Journal of Nuclear Science and Technology

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Research Laboratory for Nuclear Reactors , Tokyo Institute of .... preliminary result of 24-keV cross sections for several nuclei was given in the previous report'". .... 2 Fe-filtered neutron beam ... The abundance of 235u ... E.4-803MeoV ... where superscript r means the quantities at the black resonance, and subscripts S and B.
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Measurement of Neutron Capture Cross Sections with Fe-Filtered Beam a

a

a

Nobuhiro YAMAMURO , Takeshi DOI , Toshiharu MIYAGAWA , b

b

Yoshiaki FUJITA , Katsuhei KOBAYASHI & Robert C. BLOCK

b

a

Research Laboratory for Nuclear Reactors , Tokyo Institute of Technology , Oh-okayama, Meguro-ku, Tokyo b

Research Reactor Institute, Kyoto University , Kumatori-cho, Sennan-gun, Osaka Published online: 15 Mar 2012.

To cite this article: Nobuhiro YAMAMURO , Takeshi DOI , Toshiharu MIYAGAWA , Yoshiaki FUJITA , Katsuhei KOBAYASHI & Robert C. BLOCK (1978) Measurement of Neutron Capture Cross Sections with Fe-Filtered Beam, Journal of Nuclear Science and Technology, 15:9, 637-644 To link to this article: http://dx.doi.org/10.1080/18811248.1978.9735566

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Journal of NUCLEARSCIENCE and TECIIKOLOGY, 15(9], pp. 637-644

(September 1978).

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Measurement of Neutron Capture Cross Sections with Fe-Filtered Beam* Nobuhiro YAMAMURO, Takeshi DOI, Toshiharu MIYAGAWA, Research Laboraiory f o r Nuclear Reactors, Tokyo Institute of Technology“”

Yoshiaki FU JITA, Katsuhei KOBAYASHI and Robert C. BLOCK’ Research Reactor Institute, Kyoto University$*

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Received April 3 , 1978 T h e neutron capture cross sections of “Nb, Y n , 1271, ’“Ho, IslTa, y J T h and rsaU were measured using the Fe-filtered beam. A 15-cm thick Fe filter was placed in the neutron beam produced by the KUR 46-MeV electron Linac and capture 7-rays were detected by two C,F, scintillation detectors located a t a n 11.7111-flight path. T h e pulseheight weighting technique w a s used to determine the relative capture p r a y detection efficiency. T h e neutron flux w a s measured by the same detectors, whose detection reaction w a s calibrated by the satuefficiency for t h e 480-keV r-ray f r o m the ‘OB(W,U~T) rated resonance capture in Ag a t 5.2-eV. Self-shielding and multiple scattering corrections were applied to the data. T h e results of 24-keV capture cross sections a r e 340, 770, 780, 1,280, 880, 520 and 520 mb f o r g3Nb, T n , leiI1 1 0 6 H ~ I, T a , 232Thand 238U, respectively. Total errors are 5 to 876, with a n estimated systematic e r r o r of 4%. T h e discrepancy between the present results and other data measured recently is within 10!6.

K E Y W O R D S : neutron capture cross sections, niobium 93, indium 1 1 5 , iodine 1 2 7 , holmium 1 6 5 , tantalum 181, thorium 232,uranium 238, Fe-filtered neutron beam, d a t a , errors

I. INTRODUCTION A “point” cross section measurement using a filter is an attractive method to determine cross sections accurately because a low background neutron flux can be obtained. If a suitable filter which has a neutron cross section minimum is placed into a pulsed white or a reactor beam, a monochromatic neutron beam is transmitted from the neutron source to the experimental area. In case of a pulsed white beam produced by a target-moderator assembly bombarded by high energy electrons, the monochromatic neutron beam which has a peak at 24.3keV and a full width at half maximum of about 2keV was obtained after passing through an iron plate of 30-cm thickness“). The background level near the 27-keV peak in the iron cross section was below about 0.15% of the peak counting rate at the - 1 -

24.3 keV resonance-potential interference minimum. When a filter composed of 23-cm of iron, 35-cm of aluminum and 5-cm of sulfur was inserted into a reactor beam, a similar monochromatic beam has also been obtained(e). Using these monochromatic neutron beams, a precise absolute 24-keV cross section can be measured, and relative energy dependent measurements can be normalized to this “point” cross section to provide an accurate determination of the cross section in the keV region. With the aid of the Fe-filtered beam,

* T h i s s t u d y w a s financially supported by the ** :* t

Visiting Researches Program of Research Reactor Institute, Kyoto University. Oh-okayama, Meguro-ku, Tokyo. Kumatori-cho, Sennan-gun, Osaka. Visiting Professor from Rensselaer Polytechnic Institute, U.S.A.

J . N u c l . Sci. Technol.,

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the neutron capture cross sections of In, T a and depleted U were measured at the RPI 1.25-m diam. liquid scintillation spectrometer (9)(0. In the present experiment, the 24-keV neutron capture cross sections of "Nb, II5In, lZiI, IGGHo,IslTa, 232Th and 23*U have been measured with a 15-cm thick Fe-filtered beam and a different y-ray detection technique. The preliminary result of 24-keV cross sections for several nuclei was given in the previous report'".

II. EXPERIMENTAL METHOD 1. Apparatus The experimental arrangement is shown in Fig. 1. The neutrons were generated by a tantalum photoneutron target with a 5-cm thick polyethylene moderator irradiated with electrons from the KUR 46-MeV linear accelerator. The accelerator was typically operated with a 0.6-psec electron pulse width and a 140-Hz repetition rate.

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ONCRETE WALL

Pb

PARAFFIN - Lizc03 46MeV

Fig. 1 Experimental arrangement of 24-keV capture cross section measurement

A 5-cm thick iron filter was placed at the entrance to the evacuated flight tube, about 2.5m from the neutron source. Another 10 -cm thick iron filter was placed at about a 7-m flight distance. The Fe-filtered neutron beam was collimated to 46-mm diameter onto a sample, which was mounted on a 4-position automatic sample changer. The neutron flight path was 11.7 m. Two C,F, liquid scintillation detectors of 10-cm diameter by 4-cm thick were located face to face and were shielded from y-ray background by a 5-cm or a 10-cm thick lead wall. The signals from the scintillation detectors were fed into a linear amplifier and the amplified signals were passed to a timing single channel discriminator whose bias was set at about 150keV. The linear signals and the timing logic signals were recorded with 32channels X 32-channels pulse height and timeof-flight mode in a Nuclear Data 4420 Analyzer. - 2 -

2. Measurements By placing a thick loB sample between two C,F, detectors, the Fe-filtered beam spectrum near 24 keV was measured. In this measurement, the electron pulse width was 60nsec and the channel width of the time analyzer was 30nsec. As the neutron capture probability of thick loB sample is nearly constant for neutrons in the 24-keV band, the spectrum shown in Fig. 2 illustrates directly the spectrum of the neutron flux near 24 keV. Since the counting rate of CGFG detectors for the y-rays emitted when neutrons were captured by nuclei was not high, a 15-cm thick iron filter was used so that the energy band width of the 24-keV filtered neutron beam was broader than that shown in the Refs. (1) and (2), and the ratio of peak count to the background count is about 200. The resonance shape dips are caused by resonances in the manganese impurity in the iron filter.

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Vol. 15, No. 9 (Sep. 1978)

NEUTRON ENERGY(KeV) 20 15

30 25 I I

cept iodine which was PbI, powder capsuled into an aluminum can. The thickness for each sample used in the present experiment is shown in Table 1. The abundance of 235u in the sample of depleted uranium was less than 400ppm. When the data were acquired, ordinarily three samples and the reference sample were mounted and samples were cycled into the neutron beam one by one by the sample changer such that a cycle was repeated every 15 to 20 min. T h e signals for each sample were stored automatically into the corresponding position of memories in the analyzer. Although the beam intensity varied gradually in the course of an experimental run of 10 to 20 hr, the total neutron flux impinging on samples was proportional to the irradiation time of each sample within less than 1% error, because the samples were irradiated repeatedly over many cycles. This fact was well-ascertained by a 20-hour test run. Figure 3 shows the time-of-flight spectra for 24-keV neutron capture in PbI, and * W

10

I

1

; \; BURST

WIDTH Wns

CHANNEL WIDTH 30ns

J

w z z a I u

g lo2 a

r

z 10' ::.=I

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8

.. . . , .. . . ...... . ., ......

.:

-

. . . ....... -..-

.- .. . ..-. -. . . I

200

. ..-

I

I

250 300 CHANNEL NUMBER

T h e straight line in the figure indicates the background counts which are determined by connecting both wings of the peak count. Resonance shape dips near the peak are caused by the manganese impurity in the Fe filter.

Fig. 2 Fe-filtered neutron beam time-of-flight spectrum

The incident neutron flux was measured with a loB sample because the 480-keV y-rays emitted from the l0B(n, a , ~ reaction ) can be detected with the C,F, detectors. Boron sample was made of boron powder packed into an aluminum capsule. The sample was chemically analyzed for boron and was isotopically analyzed for log. It was found to be 93.93 % B of which 93.27% was log. About 1776 of the incident 24-keV neutrons were captured by this sample. The counting rate of the 480keV y-ray was apt to vary with the fluctuation of the discrimination level of about 150 keV during a long time experimental run. Thus, in the first run, a reference sample (a 2.5-mm thick A p plate) was mounted in addition to the loB sample, and a careful comparison was made between the 1°B and reference samples. After this run, the count for the reference sample was used as the neutron monitor. Metallic samples were used as targets ex- 3 -

T h e channel width of the time analyzer used is 0.25 psec. T h e regions between the solid lines represent the interval over which the data are integrated.

Fig. 3 Typical time-of-flight spectra for 24-keV neutron capture i n samples of PbIz and 238U

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samples. Figures 4(a) and (b) show the pulse height spectra for each sample of Pbl 2 and 238 U, respectively. In Figs. 4(a) and (b), the solid dots indicate the background subtracted pulse height spectra. These r-ray spectra were multiplied by the weighting function of the C6F 6 detector obtained previously