Download Solid State Nuclear Track Detectors: Detection and Evaluation of Nuclear Radiations and more Exercises Advanced Physics in PDF only on Docsity!
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:
- 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.
- 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: C 12 H 18 O 7 ]. 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.
- 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.
- 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:-.
- Relative inexpensiveness: They consist of cheap, simple materials.
- 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.
- 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.
- They can be, used over a long time-period, without maintenance, significant background or noise problems, or fading at normal temperatures.
- Track records in geological and extraterrestrial samples (meteorites) remain intact for millions, indeed billions, of years.
- They are small, durable and unobtrusive and thus can be used in homes as well as in application where small geometry is important.
- Heavily charge particles (such as protons, alpha particles and fission fragments) can be distinguished from one another.
- 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.
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 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.
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.
8 6 5 3 2 9 5 5 3 1.
8 7 7 3 1.
Average Diameter (^) 8.7 μm 6.2 μm 4.6 μm 2.9 μm 1.8 μm
