A facility for measurements of (n,γ) cross-sections of a nucleus in the range 0.008≤En<20 MeV

ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 618 (2010) 153–159

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

A facility for measurements of (n,g) cross-sections of a nucleus in the range 0.008 r En o20 MeV M. Segawa a,n, Y. Toh b, H. Harada b, F. Kitatani b, M. Koizumi b, Y. Hatsukawa a, T. Fukahori b, H. Matsue a, M. Oshima b, Y. Tanimura c, M. Tsutsumi c, Y. Nagai b,d a

Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan c Department of Radiation Protection, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan d Research Center for Nuclear Physics, Osaka University, Ibaraki, Osaka 567-0047, Japan b

a r t i c l e in fo

abstract

Article history: Received 19 December 2008 Received in revised form 17 March 2010 Accepted 19 March 2010 Available online 8 April 2010

A new measurement system to determine the neutron-capture cross-sections of a nucleus at 0.008 r En o 20 MeV has been installed at the 4 MV Pelletron accelerator laboratory at the facility of radiation standards at Japan Atomic Energy Agency. The performance of the new system was studied by measuring the neutron-capture reaction cross-section of natPb using pulsed neutrons from 7Li(p,n)7Be at 12 rEn r 103 keV. Prompt g-rays from the reaction were detected by means of a highly sensitive antiCompton NaI(Tl) spectrometer combined with a time-of-flight method. The obtained result demonstrated good sensitivity of the new system to determine the neutron-capture cross-sections of a nucleus at kiloelectron volt neutron energy. & 2010 Elsevier B.V. All rights reserved.

Keywords: Radiative capture Spectroscopy with light ions Neutron spectroscopy X- and g-ray spectroscopy Scintillation detectors

1. Introduction Fast neutrons in the energy region En 41 keV have been used as a unique probe in science fields, such as nuclear astrophysics, nuclear physics, and nuclear engineering. In nuclear astrophysics, models of stellar nucleosynthesis play an important role in tracing the history of the Galaxy by studying the abundance of various elements as a function of the metallicity of stars [1]. The neutroncapture cross-section of a nucleus at an astrophysics relevant energy, 1oEn o500 keV, is one of the fundamental input parameters for constructing models of stellar nucleosynthesis of heavy elements, which were produced via slow (s-) and rapid (r-) neutron-capture processes [2,3]. In nuclear physics, many interesting phenomena, such as non-resonant s- and/or p-wave direct capture processes [4–6], doorway states [7,8], Pigmy [9], magnetic dipole (M1) [10], and electric quadrupole (E2) resonances [11,12], were observed by fast neutron-capture reaction of a nucleus. These phenomena provided crucial information to understand the nuclear reaction mechanism and the nuclear structure relevant to the reactions. In applications of nuclear engineering, the precise nuclear data of the cross-section of a nucleus, of fission yields, and

n

Corresponding author. Tel.: + 81 29 282 5441; fax: + 81 29 282 5922. E-mail address: [email protected] (M. Segawa).

0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.03.127

of an emitted number of neutrons are required in the neutron energy range from thermal to several megaelectron volt. As far as astrophysics interest is concerned, the neutron-capture crosssections have been measured at astrophysics relevant energy by using quasi mono-energetic [13] and/or continuous energetic pulsed neutrons [14]. The quasi mono-energetic pulsed neutrons can be obtained by the 7Li(p,n)7Be reaction using protons of about 1.9–2.5 MeV, in which only two reaction channels, such as 7 Li(p,n)7Be and 7Li(p,g)8Be, are open when one uses a Li metal as neutron production target. Intense g-ray events due to 7 Li(p,g)8Be can be discriminated from true events due to neutron-capture by a sample nucleus with a time-of-flight (TOF) method, which allows us to install a low-background measurement system to accurately determine the small neutron-capture cross-section of a nucleus with a mass number A, sg(A), on the order of a few microbarn. The continuous energetic pulsed neutrons have been produced by bombarding a heavy element, such as Hg, Ta, and Pb, with high-energy electrons [14] and/or protons [15]. Using thus-produced neutrons with a continuous energy spectrum, one can measure sg(A) by using the same experimental setup in a wide energy range, which could allow us to determine the cross-section with a small systematic uncertainty. Recent progress using a high intensity proton beam accelerator could allow us to measure sg(A) of a rare abundant nucleus and/or a radioactive nucleus [16].

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However, the continuous energetic pulsed neutrons are produced together with various background particles, such as high-energy g-rays, muons, and pions by spallation reactions. One needs to prepare well-designed heavy shields against these background particles to obtain true events from (n,g) of a sample nucleus with a good signal-to-noise ratio. Despite the progresses so far made, further progress is required to reduce these background events for measuring neutron-capture cross-sections as small as a few microbarn. In order to accurately measure not only the sg(A), but also a neutron inelastic scattering crosssection of a nucleus with a mass number A, sn0 g(A), using quasi mono-energetic neutrons in the energy range of 8 keVrEn o20 MeV from the mentioned interesting points of view, we installed a new measurement system at the 4 MV Pelletron accelerator laboratory in the facility of radiation standards (FRS) of Japan Atomic Energy Agency (JAEA). A new system is described in Section 2, its performance study by measuring the neutroncapture cross-section of natPb is described in Section 3, the present result is given in Section 4, and we conclude our description of the present study in Section 5.

2. Measurement system The 4 MV Pelletron accelerator (4UH-HC) was made by the National Electrostatics Corp. and is equipped with a Duoplasmatron ion source. A pulsed beam was generated using a sweep and a klystron buncher. Quasi mono-energetic pulsed neutrons in the neutron energy range from 8 keV to 14.8 MeV were already produced using 45Sc(p,n)45Ti, 7Li(p,n)7Be, 2H(d,n)3He, and 3 H(d,n)4He reactions. So far, these neutrons have been mostly used to test properties of various types of neutron counters. The accelerated proton beam energy was calibrated using the 7 Li(p,n)7Be reaction at the threshold energy within an uncertainty of 1 keV. The typical averaged proton beam current was about 5 mA at the repetition rate of 4 MHz. The pulse width of the proton beam was measured to be 3.3 ns (FWHM), as discussed later. Although the proton beam current was smaller by a factor two and the pulse width was about two times wider than the beam qualities of the 3 MV Pelletron accelerator at the Tokyo Institute of Technology (TIT), the mentioned beam qualities should be improved in near future. 2.1. g-ray detector We used an anti-Compton NaI(Tl) spectrometer to detect discrete g-rays promptly emitted from the neutron-capture of a nucleus, since the electromagnetic multipolarity of a discrete g-ray provides direct insight into the reaction mechanism of neutron-capture as well as nuclear structure. Such information is essential to improve the prediction of a theoretical model to calculate the (n,g) cross-section of an unstable nucleus from nuclear astrophysics interest. It should be added that when one knows level schemes of a neutron capturing nucleus, a discrete g-ray emitted from a neutron capturing state to low-lying states including the ground state uniquely characterizes a final nucleus, which could allow one to determine the sg(A) with a small systematic uncertainty. An anti-Compton NaI(Tl) spectrometer used in the present study was developed by Ohsaki et al. [17] including one of the present authors to measure a neutron-capture cross-section of a nucleus at 5oEn o600 keV at TIT. Since we plan to measure the sg(A) above 600 keV in near future, we moved the spectrometer from TIT to the FRS. The spectrometer consisted of a central NaI(Tl) detector with a diameter of 22.9 cm and a length of 20.3 cm and an annular

NaI(Tl) detector with an outer diameter of 33.0 cm and a length of 27.9 cm to detect a discrete g-ray with a good signal-to-noise ratio [17]. Low-background photomultiplier tubes were used for the anti-Compton NaI(Tl) spectrometer to reduce any background events due to the g-rays from the b-decay of 40K (at 1.461 MeV). Here four (eight) photomultiplier tubes were used for the central (annular) NaI(Tl) detector. Since a NaI(Tl) detector is known to be quite sensitive to neutrons, the spectrometer was heavily shielded with 6LiH and borated polyethylene blocks to prevent neutrons scattered by a sample from entering into the central NaI(Tl) detector, and Pb against g-rays produced by thermalized neutroncapture reactions by various materials placed in the measurement room. Concerning scattered neutrons mentioned above, it should be mentioned that 6LiH with a thickness of 30 cm played an essential role to attenuate scattered neutrons via the 6Li(n,a)3H reaction without emitting any extra g-rays. The thickness was also determined by considering the attenuation of intensities of g-rays from the neutron-capture reaction of a nucleus. The energy resolution of the central NaI(Tl) detector was 7% at 662 keV. The threshold energy level of an annular detector and the discrimination level of the constant fraction discriminator for the g-rays detected by the central NaI(Tl) detector were respectively set at  30 keV to effectively suppress any Compton background g-rays escaping from the central NaI(Tl) detector and at about 600 keV to reduce the total count rate of the data-taking system. The spectrometer was set at 1001 with respect to the proton beam direction. The distance between the sample position and the front face of the central NaI(Tl) detector was 119 cm. Note that since the low energy neutrons were emitted from the 7Li neutron production target within a narrow cone of about 501 with respect to the proton beam direction, they did not hit the spectrometer directly. A schematic view of the experimental setup is shown in Fig. 1. The floor of the measurement room was made of an Al grating to reduce any background due to scattered neutrons from the floor.

3. Performance of the new measurement system The performance of the new system was studied by measuring the neutron-capture cross-section of a natural lead sample, sg(natPb), using neutrons from 7Li(p,n)7Be in the neutron energy 12rEn r103 keV. We used a natPb sample, since a high sensitive measurement system could allow us to detect a discrete g-ray from a state populated by natPb(n,g) to a low-lying state of a neutron capturing nucleus and thereby to determine a small sg(natPb) at non-resonance neutron energy. Note that the neutron-capture cross-sections of 206Pb [18–23], 207Pb [18,19,21, 24–26], and 208Pb [27,28] at resonance energies were measured using neutrons with high energy resolution, although there remain discrepancies of the neutron-capture cross-sections of 206 Pb and 207Pb between the different data sets, as discussed in Refs. [20,24]. The cross-sections between the resonances, however, are poorly studied [22,29,30]. Note that the broad-energyaverage cross-section, including that at non-resonance neutron energy, is of importance for nucleosynthesis application. Hence, an evaluated cross-section of a nucleus, which is given in JENDL [31] and/or in ENDF [32] has been used in stellar nucleosynthesis calculations. However, an evaluation would be quite difficult, especially when there are many resonances, which interfere with the non-resonant capture reaction process. In the present study, therefore, we aimed at determining the neutron-capture cross-sections of natPb in the mentioned neutron energy using a natPb sample with a diameter of 90 mm and a thickness of 8 mm. A gold sample with a diameter of 90 mm and a

ARTICLE IN PRESS M. Segawa et al. / Nuclear Instruments and Methods in Physics Research A 618 (2010) 153–159

155

10

Counts/keV [arbitrary unit]

8

6

4

2

0 0

20

40 60 80 Neutron Energy [keV]

100

120

Fig. 2. Neutron energy spectrum taken by a 6Li-glass scintillation detector. The obtained neutron energy corresponds to astrophysics relevant energy.

Fig. 1. Schematic view of the experimental setup. The spectrometer was set at yg ¼ 1001 with respect to the proton beam direction. We plan to observe g-rays promptly emitted from the neutron-capture of a nucleus at yg ¼ 1251 in the near future.

thickness of 2 mm was used for normalization of the absolute capture cross-section of natPb, since the cross-section of Au is well known with a small uncertainty of 3–5% [32]. The blank sample (without any sample) was also used to monitor any changes of the incident neutron energy and the neutron flux. A 7Li neutron production target with a thickness of about 100 mg/cm2 was made by evaporating lithium fluoride on a copper disk with a thickness of 0.5 mm, which was cooled by circulating oil to prevent 7Li from evaporation [33]. Neutron scattering effects due to copper and oil would not make a problem in the determination of the neutron-capture cross-sections, since we used measured neutrons at the sample position for a normalization of the cross-sections, as mentioned above. The time-of-flight (TOF) spectrum of neutrons produced by the 7Li(p,n)7Be reaction was measured by the efficiency calibrated 6 Li-glass scintillation detector with a diameter of 50 mm and a thickness of 12 mm. Note that the neutron TOF (t) is defined as a time difference between tc (a time of capture) and tp (a time of neutron production), and is related to neutron energy as follows: t(ns)¼72.3  L/(En)1/2. Here, L and En are the distances (in units of meter) between the 7Li neutron production target and the 6 Li-glass detector, and the neutron energy (in units of megaelectron volt), respectively. The 6Li-glass detector was placed 30 cm downward from the 7Li neutron production target position with

respect to the proton beam direction. A typical neutron energy spectrum, which was obtained by a blank sample, is shown in Fig. 2. nat Pb and Au samples were placed 12.7 cm downward from the 7 Li(LiF) neutron production target position with respect to the proton beam direction. This distance was necessary to determine sg(A) as a function of the kiloelectron volt neutron energy with a TOF method by separating intense g-rays from the 7Li(p,g)8Be and 19 F(p,ag)16O reactions at the target position, which passed through a shadow bar of lead with 0.15 m3. The shadow bar of 15 cm in width and length and 20 cm in height was designed not to be irradiated with neutrons emitted in a narrow region at a forward angle within about 501 with respect to the proton beam direction and not to attenuate the g-ray intensity emitted from samples as shown in Fig. 1. Note that a distance of about 35 cm would be necessary for the measurement of the neutron-capture cross-sections in the neutron energy of 1 MeV. The event rates for these samples were connected to the neutron counts detected by the efficiency calibrated 6Li-glass detector mentioned above. Twodimensional data on the timing of neutron TOF and g-ray energy of the anti-Compton NaI(Tl) spectrometer were stored on a personal computer in the list mode. The energy calibration of the central NaI(Tl) detector was made using g-ray standard sources, such as 22Na, 60Co, and 88Y, and g-rays from the 1H(n,g)2H and 56 Fe(n,g)57Fe reactions. A typical TOF spectrum for a blank sample taken by the central NaI(Tl) detector is shown in Fig. 3, in which a sharp peak at 0 ns was due to g-rays from 7Li(p,g)8Be and 19F(p,ag)16O. The width of the proton pulses was determined to be 3.3 ns at FWHM. For a sample thickness of 8 mm and a flight path of 12.7 cm the corresponding resolution in neutron energy is 4.2 keV at 30 keV. The event rate in a region other than the 0 ns peak region was shown to be independent of time, since the proton beam with the width of 3.3 ns was irradiated on the 7Li neutron production target every 250 ns. Note that any time-dependent background events due to incident neutrons would cause a systematic uncertainty in determining the neutron-capture cross-section of a nucleus, since they would be hardly separated from real events

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from (time dependent) neutron-capture by a sample nucleus at kiloelectron volt neutron energy. A typical TOF spectrum from natPb(n,g) at 12 rEn r103 keV is shown in Fig. 4(a), in which we clearly observed several peaks at t ¼35, 45, and 70 ns. These peaks correspond to resonances at En  66 (at 35 ns), 41 (45 ns), and 16 keV (70 ns), which were identified in previous studies of the (n,g) reactions of 206Pb and 207 Pb [18–20,24], as discussed later. Foreground (including background) and background g-ray energy spectra are shown in Fig. 4(b), in which the former spectrum (solid line) was obtained by putting the gate in a proper region (F) on the TOF spectrum in Fig. 4(a) between 30 and 90 ns. The latter spectrum (dotted line) was obtained by putting the gate in a region (B) on the TOF spectrum between about 100 and 125 ns, in which we can see background g-rays from the b-decays of 40K (at 1.461 MeV) and 208Tl (at 2.614 MeV), and the 2.224 MeV g-ray from 1H(n,g)2H. Hydrogen was contained in 6LiH placed in front of a NaI(Tl) detector, and in a boron-doped polyethylene

placed around an annular NaI(Tl) detector. Here, it should be noted that background events on the TOF spectrum depended very weakly on the TOF time between 50 rt r130 ns, and increased with increase in t at t 4130 ns, as shown in Fig. 4(a). We confirmed that an event rate of the background g-ray energy spectrum, which was obtained by putting the TOF gate between  50 rt r  25 ns was the same as that obtained at 100 rt r125 ns within statistical uncertainties after correcting for the weak dependence of the background events on t. An origin of the TOF spectrum at t4130 ns remains to be open. Here, it should be mentioned that in order to obtain information about the resonance energy of the observed g-rays, we put several gates at 6.7rEg r7.0 MeV and at 7.1 rEg r7.4 MeV on the g-ray energy spectrum in Fig. 4(b). The obtained TOF spectra are shown in Fig. 5, in which we can see that the signal-tonoise ratio of the TOF spectra was much improved compared to the TOF spectrum in Fig. 4(a). This was due to the fact that the 1.461 MeV intense g-ray background from the b-decay of 40K

104

All Eγ =7.1-7.4MeV Eγ =6.7-7.0MeV

Counts [arbitrary unit]

Counts [arbitrary unit]

102

103

2

10

0.1

101

101 50 TOF [nsec]

100

100

150

Fig. 3. A TOF spectrum for a blank sample, which was taken by the central NaI(Tl) detector. The full width at half maximum of the peak at t¼ 0 ns was 3.3 ns.

0

50 TOF [nsec]

2.5

10

2

FORE BACK

104 natPb(n, γ )

103

206

Pb(n, γ) Pb(n, γ)

207

H(n,γ) H 208 Tl

B Counts/50 keV

F

150

3

K

5

10

100

Fig. 5. TOF spectra obtained by gating the g-ray at 6.7 r Eg r 7.0 MeV (solid line) and at 7.1 r Eg r 7.4 MeV (bold line).

7L i( p , γ) 8Be

Counts [arbitrary unit]

-50

40

0

1.5

2

-50

1 0.5

102 -50

0

50 100 TO F [nsec]

150

0

1

2

3

4 5 Eγ [MeV]

6

7

8

Fig. 4. (a) TOF spectrum of natPb(n,g) at 12r En r103 keV and (b) foreground (including background) and background g-ray energy spectrum obtained by gating on a proper region of foreground (F) and background (B) on the TOF spectrum (a). Discrete g-rays from 206Pb(n,g)207Pb and 207Pb(n,g)208Pb are shown by circle and black diamonds, respectively.

ARTICLE IN PRESS M. Segawa et al. / Nuclear Instruments and Methods in Physics Research A 618 (2010) 153–159

deteriorated the signal-to-noise ratio of the TOF spectrum. In Fig. 5, we can clearly see that the mentioned g-rays are connected to the resonances at En  16 (at 70 ns), 41 (at 45 ns), and 66 keV (at 35 ns) in 207Pb and/or 208Pb. Next, we obtained a background-subtracted (net) g-ray spectrum by putting the gate in a proper region on the TOF spectrum in Fig. 4(a) between 40 and 48 ns, corresponding to the neutron resonance at 41 keV in 208Pb, as shown in Fig. 6. Here, we can clearly see the 7.41 MeV g-ray from 207Pb(n,g)208Pb to the ground state of 208Pb and several discrete g-rays at 6.78, 6.21, and 5.88 MeV. These g-rays were also observed in a previous study of radiative transitions in 207Pb and 208Pb using a NaI(Tl) detector, which was placed at 901 with respect to a proton beam direction [19]. Note that since the energy resolution of the present system was 2.6% (FWHM) at 6.1 MeV, two times better than that of the previous one of 5.5% [19], we could accurately determine the gray strength of the peaks mentioned above. A g-ray yield in the net spectra thus obtained is derived by applying the pulse height weighting technique developed by Macklin and Gibbons [13] using the experimental response function of the NaI(Tl) spectrometer of Ref. [17]. Using a g-ray yield, Yg(APb), and assuming isotropy of the emitted g-rays [34], the capture cross-section sg(APb) is given by

sg ðA PbÞ ¼

Yn ðAuÞ PðA PbÞ CðAuÞ ðr 2 nÞAu Yg ðA PbÞ sg ðAuÞ Yn ðA PbÞ PðAuÞ CðA PbÞ ðr 2 nÞA Pb Yg ðAuÞ

ð1Þ

Yn(Au) {Yn(APb)} and P(Au) {P(APb)} are the number of neutron counts measured by a 6Li-glass detector during the measurements of gold (natPb) and the neutron transmission of a gold (natPb) sample, respectively; r and n are the radius and thickness (atoms/ barn) of the sample, respectively; and Yg(Au) {Yg(APb)} and sg(Au) {sg(APb)} are the g-ray yield and the absolute capture crosssection for Au {APb}, respectively. A correction factor C(Au),

157

Table 1 The isotopic composition of the natPb sample determined by measuring isotope ratios of both the sample and a lead isotope reference material SRM-981 (NIST, Gaithersburg, MD, USA) using a double-focusing sector type inductively coupled plasma mass spectrometry (ICP-MS), in which that of ‘‘normal lead’’ is also shown [37]. CV is a coefficient of variation. Isotope

Present lead Abundance

Pb-204 Pb-206 Pb-207 Pb-208

0.0138 0.2479 0.2150 0.5233

Error 0.0003 0.0020 0.0016 0.0041

Normal lead CV (%) 0.21 0.79 0.73 0.78

Abundance 0.014 0.241 0.221 0.524

comprising four factors (Cnm, Cns, Cga, and Cgg), is defined as follows: CðAuÞ ¼ ðCnm Cns Cga Cgg ÞAu

ð2Þ

Here the parameters Cnm and Cns are introduced to correct for the overestimation of the g-ray yield due to the multiplescattering effect and the shielding of the incident neutrons in a sample. Note that the flux of the incident neutrons is attenuated due to scattering and/or absorption in a sample. The parameters Cnm and Cns were calculated using the Monte-Carlo code, TIMEMULTI [35]. The parameters Cga and Cgg are for the g-ray absorption by the sample and the finite size of the sample, respectively. They were calculated by a Monte-Carlo method. Here it should be mentioned about the isotopic composition of the ‘‘natural lead’’ target, whose accurate value is important in deriving cross-sections from the measured g-ray data. It has been known that the isotopic composition of lead depends on its origin and the amount of ‘‘radiogenic lead’’ in the sample. Namely, they have a systematic variation reflecting the geological evolution of the sample [36]. Thus, we verified the isotopic composition of the lead sample by measuring isotope ratios of both the sample and a lead isotope reference material SRM-981 (NIST, Gaithersburg, MD, USA) using a double-focusing sector type inductively coupled plasma mass spectrometry (ICP-MS). The results corrected for the mass bias are shown in Table 1 together with those of ‘‘normal lead’’ [37]. The isotopic fractions of 206Pb and 207Pb of the present lead sample are higher by 3% and lower by 3% than those of ‘‘normal lead’’, and those of 204Pb and 208Pb are the same as those of ‘‘normal lead’’ within their uncertainties.

4. Results The total capture cross-sections of natPb as well as the partial capture cross-sections corresponding to discrete g-ray transitions from capture states of 206Pb and 207Pb to the low-lying states of 207 Pb and 208Pb in the natPb(n,g) reaction were obtained at average neutron energy, /EnS. Note that we identified a reaction channel of (n,g) of natPb by referring to energies of observed discrete g-rays. Here, /EnS in the neutron energy range E1 rEn rE2 is defined as follows: R E2 E fðEn Þ  sg ðEn Þ  En dEn ð3Þ o En 4 ¼ 1R E 2 E1 fðEn Þ  sg ðEn ÞdEn

Fig. 6. . Net g-ray spectrum obtained by gating a proper region on the TOF spectrum in Fig. 5, including 41 keV resonance. The g-rays from 206Pb(n,g)207Pb and 207Pb(n,g)208Pb are shown by circles and black diamonds, respectively.

Here, the parameter f(En) denotes the intensity of the incident neutron at the energy En, and the parameter sg(En) is the neutroncapture cross-section at En. The parameter C is the correction factor mentioned above.

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Table 2 The total capture cross-sections of natPb(n,g) and the partial capture cross-sections corresponding to discrete g-ray transitions from natPb(n,g) to the low-lying states of 207 Pb and 208Pb are given. Here En, /EnS, C, and Eg indicate neutron energy, average neutron energy, a correction factor, and g-ray energy, respectively. En (keV)

oEn 4 (keV)

C

Total sg (present) (mbarn)

Partial Pb (res.)

12–30

20

1.63

13.8 7 0.8

207

Pb

208

Pb

30–50

41

1.23

17.9 7 0.7

207

Pb

208

Pb

50–103

74

1.17

6.2 7 0.3

207

Pb

208

Pb

The total cross-sections of natPb were accurately determined at /EnS¼20, 41, and 74 keV, as given in Table 2. The quoted uncertainty is the result of the combined uncertainties on the statistics of the g-ray yield (2–5%) and the absolute cross-section of Au (3%). Here, we note that a sum of the partial cross-sections of natPb corresponding to the 6.76, 6.19, and 5.86 MeV g-ray transitions in 207 Pb is 11.5 70.8 mb, and that of the partial cross-sections of nat Pb corresponding to the 7.39 and 4.77 MeV g-ray transitions in 208 Pb is 2.0 70.4 mb. Consequently, a sum of the partial crosssections mentioned above, given as 13.5 70.9 mb, is almost equal to the total cross-section of natPb of 13.870.8 mb. Namely, the total cross-section of natPb in the present neutron energy region is dominated by the partial cross-sections corresponding to observed discrete g-ray transitions. Similarly to the case mentioned above, the total cross-sections of natPb at /EnS¼41 (74) keV is equal to the sum of the partial cross-sections of natPb corresponding to observed discrete g-ray transitions at 41 (74) keV.

5. Conclusions A measurement system of the neutron-capture cross-sections of a nucleus at 0.008rEn o20 MeV with a TOF method has been newly installed at the facility of radiation standards at Japan Atomic Energy Agency. The performance of the new system was successfully studied by measuring discrete g-rays promptly emitted from resonance states populated by the natPb(n,g) reaction using pulsed neutrons from 7Li(p,n)7Be by means of an anti-Compton NaI(Tl) spectrometer. The time-dependent neutron-induced background was shown to be negligibly small, comparable to the time independent neutron background in the present study, which demonstrates high sensitivity of the new system to measure the mentioned cross-sections using fast neutrons. With use of the measurement system, successful measurements of discrete g-ray spectra from the neutron-capture by natPb in kiloelectron volt energy region enables us to accurately determine the cross-sections with a small systematic uncertainty.

Acknowledgements We would like to thank H. Yokomizo, K. Asahi, M. Igashira, Y. Yamaguchi, S. Shimizu, M. Yoshizawa, and M. Matsubayashi for

Eg (MeV)

sg (present) (mbarn)

i-f 

6.76 6.19 5.86 7.39 4.77

c.s-gnd (1/2 ) c.s-1st (5/2  ) c.s-2nd (3/2  ) c.s-gnd (0 + ) c.s-1st (3  )

5.27 0.6 3.47 0.4 2.97 0.4 1.47 0.3 0.67 0.2

6.78 6.21 5.88 7.41 4.79

c.s-gnd (1/2  ) c.s-1st (5/2  ) c.s-2nd (3/2  ) c.s-gnd (0 + ) c.s-1st (3  )

5.07 0.3 1.37 0.2 3.07 0.2 8.67 0.5 0.57 0.1

6.81 6.24 5.91 7.44 4.82

c.s-gnd (1/2  ) c.s-1st (5/2  ) c.s-2nd (3/2  ) c.s-gnd (0 + ) c.s-1st (3  )

1.67 0.2 0.67 0.1 2.37 0.2 0.27 0.1 0.47 0.1

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