or fractionally charged particles

NUCLEAR INSTRUMENTS AND METHODS t[ 5 U974) 245-252, © NORTH-HOLLAND PUBLISHING CO.

A COSMIC RAY TELESCOPE USED TO SEARCH FOR MASSIVE AND/OR FRACTIONALLY CHARGED PARTICLES C. R. ALCOCK*, A. CHISHOLM, M. TYNDELt and P. C M. YOCK Physics Department, Universtty of Auckland, Auckland, New Zealand

Received 20 August 1973 A telescope used to search for massive and/or fractionally charged particles m the cosmic radiation is described. The telescope uses plastic scintillators (for ionization and time-offlight measurements), wide gap spark chambers (to fix parncle trajectories) and lead/iron absorbers (for range measurements). ~Ilmlng and ionization measurements are recorded using a fast

oscilloscope. Measurements are recorded for slow particles (0 2 c
1. Introduction

of charged cosmic ray pamcles that are incident on it with speeds between 0.2c and 0"55c. F r o m the speed and Ionization measurements, charge measurements are made (to an accuracy of about +0.05e). Thus particles with fractional charge (not necessarily third integral) may be identified, if they are present. Likewise, from the speed and range measurements, mass measurements are made, and pamcles of abnormally high mass may be identified, if they are present. The range of speeds studied (VlZ. 0.2c to 0"55c) was determined as follows. The range measurements are performed with the use of lead/iron absorbers of thickness ~ 30 g/cm 2 This thickness was chosen so as to ensure that nuclear interactions of strongly interacting particles would not occur, in most cases, within the absorbers. This thickness determines the incident speeds that particles must have for the telescope to be able to provide meamngful information on their masses. Since the telescope was designed to search for heavy particles (i.e. of mass > 3 GeV/cZ), it was necessary that protons and deuterons [and tritons, although these are rare in the cosmic rays9)] had to be identifiable. The range of a proton with v ~ 0 . 5 c is 30 g/cm z, and that of a deuteron is twice this. Thus only p's and d's incident on the telescope with speeds <0.5c are slowed significantly by the absorbers and therefore distinguishable from heavier particles incident with the same speeds. The electronics was consequently set up to record data only for particles incident with speeds between approximately 0.2c and 0-55c. The lower limit of 0.2c was chosen so that a reasonable band of speeds would be examined. Clearly, the flux of heavy particles must (assuming it is non-zero) decrease with speed for non-relativistic speeds.

Recently, many theories have been proposed which assume or suggest that a class of subnucleonic particles exists. The most well-known of these theories is, of course, the quark model, in which it is assumed that the subnucleonic particles have charges __+½e and +~e. Many experiments have been set up to search for third-integrally charged particles but they have not yielded reproducibly positive results1). In view of the many theories a-7) that have been put forward which do not require that subnucleonic particles be thirdintegrally charged, a cosmic ray telescope has been set up at the University of Auckland to search for heavy, strongly-interacting, long-lived particles with charges not necessarily equal to _+½e or +_2e. The purpose of this paper is to describe the construction, calibration, and operation of the telescope. The telescope was designed specifically to search for stable or long-hved cosmic ray particles at 6 m.w.e. underground w~th the following properties: a) mass > 3 GeV/c 2, b) charge between approximately 0"5e and 2e in magnitude, not necessarily integral or third integral, c) mean free path for nuclear interactions > 30 g/cm 2, d) velocity between approximately 0"2c and 0.55c, and within 15 ° of the zenith. The operatton of the telescope may be summarized as follows. It records the speeds, ionizations and ranges * Address after September 1. Astronomy Department, California Institute of Technology, California, U.S.A. t Address after September 1" Cavendish Laboratory, Cambridge University, Cambridge, England. 245

246

c.R.

A L C O C K et al.

The telescope consists of two wide-gap spark chambers, five plastic scintillators with which time-offlight and ionization measurements are made, and two lead/iron absorbers with which the range measurements mentioned above are made. The methods of data acquisition and analysis that are used are quite simple, but somewhat unconventional, and these are described here in detail In its first 1900 h of operation the telescope was traversed by two particles which had the characteristics of massive particles. This has been reported elsewhereS). The flux of medmm energy deuterons at 6 m.w e. underground has also been measured and reportedg).

2. The telescope The telescope is shown in figs. 1 and 2. It is situated in the basement of a multi-storey building - at sea

cxPl02I: ~

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/I

SPARK CHAMBER

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SICINTINo, I

level under approximately 600 g/cm z of concrete. The temperature of the room is kept between 24"5°C and 27 °C There is an air gap of 2'1 m between the top of the telescope and the bottom surface of the concrete. The telescope is directed vertically. It has a crosssectional area of 0.164 m z and subtends a solid angle of 0.057 sr. Some discussion of the above described location of the telescope is necessary. If heavy hadrons exist in the cosmic rays then they would be produced in high energy interactions of cosmic rays at high altitudes in the atmosphere The depths to which they would then penetrate on average before stopping, assuming they are sufficiently long-lived so as not to decay m flighl, would depend on their masses, mean free paths, and interaction melasticmes Similarly, the best altitude for locating a detector of slowing 0 . 2 c < r < 0 - 5 5 c )

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Fig. 1 Front vlew of telescope (not to scale).

Fig 2. Side view of telescope (approxtmately to scale).

A COSMIC RAY TELESCOPE

247

<1-8 ns (Philips XP1021). The anode outputs of the heavy particles depends on thexr masses, mean free photomultipliers are delayed appropriately and dispaths, and Interaction melastw~ties. These quantxties played sequentially on a single sweep of the oscilloare not, a priori, known Consequently, the location scope. The trace is p h o t o g r a p h e d In this way tlme-ofof the present experiment (i.e. 600 g/cm z below sea level) should be regarded as a real location only. It as, fltght and lomzatlon measurements are made. The oscdloscope is a Hewlett-Packard 183 with an in fact, about an o p t i m u m depth for searching for 1830 amphfier and 1840 timebase, and has a bandwidth slowing, long-hved (lifetime > 1 0 . 6 s) hadrons with of approximately 400 MHz. The tImebase of the masses ~ 2 5 GeV/c 2, mean free paths ~ 120 g/cm 2, and melasticmes ~0.4. 4~oscdloscope is 10 ns/cw. This was calibrated to an accuracy of -+ 0"5% using a crystal controlled oscillator The five (tdentical) scmtdlators shown in figs. 1 and 2 h a w n g a period of 10.000 ns are N E 1 0 2 A plastic and are 40-6 x40-6 cm 2, 0.64 cm The oscilloscope trace m the single-sweep mode is thick. The light pxpes are adiabatic. The spark chambers are 10.2 cm thick and are filled with a mixture of rather faint, By using the blue PI1 phosphor, an f2 permanently open camera aperture, pulsed flood-gun 99% helium and 1% alcohol v a p o u r at atmospheric fogging, Polaroid type 410 film and a demagnificatlon pressure. Absorber no. ! consists o f 15"7 g/cm 2 Pb ratio of 0'5, satisfactory photographs are obtained. and 2.5 g/cm 2 Fe. This absorber prevents the telescope While the telescepets being operated m search for slow from being m g g e r e d by slowing light particles (e.g. parttcles the tugger rate ~s rather low (about once per muons) and gl~es a lower hmlt to the mass of any 6 h) and this necessitates t a k m g long exposures. To particle recorded. Absorber no. 2 was 32'g g/cm 2 Pb plus 30"0 g/cm 2 Fe for 500 h of operatton, a n d ~ a v o l d supertmposmg the traces for different eyents on 5.0 g/cm 2 Fe for the remainder This absorber gwes a"~ "one another the spot of the oscilloscope is scanned further range measurement on any p a m c I e recorded, vertically at the rate o f 1 division per 6 h. To avoid permitting a more precise mass hmlt calculation, excesswe fogging from the spot it was partially blanked The electomcs are shown m fig. 3. The photomultiwhen scanning. A n example of a 17 h time exposure is pliers have riseumes of 2 ns (Phihps 56 AVP) and shown m section 4. DELAY

.~'i~l

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Fig. 3. The electromcs. All delays (except for the T A C. strobe) are 50 (2 cables All fan-outs and mixers are passwe, symmetrical and of 50 -(2 impedance. All attenuators are passive, 2 x and of 50 -Q impedance. Pulse chppmg is achieved via a symmetrical matched. tee and a t 9 ns shorted cable. The oscilloscope is operated m the " A + B " mode.

248

c.R.

ALCOCK

Time-of-flight and ionization measurements are taken from such photographs using a travelling microscope. All timing measurements are taken from the leading edges of the pulses, at half maximum. The pulse heights are measured to evaluate ionizations. The oscilloscope is triggered by a 3-fold coincidence as shown m fig. 3. The constant fraction timing discriminators are O R T E C 463, the third discriminator is a Chronet~cs 164. In the slow particle work a pulse from PM no. 1 must precede a pulse from PM no. 5 by 11 to 3I us, and a pulse from PM no. 3 must occur within + 30 ns of the pulse from PM no. 1 in order for a coincidence to occur. This coincidence requirement accepts pamcles which penetrate the telescope at speeds between 0.2c and 0.55c, approximately. The time-to-amplitude converter (ORTEC 437A) and multichannel analyser (having 1024 channels) give an independent check on some of the timing measurements, as indicated in fig. 3. It is found that the relevant measurements coincide to within +_0.4 ns for more than 90% of muons, and better than this for slow ( ~ 0-5c) particles. The spark chambers are operated at 27.5 kV (pulsed). For each event, the high-voltage pulse is applied 1.1 ps after completion of the oscilloscope sweep, and the T.A.C. is strobed 100 ps after the high-voltage pulse is applied. This avoids interference from the spark chambers affecting the oscilloscope and multi-channel analyzer data.

3. Calibration of the telescope The time scale on the photographs is cahbrated in terms of the speeds of cosmic ray muons. These muon

et al.

pulses are also used to convert observed pulse amplitudes into energy losses in the scintillators. Muons penetrate the telescope at the rate of about one per two seconds. By adjusting relative delays to the coincidence circuit, the tugger circuit of the telescope was modified to accept fast pamcles (v ~c), and over 200 muon events were recorded at various times. The oscilloscope traces for 3 muons are shown in fig. 4. These observations, as shown below, yielded an absolute calibration for speed and ionizauon measurements. 3.1. SPEED MEASUREMENTS Using the known muon spectrum t°) the average speed of muons penetrating the telescope was calculated and found to be (0.991_+0.002)c. Using this number the telescope was cahbrated absolutely as a speed measuring instrument, as shown here. Let Tt = time particle passes through tth scintillator of fig. 1, t~ = (average) time photons generated at T~ reach photocathode of ith photomultiplier of fig. 1, z~ = time pulse from ith photomultipher reaches half its maximum height on oscilloscope screen of fig. 3, and then define T o = T j - Ti, hj = t j - t , , "Frj ~

j > i

7/3-~t"

Then clearly -c,~= t~j+6,~ (for all j > i ) , where the 6,1 are delays introduced by the electronics. The mean

Ftg. 4. The oscdloscope traces for three muons. The vertical gain for this exposure was 100 mV/div, and the time-base 10 ns/div. The pulses are negaUve (i.e. anode) pulses.

249

A COSMIC RAY TELESCOPE

of any ,~j could be measured for the muon events. Also, the corresponding mean value for t~j could be calculated assuming that the mean muon velocity was 0.991 c. In this manner 6 t 3,334 and 3z 5 were measured. The fwhm for the distributions of *~3, z34 and z25 were 2.0 ns, 2.0 ns and 3.6 ns, respectively. The fwhm for the distribution of½(zl4 + zas) was only 1.1 ns - the usefulness of this result becomes apparent below. Using the measured values of 314( = 313 + 834) and 625 it is possible to evaluate t14 and t25 for individual events. To measure actual transit times for particles passing through the telescope the following method is used. The geometry of tile telescope (see fig. 1) implies that

t14+t25 = T ~ 4 + T 2 5 . Also, unless the particle slows in absorber no. 1, T1, = T25. Since absorber no. 1 is only 8 cm above scintillator no. 5, T~, ~ Ta 5, and the error in assuming T1, =T25 is easily shown to be negligible. Thus the velocity of the particle can be calculated using the relation: (X 2 _}_y2 "I- 12)½

½(t1,+t25) where l = distance between scintillators 1 and 4 (or 2 and 5), x = horizontal particle displacement (as shown in fig. 1), y = horizontal particle displacement perpendicular to x (as shown in fig. 2). In the above equation l is, of course, known and x is measured for each event (via the spark chambers or the relative times of photomultiplier pulses 1, 2, 4 and 5.) In this experiment y was not measured for each event. Instead, the average value for y2 for a large number of muons was inserted into the above equation for each event. It was found that y2 ~ (12.7 cm)_2, and it was shown that the replacement of y2 by 2 2 introduced a negligible error (~<0.5%) in individual speed measurements. The accuracy of speed measurements made as above may be evaluated from the fwhm of the distribution of ½(z14+z25) for muons. This follows because the quantity ½ ( z l , + z 2 s ) = a constant plus the particle transit time from scintillator 1 to 4 (or 2 to 5). Since the fwhm for ½(zl, +z25) was found to be 1.1 ns for muons it follows that speed measurements may be made to an accuracy of better than + 5% for particles with speed v < 0.5c.

3.2. IONIZATION MEASUREMENTS

Absolute ionization measurements were made from the anode pulses recorded by the oscilloscope as described above. The average pulse heights for muons were measured and found to be 29 mV, 28 mV, 35 mV, 31 mV and 43 mV for scmtdlators 1-5, respectively. For any particular event the ionization in each of the five detectors is measured as a multiple of the average muon ionization. These measurements are then converted to absolute energy-loss measurements by noting that the average energy loss for muons in the five detectors is 2.23 MeV g-1 cm a. This figure was calculated from published energy loss tables for plastic scintillators la) and the known vertical muon energy spectrumS°). To obtain greater accuracy the above procedure was refined as follows. First, the slightly non-linear relationship connecting energy-loss and light output was incorporated in the manner of Goodlng and Pugh12). Second, small drifts in the photomultiplier gains were allowed for. These drifts were detected by measuring the average muon pulse heights for each detector at various times through the experiment. (The values quoted above were the average pulse heights at the beginning of the experiment.) Third, the linearities of the PM anode outputs (for large pulses) were checked by comparing them with dynode outputs. The accuracies of ionization measurements made as above were estimated as follows. The fwhm for the pulse height distributions for muons for the five scintillators were found to be 40%, 30%, 40%, 40% and 35%, respectively, and these were used to estimate the statistical uncertainties associated with any particular ionization measurement. Systematic errors (such as might be present in the calculated value of 2-23 MeV g-lcm2 for the average muon energy-loss in the scintillators) were expected to be swamped by the statistical uncertainties. As an independent check on the accuracy of the above speed and ionization measurements the following procedure was carried out. A sample of 78 particles that were observed to penetrate the telescope with speeds between 0.4e and 0.55c and which (using the methods described in the next section) were all identified as being protons, deuterons or tritons was considered. For each of these 78 particles the charge was calculated in terms of the measured values of speed and ionization using the equation dE dx

= zZf(v),

250

C.R.

ALCOCK

where f ( t ) is a well-known function that has been tabulated for NE 102 A plastic scintillator by Paul~). The distribution of charges so obtained had a mean value of 0'99e and a fwhm of 0-10e. These results are consistent with what would be expected in terms of the observed fwhm for the pulse height distributions for muon aonlzatlons and for the distrlbution of 1(z14+'c25 ) for muons. They indicate the accuracy with which charge measurements may be made with the telescope. The effective speed of hght allowing for reflections in the plastic scintillators (Cp) was also measured, by using a least squares fit of the muon data points to

Cp(t14--t25)

=

2l sm qS,

where ~b is the angle shown m fig. 1. The angles were obtained from the spark chamber photographs, t ~ a n d t25 as described above. The result w a s Cp ~ 0"58C. This result is used m the next section. In the anal~sls of'" slow particle events', as described m the following section, the angle ~b is needed for each event However, in many slow pamcle events, no spark ts produced m the bottom spark chamber. The angle of ~he spark m the top chamber (qS) was measured to give an estimate of ~b for these events Comparison of ~b and q5 for the muon events (where in essentially all cases sparks occurred m both top and bottom spark chambers) showed that ~b and q5 coincide to within a standard devlanon of 2 °.

4. Method of analysis for slow particle events For the events that are recorded when the trigger clrcmt is set up to accept slow pamcles, as discussed in the previous section, the follo~vmg selection criteria are apphed to determine which events are genuine slow pamcle e~ents (1 e. not multi-particle events.) 1) The spark chamber sparks, and also the pulse heights in scintillators 1-5, have to be consistent with what is produced by a single particle penetrating scintillators 1-5 2) Itl3-t3¢l < 0 . 6 n s . This is an obvious check since scintillators l, 3 and 4 are evenly spaced. This test clearly serves to identify multi-particle events. 3) Since the distances between scintillators 1 and 4, and between 2 and 5, are the same we must haYe T ~ = T2s for single pamcle events. In analysing the data this test may be apphed, taking account of experimental errors, by requiring that [tS"- t~4+ t25 [ < 0"7 ns,

et al.

where 21 sin

t~" = - - , Cp

and ~, l and c v have been defined previously. This test may be used to identify events involving heavy particles (mass > 3 rap) which stop in scintillator no. 5. Approximately 25% of all events are accepted as single particle events. Nearly all events whlch fail to be accepted do so very obviously [i.e., they clearly fail to satisfy (1)]. Once an event has been accepted for analysis the speed and ionization of the corresponding particle are measured as described in sect. 3, and thus the charge is obtained by application of the previously mentioned equation dE/dx = z 2 f(t ) The mass of the particle is obtained as follows. The range R(M,z,L) in a given material of a particle of mass M, charge z, and vetocity r is found using

R(M,z,c)

=

-M - (~) 2 Rp@), Alp

where Rp(t,) is the range of a proton of velocity L~in the same material and Alp is the proton mass. Using, for example, the tables of Rp hsted by Janni13), R(M,z,t) can be found for any particle in the absorbing materml used in the telescope, if M. z and r are known. The charge of a given particle and ~ts velocity above absorber no. 1 are measured. For any " r e a l " ~.alue of M, R(M,z,c) can then be calculated. By subtracting the thickness of absorber no. l, R'(M,z,v') - the range of the particle below absorber no. 1, can be obtained The speed, ~', of the pamcle below absorber no. 1 may now be calculated, and thereupon the ionization produced m scintillator no 5 may be predicted [using the equation dE/dx = z 2 f ( t ')] and compared w~th the measured value. In this way a best estimate of the mass, M, of a pamcle is obtained Confidence levels for each mass measurement made in this manner may be deduced from the accuracies of the speed and iomzatlOn measurementsS). Each mass measurement may be checked to see that st is consistent with the data obtaSned with the bottom spark chamber. If a measured mass is such that R(M,z,c) exceeds the combined thicknesses of absorbers 1 and 2, then clearly a spark should be visible in the bottom chamber which as collinear with the spark m the top chamber, and vice versa.

Two oscilloscope photographs obtained under ""slow particle" operating conditions are shown in figs. 5 and 6. The greater mnlzalions and slower speeds ot these particles compared with those of muons (see

A COSMIC RAY TELESCOPE

fig. 4) are clearly a p p a r e n t . Fig. 5 shows the trace p r o d u c e d by a particle identified as a p r o t o n which was incident on the telescope with an energy o f ( 1 2 2 + 12) MeV. Fig. 6 is the trace for a d e u t e r o n of (196_+20) MeV. A l t h o u g h the d e u t e r o n was incident at a slower speed, it p r o d u c e d less ~onlzatlon in scintillator no. 5 t h a n did the p r o t o n . F o r b o t h o f these events a b s o r b e r no. 2 c o n s i s t e d o f 5.0 g/cm z Fe. The p r o t o n did n o t penetrate this a b s o r b e r ; the d e u t e r o n did.

251

with mass > 3 mp 0 f present) m a y be identified. The a p e r t u r e o f the telescope is such that it is sensitive to fluxes > 10 - l ° p a m c l e s / c m 2 s sr assuming an operating p e r i o d o f ~ l y . The telescope is sensitive to c h a r g e d particles, which m a y or m a y not also interact strongly.

The a p p a r a t u s described is a m o d e r a t e l y wide aperture cosmic ray telescope, sensitive to long-hved, slowing, heavy particles. Charge m e a s u r e m e n t s are possible to an accuracy o f a b o u t 0.05e, a n d p a m c l e s

Two other g r o u p s are c o n d u c t i n g similar searches (for heavy p a m c l e s ) using cosmic ray telescopes. A D u r h a m g r o u p 1~'1s) uses a range telescope in which the speeds o f cosmic ray particles are m e a s u r e d using the response curves o f C h e r e n k o v counters. ~Fhls technique is m o r e sensitive for higher speeds (0"7c-0'8c) t h a n the present telescope. Partly as a consequence, the D u r h a m telescope contains much more materml t h a n the present telescope ( ~ 200 g/cm z as o p p o s e d

Fig. 5. The oscilloscope trace for a proton of (1224-12)MeV

The vertical gain Is 200 mV/dlv, and the time-base 10 ns:'dlv

g. Conclusion

:

Fig. 6 The oscilloscope trace for a deuteron of (1964-20) MeV. Theve mcal gain is 200 mV/div, and the time-base 10 ns,'dlv. Also shown on this photograph is the trace of a multi-particle event.

252

c . R . ALCOCK et al.

to 30 g/cm 2) which greatly increases the chance o f h a d r o n i c interactions. F o r this reason the p r e s e n t telescope m a y be m o r e efficient for detecting new h a d r o n s . C h a r g e m e a s u r e m e n t s m a y be m a d e with the D u r h a m telescope, b u t with less precision t h a n is possible with the present telescope. The larger a p e r t u r e o f the D u r h a m telescope partly c o m p e n s a t e s for these dlsadvantages. A M o s c o w g r o u p 16'1v) uses a range telescope in which the speeds o f the particles are n o t m e a s u r e d , except t h r o u g h the use of C h e r e n k o v anticoincidence counters. Three thick a b s o r b e r s serve to distingmsh p r o t o n s , d e u t e r o n s a n d heavier particles. C h a r g e m e a s u r e m e n t s are n o t possible with this telescope, a n d the small a p e r t u r e w o u l d require very long o p e r a t i n g p e r i o d s to detect rare events. F u r t h e r m o r e , the t o t a l thickness o f the a b s o r b e r s used ( ~ 200 g/cm z) renders it less sensitive to h a d r o n s . A f t e r c o n s t r u c t i o n o f the present telescope was c o m p l e t e d we were i n f o r m e d o f two further telescopes which are being c o n s t r u c t e d to related designs a n d for a similar purpose. One is to be used at the C E R N Intersecting Storage Rings 1s) a n d the other b y a Sydney cosmic r a y group19). This w o r k 1s s u p p o r t e d by grants from the N e w Z e a l a n d University G r a n t s C o m m i t t e e , the G o l d e n K i w i L o t t e r y C o m m i t t e e a n d the A u c k l a n d University R e s e a r c h C o m m i t t e e . The s p a r k c h a m b e r s were k i n d l y supplied by the University o f M m h i g a n via P r o f L. Jones.

References 1) L. W. Jones, Physics Today (May 1973) p. 30. 2) Z. Maki, Prog. Theor. Phys. 31 (1964) 331. 3) F. Gursey, T. D. Lee, and M. Nauenberg, Phys. Rev. 135B (1964) 467. 4) H. Bacry, J. Nuyto, and L. Van Hove, Phys. Lett. 9 (1964) 279. 5) j. Schwinger, Phys. Rev. 135B (1964) 816. 6) M. Han and Y. Nambu, Phys. Rev. 139B (1965) 1006. 7) p. C. M. Yock, Ann. Phys. 61 (1970) 315; and to be pubhshed. s) C. R. Alcock, A Chisholm M. Tyndel and P. C. M. Yock, 13th Int. Conf. Cosmw Rays, Denver (August 1973), paper 409. 9) C. R. Alcock, M. Tyndel and P. C. M. Yock, submitted to Nuovo Clmento (August 1973). lo) O. C. Allkofer, K. Carstensen and W. D. Dau, Paper MU-4, 12th Int. Conf. Cosmic Rays, Hobart (1971). J. M. Paul, Nucl. Instr. and Meth. 96 (1971) 51. 12) T. J. Gooding and H. G. Pugh, Nucl. Instr. and Meth. 7 (1960) 189. J. F. Janni, U. S. Air Force Report AFWL-TR-65-150 (1966). F. Ashton, H. G. Edwards and G. N. Kelly, Nucl. Instr. and Meth. 95 (1971) 109. 15) F. Ashton and G. N. Kelly, Proc. llth Int. Conf. Cosmlc Rays, Ed. A. Semegyi, Supp. Acta Physica (Hungary) 29 (1970) 19. 16) A. M. Galper, V. A. Gomezov, V. G. Klrillov-Vgryumov, Yu. D. Kotov, B. I. Lutchkov and A. M. Rogovski, Soy. J. Nucl. Phys. 10 (1970) 193. 17) A. M. Galper, V. A. Gomozov, V. G. Kirfllov-Vgryumov, Yu. D. Kotov, B. I. Lutchkov and V. N. Yurov, 12th Int. Conf. Cosmic Rays, Hobart (1971) Paper HE-48. 18) A. Zichichi, Invited Talk - 5th Plen0ry Session Int Conf. Instr. High Energy Physics, Frascatl (May 1973). 19) L. S. Peak, private eommumcation.