study of chemical etching behavior of lr- 115 type ii...

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CHAPTER 6

STUDY OF CHEMICAL ETCHING BEHAVIOR OF LR-

115 TYPE II SOLID STATE NUCLEAR TRACK DETECTOR

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CHAPTER 6

STUDY OF CHEMICAL ETCHING BEHAVIOR OF LR-115 TYPE II SOLID STATE NUCLEAR TRACK DETECTOR

6.1 Introduction

The discovery, development and application of Solid State Nuclear Track

Detectors (SSNTDs) within the last two decades, is in many ways a good example of the

birth and growth of a new discipline. Already these detectors are in use in many diverse

branches of science from nuclear physics to geology and from space physics to

archeology. Solid State Nuclear Track Detectors (SSNTDs) are well known for the

detection of ionizing radiation through track formation of heavy ionizing particles.

Relative sensitivity of different SSNTDs is different. Plastic detectors are most sensitive

whereas glass and minerals detectors are least sensitive.

This chapter deals with the basics of track formation, its revelation by chemical

etching, conditions during track formation and the use of an enhancement tool for track

revelation in LR-115 type II detectors along with exploring a novel method of % mass

change of the polymeric sample to determine the bulk etch rate of the samples. The

results of both pre-UV and post-UV exposure to alpha irradiation on the detector are

compared with the results of normal etching conditions.

6.2 Formation of Particle Tracks

The passage of heavily ionizing nuclear particles through most insulating solids

creates narrow paths of intense damage on an atomic scale. These damaged tracks may be

revealed and made visible in an ordinary optical microscope by treatment with a properly

chosen chemical reagent that rapidly and preferentially attacks the damaged material. It

less rapidly removes the surrounding undamaged matrix in such a manner to enlarge the

etched holes that mark and characterize the sites of the original, individual, damaged

regions. This simple technique of observing particles has been used in wide variety of

technical fields that range from nuclear science and engineering to cosmic ray

astrophysics and from geology, archaeology, and sub oceanic geophysics to lunar sites

and meteoritics.

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6.2.1 Energetic Ion Interactions with Polymers

When a charged particle passes through a polymeric material, it loses its energy

mainly through four processes: electronic stopping (ionization and excitation), nuclear

stopping (displacements), radiative losses (bremsstrahlung and Cherenkov) and phonon &

plasmon decay.

(i) Fleischer et al. (1975) suggested that due to irradiation, a burst of ionization takes

place along the path of a charged particle to create an electro statically unstable array

of adjacent ions which eject one another from their normal sites into interstitial

position. Due to this primary ionization an array of interstitial ions and vacant lattice

sites is produced by the coulomb energy of the ions that give rise to space charge

polarization of the sample. We can also say that a fast charged particle will eject

electrons from atoms that were close to its path, leading to the polarization of sample

induced by irradiation. After this, elastic relaxation diminishes the acute local stress

by spreading the strain more widely. So it creates a long range chain that makes

possible the direct observation of unetched tracks in crystals by transmission electron

microscopy. The electronic stopping process results from the interaction of incident

ion with target electrons. The primary products produced during this process are

electrons, ions, atoms in excited states, free radicals and molecules. The energy of the

incident ions is distributed by the excited molecule among its neighbors as phonons

and excitons. These are the processes of ionization and excitation respectively. In

addition the chemical bonds may also break (chain scission), when the excited energy

is localized in a particular chemical bond. Such radiolysis scission frequently causes

the loss of side groups, such as hydrogen. This reaction produces unsaturated bonds

in polymer chain or cross-linking when the cleavage of C-H bonds occurs on adjacent

molecules. The electronic processes involve electrostatic forces between the incident

particle and the electrons surrounding the target nuclei leading to the stripping of

these electrons from their orbits or to raising the electrons to less tightly bound states.

(ii) The nuclear stopping processes involve electrostatic forces between the moving ion

and the target nuclei resulting in the ejection of target atoms from lattice sites or out

of the molecular chains. This chain scission leads to the formation of large

molecular species such as hydrocarbons. When scission occurs at pendent atoms or

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side groups, unsaturation or cross linking may occur.

(iii) In radiative losses, a charged particle while passing close to the field of a nucleus

gives rise to the emission of electromagnetic radiation known as bremsstrahlung.

Bremsstrahlung is an important means of energy loss only for light particles such as

electrons. At a given energy of the incident particle the energy loss through

bremsstrahlung emission is proportional to (Z1/ml) Z2, where Z1, m1 are the charge

and mass of the incident particle and Z2 is the charge on the target nucleus. In

addition, at velocities greater than the phase velocity of light in the stopping

medium, the moving charge causes polarization of atoms near to the particle

trajectory and a coherent wave front of radiation is formed known as Cherenkov

radiation. For non- relativistic incident particles, radiative losses are negligible.

(iv) The phonon or plasmon decay mainly constitutes thermal energy losses by atoms

and electrons. These do not have any significant impact on materials properties.

Among these processes, the first two produce most significant effects in changing

material properties. Both electronic and nuclear processes contribute to cross-linking and

scission in varying degree. Experimental evidences indicate that electronic processes are

responsible mainly for the formation of unsaturated bonds and cross linking while the

nuclear processes are the main cause of chain scission. Among many other processes that

may occur during irradiation, cross-linking, unsaturated bond formation and chain

scission are considered to be the most important processes for modifying the polymeric

materials. At the energies, typically more than 1Mev/amu the velocity of the ions or the

charged particle is comparable or higher than the Bohr electron velocity. The ions with

such high energies are referred to as swift heavy ions (SHI). Nuclear energy losses

dominate at lower energies whereas the electronic energy losses dominate at higher

energies. The other important change in polymeric properties due to irradiation comes

from the compositional modification, loss of gaseous molecules such as hydrogen oxygen

and nitrogen alters the chemical composition of polymers drastically. Injected ion species

may be trapped at certain preferred sites in the matrix, and can form precipitates by self-

clustering or by reacting with host elements, or assist in cross-linking by forming

chemical bonds with the polymer chains.

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6.2.2 Penetration Range

The distance that the ion travels before coming to full stop is called penetration

range R. Penetration range can be calculated with different available codes as for example

TRIM (Transport of Ions in Matter). From calculation with this program it is found that

ions with energy of 11.4 MeV/nucleon can travel 100-150 µm in polycarbonate before

their energy reaches the nuclear stopping range.

6.2.3 Latent Tracks

The latent track consists of a track core and a track halo. The track core has a

diameter less than 10 nm and is surrounded by the track halo, whose diameter is defined

by the maximum range of the delta (δ) electrons and may reach a size up to 1000nm. The

specific damage contained in the latent track depends on the kind of material.

Amorphisation of crystalline targets, creation of defects or outgasing processes, are some

of the presently known irradiation effects. In the case of polymers, the track halo has a

special relevance. Polymers are, together with for example alkali halides and biological

objects, sensitive to radiolysis. For this reason, the δ-electrons may cause in these

materials further direct damage in a similar way as under electron irradiation. In particular

polymers, chemically activated species may undergo secondary reactions on a time scale

of milliseconds to hours. Reaction with light or other ambient air also take place in weeks

or months. The first ion track was observed in mica by Silk and Barnes with a

transmission electron microscope (TEM). Since then, tracks in different materials have

been revealed with atomic resolution by employing different techniques.

6.3 Procedures for Revelation of Tracks

The quality of information that can be acquired on the nature of tracks depends on

the means of observation. So here are the several methods used to or attempted for seeing

the tracks.

6.3.1 Chemical Etching

Chemical etching is the most preferred way of observing the particle tracks in

solids. This is the technique of preferential chemical attack to enlarge and display tracks.

The utility of the technique derives from its simplicity, that only common chemicals

being needed and from the effective magnification that results from enlarging etched

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tracks to sizes where they can be viewed with an ordinary optical microscope. The

procedure has allowed tracks to be observed in many dozens of substances. It is also the

only procedure that has succeeded in revealing tracks of extremely low energy nuclei in

solids.

6.3.2 Transmission Electron Microscopic (TEM) Observations

It can be used both as a diffraction contrast mode and a thickness contrast mode.

The observation of fission fragment tracks in mica was made with a transmission electron

microscope (TEM), using the electron beam in a diffraction contrast mode to observe

tracks as dark lines where crystal planes are bent sufficiently to scatter electron out of the

direction of the Bragg reflection that is imaged. The tracks can also be seen with the TEM

by using thickness contrast in viewing etched sample. Here they appear as light

lines.TEM observations of un-etched tracks by diffraction contrast are inherently limited

to crystalline materials. They are conveniently scanned when very high track densities are

present.

6.3.3 Decoration Technique

A variety of methods have been used to nucleate precipitates of a second phase

along the damaged track with in the primary phase. In general the methods have the virtue

that they allow tracks to be displayed throughout a large volume, so that the full lengths

of track can be measured even though the tracks fail to reach a surface of detector.

6.3.4 Detection by Colour Change

The color centers present in a photo chromatic material can be used to display

particle tracks.

6.3.5 Detection with X-rays

In principle it should be possible to image individual tracks in solids by means of

their strain fields, in a manner similar to the X-ray topographic methods of imaging

dislocations. As yet this has not proved possible, possibly because the strain around a

track is expected to decrease as r-2 with radial distance as compared to r-1 for the strain

field of a dislocation.

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6.4 Etching

Etching is the process of modifying the flat smooth surface to identify the structure,

phases, and other effects such as orientation of grains, deformation and distribution of

solute elements.

6.4.1 Visual Examination of Sample

It is always preferable to look at a specimen in the unetched condition in the first place,

particularly when the specimen has been taken from a component, when it is being used

to sample the quality of material.

6.4.2 Depth of Etching

In general, etching should be kept to a minimum. For macro-examination, etching has to

be fairly deep in order to reveal the features of interest. As the magnification and

resolution of the optical microscope are increased, the depth of the attack has to be

reduced; otherwise the largeness of attack could impose a structure (such as resolvable

pits) upon the true structure of the alloy. Furthermore, a deeply etched specimen

examined at higher magnifications could also have changes of level outside the depth of

the field of the objective. With phase-contrast illumination the desirable depth of etching

is usually even smaller.

6.4.3 Types of Etching

6.4.3.1 Chemical Dissolution

Probably the majority off etched specimens is prepared by dissolution of the specimen;

the solutions used being selected because they dissolve various constituents differently.

a. Strong Oxidizing Agents- These are strong acids like nitric acid, sulfuric acid and

chromic acid. They are usually used in diluted form.

b. Organic Acid- Acids like citric acid, picric acid, acetic acid and oxalic acids are

extensively used. They are weak acids and are used in high concentrations.

c. Diluents- These are non-aqueous such as ethyl alcohol, glycerol or one of the more

complex glycols. They dilute the active agents without pitting and grooving complex

aqua-ions would not be formed with ions of metal oxides.

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d. Reagents- Ferric chloride and hydrochloric acid. They give differential staining of

constituents by depositing colored or finely divided particles of some reaction

product on the phase examined.

6.4.3.2 Tinting

Tinted layers on the specimen are given such that the differentiation in color (or

depth of particular color) from one constituent to another. There are two methods of

tinting.

a. Chemical controlled oxidation

b. Sulfide formation

They are capable of sensitively differentiating between phases. The specimen is allowed

to oxidize in air by heating oxides giving interference colors.

6.4.3.3 Electro-etching

This is essentially a chemical etching where one uses reagents to dissolve phase of

the sample to be examined on the application of current. Metal ions are formed at the

anodes and electron appears at cathode. Different metal regions may be are more anodic

than the other so they dissolve preferentially.

Electro-etching is capable of more precise control than chemical etching. The potentiostat

is device which enables voltage in the electrolytic cell to be maintained at a

predetermined level, so that particular anodic (or cathodic) reactions may be isolated.

Thus it is possible to etch various phase selectively. Once these voltages are known for a

particular system, the voltage can be used to identify constituents.

6.4.3.4 Vacuum Etching

Important for study of grain boundaries, grains, twins and other features are

revealed by suitable specimen in vacuum. Structure is revealed by ridges or pits which are

related to specific crystal planes.

6.4.3.5 Cathodic Etching

Used for alloys or samples comprising different layers of very dissimilar alloys.

The specimen is made the cathode in a discharge tube and bombarded with ions of the gas

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in the tube. The discharge tube is taken in the form of a bell jar. The anode is placed a few

inches away, often above the specimen. The rate of etching is controlled by the energy

and size of the bombarding ions by the current density they impose. A typical current

density is 20mA/cm2 and typical ion energy is 5keV. The bell jar is pumped out to 10-3

torr and the gas (say argon) is admitted to a pressure of 10-2 torr.

6.4.3.6 Etch Pitting

Chemically etched surface may be obtained by using reagent that are very

selective and attack only the sites of emergent dislocations. Crystallographic pits may

then be formed and give information about dislocation densities, sub grain boundaries and

orientation etc.

6.5 Materials and Method

There are various methods to measure concentrations of 222Rn and its progeny,

among which the SSNTDs, particularly cellulose nitrate (CN), are used. Alpha particles

emitted by 222Rn and its progeny hit the detector and leave the latent tracks in it, which

can be made visible under the optical microscope after chemical amplification via

etching. Chemical etching is a process of pore formation, during which a suitable etchant

attacks the detector isotropically at a sufficient speed and the damaged regions along the

ion trails (latent track) are preferentially dissolved, removed and get transformed into a

hollow channel (Fleischer et al., 1964).

6.5.1 Effect of Temperature and Normality on Chemical Etching Behavior of LR-115

Track development in an SSNTD is based on two parameters Vt (trach etch rate)

and Vb (bulk etch rate). Therefore a precise control of the bulk etch rate, among others, is

crucial for a correct measurements of the 222Rn concentrations. As it is known that Vb

depends on many factors like structure, chemical composition and preparation of the

detector, etching conditions (like chemical nature of etchant, temperature, concentration

as well as the stirring conditions) aging, pre-irradiation treatment and storage conditions,

environmental conditions during irradiation (ionizing as well as the non ionizing

radiation) etc. (Fujii et al., 1991). The chemical etching behavior was studied by

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measuring the bulk etch rates of LR-115 under the effect of different etching temperatures

and different normalities of the etchant.

6.5.2 Results and Discussion

The cumulative thickness removed of the samples after etching was shown in the

table 6.1 for etching in 2.5 N aqueous solution of NaOH at three different temperatures

viz. 40 ˚C ± 0.5 ˚C, 50 ˚C ± 0.5 ˚C and 60 ˚C ± 0.5 ˚C. Fig. 6.1 show the graph of the

cumulative thickness removed of pristine LR-115 samples in 2.5 N aqueous solution of

NaOH at three different temperatures viz. 40 ˚C ± 0.5 ˚C, 50 ˚C ± 0.5 ˚C and 60 ˚C ± 0.5

˚C. It also shows the bulk etch rate of the sample for these three different cases.

For studying the effect of normality the cumulative thickness removed of the

samples after etching was shown in the table 6.2 for etching in 1.5 N, 2.5 N and 3.5 N

aqueous solution of NaOH at 60 ˚C ± 0.5 ˚C. Fig. 6.2 show the graph of the cumulative

thickness removed of pristine LR-115 samples in 1.5 N, 2.5 N and 3.5 N aqueous solution

of NaOH at 60 ˚C ± 0.5 ˚C. It also shows the bulk etch rate of the sample for these three

different cases.

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Table 6.1: Cumulative thickness removed (in µm) in 2.5 N NaOH with time at 40 ˚C, 50 ˚C and 60 ˚C ± 0.5 ˚C

Table 6.2: Effect of normality of NaOH on cumulative thickness removed with time at 60 ˚C ± 0.5 ˚C

Time (min.) Cumulative thickness removed (m)

40 oC 50 oC 60 oC

20 0.0 0.0 1.0

40 1.0 1.0 2.0

60 1.0 2.0 4.0

80 2.0 3.0 5.0

100 2.0 3.0 7.0

ime (min.) Cumulative thickness removed (m)

1.5 N NaOH 2.5 N NaOH 3.5 N NaOH

20 1.0 1.0 1.0

40 2.0 2.0 2.0

60 3.0 4.0 6.0

80 4.0 5.0 7.0

100 5.0 7.0 8.0

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Fig. 6.1: Cumulative thickness removed and the bulk etch rate of pristine LR-115 samples in 2.5 N NaOH with time at 40 ˚C, 50 ˚C and 60 ˚C ± 0.5 ˚C

Fig. 6.2: Cumulative thickness removed and the bulk etch rate of pristine LR-115 samples in 1.5 N, 2.5 N and 3.5 N NaOH with time at 60 ˚C ± 0.5 ˚C

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6.5.3 Determination of Bulk Etch Rate: Analytical Method

Various techniques have been used for the determination of bulk etch rate (Yu et

al., 2004; Yip et al.,2003, 2006; Salama et al., 2006; Tse et al., 2007).. Also there are

several methods used for the direct measurement of the bulk etch rate viz., digital

micrometer, AFM or surface profilometer, through measurements of track-parameters etc.

This work is devoted to determine the bulk etch rates of the LR-115 detectors (Type II,

non-strippable, purchased from DOSIRAD, France) under different etching conditions by

exploring a novel method of % mass change of the polymeric sample.

The bulk etching behavior of LR 115 type-II polymer was studied by measuring

the % mass change in the samples after chemical etching in 1.5 N NaOH and 2.5 N

NaOH solutions. The sample was subjected to etching by submerging it in the chemical.

The etching process starts uniformly on the whole surface layer of polymeric samples

(but not on the polyester base as it is not etched by NaOH solution). The active layer

starts coming out of the sample and leads to the reduction in the mass of the sample. The

proposed method of % mass change of the sample was related to the sample thickness

removed using the relation

ρ = = × ×

--------------------------- (6.1)

Where ρ stands for density and 푙, 푏, 푡 stands for length, breadth & thickness respectively.

However to check the suitability of the method the bulk etch rates of these

samples were also measured by another method. The details of this technique has been

discussed in Chapter 3 (Section 3.3.5 “Technique for Measurement of Bulk Etch Rate,

Track Etch Rate and Track Sensitivity”)


The % mass change in the samples after etching was shown in the table 6.3 for

etching in 1.5 N and 2.5 N aqueous solution of NaOH. Table 6.3 also shows the estimated

thickness removed using equation (6.1). Fig. 6.3 show the graph of the estimated

thickness removed of pristine LR-115 samples in 1.5 N and 2.5 N NaOH with time at 60

˚C ± 0.5 ˚C.

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Table 6.4 shows the cumulative thickness removed measured by a digital

micrometer. Fig. 6.4 show the graph of the cumulative thickness removed of pristine LR-

115 samples in 1.5 N and 2.5 N NaOH with time at 60˚C ± 0.5 ˚C. Table 6.5 shows the

coefficients (A and B) for the linear regression equation (y = A + Bx) for the relationship

between estimated thickness removed from the % mass change (y) [cumulative thickness

removed(y) in second method] and the etching time (x) for 1.5 N and 2.5 N NaOH at 60

˚C ± 0.5 ˚C and the corresponding determined bulk etch rates (Vb in µm h-1). The bulk

etch rates for the pristine LR-115 was found to be 3.02 µm h-1 and 4.01 µm h-1 with the

estimated thickness removed from the % mass change method in 1.5 N NaOH and 2.5 N

NaOH at 60 ˚C ± 0.5 ˚C respectively. The bulk etch rates for the pristine LR-115 was

found to be 3.00 µm h-1 and 4.20 µm h-1 with the cumulative thickness removed from the

measurements done by a digital micrometer in 1.5 N NaOH and 2.5 N NaOH at 60 ˚C ±

0.5 ˚C respectively. A good correlation was found between the bulk etch rates determined

by using two different methods.

6.5.5 Determination of Track Sensitivity

Normally the radon monitoring is done by employing the detector in the natural

environmental conditions that can change the detectors properties. These environmental

effects are more prominent in countries like India, where temperature levels are high and

solar radiations are very intense. As the track registration and development in an SSNTDs

can be affected by these parameters so the study of surface chemical etching behavior of

this polymer by giving pre-UV and post-UV exposure to alpha irradiation on it is an

important aspect.

The measurement of radon values depends upon the exact counting of the tracks produced

by alpha particles on these detectors. The counting of these tracks is normally done by an

optical microscope of suitable magnification. So the quality of visualization of these

tracks is also an important factor. Some of the workers have reported the methods of

enhancing the revealed tracks for registration of alpha particles (Sohrabi and Khajeian,

1984; Su, 1988). The ultrasonic beam is employed to the etchant to find its use as a tool

for enhancing the revelation of the alpha particle tracks in the detector. The track

sensitivity (Vt/Vb) describes the shape and size of the track and depends on both bulk etch

rate and track etch rate. For determination of the bulk etch rate we used direct

151

measurement technique. (Mehta et al., 2009; Shikha et al., 2009). The track etch rates

were also determined for finding the track sensitivity. The track sensitivity and effect of

UV and Ultrasonic beam on LR 115 type-II polymer was studied by chemical etching by

giving pre-UV and post-UV exposure to alpha irradiation on it, by the application of

ultrasonic beam on the etchant after alpha irradiation and under normal conditions. The

effects are discussed in terms of bulk etch rate Vb, track etch rate Vt, and track sensitivity.

The details of this technique has been discussed in Chapter 3 (Section 3.3.5 “Technique

for Measurement of Bulk Etch Rate, Track Etch Rate and Track Sensitivity”)

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Table 6.3: % mass change of pristine LR-115 samples in 1.5 N and 2.5 N NaOH with time at 60 ˚C ± 0.5 ˚C and estimated thickness removed

Table 6.4: Cumulative thickness removed of LR-115 samples in 1.5 N and 2.5 N NaOH with time at 60 ˚C ± 0.5 ˚C

Time (min.) Cumulative thickness removed of pristine LR-115 samples (µm)

1.5 N NaOH 2.5 N NaOH

20 1 1

40 2 2

60 3 4

80 4 5

100 5 7

Time (min.) % mass change of pristine LR-115 samples

Estimated thickness removed (µm) from equation (6.1)

1.5 N NaOH 2.5 N NaOH 1.5 N NaOH 2.5 N NaOH

20 0.89 1.05 0.98 1.15

40 1.78 2.22 1.96 2.44

60 2.50 3.18 2.75 3.50

80 3.74 4.94 4.11 5.44

153

0 20 40 60 80 1000

1

2

3

4

5

6at 60 oC for pristine in 1.5 N NaOHa

Est

imat

ed T

hick

ness

Rem

oved

(m

)

Time (min)

0 20 40 60 80 1000

1

2

3

4

5

6

7 b at 60 oC for pristine in 2.5 N NaOH

Estim

ated

Thi

ckne

ss R

emov

ed (

m)

Time (min)

Fig. 6.3: Estimated thickness removed of pristine LR-115 samples in (a) 1.5 N and (b) 2.5 N NaOH with time at 60 ˚C ± 0.5 ˚C

0 20 40 60 80 1000

1

2

3

4

5 a

Cum

ulat

ive

Thi

ckne

ss r

emov

ed (

m)

at 60 oC for pristine in 1.5 N NaOH

Time (min)0 20 40 60 80 100

0

1

2

3

4

5

6

7

8

b at 60 oC for pristine in 2.5 N NaOH

Cum

ulat

ive

Thi

ckne

ss r

emov

ed (

m)

Time (min)

Fig. 6.4: Cumulative thickness removed of pristine LR-115 samples in (a) 1.5 N and (b) 2.5 N NaOH with time at 60 ˚C ± 0.5 ˚C

154

Table 6.5: The coefficients (A and B) for the linear regression equation (y = A + Bx) for the relationship between the estimated thickness removed from % mass change (y) [cumulative thickness removed(y) in second method] and the etching time (x) for 1.5 N and 2.5 N NaOH at 60 ˚C ± 0.5 ˚C and the corresponding determined bulk etch rates (Vb in µm h-1)

Table 6.6: Sample condition, etching time, bulk etch rate, track etch rate and the track sensitivity for the etching of different samples in 2.5 N NaOH at 60˚C ± 0.5 ˚C

S. No.

Sample condition Etching Time (min)

Bulk Etch Rate Vb

(m h-1)

Track Etch Rate Vt

(m h-1)

Track Sensitivity

Vt / Vb

1 Only alpha irradiated (Set A)

100 4.2 8.51 2.03

2 Post irradiation (alpha irradiation + 2 h UV

exposure) (Set B)

80 4.8 9.27 1.97

3 Pre irradiation (2 h UV exposure + alpha irradiation) (Set C)

80 4.7 8.44 1.76

4 Alpha irradiated + Ultrasonic etched

(Set D)

80 5.33 12.68 2.38

Normality With estimated thickness change from % mass change

With cumulative thickness removed

1.5 N NaOH A = -0.05 ±.002 B = 0.0504 ± 0.0016

Bulk etch rate(Vb) = 3.02 ± 0.09 µm h-1

A = 0 B = 0.05 ± 0.001


2.5 N NaOH A = -0.15429 ± .004 B = 0.06669 ± 0.003


A = -0.3333 ± 0.003 B = 0.07 ± 0.004


155


The surface chemical etching behavior of LR 115 type-II was studied by measuring

the cumulative thickness (total etched-out thickness of the sample, as measured by the

digital micrometer before and after etching), removed. Table 6.6 shows the results

obtained for the present study for etching in 2.5 N NaOH at 60 ˚C ± 0.5 ˚C for different

sample etching conditions (Set A, B, C and D). It shows the values of bulk etch rate, track

etch rate and the track sensitivity for all the sets of the samples. The bulk etch rate and

track etch rate were calculated using the formulas given elsewhere for CR-39 detector

(Ram and Bose, 1984)

for the bulk etch rate Vb:

Vb = Δ (6.2)

where Δx is the thickness of the detector removed during the etching in time t.

for the track etch rate Vt:

Vt = Vb (6.3)

where Y = Δ

, d is the track diameter and Δx is the thickness of the detector removed

during the etching in time t.

However in our case of LR-115 type II detector, this detector has sensitive layer on only

one surface and polyester layer on other surface i.e. resistant to etching in our case (the

polyester layer was obtained by removing the active layer of LR-115 using a razor and

then employed for etching in 2.5 N NaOH at 60 ˚C ± 0.5 ˚C, we found no removal of the

layer for 100 min of etching time), so we have omitted the factor 2 for the calculation of

these parameters (Fleischer et al., 1975; Durrani and Bull, 1987). The passage of charged

particles through the polymer leads to the chain scission and creates narrow paths of

intense damage. Fig. 6.5 shows the photomicrographs of the samples of different sets

etched under different conditions revealing the alpha particles tracks in LR-115.

156

i) Effect of UV exposure: Table 6.6 clearly shows the effect of exposure of UV light for

both pre-irradiated and post-irradiated samples. It can be seen from that with the exposure

of UV light to samples the track sensitivity of the samples reduces than the track

sensitivity of the normal condition samples irrespective of that the UV light exposure

being done prior to alpha irradiation or after the alpha irradiation. The decrease of track

sensitivity can be understood in terms of slight increase in the bulk etch rate of the

samples and not much change in the track etch rate of the samples. The UV exposure of

the polymer samples in the presence of oxygen causes deterioration of its properties. The

possible method of this photo-oxidation is a free radical chain mechanism that goes with

initiation, propagation, termination and chain branching. In case of LR-115 type II

detector a die is being added to provide the detector a red color.

However the use of dies can minimize photo-oxidation in the polymer and thus

reduce the polymer degradation. So the increase in bulk etch rate may be due to thermo-

oxidation whose process is almost similar to photo-oxidation. Normally there is sharp

increase in bulk etch rate with UV exposure for CR-39 SSNTD case (Wong and Hoberg,

1982), but in our case the increase is not very sharp as explained above. The track

sensitivity of set C (2 h UV exposure then alpha irradiated) samples is less than the track

sensitivity of set B (alpha irradiated then 2 h UV exposed) samples and so the efficiency

of Set C samples would be higher than that for set B samples. This can be explained due

to more track etch rate in post-irradiation case. The reason behind that may be the

creation of some preferential sites for the absorption of UV light quanta due to the

damage created by alpha particles. Those preferential sites are created at the track sites

that lead to the little increase in the track etch rate and hence the track sensitivity. This

result is in agreement with the suggestions given in (Henke et al., 1970). However the

decrease in track sensitivity is not drastic for both pre-irradiated and post-irradiated

samples of LR-115, which shows that the sensitive layer of this detector is not much

affected by the exposure of UV beam for the 2 h of exposure time. This may be due to the

protective behavior of its red dye against UV exposure.

ii) Effect of ultrasonic etching: Fig. 6.5 (a) and (d) shows the tracks of the alpha

particles revealed by the normal etching method and ultrasonic etching method

respectively. In both the cases the tracks were round but latter were more pronounced and

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easy to identify and have slightly more diameter than the former one. The ultrasonic

etching process reveals the tracks in less time than the normal etching case.

The rate of dissolution of the LR-115 in NaOH depends on the concentration of

the active layer of cellulose nitrate in the etchant. It becomes slower as the concentration

of the cellulose nitrate in the etchant increases. Under normal conditions, if the bulk

etching rate is slower than the rate of etching along the radiation damaged region, alpha

tracks can be etched out. However, as the concentration of cellulose nitrate dissolved in

the track core region becomes higher, the rate of dissolution becomes slower; so the bulk

etching rate may be in comparison to the track etching rate. This explains the situation of

the normal etching in which tracks are revealed but are not of good quality.Both the bulk

etching rates and track etching rates were found to be more in ultrasonic etching condition

than in the normal condition. However the track etching rate increases more than the bulk

etching rate in ultrasonic etching condition that leads to the increase in the track

sensitivity. This suggests that the track etching response is better in ultrasonic etching in

comparison to normal etching. To understand these results we have to look into the

mechanism of ultrasonic etching. Ultrasonic’s travel in the form of compression and

rarefaction in the etchant, they produce rapidly changing pressure in the liquid that causes

the formation of cavities where the pressure is relatively low. These cavitation bubbles of

the etchant are supported only by the negative pressure or rarefaction of the ultrasonic’s

surrounding them. This bubble, subjected to alternating compression and rarefaction,

starts to oscillate in resonance with ultrasonic’s and grow larger and finally results in a

violent collapse or “implosion”. During implosion, the inward rush of the bubble upon

itself driven by the compression of the ultrasonic’s results in form of energy of collapse

concentrated on a very small area. The energy of this collapse is directed towards the

sample. These implosions generate, momentarily, local pressures upto several GPa and

temperatures at the centre of bubble of the order of 104 - 106 K. Ultimately the passage of

ultrasonic’s through etchant creates the cavitation-implosion process, and many tiny

vacuum regions on the surface of the sample. The action of the vacuum on the track core

region will suck out the solution saturated with the dissolution of active layer of cellulose

nitrate and replace it by fresh solution. Hence the tracks etch rate increases more than the

bulk etch rate in the case of ultrasonic etching.

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Fig. 6.5: Photomicrographs for the revelation of the alpha particle tracks in the LR-115 polymer for different sets of samples [a] normal alpha irradiated sample (set A), [b] alpha irradiated then 2h UV exposed (set B)

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Fig. 6.5: Photomicrographs for the revelation of the alpha particle tracks in the LR-115 polymer for different sets of samples [c] 2h UV exposed then alpha irradiated (set C) and [d] alpha irradiated then ultrasonically etched (set D).

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Fig. 6.6: Track sensitivity of different samples

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6.6 Conclusions

Following important conclusions had been drawn from the study:

The surface chemical behavior was studied by etching process. The thickness of

polymeric sample with etching at different temperatures and normality was studied.

The results indicated an increase in etch rate with temperature and normality.

Investigation of the suitability of a method of % mass change for determining the

bulk etch rate of a polymeric sample have been reported. For checking the validity of

the method another well known method of cumulative thickness removed was used to

find the bulk etch rates of the same samples. The bulk etch rates found were 3.02 µm

h-1 and 3.00 µm h-1 with the two methods in case of etching in 1.5 N NaOH at 60 ˚C

± 0.5 ˚C. The bulk etch rates found were 4.01 µm h-1 and 4.20 µm h-1 with the two

methods in case of etching in 2.5 N NaOH at 60 ˚C ± 0.5 ˚C. These results showed a

good correlation between the two methods. The % mass change method has the

advantage over some other methods as it is a non-destructive method.

Our study shows that there is an increase in bulk etch rate of UV exposed samples

than the normal samples. However, the track etch rate of UV exposed samples does

not show this behaviour. Although there is loss of track sensitivity of the UV exposed

samples, but that change is not too drastic so as to decrease the efficiency of the

samples drastically. The LR-115 detectors can be used effectively in the radon

measurement study in natural environment where the exposure of the samples to

sunlight is for a small time (1-2 h) during the whole day. Our study also indicate that

the geometrical shapes of the etched tracks due to alpha particles in LR-115 detector

registered from Am241 source are more sharper and easily identifiable in ultrasound-

induced etching than those obtained by conventional etching. Fig. 6.6 shows the

variation in track sensitivity of all the sets of samples. This tends to suggest that the

track etching response is better in ultrasonic etching as compared to normal etching.

The result supports the well established idea that heat enhances the chemical etching

of particle tracks and agitation is required for removing the etch products.

study of chemical etching behavior of lr- 115 type ii...

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