Galileo Spacecraft Pages
Source: JOMPI Proposal, November 1976
The requirements for good energy resolution and good timing resolution place conflicting constraints on the pulse shaping electronics. Energy resolution improves with (t)-½ in the region where the preamp noise is dominant (t is the shaping time-constant). For all t of interest for this experiment and for the detector capacitances involved, the preamp noise will dominate. On the other hand, timing resolution tends to improve with (t)+½ . The J and K detectors thus require separate signal processing for optimum resolution in both timing and energy. Preamp modules (within the telescope housing) initiate separate signal processing for timing and energy signals from each detector. Basically two outputs are produced, a <100 ns rise time charge sensitive output and a 2 ns rise time time-pick-off (TPO) output.
For energy analysis, each detector's charge sensitive preamp output (J,K,L) feeds a chain of amplifiers, pulse shaping (t = 100 nsec), and discriminators. Using particle energy, TOF and energy-loss characteristics, coincidence and anticoincidence conditions are established between the three detector elements to define a set of discrete channels. The basic coincidence condition is a single event in a single J detector in coincidence with K and anticoincidence with L(J7K7(L)). In addition, all events above thresholds in a single J detector are measured in the single-parameter channels. Together with a priority logic system the discriminator coincidence conditions with the basic J7K7(L) format are used to determine if a particular event will be analyzed in the pulse height analyzer (PHA). In addition to establishing the priority system, the discrete channels are monitored to give count rate data over relatively broad energy and particle-species bands.
For timing analysis, each preamp TPO output feeds a separate constant-fraction-of-pulse-height timing discriminator optimized to the capacitance and low energy limit for each detector. This optimization involves subtle trade-offs between best timing resolution at high energies and bounding the timing error at the lowest energy.
The timing analysis determines the particle's time-of-flight over the 7.5 cm between J and K. A timing signal from one of the J timing discriminators provides the stop pulse for the time-to-amplitude converter and the timing signal from K provides the start pulse. The basic requirement for a valid TOF signal is that there be one (and only one) event in a single J detector, followed within 100 ns by a single event in K. Pile-up discriminators monitor pulse width of the events in each timing channel and will veto pile-up events caused by particles hitting the same detector with a separation of > 50 ns. In the normal mode, an event must have a valid TOF signal, in addition to meeting the priority and coincidence requirements discussed above, to enable full analysis of the three PHA signals (DEJ, DEK and TOF). (This requirement may be removed by command in the unlikely event of a failure in the TOF electronics.) The effect of pile-up in the longer time constant J and K energy channels is minimized by imposing the condition that no more than one event as detected in the fast channels (50 ns worst case resolution) may occur within the energy channel pair-resolution time after the event "processing" has begun.
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Updated 2/24/04, T. Hunt-Ward