Passive spectrometry of linear energy transfer: development and use

A linear energy transfer (LET) spectrometer based on the evaluation of particle track parameters in a chemically etched polyallyldiglycolcarbonate (PADC) track detector has been developed at our laboratory. It permits us to determine LET spectra between 10 and 700 keV microm(-1) in tissues. The LET spectra obtained permit us to calculate total dose and dose equivalent corresponding to particles with etchable tracks also. We have recently been able to verify the calibration curves used by using C, Mg, Ne, Si and Fe ion beams with different energies. The calibration curves obtained are presented and compared with those originally used, and a good correlation is found. The LET spectrometer with new calibration was used to analyse the radiation quality of the radiotherapy proton beam at the Joint Institute for Nuclear Research (JINR). The radiation quality was studied along the proton's range, particular attention being devoted to the region of the Bragg peak. It was found that the biologically weighted effective dose (BWE) reaches a value of about 1.25 at the Bragg peak region. At the beam entrance this value increases to about 1.02 due to secondary particles created through primary proton nuclear reactions in tissues.     

Doses and LET spectra in the beam of 12C with energy 500 MeV amu-1

This paper describes the results of experimental studies performed at the Joint Institute for Nuclear Research (JINR) in a (12)C ion beam with the primary nominal energy 500 MeV amu(-1). Data measured by means of a diamond detector and a spectrometer of linear energy transfer (LET) based on chemically etched track detectors are presented, analysed and discussed. LET spectra are also calculated by program SRIM.     

Dosimetry of low energy ions by means of solid state nuclear track detectors

This paper concerns applications of solid state nuclear track detectors (SSNTDs) of the CR-39 type for measurements of ion streams emitted from the rod plasma injector (RPI). The main diagnostic tool was a Thomson-type mass spectrometer, To detect low energy ions use was made of an additional ion-acceleration system, which enabled the registration threshold to be lowered to about 25 keV. Simultaneously with the time-integrated measurements, time-resolved studies were performed of ion streams by means of Faraday-type collectors. It was shown that the investigated plasma-ion source generates also high energy ions, e.g. protons of energy of 2.6-3.7 MeV. For the analysis of the irradiated and etched CR-39 detectors use was made of a computerised system, which enabled determination of the dimensions of the registered tracks produced by low energy protons, deuterons and nitrogen ions. It has extended the known characteristics of the CR-39 detector.     

Nuclear tracks in CR-39 produced by carbon,oxygen, aluminium and titanium ions

This work describes the response of CR-39 (allyl diglycol polycarbonate) to different ions (C, O, Al and Ti) produced by the Instituto de Fisica 3 MV 9SDH-2 Pelletron accelerator and backscattered from a thin Au film on a C support. The ion energies were chosen in series such that the ranges of the different ions in the detector were 2, 3, 4, 5, 6, 7 and 8 microm respectively for each series. Once exposed, the detectors were etched with a solution of 6.25 M KOH at 60 degrees C, and the reading was carried out using a digital image analysis system. An analysis of the measured track diameters of all the types of ions indicates that, for a given range, track kinetics are independent of type of ion, energy and stopping power.     

Contribution of secondary particles to the dose in 12C radiotherapy and other heavy ion beams

The results of experimental studies performed in a radiotherapy (12)C ion beam with a nominal energy of 500 MeV/amu and in (16)O and (56)Fe ion beams with a nominal energy of 1 GeV/amu have been described. Linear energy transfer (LET) spectra have been established by means of an LET spectrometer based on a chemically etched track detector, and the measured results were also compared with theoretical calculations obtained using the program Stopping and Range of Ions in Matter (SRIM). It was observed that with increasing depth in a beam, the LET spectra are shifted towards higher values of LET; one can also observe an important widening of the spectra along the range, as well as an increasing amount of nuclear reaction products and/or of fragments in the spectra. The relative contribution of these secondary particles to the total absorbed dose was assessed.     

Semi-automated,three-dimensional measurement of etched tracks in solid-state nuclear track detectors

We describe a new method for measuring the shapes and sizes of etched tracks in plastic or glass detectors, using a commercial digital image processing system. The method exploits the fact that in transmitted light any portion of the microscopic image of the track is in best focus when it is darkest. A minimization algorithm is used to produce a two-dimensional projected image of the three-dimensional etched track. The locus of points of maximum gradients in the image leads to a closed contour of the projected image. From this gradient image the profile, length and width of the conical etched track are automatically determined. The method is also applicable (a) to a track that penetrates a detector and has been etched until the cones from opposite surfaces are connected and (b) to a track with a curved profile due to a strongly slowing particle. The method is illustrated with measurements of tracks of 1 A GeV uranium ions in Tuffak polycarbonate and in CR-73 polycarbonate and of cosmic ray tracks in a CR-39 detector.     

Identification of low energy ions (6 ≤ Z ≤ 10) in a plastic track detector CR-39

A method is developed for identifying low energy ions (6 ≤ Z ≤ 10) in a plastic track detector CR-39. After initial etching, rinsing in water and alcohol and final chemical etching of the detector, all in an ultrasound field, the etch rate ratio ($\text{V}{}_{\mathrm{<mtext>T</mtext>}}\text{V}{}_{\mathrm{<mtext>B</mtext>}}$) is directly measured at a defined distance from the end of the track. This method can be used to determine the etch rate ratio at several points along the track. The plastic track detector CR-39 is calibrated using ¹²C, ¹⁶O, ¹⁹F and ²²Ne ion energy in the interval from 2–7 MeV/amu.     

The comparison of calculated and experimental microdosimetric distributions for carbon ions

The aim of this work is to present microdosimetric characteristics of 400 MeV amu(-1) and 500 MeV amu(-1) carbon ions obtained by theoretical calculations and to analyse them with respect to experimental data obtained by tissue-equivalent proportional counter in a scope of project ICCHIBAN and by etched track detector CR 39 Page irradiated by LHE nuclotron at JINR, Dubna, Russia. Track structures provided by Monte Carlo code TRIOL are used as an input data for calculations of energy distributions. The calculations of frequency f(y) and dose d(y) distributions are performed using own developed programs.     

Modifications induced by swift heavy ions

On their way through matter, energetic heavy ions induce a continuous trail of ionization and excitation. A narrow path of irreversible physical, chemical and structural changes, the latent track, is formed. In this report, some of the most important techniques (transmission electron and atomic force microscopy, small-angle X-ray scattering, chemical etching) to study and to characterize ion induced modifications will be presented. Furthermore, selected examples for application oriented projects will be given.     

The STARTRACK experiment

STARTRACK is the acronym of an experiment that aims to measure ionisation cluster distributions in nanometric sites placed at different distances from an accelerated charged particle track. STARTRACK will first use the 20 nm wall-less detector, already used for studying the nanometric track structure of an alpha particle, and then a new detector designed for measuring ionisation clusters in 10 nm sites. The experiment is mounted on the beam line of the Tandem-Alpi accelerator facility of the Legnaro Laboratories, which supplies ion beams from hydrogen to gold up to energy ranging from 7 to 28 MeV/amu. Track nanodosimetry aims to measure ionisation clusters along and aside the ion track down to occurrence probability of 10(-5). To reach such a goal at least 10(6) events have to be collected and the cluster pile-up probability has to be reduced to less than 10(-6). By using a beam profile detector, which is sensitive to very weak currents, and an event rejector counter, such an aim is feasible.     

The contribution of collision cascades to sputtering and radiation damage

Measurement of the velocity distribution of sputtered atoms has tested theories of sputtering and radiation damage. Experiments measuring the time of flight of sputtered atoms in precisely defined directions from the emitting surface, from which the related velocity and energy distributions may be deduced, are described. The energy distribution from polycrystalline targets often fits the form E/(E + Eb)3 derived from a theoretical model in which the bombarding ions initiate collision cascades that eject atoms through the surface. It is assumed that atoms are bound to the surface by a binding force normal to the surface, represented as a binding energy, in the formula. It is shown how this theory relates to that of radiation damage. Departures from the formula seem to correlate with high-energy density in cascades and/or low values of Eb/kT0, with T0 the target temperature. A second component then appears in the energy spectrum approximated by [formula: see text], where deltaT is an effective local temperature rise induced by cascades. The velocity distributions from single crystals are strongly affected by both the direction of ion incidence, indicative of ion channelling, and by emission in directions close to simple crystal axes, indicative of momentum focusing within cascades. Models of the cascade region, and the local heating it causes, have been deduced from sputtering experiments and have advanced our understanding of defect structures caused by radiation damage. Momentum focusing processes are active in creating interstitial-vacancy pairs in both radiation damage and sputtering and their properties have been deduced from these experiments. It is shown how the study of sputtering has enhanced the understanding of radiation damage.     

Etched track detectors and the low dose problem

The risk to human health of exposure to low-level radiation is not precisely known yet. One way of studying this is to carry out in vitro biological experiments with cell cultures and to extend the conclusions to biological models. To relate the macroscopically deteminable 'low dose' to the damage of cells caused by a certain type of ionising particle is nearly impossible. therefore the number of hits and the imparted energy are the significant quantities. They can be estimated by particle transport calculations and by direct measurements. The effect of low dose was investigated in radio-adaptation experiments when mono-layers of different unsynchronised cell cultures were irradiated by neutrons produced in the filtered beam of the Budapest Research Reactor (BRR). The energy deposition was investigated by replacing the mono-layers with etched track detectors of the CR-39 type.     

Novel Al2O3:C,Mg fluorescent nuclear track detectors for passive neutron dosimetry

The latest advances in the development of a fluorescent nuclear track detector (FNTD) for neutron and heavy charged particle dosimetry are described and compared with CR-39 plastic nuclear etched track detectors (PNTDs). The technique combines a new luminescent aluminium oxide single crystal detector (Al(2)O(3):C,Mg) with an imaging technique based on laser scanning and confocal fluorescence detection. Detection efficiency was obtained after irradiations with monoenergetic neutron and proton beams. Dose dependences were measured for different configurations of the detectors exposed in fast- and thermal-neutron fields. A specially developed image processing technique allows for fast fluorescent track identification and counting. The readout method is non-destructive, and detectors can be reused after thermal annealing.     

Measurement of parameters of tracks in CR-39 detector from replicas

In determining the etched track rate in solid-state nuclear track detectors, track lengths should be determined accurately. A method based on surface profilometry is proposed to determine the track lengths in CR-39 detectors through measurements of their replicas. Tracks from alpha particles with an incident energy of 4 MeV have been chosen to demonstrate the method. After irradiation and chemical etching, resin replicas were made from the tracks, of which the heights were measured by the Form Talysurf PGI Profilometer. The results showed that the surface of the replicas were smooth and the heights of the replicas were uniform, so the replicating fluid should have filled the tracks completely and the replicas truly reflected the dimensions of the tracks. The heights of the replicas were conveniently determined from the lateral view of the replicas generated by the Form Talysurf PGI Profilometer.     

Dry etching and sputtering

Dry etching is an important process for micro- and nanofabrication. Sputtering effects can arise in two contexts within a dry-etch process. Incoming ions cause removal of volatile products that arise from the interaction between the dry-etch plasma and the surface to be etched. Also, the momentum transfer of an incoming ion can cause direct removal of the material to be etched, which is undesirable as it can cause electrical or optical damage to the underlying material. This is largely avoided in dry-etch processes by use of reactive chemistries, although in some processes this component of the etching can be significant. Etch processes, both machine type and possible etch chemistries, are reviewed. Methods of characterizing the electrical and optical damage related to ion impact at the substrate are described. The use of highly reactive chemistries and molecular constituents within the plasma is best for reducing the effects of damage.     

CHICSi—a compact ultra-high vacuum compatible detector system for nuclear reaction experiments at storage rings. II. Detectors

We describe the detectors for identification of charged particles and fragments in CHICSi, a large solid angle multi-telescope system mounted inside an ultra-high vacuum (UHV), cluster-jet target chamber. CHICSi performs nuclear reaction experiments at storage rings. The telescopes consist of a first very thin, 10–14 μm Si detector, a second 300 μm (or possibly 500 μm) ion implanted Si detector supplemented by a 6 mm GSO(Ce) scintillator read out by a photodiode (PD) or by a third 300 μm Si detector. The telescopes provide full charge separation up to Z=17 and mass resolution up to A=9 in the energy range 0.7–60A MeV. The thin p-i-n diode detector, etched out from a 280 μm Si wafer, and the GSO/PD detector, both exclusively developed for CHICSi, provide an energy resolution 8%, while the standard 300 μm detectors have 2% energy resolution. Radiation stability of the Si detectors is confirmed up to an integrated flux of 10¹⁰ alpha particles. The GSO detector has 70% light collection efficiency with the optical coupling to the PD a simple open, 0.2 mm, gap. A new method, developed to perform absolute energy calibration for the GSO/PD detector is presented.     

Study of 132Xe ion tracks in a soda glass detector

Soda glass track detectors have been exposed to ¹³²Xe ion cyclotron beams at the Joint Institute for Nuclear Research, Dubna (USSR). The etch pit diameters are measured for different etching times after etching the detectors in a ‘new etchant’ free of the adverse effect of the etch product layer. The computed values of energy loss, $\text{d}\text{E}\text{d}\text{x}$, in soda glass have been correlated with the track diameter. The energy resolution of soda glass and the critical angle for etching of ¹³²Xe ion tracks in glass detectors have also been determined. The maximum etched track length of Xe ions in soda glass has been compared with the theoretical range. The effects of different annealing conditions on the bulk etch rate, the track etch rate, the etching efficiency, the track diameter and on the range of Xe ions in soda glass have been studied.     

Detektion der Spiralbahnen in einem Klein‐Zyklotron

Detection of the helical ion‐paths in a small cyclotron The small cyclotron COLUMBUS, which was developed by Gymnasium Ernestinum in cooperation with the University of Applied Sciences of Coburg, is a particle accelerator for education and teaching purposes. As a result of a magnetic field, ions move on helical paths, which are accelerated at constant intervals by a high‐frequency electric field and so gain in kinetic energy. A Linear Translator moves an ion‐detector across these paths; while the current position is always assigned to the corresponding ion current. The result is a specific spectrum; from the position of each peak the radii of the spiral track can be calculated as well as the energy of the ions. It is planned to equip the measuring system with a user‐friendly inter‐face to meet the educational requirements of the cyclotron.     

Ellipsometric study of the influence of chemical etching on thin porous silicon structures

The effect of chemical etching on Porous Silicon (PS) samples is studied and quantified by using variable angle spectroscopic ellipsometry (VASE). The main aim of this work is to assess the impact of such etching on the physical properties of electrochemically etched, thin PS antireflection coatings (ARC) for solar cell applications. In this study, detailed models of PS layers etched at constant current densities are created using a graded uniaxial Bruggeman Effective Medium Approximation (BEMA). Changes in porosity, thickness, and optical anisotropy of the PS samples due to chemical etching are determined as a function of etching time after PS formation. Three series of PS films, etched at three different current densities, are investigated. It is shown that significant changes in physical properties occur for chemical etching times longer than ~ 60 s. The anodic etching process for fabricating PS ARC structures can be performed in less than 10 s. Therefore, chemical etching does not lead to significant deviations from the intended PS structure and is not seen as a hindrance to accurate control of processes for fabricating thin PS ARCs.