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Presentationby

M.I. Al-JarallahDepartment of Physics

King Fahd University of Petroleum & MineralsDhahran

Saudi ArabiaContact: 009 663 860 2281

Email: mibrahim@kfupm.edu.saHome Page:

http://faculty.kfupm.edu.sa/phys/mibrahim/

Solid State Nuclear Track Detectors: Principles and

Applications

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Table of Content

1. Introduction

2. The basic principles of SSNTD technique

3. Types of SSNTD

4. Physics and Chemistry of Nuclear Tracks

5. Measurements and Applications

6. Conclusion

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1. Introduction• Since its discovering in 1958, the technique has over the

last few decades, become a popular and well established method of measurement in a large number of fields involving different aspect of radioactivity or nuclear interactions. It has grown to such an extent that now there is hardly a branch of science and technology where it does not have actual or potential applications. Fields where well established applications of this technique already exist include fission and nuclear physics, space physics, the study of meteoritic and lunar samples; cosmic rays; particle accelerators and reactors; metallurgy and archeology; medicine and biology.

• In the seminar I will present the principle of the SSNTD technique and some of the applications.

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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• They do not need any electronic/electric

instrumentation; they can be deployed under field conditions and in remote fairly inaccessible places for long durations of time without the need for human intervention or back up except for initial placement and final retrieval.

• Their rigidness

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2. 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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3. Types of SSNTD

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

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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.

• 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: C12H18O7]. It can record all charged nucleons, starting with protons.

• Earth scientists who use mineral crystal as natural detectors for, e.g., age determination of rocks, or research workers in the field of planetary sciences applications, have to utilize specialist machinery and techniques for crystal cutting, grinding and polishing; heavy-Liquid and magnetic separations of minerals. Special microscopes for minerals identifications: sample counting and replication methods etc.

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4. Physics and Chemistry of Nuclear Tracks:

a) Formation of Latent Tracks:

• There are no universally accepted models for the formation of Latent tracks in dielectric solids [e.g. mineral crystals, glasses]. In polymers, two processes are believed to determine the formation of a latent track:

1. Defect creation

2. Defect Relaxation

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The defect creation process can be subdivided into the following steps:• The primary interaction between the passing particles

and the atoms of the medium which takes place over a very short time (of the order of 10-17s for 1 MeV -particles).

• The electronic collision cascade process, which spreads out from the particle trajectory: it leaves behind a positively charged plasma zone, and produces chemically activated molecules outside this zone. The process lasts approximately 3 orders of magnitude longer than the primary interaction (i.e.~ 10-14 s).

• The atomic collision cascade is the next process, which occurs owing to the “Coulomb explosion” of the remaining charged plasma. The process takes place within a timescale of ~10-12s.

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The defect relaxation can be subdivided into two processes:

• Aggregation of the atomic defects within the depolymerized zone (track core) into an extended defect over a timescale of about 10-10s.

• Relaxation of molecular defects via secondary reactions of chemically activated species in the partly depolymerized zone (track halo). This process occurs on a timescale of ~1 s.

• Axial and radial sections through a latent track are shown in Fig. 3.1.The track core, ~10 nm in diameter, corresponds to the range of the atomic collision cascade. In this zone the molecular weight is drastically reduced. The track core is surrounded by a track halo, 100-l000nm in diameter, corresponding to the electronic collision cascade with modified chemical properties.

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unetched tracks in polymers 10 – 100 nm

chemically etched tracks = 1m

ECE etched tracks = 100 m

b) Visualization of Tracks by Chemical Etching:

The etching is usually carried out in thermostatically controlled baths

There is also Electro Chemical Etching…

13Figure: Chemical Etching of SSNTD

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c) 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

• What is important is to ensure that no tracks are counted more than once and none left out through any bias. Any lower limit on the size of acceptable etch pits must be consistently imposed by a given observer. Criteria for genuine tracks [whose pits have regular shapes – whether circular or conic sections in the case of CR – 39, and whose conical bottoms appear as pinpoints of light by moving the objective up and down] as against defects, scratches and other artifacts, have usually got to be learnt by new workers, who should first familiarize themselves with detectors artificially irradiated with particles or fission fragments and etched with care.

• 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.

• There are also spark counting and automatic track evaluation.

18Figure: Track analysis of charged particle on SSNTD after chemical

etching

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5. Measurements and Applications:

5.1. Earth and Planetary Sciences

• a) 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.

21Figure: Measurements of radon exhalation rate from granites using

SSNTD with sealed vessel.

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5.2 Fission Track Dating• Fission track dating (FTD) is one of the earliest applications of the SSNTD

technique. The idea is to use mineral crystals themselves-which are to be dated-as the natural track detectors. The dating is based on the fact that all mineral crystals contain some uranium as a trace element-ranging from parts per billion (ppb) to several thousand parts per million (ppm) by weight-a typical value being a few ppm. The 238U component (natural abundance, 99.3%) of the U-content undergoes natural fission at a fixed rate (with a fission half-life of ~1016 year, i.e. a fission decay constant f of ~ 7 x 10-17 year-1). This leaves latent fission tracks-produced by the energetic fission fragments-in the body of the crystal at a known time-rate, which can be easily revealed by etching the crystal (after grinding and polishing its surface to eliminate scratches, etc.) in an appropriate reagent. If, then, one knew the uranium content of the crystal, it would be easy to calculate the time elapsed (since the crystal had last solidified) that had resulted in the number of fission tracks actually observed in the crystal. The uranium content is actually determined by irradiating the crystal-after having eliminated the pre-existing natural tracks by heating it to a high temperature-with a known fluence of thermal neutrons in a reactor. The thermal neutrons produce induced fission in the 235U component of the uranium content; and since the thermal fission cross section is known, this would reveal the 235U content and hence the 238U content.

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5.3 Planetary Science

a) Lunar Samples• In the heyday of lunar research (early 1970s), the

SSNTD technique played a prominent role in helping the scientists unravel the radiation history of the moon. The method could work wonders with minuscule quantities of lunar material: grains some hundreds of m across-and weighing merely some tens of g-allowed information stored in them over hundreds of millions of years to be decoded. Example: The fact that near-perfect vacuum prevailed on the moon means that even low-energy (solar) cosmic rays had been able to reach the surface of the moon. Unetched lunar grains, subjected to transmission electron microscopy, showed up enormous track densities (up to 1010-1011 tracks cm-2).

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• One important and unexpected result was the finding-made on samples collected from the surface of the moon as well as from varying depths down to ~3m below the surface by both manned (US) and unmanned (Russian) missions, using drilling devices-that the track density histograms in lunar grains at all depths (down to ~3 m) were roughly the same. This-combined with the fact that even grains ~ 400 m across, found at depths to 3 m, sometimes showed track density gradients across their surface (indicating that at one time they must have lain right at the top of the moon for these low-energy cosmic rays to undergo appreciable attenuation over such tiny-viz., some hundreds of m-distances)-gave rise to the concept of “cosmic gardening” on the surface of the moon. Thus, it was postulated that a churning and mixing of the soil in the top several meters-probably caused by micrometeoritic bombardment-took place, such that over a timescale of some hundreds of millions of years, the top soil was completely turned over.

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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

• The application of SSNTD in the field of charge particle identification was initiated in 1967. the ability to extract quantitative information about individual particles soon led to its use in cosmic ray measurements.

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• The e so-called “multiple-sheet method” is illustrated in Fig. 3.12, in which )a particle that crosses five detector sheets comes to rest in the sixth sheet.After exposure to cosmic rays in space, all six sheets are etched. Since the rate of ionization increases downward, i.e. along the direction of the particle’s progress, the cone-shaped etch-pits steadily lengthen; the final etched shape (in sheet 6) is cylindrical or test-tube like because preferential etching (with a velocity VT) ended at the site where the particle came to rest. The length of each of the ten cones gives the localized value of the ionization rate; and the distance from each cone to the final rounded-out location gives the 10 residual ranges of the particle-providing, in this case, a tenfold redundancy that improves the quality of the measurements of the cosmic-ray charge and energy.

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5.4 Physical Sciences:

• The SSNTD technique has been used in a variety of nuclear physics and related studies comprising: neutron induced fission, charged particle induced fission; photo fission and electrofission; high energy reaction; exotic modes of decay; development of accelerator driven systems.

• a) Neutron Measurements:• Ever since their discovery, SSNTDs have been extensively

applied to the study of the complex problems of neutron dosimetry. Neutron, being electrically uncharged, cannot produce etchable tracks directly and therefore are usually detected via charged nuclear reaction products using appropriate neutron converter. There exist a qusi-linar relation between the observed track density and the neutron fluence.

• Generally speaking, there are two approaches: either one observes direct neutron effects in the detector such as the H(n,) intrinsic reaction; or one observes induced reaction products, from a “converter screen” placed in closed contact with the detector, using a reaction such as 6Li (n,)3H.

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b) Nuclear and Reactor Physics• SSNTDs have been used in about 100 nuclear laboratories

worldwide, many of which have their own accelerators and/or nuclear reactors.

• SSNTDs are particularly widely used in a variety of nuclear physics experiments e.g. for the recording of rare events such as spontaneous fission radioactive decay with spontaneous emission of particles heavier than ’s was predicted in 1980, and such an “exotic nuclear decay mode was first observed by Rose and Jones in 1984.

• Systematic of experimental results obtained by SSNTDs and other detectors lead to know until now 19 nuclides having heavy fragment radioactivity with the emission of 14C, 20O, 19F, 24,25,26Ne, 28,30Mg.

• 242Cm 34Si + 208Pb T½ = (1.4 03) x 1023s (1)• 230U 22Ne + 208Pb (2)• The branching ratio for the emission of heavy ions to - particles in

(2) was found to be (1.3 0.8) 10-14 the primary branching ratio relative to - decay and relative to spontaneous fission in (1) were found to be 1.0 x 10-16 and 1.6 x 10-9 respectively.

• SSNTDs have been used to measure cross sections to 10-35 cm2.

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c) Radiography• An important property of SSNTDs is their ability to register

and localize individual radiation events. Thus, an image of the objects emitting or transmitting radiations is formed on the SSNTDs exposed to them. A number of radiographic techniques have been developed for the physical and chemical characterization of materials. The application of SSNTDs as a research tool in the laboratory as well as for large-scale analytical purposes (e.g. for non destructive imaging for industrial use) is being explored on an on-going basis. Such applications can be classified into two categories: (i) autoradiography, and (ii) transmission radiography. According to the nature of the detected radiation and/or experimental setup, autoradiography can be subdivided into (i) autoradiography based on natural radioactivity; (ii) neutron-induced autoradiography; (iii) ion-induced autoradiography; (iv) photon-induced autoradiography; and (v) ion or neutron activation autoradiography. Transmission radiography can also be subdivided into: (i) neutron radiography” (ii) ion radiography; (iii) ion lithography; and (iv) ion channelography.

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d) Elemental Analysis and Mapping

• An interesting use of the SSNTD technique has been its application in measuring the amount and spatial distribution of certain types of elements in a sample.

• Here there are two possibilities. First is where the element in question is radioactive in itself-giving out, say, -particles or fission fragments. The second is that exposure to, say, thermal neutrons can produce a reaction in the given isotope, leading to the emission of charged particles such as ’s or fission fragments.

• Geologists who may, for instance, wish to determine the uranium content of a rock, routinely use the first approach. Here the only requirement is to place an -sensitive plastic detector in contact with a roughly polished surface of the U-containing rock for an appropriate length of time. The detector is then removed, etched, and -counted under an optical microscope. A simple equation then gives the U content.

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• As an alternative to the above-where spontaneous production of ’s is taking place from the decay of an element (238U and its descendants in our case)-one could induce fission in the 235U content by thermal neutron bombardment in a reactor.

• A third scenario for elemental analysis when, for instance, thermal neutrons are used to produce an (n,) reaction in a given element (or isotope) distributed in the main matrix of the sample Here, the situation is analogous to that of induced fission—except for the fact that a now is the cross section for the (n, ) reaction, and only one is emitted per reaction

• It may be worth emphasizing the fact here that, in the case of all the above-described approaches, one not only obtains an estimate of the quantity of the element or isotope under examination in a given sample but also a replica of the distribution pattern of that element in the sample; hence the word “mapping” in the title of this Subsection.

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5.5 Biological and Medical Sciences

• a). Radiation Protection Dosimetry/Health Physics

• Measuring doses of radiation to which humans have been exposed is important for their biological safety. Among topics related to the application of SSNTDs in radiation protection (or health physics) are: (i) radon dosimetry (in homes, workplaces, mines); (ii) neutron dosimetry (especially around nuclear or accelerator facilities); and (iii) heavy ion dosimetry (space missions; supersonic air travel; personnel dosimetry of regular crew members of high-altitude aircraft).

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b) Environmental Sciences

• The best applications of the SSNTD technique to environmental studies are obviously those that exploit its strongest suits, namely where integration of the effects in question is advantageous [e.g. when the signal is weak in terms of intensity…]; the phenomenon contains charged – particles…:

• Areas of successful SSNTD applications: 1) Measurement of Ur and Ra concentrations in Water,

Milk, Soil, Plants, etc.• The methods are straight forward. Plastic detector foils

are either left immersed in water or in contact with the samples in questions, or implanted in the soil and left undisturbed for a period of days or weeks (depending on the intensity of the signal). After exposure, the detectors are retrieved, etched and counted for - particles.

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2) Plutonium in the Environment

• Environmental hazards of the long lived (T½ = 24100 y) radioisotope 239Pu, forming a part of the nuclear waste generated all over the world by nuclear power plants from their 238U – containing fuel, have highlighted the need for strict surveillance of plutonium in the environment. One method, entailing thermal neutron fission of 239Pu – aim of attaining a measurement sensitivity of 10-14 to 10-15g of Pu per gram of human tissue. NTDs are used to detect the fission fragments of Pu.

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3) “Hot particle” Measurements

• The NTD technique has proved to be a very suitable method of measuring the effects of hot particle in the environment, (Fig.3.19).

• NTD radiography was applied to identify the aerosol – contained hot particles from Chernobyl fallout. Fission was induced in the transuranium elements deposited on aerosol filters, using (n, ) and (,f) interactions. The resulting clusters of fission fragments where the detected and mapped by NTDs so were the -emissions from the heavy radioisotopes involved.

• Quantitative measurements of the environmental effects of nuclear accidents are obviously, of great importance; and NTD technique provide a means for simple, inexpensive and widespread surveys of such effects.

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c) Cancer Diagnostic and Therapy

• Nuclear Therapy has encompassed the use of photons, electrons, ions as well as neutrons. Recent addition to this list are (i) radiotherapy with light ions (carbon, oxygen, neon); and (ii) boron neutron capture therapy (BNCT). BNCT has been revitalized during the past few years. The treatment relies on selective accumulation or retention of boron compounds in tumor tissue, and the subsequent exposure to thermal neutrons.

• The tumor tissue gets irradiated by the 10B distribution in the tumor is essential for evaluating the potential usefulness of various 10B delivery compounds. For this purpose, the neutron induced autoradiography with SSNTDs has been found to be the most powerful technique. The reasons for this include:

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I. high concentration sensitivity (average boron concentration down to ppm range can be measured. Local concentration in structural detail (cells) as small as 100 – 1000 m2 can be measured with statistical errors of about 10%;

II. high spatial resolution (a few m);

III. ability to selectively image boron distribution in a whole body section.

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6. Conclusion

• It is hoped, that the seminar have demonstrated what a versatile and power technique the SSNTD methods is.

• Despite its versatility, the technique is relatively simple, and at its basic level inexpensive – which make it particularly attractive for the Third World Laboratories.

• Presently over 300 papers per year are being published globally in the discipline, covering various topics in science and technology, both on Earth and Space.

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Thank you