Continuous monitoring of the radiation background is a key requirement in many applications. This is usually achieved by simultaneously using two types of detectors: spectrometers and dosimeters. While dosimeters have omnidirectional detection characteristics, they provide no information about single particles. Spectrometers can identify particle species and energies, but have narrow opening apertures, limiting their fields of view. To record the complete flux profile of an anisotropic radiation environment, either several spectrometers must be used, or a single device must be repeatedly turned in order to cover the complete solid angle. Instantaneous measurements of the radiation environment thus requires a large number of detectors.
Our group tries to overcome these problems by combining the advantages of both detector systems within one instrument: The Multi-purpose active-target particle telescope (MAPT). MAPT combines particle spectroscopy of single particles with a high angular acceptance. Particles arriving from all directions can be measured simultaneously. The active volume of the detector consists of 900 scintillating plastic fibers with a square cross section of 4 mm² and a length of 71 mm. The scintillators are stacked in 30 layers à 30 fibers each. Each layer is rotated by 90° with respect to its neighboring layers. This arrangement allows 3D-tracking of particles entering the active volume of the detector.
The scintillation light of the fibers is read out by 900 silicon photomultipliers (SiPM). SiPMs are compact semiconductor detectors with intrinsic amplification. It is possible to measure single photons using this kind of detectors. The analog signals created by each of the 900 SiPMs is read out individually by a combination of ASICs and FPGAs and combined to a particle event. The parallel read out allows continuous data taking even in high rate environments (as it can appear in the radiation belts of Earth or during solar events).
MAPT fits into a cube of 10 cm length and weighs about 1.5 kg. This compact design enables the operation of the detector on different spacecraft and satellites.
The detection principle for charged particles with MAPT is based on the measurement of the energy loss of the particles in the detector material. The slowing-down process of the particle is specific for a certain particle species and energy. By measuring the course of the slowing-down process we can measure these characteristics. For heavy-ions like protons or alpha particles the slowing-down process can be described by the so-called Bragg Curve.
The characteristic shape of the Bragg Curve depends on: energy, mass, and charge of the particle. Thus, by comparing the measured curve to the theoretical Bragg Curve, the particle can be identified.