Section 7.3.4. Electronic aspects of neutron detection

Each collected burst of electrons, corresponding to one captured neutron, will be successively amplified, identified (discriminated), and transformed to a well defined signal, by a chain of electronic devices that are represented in Fig. 7.3.4.1 . In the case of the gas detector, the complete electronic chain is generally contained in a grounded metallic box acting as an electrical shield. The detector is connected to this box via a coaxial cable as short as possible to avoid noise and parasitic capacitance. A high-voltage power supply feeds the detector anode through a filter. The charge-sensitive preamplifier contains a field-effect transistor (FET) to minimize the background noise, since, from the detector up to this stage, the electronic level is very low. At the output of the FET, the pulse corresponding to one neutron has an amplitude of about 20 mV. This pulse enters an operational amplifier with adjustable gain G, which delivers a signal of about 2 V, the analogue signal (ANA). The electronic pulse-rise time (0.5 to a few µs) is adapted to the detector electron-collecting time, i.e. its amplitude is roughly proportional to the number of electrons collected at the anode. The last part of the electronic chain is a discriminator with an adjustable threshold, followed by a trigger delivering a calibrated signal (e.g. +5/0 V), called the logic signal (LOG), which is sent to a scaler.

Figure 7.3.4.1| top | pdf | Electronic chain following (a) an 3He gas detector and (b) a scintillation detector.

In the case of the scintillator, the photomultiplier ensures the conversion of light to electrons and produces a strongly amplified electronic signal that is processed through a discriminator and trigger as for the gas detector.

For the gas detector, there are basically three parameters to be adjusted, the gas-amplification coefficient M (a function of the detector high voltage), the electronic amplification gain G, and the discriminator threshold T. Since these adjustments are interactive, the high voltage is initially set at the value given by the manufacturer. The gain G is adjusted in order not to saturate the amplifier. When the electronic amplifier power supply voltage is 5 V, a typical setting of the pulse maximum amplitudes is about 3 V.

We present now the three types of operation necessary to adjust the electronics.

The electronic adjustments and controls of types of detector other than ¹⁰BF₃ gas detectors are basically the same once the changes in the amplitude spectrum have been taken into account. We present in Fig. 7.3.4.2(e) the amplitude spectra for an ³He gas detector with significant wall effects, for a ¹⁰B solid-deposit detector with very low efficiency, and for a scintillator. The energy of the secondary particles produced in an ³He gas detector is 765 keV, about three times less than in ¹⁰BF₃, reducing the signal-to-noise ratio; the relative importance of the wall effect is greater and extends to A₀/4. In the case of the ¹⁰B deposit detector, only one of the secondary particles escapes the foil, so that we do not detect an amplitude A₀ corresponding to the full capture-reaction energy, but only that corresponding either to an average α or Li trace. The quality of the valleys depends on the t/r (foil thickness/particle range) ratio in the ¹⁰B solid (see Fig. 7.3.3.2). The figure corresponds to a monitor where . For the scintillator, the valley in the amplitude spectrum is not very good, even for good glasses and without γ radiation. The discrimination is therefore always much inferior to that of a gas detector. Moreover, the gain of the photomultiplier is very sensitive to the high voltage and has long-term stability problems.