Mass distribution in the fission of 237Np with epicadmium neutrons

J, inorg,nucl.Chem..1969.Vol. 3I. pp. 3739to 3745. PergamonPress. Printedin GreatBritain

MASS

DISTRIBUTION IN THE FISSION WITH EPICADMIUM NEUTRONS*

OF

~37Np

R. STELLA, L. G. MORETTO, V. MAXIA, M. DI CASA, V. CRESPI and M. A. ROLLIER Laboratorio di" Radiochimica, Istituto di Chimica Generale, Universitb. di Pavia, 27100 Pavia, Italy

(First received 24 March 1969; in revised form 2 May 1969) Abstract-The fission yields of seventeen mass chains in the mass region 93-153 have been determined for the fission of 2ZTNp with epicadmium neutrons using radiochemical techniques. These yields have been determined by comparison with the corresponding yields in ~ U neutron induced fission using 14°Ba as internal standard. The mass yield distribution constructed with these data shows the two familiar broad peaks centered approximately at mass numbers 98 and 139. Such a distribution is compared with the corresponding distribution for 2~5U and 239Pu thermal neutron fission. INTRODUCTION

THE STUDY of the neutron induced fission of ~3rNp presents some interest because the properties of this nuclide are intermediate between the "fissile" nuclides like 23~U, ~3~U, 239pu and the "threshold-fissioning" nuclides like ~32Th, ~3gU etc. In fact 237Np present a slight fission component in the thermal region. The fission cross section for thermal neutrons is 0.019 barns to be compared with the capture cross section which is 170 b. The neutron induced fission cross section rises to a sizable fraction of the total reaction cross section at neutron energies above 0.3 Mev,[1] reaching a value of 1.4 barn which stays essentially constant from 1 to 6 MeV neutron energy [2]. Few data are available on the mass distribution of neutron induced fission of 237Np. The first measurements were reported by Ford and Gilmore[3] who studied the fission induced by degraded fission spectrum neutrons and later Coleman et al. [4] determined the mass yield curve for 14-5 MeV neutrons. More recently lyer et al. [2, 5] reported the fission yields of several nuclides for reactor neutron induced fission. While these authors used radiochemical procedures, Bennet and Stein[6] used solid state detectors to obtain the spectra of the fragment masses and energies in the fast neutron induced fission of 237Np. The ~3~Np fission yields so far available are often the results of few determinations. In this paper the fission yields data for 17 nuclides in the regions A = 93-99, A : 111-115 and A = 131-153 are reported, for ten of them for the first time. The mass distribution curve is also presented. *This work was supported by the Consiglio Nazionale delle Ricerche. 1. E. K. Hyde, The Nuclear Properties of the Heavy Elements, Vol. Ili, p. 52. Prentice-Hall, New York (1964). 2. R. S. lyer, H. C. Jain, M. N. Namboodiri, M. Rajagopalan Rajkishore, M. V. Ramaniah. C. L. Rao. N. Ravindran and H. D. Sharma, IAEA Symp. Physics and Chemistry of Fission, SM-60[30. Salzburg, Austria (1965). 3. G. P. Ford and J. S. Gilmore, LosAlamos Scientific Laboratory, Rep. No. LA-1997 (1956). 4. R. F. Coleman, B. E. Hawker and J. L. Perkin,J. inorg, nucl. Chem. 14, 8 (1960). 5. M. N. Namboodiri, N. Ravindran, M. Rajagopalan and M. V. Ramaniah, J. inorg, nucl. Chem. 30.2305 (1968). 6. M.J. Bennet and W. E. Stein, Phys. Rev. 156, 1277 (1967). 3739

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R. STELLA et al.

EXPERIMENTAL The experimental procedures have been previously reported in detail [ 7 - l 0]. Neptunium dioxide from O a k Ridge National Laboratory was used without preliminary purification. About l 0 m g of NpO2 and 0.6 mg of 93% enriched uranium in form of A1 alloy wrapped in a 2 m m thick cadmium container were irradiated for 3 hr in the central channel of the Triga Mark II L E N A reactor of the University of Pavia, where the thermal neutron flux is about 101an cm-2sec -1 and the c a d m i u m ratio for 19~Au is about 2. The 2 m m cadmium thickness was chosen to eliminate the thermal neutron capture and to minimize the neutron capture due to the 237Np resonance at 0.5 eV. After a cooling time ranging from 8 to 15 hr the sample was dissolved in 6 N HCI and accurately weighed portions of the solution were taken. One of these was used directly without any previous chemical separation to count the reference nuclide 14°La in equilibrium with ~4°Ba. The 1.6 M e V X4°Lay ray, which can be easily identified in a fission product spectrum obtained with a NaI(TI) crystal, was used. In the other remaining fractions all the investigated elements were separated after carrier addition, with the exception of Zr which was isolated cartier-free.

RADIOCHEMICAL

PROCEDURES

Zirconium was separated carrier-free on a D o w e x 2 × 8 column with 12 N HCI and 0.06 N HF[1 1] after the previous separation of Np, U and Pu on a D o w e x 1 × 8 column in 8 M HNO3, and the distillation of ruthenium[12]. The chemical yield of this separation has been evaluated from the 65 day activity of 95Zr in a weighed aliquot of the initial fission product solution after decay of the interfering activities. Molybdenum was separated using the Scadden [13] method by precipitation with a-benzoinoxime. The chemical yield was determined by weighing the final precipitate of PbMoO4. Silver was precipitated as AgCl from a nitric solution following the method described by Glendenin[14]. The chemical yield was calculated by weighing the AgCl precipitate. Cadmium was separated by repeated precipitations of CdS with H2S in diluted acid solutions according to the method of Glendenin [ 15]. The chemical yield was evaluated by weighing the final precipitate of CdNH4PO4. Iodine was separated by a number of redox and solvent extraction cycles following the method of Glendenin and Metcalf[16]. The chemical yield was determined by weighing the AgI precipitate. The rare earth group was separated by successive precipitations as fluorides and hydroxides and purification on a D o w e x A 1 × 8 column [ 17]. The rare earths 7. 8. 9. 10. I I. 12. 13. 14.

R. Stella, V. Crespi and V. Maxia, Ricerca Scient 37,347 (1967). R. Stella, M. Di Casa and V. Maxia, R icerca Scient 37, 354 (1967). R. Stella, M. Di Casa and V. Maxia, Ricerca Scient 37,357 (1967). R. Steila, V. Maxia and L. Moretto, Ricerca Scient 38, 1190 (1968). L. Wish,Analyt. Chem. 31,326 0959). L. Wish, and E. C. Ereiling, Rep. No. U S N R D L 464 (1960). E. M. Scadden, Nucleonics 15, 102 (1957). L. E. Glendenin, NNES-Radiochemical Studies: The Fission Products, Vol. 9, Book 3, p. 1580. McGraw-Hill, N e w York (1951). 15. L. E. Glendenin, NNES-Radiochemical Studies: The Fission Products, Vol. 9, Book 3, p. 1575. McGraw-Hill, N e w York (1951). 16. L. E. Glendenin and R. P. Metcalf, NNES-RadiochemicalStudies: The Fission Products, Vol. 9, Book 3, p. 1625. McGraw-Hill, N e w York (1951). 17. P. C. Stevenson and W. E. Nervik, USAEC Rep. No. N A S - N S 9020, p. 186 (1961).

Mass distribution in ~37Np neutron fission

3741

were separated from one another on a Dowex 50 W × 4 cation exchange column by elution at room temperature with 0.5 M a-hydroxyisobutyric acid at pH varying from 4.2 to 3.4118]. The chemical yields were determined by a spectrophotometric method using Cu-EDTA-PAN (1-2 pyridylazo-2 naphthol)[ 19]. Counting Procedures Except for silver, the activities were gamma counted using a 3 × 3 in NaI(T1) crystal connected to a 400 channel pulse-height analyzer. 11lAg was beta counted in an end window Geiger counter. The activities of the same fission product from 237Np and from 235U respectively were measured under the same geometry, and corrected for chemical yield and, when needed, for decay. The mAg activities were also corrected for difference in thickness. 97Zr, 99M0 and "5Cd were determined from the daughters activities after their growth into equilibrium with their parents. The 135I yield was evaluated from the activity of 135Xe daughter formed. Data Treatment To avoid absolute disintegration rate determinations we compared the relative disintegration rate for each nuclide with the corresponding value from 235U thermal neutron fission. The reported yields for 237Np are relative to the one of 14°BaJ4°La which was assumed to be equal to that for 235U. If one irradiates simultaneously samples of 237Np and "35U and counts the same couple of nuclides from both samples at the same time and under the same geometry, the ratio R = (Ax/As)Np

(.4~/As)u (where A x and A s are the activities of the two nuclides X and S) does not depend upon the irradiation time, the decay time, the half lives of the nuclides and the counting efficiencies. It follows that the yield (Yx)Np can be calculated from the equation: (Yx)Np -~ R (Ys)Np ( r s ) u " (Yx)u.

Having assumed (Ys)Np/(Ys)u = 1, we obtain: (Yx)Np = R ( Y x ) v .

The 235U fission yields were taken from Katcoff[20] assuming the 235U epicadmium fission yields to be equal to those of thermal fission even in the valley region. The results of the Los Alamos Radiochemistry Group [21,22] show that in the K. Wolsberg, Analyt. Chem. 34, 518 (1962). J. Inczedy, G. Nemeshegyi and L. Erdey.Acta chim. hung. 43, 1 (1965). S. Katcoff, Nucleonics 18, 201 (1960). G. A. Cowan. A. Turkevich, C. I. Browne and Los Alamos Radiochemistry Group, Phys. Rev. 122, 1286 (1961). 22. G. A. Cowan, B. P. Bayhurst and R. J. Prestwood, Phys. Rev. 130, 2380 (1963). 18. 19. 20. 21.

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R. STELLA et al.

Table 1. Relative fission yields in the epicadmium fission of 237Np

Mass chain

Nuclide measured

Half-life

93 95 97 99 111 115 131 132 133 135 140 141 143 144 147 149 151 153

93y 95Zr 9rZr 99Mo 11lAg 115Cd 1311 13'ZTe

10.00 hr 65.00 day 17.00 hr 66.00 hr 7.50 day 2.30 day 8.06 day 78.00 hr 20'80 hr 9.20 hr 40.00 hr 32.50 day 33.40 hr 284.00 day I 1.10 day 53.00 hr 28.40 hr 47.00 hr

133]

135Xe 14°La 141Ce 143Ce 144Ce 14TNd 149pm 151pm 1~3Sm

No. of determinations

Fission yield (%)

5 8 4 4 5 5 9 4 10 4

5.72 + 0.08 5.70 + 0.12 6.04 + 0.11 6.78 + 0.24 0.122 + 0.007 0.053 +0.003 3.47 + 0.28 6-72+0.22 6"42 + 0'58 5.35 + 0.13 6_35 (*) 6.67 + 0.11 5.54 + 0.12 5.07 + 0.17 3.21 + 0.15 1.88 + 0.14 1.07 + 0.05 0-44 + 0.07

5 5 3 5 5 5 5

*Assumed value.

c 0"51-

i 160

Moss number

Fig. 1. Mass distribution in the fission of 237Np with epicadmium neutrons, x, data point; o, mirror point.

Mass distribution in 237Np neutron fission

0"~ ~m

l

b. 0"01

005 x

]; p~mnt

0.01

v,ork ~Nomboodir i ._Bet, neff and Stein x Ford ard Gilrnore ~ i i

80

,~o

,~o

i

~o

i

,~o

Moss Fig. 2. M a s s d i s t r i b u t i o n in n e u t r o n induced fission o f 237Np f r o m d i f f e r e n t authors.

I0 5

I

~

0-5

! O.I 0-05

0.01

Fig. 3. Mass distribution in the fission of 237Np with epicadmium neutrons compared with the mass distributions of thermal neutron induced fission of 23sU and 2~pu [20].

3743

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R. STELLA et al.

valley region there are slight differences in fission yields from resonance to resonance; however the overall 235U epithermal fission can be considered as an average of many resonances weighed over the epithermal 1/E neutron spectrum. Therefore it is reasonable to expect that the slight differences in mass distribution from resonance to resonance may be washed out. RESULTS

The cumulative fission yields of 17 mass chains are given in Table 1. Also reported are the number of determinations and the standard deviations of the means. Table 2. Absolute fission yields in the epicadmium fission of ZarNp

Yields % Nuclide

Present work

aaBr saSr

Ford and Gilmore*

1"3 [1]

91Sr any

95Zr 97Zr 99Mo l°aRu

5"15---0'07 5"13-+0.10 5.44-+0.10 6.11-+0.21

[5] [8] [4] [4]

5.7 [1] 6.14 [1]

0.110-----0.006[5]

0'077 [1]

0.045---0"003 [5]

0'036 [1]

l°SRh

l°eRu lo9pd mAg H2Pd 113Ag HsCd ~21Sn

0.11 [1] 0.34 [1]

~25Sn

~2rSb 12~Fe 131| ~32Te laa| la~Xe l~Ba

3"06-+0"25 5'92+--0"19 5'66-+0"51 4.71-+0.11 5"60*

[9] [4] [10] [4]

Namboodiri

et

al.

0"265-+0"009 [2] 2"040-+0"080 [5] 4.040-+0.103 [2]

6.950±0.165 [2] 6.980-+0.004 [2] 4"040-+ 0"270 [2] 2"750-+0"164 [3] 1'560-+ 0'020 [2] 0"299-+0'003 [2] 0"085---+0"0012 [2] 0'072---0"003 [2] 0'045-+0'003 [2] 0'041-+0'001 [3] 0.047-+0.0013 [2] 0.126-+0.003 [2] 0.916___0.040[2] 2.600 [i]

5"1 [1]

6"330----_0"036[3]

5.0 [1]

5.30 internal standard

~41Ce 143Ce 144Ce 147Nd 149pm lslPm x~aSm I~Eu

5.88-+0.10 [5] 4"88-+0"11 [5] 4.47___0.15 [3] 2"83-+0"13 [5] 1"66---+0"12 [5] 0.94-+0.04 [5] 0'39-+0.04 [5]

4.970-+0.234 [2] 3.7 [1]

4.310-+0.158 [3] 2.350-+0.013 [3]

0.23 [1]

0.090--+0.0012 [2]

*These data are normalized by taking as yield of agMo that of thermal neutron induced fission of 235U.

3745

Mass distribution in 237Np neutron fission Table 3. Characteristics of mass yield curves of different neutron fissionable nuclides Most probable mass number Fissile nuclide Z32Th* zzau 2a~U ZZrNp 23sU 2agPu

Bombarding neutrons

Light

Heavy

Fission spectrum Slow neutrons Slow neutrons EpiCd neutrons Fission spectrum Slow neutrons

92 94 95 98 98 99

139 138 139 139 139 138

Mass Ratio width of most at half probable height masses 14 14 15 16 16 16

1.51 1.47 1.46 1.42 1.42 1.40

Ratio of peak to trough yields 115 450 650 136 200 150

*E. K. Hyde, The Nuclear Properties of the Heavy Elements, Vol. I11, p. 111. Prentice-Hall, New York (1964).

DISCUSSION

A detailed mass yield curve has been plotted using the experimental yield values and the mirror points calculated assuming that a constant number of neutrons, equal to 2.8 are emitted [23]. The relative yields reported in Table 1 were converted to absolute values by normalizing the area under the fission yield curve to 200 per cent. Using the absolute values the mass yield curve shown in Fig. 1 was obtained. It has the two familiar peaks at mass numbers 98 and 139. This 41 mass number interval and the peak to valley ratio, that is found to be about 136, are in good agreement with the Iyer's results [3]. In Fig. 2 the radiochemical results of Namboodiri [5], Ford and Gilmore[3] and the recent physical measurements of Bennet and Stein[6] are plotted to allow a comparison with our results reported as points. In Fig. 3 a comparison is made of the mass distribution we obtained for 2aTNp with the corresponding distribution for zasu and 2aspu thermal neutron fission. It can be noticed that the heavy peak of the 2ZTNp mass distribution essentially coincides with those of 235U and 2agPu while the light peak has an intermediate position between those of zasU and zagPu. This is in agreement with all previous observations in nuclei undergoing asymmetric fission, namely that the heavy peaks tend to be fixed while the light complementary peaks shift their position accordingly. In Table 3 the main features of the mass distribution of zaTNp are compared with those of other neutron fissionable nuclides. 23. E. K. Hyde, The Nuclear Properties of the Heavy Elements, Vol. I 11, p. 215. Prentice-Hall, New York (1964).