551 Timing Single Channel Analyzer

With SCAs that utilize leading-edge timing, the rise time of the input pulses causes degradation of time resolution because the pulses have varying amplitudes. 

Constant-fraction timing compensates for varying amplitudes and essentially eliminates this timing shift, giving consistently better timing results.

For the internally set 50% fraction, the output occurs soon after the midpoint on the linear input trailing edge to facilitate gating and accumulation of data at very high input rates. This technique also minimizes timing shift and dead time when used with sodium iodide, silicon, and germanium detectors, thereby allowing better system time resolution and higher counting rates.

The constant-fraction technique makes it possible to realize significant improvements in time resolution in most timing applications. Notice that analysis is made of the main amplifier output. This technique allows optimization of time resolution and extension of dynamic range for neutron-gamma discrimination and other timing applications. Walk of <3 ns for 100:1 dynamic range using input pulses from a pulser is possible.

The Model 551 is versatile, with three basic operating modes provided. In the Window mode, the unit operates as a high-resolution, narrow (0 to 10%) window, single-channel analyzer. For wide-window applications, the Normal mode is used. In this mode the upper-level and lower-level controls are independently variable from 0 to 10 V, and an output is generated for pulses analyzed between the levels. Through use of the separate rear-panel LL Out and UL Out outputs, the unit can operate as a dual wide-dynamic-range integral discriminator for leading-edge timing or for pulse routing.

The dc-coupled input of the Model 551 makes it possible to take full advantage of the baseline restoration of the main amplifier for maximum performance at widely varying counting rates.

The continuously adjustable output delay (two ranges covering 0.1 to 11 µs) makes it possible to align output signals that have actual time differences without a need for additional delay devices or modules. Alternatively an External strobe input can be used to cause an SCA output at the desired time.

For an application where it is desirable to scan an entire spectrum, an external base-line sweep input is provided via the rear-panel LL Ref Ext BNC connector. In this mode of operation, the baseline (lower-level threshold) on which a window is riding is swept through an energy range and the count rate is recorded as a function of energy.

Performance

Dynamic Range	200:1.
Pulse Pair Resolving Time	Output pulse width plus Delay (as selected by the front-panel Delay controls), plus 100 ns for fast NIM output or plus 200 ns for positive NIM output. Minimum resolving time for negative output 220 ns; for positive output 800 ns.
Threshold Temperature Instability	≤±0.01%/°C of full scale, 0 to 50°C using a NIM Class A power supply (referenced to –12 V)
Discriminator Nonlinearity	≤±0.25% of full scale (integral) for both discriminators.
Delay Temperature Instability	≤±0.03%/°C of full scale, 0 to 50°C.
Delay Nonlinearity	<±2% of delay range.
Window Width Constancy	≤±0.1% variation of full-scale window width over the linear range 0 to 10 V.
Minimum Input Threshold	50 mV for lower-level discriminator.
Time Shift vs. Pulse Height (Walk)	Walk (ns)    Dynamic Range  System A    System B  ±1.0 ±2.5 ±3.0 ±2.0 ±4.0  ±4.0 10:1 50:1 100:1 System A: Using an ORTEC Model 460 Amplifier, single delay-line mode, integrate ≤0.1 µs with 1-µs delay line. System B: Using an ORTEC Model 570, 571, or 572 Amplifier, unipolar output with 0.5-µs shaping time. Input from ORTEC Model 419 Pulser.