0.2mm
F5 Aℓ0.5mm+Cu0.3mm
6 Air kerma
Approx. 8.5Gy/min
(at 160kV,18.7mA, filter F1, 250mmH)
Irradiation system radiate X-ray by Bremsstrahlung effect which produces a
continuous spectrum below:
14
Figure 2. 2 Irradiator‘s spectrum at 160kV, 18.7mA with different filter
2.2 Dosimetry
2.2.1 Fricke dosimetry
Since Fricke ferrous solution is sensitivity to impurity, Pyrex tube is used as
standard (Figure 2.3).
15
Figure 2. 3 Pyrex tube (left) and Eppendorf tube (right)
Pyrex tube was made of Borosilicate with a length of about 10.9 cm and a
diameter of 15.4 mm. Eppendorf tube was made of Polypropylene with length
around 3.8cm and diameter of 9.1 mm (11.2 cap diameter).
The dose uniform area of the irradiator is a circle with 8 cm diameter at
150mm from the window. Pyrex tube is slightly longer than this and it has a large
volume which requires prepare a great deal of microorganism solution. Therefore,
the Eppendorf tube is used for microorganisms test due to its small volume and
availability. Eppendorf tube is made of plastic instead of glass which some low
energy X-rays can penetrate.
The height of turntable was adjusted to both of tubes center at the same
distance from the source (Figure 2.4).
16
Figure 2. 4 Dosimetry diagram
After irradiation, the Fe3+ concentration was measured by UV-VIS
spectrometer by measuring Fe3+ absorbance at 304nm wavelength. The absorbed
dose in solution was calculated by equation:
D (Gy ) 
2.77 10 2 A
1  0.007(T  25)
Wher D is absorbed dose
A is absorbance difference between irradiated solution and control solution
T is the temperature of solution in Celsius degree when measure the
absorbance
2.2.2 Gafchromic film dosimetry
Ashland Gafchromic film HD-V2 has a dynamic dose range from 10 to 1000
Gy, high dose rate and energy independence. After irradiation at the center of the
turntable, the film is kept for 24 hours and the color change was scanned by scanner
KONICA MINOLTA Biz195 at 24-bit color resolution (8-bit per channel). Then
the optical density was analyzed from the scan picture by ColorPic v4.1 software.
The measured optical density is used to calculate dose by using the equation 1.5.
To identify the dose distribution in material, the film was sandwiched
between PMMA plates Due to living organic material and PMMA have the
17
component ratio of carbon, hydrogen and oxygen, the PMMA can be used as an
organic material phantom.
2.3 Microorganisms experiment
2.3.1 Microorganism sample preparation
E.coli K12 strain is obtained from Osaka Prefecture University and this
strain is harmless to human. The growth medium for E. coli is the Lysogeny broth
(LB) which contain 0.5g sodium chloride, 0.5g Yeast extract, 1g Peptone per 100
ml. For the preparation of irradiation sample, E. coli was incubated in LB broth at
30 oC for 15 hours.
2.3.1.1 Microorganism sample in Eppendorf tubes
As the broth affects the radiation sensitivity of E. coli, the cell suspension
was centrifuged at 5,000 rpm for 5 minutes and washed three times by sterilized
distilled water. The pure final microorganism liquid was put into the sterilized
Eppendorf tubes and sealed with parafilm. These prepared samples are used for
irradiation.
2.3.1.2 Plastic bag sample
To estimate the depth dose effect of radiation, a thin layer sample was
prepared by using a heat-resistant plastic bag (Figure 2.5).
Figure 2. 5 Plastic bag microorganism container
The microorganism suspension was put into these sterilized bag without
centrifuging. The bag with 200l microorganism was heat-sealed completely.
PMMA plates were used as a phantom to study the depth dose effect. The plastic
18
bag samples were placed at the top, middle and bottom of the phantom. Then the
phantom system was irradiated at 150mmH, 160kV, 18.7mA, F0.
19
Chapter 3: Results and discussion
3.1 Dosimetry
3.1.1. Fricke dosimetry
As the Fricke dosimeter is a most common liquid dosimeter, it was used as a
standard measurement because of its precision and wide measurable dose range.
Additionally, food and agriculture products contain water, and microorganism
samples are suspension in water. Fricke dosimeter is used to determine the
absorbed doses in the samples.
3.1.1.1 Absorbed dose measurement
The Fricke dosimetry solution is highly sensitive to impurities,
particularly organic impurities. Pyrex tube is recommended to make Fricke
dosimetry system since Pyrex tubes were made of high-quality glass.
- Dose rate at different height of irradiation position
Absorbed doses are measured at different height of irradiation table
because the dose is decreased by the inverse square law. Figure 3.1 shows
the relationship of irradiation time vs dose.
F1 250mmH
F1 150mmH
200
y=16.828x-2.409
R2=0.9991
Dose (Gy)
150
100
y=6.335x-0.662
R2=0.9998
50
0
0
5
10
15
20
25
30
Time (Mins)
Figure 3. 1 Dose at 250 mm H (height) of the irradiation table
(160kV, 18.7mA, filter F1)
The data fit well in linear (R2 = 0.9998) and the dose rate at 250mm H
obtained from the equation is 6.335 Gy/minute. The dose rate at 150mm H is
16.828 Gy/minute is about 2.66 times higher than that at 250mm H. It nearly fits
20
with the ratio 2.78 of inverse square law. The ratio measured is less than theory
because the X-ray beam goes through air inside the irradiator chamber, so some
energy is transferred to air molecules.
- Effect of filter to cut the low energy X-rays
The filters block low energy of X-rays emitted by the Röntgen tube and
some proportion of higher-energy X-rays is also absorbed. The decrease of dose by
various filters is shown in Table 3.2.
Table 3. 1 Dose and dose rate measured with different filter
(160keV, 18.7mA, 150mm H)
Time
(minute)
5
10
Dose rate
(Gy/min)
Dose (Gy)
F0
109.10
F1
F3
F4
F5
58.57
43.57
33.95
40.53
208.65 170.047 111.48
95.63
65.93
86.04
9.3
6.64
8.82
21.01
84.83
F2
16.8
11.34
The dose rate decrease with the thickness and the density of the filter. The
dose rate with F5 is higher than that of F4. This might be an experimental error by
using the distinct spectrometer.
-
Energy and intensity effect
Absorbed dose depend on the voltage and current of the irradiator due to
X-ray spectrum will be shifted when they are changed. In other words, the
voltage determines X-ray average energy emitted and the current determine
number of photons are given off at certain energy or intensity of the beam for
short. Therefore, adjust these specifications can affect the energy absorbed in
the material, the dose maybe not the same even at the same power with different
electron speed or intensity.
21
Number of photon
4000
50kV
100kV
160kV
3000
2000
1000
0
0
20
40
60
80
100
120
140
160
X-ray energy (keV)
Figure 3. 2 X-ray irradiator spectrum at 50kV, 100kV, 160kVat same current
As figure 3.2, adjust the voltage of the X-ray tube affect the number of
photons emit at the same energy channel.
160kV 18.7mA
100kV 30mA
50kV 15mA
Dose (Gy)
200
y=17.016x-0.156
R2=0.9999
y=9.773x-0083
R2=0.9971
100
y=2.982x-0.02
R2=0.9985
0
0
5
10
TIme (Minute)
Figure 3. 3 Change in doses by energy with 50, 100 and 160 keV
(150mm H, F1)
22
As shown in Figure 3.3, dose and dose rate are changed with the voltage and
current of the X-ray tube. The dose rate at 160 kV, 18.7 mA is highest and around
17.02 Gy/minute. This value was dropped nearly a half to 9.77 Gy/min with the
voltage changed to 100kV, 30mA. This means that at the same power 3kW the dose
rates are different. Decreasing the power to half by 50 kV, 30 mA, the dose rate is
one-third of the dose rate at 100kV because the low energy X-ray intensity at 50kV
is much higher than at 100kV and this low energy X-ray is absorbed completely in
1 mm aluminum sheet.
3.1.2 Absorbed dose in Eppendorf tubes
250mmH
150mmH
300
y=18.628x-0.595
R2=-.9964
Dose(Gy)
200
y=7.758x-0.526
R2=0.9975
100
0
0
5
10
15
20
25
30
Time (Minute)
Figure 3. 4 Dose and dose rate in Eppendorf tube at 150 and 250 mm H
(160kV, 18.7mA, filter F1)
The dose rate in the Eppendorf tube is higher than in Pyrex tube because the
diameter of Eppendorf is smaller and the denser glass than plastic of Pyrex tube
block more low energy X-ray than the Eppendorf. Doses in Eppendorf tube were
measured using different filters and the result is shown in Table 3.5.
23
Table 3. 2 Dose and dose rate in Eppendorf tube at 150 and 250 mm H
(160 keV, 18.7 mA, 150mm H)
Dose (Gy)
Time (minute)
F0
F1
F2
78.00
56.51
41.00
38.50
48.33
10 528.03 184.92 124.10
84.49
79.22
96.67
5 251.74
F3
F4
F5
Dose rate (Gy/min)
60.33
18.67
11.83
8.33
7.83
9.67
Dose rate in Pyrex tube
(Gy/min)
21.01
16.8
11.34
9.3
6.64
8.82
Dose rate difference (%)
65.19
9.82
4.23
-12.00
14.89
8.62
The dose rate in the Eppendorf tube is decreased when the filter is increased
and the dose rate in Eppendorf tube is nearly 3 times higher than the dose rate in
Pyrex tube because Pyrex tube have bigger diameter than the Eppendorf tube and
Pyrex tube was made of glass which is denser than plastic.
Doses in Eppendorf tube were measured using different energy and the
result is shown in Figure 3.5.
160kV
100kV
50kV
Linear Fit 160kV
Linear Fit 100kV
Linear Fit 50kV
Dose (Gy)
200
y=18.628x-5.955
R2=0.9964
y=12.227x+1.709
R2=0.9987
100
y=4.138x+1.524
R2=0.9908
0
0
5
10
Time (Minute)
Figure 3. 5Dose in Eppendorf tube at different voltage
(150 mm H, F1)
24
The doses and dose rate are changed with the current and voltage. The
results are similar in the Pyrex tube but the dose rates measured in the Eppendorf
tube are higher than the Pyrex tube because the diameter and material are different.
3.1.3 Gafchromic film dosimetry
3.1.2.1 Gafchromic film calibration
Gafchromic film dosimetry has good energy independence and Ashland
Company provides a high-quality dependence equation between dose and optical
density. It is necessary to make calibration because the X-ray irradiator gives off a
wide range of energy electromagnetic wave. The film color changed over time that
required to build a new equation for a 24-bit scanner with the X-ray irradiator. To
get a good equation, Fricke dosimeter is used as a standard because it is a precise
measurement in solution. The dose rate obtained by Fricke dosimetry was used to
estimate the dose absorbed by film. The irradiated Gafchromic film was scanned
after 24 hours. The optical density (OD) of each film was recorded by ColorPic
software to make a calibration equation.
Ashland Company recommends to fit the calibration data to a function
having the form:
d x ( D)  a  b / ( D  c )
Table 3. 3 Parameters of fitting equation with different measured dose rate
(160kV, 18.7mA, 150mm H)
Optical density
Filter
Parameter Green
a
F1
F5
F1 Air
kerma *
b
Red
-13.826
Blue
-24.401
-7.480
3339.650 12481.750
7708.734
c
-28.099
-77.778
-210.958
a
-9.538
0.759
10.863
b
3190.135
7995.522
2116.031
c
-26.684
-56.956
-91.362
a
-13.825
-24.401
-7.480
b
c
4553.167 17017.200 10509.820
-38.310
-106.039
*Note: Air kerma is obtained from Hitachi.
25
-287.612
All three Gafchromic film calibration equations are reliable with R2 value
close to 1. The F1 equations are more precise because the doses calculated for the
other condition from these equations are nearly the same.
Optical density (8-bit color)
150
Channels
Red
Green
Blue
100
y=-24.401+7708.734/(x+77.777)
R2=0.9989
50
y=-13.825+3339.650/(x+29.099)
R2=0.9992
y=-7.480+7708.734/(x+210.968)
R2=0.9876
0
0
50
100
150
200
Dose (Gy)
Figure 3. 6 Optical density of each color channel
(160 kV, 18.7 mA, filter F1, 150 mm H)
Figure 3.6 shows the red channel is quite sensitive to radiation, the original
curve of Ashland is also the same. To measure a dose above 150 Gy, green and
blue channels are a better option because the red channel is saturated. However,
since the red channel have high dose responsibility at the dose lower than 150 Gy,
it is an optimal equation to measure the dose rate due to 8-bit scanner disadvantage.
Particularly, the limit of 8-bit scanner that the OD range is from 0 to 255 (28) and
the 16-bit scanner OD range is 0 to 65535. The 16-bit scanner has a higher
resolution. So the 8-bit scanner measure less precise than the 16- bit with the same
film. Moreover, the 8-bit scanner cannot measure the small change in color when
the channel has a low response to radiation.
26
3.1.2.2 Filter and energy effect
Table 3. 4 Dose rate measured by Gafchromic film at 150 mm H at different
condition
Energy
& Current
160kV, 18.7mA
100kV,
30mA
Filter
F0
F1
F2
F3
F4
F5
F1
Dose rate (Gy/min)
75.33
16.83
15.33
14.17
10.50
8.17
13.17
21.01
16.8
11.34
9.3
6.64
Dose rate in Pyrex
tube (Gy/min)
8.82
9.77
Dose rates are decreased when filter thickness and density are increased
(Table 3.4). The non-filter (F0) dose rate is about 4.5 times higher than F1 since the
low energy X-rays have high linear energy transfer. The dose rate measured by
Gafchromic film is higher than the dose rate measured by Fricke dosimeter in
Pyrex tubes. The dose rate of F3, F4 and F5 is fluctuated and the further experiment
is required.
3.1.2.3 Dose distribution in PMMA
Polymethyl methacrylate (PMMA) has comparable density to water and
organic materials. Therefore, PMMA is applied to measure the dose rate
distribution in foods as phantom.
Table 3. 5 Dose rate distribution in PMMA with different filters
(160keV, 18.7mA)
Dose rate (Gy/min)
Sample
Thickness
(mm)
1
0
77.500 15.320 14.233 10.556 6.982 5.865
2
1
56.251 13.374 13.413 11.177 7.332 6.159
3
2
40.135 12.806 11.988
9.471 6.075 5.103
4
3
32.405 11.772 11.988
9.471 6.658 5.593
5
4
21.492 11.300 11.988
8.553 6.982 5.865
6
5
11.492 10.853 11.988
8.553 6.357 5.340
7
6
10.748 10.432 10.792
8.553 5.812 4.882
8
7
10.116 10.032 10.264
9.471 5.812 4.882
9
8
F0
9.573
F1
9.653
27
F2
9.322
F3
F4
F5
7.768 5.566 4.675
10
9
9.091
8.625
9.775
7.416 5.566 4.675
11
10
8.660
7.734
8.505
7.088 5.566 4.675
12
20
5.823
4.621
5.248
4.538 3.243 2.725
13
30
4.162
2.657
3.217
2.681 2.077 1.745
14
40
1.991
1.802
1.738
1.979 1.429 1.200
15
50
1.383
1.007
1.289
1.033 0.995 0.836
The dose rate decreases while the PMMA thickness is increasing. The dose
rates with F1 to F5 are gradually decreased but the dose rate of F0 is rapidly
decreased in the thin layer because the low energy X-rays lose a large amount of
their energy while they penetrate into material. Dose rate distribution in PMMA
irradiated with F0 and F1 is shown in Figure 3.7.
F0
F1
100
Dose(Gy/min)
y=77.907e-0.327x
R2=0.9956
y=13.936e-0.0448x
R2=0.9937
10
y=14.515e-0.0570x
R2=0.9931
1
-5
0
5
10
15
20
25
30
35
40
45
50
55
Thickness (mm)
Figure 3. 7 Dose rate distribution in PMMA
The dose rate of F1 decreases exponentially with increase of PMMA
thickness while the dose rate of F0 decreased markedly from 0 to 5mm thickness of
PMMA. At 5mm PMMA thickness, the dose distribution is nearly the same with
other filters because the higher energy X-rays can through the filters. They have
high penetration and the dose rates distribution nearly the same among different
filters.
28
3.2 Disinfection of E. coli by radiation
3.2.1 Filter effect on E. coli radiosensitivity
Filters absorb the low energy of X-rays, and their intensity so the effect of
radiation maybe not the same with different filters. The dose rate is used for
calculating in this part is measured by the Fricke dosimeter in Eppendorf tube at
each condition since the microorganism samples contained in Eppendorf.
1
y=e-0.0299x
R2=0.9921
0.1
Survival rate
Survival rate
Low dose survival curve
High dose survival curve
y=0.1291e-0.0092
R2=0.9937
0.01
1E-3
1E-4
0
200
400
600
Dose (Gy)
Figure 3. 8 F0 Survival curve
(160kV, 18.7mA, 150mmH, filter F0)
Figure 3.8 shows that the survival rate of E.coli decreases dramatically from
0 to 100 Gy, and it decreases gradually at the higher dose range. This can be
explained by the penetrate of low energy X-ray is shorter than the high energy Xray. Therefore, dose distribution inside the Eppendorf tube is not even, the upper
part of the tube exposed to more radiation than the bottom part. The survival rate
when the tube is mixed is average of the survival rates at two parts of the tubes.
This average survival rate will drop rapidly when the high dose region survival rate
decrease more dramatically than the low energy region. At the higher dose range
above 100 Gy, the survival rate of high dose part of the tube is much less than the
low dose counterpart, so the average survival rate is mainly determined by the low
dose region.
29
F1
Low dose curve
High dose curve
1
y=e-0.0265x
R2=0.9987
Survival rate
0.1
y=0.0492e-0.009x
R2=0.9926
0.01
1E-3
1E-4
0
200
400
600
Dose (Gy)
Figure 3. 9 F1 Survival curve
(160kV, 18.7mA, 150mmH, filter F1)
At this condition, the shape of the survival curve is the same as the F0 curve,
but the decreasing rate is slightly lower in the low dose range, and the high dose
range curve is nearly the same as F0 condition. The small difference between the
two condition that the low dose range of F1 is wider than the F0 one due to great
amount of X-ray below 15keV have been absorbed by the filter F1.
F2
F2 Survival curve
1
Survival rate
0.1
y=e-0.02438x
R2=0.9991
0.01
1E-3
1E-4
0
100
200
300
Dose (Gy)
Figure 3. 10 F2 Survival curve
(160kV, 18.7mA, 150mmH, filter F2)
30
400
The figure 3.10 show that the survival rate of microorganism decrease not
fast as the low dose part of F0 and F1 survival curve due to the low energy X-rays
are completely shielded. The same thing happened with the other filters, these
curves are slightly differences.
F3
F3 Survival curve
1
0.1
Survival rate
0.01
y=e-0.0247x
R2=0.9998
1E-3
1E-4
1E-5
0
200
400
600
Dose (Gy)
Figure 3. 11 F3 Survival curve
(160kV, 18.7mA, 150mmH, filter F3)
F4
F4 survival curve
1
0.1
Survival rate
0.01
y=e-0.0245x
R2=0.9987
1E-3
1E-4
1E-5
0
100
200
300
400
Dose (Gy)
Figure 3. 12 F4 Survival curve
(160kV, 18.7mA, 150mmH, filter F4)
31
F5
F5 survival curve
1
0.1
Survival rate
0.01
1E-3
y=e-0.0225x
R2=0.9969
1E-4
1E-5
0
200
400
600
Dose (Gy)
Figure 3. 13 F5 Survival curve
(160kV, 18.7mA, 150mmH, filter F5)
The filter effect is dominated with filter F0, F1, this effect becomes less
important with the thicker and denser filters. For sterilizing E. coli, depend on the
thickness different filters can be used to get optimal penetration and time.
3.2.2 Energy effect
Voltage changing also can affect X-rays energy emitted by the tube, it is
similar to filter but on the reverse way. Particularly, changing voltage will reduce
or increase the highest energy and average energy, intensity of photon coming out
at certain energy. This effect causes not only dose variation but also make the result
different since different wavelength photons transfer not the same amount of
energy to material. This effect was shown in Fig. 3.14, 3.15
32
F1 50kV 30mA
Low dose curve
High dose curve
1
Survival rate
0.1
0.01
1E-3
y=0.061e-0.0105x
2
=0.9958
-0.02314x
y=e
R2=0.99996
1E-4
0
200
400
600
Dose (Gy)
Figure 3. 14 Survival curve at 50 keV
( 30mA, 150mmH, filter F1)
F1 100kV 30mA
High dose curve
Low dose curve
1
Survival rate
0.1
0.01
1E-3
y=e-0.03x
R2=0.99796
y=e-0.0083x
R2=0.8627
1E-4
0
200
400
600
Dose (Gy)
Figure 3. 15 Survival curve at 100 keV
(30mA, 150mmH, filter F1)
There is a similar survival curve of E. coli between 160kV, 18.7mA; 100kV,
30mA; and 50kV, 30mA curves at the same condition. In three lines, survival rate
33
decreases rapidly in low dose range and drop gradually in high dose range.
160kV 18.7mA
100kV 30mA
50kV 30mA
1
0.1
Survival rate
0.01
1E-3
1E-4
1E-5
0
200
400
600
Dose (Gy)
Figure 3. 16 Survival rate at different voltage at 150mm from the source
-
Survival rate at the different distance from
source\
y=e-0.0265x
R2=0.9987
1
250mmH
150mmH
0.1
Survival rate
0.01
y=e-0.03931x
R2=1
y=0.0492e-0.009x
R2=0.9926
1E-3
1E-4
y=0.059e-0.0188x
R2=0.9975
1E-5
0
200
400
Dose (Gy)
34
600
Figure 3. 17 Survival curve at 150mm and 250mm height
(160kV, 18.7mA, filter F1)
The 250mmH survival rate decrease sharply than the 150mmH survival rate
due to distance adjustment change the dose rate which requires long time
irradiation. Particularly,14 minute irradiating at 250mmH and 6 minute irradiating
at 150mmH is necessary for the samples expose to 100Gy.
3.2.3 Effect of thickness of the sample
Food is contaminated not only on the surface but also deep inside the food.
To analyze the depth dose distribution, PMMA is used as a phantom. Table 3.4
shows the survival ratio of E. coli at 6 mm depth of PMMA phantom. The survival
ratios increase when the thickness of PMMA layers increase because of dose
attenuation. However, the data were varied and the further experiment is required to
clarify the depth effect.
Table 3. 6 Survival ratio inside the PMMA phantom
( 160kV, 18.7mA, 150mmH, filter F0, 2 minutes irradiating)
Sample
Position
(mm)
Dose
(Gy)
Control
0
0
0
2 mm
4 mm
6 mm
Dilution
Plate 1
Plate 2
Plate 3
CFU/ml
Survival
ratio
5
291
480
252(C)
6
35
20
37
3.49E+0
8
1.00E+0
0
155
4
32
42
34
3.60E+0
6
1.03E-02
1
112
4
C
C
252
2.52E+0
7
7.23E-02
2
80
5
84
92
79
8.50E+0
7
2.44E-01
0
155
3
1402
1616(C)
1586
1.53E+0
7
4.40E-02
2
80
4
317
540
429
4.29E+0
7
1.23E-01
4
43
4
449
540
628(C)
5
55
52
33
5.37E+0
7
1.54E-01
0
155
2
133
139
219
1.64E+0
5
4.69E-04
35
3
64
4
1200
1002
1160
1.12E+0
8
3.21E-01
6
20
5
105
81
113
9.97E+0
7
2.86E-01
*Note:
C: the plates are contaminated by another microorganism
The dose is calculated from the Gafchromic film dose distribution in PMMA.
36
3.3 Discussion
Fricke dosimeter, Gafchromic film are used to measure the dose, dose rate
and dose distribution inside the organic material. It is necessary to measure again in
the future experiment for more precise dose, dose rate. Measurement of low energy
X-ray dose is a complicated and sophisticated process because low energy X-rays
(<20keV) dosimetry can be contaminated with the secondary electron and higherenergy X-rays can have a big backscatter factor.
The X-rays emitted by the MBR-1618R-BE Hitachi irradiation system can
be applied to the disinfection of microorganisms in foods. The filter is an important
factor because the filter cuts off the low energy parts of X-rays. The survival
curves of E.coli are exponential with higher filters such as F3 – F5 but the survival
curve of F0 and F1 composed with two parts of sharp decrease in low dose range
and slow decrease in high dose range. The low energy part of X-rays is useful for
the disinfection of thin foods or small granules such as peppers.
X-ray tube irradiator is compact and inexpensive. For the industrial scale
irradiation, high dose rate and good dose uniformity ration (Dmax/Dmin) are
important.
It is considered that the low energy X-ray radiation without filter was
effective for the surface (5 mm thickness) irradiation. For the irradiation of thick
materials, the filter such as 1.0 mm - 2.0 mm Al is necessary to obtain a better dose
uniformity ratio.
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CONCLUSION
Although dose measurement for X-ray irradiator has many disadvantages
and errors, the dosimetry results are still precise enough for the microorganism
control experiment. The absorbed dose measured by Fricke dosimetry at 250mmH,
160kV, 18.7mA filter F1 is about 1,01 Gy/h. The Eppendorf dose is slightly higher
than Pyrex dose because of its lighter material and smaller diameter. The
distribution of dose inside material, the PMMA is used to measure the distribution,
the dose and dose rate decrease exponentially when X-ray penetrates into PMMA.
The thicker filter, the more uniform dose becomes, so it is necessary to use filter for
high dose uniformity ratio.
The X-ray is effective to disinfect microorganism, the D10 value is around
100Gy for E.coli K12. For comparison, this value with the gamma source is about
200Gy. Moreover, an X-ray irradiator is very effective to sterilize the surface of the
food and the optimal thickness of the irradiated object is 5mm with filter F0. To
disinfect the thicker food, different filters can be used to get a uniform dose, but the
effectiveness will be decreased and the time irradiate will also increase. Moreover,
the X-ray emitted has low penetration so the size of the food is limited, and the
dose rate is lower than Co60 gamma source so it takes a longer time to irradiate
food.
All in all, MBR-1618R-BE Hitachi irradiator has good disinfection ability,
especially disinfect thin or small grain food such as pepper, powder, milk, etc.
However, this research just experimented on E.coli but there are many
microorganisms that resist radiation and it is necessary to study more about the
effect of X-ray on these microorganisms.
38
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