Nuclear Tracks

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Nuclear Tracks
Sup. P.J.Apel
4/4/2010
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Solid-state nuclear track
detector
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.
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.
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a
b
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The electronic energy loss can be described by the Bethe-Bloch
formula
where
e
Zeff
Zt
N
me
v
I
β
δ
U
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elementary charge
effective charge of the projectile
atomic number
number of target atoms per unit volume
electron mass
velocity of the ion
ionization energy
v/c
relativistic correction
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 SSNTD
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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
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Figure: Track analysis of charged particle on SSNTD after chemical
etching
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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
Lung Cancer, and a helpful resource –
uranium
exploration
and
tentatively
e.g.
e.g. means for
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
a) Lunar Samples
b) Meteoritic 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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