Excitation functions for proton-induced reactions of 140Ce and 142Ce up to Ep = 15 MeV

2.A.I 2.D] I

Nuclear Physics A67 (1967) 253--260; (~) North-Holland Publishiny Co., Amsterdam N o t to be reproduced by photoprint or microfilm without written permission f r o m the publisher

EXCITATION FUNCTIONS FOR P R O T O N - I N D U C E D R E A C T I O N S OF 14°Ce AND 142Ce UP TO Ep -- 15 MeV MICH[AKI FURUKAWA Department of Chemistry, Faculty of Science, Unicersity of Tokyo, Honoo, Tokyo, Japan

Received 25 July 1966 Abstract: Excitation functions for the reactions la°Ce(p, n)~4°Pr, l~°Ce(p, 2n)~39Pr and ~4'-'Ce(p,n)mPr have been measured by the activation method. The sum of the measured cross sections for 14°Cc is compared with the theoretical total reaction cross section. Below 10 MeV the sum is reproduced by the theoretical values, when r 0 is assumed to be 1.5 fm. The measured cross sections for each reaction are compared with the predictions from the statistical theory of nuclear reactions by use of a level density expression of the form W(E) - C exp [4a(E-- 6)]t-. The experimental cross sections and the theoretical values agree fairly well, when different values of a are applied to the diffcrent targets. E I

I

NUCLEAR REACTIONS x4°Ce(p, n), (p, 2n), l'2Ce(p, n); E = 5--15 MeV; measured
1. Introduction T h e (p, n) cross sections have been extensively measured by several investigators (refs. 1-8)). However, little i n f o r m a t i o n is available on the (p, n) cross sections in the rare-earth region. The present a u t h o r and his c o l l a b o r a t o r s have m e a s u r e d excitation functions for a l p h a - p a r t i c l e - i n d u c e d reactions on ~38Ba (ref. 9)), 139La (ref. 10)), 140Ce (ref. 9)) and t42Ce (ref. 9)). Reliable values o f total reaction cross sections o f p r o t o n s are necessary in the analysis o f the e x p e r i m e n t a l results. Since c h a r g e d - p a r t i c l e emission is i m p r o b a b l e , the sum o f the (p, x n ) reaction cross sections is assumed to be the total reaction cross sections. In the present study, excitation functions for 14°Ce(p, n)t4°Pr, aa°Ce(p, 2n)139pr and 142Ce(p, n)142pr reactions were measured up to Ep = 14.8 MeV. The 14°Cc target is one o f the most suitable for the a b o v e purpose, since the residual nuclei o f the (p, n) and the (p, 2n) reaction can be easily measured radiochemically.

2. Experimental procedure T h e i r r a d i a t i o n s were p e r f o r m e d with the deflected p r o t o n b e a m o f the I N S 160 cm cyclotron. T h e targets were i r r a d i a t e d in a previously described target assembly i t ) , and the b e a m was collected in a F a r a d a y cup. T h e total charge was m e a s u r e d by a 100 % feed-back type current integrater. Several targets were i r r a d i a t e d c o n c u r r e n t l y by means o f the stacked-foil technique. A l u m i n i u m foils placed between the targets 253

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M. FURUKAWA

degraded the energy of the beam. The bombarding energy was determined in each case by using the values of the stopping power of each material. The stopping power for aluminium was taken from the compilation of Sternheimer ~2), whereas those for other elements were calculated from the stopping power formula of Bethe et al. ~3) using the mean excitation potential I = 13Z (eV). The incident energies of the protons were 14.8, 14.1 and 10.0 MeV. In subsequent experiments, the energy regions overlapped by a few MeV. The targets consisted of ceric oxide or cerous fluoride deposited on aluminium foil by a previously-described sedimentation technique 14). Targets were prepared from natural ceric oxide to a thickness of about 5 mg/cm 2. They were found to be uniform to within 10 ~o in all cases. The targets were 3 cm × 3 cm in area, and the beam was collimated to an area of 1 cm x 1 cm ensuring that the targets intercepted all the beam including any scattered protons. The target foils were positioned so that the forward recoils were caught in the aluminium backing. Irradiation times ranged between 0.5 min and 2 h, and the beam current was about 0.5 #A. After irradiation, the target was dissolved in fuming nitric acid containing a few drops of 30 ~o H 2 0 2 and lanthanum carrier. Cerium was separated from other rare earths by means of Ce(IO3) 4 precipitation. Then Ce(IO3) 4 was dissolved in cone. HC1 and converted to CeC13. Rare earths other than cerium were recovered on a La(OH)3 precipitate. Both the cerium and lanthanum fractions were made up to volume, and aliquots were taken for counting. The disintegration rates of the samples were determined in a variety of ways; (i) The 14°pr nucleide was counted without chemical separation. The two 0.51 MeV g a m m a rays from positon annihilation were counted in coincidence by two 3.8 c m x 2.5 cm N a I scintilation counters. The counting efficiency of the equipment was determined with a 22Na standard source; its disintegration rate had been determined by ~-~ coincidence counting. The decay was followed for 30 min. (ii) Negatons from t*2pr were counted with a calibrated G M counter; its efficiency had been determined by various beta emitters of known disintegration rates. The decay was followed for a week, and the decay curve was analysed. (iii) The 139pr nucleide was allowed to decay to its 139Cedaughter. G a m m a rays from the latter nucleide were counted with a 4.4 cm × 5.1 cm well-type NaI scintillation counter connected to a 10-channel pulse-height analyser. The counting efficiency was assumed as 0.80. 3. Results and discussion

Cross sections were calculated directly using the data shown in table 1. Most of the data were taken from the N R C compilation 15). The cross sections and the energies at which they were measured are presented in table 2, the threshold values of the reactions are also presented. Threshold values were calculated from the atomic mass data of Mattauch et al. 17). The excitation functions are plotted in figs. 1 and 2

255

PROTON-INDUCED REACTIONS TABLE I

Decay properties o f residual nuclei Nucleide

14°Pr l~Pr aagCe

Half-life

Radiation detected

3.4 min 19.2 h 140

d

Branch abundance (~)

fl~ /3

54 a) 100

0.166 MeV

82 h)

a) F r o m the data o f Biryukov et al. 16). h) Conversion electron emission is taken into account. TABLE 2 Experimental cross sections (or in m b ) as a function o f b o m b a r d i n g energies (MeV) in the laboratory system vt0Ce(p, n)14opr (Eth 4.18 McV) Ep

140Ce(p, 2n)tsgpr (Ett t 11.90 MeV) E~

142Ce(p, n)t4Zpr (Eth 1.58 MeV) Ep

14.59

558--100

14.58

447--67

14.58

29.5

13.87

772

14.04

281 fl-42

14.04

29.44-5.0

13.33

815+145

13.28

91+15

13.28

34.2

12.68

946

12.49

33.0

11.83

950

11.66

48.6

11.31

807 ! 130

11.00

51.3 !:7.3

10.05

563

10.04

83.5

9.72

426

9.74

80.7

8.68

225

9.37

9 5 . 5 ~ 13.5

8.19 7.50

160+25 91

9.21 8.67

87.0 108.0 114.5.'.-16.0

7.18

86

7.97

6.64

41

7.09

58.6

6.47

21.8

5.57

7.6-1 2.0

5.76

6.9 :v: 1.4

with the estimated standard deviations for some experimental points. The standard deviations were estimated from uncertainties in the determination of target thickness, measurement of beam currents and determination of disintegration rates of residual nuclei. The cross sections for the l*°Ce(p, n)t4°Pr and the t*ZCe(p, n)t42pr reactions were measured by Blaser et al. 2) at 6.7 MeV. The cross section for the l*2Cc(p, n) t42pr reaction was measured by Blosser and Handley 4) at 12 MeV. These results are also presented in figs. 1 and 2. However, the agreement between the present results and the previous ones is poor. The reason for this discrepancy remains unknown. In fig. 3, the sum of the measured cross sections for l*°Ce is compared with the total reaction cross sections calculated on the basis of the continuum theory (accord-

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BLASER etal (pn)

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,

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l

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/ I

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Fig. I. Excitation functions for the reactions ~°Ce(p, n)'4°Pr and ~4°Ce(p, 2n)I'~'Pr. The experimental value of Blaser et al. 2) is also shown. . . . .

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l0 l E~, (MeV, lab) Fig. 2. Excitation function for the '42Ce(p, n)X4~Prreaction. The experimental values of Blaser et al. 2) and Blosser et aL 4) are also shown.

257

PROTON-INDUCED REACTIONS

ing to Shapiro) is). A value of ro = 1.5 fm appears to give the best fit to the sum of the measured cross sections at lower energies. However, above 10 MeV the discrepancy from the experimental data becomes significant. I l l ,

l

'

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J

t

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ro~17

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1

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1

~

10 M e V lab)

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1

15

Fig. 3. Total reaction cross sections. T h e theoretical curves are obtained from the calculation of Shapiro le).

The experimental cross sections for each reaction may be compared with the predicted values from the statistical theory. The calculations were performed in the usual manner using the following assumptions: (i) Protons, neutrons and alpha particles were the only particles involved in the calculation. The binding energies of various particles were obtained from the compilation of Mattauch et al. 17) and are listed in table 3. TABI.E 3 Binding energies (in McV) Nucleidc rl~pr 14~Pr ;4~Pr l~°Pr raCe l~°Ce

Bn

Bp

B~

7.32 5.85 9.39 7.67 7.21 9.04

5.76 5.65 5.24 4.89 8.86 7.99

-1.90 --0.43 0.98 0.91 -- 1.43 1.41

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l o :¢ --

'

'

'

l

'

'

'

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l

'

'

'

'

I

Exp. value

.g

~0--

(p,n)

--A--

(p,2n)

E

b

/

10 2

//.

7 Calc. value lO

a-2

•

~8

5

lO (MeV, lab)

F,,

15

Fig. 4. Experimental and calculated excitation functions for the reactions ta°Ce(p, n)~°Pr and 14oCe(p, 2n)lagpr.

1 0 :~

a=2

10 2

0

0 10 :a:8:

0

1

I

I

L

I

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5

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I

l0 Ep (,k,h,\: lab)

I

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I

T

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15

Fig. 5. Experimental and calculated excitation function for the *a-~Cc(p, n)'~:Pr reaction.

PROTON-INDUCED REACTIONS

259

(ii) The calculated values of capture cross sections by Shapiro t8) were not reliable as shown in fig. 3. Since no optical-model calculation has been published in a generally accessible form, the sum of the measured cross sections for the 14°Ce(p, n) and 14°Ce(p, 2n) reactions was assumed as the capture cross section for protons. The capture cross sections of alpha particles were obtained from the calculation of Igo 19). The neutron capture cross sections were assumed as the geometric cross section, when the nuclear radius was expressed as 1.5 A ~ fm. (iii) The dependence of the level density on the excitation energy of the residual nucleus E was assumed to be W(E) = C e x p [4a(E-/~)] ~, where C is a constant, a the level density parameter and 6 a pairing energy term. The values of 6 were taken from Cameron's table of pairing energies 2o). Only the parameter a was varied in the calculation. 10 2

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O--Exp. value

a**2 ==4

10

----8

&d.

a~2 ~4

\

0.1

I

10

I

I

12 Ep

I

14 (MeV, lah~

~8

\ I

I

16

Fig. 6. Experimental and calculated cross section ratios ~r(p, n)/a(p, 2n) for aa°Ce. The dashed curves represent theoretical values obtained using the atomic mass data of Mattauch et al. ~7). The solid curves represent theoretical values based on the corrected threshold value. (See text).

The calculations were carried out at 1 MeV intervals above the thresholds of the (p, 2n) reactions. The parameter a was varied between 2 and 8 M e V - 1. The calculated results are compared with the experimental data in tigs. 4 and 5. The emission of charged particles was practically negligible, since it did not exceed 5 % of the total reaction cross section. For the 14°Ce target, fair agreement is obtained using a = 2

260

M. FURLIKAWA

M e V - i . On the contrary, the calculated cross sections for the ~42Ce(p, n) reaction agree with the experimental results for a = 8 MeV -1. Different values of a were necessary to fit the results of different targets. However, these calculations are very sensitive to the values of the binding energies of various particles. The experimental and calculated values for the ratio of cross sections for the t4°Ce(p, n)14°Pr and 14°Ce(p, 2n)139pr reactions are presented in fig. 6. The threshold for the latter reaction was estimated to be 12.7_ 0.2 MeV from the present experiments. This value is higher than that obtained from the atomic mass data by 0.8 MeV. If the corrected threshold value is used in the calculation, the use of a = 4 gives the best fit to the experimental data. A more precise mass measurement is necessary to obtain a definite conclusion as to the choice of the parameter a. Nevertheless, the present results may indicate the validity of the compound nuclear hypothesis in this mass region. The apparent discrepancy may be a consequence of shortcomings in the calculation procedures. The continued collaboration and helpful advice of Drs. S. Tanaka, H. Amano, S. Iwata and M. Yagi are greatly acknowledged. The author's thanks are due to Professor I. N o n a k a and the other members of the staff of Low Energy Division, Institute for Nuclear Study, University of Tokyo, for their interest and encouragement. He is also indebted to members o f the cyclotron crew for their help in the irradiation. The continued assistance o f T . Ando, A. lguchi and M. Chiba throughout the work is also acknowledged.

References 1) J. P. Blaser, F. Boehm, P. Marmier and D. C. Peaslee, Helv. Phys. Acta 24 (1951) 3 2) J. P. Blaser, F. Boehm, P. Marmier and P. Scherrer, Helv. Phys. Acta 24 (1951) 441 3) J. P. Blaser, F. Bochm, P. Marmier and P. Scherrer, Helv. Phys. Acta 24 (1951) 465 4) H. G. BIosser and T. H. Handley, Phys. Rev. 100 (1955) 1340 5) H. A. Howe, Phys. Rev. 109 (1958~ 2083 6) S. Tanaka and M. Furukawa, J. Phys. Soc. Japan 14 (1959) 1269 7) J. Wing and J. R. Huizenga, Phys. Rev. 128 (1962) 280 8) L. F. Hansen, R. C. Jopson, H. Mark and D. C. Swift, Nuclear Physics 30 (1962) 389 9) M. Furukawa et al., to be published 10) M. Furukawa, Nuclcar Physics 77 (1966) 565 1 I) S. Tanaka e t al., J. Phys. Soc. Japan 15 (1960) 545 12) R. M. Sternhcimer, Plays. Rev. 115 (1959) 137 13) M. S. Livingston and H. A. Bethe, Revs. Mod. Phys. 9 (1937) 245 14) M. Furukawa et al., J. Phys. Soc. Japan 15 (1960) 2167 15) Nuclear Data Sheets, NAS-NRC (1964) 16) E. I. Biryukov, O. I. Gregorev, B. S. Kuznetzov and N. S. Shimanskaya, lzv. Akad. Nauk SSSR (ser. fiz.) 24 (1960) 1135 17) J. H. E. Mattauch, W. Thiele and A. H. Wapstra, Nuclear Physics 67 (1965) 1 18) M. M. Shapiro, Phys. Rev. 90 (1953) 171 19) G. lgo, Phys. Rev. 115 (1959) 1665 20) A. G. W. Cameron, Can. J. Phys. 36 (1958) 1040