3/2003 Rev 1 II.3.8 – slide 1 of 34 IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources Session II.3.8 Part IIQuantities and Measurements Module 3Principles of Radiation Detection and Measurement Detection and Measurement Session 8Nuclear Track Detectors

3/2003 Rev 1 II.3.8 – slide 2 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 3 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

3/2003 Rev 1 II.3.8 – slide 4 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 5 of 34 Relevant Quantities  Quantities of interest for neutron detection are:  Ambient dose equivalent, H* (10), for area monitoring and  Personal dose equivalent, H p (10), for individual monitoring, when track etch detectors are used as personal dosimeters (e.g. PADC)

3/2003 Rev 1 II.3.8 – slide 6 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

3/2003 Rev 1 II.3.8 – slide 7 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

3/2003 Rev 1 II.3.8 – slide 8 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

3/2003 Rev 1 II.3.8 – slide 9 of 34 L  for Dielectric Materials  Inorganic materials have a threshold L  mostly exceeding 15 MeV cm 2 /mg, corresponding to 500 keV/  m in unit-density tissue  Plastics tend to have lower thresholds

3/2003 Rev 1 II.3.8 – slide 10 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 cm 2 /mg or 100 keV/  m in tissue

3/2003 Rev 1 II.3.8 – slide 11 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 12 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 13 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 14 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 15 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 16 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 17 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 18 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 19 of 34 Neutron Track Etch Dosimetry Using Fission-Foil Converters  Number of such etchable tracks has been found to be 1.16 x 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 20 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 10 6 to 10 3 tracks/cm 2

3/2003 Rev 1 II.3.8 – slide 21 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 22 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 23 of 34 Electrochemical Etching  One of the major problems in personnel neutron dosimetry is measurement of neutrons with energies below 500 keV  A sensitive neutron dosimeter is proton-sensitive CR-39 track detectors  Electrochemical etching of CR-39 track detectors has distinct advantages over chemical etching

3/2003 Rev 1 II.3.8 – slide 24 of 34 Electrochemical Etching  When chemical etching is used, poor visibility for optical counting makes it difficult to discriminate against artefacts (e.g. scratches, dust particles)  Large discrepancies have been encountered between experimental results and the calculated response at energies below 1 MeV for chemically etched CR-39 detectors

3/2003 Rev 1 II.3.8 – slide 25 of 34 Electrochemical Etching  Electrochemical etching (ECE) makes it possible to overcome the difficulties of counting of low densities of short-range tracks  ECE typically is done under conditions of 24  C with 30 kV/cm rms and 2 kH  C  ECE typically is done under conditions of 24  C with 30 kV/cm rms and 2 kH z after 5 hours pre-etch at 60  C

3/2003 Rev 1 II.3.8 – slide 26 of 34 Electrochemical Etching  In attempts to reduce background, a temperature of 24  C is very often used for the ECE, which then is only capable of producing ECE-spots at tracks previously formed by heavy chemical etching at  C  In attempts to reduce background, a temperature of 24  C is very often used for the ECE, which then is only capable of producing ECE-spots at tracks previously formed by heavy chemical etching at 60  C  The ECE of pre-etched detectors provides excellent registration characteristics for mono-energetic charged particles

3/2003 Rev 1 II.3.8 – slide 27 of 34 Comparison of Tracks for Chemical vs Electrochemical Etching

3/2003 Rev 1 II.3.8 – slide 28 of 34 Electrochemical Etching  No single chemical pre-etching exists for the formation of all the proton-recoil tracks induced by neutrons  Of all the pre-etched tracks, only a small fraction is enlarged by ECE processes since the formation of tree spots occurs preferentially at pointed-shape tracks

3/2003 Rev 1 II.3.8 – slide 29 of 34 Electrochemical Etching  This results in non-linearity of the ECE response with neutron energy, after pre- etching  This result is shown in the next figure by the dashed line

3/2003 Rev 1 II.3.8 – slide 30 of 34 Linearity of CR-39 Response Following Electrochemical Etching

3/2003 Rev 1 II.3.8 – slide 31 of 34 Electrochemical etch

3/2003 Rev 1 II.3.8 – slide 32 of 34 Electrochemical etch + chemical etch

3/2003 Rev 1 II.3.8 – slide 33 of 34 Electrochemical etch + chemical etch

3/2003 Rev 1 II.3.8 – slide 34 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 cm 2 /g or 10 KeV/  m in tissue

3/2003 Rev 1 II.3.8 – slide 35 of 34 Track Etch Detector System – Autoscan 60

3/2003 Rev 1 II.3.8 – slide 36 of 34 Track Etch Detector System - Autoscan 60 Reader

3/2003 Rev 1 II.3.8 – slide 37 of 34 Track Etch Detector System – Polymer Detectors

3/2003 Rev 1 II.3.8 – slide 38 of 34 Track Etch Detector System

3/2003 Rev 1 II.3.8 – slide 39 of 34 The PADC Neutron Dosimeter

3/2003 Rev 1 II.3.8 – slide 40 of 34 The PADC Neutron Dosimeter

3/2003 Rev 1 II.3.8 – slide 41 of 34 The PADC Neutron Dosimeter

3/2003 Rev 1 II.3.8 – slide 42 of 34 The PADC Neutron Dosimeter - Fluence Response

3/2003 Rev 1 II.3.8 – slide 43 of 34 The PADC Neutron Dosimeter - Response to Fast Neutrons

3/2003 Rev 1 II.3.8 – slide 44 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

3/2003 Rev 1 II.3.8 – slide 45 of 34 Typical Track Etch Pit

3/2003 Rev 1 II.3.8 – slide 46 of 34 Etch-pits In the Trek Detector Produced By Cosmic Rays

3/2003 Rev 1 II.3.8 – slide 47 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 48 of 34  Knoll, G.T., Radiation Detection and Measurement, 3 rd 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, 2 nd Edition, Technical Reports Series No. 277, IAEA, Vienna (1997) Where to Get More Information

3/2003 Rev 1 II.3.8 – slide 49 of 34  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) Where to Get More Information

3/2003 Rev 1 II.3.8 – slide 50 of 34  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) Where to Get More Information

3/2003 Rev 1 II.3.8 – slide 51 of 34 Where to Get More Information  Cember, H., Introduction to Health Physics, 3 rd Edition, McGraw-Hill, New York (2000)  Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8 th 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)