Studies of high-energy nuclear fission by means of mica detectors

I

2.J

I

NuclearPhysics A90 (1967) 177--185; ~ ) North-HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout written permissionfrom the publisher

STUDIES OF HIGH-ENERGY NUCLEAR FISSION BY M E A N S O F M I C A D E T E C T O R S CERN - HEIDELBERG - NAPLES - WARSAW COLLABORATION R. BRANDT t, F. CARBONARA it, E. CIESLAKttt, M. DAKOWSK1 +~, Ch. GFELLER ~t, H. PIEKARZ ++, J. PIEKARZ ttt, W. RIEZLER .+t++, R. RINZIV1LLO tt, E. SASSI ~-t, M. SOWINSKI ~t and J. ZAKRZEWSKI *

CERN, Genet'a, Switzerland Received 3 June 1966 Abstract: Nuclear fission of heavy nuclei bombarded with high-energy protons of the CERN Proton Synchrotron was investigated by means of mica sandwiches. Apart from single tracks resulting from spallation reactions, and correlated pairs of tracks due to binary fissions, spatial coincidences of three fragment tracks were observed and interpreted as possible examples of ternary fissions of heavy nuclei. 1. Introduction

Recently the C E R N - H e i d e l b e r g - N a p l e s - W a r s a w C o l l a b o r a t i o n 1) has b e g u n a series of experiments 2 - 4 ) with mica detectors to study nuclear fission of heavy nuclei in the high-energy region. These experiments rely u p o n the basic fact that such detectors are insensitive to light nuclei 5), thus p e r m i t t i n g the use of high-intensity p r o t o n beams which can be m o n i t o r e d by radio-chemical m e t h o d s z). I n mica, only nuclei with mass n u m b e r exceeding a b o u t 30 can be registered 6). I n order to study nuclear fission at high energies, i.e. nuclear b r e a k - u p into two (or more) parts of c o m p a r a b l e masses 7), a p h e n o m e n o l o g i c a l distinction must be made between events in which two (or more) heavy fragments are produced (tentatively ascribed to "fission") a n d those in which there is only one heavy p r o d u c t nucleus (tentatively ascribed to " s p a l l a t i o n " ) . This m a y be achieved by using a " s a n d w i c h " composed of two sheets of mica with a thin target layer between them. A n event will be taken as due to b i n a r y fission, say, if one observes a spatial coincit CERN and Max-Planck-Institut ftir Kernphysik, Heidelberg, Germany. tt Istituto di Fisica Superiore dell'Universit/L Napoli, Italy. Istituto di Fisica Nucleare, Sezione di Napoli, Italy. *tt University of Warsaw, Warsaw, Poland. Institute for Nuclear Research, Warsaw, Poland. ~; CERN; present address: Technikum Burgdorf, Switzerland. ¢¢** CERN. * Visiting scientist at CERN and University of Warsaw, Warsaw, Poland; present address: Enrico Fermi Institute of Nuclear Studies, University of Chicago, Chicago, Ill., U.S.A. 177

178

P,. BRANDTe t

al.

dence of two heavy fragment tracks, generally one in each of the mica sheets forming a sandwich. In this report, qualitative features are discussed of the results obtained by exposing mica sandwiches to extracted proton beams at the C E R N PS.

2. Experimental procedure Several methods of preparation of mica sandwiches were described in the previous reports 3, 4) of this work. In the work presently described, sandwiches were made of single, partially cleaved pieces of natural mica * of about 100 #m thickness 3, s). Thin target layers were obtained by thermal evaporation, or electro-spraying (of uranium), onto an aluminium foil of thickness about 200/~g/cm 2, the foil then being inserted between the two mica sheets thus forming a sandwich. I f the inner mica surfaces were clean enough (no dust, fingerprints or scratches) then the application of a slight pressure was sufficient to close a sandwich. Care had to be taken, however, that no air channels connected the target area with the outside. Mica sandwiches were irradiated in the extracted proton beams of energy about 20 GeV at the C E R N Proton Synchrotron 9). The beams were monitored with aluminium foils in which the amount of 24Na, produced in the reaction 27Al(p, 3pn)24Na, was measured 2). In order to determine the possible contribution of particles other than the primary high-energy protons, the ratio, S = ~Al(18F)/(rAI(24Na), was investigated. It has been shown 7) that under the prevailing conditions, the reaction 2VA1 ~ 18F is insensitive to such background particles, whereas the reaction 27Al--+ 24Na is very sensitive to them, especially to neutrons of energy about 10 MeV 1o). Since the values of S found for the monitoring foils have agreed, within the errors, with those obtained at about 20 GeV for thin targets 11, 12), i.e. So = 0.74_+0.02, it is reasonable to expect that in the present experiment any low-energy background particles have played an insignificant role in the production of 24Na from AI. After exposure, the sandwiches were opened to remove the target foils** and placed for 20 to 40 minutes in hydrofluoric acid at room temperature to reveal the tracks. After rinsing in water and alcohol and drying, the sandwiches were observed under an ordinary optical microscope.

3. Results Results presented below were obtained from exposures in which protons were incident perpendicularly on mica surfaces. Figs. la, b, and c show, as an example, microphotographs of typical spatial coincidences of tracks observed in sandwiches with uranium, bismuth, and gold, respectively, irradiated with the proton ÷ Some mica samples were annealed at high temperature to remove the background of "fossil" tracks 5). ~÷ The removed foils could be analysed chemically to determine the amount of the target metal.

Facing

Fig.

p. 178

la) typical

binary

fission

events

of uranium.

Fig.

I b) typical

binary

fission

events

of bismuth.

Fig. I. Microphotograph of events registered in mica sandwiches exposed to an extracted beam of protons of about 20 GeV energy (both the inner surfaces of the mica sandwich are in focus at the same time).

Fig.

Ic) typical

binary

fission

events

of gold.

Fig.

Id) typical

Fig.

Fig.

le) typical

“fission

in flight”

(as described

“fission

in Right”

(as described

If) a possible

example

of a ternary

in the text)

for uranium

in the text) for bismuth.

fission

event

in uranium.

HIGH-ENERGY

NUCLEAR

179

FISSION

beam of about 20 GeV. In the photographs both Of the inner surfaces of the mica sandwiches are in focus at the same time. Similar events, observed in experiments 3, 4) in which mica sandwiches with uranium were irradiated with low-energy neutrons, were shown to be caused by low-energy fission of uranium nuclei sandwiched between the two mica sheets. It is clear, therefore, that events seen in fig. 1 must be due to binary fissions induced in respective heavy nuclei by the incident high-energy protons. o/.

(a)

8O

70

6o

"s 50 E

- -

~20OeV

protons

on U

....

550MeV

protons

on U

neutrons

on U

........ t h e r m a l

D

c 41 L

r--n

180

t60

140

120

tO0

Fig. 2. Distribution of angles less than 180° between the correlated tracks of fission fragments in projection onto the mica plane (the bombarding energy is given in the text). a) U (426 events), a comparison with 550 MeV p (130 events) and thermal neutrons (250 events) is given. The distributions of angles less than 180 ° between the correlated tracks of fission fragments, in projection onto the mica plane, have been investigated. Fig. 2a shows such a distribution for uranium irradiated with thermal neutrons; the spread in the angles reflects the experimental uncertainties inherent in this method (much larger errors affect dip angle measurements 3,4)). Fig. 2a additionally shows similar distributions for uranium bombarded with protons of energy 550 MeV 13) i and 21 GeV, respectively. It is seen that these distributions become broader with the increase of the bombarding energy. This can be explained as the result of fission of nuclei which received a sufficiently high momentum transfer, thus giving rise to V-like t

F u r t h e r details on the expost~res at 550 MeV will be publ i s he d elsewhere.

R. B R A N D T e t

180

aL

coincidences of tracks, shown in figs. ld and le. Similar distributions for Bi and Au at 21 GeV and 550 MeV are shown in figs. 2b and c. (b)

t

% m

~3o "6 .D

E

20

- -

~20 OeV protons on Bi

.....

550 MeV protons on Bi

-I

1

I r-~

180

160

140

120

100

80

60

I

I

I r-~

40

20 °

Fig. 2b. Bi (443 events), a comparison with 550 M e V p (300 events) is given.

(c)

,n 3C E --~20 "o

"~7

GeV protons on Au 550 MeV protons on Au

I

~2c

:

T1(

L

1 LJ k.~ 180

160

140

120

i-~._,~q r-.% 100 80 60

£ 0 ~ 20°

Fig. 2c. A u (250 events), a comparison with 550 MeV p (191 events) is given.

Figs. 3a, b, and c give the distributions of the ratio of the horizontal projections, ll/l a, of the correlated fission fragment tracks obtained in the irradiation of U, Bi and Au with protons of about 20 GeV and 550 MeV and, for uranium, also with thermal neutrons.

HIGH-ENERGY

NUCLEAR

181

FISSION

(a)

--3

% 40 c >

"*6

t

30

oa aQ

E

~20

OeV

protons

on

Uranium

.......

550

MeV

protons

on

Uranium

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

thermal

neutrons

on

Uranium

,..,:

20

lO

~ 1.0

2.0

3.0

,r-I

6.0

5.0

4.0

---~,_~

;....t

7.0

11 / 12

Fig. 3. Distributions o f the ratio o f the projected length, ll/12 o f the correlated fission f r a g m e n t tracks. a) U (30• events), a c o m p a r i s o n with 550 M e V p (200 events) a n d thermal n e u t r o n s (196 events) is given. %

lb)

4C i i i I i i

3c

--",,20GeV

protons

on Bi

.....

protons

on Bi

i J

550MeV

c 2C

10

1.0

2.0

3.0 --=~,-

4.0 11 l i 2

Fig. 3b. Bi (152 events), a c o m p a r i s o n with 550 M e V p (300 events) is given.

R, BRANDT et al.

182

Apart from coincidences of fission fragment tracks, many single, uncorrelated tracks were observed on both the inner mica surfaces in the target area of a sandwich. The ratio of the total number of such tracks, with projected length greater than 5/~m, to the number of binary fissions, noted in the same target area, is about 0.26 for uranium. This ratio is much larger than that observed in the experiment 3) on the thermal fission of uranium, where it was less than 0.04. In the latter case such single tracks were the result of the absorption of their fission partners in the supporting aluminium foil. In the present experiment, therefore, the majority of these tracks must be interpreted as due to the high-energy spallation reactions. °/.

40

(c)

30 -

-

~20

GeV p r o t o n s

on Au

5 5 0 MeV p r o t o n s

on Au

--i .......

1 20

1l _ ~

10

r-T-n 1.0

2.0

3.0

5.0

4.0 ~,~

6.0

|~/I 2

Fig. 3c. A u (152 events), a c o m p a r i s o n with 550 MeV p (190 events) is given.

The fraction of such single tracks increases when the mass of the target nucleus decreases: it is 1.0 for Bi and 1.6 for Au. Such an increase can be understood qualitatively as being due to the fact that the cross section for fission decreases rapidly with the value of Z2/A for the target nucleus ~4), while that for spallation does not. The observed large values of the fraction of single tracks with ranges comparable with those of the fission fragments indicate the necessity of using mica sandwiches (i.e. a 4n geometry) rather than single mica sheets (i.e. a 2re geometry) in studies of high-energy nuclear fission. 4. Observation of ternary fission Fig. I f gives a microphotograph of a three-pronged event observed at 18 GeV in uranium. A number of such events have been observed in this work and only those

HIGH-ENERGY NUCLEAR FISSION

183

251

{a)

•, expected 2C ......

experimental (118 X2= 585

T+

events)

(v = 2 )

"s P(X 2)

<< 1%o

E 1C

I

260

240

220

200

]

_~

180

L_

160

140

120°

25 (b) expected

I

20

I

.......

experimental (18T- events)

~(2 =1.15 46 ~' 15

I

(v = 2 )

p(~2 ) = 57°/°

l

A i r

260

240

220

~. . . . . .

'

200

180

"

160

140

120°

Fig. 4. Projected biggest angle distributions f o r three-prong events observed in u r a n i u m sandwiches exposed at 18 OeV: a) for T + events (defined in the text); b) for T - events (defined in the text).

184

R. BRANDTet al.

events have been selected for which the distance between the common intersect of the projected directions of the three tracks and the beginning of each track is less than 20/~m, and the length of each track is longer than about 3/~m. No such events were seen in previous experiments on uranium irradiated with low-energy neutrons 3, 4) (about 5 000 binary events) and with 550 MeV protons (about 5 000 binary events). In all, 118 events were observed for which all three tracks have such dip angles (points of entry into the mica) as to indicate that they may have emerged from a common centre (T + events). Clearly, among the T + events there may be examples of genuine ternary fission (i.e. nuclear break-up into three fragments, each with a mass number exceeding 30. It should be pointed out that the term "ternary fission" is used here without implication of any particular mechanism by which the reaction proceeds). In addition, 18 events have been observed whose dip-angle measurement indicates that they have not emerged from a common centre. These events are called T - and are an accidental coincidence of a binary fission with a single track in each case. Fig. 4 shows the distribution of the largest angle ~., in projection onto the mica plane, between every pair of tracks separately for the T + and T - events. The distribution expected for accidental coincidences is also shown. Both distributions are similar in the case of the T - events; however, the distribution for the T + events exhibits a relative excess of events with the angle c~ < 160 ° (47 events). A )~2 test comparing the shapes of the curves gives for two degrees of freedom, the value of 585 for T + distribution, and 1.15 for the T - one, thus supporting the interpretation given above. The ratio of the number of T + events, with angle e < 160 °, to that of binary fissions is 1 : (760 +__150). The relative frequency of ternary fission found in this work is smaller than that observed by Fleischer et al. ~ ) in an experiment on Th nuclei bombarded with 400 MeV argon ions. It is, however, much higher than in the experiments with 235U irradiated with thermal neutrons 16) (see ref. 15) for a comprehensive survey of the experimental results obtained until now). This presumably indicates the dependence of ternary fission on the excitation energy of the fissioning nuclei. The authors wish to thank Dr. W. O. Lock (CERN), Prof. G. Cortini (Naples), Prof. M. Danysz (Warsaw), Prof. W. Gentner (Heidelberg), Prof. W. Paul (CERN), Prof. P. Preiswerk (CERN), Dr. A. Kjelberg (CERN), Dr. G. Rudstam (CERN), and Dr. S. G. Thompson (Berkeley) for their interest and continued support of this work. It is a pleasure to acknowledge many helpful discussions with our colleagues from Strasbourg (see following publication).

HIGH-ENERGY NUCLEAR FISSION

185

References 1) CERN-Naples-Warsaw Collaboration, CERN Internal Report EmC 64/19 (1964) 2) R. Brandt, Ch. Gfeller and J. Zakrzewski, CERN Report 64-49 (1964) 3) E. Cieslak, J. Piekarz, J. Zakrzewski, M. Dakowski, H. Piekarz, and M. Sowinski, Nucl. Instr. 39 (1966) 224 4) F. Carbonara, P. Cuzzocrea, L. Ferorelli, R. Rinzivillo and E. Sassi, I N F N 65/4 TC-65/15, Napoli (1965) 5) R. L. Fleischer, P. B. Price, and R. M. Walker, Ann. Rev. Nucl. Sci. 15 (1965) 1 6) R. L. Fleischer, P. B. Price, R. M. Walker and E. L. Hubbard, Phys. Rev. 133A (1964) 1443 7) G. Friedlander, Proc. Symposium on the Physics and Chemistry of Fission, Salzburg, 1965 (IAEA, Vienna) Vol. II, p. 265; also R. Brandt, ibidem, Vol. 1I, p. 329 8) P. B. Price, R. L. Fleischer, R. M. Walker and E. L. Hubbard, in Proc. Third Conf. on Reactions between Complex Nuclei (University of California Press, Berkeley, 1963) p. 332 9) C. Bovet, A. Nakkasyan and K. H. Reich, Proc. Fifth Intern. Conf. on High-Energy Accelerators, Frascati, September 1965 (to be published) 10) K. Goebel, H. Schultes and J. Z/ihringer, CERN Report 64-12 (1964) 11) J. B. Cumming, J. Hudis, A. M. Poskanzer and S. Kaufman, Phys. Rev. 128 (1962) 2392 12) R. Brandt, Ch. Gfeller and W. Riezler, to be published 13) H. Faissner and H. Schneider, Nuclear Physics 19 (1960) 346; H. G. de Carvalho, G. Potenza, R. Rinzivillo, E. Sassi and G. Vanderhaege, Nuovo Cim. 25 (1962) 880 14) N. A. Perfilov, ZhETF (USSR) 41 (1961) 871; JETP (Soviet Physics) 14 (1962) 623 15) R. L. Fleischer, P. B. Price and R. M. Walker, Phys. Rev. 143 (1966) 943 16) L. Muga, Proc. Symposium on the Physics and Chemistry of Fission, Salzburg, 1965 (IAEA, Vienna) Vol. II, p. 409