Performances of a β -delayed neutron detection array at Peking University

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 606 (2009) 645–650

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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima

Performances of a b-delayed neutron detection array at Peking University Jianling Lou 1, Zhihuan Li , Yanlin Ye , Hui Hua, Q.J. Faisal, Dongxing Jiang, Xiangqing Li, Shuangquan Zhang, Tao Zheng, Yucheng Ge, Zan Kong, Yushou Song, Linhui Lv, Chen Li, Fei Lu, Fengying Fan, Zhongyu Li, Zhongxin Cao, Liying Ma, Qite Li, Jun Xiao School of Physics and State Key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China

a r t i c l e in fo

abstract

Article history: Received 18 November 2008 Received in revised form 1 April 2009 Accepted 8 April 2009 Available online 24 April 2009

A b-delayed neutron detection array composed of a neutron sphere and two neutron walls was constructed in the State Key Laboratory of Nuclear Physics and Technology at Peking University. Recently the performances of this detection array were largely improved and tested with a 60 Co source, cosmic rays and 16 C and 17 N radioactive beams. The Tyvek 1056D paper and silicone grease were chosen for the reflection and coupling materials, respectively. For the neutron sphere with large detection solid angle (30% of 4p steradian), the intrinsic efficiency is about 14:1% at a neutron energy of 1 MeV and the detection threshold is about 350 keV; for the neutron walls with flexible setup, these values are 36:5% and 200 keV, respectively. The combined array of neutron sphere and neutron walls has successfully been applied to measure the b-delayed neutrons emitted from neutron-rich unstable nuclei. & 2009 Elsevier B.V. All rights reserved.

Keywords: b-delayed neutron detection array Time-of-Flight (ToF) Energy resolution Intrinsic efficiency Detection threshold

1. Introduction

b-delayed neutron emission is often the dominant decay mode for neutron-rich nuclei near the drip-line. The large decay energy (Q value) is in favor of the population of highly excited states of the daughter nucleus, providing a special tool to study the structure of unstable nuclei. Therefore in recent years several b-delayed neutron detection arrays, which were designed to measure the discrete energies of b-delayed neutrons using Timeof-Flight (ToF) technique, have been constructed, such as TONNERRE (TONneau pour NEutrons REtarde´s) at GANIL [1], neutron ball at MSU [2,3], neutron wall at RIKEN [4], and a combined setup composed of a neutron sphere and two neutron walls at Peking University [5]. Over past few years the performance of the Peking University array has been improved due to the replacement of some PMTs (Photo-Multiplier Tubes), application of new light reflection and coupling materials, and a new design of the whole mechanical configuration. The array was tested by radioactive sources, cosmic rays and radioactive nuclear beams. The overall results are reported in this article.

 Corresponding author. Tel.: +86 1062751883, fax: +86 1062751873.  Corresponding author. Tel.: +86 1062761193, fax: +86 1062751615.

E-mail addresses: [email protected] (J. Lou), [email protected] (Z. Li), [email protected] (Y. Ye). 1 Supported by the National Basic Research Program (973 program) of China (2007CB815002, 2005CB724804), the National Natural Science Foundation of China (10775003, 10475004, 10735010, 10775005, 10875002, 108275002, J0730316). 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.04.018

The detector array at Peking University is schematically shown in Fig. 1. The sphere and the walls are all made of plastic scintillator BC408. The sphere is composed of eight large-area paddles. Each paddle is a wedge-shaped scintillator with a length of 157 cm and curved to a radius of 100 cm in order to keep the same flight distance for neutron ToF measurement. The thickness of each paddle is 2.5 cm, and the width is 40 cm at the middle and reduced to 20 cm at both ends. Each end of the scintillator is connected to an EMI-9214B PMT via a light guide of 30 cm long. The diameter of the active area of the PMT cathode is 46 mm. The solid angle acceptance of one paddle is about 3:75% of 4p steradian. The neutron walls are consisted of 20 short plastic scintillation bars, each with a length of 40 cm and a cross-section of 4:5  2:5 cm2 . Each end of the bar is connected to an XP2020 PMT via a 10 cm long light guide. Application of the XP2020 PMT here is due to its higher gain and better time resolution in comparison to EMI-9214B PMT. The wall is designed to have a flexible mechanical configuration in order to be arranged in different combinations and at different places according to the requirement of physics experiments. When the central flight path is set at 100 cm, the solid angle coverage of the total neutron wall is about 2:9% of 4p steradian. Compared to the paddles of the neutron sphere, the neutron walls are expected to have higher detection efficiency at low energies due to the lower detection threshold resulting from a smaller scintillator and higher gain PMTs. This performance is extremely important for the measurement of low energy neutrons emitted from the energy levels just above the particle unstable threshold of the nucleus.

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J. Lou et al. / Nuclear Instruments and Methods in Physics Research A 606 (2009) 645–650

Fig. 1. Schematic view of the b-delayed neutron detection array at Peking University. One paddle at the left side of the neutron sphere is not shown here in order to open the view of the internal equipments.

Performance measurements using a 60 Co radioactive source and cosmic rays are reported in Section 2, the tests with 16 C and 17 N beams are described in Section 3, and the overall results are summarized in last section.

2. Tests with a

60

Co source and cosmic rays

It is well known that light transmission in the plastic scintillator affects significantly its intrinsic detection efficiency, especially in the case of long and large-area scintillators. In addition to the properties of the scintillation material itself, the reflection layer covering the surface of the scintillator and the coupling layer between the light guide and PMT also plays important roles in light transmission, and therefore need to be selected carefully for each specific detector material and shape.

Fig. 2. QDC amplitude (a) and time difference (b) spectra for a large-area plastic scintillator wrapped with aluminum foil or Tyvek 1056D paper, measured in the test experiment applying cosmic rays at the center of the scintillator.

2.1. Comparison of different reflection materials Five commonly used reflection materials, namely aluminum foil, Tyvek 1056D paper (with three-layer thickness) [6], Aluminized mylar foil, ordinary A4 paper (with three-layer thickness), ESR (Enhanced Specular Reflector) [7], were studied and compared to each other by using the radioactive source 60 Co. g rays at a mean energy of 1.25 MeV emitted from the source generate the so-called Compton edge in the energy spectra detected by the plastic scintillator. Keeping all other conditions unchanged, this edge attained maximum channel when Tyvek 1056D paper was wrapped to the surface of the scintillator. Cosmic rays were also used to test the effect of the reflection layers. Two small plastic scintillation bars, placed above the detector to be tested, were used to select a cosmic ray and to provide the trigger signal. The output signal of each PMT of the tested scintillator was split into two, one was sent to TDC (Timeto-Digital Convertor) via a CFD (Constant-Fraction Discriminator) to measure the time resolution and the other one was fed into QDC (Charge-to-Digital Convertor) to measure the total charge of the signal. As an example, Fig. 2 shows the amplitude of the QDC signal for a neutron sphere scintillator wrapped with Tyvek 1056D paper or aluminum foil, and the corresponding time difference spectra coming from both ends of one scintillator. The amplitude for a scintillator wrapped with Tyvek 1056D paper is nearly two times larger than that wrapped with aluminum foil, indicating a much better reflection performance for the Tyvek 1056D paper. The time resolution is 850 ps (FWHM) for the neutron sphere

scintillation detector wrapped with Tyvek 1056D paper, a little larger than that wrapped with aluminum foil. The effective light propagation velocity along a scintillation bar can be determined by moving the position of the cosmic-ray trigger detectors and measuring the corresponding shift of the peak position in timing spectrum. This velocity was found to be 17.0 cm/ns for the long curved scintillator and 15.4 cm/ns for the short bar. The time resolution can then be converted into position resolution, which varies with position along the scintillator. For a long and wedge-shaped paddle of the neutron sphere, the position resolution is about 10.2 cm (FWHM) at either end of the scintillator and increases to about 14.4 cm (FWHM) at the midposition of the paddle. For a short bar of the neutron wall, the position resolution is about 3.1 cm (FWHM) and keeps almost constant along the bar. 2.2. Selection of coupling materials Four different optical coupling materials between PMT and light guide, such as silicone oil, silicone grease, silicone rubber sheet and air, were used to test the light output. The result shows that the light output obtained with silicone oil coupling is almost the same as that with silicone grease coupling, but is twice of that with air coupling and 20% larger than that with silicone rubber sheet coupling. Considering the long time stability, silicone grease was finally chosen for the present neutron detection array.

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3. Tests with

17

N and

16

C beams

3.1. Experimental setup The experiment was carried out at HIRFL (Heavy Ion Research Facility in Lanzhou). The main goal was to determine the energy resolution, the intrinsic detection efficiency and detection threshold of the combined neutron detection array under the real experimental conditions. Both 17 N and 16 C radioactive beams, with well known b-delayed neutron emission energies and branching ratios as shown in Table 1 [8–10], were used. The primary beam of 26 Mg at 68.8 MeV/nucleon was provided by HIRFL and impinged on a 9 Be primary target. The produced fragments were separated, purified, and collected by RIBLL (Radioactive Ion Beam Line in Lanzhou) [11]. The experimental setup was as presented in Fig. 1. The secondary beams (17 N or 16 C) were identified by the implantation detection system composed of one upstream silicon surface-barrier detector, one aluminum energy degrader with adjustable thickness, one implantation plastic scintillation detector (referred to as the b detector) and one downstream silicon surface-barrier detector [12,13]. The average intensities of 17 N and 16 C beams were about 1900 and 160 pps with purities of 86% and 46%, respectively. The main impurity in17 N beam was 18 Oð14%Þ. A small quantity of 17;18 Nð3:6%Þ and a larger quantity of 19 Oð48%Þ were found in 16 C beam. It should be noted that the b decay of contaminant 18 O or 19 O has no delayed neutron emission. Neutrons emitted following the b decay of 17 N or 16 C, together with other radioactive species stopped in the implantation

647

detector, were measured by surrounding neutron detection array. The starting time is provided by the b detector and the stopping time is the average of two timing signals from both ends of a scintillator in the neutron sphere or the neutron wall (ðT 1 þ T 2 Þ=2). Four high-purity germanium fourfold segmented clover detectors, produced by Eurisys [14], were also used to detect emitted g rays. 3.2. Neutron Time-of-Flight spectra In Fig. 3 is shown the b-delayed neutron ToF spectra for 17 N. In order to illustrate the results in a comparable way, the flight paths of all neutron detectors were scaled to 1 m in the display. A sharp peak resulted from relativistic electrons, which provides the time zero reference, was identified in the experiment but not shown in the figure. The spectra were fitted with an asymmetric Gaussian function plus a cubic background, using the nonlinear fitting program PeakFit. The asymmetric Gaussian function is given as follows: 2

for xox0 ;

Y L ¼ A0  e2:773ðxx0 Þ

for xXx0 ;

Y R ¼ A0  e2:773ðxx0 Þ

=G2

(1)

=G2 ½1þSðxx0 Þ

(2)

2

where Y L and Y R are left and right side amplitude of the peak with the centroid at x0 , G the FWHM of the peak, and S the asymmetric factor applying only to the right side of the peak. In Table 1 is shown the fitted x0 , G and S factors for the peaks in ToF spectra measured by the neutron sphere and the neutron wall,

Table 1 b-delayed neutron ToF peak parameters for the neutron sphere and the neutron wall. Nucleus

17

16

N

C

Energy (MeV)

BR (%)

0.383

38

1.171 1.700

50.1 6.9

Neutron sphere

Neutron wall

x0 ðnsÞ

G ðnsÞ

S

x0 ðnsÞ

G ðnsÞ

S

118.4

11.0

0.8

116.9

10.7

0.23

66.8 55.3

3.9 3.2

0.6 0.4

66.5 55.6

5.1 3.9

0.28 0.29

0.808

1.0

80.6

4.1

0.6

80.2

5.6

0.23

1.715 3.29

15.6 84.4

55.2 40.2

2.9 2.7

0.4 0.2

55.6 40.2

3.4 2.0

0.28 0.2

x0 , G and S represent the centroid position, the FWHM and the asymmetric factor of each peak, respectively.

Fig. 4. Comparison of the original ToF spectrum from the decay of spectrum gated on pulse amplitude (4200 channels).

17

N to the

Fig. 3. Neutron ToF spectra from the decay of 17 N detected by the neutron sphere (a) and the neutron wall (b). The experimental data were fitted by asymmetry Gaussian function plus a cubic background. The dashed line, the dotted line and the solid line represent fitted experimental data, background and fitted neutron peaks, respectively.

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respectively. The value of G increases with the decreasing neutron energy, for both neutron sphere and the neutron wall, which might be explained by the larger timing fluctuation in the detection material for lower energy neutrons. The asymmetry factor S also increases with decreasing neutron energy for the neutron sphere, but keeps almost constant (0.2–0.3) for the neutron wall. The asymmetric property of the peaks was further studied for the neutron sphere. In Fig. 4 is shown the ToF spectrum from the pffiffiffiffiffiffiffiffiffiffiffiffi decay of 17 N gated by the average pulse amplitude (Q ¼ Q 1 Q 2 ) being larger than 200 channels, in which the shape of the neutron ToF peak is much more symmetrical but at an expense of decreasing the number of counts especially for the neutron

Fig. 5. The ToF peak position of 1.171 MeV neutron group as a function of pulse amplitude.

peaks at lower energies or larger ToF. To further clarify this amplitude dependent property, we draw in Fig. 5 the ToF peak position of the 1.171 MeV neutron group as a function of the pffiffiffiffiffiffiffiffiffiffiffiffi average pulse amplitude recorded by QDCs (again Q ¼ Q 1 Q 2 ). The peak position is almost constant for large pulse amplitude but shifts dramatically to larger channels when the amplitude is lower than 800 channels or so. As a matter of fact, due to the relatively long propagation path of the scintillation light in a long curved scintillator of the neutron sphere, the pulses reaching the PMT are largely attenuated, and many of them would cause ToF shifts which form the asymmetrical tail at the right side of each ToF peak. In contrast, in a short scintillation bar of the neutron wall, the effect of light attenuation is much smaller and therefore the asymmetrical property of the peak is trivial. The same phenomenon was also found by L. Heilbronn et al. [15]. The above observed ToF shift related to the small pulse amplitude might be explained by the working mode of the CFD attached to each PMT. It is well known that the CFD generate precise timing signals only for the input signals having consistent rise time. But the rise times of most small pulse signals have large fluctuations which may cause random delay of discrimination. Furthermore, the zero-crossing comparator in the CFD requires a minimum amount of charge to switch its output from ‘‘0’’ to ‘‘1’’ state [16]. The uncertainty in accumulating the small amount of charge might also cause a time walk. Therefore the small pulse amplitude resulting from the long transmissionpath attenuation via multiple reflections in a long curved scintillator and the working mode of the CFD might be the main reasons to explain the asymmetrical tail aside each neutron ToF peak.

Fig. 6. The energy spectra measured by the neutron sphere and the neutron wall. (a) and (c) are from the decay of

17

N, and the other two from the decay of

16

C.

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3.3. Energy response function In Fig. 6 the ToF spectra are converted to neutron energy spectra for the decay of 17 N and 16 C. Again the spectra were fitted with an asymmetric Gaussian function plus a cubic background using programm PeakFit. The absolute energy resolution factor (G) varies with neutron energy as shown in Fig. 7. The data are well described by second-order polynomial function. This function can then be applied as an energy response function to fit the unknown b-delayed neutron energy spectrum detected by neutron sphere or neutron wall, respectively. The absolute energy resolution depends almost linearly on the neutron energy in this a few MeV energy range, corresponding to an almost constant relative energy resolution (G=E). The relative energy resolution at 1 MeV is 10:9% for the neutron sphere and 14:7% for the neutron wall, respectively.

3.4. Intrinsic efficiency and detection threshold The intrinsic detection efficiency was also calibrated with 16 C and 17 N beams. The results are obtained by using the net neutron peak counting, the known branching ratio listed in Table 1, the total number of observed decay events and the geometrical efficiency of the corresponding neutron detector. The dead time of the data acquisition system is taken into account and the proportion of various accompanying isotopes in the beam were checked by the characteristic g rays. The experimentally determined intrinsic efficiencies for the neutron sphere and the neutron walls are plotted in Fig. 8(a) and (b), respectively. In

Fig. 8. The intrinsic detection efficiency of the neutron sphere (a) and the neutron wall (b) obtained from beam test. The solid curves are fitted to the experimental and simulation data points.

order to reasonably extend the efficiency curve to a wider energy range, in addition to six experimental data points in the range of 0.383–3.29 MeV, three Monte Carlo simulation points at 1.7, 2.5, 4.5 MeV, calculated by the programme KSUEFF [17], are also shown in the figure. These simulation points were derived from the same electron-equivalent threshold which is adjusted until the simulation value at 1.7 MeV is matched to the experimental value. The data points were then fitted by an efficiency curve. For the neutron sphere, the efficiency rises at energies from 0.383 to 1.17 MeV and then decreases slowly above 1.700 MeV. For the neutron wall, the efficiency is about 36:5% at 1 MeV and is still high at 0.383 MeV (about 26:5%). To determine the detection threshold of the neutron walls we put one more Monte Carlo data point at 0.25 MeV, assuming the same electron-equivalent threshold in the calculation. From the efficiency curve we estimate that the detection threshold at low energy side is about 350 keV for the neutron sphere and about 200 keV for the neutron walls.

4. Summary

Fig. 7. The absolute energy resolution for the neutron sphere (a) and the neutron walls (b) as a function of neutron energy. The experimental data are fitted by a two order polynomial function.

A combined b-delayed neutron detection array composed of a neutron sphere and two neutron walls was built in the State Key Laboratory of Nuclear Physics and Technology at Peking University. Five different reflection materials for the scintillator and light guide and four different optic coupling materials used between the light guide and PMT were tested by using a 60 Co source and cosmic rays. Finally the Tyvek 1056D paper and silicone grease were chosen for the reflection and coupling

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Table 2 Properties and parameters of the combined b-delayed neutron detection array running at Peking University. Array

Scintillator

Sizea (cm3 )

Units

Rb (cm)

DOc (%)

Timed resolution (ns)

Energye resolution (keV)

Thresholdf (keV) (En)

xn g (%)

Sphere Wall

BC408 BC408

157  40ð20Þ  2:5 40  4:5  2:5

8 20

100 62

30 7.6

0.85 0.2

109 143

350 200

14.1 36.5

a

Size: length  width  thickness, for the neutron sphere, the width is 40 cm at the middle and reduce to 20 cm at both ends. R: the flight distance of b-delayed neutron. c DO: the covered solid angle relative to 4p steradian. d Time resolution: the FWHM of a peak in time different spectrum when impinging cosmic rays at the center of the neutron detector. e Energy resolution: the FWHM of a peak in the neutron energy spectrum at around 1 MeV. f Threshold: the detectable lowest energy limit for b-delayed neutrons. g xn : the intrinsic efficiency for 1 MeV neutrons. b

materials, respectively. The effective light propagation velocities and the position resolutions were also obtained based on the test. Testing with 16 C and 17 N beams were carried out to determine the energy resolution, intrinsic detection efficiency and detection threshold, and the results were summarized in Table 2. The intrinsic efficiency at a neutron energy of 1 MeV attains 14:1% for the neutron sphere and 36:5% for the neutron wall. Most importantly, the detection threshold for the neutron wall is estimated to be as low as 200 keV, the applicability of the array to be extended to lower energy neutrons emitted just above the particle unstable threshold of the nucleus. In comparison to currently existing similar arrays [1–3], the combination of the Peking University neutron sphere and neutron walls gives excellent performances for measuring the b-delayed neutrons. With the recently upgraded version of the neutron detection array, b-delayed neutrons and g rays from the decay of neutronrich nucleus 21 N were measured. Thirteen new neutron groups ranging from 0.28 to 4.98 MeV were observed for the first time (the further data analysis are underway and will be reported elsewhere), indicating that the combined neutron detection array is an adequate tool to study the highly excited states of neutronrich unstable nuclei.

Acknowledgments We would like to acknowledge the staff of HIRFL for providing the 26 Mg primary beam and the staff of RIBLL for all kinds of assistance. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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