A measuring system for time of flight spectrum of neutron based on pulse time sequences detection

Applied Physics Frontier August 2013, Volume 1, Issue 3, PP.27-31

A Measuring System for Time of Flight Spectrum of Neutron Based on Pulse Time Sequences Detection Yong Ren 1†, Peng Feng 2, Fan Yang 2, Jiansheng Li 3, Deling Mi 2 1. College of Communication Engineering, Chongqing University, Chongqing 400044, China 2. Key Laboratory of Optoelectronics Technology and System, Ministry of Education, Chongqing University, Chongqing 400044, China 3. China Academy of Engineering Physics, P.O. Box 919-210, Mianyang 621900, China †Email:

renyong@cqu.edu.cn

Abstract To measure the time of flight (TOF) spectrum for the

252

Cf fission neutron, a TOF measuring system based on the detected time

sequences of neutron pulse has been proposed and set up. A 252Cf fast ionization chamber combined with traditional electronic circuit is applied in the system to form the neutron flight pulse signals. It can not only detect the pulse sequence on-line with 1 nanosecond precision, but also realize time-digital-convertion simultaneously. The cross correlation function is to calculate the measured spectrum and the numerical statistics of the flight time of neutron and  photons are sent to PC and obtained through data processing. Meanwhile, the timing precision of the system can be judged by the -shape. The distinguishing neutron vs. -photons (n-) can also be achieved by setting a delaying factor. Experimental results show that the proposed system can accurately obtain the TOF spectrum whose precision is better than that of the traditional measuring system.

Keywords: 252Cf Source Neutron; TOF Spectrum; Nanosecond Precision; Pulse Sequences Detecting; Correlation Function

1 Introduction As an effective measuring method for fast neutron energy spectrum, Neutron Time of Flight Spectrum (NTFS) measurement is to obtain the flight time distribution of neutrons. Traditional NTFS measuring methods of the 252Cf spontaneous fission neutrons (accompany with γ photons)[1] applying time to amplitude converter(TAC) and multichannel pulse analyzer[2-3] have disadvantages including fixed trigger-time, statistical approximation, absence of original data, combined with -photons interfering, hindering the improvement of time-precision for such kind of method. A new approach to measure NTFS more precisely has been proposed, which is originated from 252Cf source driven power spectral density analysis method[4-5], and utilizes pulse time sequence detecting and correlation function to obtain nanosecond-level TOF. The outline of this paper is as follows: In Section 2, the construction of NTFS measuring system is briefly depicted. Section 3 concretely describes how to detect pulse time signal and process the acquired time sequence with correlation function. Section 4 illustrates the method of discriminating neutron and  photons. Experimental results and performance analysis are presented and discussed in Section 5.

Section 6 concludes this paper.

2 Constructions of NTFS Measuring System Fig. 1 is the schematic diagram of the proposed NTFS measuring system. It is consisted of a 252Cf fast ionization chamber, a charge-sensitive-preamplifier, a fast amplifier, two constant ratio timer I and II, a liquid scintillator (BC501), a photomultiplier tube(PMT), a high speed data acquisition card (DAQ, pulse time sequence detector) and a PC. - 27 http://www.ivypub.org/apf/

252

Cf fast ionization chamber

Charge-sensitive

Fast

Constant ratio

-preamplifier

amplifier

timer I

High speed data acquisition card

Flight distance (e.g. 40 cm)

(Pulse time

PC

sequence BC501

Constant ratio

PMT

timer II

detector)

FIG.1 THE SCHEMATIC DIAGRAM OF NTFS MEASURING SYSTEM

To obtain the TOF of the neutron, the departure and arrival time must be measured accurately. As one part of NTFS system, the 252Cf fast ionization chamber takes the fission fragments and prompt neutrons into account and obtains accurate fission time (1 nanosecond-level) by detecting the fission fragments. Hence, the output of the constant ratio timer I is the departure time of the neutrons and γ photons. Moreover, by separating the 252Cf fission source and the liquid scintillator with a distance, a part of fission neutrons and γ photons can be detected by the liquid scintillator and the arrival/flying time are output of constant ratio timer II. 252

Cf source spontaneous fission neutrons and γ photons can be detected and formed as a random pulse sequence if they are input into some joint electronics circuits mentioned in Fig 1. The neutron intensity in the fast chamber can reach the level of 105 cps (counts per second). By detecting the spontaneous fission fragments, the precision of prompt neutron’s departure time is nanosecond-level[6], and obviously the precision of corresponding pulse signal from constant ratio timer II is also nanosecond-level. Generally, the parameters of the pulse signal which can reflect the neutrons’ departure and arrival time are: the FWHM (~5ns), the rising edge (~3ns), the falling edge (~3ns), the minimum pulse spacing (~10ns) and the maximum of pulse peak (800 mV), with weak random fluctuation (mV-level).

The above-mentioned detecting elements send the departure and the arrival time pulse signal to high speed DAQ to detect, record and transmit. Finally, by analyzing and calculating the data, the measurement result of the 252Cf spontaneous fission neutron energy spectrum will be achieved and stored for future processing. [7]

3 TOF measurements TOF measurement, which requires nanosecond-level precise time accuracy in fission neutrons and γ photons pulse sequence time detection, combined with online detection, high speed and capacity, synchronization accuracy, is difficult to realize. Therefore, the time detecting process which traditionally is named as Time-Digital-Convert (TDC), is the most important part in random signal analysis and application. To accurately and efficiently obtain the nanosecond-level time sequence signal for prompt neutrons and -photons, the proposed NTFS measuring system utilizes an high precision TDC module which has been applied successfully in other applications [8]. Actually, This TDC module is an embedded high speed DAQ installed in a PC. The key parts of this DAQ include a dual channel, 1 Giga Hz A/D (Analog to Digital) conversion unit and a high performance FPGA processor unit.

In the actual measurement, the detected neutron and γ photons pulse signals from 252Cf fast ionization chamber which represent the neutron’s departure time are sent into the channel #1 of high speed DAQ and the pulse signals from liquid scintillator BC501 which represent the neutron’s arrival time are sent into channel #2. Then PC processes all acquired data streams which are recorded as occurring time of a pulse, respectively. After the processes including identification, recovery, split as block, the original data streams are transferred to a series block whose elements is ‘0’ and ‘1’ time pulse where ‘1’ indicates there is a neutron or γ pulse in block and ‘0’ means nothing occurred.

Normally, the number of fission neutrons and γ photons produced by a 252Cf spontaneous fission neutrons source obeys a Poisson distribution [9], which means we can use the stationary-random-process related method to calculate inner property of these detected pulse signals via correlation function [15]. In this case, correlation function reveals - 28 http://www.ivypub.org/apf/

the time similarity between two detected events [10] and its distribution fully corresponds to the neutron and γ TOF spectrum. Then this NTFS measuring system splits the data sequences as blocks, each of which is a part of sequences and each channel has 108~109 blocks. The length of the block is 1024 time bins, and the accuracy of time bin is 1ns.

The measurement of the TOF spectrum based on the correlation function mainly uses the cross-correlation function between the two channels. After discretized and split, the cross-correlation function will be processed as follows: Step 1: M 1

CC12 ( )   CC12i ( )  i 0

1 N

M 1 N 1

 X (k )Y (k   ) i

(1)

i

i 0 k 0

where CC12i ( ) is the cross-correlation function between channel #1 and #2 for ith block, the accumulation of i CC12i ( ) is the total cross-correlation function CC12 ( ) , N is the length of a block, M is the number of blocks, X (k) is the k moment value in the ith block from channel #1 (fast ionization chamber), Yi(k+τ)is the (k+τ)th value in ith block from channel #2 (BC501 liquid scintillator), τ is correlation time delay of  (N 1)  τ  N 1 Based on the former correlation function, the neutron and  TOF information can be obtained.

4 Discrimination of neutron and γ photon Each 252Cf spontaneous fission emits about 4 prompt neutrons and 6 γ photons once. Within the same flight distance, the flight time of the γ photon (light velocity, 30cm/ns) can be regarded as the same. However, the neutron flight time determined by its energy is generally longer than that of the γ photon [11].

3500

TABLE 1 VALUE DISTRIBUTION OF TOF SPECTRUM

3000

Flight

γ peak

γ peak

Neutron peak

distance/cm

position/ns

FWHM/ns

position/ns

40

22

<2

40

2500

2000 系列1 1500

30

22

<2

35

1000

20

22

<2

31

500

10

21

<2

29

time/ns

0 1

5

9

13

17

21

25

29

33

37

41

45

49

53

57

61

65

69

73

77

81

85

89

93

97

FIG.2 TOF SPECTRUM OF NEUTRON AND Γ (40CM)

The TOF distribution of the neutrons and γ photons measured by NTFS measuring system is shown in Fig. 2. Flight time of γ photons should be a constant if the distance between 252Cf fast ionization chamber and BC501 is unchanged.

It means that ideally, a fixed peak corresponding γ photons should appear in the TOF spectrum. But, affected by several factors such as the time jitter of measuring system, this peak becomes a narrow peak with small width.

Fission timing signal shows emitting time of γ photons and FWHM of this narrow peak indicates the timing accuracy of this NTFS measuring system [12]. Undoubtedly, the timing accuracy is a crucial performance index; especially the FWHM of γ peak is the key parameter of NTFS measuring system. Each part of this system is carefully adjusted to couple with nanosecond-level timing. Therefore, any error from other components will be reflected by the γ peak quickly and accurately. Hence, the FWHM of the γ peak can precisely reflect the system timing accuracy and flight calculating result. There are always two peaks which will appear in the cross-correlation function CC12, the narrow one is for γ photons, while the wide one is for neutron’s NTFS. The statistical average value of several repeated measurements are given in Table 1. From the TOF spectrum and Table 1, it can be observed that γ peak concentrates in a narrow range of FWHM<2ns, which is a much better result than that of the Mihalczo’s result (FWHM=2.7ns) [13] . A better convergence degree of the measured γ peak means a better timing accuracy of the measuring system, which can be used to test the timing performance of the 252Cf random pulse source measuring system and the time response characteristics of nuclear detecting system [14].

Considering the γ distribution characteristics of

252

Cf spontaneous fission neutron and the TOF of γ, a factor in

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introduced to adjust the time delay in the correlation function accumulation calculation, by which the correlation time delay is modified as: x  τ  N 1

(3)

Where x is the lower limit of time delay factor, its value can be calculated as γ peak position plus the FWHM of the γ peak. This algorithm can achieve n-γ discrimination quickly and is also simple to obtain NTFS individually.

5 Results and Discussion 3000

2500

2000

1500

系列1

1000

500

cross-correlation function CC12

cross-correlation function CC12

3000

2500

2000

1500

系列1

1000

500

0 1

5

9

13

17

21

25

29

33

37

41

45

49

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69

73

77

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97

0

time/ns

1

5

9

13

17

21

25

29

33

(a)40cm 3000

2500

2000

1500

系列1

1000

500

0

cross-correlation function CC12

cross-correlation function CC12

3000

37

41

45

49

time/ns (b)30cm

53

57

61

65

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81

85

89

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97

2500

2000

1500

系列1

1000

500

0 1

5

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81

85

89

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1

5

9

13

17

21

25

29

33

37

41

45

49

time/ns

time/ns

(c)20cm

(d)10cm

53

57

61

65

69

73

77

81

85

89

93

97

FIG. 3 EXAMPLES OF FLIGHT TIME SPECTRA OF NEUTRON VIA NTFS MEASURING SYSTEM

After confirmed the validity of the system timing accuracy by observing the distribution status of γ peak of the flight spectrum, we measured and obtained a large amount of TOF spectra data with various chamber-scintillator distances, and then modified the distance between 252Cf source fast ionization chamber (channel #1) and BC501 scintillator (channel #2), used traditional TOF measuring system and NTFS with several comparison experiments obtained under the same condition.

The total number M of the blocks is larger than 3×108 with 1ns-length time bin. Figure 3 shows the neutron flight time spectrum with the distances of 40cm, 30cm and 20cm, respectively. The longitudinal axis indicates the statistical results of the cross-correlation function CC12, and the lateral axis indicates the time with the unit of ns.

After γ peak is discriminated and removed from CC12, the distribution of the 252Cf fission neutron spectrum is continuous. With the reduction of the distance between the BC501 and the fission material, the concentrated area of the spectrum converges and the neutron peak approaches to the γ peak, and a little bit overlapped part of the neutron peak is cut. These phenomena match the theoretical prediction. The neutron distribution area is clear as shown in Figure 3, indicating that it is almost impossible to mistaken identify the γ signal to be neutron.

6 Conclusions Based on the pulse time sequence detecting and correlation function method, a new nanosecond-level TOF spectrum measuring system for 252Cf neutron source, which is named as NTFS measuring system, has been designed and set - 30 http://www.ivypub.org/apf/

up. NTFS measuring system applies a new approach to measure the TOF of neutron that is different from the traditional TAC and multichannel analysis method. Combined with a high speed DAQ card, NTFS measuring system can transmit a series of detected neutron and γ pulse signals into PC and analyze them with cross correlation function to get TOF spectrum. NTFS measuring system has already been applied in certain practical measurement to detect and separate neutron and γ photons successfully; the corresponding spectra of neutron TOF also provide a helpful data basis for further measurement and analysis.

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AUTHORS Yong Ren received the B.S. and M. S. degree in electronic and computer science from Xi’an Jiaotong University, Xi’an, China, in 1988 and 1991, respectively. Currently, he is an Associate Professor at the Department of Communication, Chongqing University. His research interests include nuclear signal detecting and analysis, digital image processing.

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