The Basis for Design and Manufacture of a DSP-Based Coincidence Spectrometer

doi:10.4236/jasmi.2013.33018

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Journal of Analytical Sciences, Methods and Instrumentation, 2013, 3, 152-157

http://dx.doi.org/10.4236/jasmi.2013.33018 Published Online September 2013 (http://www.scirp.org/journal/jasmi)

The Basis for Design and Manufacture of a DSP-Based

Coincidence Spectrometer

Nguyen Xuan Hai1*, Pham Ngoc Tuan1, Nguyen Nhi Dien1, Dang Lanh1, Tuong Thi Thu Huong1,

Pham Dinh Khang2

1Nuclear Research Institute, Dalat, Vietnam; 2Nuclear Training Center, Hanoi, Vietnam.

Email: *[email protected]

Received July 1st, 2013; revised August 8th, 2013; accepted August 17th, 2013

cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

The coincidence technique and the coincidence spectroscopy have been developed and applied for over 40 years. Most

of popular coincidence measurement systems were based on analog electronics techniques such as time to amplitude

conversion (TAC) or logic selecting coincidence unit. The above-mentioned systems are relatively cumbersome and

complicated to use. With the strong growth of digital electronics techniques and computational science, the coincidence

measurement systems will be constructed simpler but more efficient with the sake of application. This article presents

the design principle and signal processing of a simple two-channel coincidence system by a technique of digital signal

processing (DSP) using Field Programmable Gate Arrays (FPGA) devices at Nuclear Research Institute (NRI), Dalat.

Keywords: Coincidence Spectrometer; Digital Signal Processing; FPGA

1. Introduction

The background reduction measurement systems with

active methods are based on coincidence or anti-coinci-

dence techniques. They are mainly built from proper

functional electronics modules in NIM or CAMAC stan-

dards [1,2]. They allow us to identify coincidence or anti-

coincidence events via main electronics blocks called

coincidence unit or time to amplitude converter. Nor-

mally, basic configuration of a coincidence measurement

system will, at least, consist of two channels, and the

selection of coincidence event pairs depends on the de-

fining moment of the pulses appearing at “Timing” out-

put of the pre-amplifier. Obvious drawback of this sys-

tem is cumbersome in size, adjusting operation and syn-

chronizing signals among electronics stages.

The growth of computer engineering and programma-

ble devices that are capable of operating at high frequen-

cies has allowed us to design a new spectrometry genera-

tion. The spectrometry generation is compact on size,

simple in terms of connectivity and using [2-4].

At Nuclear Research Institute (NRI), Dalat, a number

of researches and construction of Compton suppression

as well as event-event coincidence systems were perfor-

med in 1990s. The results of these studies were reported

in [5]. In the recent period at NRI, further studies on the

coincidence spectrometer were presented in several pub-

lications [6-8]. Overall, although there have been sig-

nificant improvements on acquiring as well as processing

data, this system is still based on the traditional way in

obtaining signals under the operation of a coincidence

unit or TAC. With the research results gathered during

the installation, investigation and exploitation of an

“event-event” coincidence spectrometer on digital signal

processing (DSP) at NRI, we have now proposed a new

approach for a design of the DSP-based multi-application

coincidence technique through Field Programmable Gate

Arrays (FPGA) devices. The basis for the design of this

spectrometer will be presented within the framework of

this article.

2. The Basis and Method for the Design

2.1. Design of Multi-Channel Analyzer

The block diagram of MCA is presented in Figure 1. The

functions of the diagram can be summarized as follows:

The pre-filter works as high-pass filter giving an out-

put signal with a constant shaping time. In addition, the

pre-filter has amplification function to generate the ap-

propriate signals for ADC conversion.

The pre-filter’ output signals are sent to flash ADC for

*Corresponding author.

The Basis for Design and Manufacture of a DSP-Based Coincidence Spectrometer 153

Figure 1. The MCA block diagram.

The Basis for Design and Manufacture of a DSP-Based Coincidence Spectrometer

154

sampling. The ADC’ output signals are digital copies of

input’ analog signals.

The transfering “Exponential Decay to Rectangular

Shape” circuit converts exponential decay to rectangular

pulse, and its function is the opposite of high pass de-

convoler (HPD). The transfering “Rectangular Shape to

Trapezoidal Shape” circuit converts shape of rectangular

to trapezoidal pulse, and its function is the low pass filter

(LPF). The circuits of pulse pile-up rejection, base line

restoration and built-in configuration are also designed in

the main board.

The pulse pile-up rejection circuit detects the pile-up

pulses in duration from rise time to half of the flat width,

in slow channel. In case of non-overlapped pulse is de-

tected in monitoring duration, the right pulse will be

analysis. A dual port memory is integrated in FPGA de-

vice for buffering data. The preset time is set by user

with capacity up to 4.2 × 109 seconds. The interface cir-

cuit is integrated in the main board and connected to PC

through a USB-RS232 bridge.

2.2. Design of the “Event-Event” Coincidence

Spectrometer

The block diagram of the “event-event” coincidence

spectrometer is presented in Figure 2. According to the

block diagram, the coincidence spectrometer consists of

three parts; they are two DSP-based multi-channel ana-

lyzers and one channel of timing discriminators. Two

MCA channels are used for getting the energy spectrum

from the two detectors. Timing discrimination circuits

are used for the time interval measurement. The pre-filter,

Fast ADC and DSP MCA are the components of the en-

ergy analysis channel. The FAST COMPARATOR,

TIME COUNTER, TIME COMPARATOR and COIN-

CIDENT CONTROLLER are the components of the

coincidence channel.

Except for the PRE-FILTER, FAST ADC and DDR2

MEMORY, almost other components are designed and

integrated into one FPGA device which is named

XC6SLX16.

Figure 2. The block diagram of the “event-event” coinci-

dence spectrometer.

The external DDR2 MEMORY is used to save the

measured results.

The interface circuit is integrated on the main board

and connected to PC through USB-RS232 bridge.

Principal Operation

Two MCAs receive pulses from the preamplifier’s en-

ergy outputs, the pulses’ amplitudes are analyzed (by

DSP techniques); simultaneously, the preamplifier’s

output pulses are connected to fast comparator circuits.

The output signals of these comparator circuits are used

to start/stop a time counter. The used clock frequency is

400 MHz (2.5 ns period). The contents of the counter

(time interval between two events) are compared with

values of preset time interval; if the measured time is in

the selected range, the amplitudes of pair of events will

be recorded. Three values E1, E2 and T will be stored

into memory; in which E1 and E2 are energy values of

pair of coincidence gamma rays, and T is time interval.

The measured results are read by data acquisition pro-

gram and saved to file on the hard disk.

3. Fabricating

3.1. Hardware

The entire design is programmed and configured in the

FPGA chip XC6LX16-CS324. The used main board is

SP601 kit supplied by Xilinx as Figure 3.

3.2. Data Acquisition Program

The data acquisition program for energy and timing

spectra was written under LabWIEW 8.5. The functions

of program are as follows:

+ Connecting peripheral devices to PC and interfacing

to PC through USB port.

+ Control of data acquisition for multi-MCA mode:

start/stop data acquisition, preset measurement time, data

saving…

+ Control of data acquisition for “event-event” coin-

cidence mode: start/stop data acquisition, preset meas-

urement time, data saving…

+ The base data analysis: display spectra, counts per

channel, energy calibration, zoom in and zoom out spec-

tra.

3.3. The Main Features of Spectroscopy

1) The multi-channel analysis:

- Input: accept the preamplifier’s output pulses from

semiconductor detector, the amplitude from tens to 500

mV;

- Resolution: 8192 channels;

- Capacity of counts/channel: 232 − 1;

- Preset time capacity: 232 − 1 seconds with 1 second

The Basis for Design and Manufacture of a DSP-Based Coincidence Spectrometer 155

Figure 3. The main board.

step;

- The integral nonlinearity: <±0.05%;

- Shaping time: 0.4; 0.8; 1.6; 3.2; 6.4; 12.6; 25.2 μs;

- Coarse gain: 1, 2, 5 and 10;

- Fine gain: 0.75 ÷ 1.25, step 0.01;

- Displaying and rejecting the pile-up pulses, baseline

correction and dead time correction.

2) Event-event coincidence:

- Preset coincidence time window: 5 ÷ 10,000 ns, res-

olution step 5 ns;

- Mode: coincidence, PHA;

- Memory capacity: 16 M events.

3) Power supply:

+12 V/800 mA,

−12 V/50 mA,

+6 V/300 mA,

−6 V/100 mA.

4) Dimensions: width NIM 2M.

4. Testing and Results

The experiment configuration is showed in Figure 4. The

disk-shaped 60Co source with activity of 24 kBq was

placed between the two detectors (GMX35). The dis-

tance from source to the detectors’ surfaces was 4 cm. In

front of each detector, a sheet of lead 2 mm in thickness

Figure 4. Experiment configuration.

was placed to reduce the effect of back scattering from

the detectors to each other.

Data were collected during 2 h. The interfacing pro-

gram was set up in event-event coincidence mode. The

data files were saved to hard disk automatically in all

4096 coincidence events. The Figures 5-7 are energy

spectra of each channel, summation spectra and timing

spectra respectively.

Figure 6 shows the 60Co summation spectra, with only

one peak and Compton scattering background. The coin-

cident spectrum (Figure 5) of each channel contains two

The Basis for Design and Manufacture of a DSP-Based Coincidence Spectrometer

156

Figure 5. Energy spectrum of 60Co.

Figure 6. Summation spectra of 60Co.

Figure 7. Timing spectra (one clock unit equivalent 2.5 ns).

peaks (1173 and 1332 keV) as in MCA mode with gate

control. The measured energy resolution of 1332 keV

peak was ~8.5 keV, which was worse than three times in

comparison with detectors’ nomination value.

Figure 7 shows the timing spectra measured with 60Co

source. The time resolution and the peak/background

height ratio were ~12.5 ns and 2453:220 respectively.

The timing resolution was good but the spectrum shape

was not smooth.

Figure 6 shows only the full-energy peak of



-rays of

cascade. In practice, the nuclide 60Co decay



-emission

to 2.5 MeV level in 60Ni. The 2.5 MeV level is de-excited

by a cascade of 1.17 and 1.33 MeV



-rays to the ground

state. None of cross-over is observed. This showed that

the data acquisition algorithm and program for DSP co-

incidence spectrometer are exact but the energy resolu-

tion and timing spectra are not good.

5. Conclusions

The design of systems based on DSP techniques using

FPGA allows constructing a simple, compact and impact

coincidence spectrometer in which all of parameters are

selected and controlled by software.

Sample frequency is very important and reduced noise

is a big problem. These are problems that should be

solved in designing and manufacturing DSP coincidence

spectrometer.

Currently, the reduced noise for DSP coincidence

spectrometer has been studying at the Department of

Nuclear Physics and Electronics, NRI, Dalat. It is hope-

ful that, in the near future, the design of system might be

the basis for development and application of coincidence

measurement techniques in the field of physics research

and applications.

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