The outstanding advantage of digital signal processing (DSP) techniques and Field Programmable Gate Array (FPGA) technology is capable of improving the quality of the experimental measurements for nuclear radiation. In this article, a compact DMCA 8 K was designed and manufactured using DSP technique based on FPGA technology. In particular, the output of the preamplifier is completely processed by digital techniques which are obtained from the analog-to-digital converter (ADC) to calculate the baseline, DC offset, energy peaks, pile up, threshold discrimination and then the form of energy spectrum. The Spartan-6 board is used as a hardware for the development of the digital multichannel analyzer (DMCA), which is equipped with the 14-bit AD6645 with 62.5 Msps sample rate. The application software for instrument control, data acquisition and data processing was written under C++ builder via the RS-232 interface. The designed DMCA system has been tested with a HPGe detector using gamma sources of 60Co and 137Cs and a reference pulser.
The task of electronics in systems of nuclear physics is to collect the electrical charge deposited in a radiation detector and to convert it into a digital code to be further analyzed by digital elements. In most cases, the significant quantities are the particle energy and the time of particle occurrence. The traditional spectroscopy systems for the particle detectors have been made of almost all analog parts constituting a chain of the preamplifier, shaping amplifier and peak sensing ADC configured as separate units. In the recent period, most of the radiation measurement systems designed at Dalat Nuclear Research Institute have been digitalized such as the FPGA-based MCA-8K [
- Digital implementation of the shaping amplifier and peak sensing ADC,
- Charge sensitive preamplifier directly connected to the digitizer,
- Providing pulse height, time-stamp (16 ns) and optionally raw data,
- Pile-up rejection, baseline restoration, ballistic deficit correction,
- Best suited for high-resolution spectroscopy (HPGe and Si detectors).
For such purposes, a set of algorithms used in the digital spectrometric channel containing all necessary data acquisition elements have been developed.
The block diagram of DMCA is shown in
The analog signal from particle detector is transferred to the high-speed sampling ADC for analog-to-digital conversion in real time into FIFO (First-in First-Out) within FPGA. In this design, 14-bitAD6645 is used as a waveform digitizer at a sampling rate of 62.5 MHz.
The output of collected data from FIFO is an exponential signal with long decay time (50 µs) which is shortened to 3 µs by C-R differentiator. However; it creates an undershoot. A pole-zero cancellation (PZC) circuit is often applied to eliminate undershoot in many radiation detectors.
For controlling of data acquisition, setting of parameters and interfacing with the computer, an application program was developed, achieving lots of functions including RS-232 serial communication, system parameter settings, spectrum display and spectrum data management, energy calibration, etc.
With DMCA system, precise energy calibration of a spectrum depends on the linearity of the system. To test the integral nonlinearity of DMCA, an experimental configuration is performed with reference pulser DB-2 generating a positive nuclear tail pulse with the rise time of 100 ns and fall time of 50 μs. Changing the incremental steps from 0 to 1100 mV, with the 44 mV step up. The corresponding channel-voltage value pairs are listed in
I N L D M C A = Δ Y max Y max × 100 % = 22.354 7777 = 0.28 %
Peak channel of the generated pulse resolved by the DMCA is shown in
No. | X (mV) | Yr | Yi | ∆Ymax | No. | X (mV) | Yr | Yi | ∆Ymax |
---|---|---|---|---|---|---|---|---|---|
1 | 197 | 48 | 219.354 | −22.354 | 23 | 630.4 | 3912 | 635.538 | −5.138 |
2 | 216.7 | 109 | 225.925 | −9.225 | 24 | 650.1 | 4095 | 655.249 | −5.149 |
3 | 236.4 | 263 | 242.512 | −6.112 | 25 | 669.8 | 4271 | 674.205 | −4.405 |
4 | 256.1 | 429 | 260.391 | −4.291 | 26 | 689.5 | 4462 | 694.778 | −5.278 |
5 | 275.8 | 615 | 280.425 | −4.625 | 27 | 709.2 | 4644 | 714.380 | −5.180 |
6 | 295.5 | 759 | 295.935 | −0.435 | 28 | 728.9 | 4823 | 733.660 | −4.760 |
7 | 315.2 | 941 | 315.538 | −0.338 | 29 | 748.6 | 4947 | 747.016 | 1.584 |
8 | 344.9 | 1118 | 334.602 | 0.298 | 30 | 768.3 | 5130 | 766.726 | 1.574 |
9 | 354.6 | 1297 | 353.882 | 0.718 | 31 | 788 | 5230 | 777.497 | 10.503 |
10 | 374.3 | 1458 | 371.223 | 3.077 | 32 | 807.7 | 5476 | 803.993 | 3.707 |
11 | 394 | 1655 | 392.441 | 1.559 | 33 | 827.4 | 5671 | 824.996 | 2.404 |
12 | 413.7 | 1795 | 407.520 | 6.180 | 34 | 847.1 | 5858 | 845.138 | 1.962 |
13 | 433.4 | 1963 | 425.615 | 7.785 | 35 | 866.8 | 6105 | 871.742 | −4.942 |
14 | 453.1 | 2161 | 446.941 | 6.159 | 36 | 866.5 | 6308 | 893.606 | −7.106 |
15 | 472.8 | 2330 | 465.144 | 7.656 | 37 | 906.2 | 6517 | 916.117 | −9.917 |
16 | 492.5 | 2498 | 483.239 | 9.261 | 38 | 925.9 | 6696 | 935.397 | −9.497 |
17 | 512.2 | 2698 | 504.781 | 7.419 | 39 | 945.6 | 6880 | 955.215 | −9.615 |
18 | 531.9 | 2906 | 527.184 | 4.716 | 40 | 965.3 | 7058 | 974.387 | −9.087 |
19 | 551.6 | 3183 | 557.019 | −5.419 | 41 | 985 | 7239 | 993.883 | −8.883 |
20 | 571.3 | 3375 | 577.699 | −6.399 | 42 | 1004.7 | 7408 | 1012.085 | −7.385 |
21 | 591 | 3551 | 596.656 | −5.656 | 43 | 1024.4 | 7595 | 1032.227 | −7.827 |
22 | 610.7 | 3727 | 615.612 | −4.912 | 44 | 1044.1 | 7777 | 1051.830 | −7.730 |
Count rate (Kcps) | 1 µs | 2 µs | 3 µs | 4 µs | ||||
---|---|---|---|---|---|---|---|---|
Peak channel (ch) | Peak shift (%) | Peak channel | Peak shift | Peak channel | Peak shift | Peak channel | Peak shift | |
0.1 | 3485 | 0 | 3185 | 0 | 2903 | 0 | 2785 | 0 |
0.5 | 3485 | 0 | 3185 | 0 | 2903 | 0 | 2785 | 0 |
0.7 | 3484 | 0.02 | 3184 | 0.03 | 2901 | 0.07 | 2784 | 0.04 |
10 | 3478 | 0.2 | 3148 | 1.2 | 2892 | 0.3 | 2775 | 0.4 |
15 | 3466 | 0.5 | 3132 | 1.7 | 2876 | 0.9 | 2758 | 0.9 |
20 | 3439 | 1.3 | 3098 | 2.7 | 2838 | 2.2 | 2722 | 2.2 |
30 | 3375 | 2.3 | 3021 | 4 | 2757 | 4.7 | 2641 | 4.8 |
40 | 3285 | 5.2 | 2910 | 7.1 | 2644 | 8.1 | 2530 | 8.2 |
50 | 3220 | 6.3 | 2829 | 8.7 | 2564 | 9.6 | 2453 | 9.8 |
The energy spectra collected from the 60Co are shown in
This work carried out a design and construction of a compact DMCA-8K based on FPGA for high resolution gamma spectroscopy. This article also conducts an observation on the performance and quality of this system such as input count rate, FWHM, peak shape, peak position, pile up rejection, integral nonlinearity. Main technical characteristics are as follows:
• ADC conversion bit: 14-bit, sampling rate: 62.5 MHz
• Resolution: 8192 channels,
• The integral nonlinearity: 0.23%,
• Maximum count capacity per channel: 232−1,
• Shaping time: 1; 2; 4; 6 and 12 µs by software,
• Course gain: 0.3; 0.5; 1; 2 and 3,
• DB9 connector for preamplifier power supply
• On-line pile-up rejection,
• The input receiving positive, unipolar pulse peak amplitude from 0 to 1.1 V,
• Data acquisition program MCA DAQ has written in CBuilder ++ 6.0, self-executing under Windows XP environment.
The authors declare no conflicts of interest regarding the publication of this paper.
Quy, D.H.N., Tuan, P.N. and Dien, N.N. (2019) Design and Construction of a Digital Multichannel Analyzer for HPGe Detector Using Digital Signal Processing Technique. Journal of Analytical Sciences, Methods and Instrumentation, 9, 22-29. https://doi.org/10.4236/jasmi.2019.92003