Volume 124B, number 3,4 PHYSICS LETTERS 28 April 1983

TOTAL KINETIC ENERGY RELEASE AND MASS DISTRIBUTIONS

IN PROMPT AND DELAYED MUON INDUCED FISSION OF 238U

P. DAVID a, j . HARTFIEL a, H. JANSZEN a, T. JOHANSSON b, j . KONIJN 1, T. KROGULSKI c, T. MAYER-KUCKUK a, C. PETIT JEAN d, S. POLIKANOV c, H.W. REIST d and G. TIBELL b

a Institut ffir Strahlen- und Kernphysik, University o f Bonn, Bonn, Fed. Rep. Germany b The Gustaf Werner Institute, Uppsala, Sweden c Institute for Nuclear Research, Swierk, Poland d SIN, Villigen, Switzerland e CERN, Geneva, Switzerlandand GSI, Darmstadt, Fed. Rep. Germany

Rcceived 2 February 1983

The total kinetic energy release and mass distributions in prompt and delayed muon induced fission of 238U were mea- sured. A yield of prompt symmetric fission (116 ~< A ~< 122) of (0.26 -+ 0.10)% with a peak to valley ratio of 25 in the mass spectrum was observed. A ratio of prompt to delayed fission yields of (8.8 -+ 0.3)% is deduced.

Negative muons after having formed muonic atoms induce fission in heavy nuclei in two different modes. These can be separated experimentally into a prompt component caused by radiationless transitions in the muonic cascade, and a delayed part due to nuclear cap- ture of the muon. In the prompt part the muon influ- ences important fission parameters, which open new possibilities to understand the fission process. Some effects like the augmentation o f the fission barrier are well established both experimentally and theoretical- ly [1,2].

The p r o m p t / 2 - induced fission of 238U was shown to be due to the radiationless 3d ~ ls transition in (74 -+ 14)% of all events [2], implying that the en- trance channel is dominated by a quadrupole excita- tion (E.y ~ 9.4 MeV). In photofission on the other hand, dipole absorption is the most common process, whereas fission induced by hadron scattering involves several angular momenta.

By measuring the total kinetic energy (TKE) release and mass distributions in muon induced fission for the prompt and delayed components separately, one obtains interesting data for a comparison. In the

1. NIKHEF-K, Amsterdam and CERN visitor.

prompt part the muon may to some extent affect the transition from the saddle point to scission. By extend- ing the comparison to results obtained with other probes one might also see the effects of other differ- ences in the entrance channels, both with regard to energy and angular momentum of the observed radia- tion.

Here we present results from such a measurement in 238U made under much more favourable experimen- tal condit ions than in the only one previously report- ed [31.

The experiment was performed at SIN using a nega- tive muon beam with a momentum of 53 MeV/c, a Ap/p of 3% and an intensity of 105/2 /s, as defined by a two-scintillator telescope (area 38 cm2). The ex- perimental arrangement is shown in fig. la. Fission fragments were detected in coincidence in two Si sur- face-barrier detectors, F 1 and F2, each 9 cm 2 large and 60/2m thick, mounted opposite to each other and aligned parallel to the target. The detectors were placed at distances 5 and 55 mm from the target, re- spectively. The reason F 2 was placed further from the target was to reduce the energy loss dispersion o f the fragments. The uranium target was 4 × 4 cm 2 in area and consisted of a layer 380/2g/cm 2 thick. It was

0 031-9163 /83 /0000-0000 /$ 03.00 © 1983 North-Holland 161

Volume 124B, number 3,4 PHYSICS LETTERS 28 April 1983

S I

2(m,

S~MI M2

F~T

a

F2

10 ~

10:

10

, , , , , , , , , , , , . , , , , , , , , , ,

b Z)~U( I j ] f )

/1 L . . . . . . . . . . . . . . . . . • . . * . . . . . . . , . . . . . . . . . . 1 + ' ' " " " 1 1 " * : + * ' Z ' l l l *

I = 7 8 ± 2 I n s l

l i i i L i I i , i , , , ~ i , L i

0 5 10 15 t [ns]

Fig. 1. (a) Experimental set-up consisting of the two (S1, $2) scintillator muon telescope and the vacuum chamber contain- ing the target T and the fission detectors F1 and F2. M1 and M2 are A1 moderators. (b) Time spectrum t (g-, FI).

made by evaporating natural uranium as UF 4 on a 250 gg/cm 2 Ni foil. The muon beam passed through F 1 before reaching the target. A time resolution of 1.2 ns (FWHM) was achieved in the measurement of the time difference between pulses from the muon telescope and detector F 1 (see fig. lb) .

The lifetime of muonic 238U in the ls state was determined from the t ime distr ibution of delayed fis- sion fragments. It was found to be (78 ± 2) ns in agree- ment with previous results [ 4 - 6 ] .

The fission detectors were calibrated with a 252Cf source. Mass and kinetic energy distributions of the correlated fragments were determined using the i teration procedure developed by Schmitt et al. [7]. Corrections for the emission of neutrons were taken into account according to ref. [8], From the 252Cf calibration data we estimate a mass resolution of 5 u.

Two mass distributions were reconstructed in the iteration process: one for prompt and the other for delayed events. The "p rompt" distribution had to be corrected for the admixture of delayed events in the chosen time interval of -+ 1.8 ns. The amount of these

6 Y

1%]

4

a

, , , , , , ,

238U (~J: f )prompt

+++ +++++ ++ + +

{ + + +

f ÷ ÷ ÷

I I I I I I I

80 100 120 140 160

6 Y

[%]

t,

b

i ~ , l l ,@ l ° I

80

238U(~-,f) detayed

O O

O • o

@ @ O

i I I I I ~ * - '

100 120 %0 160

Fig. 2. (a) Mass distribution of prompt fission events. (b) Mass distribution of delayed fission events.

delayed events was found by decomposing the time spectrum (fig. lb) into a gauss±an and an exponential component with a gauss±an time resolution curve folded in. After normalization to the number found in the " p r o m p t " window, the mass distribution of the delayed fission events was subtracted to produce the final mass distribution of prompt fission events (fig. 2a). The mass distribution for delayed events is plot ted in fig. 2b. For the ratio of prompt to delayed fission events our analysis yields a value of 0.088 + 0.003. This value is in general agreement with all previous results as listed in ref. [6] except for the value given in ref. [ 1 ]. The time resolution in the ex- perinaent of ref. [1], however, is a factor of three worse.

A striking difference is observed between the two mass distribution with regard to the yields of symmet-

162

Volume 124B, number 3,4 PHYSICS LETTERS 28 April 1983

I0- i

Y[O/o]

I.

0.1

I i I r I [ I i I F ; p

zx o

t{ o zx

238 U ( o., O..' f ) o

( ' y B r e m s , f ) z,

I I I ~ i 1 i

80 I00 120 I/.0 160

180

170 >~ z

150

80 90 100 110 A i i

o o

o ,7 ,++

0 ~ ii *

+ ,~. i, " Z38U(~u-,f)Pr'°mP f • 238U (,u-, f ) delayed

• o 23aUla ,af i ,Ea=120He V

i i I i 0.S 0.6 07 0.8 09 ALLAH I~

Fig. 4. TKE of prompt (dots) and delayed (triangles) fission events versusAL/AH, the mass ratio of light and heavy frag- ments. The open circles are the data from the reaction 238U (~, c~'to of ref. [10].

Fig. 3. Prompt/a- induced fission mass yields (dots) together with the yield from the reaction 238U(c~, c~'f) fo re x = 5-10 MeV [10] (open circles) and the post-neutron yield from the

fFmax - reaction 238U (% t0 with bremsstrahlung o__,, t - 12 MeV [ 11 ] (triangles).

ric fission. This difference seems to be mainly deter- mined by different excitat ion energies as will be dis- cussed for the TKE below. Our result for the sum of prompt and delayed fission yields is in good agree- ment with previous results for muon induced fission obtained at SIN [9] by radiochemical methods, where the two contributions cannot be separated.

In fig. 3 the prompt mass distribution is plot ted together with data from inelastic a-scattering on 238U for excitat ion energies from 5 to 10 MeV [10] and the post-neutron mass distribution obtained in brems- strahlung induced fission of 238U with E~ nax = 12

MeV [ 11 ]. Our results for the symmetric mass yield for 116 ~< A ~< 122 is (0.26 + 0.10)%, in agreement with the results from the other measurements display- ed in the figure. For this comparison the result from photofission was corrected for the emission of neu- trons. The accuracy of the symmetric yield measured for prompt fission is limited not only by the subtrac- t ion of delayed events (included in the statistical er- ror) but also by the energy loss of fission fragments in the target.

In fact, within the present accuracy the three mass distributions are quite similar over the whole mass range. Thus, the presence of the muon and the large admixture of quadrupole excitation in the entrance channel do not give noticeable effects on the prompt mass distribution in muon induced fission.

Three TKE distributions are presented in fig. 4. One is from 238U fission induced by inelastic a-scat- tering in the excitation energy region 5 to 10 MeV [10]. The other two are from the present experiment. They correspond to mean excitation energies of about 9 MeV in the prompt case, assuming 2p -+ ls and 3d --* 1 s radiationless transitions to be responsible in the proport ion 1 : 3 [2], and between 18 and 12 MeV in the case of delayed fission due t o / a - capture. These values correspond to first and second chance fission, respectively [ 12].

Before we compare the three distributions it should be pointed out that there is an uncertainty o f about -+4 MeV in the absolute values of the TKE in our mea- surements due to corrections of energy losses in the target, including the backing material. Since the prompt and delayed distributions are affected in the same manner the comparison between these two is not disturbed by this uncertainty.

As seen in fig. 4 these latter distributions are iden- tical for mass ratios in the region 0.5 to 0.7. From a ratio of 0.75 up to the symmetric case there are large

163

Volume 124B, number 3,4 PHYSICS LETTERS 28 April 1983

differences. These might be caused by a combinat ion of effects from the difference in the average excita- t ion energy of the fissioning nucleus, or, in the case of prompt fission, the dominance of quadrupole exci- ta t ion and the presence of the muon.

A comparison with the results from c~-induced fis- sion is more difficult, due to the above mentioned un- certainty in the TKE scale. However, there is a clear slope difference for mass ratios below 0.75 between ceinduced fission on the one hand and the two muon distributions on the other. In addition, the prompt TKE distribution peaks at a mass ratio which is larger than for the other two.

In order to shed more light on the observed differ- ences in the TKE distributions it would be very help- ful to have such data also from photofission of 238U for several well defined excitat ion energies.

It is a pleasure to thank Professor Blaser and his staff for their encouraging support and for the excel- lent working conditions at SIN as well as Dr. A.M. Wetherell, EP Division, CERN for his continuing sup- port. We appreciate very much the help of Mrs. H. Hodde (ISKP) and Mr. J. Pfeiffer (KFA Jfilich) in pre- paring the 238U target for these measurements. We are indebted to the following institutes or organiza- tions for financial support: ,Bundesministerium far

Forschung und Technologie der Bundesrepublik Deutschland (PD, JH, H J, TM-K), the Swedish Natu- ral Science Research Council (T J, GT), NIKHEF-K, Amsterdam (JK) and the Insti tute of Nuclear Research, Swierk (TK).

References

[1] D. Chultem et al., Nucl. Phys. A247 (1975) 452. [2] T. Johansson et al., Phys. Lett. 97B (1980) 29;

A.H. Blin and G. Wolschin, Phys. Lett. 112B (1982) 113, and references therein.

[3] B. Budick, S.C. Cheng, E.R. Macagno, A.M. Rushton and C.S. Wu, Phys. Rev. Lett. 24 (1970) 604.

[4] Dz. Ganzorig et al., NucL Phys. A350 (1980) 278. [5] W.W. Wilcke et al., Phys. Rev. C21 (1980) 2019. [6] S. Ahmed et al., Phys. Lett. 92B (1980) 83. [7] H.W. Schmitt, W.E. Kiker and C.W. Williams, Phys. Rev.

137 (1965) B837; H.W. Schmitt, J.H. Neiler and F.J. Walter, Phys. Rev. 141 (1966) 1146; F. Plasil, R.L. Ferguson, F. Pleasonton and H.W. Schmitt, Phys. Rev. C7 (1973) 1186.

[8] P. David et al., Nucl. Phys. A380 (1982) 27. [9] P. Baertschi et al., Nucl. Phys. A294 (1978) 369.

[10] B.B. Back et al., Phys. Rev. C23 (1981) 1105. [11] E. Jacobs et al., Phys. Rev. C19 (1979) 422. [12] J. Hadermann and K. Junker, Nucl. Phys. A271 (1976)

401.

164