Pileups

On-demand beam pulsing has been shown to be effective for eliminating pulse pileup in the X-ray detection system, minimizing the energy dissipated in delicate specimens, yet maximizing the data throughput of the overall system. In essence. 

When particle impacts with a solid surface, the atoms of the surface layer undergo crystal lattice deformation, and then form an atom pileup on the outlet of the impacted region. With the increase of the collision time, more craters present on the solid surface, and amorphous transition of silicon and a few crystal grains can be found in the subsurface. 

The cross section of the collision region that the particle impacts with the Si surface with an incident angle of 45° at a speed of 2,100 m/s is shown in Fig. 16 [28]. As the particle impacts into the Si surface layer, the contact region of the Si surface layer transforms from crystal into amorphous phase immediately. The area of the depressed region and the thickness of the amorphous layer increase with the penetration depth of the particle (Figs. 16(a)-16(c)). After it reaches the deepest position, the particle then moves both upwards and rightwards, and some silicon atoms ahead of the particle are extruded out and result in a pileup of atoms. Then the released elastic deformation energy of the Si surface pushes... 

For cell isolation purposes an important cytometer sort feature is pulse-pileup also referred to as peak-pileup or PPU. This recognizes split peak intensities arising from two or more cells in the same droplet that are strung together in chains or that coincidentally partially eclipse the laser beam. The two split peaks would each have the same forward- and side-scatter... 

Importantly, the dead-time of TACs and TDCs is comparatively long, typically 125-350 ns. When a photon arrives within this time interval after the detection of a photon, it will not be observed. Therefore, care must be taken that the count rate of the experiment is sufficiently low to prevent this pulse-pileup. TACs and TDCs usually operate in reversed start-stop geometry. Here, the TAC is started by the fluorescence signal and stopped by the laser trigger. 

Fig. 3.7. The detection efficiency of a system with a dead-time 350 ns as a function of the incident count rate. At high count rates the detection efficiency reduces due to pileup effects.

SPC techniques are hardly affected by additive noise and multiplicative noise is absent. However, subtractive noise due to the collection efficiency and transmission of optics and the quantum efficiency of the detector do play a role. In addition, at high count rates, the efficiency goes down due to pileup effects. 

A common cause of inaccuracy in SPC-based time domain detection is pulse-pileup, that is, the arrival of photons during the dead-time of the detection system. Because the higher probability of emission (and detection) in the earlier part of the decay, pulse-pileup is more probable in this part of the decay. Consequently, the decay will be distorted and the lifetime will be biased towards higher values. Moreover, pulse-pileup will also result in a reduction of the detection efficiency (see Fig. 3.7 and Eq. (3.4)). Therefore, care should be taken to avoid excitation rates too close to the efficacy count rate (i.e., the inverse of the dead-time) in order to minimize these effects. 

List /usr/users/snith/opsd pileup.list accelrys... 

It is also interesting to note that the limitations of most neutron systems lie not in the neutron source(s) but rather in the (gamma ray) detectors and subsequent data acquisition system. The obvious need is for development of detectors with higher rate capability and improved energy resolution. All modern digital data processing techniques have allowed significant improvement in the performance of conventional scintillation detectors with respect to rate and pileup rejection. 

Dislocations climb over obstacles in the slip plane. This process lessens hardening by reducing the drag on moving dislocations and by relieving the back stress associated with a pileup of dislocations, thereby allowing the dislocation source to continue to emit dislocations. 

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