Charge-sensitive preamplifiers

An external charge-sensitive preamplifier (type LE 14551) also designed by A. Bradley (3, 4), completes the preamplifier loop. 

The light decay time constant in Nal is about 0.25 (is. Typical charge sensitive preamplifiers translate this into an output pulse rise time of about 0.5 (is. Fast coincidence measurements cannot achieve the very short resolving times that are possible with plastic, especially at low gamma ray energies. 

The rise time T, of the pulse generated by a semiconductor detector can be measured at the output of a charge-sensitive preamplifier. If the preamplifier is sufficiency fast, T, is determined by the following factors ... 

There are three basic types of preamplifiers charge-sensitive, current-sensitive, and voltage-sensitive. The voltage-sensitive preamplifier is not used in spectroscopy because its gain depends on the detector capacitance, which in turn depends on the detector bias. The charge-sensitive preamplifier is the most... 

Figure 10J5 The first stage of a To next stage charge-sensitive preamplifier dc-coupled to the bias circuit.

The noise of the charge-sensitive preamplifier depends on three parameters the noise of the input FET, the input capacitance C , and the resistance connected to the input. The noise can be determined by injecting a charge Q, equivalent to E, into the preamplifier and measuring the amplitude of the generated pulse. Commercial preamplifiers are provided with a test input for that purpose. In general, the noise expressed as the width (keV) of a Gaussian distribution increases as input capacitance increases (Fig. 10.36). 

The sensitivity (or gain) of a charge-sensitive preamplifier is expressed by the ratio V/E, where V is given by Eq. 10.31. For a 1-MeV particle in a germanium detector, the sensitivity is (using Cf 5 pF)... 

The capacitance of the detector has an effect on the energy resolution because it influences the performance of the charge-sensitive preamplifier that accepts the detector signal. The contribution of the preamplifier to the value of Fg increases with the input capacitance. One of the manufacturers, Canberra, reports a 0.570-eV F with zero input capacitance and a slow increase with higher values as shown in Fig. 12.32. Clearly, the resolution improves if the capacitance is kept low. The other component of the input capacitance comes from items like connectors and cables. Reduction of the length of input cable and of connectors capacitance is helpful. For the best resolution with a given system, the preamplifier should be located as close to the detector as possible. 

Figure 12.33 The dependence of on input capacitance for a charge-sensitive preamplifier (from Ref. 2).

Fig. 3. X-ray spcctmm at room temperature detected using a PIN plioto< ode (without scintillator) connected to the Amptek A250 Charge-Sensitive Preampliiier and the X-ray spectrum at room temperature detected using an Avalanche Photodiode (APD) connected to the Amptek A250 Charge-Sensitive Preamplifier.

In preliminary tests performed on the X-ray camera a subarray of 5x5 strips was used to obtain a 25 pixel detector each strip coupled to a hybrid charge sensitive preamplifier (CSP), model CS 507, produced by Clear Pulse (Tokyo) in a modified version with external input FET and a resistive feedback loop CSP is characterised by an equivalent noise (r.m.s.) of about 1 keV. Pulses from CSP... 

Fig. 1. (a) A back illuminated view of the Hgl2 X-ray camera integrated on the ceramic support showing the p-strip grid (b) Scheme of the electronics used for testing the central 5x5 pixels. PA s are the charge sensitive preamplifiers, C.L.U. represents the coincidence logic unit, P.H. is the Peak and Hold module. 

Figure 4,11 Pulse shapes observed with a proportional counter and spectrometer electronics. The detector is responding to Ag and Mn K lines from radioisotopes, (a) The charge-sensitive preamplifier output, (b) The output from a semigaussian shaping ampli-fler with a -fxs shaping time constant, (a) and (b) are multiple traces from an oscilloscope, (c) The energy (pulse height) spectrum obtained by analyzing the amplifier output on a multichannel pulse height analyzer. (Reprinted by courtesy of EG G ORTEC.)...

Figure 6.6 The FWHM noise due to capacitance effects in a charge-sensitive preamplifier (a) a typical preamplifier, according to ORTEC (b) a particularly low noise preamphfier, according to Canberra, who also provided the range of detector capacitances. Reproduced by permission of Canberra.

As with all detectors, the pulse of current at the output, in this case the PMT anode, must be integrated to provide the signal. Because electronic noise is usually not a problem, preamplifiers for scintillation systems need not have a particularly low noise specification. AU three types of preamplifier - voltage, current and charge-sensitive - are in common use. Charge-sensitive preamplifiers are often offered for routine use but low cost voltage-sensitive types are also common. For normal gamma spectrometry... 

CHARGE SENSITIVE PREAMPLIFIER A preamplifier for which the output signal is proportional to the charge collected at the input. 

The ( harge from the diode will be collected with a charge sensitive preamplifier, mounted in close proximity to the diode. Both the diode and the preamplifier wall be shielded to eliminate electronic noise pickup. The required noise characteristics of the preamplifier... 

Existing commercial charge-sensitive preamplifiers fall into... 

Schematic of the CANBERRA charge-sensitive preamplifier Model 2004...

Charge sensitive preamplifiers are normally used with BF3 detectors to integrate the charge from the detector. They work satisfactorily with input capacitance in the order of 1 pF. The reliability of counting with charge sensitive preamplifier is limited to 10 pulses per second. 

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