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Introduction to Charged-Particle Detectors

Energy Resolution and Noise

A typical nuclear electronic chain is shown in Fig. 1. For alpha spectroscopy such electronics have long been available in NIM format; e.g., the SOLOIST or 576A, or more recently in a completely integrated, computer controlled multi-input system, the OCTĘTE-Plus.

The warranted energy resolution is measured in keV FWHM using a thin-window, 5.486-MeV 241Am alpha particle point source placed at a distance from the detector equal to at least twice the detector diameter. The time constant in the main amplifier is also indicated: ORTEC uses 0.5 µsec pulse width at half the maximum pulse height for surface barrier detectors and 1 µsec for ULTRA detectors. The electronic noise of the detector is measured with the chain of Fig. 1, with an electronic pulser replacing the alpha source. This noise has historically been referred to as the "Beta resolution" because when the detector is used with very low specific ionization particles such as conversion electrons (Fig. 2), the energy resolution is approximately equal to the noise. The energy resolution, measured as described above, depends on a number of factors; the most important are:

a) Electronic noise due to the detector leakage current and capacitance. This noise component can be minimized by choice of preamplifier and by optimum amplifier time constant selection. The time constant is preset at its optimum value in all ORTEC alpha spectrometers.

b) Electronic noise due to the bias resistor. This noise component increases with decreasing values of the bias resistor. Typically, the value of the bias resistor is sufficiently high to make this component negligible. However, at elevated detector temperatures, it may become necessary to decrease the value of the bias resistor, with a concomitant noise increase.

c) Energy loss and straggling in the detector entrance window. This factor is important when striving for high geometrical efficiency with the alpha source positioned as close as possible to the detector entrance contact. In this situation, alpha particles emitted perpendicular to the detector front contact pass through the entrance dead layer and undergo the minimum energy loss; alpha particles that enter the front contact at an angle pass through a thicker dead layer, thereby losing more energy in the contact. The energy resolution is thus degraded.

fig6.jpg (16535 bytes)Fig. 1. Typical System for Charged-Particle Spectroscopy.

 

 

 

fig7.jpg (25888 bytes)

Fig. 2. Specific Energy Loss for Electrons in Silicon.
NOTE: Channeling of ions between crystal planes can result in significant variations from the data shown here.