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For both of these techniques, a detector material is needed that measures the deflection of the particle and/or its stopping range. Three detector materials - commonly used in laser-ion acceleration - are introduced here, the radiochromic film, the nuclear track detector CR39 and the image plate (where only the latter two have been used in the experiments described in this thesis).

Radiochromic film

Radiochromic film (RCF) is a film widely used in medicine for dosimetry, com- mercially available from GafChromic. It is mainly sensitive to ionizing radiation, such as electrons, protons and other ions. On impact, it instantly turns blue due to a chemical process, where the color density depends on the absorbed dose. Stacks of RCF layers can be used to measure the energy distribution of laser- accelerated proton beams. Each proton deposits most of its energy - according to its stopping range/Bragg peak - in the corresponding layer of the RCF-stack. From the color density in each layer, an energy spectrum of the proton beam can be reconstructed. The full proton beam can be analyzed at once, but with poor energy resolution (depending on the numbers of layers in the stack). Other species, such as carbon, have a very short stopping range compared to the protons and are, if present, usually stopped within the first few layers of such a stack. Hence, in experiments, where other species are of interest, additional detectors or a completely different detection system is necessary, such as for this thesis. Nuclear track detector, CR39

The nuclear track detector CR39 [144] (Columbia Resin #39) is widely used for eyeglass lenses and in astrophysics for detection of high energy particles in space [145]. CR39, a allyl diglycol carbonate polymer, is commercially avail- able, for instance from TasTrack that supplies plates of CR39 with a density of

a)

b)

Figure 3.13: Image of an scanned CR39, showing parabolic particle tracks ob- tained from a Thomson parabola (see Sect. 3.4.2 for details), where in a) eccen- tricity is color coded in the z-axis and in b) the enclosed area of the pits (in arbitrary units).

1.3 g/cm3_{. The plate dimensions used in the experiments are 100 mm by 50 mm}

by 1 mm. CR39 is not sensitive to electrons and photons, it is however an ex- cellent detector material for protons and higher Z ions. On impact, the particle creates damage tracks in the polymer structure along its flight path; the track diameter directly depends on the energy loss of the particle and hence grows larger towards the Bragg peak and vanishes afterwards. In order to make these tracks visible the CR39 is etched in a solution of sodium hydroxide at 80 C◦_{; the} track diameter grows linearly with the etching time. Once the track diameter has passed the threshold for “visible” detection, a microscope can be used to count the particle tracks.

The scanning microscope is controlled by the pattern recognition softwareSAMAICA; this software recognizes the track entrances, the so-called pits, on the CR39 sur- face, by identifying a number of different parameters, such as the pit diameter, its eccentricity, its enclosed area and its relative position on the CR39 with re- spect to the scanning area. These parameters strongly depend on the deposited energy per unit length and thus the particle species and can later be used to distinguish pits of different species and for background elimination. For example, the eccentricity of pits created by “real” particles is typically close to 1 for normal incidence, while pits of other origin have a much lower ellipticity value.

in MatLab that allows to clean the scanned images from background and also to isolate single particle species for accurate analysis. Fig3.13shows the image of an scanned CR39, with parabolic particle tracks obtained from a common Thomson parabola (see Sect. 3.4.2 for details). In frame a) the eccentricity is color coded in the z-axis and in frame b) the enclosed area (x and y are spatial dimensions of the scan area); both parameters change for the different parabola traces, which correspond to different charge to mass ratios of the particles and are clearly dis- tinguishable from the background. The parameters change also along the single parabolic traces, which corresponds to a change in incident particle energy. The CR39 nuclear track detector has been used in most of the experiments described in this thesis.

Image plate

Image plates (IP) are commonly used in medicine (dental X-rays), material re- search and in biology and have almost completely replaced photographic X-ray films. The active layer of an typical IP consists of crystals of barium fluorohalide phosphor (BaFBr:Eu2+_{) and is supported by a magnetic support layer. This ac-}

tive layer has a linear response to X-rays over a large wavelength range starting at a few nm [146], but is also sensitive to electrons and ions. When such a particle or photon hits the active layer of the IP, electron hole pairs are created and trapped in lattice defects. In order to retrieve the data from the IP, it is processed by a special IP scanner (a FLA-7000 has been used in the framework of this thesis). The scanner illuminates the IP with laser light of wavelengthλ = 632.8 nm, which causes recombination of the trapped electron hole pairs under emission of a pho- ton. This process is called photo stimulated luminescence (PSL) and is detected by the scanner to generate an the digital image of the irradiated IP within only a few minutes. The IP can also be erased with bright white light and be reused many times.

The data is stored in a logarithmic gray scale, usually at a spatial resolution of 25µm. The data can be transformed into the linear PSL scale via

P SL= psize 100 2 4000 S 10 L(x/G−0.5) _(3.9)

with psize the pixel size, i.e., the resolution of the scanner, L the latitude (typ- ically 5), S the sensitivity of the scanner (with values from 103 _{to 10}4_{), x the}

logarithmic gray scale value of the scanned pixel and G the gradation or the dynamic range of 216_{. It should be noted here that the trapped electrons decay}

naturally[147], for which it is necessary to scan the IP at a fixed time after their exposure to obtain meaningful and comparable data.

A general problem of IP is to obtain an absolute calibration of PSL to particle or photon number, which is in addition complicated by the huge number of dif- ferent IP and IP scanners. In Ref. [148] and Ref. [149] calibrations for IP and protons are published each for a specific type of IP and scanner (both different from the ones used for this thesis). The calibrations differ by about one order of magnitude with 0.01 and 0.08 PSL/proton in the more or less constant tail of the calibrations far behind the Bragg-peak.

One can speculate that the calibrations for other IP and scanner combinations should be somewhere in this region (they are after all a standard tool for many physicians). They should, however, only be used (if at all) as a rough guide for the data shown here, especially since the IP used in the framework of this the- sis are BAS-TR2020, which have no protective layer on top of the active layer (of _∼ 50µm thickness, making them much more sensitive) as compared to the calibrations presented in the afore mentioned references.