Nuclear tracks: present and future perspectives

Current research work related to the development of nuclear tracks comprising: (i) fundamental principles (nuclear track physics and chemistry, as well as development of track detectors and the relevant hard- and software), (ii) development of nuclear instruments and methods (etch track radiometers for ions, neutrons and cosmic rays, radon monitoring devices, radiography and fission track dating) is briefly outlined. The paper concentrates on a literature survey of applications of nuclear tracks in (iii) physical sciences (high-energy physics, nuclear physics and earth sciences), (iv) biomedical sciences (radiation protection, environment, cancer therapy), and (v) technological sciences (materials, nano-technology and nuclear technology).

Presently about 350 papers per year are being published in this field. Increased activity is noted in ion track technology (track-made membranes, modern nano-tech methodology including biological and biological-like samples, nano-electrode bio-electrochemistry, bio-magnetic assays and probes). New applications of nuclear tracks in fundamental (possibility of the detection of neutron quantum states in a gravitational field, nucleus–nucleus interactions, search for new chemical super-heavy elements) and applied science (precise measurements of the behaviour of radiation in human tissue in connection with of long term space missions and treatment of cancer) are surveyed, and possible research in the next decades is presented and examined in this review paper.     

Nuclear tracks in solids: registration physics and the compound spike

Observations of GeV heavy ion and MeV cluster-ion tracks in crystalline solids give us new insight into registration physics. Thermal and ion explosion spikes no longer compete; a ‘compound spike’ accounts for both. Ion explosion dominates for surface tracks (electronic sputtering). And there can also be transient plasma stopping in the bulk. For clusters there are ‘vicinage effects’—both electronic and nuclear—which can influence track dimensions and structure. Displacement cascades in large energetic clusters may lead to projectile “fission” and coherent flow into sub-tracks. The absence of tracks in certain targets, and their size/structure in others, leads to a model of projectile assisted prompt anneal (PAPA) in 10⁻¹¹s, either partial or complete, often by swift epitaxy, on elemental lattices (e.g. silicon) or on compound sublattices (e.g. fluorite). Phase transformations are important, but simple target amorphization is rare—the exception, not the rule. For many targets the thermal spike (macroscopic) fails, since ‘point’ defects (atomistic) characteristic of the target, their motion, and the electronic band structure, determine latent track detail. Circumstances in which the Bragg Rule of Additivity fails completely are revealed, and the kinetic threshold for constructive phase transitions in tracks described. This same track physics applies generally also to geothermometry—the opposite time extremum (10⁺¹¹s)—where annealing is due to defect assisted delayed anneal (DADA). Differences between etching rates of induced and spontaneous fission tracks can be explained. The geothermobarometric “Wendt/Vidal effect” (2002)—combined pressure, temperature and stress (with time) influences on fission track annealing (in e.g. apatite)—is briefly discussed.     

Cosmic history of some pallasites based on fossil track studies

Investigation of fossil charged-particle tracks in various mineral phases of extraterrestrial samples is a powerful method for research the early stages of the solar system. Examination of fossil charged-particle tracks in the phosphate and olivine crystals of 5 pallasites coupled with U content determination allowed the estimation of the contributions of all possible track sources to the total track density and calculation of a value of model fission-track age.

Phosphate crystals from all studied pallasites were established to contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); induced fission of U and Th by cosmic rays; spontaneous fission of ²³⁸U; spontaneous fission of extinct short-lived ²⁴⁴Pu nuclei presented in significant quantities in the early solar system. The discovery of a high track density attributed to the extinct ²⁴⁴Pu pointed obviously to the high value of the fission-track age. Model fission-track ages of (4.37±0.02) Gyr for the Marjalahti pallasite; (4.37±0.01) Gyr for the Omolon pallasite; (4.19±0.02) Gyr for the Bragin pallasite; (4.18±0.03) Gyr for the Krasnoyarsk pallasite; (4.21±0.02) Gyr for the Brenham pallasite were calculated. The comparison of represented data with petrographic analyses allowed us to interpret a value of fission-track age as the time of the last intensive shock/thermal event in the cosmic history of a pallasite.     

Fission-track analysis of meteorites: Dating of the Marjalahti pallasite

The results of the Marjalahti pallasite fission-track age determination are presented. Thorough examination of fossil tracks in the phosphate (whitlockite) crystals coupled with U-content determination in whitlockites can make it possible to estimate the contributions of all possible track sources to the total track density and to calculate a model fission-track age.

It is found that whitlockite crystals of the Marjalahti pallasite contain fossil tracks due to galactic cosmic rays (VH, VVH nuclei); fission of U and Th induced by cosmic rays; spontaneous fission of ²³⁸U; and spontaneous fission of extinct, short-lived ²⁴⁴Pu present in significant quantities in the early solar system.

A great track density attributed to the extinct ²⁴⁴Pu testifies to the high fission-track age. The model fission-track ages of (4.31±0.02)×10⁹ yr for the Marjalahti pallasite are calculated. Petrographic studies allow us to interpret the fission-track age as the time of the last shock/thermal event in the cosmic history of the pallasite.     

Cosmic history of the Bragin pallasite: evidence from the fission-track analysis

Chronology is rather a weak point in the investigation of pallasites, the stony-iron meteorites. No chronological data are known for the Bragin pallasite. Our attempt to reconstruct its cosmic history was based on the interpretation of fission-track analysis data. To apply this method only uranium-rich phosphates can be used. Extremely rare grains of stanfieldite were extracted from the silicate sawing residue and from the pallasite sample directly.

The researches pursued by us made it possible to find two populations of fossil tracks in stanfieldite grains. The tracks of these populations strongly differed both in size, shape and character of distribution. The first population, consisting of short (L2–6 μm instead of L8–12 μm for induced fission tracks), round-shaped tracks irregularly distributed, as we suppose, suffered an intense heating process, which caused a significant amount of partial annealing. The second population, consisting of longer (L8–12 μm), rhombic-shaped tracks homogeneously distributed, occurred after this thermal event. Only the second population track density was used for the fission-track age calculation.

After correction of the fossil track density, consisting of the second population tracks, for other possible track sources, the revealed tracks were unequivocally identified as those due to the spontaneous fission of ²⁴⁴Pu and ²³⁸U. The largest part of them was attributed to the spontaneous fission of ²⁴⁴Pu; ρ_Pu/ρ_U≈3. The model fission-track age of the studied pallasite turned out to be 4.20 Gyr. This value fix the time of the last shock/thermal event in the cosmic history of the Bragin pallasite, which had caused the partial annealing of tracks presented to that time and “fission-track clock” reset.     

Some contributions from SSNTD towards nuclear science: from multifragmentation towards accelerator driven systems (ADS)

Three examples are chosen to show the importance of SSNTD as one of the essential tools in nuclear science:

(1) Multifragmentation into more than two heavy reaction products: Starting with the observation of three heavy reaction products in the interaction of relativistic protons or 414 MeV ⁴⁰Ar with actinides in the early 1960s, up to the observation of five heavy reaction products in the interaction of 2400 MeV ²³⁸U with uranium, SSNTD had a leading role in this research.

(2) In the search for superheavy elements (SHE: Z around 114): Many different techniques are used. However, SSNTD are exclusively decisive in the possible observation of SHE within the heavy element component of galactic cosmic rays.

(3) Accelerator driven systems: They are increasingly important in the discussion of energy producing nuclear power stations and in the corresponding ability to transmute long-lived poisonous radioactive materials (above all plutonium) into shorter lived fission fragments or stable nuclides. SSNTDs play an important role in the determination of the energy dependent neutron fluence in small volumes (≈cm³) or in the exact beam profile determinations of the primary proton beams.

This contribution ends with an outlook into possible future fields of physics research: With the advent of a new generation of relativistic heavy ion accelerators, such as the NUCLOTRON at the JINR in Dubna, Russia, and RHIC in Brookhaven in the United States, one can continue to study (and finally confirm or disprove) all phenomena mentioned already, as well as additional controversial phenomena, such as “enhanced nuclear cross-sections over short distances”, called colloquially “anomalons”. Again SSNDT can be used in at least a twofold manner as an important tool: (a) the enhanced neutron production with ¹²C ions or heavier ions in thick targets at energies above approximately 50 GeV and (b) the reduced “mean-free-path” of secondary fragments produced by the same heavy and energetic ions.     

Fission fragment track etching modelling for the cellular localisation of Neptunium-237 nuclei in rat tissues

Heavy ion recording in dielectric isotropic detectors has a wide range of applications in uranium cartography or dating. In this study, solid state nuclear tracks detectors (SSNTDs) have been applied to localise neptunium-237, in vivo, by means of the neutron-induced fission cartography of neptunium-237 nuclei in organ sections. At the cellular level, a precise localisation can only be achieved with a good understanding of the etch pit evolution during the chemical etching process.

A tailor-made software for modelling the etching process has been developed to simulate the profile of an etched track produced by a neutron-induced fission fragment. This software is based on a model that considers the evolution of the track etching velocity along the damage trail in order to perfectly model the shape of the track.     

Science and technology with nuclear tracks in solids

Fission track dating has greatly expanded its usefulness to geology over the last 40 years. It is central to thermochronology—the use of shortened fission tracks to decipher the thermal history, movement, and provenance of rocks. When combined with other indicators, such as zircon color and (U–Th)/He, a range of temperatures from C to C can be studied. Combining fission track analysis with cosmogenic nuclide decay rates, one can study landscape development and denudation of passive margins. Technological applications have expanded from biological filters, radon mapping, and dosimetry to the use of ion track microtechnology in microlithography, micromachining by ion track etching, microscopic field emission tips, magnetic nanowires as magnetoresistive sensors, microfluidic devices, physiology of ion channels in single cells, and so on. In nuclear and particle physics, relatively insensitive glass detectors have been almost single-handedly responsible for our knowledge of cluster radioactivity, and plastic track detectors together with automated measuring systems have been used at the Bevalac, Brookhaven, CERN, and GSI, mainly to study fragmentation of high-energy heavy nuclei. Almost everything we know about the ultraheavy cosmic rays has been learned using Lexan on the Long Duration Exposure Facility and BP-1 phosphate glass on the Mir Station. New topics include development of calorimetric aerogels capable of measuring kinetic energies of hypervelocity interstellar and interplanetary dust grains in space and research on identification of strains of Bacillus spores by measurements of their size and swelling rates when humidified.     

Thermal spike analysis of low energy ion tracks

The theoretical track diameter of low energy ions in organic materials is usually estimated through the model of dose deposition by delta rays, with results remarkably lower than the experimental values obtained via a replica method and electron microscopy. The track detector used here is Makrofol-E and the ions studied have specific energies between 1.4 and 100 keV/n. To evaluate the problem from another point of view, thermal effects for track formation, a modified version of the “liquid drop model” for insulators was applied. The electronic as well as nuclear energy deposition by an individual ion are considered and the thermal spike evolution is studied. The model allows for the formation of ion tracks in a range of energies previously considered as “forbidden”. There still exists a discrepancy between the experimental data and the track diameters predicted by the model, and although this difference is smaller than the obtained with previous calculations, it suggests the necessity of further adjustments.     

Airborne radioactivity levels in dwellings of Khammam district, Andhra Pradesh, India

Radon/thoron levels are estimated in Khammam district as a part of “Radon/thoron mapping” program in Andhra Pradesh, India. About 100 dwellings were chosen at random covering most parts of Khammam district. Twin chamber cup dosimeters consisting of solid-state nuclear track detectors (SSNTDs, LR-115, Type-II) were used to evaluate the radiation levels at quarterly intervals for a period of one calendar year. The average radon and thoron concentrations were found to be 39.1±13.3 and 19.4±14.9 Bq/m³, respectively. Seasonal variations of radon/thoron levels were also studied. Variations of radon/thoron concentration levels with different types of dwellings have been studied. In the bare flooring dwellings thoron levels were found to be highest in comparison to concrete, limestone and marble floorings.     

Uranium analysis by neutron induced fissionography method using solid state nuclear track detectors

In this study total twenty samples (eight reference materials and twelve sediment samples) were analysed for their uranium content which is in the range of 1–17 μg/g, by neutron induced fissionography (NIF) method using solid state nuclear track detectors (SSNTDs) in comparison with the results of neutron activation analysis (NAA), delayed neutron counting (DNC) technique or fluorometric method. It is found that NIF method using SSNTDs is very sensitive for analysis of uranium.     

Anomalous cosmic ray carbon and oxygen tracks in CN-Kodak

For observation of low energy cosmic ray particles we used CN-Kodak nuclear track detectors on Cosmos satellites. In solar quiet periods during solar minima conditions the detectors registered anomalous cosmic rays (ACRs). The ACRs are characterized by flux enhancements of several elements and it is known that the carbon enhancement is small compared with that of oxygen. In all of our quiet-time exposures the relation between carbon and oxygen was extremely small (C/O 0.03). But in two quiet-time periods of 14.03.96–11.06.96 and of 15.12.97–14.04.98 we have identified many tracks as carbon in a L–R diagram. As a result the observed C/O ratio appears to be more than 0.5, whereas other experiments show no evidence of enhanced flux of carbon during these periods. The reason for the unexpected response of CN-Kodak is discussed.     

Alternative procedure for LR 115 chemical etching and alpha tracks counting

An improved procedure for etching and analysis of alpha tracks induced in LR-115 detectors is proposed with the advantages of simplicity and its relatively low cost. A new type of detector holder was designed to etch and rinse efficiently up to 100 detectors. We develop a simple and reliable methodology with a semiautomatic track count using a Nikon digital camera coupled to a PC and employing software “SCION” freely available on the Internet. Track images are binarized prior the application of software “SCION” so that original track shapes are not distorted, space resolution is improved and track counting has low dependence on focus and illumination level. High discrimination for tracks is achieved when marks and rips perturb the detector surface. An image generator of nuclear tracks is included to study the effect of track overlapping effect on counting.     

Search for massive rare particles with the MACRO track-etch detector at Gran Sasso

Searches for massive penetrating particles in the cosmic radiation have been performed with the MACRO nuclear track detector used as a “stand alone” detector. The complete experimental procedure is presented. In absence of candidates, updated estimates of the flux upper limits both for the CR-39 “stand alone” detector and for the global MACRO detector are presented for magnetic monopoles, nuclearites and charged Q-balls.     

Effect of thermal annealing on pore density,pore size and pore homogeneity of polycarbonate NTFs

When an energetic and electrically charged beam of particles penetrates a nonconducting material, like crystal, polymer, etc., a permanent change in the nature of the chemical matrix along its path occurs giving rise to damaged sites known as latent tracks having diameter of the order of 10 nm. These tracks show an enhanced chemical reactivity as compared to the surrounding material; and, on etching, the damaged sites are enlarged forming nuclear track filters (NTFs). The NTFs so formed find applications in the fields as nuclear physics, geology, radiation dosimetry, material research, medicine and environmental studies. The pore density and pore size homogeneity are important traits of NTFs. These NTFs are also employed for the production of micro/nanostructures through template synthesis which finds various applications for the fabrication of sensors, devices and in the field of opto-electronics. The pore size of the NTF is generally not uniform due to various reasons like material defects in solid state nuclear track detectors (SSNTDs), nonuniform flux density of irradiating beam, variation in the angle of exposure during irradiation, temperature gradient in the etching solution, etc. The present work reports the development of NTFs and study of the effect of pre-etch thermal annealing of irradiated polycarbonate thin films (SSNTDs) on the density and homogeneity of etched pores in developed NTFs. NTFs were developed by irradiating Makrofol (KG) with heavy-ion beam at the UNILAC, Gesellschaft fur Schwerionenforschung (GSI), Darmstadt, Germany and then chemically etching of annealed and unannealed irradiated samples. The pore size, pore density and pore size homogeneity of the NTFs were compared for annealed and unannealed samples. The study indicates decrease in pore size, pore density and increase in homogeneity of pores with post-irradiation and pre-etch thermal annealing. The uniformity of the pores of NTF will lead to uniformity of micro/nanostructures fabricated by using the pores as templates. These uniform micro/nanostructures can act as excellent charge sprayers in the cold cathode emitters.     

Study of fossil tracks due to VH and VVH groups of cosmic ray nuclei in olivine crystals from Luna-16 and Luna-24

The olivine crystals from lunar regolith samples taken by the Soviet unmanned spacecrafts Luna-16 and Luna-24 were investigated. Eleven 0.5 –1.0 mm size olivine crystals were mounted in epoxy, polished and then etched in modified WO₄ solution. The Fe-group track densities up to 10⁸ tracks.cm⁻² (Fe-group) were measured under optical microscope. The tracks of length greater than 30 microns due to Z ≥ 36 cosmic ray nuclei are counted for VVH tracks density for all the crystals. The VVH / VH track densities ratio for these lunar olivine crystals varies from 1.25×10⁻⁴ to 2×10⁻³. It corresponds to the averaged depth of these crystals in lunar soil of 2–8 cm during galactic cosmic ray exposure. Lunar crystals are well suited for VVH track studies due to a very high track density. Two crystals were annealed at 430°C for 32 hrs. This procedure eliminates iron group tracks completely and leaves etchable tracks of nuclei with Z 50 even in the olivine crystals with Fe-group tracks up to 1–2×10⁸ tracks cm⁻². We were able to measure two tracks with the length 195 and 210μm which were produced by Th---U group of Galactic cosmic ray nuclei.     

Nuclear track radiography of “Hot” aerosol particles

Nuclear track radiography was applied to identify aerosol “hot” particles which contain elements of nuclear fuel and fallout after Chernobyl NPP accident. For the determination of the content of transuranium elements in radioactive aerosols the measurement of the -activity of “hot” particles by SSNTD was used in this work, as well as radiography of fission fragments formed as a result of the reactions (n,f) and (γ,f) in the irradiation of aerosol filters by thermal neutrons and high energy gamma quanta. The technique allowed the sizes and alpha-activity of “hot” particles to be determined without extracting them from the filter, as well as the determination of the uranium content and its enrichment by ²³⁵U, ²³⁹Pu and ²⁴¹Pu isotopes. Sensitivity of determination of alpha activity by fission method is 5×10⁻⁶ Bq per particle. The software for the system of image analysis was created. It ensured the identification of track clusters on an optical image of the SSNTD surface obtained through a video camera and the determination of size and activity of “hot” particles.     

On search and identification of relatively short-lived superheavy nuclei (<Emphasis Type="Italic">Z</Emphasis> ≥ 110) by fossil track studies of meteoritic and lunar olivine crystals

The main goal of the present work is the search for and identification of relatively stable nuclei of superheavy elements (SHE) (Z>110) in galactic matter by fossil track study of nonconducting crystals from the surface of meteorites and rocks from the lunar regolith. Nuclei of SHE are thought to be the products of nucleosynthesis in explosive processes in our Galaxy (supernova r-process nucleosynthesis and, especially, neutron-star formation, etc.). When accelerated to relativistic energies in the Galaxy, they can produce extended trails of damage in nonconducting extraterrestrial crystals. The lifetime of such SHE in galactic cosmic rays will range from 10³ to 10⁷ yr to be registered in extraterrestrial crystals. To search for and to identify the superheavy nuclei in the galactic cosmic rays, it was proposed to use the ability of nonconducting extraterrestrial crystals such as olivines, pyroxenes, and feldspars to detect and to store for many millions of years the trails of damage produced by fast Z≥23 nuclei coming to rest in the crystalline lattice. The track lengths of fast Z≥23 nuclei are directly proportional to Z² of these nuclei. The nuclei of SHE produce, when coming to rest in a crystal volume, tracks that are a factor of 1.6–1.8 longer than the tracks due to cosmic-ray Th and U nuclei. To identify the tracks due to superheavy nuclei, calibrations of the same crystals were performed with accelerated Au, Pb, and U nuclei. For visualization of these tracks inside the crystal volume, proper controlled annealing and chemical etching procedures were developed. Since 1980, fossil tracks due to Th and U nuclei have observed and unambiguously identified (1988) by subsequent calibrations of the olivine crystals with accelerated U, Au, and Pb ions. The number of tracks of Th and U nuclei measured in olivine crystals totaled more than 1600, as compared with the prior 30 events. The other approach to identifying SHE in nature is to search for tracks in phosphate crystals from spontaneous fission of Z ≥ 110 nuclei; these produce two-prong and three-prong fission fragment tracks and differ significantly from the tracks from spontaneous fission of ²³⁸U and ²⁴⁴Pu nuclei. Extraterrestrial phosphate crystals of lunar and meteoritic origin will be investigated. Such SHE nuclei can survive in crystals of extraterrestrial rocks and produce spontaneous fission tracks, if the lifetime is more than 5×10⁷ yr.     

Tracks to therapy

Studies of the structure of particle tracks have led to models of track effects based on radial dose and radiobiological target theory that have been very successful in describing and predicting track effects in physical, chemical, and biological systems. For describing mammalian cellular inactivation two inactivation modes are required, called gamma-kill and ion-kill, the first due to synergistic effects of delta rays from adjacent ion paths thus resembling the effects from gamma rays, and the second to the effects of single ion transits through a cell nucleus. The ion-kill effect is more severe, where the fraction of cells experiencing ion kill is responsible for a decrease in the oxygen enhancement ratio, and an increase in relative biological effectiveness, but these are accompanied by loss of repair, hence to a reduction in the efficiency of fractionation in high LET therapy, as shown by our calculations for radiobiological effects in the “spread out Bragg Peak”.     

On methodical problems in estimating geological temperature and time from measurements of fission tracks in apatite

The results of apatite fission-track modelling are only as accurate as the method, and depend on the assumption that the processes involved in the annealing of fossil tracks over geological times are the same as those responsible for the annealing of induced fission tracks in laboratory experiments. This has hitherto been assumed rather than demonstrated. The present critical discussion identifies a number of methodical problems from an examination of the available data on age standards, borehole samples and samples studied in the framework of geological investigations. These problems are related to low- (<60°C) and high-temperature (>60°C) annealing on a geological timescale and to the procedures used for calculating temperature–time paths from the fission-track data. It is concluded that it is not established that the relationship between track length and track density and the appearance of unetchable gaps, observed in laboratory annealing experiments on induced tracks, can be extrapolated to the annealing of fossil tracks on a geological timescale. This in turn casts doubt on the central principle of equivalent time.

That such uncertainties still exist is in no small part due to an insufficient understanding of the formation, structure and properties of fission tracks at the atomic scale and to a lack of attention to the details of track revelation. The methodical implications of discrepancies between fission track results and the independent geological evidence are rarely considered. This presents a strong case for the re-involvement of track physicists in fundamental fission track research.