5
Ann. nucl. Eneryy, Vol. 17, No. 2, pp. 95 99, 1990 0306-4549/90 $3.00+0.00 Printed in Great Britain. All rights reserved Copyright ~? 1990 Pergamon Press plc MEASUREMENTS OF 1225n NEUTRON RESONANCE PARAMETERS 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-of- flight 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+°:0182and 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. 1. INTRODUCTION s-Wave neutron strength functions of Sn-isotopes are interesting in view of the doorway state model. Sys- tematic measurements of neutron resonance par- ameters 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 depen- dence obtained from calculation with the con- ventional optical model. This could not be explained within the framework of the optical model. Shakin (1963) succeeded in reproducing these values quan- titatively by using a two-particle one-hole state (door- way 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 sec- tions (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 s- wave neutron strength function. Preliminary results 95 of the present study have been presented previously (Nakajirna et al., 1985). 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 time- of-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 water- cooled Ta target. The neutrons were moderated in a B-loaded (5%) 20 cm dia× 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 Isotopes Atomic weight (%) Precision (%) 112 <0.03 114 <0.03 115 <0.03 116 0.34 + 0.02 I17 0.30 +0.02 l 18 0.95 + 0.03 119 1.01 +0.03 120 3.78 + 0.05 122 92.20 ±0.12 124 1.36 _+0.03

Measurements of 122Sn neutron resonance parameters

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Page 1: Measurements of 122Sn neutron resonance parameters

Ann. nucl. Eneryy, Vol. 17, No. 2, pp. 95 99, 1990 0306-4549/90 $3.00+0.00 Printed in Great Britain. All rights reserved Copyright ~? 1990 Pergamon Press plc

M E A S U R E M E N T S OF 1225n N E U T R O N R E S O N A N C E P A R A M E T E R S

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-of- flight 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 ~22Sn is substantially larger than the theoretical prediction of the doorway state model.

1. INTRODUCTION

s-Wave neutron strength functions of Sn-isotopes are interesting in view of the doorway state model. Sys- tematic measurements of neutron resonance par- ameters 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 depen- dence obtained from calculation with the con- ventional optical model. This could not be explained within the framework of the optical model. Shakin (1963) succeeded in reproducing these values quan- titatively by using a two-particle one-hole state (door- way 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 ~22Sn was 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 sec- tions (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 s- wave neutron strength function. Preliminary results

95

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

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 time- of-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 water- cooled Ta target. The neutrons were moderated in a B-loaded (5%) 20 cm d ia× 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

Isotopes Atomic weight (%) Precision (%)

112 <0.03 114 <0.03 115 <0.03 116 0.34 + 0.02 I17 0.30 +0.02 l 18 0.95 + 0.03 119 1.01 +0.03 120 3.78 + 0.05 122 92.20 ±0.12 124 1.36 _+0.03

Page 2: Measurements of 122Sn neutron resonance parameters

96 Y. NAKAJIMA et al.

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 sample- out 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. No sample changer was used in this experiment. The neutron

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 sep- arated isotope enriched to 92.20% t22Sn which was loaned in the form of oxide power from the Oak Ridge National Laboratory. The isotopic composit ion of the sample is given in Table 1. The sample was encap- sulated in a cylindrical 0.3 mm thick walled A1 case

1 ,00

0.80 C 0

o~ 0.60 03

E 03 [- 0 .40

L

I - - - 0.20

0.00 -

6'700 6'750 6800 6850 6900 6950 N e u t r o n E n e r g y ( e V )

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

1 . 0 0

O .BO C 0

m 0.60 69

E 0) r- 0.40

o

0.20

0.00

I I P I J I , , , , I I i I I I I , i i I i I l f I s i I I I i I

26000 26500 2"/000 27500 28000 28500 29000

N e u t r o n E n e r g y ( e V )

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

Page 3: Measurements of 122Sn neutron resonance parameters

M e a s u r e m e n t s o f m S n n e u t r o n r e s o n a n c e p a r a m e t e r s

Table 2. The resonance parameters of mSn

9 7

Present

Eo (eV) 9F, (eV)

Fuketa et al. (1963) Adamchuk e t al. (1966)

E 0 (eV) r , (eV) E 0 (eV) r , (eV) Carlton et al. (1977)

E0 (eV)

1751.2±0.07 3.03±0.04

3452.9 ± 0.2 0.23 ± 0.02 3896.7±0.2 0.19±0.02 4838.1 ±0.2 0.37±0.03

5688.3 + 0.3 0.31 + 0.04

6817.2±0.3 18.4±0.4 6921.8±0.4 0.45±0.06 7973.7 ± 0.6 0.29 ± 0.06 9446.5 ±0.4 0.86+0.11

11,824.0 ± 0.4 1.89 + 0.19 13,234.0 ± 0.7 1.84 ± 0.26 13,819.0±0.7 1.81 ±0.26 15,084.0 ± 0.6 2.5±0.3 18,102.0_+ 1.4 1.5 ±0.4 18,967.0±0.7 6 .0+0.6 20,644.0 ± 1.7 2.3 ± 0.5 22,902.0± 1.0 8.9±0.9 24,129.0± 1.8 3.4±0.7

27,100+2 31.9±2.4 28,280 ± 2 10.4 + 1.4 28,419±2 6 .7± 1.2

106.9 0.00077 ±0.00008 259.9 0.00175 + 0.00012

1750 3.20 ±0.32

3450 0.12 +0.10

6850 16.0 ± 3.2

1760±8 4.1 ±0.5 1737 2073 3138 3425 3868 4799

5400± 80 5 ± 2 5654 6754

6880± 30 23 + 4 6858

7889

11,740

14,900

' I ' ' I

1228 n + n ,- --~ 25

20 ×o&~.'~ . . .

. t ~ / 15 / / ,

/ , / /w z- "~iO

z / . . / / / / / 5 / / / / z / / /

,, I I I I I I O0 I0 20 50

Neutron Energy (keV)

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

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

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 e t al., 1978) based on the multi-level Breit- Wigner formula to obtain resonance parameters for the individual resonances. Examples o f the shape analysis are shown in Figs la and b. For these analy- ses, radiative width for all resonances was assumed to be 110 meV from the systematics o f the s-wave resonances (Mughabghab e t al. , 1981). Potential scat- tering radii were obtained from the analyses o f the large resonances at 1.75 and 6.82 keV, and their aver- age value was used for the analysis o f the other res- onances. Statistical spin factor was assumed as 9 = 1 for all resonances, a l though 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

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

Page 4: Measurements of 122Sn neutron resonance parameters

98 Y. NAKAJIMA et al.

9 0 0

800

700

600

> 500 D

4 0 0 L

N 3oo

g /

/ •

/ i I

I

1 I

I J22~ n- + 13 / /

/ / / / •

IIII1

/ 0

, ' 3 "

I I I I I t I I

I i

f

11 ~

b J

0 b¢~- I P I I I I 0 5 IO 15 2 0 25 3 0

t~ulrt~, Erergy ( keY )

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 5.60 + 0.05 Nikolenko et al. (1982) 5.55+0.21 Mughabghab et al. (1981) 5.7_+0.3

Table 4. s-Wave neutron strength functions of tnSn

Author So x 104 Energy range (keV)

Present 0 30 +°~2 1.5 30 • 008 N i k o l e n k o et al. (1982) 0 .17_+0 .05 1 200 A d a m c h u k et al. (1966) 0.49~°622 0.1 7 F u k e t a et al. (1963) 0 .20_+0 .10 0 . 1 - 7

scattering. The deduced resonance parameters are listed in Table 2 a long with previous results. The errors of the present results given in the table conta in only statistical cont r ibu t ions resulting from the least squares analyses. The resonances at 1751.2, 6817.2 and 27,100 eV alone were assigned to be the s-wave levels f rom the interference between the potent ia l and resonance scattering. The pari ty ass ignment of the o ther levels could not have been made positively.

The present values are in agreement with those of Fuketa e t al. (1963) and o f A d a m c h u k e t al. (1966) for the large resonances at 1.75 and 6.82 keV within experimental errors. The 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 ther experiments. The evaluat ion of BNL-325 ( M u g h a b g h a b e t al. , 1981) adopted this resonance based on the measurements of A d a m c h u k e t al. (1966). Car l ton e t al. (1977) measured the capture da ta with the Ge(Li) detector which has very high detectabil i ty of the neu t ron resonances in the lower energy region, but they did not observe this resonance. Therefore, it can be concluded tha t this resonance is no t due to t22Sn, bu t due to some impuri ty or spurious. The resonances at 2073, 3138 and 6754 eV repor ted by Car l ton e t al. (1977) can be though t to be surely due to ~22Sn, but too small to be detected in the present experiment.

We did not measure the t ransmiss ion wi thout the A1 and Bi filters. There is some possibility tha t the large resonances of 27A1 and 209 Bi shield the res- onances of 122 Sn. For tuna te ly no resonance of 122Sn

was no t shielded below 15 keV, as can be seen from Table 2. There is no large resonance of 2VA1 and 2°9Bi between 15 and 30 keV which completely shields the resonances of 122Sn .

The staircase plot of the resonances is shown in Fig. 3. Assuming all resonances to be the s-wave, average level spacing is D 0 = 1.17 + °i0 ° 9 keV below 15 keV. The error was est imated with a me thod described in Liou and Rainwater (1972), and was shown with dashed lines in Fig. 2.

Page 5: Measurements of 122Sn neutron resonance parameters

Measurements of 12ZSn neutron resonance parameters 99

Figure 3 shows the staircase plot of the reduced neu t ron widths assuming all levels are s-wave, s-Wave neu t ron s t rength funct ion was obta ined f rom average slope in Fig. 2, which is no t sensitive to the con- t amina t ion of the p-wave resonances. The error was also es t imated with the me thod described in Liou and Rainwater (1972), and was shown with dashed lines in Fig. 3. The potent ia l scattering length is compared to previous da ta in Table 3. The present potent ia l scattering length is in good agreement with tha t of Nikolenko et al. (1982). The s-wave neu t ron s t rength funct ion is listed in Table 4 a long with previous values. Those of Fuke ta et aL (1963) and A d a m c h u k et al.

(1966) were ob ta ined from the individual resonance parameters , but the measured numbers of the reson- ances are smaller than ours. Therefore the present s- wave s t rength funct ion has much smaller error than the previous results.

The present s-wave s t rength funct ion is larger than the value deduced f rom the measurements of the aver- age differential elastic scattering cross section by Nikolenko et al. (1982). The i r value is s trongly depen- dent on the absolute values of the measured differ- ential elastic scattering cross sections, which is difficult to deduce. At present, the measured s-wave s t rength funct ions of the Sn isotopes are smaller than the pre- dict ion of the doorway state model with A = 3 MeV (averaging energy interval of the doorway states) for the isotopes below A = 1 18 and larger, above A = 120 ( M u g h a b g h a b et al., 1981). If a larger A is used for

the calculation, the mass dependence of the s-wave s trength funct ions for the Sn isotopes [see Fig. 4 in M u g h a b g h a b et al. (1981)] becomes milder, and the agreement between the experimental and 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.