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CHAPTER -3

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IRRADIATION: Material Modification

3.1 Irradiation equipments

In recent years, electron beams of energy keV to MeV range have been extensively used

in material science studies. Such beams are available from a linear accelerator which can

produce electron beams up to a few MeV energy. Such a linear accelerator (LINAC) has

been used our studies.

3.1.1 Modern Linear Accelerator

A Linear Accelerator is a complicated device which can produce ion beams, electron

beams and X-ray photon (MV) beams of variable energy. It accelerates charge particles

along a straight line. An electric field is employed to accelerate them towards the first

drift tube. This drift tube shields them from external electric fields and enables them to

drift with a certain velocity. They get accelerated at the first gap and enter into the next

drift tube. This process is continuously repeated and the particles gain energy when

they cross each gap. Finally when they attain the required energy, they are allowed to

exit from the accelerator.

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The applied alternating field tries to alternately accelerate and decelerate the particles.

But the drift tubes shield the beam from getting decelerated by the field. So drift tubes

play an important role during acceleration of the particles.

Figure 2 : Wave guide of a Modern Linear Accelerator

Broadly a LINAC consists of following components which are helpful for accelerating

the charge particles.

1. The electron gun

2. The buncher

3. The LINAC itself

1. The Electron Gun

The first stage of acceleration begins here. A thermionic material is used as the cathode

and has negative charge. Electrons are emitted on heating the cathode in the electron

gun. Thorium, barium aluminate etc are such materials which are used as cathode.

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When the cathode is heated, the electrons gain energy and come out of their parent

atoms. The anode is generally made of a copper screen called as grid and is positively

charged. This anode acts as a gate. In some particular interval of time like 2x10 -9 sec, a

flash of positive charge is given to the anode. Electrons get attracted and rush towards

the anode in large numbers. Near the gate, the strength of attraction increases and they

cross through the gate. The pulsing frequency of the gate is 500 MHz. And the electrons

arrive at the anode in bunches every 2x10 -9 sec. The shape of the anode is such that it

creates an electromagnetic field. This field helps to guide the electrons to the buncher.

2. The Buncher

When the electrons emerge out of the electron gun, they are packed into small bunches. This is

done by the buncher and hence the name. A klystron or a magnetron delivers microwave

radiation to the buncher. In the process, electrons also get accelerated in a wavy manner as

shown in the following figure 3. The disks represent the bunches of electron in the buncher. The

electrons which are nearer to the crest receive more energy. So the higher up electrons catch up

with them who are lower. The wave form on the right represents the same electrons a fraction

of a second later. In this manner, electrons in the form of bunches get accelerated till they come

out of it.

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Figure 3 : Micro wave structure

3. The LINAC

The loosely packed bunches are packed tighter by the LINAC with the help of

radiofrequency waves. When the electrons come out of the buncher and enter the LINAC,

they have a speed of about 0.6c.

Description of LINAC:

Through continuous development in technology and models, linear accelerators have

overcome the difficulties faced by the age old cobalt-60 machines. The main advantage

of LINAC is that by using this machine, required dose can be given to the treatment

area only and a negligible portion of the radiation goes to other unaffected areas. The X-

ray beam also suffers negligible scattering outside the beam. Whenever required, this

LINAC can be adjusted to work with electron beams rather than X-rays. In an isocentric

mount, the distance between the target for X-rays and the rotational axis is about 100

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cm. So the material or patient which is supposed to receive the dose is placed on this

axis. Various angular positions can be used by turning the unit but not disturbing the

material or patient. For this purpose, there is provision for shortening the accelerating

tube when the working range of energy is between 4 to 6 MeV. X-rays are produced

when these electrons pass through the tube and collide with the target. X-rays thus

produced are symmetric about the axis i.e. about the line joining the source and the

machine. When the target is shifted to one side the beam of electrons directly emerge

out through a thin window. The accelerating structure tube is larger in high energy

machines. The klystron delivers the RF power to the wave guide along the machine

axis. A rotating vacuum seal helps in the process. The quadrupole focusing magnets

receive the electron beam at 600. They are further bent by 900 to be delivered to a target

or emerge out. The modern LINACs can be used in both electron and photon modes. A

thick target is used to stop high energy electrons coming from the wave guide. During

operation in electron mode, the target is moved away from the path and the electron

beam moves ahead undeviated. There is a movable mount placed below the target

which contains a number of foils know as scattering foils. For different energies,

different foils are used [1].

The primary collimator is made of a heavier metal. The beam passes through a hole in

this collimator. There is a tungsten filter below this. The ion chamber monitors the dose.

It is, in turn, connected to a quadrant to regulate the beam optics. Calculation of beam

optics plays a very important role in any sort of beam aligning . Just below the ion

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chamber, a light localizer is positioned. There is a pair of collimators at right angles.

Square and rectangular field can be obtained by rotating them. They are positioned at a

distance of about 50 cm from the target.

The major parts of a LINAC are mainly (1) Gridded Electron Gun, (2) Energy Switch,

(3) wave guide, (4) achromatic 3 field bending magnet, (5) real time beam controlling

steering system, (6) focus solenoid, (7) 10 port carousel, (8) ion chamber, (9) asymmetric

jaws, etc.

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Figure 4: Partial lay out of a LINAC showing major parts.

(Source: http://its.uvm.edu/medtech/tmodule.html)

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Multi-Leaf Collimator (MLC)

Just before the exit port of the LINAC through which the beam comes out there is a

multi-leaf collimator (shown as 10 in Fig. 4). It consists of a number of leaves made of a

high -Z material like tungsten. They can be moved into or away from the path of a beam

whenever required. They provide conformal precise shaping of mega voltage beams for

radiotherapy treatment. In Conformal Radiotherapy (CRT) and Intensity Modulated Radiation

Therapy (IMRT) dose is delivered using MLC. Our LINAC has static 80 leaves MLC with 40

numbers in each bank. The width of MLC is 1 cm at isocentre for precise shaping of beam for

IMRT . For delivering exact dose to any irregular shape, MLC has a major role in LINAC. So it

needs proper quality assurance checks before delivering any precise radiation dose.

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Figure 5: Elekta Modern Linear accelerator with MLC and Portal vision

(Own photograph of the LINAC at our Centre, HHRC, Bhubaneswar)

Wave guide (WG) is an integral part of a LINAC where electrons are accelerated by

using microwaves. The WG structure is energized at microwave frequency most

commonly at 3000 MHz (100 mm wave-length in free space) [2] ; then these accelerated

electrons are allowed to strike a heavy metal target. It results in emission of high-

energy X-rays (transmission type). Also without this target, electron beam can be

emitted with fine control. Modern high energy LINACs typically provide, in addition to

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two megavoltage photon energies, electron beams of energy varying from 4 to 22 MeV.

The energy dispersion in the beam is almost negligible when it leaves the accelerator.

However, the beam while emerging out through the window may interact with

scattering foils, collimators, air etc. Outcome of this radiation from LINAC is directly

related to the accuracy of beam data used for clinical and other scientific research

applications [3].

The LINAC can be used for therapy (treatment) and research purpose after

completion of some satisfactory scientific tests called as pre-commissioning testing. It

includes machine specific mechanical tests, radiation specific dosimetric tests and

radiation safety tests. Satisfactory acceptance testing is done to ensure that the LINAC is

working as per the specifications and all safety requirements are met.During

commissioning a LINAC for clinical use , all dosimetric parameters need to be

measured accurately because they are of crucial importance in treatment planning

system (TPS) for individual patients [4]. So, it becomes necessary to obtain a dataset

which considers percentage depth dose (PDD), isodose distribution and output

parameters for variour field sizes (FSs).

In our work, we have used the LINAC of Hemalata Hospitals & Research Centre

(HHRC), Bhubaneswar. We installed and commissioned this LINAC (Elekta, Crawelly ,

UK) which is the first of its kind for therapeutic use in the state (Given in figure 5). In

this work we have showed all the data obtained during commissioning. This LINAC

has dual energy photon beam (6MV and 15 MV) and multi energy electron beam (4, 6, 9

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and 12 MeV) with Multi Leaf Collimator (MLC) facility. The details of the LINAC

parameters are given in Table 1. The installation has been followed by acceptance

testing to ascertain that the product specifications along with any other requirement are

satisfied properly. All these tests have been carried out according to the acceptance

testing procedure mutually agreed upon by the supplier and our research centre[5]. In

this work we used some dosimetric equipments like 3D water phantom scanner with

computer interface called as radiation field analyzer (RFA-300), ionization chambers

(two cylindrical and one flat), and Dose-1 electrometer (all instruments are from

Scanditronix - Wellhofer Company, Germany), solid phantom (an assembly of tissue

equivalent solid plates of different thickness), radiochromic films (EDR2), film laser

scanner, barometer, chronometer and thermometer. The 3D water phantom called as

radiation field analyzer (RFA-300) and controlled by OmniPro-Accept computer

software was used for depth-dose, beam profile, penumbra, and isodose measurements.

The RFA-300 consists of a cubic water tank with inner dimensions of 58 × 58 × 58 cm3.

All measurements were done in the water medium as our human body contains 80% of

water and it is a standard protocol worldwide. Film scanners are used to convert the

film data to digital data by using computer soft ware, which gives finally the intensity

of the incident beam at various points in the radiation field. The positional accuracy of

the drive mechanics of the water phantom was ± 0.5 mm, and the reproducibility was

0.1 mm (supplied value, RFA-300 Plus System Manual, 1998).

Air cavity ionization chamber is the detector of choice for the radiation measurement,

as its measurement response is independent of the fluence of the beam characteristics

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(beam quality, dose and dose rate) and shall possess good reproducibility and

repeatability. The radio chromic film gives practical and rapid indications of the dose

distribution in a plane. The advantage of the film lies in its high spatial resolution,

which is particularly useful in regions where dose gradient is very high or very low. As

per standard IAEA protocols following tests have been done for LINAC commissioning

for its both photon and electron beams.

3.1.1.1 Machine specific tests

Testing of this accelerator machine has been broadly classified into two types, i.e., a)

Electrical tests and b) Mechanical tests. Electrical tests involve the testing and proper

functioning of all interlocks and emergency cut-off switches and mechanical tests

involve patient treatment couch movement, collimator rotation, gantry rotation, optical

distance indicator (ODI) scale verification and position of isocentre mechanically.

The mechanical device MLC is of different sizes (regular, mini and micro). Their

usage depends on their width. The mechanical stability and characteristics of the above

device and their suitability for this LINAC were checked and compared with reported

parameters of manufacturer [6].

3.1.1.2 Radiation specific tests

The basic parameters like percentage depth dose (PDD) and profiles (in-plane and

cross-plane) of photon beams at a variety of depths for open and wedge fields, energy

stability verification and data related to MLC such as inter and interleaf leakage,

penumbra, tongue and grove effect come under this test group for photon beam.

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However , PDD, profiles analysis and output factors for different applicator size (AS)

are essential tests for electron beam in radiation specific test group.

It should be ensured that for a given nominal beam energy, the radiation monitor

response is independent of parameters like dose rate, the direction of the radiation

beam, temperature and pressure. However, the variation of monitor response is related

to the ion recombination in monitor chamber due to inadequate voltage supply to the

chamber. Hence, monitor should be carefully calibrated for each dose rate in clinical

use. For checking of monitor and chamber response, like reproducibility, linearity,

dependence on gantry rotation, dependence on the field shape and stability with time

have been properly checked before starting the detailed procedures.

It is important to measure the leakage radiation of LINAC installation bunker on the

outside walls including top and bottom sides indicating clearly the area occupied by

radiation and non-radiation workers (including public) with gantry positions at 0°, 90°,

180° & 270° . These tests were done for radiation safety integrity of LINAC installation

by the use of water phantom and radiation survey meter with nominal photon energy.

The measurements of radiation dose at various points of the outside walls of bunker

were carried out using radiation survey meter (Victoreen, model-451P RYR, Fluke

biomedical, USA). But as per Atomic Energy Regulatory Board (AERB) Mumbai,

recommended value of safety radiation level at outside area of the bunker is up to 2

mR/hr (maximum). Our measured values at different positions of outside bunker are

less than 1 mR/hr. The radiation exposure due to leakage at any point outside the

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useful beam but inside a plane circular area of radius 2 meters centered around and

perpendicular to the central axis of the beam at normal treatment distance (100 cm)

needs to be measured from point of view of radiation safety of patients. Its tolerance is ≤

0.2% of the radiation exposure on the beam axis at same distance [7].

3.1.2 Low temperature Liquid Irradiation cell (LTLIC)

A multi-purpose cryostat for low temperature electron irradiation for

semiconductors (solids) has been reported in late 1978 by Gmelin et al. [8]. Again,

sample holder for low temperature gamma irradiation of crystalline solids has been

reported [9] for electron paramagnetic resonance (EPR) analysis. Blood irradiation by

ultra violet radiation at room temperature was reported in a number of U.S Patents [10].

Again, a liquid cell for X-ray fluorescence analysis at room temperature has been

patented by Hosokawa et al [11]. A temperature controlled low-temperature liquid cell

(a sealed container) for photo-acoustic spectroscopy has been reported [12] which is not

suitable for MeV electron irradiation. Though the multi-purpose cryostat is designed for

low temperature irradiation study, it is only suitable for solid sample, and fails to take

liquid samples. Hence, the need arises to design a liquid cell for the biofluid material

irradiation at low temperature.

Our LINAC provides vertical beams; hence is very interesting and useful for the

modification of materials in general and special materials like bio-fluids (liquid) in

particular. The MeV radiation beams (X-rays and electrons) produced by this

accelerator are also useful not only for modification but also for synthesis of new

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materials, polymers and complexes. In recent time, biomaterials, in particular liquid

biomaterials, have aroused a lot of interest among material researchers. The irradiation

of liquid samples with high energy electrons is a difficult process since its

incompatibility in vacuum and difficulty in mounting the sample in a particular angle

/axis because of its flowing nature.

Electron beam irradiation technologies in keV range have long been used for

biomaterial applications in different direction [13]. However, it is interesting to see the

changes developed in such materials when exposed to highly energetic electrons. The

MeV electron beam produced from a modern-LINAC can also be used for irradiation of

various materials in medical science as well as in material science to study the

modifications produced.

Material gets modified due to interaction between high energy electrons and matter.

However, irradiation of liquid samples with MeV electron beam obtained from a

traditional Linear Accelerator (LINAC) is a difficult process. Again, to design a low

temperature liquid irradiation cell (LTLIC) or cryostat is technically difficult and

challenging task for technologists. Though the multipurpose cryostat is designed for

low temperature electron irradiation study of solids, it is not suitable for liquid samples.

Hence, it is necessary to design an instrumental attachment or a special type of cell to

facilitate irradiation of liquid samples at low temperature. In this work, we design and

report the technical details and merits of an indigenously fabricated setup to facilitate

irradiation of liquid samples by MeV electron beam at low temperature (liquid

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nitrogen) to solve those difficulties. The merits over other liquid cells reported are also

highlighted.

Advantages over the prior art

The device designed for this purpose has following advantages over the prior art : a)

This low temperature liquid irradiation cell (LTLIC) can be successfully used for

irradiation induced modification of biofluids / liquid samples at LN2 temperature

(shown in figure 6 ).

b) It is suitable for low temperature experimental use.

c) This low temperature liquid irradiation cell is functioning properly to irradiate

the liquid samples at different energy/fluences.

d) It is economical in maintenance.

Figure 6 : Diagram of Low temperature liquid irradiation cell

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We report:

1. A method of irradiation of liquid samples which comprises:

i) Liquid sample taken in hard glass cylindrical tube ;

ii) An energetic electron beam collimated by the help of applicators of different

sizes after falling on scattering foil.

It is worth mentioning that the total irradiation work has been carried out in liquid

nitrogen temperature environment.

2. In the method of irradiation of liquid samples mentioned above, the radiation

heating of the sample can be minimized at very low temperatures, which will also

reduce the cell death and biological defects in the liquid sample.

3. The scattering foil acts as an intensity modulator for the whole collimated beam with

uniform intensity distribution.

4. Very less amount (microliter)) of liquid sample can be mounted for irradiation with

the help of a micro syringe.

5. High energy electron beam of energy 4 MeV has been used to irradiate the liquid

biomaterial.

6. The device for irradiation of liquid samples comprises the following.

a) a water tube,

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b) perspex (C5H8O2) cell ,

c) volume adjusting syringe, and

d) LN2 container.

The liquid sample was filled in the cylindrical perspex container with variable volume

adjustment and fixed with Styrofoam in front of the beam line.

7. In this device , LN2 was allowed to flow around the sample. As a result, the sample

became solid during irradiation.

8. In this device, a pulse beam of high energy electrons (4 MeV) was used to irradiate

the bone marrow (rabbit) fluid at a dose rate of 300 MU/min.

9. All these have been presented in the drawings (Fig 6 and 7a & b).

3.1.2 .1 Engineering design of LTLIC

A specially designed cell for liquid sample irradiation is shown in Figure 7 (a,b).

The major parts of the special cell consist of a water tube, perspex (C5H8O2) cell, volume

adjusting syringe and LN2 container. The “water tube” is made of a plastic cylinder

(special grade and non reactive to acid/base medium) with 0.5 mm thickness ,9 mm

internal diameter,10 mm external diameter and 15 mm length (graduated). One side

of the water tube has been closed with a 3 thick mylar film with the help of adhesive

(Fevikwik). The cell is made of a perspex sheet which is 10 mm thick and 50 50 mm2

area. A 10 mm free-hole was drilled at the centre of the perspex sheet to hold the

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volume adjusting syringe, a commercially available special type of syringe (3 ml

volume with least measurement unit of 0.1 ml ) , which is chemically inactive. The

advantage is that, because of adjusting provision very less amount ( l ) of liquid

sample can be mounted for irradiation with the help of a micro syringe. Again a

concentric blind hole of 30 mm diameter and 8 mm depth was drilled to make a

circular path for flow of LN2 . A thin steel pipe of outer diameter 3 mm, internal

diameter 2 mm, and length 50 mm was taken to connect liquid cell with the LN2

container. This makes a continuous flow of liquid nitrogen around the sample in a

controlled manner. The LN2 container of the LTLIC as shown in Fig. 6 is made up of

thermocool material which can hold LN2 for 6-8 hours easily. The thermocool box of is

of internal volume 101014 cm3 and outer volume 140 140 160 cm3 with a capacity

of 1 liter LN2. Once filled, it can cool the materials in LTLIC for 1 hour during

irradiation.

This LTLIC design and work have been patented (Deposit No: No.935/KOL/2013 on

08.08.2013, Serial no. 3 in publication list )