Session II.3.8
Part II Quantities and Measurements
Module 3 Principles of Radiation
Detection and Measurement
Session 8 Nuclear Track Detectors
3/2003 Rev 1
IAEA Post Graduate Educational Course
Radiation Protection and Safe Use of Radiation Sources
II.3.8 – slide 1 of 34
Overview
 Nuclear track etch detectors and principles
of detection will be discussed
 Students will learn about etchable plastic
foils, detection thresholds for various
materials, neutron detection, approaches to
track etch detection, proton detection, and
types of track etch detector systems
3/2003 Rev 1
II.3.8 – slide 2 of 34
Content
 Etchable plastic foils
 Detection thresholds for various track
etch materials
 Energy window of detectability
 Neutron detection
 Stability of track damage
 Chemical etching procedures
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II.3.8 – slide 3 of 34
Content
 Fission-foil converter approach to
track etch detection
 Cellulose nitrate approach to track
etch detection
 Proton detection
 Types of track etch detector systems
3/2003 Rev 1
II.3.8 – slide 4 of 34
Etchable Plastic Foils
 During the 1960s, investigators found that dielectric
materials became more chemically etchable by
exposure to ionizing radiation
 Materials include glass, mica and plastics
 Radiation induction of etchability requires a certain
energy deposition “threshold” per unit mass along
the track of high-LET particles
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II.3.8 – slide 5 of 34
Etchable Plastic Foils
 After radiation exposure, chemical action by an
etchant (e.g. HF, NaOH) causes removal of the
radiation-damaged volume
 This removal leaves a conical or cylindrical pit or
hole
 This hole is large enough to see with low
magnification and to count and interpret
dosimetrically
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II.3.8 – slide 6 of 34
Etchable Plastic Foils
 For the material to be etchable, the specific energy
imparted in the particle track must exceed some
threshold
 The threshold is characteristic of each dielectric
material
 Threshold minimum specific energy is given in
terms of dT/dx or L of the charged particle for a
given material
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II.3.8 – slide 7 of 34
L for Dielectric Materials
 Inorganic materials have a threshold L
mostly exceeding 15 MeV cm2/mg,
corresponding to 500 keV/m in unitdensity tissue
 Plastics tend to have lower thresholds
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II.3.8 – slide 8 of 34
L for Dielectric Materials
 A common radiation-vulnerable plastic in
use is cellulose nitrate
 Cellulose nitrate has a threshold L as low
as 1 MeV cm2/mg or 100 keV/m in tissue
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II.3.8 – slide 9 of 34
Energy Window of Detectability
 Kinetic energy of a given heavy charged
particle cannot be too high, since L would
be too low in this case
 However, the kinetic energy must be high
enough for the particle to have sufficient
range in the material to have recognizable
tracks after chemical etching
3/2003 Rev 1
II.3.8 – slide 10 of 34
Energy Window of Detectability
 This situation creates an energy “window”
of detectability
 Window is different for each combination of
charged particle, material, irradiation
conditions, and etch processing
 In many cases, this window is closed and no
detection is possible
3/2003 Rev 1
II.3.8 – slide 11 of 34
Energy Window of Detectability
 Because of the energy window, nuclear track
etch radiation detection has not generally
been practical for electrons or gamma rays
 Its greatest application has been in the area
of neutron dosimetry
3/2003 Rev 1
II.3.8 – slide 12 of 34
Threshold L vs RBE
 The lowest threshold L in tissue
(= 100 keV/m for cellulose nitrate) occurs
around where the RBE reaches a maximum
in many mammalian biological systems
 The RBE range of greatest interest occurs
from L = 0.2 keV/ m to L = 100 keV/ m
3/2003 Rev 1
II.3.8 – slide 13 of 34
Stability of Track Damage
in Plastics
 Latent-damage track in plastics is stable
against spontaneous repair at storage
temperatures below 50° C
 Approaching the softening temperature of a
plastic results in rapid annealing and repair,
thus removing etchability
3/2003 Rev 1
II.3.8 – slide 14 of 34
Chemical Etching Procedures
 Becker (1973) has reviewed many details of
various etching procedures
 Combination of an etchant with high-voltage
AC across the film has been found to
improve etching speed and reproducibility in
some cases
3/2003 Rev 1
II.3.8 – slide 15 of 34
Two Approaches to
Track Etch Detection
 Fission-foil converters
 Direct interaction of neutrons in
etchable film and overlying plastic
layer
3/2003 Rev 1
II.3.8 – slide 16 of 34
Neutron Track Etch Dosimetry
Using Fission-Foil Converters
 When a foil of fissionable material is struck
by neutrons of energies above its fission
threshold energy, very heavy energetic
charged particles are generated
 If the foil is adjacent to a plastic film,
etchable damage tracks are efficiently
produced
3/2003 Rev 1
II.3.8 – slide 17 of 34
Neutron Track Etch Dosimetry
Using Fission-Foil Converters
 Number of such etchable tracks has been found to
be 1.16 x 10-5 per neutron barn
 This simple relationship arises from the nearly
constant detection efficiency of plastics for fission
fragments,and
 The fact that the common fissile materials yield
similar distributions of fragment species
3/2003 Rev 1
II.3.8 – slide 18 of 34
Neutron Track Etch Dosimetry
Using Fission-Foil Converters
 A typical chemical etching scheme,
used with polycarbonate films,
consists of a bath in 30% KOH at 60C
for 5 to 50 min
 Track concentrations range from 106 to
103 tracks/cm2
3/2003 Rev 1
II.3.8 – slide 19 of 34
Track Etch Using
Cellulose Nitrate
 Neutron dosimetry can also be
performed by neutron interactions in
plastic films
 Fast neutrons striking cellulose nitrate
films cause recoiling C, O, and N
atoms
3/2003 Rev 1
II.3.8 – slide 20 of 34
Track Etch Using
Cellulose Nitrate
 Tracks are inconveniently small under
ordinary etching procedures
 Track size can be greatly enhanced by
application of an AC voltage (2000 V at
1 kHz) across the film in the etching
bath
3/2003 Rev 1
II.3.8 – slide 21 of 34
Proton Detection Using
Commercial Plastics
 A commercial plastic called CR-39 has a low
enough L to detect protons resulting from
elastic collisions with hydrogen
 The L  threshold, below which the proton
tracks are not dense enough to cause
etchable damage, is about 100 MeV cm2 /g or
10 KeV/m in tissue
3/2003 Rev 1
II.3.8 – slide 22 of 34
Track Etch Detector System –
Autoscan 60
3/2003 Rev 1
II.3.8 – slide 23 of 34
Track Etch Detector System Autoscan 60 Reader
3/2003 Rev 1
II.3.8 – slide 24 of 34
Track Etch Detector System –
Polymer Detectors
3/2003 Rev 1
II.3.8 – slide 25 of 34
Track Etch Detector System
3/2003 Rev 1
II.3.8 – slide 26 of 34
Glass Track-Etch Detectors
 Developed by University of California at
Berkeley in the United States
 Called BP-1 (barium-phosphate) glass and
has high sensitivity and resolution
 Used in experiments in high-energy
physics, and cosmic-ray physics in outer
space
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II.3.8 – slide 27 of 34
Typical Track Etch Pit
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Etch-pits In the Trek Detector Produced
By Cosmic Rays
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II.3.8 – slide 29 of 34
Summary
 Nuclear track etch detectors and principles
of detection were discussed
 Students learned about etchable plastic
foils, detection thresholds for various
materials, neutron detection, approaches to
track etch detection, proton detection, and
types of track etch detector systems
3/2003 Rev 1
II.3.8 – slide 30 of 34
Where to Get More Information
 Knoll, G.T., Radiation Detection and
Measurement, 3rd Edition, Wiley, New York (2000)
 Attix, F.H., Introduction to Radiological Physics
and Radiation Dosimetry, Wiley, New York (1986)
 International Atomic Energy Agency,
Determination of Absorbed Dose in Photon and
Electron Beams, 2nd Edition, Technical Reports
Series No. 277, IAEA, Vienna (1997)
3/2003 Rev 1
II.3.8 – slide 31 of 34
Where to Get More Information
 International Commission on Radiation Units
and Measurements, Quantities and Units in
Radiation Protection Dosimetry, Report No. 51,
ICRU, Bethesda (1993)
 International Commission on Radiation Units
and Measurements, Fundamental Quantities and
Units for Ionizing Radiation, Report No. 60, ICRU,
Bethesda (1998)
 Hine, G. J. and Brownell, G. L., (Ed. ), Radiation
Dosimetry, Academic Press (New York, 1956)
3/2003 Rev 1
II.3.8 – slide 32 of 34
Where to Get More Information
 Bevelacqua, Joseph J., Contemporary Health
Physics, John Wiley & Sons, Inc. (New York,
1995)
 International Commission on Radiological
Protection, Data for Protection Against Ionizing
Radiation from External Sources: Supplement to
ICRP Publication 15. A Report of ICRP
Committee 3, ICRP Publication 21, Pergamon
Press (Oxford, 1973)
3/2003 Rev 1
II.3.8 – slide 33 of 34
Where to Get More Information
 Cember, H., Introduction to Health Physics, 3rd
Edition, McGraw-Hill, New York (2000)
 Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds.,
Table of Isotopes (8th Edition, 1999 update), Wiley,
New York (1999)
 International Atomic Energy Agency, The Safe Use
of Radiation Sources, Training Course Series No. 6,
IAEA, Vienna (1995)
3/2003 Rev 1
II.3.8 – slide 34 of 34