Radiation Measurements

P E R G A M O N Radiation Measurements 31 (1999) 25-36

S S N T D A P P L I C A T I O N S IN S C I E N C E A N D T E C H N O L O G Y -

A BRIF~F R E V I E W

H.A. KHAN AND I.E. QURESHI

Pakistan Institute of Nuclear Science & Technology (PINSTECH), P.O. Nilore, Islamabad, Pakistan

ABSTRACT

The technique of Solid State Nuclear Track Detection (SSNTD) has matured since long as a viable method of charged particle detection. The usage of this method has been successfully extended to neutron detection and gamma dose measurements as well. The etch-track mechanism has been fitrther exploited to generate a major application area of nuclear track filters. In spite of the remarkable diversity of SSNTD applications that have emerged over the years in different fields, its potential is by no means saturated. In this article, a brief review of SSNTD applications is presented with reference to contemporary interests in science and technology. For convenience, the coverage of topics is organized under broad categories of Nuclear Physics, Materials Research, Geology, Environmental Science and allied technologies. While identifying high interest areas, those with limited but innovative applications are also mentioned. In some cases, the important results are quoted for the purpose of illustrating the strength of track detection method, hi general, the presentation is aimed at providing a broad perspective of current SSNTD uses instead of detailed description of individual applications. The coverage is selective rather than exhaustive and portrays authors' preferences. Some comments related to the adoption of this tecInfique as a mainstream method of detection are also given.

KEYWORDS

SSNTDs; applications of track detectors; nuclear science; material science, environmental science.

INTRODUCTION

The emergence of etch-track detectors as a new means of charged panicle detection in the late fifties (Young, 1958; Silk and Barnes, 1959) was followed by a phase of rapid expansion in the applications of this technique. The pioneers of this field, specially the famous trio of Fleischer, Price, and Walker (Fleischer et al., 1969), worked with zeal and imagination to introduce ever new uses of track detectors in different areas of science and technology. When the classical monograph, 'Nuclear Tracks in Solids', appeared in 1975 (Fleischer et al., 1975), it already contained references to a very broad spectrum of works in diverse areas, such as Archaeology, Astrophysics, Biophysics, C~ology, Elementary Particle Physics, Geochronology, Metallurgy, Nuclear Physics, Dosimetry, hnaging, and many other fields. The upsurge in new applications subsided during eighties, but the consolidation of new avenues in terms of the improvement of precision and better methodologies continued during this period. In more recent times, a discernible tilt has been observed towards technological applications of track detectors, as compared to their use in basic research work. The pace of activity in the field of nuclear track detectors has been kept at a significant level by holding regular biennial conferences and the publication of their proceedings since early sixties (e.g. Ilid et al., 1997), which is a tribute to the International Nuclear Track Society. These conferences also helped promote a community of track detector practitioners. Apart from these major regular events, there have been a number of workshops, meetings and mini-conferences on specialized topics, from time to time (e.g. Durrani,1981; Furlan and Tommasino, 1991). While the series of conference proceedings do provide a collage of track

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26 H.A. Khan, L E. Qureshi / Radiation Measurements 31 (1999) 25-36

detector works, the need of good text-book material cannot be over emphasized. So far, apart from the pioneering treatise of Fleischer, Price, and Walker, there are only two texts available for the newcomers, one by Durrani and Bull (1987) and the other by Spohr (1990). The gap has been somewhat filled by the release of a new book by R.L. Fleischer (1998) and a reference treatise relevant to radon measurements edited by Durrani and I1i6 (1997). All the on-going work in different laboratories using track detectors cannot be obviously covered in the conferences and textbooks. The reporting of this work in different scientific journals is a natural source of information for the scientific community at large. Although there is no single journal dedicated to the specialized field of SSNTD, however, extensive coverage is done by the journals, ~Radiation Measurements', 'Radiation Protection Dosimetery', and 'Nuclear Instruments and Methods in Physical Research'. Some of the important reviews with extensive coverage of subject material have been published in these journals (e.g. Khan et aL, 1993; Durrani, 1993; Ilid et aL,

1993). Apparently there is a lack of representation of SSNTD-based work in the core journals of wide circulation in physics community, some important results of wider interest have been published over the years in journals like 'Nuclear Physics'(e.g. Qureshi et al., 1988a; Price, 1989), 'Physical Review' (e.g. Gottschalk et al., 1983; Khan et al., 1984), 'Zeitschrift Physik' (e.g. Haag et aL, 1984; Peterson et al., 1995) etc. It is often necessary to link various strands of interest and see if there is an emerging pattern in track detector applications. This has been done by R. L.Fleischer himself in some conferences (Fleischer, 1991; Fleischer, 1997). The present article may seem as yet another attempt to put together the multifarious uses of track detectors. However, our aim is to critically assess the most recent applications with a view to identify growth points in this still fertile techniqu'el Because of the article length constraint, the choice of topics can only be selective and the scope of discussion has to be limited. No claim is made of complete coverage of each and every significant work owing to the inevitable personal preferences where choices have to be made. In the following sections, we have treated closely related fields of research jointly, for example, Nuclear Physics, Particle Physics, and Astrophysics are treated under one heading of Nuclear Science. Other principal topics covered below include applications in Geology/Geophysics, Dosimetry, Environmental Science, and Material Science. The basic scientific issues and their technological offshoots are combined to give a coherent view of the subject. Results are quoted only to the extent that they help to emphasize the significance of the relevant application. A critique of the present situation is given in the last section

NUCLEAR SCIENCE

Low-energy heavy ion reactions

As is well known, the genesis of Solid State Nuclear Track Detectors is linked with Nuclear Physics (Silk and Barnes, 1959). It is also this area which continues to provide greatest impetus to growth in track detection methodology. Of major Significance is the progress made in quantitative analyses based on geometrical parameters of observed tracks in solids. The crucial factors for a reliable detection system are the reproducibility of the results, statistical significance of the data obtained and the accuracy of measurements. Once all the features and procedures of the detection process are well- determined, one is then only interested to see what information of physical importance can be derived concerning the investigated problem. In the case of SSNTDs, the methodology has been developed enough to justify its use in the detailed study of Nuclear Physics issues. The quantitative analyses of heavy ion reactions initiated by Gottschalk et al. (1983) and strengthened by the work of Haag et al. (1984), Khan et al. (1985), Vater et al. (1986), and Qureshi et aL (1988a) can be trul3; regarded as an important milestone in the use of SSNTDs in Nuclear Physics. A typical heavy ion reaction in the low energy region can be investigated with SSNTDs in fairly great detail. Gottschalk et al. (1996) have recently published an extensive review in which a large number of heavy ion nuclear reactions have been reported with data for, (i) total and partial cross sections, (ii) elastic scattering angular distributions and derivation of quarter point angles, (iii) determination of reaction mechanism such as deep inelastic scattering and sequential fission, and (iv) masses, kinetic energies, and angular distributions of the reaction products in intermediate and final reaction steps.

H. d. Khan, I. E. Qureshi / Radiation Measurements 31 (1999) 25-36 27

These observable lead to detailed investigations of reaction Q-values, kinetic energy losses, mass transfers and relative velocities as well as in-plane and out-of-plane angles of reaction products, resulting in the complete kinematical analysis of the given nuclear reaction. The technique has been adapted by Qureshi et al. (1996a) to study in-flight fission of heavy projectiles within the matrix of an insulating solid. Such a process is obviously not prone to investigation with electronic devices.

Pion- induced f i ss ion studies

SSNTD applications in basic research relevant to Nuclear Physics have been dominated by fission studies. Beginning with the earliest ground breaking observations of fission tracks in mica, the track detectors were used to generate the data of historical importance on spontaneous fission half-lives, life-times of compound nuclei, fission cross sections and fission barrier heights. This phase of work was followed by the study of fission using a variety of projectiles including heavy ions and other charged particles. In particular the pion-induced fission has been pursued in order to understand high excitation fission dynamics. Pion being the field particle of nuclear force is fully absorbed in the nucleus, thereby depositing its rest mass energy (~ 140 MeV) and kinetic energy within the nucleus. Fission cross sections and binary fission probabilities have been studied with negative and positive pions of energies in the range 80 - 500 MeV, using a wide variety of targets, from Fe to U. The cumulative results of fission probabilities are shown in Fig. 1. In this figure, the solid line is the parameterized mass dependence curve for fission probabilities (Peterson et al., 1995),

P~= 0.00133 + 0.999 [1+ exp {(35.12 - f)/0.9215}1 (1)

where 'f' is the fissility defined as f = (Z+I)2/A for n + interaction.

/ t . a

lo'" ,7 ° ]

Io'" 15 20 25 30 35

FISSILITY

Fig. 1. The combined data of ~+ induced fission probabilities for nuclides having a wide distribution of fissilities [(Z+I)2/A]. The data'iS fitted by a four-parameter function of fissility

(see text).

28 H.A. Khan, 1. E. Qureshi / Radiation Measurements 31 (1999) 25-36

The on-going extensive work on pion induced fission studies using negative pions of energies 500, 672, 1068, 1665 MeV was partially reported in the 18 th IC NTS (Khan et al., 1997). Fission and fragmentation studies with elementary particle projectiles as well as heavy ions are profoundly interesting from the point of view of basic research in nuclear physics. This work is expected to gain momentum and spread beyond the presently limited number of laboratories such as Marburg, Giessen, Dubna, Islamabad, etc.

High-energy heavy ion reactions

The successful applications of SSNTDs in the kinematical analyses of heavy ion reactions at low energies have been admirably extended to the high-energy region by the Siegen group (Brechtmann and Heinrich, 1988). Their method is based on the calibration in terms of etch- cone diameters, which are measured with high statistical significance using automatic image analysis system. Excellent charge resolution (C~z = 0. le) could be obtained with this technique. (Fig. 2). The problems investigated include the search for projectile fragments with fractional charges, mean-free paths of relativistic heavy ion fragments, charge correlation and transverse momenta in the high- energy heavy ion frgmentation. A noteworthy feature of these experiments is the enormity Of measured track data. In one case (Dreute et aL, 1991) 180 foil sides were measured while each surface contained - 30,000 tracks. The Au + Ag and Au + CR-39 interactions yielded 6610 events in which all projectiles in the range 6< Z < 65 were detected. Conclusions of foundamental importance were drawn from the observation of charge correlations and transverse momenta in multifragmentation reactions. The work was extended to study multifragmentation of Au incident on thick targets (Rusch et aL, 1994). The study of relativistic ion fragmentation was made possible by concurrent developments in automatic measuring systems. The high statistics required for such a work is possible only when the data is collected automatically by image analyzers. The number of automatic systems available in SSNTD laboratories is on the rise. However, the price tag is still too high to be affordable for low budget laboratories. Encouraging progress for low-cost automatic systems has been reported by Hashemi-Nezhad and Dolleiser (1997).

Q .Q

E c

4000

~000

2000

|000

0

3 2 S÷ [CR-39]

5 I0 Fragment charge

I

15

Fig. 2. High resolution charge identification for the fragmentation of 720 MeV 32S m CR-39, obtained by measuring areas of track openings (figure by Brechtmann & Heinrich, 1988).

H. A. Khan, L E. Qureshi / Radiation Measurements 31 (1999) 25-36 29

Complex radioactivity

A typical example of a sudden resurgence in the use of SSNTDs is the case of Complex Radioactivity. Capitalizing on the strong points of track detectors, Price et al. (1985) used the method effectively to study spontaneous emission of heavy ions from certain nuclides soon after the discovery of this radioactive mode. Complex radioactivity or cluster radioactivity occurs in competition with o~-decay with relative branching ratios of 10 -9 or lower. The radioactive decays involving emission of carbon, neon, magnesium and silicon ions have been observed from elements in the trans-lead region. This process is tailor-made for the most effective utilization of track detectors because a rare decay mode in the presence of high background of s-decay is to be investigated reliably. It is, therefore, not surprising that the majority of cluster decay modes observed to-date, were reported on the basis of SSNTD data or higher accuracy were obtained with this technique. A detailed overview of cluster radioactivity results based on SSNTDs has been given by Qureshi (1996b).

Astrophysics and Cosmic Rays

The SSNTDs have been extensively pressed into service for the search of magnetic monopoles. The MACRO collaboration has been using a 'stand-alone' etch track detector system since 1991. The evidence for primordial monopoles is being sought by exposing CR-39 sheets in the underground Gran Sasso laboratory in Italy. The upper bound for an isotropic flux of bare g = gD magnetic monopoles in the range 10 -4 < 13 < 10 .3 with 90% CL level has now reached the Parker bound as reported in the 18th IC NTS (Popa et al., 1997). The interest in the detection of magnetic monopoles is so general that it merited a comment in the popular British journal 'The Economist'. In its first issue of Nov. 1997, the negative result of five-year long study by Yudong He of the University of California was reported in the science and technology section. It was pointed out that the method of study, based on barium-phosphate glass is a particularly good technique for detecting extremely rare particles with unusually large charges. An area of intense SSNTD usage has been the study of elemental and isotopic composition of primary galactic cosmic rays. A long series of experiments on board cosmos series of Russian satellites have been monitoring cosmic ray heavy nuclei with energies from 100 to 800 MeV/nucleon, since 1973. A compact summary of their results has been given by Marenny (1995). The data obtained were used to construct a modified model for the fluxes and spectra of cosmic ray protons and heavy nuclei to be calculated with high accuracy. Among the on-going projects, one of the most note-worthy is the Ultra Heavy Cosmic Ray Experiment on board NASA's Long Duration Exposure Facility. As a result of about 40% of the analysis done up to 1997 (Keane et al., 1997), 3000 nuclei with Z>65 have been observed in an array of 100 m 2 Lexan Polycarbonate exposed for 69 months in space during 1984- 1990. The important features of their data samples include, (a) a prominent peak around Pt region, (b) a depleted peak in the Pb region, and (c) a significant number of actinide elements.

GEOLOGY/GEOPHYSICS

Fission track dating

A well-tested method of sound physical basis and practicality is the one used for dating geological samples, The so-called 'fission-track-dating' method has been applied in so many cases, that it is now considered as a technique of routine usage. The fission track dating requires the availability of a research reactor and consequently some laboratories may be handicapped owing to the absence of such a facility. The use of age equation involving a number of parameters, apart from fission fragment statistics, presents certain hazards, which have to be carefully handled, The neutron fluence is a crucial factor since the age of a sample is calculated from an expression which is proportional to this very big number (N 1012 _ 10~5). The relative abundance of U235/U 238 and spontaneous fission decay constant of 238U are well known factors but probably re-evaluations with better statistics will be helpful to increase the accuracy. In any case, it is to be remembered that the fission tracks in crystals represent the age when the crystal was cooled below the closure temperature. Different crystals within

30 H.A. Khan, L E. Qureshi ~Radiate'on Measurements 31 (1999) 25-36

a given rock would yield widely different ages of the rock if there were significant difference of closure temperature. This fact can also be used advantageously to estimate cooling rate of the rock and the uplift history of mountain ranges. Moreover, the subsequent annealing results in the fading of tracks, which seriously affects track statistics. Careful analysis of the correction factor due to track fading is a prerequisite for reliable age determination. An example of a good age determination isthe work reported by Guo et al. (1997). This work is also relevant to Archaeology since tektites were found near the habitat Chinese ancient man. The technique of fission track dating is widely used for the study of archaeological remains of old civilizations (Wagner, 1978). The track dating method applied on lunar samples, meteorites and tektites have also yielded a wealth of information on heavy components of cosmic rays, lunar surface erosion and thermal histories of inter-stellar matter (Fleischer et al., 1967).

Mineral exploration

The thermal history of rocks studied with SSNTDs has been identified by R.L. Fleischer (1998) as one of the most potentially profitable commercial use of track detectors in view of its relevance to petroleum geology. The tracks of spontaneous fission fragments in minerals are shortened and the track etching rate is reduced due to high temperature episode experienced by a given rock formation. Such rocks are not likely to bear oil deposits. Therefore an expensive deep drilling process can be avoided in regions with unfavourable the.rmal history. Perhaps this kind of work has not gained as much attention as it deserves. There have been hardly any relevant papers reporting successful or unsuccessful application of this method in the recent past. A more direct method of oil/gas prospecting, based on the pattern of radon emissions from earth's crust has also been discussed in the literature, but convincing evidence of its successful application is lacking. One example where radon emanations do provide information about underlying mineral deposits is that of uranium ore. Since radon is a progeny of uranium, there is a direct relationship between its outflow and presence of ore- bodies. The measurement of radon anomalies mapped by surveying an extended area in a uniform grid pattern is a proven method for uranium prospecting. Qureshi et al. (1988b) have performed a remarkable study (Fig. 3) in which the displacement of uranium ore-body from its original location was correctly identified on the basis of radon levels at the site of interest. Owing to the lowering of water table, an ore body was mobilized downward along a shale incline. The gamma-counters and logging units indicated a false uranium anomaly at its old site. However, radon measurements with SSNTDs in the suspected as well as adjoining areas gave two impressions; one corresponding to the older position and one to the new. The ore body mobilization was confirmed by drilling.

Fig. 3. Thc mobilization of uranium ore body detected by radon measurements using SSNTDs (Qureshi et al., 198g b).

H. A. Khan, I. E. Qureshi / Radiation Measurements 31 (1999) 25-36 31

Geophysical applications

Apart from fission track dating and mineral exploration, a number of other geological studies are prone to track detection technique, e.g. searching for geothermal energy sources, location of main boundary faults and prediction of earthquakes. Unfortunately, not much work has been reported on these topics in recent years. All these investigations involve the monitoring of radon outgassing which is affected in characteristic ways depending on the underground geological structures. Extensive radon mapping in five Mexican geothermal energy fields by Balcazar et al.

(1990) established a positive correlation between radon emanation and active regions. It was shown in these studies that the data of radon yields combined with other geophysical data could be used to locate areas for in depth flow testing, in order to find potential targets for geothermal energy sources. To locate hidden crustal structures such as geological faults, has been among the most successful applications of radon measurements using track detectors. The work of King (1978) and Tanner (1980) was instrumental in the location of subsurface active and dead geological faults in the USA, over San Andreas fault nmning through California. The main boundary faults in selected regions were demarcated with alpha sensitive plastic detectors in India by Ramola et al. (1988), and in Pakistan by Qureshi et al. (1991). In these studies, track detectors were placed in tubes, which were buried at regular internals along a line crossing the expected fault zone. The detectors closest to fault region showed alpha track density well above the background level indication well defined radon anomaly. The question of earthquake prediction using radon level measurements is somewhat controversial at present. This by no means indicates the failure of the technique. It only indicates that no single precursor may be enough to foretell the geological upheavals leading to the onset of an earthquake strong enough to cause human and material loss. By its very nature the work has to be spread over many decades in order to define radon emission systematics and to project future events on the basis of observed data. Qureshi et al. (1989) established two stations in Pakistan and observed radon data using cellulose nitrate based detectors buried in holes 50 cm deep. The experiment was repeated twice each with a span of three months. The data gave non-conclusive direct correlation with high radon yield and imminent earthquake event of magnitude 2 or more on the Richter scale.

ENVIRONMENTAL SCIENCE

Radon dosimetry

Measurements of radon levels to monitor and control indoor radioactive pollution continues to be a significant activity pursued by the users of SSNTDs. Most of the European countries have completed their radon level surveys on national scale and created their countrywide radon profiles. The current status of SSNTD technology for this kind of work seems to be adequate. Therefore very little developmental work on radon dosimetry has appeared in the literature recently. However, there are a number of issues related to the understanding of passive dosemeter response, which still remain to be addressed. The 'radon level' issue has on a large scale several components which have attracted varying amount of attention. To start with, it is to be proven in more quantitative terms that the effects of a certain indoor radon level involves health risks of a certain nature and severity. Epidemiologieal studies in this respect are seriously lacking and simulations are beset with problems of synergetic effects due to other non-radioactive influences giving rise to a given health effect. Users of track detectors, in any case, feel less concerned about this aspect although ultimately it is the availability of such data that justifies the radon level surveys on a wider scale. The second aspect is the use of properly calibrated dosemeters. Owing to a number of inter-calibration exercises conducted by commission of European countries, it is now agreed that SSNTD data is consistent and reliable for integrated radon level measurements. Various types of dosemeters in use include, diffusion samplers, permeation samplers, and bare detector dosemeters. The third aspect is related to radon mitigation. Again the track detector community is not directly involved in the relevant technology. However, in order to implement any solution, it remains to be

32 H. A. Khan, I. E. Qureshi / Radiation Measurements 31 (1999) 25-36

proved that the technique is successful. This involves post-implementation surveys preferably under the same conditions and identical dosemeters as done in the first place.

Neutron dosimetry

This is an area of track detector applications, which must be mentioned in any review, both because of its importance to nuclear technology as well as because of its widespread usage in nuclear establishments. The neutron fluxes at irradiation chambers of a reactor are required to be monitored periodically for their effective utilization in different experiments. Also, to avoid the risk of neutron exposure to radiation workers, it is desirable to use personnel neutron dosimeters. In both instances etch track detectors have been found to be effective and reliable. The neutrons themselves do not leave tracks in dielectric solids, however, their secondary effects, in the form of the recoiling atoms of the detector under neutron impact, or the induction of nuclear reactions in suitable converters leading to charged particle production, are readily observable. The former method is suitable for high-energy neutrons. The response of the detector has been shown to be smooth enough, in the form of track density for dose measurements, and in the form of track lengths for the identification of neutron energy bins. Slow neutrons are best detected by the neutron-induced fission in 235U, whereby fission fragmentsare registered in the track detector. Neutrons may also induce (n, o 0 reactions in certain materials, such as lithium and boron. The resulting o~-particles are registered in the detector giving a track count in direct proportion to the neutron flux. Neutron dosemeters for low as well high-energy neutrons have been put to use in a variety of personnel exposure situations. However, for good accuracy and reproducible results, it is necessary to use direct calibration as well as simulation of detector response. The direct calibration of the detector is achieved experimentally by irradiation with a predetermined neutron dose or else with neutron groups of known energies and fluxes. DOrschel et al. (1997) have also used calibrations with secondary particles such as protons and s-particles for adjusting the parameters of a model for the simulation of detector response. These parameters depend on factors such as restricted energy loss and critical angle of particle incidence, which are determined experimentally.

Fil ters

One of the earliest uses of track etching technique was to produce extra fine sieves having pore sizes as small as molecular dimensions. This development was based on the observation that after the passage of energetic charged particles across the thickness of a thin detector, it is possible to produce through-holes under appropriate etching conditions. The detector membranes perforated in this manner have been put to various uses, resulting in a significant widening of the scope of SSNTD applications. The basic work on pore sizes and shapes in different materials for different etching conditions has been accomplished long ago and presently only the technological applications are mainly reported in the literature, e.g. cytology, water purification, aerosol monitoring, reactor effluent assay, separation of liquid mixtures and mixed gases, etc. As a powerful and versatile tool the microfilters offer opportunities of environmental research applications, limited only by the imagination of the user. On the front of filter methodology, an important development is related to the fabrication and use of a single hole with desired size and shape. The production of such an isolated hole is feasible by using a dispersed heavy ion beam of required ion mass and energy. Perhaps the most spectacular uses of single ion etch track filter has been in the field of flow cytometry. Using the resistive pulse technique, DeBlois and Bean (1970) were able to count and measure the size of red blood cells. Moreover, the rigidity of red cells related to certain diseases can also be ascertained by letting the blood pass through pores of sizes somewhat smaller then the normal healthy cell size. A novel form of etch track membranes has been reported by Yoshida et al. (1997) in the Cario conference. They have developed membrane of CR-39 which were chemically modified by radiation induced grafting of a polymer gel (Acryloyl-L-proline methylester). The gel exhibits a characteristic

14. ,4. Khan, L E. Qumshi / Radiation Measurements 31 (1999) 2.5-36 33

phase transition at a lower temperature of 14°C. Thus the pore sizes become thermo-responsive, much like the surface membranes of living cells (Fig. 4).

Fig. 4. The thermo-representative synthesized nuclear pore filters by gratted membrane tecbafique (a) 0 ?C and (b) 30 °C (figure by Yoshida et aL, 1997).

MATERIAL SCIENCE

Radiography~Imaging

Radiography plays an important role in radiobiological studies and materials research. The potential uses of etched track detectors in these areas have been identified anal successfully demonstrated. However, there appears to be a lack of full exploitation of this technique both in the industry as well as in the basic research centres. The imaging with track detectors takes several forms. Firstly, a sample may itself contain o~-emitting radionuclides in which case an appropriate detector such as cellulose nitrate in direct contact with the material will register or-tracks of varying track density leading to the formation of an image. Secondly, the sample may be exposed to thermal neutrons, which would trigger nuclear reactions such as (n, o~) and (n, f) at specific locations. When these secondary s-particles or fission fragments enter the detector placed in contact with the sample an image would be formed. Thirdly, a charged particle beam, having different penetration in different parts of a thin sample, may be allowed to enter the detector placed behind the sample. The tracks induced by such a beam will have different densities at different positions and would produce different gray levels after etching; thereby an image much like the X-ray image will be produced. A novel method of radiography, dubbed as "Radongraphy' has been reported by Skvar~ and Ili6 (1994). In this method, radon is allowed to diffuse into a material, so that after some passage of time, the or- emission by radon daughters from the surface of the sample serve as a radiograph of the sample's top layer.

Ion lithography

Ion lithography is an apt choice to end this brief overview, since it reflects the present potentials as well as future possibilities of using track etch methodology in a yet another area of technology. Not surprisingly, the lead in this field is being provided by GSI ( Darmstadt, Germany), which houses excellent hea~3z ion accelerator facilities with wide variety of ion sources and energy ranges. The monograph by Spohr (1990) gives fairly good coverage of the relevant details. It suffices here to point

34 H.A. Khan, I. E. Qureshi / Radiation Measurements 31 (1999) 25--36

out that heavy ion lithography enlarges the scope of conventional lithographies in several ways. Firstly, the irradiation is controlled in a precise manner in terms of its spatial distribution because heavy ion beams can be controlled in a precise manner. Secondly, the irradiation by heavy ions leads to energy deposition at sufficient depth of the resist material, making it possible to create three- dimensional structures, during the process of development. Thirdly, the range of materials susceptible to lithography is considerably extended because of high energy doses imparted by heavy ion beams. The need of higher and higher packing densities of transistors in the integrated circuits has practically reached the limit of miniaturization with conventional lithographies. In order to produce patterns of finer line-work it is necessary to use extremely short wave-lengths, which are provided by beams of heavy ions.

CONCLUDING REMARKS

From the foregoing discussion, it is amply obvious that track method has now matured into a well- established scientific tool with wide variety of applications in diverse areas of science and technology. There is still scope for expanding its usage, while strenuous efforts have to be continued to consolidate the established areas of applications. In particular, quantitative analyses with higher statistics would be a key factor for its adoption as a general-purpose instrument in nuclear physics research. It is worth recalling that the major advantages of track detectors, namely, cost-effectiveness, ease-of-use, off-line nature of data taking, and threshold mode of detection, played a major role in its acceptability during the early phases of development These features would continue to be important factors for its growth in future. However, there are certain aspects of etch-track methodology that have to be addressed urgently and'with concerted efforts of the SSNTD community. At the most fundamental level, it has yet to be elaborated in sufficient detail, what precisely is the mechanism of latent track formation. The track morphology has to be studied more extensively using electron, or atomic force, microscopy along with any other method of analysis at hand, such as delta- electron detection etc. The next step, involving the process of chemical etching, has to be elevated from an art form to a proper science by a thorough investigation of chemical kinetics in the detector matrix. The limitations would still persist if standardization of track detector materials and automation of track counting and measurements were not adopted in more laboratories. It is our considered opinion that all sciences, track science being no exception, flourish in an open and cooperative atmosphere. If access to accelerators and calibration facilities is offered to all members of International Nuclear Track Society, it is conceivable that this method may become indispensable for scientific workers across the globe.

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