The experiment took place at the lab of Biomedical Engineering and Environmen-tal Science Department at NTHU. The electronics setup is shown in Fig. 3.1 and Fig. 3.2, and the detector setup is shown in Fig. 3.3. As one can see, there are two detecting systems in this experiment, NE213 scintillator and3He proportional tube. The neutron source was hanged for reducing the neutron scattering. The distance between neutron source and detector is 20 cm.
Figure 3.1: The electronics setup: NE213 liquid scintillatior system.
Figure 3.2: The electronics setup: 3He proportional tube system.
148.5 cm 20.0 cm
20.0 cm
NE213 detector
3He detector
Figure 3.3: The experiment setup.
The keywords of the electronic setup system:
(a) NE213 liquid scintillatior system
1. H.V 3002D ：High voltage power supply, Module 3002D, CANBERRA.
2. PreAmp. ：Preamplifier, Module 2005, CANBERRA.
3. Amplifier 2026： Amplifier, Module 2026, CANBERRA.
4. 2160A：Pulse Shape Discriminator, Module 2160A, CANBERRA.
5. CFD 2126：Constant Fraction Discriminator, Module 2126, CANBERRA.
6. TAC/SCA 2145：Time-Amplitude-Converter/ Signal-Channel-Analyzer, Mod-ule 2145, CANBERRA.
7. ADC 8701：Analog-to-Digital Converter, Module 8701, CANBERRA.
8. MCA：Multiple Channel Analyzer
(b) 3He proportional tube system
1. H.V 3102D ：High voltage power supply, Module 3002D, CANBERRA.
2. PreAmp. ：Preamplifier, Module 2006E, CANBERRA.
3. Amplifier 572： Amplifier, Module 572, ORTEC.
4. MCA：Multiple Channel Analyzer
The scintillator is in contacts with PMT (Photo Multiplier Tube ) and the high voltage applied to the PMT was -2000 V. The signal output from PMT can be divided into two parts, one is for the energy spectrum, and another is for the pulse shape discrimination. The former signal goes through the preamplifier, the amplifier, finally, to the ADC (Analog-to-Digital Converter). The latter one is an anode output. The signal was divided into two; one is fed into the constant fraction discriminator (Module 2126), another one is fed into pulse shape discrimi-nator (Module 2160A). The modules will register the signals and then output step signals. These step signals were fed into TDC (Time-to-Digital Converter). The signal from Module 2126 is the start signal, and the signal from Module 2160A
is the stop signal. We can select any time window we want, and then the TDC module will output a gate signal. Finally, we fed this gate signal into ADC module to perform coincident with the signal from amplitude. After coincident method, the signal will be sent to MCA.
For 3He proportional tube detection system, the high voltage supply to the anode of the proportional tube was +1300 V. There is no need to do the pulse shape discrimination, since the reaction in the tube is mainly depending on (n, p) reaction. The energies, deposited by proton and triton are proportional to the pulse height. Signals passed through the preamplifier first, and were then fed into the amplifier. After that, signals were sent to MCA.
3.1.1 Preamplifier
The purposes for using preamplifier is for a good coupling between the detector and the counting system and a minimization of the noise. There are three type of preamplifier, charge-sensitive, current-sensitive, and voltage-sensitive.
For the NE213 detecting system, the preamplifier is Model 2005 which is a charge-sensitive preamplifier, the functional schematic is shown in Fig. 3.4 There are three parts in the Model 2005, integrator, P/Z (pole/zero), and buffer. In the integrator circuit, the feedback capacitor of the preamplifier collects the charges from the detector, the output voltage is proportional to the charge from the de-tector since V0=Q/C. The P/Z circuit is for optimizing the signal performance.
The differentiator is made of a capacitor of the P/Z circuit and the resistor. It provides the 50 µs tail pulse. The capability of the buffer circuit is to avoid the pulse degradation. The features of this preamplifier are low noise, high charge rate capacity (up to 9 µC/s), and fast risetime (less than 15 ns).
For the He3 detector, the preamplifier is Model 2006. This preamplifier is also the charge-sensitive preamplifier which is shown in Fig. 3.5. But this model is for the proportional counter.
Figure 3.4: The functional schematic of the Model 2005 preamplifier.
Figure 3.5: The functional schematic of the Model 2006 preamplifier.
3.1.2 Amplifier
The signals output from the preamplifier can not be sent too far away since they are just a few millivolts. In general, the amplifier is used to amplify and shape the signal. It can avoid the signal pile-up and improve the signal-to-noise ratio.
In the NE213 detecting system, we use the Model 2026 Spectroscopy Amplifier.
There are three pulse shape modes, Gaussian, Triangular and bipolar. For shaping time selection, there are six-position switch we can select. They are 0.5, 1, 2, 4, 6, and 12 µs. The gain range is variable from 2.5 to 1500 with the eight-position switch, the coarse gain (×5, ×10, ×20, ×50, ×100, ×200, ×500, and ×1000), and ten-turn potentiometer, the fine gain which is variable from ×0.5 to ×1.5. In the experiment, we selected the Gaussian mode and set the shaping time at 1.0 µs.
The gain is 100 × 7.55 ( coarse gain × fine gain).
For the He3 proportional tube, we use the ORTEC Model 572 Amplifier to shape the signal. The pulse shape output from the amplifier is semi-Gaussian.
There are two shaping mode to choice, unipolar and bipolar. The gain range is variable from 1 to 1500 with the six-position switch, coarse gain (×20, ×50, ×100,
×200, ×500, and ×1000), and ten-turn potentiometer, the variable gain from ×0.5 to ×1.5. There are also six position, 0.5, 1, 2, 3, 6, and 10 µs, which we can choose for shaping the time.
3.1.3 Discriminator
The discriminator is used to determine the signals that we want. In general, we use some conditions, called threshold or discrimination level, to determine the signals. If the amplitude of an input signal is larger than the threshold value, the discriminator produces a logic signal, and vice versa. In this experiment, we use two discriminators, the Model 2126 and the Model 2160A, to find the neutron signals.
The Model 2126 accepts the negative signals and the output signal are both positive and negative. The count rate is 200 MHz. There are three timing modes,
constant fraction function, leading-edge, and slow rise time rejection. We can select one of them on the front panel. In addition, the threshold level is also selected on the front panel, it is the ten-turn potentiometer and the range of the threshold level are between -5 mV to -1 V. In the experiment, we selected the leading edge timing method and the threshold level is set at the scale 0.3.
The Model 2160A also accepts the negative signals and the output signal are both positive and negative. The timing method of the Model 2160A discriminator used is zero-crossing. Since this module is used under the zero-crossing method, it does not need to set the threshold level.
3.1.4 Time to Amplitude Converter and Single Channel Analyzer
The Model 2145 includes two parts, time-to-amplitude converter (TAC) and single-channel analyzer (SCA). The purpose of this module is to separate different par-ticles, such as neutron and gamma.
The TAC part is used to analyze the time constant of different particles. The two signals are fed into this module, and then TAC circuit part generates a signal with an amplitude proportional to the time interval. For the same particle, the time interval of the signals are almost close, that is the amplitude of the TAC output is almost the same, which is called the time constant. The usable time range is between 5 ns and 1 ms.
The other part of the Model 2145 is SCA. It is also a discriminator, which requires two conditions to determine the signals.† It produces a logic signal only when the amplitude of an input signal is larger than the lower level threshold and smaller than the upper level threshold.
After the signal passes through the TAC part, it flows to the SCA part. The
†The discriminator we have discussed in last subsection is also called integral discriminator, comparing to this, the single channel analyzer is called differential discriminator. The former uses one condition to determine the signals. The later one uses two conditions, lower-level discriminator (LLD) and upper-level discriminator (ULD), to determine the signals.
module produces the logic signal when the amplitude of the TAC signal locates in the interval of the threshold levels. This signals are fed to the ADC module and coincident with the signal from the amplifier. The threshold levels can be select at the front panel controls.
3.1.5 Analog to Digital Converter
The analog-to-digital converter (ADC) is used to convert a continuous signals to discrete signals. After a signal passes through the ADC, the ADC produces a number that is proportional to the pulse height of the signal. In general, the number is called channel number.
The Model 8701 is Wilkinson type analog-to-digital converter. This ADC is used at the peak detection method. The clock of this model is 100 MHz and the resolution is up to 8192 channels. There are two conversion modes, Pulse Height Analysis(PHA) and Sampled Voltage Analysis(SVA). The former method is usually used. For peak detection, there are also two modes, automatic and delayed. In the automatic mode, when the signal above the threshold, the linear gate opens. When the amplitude of the signal is below 90% of its pules height, the linear gate closes. However, when the wide pulses come, the automatic detection may contain uncertainty. Therefore, the delayed mode could be used for solving this problem. In this mode, the linear gate closes at the end of the selected delay time.
In this experiment, we set the resolution to 256 channels. In addition, we use automatic peak detection and PHA mode conversion.
3.1.6 Multiple Channel Analyzer
The instrument used to record the number is called the Multiple Channel Analyzer.
In the experiment, there are two different MCA software for two systems. One is called System 100 (S100); the other is called ACCUSPEC. The former is for NE213 detection system and it operates under the windows 3.0 OS. The later one
is for3He proportional tube, it operates under the dos OS.