CR39, a solid state nuclear track detector (SSNTD), is widely used for quantitative ion de- tection in laser–acceleration experiments [34]. It offers two major advantages in the mixed radiation background of these experiments: first, it is insensitive to electrons, gammas or light, thus, only detecting the ion signal, and, second, even sensitive to single ions. Depend- ing on etched track diameter and microscopic resolution, spatial resolutions is in the order
of 1µm.
Track formation in CR39
In the scope of this work, 1 mm thick plates of CR391, a clear plastic solid of poly allyl
diglycol carbonate (PADC,C12H18O7,ρ = 1.3 g/cm3) were used [107].
Tracks are formed, if the energy deposition of a charged particle exceeds a material dependent threshold value. For CR39, this is the case for protons with energies exceeding 20 keV [108, 109]. The majority of the insulating solids, representing the group of SSNTDs, is only sensitive to heavy ions, a possible explanation for the track formation under these conditions is given by the ion explosion spike mechanism [110]. CR39 is among a small group of polymers which are also sensitive to protons or deuterons. In this case, breaking of covalent bonds in the polymer is the main mechanism of track formation. Track diameters are in the order of 50 ˚A. For efficient track counting under a microscope, enlargement by chemical etching is necessary. The minimum required pit diameter for microscopic analysis of about 1µmimposes an upper limit on the detectable particle energy. The track diameter distribution for protons has a maximum well below 1 MeV for all practical etch times of up to 20 hours [108, 111]. A decrease in proton track diameter with increasing proton energy, yields a track diameter of only about 2µmfor the highest investigated energy of 6 MeV [112]. For CR39, efficient microscopic analysis is, thus, limited to proton energies below 4 MeV. A second, higher energy range becomes accessible by back–side analysis of the CR39, provided the residual energy of particles at the back surface is sufficiently low to allow analysis of etched tracks.
Track etching
A qualitative picture of the track etching mechanism is depicted in fig. 3.1a. The etching rate of the track differs from the one in the undamaged bulk region, which is approximately one order of magnitude smaller [39]. Hence, material removal is much faster along the track, yielding a pit in the surface of the etched nuclear track detector.
However, this is only true, if the particle’ s angle of incidence θ is not too shallow. Track visibility requires, that the track depth dτ, obtained in a etch time τ, exceeds the thickness of removed bulk surface bτ. Neglecting changes in the track etching rate with increasing particle range, a minimum angle of incidence θ can be determined from the simple picture of fig. 3.1a. θ >arcsin v0 vtrack (3.1) A track corresponds to a densely damaged region and can, thus, anneal with time. Therefore, irradiated detector plates were etched within a few days after exposure. As etching reagent a six–molar caustic soda (NaOH) solution was used. A constant temperature of 80℃ was maintained during the etching process. A magnetic stirrer ensured a homogeneous distri- bution of both, temperature and reagent concentration, during the 90 minute long etching
(a) (b) Figure 3.1:CR39 track etching
a) The visibility and form of etched tracks is related to a difference in etching velocities of bulk material and track as well as angle of incidence of impinging particles.
b) Microscopic image of etched proton tracks under normal incidence. Green frames around the tracks mark elliptical fits of the measurement system on the automatically detected tracks. Coloured arrows mark errors in the automatic track analysis, introduced by track overlapping (red) and bad contrast (blue).
process.
Track analysis
Manual track counting is impractical for large numbers of tracks. Therefore, an automatic track counting system has been employed for track analysis. The used system is an improve- ment of a system that was originally developed in Siegen [113, 114]. It consist of a Zeiss Axiotron II microscope [115] equipped with a CCD camera and motorized stage. Minimum required track diameters are about 1.5–2.0 µm. The scanning procedure is controlled by the pattern recognition software,Samaica [116]. During this procedure, images acquired by the microscope’ s CCD camera, are immediately analysed. Recognized particle tracks are fitted with ellipses (fig. 3.1b) and fit parameters are stored with additional track information such as e.g. brightness, position or ellipticity for further analysis. During analysis of etched CR39, problems related to the automatic focus control of the microscope, initiated regular dead locks of the program. In an early version ofSamaica a restart of the scan was necessary.
In particular for analysis of large CR39 areas (i.e. 25 cm2), where a complete scans requires approximately 24 hours to complete, this was not tolerable. Although a software upgrade allowed to continue the scan after the program freeze, the problem was finally solved by a regular reset of the auto–focus control. Another problem of long–term scans is associated with dust deposit on the CR39 surface, particularly as the automatic track counting system is not located in a dust–clean environment. However, dust–introduced track artefacts can typically be filtered from the data, as the pit form of true tracks, which depends on particle type, energy and angle of incidence, differs significantly.
Information on the incident radiation can be gained by analysis of the track pit, if the etch- ing channel does not exceed the range of the particle. In this case, the track pit has a cone shaped form. The angle of incidence can be obtained by measuring the ellipticity of the cone’s basal plane. For normal incidence, circular pits are observed (fig. 3.1b), distorting into elliptically ones with decreasing angle of incidence. The track etching velocity, vtrack depends on the number of broken bonds and thus, on particle type and associated energy loss. If the etching velocity v0 of the undamaged bulk is known, simple geometrical consid-
erations yield the track etch rate and the corresponding energy loss [13].
Unambiguous distinction of individual particle tracks is essential for track analysis. A SS- NTD is saturated if the majority of particle tracks overlaps (fig. 3.1b), posing a major problem for automatic track counting [109]. The saturation fluence depends on the pit di- ameter and, thus, also on etching time. For the used proton etching procedure, an etch time of 90 minutes results in typical track diameters of 2–3µmin diameter, yields a satura- tion fluence of up 108 particles/cm2 [105]. However, practical saturation levels are generally further limited by the increase of the track overlapping probability with fluence.