Lower level discriminator

The current pulse is electronically amplified, passed through a lower level discriminator, and counted by a multichannel scaler. The lower level dis-... 

Fig. 10. Single-photon response of a RCA 8850 photomultiplier demonstrating the principle of two-channel sampled timing. In all three cases, the channel 1 sampled voltage exceeds the lower level discriminator. In the top example, the channel 2 sample is nearly equal and is accepted. In the other two examples, the current pulses arrive 1 ns late and 1 ns early and the large differences between the two sampled voltages causes these photons to be rejected. (After ref. 57.)...

This resolution is measured by accumulating a peak in the energy spectrum and measuring the full width of the peak at one-half its maximum intensity. For a very good counter assembly, k can be as low as 2.46 in units of (keV) /. Worse resolutions are common, particularly if the counter has aged, has been damaged by a rapid temperature transition (temperature changing faster than 10 C/h should be avoided), or has been exposed to a direct x-ray tube beam. Because of the poor resolution, it is very difficult to set the lower-level discriminator in the pulse... 

A second discriminator level Vu has been added to the lower-level discriminator Vl normally used in the integral mode. In the window mode the pulse height selector produces a standard output logic pulse only if the amplifier pulse amplitude... 

Next the multichannel analyzer must add one count to memory location Nc. This is carried out during the memory cycle time [Fig. 4.28(e)]. The memory location whose address or channel number is Nc is identified and its present contents are read. If memory location Nc presently contains m counts, the number m + 1 is written back in. At the end of the memory cycle the multichannel analyzer is free to process another amplifier pulse. If a pulse is already present above the lower-level discriminator level at the linear gate input, the linear gate remains closed until the pulse drops below the discriminator threshold. At this point the linear gate opens and the multichannel analyzer is ready to process the next amplifier pulse that exceeds the lower-level discriminator threshold. The measurement process is repeated on a pulse-by-pulse basis to build up the energy spectrum histogram in memory. 

It may not always be appropriate to use the lower level discriminator to remove gross numbers of counts from the low-energy end of the spectrum. In measurements of plutonium isotopes, the spectra will be dominated below... 

Lower level discriminator (LLD) - pulses below this level will not be analysed. Use this to reject electronic noise and low-energy X-rays. 

LLD Lower Level Discriminator A threshold that is used to eliminate noise and small pulses. 

---