Chang Shu Wing

8/14/2019 Chang Shu Wing

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1.CITY UNIVERSITY OF HONG KONGDEPARTMENT OF

PHYSICS AND MATERIALS SCIENCE

BACHELOR OF SCIENCE (HONS) IN APPLIED PHYSICS 2008-2009

PROJECT REPORT

Measurement of Ultraviolet Radiation with Radiochromic Film

by

CHANG SHU WING

March 2009


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Measurement of Ultraviolet Radiation with Radiochromic Film

by

CHANG SHU WING

Submitted in partial fulfilment of the

requirements for the degree of

BACHELOR OF SCIENCE (HONS)

IN

APPLIED PHYSICS

from

City University of Hong Kong

March 2009

Project Supervisor : Dr. T. CHEUNG


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Acknowledgement

Thanks to the Department of Physics and Materials Science of City University of

Hong Kong for giving me a chance to conduct the final year project, in particular, I

would like to thank my supervisor Dr. T. Cheung for giving me encouragement,

guidance and much useful information. This project could not been successful

without the help of Dr. Cheung. I also would like to thank Dr. R.Q. Zhang for

providing me much valuable suggestion. Thanks to all AP staff for their support.

Last but not least, I would like to thank Mr. Paul Tsui for helping me to proofread

the report.


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TABLE OF CONTENT

Cover page ...I

Tittle page . II

Acknowledgement .III

Table of contentIV - V

List of figures... VI - VII

List of tablesVIII

Abstract....IX - X

1. Introduction and objective

1.1 Introduction 1 - 3

1.1.1 Introduction to UVR

1.1.2 Measuring device of UVR1.1.2.1 Physical detector

1.1.2.2 Biochemical detector

1.2 Objective.4

2. Literature Reviews

2.1 Ozone depletion5 - 7

2.2 Effect of UVR on human.............7 - 11

2.1.1 Beneficial effect of UVR

2.1.2 Detrimental effect of UVR

2.1.2.1 Effect on eyes

2.1.2.2 Effect on skin

2.3 The variation of UV level from natural source11 - 13

2.4 Health protection..14 - 16

2.4.1 SPF and PA

2.4.2 UV Index

2.5 Introduction to Gafchromic MD-55-2 film17 - 20


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3. Methodology

3.1 Construction of a UV meter..21 - 23

3.1.1 Densitometer

3.1.2 Use of radiochromic film

3.2 Measure the UV dose of a UV sterilizing lamp23 - 24

3.3 Measure the UV dose of a banknotes identifier24 - 25

3.4 Compare the efficiency of different brand of sunscreen

lotion ..25 - 26

4. Results and discussion

4.1 Calibration of the densitometer.27 - 29

4.2 The UV dose of a UV sterilizing lamp..29 - 31

4.3 The UV dose of a banknotes identifier..31 -324.4 The efficiency of different brand of sunscreen lotion...32 - 34

5. Conclusion.35

6. Reference 36 - 37

Appendix A. Occupation potentially associated with UVR exposure .. 38

Appendix B. Six types of skin39


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List of figures

Figure 2.1 Concentration of CFCs in the atmosphere of recent years

Figure 2.2 The ozone hole in stratosphere

Figure 2.3 Banknotes identifier

Figure 2.4 UV sterilization lamp

Figure 2.5 Tanning room

Figure 2.6 UVR photon direct and indirect action to the DNA

Figure 2.7 DNA replication

Figure 2.8 Single-strand break & double-strand break of DNA

Figure 2.9 Reflectance of UV

Figure 2.10 UV index table

Figure 2.11 UV int. across the UV spec. & the erythemal action

spectrum

Figure 2.12 Total erythemally weighted UV intensity

Figure 2.13 Structure of MD-55-2 radiochromic film

Figure 2.14 Response dose of MD-55-2 radiochromic film

Figure 2.15 Sensitometric response of MD-55-2 radiochromic film

Figure 3.1 Circuit diagram of the densitometer


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Figure 3.2a External appearance

Figure 3.b Internal structure

Figure 3.3 UV sterilizing lamp

Figure 4.1 Calibration line

Figure 4.2 Modern banknote identifier

Figure 4.3 Environment of Sai Sha pebble beach

Figure 4.4 Platform of measurement


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List of tables

Table 3.1 Specification of ASL UV sterilizing lamp

Table 3.2 Specification of K&C banknotes identifier

Table 3.3 Comparison of the sunscreen lotions used in this project

Table 4.1 Calibration with standard doses

Table 4.2 Densitometer response

Table 4.3 Result by the UV sterilizing lamp

Table 4.4 Result of banknotes identifier

Table 4.5 Result of different brands of sunscreen lotion


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Abstract

This is not a new topic that people discuss about the ozone hole since the late of

1970s, people most concern about that more harmful radiation from the universe

will enter to the ground.

The ozone depletion is happened in the layer of lower stratosphere, which are

destroyed by the increasingly spew out of CFCs of human activities. The ozone hole

not only reduce the aborption of solar enery in the atmosphere, which is a source of

global warming, but also let more UV reach to the ground, since the ozone layer

prevents most of the UVB (wavelenth 270 315 nm) of ultraviolet from entering the

earth.

There are three types of ultraviolet, named UVA, UVB and UVC, all of these UVR

are harmful, the higher dose of it in the ground, the greater chance will cause

damaging effect to humans, animals and plants. In view of this, it is necessary to

monitor with the UV dose. There are many types of UV detectors, different types of

detector has it particular application which depend on the need of user and the

source of the radiation. From the point of view of Physics, the ideal UV detector is

that able to measure the absorbed dose, so as to help us to evaluate the risk degree of

UV radiation.

Gafchromic MD-55-2 dosimetry film (product of ISP Technologies Inc.) is widely

used in medical application and it is designed for the measurement of the absorbed

dose of high energy photon. The MD-55-2 dosimetry film is tested to be very

sensitive to UVA radiation, but has low response to UVB, visible light and infrared.


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When the film is exposed under UVA radiation, the color will change from colorless

to blue the higher is the dose, the blue will be darker. As the radiochromic film is

very convenient to use and there is no waste product after usage, thus, it is an ideal

detector of UVA.

No matter natural and artificial sources of UV radiation, the same UV absorbed dose

will have the same effect. In this project, I made use of the property of Gafchromic

MD-55-2 dosimetry film to investigate the UV dose in various cases:

i.) To compare the efficiency of different brand of sunscreen lotion.ii.) To evaluate the risk of using UV banknotes identifier.iii.) To evaluate the risk of using UV light for sterilization.

As it is necessary to change the result (color of the radiochromic film) to numeric

data, a portable LED densitometer was constructed. The densitometer was calibrated

by a set of radiochromic film with exposed to various standard doses respectively.

As a result, a linear graph was obtained and I have just made use of the graph, to

estimate the UV dose absorbed and evaluate the harmful effect of each cases.


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1. Introduction and objective

1.1Introduction1.1.1 Introduction to UVR

There are two main sources of ultraviolet radiation (UVR) natural and artificial

sources of UVR. The sun is the principle natural source and artificial sources are

widely applied in different industries. In short, exposure to UVR may be detrimental

or beneficial.

UVR can be further classified into UVA, UVB and UVC. They are defined by the

wavelength region:

UVA (320nm 400nm), this spectral region has been called the blacklight

region, because it has been used for many years to excite fluorescent and

phosphorescent substances that reradiate the absorbed energy as visible light. It

has relatively lesser biological effect compare with UVB or UVC, but excess

dose of UVA is also carcinogenic.

UVB (280nm 320nm), it has been observed to have the most biological

effects and it is extremely dangerous to living organisms. Nevertheless, living

organisms could able to avoid from being harmed by the UVB by feathers, fur

or pigments which will absorb the radiation before it reach the physiological

target.

UVC (200nm 280nm), this spectral region is all artificial in the earth, and the

main purpose of using UVC is sterilization. UVC can cause little damage to our


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skin and eyes, since it is very easily absorbed by the nucleic acid in cells, but

the overlying dead layers of skin can absorb much of the UVC, such that at a

level of only causing mild erythema, even repeating exposure.

For solar radiation, UVR below 290nm are almost absorbed by the stratosphere,

over 98% of UVR reach the ground is UVA, only little of them are UVB (less than

2%) and there is no UVC from natural in earth.

1.1.2 Measuring device of UVR

As human eyes are invisible to ultraviolet, therefore, we need devices to monitor the

UV dose. Regarding an ideal dosimeter should have particulars features, summary is

as following:

Accuracy The ability of measure the amount dose correctly.

Precision The measuring data should be consistence for the same sample.

Detection limit It is a guide to the lowest detectable dose with certain

dosimeter type.

Measurement range It is the lowest and highest detectable value that it could

measure.

Dose response It should be independent of dose rate of delivery, such that it

could measure a pulse of high dose, even in a very short time.

Energy dependence For different radiation qualities, the dose response

should be the same.

Spatial resolution Ideally, a small volume or even a point of dose should be

detectable.

Ease of handling The device should be simple to use and enough for clinical

and industrial use on a routine basis.


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There are mainly two types of detectors physical detector (which principle base on

physical theory) and the biochemical detector (which principle on a chemical or

biological reaction) The physical detectors are mainly apply on measuring

instantaneous irradiance while the biochemical detectors are often used to measuring

the dose of UVR.

1.1.2.1 Physical detector

This type of detectors depends on their response to radiation energy (which measure

the temperature change inside the meter) or the quantum effect (such as

photoelectric effect, which measure the current generated by the incident UVR).

1.1.2.2 Biochemical detectorThe human skin can be used as a basic UVR dosimeter, as it can burn our skin

caused the erythema. Some detectors make use of the response to UVR of

micro-organism to measure the dose.

Chemicals irradiated by the UVR could be analyzed the change in laboratory,

common methods are the acetone-methylene blue reaction and photochemical

decomposition of oxalic acid in the presence of uranyl acetate.

The most important type of biochemical detector is photographic plates which is

also the measuring method of this project. The degree of how dark is the plant

indicates the irradiation dose and by means of using a densitometer, the UVR dose

could be found.


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1.2 Objective

Since most of the UVR around us is UVA, thus, in this project I used the

radiochromic film to study the UVA dose in three cases:

Case 1: Sunscreen lotion

To compare the shielding ability of UVA of different brands of sunscreen lotion

Case 2: UV sterilizing lamp

To evaluate the risk of using sterilizing lamp

Case 3: Banknotes identifier

To evaluate the risk of using banknote identifier

As I only used the Gafchromic MD-55-2 film as detector, to get numeric data from

the film for analysis; the first step of this project is to construct a calibrated and

portable densitometer.


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2. Literature Reviews

2.1 Ozone depletion

Ozone depletion is a result of damaging human activities. Ozone in the stratosphere

is response to absorb the UV radiation, after that, ozone will break into oxygen atom

and oxygen gas: O3+ UVR O + O2. In the ozone cycle, the oxygen atom will

combine with the oxygen gas to form ozone again: O + O2 O3. The ozone layer

acts as a protective layer to filter the harmful UVB. Since ozone can also easily be

destroyed by the free radical such as OH-, Cl

-or Bl

-and the free number radical Cl

-

is continuously increasing since 1970s, it is mainly due to the increase of CFCs in

atmosphere.

CFCs is a man-made compound Chlorofluorocarbons, which could not be found

in natural; people widely use it as solvent, spray can popellants, refrigerator/ air

conditioning fluid, etc. CFCs do not stay in the ground and it will reach the ozone

Figure 2.1 Concentration of CFCs in the atmosphere from 1976 - 2004


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layer finially. In stratroshpere, CFCs react with ultraviolet radation to form the free

radiacal Cl-: CFCl3+ UVR CFCl2+ Cl

-. Then the free radical Cl

-will react with

the ozone molecule to form the chorline monoxide: Cl-+ O3ClO + O2. The

chlorine monoxide can further react with the ozone molecule to form the free radical

atomic chlorine again: ClO + O3Cl-+ 2 O2.

As a result, these reactions dramatically decrease the amount of ozone molecule in

the stratosphere the big ozone hole is formed.

Figure 2.2 The ozone hole in stratosphere

Consequently, more harmful radiation, such as UVB, can reach surface of the

ground easily. Many people easily draw a conclusion that people will have much

higher risk of sun-burn or skin cancer if exposing to sunlight. However, many

scientific researches show that the cases of sun-burn or skin cancer do not have a

sharp increase. The reason is that our atmosphere couldnt filter the UVA efficiently,

the increase of UVB is just a very little part compared with UVA. Moreover, the


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affect from the ozone hole is during winter, and then people usually put on more

clothes, that UVB couldnt penetrate into our skin. Despite that, the ozone hole also

affects us in other area, such as, global warming.

2.2 Effect of UVR on human

2.1.1 Beneficial effect of UVR

UVR has been widely applied on different scientific, industrial or commercial field,

such as analyzing minerals (distinguish the materials), sterilization (UV sterilizing

lamp in restaurant), authentication (banknotes identifier), commercial activities

(tanning room) and medical application (treatment of skin curing the Psoriasis and

Vitiligo), etc.

Figure 2.3 Banknotes identifier Figure 2.4 UV sterilizing lamp

Figure 2.5 Tanning room

Other than particle applications, one of the beneficial effects of UVR to human body

is the conversation of 7-dehydrocholesteol to vitamin D. Lacking of vitamin D


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affects the aborption of calcium in our body. Many surveys discover that large

amount of people are suffering from vitamin D deficiency, which leads to rickets,

dental caries or other skin dieases, etc.There is a widespread feeling that sunlight is

good for health, but exposuring in sunlight will also increase dose of UVR (increase

the chance of having biologic effect) besides vitamin D. In other words, excess dose

of UVR is definitely harm more than good.

2.1.2 Detrimental effect of UVR

Since UVR penetrates essentially only into our eyes and skins, the biological effect

in the two parts is more obvious than other organs.

2.1.2.1 Effect on eyes

The ocular tissues of human eyes will be damaged for certain degree when exposure

in UVR. The UVR primarily damage the tissues at cornea and lens, which may be

classified as acute effect and chronic effects.

Acute effect

When expose the UVR in a short time with high dose, it may cause a painful ocular

condition (medical term: Welders flash/ Arc eye).

Chronic effects

When the eye is prolonged exposure in UVR, and it may cause certain eyes diseases,

such as Cataract a clouding in the eyes lens that affects vision, Pterygium

inflammation of the conjunctiva, Pinguecula some yellow spots appear on the

white of the eyes (which may also appear due to aging), etc.


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2.1.2.2 Effect on skin

Other than sunburn of the skin; death, mutagenesis and malignant transformation are

the most obvious effects of UVR on cells. Many researches have confirmed that

UVR is the main cause of skin cancer. According to Japanese scientists - Matsumura

Y and Ananthaswamy HN from the department of Dermatology of Kansai Medical

University, they stated that UVR present in sunlight is an environmental human

carcinogen and both UVR source natural sunlight and therapeutic artificial lamps are

a major concern for human health. All in all, UVR is harmful to skin and the effects

of UVR are also classified to acute and chronic.

Acute effects:

Erythema affected area will be pained and reddening, those symptoms confining to

the exposed area where reflects blood vessel dilation and increases blood flow in the

skin. Scientists believed that the initial photochemical reaction between the UV

photon and cell is in the epidermis where will release intracellular substances which

diffuse into the papillary dermis to cause the dilation of blood vessel, however, there

may be some cases that is direct injury to the dermis.

Tanning It is the pigmentation of the skin, the symptom will become noticeable

about 48 hours after irradiance to the UVR, and the color of the skin become darker.

Tanning is partly due to the migration of the pigment melanin which already exist in

the basal cells to the upper layers of the skin, and partly due to the formation of the

new pigment melanin.

Chronic effects:

Skin aging UVR can damage the collagen fibers which responsible for the skin


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strength and elasticity. The damage of the collagen fibers will finally cause the skin

loss of elasticity, hyper-pigmentation, and a dry, dull, leathery texture.

Skin cancer there are three common types of skin cancer, which are basal cell

carcinoma, squamous cell carcinoma and malignant melanoma. High energy

radiation can damage the cellular components, including DNA, and the radiation

damage the DNA in two ways.

Direct action and indirect action

UVB and UVC damage to the DNA directly by ionizing or exciting the DNA

molecules in skin cells, this will cause aberrant covalent bonds to form between

adjacent cytosine bases, producing a dimer which will cause the mutation and result

cancer.

UVA damages to the DNA by an indirect way by interacting with the atoms or

molecules of the cells (particularly water) and then produce some highly reactive

chemical intermediates, such as hydroxyl and oxygen radicals which may reach and

damage the critcial target of DNA.

Figure 2.6 UVR photon direct and indirect action to the DNA

(Pictures are sourcing from: www.nias.affrc.go.jp)


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DNA damages have two cases single strand break (SSB) and double strand break

(DSB). All organisms have the ability to repair the DNA damage. As DNA has four

kinds of nitrogenous base A-T and C-G, and repair or replication depend on specific

base pairing, therefore most of the SSB could be repaired to the original gene coding.

However, for the case of DSB, the chromatin snaps into two pieces, that will usually

leads to the dead of cell, mutation or even cancer cell.

Figure 2.7 DNA replication

Figure 2.8 Single-strand break & double-strand break of DNA

(Pictures are sourcing from: www.nias.affrc.go.jp)

2.3 The variation of the UV level from natural source

The ground level UV intensity is proportional to the solar spectral irradiance at the

upper layer of the atmosphere and the absorption and scattering of UVR by the

atmosphere and that is related to some factors.


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Position of the sun

For the higher position of the sun, UV intensity in the ground surface will be higher.

It will change with the season, time of that day and geographic latitude.

The equation of calculating the angular position of the sun is:

where, 0is the angular position of the sun

is the solar declination = 0.006918 0.399912cosn+ 0.070257sinn

0.006758cos2n+ 0.000907sin2n

0.002697 cos3n 0.001480sin3n

n= 2dn/ 365 , dnis 0 for 1-Jan & 364 for 31-Dec

is the latitude (positive in the northern hemisphere & negative in the

southern hemisphere)

this the local hour angle, th= (GMT/ 12 1 + / 180) + EQT

GMT is the Greenwich Mean Time

is the longitude (positive east of the Greenwich meridian, negative west)

EQT is the equation of time = 0.000075 + 0.001868cosn 0.032077sinn

0.014615cos2n+ 0.0040849sin2n

Amount of ozone

Ozone layer is response to absorbs UV radiation, so the higher concentration of the

ozone molecule in the atmosphere, the amount of UV radiation reaching the Earth's

surface will lesser.

Ground reflection

Different substances on ground can reflect the UVR to greater or lesser intensity,

cos0= sinsin+ coscoscosth


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below table show the UV reflectance on different surface:

Figure 2.9 Reflectance of UV

Sea level

The higher is the altitude about the sea level, the higher is the UV intensity, because

the amount of ozone available to absorb UVR is decreasing with the depth of the

atmosphere.

Clearance of the sky

The UVR intensity could be influenced by air and aerosols, since the molecules of

clouds, haze, floating pollutants and smog could scatter, absorb or reflect the photon

of UVR.


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2.4 Health protection

The biological effect is proportion to the wavelength and the dose of UVR. However

the effect of UVR to different people is various, since there are different types of

skin (details see Appendix B). In order words, the same doses are in the skin to

different people, the chances of causing skin cancer are also different. Despite of the

fact that it is difficult to measure the effect of UVR to each person, our attitude

toward the UV dose is that keeping it as low as reasonably achievable. For instance,

when we are located at outdoor in a sunny day, we may wear sun glasses to protect

the eyes and cover our skin with white clothing (as darker color absorbs more

radiation) to protect ourselves. Moreover, we may apply the sunscreen lotion

(principle on spreading a layer which could effectively absorb the UVR) with

suitable SPF/ PAvalue on skins or avoid staying outdoors when the UV indexis

high.

2.4.1 SPF and PA

SPF (Sun Protection Factor) introduced by United States, the value tells how long

you may stay in the sun without burning from UVB light. For example, peoples

skin will produce redness under sunlight in 10 minutes on average, if the value of

sunscreen lotion is SPF 15, which means you may stay in the sun without burning

for 15 x 10 = 150 minutes. SPF is only a sunburn meter allowing tanning with

enough sun exposure. However, UVA has not yet been tested to acquire a SPF rating;

we couldnt only use the SPF to evaluate the time of exposing sunlight.

PA (Protection grade of UVA) introduced by Japan Cosmetic Industry Association,

which divides the sunscreen lotion to three grades: PA+, PA++ and PA+++. For

more of the symbol +, indicating that the protection of UVA is higher.


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2.4.2 UV index

The UV index is a measurement of the UVR intensity at the ground surface relevant

to the effect on human skin and the respective exposure levels as categorized by the

World Health Organization (WHO) are shown in the table below:

Figure 2.10 UV index Table

Below is the standard method of calculating the UV index suggested by WHO:

Firstly measure the intensities of UVR at different wavelengths up to 400 nm (Blue

line in Figure 2.11), and then multiply these intensities by the weighting factors at

the corresponding wavelengths in the erythemal action spectrum (Red line in Figure

2.11) to reflect the human skin's response to each wavelength.

Secondly, sum up the products obtained above to get the total erythemally weighted

UV intensity in mW/m2, which is the area under the curve in Figure 2.12.

Lastly, multiply the total erythemally weighted UV intensity (area of the curve) by

0.04 to obtain the UV index.


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Figure 2.11 UV int. across the UV spec. & the erythemal action spectrum

Figure 2.12 Total erythemally weighted UV intensity (Area under the curve)

(Data Source: The National Oceanic and Atmospheric Administration)

For the UV level index is higher, which means the higher chance of getting sun-burn.

We may use the UV index as reference to adopt respective measure to protect

ourselves.


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2.5 Introduction to Gafchromic MD-55-2 film

Radiochromic reactions are defined as a direct coloration of a media by the

absorption of radiation without involving additional treatment. This type of reaction

were first observed and recorded by Niepce in year 1826.

The change of color after irradiating by radiation could be on both organic and

inorganic media. According to a research by M.J. Buston et al., he said

Radiochromic organic image-forming systems can involve cis-trans isomeric

dissociations or conversions, which can result in ketonic, anilic and enolic bonds.

These tautomerisations can lead to double-bonded colorations of anils organic acids,

stilbenes and other polycyclic compounds. Other radiochromic effects include

radiation-induced vesicular films giving light scattering properties or

radiation-induced hardness of polymeric microcapsules, containing diffusible dyes

which are released mechanically. Another image-forming technique is the use of

organic-free radical imaging medium which combines photo-polymerisation with

leuco dyes that produce color upon irradiation. This results in the pairing of free

radicals to turn radiation-induced cross-linked carbon chains which result in

covalently bonded growing chains.

Nowadays, most type of radiochromic materials are in form of film. The

radiochromic film is used as dosimeters for industrial and medical application. The

original film is clear and transparent, but it changes its color to blue after the high

energy photon irradiation. The darkness of the blue of the film is linear with the

radiation doses, we could use calibrated densitometer to measure the optical density

of the film, and the doses of radiation could be calculation. As there are many

different types of radiochromic film available in the market, for convenience, we

used the CAFCHROMIC MD-55-2 in this project, because it has most typical

structure and properties of radiochromic film.


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GAFCHROMIC MD-55-2 is a radiochromic film is a product of International

Specialty Products (ISP). The product is designed for measuring the absorbed dose

of high energy photons and it has been widely applied by the medical physicist and

dosimetrist working in the radiotherapy environment. It also has other applications

in external beam radiotherapy, particularly intensity modulated radiation therapy

(IMRT) and brachytherapy. Moreover, with the property of water resistance, it also

could use to study the UVR dosimetry under water. The structure of

GAFCHROMIC MD-55 radiochromic dosimetry film is shown in figure 2.13.

Figure 2.13 Structure of MD-55-2 radiochromic film

The standard construction of the MD-55-2 radiochromic film is that consisting of 2

clear polyester layers, 2 active layers, 2 adhesive layers and central clear polyester.

The thick of the active layer is about 16m and it is coated onto a 2.6 mils (about

67m) clear polyester, two pieces of this film construction are laminated to the

central clear polyester with 1 mil (about 25m) thickness by 1 mil (about 25m)

adhesive layer. Below is the summary of the composition in each layer:

Clear polyester: Carbon (45%), Hydrogen (36%) and Oxygen (19%)

Active layer: Carbon (31%), Hydrogen (51%), (Nitrogen (5%) and Oxygen (8%)


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Adhesive: Carbon (33%), Hydrogen (50%) and Oxygen (17%)

One of the advantages of radiochromic film is can reusable, so that it could measure

the fractionated doses in different situation, but on the contrary, due to its sensitivity

of UVR, the storage of the MD-55-2 radiochromic film should be away from the

UVR sources such as sun and fluorescent light, so as to prevent the unwanted

coloration before use. According to specification by ISP, the most suitable range of

the dose measurement of MD-55-2 radiochromic film is from 3 100Gy. Other

research by M.J. Buston found that the measurement dose rate range is 0.034Gy/min

to 80Gy/min and they also tested the response of various dose (UVB, UVA, visible

radiation, infrared and solar radiation) to the film, proved that it is very sensitive to

solar radiation and UVA, following is the result of the test.

Figure 2.14 Response dose of MD-55-2 radiochromic film

(Data source from: UVR dosimetry with radiochomic film by Martin J. Buston, T.

Cheung, Peter Yu, Donna Abbati & Greennoak)

The sensitometric response of MD-55-2 radiochromic film is also ideal for


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measurement of UVR dose because it has the linear relation of the dose of optical

density. Following data (from ISP) is obtained from a radiochromic densitometer

model 37-443. It shows the linear relation between the doses and density.

Figure 2.15 Sensitometric response of MD-55-2 radiochromic film


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3. Methodology

3.1 Construction of a UV meter

UV meter is a device that could measure the intensity of UVR. In an ideal case, the

meter should be able to measure the energy of the UVR per unit mass. Radiochromic

film will change its color after irradiance of high energy photons and very sensitive

to UVA, it is very useful to be the UV meter in this project. On the other hand, the

optical density of the radiochromic film could be found by a calibrated densitometer.

Traditional measuring device consists of three major parts i.) traducer ii.) container

and iii.) recorder. The radiochromic film plays the role of traducer which response to

sense the signal, and the densitometer plays the role of both container and recorder

to process and display the signal. Furthermore, one precaution of using the

radiochromic film for measurement is that we should avoid leaving finger print on it,

as it will affect the optical density. Every time I handle the experiment, I use glove

to avoid this problem.

3.1.1 Densitometer

To construct a densitometer is one of an important procedure in this project.

Following are the apparatus I used to make the densitometer:

1.) A red light LED as suggested by ISP, to monochromic red light source should

be used in the densitometer, LED does not have any heat or UVR in the light beam,

and it could produce a monochromic light without using any filter. Moreover, the

failure rate and energy consuming is very low, therefore, red light LED is very

suitable to be light source of densitometer.

2.) A photodiode in this project, BPW21 photodiode was used, it is similar to the

solar cell, which could generate a small current when illuminated by a beam of


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photons. When a piece of film is placed in between the LED and the photodiode, the

film will absorb some of the photon, and thus the current generated by the

photodiode will be smaller. With this characteristic, the current generated by the

photodiode current can used to calculate the doses absorbed by the radiochromic

film.

3.) A digital multi-meter this device can measure the magnitude of current

generated by the photodiode, and displays the data in digital format.

4.) A plastic case so as to avoid the background influence by surrounding and

protect the delicate parts of the densitometer, all the components were placed inside

a plastic container.

The circuit diagram of the densitometer is shown below:

Figure 3.1 Circuit diagram of the densitometer

Below are the external appearance of the densitometer (Figure 3.1a) and the internal

structure of the densitometer (Figure 3.1b). The operating keys of the multi-meter

are in an opened window, and there is a switch on the bottom of left hand corner,

also the slit for inserting the radiochromic film is on the bottom of the right hand

A


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corner. The front cover could be opened by removing the magic tape. This is

designed for replacement of any parts such as batteries, LED or photodiode in a

more convenient ways.

Figure 3.2a External appearance Figure 3.2b Internal structure

Before using the densitometer for measurement, it needs to calibrate. I used a set of

radiochromic film which has been exposed to UVA with known doses. The doses are

from the range of 0Gy 32.5Gy and difference between each dose is 2.5Gy, and a

calibrated line had got at last. All the measurements and calculations in this project

will base on the calibrated line as standard.

3.1.2 Use of radiochromic film

In this project, we used the Gafchromic MD-55-2 radiochromic film to measure

the UV dose. As the original size of the film is 5 x 5 but it is a waste of using a

whole film to measure in one situation, since the film is very expensive. Therefore, I

cut it into a smaller size and in form of strip so that it could be inserted to the slit of

the densitometer.

3.2 Measure the UV dose of a UV sterilizing lamp

Using the UV lamp for sterilizing is a very common method. However, the UVR


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released by the lamp may be harmful to us. In this project, I measured the UVA

doses released by a UV sterilizing lamp which specification is as below:

Table 3.1 Specification of ASL UV sterilizing lamp

Figure 3.3 UV sterilizing lamp

So as to distinguish the difference of absorbed doses of different distances from the

source, one piece of radiochromic film was in a distance about 5cm from the lamp,

another was in a distance about 20cm. The duration of this experiment is 60 minutes

and location is on the desk of the kitchen of my apartment (dimension: 1.7 m X 1.9

m X 1.9 m).

3.3 Measure the UV dose of a banknotes identifier

The problem of fake banknotes has never been stopped; so as to prevent financial

loss, using a banknotes-identifier is the simplest way for a genuine banknote has


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fluorescent mark, it is only visible under ultraviolet light. Cashiers need to use the

banknotes identifier everyday, they are most like to have more absorbed dose of

UVR. In this project the specification of the banknotes identifier I used is as below:

Brand K & C

Model AV-14V-SF

Country of origin China

Voltage 220 ~ 230 V / 50 Hz

Power 6W

Dimension 10cm x 14cm x 28cm

Table 3.2 Specification of K&C banknotes identifier

There is no cashier who is exposed to UVA continuously, however, the

Gafchromic MD-55-2 radiochromic film could measure the dose fractionation

(separating doses) accurately, and this property could be used to find the daily

accumulated dose of a cashier.

The duration of the measurement is 30mins I assumed the working hour of a

cashier is 10 hr each day, for each hour he/she uses the identifier for 40 times and

each time for 5s, i.e. 10 x 40 x 5 = 2000sec (approximate 30 mins).

3.4 Compare the efficiency of different brand of sunscreen lotion

Many peoples enjoy sun shower, most of them would use the sunscreen lotion to

prevent sun-burn. As there are many brands of sunscreen lotion in the market,

different sunscreen lotions also have its particular SPF value and PA grade. In this

project, I used 5 different sunscreen lotions to compare their efficiency of shield the

UVA from the sun.


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To perform this experiment, a sunny day should be chosen, so that the result will be

more accurate and the different efficiency of each brand will be more obvious.

Following is a summary of the sunscreen lotions I used in the experiment.

Table 3.3 Comparison of the sunscreen lotions used in this project

Five pieces of radiochromic film were clothed with the different sunscreen lotions

respectively. One more film would not cloth anything as the control experiment, so

as to find the UVA absorbeddoses on that date.


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4. Results and discussion

4.1 Calibration of the densitometer

For calibrating the densitometer, a set of Gafchromic MD-55-2 radiochromic film

with known dose are used, each film was measured 5 times and the means are used

to plot the graph and following table are the results in details:

A

A

Table 4.1 Calibration with standard doses

Figure 4.1 Calibration line


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The X-axis represents the absorbed UVA dose while the Y-axis represents the current

generated by the photodiode. The calibrating function could be approximated by 2

straight lines and result as below:

For, 0 X 7.5 Y = 2.09X + 30.51

X = 7.5 Y = 17.31

7.5 X 32.5 Y = 0.29X + 17.25

The above method could apply and find the dose directly. However, as to make the

result more accurate, in the following experiment I will use the measured value of

current to do a similar calibration again, so as to make the result more accurate.

Discussion:

The accuracy and precision of the densitometer are good. However, when measuring

the current without inserting any film, the result is different from time to time:

Measured current (A)

1st 64.12

2nd 63.68

3rd 63.52

4th 63.435th 63.26

Table 4.2 Densitometer response

This frustrating phenomenon is due to some reasons:

i.) The interaction between the photons and the photodiode is not uniform.

ii.) The red light LED does not produce the same amount of photons at all the time.


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iii.) The output of the power source is not the same at all the time.

iv.) Batteries are consumable, thus, the output power is slightly decreasing with

time.

Other then the frustrating phenomenon, there are also some instrumental errors due

to the multimeter, LED and the photodiode. To improve the accuracy of the

densitometer, better multimeter and photodiode could be used.

For the radiochromic film, it has a serious problem, as the 0Gy of the standard film

have been slightly change to blue color due to exposure of UVA. However, the line

plotted by the standard film showed that, the whole set of data is in linear relation, it

implied that the whole set of film were exposed to an additional amount of UVA

dose. This may be due to the film has been used for a long time some people let it

expose to UVA accidentally. In view of this, I used the 0Gy film and one new film to

do a calibration, and found that, their difference of the measured current is about

15A. Therefore, for all the data measured by using the new radiochromic film, in

this project, the value will minus 15A as an adjustment.

4.2 The UV dose of a UV sterilizing lamp

Measuring location: Apartment kitchen

Duration: 1 hour

Following is the result from the 2 pieces of radiochromic films:


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Table 4.3 Result by the UV sterilizing lamp

Both of the radiochromic films indicated the UVA dose from the UV sterilizing lamp

is very low, both are near tozero.

Discussion:

I have performed the experiment for twice times, as the 1sttime of doing the

experiment, I also got the same result, I doubt there is some serious errors lead both

film could not detect any UVA from the lamp. After that, I used other 2 pieces of

radiochromic film for the 2nd

time of the experiment, the result is also indicate that

there is no UVA. Therefore, I conclude that the UV sterilizing lamp does not

produce UVA, but other higher energy photons (UVB or UVC).

After the experiment the kitchen was full of bad smell, with reference to the

genertatin process of ozone in the stratosphere, the smell is from ozone gas. In

statrosphere UVC reacting with oxygen: O2+ UVC 2 O-and due to the free

radical of O-is highly reactive with the oxygen gas: O

-+ O2O3.

The zero dose of UVA does not mean the UV sterilizing lamp is a safe device for

using. On the contrary, it is very dangerous, as it produces the high energy

ultraviolet photons which could cause damaging biological effect. The working

principle of a UV sterilizing lamp is that on one hand releasing UVB/UVC to keep

the bacteria, on the other hand, the UVC could produce ozone (a toxic gas) to kill

micro-organisms.

The UV lamp is efficient for sterilization, but this device should be used in a place


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where should avoid contacting with human.

4.3 The UV dose of a banknotes identifier

Measurement location: The platform inside the banknotes identifier

Duration: 30 minutes

The measuring result is as below:

Table 4.4 Result of banknotes identifier

By using the calibrated line: Y = 1.536X + 28.18, the absorbed dose could be

calculated which is 2.92Gy.

Discussion:

The UVA absorbed dose of using this type of banknotes identifier is low. In other

words, it is safe to use. However, modern design of the banknotes identifier could

much reduce the user of contacting the UVR by building the light tube inside.

Certainly, it also could be used for counting the banknotes and with higher

identifying ability than the traditional one. Following figure shows the modern

design of the banknote identifier.


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Figure 4.2 Modern banknote identifier

4.4 The efficiency of different brand of sunscreen lotion

Measurement location: Sai Sha pebble beach

Experiment date: 14-Mar-2009

Time: 15:00 16:00

(A)

Table 4.5 Result of different brands of sunscreen lotion

The film without using the sunscreen lotion measure value 14.71A, using the

calibrated line: Y = 0.648X + 22.02, the total absorbed dose is 11.28Gy

For Coppertone SPF 4, using the calibrated line: Y = -2.092X + 30.51, absorbed

dose is 2.45Gy, which shield 78.26% of UVA.

For Coppertone SPF 50, using the calibrated line: Y = -2.092X + 30.51,absorbed


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dose is 0.16Gy, which shield 98.56% of UVA.

For COS MED, using the calibrated line: Y = -2.092X + 30.51, absorbed dose is

1.33Gy, which shield 88.18% of UVA.

For Sun Bears, using the calibrated line: Y = -1.716X + 28.63, absorbed dose is


For Whos, using the calibrated line: Y = -2.092X + 30.51, absorbed dose is


Discussion:

Figure 4.3 and figure 4.4 show the environment of the experiment, which may

influence the experiment, such as the water, stone, grass and stand

Figure 4.3 Environment of Sai Sha

Figure 4.4 Platform of measurement


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From the result, which showing that the different brands of sunscreen lotion have

different ability to shield the UVA, even the PA grade is the same (Coppertone SPF

50 and Sunbear). The best performance if Coppertone SPF 50, while the worst

performance is Sunbear.

Improvement of the experiment

Other than the surrounding materials are source of the error. Other errors are that it

is difficult to cloths the radiochromic film with the sunscreen lotion liberally, and the

radiochromic film should clear with water thoroughly before inserting it to the

desitometer, otherwise the residue of the sunscreen lotion in the film will affect the

optical density.


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5. Conclusion

Gafchromic MD-55-2 radiochromic film and the densitometer is a good UVA dose

meter after calibrated against a primary standard. However, when handling the

radiochromic film, we should avoid leaving finger print on it, as it will affect the

optical density.

For the UV sterilizing lamp, it doesnt emit UVA, but evidence showed that, it emits

the higher energy photon UVB/UVC, it will cause more biological effect and release

toxic gases ozone, it is a dangerous and should handle with great care when using it.

For the banknotes identifier, the dose is very low, very little chance to cause any

biological effect, and so it is safe for using, but modern banknotes identifier design

is much more safe and reliable than that used in the experiment.

Some of the sunscreen lotion available in market dont use PA grade, but all of them

have the SPF value. SPF value doesnt indicating of the protection of UVA. In this

project, the best performance sunscreen lotion in the experiment is SPF50 and PA++,

the worst performance if SPF30 and PA++, which implied that even the same PA

grade, but the efficiency is not the same.


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6. Reference

1. Dr. J. Chavaudra, Dr. M. Faber, Dr. C.Frohlich, Professor F. Stenback, Professor F.

Urbach, Professor M. Wassermann, Dr. R. D. Bojkov and M. Malone, 1979,

Environmental Health Criteria 14, Ultraviolet Radiation

2. Roger L. Clough and Shalaby W. Shalaby, 1994, Irradiation of Polymers

3. Manfred Tevini, 2000, UV-B radiation and ozone depletion effect on humans,

animals, plants, micro-organisms, and materials

4. Roger L. Clough and Shalaby W. Shalaby, 1990, Radiation effects on Polymers

5. Francesco Ghetti, Giovanni Checcucci and Janet F. Bornman, 2001,

Environmental UV radiation: impact on ecosystems and human health and

predictive models

6. Christos S. Zerefos and Alkiviadis F. Bais, 1997, Solar Ultraviolet Radiation

7. John A. Parrish, R. Rox, Frederick Urback and Donald Pitts, 1978, UVA

biological effects of ultraviolet radiation with emphasis on human responses to

longwave ultraviolet

8. Buston M.J., Cheung Tsang, Yu Peter. K.N. Abbati D. & Greenoak G..E. (2000)

Ultraviolet radiation dosimetry with radiochromic film. Phy Med. Biol. 45 1863 -


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1868

9. http://copublications.greenfacts.org/en/sunbeds/l-3/7-conclusion.htm#0p0

10. http://www.gafchromic.com/

11. http://www.harpell.ca/manufacturer/isp/gafchromic-ebt

12. http://www.hos.org/hpspublications/articles/uv.html

13. http://www.webshade.com.au/ShadeInfo/ShadeFacts/factors.html

14. http://en.wikipedia.org/wiki/Ultraviolet


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Appendix A

Occupation potentially associated with UVR exposure:


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Appendix B

Six types of skin

Classification based on the sun-burning and tanning response to sunlight

Skin type Typical featuresSunburn

suscep-tibilityTanning ability Skin cancer risk

IPale white skin, blue/

hazel eyes, blond/ red hairHigh None High

II Fair skin, blue eyes High Poor High

III Darker white skin Moderate Good Low

IV Olive skin Low Very good Low

V Brown Very low Excellent Very low

VI Dark brown or black skin Very low Excellent Very low

Source from: GreenFacts

Chang Shu Wing

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