2.2 Electronic detector systems
2.2.4 Other detector types
Due to the high pulse flux of laser–accelerated ion beams the majority of today used detectors are passive devices. However, increasing repetition rates of laser systems used for ion acceler- ation require detectors with a prompt quantitative response. A summary of other electronic detector systems is given which are currently investigated for detection of laser–accelerated ion pulses.
Scintillation detectors
The principle of a scintillation detector is based on the conversion of deposited particle energy into visible light, subsequently detected for instance by a photomultiplier or photodiode. Two groups of scintillating materials are used, organic and inorganic scintillators. To be appropriate as scintillation detector, several requirements have to be fulfilled, regardless of the type of scintillator. A scintillator has to be transparent for the wavelength of scintillation light emission and a large fraction of the deposited energy should be converted into a prompt fluorescence signal.
In a large number of organic scintillators, molecules have a π–electron structure. Incident radiation causes excitation of higher electronic states. Transitions from the short–lived first excited singlet state to the ground state and corresponding vibrational levels, leads to emis- sion of prompt scintillation light. However, a small amount of excitation energy is transferred to states with longer lifetime, resulting in a delayed light emission. Therefore, time response of a scintillator is characterized by the three time constants. First, the time required to populate the upper transition stateτp, second, the time for the prompt decay of the excited state τf d and finally, the time for the delayed decay τsd. Typical decay constants of organic scintillators for the fast decay are in the order of 1–4 ns.
In case of an ideal scintillator, the amount of luminescence per unit path length dLdx is pro- portional to the energy loss dEdx. In a real scintillator a finite probability for radiation less transitions, summarized as quenching, results in a smaller scintillation efficiency. The rela- tion of luminescence depending on specific energy loss is is described by Birks formula.
dL dx =
SdEdx
1 +kBdEdx (2.23)
S is the efficiency factor in absence of quenching and the parameterkB accounts for quench- ing losses. The light yield of an organic scintillator is, thus, LET dependent. As a result, higher values of the light yield for electrons compared to heavy charged particles of the same energy are observed [39].
In an inorganic scintillator impurities, denoted as activator centres, are responsible for the emission of scintillation light. Activator states are situated in the band gap of the crystal lattice. Lifetimes of these states are in the order of several 10–100 ns, as a result, the re- sponse of inorganic scintillators is a factor of 10–100 times slower than for an organic one. The light yield by the slow decay component is also more pronounced resulting in a higher background during measurements. But inorganic scintillators are less prone to quenching effects, resulting in better proportionality of the light yield with deposited energy.
For detection of laser–accelerated ions, organic scintillators are investigated as position– sensitive elements in the spectrometer plane [38] as well as a scintillator stack for ion beam profile measurements [33, 99].
Micro–channel plate (MCP)
A micro–channel plate is an electron multiplier built from a bundle of parallel channels, usually tiled at a small angle of about 8 ° with respect to the MCP surface (fig. 2.8) [100,101].
Many different geometries covering large areas of several cm2 are, therefore, possible. Secondary electrons, generated by collision of the incident particle with the channel wall,
Figure 2.8:Schematic sketch of a micro–channel platefrom [101].
A micro–channel plate is first and foremost an electron multiplier. An incident electron, accelerated in the applied electric field of an individual MCP channel, generates secondary electrons by wall collisions. This process repeats itself, for both, primary and secondary particles, during their drift along the channel, thus, amplifying the electron number.
made of a highly resistive material, are accelerated in the electric field which is applied along the channel length. Secondaries experience several wall collisions themselves during their drift along the channel yielding multiplication factors as large as 107 [100].
The applied potential difference across the channel length as well as the length to diameter ratio α (typically ∼ 40–100) determine the gain of the multiplier which is limited by space charge saturation at the channel output.
The MCP operation requires a vacuum better 10-6 mbar. However, collisions of electrons with residual gas molecules are able to generate positive ions, in particular in the high charge density region a the channel end. To prevent ion feedback from ions drifting towards the channel input, a two stage MCP, the so called Chevron, is typically used. Ion feedback is suppressed by introduction of a directional change of the channel orientation between both MCP stages [100].
When used as particle detector, MCPs are usually coupled to a phosphor screen to convert the amplified electron distribution into visible light which is in turn detected by a CCD camera system. The high gain of the MCP enables not only low signal levels, but in principle even single particles to be detected. However, the efficiency of single particle detection strongly depends on the secondary electron emission probability which scales with the energy loss and, hence, is particle dependent [102]. The maximum probability is, therefore typically in the keV range. For protons of 230 MeV, the emission probability is, for instance, as low as 4–5 % [103].
photons and electrons are present, this can be a major limiting factor for their applicability. Nevertheless, the combination of a Thomson spectrometer and a MCP coupled to a phosphor screen has been investigated for prompt ion diagnostic in laser–plasma experiments in many cases [34, 104–106].
analysis
The Tandem accelerator of the Maier–Leibnitz–Laboratory (MLL) in Garching offers unique possibilities to investigate detector response under different irradiation conditions, allowing exposure to a single ion as well as up to 109 protons/cm2 within a single ns–pulse. The latter, thus, offers comparable pulse intensities as a laser–accelerated ion pulse in a few MeV broad energy interval, but with major advantages of no parasitic background radiation what- soever and full beam control. For bio–medical experiments, accomplished at the Tandem accelerator as well as ATLAS laser, a film based dosimetry has been established. Calibration measurements span the gap from low–energy 3 MeV protons, at the Tandem accelerator, up to therapeutic relevant energies of about 200 MeV, available at the Rinecker Proton Therapy Center (RPTC).
Test measurements of the developed online diagnostic system were accomplished in a laser– accelerated proton beam at the ATLAS and DRACO laser, respectively.
3.1 Detector systems
Different types of detector systems, non–electronic and electronic, have been investigated and calibrated for detection of laser–accelerated proton beams.
CR39, a nuclear track detector for protons, is employed for absolute fluence determination in calibration measurements. IPs were wildly used in all Munich laser–acceleration experiments so far. In these measurements, different types of IP and IP readers have been employed. As IP and scanner form a measurement system, calibrations are required for any IP–scanner combination. The same is true for radiochromic EBT2 films and associated scanner systems, used for dose verification in low–energy proton irradiation of biological samples. Three pixel detectors, based on different architectures were investigated for real–time detection of laser– accelerated proton beams.