Nuclear TracksNuclear Tracks

Sup. P.J.ApelSup. P.J.Apel

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A solid-state nuclear track detector or SSNTD (also known as an etched track detector or a dielectric track detector, DTD) is a sample of a solid material (photographic emulsion, crystal, glass or plastic) exposed to nuclear radiation (neutrons or charged particles, occasionally also gamma rays), etched, and examined microscopically.

Solid-state nuclear track detector

Solid-state nuclear track detector

The tracks of nuclear particles are etched faster than the bulk material, and the size and shape of these tracks yield information about the mass, charge, energy and direction of motion of the particles.

If the particles enter the surface at normal incidence, the pits are circular; otherwise the ellipticity and orientation of the elliptical pit mouth indicate the direction of incidence.

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Charged particles which penetrate a solid, can lose their energy via

various interaction types, such as

• Excitation and ionization of target electrons (electronic energy loss)

• Projectile excitation and ionization

• Electron capture

• Elastic collisions with target atoms (nuclear energy loss)

• Electromagnetic radiation(Bremsstrahlung, Cherenkov effect)

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The energy loss depending on the specific energy of the incoming

ion is displayed in fig. (1) for a uranium ion passing through

polyimide, calculated using the SRIMo3 code.

It is a characteristic of fast ions that the maximum of the irradiated

electronic energy loss occurs shortly before the particle is stopped,

because their interaction cross section for these processes increases

with decreasing velocity.

a b4/4/2010 4

The electronic energy loss can be described by the Bethe-Bloch formula

wheree elementary chargeZeff effective charge of the projectileZt atomic numberN number of target atoms per unit volumeme electron massv velocity of the ionI ionization energyβ v/cδ relativistic correctionU correction taking in to account screening of inner electrons

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The reasons for the widespread use of SSNTD include:

The basic simplicity of its methodology The low cost of its materials The great versatility of its possible applications The small geometry of the detectors Their ability in certain cases to preserve their track

record for almost infinite length of time Their rigidness

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The basic principles of SSNTD technique

When heavy charged particles [proton upward] traverse

a dielectric medium, they are able to leave long lived

trials of damage that may be observed either directly by

transmission electron microscope [TEM] provided that

the detector is thick enough, viz. some m across or

under ordinary optical microscope after suitable

enlargement by etching the medium.

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

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

2) Natural minerals crystals (and glasses): That have imprinted within them, a record of their

radiation (and thermal) history over the icons. These find their greatest application in fields such as geology, planetary sciences [especially lunar and meteoritic samples], oil exploration etc.

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Figure: Chemical Etching of SSNTD4/4/2010 10

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Track Evaluation Methods:

1) Manual (Ocular) Counting:

Manual [or more accurately, ocular: eye] counting denotes

non-automatic counting of etched tracks generally using an optical

microscope, with a moving stage, and two eye pieces

Figure: Track analysis of charged particle on SSNTD after chemical etching4/4/2010 13

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Measurements and Applications:

1.Earth and Planetary Sciences

Radon Measurements:

Radon measurements are one of the most widely

used application of SSNTDs today. Radon is

naturally occurring radioactive gas that constitutes

both a hazard e.g. Lung Cancer, and a helpful

resource – e.g. means for uranium exploration and

tentatively for earthquake prediction.

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Figure: Measurements of radon exhalation rate from granites using SSNTD with sealed vessel.

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2. Fission Track Dating

3. Planetary Science

b) Meteoritic Samples:

a) Lunar Samples

1) Age determination

2) Cooling-down of the early solar system

3) Determination of pre-atmospheric size of meteorites

4) Cosmic Ray Measurements: Particle Identification

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