Gamma rays of primary fission products from 235U(n, f) and 239Pu(n, f)

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2.--~--[

Nuclear Physics A206 (1973) 374--384; (~) North-Holland Publishing Co., Amsterdam

I

Not to be reproduced by photoprint or microfilm without written permission from the publisher

GAMMA RAYS OF PRIMARY FISSION PRODUCTS FROM Z3SU(n, f) AND 239pil(n~ f ) W. J. SCHINDLER and C. M. FLECK Atominstitut der Osterreichischen Hochschulen, Vienna Received 5 January 1973 (Revised 19 February 1973) Abstract: Gamma rays of primary fission products in thermal-neutron-induced fission of 23~U and 239Pu were investigated. Isotopic assignments of several lines were made by comparing fission yield ratios and relative y-ray intensities of the two fissioning nuclei. Differences between 235U and 252Cf fission product 7-ray lines are discussed.

E

NUCLEAR FISSION 235U, 239pu(n, f), E = thermal; measured E.:, I~,, (fission) y-delay; deduced relative fission yields (U+n)/(Pu+n).

1. Introduction Most important for a better understanding of some nuclear properties seem to be the studies of nuclei far from the stability line. The only simple way to produce a large excess of neutrons in nuclei is the fission process, and an important means for studying their properties is v-ray spectroscopy. A bulk of data of longer-lived fission products has been accumulated already by application of rapid chemical separation techniques and subsequent investigation of the radioactive decay. It is desirable also to study the de-excitation of primary fission fragments, a process usually taking place after fission within some nsec, or within even less time. Since experimental evidence for a new region of stable ground state deformation was established 1), interest has increased in these nuclei exceedingly rich in neutrons with mass number around 100. In the last few years, since good semiconductor detectors became available, many experiments with z s2cf(sf) sources have been carried out leading to proper assignments of fission v-rays to individual isotopes and to the construction of level schemes of these isotopes [see, e.g., refs. 2- 6)]. Rather few investigations of thermal-neutroninduced fission of 235U and 239pu have been performed 7-9), mainly due to the inherent experimental difficulties. Further measurements seem to be important in order to compare the results with those obtained from 252Cf(sf) and to extend the data to nucle~ with mass 85-95, produced with considerable mass yield especially by the 2 3 5 U a n d 239pu f i s s i o n r e a c t i o n s

(fig. 1). 374

23sU, 239pu{,n, f) FISSION PRODUCT 7-RAYS

375

In the present paper, measurements on ),-rays emitted less than 30 see after the fission of 23sU and 239pu are reported. Although mass and charge measurements were not possible due to the experimental restrictions (see sect. 2), we were able to Fission Yield 10%

/

U(n.f/ Pu(n,f) / / ~_. .~ f / Cf(s f)

"-,

/,'

,

,

I

I

i

t

I

80

I

120

I40

A

Fig. 1. Comparison of fission yield in Cf spontaneous fission (Cf(sf)) and the thermal-neutroninduced fission of 235U (U(n, f)) and 239pu (Pu(n, f)).

establish isotopic assignments to some v-ray lines by comparing the v-ray intensities of U and Pu fission. The differences between 235U and 252Cf fission product y-ray spectra are discussed.

2. Experiments For the measurement of the v-ray spectra, thick targets of 99 % enriched metallic 2a 5U and Pu(NO3)4 were used. The application of targets thin enough to allow fission fragment detection was impossible because o f the rather low neutron flux of the T R I G A Mk-2 Reactor in the Atominstitut l o). The spectra were observed in a Ge(Li) diode with 8 % efficiency relative to a 7.6 cm x 7.6 cm NaI(T1) detector. Coincidence measurements were performed with an N a I crystal as a second y-ray detector, leading to a gross correlation of individual lines and quanta of a certain energy interval. The spectra were recorded in a 8 x 1000 channel matrix. In an alternative run the N a I crystal was used as a detector for y-rays de-exciting the high-lying levels of fission fragments, usually having an energy between 1000 and 4000 keV. It served as a start detector for delayed-coincidence measurements, covering an angle of approximately 4W6. The interval of time-to-pulse-height conversion was 100 nsec. As can be seen from the mass- and charge-distribution lines for 235U and 239pu fission [see, e.g., ref. 11)], the "secondary fission products" have half-lives longer

376

W.J. SCHINDLER AND C. M. FLECK

than 30 sec. Nuclei are called "secondary fission products", provided that they are formed by fl-emission of primary fission products, and that their yield in the primary fission process is less than 1 ~o- They could easily be distinguished from the interesting g-ray lines of the primary fragments because of the long half-lives. After activating for a certain time the target was moved out of the beam and spectra of several time intervals were recorded. By this means, energies, intensities and half-lives of the longlived daughter nuclei were measured resulting in good agreement between the values obtained as those in refs. 12-1¢). All y-transitions observed in the prompt U and Pu spectrum, but not however in the delayed spectrum, are assumed to be due to primary fission fragment g-rays. Some peaks were present in the U or Pu spectrum only. Most of them could be assigned to 235U(n, g) [refs. 15-t7)] and 239pu(n, g) [ref. 17)], respectively. For three lines no ossignment to fission or capture could be made. 3. Results

In table 1 the energies (in keV) of the g-ray lines assigned to fission fragments are noted in the first column. Below 300 keV no spectra were measured because of the high natural activity of U and Pu, the daughter nuclei of which emit a large number of low-energy g-rays. An L in column 1 indicates that a secondary fission fragment with an intense transition at the same energy exists. Column 2 gives the results of refs. 7-9) concerning thermal-neutron-induced fission of 235U. At present only one work about the g-rays from 239pu(n, f) has been published, and it presents no table, but only a drawing of the spectra. An x in column 3 indicates agreement after comparison with our spectra. Obviously this agreement can only be a tentative one. The 2s2Cf(sf) data and the Z- and A-assignments established in the references are listed in columns 4 and 5 respectively. The element symbol is written between Z and A if the assignment se eros to be definitely fixed. The half-life in column 6 was taken from ref. 6) for values up to 614 keV and over 1000 keV. Our values are recorded from 614 keV to 900 keV. The symbol p indicates a " p r o m p t " transition that is less than 10 nsec in the interval covered by our measurements, and less than 1 nsec, if the value was taken from refs. 4, s, s) (below 600 keV). Half-life values in brackets indicate that one author found a prompt transition while another author found a delayed one. The last column gives references. Table 1 shows that quite a lot of the y-ray lines have been assigned to individual isotopes, but there are also many transitions for which only the mass number of the emitting nucleus is known. We tried to find the Z-assignments for these lines with the help of the isotopic yields of the fragments and the measured intensities of the g-ray lines in 235U and 239pu fissions. Fission g-ray spectra are very complex. Therefore, it is obvious that only the strongest transitions in the de-excitation of the fragments can be observed. These transitions

Z

80

130

I"

,

,

I 90

140

•

,

"

0.5

A

t3

150

1 A

heavy fragment'~

I I00

ligh

Fig.2. Conlour diagram of isotopic fission yield in neutron induced ===U fission

Fig. 2. Contour diagram of isotopic fission yield in neutron-induced fission of 235U. The dots represent the position of isotopes and need not lie on the contour lines. The straight line is the smoothed line of most probable mass of the fission product elements.

50

55

60

33

35

40

43

Z

-

i

130

80

° "~

= ////~/I

J"

., :

~

140

.;

90

I

~ 0 : °

Contour diagram of the ratio of isotopic yield

I

°

"

,

I

150

.0.

A

A

ment

.025

ilight frog 100

.

o %~-..-~ ~25

Fig. 3. Contour diagram of the ratio of isotopic yield, for the two fissioning systems 235U and 239pu ' f y (U)/fy (PH). Circles around points represent isotopes for which ~-rays are known.

50

55

33

35

43

,.<

C

0

O Z

~-

XXX

X

XX

XX

X

~

-

~

~

~-

(D

5

r~

> Z

[; [-

235U, 239pu(n, f) FISSION PRODUCT y-RAYS

379

TABLE 1 (continued) 235U, 239ptl

235U

239pu

2s2Cf

Z

A

T

961 965

969.2 L 979.5 987.2 989.7 1036 1045 1089.9 1103.6 1118.9

x

974

131

s, 9)

974

52

T1

132

5.7)

990

52

Te

132

~)

9)

x

9) 9)

x x

so)

1124

1138 1152.7 1178 1182 1218.7 1223.1 1238 1278.9 1303.7 1307.7 1312.7 1322.2 1427

Ref.

9)

x 1151.6

134

100

1181

135

600

1178 1217

1278

1279

1313.3

6)

7) 6)

7)

5)

40

Zr

98

32

Te

134

(160)

s-7)

137

3000

6)

usually lead to the ground state or at least tlaey either come from or go to rather lowlying states, below 2000 keV. Consequently, the intensities of the lines are independent of the energy level in which the fragment is produced, and which might be considered to be different in U and Pu fission. For this reason the ratio of the isotopic yield in U and Pu fission is equal to the ratio of the absolute intensities in the U and Pu spectra, and proportional to the relative intensities, f y (U)/fy (Pu) = k . r~y (U)/r~y (Pu). The isotopic yields were taken from the excellent work by Reisdorf et al. 11). The only drawback in this work was the fact that not the direct values, but omy the most probable mass for each charge and the width of the distribution, assumed to be Gaussian, are reported. Therefore, the yields for individual isotopes had to be recomputed. The result for U is shown in the contour diagram of fig. 2. Points represent the position of isotopes and do not necessarily lie on the contour lines. The straight line is Ap = bZp + c, with Ap and Zp representing the most probable mass and charge, respectively, and where b and c are parameters. In fact this expression is not correct but is only an approximation [see ref. 11)], since the relation between Ap and Zp is exactly represented not by a straight, but by a slightly S-shaped line. For this reason the contour lines do not appear symmetric to the straight line. A similar diagram exists for Pu, in which the maximum is shifted to a higher A-value, the parameters b and c being different

W. J. SCHINDLER AND C. M. FLECK

380

TABLE 2 Comparison of fission yield and relative y-ray yield ratios E(keV) 296.5 325.5 351.6 368.8 375.8 381 393 431.1 446.5 456.5 482 551 554.7 560.1 570.5 580.3 584.6 605 613.9 617.5 625.3 631 634 655.4 682.4 707.1 729.2 734.1 762.4 775.4 781.5 787 799.9 809 826.9 837.8 880 961 969.2 979.5 987.2 989.7 1036 1045 1089.9 1103.6

Z

A

32 40 40 38 42 39

Te Zr Zr Sr Mo Y

54 55 56

Xe Cs Ba

54 54

fy(u) fy(Pu)

134

1.2

1.9

Xe Xe

102 100 96 104 96 132 136 139 144 147 140 138

42 56

Mo Ba

104 142

0.23 0.7

54 42 54

Xe

139 104 138

1.3 0.23 1.0

Xe

1

0.26 0.23 0.82 0.43 0.5 2.0 1.9 1.0

100 40 56

Zr Ba

36

100 142

1.05 0.7

91

4.5

95

kryY(U) rTy (Pu)

0.6 4-0.12 1.1 5:0.2 0.6 i 0 . 1 2

0.555:0.1 2.2 -4-0.5 0.7 :~0.25 2.0 5:0.4 0.5 5:0.1 1.4 5:0.4 0.265:0.05 0.5 5:0.1 0.8 5:0.15 0.954-0.11 1.0 ±0.15 1.4 5:0.4 0.855:0.13 0.7 5:0.25 0.954-0.2 0.95 +0.2 0.5 4-0.3 0.855:0.17 1.0 4-0.25 1.5 4-0.3 1.0 5:0.3 0.85dz0.25 0.8 4-0.2 2.0 5:0.6 0.9 4-0.25 0.534-0.3 1.3 4-0.4 0.8 4-0.2 0.7 5:0.2 1.354-0.25 0.654-0.14 0.954-0.3

52

Te

132

0.76

0.454-0.15 1.1 5:0.5 0.654-0.2 0.95-4-0.35 1.054-0.3

235U, 239ptl(n, f) FISSION PRODUCT 7,-RAYS

381

TABLE 2 (continued)

E (keV)

Z

A

fy (U) fy (Pu)

1118.9

kr2,,y(U)

rey fPu)

3

1124

3

1138 1152.7 l 178 1182 1218.7 1223.1 1238 1278.9 1303.7 1307.7 1312.7 1322.2

40

Zr

98

0.55

0.5 4-0.2 2.5 0.43 4- 0.2 2.0 -4-1.3 0.9 4-0.25 1.4 4-0.35

32

Te

134

1.2

1.554-0.4

134 135

137

1,4 4-0.55 0.9 4-0.23 0.95 4-0.3

as well. The important contour diagram o f f y (U)/fy (Pu), therefore, has the form of fig. 3. Here a circle around a point represents isotopes, the transition lines of which are known and have been detected in our measurements. The yield ratios of these isotopes were used for finding the proportionality factor k (for our measurements 1.5 + 0.2). Table 2 shows that most experimental yield ratios agree well with the computed ones. The exceptions are easily explained as follows. The data used do not reveal any fine structure in either the mass or charge yield. According to refs. 11, is), in general no fine structure occurs, but mainly the magic nuclei 134Te and ~36Xe are formed. In 23sU fission the corresponding light fragment 9SZr is more likely produced than l°2Zr therefore, while in 239pu fission l°2Zr is predominant, because the corresponding heavy fragment is magic ~36Xe. It cart be seen from table 2 that the yield ratios measured for 1°2Zr and fbr 98Zr a r e too low and too high respectively, with reference to our foregoing consideration. The other case where no agreement could be found is the 707 keV line. For 9~Kr the isotopic ratio is 4.5; the measured ratio for the line is 1.5. But attention must be drawn to the fact that Horsch and Michaelis s) have measured the mass value for this line only, and assigned the most probable charge. If Z = 37 is taken instead of Z = 36, the isotopic ratio is 1.7, which agrees, taking into account the error limits, with our measurements. It is concluded, therefore, that the intense 707 keV line represents a transition in 91Rb rather than in 91Kr. With the help o f table 2 some other assignments cart be made: apparently the 446.5 keV line is a transition in ~4VCe, and the 619 keV line a transition in l°°Zr. The assignment o f the 837.8 keV line is difficult, Z = 38 being more likely than Z = 39. For the y-rays o f more than 1 MeV no proper assignments can be made due to the heavy errors arising from the low response o f the Ge(Li) detector for high-energy ~-rays.

382

W . J . SCHINDLER AND C. M. F L E C K

By means of the method shown, the problem of isotopic assignment to individual lines could be solved requiring comparatively simple experimental equipment. If 7rays of an isotope were detected to coincide with X-rays, arising from converted transitions in the sasne nucleus, the Z-assignment was rather simple and very accurate. Also an A-assignment could be established in the way just described. For this measurement, thin targets are not necessary, and therefore no high-flux reactor is needed.

4. Discussion An interesting feature of ~-ray energies in Cf(sf) needs special discussion. Table 1 shows that up to now just two lines between 650 and 1150 keV have been measured in Cf(sf), while in 235 U and 2 3 9 p u thermal-neutron-induced fission many transitions keV 2000

?500

5m 500

~

'/3

140

~

150

160 A

Fig. 4. The low-lying levels of doubly even heavy Cf(sf) fragments, according to ref. 5). Points are connected by lines, if they represent equivalent states (e.g. first rotational 2 + ) in different isotopes of the same element. keV

/

6*

tO00

/ / 500 2*

° Sr

r

90

=

xZr

I

tO0

=t4o

=

* Ru

x Pd

=

110

A

Fig. 5. The low-lying levels of doubly even light Cf(sf) fragments, according to ref. 4). Solid lines connect equivalent states in different isotopes of the same element (notice the high-lying first 2 + state of 9SZr). The broken line shows the systematics of the equivalent states from the heavy (A = 116) to the light (A = 96) parts of the fragments. The dottea line is an extrapolation of the broken line, assuming symmetry around 102Zr"

235U, 239pIl(R,f) FISSION PRODUCT y-RAYS

383

in this energy interval have been found. For most of these y-ray lines mass assignments are unknown, but the fact that the transitions were not found in Cf(sf) implies that they arise from nuclei with masses 85-95 (for comparison see fig. 1). It is also of interest that nearly all transitions found in U and Pu fission have energies in the interval 650-1150 keV only (and perhaps below 300 keV). This circumstance may be interpreted in the following way: The low-lying levels of the heavy doubly even fragments have been carefully studied by Wilhelmy et al. 5). Fig. 4 shows the transition from magic nuclei (134Te, 136Xe) to pure rotators (Nd, Sin), all values having been taken from table 1 in ref. 5). In the same manner the lowlying levels of the light doubly even fragments have been studied by Cheifetz et al. 4) and the level systematics are shown in fig. 5 [values taken from table 1 in ref. 4)]. The nuclei xO0Zr and ~°2Zr are almost perfect rotors while the Ru and Pd isotopes show a strong vibrational admixture. Still, the transition energy between 6 + and 4 + level does not exceed 600 keV. The strongest deformation is in the region of 9s-x°2Kr [ref. 19)]. These nuclei are not formed with appreciable yield in the fission process. The ~°2Zr nucleus is closest to this region, and consequently symmetric level systematics around this nuclide are assumed, as indicated by the dotted line in fig. 5. But this assumption fails to reveal the measured transition energies, and therefore must be assumed wrong. For a correct interpretation of the experimental values and to be in accordance with the properties of the nuclei known up to now, it is necessary to take for granted an abrupt change from rotational to pure vibrational nuclei. A similar systematic behaviour has already been detected in the rare-earth region 17), for exarople in xs2~nss,.,,,6,(E4/E2 = 2.2) and ls*c:a9o,.,,,6,(E4/E2 = 3.0). Another proof that the assumption of an abrupt change is probably right is found in the nucleus 9SZr, where the first excited level E2 has an energy of 1223 keV, while for l°°Zr, with rotational properties, it is 213 keV. The fact that nuclei with mass 85-95 preferentially emit quanta with an energy between 650 and 1150 keV could easily be explained by our assumption. These energies are usually found in spherically symmetric vibrational nuclei. The coincidence measurements also prove that this interpretation is right: if lines in the considered energy interval are found to be coincident with others, they have approximately the same energy. This property could be easily and consistently interpreted as indicating, that the transition from the two-phonon state to the one-phonon state has approximately the same energy as the transition from the one-phonon state to the ground state. Although the results of our analysis cannot be regarded as absolutely fixed on an experimental basis, we wanted to present them in order to encourage further measurements. Taking into account the rapid progress in experimental techniques, we hope that the validity of our results will soon be ascertained.

384

W . J . SCHINDLER A N D C. M. PLECK

References 1) S. A. Johansson, Nucl. Phys. 64 (1965) 147 2) H. R. Bowman, S. G. Thompson, R. L. Watson, S. S. Kapoor and J. O. Rasmussen, Proc. Syrup. on physics and chemistry of fission, Salzburg, 1965, vol. 2 (IAEA, Vienna, 1965) p. 125 3) R. L. Watson, J. B. Wilhelmey, R. C. Jared, C. Rugge, H. R. Bowman, S. G. Thompson and J. O. Rasmussen, Nucl. Phys. AI41 (1970) 449 4) E. Cheifetz, R. C. Jared, S. G. Thompson and J. B. Wilhelmey, Phys. Rev. Lett. 25 (1970) 38 5) J. B. Wilhelmey, S. G. Thompson, R. C. J'ared and E. Cheifetz, Phys. Rev. Lett. 25 (1970) 1122 6) W. John, F. W. Guy and J. J. Wesolowski, Phys. Rev. C2 (1970) 1451 7) I-L Weigmann, ~I. Winter and M. Heske, Nucl. Phys. A134 (1969) 535 8) F. Horsch and W. Michaelis, Proc. Symp. on physics and chemistry of fission, Vienna, 1969 (IAEA, Vienna, 1969) p. 537 9) P. Matussek, H. Leuschner, W. Michaelis, I-L Ottmar, C. Weitkamp and H. Woda, paper presented at the Int. Meeting on non-destructive measurement and identification techniques in nuclear safeguards, Ispra, 20-22 Sept. 1971 10) R. Muchsel, thesis, Atominstitut der Osterreichischen Hochschulen, Vienna, 1969 11) W. Reisdorf, J'. P. Unik, I-I. C. Griffin and L. E. Glendenin, Nucl. Phys. A177 (1971) 337 12) L. U. East and G. Keepin, Proc. Symp. on physics and chemistry of fission, Vienna, 1969 (IAEA, Vienna, 1969) p. 647 13) F. Adams and R. Dams, J. Radioanalytical Chem. 3 (1969) 271 14) M. Mantel, J'. Gilat and S. Amiel, J. Radioanalytical Chem. 2 (1969) 395 15) W. R. Kane, Phys. Rev. Lett. 25 (1970) 935 16) A. B~icklin, B. Fogelberg and E. Falkenstr~m-Lund, Proc. Int. Symp. on neutron-capture gamma ray spectroscopy, Studsvik, Aug. 1969, (IAEA,. Vienna, 1969) p. 141 17) C. M. Lederer, J. Hollander and I. Perlman, Table of isotopes (Wiley, New York, 1970) 18) A. C. Wahl, J. lnorg. Nucl. Chem. 6 (1958) 263 19) D. A. Arseniev, A. Sobiczewski and V. G. Soloviev, Nucl. Phys. A139 (1969) 269 20) V. V. Verbinski, H. Weber and R. E. Sund, Trans. Am. Nucl. Soc. 14 (1971) 391