Observation of fisson events in mica sandwiches

NUCLEAR

INSTRUMENTS

AND

METHODS

39 (1966) 2 2 4 - 2 3 1 " .(: N O R T H - H O L L A N D

PUBLISHING

CO.

O B S E R V A T I O N OF F I S S I O N EVENTS IN MICA S A N D W I C H E S E. C I E S I . A K , J. P 1 E K A R Z and J. Z A K R Z E W S K I

Inls'titutc of l','xperimenlal Physics, University of War,saw, I'Var,~aw and M. D A K O W S K I , II. P I E K A R Z and M. S O W I N S K I

btstitute of Nuch,ar Research, Warsaw Received 4 A u g u s t 1965 A m e t h o d is described of preparation of mica sandwiches that could be used in experiments to study the cross-section of heavy nuclei for the high-energy fission. Mica sandwiches with natural uranium, prepared by this m e t h o d , were exposed to thermal neutrons from a reactor. After appropriate chemical treatment,

fission fragment tracks were observed under an optical microscope. T h e average efficiency of observation of fission events in such sandwiches was estimated with the help of s e m i c o n d u c t o r detectors.

I. Introduction

wich prepared by partially cleaving a slab of mica. The aim of the present work was to establish a method of preparation of mica sandwiches that could be used in experiments to study the cross-section t\~r high-energy fission. Mica sandwiches with natural uranium, prepared by the method described below, were exposed to thermal neutrons from a reactor. After appropriate chemical treatment, fission fragment tracks were observed under an optical microscope. The efficiency of observation of fission fragment coincidences in such sandwiches was then investigated.

The experimental data orl the cross-section for fission of heavy nuclei bombarded with protons of energy greater than 1 GeV has been obtained until now by means.of experiments using the emulsion techniqueS' 2). Recently an effort has been made by the C E R N Naples-Warsaw collaboration 3) to study this problem with mica detectors. Mica will register tracks of heavy nuclei, such as fission fragments, but is insensitive to light nuclei with mass number less than about 30, ref. 4). For this reason, it has found wide-spread use for registering the tracks of tission fragmentsS). In order to determine the cross-section for fission in a high-energy reaction it is necessary, however, to distinguish fission events from those in which single heavy fragments are produced. This can be achieved with the use of a "sandwich" composed of two sheets of mica with a thin target layer between them3). For every fission fragment registered in sheet I, there should be present its partner in sheet 2. Only such events* should be taken as due to the fission process. With this idea in mind, Brandt et al. 6) exposed mica sandwiches with heavy target elements to the highenergy proton beams at the C E R N PS. Every sandwich was composed of a pair of separate mica sheets glued together with araldite at one side. A layer of a heavy element evaporated into the inside surface of one of the mica sheets served as a target. However, first attempts to find spatial coincidences of fission fragments were not successful. Another method of sandwich preparation was employed by Price et al. 7) in their study of fission of heavy nuclei induced by heavy ions. These authors placed a drop of solution containing a heavy element in a sand* Hereafter referred to as "fission fragment coincidences". 224

2. Preparation of mica sandwiches Natural mica, neither pre-etched in hydrofluoric acid, nor annealed at high temperatures, was used in this

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OBSERVATION

OF F I S S I O N

225

E V E N T S IN M I C A S A N D W I C H E S

work (the density of the "fossil" tracks 5) was about 0.9 x 102 tracks/cm2). Sandwiches were constructed from pieces of mica of dimensions 2.5 cm x 2.0 cm × 0.01 cm. Every piece was partially cleaved and an aluminium foil of dimensions 1.0 cm × 0.5 c m x 200 pg/cm 2 was inserted between the two mica sheets. The foil was covered on one side with a thin layer of a target element. Layers of target elements, about 20 pg/cm 2 thick, were obtained either by electrospraying 8) (in the case of natural uranium) or by thermal evaporation (in the case of other elements such as bismuth, lead, gold and silver). The thicknesses of the uranium targets were estimated from the zt particle decay rate. The number of ~e particles was determined with the use of a semiconductor spectrometer with a multichannel analyser. Fig. 1 shows a typical energy spectrum of these particles: separate lines belonging to the 2341.j, 235 U and Z38U isotopes arc clearly seen. The accuracy of this method of thickness determination is limited by the number of counts and by the error in the determination of the solid angle subtended by the semiconductor detector. The thicknesses of the remaining targets were determined by weighing the aluminium fifils before and after evaporating a metal on them. Rectangles of dimensions 0.5 cm 2 were cut out of larger pieces of the foil (16 cm 2) to reduce the errors in the thickness determination. In order to keep the two sheets of mica forming a sandwich close together, a drop of a liquid was placed between them. Of the various substances tested, nitro lacquer dissolved in acetone was finally used. However, it was found that the application of a slight pres-

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sure to the freshly prepared mica sandwiches also produced the desired effect. The discussion of the results obtained by both methods is given in section 4.

3. Observation of spatial coincidences Sandwiches with uranium* were exposed to thermal neutrons from a reactor of the Institute of Nuclear Research, Warsaw, at Swierk. After the irradiation, all sandwiches that were glued with nitro lacquer had to be immersed in acetone for up to several hours to remove the lacquer solution. After rinsing in water and alcohol, they were p l a c e d - t o g e t h e r with those prepared without the g l u e - i n hydrochloric or nitric acid to dissolve the targets. This procedure was repeated until the inner surfaces of the mica sheets were quite clean. The normal technique of mica processing was Fig. 2. T h e m i c r o p h o t o g r a p h o f typical coincidences obtained in a sandwich without the lacquer solution. Both the inner surfaces of the two mica sheets are in focus at the same time.

* The results of the exposures o f sandwiches with u r a n i u m a n d other elements to the high-energy proton beams at the C E R N PS will be described elsewhere.

226

r:. CIESLAK et al.

then as followsS). The sandwiches werc placed for 20 to 30 rain in hydrofluoric acid at room temperature and afterwards rinsed in water and alcohol. All the observations were made on an ordinary optical microscope. Spatial coincidences of fission fragments were observed in all u r a n i u m sandwiches irradiated with thermal neutrons. Fig. 2 shows a microphotograph of typical coincidences obtained in a sandwich without the lacquer solution. In this photograph both the inner surfaces of the two mica sheets are in focus at the same time. One can clearly see tracks of fission fragments impinging on mica at various angles of incidence. The dip angles, 0, of the tission fragment tracks were measured by several observers on a Koristka MS2 microscope and the angular distribution obtained for sandwiches without the glue are shown in fig. 3. Shaded areas correspond to single, uncorrclated tracks which will be discussed in greater detail below.

TABLF,

Experiment 1

1

Experiment 2

Experiment 3

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The efficiency of registration of u r a n i u m fission fragments as a function of their angle of incidence on natural mica was first investigated with an cxpcrimental set-up shown in tig. 4. A fission fragment beam was obtained by irradiation of an u r a n i u m layer with thermal neutrons from a reactor. The beam was monitored with a semiconductor detector placed inside the vacuum chamber. The n u m b c r of fission fragments incident on a single mica shcet, set at an anglc 0 to the bcam, was expected to be the same for every angle 0 since the n u m b e r of counts on the m o n i t o r was the same for every exposure. Three experiments were per-

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OBSERVATION

OF FISSION

EVENTS

227

IN M I C A S A N D W I C H E S

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an important role. A n y conclusions based on the measured dip angles o f tracks in mica must, therefore, be treated with caution. The efficiency o f observation o f fission fragment coincidences in a sandwich was then investigated. An uranium target, prepared as described in section 2, was exposed during a certain time to thermal neutrons from a reactor. The total n u m b e r o f fission events occuring in the target was determined from the number o f fragments emitted into a solid angle subtended by a

semiconductor detector*. This target was afterwards placed in a partially cleaved piece o f mica and irradiated with neutrons for such a time that the expected density o f fission fragment coincidences did not exceed about 10 3 per cm 2 (the fluctuations in the neutron flux were estimated to be about 2 ~ ) . Four sandwiches with lacquer solution and four without it were irradiated; thc number o f expected fission events varied from * T h e efficiency of registration of fission f r a g m e n t s in the semic o n d u c t o r detectors used in this w o r k was greatcr t h a n 99%.

E. CIESLAK et al.

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OBSERVATION OF FISSION EVENTS IN MICA SANDWICHES

220

TABLE2 The efficiency of observation of fission fragment coincidences in sandwiches

Sandwich no.

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Number of single tracks per number of expected fission events

Number of observed coincidences* per number of expected fission events

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* Corrected for the expected number of "fossil" tracks. about 700 to 2000. The numbers o f fission fragment coincidences and single tracks observed in the target area o f every sandwich are given in table 2. It is seen that the efficiency o f observation o f coincidences, i.e. the number o f coincidences observed in the target area per n u m b e r o f the expected fission events exhibits large variations for sandwiches with the lacquer solution. A great n u m b e r o f fission events was detected in every such sandwich outside the target area indicating that uranium must have been smeared a r o u n d by the lacquer solution. This fact explains why the observation efficiency differs so strongly from one sandwich to another, the smearing o f uranium being an uncontrolled effect. The results are more consistent for sandwiches without the lacquer solution, where no fission events were seen outside the target area. As an approximate estimate o f the efficiency in this case one can take the average value of (96 ___5)~o. The quoted error is a r m s error o f the average c,,dculated from the spread o f the individual values. As can be seen from table 2, in every sandwich one observes a n u m b e r o f fission fragment tracks without their partners. F o r sandwiches without the lacquer solution the fraction o f such single, uncorrelated tracks is much larger on the mica surface adhering to the target layer than on that close to the aluminium foil. This effect can be explained as a result o f the absorption in the aluminium foil o f fission fragments o f small angles of incidence (see also fig. 3). The effect o f the foil can also be seen in fig. 7, which

shows the range* distribution o f fission fragments separately for both mica sheets o f a sandwich. The average range for fragments which passed through the foil is about 1 l~m smaller than for those which did not. In this work one could not see spatial coincidences for which the distance L, between the entrance points o f the two fission fragments was greater than the field o f view o f the objective used for observation (i.e. a b o u t 250 ~m, see fig. 8). Some o f the uncorrelated tracks are, therefore, a result o f the non-observation o f spatial coincidences o f two fragments for which L was greater than about 250/~m. Adding the fraction of the uncorrelated tracks observed in mica sheets adhering directly to the target layers to the efficiency o f observation o f coincidences, one finds that the average efficiency o f registration o f fission events in sandwiches without the lacquer solution is (99 ___4) ~o +. The results obtained for sandwiches without the glue suggest that fission fragments are registered with 100~o efficiency at all angles of incidence (including those less than 6 '~ which were not studied directly in the present experiment). This agrees with a similar conclusion reached earlier by Debeauvais et al. ~) who c o m p a r e d * The range R, was defined as the distance between the tangents to the extremities of each track. ÷ It should be stressed that the figure given previously is for the efficiency of observation of coh~cidences, not the efficiency of registration of fission events.

r:. C I I ! S L A K et al.

230

the registration efficiency of fission fragments in mica with that in emulsion. As shown in fig. 8, for evcry pair of tission fragment tracks the distance H, between the two mica sheets of a sandwich at exposure is given by the formula H = L/cotgO. For isotropic angular distribution half of the events should have dip angles less than 30", i.e. L > L½ = H cotg 30 °. Hence the determination of L~, gives a rough estimate of H = Litg30 :. Measurements

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of L were made in every sandwich and the values of H so obtained are given in table 2. They were corrected for the cffcct of the non-observation of events for which L was greater than about 60/~m (i.e. the tield of view of the objective used for measurements). 5. Conclusions The main results of this work can be summarized as follows" I

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OBSERVATION

OF

FISSION

EVENTS

IN

MICA

SANDWICHES

231

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....... i_ Fig. 8. The cross-section through a pair of correlated fission fragments tracks registered in a mica sandwich. 1. A m e t h o d o f p r e p a r a t i o n o f mica sandwiches described in this p a p e r m a d e it possible to observe the spatial coincidences o f the two f r a g m e n t s arising in the thermal fission o f uranium. O f the two techniques tested, the one in which no gluing solution is used is s u p e r i o r in the present e x p e r i m e n t a l conditions. 2. The efficiency o f registration o f fission events in n a t u r a l mica was f o u n d to be the same for the e x a m i n e d angle o f incidence r a n g i n g from 6 ° to 90 °. 3. The average efficiency o f o b s e r v a t i o n o f fission f r a g m e n t coincidences was e s t i m a t e d to be (96 + 5) % for sandwiches without the gluing solution. 4. The average efficiency o f registration in n a t u r a l mica o f fission fragments o f all angles o f incidence was inferred from the a b o v e result to be (99 + 4) ~ . Some o f the a b o v e rcsults, e.g. the m e t h o d o f sandwich p r e p a r a t i o n and target thickness d e t e r m i n a t i o n , m a y be utilized in e x p e r i m e n t s investigating the crosssection o f heavy nuclei for the high-energy fission. However, the efficiency o f o b s e r v a t i o n o f coincidences m a y be lower than t h a t d e t e r m i n e d in this w o r k as a result o f the n o n - c o l l i n e a r i t y o f some fission events a n d the possible a n i s o t r o p y in the a n g u l a r d i s t r i b u t i o n o f fission f r a g m e n t s with respect to the incident p a r ticles~).

The a u t h o r s wish to t h a n k the C E R N a n d Naples g r o u p s for their c o - o p e r a t i o n . They are very grateful to Prof. M. D a n y s z for his s u p p o r t a n d interest a n d both to him a n d to Dr. W. O. Lock ( C E R N ) for discussion a n d c o m m e n t s on this work.

References l) See e.g.H.G, de Carvalho, G. Cortini, M. Muchnik, R. Rinzivillo and E. Sassi, Nucl. Phys. 53 (1964) 345. 2) See e.g.P.A. Gorichev, O. V. Lozhkin, N. A. Perfilov, JETP 45 (1963) 1784. 3) CERN-Naples-Warsaw Collaboration, CERN, EmC 64/19 (1964). 4) R. L. Fleischer, E. L. Hubbard, P. B. Price and R. M. Walker, Phys. Rev. 133 A (1964) 1443. s) See e.g.R.M. Walker, Proc. Conf. Strasbourg (1963). 0) R. Brandt, Ch. Gfeller and J. Zakrzewski, Yellow Report CERN 64 49 (1964). 7) p. B. Price, R. L. Fleischer, R. M. Walker and E. L. Hubbard, Proc. Third Conf. on reactions between complex nuclei, University of California Press (1963) p. 332. 8) M. Dakowski, H. Piekarz and M. Sowinski, Institute of Nuclear Research, Warsaw, Report 595/IA/PL (1965). 9) M. Debeauvais, M. Maurette, J. Mory and R. M. Walker, Int. J. Appl. Rad. lsot. (G.B.) 15 (1964) 289.