Mass and energy characteristics of the 241Pu(nth, f) fragments

Nuclear Physics A369 (1981) 15-24 © North-Holland Publishing Company

MASS AND ENERGY CHARACTERISTICS OF THE 24'Pu(nh, f) FRAGMENTS F. CAYTUCOLI t, C. WAGEMANS II, P. PERRIN, E. ALLAERT tt, P. D'HONDT t' and M. ASGHAR Instilut Laue-Langesin, BP 156X, 38042 Grenoble, France Received 28 April 1981 Abstract : Fission-fragment mass and kinetic energy distributions and their correlations have been studied for the thermal neutron induced fission of 2 "Pu. The global mass distribution is smooth, apart from a small shoulder at ju :zz 144 probably due to the deformed shells N .: 88 and N _> 60 in the heavy and the light fragment respectively. When low excitation events are selected, these structures become more pronounced . Furthermore, there is a sudden increase of 1-2 MeV in L(h) for masses around 85 and above 155 which is probably associated with a spherical shell N = 50 in the light fragment . Finally, the prompt neutron emission curve v(m*) is calculated .

E

NUCLEAR REACTIONS 24 `Pu(n,., f), measured complementary fragment kinetic energies ; deduced energy and mass distributions and mass-energy correlations .

1 . Introduction Fission-fragment mass and energy distributions and mass-energy correlations have been studied for a large variety of fissioning systems t-3). For the thermal neutron induced fission of Z4tPu, only a few results have been reported 4-8 ), in which no detailed mass-energy correlations were studied. In the case of 24'Pu (%, f), the amplitude of the odd-even effect has been determined to be 1 .04±0.05 compared to 1 .034±0 .05 for 239Pu(n,b, f) and 1 .25±0.05 for 235 U(nth, f) [refs. 9- '°)] . So a weak fine structure similar to that observed for 239pu(nth, f) [ref. ")] is expected for 24'Pu(n,h, f). To investigate this, we have made a detailed study of the 241pu(nth, f) fragment mass and energy distributions and mass-energy correlations, taking profit of the excellent experimental conditions available at the Institut Laue-Langevin (Grerioble). These results complete similar studies on the other common fissile isotopes 11,12) .

t It

CEN Bordeaux-Gradignan, Université de Bordeaux, France . Collaboration Nuclear Physics Laboratory, B-9000 Gent and SCK-CEN, B-2400 Mol, Belgium. 15

16

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241 Pu(n f)

2. Experimental procedure and analysis The measurements were performed at the neutron beam IH1 of the Grenoble high flux reactor. The beam was collimated to 1 cm diameter, and the equivalent thermal neutron flux at the target position was 5 x 10 9 neutrons/cm 2 . sec. with oth/4)fast ^ 104. The 241Pu target consisted of a 2.3 pg 241 pu/cm2 layer obtained via solution spraying of Pu acetate in methanol . For test purposes, an almost identical 235U target with 2.2 ttg 235 U/cm2 prepared via solution spraying of uranylacetate was used. Both targets were deposited over a diameter of 10 mm on identical backings of 32 ttg/cm 2 poly-imide coated with 21 hg Au/cm'. The isotopic composition of the Pu target was (in atom %) : 238pU (0.0263), 239pU (1 .1682), 24 °Pu (4.4822), 241Pu (91 .0193), 242pu (3.3010), 244Pu (0.0030) . Both targets were prepared by the CBNM Sample Preparation Group. The pulse heights and the time-of-flight difference were recorded for more than one million coincident 241 Pu(n,h, f) fragment pairs, and registered event by event on a magnetic tape . These pulse heights were converted into kinetic energies and provisional masses via an iterative procedure using the calibration method ofSchmitt et al. 13) , the 241pu calibration constants 4) and the mass and momentum conservation relations. The time-of-flight difference data were used as a coherence test, resulting in the elimination of poorly measured events 12,14) . 3. Results and discussion The good experimental conditions of the present experiments resulted in high resolution data. This is reflected in the various distributions and mass-energy correlations shown in figs. 1-6. These data have not been corrected for neutron emission nor for resolution effects, so the mass quantities are always provisional masses (tt) . Table 1 summarizes the main characteristics of the 241 Pu(n,, f) fragment energy and mass distributions obtained in the present work. The corresponding 233U , 235U and 239 Pu(nth, f) data obtained under the same experimental conditions are given for comparison purposes . These results underline the similarity between the 233U and 235 U(n,h, f) characteristics on one side and the 239Pu and 241 Pu(n,h, f) characteristics on the other side . 3.1 . MASS DISTRIBUTIONS AND MASS-ENERGY CORRELATIONS

In fig. 1 the 241Pu(nih, f) fragments mass yield distribution is given ; because of the symmetry only the heavy fragments peak is shown. The curve is very smooth apart from a shoulder in the mass region 143-145, probably due to the influence of the deformed shells N 88 and N >, 60 in the heavy and the light fragments. Its general behaviour is quite in agreement with the curves obtained by Neiler et al. 4) and Vorobeva et al. '). If we compare the present mass distribution with that of 239pU

F. Caitucoli et al. / 241 pu(n  J')

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TABLE 1

Main characteristics of the 233U, 233U , 23 'Pu and 241 Pu(n  ,, f) fragment mass and energy distributions (provisional mass quantities are given) 233U(n,h " f) [ref. 12 )] E,c Ur.

(MeV) (MeV) (MeV)

(MeV) UFL (MeV) EH (MeV) aE  (MeV) EL

P'L

a. p', a P/V

JEK (MeV) AC, (MeV)

176 .0±0.5

177 .5±0 .5

172 .7±0 .5')

177 .9±0 .5')

179 .4±0.5')

10 .4

10.7

11 .8

11 .6

100 .8

100.4

102 .0

101 .9

5 .10

5 .8

5 .71

70 .6

73 .9

7 .28

7 .6

8 .5

8 .39

95 .57

97 .20

100 .6

102 .83

5 .60

5 .70

6 .6

6 .27

138 .43

138 .80

139 .4

139 .17

5 .61

5 .70

6 .6

6.27

350

±20

411

±33

17 .0±1 2 .49

P/V radiochemical ') b)

171 .0±0 .5

18 .2±1

v,

241Pu(n h, f) this work

172 .3±0 .5')

69 .8

(amu) (amu) (amu)

23'Pu(nm " f) [ref. I, )]

170 .6±0 .5

5 .25

("MU)

23'U(nm" f) [ref. ")]

440

b)

554 650

±31 ±36 b)

75 .6

129

±3

265

± 16

151

±8 b)

311

+26 b)

21 .8±1 .1

19 .6±0 .4

23 .6±1 .3

20 .6±1 .1

18 .6±0 .4

22 .6±1 .3

2 .44 650

2 .88

2 .93

150

330

Corrected for neutron emission with the v, values listed in the table, the errors quoted are only relative . Normalized to the radiochemical P/V = 650 through the present results for 2"U(n,h, f) .

120

130

140 150 HEAVY FRAGMENT MASS N H lamu)

160

170

Fig . 1 . The provisional mass distribution for 241Pu(n,h, f), not corrected for resolution effects.

F. Caitucoli et al .

/ 241 Pu(n,, f)

v a N 3

0 k ai

>

~A 0 N L e, 0

~E

C

s ân L

W N

C O

A

q u 12 .

HOIsINnoo

u

N

_ 00

N

F. Caítucoli et al . / 14' pu(n,h, jJ c

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F. Caitucoli et al.

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Z°' Pu(n,, f)

(n,, f) [ref. 11 )], we observe in both cases enhanced yields for asymmetric masses (A H > 146) compared to the thermal fission of "5U . The experimental peak-to-valley (P/V) ratio is 265 ± 16. This value can be corrected for resolution effects through the radiochemical P/V = 650 for 21 'U(nth, f) and the 2"U(n ,h, f) mass distribution which we obtained under identical experimental conditions. In this way, a resolution corrected value P/V = 311 ±26 is obtained for 241 Pu(n ln, î), in perfect agreement with the radiochemical value of 330±30 [ref. ' S)] . Fig. 2 shows the mass-energy correlations calculated for five light fragment kinetic energy windows (width 2 MeV). Since the light fragment kinetic energies are rather constant and independent of the mass division in fission (cf. fig. 4), a window at the light fragment kinetic energies selects fission events with approximately the same excitation energy for all mass divisions. This can be done in a more rigourous way by calculating the quantity Q-EK for each fission event. Here Q is the overall reaction energy calculated from the data of Nix "). For a given mass ratio MLI MH, the energetically most favourable Q-value is selected. The mass distributions corresponding to the lowest windows on Q - EK (width 2 MeV) are shown in fig. 3 . Both series of mass distributions clearly reveal fine structures at hH = 134, 138, 140, 144-145, 150, 155, 160, the amplitudes of which decrease with increasing excitation energy (i.e. increasing Q-EK or decreasing EL values) as expected . These structures (or shoulders) are due to odd-even and/or shell effects. Their amplitudes are comparable to those observed in the thermal fission of 239 Pu [ref. 11 )]. 3.2 . KINETIC ENERGIES AND ENERGY-MASS CORRELATIONS

The energy spectrum for a single fission fragment is shown in fig. 4. More detailed characteristics of the energy distributions are given in table 1, which shows that the rms widths aEK , QEL, and QEH for 241Pu(nW f)arequite similar to those of 239Pu(n,h, f), i.e. much wider than the corresponding values for 233U and 235U(nth,, f). For AEK (difference between the maximum total kinetic energy value and the value at symmetry, cf. fig. 6a) we obtain a value of 22 .6± 1 .3 MeV for 211 Pu(n,h, f), compared to 21 MeV and 20 MeV as measured by Vorobeva et al. ') and Neiler et al. 4). In figs. 5 and 6 the E(u), ÉK(uH) and aEK(uH) distributions are given. These data confirm our previous observations for example for `Pu(nth, f) [ref. 11 )], i.e. a sudden increase in energy (1-2 MeV) for masses around 85 and above 155 (fig . 5). This effect is more pronounced in fig. 5a for p, ~ 155. These phenomena are probably due to the nearly spherical light fragment with IzL x 85 which has a spherical shell with N = 50, and the corresponding heavy fragments which are deformed (but stable) with masses in the rare earth region 1'). 3.3 . THE PROMPT FISSION NEUTRON EMISSION CURVE

For 241p(nth, f), no information on the fission neutron emission curve in function

F.

Caitucoli et al. /

"'Pu(n,h,

90

f)

100

21

no

120

E ( MeV ) -

Fig. 4. Single fragment kinetic energy distribution for "Pu(n,, f) .

"

> z au

_

. .. . . . . . . . . .

9t I 80 70 80

4OL 70

i

80

v. 90

100

110 120 130 140 PROVISIONAL FRAGMENT MASS N [amu)

150

160

170

Fig. 5. The single fragment kinetic energy as a function of the fragment provisional mass for za1Pu(n,, f). Notice the increase of R around R 85 and above ju -- 155.

F. Caitucoli et al. / 241 Pu(n,h, f)

22

10 . .

.

)b)

. . . . . . . . . . . . . . . .

150E

3-

w

%j

150E

140E

1301 120

130

140

150

I 170

150

HEAVY FRAGMENT[ PROVISIONAL) MASS

11,

Fig. 6. (a) The total fission fragment kinetic energy for "'Pu(n, h , f) as a function of the heavy fragment provisional mass . Notice the increase of EK for t!N 3 155. (b) The rms width of the total fission fragment kinetic energy for 2"Pu(n,h, f) as a function of the heavy fragment provisional mass.

3

2

E

70

80

90

100

110 120 130 S40 - FRAGMENT MASS (m' )

ISO NO.

150

170

Fig. 7. The calculated fission neutron curve for "'Pu(n,, f) in function of the (pre-neutron emission) fragment mass (frill line). The uncertainties are indicated by the intermittent lines.

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Pu(nth, f)

23

of the fragment mass v(m*) is available. Taking into account the great similarity of the shape of v(m*) for most fissioning systems t a), the 241 Pu(n f) v(m*) distribution is generally approximated by that of 2"Pu(n f) multiplied by v, [241Pu(n,h, f)]/vt [2"Pu(n,,, f)], i .e. the ratio of the average total number of fission neutrons emitted in both reactions. The present results combined with the post-neutron emission mass distribution as reported by Cuninghame 1 s) were used to calculates v(m*) applying the iterative procedure described in ref. 19) . v(m*) was calculated using a value a2(vt) = 1 .173±0 .004 for the variance of the total number of neutrons emitted [ref. 2 °)] ; for the average variance
24 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20) 21)

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