Measurements of 122Sn neutron resonance parameters

Ann. nucl. Eneryy, Vol. 17, No. 2, pp. 95 99, 1990 Printed in Great Britain. All rights reserved

MEASUREMENTS

0306-4549/90 $3.00+0.00 Copyright ~? 1990 Pergamon Press plc

OF 1225n N E U T R O N PARAMETERS

RESONANCE

Y . NAKAJIMA, M. OHKUBO, Y. FURUTA, M. MIZUMOTO, M. SUGIMOTO and

Y. KAWARASAKI Department of Physics, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki-ken 319-11, Japan (Received 4 October 1989)

Al~nmet--Neutron transmission measurements were carried out on a 1225n oxide sample enriched to 92.20% at a 190 m station of the Japan Atomic Energy Research Institute linac with the neutron time-offlight method. Resonance energies and neutron widths were determined for 21 resonances between 1.5 and 30 keV by a shape analysis code based on the Breit Wigner multi-level formula. The following average resonance parameters for s-wave neutrons were obtained: Do = 1.17+0°:0°9keV, So × 104 = 0.30+°:0182 and R' = 5.60 ± 0.05 fm. The present s-wave neutron strength function of ~22Snis substantially larger than the theoretical prediction of the doorway state model.

of the present study have been presented previously (Nakajirna et al., 1985).

1. INTRODUCTION

s-Wave neutron strength functions of Sn-isotopes are interesting in view of the doorway state model. Systematic measurements of neutron resonance parameters of the Sn-isotopes were made first by Fuketa et al. (1963). The measured s-wave neutron strength functions showed a general decrease with mass number, which was right contrary to the mass dependence obtained from calculation with the conventional optical model. This could not be explained within the framework of the optical model. Shakin (1963) succeeded in reproducing these values quantitatively by using a two-particle one-hole state (doorway state) concept which was introduced based on the unified theory of nuclear reactions developed by Feshbach (1958, 1962). Afterwards the neutron resonance parameters of these isotopes were measured again (Adamchuck et al., 1966), but the accuracy of the experimental s-wave neutron strength function of ~22Snwas not still enough for comparison with the theory, because a limited number of resonances were observed for 1:25n. Recently, the neutron strength functions were extracted from differential elastic scattering cross sections (Nikolenko et al., 1982). It is worthwhile to obtain a more accurate s-wave strength function of this isotope in order to refine the muclear models. In the present work, the neutron transmission of ~2:Sn was measured with higher energy resolution in a wider energy range than those in the previous experiments, which resulted in a more accurate swave neutron strength function. Preliminary results

2. EXPEI~IMENTAIL PROCEDUIiIE

The measurements of the neutron transmission were carried out at a 190 m flight path station of the Japan Atomic Energy Research Institute linac timeof-flight facility. The experimental apparatus and the procedure have been described in detail in Tsubone et al. (1984). The electron linac was used to produce pulsed neutrons by photonuclear reactions in a watercooled Ta target. The neutrons were moderated in a B-loaded (5%) 20 cm d i a × 5 cm thick polyethylene disk. The linac was operated at a repetition rate of 300 pps, an electron energy of 120 MeV with an average electron beam current of 30/~A (peak current of ~ 4 A) and a burst width of 25 ns. 7-Rays from the target were blocked by a Pb shadow bar to reduce y-flash signals in detectors. The neutron beam traveled down Table 1. Isotopic composition of the 122Sn sample

95

Isotopes

Atomic weight (%)

Precision (%)

112 114 115 116 I17 l 18 119 120 122 124

<0.03 <0.03 <0.03 0.34 0.30 0.95 1.01 3.78 92.20 1.36

+ 0.02 +0.02 + 0.03 +0.03 + 0.05 ±0.12 _+0.03

96

Y. NAKAJIMAet

to a detector station through an evacuated A1 flight tube and was collimated to 3 cm dia at samples placed in a 100 m midway station. Neutron T O F spectra for sample-in and sampleout were measured with five sets of 11.1 cm dia x 1.25 cm thick 6Li-glass scintillators placed at 190 m from the neutron producing target. The channel width of a time analyzer was 25 ns and total detection channels were 16 k. Neutron intensity was monitored with a 6Li-glass detector at a 45 m station. N o sample changer was used in this experiment. The neutron

al.

T O F spectra for the sample-in and sample-out were normalized with the monitor counts for each spectrum. For background determination° a 4 cm thick A1 plate and a 1 cm Bi plate were kept placed in the neutron beam throughout the experiment. A sample used for the measurements was a separated isotope enriched to 92.20% t22Sn which was loaned in the form of oxide power from the Oak Ridge National Laboratory. The isotopic composition of the sample is given in Table 1. The sample was encapsulated in a cylindrical 0.3 mm thick walled A1 case

1 ,00

C 0

0.80

o~ 0 . 6 0 03

E 03

[- 0.40 L I---

0.20

0.00 -

6'700

6'750 Neutron

6800 6850 6900 Energy ( eV )

6950

Fig. la. An example of the least squares shape analysis in the energy region from 6730 to 6970 eV.

1.00

C 0

m

69

O .BO

0.60

E 0)

r- 0.40 o

0.20

0.00 I

I

26000

P

I

J

I

,

26500

,

,

,

I

I

2"/000

Neutron

i

I

I

I

I

,

i

27500

Energy

i

I

i

28000

I

l

f

I

28500

s

i

I

I

I

i

I

29000

( eV )

Fig. lb. An example of the least squares shape analysis in the energy region from 26 to 29.3 keV.

Measurements

of mSn neutron resonance parameters

97

Table 2. The resonance parameters of m S n Fuketa et al. (1963)

Present

Adamchuk e t al. (1966)

9F, (eV)

E 0 (eV)

r , (eV)

E 0 (eV)

r , (eV)

1751.2±0.07

3.03±0.04

106.9 259.9 1750

0.00077 ±0.00008 0.00175 + 0.00012 3.20 ±0.32

1760±8

4.1 ±0.5

3452.9 ± 0.2 3896.7±0.2 4838.1 ±0.2

0.23 ± 0.02 0.19±0.02 0.37±0.03

5688.3 + 0.3

0.31 + 0.04

6817.2±0.3 6921.8±0.4 7973.7 ± 0.6 9446.5 ±0.4 11,824.0 ± 0.4 13,234.0 ± 0.7 13,819.0±0.7 15,084.0 ± 0.6 18,102.0_+ 1.4 18,967.0±0.7 20,644.0 ± 1.7 22,902.0± 1.0 24,129.0± 1.8 27,100+2 28,280 ± 2 28,419±2

18.4±0.4 0.45±0.06 0.29 ± 0.06 0.86+0.11 1.89 + 0.19 1.84 ± 0.26 1.81 ±0.26 2.5±0.3 1.5 ±0.4 6.0+0.6 2.3 ± 0.5 8.9±0.9 3.4±0.7 31.9±2.4 10.4 + 1.4 6 . 7 ± 1.2

Eo (eV)

3450

0.12 +0.10

5400± 80

'

I

6850

'

I

,- --~ ×o&~.'~ t

~

. .. /

//, /, /

"~iO

z

/w

z-

// . . / / / / / / / / z / //

5 ,,

O0

I

23 + 4

5654 6754 6858

14,900

'

.

5± 2

11,740

20

15

6880± 30

1737 2073 3138 3425 3868 4799

7889

1228 n + n

25

16.0 ± 3.2

Carlton et al. (1977) E0 (eV)

I I I I0 20 Neutron Energy (keV)

I

I 50

Fig. 2. The cumulative number of the resonances vs the neutron energy. o f 4 cm dia. The isotopic thickness o f the sample was 8.74 x 10 -3 atoms barn L

counts at back resonances at 35 and 87 keY for 27AI, and 0.8, 2.3, 5.1 and 12.1 keV for 2°9Bi. The neutron transmission o f ~22Sn was analyzed with a least-squares shape analysis code SIOB (de Saussure et al., 1978) based on the multi-level BreitWigner formula to obtain resonance parameters for the individual resonances. Examples o f the shape analysis are shown in Figs l a and b. For these analyses, radiative width for all resonances was assumed to be 110 m e V from the systematics o f the s-wave resonances ( M u g h a b g h a b et al., 1981). Potential scattering radii were obtained from the analyses o f the large resonances at 1.75 and 6.82 keV, and their average value was used for the analysis o f the other resonances. Statistical spin factor was assumed as 9 = 1 for all resonances, although these assumptions are valid only for the s-wave resonances. The resonances due to isotopic and chemical impurities were checked, and we could not find any trace o f the large resonances o f the impurities.

4. RESULTS AND DISCUSSIONS 3. ANALYSIS OF DATA

A dead time correction was first applied to the raw data o f the T O F spectra. The correction was less than 1% for 1 /~s dead time. Then the background was subtracted from each spectrum by interpolating the

The resonance energies and neutron widths o f 122Sn were determined for 21 resonances between 1.5 and 30 keV, only three resonances o f which were assigned to be s-wave resonances positively by interference between the potential scattering and the resonance

98

Y. NAKAJIMA et al. 900

/ i I I

800

J22~ n-

1I

+ 13

/

/

I

//

700

J

//

b

•

IIII1

600 /

0

,

> 500 D

'3"

II II

400

It II

I

i

L f

N

3oo

11

~

g

/

0

b¢~0

/

•

I 5

P I I IO 15 20 t~ulrt~, Erergy ( keY )

I 25

I 30

Fig. 3. The cumulative sum of the reduced neutron widths vs the neutron energy. In this plot all resonances were assumed to be the s-wave ones.

Table 3. Potential scattering length (fm) Present Nikolenko et al. (1982) Mughabghab et al. (1981)

5.60 + 0.05 5.55+0.21 5.7_+0.3

Table 4. s-Wave neutron strength functions of tnSn Author

So x 104

Energyrange (keV)

0•30+°~2 008 0.17_+0.05 0.49~°622 0.20_+0.10

1.5 30

N i k o l e n k o et al. ( 1 9 8 2 ) A d a m c h u k et al. ( 1 9 6 6 ) F u k e t a et al. ( 1 9 6 3 )

Present

1 200 0.1 7 0.1-7

scattering. T h e deduced resonance p a r a m e t e r s are listed in Table 2 along with previous results. The errors o f the present results given in the table c o n t a i n only statistical c o n t r i b u t i o n s resulting from the least squares analyses. The resonances at 1751.2, 6817.2 a n d 27,100 eV alone were assigned to be the s-wave levels from the interference between the potential a n d resonance scattering. The parity assignment of the o t h e r levels could n o t have been m a d e positively. The present values are in agreement with those of F u k e t a e t al. (1963) a n d o f A d a m c h u k e t al. (1966) for the large resonances at 1.75 a n d 6.82 keV within experimental errors. T h e resonance at 5.4 keV reported only by A d a m c h u k e t al. (1966) could not

be observed in the o t h e r experiments. The evaluation of BNL-325 ( M u g h a b g h a b e t al., 1981) a d o p t e d this resonance based o n the m e a s u r e m e n t s o f A d a m c h u k e t al. (1966). C a r l t o n e t al. (1977) measured the capture d a t a with the Ge(Li) detector which has very high detectability o f the n e u t r o n resonances in the lower energy region, but they did not observe this resonance. Therefore, it can be concluded t h a t this resonance is n o t due to t22Sn, b u t due to some impurity or spurious. The resonances at 2073, 3138 a n d 6754 eV reported by C a r l t o n e t al. (1977) can be t h o u g h t to be surely due to ~22Sn, but too small to be detected in the present experiment. W e did n o t measure the transmission without the A1 a n d Bi filters. There is some possibility t h a t the large resonances of 27A1 a n d 209 Bi shield the resonances o f 122Sn. F o r t u n a t e l y n o resonance of 122Sn was n o t shielded below 15 keV, as can be seen from Table 2. There is no large resonance of 2VA1a n d 2°9Bi between 15 a n d 30 keV which completely shields the resonances o f 1 2 2 S n . The staircase plot of the resonances is shown in Fig. 3. A s s u m i n g all resonances to be the s-wave, average level spacing is D 0 = 1.17 + °i0° 9 keV below 15 keV. The error was estimated with a m e t h o d described in Liou a n d R a i n w a t e r (1972), a n d was s h o w n with dashed lines in Fig. 2.

Measurements of 12ZSnneutron resonance parameters Figure 3 shows the staircase plot of the reduced n e u t r o n widths assuming all levels are s-wave, s-Wave n e u t r o n strength function was o b t a i n e d from average slope in Fig. 2, which is n o t sensitive to the cont a m i n a t i o n of the p-wave resonances. The error was also estimated with the m e t h o d described in Liou a n d R a i n w a t e r (1972), a n d was s h o w n with dashed lines in Fig. 3. The potential scattering length is c o m p a r e d to previous d a t a in Table 3. The present potential scattering length is in good agreement with t h a t of N i k o l e n k o et al. (1982). T h e s-wave n e u t r o n strength function is listed in Table 4 along with previous values. Those of F u k e t a et aL (1963) a n d A d a m c h u k et al. (1966) were o b t a i n e d from the individual resonance parameters, but the measured n u m b e r s of the resonances are smaller t h a n ours. Therefore the present swave strength function has m u c h smaller error t h a n the previous results. T h e present s-wave strength function is larger t h a n the value deduced from the m e a s u r e m e n t s of the average differential elastic scattering cross section by N i k o l e n k o et al. (1982). T h e i r value is strongly dependent o n the absolute values o f the m e a s u r e d differential elastic scattering cross sections, which is difficult to deduce. A t present, the m e a s u r e d s-wave strength functions o f the Sn isotopes are smaller t h a n the prediction o f the d o o r w a y state model with A = 3 M e V (averaging energy interval of the d o o r w a y states) for the isotopes below A = 1 18 a n d larger, a b o v e A = 120 ( M u g h a b g h a b et al., 1981). If a larger A is used for

99

the calculation, the mass dependence of the s-wave strength functions for the Sn isotopes [see Fig. 4 in M u g h a b g h a b et al. (1981)] becomes milder, a n d the agreement between the experimental a n d calculated values will be improved. Acknowledgements--This work was greatly assisted by Mr T. Shoji. We would like to express our sincere appreciation to the operating crew of the JAERI linac for supplying a stable beam. We are much indebted to Dr S. Whetstone (U.S. Department of Energy) for the loan of the enriched isotope from ORNL. REFERENCES

Adamchuk Yu. V. et al. (1966) Soy. J. Nucl. Phys, 3, 589. Carlton R. F., Raman S. and Slaughter G. G. (1977) Phys. Rev. CI~, 883. Feshbach H. (1958) Ann. Phys. 5, 357. Feshbach H. (1%2) Ann. Phys. 19, 287. Fuketa T., Khan F. A. and Harvey J. A. (1%3) Report O,RNL-3425, p. 36. Lion H. I. and Rainwater J. (1972) Phys. Rev. (26, 453. Mughabghab S. F., Divadeenam M. and Holden N. E. (1981) Neutron Cross Sections, Vol. 1, Part A. Academic Press, New York. Nakajima Y. et al. (1985) Nuclear Data for Basic and Applied Science (F. Young et al., Eds.), Vol. 1, p. 947. Gordon & t/reach,/~Iew York. Nikolenko V. G., Popov A. B. and Samosvat G. S. (1982) Nuclear Data for Science and Technoloyy (K. H. B6ckhoff, Ed.), p. 781. Reidel, Dordrccht, Holland. de Saussure G., Olsen D. K. and Perez R. B. (1978) Report ORN L/TM-6286. Shakin C. (1963) Ann. Phys. 22, 373. Tsubone I. et aL (1984) Nucl. Sci. Engng 88, 579.