Journal of Nuclear and Radiochemical Sciences,Vol. 3, No. 1, pp. 63–65, 2002

63

Systematic Studies of Asymmetric Mass Distributions in Proton-induced Fission ofActinides
S. Goto,∗,a D. Kaji,a I. Nishinaka,b Y. Nagame,b S. Ichikawa,b K. Tsukada,b M. Asai,b H. Haba,bS. Mitsuoka,b K. Nishio,b M. Sakama,c Y. L. Zhao,c,d K. Sueki,c M. Tanikawa,e K. Takamiya,fH. Kudo,a and H. Nakaharac
aDepartment of Chemistry, Niigata University, Niigata 950-2181, Japanb
Japan Atomic Energy Research Institute, Tokai, Ibaraki 319-1195, Japanc
Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, JapandInstitute of High Energy Physics, Chinese Academy of Science, Beijing 100039, Chinae
Department of Chemistry, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japanf
Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan
Received: March 22, 2002; In Final Form: May 11, 2002
In order to clarify the relationship between a mass distribution and shell structure of the fission fragments in nuclear
fission, the fragment mass and the kinetic energy distributions in the proton-induced fission of plutonium isotopes,
239,242,244
Pu, were precisely measured using a double time-of-flight system. It was found that the position of thelight side of the heavy asymmetric mass distributions shifts to a larger fragment mass number according to theneutron-proton ratio, N f /Z f , of the fissioning nucleus. The result is qualitatively explained by the change of themost probable mass number of fission fragments for Z = 50 proton shell.

1. Introduction

2. Experimental

In the low-energy fission of actinides, particularly in thethermal-neutron-induced fission and the spontaneous fission, anumber of studies of the mass distributions have been performed
for a long time. Comparing these distributions,1 it can be seenthat the peak of the heavy asymmetric mass distribution locatesat A ∼ 140 in any system and that the position of the light peakshifts with the mass number of the fissioning nucleus. These features have been qualitatively explained by the fragment shells of
Z = 50 and/or N = 82 and the deformed shell of N = 88. Until
now, however, direct evidence of the shell effects has not beenpresented.
Recently, in the low-energy proton-induced fission of actinides, it has been demonstrated that there exist at least two
independent deformation paths for fission process; one leads toa symmetrically elongated scission configuration, and the otherleads to a compact scission configuration with reflection asymmetry.2 From the detailed analysis with respect to the asymmetric mass distributions in the proton-induced fission of 232 Th,
238
U, 244 Pu, and 248 Cm, it has been found that the position of thelight side of the heavy asymmetric mass distributions converges
on A = 126–128, whereas the heavy side of those distributions
broadens with the increase of fissioning nuclear masses.3 Thesetrends are consistent with the assumption that the heavy fragments of Z = 50 or the light fragments of N = 50 are produced
preferentially.
If the systems have different neutron to proton ratios, N f /Z f ,the fragment mass number corresponding to Z = 50 and N = 50
should change with the ratios under the unchanged charge distribution (UCD) hypothesis. Therefore, it is expected that thefeatures of the asymmetric mass distribution will be affectedby the N f /Z f ratios. On the basis of this expectation, the mass
distributions in the proton-induced fission of uranium isotopes,
233,235,238
U, have been studied previously.4 As a result, it wasfound that the shell effects of fission fragments, particularly at
Z = 50, affect the mass division in the nuclear fission. To study
on this matter more systematically, in this work, the mass distributions in the proton-induced fission of 239,242,244 Pu were measured, and the influence of the shell effects of the fragments on
the mass division was discussed.

A proton beam with energy of 13 MeV was supplied fromthe JAERI (Japan Atomic Energy Research Institute) tandemaccelerator. An average beam current was 1.0–1.5 µA. The
239,242,244
Pu (∼60 µg cm−2 ) targets were prepared by electrodeposition on 0.1 µm Ni foils. Fission fragment velocities weremeasured by the double time-of-flight (double TOF) system.
The start and stop detectors of the first telescope (TOF1) placedat θlab = 50◦ were composed of a gold-evaporated thin organic
film and a microchannel plate (MCPD). A Si surface barrier detector (SSD) located behind the stop MCPD was used for thestop and energy signals. The second telescope, TOF2, was set
around θlab ∼ 128◦ at the opposite side of the beam direction inorder to detect complementary fragments. The start and stop
signals of TOF2 were taken by a MCPD and a thin (1 mm)
plastic scintillator, respectively. The flight paths of the telescopes were about 96 and 110 cm and their solid angles, 0.23
and 4.6 msr, respectively. More than 2 ×105 fission coincidence
events were accumulated in each fissioning system. Detector
calibration was performed by using 230-MeV 127 I beam on calibration targets, 89 Y, nat Ag, nat In, 141 Pr, and 159 Tb, which haveatomic and mass numbers in the regions of the fission products. Each target of them was made by vacuum evaporation
on 30 µg cm−2 thick carbon foil, and each thickness was 30–
40 µg cm−2 . The time-of-flight and the energy of the elastically scattered iodine ions and the recoiling target nuclei weremeasured for calibration. A time resolution of approximately
600–700 ps was achieved and the resulting overall resolutionof mass and total kinetic energy (TKE) were estimated to be∆M ∼ 2.0 amu and ∆TKE ∼ 2.5 MeV, respectively, which include the effects of energy loss of the particles passing throughthe target, the backing foil, and the foils of MCPDs.

∗Corresponding author. E-mail: goto@curie.sc.niigata-u.ac.jp.
FAX: +81-25-262-6116.

3. Results and Discussion
Based on the laws of the momentum and the mass conservation in the nuclear fission, the primary mass (A1 ) and the TKE ofeach fission fragment were evaluated from the measured velocities of a pair of fragments on the assumptions that no neutron
was emitted from the compound nucleus prior to fission and thatneutrons were isotropically emitted from primary fragments andhad no influence on the initial fragment velocity on the average.
The fragment mass and the total kinetic energy were calculated

c 2002 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 6/30/2002

64J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

Goto160

Fragment mass / amu

Yield / %101

10080100120

140160Figure 1. Fragment mass distributions in the 13-MeV proton-inducedfissions of 239 Pu (dashed line), 242 Pu (dot-dashed line), and 244 Pu (solidline).

1.52

1.54

1.56

1.58

1.60

Figure 3. Correlation between N f /Z f of the fissioning nucleus and themass number at the “one-fifth” yield of the peak for the heavy asymmetric mass distribution. Closed symbols represent the position of the lightside, and open symbols the heavy side. Expectation from the UCD hypothesis for Z = 50 (dashed line) and N = 82 (dotted line) is also shown.

101

170

Fragment mass / amu

Yield / %130

Neutron-to-Proton ratio of Fissioning Nucleus

Fragment mass / amu

01010-1120

1401201.50-110150

130140150160

Figure 2.
Decomposed heavy asymmetric mass distributions forthe 13-MeV proton-induced fissions of 239 Pu (dashed line), 242 Pu (dotdashed line), and 244 Pu (solid line).

by the following equations:and1,
1 + v1,cm /v2,cm(1)1(2)
A f v1,cm v2,cm ,2where v1,cm and v2,cm are the velocities in the center-of-mass system for the complementary fragments A1 and A2 , respectively,
and A f is the mass of the fissioning nucleus with a relation of
A f = A1 + A2 .
The obtained mass distributions are demonstrated in Figure 1.
A general feature of the mass distribution in asymmetric fission, as the mention above, is that the position of the light peakchanges with the mass number of fissioning nucleus, whereasthat of the heavy peak remains approximately constant. However, a detailed examination for the present fissioning systems
shows that the heavy peak slightly shifts to the heavy mass side
TKE =

150
140
130
120

232

236

240

244

248

Fissioning nucleus mass / amu

Fragment mass / amu

A1 = A f

160

Figure 4. Correlation between the mass of fissioning nucleus and themass number at the “one-fifth” yield of the peak for the heavy asymmetric mass distribution. Closed symbols represent the position of the lightside, and open symbols the heavy side. Expectation from the UCD hypothesis for the complementally light fragment of N = 50 (dashed line)
is also shown.

with the increase of the mass number of fissioning nucleus. To
examine the features of the asymmetric mass division more precisely, it is necessary to subtract the component corresponding
to symmetric mass division from the total mass distribution, although the contribution from the symmetric fission is not expected to be large in the the present systems as seen in the figure. The two component analysis in the TKE distribution foreach fragment mass was done according to Reference 4.
Figure 2 shows the heavy asymmetric mass distributions
which resulted from the two component analysis. It was foundthat the position of the light side of the heavy peak shifts to theheavy mass side with increasing A f , the same as the results inthe system of uranium isotopes. To discuss this trend quantitatively, the mass number at the “one-fifth” yield of the peak forboth sides of the heavy peak was plotted against N f /Z f and A fof the fissioning nucleus for all systems including the system

Systematic Studies of Asymmetric Mass Distributions
studied previously, p + 232 Th and p + 233,235,238 U (Ref. 4).
The results were depicted in Figures 3 and 4. The errors werecome from the ambiguity in the fit of TKE distributions forthe two component analysis. From these figures, it was foundthat the position of the light side of the heavy peak shifts certainly with increasing N f /Z f although the data disperse somewhat, while that of the heavy side shifts depending on the mass
number of fissioning nucleus, A f .
In Figure 3, for the light side of the heavy asymmetric mass
distribution, the dotted and dashed lines denote the trend of themass number expected from the UCD hypothesis for N = 82 andfor Z = 50, respectively. It was demonstrated in Figure 3 thatthe position of the light side of the heavy peak follow the mostprobable mass of Z = 50. On the other hand, as shown in Figure 4, the position of the heavy side shifts differently from theexpectation of UCD. Namely, the heavier the fissioning nucleus
mass, the larger the difference. This indicates that the yield of
the light fragment with N = 50 decreases with A f . Therefore, theshell effect of N = 50 in the asymmetric fission is not so strongas that of Z = 50.
4. Conclusions
To study the correlation between the fragment mass distributions and the shell structures of fission fragments, the fragment
mass and the energy distributions in proton-induced fissions ofplutonium isotopes, 239,242,244 Pu, were precisely measured usinga double time-of-flight method.
Obtained fragment mass distributions were decomposed into
symmetric and asymmetric mass division components by ana-J.Nucl.Radiochem.Sci.,Vol. 3, No. 1, 2002

65

lyzing the total kinetic energy distributions. The results indicatethat the position of the light side of the heavy asymmetric mass
distribution shifts to the heavy mass side with the N f /Z f valueof the fissioning nucleus. This trend is qualitatively explainedby the change of the most probable mass number of fission fragments with the Z = 50 proton shell.
On the other hand, the position of the heavy side of the heavy
peak depends not on N f /Z f but on the fissioning nucleus mass
A f . However, the positions deviate from the expectation of UCD
with A f . This indicates that the shell effects of N = 50 for fission
fragments are not so strong as that of Z = 50.
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Gorski, and R. K. Sjoblom, Proceedings of the 3rd IAEA
Symposium on Physics and Chemistry of Fission, Rochester,1973 (IAEA, Vienna, 1974), Vol. 2, p. 19.
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H. Ikezoe, Y. L. Zhao, K. Sueki, H. Nakahara, M. Tanikawa,
T. Ohtsuki, H. Kudo, Y. Hamajima, K. Takamiya, and Y. H.
Chung, Phys. Lett. B 387, 26 (1996).
(3) Y. L. Zhao, Ph.D. Thesis, Tokyo Metroporitan University,1999.
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Tsukada, M. Asai, H. Haba, S. Mitsuoka, K. Nishio, M.
Sakama, Y. L. Zhao, K. Sueki, K. Tanikawa, K. Takamiya,H. Kudo, and H. Nakahara, J. Radioanal. Nucl. Chem. (inpress).