Scintillator Detectors

    The scintillator detectors in TIGER are responsible for determining the charge of a particle that passes through the instrument.  Since the particles the pass through TIGER have a lot of kinetic energy, we can assume that there are no orbital electrons in the incoming atom.  For example, if an iron nucleus passes through the instrument, the scintillator will only "see" the 26 protons that are in the nucleus of the atom.  Since iron is the only element that has 26 protons in its nucleus, we can tell from the scintillator, with an extremely reasonable degree of accuracy, that the particle that entered was in fact iron.



    The scintillator is made of a special kind of plastic called poly-vinyl toluene (PVT).  When a charged particle passes through the PVT, it excites and ionizes the molecules in the plastic.  In so doing, electrons in the PVT molecules move up to higher orbital energy levels and release a bluish light as they drop back down.  The PVT is designed so that the light that is emitted inside the plastic is totally internally reflected.  The light bounces up and down inside the PVT until it reaches the end.  Here the light is able to escape out the side of the scintillator, where it enters another piece of plastic known as a wavelength shifter bar (WLSB).  This process is shown in the figure below.  Once inside the WLSB, the wavelength of the blue light is shifted to green, continues to reflect internally inside the WLSB, and is piped down to the photomultiplier tubes on either end of the WLSB (see photo above).  In this way, only 8 PMTs (2 on each corner) are needed to read out an entire ~1 m2 panel of PVT.


    In TIGER, there are four scintillating detectors, S1, S2, S3, and S4.  It is good to have this kind of redundancy when building particle detectors.  The sum of the signals from several scintillator detectors is always better than just the signal from one of them.  Furthermore, if one or more of the scintillator detectors fail, the experiment will only suffer a handicap, not a total failure.  The S1 and S2 scintillators are the most important for deriving the charge of a particle.  Coincidence in the detector is also determined by the scintillator detectors.  If the signal in the required detectors is high enough, the software decides that a particle has indeed passed through the detector, and proceeds to read out the light output.  For this and the last TIGER flight, the following coincidence pattern was used:  (S1 OR S2) AND (S3 OR S4).



    The plot above shows what the scintillator signal (dE/dx) would look like when plotted against the Cherenkov (energy) signal.  As the energy increases from 0, the scintillator signal drops rapidly at first.  However, as the energy of the incoming nucleus reaches relativistic proportions, it becomes a minimum ionizing particle (MIP) and the scintillator once again begins to respond.  The signal that a certain nucleus creates in the scintillator is highly sensitive to its charge, as was discussed before.  The scintillator signal, while also somewhat sensitive to the energy of the particle, is proportional to Zn, where n is between about 1.5 and 2.  Looking again at the plot to the left, as more and more particles trigger the detectors, charge bands will begin to form in the data.  Here we see where the contours of Fe, Co, Ni and Cu would lie with respect to one another.


One of the TIGER scintillators, all ready to go