Investigation of mass, charge and energy of 241Pu(nth, f) fragments with the Cosi-Fan-Tutte spectrometer

Investigation of mass, charge and energy of 241Pu(nth, f) fragments with the Cosi-Fan-Tutte spectrometer

NUCLEAR PHYSICS A ELSEVIER Nuclear Physics A580 (1994) 15-32 Investigation of mass, charge and energy of 241pu(nth, fragments with the Cosi-Fan-Tutt...

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NUCLEAR PHYSICS A ELSEVIER

Nuclear Physics A580 (1994) 15-32

Investigation of mass, charge and energy of 241pu(nth, fragments with the Cosi-Fan-Tutte spectrometer P. Schillebeeckx

a,1, C. Wagemans b,c, p.

f)

Geltenbort d, F. G6nnenwein A. Oed d

e,

a CEC-JRC, Institute for Safety and Technology, 1-21020 Ispra, Italy b National Fund for Scientific Research, Brussels, Belgium c Nuclear Physics Laboratory of the University, B-9000 Gent, Belgium d Institut Laue- Langevin, F-38042 Grenoble, France e Physikalisches Institut der Universitiit, W-72076 Tiibingen, Germany

Received 15 June 1993; revised 18 May 1994

Abstract The mass, charge and energy distributions and their correlations have been studied for the fragments from thermal neutron-induced fission of 241pu, using the fission-fragment spectrometer Cosi-Fan-Tutte. The mass and charge distributions obtained agree with the corresponding radiochemical data. A proton even-odd effect of (10 + 1.5)% on the nuclear-charge yield and 0.4 + 0.2 MeV on the total kinetic energy has been determined. A fission-mode analysis on the nuclear-charge distribution has been performed. Keywords: N U C L E A R REACTIONS; 241pu(n, F), E-thermal; measured fission light-frag-

ment mass-charge-energy correlations; deduced different mass, energy, charge yields, proton even-odd effect on yield, kinetic energy. Fission-mode analysis.

I. Introduction U p to now, the knowledge of the f r a g m e n t properties in the thermal neutron-ind u c e d fission of 241pu was fairly p o o r and incomplete. T h e mass and kinetic energy of the fragments e.g. were studied by several authors [1-4] using the double-energy m e t h o d with its inherent resolution limitations. I n d e p e n d e n t yields for the elements 35Br, 36Kr and 54Xe have b e e n d e t e r m i n e d by Brissot et al. [5]. Evaluated I This work is part of the Ph.D. Thesis of P. Schillebeeckx presented at the University of Gent, Belgium. 0375-9474/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0375-9474(94)00304-6

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P. Schillebeeckx et al. / Nuclear Physics A580 (1994) 15-32

integrated mass and nuclear charge distributions based on radiochemical data are given in Refs. [6-8]. For a detailed understanding of the fragment properties, however, a correlated knowledge of mass, charge and kinetic energy is required. So we took profit of the new opportunities offered by the high-resolution spectrometer Cosi-Fan-Tutte [9] to investigate, for the first time, mass, charge and kinetic energy simultaneously for 241Pu(n th, f).

2. Experimental setup and data recording The experiments were carried out with the Cosi-Fan-Tutte spectrometer [9] installed at the high-flux reactor of the Institut Laue-Langevin (ILL) in Grenoble. In the present experiments only one detector arm was used. A 7 ~ g / c m z 241puacetate sample (isotopic composition: 239pu (1.3%), za°pu (5.0%), 241pu (89.9%), 242pu (3.7%)) was placed in the neutron beam having a capture flux of about 5 >( 10 9 n / c m 2 . s . The plutonium was deposited via electrospraying on a 30 ~ g / c m 2 polyimide backing covered with 26 ~ g / c m 2 of gold. The thermal-fission contribution of the other plutonium isotopes (mainly 239pu) is less than 1% of the 241pu(nth, f) counting rate. The time-of-flight signal (T) is given by the start- and stop-time pick-up detectors developed by Oed et al. [10], with a velocity resolution 8 v / v = 0.2%. The linearity of the time-of-flight system was checked using the 1 GHz time calibrator of Hartman and Oed [11], allowing a simple relationship between velocity v and flight-time (T): l v

(1)

aT+b'

with l the length of the flight-path, a the conversion factor and b the offset. The conversion factor a can be obtained from the 1 GHz time calibrator. For the calibration of relation (1) two measurements with different electronic settings have been used (see Table 1). The flight-path length (l = 107.2 _+0.1 cm) was calibrated with the 6Li(n, a)t reaction products produced in a 6Li sample. The offset b for the 241pu(nth, f) measurements was calculated from the 5.554 MeV a-line originating from the decay of 241Anl present in the 241pu sample.

Table 1 Calibration constants of the TOF-system determined by the time calibrator, 6Li(ntn, a)t reaction and the zalAm a-decay a (ns/ch) b (ns) 1 (cm)

6Li(n th, a)t

Z41Am

103.26 × 10- 3 47.19 (0.05) 107.2 (0.1)

54.948 × 10 - 3 45.72 (0.05) 107.2 (0.1)

P. Schillebeeckx et al. / Nuclear Physics A580 (1994) 15-32

17

The pulse-heights (P) were determined by means of axial ionization chambers especially developed for the Cosi-Fan-Tutte spectrometer (Oed et al. [12]). This type of detector also permits to determine the nuclear charge of the fission fragments by measuring a parameter Q of the Bragg-curve via a method proposed by Oed et al. [13]. The events were recorded in list mode on a magnetic tape via a PDP dataacquisition system. During the present experiments, about 340000 correlated (T, P, Q) light 241pu(nth, f) fragments were recorded. In order to test the spectrometer and especially the method used for the data analysis, also a run with a 350 I~g//cm2 evaporated 235UF4sample was performed.

3. Data analysis To analyse Cosi-Fan-Tutte data, the three-dimensional distribution Y(T, P, Q) has to be transformed into a distribution Y(m, E, Z) with m, E and Z the mass, kinetic energy and nuclear charge respectively of the fission fragments. The mass of the fission fragments can be obtained from the Y(T, P) distribution based on the relationship

E= ½kmv 2,

(2)

k (= 1.0365) being a conversion factor to express E, m and v in MeV, u and cm/ns, respectively. Due to the high resolution of the time-of-flight system and the ionization chamber, a mass resolution of -- 0.8% can be deduced from relation (2). Only for the light-fragment group separated mass lines could be observed. This is due to both the lower absolute energy resolution of the ionization chamber and the lower kinetic energy of the heavy fragments. The yield of the different masses Y(mi)l T is obtained by unfolding the pulse-height spectrum Y(P)I T, using a weighted least-squares method:

Y ( P ) IT = ~,Ri " r ( m i ) ] T ,

(3)

i

with

1 R i

=

tr(T)(2,n.)l/2

((P-Pi) 2) exp

2tr(T) 2

.

(4)

The width (tr(T)= a(T)+ b(T)m i) is linearly related to the mass mi, and the central position Pi is directly linked with the mass m i. Using relations (1) and (2), each yield Y(mi)[ T can be transformed into a yield Y(mi)l E resulting in a distribution Y(m, E). The kinetic energy has only to be corrected for total energy loss in the sample and the start detector (--1.5 MeV [14]). The relationship between the pulse-height and the energy can be expressed by the well-known Schmitt-Neiler relation [15]:

E = (a 1 + a 2 m ) P + b 1 +bErn.

(5)

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P. Schillebeeckx et al. / Nuclear PhysicsA580 (1994) 15-32

This expression shows the mass-dependent pulse-height defect of the ionization chamber and the mass-dependency of the energy loss of the stop detector. The nuclear charge of the fission fragments is determined by the parameter Q. For a given mass m, (T, P ) combinations falling in the corresponding mass window m + 0.25 u were used to analyse the distribution Y(Q) I T,m. For a time window that corresponds to AE = 0.5 MeV, the spectra Y(Q) I T,m are unfolded as follows:

(6)

Y(Q) IT,,n = ~_,R, "Y(Zi) lr,m, i

with

1

Ri= tr(T, m ) ( 2 r r ) 1/2 exp

(

o/ 2)

2 ~ ( T , m) 2

(7)

a gaussian response function as shown in Fig. 1, resulting in the yields Y(Zi) I m,E. This analysis is only possible for events with reasonable statistics and was limited to 91 ~
(8)

which was tested for energy values between 90 and 110 MeV and for mass values between 83 and 110 u. This relation is used for the response function (7) in the analysis of the 241pu(nth, f) measurements. The width-parameter o'(T, m) was taken only to be dependent on the mass and time-of-flight. The resulting charge resolution (FWHM) as a function of the kinetic energy was for each mass approximated by a smooth function as input data for the response function (7). The correctness of our analysis method can be shown by a comparison of our results for the reaction 235U(nth, f) with the results obtained by Lang et al. [16] on the Lohengrin spectrometer. The distribution of light fragments with mass 88 ~
P. Schillebeeckx et at/Nuclear PhysicsA580 (1994) 15-32

19

"rSO

300

320

3&O

360

380

too

m=97u

300

320 CHARGE

3~0

360

PARAMETER

~0

400

IGHANNEL$I

Fig. 1. The spectrum of the nuclear-charge parameter for fixed time-of-flight Y(Q) Ir,m for masses 93 u, 95 u and 97 u. The results of the unfolding procedure using relations (6) and (7) is shown by the full line.

4. Discussion of the results T h e results o f t h e C o s i - F a n - T u t t e d a t a all r e f e r to mass, c h a r g e a n d k i n e t i c - e n ergy c h a r a c t e r i s t i c s a f t e r n e u t r o n e v a p o r a t i o n . This l e a d s to t h e s o - c a l l e d p o s t n e u t r o n characteristics. O n l y t h e e l e m e n t a l y i e l d i n t e g r a t e d o v e r t h e k i n e t i c e n e r g y is a d i r e c t p r e n e u t r o n c h a r a c t e r i s t i c o f t h e fission f r a g m e n t b e f o r e n e u t r o n e v a p o r a tion. D u e to t h e low m a s s r e s o l u t i o n for t h e h e a v y f r a g m e n t s , w e limit t h e discussion to t h e c h a r a c t e r i s t i c s o f t h e light f r a g m e n t .

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P. Schillebeeckx et al. / Nuclear Physics A580 (1994) 15-32 10 A - 99

6

2

0 80

90

100

110

120

90

100

110

120

A = 103 8

--

6

_w +

2

0 80

KINETIC ENERGY (MeVI

Fig. 2. The kinetic-energy distributions of the nuclides 99Zr and l°3Nb.

4.1 Distribution of the kinetic energy and mass Y(m, E) The comparison in Fig. 6 and Table 2 of the global-mass-yield distribution for 241pu(nth, f) obtained in this work with the results of Wahl [7] shows the good agreement between our results and those obtained via an evaluation of radiochemical data. Both distributions are normalized to 100% for the mass region 83 ~
P. SchiUebeeckx et al./ Nuclear Physics A580 (1994) 15-32

21

try97

,-I ul

Z Z "r

o at Ill I-nl

t.00

Z-.t,0

Z~39 e¢"

38O

n

38

gl 0 at "V"

36O

0

3~

32O ,

i

,

i

,

L

.

t

,

I

,

L

560 SB0 600 620 640 660 680 FLIGHT TIME ICHANNELSl

Fig. 3. The nuclear-charge parameter as a function of the flight-time for fixed nuclear charge ( Z = 38) and for fixed mass (m = 97). The full line represents the least-squares fit procedure using relation (8) in the analysis of 241pu(nth, f).

6

ca 4 ,.I

ua 2 42

0

90

95 LIGHT

FRAGMENT

100 MASS

Fig. 4. Comparison of the nuclide distribution for 235U(nth, f) obtained in this work (points) with the results of Lang et al. [16] (full line).

22

P. Schillebeeckx et al. / Nuclear Physics A580 (1994) 15-32 30

25

2o ta ...I 15 tu >.

10 5 0 32

' 34

36

38

40

I 44

42

PROTON NUMBER OF THE LIGHT FRAGMENT

Fig. 5. C o m p a r i s o n of the elemental distribution for 235U(nth, f) obtained in this work (points) with the results of Lang et al. [16] (full line).

moments of the kinetic-energy distribution Y(E) Im of the isobars are shown in Fig. 8. One observes the increase of kinetic energy and width around mass 106. Although these are postneutron data, this indicates the appearance of the standard-I and standard-II channel in that region as discussed in Ref. [18]. The same increase in (E)(m) and tre(m) is found around mass 85. This increase confirms the suggestion in Ref. [18] of the appearance of a third fission mode in that mass region in the thermal neutron-induced fission, which was not found in spontaneous

6

w

q4),. 2.

0

, 80

• 90

, 100

i 110

120

MASS NUMBER

Fig. 6. C o m p a r i s o n of the mass distribution in ~41pu(nth, f) (normalized to 100% for 83 ~< m ~< 113) obtained in this work (points) with the results of Wahl [7] (full line).

P. Schillebeeckxet al. / Nuclear PhysicsA580 (1994) 15-32

23

Table 2 The global mass distribution (normalized to 100% for 83 ~
Y(m) (%)

Y(m) (%) [7]

(z)(m)

trz(m)

83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113

0.236 (0.020) 0.392 (0.026) 0.524 (0.030) 0.678 (0.034) 0.846 (0.038) 1.075 (0.043) 1.428 (0.050) 1.741 (0.055) 2.052 (0.060) 2.395 (0.065) 2.982 (0.072) 3.637 (0.080) 4.118 (0.085) 4.394 (0.087) 4.852 (0.092) 5.040 (0.094) 5.807 (0.100) 6.266 (0.104) 6.475 (0.106) 6.679 (0.108) 6.894 (0.109) 6.953 (0.110) 6.566 (0.107) 5.780 (0.100) 4.640 (0.090) 3.323 (0.076) 2.198 (0.062) 1.210 (0.046) 0.532 (0.030) 0.209 (0.019) 0.079 (0.012)

0.201 0.352 0.412 0.604 0.765 0.986 1.227 1.549 1.881 2.354 3.008 3.451 3.994 4.457 4.728 4.909 6.107 6.308 6.328 6.700 6.781 7.143 6.288 6.167 5.111 3.662 2.274 1.338 0.573 0.241 0.101

34.941 (0.175) 35.354 (0.177) 35.761 (0.179) 36.157 (0.181) 36.526 (0.183) 37.003 (0.185) 37.358 (0.187) 37.735 (0.189) 38.138 (0.191) 38.407 (0.192) 38.832 (0.194) 39.221 (0.197) 39.560 (0.198) 39.972 (0.200) 40.422 (0.202) 40.737 (0.204) 41.153 (0.206) 41.588 (0.208) 41.928 (0.210) 42.201 (0.211) 42.654 (0.213) 43.025 (0.215) 43.536 (0.218)

0.616 (0.017) 0.602 (0.016) 0.542 (0.010) 0.587 (0.017) 0.648 (0.025) 0.693 (0.011) 0.640 (0.011) 0.606 (0.012) 0.598 (0.011) 0.645 (0.007) 0.699 (0.010) 0.794 (0.018) 0.654 (0.013) 0.652 (0.012) 0.690 (0.010) 0.724 (0.014) 0.667 (0.009) 0.665 (0.014) 0.664 (0.009) 0.592 (0.007) 0.618 (0.011) 0.675 (0.010) 0.585 (0.007)

fission. T h e fine s t r u c t u r e in the b e h a v i o u r of ( E >(m) has b e e n e x p l a i n e d in detail by Schmitt et al. [19] for Z39pu(nth, f), which has a similar behaviour. This e x p l a n a t i o n is b a s e d o n the c o m b i n a t i o n of the p r o t o n e v e n - o d d effect in the yield with the b e h a v i o u r of the average kinetic energy of the n u c l i d e s (see Section 4.2).

4.2 Distribution of the kinetic energy, mass and nuclear charge Y(m, E, Z) T h e global charge d i s t r i b u t i o n Y ( Z ) is shown in Fig. 9 t o g e t h e r with the e v a l u a t e d d a t a from W a h l [7]. Both d i s t r i b u t i o n s are n o r m a l i z e d to 100% for 34 ~< Z ~< 44. It should b e stressed that, except for the e l e m e n t s Z = 35, 36 a n d 40, W a h l ' s e v a l u a t i o n for the charge yields is b a s e d o n e l a b o r a t e m o d e l calculations. Evidently, the m o d e l p r e d i c t i o n s are o n the whole quite reliable. T a b l e 3 shows the

24

P. Schillebeeckx et aL / Nuclear Physics A580 (1994) 15-32 12 E~= 96 MeV

El = 90 MeV 10

lO

8

8

6

6

4

4

2

2

÷

0 8O

90

100

110

0 80

90

100

110

I00

II0

12 El = 110 MeV

Ez= 103 MeV

lO

10

8

8

8

4

4

2

2

C "" 80

90

1O0

110

LIGHT F R A G M E N T M A S S (U)

0 80

,. 90

LIGHT F R A G M E N T M A S S (U)

Fig. 7. Light-fragmentmass yields for different kinetic energies in 241pu(nth, f). good agreement between our results and those of Brissot et al. [5]. The global proton e v e n - o d d effect calculated in the range 36 ~
Y(Z) = EYe(Z).

(9)

i The nuclear-charge distribution Y,.(Z) for a fission mode i is defined by Y/(Z) = W/Yg,i(Z)(1 + aitSe_o) ,

(10)

P. Schillebeeckx et al. / Nuclear Physics A580 (1994) 15-32

25

102 101

!

100

99 I-,

98 i 90

970

[

,

I

110

100

8

z

_0

7

w

,,=,

~6

, 0

l 90

,

LIGHT FRAGMENT

I 100 MA88

,

I 110

(U)

Fig. 8. The average fragment kinetic energy and width (standard deviation) as a function of the light-fragment mass in 241Pu(n th, f)" T h e kinetic energy is not corrected for the energy loss in the target and the start detector.

with W,. the yield, Yg,i(Z) a normalized gaussian distribution, a i a parameter related to the global proton even-odd effect of fission mode i and Be_o the delta function, which is one for an even charge and zero for an odd charge. The gaussian distribution has the average charge ( ( Z ) i ) and dispersion (o"i) as free parameters. On the basis of such a procedure our experimental data can be well described by supposing two fission modes as shown in Fig. 10. The parameters obtained are given in Table 4 and compared with the results for 239pu(nth, f) which agreed with the theoretical expectations of Brosa et al. (see Refs. [17,18]). The fission-mode parameters (average mass and dispersion) of Ref. [18] were transformed into parameters linked with the charge distribution using a constant mass/charge ratio. The results, based on the nuclear-charge distribution, are in perfect agreement with the results obtained in Ref. [18] based on double-energy measurements for 239pu(nth, f). The dispersion deduced from the charge data is systematically lower compared to the double-energy measurements. The average difference corresponds to about three mass units which is just the average mass resolution of the double-energy method. The effect of the appearance of fission modes can also be

26

P. SchiUebeeckx et al. / Nuclear Physics A580 (1994) 15-32 20

15 ¸

10

5

o

32

!

3,

3'8

3'8

20

22

2,

,8

CHARGE

Fig. 9. Comparison of the elemental distribution summed over kinetic energy (normalized to 100% for 34 ~
illustrated by a comparison of the charge distribution f o r 241pu(nth, f) with the distribution of 239pu(nth, f) obtained by Schmitt et al. [19]. This comparison indicates an increase in yield around charge 42 (fission mode I) compensated by a decreasing yield around charge 38 (fission mode II) with increasing mass number of the compound nucleus. The same conclusion can be drawn by comparing the yields of the fission modes in Table 4. The effect has also been found by Schillebeeckx et al. [18] in a comparative study of the spontaneous fission of several plutonium isotopes. A surprising effect is the lower global even-odd effect for fission mode I compared to fission mode II, although one should consider the large uncertainties quoted.

Table 3 The global charge distribution (normalized to 100% for 34 ~
z

Y(Z)(%)

34 35 36 37 38 39 40 41 42 43 44

0.02 (0.08) 1.71 (0.23) 4.67 (0.38) 5.91 (0.42) 12.47 (0.62) 12.12 (0.61) 17.32 (0.73) 15.94 (0.70) 17.96 (0.74) 8.91 (0.52) 2.75 (0.29)

Y(Z)(%~5] 2.1 (0.3) 3.85 (0.36)

16.73 (1.0)

P. Schillebeeckxet al./ NuclearPhysicsA580 (1994) 15-32

27

20

15"

tu

10

5



x

°°.

" ~.~

0 32

34

36

38

40

42

44

46

CHARGE

Fig. 10. The elemental distribution summed over kinetic energy for 241pu(nth, f) obtained in this work (points) with the results of the fission-mode analysis (full line).

T h e p r o t o n e v e n - o d d effect as a function of p r o t o n n u m b e r of the heavy f r a g m e n t 6p(Z), calculated according to Tracy et al. [23,24], for 241pu(nth, f) is c o m p a r e d with the b e h a v i o u r for 235U(nth, f) in Fig. 11. This c o m p a r i s o n shows that the d e c r e a s e of the e v e n - o d d effect as a function of the fissility p a r a m e t e r is not due to local effects. In the whole nuclear-charge region, ~ p ( Z ) for 235U(nth, f) lies systematically above 6 p ( Z ) for 241pu(nth, f) by a factor of about two. This figure also reveals the similarity in the structure of the e v e n - o d d effect as a function of the h e a v y - f r a g m e n t nuclear charge for 235U(nth, f) and 241pu(nth, f). T h e observed structure is essentially d e t e r m i n e d by the heavy fragment: large values at Z = 52 and 56 and a m i n i m u m value at Z = 54. T h e explanation of L a n g et al. [16] for the m i n i m u m at Z - 54 in 235U(nth, f) being due to the corresponding influence of the light f r a g m e n t via the d e f o r m e d p r o t o n shell at Z = 38 should be put into question. Indeed, this would imply for the Pu isotopes a m i n i m u m around h e a v y - f r a g m e n t nuclear charge Z - - 5 6 , w h e r e a m a x i m u m value is observed. A n

Table 4 Characteristics of the standard-I and standard-II fission modes for 241pu(nth, f) and 239pu(nth, f). The values for 241pu(nth, f) are deduced from the charge distribution in this work. The values for 239pu(nth, f) are based on Ref. [18] using a constant charge/mass ratio Standard I Standard II 241pu(nth, 0 239pu(nth,0 241pu(nth, 0 239pu(nth,f) W (%) (Z h) o-z cSp (%)

36.5 (7.5) 52.15 (0.14) 1.15 (0.1) 4.2 (4.1)

24.8 (0.3) 52.3 (0.1) 1.4 (0.1)

63.5 (6.3) 54.92 (0.37) 1.91 (0.16) 13.6 (2.8)

74.2 (0.3) 54.8 (0.1) 2.5 (0.1)

28

P. Schillebeeckx et al. / Nuclear Physics A580 (1994) 15-32 40

"~ 3O

÷ t

IO 1u it u

m

20

a r~

o Z

nl

io

+

0

i

I

48

i

50

I

i

I

52

i

I

54

i

I

56

i

58

60

HEAVY FRAGMENT PROTON NUMBER

Fig. 11. Comparison of the proton e v e n - o d d (asterisks) and 241pu(nth, f) (circles).

effect as a function of the nuclear charge for

235U(nth, f)

explanation on the basis of fission modes as given in the previous paragraph appears to be more appropriate. An analysis of the elemental yields at fixed kinetic energies Y(Z) I E shows that as the kinetic energy increases, the yields of the even elements become relatively more important, especially the yield around Z = 42. The increase of the proton even-odd effect as a function of the kinetic energy 8p(E) is shown in Fig. 12 and compared with the behaviour for 235U(nth, f) from Ref. [16]. The increase with energy is similar for both reactions, but the magnitude of the proton even-odd

40

--

30

It.) itl ii i1

tu

2C

r~

o, Z

iii

io

i

5

i

,

i

90

. . . .

I

,

95

,

.

.

L

,

100

,

,

,

I

,

105

i

,

,

I

,

110

,

,

,

115

KINETIC ENERGY (MeV)

Fig. 12. Comparison of the proton e v e n - o d d (asterisks) and 241pu(nth, f) (circles).

effect as a function of the kinetic energy for

235U(nth, f)

29

P. Schillebeeckxet aL/NuclearPhysicsA580 (1994) 15-32 101.0 100.8 o

--

100.6

~ 100.4 Z lu

~I,,- 100.2 wl

~ 100.0 99.8

36

38 PROTON

40

42

44

NUMBER

Fig. 13. The average kinetic energy of the elements in 241pu(nth, f).

effect is always a factor of about two smaller for 241pu(nth, f) as compared to 23SU(nth, f). Together with the behaviour of the local e v e n - o d d effect ~p(Z) this again proves that the decrease of the proton e v e n - o d d effect as a function of the fissility parameter is a global effect. In Fig. 12 it is furthermore interesting to note that for low kinetic energies the e v e n - o d d effect appears to stay constant at a sizable non-zero value. This feature has already been noted for other fission reactions induced by thermal neutrons and discussed in terms of "cold deformed fission" [25,26]. The mean kinetic energy as a function of the nuclear charge (E)(Z) (uncorrected for the energy loss) in Fig. 13 shows that the average kinetic energy for the even-Z elements is larger than that for the odd-Z elements. The average e v e n - o d d effect in kinetic energy of the light fragment for 36 ~
30

P. Schillebeeckx et al. /Nuclear Physics A580 (1994) 15-32

0.9 0.8 z

0

0.7

I,l,I

B 0.6

0.5

0.4

I 34

K

I 36

,

I 38

,

I 40

,

I 42

,

I 44

AVERAGE PROTON NUMBER

Fig. 14. The standard deviationsof the isobaric nuclear-chargedistributions as a function of the average isobaric nuclear charge in 241pu(nth, f). The influence of the spherical proton shell Z = 50 in the heavy fragment on the outcome of fission has been brought into focus by Clerc et al. [27], and later by Schmitt et al. [19] in a very specific way. They propose to compare fractional independent chain yields (normalized for fixed chain mass) for some selected elements Z. This is done in Fig. 15 including our new results on the 241pu(nth, f) reaction. The fractional independent yields for the elements 39Y and 41Nb are seen 235 to be quite similar for the three fissioning systems shown: U(nth , f), and 239pu(nth, f) and 241pu(nth, f). The maxima for a given element are reached at approximately the same mass number. By contrast, for the element 43Tc the yield in uranium fission is definitely lower than in plutonium fission, the yields for the two Pu isotopes staying similar. For 239pu(nth, f) and 241pu(nth, f) one finds a maximum value around mass 107 of more than 50%, whereas for 235U(nth, f) for mass 107 only a yield of (15.4 + 4.9)% is obtained. This comparison clearly indicates that the low 4 3 T c yield in 235U is due to the properties of the complementary heavy fragment, in particular the stability of the proton shell Z = 50. 5. Conclusions In the present work, 235U(nth, f) data for fragment masses and charges obtained at the spectrometers Lohengrin and Cosi-Fan-Tutte of the ILL Grenoble are compared. This comparison shows that by a careful analysis of the experimental data, high-quality results can likely be obtained at the Cosi-Fan-Tutte spectrometer, for the mass as well as for the nuclear charge of the fission fragments. The main aim of the investigation, however, was the study of the 241pu(nth, f) reaction. Fairly comprehensive data for mass and charge yields and the kinetic energies of fragments from the light group are presented. Charge yields from the 241pu(nth, f) reaction were only poorly known.

P. Schillebeeckx et aL / Nuclear Physics A580 (1994) 15-32

31

80 2alPu(nth,f)

60

zl = 41

zi

40

o 90

'/

95

100

105

110

115

100

105

110

115

8O z39Pu(nth,f)

6O

g

20

90

95

23SU(nth,f)

60 zl = 39

;

zt = 41

4o

uJ

20

0 90

,

_

95

100 MASS

105

,

m

110

.... 115

NUMBER

Fig. 15. Fractional independent yields of the elements with Z = 39, 41 and 43 f o r 235U, 239pu and ~ l P u ( n t h , f) as a function of fragment mass.

The Brosa fission modes observed in Ref. [18] for 239pu(nth, f) are found back in 241pu(nth, f) by applying for the first time a fission-mode analysis on nuclear-charge data. The proton even-odd effect in yield and kinetic energy for 241pu(nth, f) follows the systematic behaviour of other thermal neutron-induced fissioning systems as already discussed by G6nnenwein [21]. Also the phenomenon of cold deformed fission is indicated for the reaction under study. The influence of the heavy fragment on some special characteristics like the local proton even-odd effect and the relative yield of 43Tc is again established by

32

P. Schillebeeckx et al. ~Nuclear Physics A580 (1994) 15-32

o u r results. T h e l a t t e r o b s e r v a t i o n s h o w s t h e i n f l u e n c e o f t h e s p h e r i c a l p r o t o n s h e l l Z = 50 o n t h e f o r m a t i o n o f t h e fission f r a g m e n t s . T h i s p r o t o n s h e l l h a s b e e n l i n k e d w i t h t h e s t a n d a r d - I fission m o d e as in R e f s . [18,22].

Acknowledgements W e a r e g r a t e f u l to A . C . W a h l w h o s u p p l i e d u n p u b l i s h e d d a t a f r o m his analysis o f m a s s a n d c h a r g e y i e l d s f o r t h e 241ptl(nth, f) r e a c t i o n , a n d to t h e I L L G r e n o b l e f o r p u t t i n g t h e C o s i - F a n - T u t t e s p e c t r o m e t e r at o u r d i s p o s a l .

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