ETCHING CHARACTERISTICS OF SOLID STATE

NUCLEAR TRACK DETECTOR

Submitted to:

Dr. Asloob Ahmed Mudassar

Submitted by:

Yasir Ali

M.Phil. Physics DPAM

PIEAS

Radioactive elements emit different kinds of radiations. These radiations may be alpha particles,

beta particles or electromagnetic gamma rays. These radiations require different kinds of

detection systems. Gamma rays detection require a special kind of detector, beta particles need

another kind of detector. Similarly alpha particles require it own detector with specific

properties. Solid state nuclear track detector is also used to detect nuclear radiations.

Solid state nuclear track detector (SSNTD):-. Fast moving charged particles are known to

produce trails of atomic disorder in an immense variety of dielectric solids and some other

materials. These dielectric solids include crystals, glasses, and high polymers, and other

materials used, include intermetallics, certain metals and amorphous metals, oxide conductors

and superconductors. Tracks produced by radiations from nucleus are known as nuclear tracks.

Nuclear tracks have found many applications in different branches of science.

A solid-state nuclear track detector or SSNTD is also known as an etched track detector or a

dielectric track detector, DTD. When it is uncovered to nuclear radiation i.e. neutrons or charged

particles, intermittently as well gamma rays, is etched, and inspected microscopically which give

information about the radiation.

History :-.D.A. Young of Atomic Energy Research Establishment (AERE) at England

discovered in 1958, the method of detecting charged particles through particle track produced in

LiF crystals. When a LiF crystal was irradiated with radiation from uranium, they penetrate into

crystal and produced a track in it. These tracks are helpful in detection of these charged particles.

In 1959 other two researchers, Silk and Barnes of AERE England, observed hair like tracks of

fission fragments. After these two events, researchers from other parts of the world started work

on this new detector and they developed this method further. Other elements were also

discovered which could have similar tracking characteristics.

Very soon after its discovery, the use of SSNTDS started in almost every field of science and

technology which includes radiation dosimetry, nuclear physics, space physics, geology,

medicine, etc. Having observed tracks in SSNTD.

How it works:-. When a heavily ionized particle falls on SSNTD, it penetrates inside it.

During the process particle strikes with the atoms of lattice of SSNTD and may knock out them

from their lattice sites. Depending on particle’s energy, it may penetrate deep inside SSNTD and

having some decrease in energy. Therefore track continuous inside the detector also in form of

cone. These tracks are used to extract valuable information about the radiation incident o

detector.

SSNTDs fall in two distinct categories: 1) Polymeric or plastic detectors: These are widely used not only for radiation monitoring and

measurement, but also in many other fields involving nuclear physics and radioactivity.

2) Natural minerals crystals (and glasses): This kind of SSNTD has greatest application in fields

such as geology, planetary sciences [especially lunar and meteoritic samples], oil exploration etc.

The most widely used SSNTDs today are plastic, which unlike mineral crystal do not require

special preparation such as grinding and polishing. They are also much more sensitive than

crystals and glasses. At present, the most sensitive and also the most widely used plastic is the

CR-39 polymer [a poly allyldiglycol carbonate: C12H18O7]. It can record all charged nucleons,

starting with protons.

Tracks visualization:-. Due to irradiation of radiation tracks are formed on SSNTD. These

tracks are of very small size and of order of 10nm. To make these tracks visible, they are passed

through a process called etching. In etching 6 Molar solution of NaOH is used and SSNTD is

placed in it for irradiation. This process make the tracks formed by radiation larger in size and

after some time their size become of the order of micrometer. Size of tracks depends on duration

of etching. In this experiment we study etching characteristics of SSNTD detector.

Tracks evaluations:-. Many methods are used to evaluate tracks so that valuable

information about the radiation is obtained.

1. Manual (Ocular) Counting:-. Manual or more accurately, ocular (with eye)

counting denotes non-automatic counting of etched tracks generally using an optical

microscope, with a moving stage, and two eye pieces. But there is a problem with this

method that some of tacks may be repeated in counting and some may be left.

2. There are also spark counting and automatic track evaluation. By these methods

we can also count tracks of pits.

From data taken track density is found. Track densities are expressed either in relative terms or

in absolute terms [tracks cm-2

] which is converted after calibrating into a dose (e.g. Bq m-3

h) or

radon concentration (e.g. Bq m-3

) by dividing by the time of exposure.

Advantages and Disadvantages of SSNTDs:-.

1. Relative inexpensiveness: They consist of cheap, simple materials.

2. They are sensitive to radiations of high linear energy transfer but are insensitive to

a background of lightly-ionizing radiations such as beta particles and gamma rays.

3. Ease of development (there is no need for darkroom processing as in the case of

TLD or nuclear emulsions). Simple chemicals (e.g. NaOH) can be used in ordinary

daylight to etch tracks in polymers, mineral crystals, glasses, etc.

4. They can be, used over a long time-period, without maintenance, significant

background or noise problems, or fading at normal temperatures.

5. Track records in geological and extraterrestrial samples (meteorites) remain intact

for millions, indeed billions, of years.

6. They are small, durable and unobtrusive and thus can be used in homes as well as

in application where small geometry is important.

7. Heavily charge particles (such as protons, alpha particles and fission fragments)

can be distinguished from one another.

8. They are passive detectors and do not require power supplies. The disadvantages of SSNTDs are that all the observations require

extensive use of microscope. This process is time consuming and human

error is always present in the observation, which may affect the results.

Automatic scanning systems are available commercially which can reduce the

observations time but they are very costly.

Objective of experiment:-. In this experiment we need to study etching characteristics of SSNTD.

We observe effect of duration of etching on tracks formed on detector.

Apparatus:-. In this experiment we used SSNTD strips which were already passed through

the process of etching and irradiation from two different radiation sources and microscope.

Procedure:-. To see the effect of etching duration of tracks we need to measure size of tracks.

For this we need calibrated microscope. So first of all we calibrated microscope.

Calibration of microscope:-. Measurement of microscopic objects requires the use of an

eyepiece micrometer and a stage micrometer. The eyepiece micrometer is a round glass disk on

which a scale has been etched. The eyepiece micrometer is inserted into one eyepiece and held in

place in the correct focal plane of the eyepiece with a small retaining ring. The eyepiece and

eyepiece micrometer can be rotated 360 degrees in the eye tube so the measuring scale can be

aligned with or superimposed over the image of your specimen. With eyepiece micrometer there

is another scale of micrometer called stage micrometer placed on stage. A stage micrometer is

simply a microscope slide with a known dimension etched upon its surface. The stage

micrometer is placed directly on the stage of the microscope and brought into focus. By rotating

the eyepiece both scales can be positioned parallel to each other. To calibrate the eyepiece

micrometer you must first find out how many intervals of the eyepiece micrometer correspond to

a certain distance on the stage micrometer. You can then calculate the value of one interval of the

eyepiece micrometer.

Our microscope had s stage micrometer with one micrometer each division. And also that had

three objectives with 40x, 100x, and 10x resolutions. Each objective needed separate calibration.

1. Objective 10x :-. In this case eyepiece 1 division were equal to 10 divisions (i.e. 10µm),

therefore each division of eyepiece had 10/1 = 10 µm length.

2. Objective 40x :-. In this case 4 divisions of eyepiece were equal to 10 divisions of stage

micrometer. Therefore each eyepiece had 10/4 = 2.5 µm length.

3. Objective 100x :-. In this case one division of eyepiece was equal to single division of

stage micrometer. Therefore each eyepiece had 1/1 =1 µm length.

We used 100x objectives therefore each eyepiece division was 1 µm in length.

Figure. 1. Calibration of microscope.

Measuring size of tracks:-. Now to get data about size of tracks, we placed different strips of

SSNTD which were irradiated from different sources and were etched for different time

durations. First of all we used strips irradiated with Americium 242 source which is an alpha

emitter. We used strips etched for different time duration. Using slide attached with stage, strip

of SSNTD could be moved in back and forth direction and also in right and left direction. Using

this facility, different pits i.e. tracks of radiation were brought on scale of eyepiece as shown in

figure, from where it size were found. Many readings were taken for each strip.

Similar procedure was repeated for Californium 252 which is an alpha emitter and also fission

fragments come out from this source due to spontaneous fission so we took different readings for

alpha particles and for fission fragments.

Figure 2. Measuring size or diameter of tracks of radiation on SSNTD.

Observations:-. First we took reading for Americium 242 element. Ten different tracks/pits

were selected and diameter of those pits was measured for five different strips. Data is given

below.

For each case we can see that etching or diameter of tracks of alpha particles and fission

fragments are increasing with time.

Table of data taken for alpha particles emitted from Americium 242 , scale is in micrometer

Etching Time

12 Hours 10 Hours 8 Hours 6 Hours 4 Hours

Diameter measured ten

different times

All readings are taken in

µm

9 9 6 4 3

9 7 5 5 3

7 8 5 5 4

8 6 6 3 3

8 7 7 4 3

10 8 5 6 3

7 8 6 4 4

10 8 7 4 3

9 7 6 5 3

9 7 6 4 2

Average Diameter(µm)

8.6 µm 7.5 µm 5.9 µm 4.4 µm 3.1 µm

Table of data taken for Fission fragments particles emitted from Californium 252 , scale is in micrometer

Etching time 10.5hrs 8.5hrs 6.5hrs 4.5hrs 2.5hrs

Diameter of tracks

measured for ten

different tracks

randomly in

micrometer

25 23 15 9 4

20 21 14 8 3

24 18 14 6 3

23 16 16 7 4

22 17 13 8 4

23 17 14 9 5

24 17 15 8 3

22 18 13 7 4

22 17 13 9 3

24 19 16 8 4

Average diameter 22.9 µm 18.3 µm 14.3 µm 7.9 µm 3.7 µm

Table of data taken for alpha particles emitted from Californium 252 , scale is in micrometer

Etching time 10.5hrs 8.5hrs 6.5hrs 4.5hrs 2.5hrs

Diameter measured in

micrometer

For ten different tracks for each

track

11 6 4 3 1.5

8 6 4 2 2

9 7 4 3 2

7 8 5 2 2

7 6 4 3 2

10 6 3 4 2

10 5 5 3 1.5

8 6 5 3 2

9 5 5 3 1.5

8 7 7 3 1.5

Average Diameter 8.7 µm 6.2 µm 4.6 µm 2.9 µm 1.8 µm