Evidence for Coulomb-field induced light-particle emission during scission

Volume 186,number 1

PHYSICSLETTERSB

26 February 1987

EVIDENCE FOR COULOMB-FIELD INDUCED LIGHT-PARTICLE E M I S S I O N DURING s c I S S I O N A. BRUCKER, B. LINDL, M. BANTEL, H, HO l, R. MUFFLER, L. SCHAD 2, M.G. TRAUTH and J.P. WURM Max-Planck-lnstitut fiir Kernphysik, Postfach 103980, D-6900 Heidelberg, Fed. Rep. Germany Received 3 September 1986

Energyand angularcorrelations of protons and ctparticles were measured in coincidencewith fission fragmentsin the reactions 200 MeV, 254 MeV 37C1+~24Snand 318 MeV 2sSi+141pr. The light-particle yield is dominated by statistical emission from composite nucleus and from fission fragments, but ct spectra also display the characteristics of evaporation during scission. We give Coulomb-fieldinduced light-particle emission as a novel and hitherto onlyconsistent explanation of this emission process.

In the last few years the investigation of fusion -fission reactions disclosed a near-scission emission mechanism (NSE) of a particles [ 1-3] resembling the characteristics of long-range a particles in ternary fission [4], besides forward peaked pretherrealization emission, sequential particle emission from composite nucleus (CNE) and from the fully accelerated fission fragments (FE). The increase of NSE multiplicity with increasing excitation energy, however, is not understood in terms of ternary fission [5,6]. To investigate the properties of NSE in more detail we have studied the fusion-fission reactions (A) 200 MeV 37C1+1245n and (B) 318 MeV 2sSi+ ~4~pr with composite-nucleus excitation energies of 100 MeV and 207 MeV, respectively, by measuring the energy and angular distribution of protons and a particles in coincidence with fission fragments. In this paper we would like to demonstrate that the characteristic features of near-scission emission evolve naturally from field-induced light-particle emission, a process similar to Stark ionization in atomic physics, which is novel to nuclear physics. The experiments were performed at the Heidelt Presentaddress: Bodenseewerk, D-7770Oberlingen, Fed. Rep. Germany. 2 Presentaddress: DeutschesKrebsforschungszentrum,D-6900 Heidelberg, Fed. Rep. Germany, 20

berg MP-Tandem-postaccelerator combination using a time-focused beam. Fission fragments were detected in an octagonal ring counter of eight identical position-sensitive parallel plate avalanche counters (PPAC) around the beam axis. The detection angles were 40 ° _+8 ° (A) and 50 ° _+10 ° (B); the detector covered a solid angle of--- 1.4 sr. Coincident protons and a particles were detected by AE-E Si telescopes and scintillator telescopes [7] at 20 (A) and 14 (B) different scattering angles. This setup allowed lightparticle correlation measurements in and out of the reaction plane defined by the fission fragment momenta and the beam axis. Symmetric fission has been selected by a window in the time-of-flight versus pulse height spectra of the PPAC. The most-probable velocity vectors of the fission fragments are indicated as arrows in fig. 1a. The mass range and the average kinetic energy were measured by a Si detector at the same scattering angle as the PPAC. Plasma delay *~ and pulse height defects [ 8 ] in the silicon detector were corrected event-byevent. In the chosen fission window we determined mass ranges of 63
Volume 186, number 1

26 February 1987

PHYSICS LETTERS B

a

b deficit

d

c i

+55 °

i

4

~

3-

+{

I

+15 °

i

I

0 4

2

- -

/

.....",0,

.,v

~ . "

÷~,

~'..fz..

+140 °

+55 °

......... 1

I

7'

-

\ t o , *....

I

_15 °

I

I

-55

0

.... "-~\ -55*

r

-140 °

°

2

_,50

I

-

!

1 1 em/ns

%

i

~'PPAc 40 °

O0

20

40

0

20

40

20

40

E~ [ i e V ] Fig. 1. Velocity diagram (a) and coincident a spectra (b, c, d) o f the 200 MeV 37C1+ 1245n experiment. (a) Full vectors denote the mostprobable velocities o f the detected and undetected fragment and the center-of-mass velocity. Circles show the most-probable velocities for a particles sequentially emitted from the fragments (..., - - ) and from the composite nucleus ( - . - ) . Shaded areas mark the irregularities in the a yield. (b, c, d) Double differential a multiplicities; full curves represent the three-source calculation of sequential emission, same notation as in (a). Detector thresholds are indicated by arrows.

ties (circles) of a particles emitted from the composite nucleus and from the fragments. Evaporation barriers have been adopted from the systematics of ref. [ 9 ]. Anomalies of a emission have been inferred by comparing the experimental spectra at various angles to spectra generated by a three-source calculation [ 2,10,11 ]. In this calculation the reaction kinematics were defined by the measured velocity distribution of the fission fragments and it was assumed that the composite nucleus and both fragments are sources of isotropic light-particle emission, parametrized as a generalized Maxwell-Boltzmann distribution. The calculated light-particle yield and spectral shape was fitted at two angles and extrapolated to the various lab angles. Here we show only a few of the measured light-particle spectra to demonstrate anomalies in the a spectra which are correlated to the scission axis as indicated in fig. la: a deficit in the yield at 0 = - 5 5 ° and a surplus of yield at/9 = + 15 ° and 0 = - 140 °. The corresponding spectra on the opposite side of the beam are well described by the three-source calculation (fig. lb, lc, ld). In the spectra both anomalies

are located in the range between FE and CNE energy. The deficit observed at 0 = - 5 5 ° cannot be caused by kinematic effects due to emission from not fully accelerated fragments: these would shift the upper part of the spectrum to lower energies resulting in an increase of yield where the deficit is observed, and in a depletion in the upper end of the spectrum which is well described by the three-source calculation. The correlation with the scission axis and the energetic position of the anomalies conclusively demonstrate that light-particle emission during scission is responsible for the observed anomalies in the spectra. In fig. 2 we present the results of the 2sSi+ i41pr experiment where, in addition, protons were analysed. Comparison of the a (top part of fig. 2) and proton (bottom part of fig. 2) spectra shows a clear difference: proton emission is reasonably well described everywhere by sequential emission, the a spectra again show the abovementioned anomalies. In order to examine the spatial distribution of the light-particle multiplicity we have detected protons and a particles in a plane perpendicular to the reaction plane and to the center-of-mass scission axis 21

Volume 186, number 1

PHYSICS LETTERS B

a

26 February 1987

b /

d

C 20

40

60

I

I

I

O~ :

0

20 I

40

i

-60 °

0~

I

0

I

20

I

-170 °

=

40

I

I

0~

I

=

6

I

I

i

+60 °

1~I,c~

=

20

°

4

%

0

*"

F.......

...-""'/_

//"

.LP

90

2',,.

\

_LY/

. . . . "/

,I .

0

I

/

Ov =

8

aane -

" . . .c.m.

:,

scission

_~_~PPAC/ - /

axis

I

I

I

I

I

I

I

16

~,~,io~'\~..':.: ::. : --.

I

- 170 °

--

Ov =

--

1)i,p

+60 ° I

23 °

=

/ ~!i:I

_~

I

,_ /

20

40

---

L

20

[

40

0

20

40

ELF [MeV]

Fig. 2. Spatial velocity diagram (a) for a emission, (b, c, d) coincident proton (lower part) and a spectra (upper part) of the 318 MeV 2sSi+ ~4~Pr experiment; same notations as in fig. 1. In the out-of-plane spectra (d) the angle to the spin axis of the composite nucleus 0LLVis indicated.

(out-of-plane measurements). For illustration the spatial velocity diagram has been drawn in fig. 2a; the corresponding diagram for proton emission is similar and therefore omitted. Due to rotation, the yield of sequential light-particle emission decreases towards the rotation axis [ 2,12 ], i.e., the normal to the reaction plane (0X,Lp= 0 ° ). In the calculation the yield parameters for the three sources were adjusted to fit the data at each O~,LV-Energy shifts in the particle spectra as a function of OLLVwere not observed. The detailed out-of-plane data and its analysis in terms of the deformation of the composite nucleus will be published elsewhere [ 13 ]. The out-of-plane measurements show a doughnutshaped surplus yield of a multiplicity around the CM scission axis; the proton spectra exhibit no anomaly. Fig. 2d displays light-particle spectra at the closest angle (0x,==20 °, OLp=23 °) to the spin axis of the composite nucleus. From the in- and out-of-plane data we have determined the a particle multiplicities for sequential emission (i.e., MCNEand MFE) and for the near-scission emission, denoted by M ~ ( s u r p l u s ) and M ~ E (deficit); these are displayed in fig. 3 for the different reactions studied, including 254 MeV 22

10 -z

~

~

Fig 10-3 NSE 10-4

I

0

50

i00

I

]

150

200

Ex [MeVl Fig. 3. a multiplicities McNE(×), MvE(e) and M ~ ( X T ) , d©f MNsE (o) versus the excitation energy of the composite nucleus for the various experiments (A: E~=100 MeV; 254 MeV 37C1+ 124Sn"Ex-- 140 MeV; B: Ex= 207 MeV).

Volume 186,number 1

PHYSICS LETTERSB

37C1 + 124Sn [ 14 ]. Plotted versus the excitation energy Ex of the composite nuclei, we observe from fig. 3 that the NSE multiplicity closely scales with the multiplicities of sequential evaporation (CNE, FE). The steep increase of NSE multiplicities, e.g. by factors of 20-50 from Ex= 100 MeV to Ex= 140 MeV for t h e 37C1-b 124Sn reaction is clearly associated with an evaporative process. In contrast, light-particle emission in ternary fission is known to be relatively insensitive to excitation energy and viewed as a nonevaporative process [ 15 ]. Besides, the observed deficit in direction of the scission axis is not accounted for in the ternary fission model. Since ternary fission does not describe the data, we now discuss the effect of a third body on the emission process itself as an explanation for the observed anomalous a emission. In fact, it is easy to realize that the mutual Coulomb field of adjacent fragments during or shortly after scission markedly influences the evaporation barrier and thus the intrinsic particle-emission probability. Trajectory calculations for ternary fission have neglected this effect; related three-body effects have been described for sequential fission [ 16 ]. Fig. 4a depicts schematically light-particle (LP) emission at an angle 0 with respect to the scission axis from the fragment B in the Coulomb field of fragment A. The Coulomb barrier V3n of the emitter in the three-body configuration is given by the Coulomb barrier VEB,describing the barrier without an adjacent fragment, and an additional angle-dependent modulation of the barrier A V(0): Van= VEn- A V(0). From the total energy prior to and post light-particle emission we get

AV(O)_

ZA(ZLI,+Za)e 2

ZAZLpe 2

ZkZae 2

R

R2(O)

R,(O)

at the barrier radius r s = [R2-Rll =const. According to the evaporation model [ 17 ] this modulates the rate of emission Pocexp[ - ( B + V2B)/T] in the twobody configuration by a factor f(0) = exp [A V(0)/T]. For symmetric fission of the composite nucleus 169Ta (experiment B) we have calculated f(0) in various static configurations (frozen distance R) with a temperature of T = 3 MeV: For large distances (R>_,25 fro) there is almost no effect on the barrier (A V - 0, f - 1 ) whereas in the case of a near-scission emission (e.g. R = 12.5 fm) the modulation of the

26 February 1987

~ B + L P

fral~nentA 90°

b

/ 180 o

\ i l **

0J

Fig. 4. Schematic diagram of the scission configuration (a) where light-particle (LP) emission from fragment B + LP is considered in the close neighborhood of fragment A. Position vectors are indicated. (b) Polar diagram of the modulation factor f(0) for a (dashed) and proton (dotted) emission from fragment B (solid circle) in the Coulomb field of fragment A (hatched). Static configuration, R = 12.5 fro.

barrier AVis non-negligible as shown in fig. 4b, where the modulation factor f is plotted for proton (dotted) and a (dashed) emission as a function of the emission angle 0. In this configuration the Coulomb barrier for a emission in the direction of the scission axis [AV(0=0 ° ) = - 3.6 MeV] and lowered perpen23

Volume 186, number 1

PHYSICS LETTERS B

dicular to the scission axis [6V(0=90 °) = + 2 . 7 MeV ]. These calculations confirm the very different emission pattern of proton and a evaporation: the surplus and deficit in the a spectra correlated with the scission axis and the nearly absent anomaly for protons. That the timescale for separating the fragments indeed is comparable to the timescale for light-particle emission and that the rotary motion o f the scissioning nucleus does not smear out the nuclear field emission ( N F E ) is verified by the observation o f a nuclear shadowing. This is a sharp depletion o f the low energetic particle yield in direction o f the asymptotic scission axis [ 2,10,11 ]. The occurrence o f this proximity effect indicates that the rapidly increasing m o m e n t o f inertia of the scissioning system results in an almost fixed configuration and that some non-negligible light-particle emission takes place during or shortly after scission. Furthermore the scaling o f NSE with excitation energy holds to good accuracy because o f the factorisation o f the emission rate. For a more quantitative analysis including the dynamics and the time dependence o f f ( 0 ) we performed three-body Coulomb-trajectory calculations including NFE during the scission process, which we simply approximate by a friction force between the scissioning fragments. Using a nuclear friction constant fl = 5 X 1021 s - 1 [ 18 ] results in a saddle-to-scission time o f 2 X 10- 21 s. These calculations yield modulation factors f ( 0 ) averaged over the scission time which are nearly those o f fig. 4b. The measured differential a multiplicities for surplus (perpendicular to the scission axis) and deficit (in direction of the scission axis) are larger

24

26 February 1987

by a factor of about 2.5 than the calculated ones. We have not attempted to fit the data by adjusting the saddle-to-scission time by changing the nuclear friction constant and/or the particle decay constants. The fact that the simple calculations undervalue the surplus and deficit yield by the same factor we regard as a strong evidence in favour o f our conception o f NFE. In conclusion we state that our data clearly exhibit a near-scission emission mechanism and all the observed features o f this emission process, including the scaling with excitation energy, are satisfactorily explained by our simple quasi-static model o f Coulomb-field induced a emission from fragments during or shortly after scission.

References [ 1] E. Duek et al., Phys. Lett. B 131 (1983) 297. [ 2 ] L. Schad et al., Z. Phys. A 318 ( 1984) 179. [ 3 ] E. Dueket al., Z. Phys. A 317 (1984) 83. [4 ] I. Halpern, Annu. Rev. Nucl. Sci. 21 (1971 ) 245. [ 5 ] B. Lindl, Ph.DF. Thesis (Heidelberg, 1985), unpublished. [6] M. Sowinski et al., Z. Phys. A 324 (1986) 87. [7] B. Lindl et al., Nucl. Instrum. Methods 224 (1984) 448. [8] S.B. Kaufman et al., Nucl. Instrum. Methods 115 (1974) 47. [9] J.M. Alexander et al., Z. Phys. A 305 (1982) 313. [ 10] G.Y. Fan et al., Z. Phys. A 310 (1983) 269. [ 11 ] H. Ho et al., Nucl. Phys. A 437 (1985) 465. [ 12] H. Ho et al., Z. Phys. A 300 (1981) 205. [ 13] A. Brucker et al., to be published. [ 14] L. Schad, Ph. D. Thesis (Heidelberg, 1983), unpublished. [ 15] R. Vandenbosch and J.R. Huizenga, Nuclear fission (Academic Press, New York, 1973) p. 376. [ 16] P. Gl~isselet al., Z. Phys. A 310 (1983) 189. [ 17] H. Ho et al., Z. Phys. A 283 (1977) 235. [ 18] H.A. Weidenmiiller et al., Phys. Rev. C 29 (1984) 879.