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IEEE Transactions on Nuclear Science, Vol. NS-27, No. 1, February 1980 DIRECT POSITION AND TIME DIGITIZER FOR POSITION-SENSITIVE PROPORTIONAL COUNTERS Frank J. Lynch Argonne National Laboratory Argonne, Illinois 60439 Abstract A circuit for directly digitizing the location and flight time of neutrons detected in a 2-dimensional position-sensitive proportional counter has been constructed and is now in use here in diffraction experiments with the pulsed neutron source. The position along each coordinate is digitized directly by counting pulses from a 250 MHz clock during the time interval corresponding to the difference in rise times of pulses from the opposite ends of a resistive cathode. In addition, a slower time base is included for digitizing the neutron time-of-flight. The circuits, on a single CAMAC board, provide 8 binary bits of x position, 8 bits of y position and 12 bits of time without adding to the system deadtime. All- digital circuitry assures -linearity, stability and reliability. Introduction The position of a neutron detected in a Borkowski- Kopp[] position sensitive proportional counter, PSPC, is usually derived from the difference in rise times of pulses from the two ends of a resistive cathode. Heretofore, this time difference was digitized by means of a time-to-amplitude converter, TAC, followed by an analog-to-digital converter, ADC. In addition to being complex and expensive, this combination imposes unnecessary limitations on counting rate and accuracy. It adds to the dead time of the system, and has the further disadvantages of analog systems: calibration may change with counting rate and temperature, and linearity depends on careful adjustment. A direct time digitizer, DTD, has obvious advantages for time measurement over a fixed time range with modest resolution. (The best time resolution reported for a DTD is lns.)[2] The DTD counts pulses from a highly stable clock during the measurement time, registering a representative number at the end of the interval. Therefore, it doesn't add to the dead time of the system. Furthermore, it is basically linear, and its calibration is stable, independent of counting rate and temperature. The DTD described here was designed to meet the particular requirements of a high pressure PSPC constructed by us recently at Oak Ridge National Laboratory under the tutelage of M. Kopp and J. Williams, but it can be adapted to other systems requiring a fixed time range. Position Digitizer The maximum difference in rise times of the pulses from opposite ends of the cathodes of the PSPC is about hus. To cover this range with an 8 binary bit scaler, a 250 MHz clock was chosen providing a time range of 1.024is. The basic schematic diagram of the digitizer for either position coordinate is shown in Figure 1. Fast NIM pulses from timing single channel analyzers mark the time that the amplified pulses from the ends of the cathode reach 50% of their Work performed under the auspices of U.S. Dept. of Energy. peak value. One pulse, the start pulse, sets the start flip flop, FF, thereby enabling the scaler to count pulses from the 250 MHz clock. The other pulse, the stop pulse, after a fixed delay of one ps, sets the stop FF which resets the start FF, thereby stopping the counting, and sends a look-at-me signal to the CAMAC crate controller signaling that encoding is complete. In the absence of a stop pulse, the overflow pulse from the position scaler resets the start FF and clears the position scaler. A gate pulse derived from single channel analysis of the amplitudes of the sum of the four cathode signals enables the start and stop pulses to make a conversion only when the amplitude of the sum pulse falls within the range of neutron pulse amplitudes. An additional gate input is provided to block conversion during and shortly after the beam pulse. READOUT Fig. 1. POSITION DIGITIZER Time Digitizer Encoding the time of detection of the neutron relative to its time of generation (neutron time-of- flight) is achieved by means of a separate crystal- controlled clock with 250 kHz frequency, 12 bit scaler and slave register. Figure 2 is a simplified schematic diagram. A signal marking the time of arrival of the proton burst at the spallation target clears the time scaler and starts it counting the clock pulses. When a neutron is detected causing a gate pulse, the output slave register, which is copying the state of the time scaler, stops and holds the digital time record until the 12 bits of time and 0018-9499/80/0200-0327$00.75©) 1980 IEEE 327
