Independent yields of 150Pm in the thermal neutron-induced fission of 233U, 235U, 239Pu and 249Cf and cross-section for the 149Pm(n, γ)150Pm reaction

J. inorg, nucL Chem.. 1976,Vol. 38, pp. 205-210. PergamonPress. Printedin Great Britain

INDEPENDENT YIELDS OF ~5°Pm IN THE THERMAL NEUTRON-INDUCED FISSION OF 2 3 3 U , 2 3 5 U , 239ptl AND 249Cf AND CROSS-SECTION FOR THE ~49pm(n,y)'°Pm REACTIONS H. GAGGELER,* H. R. VON GUNTEN*+ and H. S. PRUYSt *Anorganisch chemisches Institut, Universitht Bern, CH-3000 Bern, Switzerland and tEidg. Institut for Reaktorforschung, CH-5303 Wiirenlingen, Switzerland

(First received 3 February 1975:in revised[orm 27 May 1975) Abstract--Independent yields for the shielded nuclide ~5°Pmwere determined in the thermal neutron-induced fission of 233U, -~35U, 239Pu and 249Cf using radiochemical techniques. They were found to be (2.6-+0.2)× 10"%, (3 -+ 1) × 10-~%, (1.15 -+0.07) × 10 3% and (1.9-+0.2) × 10 2%, respectively; these values correspond to empirical Zp values of 58.66, 58.31, 58.79 and 59.18, respectively, if a Gaussian charge dispersion function with atr of 0.56 is used. A comparison of the Zp values for '~°Pm with Zp values for other nuclides in the thermal neutron-induced fission of several actinide isotopes is given. The cross-section for '49Pro(n,y)'S°Pm in a swimming pool reactor neutron spectrum was found to be (1630+ 160) barn. (E.I.R.). The isotopic compositions were measured in these materials and are shown in Table 1. An isotopically clean recoil source of ~'gCf(about 10/~g)from Oak Ridge National Laboratory was used at the Mainz (Germany) Institute for Nuclear Chemistry. (b) Irradiation. In a preliminary set of experiments solutions of uranium and plutonium (10-30/zg) were evaporated and wrapped in aluminium foils. The samples were irradiated for 6.0 hr in the swimming pool reactor SAPHIR at a thermal neutron flux of approx. 3-5 × 10'3 cm 2 sec '. For subsequent experiments 1-10 mg samples of uranium and plutonium oxides (sealed in aluminium or quartz tubes) were irradiated under similar conditions as above but only for 5-10 min. The irradiations for the 2"~Cf experiments were performed in the TRIGA Mark II reactor at the Institute for Nuclear Chemistry of the Mainz University (Germany). The irradiation time was 6.0 hr and the thermal neutron flux 6 × 101~cm-2 sec'. The fission fragments recoiling from the 249Cf-targetwere collected in a layer of ammonium nitrate. The target was protected against selftransfer of 249Cf by a 10 p,g layer of aluminium. (c) Chemical separations. The irradiated aluminium foils with the fissionable material were dissolved in 6 N HCI containing one drop of HF. The irradiated oxides of uranium and plutonium (l-10mg samples) were dissolved in 6N HNO3. Plutonium was extracted from 6 N FINO3 using TBP-heptane mixtures (1 : 1). The NH,NO~-catcher material from the 249Cf-experimentswas dissolved in H20. The carrierfree separation of promethium was started 1-2 hr after the end of the irradiation, thus leaving time for the decay of '~'Nd into '~'Pm. A first separation of the bulk rare earths elements from the majority of the fission products was performed on a Dowex-I ×8 column with 10% 6N HNO3-90% methanol. This step was followed by two reversed-phase chromatographic separations of the rare earths on diatomaceous columns containing di-(2-ethyl hexyl) orthophosphoric acid (HDEHP). The rare earths elements (La to Nd) were eluted with 0.20 N HNO,. Pm was then recovered with 0.25 N HNO317-10]. The activity of the effluent from the columns was monitored to aid in locating the

INTRODUCTION

THE RECENT availability of relatively fast separation procedures for the rare earth elements and the high resolution of modern GeLi-detectors makes these elements accessible for determinations of independent fission yields. The rare earth region is of special interest in the investigation of charge distribution in very asymmetric fission. Furthermore, the medium weight and heavy rare earth elements are sufficiently distant from any closed nucleon shell and should, therefore show an unaffected distribution of nuclear charge. The shielded nuclide 15°Pm is very suitable for such measurements due to a half-life of 2.7 hr and the well known y-ray spectrum. Its independent fission yield has already been determined indirectly in the thermal neutron-induced fission of 235U by a mass-spectrometric measurement of '~°Sm [1] and in the spontaneous fission of 252Cf by radiometric techniques [2]. Other isotopes of Pm (146, 148, 148m)[3-5] and ~6°Tb[6] have been determined in the asymmetric fission of 233U and 235U. However, large corrections were needed in order to compensate for second-chance reactions. The aim of this work was a direct redetermination of the value for the independent yield of ~5°Pmin the thermal neutron-induced fission of 235U and a determination of this shielded nticlide in other fissionable systems, EXPERIMENTAL

(a) Fissionable materials. Samples of 233U and 23~U were obtained from Oak Ridge National Laboratory. The ~39Pu was prepared by the plutonium group of the Swiss reactor institute ~:Part of Ph.D. Thesis, University of Berne, Switzerland (1973).

Table 1. Isotopic composition of 233U,23~Uand 239Puused for the determination of the independent fission yields of ~5°Pm(atom %)

233 U

99.91% o.o25 % 0.063 %

235 U

239pu

233 U

0.061%

234U

92.087 %

234 U

99.613 %

235U

7.351%

238 U

0.065 %

236U

0.563 %

0.258 % 238U 205 JINC Vo[ 38, No. 2--B

239pu 240pu 2~Ipu

H. G~,GGELERet al.

206

promethium fraction. Lanthanum carrier (20 mg) was added to this fraction, the promethium was co-precipitated with La~(C20,)3 and counted. The chemical yield was about 50%. (d) Counting• The promethium fractions from the uranium and plutonium experiments were counted on a 50cm3 coaxial Ge(Li)-gamma spectrometer• The detector was calibrated with a set of standard sources having similar geometry and thickness as the Pm sources. In the 249Cfexperiments a 44 cm3 GeLi-detector was used• The detector had been calibrated by the Nuclear Chemistry Group of the University of Mainz. Three photopeaks at 1165, 1325 and 1736keV with relative intensities of (23.3±0.9), (25.7-+1.0) and (10.2-0.5)%, respectively [11], were taken to compute the activity of the "°Pm. These relative intensities were converted to absolute values with the aid of the 333.9 keV line of "°Pro which has a relative intensity of 100%[11] and an absolute intensity of 71% [12]. The activity of the lS°Pm was compared to the activity of the 340 keV photopeak of "~Pm which has an intensity of 21%[12]. The measurements were followed during three successive periods of 10~ sec in order to check the decay of 1'°Pro (2-7 hr) and '5'Pro (28 hr)[13]. In one of the californium experiments a single measurement of 5 × 10" sec was used instead. The areas of the photopeaks of interest were computed (linear background subtraction) and converted to saturation activities. The weighted mean of the saturation activities of the photopeaks of l~°Pm was used to calculate the independent yield relative to the saturation activity of ~5'Pm, which has a well established yield[M]. In the ~'~Cf experiments the yield for mass chain 151 was obtained by averaging values from a determination of Kurchatov et al.[15] and our own measurement[16]. (e) Cross-section [or "Pm(n, y)"°Pm. From the irradiations of 235U during 10min and 6hr the cross-section for the l'9pm(n,y)lb°Pm reaction in the neutron spectrum of the swimming pool reactor SAPHIR can be evaluated. '3°Pro is formed directly by the fission process and by a second chance reaction according to the scheme shown in Fig. 1. Only the bold lines in the figure are considered for the calculation. The decay of "~Pr is neglected, since its half-life is relatively short. The burn-out of ~'gPr and 149Ndis also neglected, since no reaction cross-sections are known, Bateman-Rubinson type calculations[17] were performed as follows: N~5o = A,A~A3 , , , NI° '[ 1 • A2-A, t

1

1 A3-A, 1

1 A4-A~

e_A:

e A:

A~-A: A3-A2 A,-A2 - -1

1

1

. e-A3 =

A~-A3 A2-A3 A4-A3 1

1

1

.e

~,']

4A,-A~ A2-A, A , - ~ where: N~5o= Number of atoms of l~°Pm(obtained from the total activity of ~°Pm corrected by 6% for formation by direct fission reaction. Nfl = Number of atoms of ~35Uobtained from activity of *~Pm and known fission yield[14]. A and A* = Transfer constants with the following meaning and values: A~ = ~b. ~+ ~ 3 x 10-s A* = 4'. or1= 6. ~r~. Y,~ ~ 3"3 x 10-1° A2 = A* = 3.2 A3 = A3+
149Sm

~

~Stoble

149p m

15OSm

k Stable n,y

b49Nd

,A'-

L~ 15°pro

n,y" ~

v

n'Y

v

15°Nd Stable

149pr

~n,f

235Un, f

Fig. 1. Formation of ~5°Pmby second chance reaction. Only the bold lines are considered in the calculations. 0"3= o,., of 149Pm~ 1.7 x 10-2t cm-2[13] A4= h'~0pm= Decay constant of 15°pm= 7.1 × 10-5 sec -1 [13]. In order to perform the calculation a value of 1700 b [13] was first assumed for the estimation of A3. ~r3 was then obtained from A* applying an iterative processing. RESULTS AND DISCUSSION A typical 3,-ray spectrum of the promethium fraction obtained from a 6 hr irradiation of 10 #g of 239pu is shown in Fig. 2. All gamma lines in this part of the spectrum can be attributed to ~5°Pm with the exception of the 1461 keV-line of 4°K and an unidentified weak line at 1524 keV. The photopeaks at 1165,1325 and 1736 keV were used for the determination of the independent yields. They decayed very closely with the half-life of ~°Pm. The results of a first set of data (6 hr irradiations) for ~5°Pm and l~lpm in the thermal neutron fission of 233U, 235U, 239pu and 249Cf are given in Table 2. The saturation activities for ~°Pm have to be corrected for the contribution by the 149pm(n, 3,)iS°Pro reaction (see Fig. 1). The correction was especially large for ~35U. However, these determinations for 235U were used together with values from shorter irradiations (Table 3) to evaluate the cross-section for the second chance reaction (see Experimental e, Table 4). The correction of the measured values was performed using the experimentally determined cross-section of (1630-+ 160) barn. No correction was necessary for the values of 2'9Cf, since the contribution by second chance reactions was negligible in the neutron flux of 6 × 10" cm-' sec -~. Our preliminary results for the independent fission yields for ~5°Pm in the fission of 2~3U, 235U and 239pu were already published[18] and appeared also in the recent compilations of Wolfsberg[19] and Meek and Rider[M]. These yields were not corrected for contributions by the second order reaction and deviate from the values given here. In a second set of experiments for 233U, 235U and 239pu much shorter irradiation times of 5-10min, but larger amounts of fissionable material ( - 1 - 1 0 mg) were used, thus reducing the contribution to the production of ~5°Pm by second chance reactions to <1%. The results of these determinations are shown in Table 3. The values for the independent yields of 233U and 23+pu agree very well with the yields in Table 2. This indicates that our value of tr,.~

Independent yields of '~"Pm

207

t-

E

E

G.

in t03

_

i0 z

i

0

t)

I:". ' = ~ i T %:'%-::~i,.;.

" !

~

T :'

~

~

v -~

~ tO

b

-~

m m r,D tO ?.....

-

' I¢} :'

': :~

L::

IO

t

I

1

I

I

z

1200

1300

1400

1500

1600

1700

Energy , keV

Fig. 2. Gamma-ray spectrum of the prometium fraction from the fission of ~10/zg 239Pu. Neutron flux: 4x 10~3cm 2 sec-,; Irradiation: 6.0 hr; Cooling time: 4.25 hr; Counting: 104 sec. The photopeaks at 1165, 1325 and 1736 keV are used in the determinationof the yield of '5°Pro. Table 2. Results of 6hr irradiations for the determination of the independent yields of '~°Pm in the thermal neutron-induced fission of 23~U,23~U,23~Puand 2"~Cf Fissionable

Saturation activity of

nuclide

total(f+n,Y)a) [

Saturation activity

151pm

150pm, dps

fisslonb)

235u

dps x 105

239pu

249Cf

d)

99 + 20 101 ± 20

1.4

615 ~ 32

270 + 50

3.45 + 0.26

277 Z 15

17 Z ~

2.30 + 0.17

8 i

d)

2194 + 93

132 +- 40

2402 + 96

1570 +_ 300

2

+ 0,1

1.11 + 0 . 0 8 17.1

1550 ~ 300

12.0 + 0.9

5360 ~I000

27.7 +_ 2.0

730 +_ 49 1268 +_ 71

2.3 +_ 0.5 2.5 +_ 0.5 (0.3 + 0.1)

0 . 4 2 + 0.01

(0.3_+ 0.I) (0.3 + 0.1) 11.1 + 2.2

11.0 + 0.8

7476 +_291 730 ~ 49

0.32 +_ 0.01

+ 1.3

2466 +_101

1268 L 71

150pm x 10 -4 %

2.9 _+ 0.6

1.o9 + 0.08

208 ± 12

138 Z 9

% (14)

241 d 14

d)i

Independent yield of

151

e)

233 U

Chain yield

0.78 _+ 0.02

10.1 + 2.0 15.1 + 3.o

0.661+_ 0.037

193

+ 28

190

+ 24

1.75 +.h 0.2e) 1.17 _+ 0.02

Mean value of T-linesat 1165, 1325 and 1736 keV. Total activity from fission- and (n, 7)-second chance reaction. bValue from ~ corrected for contribution by (n, 3,)-second chance reaction with o-,..,,,gpmof (1630 - 160) barn. c 340 keV ~-line. ~Values used for the determination of ~r,.: (see text). "Mean value from [15] and [16]. used to perform the correction of the saturation activities ought to be correct and that the simplifications applied in the Bateman-Rubinson calculations were justified. The values for 2~5U should not be compared in Tables 2 and 3, since they were used to estimate ~r..,. The results of the determination of the cross-section for the 149pro(n, "y)l~°Pm reaction are given in Table 4. Our value for o-,.~ of (1630-+ 160) barn is in good agreement with the value of 1700--- 300 barn measured by Kondurov et al.[20], and agrees reasonably with a measurement of Mowatt and Walker[21] and a recommended value of

1400 barn[22]. The slightly lower value of [21] could be due to a better thermalised neutron spectrum. The errors in the saturation activities in Tables 2 and 3 include the errors from the measurements (lcr), decay schemes [11, 12], and estimated errors of about 5% for the detector calibration, but no errors for the half-lives of '~°Pm and '5'Pm [13]. The errors for the independent yields also include the uncertainties for the chain yields [14-16]. The precision of the multi-determinations of the independent yields of '~Pm was well within the error limits, with the exception of the short irradiations of 23~U.

H. GAGGELERet al.

208

Table 3. Independent yields of t~°Pm in the thermal neutron-induced fission of ~33U, ~3~U and 23~pu. Short irradiations Fissionable

Irradiation time

nuclide

Saturation activity 150pm

Saturation activity 151pm

ape a)

dps x 105

min

233 U

6183 ± 324

65 1

10

3934 ! 294

55.7 i

I0

930 I 323

Independent

151 %

yield 150pm

(14)

b)

10

Chain yield

5 0.32 + 0.Ol

235 U

I07 ± 10

10

672 ! 426

5

54086 12782

372 + 36

5

40664 12118

286 I 28

3.0 + 0.3 2.3 ± O.3

5.5

O.tt2 + 0 . o l 239pu

x 10 -4 %

136 I 12

0 . 3 7 t Q.13 0.21+_ 0.13 11.3 + 1.3

0.78 i 0.02 11.1 _+ 1.5

a) Mean value of y-lines at 1165, 1325 and 1736 keY b) 340 keV y-line.

Table 4. Determinationof the cross section for 149pm(n,7)'~°Pm. Irradiation of 99.6% 235U, 6 hr with thermal neutrons Neutron

Total

Saturation

Cross-section

flux cm-2see-1

saturation

activity of 151pm from

149pm(n,y)150pm

for

barn

barn

x 1013

activity of 150pm from Table 2

o

Mean value

Table 2 dps x 105

dpe 3.5

277

2.3

175~

3.5

138

1.1

1819

2194

17.1

1317

5

a) Literature:

1700 _ 300 barn 1000 + 400 barn

1630 + 160 a)

(2o) (21)

recommended, 1400 barn

We believe that systematic errors are small, since no errors due to the neutron flux, the amount of fissionable material, fission cross-sections and chemical yields were introduced in our comparison method. Weighted mean values were calculated for the independent yields of ~5°Pmin Tables 2 and 3. The results of this computation are shown in Table 5. Included in Table 5 are also fractional independent yields, using known chain yields. The indirect measurement made by Chu [1] in the thermal neutron fission of 235U and the measurement of '5°Pm in the spontaneous fission of 252Cf[2] are also shown. Only the two results for 235U can be compared. The indirect value of Chu[1] obtained by a massspectroscopic measurement of "°Sm is about 40 times higher than our determination. '5°Sm can also be formed by (n, 7)-reaction from '495m (Fig. 1) with a cross-section of 41,000 barn[13] and by a second chance reaction from '49pm. If corrections for these contributions are applied one arrives at a value which is comparable to ours. An accurate correction cannot be performed, since not enough information is given in Chu's[1] work. The experimental results for the independent yields of tS°Pm in the fission of 233U, 235U and 239pu (Table 5) are much smaller than the values predicted by Crouch [26] but are within the estimated range of uncertainty of Wolfsberg[19]. The value for 235U is, as expected, much smaller than the value for 233U. No calculated values are available in the fission of the californium isotopes. It is

(22).

rather surprising that the fractional independent yields are closely the same for 249Cf and 252Cf. One would expect a lower value for 252Cf. Since the measurement in the fission of 252Cf was performed with less sophisticated equipment [2] it could be in error and should possibly be repeated. As pointed out, the yields of '~°Pm should not be influenced by shell effects, since this nuclide is sufficiently away from any closed nucleon shell. Zp-values (most probable charges) corresponding to the fractional independent yields were taken from the work of Wolfsberg[19], who assumes a Gaussian charge distribution with a ~rof 0"56-+ 0"06[24]. In the calculation of Zuco (unchanged charge distribution) for ~33U, 235U and 239pu the number of neutrons vA evaporated from the fission fragments was also taken from the estimation of Wolfsberg[19]. A vA-value of 2.9--0.6 was estimated for 'S°Pm in the fission of 249Cf. This value was obtained from vA-values of 239pu[19] and 252Cf[25] using ~-values from the work of Unik et a1.[27]. The AZ-values in Table 5 deduced from Zp-ZueD are, with the exception of 252Cf, larger than the generally accepted mean value of =0.5 charge units [2, 19, 24, 28]. This indicates that the deviation of the heavy fragment from ZucD is larger in very asymmetric fission. From the work of Roberts et al.[3], Biichmann[4], Umezawa[5,29] and Fahland et al.[6] AZ-values for '*6Pm, '48Pro and l~l'b can be calculated. The results of these computations are shown in Table 6 and are

Independentyieldsof '5°Pm

209

Table 5. Independentyields for '5°Pmand correspondingZp values Fissionable[Reactioni nuclide

Fractional independent

Independent yield

of 150pm (1)

a)

Zp

vA

e)

yield

ZUC D

AZ

i)

k)

Author

233U

nth,f

(2.6~0.2)'10 -4

(5.2~.4)'I0 -4b)

58.66~0.20 1.40~0.14£

59.52 -0.86~0.21

this work

235U

nth,f

(3LI)'10 -5

nth,f

(4.6il.4)'10 -5b) 58.31Z0.2~ 1.73~0.17£ (2.1Z0.1)'10-3b) 58.90 1.73 f)

59.15 -0,84!0,25 59.15 -0.25

this work Ref. (1)

239pu

nth,f

(1.15L0.07)-10 -3 (I.16Z0.07).103~ 58.79Z0.16 2.0Z0.2 f)

59.53 -0.74Z0.~

this work

249Cf

nth,f

(1.9~0.2).I0 -2

(9.0!I.3)-I0 -3c) 59.18Z0.15 2.9+0.6 g)

59.94 -0.76~0.28

this work

252Cf

spontaneous fission

(2.0~0.9).10 -2

(8.3Z3.7).10 -3d) 59.16Z0.16~2.5~0.3 h)

59.31 -0.15!0.20

Ref. (2)

a) Weighted mean from Tables 2 and 3, values for 249Cf only from Tihle 2, for 235U only from Table 3. b) Yield for chain 150 from Meek and Rider (14), c) Yield for chain 150 (2.1Z0.2)%, averaged mean values of chains 149 and 151 from (15) and (16). d) Yield for chain 150 2.4 % (23). e) Most probable charge in a Gaussian charge distribution with o=0.56~0.06 (19, 24). f) Number of neutrons evaporated per fragment ~A from Wolfsberg (19), errors estimated by us. g) Extrapolated VA-Value, see text. h) VA-Value from Nifenecker et al. (25), error estimated by us. i) ZUC D = (A+gA)'(ZF/AF). k) AZ = Zp-Zuc D.

Table 6. Comparison of fractional independent fission yields, Zp-, Zt~cD-,AZ- and ~'A-valuesfor the shielded nuclides "~Pm, '"Pro, "°Proand ~Tb in the thermalneutron-inducedfission of 2~3U,~3~Uand 239ptl Fissionable system

233 U

235 U

239pu

Shielded

Fractional independent

Zp

~A

ZUC D

AZ

yield

a)

b)

c)

d)

nuclide

Author

148pm

(9.3 ~ 0.6) • 10 -7

57.83

1.5

58.78

-0.95

Umezawa

150pm

(5.2 g 0.4) " 10 -4

58.66

1.4

59.52

-0.86

this work

146pm 148pm

(4.2 ~ 1.2) " 10 -7

57.74

1.4

57.46

+0.29

Roberts

58.56

1.6

58.52

+0.04

B~chmann (4)

148pm

(5 ~ 3) ' 10 -8

57.52

1.6

58.32

-0.80

Umezawa

150pm

(4.6 ~ 1.4) • 10 -5

58.51

1.7

59.15

-0.84

this work

160Tb

<(7.6 ~ 5.1) • I0 -5e)

<62.38

1.4 f)

62.92

< -0.54

150pm

(1.16 ! 0.07) • 10 -3

58.79

2.0

59.55

-0.74

6.7 • 10 -5

Fahland

(5)

(3) (29)

(6)

this work

I, a) Most probable charge in a Oaussian charge distribution with o : 0.56 + 0.06 (19, 24). b) ~A from Wolfsberg (19). c) ZUC D = (A + ~A ) • (ZF/AF) d) A Z : Zp - ZUC D e) Using chain yield of 4 • IO4%from Meek and Rider (14). f) ~A-Value from Wahl (24).

compared to our AZ values for ~5°Pm. The AZ-values for ~"Pm of Umezawa[5, 29] deviate from 0.5 charge units as much as for '5°Pro. The value for J46pm[3] and the value for '48Pro of B~ichmann[4] deviate from this trend. The value for ~46pm is probably not very reliable since the irradiation and cooling history of the :35U sample was not well known. It is not clear why the measurements for '48Pm of B~ichmann and Umezawa differ by 3 orders of magnitude. For '6°Tb AZ is also smaller than -0.5 charge units, since Fahland et al. [6] consider their value for the independent yeild as upper limit. The general trend of large deviations of Zp from ZucD in asymmetric fission indicates that the heavy fragments could be enriched in neutrons. Another hypothetical explanation for the low yields in very asymmetric fission could be a narrower width of the Gaussian charge distribution. Indeed, this trend of decreasing width in charge distribution was found in the thesis of Fashing[30]. However, more measurements should be performed in very asymmetric fission for

heavy and also for very light fission fragments. In Fig. 3 a tentative plot is shown of AZ values for different heavy mass fission products vs Zi/A for several fissionable systems. With the exception of the values for '5°Pro (this work) and for '36Cs in the fission of nYTh[31], ngTh [32] and 245Cm[33], values for the fractional independent fission yields were taken from the compilation of Wolfsberg[19]. Zpovalues are also from this compilation (normal fractional yields with ~r = 0'56). UA-Values from the same work were used, where available. The ~'A-values for fissionable systems not included in this compilation were estimated. Errors for the v~-values were assumed to be between 10 and 25%, depending on the fissionable system under investigation. Due to even larger uncertainties in the v~-values we did not include light mass fission products in this plot. The AZ-values of all the investigated systems change slightly but in a similar way with Z2/A. Light fission fragments seem to behave in a similar way. However, the errors are very large as indicated for the case

H. G~GGELER et al.

210 i

i

i

i

229 Th 227Th 23fi U 233U 241pu239pu242A m245Cm

249Cf

o

la31 - - ~ - -

AI35xe o136Cs . . . . . .

oi4OLo. . . . . . . ,15Oprn-~ ,q -0.5

I

I

I

I

35

36

:57

38

I 39

z21,a Fig. 3. Plot of AZ values for '33I, '3~Xe, '36Cs, "°La and '~°Pm in different fissionable systems vs Z21A. Only those fission products were used, where independent yields for 2"Cf and at least two other fissionable nuclides are available in the literature. The fractional independent fission yields for '5°Pm are from this work, the yields for '33I, l"Xe, "6Cs and "°La are from the compilation of Wolfsberg [19]. Additional yields for 136Csin the fission of 227Th,22~I'hand 2"~Cm were taken from references [31-33], respectively. 12. C. M. Lederer, J. M. Hollander and I. Perlman, Table of Isotopes, 6th Edn, Wiley, New York (1967). 13. Nuklidkarte, 3rd Edn, Bundesministerium fiir wissenschaftliche Forschung, Bonn (1968). 14. M. E. Meek and B, F. Rider, NEDO-12154-1 (1974). 15. B. V. Kurchatov, L. N. Morozov, V. I. Novgorodtseva, V. A. Pchelin, L. V. Christyakov and V. M. Shubko, Soy. J. Nucl. Phys. 14, 528 (1972). 16. H. Giiggeler and H. R. yon Gunten, to be published. 17. W. Rubinson, J. Chem. Phys. 17, 542 (1949). 18. H. G~iggeler and H. R. von Gunten, Abstract Proc. Syrup. Physics and Chemistry of Fission 1973, Vol. II, p. 475. IAEA, Acknowledgements--The authors are grateful to Prof. G. HerrVienna (1974). mann and Dr. N. Trautmann (Mainz) for making possible the 19. K. Wolfsberg, LA-5553-MS (1974). experiments with 2"Cf. The cooperation of the SAPHIR group 20. I. A. Kondurov, L. M. Gracheva, A. I. Egorov, D. M. and the isotope production group of E.I.R. is highly appreciated. Kaminker, A. M. Nikitin and Yu. V. Petrov, Y. Nucl. Energy We would like to thank Mr. D. Iselin for performing the isotopic 20, 814 (1966). analyses and Dr. K. Bischoff for providing the 239pu samples. We 21. R. S. Mowatt and W. H. Walker, NBS Spec. Pub. 299, 1291 acknowledge Drs. P. Baertschi, A. Grfitter, M. Rajagopalan, A. C. (1%8). Wahl and K. Wolfsberg for interesting discussions, Miss E. R6ssler 22. W. H. Walker, AECL-3037, Part 1 (1969). and Messrs A. Schmid, and E. Hermes for their technical 23. W. E. Nervik, Phys. Rev. 119, 1685 (1%0). assistance. The Swiss National Science Foundation has supported 24. A. C. Wahl, A. E. Norris, R. A. Rouse and J, C. Williams, part of this work. Proc. 2nd Syrup. on Physics and Chemistry of Fission, p. 813. REFERENCES IAEA, Vienna (1%9). I. Y. Y. Chu, Ph.D. Thesis, University of California, UCRL- 25. H. Nifenecker, C. Signarbieux, R. Babinet and J. Poitou, Proc. Syrup. on Physics and Chemistry of Fission 1973, p. 117. 8926 Berkeley (1959), unpublished. IAEA, Vienna (1974). 2. H. R. yon Gunten, K. F. Flynn and L. E. Glendenin, J. Inorg. 26. E. A. C. Crouch, AERE-R 6056 (1%9) and AERE-R 7680 Nucl. Chem. 31, 3357 (1969). (1974). 3. F. P. Roberts, E. J. Wheelwright and H. H. van Tuyl, J. Inorg. 27. J. P. Unik, J. E. Gindler, L. E. Glendenin, K. F. Flynn, A. Nucl. Chem. 25, 1298 (1%3). Gorski and R. K. Sjoblom, Proc. Syrup. on Physics and 4. K. B~ichmann, Radiochim. Acta 9, 27 (1%8). Chemistry of Fission 1973, p. 19. IAEA, Vienna (1974). 5. H. Umezawa, I. Inorg. NucL Chem. 35, 353 (1973); JAERI 28. K. Wolfsberg, Phys. Rev. 137, B 929 (1%5). 1103 (1%6). 6. J. Fahland, G. Lange and G. Herrmann, J. Inorg. Nucl. Chem. 29. H. Umezawa, private communication. 30. J. R. Fashing, Ph.D. Thesis, MIT, May 1970, unpublished. 32, 3149 (1970). 31. K. F. Flynn and H. R. yon Gunten, Proc. 2nd Syrup. on 7. J. W. Winchester, J. Chromatogr. 10, 502 (1%3). Physics and Chemistry of Fission, p. 731. IAEA, Vienna 8. E. P. Horwitz, C. A. A. Bloomquist and D. J. Henderson, J. (1969). Inorg. Nucl. Chem. 31, 1149 (1969). 9. E. P. Horwitz, C. A. A. Bloomquist, D, J. Henderson and D. 32. N. Ravindran, K. F. Flynn and L. E. Glendenin, J. Inorg. Nucl. Chem. 28, 921 (1966). E. Nelson, J. lnorg. Nucl. Chem. 31, 3255 (1%9). 10. E. P. Horwitz and C. A. A. Bloomquist, J. Inorg. Nucl. Chem. 33. H. R. yon Gunten, K. F. Flynn and L. E. Glendenin, Phys. Rev. 161, 1192 (1%7). 34, 3851 (1972). 11. J. Barrette, M. Barrette, S. Monaro, S. Santhanam and S. 34. R. M. Harbour, D. E. Troutner and K. W. MacMurdo, Phys. Rev. CI0, 769 (1974). Markiza, Can. J. Phys. 48, 1161 (1970).

of "°Pm. The errors of the other systems in Fig. 3 have about the same magnitude. Harbour et a1.[34] recently investigated independent yields of 13~Xe from several fission systems. They reached, in agreement with our work, the conclusion that Zp-values are more nearly equal to ZocD as the mass and charge of the fissioning system increases. Work is in progress in our laboratory with the aim of extending this investigation of independent fission yields to other nuclides, including the light mass peak.