ORTEC High-Count-Rate Detectos - Detectors for High-Count-Rate Applications

High-Count-Rate Detectors

Detectors for High-Count-Rate Applications

Both conventional analog and the more modern digital electronics (such as ORTEC’s DSPEC or DSPEC PLUS) are covered in the following discussion. Typically, energy resolution specifications, in accordance with ANSI/IEEE Standard 325-1996, are measured with amplifier time constants that are a suitable compromise between optimum resolution and reasonable throughput (10 ľs for IGLET, IGLET-X, GLP, SLP, LO-AX; 6 ľs for GEM, GAMMA-X, and GWL detectors). SGD Series Safeguards detectors are designed to operate at higher count rates and are specified at 1 and 2 ľs.

Several factors can affect the high-count-rate performance of Si(Li) or Germanium (HPGe) detector based systems. First, as the input count rate increases, it becomes necessary to decrease the time constant of the main amplifier (be it Gaussian or triangular) in order to achieve a reasonable unpiled-up output count rate. This is shown in Fig. 24 (from Ref. 14), which contains a series of plots of the equation:

r_o = r_iexp (–T_Dr_i)

where r_ois the unpiled output count rate, r_iis the input count rate, and T_Dis the dead time or effective processing time of the amplifier.

As the input count rate increases, one must decrease the main amplifier time constant to obtain a sufficient unpiled-up output count rate. A family of curves for conventional analog electronics is shown in Figure 24.

The use of shorter time constants results in some trade-offs in energy resolution. Decreasing the time constants results in an increase in noise and a deterioration in energy resolution. An example is shown in Figure 25.

The combination of the information shown in Figure 24 and Figure 25 allows the user to make the energy count rate tradeoff most appropriate for his application. In larger detectors a short time constant may result in energy resolution degradation due to "ballistic deficit" effects (See Appendix B). Peak shift may also appear at very high count rates and short time constants. This effect, however, cannot be described by a simple equation. Gated-integrator amplifiers are best for some applications of coaxial detectors when conventional analog electronics are used.

Data obtained with a digital electronic spectrometer such as the ORTEC DSPEC Plus or DigiDART and a small detector (IGLET-11135) are shown in Figures 26, 27, and 28. Figure 29 shows resolution data obtained with a 12% coaxial Germanium (HPGe) detector.

The advantages of DSPEC and DSPEC Plus over conventional electronics are the range of time constants, which ensures availability of the optimum one, plus a choice of digital filter functions. DSPEC Plus offers a choice of 115 rise times.

Other factors limiting the high-count-rate performance of photon detectors are pole-zeroing, energy rate, and performance of the preamplifier and ADC (Appendix B). All ORTEC digital electronics feature automatic pole-zero and optimize functions.

When using a passive feed-back element, one factor is pole zeroing in the preamplifier second stage. In fact, the impedance of this feedback element (resistor) contains reactive elements which may cause ringing and other phenomena that greatly increase the difficulty of proper pole zeroing. A transistor-reset preamplifier, TRP (see Ref. 16; Fig. 30), eliminates this problem.

Energy rate considerations in detectors with resistive feedback preamplifiers are another performance-limiting factor. In germanium at 77 K electron-hole pairs are produced at the rate of one pair for every 2.96 eV (= e) of absorbed energy. As a consequence, if the incoming radiation is absorbed by the detector at a rate of r counts per second, of average energy E, then the radiation-generated current I flowing through the Resistor R_F (Fig. 30 and Table 2) is given by:

I = r (E/e) X 1.6 X 10^–19(in amperes).

The rE product is called the "energy rate" and is normally given in MeV/s. The current I causes a voltage drop IR_F across the feedback resistor R_F. If this voltage drop exceeds the dynamic range of the preamplifier, the preamplifier output becomes nonlinear. At sufficiently high energy rates the preamplifier "locks up."

Reducing the value of R_F in order to increase the energy rate results in an increase in electronic noise, thus creating a tradeoff situation as in the case of amplifier time constants being decreased to increase throughput.

Table 2 lists the energy rate performance of various preamplifiers.