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.     

Swelling and structure of radiation induced near-surface damage in CR-39 and its chemical etching

Systematic measurements of swelling of CR-39 nuclear track detector (NTD) due to irradiation with fission fragments and alpha particles over a wide range of fluences from ²⁵²Cf are presented here. Precisely designed and optimized exposure and chemical etching experiments were employed to unfold the structure of radiation induced surface damage. Delays in the startup of the chemical etching of latent tracks in low radiation fluence detectors are measured and are found to contain important information about structure of the surface damage. Simple atomic scale pictures of radiation induced surface damage and its chemical etching are developed using measurements of radiation induced swelling of CR-39 detectors and nuclear track parameters. The computer code SRIM2010 was utilized for the calculations of basic features of latent tracks of fission fragments and alpha particles in CR-39. Another computer code TRACK_VISION was used to compute parameters of etched tracks. Computations and experimental findings in the paper coherently compose a realistic picture of radiation damage.     

The effect of UV exposure on the track and bulk etching rates in different CR-39 plastics

It is observed that for both UK and Japanese CR-39 detectors the mean diameters of fission fragment and alpha tracks from a Cf-252 source become larger as a result of UV exposure; but in the case of ‘UV exposure last’ (i.e. Cf + UV) the diameters are larger than in the case of ‘UV exposure first’ (i.e. UV + Cf). The bulk and track etching rates undergo an increase with UV exposure for the two kids of CR-39.     

Reactive scattering damage to DNA components by hyperthermal secondary ions

We have observed reactive scattering damage to fundamental DNA building blocks by the type of hyperthermal secondary ions that are produced along heavy ion tracks in biological media. Reactions include carbon abstraction by N+, and hydrogen abstraction by O- and N+, at collision energies down to 1 eV. Our results show that localized reactive scattering by hyperthermal secondary fragments can lead to important physicochemical damage to DNA in cells irradiated by heavy ions. This suggests a fundamentally different picture of nascent DNA damage induced by heavy ion tracks, compared to conventional (x or gamma) radiation tracks.     

辐射致DNA损伤及相关生物-化学-物理问题(英文)

介绍了在DNA辐射损伤以及相关生物、物理、化学问题的一些研究工作。重点是径迹结构、DNA双链断裂和细胞学终点的关系。P53-Mdm2负反馈回路在DNA损伤的细胞响应方面起重要作用,用一个简单的模型研究了P53-Mdm2负反馈回路相互作用的动力学行为。Some of recent works are presented on radiation induced DNA damage being carried out at China Institute of Atomic Energy （CIAE） and related bio-chemi-physical problems. Emphasis is placed on track structure and its relation to the cell end-point, and a simple model proposed to study the dynamical behavior of P53-Mdm2 interaction which plays pivotal role in cellular response to DNA damage.     

Energy loss dependent transversal etching rates of heavy ion tracks in plastic

By the method of electrolytical etching track etching rates V_t and corresponding transversal track etching rates V_trans of single heavy ion tracks in thin Makrofol KG foils have been measured at ion energies from 10–480 MeV/u. Makrofol KG foils of 8 μm thickness were irradiated perpendicular to the surface with ⁷⁹Au and ⁵⁴Xe ions at specific energies with energy loss values of REL=(10–90) ^*10³ MeVcm²/g at GSI Darmstadt, Germany, and Lawrence Berkeley Lab., Cal., USA. Using the electrolytical etching method by measuring the resistance of the foil during the etching process (etching conditions: 6n NaOH, room temperature and controlled 50° C) the breakthrough time and track etching rates V_t, V_trans and V_m (bulk etching rate) were analysed. Response curves (V_t/V_m)-1 as a function of Restricted Energy Loss (REL), the maximum extension of the ion induced damage perpendicular to the ion path and the dimension of the ion track core depending on the deposited energy can be estimated.     

Effect of a magnetic field on the track structure of low-energy electrons: a Monte Carlo study

The increasing use of MRI-guided radiation therapy evokes the necessity to investigate the potential impact of a magnetic field on the biological effectiveness of therapeutic radiation beams. While it is known that a magnetic field, applied during irradiation, can improve the macroscopic absorbed dose distribution of electrons in the tumor region, effects on the microscopic distribution of energy depositions and ionizations have not yet been investigated. An effect on the number of ionizations in a DNA segment, which is related to initial DNA damage in form of complex strand breaks, could be beneficial in radiation therapy. In this work we studied the effects of a magnetic field on the pattern of ionizations at nanometric level by means of Monte Carlo simulations using the Geant4-DNA toolkit. The track structure of low-energy electrons in the presence of a uniform static magnetic field of strength up to 14 T was calculated for a simplified DNA segment model in form of a water cylinder. In the case that no magnetic field is applied, nanodosimetric results obtained with Geant4-DNA were compared with those from the PTB track structure code. The obtained results suggest that any potential enhancement of complexity of DNA strand breaks induced by irradiation in a magnetic field is not related to modifications of the low-energy secondary electrons track structure.     

Feasibility study for the use of PADC as a radiation detector for living cell cultures

In the framework of an ESA project, a microbiological experiment in space is planned. In this experiment a cell culture will be exposed to cosmic radiation onboard a spacecraft. Because the living cell culture will be directly on a nuclear track detector stack, this detector will be submitted to a different environment than normally used. The temperature will be 37°C and the culture will be in a biological growth medium. Tests have been conducted to assess the possible use of PADC in these conditions. For this, a series of alpha irradiated detectors have been exposed for different periods of time (up to 1 month) to these ‘biological’ conditions. The radiological properties as well as the mechanical properties (swelling…) have been investigated. Results show no influence of the biological environment on the PADC, which makes it useable under these circumstances.     

Geometry of etched charged particle tracks in crystalline detectors

Studies on the shape of etched fission tracks in crystalline detectors show that the track geometry carries valuable information on the crystal structure of the detector involved. The geometry of the etched tracks in crystalline detectors is a prism formed of certain crystal planes. In mica detectors, for given etching conditions the type and number of the planes depend on the incident angle and the extent of the radiation damage along the track. In a single track the type of crystal planes forming the track prism may change along the track resulting in dramatic variations of the track geometry at certain depth of focus.     

Studies on the nuclear tracks in CR-39 plastics

A review was given for our recent studies on the latent tracks in CR-39 nuclear track detector. The radial size of track core has been determined through UV spectral measurements combined to the model of track overlapping and by AFM observations of slightly etched detectors. The track core radius was found to be about a few nano-meters and almost proportional to the cubic root of stopping power. As a control study, the etching properties of CR-39, irradiated by low-LET radiation, has been examined. The observed depth dependence and dose-rate dependence of the bulk etch rate of the irradiated CR-39 were explained that the damage formation process was governed by the reaction between the radiation induced radicals and the oxygen supplied from the air. This indicated that latent tracks in CR-39 are produced through local radiation induced oxidation process along the ion paths. Studies on vibration spectra, near-IR, FT-IR and Raman spectra, of CR-39 have also been carried out to estimate the chemical structure of the latent tracks. The creation of OH group in irradiated CR-39 has been confirmed.     

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.     

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.     

Hollow particle tracks in AgCI-detectors

In selected Cd-doped monocristalline silverchloride detectors we observe developed tracks from particles at LET values > 300 keV/μm AgCl and charge state values > 9, without a visible central region: the “hollow track”. The microdensitometric lateral profile shows a density decrease near the track axis, up to a radial extension of some microns from highly charged particles as Fe or U-ions of several MeV/u; the tracks of fast U- or Fe-projectiles, moreover, represent an entanglement of delta-electron tracks only. A recent reanalysis of this “hollow track”-effect is suggested to be caused by recombination effects and structural distortions. They occure in areas of high delta-electron density; together with the characteristic process of track formation in crystalline AgCl/Cd they enable us to vizualise and to measure the radial range of distortion and to evaluate the threshold of the energy density above which the destruction occurs.     

Effects of target fragmentation on evaluation of LET spectra from space radiations: implications for space radiation protection studies

We present calculations of linear energy transfer (LET) spectra in low earth orbit from galactic cosmic rays and trapped protons using the HZETRN/BRYNTRN computer code. The emphasis of our calculations is on the analysis of the effects of secondary nuclei produced through target fragmentation in the spacecraft shield or detectors. Recent improvements in the HZETRN/BRYNTRN radiation transport computer code are described. Calculations show that at large values of LET (> 100 keV/μm) the LET spectra seen in free space and low earth orbit (LEO) are dominated by target fragments and not the primary nuclei. Although the evaluation of microdosimetric spectra is not considered here, calculations of LET spectra support that the large lineal energy (y) events are dominated by the target fragments. Finally, we discuss the situation for interplanetary exposures to galactic cosmic rays and show that current radiation transport codes predict that in the region of high LET values the LET spectra at significant shield depths (> 10 g/cm² of Al) is greatly modified by target fragments. These results suggest that studies of track structure and biological response of space radiation should place emphasis on short tracks of medium charge fragments produced in the human body by high energy protons and neutrons.     

X-Ray-induced low-energy electron emission from solids (one-dimensional theoretical model)

The effect of the solid surface on the X-ray induced low-energy electron emission is analysed, using the formalism of the general scattering theory, by means of an approach, called by us the ‘diagonalisation method’. Explicit analytical expressions and numerical results, obtained in the one-dimensional approximation, show great sensitivity of the shape of electron emission spectra near the threshold to the surface potential barrier height and to the presence of the amorphous ‘transition’ layer.     

Predicting ‘lifetime’ of fibre optics exposed to radiation

Fibre optics communications systems are rapidly replacing ‘wired’ links in a variety of applications. The fibre links have proven to be smaller and lighter than either simple wire or coaxial lines. In addition, the fibre systems are immune to all forms of electrical interference (RFI; EMI; static electricity, etc.). Because of the very nature of the passive fibre lines, electrical disturbances may be considered as part of a benign environment, as contrasted with a hostile counterpart, such as high-energy particle fields which are prevalent in a nuclear reactor or other radiation-producing machine. It is known that nuclear radiation may damage the optical transmission properties of fibre optics waveguides. Described herein is the relationship between radiation flux and subsequent damage to the fibre waveguides. Some data, curves and calculations and supporting references are presented. And, finally, a procedure is given as an aid to predicting the ‘lifetime’ of a fibre optics link when it is exposed to the damaging effects of radiation.     

High-resolution nuclear track mapping in detailed cellular histology using CR-39 with the contact microscopy technique

In radiation cancer therapies using energetic charged particles such as proton/heavy-ion therapy and boron neutron capture therapy (BNCT), studies on radiation-induced biological response at cellular level are important because the radiation damage from energetic charged particles is highly localized along their paths and the radiation sensitivity is quite different in each cellular organelle. In such studies the information on the position of charged particle impact in biological cells is necessary. A novel method for high-resolution nuclear track mapping in detailed cellular histology has been developed. In this technique, biological specimens mounted on CR-39 plates are exposed to energetic charged particles. The irradiated samples are exposed to UV, and then etched for a short time. Both etch pits of nuclear tracks and relief for transmission UV image of the specimen can be observed on the CR-39 surface with an atomic force microscope (AFM) at about 100 nm resolution.     

Measurement of apex angle of the prism using total internal reflection

Total internal reflection technique is examined critically for the measurement of apex angle of the prism. Recognition of the critical boundary completely specifies the apex angle ‘A’ of the prism by keeping constant the ratio of refractive index of sample fluid (n₁=1.4) to that of glass (n₂=1.7). Recording of observations in the laboratory is dependent upon whether the apex angle ‘A’ of prism is greater than or less than or equal to critical angle ‘r_c’. The minimum value of error function ‘χ’ in the proximity of Abbe angle explain the choice of Abbe type instruments and shown invariant by relating positive ‘₊’ and the negative angle of emergence ‘₋’. Using ray-tracing technique, an expression for the length of the critical boundary ‘l’ is derived and is shown as a measure of the apex angle ‘A’ of the prism.

The choice of the realistic grazing condition leads us to two turning points for which error function ‘χ’ is minimal and yields appropriate option of the measurement of apex angle ‘A’ of the prism. It is further shown that the small error ‘ζ’ crept in the apex angle ‘A’ of the prism can lead to a significant error in the design of Pulfrich type instruments.

Experimental measurements are in-line with the predictions made.     

Doping of nuclear tracks with fullerene solution

Latent nuclear tracks in polymers can occasionally be doped with fullerene solution. A new technique - ‘Fullerene Tracer Profiling’, FTP - allows us to verify the success of the doping procedure by determining the fullerene depth distribution along the tracks. The basic principle of FTP is described, and first examples are given. Fullerene solution is found to penetrate into ion tracks in polyimide, but not into tracks in polypropylene, or into unirradiated polyimide or polypropylene foils.